Indian Journal of ChemistryVol. 34A, February 1995, pp. 94-101

Solvent dependent absorption and fluorescence of a ketocyanine dye in neatand binary mixed solvents

Debashis Banerjee, Ashis Kumar Laha & Sanjib Bagchi"Department of Chemistry, Burdwan University, Burdwan 713 104, India

Received 11 March 1994; revised and accepted 23 September 1994

The environmental effect on the ground and excited state properties of a ketocyanine dye hasbeen studied by monitoring its absorption and fluorescence characteristics in various pure and bi-nary mixed solvents. The spectroscopic transition leading to the longest wavelength absorption orfluorescence has been found to involve considerable intramolecular charge transfer as evidenced bysemiempirical MO calculations at the AMI level. Ground state complexation is indicated in proticsolvents, aromatic hydrocarbons and cyclic ethers. The fluorescence maximum, E (F), shows a corre-lation with ET (30) scale of solvent polarity. A multiple linear correlation of E(F) with Kamlet-Taftparameter representing solvent dipolarity (n*) and hydrogen bonding ability (a and (3) suggests thatapart from dipolar interactions specific solute-solvent interactions are important in determining flu-orescence maxima. Study of E (F) in mixed binary solvents points to a preferential solvation of thesolute in the S1 state.

The polarity probes have found extensive applic-ations during the last couple of years, especiallyfor the evaluation of various microenvironrnentalproperties 1-5. Although a large number of miCTOP-olarity reporters involving solvatochromic spectraltransition are known, e.g., 4-methoxycarbonylpyri-dinium iodide:', betaine", 4-cyanopyridinium io-dide", etc., practically all of them are non-fluores-cent. Fluorescent polarity probes are more advan-tageous compared to non-fluorescent probes be-cause of their widespread applications in biologi-cal systems'. Merocyanine dyes" constitute an in-teresting system for solvatochromic studies. Someof these compounds show solvent-sensitive fluor-escence? Recently, Kessler and Wolfbeis" have re-ported the synthesis of a series of strongly fluor-escent ketocyanine dyes having unique solvatoch-ramie properties in both absorption and fluoresc-ence. It has been observed that these compoundsmay act as good fluorescent probes for studyingsolvent polarity. Although fluorescence character-istics of the ketocyanine dyes have been studiedin a few solvents, a systematic study of solvationcharacteristics is yet to be done. Besides this, astudy of spectral characteristics in mixed binarysolvents is of particular interest, for it may pro-vide information regarding the preferential solva-tion characteristics of the absorbing/emitting spe-cies". The objective of the present paper is tostudy the solvation characteristics of a ketocya-

nine dye in pure and mixed binary solvents bymonitoring its longest wavelength absorption andfluorescence band. The dye chosen for the pre-sent study is (1-1 ),(9-1)-di-l ,9-di-(2,3-dihydroindo-lyl)- 4,6-dimethylene- nona-l,3,6,8-tetraene-5-one(Fig. 1). Semiempirical calculations have been car-ried out to provide the optimised geometry, thecharge distribution and the dipole moment in theground state of the solute. It also gives informa-tion regarding the nature of the electronic transi-tion associated with the longest wavelength ab-sorption.

Materials and MethodsThe dye (solute) was synthesised as described

in literature". Indoline, 1,1,3,3-tetramethoxypro-pane and cyclopentanone were purchased fromSigma Chemicals (USA) and used as received.Purity of the prepared compound was checked byIR spectral data [IR bands obtained in KBr disc:1620, 1580, 1485, 1400 em -1] and from absorp-tion and fluorescence, spectral data [Amax(abs) = 525 nm; Amax (fl.)= 622 nm in ethanol]. Allthe solvents were purified and dried by standardprocedures'P'" and distilled over CaH2 immedi-ately before use to ensure the absence of perox-ides and oxidising agents. Mixed solvents andcorresponding solutions were prepared by care-fully mixing the components so as to minimisecontamination by moisture. Absorption spectra

BANERJEE et al.: ABSORPTION & FLUORESCENCE OF KETOCY ANINE DYE IN VARIOUS SOLVENTS 95

were measured on a Shimadzu UV-160A spectro-photometer provided with a peak detection algo-rithm. Fluorescence spectra were recorded on aHitachi F-30lO spectrofluorimeter equipped witha microprocessor and a chart recorder. Freshlyprepared solutions were used for each measure-ment and the concentrations were chosen to giveabsorbances less than 0.1 to avoid distortion ofthe spectra due to reabsorption of the fluoresc-ence light. The spectral studies were done usingsolutions having concentration of the dye in therange 10- 5 to lO- 6 mol dm - 3. The observed ab-sorption/fluorescence spectrum did not, however,depend on the concentration of the solute.

Theoretical calculationsSemiempirical MO calculations at the AM112

(Austin model 1) level using the MOPAC pro-gram (QCPE 355) were carried out on a PC AT/386. A fully optimized geometry indicates a plan-arity of the molecule in the ground electronicstate, except the aliphatic hydrogen atoms. Thedistribution of net charges over the atoms and therelevant bond distance is shown in Fig. 1. It ap-pears that there is a significantlitemation inthe C - C bond length for C - C bonds occurringbetween the N-atom of the five-membered ringand the carbonyl oxygen atom. The dipole mo-ment was found to be 4.02 D, directed exclusivelyin the direction of the carbonyl group. The mole-cule has a symmetry plane passing through theC = 0 bond and perpendicular to the molecularplane. The HOMO and LUMO have been found

50

(a)

to be n molecular orbitals and the relevant coeffi-cients of the p-atomic orbitals have been given inTable 1. While the HOMO is antisymmetric withrespect to the symmetry plane, the LUMO showsa symmetrical behaviour. Moreover,HOMO --> LUMO transition indicates a transfer ofelectron density from nitrogen to the oxygen atom.These findings indicate that in the ketocyaninedye (1) the shortest energy absorption band arisesdue to a n-n" electronic transition involving anintramolecular charge transfer (leT) from nitrog-en donor to oxygen acceptor through the inter-vening conjugated system. The nature of electron-ic transition is thus similar to that in the merocya-nine dyes-".

Results

Absorption spectrumThe results have been summarised in Table 2.

Fig. 2A represents absorption spectra in some

Table I--Relevant coefficients of /M>rbital appearing in LCAOfor HOMO and LUMO

Atom No. HOMO LUMO Atom No. HOMO UJMO

o.no 0.29 9 -(U3 0.162 11.00 - 0.30 18 •• 0.33 0.163 0.29 -0.2S II -O.IR -0.334 -0.29 - O.2S 20 O.IR -0.337 IUS 0.33 13 0.34 O.OR

Iti - O.IS 0.33 22 -0.34 O.OR

36

1JoO 1.40

1-/,()~~11'371'40 N

1-50 ,.46

1-48

Fig. 1- The ketocyanine dye (I) [Numbering of atoms is shown in (a), the net charges over the atoms and the bond distancesarc shown in (b)]

96 INDIAN J CHEM. SEe. A, FEBRUARY 1995

representative pure solvents. The absorptionspectra in all the aprotic solvents show similarstructures which lose their prominence as the po-larity of the solvent is increased. For aprotic sol-vents the shift of the absorption maximum [E(A)]is not very sensitive towards a change in the sol-vent polarity as measured by ET (30) parameter".Considering the ICT nature of the transition, onewould expect the more polar solvent to yield thesmaller transition energy. But the E(A} values forbenzene, toluene, 1,4-dioxane and tetrahydrofur-an are lower although these solvents are charac-terised by.lower polarity in the ET (30) scale.Rather, the E (A) values in these solvents run par-allel to the acceptor numbers of Gutmann". Thispoints to an enhanced solute-solvent interactionin the ground state possibly through complex for-mation in these solvents. This conclusion is alsoin accordance with the large solubility of the so-lute in these solvents. The solute is insoluble in n-hexane but it may be made soluble by addition ofexcess naphthalene; the solution gives similarspectrum as obtained with benzene. This also sug-gests the solute-naphthalene interaction, possiblythrough the zr-electron density of naphthalene.When n-hexane was. added to a benzene solutionof the dye a continuous blue shift was obtainedand no isosbestic- point could be detected. Thestructure and the shape of the band remained

practically unaltered. A plot of E(A) against themole fraction of n-hexane shows a deviation fromlinearity indicating that the solute is solvated pref-erentially by benzene. The preference for benzeneover n-hexane is intelligible in terms of enhancedbenzene-solute interaction through complexation.A red shift ( - 3 kcal mol- 1) is observed as we gofrom an aprotic solvent to a protic solvent. Simi-larity of absorption in various alcohol solvents (asdistinctly different from the other polar aproticsolvents) is suggestive of complex formation be-tween an alcohol and the solute in the groundstate. In a mixed solvent of ethanol with ethyl

A B

500 600 450~Inm

500 550

Fig. 2-(A) Absorption spectra of (I) in neat solvents [l-prop-anol (1), benzene (2), ethyl acetate (3) & acetonitrile (4). (B)Absorption spectrum of (I) in ethanol + benzene binary mix-ture. Volume per .cent of ethanol in the mixture: 0.2(1), 0.8(2),

2.0(3) 20.0(4) and 40.0(5))

Table 2-Relevant solvent parameters and the absorption and fluorescence energies of the ketocyanine dye in pure solvents

Solvent Absorption Fluorescence F(eun') F(e) ET (30)/kcal a fJ :rr*max/kcal max/kcal mol-I

mol-I mol-I

( I) Methanol 54.S 45.0 0.616 0.95 56.3 0.93 0.66 0.60(2) Ethanol 54.5 45.8 0.578 0.94 51.9 0.83 0.75 0.60(3) l-Propanol 54.8 46.1 0.548 0.93 50.7 0.78 0.80 0.52(4) l-Butanol 54.8 46.2 0.526 0.91 50.2 0.79 0.82 0.4'(5) l-Pentanol 54.7 46.2 0.498 0.89 50.0 0.70 0.92(6) l-Octanol 55.0 47.6 0.453 0.86 48.5(7) 2-Methoxyethanol 54.1 45.9 0.516 0.91 52.3 0.71(8) 2-Ethoxyethanol 54.1 45.9 0.554 0.94 50.8(9) 2-Propanol 53.9 47.7 0.546 0.92 48.6 0.76 0.84 0048

(10) Cyc1ohexanol 54.7 47.6 0.470 0.90 46.9(11) Water 50.3 44.0 0.640 0.98 63.1 1.17 0.47 1.09(12) Acetone 57.8 51.3 0.568 0.93 42.2 0.08 0048 0.71(13) 3-Pentanone 57.9 52.6 0.528 0.91 39.3 0045 0.72(14) 2-Heptanone 57.9 52.8 0.492 0.88(15) Dichloromethane 56.3 50.9 0.434 0.84 41.1 0.30 0.00 0.82(16) Dibromomethane 56.5 51.6 0.312 0.79(17) Acetonitrile 56.8 50.8 0.610 0.96 46.0 0.19 0040 0.75(18) Ethylacetate 58.8 53.1 0.398 0.77 38.1 0.00 0045 0.55(19) Benzene 57.8 54.6 0.005 0046 34.5 0.00 0.10 0.59(20) Toluene 57.8 55.3 0.026 0.54 33.9 0.00 O.ll 0.54(21) 1,4-Dioxan 57.8 54.1 0.042 0.45 36.0 0.00 0.37 0.55(22) Tetrahydrofuran 57.8 55.3 00418 0.81 37.4 0.00 0.55 0.58(23) Dimethylformamide 56.1 50.6 0.548 0.96 43.8 0.00 0.69 0.88

BANERJEE et al.: ABSORPTION & FLUORESCENCE OF KETOCYANINE DYE IN VARIOUS SOLVENTS 97

acetate and benzene an isosbestic point is clearlyindicated (Fig. 2B). The equilibrium constant forcomplexation with ethanol comes out to be 3.2and 4.3 when benzene and ethyl acetate respect-ively are used as cosolvents.

The spectrum of the solvents in an alcoholicsolvent also reflects the overlap of band fromH-bonded and bare forms of the solute. Absorp-tion measurements in ethanol at various tempera-tures also indicate the existence of isosbesticpoint. An increase in temperature indicates an in-crease in the intensity of the higher energy band.These facts point to the exothermic formation ofa hydrogen-bonded complex in ethanol. Formixed aqueous solvents no such isosbestic pointcould be detected. The band shows a gradual ba-thochromic shift as the mole fraction of water isincreased. A plot of the transition energy in mixedaqueous solvents is shown in Fig. 3. Owing to theextreme insolubility of the solute in water the datacould be recorded only upto a mole fraction of0~8with respect to water. It may be seen from thefigure that in all the cases linear plots are ob-tained and they converge to a value of 51 kcalmol- I for pure water. Throughout the wholerange of Xeo.olvent' Fig. 3 indicates almost similartransition energy for acetone and 1,4-dioxane, al-though the solvent polarity is greater in the for-mer case. This behaviour is in keeping with the1,4-dioxane-solute complexation as discussed ear-lier.

Fluorescence spectraThe emission may be characterised as the

S I -. So transition. Fig. 4 shows fluorescence spec-tra in some representative pure solvents. The flu-orescence spectrum at 298 K is structureless ex-cept in aromatic hydrocarbon and cyclic ethersolvents where two emission bands are clearly ob-served. For example, in benzene the observed flu-orescence spectrum indicates two bands with themaximum at 525 and 548 nm respectively(Fig. 4). The 548 nm band appears to be similarto the fluorescence band in the more polar sol-vent ethyl acetate. This indicates complex forma-tion with benzene. An interesting correlation ee-tween the fluorescence maxima is observed withDimroth-Reichardt polarity parameter ET (30) asmay be seen from Fig. 5 and this indicates thatthe nature of transition is similar in the two cases.It may be noted that the available data in neatsolvents may be best represented by two correla-tion lines. The data points for aprotic solvents fallon one line while the protic solvents form a se-parate class.

w

62.,oE 58

CJ"

.:: 51+

0·2 0·1+ 0-6

Xcosolvent

()'8 o

Fig. 3-Plot of the energy of maximum absorption of (I) inmixed aqueous solvents versus the mole fraction of the cosol-vents. [l!;.= pyridine. • = acetone, 0 = J ,4-dioxane, 0 = luti-

dine]

500 550 600'A/nm

Fig. 4-Fluorescence spectra of (I) in neat solvents [(a)l-propanol, (b) benzene, (c) ethyl acetate and (d) acetonitrile 1

'T'0E050u

~- 10 9 6

~L&J 4'5

35 60 6545 50 55-1

ET (30)/kcal mol

Fig. 5-Plots of maximum emission frequency of compound(I) versus ET (30) for neat solvents; numbers refer to the

serial no. in the Table 2

40

98 INDIAN J CHEM. SEe. A, FEBRUARY 1995

The double linear dependence of the fluoresc-ence energy on ET (30) indicates that the natureof the emitting state is different in the tWDclassesof solvents':'. For the aromatic hydrocarbons andcyclic ethers the higher energy maximum is in linewith the behaviour of other aprotic solvents, whilethe lower energy maximum falls below the line.Thus the lower energy band in these solventsarises due to a species in which the nature of so-lute-solvent interaction is different from that inthe case of Reichardt's betaine, the indicator dyefor E r (30) values. Thus, the higher energy bandis probably due to bare solute modified by dipo-lar solvation while the lower energy band is possi-bly due to complex formation in the excited state.

The sensitivity of the bands with solvent polar-ity prompted us to undertake the studies in mixedsolvents. The following mixed binary solventshave been used: protic + aprotic (ethanol + ben-zene; ethanol + acetone; ethanol + ethyl acetate;water + acetone; and water + dioxane) and aprot-ic + aprotic (acetone + ethyl acetate and ben-zene + acetonitrile). Fig. 6 shows the fluorescencespectra in a representative mixed binary solventas a function of solvent composition. The fluor-escence maximum changes continuously to thered as increasing amounts of a cosolvent with ahigher polarity [e.g. in the ET (30) scale] areadded to it. The intensity of the band increaseswith the addition of the polar cosolvent and thendecreases. In benzene and dioxane, where the flu-orescence spectrum is structured, the addition ofa polar cosolvent leads to a very rapid decreasein the relative intensity of the band around 525nm. The 548 nm band on the other hand shows arapid red shift. The variation of E12 (F), the maxi-mum energy of fluorescence in mixed binary sol-vents, as a function of the mole fraction has beenshown in Fig. 7. It may be seen that for all theprotic + aprotic binary mixtures the fluorescencemaximum changes rapidly when the mole fractionof the protic cosolvent is very small. But at highermole fractions the position of the band maximumbecomes less sensitive to the solvent composition.For aprotic + aprotic mixture the nature of varia-tion depends on the component solvents. Thus alinear variation of E12 (F) with the solvent com-position is obtained in the case of acetone + ethylacetate while for benzene + acetonitrile the varia-tion is non-linear.

Discussion

Pure solventsA spectroscopic transition, in general, involves

>-.-.....II")

zw.-z.....

550 600

A/nm700650

Fig. 6-Fluoresccnce band of (I) in mixed binary solventscontaining ethanol and ethyl acetate. The mole fraction of

ethanol (x) increase in the order l(x = 0) > 2 > ...> 8(x = 1)

;".0 A B

0.2 D." 0.1 0.'x1z-

Fig.7-Plot of E\2 (F) versus mole fraction [A: mixed binarysolvents of ethanol with benzene (0), ethyl acetate (D) andacetone (~); B: mixed binary solvents of water with dioxane(0) and acetone (~); C: benzene + acetonitrile system; D: ace-

tone + ethyl acetate system.]

BANERJEE et al.: ABSORPTION & FLUORESCENCE OF KETOCYANINE DYE IN VARIOUS SOLVENTS 99

an equilibrium initial state and a non-equilibriumfinal state. Using the formalism developed byMarcus 14 for treating polar media under equilibri-um and non-equilibrium conditions, one can par-tition the Franck-Condon energies £ (A) and£(F) as£(A)= £+(~F;~q- ~Fleq)+ £RO(O) .,. (1)E(F) = E+ (Ol·;~q- ~Ffq) - £RO(1) ... (2)where £(A) and E(F) denote the energies corre-sponding to maximum absorption and emissionrespectively. E denotes the difference in the freeenergies of the ground and the equilibrium excit-ed states. ~f;~q and (5f;cq denote the equilibriumsolute-solvent interaction free energy in the Soand S I states respectively. The third term in equ-ations (1). and (2) represents solvent reorganiza-tion energy in the So and S 1 states. If 'the reorga-nization energies are equal", we obtain£(A)+ £(F)= 2£+ 2(~f;fq - ~Flq)£(A)- £(F)= 2ERO

... (3)

... (4)Thus, the Stoke's shift is a measure of the non-equilibrium ERO term while £(A)+ £(F) termshave only the equilibrium contribution. Plots of[£(A) - E(F)] and [£(A) + £(F)] versus the ap-propriate dielectric functions= according to On-sager reaction field model" have been shown inFig. 8. It appears from both the plots that the al-kanols form a separate class and this suggests that

3.5

1°11

0.6 0.8 '·0f(E:)= 2«(-11

2(·'Fig. 8-Plot of v(abs) ± v(fl) versus appropriate dielectric

functions

'·2

specific solute-solvent interaction is important inprotic solvents. For [£(A)+ £(F)] versus[2(e-I)l(2f+ 1)] plots the difference between thelines for a particular value of the dielectric func-tion comes out to be around 10.5 kcal mol- 1

which is roughly of the order of hydrogen bondenergy. Similar results are obtained when otherdielectric functions 17 are used to represent thereaction field, or the molecule is assumed to beplaced in an ellipsoidal cavity IX. The ratio of thedipole moments in the S 1 and So states may beobtained from the ratio of the slopes of thestraight lines for the case where no complexationoccurs and this does not require the value of "a",the cavity radius 19. Thus, for the solute value of/A-/ /A-u comes out as 1.9. Using theoretically calcu-lated vakie of /A-o = 4.l)2D, we estimate the dipolemoment of excited state (/A- I) as about 7.60. Thisis consistent with the IC'F nature of the transitionas suggested by theoretical calculations. A corre-lation of £(F) with ET (30) parameter also pointsto the ICT nature of the transition.

The enhanced sensitivity of E (F) compared to£(A) towards a change in solvent polarity is intel-ligible in terms of increased solute-solvent interac-tion in the excited state (due to an increase in thedipole moment upon excitation) which, for aprotiesolvents, increases with the dipolarity of the sol-vent. For protic solvents, apart from the dipolarinteraction, tighter hydrogen bonding is expectedin the excited state solvate due to an increasedelectron density on the carbonyl oxygen. More-over, as pointed out by Kessler and Wolfbeis, anincrease in the electron density on the carbonyloxygen leads to an extension of conjugation modi-fying the energy levels". This effect would bemore prominent as the hydrogen bond donating(HBD) ability of the solvent increases. Thus, inprotic solvents the emitting state differs signifi-cantly from that in the aprotic solvents. This is inaccordance with the double linear correlation of£(F) and ET (30) obtained in the present case. Aschematic energy level diagram is given in Fig. 9.The large spectral shift (- 9 kcal mol- I) of theemission band in going from an alcohol to a non-polar solvent, e.g., benzene is thus rationalisablein terms of an increased reorganisation energy(SfC - S~) in alcohols due to HBD interaction.This is also reflected in the higher value ofStoke's shift for alcohols compared to that foraprotic solvents (Fig. 8).

In order to have an idea about the solvation ef-fects on the fluorescence maximum due to non-specific solvent dipolarityand specific hydrogenbonding ability we turned to the multiple linear

100 INDIAN J CHEM. SEC. A, FEBRUARY 1995

~iCnsiC

~c

SRI

SFC0

So So

A B

Fig. 9-Schematic energy level diagram of the solute'S' inaprotic (A) and protic (B) environments. Subscripts 0 and 1indicate ground and first singlet excited state. Superscripts 'FC'and 'R' denote Franck-Condon and relaxed states respectively

regression approach of Kamlet, Abboud andTaft". The following regression equation has beenobtained with a correlation coefficient of 0.98 us-ing fifteen solvents in the present case for whichall the three parameters are known.E(F)/kcalmol~l =57.23-7.310 a-3.606 f3

-4.036.n* ... (5)Here .n*, f3 and a represent the dipolarity, hy-drogen bond accepting (HBA) and hydrogenbond donating (HBD) ability of the solvent". Thenegative sign of the regression coefficient indi-cates greater stabilization of the excited state rela-tive to the ground state. Moreover, the ratio ofthe regression coefficients of a and n* (viz. 7.31/4.04) indicates that the specific HBD interactionterm is more important compared to the non-spe-cific interaction term.

Mixed solventsA considerable shift in the fluorescence maxi-

mum due to the addition of a very small quantityof a protic component to an aprotic one is oftentaken as evidence for the formation of excitedstate cornplex+'. In the present case an enhancedcomplexation of the S 1 state of the solute with aprotic solvent is expected in view of increased ne-gative charge on the carbonyl oxygen atom uponexcitation. In general, a non-linearity of solvatoch-romic shift of the solute's UV-VIS' absorption/emission spectra versus the mole fraction is ex-plained as due to a preferential solvation of thesolutcv+'. The nature of deviation of E12 versusmole fraction curve from linearity in the presentcase may thus be interpreted in terms of preferen-tial solvation of the solute in the S 1 state.

As is evident from Fig. 7 a preference for theprotic cosolvent has been observed in all the pro-tic + aprotic solvent mixtures. The preference forone solvent component over the other in the localenvironment of the solute may be explained interms of a difference in solute-solvent interactions

for the component solvents. Apart from thenonspecific dipolar interaction, a protic solventinteracts with the S1 state of the solute throughspecific H-bonding interaction. Thus, in additionto the general phenomenon of dielectric enrich-merit? due to a difference in the nonspecific inter-action, H-bonding plays a key role in determiningthe preferential solvation characteristics in pro-tic + aprotic mixture.

It may be noted that while acetone and ethylacetate cosolvents fall on the same line (Fig. 7A),the E 12 (F) values of benzene cosolvent are al-ways lower than those in the former case through-out the whole range of XEtOH' This can be ex-plained in terms of complex; formation of the so-lute with benzene. When the mole fraction of theprotic component is very small probably eachpro tic molecule goes to solvate solute moleculesand this explains the initial steep decrease in E 12

(F) values (Figs 7A & B). In. the region rich in theprotic component, the E 12 (F) versus mole frac-tion curve for mixed aqueous solvent is differentin nature from that in the mixed binary solventscontaining ethanol. In the latter case, the ,additionof a small quantity of an aprotic cosolventchanges the E l2 (F) values to a negligible extentonly, whereas in the former case a significantchange is observed. Thus an aprotic cosolventmodifies the solute-water H-bonding characteris-tics to a considerable extent. This type of behav-iour has been observed in other cases also andhas been explained in terms of strong self-asso-ciated structure of water due to water-water hy-drogen bonding=". The preference for acetoni-trile over benzene in their binary mixture may beexplained on similar lines. Acetonitrile has a fee-ble H-bonding ability (as evidenced by Kamlet-Taft parameter value of 0.19 compared to 0.83for ethanol and 1.17 for water) in addition to astronger dipolarity. In the case of ethyl acet-ate + acetone the observed data indicate no pref-erence of the solute for a particular solvent. Thiscan be rationalised in terms of almost similar so-lute-solvent dipolar interaction of the two sol-vents.

Results for mixed aqueous solvents need specialmention. The absorption maximum in these sol-vents varies linearly with the mole fraction (Fig..3)indicating a lack of preference for acetone or di-oxane over water by the solute in the groundstate. On the other hand, water is preferred bythe S 1 state of the solute (Fig. 7). Thus, the stateof solvation in the S1 state of the solute is differ-ent from that in the So state. An interesting obser-vation may be made from a plot of the Stoke's

BANERJEE et al.: ABSORPTION & FLUORESCENCE OF KETOCYANINE DYE IN VARIOUS SOLVENTS 101

10

o

~8CQ\I

~...!!::6:I:V>

V>·w64...V>

0.6 0.8 1.0XCOSOLVENT

Fig. lO-Plot. ·Ol Stoke's shift in mixed aqueous solvents as afunction of mole fraction of the dioxane (0) and acetone (.)

shift versus the mole fraction (Fig. 10). TheStoke's shift increases steeply when a very smallamount of water is added to. the cosolvent,reaches a maximum and then shows a decreasingtrend. As mentioned earlier, the Stoke's shift forthe solute is a function of the hydrogen bondingability of the medium. The sharp increase in theStoke's shift in the region rich in aprotic cosol-vents may thus be explained to be due to consid-erable preferential solvation of the solute by theprotic component leading to increased hydrogenbonded interaction. The decreasing trend in theStoke's shift in the higher mole fraction range ofwater is rationalisable in view of water-water hy-drogen bonding interaction which lowers theoverall hydrogen donation to the solute.

Thus the present study indicates that the keto-cyanine dye interacts with protic solvents throughhydrogen bonding interaction, the extent of inter-action being greater in the S I state. Solute-solventcomplexation is also indicated in aromatic hydro-carbon and cyclic ether solvents. The fluorescencemaximum is sensitive towards a change in envi-ronment in homogeneous solvents. The large redshift of the emission maximum in going from anaprotic to a protic solvent makes the solute agood fluorescence probe for studying hydrogenbonding interaction. The S I state is preferentiallysolvated by the protic cosolvent in a pro-tic + aprotic mixture. The nature of preferentialsolvation in aprotic + aprotic solvent mixtures ismainly determined by the polarity effects.

AcknowledgementThe authors thank Prof. Mihir Chowdhury of

IACS, Calcutta for helpful discussions. The au-thors also thank Prof. S. Ghosh of PresidencyCollege, Calcutta for extending the laboratory fac-ilities for measurement of the fluorescence spect-ra. Thanks are also due to Dr. Asit Kr. Chandra,

Department of Chemistry, Burdwan Universityfor his assistance in MO calculations. The finan-cial support from CSIR, New Delhi is also ac-knowledged.

ReferencesI Prendergast F G, Chem Phys Lipids, R3 (1989) 50 .2 Buncel E & Rajagopal S, ACC chem Research, 23 (1990)

226.3 Kosower E M, JAm chem Sac, 80 (1958) 3253.4 (a) Dimroth K, Reichardt C, Shiepmann T & Bohlmann

F, Annalen, 661. (1963) 1. Reichardt C, Solvent effects inorganic chemistry (Verlag Chemie, Weinheim, New York)1979.(b) Gutmann V, Electrochimica Acta, 21 (1976) 661.

5 (a) Medda K, Pal M & Bagchi S, J chem Sac FaradayTrans, 84 (1988) 1501.(b) Chatterjee P, Medda K & Bagchi S, J 50111 Chem, 20(19'1)1)249.

6 Brooker L G S. Keyes G H & Heseltine D W, J Amchem Soc. 73 (1951) 5350; Brooker L G S, Crieg A C,Hesltine & Jenkins P W, J Am chem Soc, X7 (1965)2443.

7 Abdel-Halern S T, J chem Sac Faraday Trans, 89 (1993)55.

s Kessler M A & Wollbeis 0 S, Spectrochim Acta, 47A(1991) 187.

9 (a) Suppan P, J chem Sac Faraday Trans 1,83 (1987) 495.(b) Lerf C & Suppan P. J chem Sac Faraday Trans. 88(1992) 963.

10 Weissberger A, Technique of organic chemistry (Intersci-ence, New York), Vol. 7,1955.

II Medda K, Studies all solute-solvent interaction, Ph.D.Thesis, University of Burdwan, 1991.

12 Dewar M J S, Zoebisch E G, Healy E F & Stewart J J P,JAm chem Sac, 107 (1985) 3902.

13 (a) Abdel-Mottalcb M S A, Sherief A M K, Ismaiel L M,Shryver F C D & Vanderauweraer M A, J chem Sac Far-aday Trans II, 85 (1989) 1779.(b) Werner T C & Lyon D B, J phys Chern, 86 (1982)933.

14 Marcus R A (a) J chem Phys, 38.{1963) 1858; (b) 43(1965) 1261.

15 (a) Van der Zwan G & Hynes J T, J phys Chern, 890985j4181.(b) Burnschwig B S. Ehrenson S & Sutin N, J phys Chem,91 (1987)4714.

16 Onsager L,J Am chem Sac, 58 (1963) 1436.17 Brady J E & Carr P W. J phys Chern, 89 (1985) 5759.18 Loufty R 0 & Law K Y, J phys Chem. 84 (1980) 2X()4.19 Suppan P, J Photochem Photobiol, 50A (1990) 293.20 Karnlet M J, Abboud J L M & Taft R W, Prog phys org

Chem; 13 (1980) 485.21 Kamlet M J, Abboud J L M, Abraham M H & Taft R W,

J org Chern; 48 (1983) 2877.22 Suppan P & Gucrry-Butty E. J Lumin, 33 (1985) 335.23 (a) Burget D & Jacques p, J chim Phys,I.H! (1991) 675.

(b) Chatterjee P & Bagchi S, J chem Soc Faraday Trans,87(1991)587;86(1990) 1785.(c) Migron Y & Marcus Y, J chern Sac Faraday Trans, 87(1991) 1339.(d) Dawber J B, Ward J & Williams R A, J. chem Soc Far-aday Trans I, 84 ( 1988) 713.

24 Chatccrjcc P. Laha A K & Bagchi S, J chem Sac FaradayTrallS,88 (1992) 1675.