1 Introduction and review of literatureshodhganga.inflibnet.ac.in/bitstream/10603/10359/8/08_chapter 1.pdf · Supramolecular chemistry has been synonymously referred as molecular

Introduction and review of literature

1

1

Abstract


««««««««««

The present chapter deals with the literature reports available for polyphenyls

including terphenyls, triphenylene and hexaphenylbenzene. A review of literature on

different fluorescent sensors reported for biologically significant cations such as Hg2+, Zn2+

and Cu2+ and anions like F- and CH3COO- is given. In addition to this, chemosensors based

upon chemosensing ensemble approach for various anions have been included. The objectives

of the present thesis are planned on the basis of these reports.

««««««««««


2

1.1 Introduction and review of literature

Supramolecular chemistry as defined by J. M. Lehn is chemistry beyond molecules

which focuses on the structures and functions of chemical systems formed by the

association of two or more species held together by means of non-covalent

interactions.1 Non- covalent interactions are significant in maintaining the three-

dimensional structures of large molecules, such as proteins and are involved in many

biological processes in which large molecules bind specifically but transiently to one

another. Supramolecular chemistry has been claimed to be an emergent field of

research taking its roots in chemistry. According to the definitions of emergences

related to hierarchy or more recently to scope, supramolecular chemistry is shown to

have bottom-up or top-down emergences. The bottom up emergence directly related to

hierarchy by definition, opens up the world of nanochemistry and nanomaterials while

the top-down one, attributable to scope due to the implication of supramolecular

biochemistry. Both emergences lead supramolecular chemistry to become

supramolecular science. Supramolecular chemistry has been synonymously referred as

molecular recognition chemistry. The aim of molecular recognition chemistry is to

design appropriate hosts with binding sites complementary to the guest molecules.

Enzymes, receptors, antibodies, membranes, cells, carriers and channels are some of

the biological examples whose actions depend on molecular recognition.2 Considering

the importance of natural receptors in chemistry, biology, medicine and environmental

studies, a number of synthetic receptors like crown ethers,3 cryptands,

4 spherands,

5

calixarenes,6 porphyrins

7, thiacalixarenes,

8 and cyclodextrins

9 etc. have been reported

in the literature. These synthetic receptors play an important role in metal ion

recognition. Crown ethers are known as phase transfer catalysts10

as their solubility

gets changed on interaction with metal ions whereas cryptands are the bicyclic ligands

offering three dimensional cavities for spherical recognition of the metal ions.4 On the

other hand, spherands5 are the special host molecules designed to offer an octahedral

array of oxygen atoms to bind to a metal centre whereas porphyrins,7

(thia)calixarenes,8 and cyclodextrins

9 have well defined conformational cavities which

have ability to encapsulate guest molecules. The extensive studies of the above

receptors led to the advancement of the systems and the field did not limit itself to

molecular recognition.11

The various concepts like templated synthesis12

, self-

assembly13

, and self-sorting14

have made supramolecular synthesis a powerful tool to


3

construct large and complex chemical architecture from simple building blocks

consisting of well-designed binding sites. Based on these concepts, functional

supramolecules like molecular switches, molecular logic gates, molecular elevators,

valves, springs and supramolecular catalysts were developed.15

Another important

aspect for molecular recognition is the signalling mechanism involved in the detection

process. Among various techniques viz. UV-Vis, cyclic voltammetry, potentiometry,

fluorimetry used for ion sensing, fluorescence is most popular technique owning to its

sensitivity, specificity, and real-time monitoring with fast response time.16

It is the

simplest technique that can transmit information on events occurring at a molecular

scale to the macroscopic world. A fluorescence sensing system must include two

components, an ionophore and a fluorophore, which can be independent species or

covalently linked in one molecule.17

The ionophore is required for selective binding of

the substrate, while the fluorophore provides means of signalling this binding, whether

by fluorescence enhancement or quenching. The design of fluorescent sensor involves

two approaches. The classical approach involves the covalent linking of a fluorescent

fragment to a receptor, which displays specific binding tendency towards a given

analyte. Another approach is a kind of competitive approach named as chemosensing

ensemble method in which the fluorescent indicator is bound to the receptor by non-

covalent interactions and the fluorescence of the indicator is enhanced or quenched by

the receptor. On displacing indicator by the analyte, the fluorescence of the indicator

recovers. The binding affinity difference between indicator receptor and analyte

receptor plays an important role in the detection of analyte in this approach.

Polyphenyls like terphenyl, hexaphenyl, and triphenylenes, having a great potential in

material chemistry18,19

owing to their role as liquid crystalline materials,16

molecular

scale devices,20

and molecular receptors21

are the another important class of molecular

scaffolds. Although they play a significant role in supramolecular chemistry but their

use as a scaffold for cation and anion sensors is limited.22,23,24

So, in the present

investigation, we planned to explore polyphenyls particularly terphenyl (o-terphenyl)

as the basic scaffold appended with appropriate fluorophores and binding sites in order

to synthesize receptors for sensing of different cation and anions. The sensing

behaviour of these receptors was studied toward various metal ions and anions by UV-

Vis, fluorescence and 1H NMR spectroscopy. Before presenting the results of our

findings, a brief review of literature about polyphenyls is given below:


4

1.2 Literature reports based on polyphenyls

Polyphenyls including terphenyls and hexaphenyls are nonplanar molecules consisting

of benzene rings linked together by single bonds. These are the simplest organic

compounds since their electron bands are related to one type of transition (p–p*). In

particular, terphenyl molecule consists of a central aromatic ring flanked by two other

aryl groups, which may be arranged in 1, 2 (ortho)- (a), 1, 3 (meta) (b)- or 1, 4 (para)-

(c) configurations (figure 1.1).

The chemical investigation of naturally occurring terphenyls started in 1877. Most of

the natural terphenyls are p-terphenyl derivatives. Very few m-terphenyl derivatives

occur naturally, and o-terphenyls have not been found in nature until now. Terphenyl

molecules are employed in a wide range of important applications, from the discovery

of novel scaffold in medicinal chemistry to the fabrication of advanced materials with

novel electrical and optical properties.

The biological significance of terphenyl based molecules has been well exploited in

the past decade. These are known to act as potent immunosuppressant, and show

neuroprotective, antithrombotic, anticoagulant, specific 5- lipoxygenase inhibitory and

cytotoxic activities.17

Compound 1 for instance, inhibited fibrin formation induced by

thrombin25

whereas 2 inhibited platelet aggregation ex vivo.26

Thus, both forms of o-

terphenyls function as anticoagulants.

1 2

OCH2COOMe

OH

Figure 1.1 Schematic representations of ortho (a), meta (b), and para (c) configurations of

terphenyls

a b c


5

A series of m-terphenyl amines 3a-h were synthesized and evaluated as a novel class

of cyclooxygenase (COX) inhibitors. Structure–activity relationships (SAR) were

investigated by functional group modification at the para-position of the C-10 and C-

20 phenyl substituents on the central aromatic ring. Some of the p-terphenyl

derivatives 4-7 were shown to exhibit antioxidant activities.27

Terprenin exhibited a

remarkable suppressive effect on in vitro IgE production of human lymphocytes.

The awareness of practicability of this moiety enhanced the quest for atom-efficient,

practical and scaleable routes to the synthesis of terphenyls for the sake of foreseeable

future. The synthesis of artificial terphenyl systems has been achieved most efficiently

using metal catalyzed cross-coupling reactions viz. Suzuki-Miyaura, Negishi, Kumada,

Stille coupling etc.28

The utilization of terphenyls in the synthetic chemistry began with the exploitation of

m-terphenyls as ligand, for stabilization of main group elements in organometallic

chemistry.29

The advantages of using these ligands were easy large-scale synthesis and

additional p-arene interactions of bonded metal atoms to the flanking aryl groups.

Considering this property of terphenyls, the chemists extended its scope to the

stabilization of late transition metal ions and lanthanides. This led to the synthesis of

terphenyl-based organolanthanide complexes.

Rabe et al. reported first example of donor functionalized terphenyl-lanthanide

complexes.30

The molecular structures of terphenyl derivatives of trivalent ytterbium,

yttrium, and samarium of composition [DanipYb(μ2-Cl)2(μ3-Cl)Li(THF)]2, 8 and

[DanipY(μ2-Cl)2(μ2-Cl)Li(THF)2]2, 9 are reported where Danip is 2,6-di(o-

anisol)phenyl. No symmetry higher than triclinic was observed in the diffraction data.

R1O OR2

R3O OR4

R5O OR6

4-7

4; R1 = R2 = H, R3 = R4 = R5 = Ac, R6 = COPh

5; R1 = R2 = H, R3 = R4 = R5 = Ac, R6 = COCH2CH2Ph

6; R1 = R2 = H, R3 = R6 = COCH2CH(OCOCH3)CH3, R4 = R5 = Ac

7; R1 = R2 = R4 = R5 = H, R3 = COCH2CH(OH)CH3, R6 = Ac

3a-h

a; R1 = R2 = H

b; R1 = CH3, R2 = H

c; R1 = R2 = CH3

d; R1 = Cl, R2 = H

e; R1 = R2 = Cl

f; R1 = C2H5O, R2 = H

g; R1 = R2 = C2H5O

h; R1 = CH3O, R2 = H

R2R1

NH2


6

The molecular structures of terphenyl derivatives of trivalent ytterbium, thulium, and

yttrium of general composition DnpLnCl2(THF)2 (Dnp) 2,6-di(1-naphthyl)phenyl]

10a-c were reported.31

The structures of monomeric complexes exhibited distorted trigonal-bipyramidal

coordination environments at the metal center, with the two oxygen atoms of the

tetrahydrofuran ligands occupying the axial positions of a trigonal-bipyramidal

coordination polyeder. Rabe and coworkers32

further extended their work and reported

a number of 2, 6-dimesitylphenyl (Dmp)-based trivalent compounds 11 and 12 of

ytterbium and yttrium, respectively which were stable in tetrahydrofuran at ambient

temperature.

Terphenyl derivatives also exhibited unique optical33

and electronic34

properties that

are widely exploited in the fabrication of advanced materials, notably as single- or

double-stranded helical polymers,35

liquid crystals for organic light-emitting diodes

(OLEDS),36

polymer-based photovoltaic cells37

and organic field effect transistors.38

[DanipYb(μ2-Cl)2(μ3-Cl)Li(THF)]2 [DanipY(μ2-Cl)2(μ2-Cl)Li(THF)2]2 [DanipYb(μ2-Cl)2(μ3-Cl)Li(THF)]2 [DanipY(μ2-Cl)2(μ2-Cl)Li(THF)2]2

8 9

DnpLnCl2(THF)2

10a-c DmpYbCl2(THF)2

11

DmpYCl2(THF)2

12


7

They also show promise (as dyes) for laser applications39

and molecular electronic

devices.40

With the beginning of 21st century, utility of m-terphenyls in host guest chemistry took

focus of chemists. Uppadine et al. reported dithiocarbamate ligands based on m-

terphenyl scaffold 13.41

These ligands self-assembled with zinc (II), nickel (II) and

copper (II) ions to afford neutral, dinuclear metallomacrocycles 13a in varied yields.

Intramolecular coordination of bipyridyl guests had been investigated with the zinc (II)

containing macrocycles. NMR spectroscopy and

FAB mass spectrometry demonstrated the formation of 1: 1 inclusion complexes with

4, 4’-bipyridyl. Farrell et al. reported another m-terphenyl based macrocyclic

dicarboxylate compound 14 which could be used as potential ligand or hosts for

transition metal ions.42

Six functionalized bis(phenylene ethynylene)-p,p-terphenyls (BPETs) 15-20 were

reported as potential molecular electronic devices.40

The molecules containing mono-

and dinitro terphenyl cores, were rationally designed based on the electronic properties

as found in oligo (phenylene ethynylene)s (OPEs). The improvement of electronic

properties in comparison to OPEs was possible by using BPETs due to a higher

rotational barrier between the central aromatic rings of the compounds prepared.

BPETs cores were functionalized with nitro groups and with different metallic

adhesion moieties (alligator clips) to provide new compounds for testing in the

nanopore and planar tested structures.

HN

HN

HN

HN

OO

O O

Na

Na

14

13

BuN NBuX

S SSSK+

K+

BuN NBuX

S SSSM M

NBuBuNX

SS S S

M = Zn2+, Ni2+, Cu2+

X = H, COOMe

13a


8

Clayden et al. reported amide-substituted terphenyl 21 which adopted a well-defined

conformation that was able to allow the transmission of stereochemical information

from a controlling centre to a reaction site 11 bond lengths away.43

The above design

provided a model of how extended polymeric systems might be used to communicate

binary information. Biphenyl-2, 2’-dicarboxamides prefer conformations in which the

amide groups lie anti across the biaryl C–C bond. The conformational uniformity of

21 indicated that the amide groups were in communication with one another and, in

principle, could form a relay system for carrying stereochemical information across

the terphenyl system if a means of controlling absolute stereochemistry were

introduced at one end of the oligophenylene chain.

It was in year 2008 when mesogenic behaviour of terphenyls was exploited. Steinke et

al. synthesized unsymmetrical crown ether derivatives 22 and 23 with one lateral o-

terphenyl unit bearing alkoxy and ester side chains, respectively, using methoxymethyl

(MOM) protected aryl bromide as a starting material.22

They studied the influence of

different substituents on the mesomorphic properties of mesogens and it was found

SAcAcS

SAc

SAc

NO2

AcS

NO2

NO2

NO2

NO2

20

18

19

SAcAcS

NO2

O2N

CN

NO2

O2N

SAc

NO2

O2N

15

17

16

N

N

N

N O

O

O

O

21

OO

OO

O

O

O

O

O

n

n

n

n

(n = 4-13)

122

OO

OO

O

O

O

O

O

O

n

O

O

O

n

n

n

223


9

that uncomplexed derivative 22 with chain lengths of atleast C12 formed smectic

mesophases, while complexation with alkali metal salts induced a transformation to

columnar mesophases. The complexes of 23 with a minimum alkyl chain length of C10

formed columnar rectangular mesophases at lower temperature and columnar

hexagonal mesophases at higher temperatures. Kissel et al. synthesized m-terphenyl

derivatives 24-26 having readily functionalizable hydroxyl group as well as symmetric

AA- type or unsymmetric AB- type halide termini which are considered to be used for

novel polymer synthesis.44

The former was copolymerizing with other BB-

type monomers having e.g. diboron functionalities, whereas the latter served as a

precursor of monomers having e.g. one bromo and one boron functionality which were

able to homopolymerize by Suzuki polycondensation (SPC). Terphenyl based

receptors 27a-c (TC) containing (polar) ethereal canopies were synthesized by Rathore

et al.45

The receptors were capable of directing potassium cation to the central benzene

ring for cation–p interaction and hence were reported as important component for

stabilization of tertiary structures of various proteins. A comparison of the relative

binding of K+ amongst the three terphenyl crown derivatives by competition

experiments (using 1H NMR spectroscopy) revealed that the binding efficiency of K

+

decreased in the order of o-TC > p-TC > m-TC.

A series of crown ethers 28a-f based on terphenyl moiety have also been

synthesized.46

Derivatives with shorter alkyl chains C5H11 – C8H17 showed different

phase behaviour as compared to the corresponding compounds with increased chain

length C9H19-C10H21. The fan shaped textures were obtained in shorter alkyl chain

derivatives only upon rapid cooling whereas derivatives with C9 and C10 side chains

formed stable columnar textures upon slow cooling. Besides, the complexation

behaviour of 28a-f with potassium significantly shifted the clearing points to higher

X1 X2

OH

24-26 24; X1 = X2 = I

25; X1 = X2 = Br

26; X1 = I, X2 = Br

a; o-TC

b; m-TC

c; p-TC

O O

O

O

O

27a-c


10

temperatures and also increased the mesophase stability in the cooling cycle. Later, the

cycloaromatization of the o-terphenyl units in 28a-f to the corresponding triphenylene-

substituted crown ethers 29a-f was done which led to improved mesophase

temperature ranges.47

Furthermore, the study of the mesophase properties of potassium

complexes of compounds 29a-f with particular emphasis on the influence of the

counterion was studied. It was found that the bridging anions like BF4- and PF6

- led to

higher aggregation in solution as compared to soft anions like Br- and Cl

- ions.

Chen et al. reported disubstituted propiolates 30a-b bearing chromophoric

terphenylene mesogenic groups, namely, 4’-cyano-4-terphenylyl-2-octynate and 4’-

methoxyl-4-terphenylyl-2-octynate, where the terphenyl groups were connected to the

C-C bond through ester linkage directly.48

The effects of the structural variations on

the mesomorphic, light emitting behaviours and thermal properties of the disubstituted

propiolates, and the influence of the structures on the polymerization had also been

investigated. The incorporation of the terphenylene groups and –C≡CCO2– unit into

the molecular structures could allow better conjugation of the monomers, which

endowed disubstituted propiolates with strong UV light absorption and high

photoluminescence.

Terphenyls are also known to be an important precursor for the synthesis of

triphenylenes which have great potential in supramolecular and material chemistry.18,19

a; R = CN

b; R = OCH3

RO

O

30a-b

28a-f

O

O

O

OO

O

OR

OR

OR

OR

RO

OR

RO

OR

a; R = C5H11

b; R = C6H13

c; R = C7H15

d; R = C8H17

e; R = C9H19

f; R = C10H21

29a-f

O

O

O

OO

O

OR

OR

OR

OR

RO

OR

RO

OR

a; R = C5H11

b; R = C6H13

c; R = C7H15

d; R = C8H17

e; R = C9H19

f; R = C10H21


11

Triphenylene is the planar analogue of terphenyl. The scientists worked a lot to

develop various methods of cyclization in order to synthesize this versatile material

from terphenyl. Scholl reaction is the one of the oldest C-C bond forming reactions

and has been used a lot for oxidative cyclodehydrogenation of a variety of o-

terphenyls to produce the corresponding planar polyaromatic hydrocarbon,

triphenylenes (figure 1.2).

Rathore et al. employed a new method of cyclization using DDQ/H+ system.

49 The

system in comparison to the most commonly utilized oxidants such as FeCl3, MoCl5,

or SbCl5 obviates many problems such as chlorination of the polyaromatic products

and the usage of a large excess of oxidants. Recently, Bhalla et al. reported new

terphenyl derivatives 31a-b having OTBS groups which can be easily converted to

hexasubstituted triphenylenes 32a-b during the deprotection of terphenyl derivatives

31a-b using tetrabutylammonium fluoride.50

It was concluded that the presence of

increased negative charge on phenolate oxygens after deprotection of OTBS groups in

terphenyls 31a-b provide an optimal amount of directing ability and electron density

to complete cyclization.

Triphenylene derivatives are most well known representatives of discotic liquid

crystals forming columnar mesophases manifesting its material properties.51

There are

also some reports in the literature which exploit metal complexation behaviour of

triphenylene. Compound 33 has been synthesized and utilized as a caffeine sensor by

Figure 1.2 Scholl reaction representing oxidative cyclodehydrogenation of

o-terphenyls to triphenylene

AlCl3

CuCl2

32a; R = H

32b; R = CH3

31a; R = TBS

31b; R = CH3

ORRO

TBSO

TBSO

OTBS

OTBS

F-

ORRO

HO

HO

OH

OH


12

Waldvogel et al.52

Compounds 33a-b comprising triphenylene based ketals were

employed as flourimetric receptor molecules. Addition of caffeine 34 to a solution of

receptor resulted in a bathochromic shift of the emission bands by 2 nm and increased

signal intensity, especially at 388–393 nm. The titration spectra for caffeine revealed

several isosbestic points, indicating the presence of two fluorescent species.

Jiang et al. reported triphenylene-fused metal trigon conjugates (TSZn) 35, where

multinuclear six metal sites were connected to one another via conjugation with a

triphenylene core.24

Compound 35 emitted green photoluminescence with a

significantly enhanced quantum yield and allowed intramolecular energy migration

between metal sites as a result of extended π-conjugation. In contrast to the 1D array

reported for triphenylene derivatives, self-assembly of 35 led to the construction of a

2D sheet.

Givelet et al. reported that triphenylene-based host 36 can selectively bind catechols in

chloroform.53

In compound 36, the bulky phosphinate outside rim (pus) prevented core

self-aggregation and favoured H-bonding. Also, a hydrosoluble multivalent

triphenylene 37 was synthesized, and its binding properties were tested towards

aliphatic ammoniums in phosphate-buffered water.54

Selective recognition was

observed for acetylcholine (ACh) and its agonists’ nicotine and epibatidine. An

infrared study of 37: ACh association indicated that ionic pairing occurred between the

guest ammonium and the host carboxylate, which was assisted by desolvation of the

N

N

N N

N

N

O

MO

M

O

O

M

OO

C12H25O

C12H25O

OC12H25

OC12H25

OC12H25C12H25O

35

34

N

N N

N

O

O

OO

O

O O

ONH HN

HNNH

O O

R R

a; R =

HN

HN

R

O

b; R =

33


13

guest ester. This might explain the fact that choline, a more hydrophilic molecule, did

not interact with host 37.

Another important class of polyphenylenes is hexaphenylbenzene (HPB) in which

multiple contiguous aryl groups are attached to the aromatic core (figure 1.3a). The

nonplanar structural conformation with large torsional angle between aryl groups

hinders the extent of conjugation and hence disfavour the intermolecular p-p and C-H...

p interactions (figure 1.3b). It lowers the degree of self association, increases HOMO-

LUMO gaps, and solubility as compared to the planar analogues such as HBC. These

characteristic properties make derivatives of HPB increasingly useful in various areas

of science and technology.55

The feasibility of modification of their peripheries to

incorporate suitable electroactive functionalities make them attractive starting for

construction of nanometre sized materials with novel light emitting and charge

transport properties.56

There are many reports in the literature where HPB is used as a

basic core for organising molecules with different electronic and optoelectronic

properties.

Rathore et al. contributed much to the exploration of material as well as the

complexation properties of hexaphenylbenzene based macromolecule 38 bearing

37

O

O

O

O

O

O COOH

COOH

COOH

HOOC

HOOC

COOH

Figure 1.3 Diagrammatic representations for hexaphenylbenzene (HPB)

b a

36

O

O

O

O

O

O

P

P

P

P

P

P

Ar

Ar

ArAr

ArAr

Ar

Ar

ArAr

Ar

Ar

O

O

O

O

O

O

Ar = -C6H4-pOMe


14

multiple redox-active units.57

The molecule acted as electron sponge towards a number

of electron donors. A dendritic molecule 39 was synthesized in which six

tetraphenylethylene moieties were connected to a central benzene ring in such a way

that one of the phenyl rings of each tetraphenylethylene was also part of the propeller

of the hexaphenylbenzene core.58

The cyclovoltammogram (CV) studies showed

multiple oxidation waves and intense charge resonance transition in near-IR region in

cation radical spectrum of compound 39 concluded the presence of the single hole

delocalization via electron transfer over 6 identical redox-active centres arranged

cofacially in circular array, which was confirmed by carrying out chemical coulometry

using a hindered naphthalene cation radical salt and comparing the CV spectrum of

monosubstituted derivative. Such materials have potential applications in photonic

devices. They also synthesized compound 40 and investigated the cyclic and square

wave voltammetry of compound 40 to confirm the ejection of six electrons (oxidation)

at a constant potential which can help in fabricating photonic devices.59

HPB

derivative 41 has been synthesized by trimerization of a diarylacetylene in which the

ethereal oxygens were tied together with a polymethylene bridge present on single

face.60

Compound 41 consisting of bipolar receptor site, allowed a remarkably

efficient binding of a single potassium ion because of its synergistic interaction with

the polar ethereal fence and with the central benzene ring via cation-δ interactions, a

phenomenon that was well established in gas phase61

and in solid state62

and was

known to play an important role in the stabilization of tertiary structures of various

proteins.63

38

OMe

OMe

OMe

OMe

MeO

OMeOMe

OMe

OMe

OMe MeO

OMe

39

PhPh

Ph

Ph

Ph

Ph

Ph

Ph

Ph

PhPh

Ph

Ph

Ph

Ph

Ph

Ph

Ph


15

Shionoya et al. reported a hexamonodentate ligand 42 in which Ag+ and Hg

2+ got

assembled on two different binding sites between the two disks of compound 42.64

These binding sites had completely different affinity for these metal ions. This

approach indicated that the systematic controlled synthetic strategies keeping the

electrostatic repulsions minimum can easily tailor the binding affinities of different

sites for different metals. Using hexaphenylbenzene, trismonodentate ligand 43 was

prepared that forms 10 structurally equivalent coordination capsules [M6L8]12+

with a

series of divalent d5-d

10 transition metal ions, M

2+ (M= Mn, Fe, Co, Ni, Pd, Pt, Cu, Zn,

Cd and Hg) through self assembly.65

Resulting complexes had an octahedral structure

in which six metal ions lay on the apices and eight sides were occupied by eight

ligands.

Same group reported the isostructural formation of M6(44)8 complexes from ten kinds

of divalent d5-d10 metal ions (M; Mn2+

, Fe2+

, Co2+

, Ni2+

, Pd2+

, Pt2+

, Cu2+

, Zn2+

, Cd2+

,

and Hg2+

), and the tightly packed capsule-shaped structure of Hg6(44)8 complex was

determined by 1H NMR, ESI-TOF mass, and X-ray analyses. The self-aggregation of

positively charged mercury cage complex of trismonodentate ligand 44 and its

32

O

O

O

O

O

O

41

Fe

Fe

Fe

Fe

Fe

Fe

40

43

N

NN

N

O N

O

N O

NO

N

ON

O

42

N

N

N

44


16

interconversion to neutral mercury capsule complex in response to 44/Hg2+

was

demonstrated. This dynamic interconversion resulted in molecular fluorescence

switching between fluorescent cage complex and non fluorescent capsule.66

The same

molecule 44 has also been reported as aggregate 446 formed from hexagram-shaped

amphiphile molecules 44 in aqueous methanol.67

As the building block 44 has a

hydrophobic hexaphenylbenzene core and three hydrophilic 3-pyridyl groups, 44

formed aggregates in H2O/CH3OH (1:3) through the hydrophobic effect, while it

existed as a monomer in pure CH3OH. A smaller sized spherical molecule,

adamantane, served as a template guest for the formation of a fourfold, tetrahedron-

shaped capsule 3ϵ444, resulting in significant changes in the capsule structure.

1.3 Polyaromatic receptors as fluorogenic cation sensors

Since the past few decades, the development of chemosensors capable of recognizing

and sensing cations has attracted attention of researchers worldwide.68

The selective

signalling of hard and soft transition metal ions has potential analytical applications in

many fields like chemistry, medicine, biology and environment.69

Among various

techniques viz. UV-Vis, cyclic voltammetry, potentiometry, fluorimetry used for ion

sensing, fluorescence is most popular technique owning to its sensitivity, specificity,

and real-time monitoring with fast response time.70

In fact, a variety of fluorescent

chemosensors of hard metal ions have already been synthesized and reported in the

literature. In order to accomplish fluorimetric detection of ions, the careful selection

of fluorophore is of prime importance. Among the numerous fluorimetric sensors for

cation recognition, the sensors that monitor toxic heavy metal ions like mercury,

copper, silver, zinc, iron (III), lead are particularly important as these are biologically

and environmentally significant. Since the work in the present investigation is focused

on the fluorogenic sensors for mercury, copper and zinc ions, a few representative

examples of chemosensors involving these metal ions are reviewed below:

1.3.1 Fluorogenic sensors for mercury ions

The development of highly selective sensors for mercury ions has attracted a great

interest as pollution due to mercury poses severe risks for human health and

environment.71

Typically, mercury exists in inorganic forms or as organic mercury

compounds (RHg(II), R = Me, Et, Ph). Compared with inorganic analogues, organic

mercury species are much more neurotoxic and can easily penetrate the blood–brain

barrier.72

Therefore, monitoring mercury is important for both environment and


17

human health. Several sensors have proven to be effective tools for monitoring

different forms of mercury including biosensors, chemical sensors, nanosensors,

microcantilever sensors and piezoelectric sensors. But owing to the importance of

fluorimetry, in the present review we shall focus on fluorescence based sensors.

Depending upon the fluorescence response, these sensors have been divided into two

categories (i) ‘turn-off’ and (ii) ‘turn-on’ sensors in which fluorescence of the receptor

is switched ‘off’ and ‘on’, respectively on addition of external analyte.

‘Turn-off’ mercury sensors

Wang et al. prepared 2, 3-bis(1H-pyrrol-2-yl)quinoxaline-functionalized Schiff bases

45 and 46 and characterized them as fluorescent sensors for mercury(II) ions.73

The

binding properties of 45 and 46 toward different cations examined by UV-vis and

fluorescence spectroscopy indicated the formation of 1: 1 complex between compound

45/46 and mercury (II) ion with association constant of (3.81 ± 0.7) × 105 M

−1 for 45

and (3.43 ± 0.53) × 105 M

−1 for 46. Both compounds displayed selective and sensitive

fluorescence quenching responses toward Hg2+

ions in aqueous solution. Moon et al. 74

reported a Hg2+

-selective cyclam derivative 47 which showed an ‘on-off’ type

signaling behaviour due to the conformational changes induced by metal ion from

folded to open-winged conformations by exploiting the two nearby appended pyrenyl

fluorophores.

A chemically programmed antibody sensor 48, consisting of a monoclonal antibody

EP2-19G2 (a) and stilbenyl boronic acid cofactor (b), has been utilized as sensor for

mercury detection.75

The fluorescent antibody sensor generated an intense powder blue

fluorescence when bound to the stilbenyl boronic acid cofactor; however, it was

quenched in the presence of Hg2+

ions.

47

N N

NN

HN

NH

O ON

N NH

NH

N

Nn

45; n = 1

46; n = 2


18

Corroles are the potential carriers for preparation of optical chemical sensors. On the

basis of this, optode 49 based on 5,10,15-tris(pentafluorophenyl)corrole [H3(tpfc)] as a

fluorophore has been reported as a Hg2+

sensor.76

The fluorescence intensities of the

optode membrane decreased with the increase in the concentration of Hg2+

ions.

Costero et al. reported 4, 4’ substituted biphenyl coronands 50-53.77

Among these, 52

showed high selectivity for Hg2+

ions. The presence of six oxygen atoms in the cavity

gave rise to stronger complexes with mercury whereas ligand 53 where two oxygen

atoms have been substituted by nitrogen gave rise to its stronger complexes with Cd2+

and Ni2+

ions.

Praveen et al. reported thiacalix[4]arene derivative 54 of 1, 3-alternate conformation

bearing four quinolinoloxy groups through propyl chains. The metal ion-binding and

fluorescence-sensing properties were investigated in both THF and 10% H2O-THF

systems.78

The designed ligand exhibited pronounced Hg2+

-selective ‘on-off’ type

fluoroionophoric properties among the representative transition and heavy metal ions.

Ha-Thi et al. reported the photophysical and complexing properties of another

molecule 55 having phosphane sulphide group which was found to show high

detection sensitivity for Hg2+

ions.79

Addition of mercury ions induced a 15 nm

50 51 52 53

COO

COO

O

O

O

O

NMe2

NMe2

COO

COO

O

O

N

N

NMe2

NMe2

O

O

O

O

NMe2

NMe2

O

OO

O

N

N

NMe2

NMe2

O

O

49

NH HN

NNH

F

F

F

F

F

F

F

F

F

F

F F

F

FF

48

NH

OH

O O

B

OH

OH

a

b


19

bathochromic shift of the absorption spectra however a strong decrease of the

fluorescence was observed upon mercury complexation, which can be explained in

terms of electron transfer from the excited fluorophore to the complexed mercury

cation.

A perylene based molecule, N, N’-dideoxythymidine-3, 4, 9, 10-perylene-

tetracarboxylic diimide (TT-PTCDI) 56 has been reported as a ‘turn-off’ Hg sensor.80

The strong intermolecular π-π interaction between the PTCDI backbones was

responsible for fluorescence quenching which was further caused by complexation

with Hg2+

ions. The selective complexation between thymine and Hg2+

has been

employed successfully to develop selective sensors for the detection of Hg2+

ions

based on fluorescence resonance energy transfer (FRET) and a colorimetric method.

The sensing mechanism was primarily based on fluorescence quenching induced by

molecular association and aggregation, which in turn was caused by linear

complexation with Hg2+

ions.

A visible near-infrared chemosensor 57 based on heptamethine cyanine dye (MCy-1)

for mercury ions was successfully devised and characterized by Zhu et al.81

A large

red-shift (122 nm) of the absorption maximum of MCy-1 was observed making the

naked eye detection possible. The fluorescence of compound 57 quenched markedly

with the addition of Hg2+

ions.

54

S

OO

OOS SS

O ON N

O O

N N

NN

O

O

O

O

O

O

OH

N

HN

HO

O

OCH3

H3CO

O

56

P P

O

O

O

O

S S

COOEt

EtOOC

COOEt

EtOOC

33

3

3

Hg

55


20

Kumar et al. reported compound 58 as a ratiometric chemosensor for Hg2+

based on a

calix[4]arene of partial cone (paco) conformation possessing a dansyl moiety.82

In the

presence of Hg2+

ions, the receptor 58 underwent fluorescence quenching at 502 nm

and showed the formation of two new blue-shifted bands at 412 and 435 nm. The

compound behaved as a fluorescent molecular switch upon chemical inputs of Hg2+

and Cu2+

ions. They also synthesized compound 59 as ‘on-off’ switch based on

thiacalix[4]arene of 1,3-alternate conformation appended with pyrene as

fluorophore.83

In the presence of various metal ions, the fluorescence of receptor was

found to be quenched by Hg2+

ions. Compound 60 based on thiacalix[4]arene of 1,3-

alternate conformation bearing two dansyl groups and a crown-5 ring behaved as an

‘on–off’ reversible switch for two chemical inputs Hg2+

and K+ ions and mimicked a

molecular level keypad lock in the presence of F- ions.

84 8-hydroxyquinoline based

chemosensor 61 has been reported by Keranen and Vaswani as a highly selective ‘on-

off’ fluorescent sensor for Hg2+

ions in unbuffered water.85

On addition of only 4

equiv. of Hg2+

ions, the fluorescence of compound was fully quenched.

58

OR

O

OH OR

HN

S

N

OO

R = -CH2CH2CH3

61

NO

OCH3O

n

HN NH

O O

OK

O

OK

O

OO

OK OK

62

59

S

OO

O

But

But

But

But

O

SSS

NN

O

O

O

57

S

N

S

OO

N N

-O3S SO3H

60

S

OO

O

But

But

But

But

OS SS

NH HN

OO

O

S SO O O O

N N


21

A novel water-soluble polyfluorene (PFA) 62 with two anionic L-aspartic acids in its

side chain of every repeated fluorene unit has been reported by Qin et al.86

PFA

showed excellent selectivity for Hg2+

ions over most alkaline-earth cations, first-row

transition-metal ions, group 12 metal ions, and common heavy-metal ions.

Niamnont et al. reported water-soluble fluorescent dendritic compounds 63-65

composed of phenylene-ethynylene repeating units and anionic carboxylate or cationic

ammonium peripheral groups.87

The effects of surfactants on the photophysical

properties of 63-65 were studied. The compounds 63-65 exhibited emission peaks at

454, 485, and 489 nm in the absence of Triton X-100. The addition of surfactant

caused the emission bands to blue-shift by 20, 47, and 68 nm, respectively. Besides the

effect of metal ions on the fluorogenic behaviour of compounds were also studied.

Only the fluorescence of compound 63 got significantly quenched by Hg2+

ions. The

quenching effect may involve selective formation of 63·Hg2+

complex and efficient

energy transfer between the fluorescent units in compound 63 to this complex at the

periphery.

Cheng et al. reported two ratiometric fluorescent chemosensors, 66 and 67 based on

intramolecular charge transfer (ICT) as a signaling mechanism.88

Upon addition of

Hg2+

ions, both probes displayed apparent luminescence colour changes, which could

be observed by naked eyes under a UV lamp. Dai et al. reported the azathia-crown

ether based receptor 68 having dansyl moiety as a fluorophore.89

The ion selective

signaling behaviour of the sensor 68 was investigated in CH3CN-H2O (1:1, v/v) by

N

N

N

N

3I-

65

N

HOOC

COOH

COOH

N

COOHHOOC

COOH

COOHCOOH

HOOC

63

64


22

fluorescence spectroscopy. It exhibited remarkable fluorescence quenching upon

addition of Hg2+

, which was attributed to the 1:1 complex formation between 68 and

Hg2+

, while other selected metal ions induced basically no spectral changes. Upon

excitation by light, there was an ICT from the electron-rich group (dimethylamino) to

the electron-withdrawing group (azathia-crown ether) before addition of Hg2+

resulting

in strong fluorescence. After addition of Hg2+

ions, owning to the interaction between

the fluorescent sensor 68 and Hg2+

, the above-mentioned ICT was suppressed. As a

result, the fluorescence quenching was observed.

Ruan et al. reported 7-nitrobenzo-2-oxa-1,3-diazole (NBD) derived compound 69a-b

bearing triazole binding site which worked as colorimetric and fluorescent sensor for

Hg2+

ions.90

Among the metal ions examined, only Hg2+

caused significant

fluorescence quenching in EtOH/HEPES.

A highly sensitive and specific terbium chelate probe 70 comprising of a quinolone

based dye molecule (cs124) as the light-absorption antenna and a

polyaminocarboxylate-based chelator (DTPA) for binding metal and lanthanide ions

was reported by Tan et al.91

Cs124–DTPA–Tb exhibited high luminescence in

aqueous solution along with high interacting tendency with Hg2+

ions. In the absence

of Hg2+

, cs124–DTPA–Tb emitted typical spectra of Tb3+

via an intramolecular energy

transfer from cs124 to Tb3+

. While in the presence of Hg2+

, Tb3+

got replaced by Hg2+

to form a cs124–DTPA–Hg chelate, resulting in fluorescence quenching of cs124–

DTPA–Tb.

Thakur et al. reported two simple triazole based multisignalling compounds 71 and 72

for selective detection of Hg2+

ions.92

Both compounds 71 and 72 behaved as very

selective redox (ΔE1/2 = 217 mV for 71 and ΔE1/2 = 160 mV for 72), chromogenic, and

fluorescent chemosensors for Hg2+

ions in an aqueous environment.

S

N

N

S

O

S

O

O

68

NO

N

O2N NH

N

NN

R

COOMe

NHBoc

NHCbz

COOMea; R =

b; R =

69a-b

N

S

S

R

R

66; R = C2H5O

67; R = CN


23

Koteeswari et al. recently reported amidothiourea linked acridinedione derivatives

ADD 73-75 which selectively detected Hg2+

in unbuffered aqueous solution under

broad pH range with both single- and two-photon excitation.93

Addition of Hg2+

ions

to an aqueous solution of ADD did not lead to any significant change in the longer

wavelength absorption maximum indicating the absence of any ground-state

interaction between the Hg2+

reaction centre and ADD chromophore. The

corresponding fluorescence spectrum showed a 92-fold fluorescence quenching,

without any spectral shift. The high thiophilicity of Hg2+

promoted desulfurization

leading to the formation of dosimetric product, 1, 3, 4-oxadiazole. Due to the removal

of thiocarbonyl group, electron density around the aniline moiety gets increased,

which promoted an efficient intramolecular through-space photo-induced electron

transfer (PET) from the aniline moiety to the relatively electron deficient excited state

ADD fluorophore.

A (S)-BINOL-based sensor 76 incorporating triazole moieties was prepared via click

reaction, which exhibited the Hg2+

selective switching-off type and Ag+ selective

76

O

O

N

NN

NN

N

73-75

N

O O

R

HNO

NH

CNH

S

X

73a; R= -H, X= -H

73b; R= -H, X= -OCH3

73c; R= -H, X= -NO2

74; R= -CH3, X= -H

75; R= -p tolyl, X= -H

NH

HN

CH3

ON

COO-

N COO-

N

COO-

COO-

O

Tb3+

70

71

Fe

O

N N

N

O

N N

N

H

H

NH

OEt

O

O

HN

OEt

O

O

72

Fe

O

N N

N NH

OEt

O

O


24

switching-on signalling behaviour.94

The fluorescence quenching mechanism of 76 by

Hg2+

may be attributed to the heavy atom effect or the ‘on-off’ PET (photo-induced

electron transfer).95

The fluorescence enhancement mechanism of 76 by Ag+ is

probably regarded as the off-on PET mechanism. When Ag+ is bound by two triazole

nitrogen atoms of the receptor, the anthracene units may behave as PET donors

whereas the triazole group acts as electron acceptors. Thus 76 acted as a molecular

switch for Hg2+

or Ag+ ions detection, exhibiting Hg

2+-selective switching-off and Ag

+

selective switching-on type signaling behaviours. Two indole-based, Hg2+

-selective,

fluorescent sensors 77 were designed and synthesized by coupling indole and ethylene

glycol moieties by Huang et al.96

Both 77a and 77b showed selectivity for Hg2+

ion

over other metal ions.

Computational calculations provided evidence that a sandwich-coordinated Hg2+

ion

center was formed and the polyoxyethylene spacer acted as a scaffold for bringing

functional ligands into a suitable geometry.

An excellent red-emissive BODIPY dye 78 containing a benzo[2,1,3]thiadiazole

bridge was synthesized, and its sensing ability toward metal cations was investigated

by Sun et al.97

The response of dye 78 to Hg2+

was investigated through absorption

and emission spectra. Upon addition of Hg2+

ions, the absorbance of 78 at 568 nm

progressively decreased, while a new band at 501 nm appeared and its absorbance

increased gradually, corresponding to a λmax (abs) blue-shift of 67 nm, which induced

an evident change of solution colour from purple to yellow. In the fluorescence

spectra, emission band at 603 nm decreased and several new bands at 430–550 nm

appeared, realizing ratiometric detection.

The above mentioned mercury based sensors were ‘turn-off’ type i.e. the fluorescence

of the receptor gets quenched on addition of mercury. The mercury induced non-

specific fluorescence quenching of the receptor occurs via spin orbit coupling98

NB

N

N NB

F F

F F

OC6H17

OC6H17

NS

N

78

77a-b

NH

NH

O O

OO

O n

a; n = 2

b; n = 3


25

associated with the heavy atom effect, which facilitate the intersystem crossing

process. Fluorescence quenching is not only disadvantageous for a high signal output

upon recognition but also hampers temporal separation of spectrally similar complexes

with time resolved fluorometry.99

Fluorescence enhancement on the other hand is an ideal phenomenon for the metal ion

recognition. However, the designing of fluorescence ‘turn-on’ type sensors upon

mercury binding is a challenging issue and only some fluorescence ‘turn-on’ sensors

for mercury ions have been reported. Herein, we have reviewed some of the literature

reports where the fluorescence is ‘turned on’ in the presence of mercury ions.

‘Turn-on’ sensors for mercury

Caballero et al.100

reported compounds 79 and 80 based on ferrocene and pyrene,

respectively which operate through two different channels (79: optic/redox, and 80:

optic/fluorescent) exhibiting higher sensitivity and selectivity for Hg2+

in aqueous

environment. The cyclic voltammetric (CV) and differential pulse voltammetric

(DPV) analyses in acetonitrile showed two almost-overlapped one-electron oxidation

processes for 79, whereas in acetonitrile/water (7:3), only one oxidation peak around

0.65 V was observed, versus decamethylferrocene. The ability of the azine bridge to

complex Hg2+

ions selectively was checked with compound 80, which has a

fluorogenic instead of a redox signaling unit in 79. Addition of only small amounts of

Hg2+

to the solution dramatically increased the excimer emission band at 510 nm. Ou

et al. reported a Schiff base 81 having quinoline group and a water-soluble D-

glucosamine in a single molecule. 101

The compound was highly soluble in water and

highly selective and sensitive to mercury with the low concentration limit being 0.5

mM. The sensor responded to Hg2+

ions through a combined signaling of both PET

and PCT mechanisms.

80

OCH3

NN

81

N

N

H

O

H

H

H

OH

H

HO

H

OH

OH

79

Fe

Fe N N


26

Two regioisomeric fluorescent PET Hg (II) sensors 82 and 83, which contain a novel

o-phenylenediamine based triamide receptor, have been reported by Wang et al.102

These were used to detect Hg2+

ions with exclusive selectivity, high fluorescence

enhancement, and fast and reversible response in a neutral buffered aqueous solution.

The same group reported another report on coumarin based receptor 84 which showed

ratiometric behaviour in the presence of Hg2+

ions.103

The sensor had good capability

of detecting Hg2+

in aqueous media employing ICT mechanism. The deprotonated

amide groups cooperated with the two o-phenylenediamine nitrogen atoms to form a

tetrahedral ligand atmosphere for Hg2+

ions. The other two unbound amide arms might

exert steric effects, which restrict the free rotation of the amide arms and favour the

Hg2+

-84 complexation.

Rhodamine fluorochrome, as a fluorophore and chromophore probe has attracted

considerable interest from chemists on account of its excellent photophysical

properties.104

Rhodamine derivatives are nonfluorescent and colourless, whereas ring-

opening of the corresponding spirolactam gives rise to strong fluorescence emission

and a pink colour.105

In general, a rhodamine derivative displays a pink colour change

and strong fluorescence in acidic solution by activating a carbonyl group in a

spirolactone or spirolactam moiety. In a similar way, an appropriate ligand on

84

S

N

O

NN

O

HN

OH

NH

OHHNHO

HN

HO

O

O

O

O

NNNH

O

OOH

NH

OH

NH

HN

O

HO

N

O

HO

O

O

83

82

NNNH

O

OOH

NH

OH

NH

N

O

HO

O

O

NH

O

OH


27

spirolactam ring can induce colour change as well as fluorescent change upon the

addition of metal ions even though this process is somewhat dependent on the solvent

system. The year 2007 brought a revolutionary augmentation in the rhodamine based

sensors for mercury. Among various reports on rhodamine sensors, mercury based

sensors acquired a major share. Lee et al.106

reported a rhodamine based derivative 85

which in the presence of Hg2+

ions exhibited a significant colour change and

fluorescence emission.

Rhodamine based multisignalling sensor having ferrocene unit 86 was reported by

Yang et al. which showed extreme selectivity for Hg2+

ions.107

The complexation of

Hg2+

by compound 86 was investigated by fluorescence in ethanol/HEPES buffer (1:1,

v/v, pH 7.2). Upon addition of Hg2+

ions, a new emission band of compound 86

showing a maximum at 580 nm appeared with an intense red fluorescence. Confocal

laser scanning microscopy experiments had shown that sensor can be used to detect

Hg2+

ions in living cells and map its subcellular distraction. Wu et al. reported N-

(Rhodamine-6G) lactam-N-phenylthiourea-ethylenediamine 87 as a fluorescent and

colorimetric chemodosimeter in aqueous solution with a broad pH span (5-10).108

The

compound showed high selectivity toward Hg2+

ions but no significant response

toward other competitive cations, such as Fe2+

, Co2+

, Ni2+

, Cu2+

, Zn2+

, Pb2+

, Cd2+

,

Ca2+

, Mg2+

, K+, Na

+, etc was observed. The Hg

2+-promoted ring opening of

spirolactam of the rhodamine moiety induced cyclic guanylation of the thiourea

moiety, which resulted in the dual chromo- and fluorogenic observation (off-on).

Othman et al. reported a calix[4]arene-based chemosensor 88 appended with

rhodamine moieties which undergoes Hg2+

-induced fluorescence resonance energy

85

ON N

N

O

N

NH

NH

S

O

O

S OO

87

O

N

O

NH

NH

HN

HN

S

86

O

N

N

N

O

N

O

Fe

N


28

transfer (FRET).109

Addition of Hg2+

ions to a CH3CN solution of 88 gave a

significantly enhanced fluorescence at 575 nm via energy transfer (FRET-ON) from

the pyrenyl excimer to a ring-opened rhodamine moiety. In contrast, addition of Al3+

ions induced a distinct increase of pyrenyl excimer emission while no obvious FRET-

on phenomenon was observed.

Wu et al. reported a highly sensitive fluorescent probe 89 based on rhodamine for

selective detection of Hg2+

ions in mixed N, N-dimethyl formamide aqueous media.110

Upon addition of mercury ions, a new band at 560 nm appeared along with the

increase in the intensity. The detection limit was found to be as low as 2 ppb. A

rhodamine ferrocene hybrid (RF1) 90 was also reported as a Hg sensor.111

The

response of RF1 was Hg2+

specific, and the chemosensor exhibited high selectivity

towards Hg2+

over alkali, alkaline earth metals, and most of the divalent first-row

transition metals. The RF1-Hg2+

complex was successfully isolated and the Hg2+

-

binding was found to be reversible.

89

ONH

NH

N

O

N

N

88

OHOH OOOHOH OO

HN

N

NH

O O

N

O

N

NO

HNNH

OO

ON N

N

O Fe

90

92

O

H

O

H

O O

N

NN N

N

N

OOO O OMeO

O

N

OO

O

N

N

N

OH

HO

91

93

O

H

O

H

O O

N

NN N

N

N

OOO O OMeO


29

Ho et al. reported 8,8’-(1,4,10,13-Tetraoxa-7,16-diazacyclooctadecane-7,16-

diyl)bis(methylene)diquinolin-7-ol (TDBQ) 91 as a mercury sensor.112

Compound 91

sensed Hg2+

by the reduction of Hg2+

to yield Hg22+

–TDBQ complex. In this approach,

two 7 hydroxyquinoline moieties not only act as dual sidearms attached to diaza-18-

crown-6, forming pseudo-cryptand that greatly enhanced the binding strength, but also

serve as a signal transducer.

Kumar et al. synthesized 1, 2, 3-Triazole-based sensors 92 and 93 which showed high

selectivity and affinity for Hg2+

ion.113

In addition, these systems also exhibited logic

gate properties showing NOR and OR type logic gates on using Hg2+

and H+ as inputs.

A selective redox and chromogenic probe for Hg2+

ions based on a ferrocene-

azaquinoxaline dyad 94 has been reported.114

The compound was studied by

electrochemical, spectral, and optical techniques. Receptor 94 showed similar sensing

behaviour toward Hg2+

, Pb2+

, and Zn2+

in CH3CN solution. The oxidation redox peak

was anodically shifted, and a high fluorescence enhancement (>90-fold) with visible

colorimetric changes was observed. Li et al. reported a ratiometric fluorescent sensor

95 based on thiourea-thiadiazole-pyridine (TTP) linked organic

nanoparticles.115

Transfer of TTP to nanoparticles in aqueous media led to Hg2+

ion

detection in H2O system due to the parallel alignment of thiourea and thiadiazole

groups which effectively complex with Hg2+

ions. The addition of Hg2+

ions led to

fluorescence enhancement along with the remarkable shift in the fluorescence

emission. Al-Kady et al. synthesized 8-alkyl thiourido-7-ethoxy-4-methyl coumarin

derivatives 96a-c which showed fluorescence enhancement in the presence of Hg2+

,

Ag+, and Ag nanoparticles.

116 Chelation enhanced fluorescence (CHEF) mechanism is

responsible for the formation of 1:2 complexes in Hg2+

/coumarin derivatives and 1:1

complexes in Ag+/coumarin derivatives.

N N

N

Fe F

e

94

95

Cl NH

NH

O S

N N

SN O O

NHCSNHR

C2H5O

96

a; R = C2H5

b; R = C6H5

c; R = C6H11


30

A boron-dipyrromethene (BODIPY)-based fluorescence probe with a N,N′-(pyridine-

2, 6 diylbis(methylene))- dianiline substituent 97 was prepared by Lu et al.117

The

fluorescence properties of compound were studied and compound 97 showed highly

selective fluorescent ‘turn-on’ response in the presence of Hg2+

over the other metal

ions, such as Li+, Na

+, K

+, Ca

2+, Mg

2+, Pb

2+, Fe

2+, Co

2+, Ni

+, Cu

2+, Zn

2+, Cd

2+, Ag

+,

and Mn2+

. For 97-Hg2+

complex, both the reductive and oxidative PETs were

prohibited leading to strong fluorescence emission from the fluorophore. Wu et al.

reported compound 98 based on dansyl- L- tryptophan methyl ester which was a

hypersensitive water-soluble fluorescent probe.118

The compound 98 showed 35 fold

fluorescence enhancement on addition of Hg2+

ions in aqueous media with change in

fluorescence colour from brown to green.

In virtue of the thiophilic nature of Hg2+

, three sensors 99, 100 and 101 that combine a

thiophene group and one or two rhodamine choromophores, or a thiospirolactam

rhodamine chromophore, were designed and prepared for the selective detection of

Hg2+

in aqueous media, respectively by Huang et al.119

These sensors displayed good

brightness and fluorescence enhancement following Hg2+

coordination with limits of

detection for Hg2+

at the ppb level. Sensor 100 which contained two rhodamine

carboxyhydrazone arms exhibited better selectivity, compared to those of 99 and 101.

97

N

NH HN

N

BN N

B

NF

F

F

F

98

NH

NS

HN

O

O

O O

99

ONH

NH

N

O

N

S

100

O

HN

HN

N

O

N SO

NH

NH

N

O

N

101

ONH

NH

S

N N

S


31

Lee et al. reported FRET based calix[4]arene derivative 102 locked in the 1,3-

alternate conformation bearing two pyrene and rhodamine fluorophores.120

On

addition of Hg2+

ions to the solution of 102, a significantly enhanced fluorescence at

576 nm via FRET was observed. The sensing was based on FRET from pyrene

excimer emissions to ring-opened rhodamine absorption upon complexation of the

Hg2+

ion. A multifunctional sensor 103 bearing rhodamine moiety responsive to pH,

temperature, and Hg2+

ions has been reported.121

The thermo-induced aggregation of

the compound 103 was investigated by temperature- dependent optical transmittance,

laser light scattering (LLS).

Dong et al. reported a naphthalimide and alkyne conjugate 104 which has been

utilized for ratiometric sensing of Hg2+

and Au2+

ions.122

Upon the addition of Hg2+

to

the solution of 104, a significant decrease of the fluorescence intensity at 543 nm and

an increase of fluorescence emission band centered at 486 nm were observed with an

isoemission point at 509 nm, which indicated a clear ratiometric fluorescence change.

Yao et al. synthesized a pyrene-based derivative 105 bearing an azadiene group which

showed fluorescence enhancement on adding Hg2+

ions.123

The ‘off-on’ type signalling

106

N

N

S

S

HN

NH

NH2

NH2

O

O

S

S

O

O

O

O105

O O NN

NO O

NH2

104 103

ON N

N

O

NH

NH

S O

OO

OO

NHHN

N

NO

O

N

N

O O NHHN

O O

102


32

behaviour of fluoroionophore was due to the metal ion induced conformational

changes from the weak pyrene monomer emissions to strong pyrene excimer emission.

Dansyl labeled dimerized cysteine residue 106 has been reported as a selective sensor

for Hg2+

ions by Joshi et al.124

This sensor exhibited a high selectivity and sensitivity

towards Hg2+

ion over a wide range of metal ions in 100% aqueous solution via a

‘turn-on’ and ratiometric response in 100% aqueous solution. Moreover, the presence

of other metal ions did not interfere with the detection of Hg2+

ions.

The metal coordination-inhibited spiroconjugation-like charge transfer emission has

been reported as a new methodology for designing Hg2+

sensor.125

Two terfluorenes

107 and 108 exhibited high sensitivity and selectivity for the detection of Hg2+

ions

with ratiometric fluorescence response. The sensory mechanism also verified

assignment for the dual fluorescence emissions of the terfluorene derivatives.

A rhodamine-based sensor 109 was reported by combination of the thiospirolactone

chromophore and the thiophene ring with high affinity to Hg2+

ions.126

The

fluorescence imaging showed that compound can be used for low cytotoxicity

detecting changes in Hg2+

levels within living cells. King et al. reported a series of

molecular gelator 110a-c exhibiting one dimensional intermolecular interaction in the

solid state.127

The presence of Hg2+

ions promoted 1D self assembly and gelation of an

108 107

N NN NHO

OH HO

OH

SN

HN

OCH3

O O

O

OCH3O

111 109

ON N

N

S

N

S

a; X = N

b; X = CH

c; X = C-CH3

NH

X

O

110a-c


33

aromatic molecule via cation–π interactions. The gel microstructure consisted of

bundles of high-aspect ratio fibers as shown by scanning electron microscopy.

Li et al. reported a fluorescent probe dansyl-L-glutamic methyl diester 111 which was

found to be highly selective for Hg2+

ions.128

Compound 111 possessed a weak

fluorescence emission band at 550 nm in solution because of the internal charge

transfer (ICT) between the amino group and the dansyl moiety. Addition of Hg2+

ions

led to a significant emission band at 475nm. When coordinated with Hg2+

ion, the

amidocyanogen lost its ability to donate an electron to the dansyl moiety.

Consequently, the involved ICT between the amino group and the dansyl moiety was

inhibited, the emission was released and the band shifted toward the blue region of the

emission spectrum.

The first intermolecular reaction-based fluorogenic chemodosimetric probe system

112 for Hg2+

ion recognition has been reported by Ren et al.129

High and low

concentrations of Hg2+

ions gave different fluorescence responses that could easily be

distinguished by the naked eye. In the presence of a low concentration of Hg2+

ions (0–

1.4 equiv), the fluorescence emission intensity at 492 nm was enhanced with gradually

increased Hg2+

ions. Upon further addition of Hg2+

ions (>1.4 equiv), the emission

band at 492 nm gradually decreased and concomitantly blue shifted to 454 nm.

A squaraine-based chemosensor 113 has been synthesized by Chen et al. and its

sensing behaviour toward various metal ions was investigated by UV-Vis and

fluorescence spectroscopy.130

Addition of Hg2+

ions led to ‘turn-on’ fluorescence

change along with naked eye detection which was attributed to “H”-aggregation

deaggregation process. Tang et al. reported rhodamine B based sensor, rhodamine B

hydrazide methyl 5-formyl-1H-pyrrole-2- carboxylate 114 capable of detecting both

Cu2+

and Hg2+

using two different detection modes.131

Addition of Cu2+

ions led to the

112

O

SS

NH2

NH2

+

113

N N

O-

O-

S

S

N

N

S

S

S

S

N

N

S

S

114

ON N

N

O

N

HN O

MeO


34

visible colour change from colourless to pink whereas addition of Hg2+

ions led to the

fluorescence enhancement attributed to ring opened form of rhodamine.

Samb et al. reported a phosphorus-selenium moiety 115 which allowed a selective

mercury salt complexation, followed by the formation of phosphane oxide, leading to

a ‘turn-on’ of the fluorescence.132

Upon addition of mercury salt to a solution of 115 in

CH3CN/H2O 80:20, a 100-fold enhancement of the fluorescence was observed.

Yang et al.133

reported amino acid based sensor 116 possessing pyrene which upon

addition of Hg ions exhibited a considerable excimer emission at 480 nm along with a

decrease of monomer emission at 383 nm. Mitra et al. reported anthracenyl–imino–

glucosyl conjugate 117 which selectively sensed Hg2+

ions and undergo turn-on

fluorescence enhancement by 13 fold forming a 2: 1 complex.134

The compound

showed its sensitivity towards Hg2+

ions in the presence of albumin proteins and in

blood serum and milk.

Fang et al. reported a new strategy for Hg2+

sensing by synthesizing compounds 118a-

b and 119a-b and utilized the concept of imide Hg imide complexes for ion sensing.135

Various techniques like X-ray photoelectron spectroscopy, IR, and mass had been used

to characterize the above complexation. Besides the fluorescence studies have also

120

N

N

Se

Se

OH

OH

BF4-

118a-b

NO O

NH

R

H

a; R = CH2CH2OH

b; R = CH2CH2NH2

119a-b

a; R = CH2CH2OH

b; R = CH2CH2NH2

NO O

NH

R

CH2CH2OH

O

EtOOC

Se=P

3

115

SNH

NH2

OO

O

S

116

N

O

HO

HO

OH

OH

117


35

been done. Hg2+ was able to quench the fluorescence of 118a and 118b by up to 90%

but did not quench the fluorescence of N-substituted naphthalimides 119a and 119b.

Li et al. reported a water soluble Hg2+ selective chemosensor 120 with hemocyanine as

reporting unit and NO2Se2 as an ion binding unit.136 The chemosensor showed a

remarkably high ability to discriminate between Hg2+ and chemically similar ions (K+,

Na+, NH4+, Al3+, Co2+, Fe3+, Pb2+, Ag+, Cu2+, Zn2+, Cd2+, Ni2+, Ca2+, Mg2+ and Hg2+)

in conjunction with a visible colorimetric change from red to colourless, leading to

both “naked-eye” and fluorimetric detection of Hg2+ cations.

Intramolecular charge transfer (ICT) based fluorescent reagent containing a dansyl

fluorophore has been synthesized and characterized. The reagent 121 and its complex,

121+Hg2+ in sodium acetate buffer (pH 6.7) revealed considerable fluorescence

enhancement (‘turn-on’) in the presence of bovine serum albumin (BSA) with 10 ppb

detection sensitivity.137 In another report by Lu et al., click reaction was used to

synthesize fluorescent self-assembled monolayer 122, which was used to detect Hg2+

in both water and organic solutions.138 The resulting fluorescence sensor exhibited a

rapid response and high sensitivity to Hg2+ ions due to the synergy effect between the

nitrogens of a triazole group and a rhodamine moiety. The mercury sensing behaviour

was stable over a wide pH range in aqueous solutions.

Homogeneously sized nanoparticles were successfully constructed based on

amphiphilic porphyrin-cholesterol arrays by Liu et al. 139 The compounds 123 and 124

showed unique spectral and colorimetric response to organic mercury in water, even in

the presence of Hg2+ ions.

Xie et al. reported visible light excitable Hg2+ sensors, 125 and 126, prepared by

bridging a 4-amino-7- nitro-benzoxadiazole (ANBD) fluorophore with thiaazacrown

O

N

N

NO

O

N

N

N

O

ONNN

O

NN

N

Si

Si

10

10O

O

O

OO

122

N

S OO

HNOH

O

121


36

ether via an ethylene spacer.140 Besides the specific Hg2+-induced absorption shift

from 466 to 513 nm, compound 126 exhibited the specific Hg2+ induced emission

enhancement (29-fold) and fluorescent pH-independence from pH 6 to 13.

Recently, Yang et al. reported a dansyl-labeled methionine 127 which was found to be

a highly sensitive and selective sensor for Hg2+.141 The sensor sensitively detected

Hg2+ ions by a ‘turn-on’ response in aqueous solution; however, ‘turn-off’ response

was observed on adding Hg2+ ions in organic and mixed aqueous–organic solutions.

The rhodamine based ‘turn-on’ chemosensors 128 and 129 with different coordination

ability and sensing behaviour that combine a furaldehyde and a rhodamine

chromophore or a thiospirolactam rhodamine chromophore had been reported

recently.142 The introduction of the thiol atom in 128 led to prominent absorption and

fluorescence enhancements to Hg2+ ions with a particular selectivity and excellent

sensitivity and could be used for naked-eye detection. This sensor was also applied for

in vivo imaging in rat schwann cells to confirm that compound 128 can be used as a

fluorescent probe for monitoring Hg2+ in living cells.

N

S NH

O

O

S

NH2

O

127

126 125

N

O

N

NS

O

O

S

NO2

N

O

N

NO2

N S

O

O

S

N

NH N

HN

COO-

OMe-OOC

COO-

123

N

NH N

HN

COO-

O-OOC

COO-

O

H

H

124


37

A new strategy for the ratiometric detection of toxic Hg2+ ions using a semiconductor

nanocrystal energy transfer donor coupled to a mercury-sensitive ‘‘turn-on’’ dye

acceptor 130 has been given by Page et al.143 The results demonstrated a new

paradigm of toxic metal sensing that resolved the difficulties with the use of

semiconductor nanotechnology for this purpose. The sensing capability of a

thiosemicarbazide functionalized rhodamine B dye—CdSe/ZnS NC coupled

chromophore has been examined. The pendant thiosemicarbazide group caused

rhodamine B to be optically inert at visible wavelengths due to the disruption of the

delocalized electronic structure of the dye. Upon exposure to Hg2+ ions, the subsequent

desulfurization reaction returned the visible optical properties of ‘turn-on’ dye that

became an efficient energy transfer acceptor to a CdSe/ZnS NC donor.

1.3.2 Fluorogenic sensors for copper ions

The selective sensing of copper, which is third in abundance among the essential

transition metal ions in human body has gained attention due to its significance in

biological systems.144 Copper kills a variety of potentially harmful pathogens and

hence has antimicrobial effect against MRSA, Escherichia coli and other pathogens.145

But over accumulation of copper in body produces severe or lethal intoxications.146

Duan et al. synthesized Hg/Cu based chemosensor 131 utilizing the displacement

approach.147 Compound 131, a copper based compound in which the fluorescence due

to naphthol rings was quenched due to Cu2+ ions. On addition of Hg2+ ions, a dramatic

increase in intensity of ligand occurred. Zong et al. reported L-phenyl alanine based

molecular half subtractor 132 by utilizing fluorescence and absorption behaviour on

adding Cu2+ ions.148 The copper complex could perform simultaneously the functions

of an ‘INHIBIT’ gate and an ‘XOR’ gate, capable of operating as a half-subtractor

with acid and base as inputs, by monitoring the fluorescence and absorbance modes,

respectively.

129

ONH

NH

N

S

N

O

128

ONH

NH

N

O

N

O O NN

N NH

O S

NH

130


38

Jung et al reported a pyrenylquinoline derivative 133 which upon addition of Cu2+ ions

exhibited a strong static excimer emission at 460 nm, along with a weak monomer

emission at 388 nm.149 The excimer emission intensity induced by the Cu2+ ions

quenched as the spacer length between the pyrene and quinolinylamide unit increased.

Rhodamine based compound 134 bearing pyrene groups acted as a ratiometric and

‘off-on’ chemosensor for Cu2+ ions.150 It displayed a selective and chelation enhanced

ratiometric fluorescence change (CHEF) and colorimetric change with Cu2+ among the

various metal ions examined. Jung et al. reported a fluorescent receptor 135 based

upon a benzimidazole moiety in a dipodal framework.151 The receptor showed a dual

fluorescence emission which was quenched upon addition of Cu2+ or Fe3+. The

receptor showed a ratiometric property and demonstrated ‘OR’ logic gate with Cu2+

and Fe3+ ions. Aksuner et al. reported compound 136 as a highly selective and

sensitive optical sensor for Cu2+ ions.152 The dye-doped membrane exhibited

remarkable fluorescence intensity quenching upon exposure to Cu2+ ions at pH 6.0.

Cu2+ ions form a non fluorescent complex with PCT dye corresponding to the M(L)2

with a suggested square-planar geometry and hence quenching occurred in ground

state.

131

N N

N N

HO

HO

132

NH

HOOC

133

NH

N

Br

HO

N

N

S

CH3

136

134

ON N

NN

O

OH

135

NN

HN OH

NN

HN

OH


39

Guliyev et al. reported boradiazaindacene derivative 137 which was shown to form

complex with Cu2+ ions and was non fluorescent. 153 The addition of cyanide to the

above complex led to decomplexation of Cu2+ from receptor and hence the

fluorescence got ‘turn-on’. Wu et al. reported a pyrene derivative 138 containing a

diaminomalononitrile moiety exhibited high selectivity for Cu2+ detection.154

Significant fluorescence enhancement was observed with chemosensor 138 in the

presence of Cu2+ ions. Helal et al. synthesized coumarin based sensor 139 for copper

ions.155 The introduction of an electron-donating diethylamino group made it

chromogenic and increased the binding affinity for Cu2+ by modifying the ICT

phenomenon. This increase in CT character due to large change in the dipole moment

upon excitation from the ground to the excited state resulted in very large Stokes’

shifts between their absorption and fluorescence maxima, increasing the molar

extinction coefficient of the sensor in the free and complex forms.

Xu et al. reported compound 140 as a highly sensitive and selective colorimetric and

‘off–on’ fluorescent chemosensor for Cu2+ ion.156 Among the various metal ions,

sensor 140 exhibited remarkably enhanced absorbance intensity and colour change

from colourless to pink in DMSO and MeCN aqueous buffer solution or pure MeCN,

and showed significant ‘off–on’ fluorescence accompanied with colour changes from

colourless to orange in MeCN.

N OH

NNH

NH

S

N

NHO

141

N OH

NNH

NH

O

N

NHO

142

N

S

O

O

NEt2

OH

139

N

NH2

N

N

138

NB

N

NN

N

F F

137

ON N

N

O

N

HO

OH

140


40

Maity et al. reported Schiff base ligands julolidine–carbonohydrazone 141 and

julolidine–thiocarbonohydrazone 142 for selective detection of Cu2+ in aqueous

medium.157 These molecular probes not only detected copper by naked eye but also the

detection of varying micromolar concentrations of Cu2+ ions due to appearance of

distinct colour. Ligand 141 did not show any specific changes in the fluorescence

emission in the presence of Cu2+ and other metal ions used upon excitation at 402 nm.

On the other hand, ligand 142 showed strong fluorescence emission around 535 nm

upon excitation at 430 nm. The fluorescence intensity around 535 nm was quenched in

the presence of only Cu2+ ions on excitation at 430 nm. The fluorescence-quenching

behaviour can be accounted for primarily due to the excited-state ligand 142 to metal

(Cu2+) charge-transfer (LMCT) processes.

A coumarin-based fluorescent sensor iminocoumarin (IC) 143 was reported by Ko et

al.158The compound displayed high selectivity for Cu2+ over a variety of competing

metal ions in aqueous solution with a significant fluorescence increase. IC was also

used for in vitro biological test which displayed a non-fluorescent image in the

absence of Cu2+, whereas a strong confocal image with green fluorescence upon

addition of Cu2+ ions. In addition to in vitro studies, in vivo evaluation was also done

in mice which indicated that IC had a capability to sense Cu2+ accumulation in specific

organs.

Liu et al. reported 3-formylquinoline-2(1H)one, bearing 1H-benzotriazol-1-acetic acid

hydrazide 144 as a fluorescent sensor for copper ions.159 According to X-ray crystal

structure analysis, the coordination of receptor 144 with Cu2+ exhibited an interesting

1D chain polymer framework ([Cu(153)]n). Compound 144 showed a strong emission

band at 460 nm when excited at 380 nm. Upon addition of Cu2+ ion (0–20 mM),

remarkable fluorescence quenching of compound was observed.

NH

N

HN

OO

N

N N

144

143

O ON

NN

N

N

SH


41

1.3.3 Fluorogenic sensors for zinc ions

Zinc is a ubiquitous, essential metal ion found in every cell in the human body.

Though, it is required as a key component of numerous enzymes and transcription

factors, failure to maintain an adequate zinc level in a body, organ or cell may lead to a

number of severe neurological diseases, developmental defects and malfunctions.

Some of the chemosensors for zinc ions reported in the literature are reviewed below:

Zhang et al. reported two similar ligands 145a (BAEDA) and 145b (BAPDA) based

on anthracene. 160 In pure organic solvents, 145b displayed strong fluorescence

enhancement on titration with Zn2+, but the emission of the complex could be

remarkably quenched by a trace amount of water while 145a was found to show

excellent fluorescent response for Zn2+, which could operate in a HEPES buffer. Thus

145a acted as a better sensor for Zn2+ ions.

Wang et al. reported a quinoline based compound 146 which showed 14-fold

enhanced quantum yield upon chelation to zinc ion and also exhibited high selectivity

to zinc ion over other physiological relevant divalent metals in the presence of

EDTA.161 Park et al.162 reported a calix[4]arene-based fluorescent chemosensor 147

which showed response toward Zn2+ and Cd2+ ions over the other metal ions. On

excitation at 343 nm, the maximum absorption wavelength of the pyrene in 147

displayed both monomer and excimer emissions at 395 and 476 nm, respectively

which was caused by intramolecular interaction between Py (the ground state

pyrene) and Py* (the photo-induced excited state pyrene) where two pyrenes are likely

to be in parallel. Addition of Zn2+ or Cd2+ ion to the CH3CN solution of 147 bearing

pyrene-triazoles as a metal ligating group induced a remarked ratiometry where the

monomer emission increased as its excimer emission quenched.

146

N

N

N

N

O

COONa

147

OHOH OO

NN N N N

N

145a-b

NH HN

n

a; n = 2

b; n = 3


42

Xue et al.163 reported compound 148 based on acetamidoquinoline which can

distinguish Cd2+ from Zn2+ via two different sensing mechanisms (PET for Cd2+; ICT

for Zn2+), and the discrimination is even possible by ‘naked eyes’. Wang et al.

reported binaphthyl-derived salicylidene based compound 149 as a sensitive

colorimetric and fluorescent chemosensor for the detection of Cu2+ and Zn2+ bi-

functionally.164 The compound presented a tunable system integrated with one OR

logic gate as well as one INHIBIT logic gate with Zn2+ and Cu2+ as chemical inputs by

monitoring fluorescence and absorbance as output signals. Also, one IMPLICATION

gate operating in fluorescence mode with Cu2+ and EDTA as chemical inputs, based

on their different binding capabilities, is reported. Hu et al. reported

dipyrrolylquinoxaline (DPQ) based chemosensors 150a-b.165 These compounds

displayed good sensitivity toward transition metal ions with Cd2+, Zn2+ ‘turn-on’ and

Cu2+, Hg2+ ‘turn-off’ in fluorescence. Zn2+ and Cd2+ ions coordinated to ligands 150a

or 150b are generally emissive species, causing a CHEF effect. This was explained by

assuming that the deprotonation and complexation of pyrrole with Zn2+ and Cd2+ block

its lone pair of electrons from transferring to the DPQ moiety via PET (photo-induced

electron transfer).

NH

Cl

Cl

HN

Cl

Cl

HN

NH

NH

HN

O

O

152

148

N

NH

O

O

N

N

N

149

N N

Cl OH HO Cl

Cl Cl

a; R = OMe b; R = H

N N

NH HN

N N

RR

150a-b

N

NH

O

N

N HN

O

N

151


43

Jiang et al. reported 2, 2’-(piperazine- 1,4-diyl)bis(N-(quinolin-8-yl)acetamide) (QA)

151 as an efficient sensor for Cu2+ and Zn2+ based on different binding modes.166

Compound 151 displayed an efficient ratiometric response for Zn2+ and dual-mode

selectivity for Cu2+ ions. The receptor 151 adopted an imidic acid tautomer with Zn2+

ions; and an amide tautomer accompanied with the deprotonation of NH groups with

Cu2+ ions. As a result, QA showed an efficient ratiometric and ‘off–on’ response for

Zn2+ and dual-mode selectivity behaviour for Cu2+ via a colorimetric method and

fluorescent displacement approach. Ahmed et al. reported a carbazole based

chemosensor 152 which in its deprotonated form senses Cu2+ and Zn2+ ions.167 The

deprotonated carbazole–urea receptor was used for highly selective ratiometric sensing

of Zn2+ ions by means of fluorescent changes and of Zn2+ and Cu2+ by means of

colorimetric changes.

Peng et al. reported aroylhydrazone derivative H3L 153 as a fluorescent sensor for zinc

and copper and its complexes [Zn(HL1)C2H5OH]• (H3L1) and [Cu(HL1)(H2O)]CH3OH

(H3L2) have been synthesized.168 H3L

1 displayed high selectivity for Zn2+ over Na+,

K+, Mg2+, Ca2+ and other transition metal ions in tris–HCl buffer solution (pH = 7.13,

EtOH–H2O = 8 : 2 v/v). Upon addition of 10 equivalents of Zn2+ ions, the fluorescence

intensity of H3L1 increased by 25-fold, whereas addition of Cu2+ led to fluorescence

quenching. Chen et al. reported [2-(2’-aminophenyl) benzoxazole-amide-2-

picolylamine] 154 as a fluorescent sensor for Zn2+ ions.169

Sensor showed a very weak fluorescence in Tri-HCl buffer (10 mM, pH 7.2) solution.

Upon addition of Zn2+ ions, the fluorescence intensity increased remarkably and a

fluorescence enhancement factor at 445 nm of approximately 25-fold was estimated.

The weak fluorescence of compound 154 in the absence of Zn2+ might be attributed to

radiationless channels from the nπ* state. In the presence of Zn2+ ions that coordinated

with the lone pair of the carbonyl oxygen (O2), the energy of the nπ* state would be

153

N

HN

O ROH

H3L1, R = OH

H2L2, R = H

154

N

O HN

O

NH N


44

raised so that the ππ* state became the lowest excited state, leading a substantial

increase in the fluorescence intensity.

1.4 Anion based sensors

The coordination chemistry of cations attracted most interest in the 1970s and

consequently cation recognition is now a well-developed and mature area of

supramolecular chemistry. In contrast, the coordination chemistry of anions received

little attention and it has only been in the last twenty years that sustained effort has

been applied to the problems inherent in binding anions. The ubiquity of inorganic

anions such as fluoride, chloride and phosphate in nature, their importance as food

additives, agricultural fertilizers and industrial raw materials, commands considerable

attention of the scientific community. The design of anion receptors is particularly

challenging attributed to various reasons. Anions are larger than isoelectronic cations

and therefore have a lower charge to radius ratio. This means that electrostatic binding

interactions are less effective than they would be for the smaller cation. Additionally

anions may be sensitive to pH values (becoming protonated at low pH and so losing

their negative charge), thus receptors must function within the pH window of their

target anion. Anionic species have a wide range of geometries and therefore a higher

degree of design may be required to make receptors complementary to their anionic

guest. The above factors made sensing of anions an intricate job.

It is remarkable that there are relatively few examples on anion sensing when

compared with the literature devoted to cation sensing. However, the number of papers

on anion sensing has grown considerably. A wide range of different ligands for anion

coordination are described in the literature till date based on different concepts viz.

binding site-signaling subunit approach, displacement approach, chemodosimeter

approach etc. Among various anions, F- and OAc- ions are biologically important and

significant. Some of the recent representative examples based on sensing of these

anions are reviewed below:

The core-substituted NDI sensor appended with sulphonamide groups 155 was

reported by Bhosale et al.170 On addition of Bu4NF (0- 2.0 equiv) to 155, the intensity

of bands at 559 and 609 nm steadily decreased with the appearance of new bands at

570 and 622 nm. In the presence of 3 equiv of F-, the bands at 570 and 622 nm

predominated with three clear isosbestic points.


45

The imidazolium-functionalized BINOL fluorescent receptors 156a-b were developed

as a multifunctional receptors for both chromogenic and chiral anion recognition.171

On treatment of compound 156a with various anions, the fluorescence spectra showed

a distinct and intense peak at 454 nm with CH3COO- ions and at 474 nm with F-

together with the quenching of original peaks, whereas H2PO4-, a slightly intense peak,

appeared at 405 nm. Around 140-fold and 585-fold selectivities for F- over Cl- and Br-

were observed, respectively.

Lin et al. reported a series of acridinium salt-based probes based on design 157a-d

which were capable of detecting fluoride and acetate anions via a nucleophilic attack

at the C9 position of the acridinium moiety.172 Among the 11 anions F-, Cl-, Br-, I-,

CN-, SCN-, AcO-, NO3-, ClO4

-, HSO4-, and H2PO4

- screened, the halide anions

quenched the fluorescence of probes 157a and 157b to a slight extent, and the

quenching efficiency increased in the order from Cl- to Br- to I-. On addition of F-,

CH3COO-, CN-, or H2PO4-, the fluorescence emission intensity at 495 nm of 157a was

quenched completely, whereas probe 157b displayed a fluorescence decrease at 536

158

N NHN NH

O2N

NO2

NO2

NO2

NO2

159

O

O

N

OHO

157a-d

N

R

Si(OiPr)3

BF4-

OHO

O OO

a; R =

b; R = d; R =

c; R =

N

N

NH

HN

OO

OO

NH

NH

(CH2)7CH3

(CH2)7CH3

Br

Br

S

S

O

O

O

O

155

156a-b

N

OR

OR

NN

N

2PF6-

a; R = H

b; R = Me


46

nm with a concomitant increase at 415 nm. The disappearance of the three

characteristic absorption peaks corresponding to the acridinium salts and the newly

formed peaks at 280 nm for 157a and at 288 nm for 157b strongly suggested

transformation of the acridinium moiety into the corresponding acridane.

Li et al.173 synthesized 1,3-di(2’, 4’-dinitrophenylhydrazone)-5- nitrobenzene receptor

158 which showed a higher affinity to F-, CH3COO- and H2PO4-, but no evident

binding with Cl-, Br-, and I-. Upon addition of the three former anions to the receptors

in dimethyl sulphoxide (DMSO), the solution exhibited an obvious colour change

from yellow to mauve that could be observed by the naked eye. Addition of CH3COO-

resulted in decrease at 395 nm and increase at 517 nm. The significant bathochromic

shift (122 nm) in the optical spectra was due to the hydrogen bond formation between

the –NH fragment and anions added.

A 3-hydroxyflavone derivative 159 was synthesized and its red-emitting ternary

complex, composed of Zr–EDTA and 159, was developed as a highly selective and

sensitive fluorescent sensor for ratiometric detection of F− in aqueous solution.174

Addition of F− to the solution of Zr–EDTA–159 induced a notable fluorescent change,

whereas other diverse anions did not lead to distinct fluorescence changes. This result

indicated that Zr–EDTA–159 has a higher affinity toward fluoride over other anions in

ethanol–water solution. The ratiometric signal reporting for fluoride-selective sensing

was based on the inhibition of ESIPT process arisen from the ligand exchange reaction

between fluoride anions and flavonol coordinated to Zr–EDTA.

Three catechol based dyes 160-162 reported by An et al. were utilized for F- ions

sensing in the presence of other halides.175 The sensor with three thiophene units, (E)-

2-(2,2’- terthiophen-5-yl)-3-(3,4-dihydroxyphenyl) acrylonitrile 162, gave the best

response to fluoride.

HO

HO

NC

S

S

SHO

HO

NC

S

HO

HO

NC

S

S

These results showed that extension of π-conjugation length enhanced the interaction

of the sensing molecules with fluoride due to increased acidity of the catechol, and that

160 161

162


47

the presence of fluoride caused a distinctive colorimetric and fluorescence change. The

sensing could also be achieved not only with the dye in solution but also in the solid-

state with dye adsorbed onto alumina. Besides, the luminescence of the dyes was also

checked on addition of analyte. The results indicated that the compound 162 was the

most luminescent among three dyes.

Chen et al. reported 6, 6’-bis(triphenylamine)-1,1’-binaphthyl-2,2’-diol 163 as

fluorescent chemosensor for F- ion detection.176 The two electron rich Ph2NC6H4-

groups at the 6,6’-positions were introduced to the BINOL system which led to the

different binding behaviour of the present system from the 3-substituted and 3,3’-

disubstituted BINOLs. When the compound bound with anions, the π-conjugation

system of 163 was perturbed, leading to a change of the spectral response. Among

various anions (F-, OH-, Cl-, Br-, I-, OAc-, HSO4- and H2PO4

-) tested as their

tetrabutylammonium (TBA) salts, F- was found to be the most effective fluorescence

quencher. A pyrazole-based fluorescent sensor, 5-amino-3-(5-phenyl-1H-pyrrol-2-yl)-

1H-pyrazole-4 carboxamide 164 was reported as ‘no-yes’ detection sensor for fluoride

ions177. This compound displayed both changes in UV–Vis absorption and

fluorescence emission spectra upon addition of F−. With increasing amounts of F− ion

concentration, there was drastic increase in fluorescence emission and reached

saturation with 607-fold enhancement at 424 nm.

In recent years, functionalized quantum dots (QDs) have been developed as sensors to

detect different cations and anion sensors.178 Based on this concept, Xue et al. reported

a QD based anion sensor 165 for fluoride ions.179 Specific hydrogen bond breakage by

fluoride anions was observed in a simple FRET system formed by thioglycolic acid

modified CdTe quantum dots and citrate-capped gold nanoparticles. F- ions reacted

OH

OH

N

N

163

S

S

O

O-

O

O

H COO-

COOHCdTc

Au

FRET

165

NH

N

HN

NH2

NH2

O

164


48

with the H nucleus and disassembled the AuNPs segment, resulting in the fluorescence

recovery of the quenched QDs.

Padie et al. reported benzothiazole substituted maleimide 166 as fluorochrome for

selective sensing of F- ions.180 In this report, Baylis Hillman reaction has been utilized

as an idea for fluoride detection. Upon addition of a solution of TBAF to a solution of

compound 166 in DMSO, the mixture turned dark red immediately and precipitates

appeared. This visible effect of addition of fluoride to 166 also caused 10 fold

fluorescence enhancements. The intensity response of the dye was proportionally

related to the amount of fluoride present in the solution.

A simple assay for the detection of fluoride in water by fluorescence spectroscopy was

developed by Rochat et al.181It was based on the concept of anion based masking of

cation which further formed insoluble precipitates. 1,2-bis(o-aminophenoxy)-ethane-

N,N,N’,N’-tetraacetic acid (BAPTA) 167 was reported as a Ca2+ sensor. The

fluorescence got quenched on calcium binding. The intensity of the emission was

found to be dependent on the concentration of fluoride present in the initial solution.

Ghosh et al. synthesized another anion sensor 168 having o-phenylenediamine based

cleft for sensing H2PO4- and ATP.182 The chemosensor 168 selectively bound H2PO4

-

in CH3CN showing excimer emission at 456 nm due to π- π stacking between the

pendant naphthalene units. Among all the anions studied, only H2PO4- perturbed

emission significantly. On complexation of H2PO4- with compound, the monomer

emission at 350 nm was little perturbed, a new broad emission at 456 nm appeared

with significant intensity. The sensor also fluorometrically distinguished ATP from

ADP and AMP in CH3CN–H2O. Addition of ATP to the receptor led to comparatively

more intense band formed due to favorable stacking of the adenine ring in between the

pendant naphthalenes in case of ATP.

O O

COO-

COO-

-OOC-OOC

167

168

NH HN

O O

N N

NH HNO O

H H 2PF6-

S

N

N

O

O

166


49

Quantitative determination of fluoride ions has been done by Sokkalingam et al. using

fluoride driven silyl deprotection strategy.183 One-step reaction of commercially

available 7-hydroxy-4- trifluoromethyl coumarin with TIPS-Cl provided compound

169. In the fluorescence spectra, the appearance of strong fluorescence emission at 500

nm from nonfluorescent 169 upon titration with TBAF clearly indicated that F-

triggered the removal of the TIPS moiety from compound 169, producing the highly

fluorescent chromenolate anion. The system provided chromogenic and fluorogenic

dual signals by displaying (i) a bright yellow colour and (ii) a strong green

fluorescence from an initially colourless and nonfluorescent solution, upon exposure to

fluoride. Zhang et al. reported a naphthalimide-based highly selective colorimetric and

ratiometric fluorescent probe 170 for the fluoride ion which displayed both one- and

two photon ratiometric changes.184 Upon reaction with the F- (TBA+ and Na+ salts)

anion in CH3CN as well as in aqueous buffer solution, probe 170 showed dramatic

colour change from colourless to jade-green and remarkable ratiometric fluorescence

enhancement signals. This can be ascribed to the F- promoted cleavage of the Si-O

bond. A new desilylation based sensor 171 for fluoride sensor was given by Bao et

al.185 In the absorption spectra; the peak at 313 nm decreased and new peaks

emerged at 363 and 410 nm with isosbestic point at 340 nm. In the fluorescence

spectrum, the blue fluorescence turned to yellow green on addition of F- ions which

was ascribed to desilylation. Another fluoride induced chemodosimeter 172 based on

benzothiazolium moiety has been reported by Zhu et al. which behaved in the similar

manner186 The chemodosimeter served as a ‘naked-eye’ probe for F- ions and also

detected F- quantitatively by a ratiometric fluorescence method in buffered aqueous

solution. The amphipathic chemodosimeter was successfully applied to the ratiometric

fluorescence imaging of F- in living cells. Rajamalli et al. reported poly(aryl ether)

dendron 173 with an anthracene moiety which formed gels in CHCl3/MeOH.187 The

resulting gel underwent a gel-to-sol transition, accompanied by a colour change from

deep yellow to bright red, in the presence of fluoride ions. The 590-fold enhancement

169

N

Br

O

Br

Si

170

O

CF3

OOSi

171

N

O

O

NH

O

O

O

Si


50

in the excimer emission of anthracene during gelation indicated that a gelation induced

emission enhancement (GIEE) mechanism controlled the emission properties in the

system.

Panzella et al. synthesized an acetyl derivative 174b of 2-(2-amino-4,5-

dihydroxybenzyl)-6,7-dihydroxy-3-(5,6-dihydroxyindol- 3-yl)quinoline 174a obtained

by mild acid-promoted polymerization of 5,6-dihydroxyindole.188 The acetylated

derivative 174b exhibited a remarkable fluorescence enhancement upon addition of F-

ions. Addition of the anion to the solution caused the emergence of a distinct emission

band at 489 nm following excitation at 414 nm.

Dey et al. reported a dinitrophenyl functionalized tris-(amide) receptor 175 which

behaved as a selective chemosensor for fluoride ions.189 The encapsulation of ion

within the tripodal pseudocavity in polar aprotic solvents was responsible for

solvatochromism and solvatomorphism. Addition of F- ions led to intense colour

change with emergence of new bands in the optical spectral region which was ascribed

to the strong anion-π charge-transfer interactions involving F- ion and π-acidic

dinitrophenylamide receptor 175.

ONH

N

HNO

O2N NO2O2N NO2

HN

O

O2N

NO2

175

N

HN

HN

O NH

NH

N

O

176

NS

O

Si

I-

172

N

HNCO

O

O

O

173

NRO

RO

OR

OR

RHN

NH

ORRO

a; R = Hb; R = Ac

174


51

Abraham et al. reported a cyclo[2]benzimidazole based receptor 176 for anions that

exhibited a dramatic luminescence ‘turn-on’ upon binding fluoride, bifluoride and oxo

anions dihydrophosphate and benzoate.190 The absorption and emission spectra of

receptor in 0.1% H2O: DMSO revealed a large Stokes shift which was attributed to the

significant structural reorganization upon photo excitation associated with an excited

state proton transfer (ESPT) process. Addition of fluoride induced the formation of a

new absorption peak at 340 nm, while the peak at 309 nm decreased. Furthermore, the

emission peak of receptor at 412 nm, recorded with excitation at the isosbestic point

(322 nm), increased by up to 150 times while the broad emission peak at 470 nm was

practically unchanged.

1.5 Chemosensing ensemble based reports

The classical approach to design a fluorescent sensor involves the covalent linking of a

fluorescent fragment to a receptor, which displays specific binding tendencies towards

a given analyte. Taking inspiration from antibody-based biosensors in immunoassays,

Anslyn and co-workers191 developed another efficient competition approach to the

design of chemosensors, according to the so-called chemosensing ensemble approach

(figure 1.4). In this method the fluorescent indicator is bound to the receptor through

the non-covalent interactions and the fluorescence of the indicator is enhanced or

quenched by the receptor. When the analyte displaces the indicator, the fluorescence

of the indicator recovers.

Two main requirements have to be fulfilled for chemosensing approach: the

receptor/indicator interaction must not be too strong and the indicator must show

intrinsic chemosensor

conjugate chemosensor

Chemosensing ensemble

template-assisted chemosensor

template template

Figure 1.4


52

different optical properties when bound to the receptor and when dispersed in solution.

Thus, the indicator is displaced from the host cavity on titration with the desired

analyte and released to the solution, where it displays drastically different optical

features. Hence, the occurrence of the recognition event is communicated by either a

substantial colour change or a dramatic modification of the light emission.

Anslyn and other research groups have reported the realization of chemosensing

ensembles for the detection of several organic or inorganic substrates, such as

tartarate,192 gallic acid,193 heparin,194 phosphates,195 carbonate,196 amino acids,197 and

short peptides.198

The first report by Anslyn and co-workers for ensemble was based on receptor 177 for

the detection of citrate in aqueous media.199 Receptor 177 was found to be selective for

citrate over dicarboxylates, phosphates, sugars, and simple salts in water. Due to the

preorganization of the three guanidinium moieties on the same face of the receptor and

on the ability to form multiple hydrogen bonding and charge-pairing interactions, it

binds citrate better than simple dicarboxylic and monocarboxylic acid by factors of

around 35 and 700, respectively. The anionic fluorescent dye 5-carboxyfluorescein

was used as indicator in a methanol/water solution buffered at pH 7.4.

The ensemble was used to determine citrate concentration in commercial beverages

which contain high concentrations of potentially competitive anions, including malate,

ascorbate, lactate, benzoate, and phosphates. Inspired from the work by above group,

nowadays different groups reported new designs based on this concept. Recently,

different metal complexes have been utilized as the chemosensing ensemble for the

anion detection.

OO O

COO

COO

NH

N

NH HN

NH

HN

H HN

NH

HN

177


53

A pyrene–terpyridine–Zn conjugate 178 has been synthesized and characterized as a

first ratiometric fluorescent sensor to selectively detect phosphates and pyrophosphates

in aqueous conditions.200 In the conjugate structure, Zn2+ acted as an electron acceptor

to enhance molecular ICT. The selective response to phosphates or pyrophosphates

among various anions involved ICT and ligand competition processes. In the

recognition process, Zn2+ was dissociated from the sensor by the action of phosphates

or pyrophosphates.

A self-organized ensemble of fluorescent 3-hydroxyflavone- Al (III) complex 179 as

sensor for fluoride and acetate ions has been reported by Sathish et al.201 The addition

of aluminium chloride resulted in a self organized ‘turn-on’ fluorescence of 3-

hydroxyflavone (3HF) by complexation reaction in MeOH. The ligand exchange

reaction of this chelate with fluoride ion, released both the 3HF molecules with a rapid

change in the absorbance and fluorescence intensity involving an intermediate

pathway, while only one 3HF moiety is released with that of acetate ion delivering a

quantitative estimation route for F− and OAc− ions in the concentration range from 6

μM to 50 mM and between 0–68 μM respectively.

178

N

N

N

Zn2+

O

OO

OO

O

AlH3C

O

H

CH3

O

H

179

HNO

COOH

COOH

182

HNO

COOH

COOH

181

NN

CuN

N

NCl

O

O

N

Cu

O

O

Cl

N NNH

HN NH

HN

180


54

Tang et al.202 prepared a naphthalene based ligand 180 and its dinuclear Cu (II)

complex [Cu2(180)]4+ which bound oxalate about 4-, 50-, and 200-fold more tightly

than malonate, succinate, and glutarate, respectively. The equilibrium constant for

binding of acetate to the receptor is too small to be measured accurately by the above

method (Ks < 102 M-1). The dinuclear copper complex represents a relatively simple

receptor that binds tightly and selectively to oxalate over other dicarboxylates

(malonate, succinate, glutarate). The two metal complexes in the crystal structure

(Figure 1) appear to be ideally positioned for binding oxalate with an intermetallic

distance of about 5.2 Å.

Lohani et al. synthesized simple anthracene-based chemosensors 181 and 182

containing aspartic glutamic acids in solid-phase synthesis.203 The sensor selectively

and sensitively detected

Fe3+ ions among various metal ions in 100% aqueous solution, and also in mixed

solvent system. 181–Fe3+ and 182–Fe

3+ complexes has also been utilized as

chemosensing ensemble for detection of fluoride ion.

Yang et al. synthesized a rhodamine- sugar probe 183 utilizing this method to detect

thiol containing amino acids.204 A complex of Au+ and a rhodamine hydroxylamine

having 2-deoxyribose could selectively detect cysteine and homocysteine in water.

They proposed the ternary complex structure between 183–Au+ and cysteine.

Therefore, binding of the thiol group of Cys to 183–Au+ made the thiol proton more

acidic and the released proton stabilized the open form of the rhodamine probe. The

binding stoichiometry of 183–Au+ with Cys proved to be 1: 1 and the detection limit

for cysteine was found to be 100 nm.

Lohani et al. synthesized a dual signal ensemble system based on the complex

between a rhodamine derivative 184 and Al3+ for the detection of pyrophosphate (PPi)

183

O NHEt

N

O

OMe

EtHN

Me

O

OH

184

ON N

N

O

NH2

185

NN

O

O O

O

H H


55

in 100% aqueous solutions.205 The ensemble showed highly sensitive and selective

fluorescent and colorimetric response to pyrophosphate among the anions in 100%

aqueous solutions and no interference of the potent biological competitors including

ATP, ADP, and phosphate for the detection of PPi in 100% aqueous solutions at pH

7.4. Ruan et al. reported a compound 185 as Hg2+ based chemosensing ensemble with

perylene bisimide (PBI).206 Hg2+ selectively bound to PBI that bears an imide group

similar to that of thymine and specifically induced aggregation of PBI that led to a

dramatic quenching of PBI fluorescence, thereby allowing for a sensitive and selective

Hg2+ sensing. Addition of thiol–containing amino acids i.e. cysteine to the Hg2+–PBI

ensemble solution induced dissociation of the aggregates and hence recovery of the

fluorescence.

A copper based ensemble {[Cu2(H2L)(OH)(H2O)].(ClO4)2(H2O)} 186 served as a

selective fluorescent sensor for azide ions in aqueous medium. 207 Compound 186

bound with N3- to give [Cu6(HL)2(μ1,1-N3)6] which imposed rigidity and decreased the

non-radiative decay of the excited state to give rise to fluorescent enhancement.

Compound 186 is highly selective for N3- over other anions in aqueous medium. The

fluorescence spectrum of 186 on excitation at 440 nm exhibited a fluorescence

maximum at 503 nm Addition of the N3- ion caused the fluorescence intensity to

increase and the fluorescence maximum underwent a red shift of 25 nm to form a new

band at 528 nm.

Chemosensing ensemble based receptors 187 and 188 has been reported for the

fluorimetric detection of chloride in water at near physiological pH.208 The sensors can

be obtained in situ by mixing a rhodium complex 187/188, a bidentate N, N-chelate

ligand, and a fluorescent dye 189 in buffered aqueous solution. Upon mixing the

N N

O O

O

Cu Cu

H H

O

H

O

HH

186

SO3-

OH-O3S

-O3S

3Na+

189

RhN

NOH2

N

N

SO3-

SO3-

2+

187

N

Rh2+

NOH2

188


56

sensor components, the rhodium complex binds to the N, N-chelate ligand to yield a

metal-based receptor. An ensemble composed of 187 (500 μM) and 189 (50 μM) in

MOPS buffer solution (100 mM, pH 7.0) was prepared. On excitation at λex 480 nm,

weak fluorescence emission was obtained since about 70% of the fluorescent dye is in

its non-emissive complexed form. Addition of Cl- (30.0 mM) then restored the original

fluorescence, amounting to an increase of about 325% in emission intensity.

Amendola et al. reported a dimetallic cryptate [Cu2(190)]4+ which selectively

recognized guanosine monophosphate with respect to other nucleoside monophospates

(NMPs) in a MeOH/water solution at pH 7.209 Recognition is efficiently signalled

through the displacement of the indicator 6-carboxyfluorescein bound to the receptor

and emitted yellow fluorescence.

Zhou et al. reported a ratiometric fluorescence sensing based ensemble Cd2+–191.210

To evaluate the selectivity and sensitivity of Cd2+–191 for anions, absorption and

fluorescence intensity changes upon addition of other anions were measured. Except

PPi, other anions including ADP, AMP, H2PO4-, HPO4

2-, I-, Br-, Cl-, F-, NO3-, HSO4

-,

CH3COO-, oxalate and citrate, did not cause any significant change in the emission

spectra. The outstanding features of the probe were characterized by its ratiometric

fluorescent outputs and good selectivity between ATP and ADP or AMP in aqueous

solutions.

1.6 Observations drawn from literature

From the detailed literature review, the following conclusions were drawn:

v There is a great need for synthesis of receptors for transition metal ions like

mercury, copper, zinc which are biologically and environmentally significant.

To achieve the binding ability of a receptor toward soft metal ions, the

O O

NH

HN

O O

NH

NH

N N

NH

NH

190 191

NH

NH

NH

HN

N

N

O

O


57

incorporation of soft ligating sites such as nitrogen and sulphur in the design of

the receptor is very important.

v On the other hand anion sensing continues to be a very vigorous area of

research, mainly due to the important role of anions in biological systems and

also because of their toxic and deleterious effects, e.g., as environment

pollutants. Among various anions, F-, H2PO4-, and CH3COO- ions are of great

concern.

v The sensing of anions basically requires the introduction of acidic protons for

ion recognition.

v Different scaffolds like crown ethers, cryptands, spherands, calixarenes,

porphyrins, thiacalixarenes, and cyclodextrins etc. have been used for sensing

of different types of metal ions and anions.

v Polyphenyls like terphenyls, hexaphenyl and triphenylene having a great

potential in material chemistry owing to their role as liquid crystalline

materials, molecular scale devices, and molecular receptors are an important

class of molecular scaffolds.

v There was no report in the literature where terphenyls were used for sensing of

cations and anions. So, there is a lot of potential to explore the terphenyl

derivatives and evaluate their sensing behaviour toward soft transition metal

ions and different anions.

v Among various techniques utilized in ion sensing, fluorescence has several

advantages over others given their sensitivity, specificity, and real time

monitoring with fast response time.

v The design of fluorescent sensor involves two approaches. The classical

approach involving the covalent linking of fluorescent fragment to the receptor,

which displays specific binding tendency towards a given analyte. Another

approach is a kind of competitive approach named as chemosensing ensemble

method in which the fluorescent indicator is bound to the receptor by non-

covalent interactions and the fluorescence of the indicator is enhanced or

quenched by the receptor.

v Design of molecular switches, logic gates, molecular elevators, valves, springs

and supramolecular catalysts has enhanced the efficacy of designed receptors.


58

1.7 Objectives of the present work

Based on the above observations drawn from literature, in the present investigation we

have designed and synthesized a few terphenyl and hexaphenyl based derivatives and

evaluated their recognition behaviour towards different cations and anions using

various spectroscopic techniques. A relatively new concept ‘chemosensing ensemble

method’ has also been exploited for molecular recognition using terphenyl based

receptors.

For the facility in presentation, the results of our work have been discussed in

following two chapters.

Chapter 2: Design, synthesis and evaluation of fluorogenic receptors based on

terphenyl having imine moieties

Chapter 3: Design, synthesis and evaluation of fluorogenic receptors based on

terphenyl having amide moieties

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1 Introduction and review of literatureshodhganga.inflibnet.ac.in/bitstream/10603/10359/8/08_chapter 1.pdf · Supramolecular chemistry has been synonymously referred as molecular