15
Journal of Photochemistry and Photobiology C: Photochemistry Reviews 5 (2004) 139–153 Review Bis(azacrown ether) and bis(benzocrown ether) dyes: butterflies, tweezers and rods in cation binding Suzanne Fery-Forgues , Fatima Al-Ali Laboratoire des Interactions Mol´ eculaires R´ eactivit´ e Chimique et Photochimique, UMR 5623 au CNRS, Universit´ e Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex, France Accepted 9 July 2004 Abstract This paper focuses on various chromophores that have been covalently linked to two azacrown or benzocrown ether units. These compounds can interact with near-UV or visible light. By comparison with monocrown dyes, they display particular behaviour in the presence of cations. The crowns may have the possibility to act in concert, thus forming sandwich complexes around the guest. This often leads to increased selectivity towards large cations. This is the case for photochromic crown ethers, which allow the photochemical control of cation binding, as well as for some optical sensors, designed for cation recognition. Biscrown dyes can also be rigid structures in which the two spatially separated crowns cannot co-operate intramolecularly. However, intermolecular sandwich complexes can be detected. They range from discrete face-to-face arrangements to extended supramolecular architectures, which can generate nanosized aggregates and materials. The variety of possible combinations with cations bestows a particular interest on biscrown dyes. © 2004 Japanese Photochemistry Association. Published by Elsevier B.V. All rights reserved. Keywords: Biscrown; Photochromism; Chemosensors; Fluorescence; Luminescence; Sandwich complex; Supramolecular assembly Contents 1. Introduction ..................................................................................................... 140 2. Cation selectivity and stoichiometry ................................................................................ 140 3. Photochromism and photochemical control of cation binding .......................................................... 141 4. The “biscrown” effect in optical sensors ............................................................................ 143 4.1. Rigid binding pockets ...................................................................................... 143 4.2. Pivot-centred molecules .................................................................................... 144 4.3. Molecular tweezers ........................................................................................ 145 5. Rigid dyes with remote crown moieties ............................................................................. 145 5.1. Assuming one cation per crown ether ........................................................................ 145 5.2. From dicomplexes to discrete intermolecular sandwich complexes .............................................. 146 5.3. Ion-induced formation of extended supramolecular architectures ................................................ 149 5.4. From supramolecular assemblies to nanomaterials ............................................................. 149 6. Conclusions ..................................................................................................... 150 References ........................................................................................................... 151 Corresponding author. Tel.: +335 61 55 68 05; fax: +335 61 55 81 55. E-mail address: [email protected] (S. Fery-Forgues). 1389-5567/$20.00 © 2004 Japanese Photochemistry Association. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jphotochemrev.2004.07.001

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Page 1: Bis(azacrown ether) and bis(benzocrown ether) dyes: butterflies

Journal of Photochemistry and Photobiology C: Photochemistry Reviews5 (2004) 139–153

Review

Bis(azacrown ether) and bis(benzocrown ether) dyes:butterflies, tweezers and rods in cation binding

Suzanne Fery-Forgues∗, Fatima Al-Ali

Laboratoire des Interactions Mol´eculaires R´eactivite Chimique et Photochimique, UMR 5623 au CNRS,Universite Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex, France

Accepted 9 July 2004

Abstract

This paper focuses on various chromophores that have been covalently linked to two azacrown or benzocrown ether units. These compoundscan interact with near-UV or visible light. By comparison with monocrown dyes, they display particular behaviour in the presence of cations.The crowns may have the possibility to act in concert, thus forming sandwich complexes around the guest. This often leads to increasedselectivity towards large cations. This is the case for photochromic crown ethers, which allow the photochemical control of cation binding,

spatiallym discrete

he variety of

C

as well as for some optical sensors, designed for cation recognition. Biscrown dyes can also be rigid structures in which the twoseparated crowns cannot co-operate intramolecularly. However, intermolecular sandwich complexes can be detected. They range froface-to-face arrangements to extended supramolecular architectures, which can generate nanosized aggregates and materials. Tpossible combinations with cations bestows a particular interest on biscrown dyes.© 2004 Japanese Photochemistry Association. Published by Elsevier B.V. All rights reserved.

Keywords:Biscrown; Photochromism; Chemosensors; Fluorescence; Luminescence; Sandwich complex; Supramolecular assembly

ontents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

2. Cation selectivity and stoichiometry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

3. Photochromism and photochemical control of cation binding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141

4. The “biscrown” effect in optical sensors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1434.1. Rigid binding pockets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1434.2. Pivot-centred molecules. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1444.3. Molecular tweezers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

5. Rigid dyes with remote crown moieties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1455.1. Assuming one cation per crown ether. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1455.2. From dicomplexes to discrete intermolecular sandwich complexes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1465.3. Ion-induced formation of extended supramolecular architectures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1495.4. From supramolecular assemblies to nanomaterials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

6. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

∗ Corresponding author. Tel.: +335 61 55 68 05; fax: +335 61 55 81 55.E-mail address:[email protected] (S. Fery-Forgues).

1389-5567/$20.00 © 2004 Japanese Photochemistry Association. Published by Elsevier B.V. All rights reserved.doi:10.1016/j.jphotochemrev.2004.07.001

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140 S. Fery-Forgues, F. Al-Ali / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 5 (2004) 139–153

1. Introduction

Since their discovery by Pedersen in 1967[1], crownethers have proved to be excellent tools for capturing alkaliand alkaline earth cations, for which very few specific re-ceptors existed. They are actually “hard bases”, which ef-ficiently complex with “hard acids” such as groups I andII metal ions[2]. The covalent association of crown etherswith various dyes has given rise to a new generation of com-pounds. The crown ether moiety plays the role of ion recep-tor, while the dye allows the system to interact with light.Two main classes of compounds can be distinguished, de-pending on whether a photochemical reaction is involved ornot. If it is, the question essentially concerns photochromiccrown ethers. They can be reversibly interconverted betweendistinct forms by light irradiation[3,4]. Their photochemi-cal properties govern their ability to complex cations, andvice versa. Therefore, they are key materials for the photo-chemical control of ionic conduction, and the elaboration ofionic photoswitchable devices[5–7]. When no photochemi-cal reaction is involved, the dye is used as a signalling unit,allowing the property changes of the system to be monitoredwith optical methods. This is the field of crown ether basedsensors, dedicated to the detection of ions in solution or atinterfaces. Very often, the dye displays luminescence prop-

r-lly

has

icavy

es-meey

heyheari-in-ded

ithhlye of-owanoin-edughiten-

p-nedanin-es

2. Cation selectivity and stoichiometry

To understand the behaviour of biscrown dyes, it is impor-tant to consider the different complexes that can be formed.It has long been acknowledged that the selectivity for a givencation depends on many factors, including the nature of theheteroatoms borne by the crown ether, the cation charge, andthe way the cation size matches the internal diameter of thecrown[15–17]. This latter parameter will be here of the ut-most importance, because it will determine the type of com-plex formed.

As illustrated inFig. 1, the simplest complexes are ofthe 1:1 type, meaning that one cation is inserted into onecrown. This is the case, for example, for benzo- or aza-15-crown-5 (ring size = 1.7–2.2A) [18] in the presence of Na+cations (diameter 1.98A) [18,19]. This type of complex canof course be encountered for biscrown dyes. Their formationis characterized by an equilibrium constantK1, expressed asfollows:

M + L � ML, K1 = (ML)

(M)(L)(1)

where M is the metal cation, L the organic receptor moleculeand ML the complex formed.

At high cation concentrations, 1:2 host–guest complexesc atione

M

Fa

l

-

an also appear, with the two crowns occupied by one cach. The equilibrium equation can then be written as:

L + M � M2L, K2 = (M2L)

(ML)(M)(2)

ig. 1. Different types of complexes formed between biscrown derivativesnd cations.

erties, allowing analysis by fluorimetry, a real-time monitoing technique whose high sensitivity yields an exceptionalow detection threshold. This concept of fluoroionophoresbeen extensively reviewed in the literature[8–13]. Potentialapplications encompass the areas of medical and biochemanalysis, cell biology, and the control of pollution by heametals.

Among the multiple structures that have been invtigated when combining dyes with crown ethers, sobear two crown ethers, and are often symmetrical. Thshare common features with monocrown dyes, but talso display particular complexing behaviour, due to tpresence of two receptors in the same molecule. A vety of possible arrangements is allowed, ranging fromtramolecular sandwich complexes to ion-induced extensupramolecular architectures, which are not conceivable wmonocrown dyes. Biscrown dyes therefore lead to higion-selective photochemical systems and sensors, somfering new tactics for ion-detection. We shall also see hion-induced superstructures can be used to generate nmaterials. This article is aimed at showing the specialterest of biscrown dyes. Its scope is intentionally limitto monoazacrowns and benzocrowns, which offer enoexamples to give a good overview of this field. Despthat, it is not intended to be exhaustive. Only a brief metion will be given to biscrown dyes whose optical proerties have not been exploited. It must also be underlithat bis(benzocrown ether)s were the topic in 1994 ofexcellent review by Bradshaw’s team, and the reader isvited to refer to it, to get a wider idea of related structur[14].

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S. Fery-Forgues, F. Al-Ali / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 5 (2004) 139–153 141

As a matter of fact, it is advisable to wonder whether the in-sertion of a second cation is easier or more difficult than thatof the first. In conjugated molecules, the first cation, which iselectron withdrawing, can modify the electron environmentof the second crown. Electrostatic repulsion can also hap-pen when the complexed crowns lie in close proximity. Thisco-operation effect has been well discussed in the literature[20–23]. It is now accepted that the two crowns act indepen-dently with respect to complexation when the ratio of theassociation constantsK2/K1 is equal to 0.25. A negative co-operation effect is encountered when this ratio is lower than0.25, and a positive effect is said to happen in the oppositecase.

When a cation is too big to be accommodated by only onecrown, sandwich complexes can be formed, with two crownethers co-ordinating the same cation (Fig. 1) [24]. For in-stance, the crowns with five ether links form sandwich com-plexes with K+ (2.66A) and Ba2+ (2.68A) [18,19], whilecrowns with six ether links form sandwich complexes withCs+ and Sr2+. The pioneering work of Smid and co-workersshowed that such complexes easily form intramolecularlywhen two crown ethers are connected by a tether[25,26].Such ’clam’ molecules are particularly effective due to en-tropic gain in the process of bringing two macrocycles to-gether around a captured ion. This “biscrown” concept hasb struc-t orted[ -p theflc rgec e se-l s.F beent -di earr

uchim d form sts

M

I triesa twom cts lso bf

dd ofv thec

3. Photochromism and photochemical control ofcation binding

Let us first consider the case where crown ethers are asso-ciated to a photochromic moiety. As far as biscrown structuresare regarded, the strategy is generally to switch from a form inwhich the two crown ethers are too far apart to co-operate, toa form in which they face each other and can give a sandwichcomplex with an ion.

In the early eighties, Shinkai et al. first reported that theazobenzene moiety, which isomerizes photochemically be-tween its trans and cis forms, can be used for changingthe bis(crown ether) configuration[39–41]. Compoundcis-1 (Fig. 2) reverts thermally to thetrans isomer. The au-thors compared this interconversion to the motion of a but-terfly. The roughly lineartrans isomer shows high affinityfor sodium cations, while thecis isomer permits the crownethers to act in concert in the binding of K+ and larger cations.Cation extraction and transport through a liquid membrane[41] or a composite membrane[42] was shown to be con-trolled by light. Thetrans andcis isomers of the stilbenicderivative (2) of Lindsten et al.[43] also display markeddifferences in their binding behaviour, although in thecis-isomer, the crown ether does not have the correct alignmentto form a tight sandwich complex with a potassium ion. Thiss lowst uir-i nota

ened n-dg ules,r io-p rma-t rowne con-f cturei n re-a usb t ex-p ens

con-f ouldn ob-s nt pho-t kingti rtedf ndb o thee tlev rtantr

een subsequently developed by many authors. Crystalures, although sparse, and NMR evidence have been rep14,27–31]. The formation ofintramolecularsandwich comlexes depends on the proximity of the crowns, and onexibility of the molecule. It obeysEq. (1). This sandwichomplexation is reflected in enhanced selectivity for laations. This property can be of major interest, becausectivity is difficult to achieve with monocrown moleculeor example, until now, monocrown chemosensors have

hwarted in detecting selectively K+ under physiological conitions, that is in the presence of a 30-fold excess of Na+, or

n detecting Cs+ (one isotope of which appears in nucleactor waste) among other alkali cations.

In rigid molecules where the crowns lie far apart, sntramolecular sandwich complexes cannot form. But,inter-olecularcomplexes may appear, as already evidenceonocrown dyes[32–37]. For instance, for 2:1 host–gue

pecies, the equilibrium can be written as:

L + L � ML2, K3 = (ML2)

(ML)(L)(3)

n the precise case of biscrown dyes, other stoichiomere possible, for example, that of 2:2 species, in whicholecules share two cations[31,38]. Besides these distin

pecies, extended supramolecular arrangements can aound, as will be seen below.

It must be noted that constantsK2 andK3 can be defineifferently. So it is important when comparing the workarious authors to know exactly to which equilibriumonstants are related.

e

hows that even if the symmetry of the structure often alhe two parts of the tweezer to fit together perfectly, reqng a minimum additional conformational change, this islways the case.

In contrast with the former structures, the dithienyletherivatives3 (Fig. 3), 4 and5 (Fig. 4), developed indepeently by Takeshita and Irie[44,45] and by Kawai[46],ive thermally stable photoproducts. In these molecotation is allowed around the bond which links the thhene and the cyclopentene group, so that two confo

ions are encountered. In the parallel conformer, the cthers face each other. In contrast, in the anti-parallel

ormer, the crown ethers are held apart. The latter strus photochemically reactive and undergoes a cyclisatioction upon irradiation with UV light. The molecule is thlocked in a coloured, planar form. Upon subsequenosure to visible light, it reverts fully to the initial optate.

Consequently, it was expected that the open parallelormer captures large metal ions co-operatively, which shot happen with the closed structure. This was actuallyerved for molecules of type3. Conversely, it was also showhat the presence of large cations strongly influences theochemical quantum yield of these compounds, by bloche structure in the photoinactive parallel conformer[45]. Its interesting to note that different behaviour was repoor molecules4 and5. Only a weak difference was fouetween the open and the closed forms with respect txtractability of metal picrates[46]. This suggests that subariations in the structure of the molecules play an impoole in this mechanism.

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142 S. Fery-Forgues, F. Al-Ali / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 5 (2004) 139–153

Fig. 2. Chemical structure of compounds1 and2.

Fig. 3. Chemical structure of compound3and mechanism for photorespon-sive tweezers in the dithienylethene series.

The Malachite Green leuconitrile derivatives (6) ofKimura et al. combine the “biscrown effect” with an elec-trostatic effect. They belong to the triphenylmethane dyes,many of which are known to ionise to a quinoid cation andcyanide anion upon UV-light irradiation (Fig. 5). Then, theyreturn thermically to the neutral species. It was shown thatthe electrically neutral form strongly binds a potassium cationby co-operation of the two neighbouring crown ether rings.The ionic form releases the metal cation by electrostatic re-pulsion, the nitrogen atom in the azacrown ether ring be-ing positively charged[47]. This system displays a clear-cut

Fig. 4. Chemical structure of compounds4 and5.

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S. Fery-Forgues, F. Al-Ali / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 5 (2004) 139–153 143

Fig. 5. Chemical structure of compound6.

difference between cation binding and cation release. It couldbe convenient for applications that require the all-or-nonetype of switching, for example, optical data storage. It canalso be noted that, by adding a monoaza-15-crown-5 etheron the vacant phenyl group of6, a triscrown structure wasobtained. This structure was reported to capture the bulkycesium ions, at the expense of a remarkable conformationalchange, and to form multinuclear complexes with other ions[48,49].

A polymer was obtained that carries on the side chainthe Malachite Green leuconitrile derivatives provided withtwo crown ethers. Curiously, the polymer behaved very dif-ferently from the corresponding monomeric analogues. Thecation selectivity was modified by a co-operative action ofthe spatially close crown ether rings, and the photochromismwas governed by the polymer rheology in solution[50].

It must now be noted that the butterfly motion of pho-tochromic crown ethers can also be exploited for synthesispurposes. This is the case for the photocatalyst recently re-ported by Cacciapaglia et al. for the basic ethanolysis of estersand anilides[51]. Its structure is that of compound1, bearingtwo benzo-18-crown-6 ether moieties, each complexed by abarium atom. One of the metal ions acts as a binding unitfor the substrate, and the other delivers an activated ethoxidei cavec nk

F rs anda

4. The “biscrown” effect in optical sensors

The concept of co-operative binding has also been ex-ploited in the field of optical sensors, and more particularlyin that of fluoroionophores. It can be recalled that, to be at-tractive as a fluorescent chemosensor, a molecule must showa significant change in emission behaviour in the presence ofanalytes. In the most commonly encountered structures, themechanism involves photoinduced electron transfer, pertur-bation of the internal charge transfer, or excimer formation.

4.1. Rigid binding pockets

Among bis(crown ether) dyes, the simplest structure ismade of a single chromophore bearing two arms terminatedby a crown ether. In other molecules, two dye moieties, eachbearing a crown ether, pivot around a common axis or arelinked by a flexible spacer. Molecule7 (Fig. 7), built fromanthracene substituted at the 9 and 10 sites, is an archetype ofthe first category. It adopts the classical fluorophore-spacer-receptor model, in which the fluorescence of the dye in theabsence of ion is quenched by photoinduced electron transfer(PET), an electron of the nitrogen lone pair being transferredto an orbital of the excited chromophore. Complexation of ametal ion by the receptor switches off the quenching process,a and1 wichc trongfl plex-a ob-s gestst l, ort

neb pona ons( rgec hibi-t thec n thet dingp -c

on. The reactants are in close proximity when the conis form of the catalyst is involved (Fig. 6), and the reactioinetics is strongly accelerated in this case.

ig. 6. Catalyst–substrate complex for the basic ethanolysis of estenilides.

nd induces fluorescence revival. Complexes of the 1:1:2 host-guest type, as well as 1:1 intramolecular sandomplexes, were reported to be detected. However, the suorescence enhancement that was expected upon comtion of the two crowns in a sandwich complex was noterved. The moderate spectroscopic effect obtained sughat the proportion of sandwich complex formed is smalhat PET is not inhibited in this complex[52].

In compounds of type8, the crown moieties are bory the 1 and 8 positions of the anthracenyl group. Uddition of linear dications such as alkylammonium i+NH3 (CH2)n NH3

+), these compounds displayed lahelation-enhanced fluorescence effects, due to the inion of the PET process. The selectivity with respect toation’s size can be explained by the distance betweewo crowns, the presence of which generates a rigid binocket. Among metallic cations, only CuII leads to a signifiant optical effect[53].

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144 S. Fery-Forgues, F. Al-Ali / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 5 (2004) 139–153

Fig. 7. Chemical structure of compounds7–9.

Molecule9 was not a good sensor for metallic cationseither. However, an NMR study gave evidence for a peculiarphenomenon, called “play catch” by the authors. When a 1:1complex forms between9 and a sodium cation, the cationdoes not seem to be fixed into one crown moiety, but movesslowly from one crown to another. In contrast, potassiumions are conventionally sandwiched between the two crownmoieties[54].

4.2. Pivot-centred molecules

Regarding molecules whose two moieties move around ahinge, the first example that comes to mind is that of crown-containing bis-aryl derivatives. In10 (Fig. 8), the (naphthocrown)ether moieties serve as jaws, which close around K+[28] and organic cations[55], but to our knowledge, the be-haviour of these structures in optical spectroscopy has not yetbeen reported.

Beer’s bipyridyl bis(crown ether)11 leads to a sandwichcomplex with the planar diquat dication. This complex is ev-idenced by a charge transfer absorption band which arisesfrom intermolecular interaction between the two partners.Such a complex does not form with Na+ cations, which pre-fer being fully inserted into each of the two crowns. But, co-ordination with a ruthenium salt leads to a more rigid confor-m of

Fig. 8. Chemical structure of compounds10–13.

intramolecular sandwich complexes with a sodium ion[56].Again, the optical properties have not been described.

In the next example, the hinge is formed by a ferroceneunit [57,58]. The aim of Delavaux–Nicot was to associate afluorescent unit to an electroactive unit, so that the presence ofthe cation is detected by both optical spectroscopy and elec-trochemistry. As we recently explained in a review, very fewexamples of this type of compound exist, because ferrocenederivatives are classical quenchers of excited states, knownto inhibit the luminescence of the dyes they are associatedto [59]. However, in the particular case of13, ferrocene isfar from being a fluorescence quencher, and acts as an aux-ochrome. It is interesting to note that the presence of the twofluorescent crown-containing units is not only useful to en-sure a pincer structure. It is also necessary for the opticalproperties of the compound to be retained, since the ana-logues bearing only one branch were not fluorescent. The in-tensity of the fluorescence signal of13 is markedly decreasedupon addition of Ca2+ and Ba2+ cations. An electrochemi-cal study of closely related compounds revealed that electron

ation (12), which is favourable for subsequent formation
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S. Fery-Forgues, F. Al-Ali / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 5 (2004) 139–153 145

Fig. 9. Chemical structure of compounds14–16.

communication could occur through the fluoroionophore linkbetween the complexing site and the redox centre.

4.3. Molecular tweezers

The two crown-containing dye moieties can also be con-nected by an aliphatic spacer. For instance, a Russian team[60,61]presented a bis-styryl dye of the benzothiazole series(14, Fig. 9). For the monostyryl analogue, cation binding per-turbs internal charge transfer (ICT). The selectivity towardscations is poor, and rather low complex stabilities were ob-tained, probably due to the electron-withdrawing effect of thebenzothiazolium moiety and the electrostatic repulsion be-tween the positively charged dye and the cation. Linking twomonomers by an aliphatic chain results in a dye preorganizedfor the formation of sandwich complexes with bulky cations.A dramatic increase of the stability constant was reported forBa2+ and Sr2+ in acetonitrile. Besides the decrease in fluo-rescence quantum yield observed in the presence of cationscharacteristic excimer (excited dimer) emission was detectedwith Ba2+, clearly supporting the cation-induced molecularstacking of the chromophoric units[61]. The head-to-headalignment of the dye fragments upon complexation also pro-vides an opportunity to control the stereoselectivity and ef-ficiency of the photocycloaddition that takes place betweent

Fig. 10. Chemical structure of compounds17and18.

The system of Otsuki et al.[62] comprises twophthalimide-fused crown ethers (15). Interestingly, incre-mental addition of Cs+ induces a two-stage change in thefluorescence spectrum. The intensity firstly decreases, be-cause the fluorescence is quenched by stacking of the chro-mophores in the sandwich complex. With further addition ofions, each crown accommodates a cation, and the moleculeunfolds. The fluorescence intensity then increases above thatof the free sensor, due to the rigidification of the molecularframework. These results are supported by NMR measure-ments. The two latter examples show that it is possible to dif-ferentiate an intramolecular sandwich complex from a usual1:1 or 1:2 host–guest complex by means of fluorescence.

In this respect, the approach of Yam’s team is particularlyingenious. The polynuclear gold complex16does not containany dye resembling those used in classical organic chemistry[63]. However, it absorbs in the near UV and emits phos-phorescence around 500 nm. Upon addition of K+ ions, asandwich complex is formed, bringing the two AuI centresinto close proximity. The subsequent Au–Au interaction theninduces strong red emission at 720 nm. This behaviour is in-teresting, since systems which give signal enhancement uponcomplexation are the most suitable for practical applications.

5

c liea Thee pear.Hc sen-s

5

tiesh (F ronc

he two subunits[60].

,

. Rigid dyes with remote crown moieties

In some cases, the formation ofintramolecularsandwichomplexes with cations is impossible. The two crownspart from each other on a rigid chromophoric system.xpected 1:1 and 1:2 host–guest complexes can still apowever, the possibility to formintermolecular2:1 sandwichomplexes, where two crowns belonging to two differentors share the same cation, is now increased.

.1. Assuming one cation per crown ether

In the example below, the variations in optical properave not been reported. The phenanthroline derivative17,ig. 10) of Schmittel and Ammon was used to form an iomplex surrounded by six crown ethers[64]. It was shown

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by cyclic voltammetry and NMR that this complex recog-nised alkali and alkaline earth cations. In particular, ratherstrong binding was encountered (forn= 2) with barium salts.It was proposed that every crown captures a barium cation.When Coulomb repulsion becomes too strong, the bariumcations could shift into the crown ether, as far from the nitro-gen atom as possible.

Molecule18, presented by Ataman and Akkaya, is de-signed to give a chromogenic response in the presence ofcations. It is a combination of indoaniline, an intense bluedye, with calixarene. It offers two potential sites of com-plexation. The harder calixdiquinone site preferentially bindsharder cations, such as EuIII , while the azacrown units pref-erentially interact with Na+. Opposite spectral effects areobtained according to the site occupied[65].

The organometallic complexes19 and20 (Fig. 11), de-signed by the teams of Martınez-Manez [66] and Beer[67–69], respectively, can be considered as luminescent sen-sors. They are based on the well-known ruthenium terpyri-dine and bipyridine cores, and bear two peripheral crownethers, which can be part of the electron conjugated system

or not. Their luminescence quantum yield is low, around 5×10−5 for example, for19 [66]. This has been attributed to aquenching effect by a photoinduced electron transfer (PET)from the azacrown ether[66,68], and more unusually fromthe benzocrown ether[68], to the excited chromophore. Inthe presence of metal salts, the PET process becomes moredifficult, and a luminescence increase is observed. This is thecase, for instance, for19 in the presence of heavy metals suchas Hg2+ [66]. However, the magnitude of this effect variesat the best by a factor of 2, so the actual luminescence inten-sity remains very low. This explains that complexation canbe better studied by other methods.

Among ruthenium derivatives, compound21 deservesspecial attention, because another mechanism is proposed[70]. This compound has the particularity to bind alkalications with its crowns, and to capture a chloride anion thatestablishes a hydrogen bond with the amido groups. No co-operative effect between cation and anion binding was found.The increase in emission intensity was attributed only to thebinding of the chloride anion, which imparts some rigidityon the receptor, inhibiting non-radiative decay processes insolution.

In contrast, good emission efficiency was claimed by deSilva et al.[71], regarding the delayed luminescence of aeuropium complex of22. The system is still based on PETi + + sp onee d lu-m neo y thea ound0 -iont

5s

thef lex.A lexc ther is il-l ionv wnr

builtf cule

Fig. 11. Chemical structure of compounds19–22.

2 ina then tionso 1:2h stemi e oneh epul-s rga-n ning

nhibition, the insertion of Na or K ions into the crownreventing the crown nitrogen atom from transferringlectron to the aromatic moiety. In this case, the delayeinescence of22·EuIII (n = 1, 2) is enhanced by over order of magnitude when the azacrown units are bound blkali cations, and the quantum yield reaches values ar.47 in methanol. This was the first example of metal

riggered metal-centred emission.

.2. From dicomplexes to discrete intermolecularandwich complexes

In the following examples, the authors investigatedormation of types of complexes other than the 1:1 compctually, it must be kept in mind that many types of compan co-exist in a solution, their proportions depending onespective concentrations of cations and receptors. Thisustrated byFig. 12, which shows how the species distributaries as the Ca2+ concentration is increased in a biscrood solution.

Ostaszewski et al. present different rod-like systems,rom an anthracene chromophore, like, for example, mole3 (Fig. 13) [72,73]. The complexation of alkali cationschloroform/methanol mixture depends significantly onature and length of the alkyl chain at the 9- and 10-posif the anthracene moiety. Complexes of the 1:1 andost–guest type were proposed to be formed. The sy

s characterised by a negative co-operation effect. On thand, the two complexed cations undergo electrostatic rion. On the other hand, the first complexed cation reoizes the electron density over the whole structure, weake

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S. Fery-Forgues, F. Al-Ali / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 5 (2004) 139–153 147

Fig. 12. Concentration of free sensor and host–guest complexes versusCa2+(ClO4)2− concentration. This scheme was obtained from processingthe absorption data reported for28 (2.7× 10−6 M, in acetonitrile) in[83].

the interaction of the free crown ether with the second cation.The formation of sandwich complexes was ruled out on thebasis that no excimer emission was detected, as observed forother anthracene receptors[8,74].

Squaraine derivatives24 and25were the first linear bis-crown structures to be studied. They display intense absorp-tion and emission bands in the near infrared region, a propertythat is in high demand for biological applications. It has been

proposed that in squaraine dyes the first singlet excited statearises from a charge transfer transition, mainly involving theoxygen atoms and the central cyclobutane ring, with only aminor contribution from the amino moiety[75]. However,the insertion of a metal cation into the crown causes an ab-sorbance decrease also paralleled by a fall in the fluorescenceintensity. A rather weak effect was reported by the team ofGeorge and Das, regarding compound24, placed in the pres-ence of alkali metal salts in an acetonitrile/toluene mixture[76,77]. The formation of 1:1 and 1:2 host–guest complexeswas evoked, but theK2 association constant was not calcu-lated. In contrast, Akkaya obtained strong variations using25 (n = 2) with alkaline earth cations in pure acetonitrile.The formation of 1:2 host–guest dicomplexes was not con-sidered, but 2:1 sandwich complexes were detected[78,79].Barium salts seemed to favour the formation of a blue-shiftedaggregate that was assigned to the stacking of two sensors.Magnesium complexes also slowly developed similar char-acteristics after standing overnight at room temperature. Itmust be noted that squaraines undergo reversible redox pro-cesses, so that the presence of ions can also be detected byelectrochemistry[77]. Compound25 (n = 1, 2) was used forreversible sensing of sodium and potassium, in plasticizedPVC matrix, with a detection limit of 1× 10−9 M [80]. Othersquaraine derivatives26have been reported, the fluorescenceo lexf

bis-c dm or–a ghlys hlo-r in-

Fig. 13. Chemical structure of compounds23–26.

f which is enhanced with lithium, but the type of compormed was not mentioned[81].

In our group, attention was focused on symmetricalrown ketocyanine dyes27–29 (Fig. 14). They are gooodels of fluoroionophores built on the electron doncceptor–donor pattern. Their behaviour was thoroutudied in the presence of alkali and alkaline earth percates in acetonitrile[82,83]. The presence of the cations

Fig. 14. Chemical structure of ketocyanine derivatives27–29.

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148 S. Fery-Forgues, F. Al-Ali / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 5 (2004) 139–153

duces strong variations in the absorption spectrum and adrastic decrease in fluorescence intensity. The analysis ofthe absorption data revealed that complexes of different sto-ichiometries were formed as the cation concentration wasincreased (Fig. 12). These complexes were also detected bymass spectroscopy. The analysis of the association constantsrevealed that in 1:2 host–guest dicomplexes the two crownsindependently bind one cation, although slightly differentconclusions were reported by another group concerning com-pound28 [84,85]. Interesting photophysical behaviour wasevidenced with the 1:2 dicomplex of28. After light ab-sorption by the excited dicomplex, one of the cations israpidly ejected into the bulk solution. The 1:1 complex thusgenerated is then responsible for the fluorescence detected[86].

Clear evidence for the formation of 2:1 intermolecularsandwich complexes was also given[82,83]. Complexes wereformed with the bulky cation Ba2+, but surprisingly, very sta-ble structures were also obtained with Ca2+. It is possible thatthe stacking of the two sensor molecules stabilises the sand-wich complex, which could explain that theK3 associationconstant was particularly high. Only small differences werefound between the three dyes regarding the formation of thesandwich complex. However,28and29are rigid structures,while compound27displays possibilities of rotation aroundt ti don om-p

didn( alim ingt cula-t fromp cientt d outw d thef bis-cs st di-c ge,w oundc essest ech-n notett cu-l ctionm

llowo rownd , andt d co-w d2 f

Fig. 15. Chemical structure of compounds30–33.

viologen-like bis-ammonium compounds[91,92]. This inter-action results in total fluorescence quenching. However, thepresence of metal cations such as Ba2+, which compete forbinding into the crown, leads to the disruption of the com-

Fig. 16. Chemical structure of sandwich complex34 formed between bis-crown stilbene and bisammonium viologen salt.

he carbonyl-carbon quasi-single bond[87]. This shows than this type of molecule, small conformational variationsot play an important role in the formation of sandwich clexes.

According to a first study by a Texan team, thisot seem to be the case for cleft-like molecules30 and31Fig. 15), which showed no ability to extract large alketal ions[88]. No sandwich complex was detected us

he extraction method. Since molecular mechanics calions indicate that the molecules show a slight deviationlanarity, the authors suggested that this could be suffi

o prevent stacking. However, a subsequent study carrieith electrospray ionisation mass spectrometry reporte

ormation of distinct 2:2 host–guest complexes betweenrown clefts30–32 and large alkali metal ions[89]. Theame paper shows that the formation of 1:2 host–gueomplexes is favoured by the rigidity of the quinoline bridhich reduces the electrostatic repulsion between the bations. The comparison between the two reports strhe importance of experimental conditions and of the tique used for the delicate complexation studies. Let us

hat in contrast, tetra-crowned clefts such as33, in whichhe crowns are spatially proximate, easily form intramolear complexes with large cations, detected by the extra

ethod[88,90].The molecules that give sandwich complexes also a

riginal sensing strategies to be achieved. Linear biscyes tend to associate with structures of similar shape

his feature has been smartly exploited by Ushakov anorkers. Biscrown stilbenes2 were shown to form 1:1 an:1 host–guest complexes (such as34,Fig. 16) with a series o

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S. Fery-Forgues, F. Al-Ali / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 5 (2004) 139–153 149

Fig. 17. Chemical structure of compounds35–38.

plex. Fluorescence is then restored, through the formation ofthe metal-complexed sensor[92].

5.3. Ion-induced formation of extended supramoleculararchitectures

The formation of intermolecular sandwich complexes in-duces the assembly of two sensor molecules. In quite thesame way, large supramolecular assemblies can be made (seFig. 1).

For instance, distyrylbenzene35 (Fig. 17) investigated bySchwoppe and Meier generally gave 1:1 complexes with al-kali metal picrates. However, an NMR study revealed thatall the complexes formed with K+ and one of those formedwith Rb+ gave aggregates, for which a stairway structure wasproposed[93].

Evidence is given for the same kind of architecture inthe work of Xia et al.[94–96]. The authors use NMR and

mass spectrometries, but their best argument comes from acrystallographic study, which revealed that in the presenceof lead trifluoroacetate, compound36 (n = 1) forms an al-ternating stairway structure. This is in line with the fluores-cence data. In fact, an unusual mechanism for cation sens-ing was implemented here. It has been shown recently thatfluorescence enhancement can be a response to rigidifica-tion of a chromophore upon substrate binding[97,98]. Con-sequently, it was imagined that custom designed biscrowndyes could become fluorescent only upon sandwich complexformation, a process referred to as self-assembling fluores-cence enhancement (SAFE). For instance, the fluorescenceof distyrylbenzene derivatives36 (n = 0–2) was drasticallyincreased in the presence of Na+, K+ and Cs+ salts, re-spectively. The fluorescence data could be fitted with overall1:1 stoichiometry, that is considering either 1:1 or 2:2 stoi-chiometries. In contrast, a monocrown analogue of36 (n =1) did not show fluorescence enhancement in the presence ofK+, the cation capable of forming sandwich complexes withthe crown. It is deduced that discrete sandwich complexesare not formed for the biscrown dye, which rather leads toextended complexes. This paper also underlines a very in-teresting point, that is, the stoichiometry of the complexesformed depends on the nature of the counterion used (BF4

−,ClO4

− or CF3COO−).ed in

e thalo-c gateh olec-u ey area a re-p am merw -s -p gate[

alsob por-p dt

5

micc re-a llows with arsb sen-t tiest res-c lyfi ifi-c st in-

e

Stilbenic dyes are not the only ones to be encounterxtended supramolecular assemblies. Porphyrin and phyanine derivatives that have the property to self-aggreave been extensively studied for their potential use in mlar electronics and optoelectronics, and also because thttractive for solar energy harvesting. Shinmori and Osukorted evidence that when Zn-porphyrin37was placed inixture of chloroform and acetonitrile, a face-to-face dias formed upon addition of K+ ions. It is interesting to oberve that in the same conditions,meso–mesocoupled diporhyrins38were proposed to form a huge ordered aggre99,100].

The formation of cofacial dimers and columns haseen investigated extensively in tetra(crown ether)hyrins and phthalocyanines[14,101–105], but this is beyon

he scope of the present review.

.4. From supramolecular assemblies to nanomaterials

This overview on biscrown dyes began with photochrorown ethers and will end with other photochemicallyctive compounds. Actually, photochemical reactions aupramolecular assemblies to generate nanomaterialsobust architecture. Molecules of type39 (n= 1, 2) (Fig. 18),tudied by Strehmel et al., resemble compounds35 and36,ut are substituted by four fluorine atoms that are esial for new solvatochromic and photochemical propero emerge[106]. These compounds could be used as fluoent sensors for Li+, Na+ and Ca2+ cations that completet into the crown cavity. Larger metal ions do not signantly change the fluorescence spectrum. But, the mo

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150 S. Fery-Forgues, F. Al-Ali / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 5 (2004) 139–153

Fig. 18. Chemical structure of compound39and example of structure for photoproduct40.

teresting point is that the presence of ions directs the photo-chemical behaviour of these compounds. With small cations,isomerisation of the ethylenic bond is observed, while largerions such as K+, Rb+, Sr2+ and Ba2+ cause photocycload-dition. The photoproduct (40) obtained with39 (n = 1) inthe presence of Sr2+ was analysed by mass spectrometry. Itis a compound of high molecular weight, whose mass cor-responds to that of seven units of39. According to NMRdata, it is formed from stacked supramolecular assemblies,as pictured inFig. 18. This example shows that biscrowndyes can be useful in the manufacture of nanosized materials,and could find interesting applications in this new emergingfield.

6. Conclusions

It appears from this short review that biscrown dyes arefull of possibilities. We focused our attention on the two

fields where research has been the most active, namely pho-tochromic crown ethers and sensors. It is likely that in thenext years, the design of these molecules will be improvedto insure better recognition of large cations. New develop-ments are also emerging. For example, polymers that displaya sensory response are in high demand for the manufactureof electrodes. It has been shown that crown-containing con-jugated polymers are highly selective fluorescent chemosen-sors, the selectivity of which is based on the tweezer effect[107]. Since biscrown derivatives can be introduced in plasti-cized membranes,[108] or polymerized[109,110], it can beimagined that the properties of such systems could be tunedby changing the nature of the biscrown dye or that of thepolymer backbone.

The use of crown ethers at the interface of inorganicchemistry also opens new perspectives in the field of sen-sors. Mechanisms that differ from the classical absorp-tion/emission of organic dyes are therefore involved. Agood example is the following. Crown ether derivatives are

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S. Fery-Forgues, F. Al-Ali / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 5 (2004) 139–153 151

known to adsorb onto metallic surfaces[110,111] and onnanoparticles[112], where they keep their binding prop-erties. It has been shown that crown-functionalised goldnanoparticles give a dispersion in the absence of ions, whilethe presence of K+ induces their aggregation via the for-mation of sandwich complexes. This transition induces adistinct change in colour, in response to surface plasmonabsorption.

In another area, the introduction of biscrown dyes intoorganised media has just begun, but seems promising. Forexample, it has been shown that triazolehemiporphyrazines,with two crown ethers and two fatty chains, can organ-ise into Langmuir–Blodgett films[113,114]. The interactionwith ions has not been regarded yet, but this type of as-sembly could be a first step towards ionic channels in thismedium.

Finally, we should not forget that biscrown dyes are alsouseful for other applications. For instance, Xu’s team reporteda series of bis(benzocrown ether)-substituted cyanine dyes,which act as sensitisers in photographic materials. Their sen-sitivity and storage ability were high compared to that ofconventional cyanine dyes[14,115,116].

As a matter of fact, there is no doubt that biscrowndyes will lead to very interesting applications in the closefuture.

R

45:dam,

and

byers,

tives,

xley,997)

forer-

luo-nsing,

.

94)

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S. Fery-Forgues, F. Al-Ali / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 5 (2004) 139–153 153

Suzanne Fery-Forguesgraduated from the PaulSabatier University of Toulouse, where she alsoreceived her Ph.D. under the guidance of Dr.Nicole Paillous. After a postdoctoral stay in Lon-don (UK) at the Royal Institution, she obtaineda permanent position at CNRS in 1986 in theteam of Pr. Bernard Valeur (Conservatoire Na-tional des Arts et Metiers, Paris). She turnedback to Toulouse in 1990, where she is now Re-search Director at the CNRS. Her scientific in-terest includes the study of fluorescent probes

for applications in bioanalysis, supramolecular devices and molecularmaterials.

Fatima Al-Ali was born in Lebanon and receiveda master degree in chemistry from the LebaneseUniversity in 1998. She obtained her Ph.D. degreefrom the Paul Sabatier University of Toulouse,France, in 2002 under the supervision of Dr.Guita Etemad-Moghadam. She worked as a post-doctoral fellow in the same university in the re-search group of Dr. Suzanne Fery-Forgues. She ispresently a post-doc in Polytechnic University ofMunich, Germany. She is currently interested insynthesis and molecular aggregations properties

of multifunctional phosphorus acid amphiphiles, as well as in synthesisof fluorescents probes for use in biology.