Photoluminescence, optical absorption and hypersensitivity in mono- and dinuclear lanthanide (TbIII and HoIII) β-diketonate complexes with diimines and bis-diimine bridging ligand

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Photoluminescence, optical absorption and hypersensitivity in mono- anddinuclear lanthanide (TbIII and HoIII) b-diketonate complexes with diiminesand bis-diimine bridging ligand

Mir Irfanullah, K. Iftikhar n

Department of Chemistry, Jamia Millia Islamia, New Delhi 110 025, India

a r t i c l e i n f o

Article history:

Received 24 February 2010

Received in revised form

5 May 2010

Accepted 14 May 2010Available online 16 June 2010

Keywords:

Terbium

Holmium

Luminescence

4f–4f absorption

Hypersensitivity

Solvent effect

a b s t r a c t

The photoluminescence properties of three Tb(III) complexes of the form [Tb2(fod)6(m-bpm)], [Tb(fod)3

(phen)] and [Tb(fod)3(bpy)] and optical absorption properties of their Ho(III) analogues (fod¼anion of

6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione, bpm¼2,20-bipyrimidine, phen¼1,10-phenan-

throline and bpy¼2,20-bipyridyl) in a series of solvents are presented. The luminescence of the

complexes is sensitive to changes in environment (ligand/solvent) around Tb(III) and co-sensitization of

the ancillary ligands. The enhancement of the luminescence intensity in coordinating solvents is

attributed to the transformation of eight-coordination into less symmetric nine-coordination structure

around Tb(III). Among phen and bpy, the phen is better co-sensitizer while bpm has been observed as

poor co-sensitizer. The enhancement of the oscillator strength of 5G6’5I8 hypersensitive transition in the

4f–4f absorption in some coordinating solvents is attributed to decrease in the symmetry of the field

around Ho(III) ion. The [Ho(fod)3(phen)] is inert towards the solvents and retains its bulk structure and

composition in solution. The transformation of the holmium complexes in DMSO into [Ho(fod)3(DMSO)2]

species is found. The results reveal that the luminescence and 4f–4f absorption properties of lanthanide

complexes in solution can be modulated by tuning the coordination structure through ancillary ligands

and donor solvents.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

The study of lanthanide (LnIII) ions with organic ligands is oneof the most fascinating areas of contemporary research because oftheir unique luminescence and magnetic properties [1–3].Although the intra-configurational f–f transitions are parityforbidden, making direct photo-excitation of Ln(III) ions verydifficult, this problem can be overcome by the use of organicligands containing suitable chromophores attached to the lantha-nides [4]. These ligands act as antenna by absorbing light andtransforming the absorbed energy to the emitting level of thelanthanide ion. A number of organic ligands, which act as antennaunits, have been designed to study their complexes withlanthanides [3–5]; however, the complexes with b-diketones areof particular interest since they (especially Eu(III), Tb(III), Sm(III)and Dy(III) complexes) posses brightest visible luminescence dueto their radiative f–f transitions in solution and solid state [6].The b-diketone ligands possess a strong p–pn absorption band inthe UV region, which is advantageous to efficiently transfer this

absorbed energy to the emitting levels of Ln(III) ion by theantenna effect. Luminescent lanthanide b-diketonate complexeshave significant applications, particularly as emitting materialsfor light emitting devices [7], sensors [8,9] and lasers [10].

The 4f–4f absorption and luminescence spectra of thelanthanide complexes exhibit narrow, line like intra-configura-tional 4f–4f transitions which are very unique from other metalcomplexes and molecular species since the 4f electrons areshielded from the ligands by the outer 5s and 5p shells. However,few transitions of the lanthanide complexes are very sensitive tothe changes in the coordination sphere and symmetry around themetal ion. These are termed as hypersensitive transitions [11,12].The alterations in the spectral intensities and band shapes of thehypersensitive transitions can be used to probe complex forma-tion, changes in coordination geometry and complex–ligand/complex–solvent interactions in solution [13–17].

The lanthanide tris b-diketonates are electrically neutral andcoordinatively unsaturated and rapidly increase their coordina-tion number when allowed to react with neutral ancillary ligandssuch as 2,20-bipyridine (bpy) or 1,10-phenanthroline (phen) [18].The ancillary ligands modify the overall properties of lanthanideb-diketonate complex by improving its thermal stability, filmforming ability (for fabrication of light emitting devices) andprotecting the inner coordination sphere of Ln(III) ions from

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/jlumin

Journal of Luminescence

0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved.

doi:10.1016/j.jlumin.2010.05.015

n Corresponding author. Tel.: +91 11 26837297;

fax: +91 11 26980229/26982489.

E-mail address: [email protected] (K. Iftikhar).

Journal of Luminescence 130 (2010) 1983–1993


solvent quenchers. Furthermore, they may also improve theoverall sensitization of the lanthanide ions if it posses suitableenergy level to effectively transfer the energy to them [19,20].

As part of our continuing research programme, we present hereour findings on the luminescence and 4f–4f absorption properties, atroom temperature, of terbium and holmium complexes of6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione (Hfod) withthree different ancillary ligands, i.e. bpy, phen and 2,20-bipyrimidine(bpm). These complexes are [Ln(fod)3(bpy)], [Ln(fod)3(phen)] and[Ln2(fod)6(m-bpm)] (Ln¼Tb(III) and Ho(III)). The phen and bpy arediimine ligands while bpm is a bis-diimine ligand which also acts asbridge to connect two lanthanides to form dinuclear complexes [21].The effect of varying the ligand/solvent environment around thelanthanide on the (i) luminescence properties of Tb(III) complexesand (ii) absorption intensity (oscillator strength) and band shape ofthe 4f–4f hypersensitive transitions of Ho(III) complexes arepresented.

2. Experimental

2.1. Materials

The commercially available chemicals that were used withoutfurther purification are: Ln2O3 Ln¼Tb and Ho 99.99 % fromAldrich. These oxides were converted to their correspondingchlorides by the standard procedure. 6,6,6,7,7,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione (Hfod) was purchased fromLancaster, 2,20-bipyrimidine, 2,20-bipyridyl and 1,10-phenanthro-line were purchased from Aldrich. The solvents used in this studywere of AR/spectroscopic grade.

2.2. Methods

Infrared spectra were recorded on a Perkin-Elmer spectrum RX1FT-IR spectrophotometer as KBr disc in the range 4000–400 cm�1.Elemental analyses were performed by sophisticated analyticalinstrumentation facility (SAIF), Punjab University, Chandigarh, India.Melting points were recorded in a laboratory designed apparatusand confirmed by the DSC 6220 Exstar 6000 instrument from SIINT,Japan. The thermograms (TGA/DTA) of the complexes were recordedon TG/DTA 6300 Exstar 6000 from SIINT, Japan, under both air andnitrogen atmospheres at a heating rate of 10%

oC/min. The electronic

spectra of the complexes (1�10–3 to 6�10–3 M solutions) wererecorded on a Perkin-Elmer Lambda-40 spectrophotometer, with thesamples contained in 1 cm3 stoppered quartz cell of 1 cm pathlength, in the range 200–1100 nm. The NMR spectra were recordedeither on a Bruker Avance II 400 NMR Spectrometer or BrukerDPX-300 spectrometer. Steady state room temperature excitationand luminescence spectra of the complexes (2.6�10–3 M) wererecorded on Jobin Yvon Flourolog 3-22 Spectrofluorimeter using450 W xenon lamp as the excitation source and R928P PMTas detector.

2.3. Syntheses

The Tb(fod)3 and Ho(fod)3 chelates were synthesized accordingto a published procedure with slight modification that lanthanidechlorides were used in place of lanthanide nitrates [22]. Thecomplexes [Ln(fod)3(bpy)] [18] and [Ln(fod)3(phen)] [18](Ln¼Tb(III) and Ho(III)) were synthesized by reacting Ln(fod)3

chelate and diimine ligand (phen or bpy) in 1:1 mole ratio inethanol. The dinuclear complexes, [Tb2(fod)6(m-bpm)] [21] and[Ho2(fod)6(m-bpm)] were synthesized by reacting respectiveLn(fod)3 chelate and bpm (bis-diimine ligand) in 2:1 mole ratio

in ethanol. The resulting solutions were continuously stirred onhot plate at 40 1C for 6 h. The final solutions were covered and leftfor slow evaporation of the solvent at room temperature. Thecrystals appeared after 24 h. The crystals were filtered off andwashed with cold ethanol. The products were further purified byrepeated crystallization from ethanol and dried in vacuo overP4O10. All the complexes were isolated in high yield (�80%). Thechelates and their complexes were fully characterized before use.

3. Results and discussion

It has been known for many years that coordinativelyunsaturated Ln(b-diketonate)3 chelates rapidly react with NNdonor heterocyclic ligands such as phen and bpy in solution toform coordinatively saturated complexes [18]. Therefore, thereaction of Ln(fod)3 chelate with an equimolar amount of phenand bpy in ethanol yields air and moisture stable complexes of thetype [Ln(fod)3(phen)] and [Ln(fod)3(bpy)], while reaction of thechelate with 2,20-bipyrimidine in 2:1 molar ratio yields homodi-nuclear complexes of the type [Ln2(fod)6(m-bpm)] (Ln¼Tb, Ho), asshown in Chart 1. The formation of these complexes wasconfirmed by melting points, elemental analysis, thermalanalysis, IR, UV–vis and NMR studies. These are knowncomplexes, hence their detailed characterization is not reported.One of us in an earlier report [18] has found the bpy complexes asdoubly hydrated ten-coordinate. However, during the presentsynthesis the ten-coordinate complexes could not be isolatedrather eight-coordinate complexes were formed.

3.1. Thermal studies

The TGA/DTA of Tb(III) and Ho(III) complexes are shown inFigs. 1 and 2, respectively. The complexes exhibit similar thermalbehaviour with one-step weight loss. The DTA curves of thelanthanide complexes show two endothermic peaks: one sharppeak at lower temperatures corresponding to the melting of thecomplex and an another comparatively broad peak at highertemperatures, which is consistent with the volatilization of thecomplexes. It can be seen from Figs. 1 and 2 that melting points ofdinuclear bipyrimidine complexes are much higher than the

N

NO

F3CF2CF2C

F3CF2CF2C F3CF2CF2C

O

3

Ln

O

O

3

Ln

N

N

3

O

O

CF2CF2CF3

NN

N NO

O

3

Ln Ln

Ln = Tb(III) and Ho(III)

[Ln(fod)3(bpy)] [Ln(fod)3(phen)]

[Ln2(fod)6(µ−bpm)]

Chart 1. Chemical structure of the complexes.

M. Irfanullah, K. Iftikhar / Journal of Luminescence 130 (2010) 1983–19931984


mononuclear bipyridine and phenanthroline analogues, whichreflect that the dinuclear complexes are thermally more stablethan their mononuclear analogues. Among mononuclearcomplexes the phen complexes are thermally more stable overbpy complexes, which is due to rigidly planar structure of phenthan flexible bpy.

4. Photoluminescence properties of Tb(III) complexes

The room temperature excitation and emission spectra of[Tb(fod)3(phen)] and [Tb(fod)3(bpy)] complexes in acetonitrile(non-coordinating solvent) and ethanol (coordinating solvent) areshown in Fig. 3. The 5D4-

7F5 line at 545 nm was monitored toobtain the excitation spectra. The excitation spectra of thecomplexes feature an intense broad band between 335 and400 nm with a maximum around 355 nm. These bands areattributed to ligand-centered (S0-S1) transitions of b-diketonateand heterocyclic ligands. The excitation bands of the complexes

(330–380 nm in ethanol) overlap well with the ligand centeredp–pn absorption bands (200–360 nm in ethanol) of the complexes,thus indicating that ligand-to-Tb(III) energy transfer proceedsthrough the triplet state. Upon excitation at S0-S1 band maxima,the complexes show green luminescence. The luminescencespectra are attributed to the deactivation of 5D4 state to 7Fj (j¼6,5, 4, 3) levels. These emission peaks have been observed at 491 nm(5D4-

7F6), 545 nm (5D4-7F5), 584 nm (5D4-

7F4) and 623 nm(5D4-

7F3). Similarly, the room temperature excitation andemission spectra of the dinuclear terbium complex, [Tb2(fod)6

(m-bpm)], were recorded using the S0-S1 excitation band maximain benzene (non-coordinating solvent) and acetone (coordinatingsolvent) and are shown in Fig. 4.

4.1. Solvent effect on luminescence properties

It is seen (Figs. 3 and 4) that luminescence intensities ofTb(III) complexes, in solution, are sensitive to the changes in

Temp Cel

350.0300.0250.0200.0150.0100.050.0

DT

A u

V

70.00

60.00

50.00

40.00

30.00

20.00

10.00

0.00

TG

%

100.0

90.0

80.0

70.0

60.0

50.0

40.0

30.0

20.0

10.0

0.0

III

III

80.2Cel4.71uV

108.7Cel17.08uV

219.5Cel20.71uV

Fig. 1. TG/DTA plots of (i) [Tb(fod)3(bpy)], (ii) [Tb(fod)3(phen)] and (iii) [Tb2(fod)6(m-bpm)].

Temp Cel

350.0300.0250.0200.0150.0100.050.0

DT

A u

V

80.00

70.00

60.00

50.00

40.00

30.00

20.00

10.00

0.00

TG

%

100.0

90.0

80.0

70.0

60.0

50.0

40.0

30.0

20.0

10.0

0.0

III

III

114.2Cel12.78uV

90.5Cel-0.88uV

225.4Cel17.78uV

Fig. 2. TG/DTA plots of (i) [Ho(fod)3(bpy)], (ii) [Ho(fod)3(phen)] and (iii) [Ho2(fod)6(m-bpm)].

M. Irfanullah, K. Iftikhar / Journal of Luminescence 130 (2010) 1983–1993 1985


environment (solvent/ligand). The intensity observed in coordi-nating solvents (ethanol/acetone) is larger than the non-coordi-nating solvents (acetonitrile/benzene). It is well documented thatthe luminescence properties of lanthanide complexes are sensi-tive to two important factors: (i) variations in the symmetry of thecoordination sphere around the Ln(III) ion [23–25] and (ii) theoverall energy match of the ligand-centered triplet state and

emissive level of the Ln(III) ion [26–28]. The changes in theluminescence intensity of the complexes in these solvents can beattributed to the change in the symmetry of the field aroundTb(III) ion since the luminescence intensities in coordinatingsolvents are significantly higher. The 4f–4f transitions of lantha-nides are forbidden for electric–dipole radiation in a symmetricenvironment. If the symmetry of the ligand field is reduced, the

320

0

1x105

2x105

3x105

4x105

5x105

6x105

7x105

8x105

9x105

1x106

1x106

1x106

1x106

1x106

2x106

2x106

2x106

Inte

nsity

(a.

u)

Wavelength (nm)

S0 S1

A

B

a

b

c

d

360 400 440 480 520 560 600 640 680

7F55D4

7F65D4

7F45D4

7F35D4

Fig. 3. Excitation (A) and emission (B) spectra of Tb(III) complexes: (a) [Tb(fod)3(phen)], (b) [Tb(fod)3(bpy)], in ethanol; and (c) [Tb(fod)3(phen)] and (d) [Tb(fod)3(bpy)],

in acetonitrile.

320

0.0

5.0x104

1.0x105

1.5x105

2.0x105

2.5x105

3.0x105

3.5x105

4.0x105

Inte

nsity (

a.u

)

Wavelength (nm)

S0 S1

BA

5D4

acetone

benzene

360 400 440 480 520 560 600 640 680

7F5

5D47F6

5D47F35

D4

7F

4

Fig. 4. Excitation (A) and emission (B) spectra of [Tb2(fod)6(m-bpm)] in different solvents.



f–f transitions are no longer strictly forbidden, since unevenligands and field components can mix with opposite-parity statesin 4fn-configuration levels. Consequently, the enhanced intensityof Tb(III) luminescence in a coordinating solvent could beattributed to a change in the structure in solution, whichleads to decrease in the symmetry of the ligand field around theTb(III) center. It is well known that eight-coordinate complexesof the form [Ln(b-diketonate)3(NN)] (NN¼phen or bpy) or[Ln2(b-diketonate)6(m-bpm)] generally possess square antipris-matic geometry in solid state [29–32] and the intense radiativeemission of these complexes is related to the low symmetryassociated with this geometry [33]. However, the coordination ofa solvent molecule to these complexes would further lower theabove symmetry and transform it to monocapped squareantiprismatic geometry [23,33,34] around the lanthanide ion.This further decrease in the symmetry of the ligand field aroundthe Tb(III) ion, due to the coordination of solvent molecule,increases the radiative transition probability and as a resultenhanced emission intensity of the Tb(III) complex is observed.However, it is important to mention here that the quenching ofluminescence by the CH/OH groups of the solvent has not beenconsidered since it is negligible in the case of terbium complexesas compared to the europium complexes. The luminescencequenching is inversely proportional to the energy gap betweenthe emitting state and the ground state of the Ln(III) ion. Theenergy of the emitting state Tb(III) (20,500 cm�1) is much higheras compared to the Eu(III) (17,250 cm�1), and as a consequencesolvent quenching of Tb(III)-luminescence is negligible [6].

4.2. Role of ancillary ligands

We compare the luminescence intensity of the three Tb(III)complexes in non-coordinating solvent. The luminescence is mostintense in [Tb(fod)3(phen)] followed by [Tb(fod)3(bpy)] and thedinuclear complex, [Tb2(fod)6(m-bpm)], is the least intense amongthe three (Figs. 3 and 4). In the three complexes, the Tb(III) centeris coordinated by six O-atoms of the b-diketonate and twonitrogen atoms of the NN donors (phen, bpy and bpm). Thus, thecoordination number and effective symmetry around Tb(III)center in each complex remain similar in non-coordinatingsolvent and the only variable is the ancillary heterocyclic ligand.The ancillary ligands may considerably increase the luminescenceintensity of the lanthanide complex through formation of intra-ligand states [19,20]. Therefore, the variation in the luminescenceintensity could be related to co-sensitizing ability of differentheterocyclic (ancillary) ligands coordinated to the Tb(III) in thesecomplexes. The substantial contribution of phen ligand to theoverall sensitization process of Tb(III)-centered luminescence in[Tb(fod)3(phen)] is proved by its higher luminescence intensity.This may also indicate a good energy match between the phencentered triplet state and the 5D4 emissive level of the Tb(III). Incase of lanthanide complexes, ligands may sensitize the Ln(III) ionby energy transfer, and probability is strongly influenced by theenergy difference between the ligand triplet state and theemissive level of the Ln(III) ion. It was earlier pointed out thatfor an effective energy transfer the energy difference betweentriplet state of the ligand and 5D4 level of Tb(III) should be at least1850 cm�1 [27]. The triplet energy states of fod (22,500 cm�1)[35], bpy (23100 cm�1) [36] and phen [36] (22,200 cm�1) arehigher than the 5D4 (20,500 cm�1) level and the energy differencebetween these triplet states and 5D4 state of Tb(III) is 2000, 2600and 1700 cm�1, respectively. Accordingly, bpy complex would beexpected to show larger luminescence intensity than phencomplex since its triplet state is more suitably placed to efficientlytransfer energy to the Tb(III) ion than the triplet state of phen.

However, the experimental finding is opposite of it, which couldbe related to rigidly planar phen ligand as compared to flexiblebpy. The presence of a rigid planar ligand in the complex allowsfeasible energy transfer due to effective overlap. This results in ahigher intensity of the sensitized luminescence [37]. In contrast,the excited singlet (S1) state of bpm was recorded at 265 nm(37,735 cm�1) from the high energy shoulder of ligand centeredp-pn absorption band (240–400 nm) of this complex. This largeenergy associated with the excited singlet state suggests that bpmhas a high energy triplet state (T1), which was also observed inother lanthanide tris b-diketonates [31]. Thus, bpm appears toplay a minor role in the thermal population of 5D4 emitting stateof Tb(III). Therefore, it is inferred that phen is a betterco-sensitizer followed by bpy while bpm, as ancillary ligand, isa poor co-sensitizer. Furthermore, the lower luminescenceintensity of dinuclear bpm complex compared to mononuclearphen and bpy complexes can also be attributed to the intramo-lecular Tb(III)–Tb(III) energy transfer in this complex. It may beemphasized that the luminescence properties of dinuclearlanthanide complexes are affected by intramolecular intermetallicenergy transfer since it is a nonradiative process and partiallycontributes to the quenching of Ln(III) ion emission [38].

4.3. Luminescence properties in pyridine

The luminescence characteristics of the complexes were alsostudied in pyridine which has a strong coordinating capability. Itwas expected that coordination of pyridine would increase theluminescence properties of these complexes due to the transfor-mation of eight-coordination structure to less symmetric nine-coordination structure [23]. However, the observed results are notin agreement with the expectation and the luminescenceintensity virtually decreases. It is observed that the luminescenceof Tb(III) complexes (in pyridine) is dominated by a broad bandemission, which can be assigned to the ligand centered fluores-cence (Fig. 5). This suggests that the efficient energy transfer fromligands to the Tb(III) center does not take place and back transferof energy from Tb(III) ion prevails. These results could not beexplained on the energy ground, i.e. close energy differencebetween the ligand based triplet state and the emissive level ofthe Tb(III) ion, which results in back-energy process, sinceefficient green luminescence has been observed in othersolvents. This exceptional luminescence behaviour of thesecomplexes, in pyridine, could also be explained on the ground

4000

1x105

2x105

3x105

4x105

5x105

b

a

Inte

nsity

(a.

u)

Wavelength (nm)

Ligand fluorescence

5D47F5

5D47F3

420 440 460 480 500 520 540 560 580 600 620 640 660 680 700

Fig. 5. Emission spectra of (a) [Tb(fod)3(bpy)] and (b) [Tb(fod)3(phen)] in pyridine.



of symmetry around Tb(III) center in pyridine rather than a closeenergy match between triplet level of the ligand and 5D4 emittinglevel. Pyridine is a known strong coordinating solvent, the resultssuggest that two pyridine molecules enter the inner coordinationsphere of Tb(III) and transform the coordination sphere into amore symmetric species, [Ln(fod)3(L)(Py)2] as compared to lesssymmetric [Ln(fod)3(L)] (where L¼phen or bpy). Thus,coordination of two pyridine molecules to the eight-coordinatelanthanide b–diketonate complexes sufficiently increases thesymmetry [25] and disallows the characteristic radiativetransitions and consequently Tb(III)-T1 back energy transfersprevails over f–f radiative transitions. Thus, ligand fluorescencedominates over the Tb(III)-luminescence.

5. The optical absorption properties of Ho(III) complexes

The intensity of the absorption band can be expressed in termsof a quantity called oscillator strength (P). Experimentally it isrelated to the integrated area of the absorption band and can beexpressed in terms of absorption coefficient e(n) and the energy ofthe transition ‘‘n’’ (cm�1) as given below [39]:

P¼ 4:31� 10�9 9ZðZ2þ2Þ2

" #ZeðnÞ dn ð1Þ

where Z is the refractive index of the solution and e(n) is the molarextinction coefficient at wavelength n.

The optical absorption spectra of the complexes were recorded(Figs. 6–8) in a series of solvents (carbon tetrachloride,dichlomethane, chloroform, methanol, ethanol, acetonitrile,nitromethane, benzene, acetone, pyridine and DMSO). Thespectrum of [Ho2(fod)6(m-bpm)] in acetonitrile could not berecorded since it is insoluble in this solvent. The absorptionspectra of the complexes contain ten multiplet-to-multiplettransitions originating from 5I8 ground state of Ho(III) to variousexcited states. These are (i) 5F5’

5I8, (ii) 5S2, 5F4’5I8, (iii) 5F3’

5I8,(iv) 5F2’

5I8, (v) 3K8’5I8, (vi) 4F7/2’

5I8, (vii) 5G6’5I8, (viii) 5G5,

3G5’5I8, (ix) 5G4, 3K7’

5I8 and (x) 5G2, 3H5, 3H6’5I8. The

oscillator strength (P) of the complexes in different solventsalong with the oscillator strength of HoCl3 in water is given inTables 1–3. The transition, 5G6’

5I8 (450 nm, 22,178 cm�1) isvery sensitive to the change in environment around Ho(III) ion.The oscillator strength of this transition in the complexes showsdrastic increase as compared to Ho(III) aqua ion, in any givensolvent. This transitions follows the electric quadrupole selectionrule 9DJ9r2, 9DL9r2 and DS¼0, and has been classified as beinghypersensitive by many previous workers [40–46]. It is noted that3H6’

5I8 absorption band observed at 360 nm (27,708 cm�1) alsoshows much higher oscillator strength in the complexes ascompared to Ho(III) aqueous system (Tables 1–3). However, theoscillator strength of this transition is �2.5 times less than the5G6’

5I8 hypersensitive transition. The 3H6’5I8 absorption

transition does not follow quadrupole selection rules, howeverit has also been classified as hypersensitive [40–43]. Thistransition, in the dinuclear complex, is masked by strong ligandcentered p–pn transitions in most of the solvents and in some ofthe solvents in mononuclear complexes too (for instance,nitromethane and acetonitrile).

It should be pointed out that the hypersensitive as well as non-hypersensitive transitions of the dinuclear complex, [Ho2(fod)6

(m-bpm)], are twice more intense as compared to theirmononuclear analogues: [Ho(fod)3(bpy)] and [Ho(fod)3(phen)](Tables 1–3). This two-fold increase in the oscillator strengths isdue to the presence of two Ho(III) metal ions in [Ho2(fod)6

(m-bpm)]. It offers remarkable application of the 4f–4f absorptionspectroscopy and the oscillator strength can be used as a tool tofind out the number of lanthanides present in analogouscomplexes. We are the first to use this correlation which is veryuseful in deciding the presence of number of lanthanide ions in amultinuclear complex.

5.1. Solvent effect on hypersensitivity

The solvents employed in this study can be broadly classifiedas (i) non-coordinating solvents (chloroform, dichloromethane,carbon tetrachloride, acetonitrile and benzene) and (ii) potentially

355.7 380 400 420 440 460 480 500 520

Wavelength (nm)

540 560 580 600 620 640 660 684.6

0.000

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.492

0.0070.02

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0.345

nm

A

5F

3

5F

2

3K

8

5G6

5G

5,

3G

5 5I 8

5I 8

5I 8

5I8

5I 8

5G

4,

3K

7

5I 8

pyridine

acetone

CCl4

methanol

5I85G6 hypersensitive transition

5G2,3H5,

3H6

5I8

A

5S2,5F45I8

5F55I8

442.0 444 446 448 450 452 454 456 458 460 462 464 466 468 470 472 474 475.0

Fig. 6. 4f–4f absorption spectra of [Ho2(fod)6(m-bpm)] in different solvents. (A) Full spectrum (B) inset showing the resolution of 5G6’5I8 hypersensitive transition. All the

transitions originate from 5I8 ground state.



354.1 380 400 420 440 460 480 500 520 540 560 580 600 620 640 662.3

0.000

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.442

5F

3

5F

2

3K

8

5I 8

5I 8

5I 8

5G6

5I8

5G

5,

3G

55I 8

5G

4,

3K

7

5I 8

442.4 444 446 448 450 452 454 456 458 460 462 464 466 468 470 472 474 476 477.3

0.002

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.447

nm

A

pyridine

CCl4

acetoneethanol

hypersensitive transition

B

5G2, 3H

5,3H

65I

8

A

A

Wavelength (nm)

5S2,5F4

5I8 5F55I8

Fig. 7. 4f–4f absorption spectra of [Ho(fod)3(bpy)] in different solvents. (A) Full spectrum and (B) inset showing the resolution of 5G6’5I8 hypersensitive transition.

All the transitions originate from 5I8 ground state.

349.1 360 380 400 420 440 460 480 500 520 540 560 580 600 620 640 660 675.0

0.000

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24

0.26

0.28

0.30

0.328

5F

3

5F

2

3K

8

5I 8

5I 8

5I 8

5G6

5I8 hypersensitive transition

5G

5,

3G

55I 8

5G

4,

3K

7

5I 8

5I8

440.3 442 444 446 448 450 452 454 456 458 460 462 464 466 468 470 472 474 476 478.0

-0.005

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

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0.24

0.26

0.28

0.30

0.320.332

nm

A

BCCl4

ethanol

pyridinebenzeneacetone

methanol

5G2,3H5,

3H6

A

Wavelength (nm)

A

5S2,5F4

5I85F5

5I8

Fig. 8. 4f–4f absorption spectra of [Ho(fod)3(phen)] in different solvents. (A) Full spectrum and (B) inset showing the resolution of 5G6’5I8 hypersensitive transition.

All the transitions originate from 5I8 ground state.



coordinating solvents (methanol, ethanol, acetone, nitromethaneand pyridine). It is noted that the magnitude of oscillatorstrengths of the hypersensitive transitions of the dinuclear

complex, [Ho2(fod)6(m-bpm)], in non-coordinating solvents(chloroform, dichlomethane, carbon tetrachloride) shows closesimilarity while in potentially coordinating solvents (methanol,

Table 1Oscillator strengths of the 4f–4f transitions of [Ho2(fod)6(m-bpm)] in different non-aqueous solvents.

Transitions5I8 (G.S)

Spectral

range (cm�1)a

Ho3 + aqua

ion P (�106)b

Solvents (P�106)c

A B C D E F G Hd

5F5 14,956–15,865 3.35 5.53 4.42 5.94 5.60 4.67 5.09 5.40 4.945S2, 5F4 17,935–18,855 5.11 7.42 6.37 7.48 6.12 6.79 6.65 7.73 6.525F3 20,210–20,840 3.09 1.18 1.36 1.68 1.72 1.54 1.39 1.86 1.385F2 20,840–21,195 0.24 0.23 0.29 0.33 0.25 0.23 0.34 0.32 0.275K8 21,195–21,460 0.15 0.21 0.19 0.25 0.26 0.15 0.21 0.19 –5G6 21,460–22,625 5.64 143.94 141.09 139.82 148.46 138.21 148.61 156.22 155.97 (76.34)d

5G5, 3G5 23,025–24,275 2.93 7.13 5.99 6.15 7.33 6.85 7.82 8.28 6.275G4, 3K7 25,315–26,427 – f f f 1.01 f 2.61 f –5G2, 3H5, 3H6 27,050–28,040 2.33 f f f 60.32 f 61.23 f 62.97

a The spectral ranges observed for the transitions vary from solvent to solvent, so the values listed here are only meant to indicate approximate location of the

bands.b Data taken from Ref. [40].c A¼chloroform; B¼dichloromethane; C¼carbon tetrachloride; D¼methanol; E¼benzene; F¼pyridine; G¼acetone; H¼DMSO.d The complex was dissolved in DMSO after heating on gas burner in the solvent.e The values in parentheses are due to Ho(fod)3 in DMSO.f These transitions are masked by the strong ligand/solvent absorption.

Table 2Oscillator strengths of the 4f–4f transitions of [Ho(fod)3(bpy)] in different non-aqueous solvents.


Spectral

range (cm�1)a

Solvents (P�106)b,c

A B C D E F G H I Jd

5F5 14,956–15,865 2.48 2.73 2.75 2.44 2.54 2.56 2.60 2.85 2.65 2.435S2, 5F4 17,935–18,855 3.62 3.82 3.55 3.53 3.57 3.63 3.27 3.14 3.53 3.015F3 20,210–20,840 0.86 0.94 0.87 0.90 0.83 0.91 0.84 0.93 0.89 0.705F2 20,840–21,195 0.20 0.18 0.16 0.17 0.16 0.19 0.17 0.12 0.17 0.203K8 21,195–21,460 0.14 0.14 0.12 0.15 0.12 0.14 0.12 0.12 0.13 –5G6 21,460–22,625 73.03 71.61 67.78 71.20 66.34 69.01 61.77 85.37 71.71 77.76 (76.34)e

5G5 , 3G5 23,025–24,275 3.84 4.57 4.05 4.34 3.90 4.08 3.81 4.70 4.24 3.275G4, 3K7 25,315–26,427 1.59 1.84 1.33 1.38 1.13 f 0.97 1.92 1.41 –5G2, 3H5, 3H6 27,050–28,040 29.30 27.16 26.47 29.11 24.21 f f 36.13 26.06 32.03


bands.b The oscillator strengths of aqua ion are given in Table 1.c A¼chloroform; B¼dichloromethane; C¼carbon tetrachloride; D¼ethanol; E¼benzene; F¼nitromethane; G¼acetonitrile; H¼pyridine; I¼acetone; J¼DMSO.d The complex was dissolved in DMSO after heating on gas burner in the solvent.e The values in parentheses are due to Ho(fod)3 in DMSO.f These transitions are masked by the strong ligand/solvent absorption.

Table 3Oscillator strengths of the 4f–4f transitions of [Ho(fod)3(phen)] in different non-aqueous solvents.


Spectral

range (cm�1)a

Solvents (P�106)b,c

A B C D E F G H I Jd

5F5 14,956–15,865 2.67 2.54 2.33 2.78 2.57 2.72 2.33 2.37 2.47 2.225S2, 5F4 17,935–18,855 3.62 3.64 3.51 3.76 3.79 3.67 3.34 3.21 3.57 –5F3 20,210–20,840 0.87 0.92 0.87 0.89 0.90 0.89 0.84 0.78 0.89 0.805F2 20,840–21,195 0.16 0.17 0.17 0.16 0.18 0.21 0.15 0.15 0.17 0.235K8 21,195–21,460 0.15 0.13 0.12 0.13 0.13 0.14 0.10 0.12 0.12 0.245G6 21,460–22,625 72.49 71.07 69.58 73.40 70.09 72.60 62.52 62.30 72.20 75.07 (76.34)e

5G5, 3G5 23,025–24,275 4.28 4.28 4.18 4.34 4.29 4.32 3.88 3.85 4.35 –5G4, 3K7 25,315–26,427 1.15 0.17 1.25 1.33 1.42 1.07 1.03 1.36 1.12 –5G2,3H5, 3H6 27,050–28,040 26.53 25.12 21.20 28.19 26.53 24.67 f 22.58 f 27.33


bands.b The oscillator strengths of aqua ion are given in Table 1.c A¼chloroform; B¼dichloromethane; C¼carbon tetrachloride; D¼methanol; E¼ethanol; F¼acetonitrile; G¼benzene; H¼pyridine; I¼acetone; J¼DMSO.d The complex was dissolved in DMSO after heating on gas burner in the solvent.e The values in parentheses are due to Ho(fod)3 in DMSO.f This transitions was masked by the strong ligand/solvent absorption.



pyridine and acetone) the oscillator strengths are �5–10% largerthan the values observed in the above mentioned solvents(Table 1). The magnitude of the oscillator strengths of thehypersensitive transition, in the case of [Ho(fod)3(bpy)], showssmall variations in most of the solvents studied; however, in caseof pyridine, a �17% increase in oscillator strength is observed(Table 2). Moreover, it is remarkable to mention that themagnitude of the oscillator strengths of the hypersensitivetransition of [Ho(fod)3(phen)] remains unaffected in any of thesolvents (coordinating or non-coordinating) (Table 3). A consider-able decrease in oscillator strength of hypersensitive transition ofall the complexes has been found in benzene as compared toother non-coordinating solvents (Tables 1–3), while in the case ofphen complex, the oscillator strength decreases in pyridine.

The changes in oscillator strengths of the hypersensitivetransition indicate a change in the inner coordination sphere ofLn(III) ion because the intensity of this transition is very sensitive tothe changes in the direct environment around the lanthanide ion.Hence, the results of our investigation reveal that a complex–solventinteraction occurs in some solutions. Accordingly, [Ho(fod)3(phen)]is found most stable among the complexes investigated and retainsits bulk structure and composition in solution since the oscillatorstrength of its hypersensitive transition does not show any change invarious coordinating and non-coordinating solvents except pyridineand benzene. The second most stable complex is [Ho(fod)3(bpy)]since it shows an increase in the oscillator strength of itshypersensitive transition only in pyridine compared to othercoordinating and non-coordinating solvents. The dinuclear complexshows an increased oscillator strength in three coordinating solvents(methanol, acetone and pyridine) compared to other solvents andhence is a least stable complex in solution among the threecomplexes. Given the complexities encountered in the structuresand species formed in the solution, it is not possible to give a simpleexplanation for the increase or decrease of oscillator strengths insolvents. The source of hypersensitivity has been proposed in manytheories but a reliable explanation has not been reached so far.Hypersensitivity in lanthanides has been shown to depend on(i) symmetry of the complex and vibronic coupling [44], (ii) degreeof the covalency of the metal–ligand bond [45], (iii) basicity of theligand [45] and (iv) non-uniformity of the electric field of theambient medium [46]. An increase in the oscillator strength ofhypersensitive transition of [Ho(fod)3(bpy)] in pyridine and[Ho2(fod)6(m-bpm)] in methanol, pyridine and acetone suggeststhat a solvent molecule enters the first coordination sphere of thesecomplexes. The addition of a solvent molecule in the firstcoordination sphere will change the geometry and ‘‘effective’’symmetry of the ligand field around the Ho(III) ion. These areeight-coordinate complexes with a square antiprismatic geometry insolid state [29–32]; the coordination of a solvent molecule willtransform them into nine-coordinate complexes (monocappedsquare antiprismatic in solid state) [23,34], which has a less effectivesymmetry of the field around Ho(III) than the original eight-coordinate complexes [33]. Therefore, the present result can beexplained as a geometry-related effect and an explanation of this canbe found in the inhomogeneous dielectric theory of hypersensitivity

proposed by Jørgensen and Judd [44]. However, addition of a ligand(solvent) may result in an increase in the degree of covalency, whichcould also lead to increase in oscillator strength of hypersensitivetransition [45].

The small variations in the magnitude of oscillator strengths ofthe hypersensitive transition of the complexes in non-coordinat-ing solvents reveal that these solvents are also responsible for thehypersensitivity, however this is an outer sphere effect on Ho(III).The results can only be explained on the basis of diverse solute–solvent interactions. The results suggest that chloroform stronglyinteracts with the solute (complex) through hydrogen–bond

formation with fluorine atoms of the complex, since it possessesan active hydrogen atom [47]. On the other hand, carbontetrachloride seems to show the least interaction as comparedto both chloroform and dichloromethane. The decrease in theoscillator strength in benzene and pyridine (in the case of phencomplex) could be due to an electron withdrawing ring currenteffect on the outer sphere field of Ho(III).

5.2. Effect of ancillary ligands

Although the complexes studied are eight-coordinate havingsimilar coordination environment in which each Ln(III) ion iscoordinated to six O-atoms from three b-diketonate moieties andtwo N-atoms from an ancillary heterocyclic ligand, even thendissimilar solvent effects on their hypersensitive transitions areobserved. It is interesting to associate these results with thebasicity of the heterocyclic ligands (bpm, bpy and phen). Phen ismore basic (pKa¼4.9) than bpy (pKa¼4.35) while bpm has thelowest basicity (pKa¼0.6). Being most basic, phen would beexpected to contribute more electron density through Ho–N bondmaking the Ho(III) ion more electron rich. As a result, the Ho(III)will show less attraction in gaining additional electron densityfrom electron donor solvents. The rigidly planar structure of phenwould also be helpful in obstructing coordination of incomingsolvent molecule to the closely packed inner coordination sphereof Ho(III) and prevents the complex–solvent interaction. The bpyis a flexible ligand and less basic than phen, thus strong donormolecule like pyridine could find an easy access to the innercoordination sphere of Ho(III) ion. Contrary to above two cases,the bpm is a much weaker base and would not transfer themagnitude of electron density as phen does. The Ho(III), aftercoordinating with bpm, would be scanty in electron density andwould seek more electron richness. Therefore, electron donorsolvents like methanol, acetone and pyridine would find an easyaccess to enter the coordination sphere of the metal ion. Thecoordination of the solvent molecule changes the symmetry of thecomplex and may introduce an additional covalence contribution,which would be considered responsible for enhanced oscillatorstrength of the hypersensitive transition [44,45]. The coordinationof solvent molecule also increases the luminescence intensity ofits Tb(III) analogues (vide supra). However, we have proposed thattwo pyridine molecules enter the coordination sphere of Tb(III)complexes and thereby increase the symmetry of these com-plexes while in Ho(III) complexes the increased oscillatorstrengths in pyridine suggest that only one pyridine moleculeenters the coordination sphere of Ho(III). This anomaly could bedue to the differences in the size of these Ln(III) ions. The Tb(III)ion being larger than Ho(III) ion could easily accommodate twopyridine molecules and expands its coordination number to ten inthe solution as compared to Ho(III), which would have a closelypacked structure due to its smaller size.

5.3. Solvent effect on band shape

The changes in band shape of the hypersensitive transitions,with a change in the environment, have been used as a qualitativeindication of symmetry and has been correlated with change incoordination geometry and symmetry of the complex [14,15,40–43]. From the absorption spectra of [Ho2(fod)6(m-bpm)] invarious solvents, it can be seen (inset in Fig. 6) that the bandshape of the hypersensitive transition (5G6’

5I8) is noticeablydifferent in methanol and pyridine than in carbon tetrachloride.The band shape of this transition in all other solvents (not shownin Fig. 6) except in chloroform are similar to the band shape incarbon tetrachloride. The noticeable change in the band shape of



this transition in pyridine and methanol corresponds well withthe increase in the oscillator strength of this transition in thesesolvents. It is amazing that the largest increase in the oscillatorstrength of this transition is noted in acetone, however it is notreflected on the band shape. Moreover, the increase in theoscillator strength of the hypersensitive transition of [Ho(fod)3

(bpy)] in pyridine is well reflected by a change in the band shapeof this transition (see inset in Fig. 7). Such increase in theoscillator strength and change in the band shape, in pyridine, hasbeen noted in lanthanide complexes [13,15]. Since the largestincrease in the oscillator strength for [Ho(fod)3(bpy)] is noted inpyridine (Table 2) with distinctively different band shape, it isreasonable to associate this with a change in geometry due tocoordination of pyridine. Pyridine coordination has been demon-strated in many lanthanide complexes [23,40]. It is furthersupported by the observation noted by Iftikhar et al. [48] whohave found proton resonances due to coordinated pyridine in thespectra of nine-coordinate Eu and Yb complexes in pyridine-d5

and have confirmed that pyridine did coordinate withoutdisplacing any of the ligands already present, thus resulting inan increase in the coordinated ligands around the lanthanides.The band shape of the hypersensitive transition of [Ho(fod)3

(phen)] remains unchanged in all the solvents except chloroform(see inset in Fig. 8). This corroborates well with unaffectedoscillator strengths of hypersensitive transition of this complex invarious coordinating and non-coordinating solvents (Table 3).Finally, it is remarkable that the hypersensitive transition of thecomplexes in chloroform displayed a fairly different band shape(Fig. 9) as compared to all other solvents. This supports ourpresumption that chloroform affects the outer sphere field ofHo(III) through strong hydrogen–bond interaction with fluorineatoms of the fod moiety and as shown recently in the case of[Ln(hfaa)3(phen)] (hfaa¼hexafluoroacetylacetonate) complexesdue to large asymmetry of the chloroform, which increasesasymmetry of the field around lanthanide, in this solvent [49].

5.4. Absorption properties in DMSO

The absorption spectra of the Ho(fod)3 chelate and threeHo(III) complexes were investigated in DMSO. The chelate ishighly soluble in this solvent while the complexes dissolve withdifficulty and are not highly soluble. However, upon heating thecomplexes on gas burner in DMSO solvent, they become soluble in

this solvent and do not reappear when the solution attainsambient temperature. The oscillator strengths of the Ho(III)chelate and the three complexes in DMSO were calculated. It isinteresting to note that oscillator strength of the hypersensitivetransition of the chelate and the complexes, [Ho(fod)3(bpy)] and[Ho(fod)3(phen)], are virtually similar, while the oscillatorstrength of this transition for dinuclear complex, [Ho2(fod)6

(m-bpm)], is twice as those observed for mononuclear complexes(vide supra). The band shape of the hypersensitive transition forall the three complexes and Ho(fod)3 is remarkably identical asshown in Fig. 10. This is only possible if structurally similarspecies exist in solution. Therefore, we believe that DMSO, being avery strong coordinating solvent (Gutmann number¼29.8) [24],invades the inner coordination sphere of Ho(III) by replacing the

441.80.000

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.557 a

b

c

A

Wavelength (nm)

444 446 448 450 452 454 456 458 460 462 464 466.0

Fig. 9. The band shape of 5G6’5I8 hypersensitive transition of (a) [Ho2(fod)6(m-bpm)], (b) [Ho(fod)3(phen)] and (c) Ho(fod)3(bpy)] in chloroform.

440.3

Ho(fod)3

Ho(fod)3(bpy)

Ho(fod)3(phen)

Ho2(fod)6(bpm)

5G65I8 Hypersensitive

transition

Wavelength (nm)

A

450 460 470 480 490

Fig. 10. Band shapes of 5G6’5I8 hypersensitive transition of Ho(III) complexes in

DMSO.



heterocyclic ligand to give [Ho(fod)3(DMSO)2] species asshown below.

HoðfodÞ3þ2 DMSO�!½HoðfodÞ3ðDMSOÞ2�

½Ho2ðfodÞ6ðm�bpmÞ�þ4 DMSO�!heat

2 ½HoðfodÞ3ðDMSOÞ2�þbpm

½HoðfodÞ3ðphenÞ�þ2 DMSO�!heat½HoðfodÞ3ðDMSOÞ2�þphen

½HoðfodÞ3ðbpyÞ�þ2 DMSO�!heat½HoðfodÞ3ðDMSOÞ2�þbpy

This result also corroborates with our earlier report [15] on[Nd(acac)3(phen)] and [Nd(acac)3(bpy)]. The [Ln(b-diketonate)3

(DMSO)2] species have been reported in the literature [50]. Byreplacing the heterocyclic ligand, structurally similar species exists insolution. Thus, these complexes, which have different structures inthe solid state, acquire similar structures in this solvent.

6. Conclusions

We have shown that photoluminescence properties of struc-turally related three Tb(III) b-diketonate complexes with differentancillary heterocyclic ligands and optical absorption properties oftheir Ho(III) analogues exhibit remarkable sensitivity to thechanges in inner coordination sphere of the Ln(III) ion. The greenluminescence intensities of Tb(III) complexes in coordinatingsolvents are enhanced as compared to the intensities in non-coordinating solvents. The transformation of eight-coordinatestructure into less symmetric nine-coordinate structure aroundTb(III) due to the coordination of solvent has been demonstrated.The substantial contribution of ancillary ligands in the overallsensitization process of Tb(III)-luminescence has been observed.The phen and bpy as ancillary ligands have been found betterco-sensitizers compared to bpm to sensitize Tb(III)-luminescence.The prevalence of ligand fluorescence over Tb(III) luminescencehas been observed in pyridine. The oscillator strength ofthe 5G6’

5I8 hypersensitive transition of Ho(III) in dinuclear[Ho2(fod)6(m-bpm)] is enhanced in coordinating solvents (metha-nol, pyridine and acetone) compared to the oscillator strength innon-coordinating solvents. The oscillator strength of the dinuclearcomplex, in any of the solvents, is twice of those noted formononuclear analogues. This confirms that two holmium centersare present in the dinuclear system. The [Ho(fod)3(bpy)] showsenhanced oscillator strength for hypersensitive transition only inpyridine while for mononuclear [Ho(fod)3(phen)], the oscillatorstrength of this transition remains unchanged in coordinating andnon-coordinating solvents. Therefore, [Ho(fod)3(phen)] is inerttowards any of the solvents and retains its bulk structure andcomposition in solution. The results give a great insight ofcontrolling the luminescence and absorption properties oflanthanide complexes in solution by tuning the coordinationstructure through ancillary ligands and donor solvents.

Acknowledgments

Part of the work was supported by the UGC Special AssistanceProgramme to the Department of Chemistry, Jamia Millia Islamia(no. F.540/17/DRS/2007/SAP-1), which is gratefully acknowledged.MI thanks CSIR (Govt. of India) for Senior Research Fellowship.

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