Solvent Dependent Absorption and Fluorescence Ofaketocyanine Dye

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

Solvent dependent absorption and fluorescence of a ketocyanine dye in neat and 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 has been studied by monitoring its absorption and fluorescence characteristics in various pure and binary mixed solvents. The spectroscopic transition leading to the longest wavelength absorption or fluorescence has been found to involve considerable intramolecular charge transfer as evidenced by semiempirical MO calculations at the AMI level. Ground state complexation is indicated in protic solvents, aromatic hydrocarbons and cyclic ethers. The fluorescence maximum, E (F), shows a correlation with ET (30) scale of solvent polarity. A multiple linear correlation of E(F) with Kamlet-Taft parameter representing solvent dipolarity (n*) and hydrogen bonding ability (a and (3) suggests that apart from dipolar interactions specific solute-solvent interactions are important in determining fluorescence maxima. Study of E (F) in mixed binary solvents points to a preferential solvation of the solute in the S 1 state.

The polarity probes have found extensive applic- nine dye in pure and mixed binary solvents by ations during the last couple of years, especially monitoring its longest wavelength absorption and for the evaluation of various microenvironrnental fluorescence band. The dye chosen for the pre- properties 1-5. Although a large number of miCTOP- sent study is (1-1 ),(9-1)-di-l ,9-di-(2,3-dihydroindo- olarity reporters involving solvatochromic spectral lyl)- 4,6-dimethylene- nona-l,3,6,8-tetraene-5-one transition are known, e.g., 4-methoxycarbonylpyri- (Fig. 1). Semiempirical calculations have been car- dinium iodide:', betaine", 4-cyanopyridinium io- ried out to provide the optimised geometry, the dide", etc., practically all of them are non-fluores- charge distribution and the dipole moment in the cent. Fluorescent polarity probes are more advan- ground state of the solute. It also gives informa- tageous compared to non-fluorescent probes be- tion regarding the nature of the electronic transi- cause of their widespread applications in biologi- tion associated with the longest wavelength ab- cal systems'. Merocyanine dyes" constitute an in- sorption. teresting system for solvatochromic studies. Some of these compounds show solvent-sensitive fluor- Materials and Methods escence? Recently, Kessler and Wolfbeis" have re- The dye (solute) was synthesised as described ported the synthesis of a series of strongly fluor- in literature". Indoline, 1,1,3,3-tetramethoxypro- escent ketocyanine dyes having unique solvatoch- pane and cyclopentanone were purchased from ramie properties in both absorption and fluoresc- Sigma Chemicals (USA) and used as received. ence. It has been observed that these compounds Purity of the prepared compound was checked by may act as good fluorescent probes for studying IR spectral data [IR bands obtained in KBr disc: solvent polarity. Although fluorescence character- 1620, 1580, 1485, 1400 em -1] and from absorp- istics of the ketocyanine dyes have been studied tion and fluorescence, spectral data [Amax in a few solvents, a systematic study of solvation (abs) = 525 nm; Amax (fl.) = 622 nm in ethanol]. All characteristics is yet to be done. Besides this, a the solvents were purified and dried by standard study of spectral characteristics in mixed binary procedures'P'" and distilled over CaH2 immedi- solvents is of particular interest, for it may pro- ately before use to ensure the absence of perox- vide information regarding the preferential solva- ides and oxidising agents. Mixed solvents and tion characteristics of the absorbing/emitting spe- corresponding solutions were prepared by care- cies". The objective of the present paper is to fully mixing the components so as to minimise study the solvation characteristics of a ketocya- contamination 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- to be n molecular orbitals and the relevant coeffi- photometer provided with a peak detection algo- cients of the p-atomic orbitals have been given in rithm. Fluorescence spectra were recorded on a Table 1. While the HOMO is antisymmetric with Hitachi F-30lO spectrofluorimeter equipped with respect to the symmetry plane, the LUMO shows a microprocessor and a chart recorder. Freshly a symmetrical behaviour. Moreover, prepared solutions were used for each measure- HOMO --> LUMO transition indicates a transfer of ment and the concentrations were chosen to give electron density from nitrogen to the oxygen atom. absorbances less than 0.1 to avoid distortion of These findings indicate that in the ketocyanine the spectra due to reabsorption of the fluoresc- dye (1) the shortest energy absorption band arises ence light. The spectral studies were done using due to a n-n" electronic transition involving an solutions having concentration of the dye in the intramolecular charge transfer (leT) from nitrog- range 10- 5 to lO- 6 mol dm - 3. The observed ab- en donor to oxygen acceptor through the inter- sorption/fluorescence spectrum did not, however, vening conjugated system. The nature of electron- depend on the concentration of the solute. ic transition is thus similar to that in the merocyanine dyes-". Theoretical calculations Semiempirical MO calculations at the AM112 Results (Austin model 1) level using the MOPAC pro- gram (QCPE 355) were carried out on a PC AT/ Absorption spectrum 386. A fully optimized geometry indicates a plan- The results have been summarised in Table 2. arity of the molecule in the ground electronic Fig. 2A represents absorption spectra in some state, except the aliphatic hydrogen atoms. The distribution of net charges over the atoms and the relevant bond distance is shown in Fig. 1. It ap- Table I--Relevant coefficients of /M>rbital appearing in LCAO pears that there is a significantlitemation in for HOMO and LUMO the C - C bond length for C - C bonds occurring Atom No. HOMO LUMO Atom No. HOMO UJMO between the N-atom of the five-membered ring o.no 0.29 9 -(U3 0.16 and the carbonyl oxygen atom. The dipole mo- 2 11.00 - 0.30 18 •• 0.33 0.16 ment was found to be 4.02 D, directed exclusively 3 0.29 -0.2S II -O.IR -0.33 in the direction of the carbonyl group. The mole- 4 -0.29 - O.2S 20 O.IR -0.33 cule has a symmetry plane passing through the 7 IUS 0.33 13 0.34 O.OR C = 0 bond and perpendicular to the molecular Iti - O.IS 0.33 22 -0.34 O.OR plane. The HOMO and LUMO have been found

50 36

(a)

1JoO 1.40

1-/,()~~11'37 1'40 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 distances arc shown in (b)] 96 INDIAN J CHEM. SEe. A, FEBRUARY 1995 representative pure solvents. The absorption practically unaltered. A plot of E(A) against the spectra in all the aprotic solvents show similar mole fraction of n-hexane shows a deviation from structures which lose their prominence as the po- linearity indicating that the solute is solvated pref- larity of the solvent is increased. For aprotic sol- erentially by benzene. The preference for benzene vents the shift of the absorption maximum [E(A)] over n-hexane is intelligible in terms of enhanced is not very sensitive towards a change in the sol- benzene-solute interaction through complexation. vent polarity as measured by ET (30) parameter". A red shift ( - 3 kcal mol- 1) is observed as we go Considering the ICT nature of the transition, one from an aprotic solvent to a protic solvent. Simi- would expect the more polar solvent to yield the larity of absorption in various alcohol solvents (as smaller transition energy. But the E(A} values for distinctly different from the other polar aprotic benzene, toluene, 1,4-dioxane and tetrahydrofur- solvents) is suggestive of complex formation be- an are lower although these solvents are charac- tween an alcohol and the solute in the ground terised by.lower polarity in the ET (30) scale. state. In a mixed solvent of ethanol with ethyl Rather, the E (A) values in these solvents run par- allel to the acceptor numbers of Gutmann". This points to an enhanced solute-solvent interaction A in the ground state possibly through complex for- B mation in these solvents. This conclusion is also in accordance with the large solubility of the solute in these solvents. The solute is insoluble in n- hexane but it may be made soluble by addition of excess naphthalene; the solution gives similar spectrum as obtained with benzene. This also suggests the solute-naphthalene interaction, possibly 500 600 450 500 550 ~Inm through the zr-electron density of naphthalene. When n-hexane was. added to a benzene solution Fig. 2-(A) Absorption spectra of (I) in neat solvents [l-propanol (1), benzene (2), ethyl acetate (3) & acetonitrile (4). (B) of the dye a continuous blue shift was obtained Absorption spectrum of (I) in ethanol + benzene binary mix- and no isosbestic- point could be detected. The ture. Volume per .cent of ethanol in the mixture: 0.2(1), 0.8(2), structure and the shape of the band remained 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 clearly indicated (Fig. 2B). The equilibrium constant for 62 complexation with ethanol comes out to be 3.2 ., and 4.3 when benzene and ethyl acetate respect- o ively are used as cosolvents. E 58 The spectrum of the solvents in an alcoholic CJ " solvent also reflects the overlap of band from .:: 51+ H-bonded and bare forms of the solute. Absorp- tion measurements in ethanol at various tempera- w tures also indicate the existence of isosbestic 0·2 0·1+ 0-6 ()'8 o point. An increase in temperature indicates an in- Xcosolvent crease in the intensity of the higher energy band. These facts point to the exothermic formation of Fig. 3-Plot of the energy of maximum absorption of (I) in a hydrogen-bonded complex in ethanol. For mixed aqueous solvents versus the mole fraction of the cosolvents. [l!;.= pyridine. • = acetone, 0 = J ,4-dioxane, 0 = luti- mixed aqueous solvents no such isosbestic point dine] could be detected. The band shows a gradual ba- thochromic shift as the mole fraction of water is increased. A plot of the transition energy in mixed aqueous solvents is shown in Fig. 3. Owing to the extreme insolubility of the solute in water the data could be recorded only upto a mole fraction of 0~8with respect to water. It may be seen from the figure that in all the cases linear plots are obtained and they converge to a value of 51 kcal mol- I for pure water. Throughout the whole range of Xeo.olvent' Fig. 3 indicates almost similar transition energy for acetone and 1,4-dioxane, although the solvent polarity is greater in the former case. This behaviour is in keeping with the 1,4-dioxane-solute complexation as discussed earlier.

Fluorescence spectra 500 550 600 The emission may be characterised as the 'A/nm S I-. So transition. Fig. 4 shows fluorescence spectra in some representative pure solvents. The flu- Fig. 4-Fluorescence spectra of (I) in neat solvents [(a) orescence spectrum at 298 K is structureless ex- l-propanol, (b) benzene, (c) ethyl acetate and (d) acetonitrile 1 cept in aromatic hydrocarbon and cyclic ether solvents where two emission bands are clearly observed. For example, in benzene the observed fluorescence spectrum indicates two bands with the maximum at 525 and 548 nm respectively (Fig. 4). The 548 nm band appears to be similar to the fluorescence band in the more polar sol- 'T '0 vent ethyl acetate. This indicates complex forma- E 050 tion with benzene. An interesting correlation ee- u tween the fluorescence maxima is observed with ~ 10 9 6

Dimroth-Reichardt polarity parameter ET (30) as -L&J 4'5 may be seen from Fig. 5 and this indicates that ~ the nature of transition is similar in the two cases. It may be noted that the available data in neat 35 40 45 50 55 60 65 -1 solvents may be best represented by two correla- ET (30)/kcal mol tion lines. The data points for aprotic solvents fall Fig. 5-Plots of maximum emission frequency of compound on one line while the protic solvents form a se- (I) versus ET (30) for neat solvents; numbers refer to the parate class. serial no. in the Table 2 98 INDIAN J CHEM. SEe. A, FEBRUARY 1995

The double linear dependence of the fluorescence energy on ET (30) indicates that the nature of the emitting state is different in the tWDclasses of solvents':'. For the aromatic hydrocarbons and cyclic ethers the higher energy maximum is in line with the behaviour of other aprotic solvents, while the lower energy maximum falls below the line. Thus the lower energy band in these solvents arises due to a species in which the nature of solute-solvent interaction is different from that in the case of Reichardt's betaine, the indicator dye for E r (30) values. Thus, the higher energy band >- is probably due to bare solute modified by dipo- ...... - II") lar solvation while the lower energy band is possi- z bly due to complex formation in the excited state. .-w The sensitivity of the bands with solvent polar- .....z ity prompted us to undertake the studies in mixed solvents. The following mixed binary solvents have been used: protic + aprotic (ethanol + benzene; ethanol + acetone; ethanol + ethyl acetate; water + acetone; and water + dioxane) and aprotic + aprotic (acetone + ethyl acetate and ben- 550 600 650 700 zene + acetonitrile). Fig. 6 shows the fluorescence spectra in a representative mixed binary solvent A/nm as a function of solvent composition. The fluorescence maximum changes continuously to the Fig. 6-Fluoresccnce band of (I) in mixed binary solvents red as increasing amounts of a cosolvent with a containing ethanol and ethyl acetate. The mole fraction of ethanol (x) increase in the order l(x = 0) > 2 > ...> 8(x = 1) higher polarity [e.g. in the ET (30) scale] are added to it. The intensity of the band increases with the addition of the polar cosolvent and then decreases. In benzene and dioxane, where the fluorescence spectrum is structured, the addition of a polar cosolvent leads to a very rapid decrease ;".0 in the relative intensity of the band around 525 A B nm. The 548 nm band on the other hand shows a rapid red shift. The variation of E12 (F), the maximum energy of fluorescence in mixed binary solvents, as a function of the mole fraction has been shown in Fig. 7. It may be seen that for all the protic + aprotic binary mixtures the fluorescence maximum changes rapidly when the mole fraction of the protic cosolvent is very small. But at higher mole fractions the position of the band maximum becomes less sensitive to the solvent composition. For aprotic + aprotic mixture the nature of variation depends on the component solvents. Thus a linear variation of E12 (F) with the solvent composition is obtained in the case of acetone + ethyl acetate while for benzene + acetonitrile the varia- 0.2 D." 0.1 0.' tion is non-linear. x1z-

Discussion Fig.7-Plot of E\2 (F) versus mole fraction [A: mixed binary solvents of ethanol with benzene (0), ethyl acetate (D) and acetone (~); B: mixed binary solvents of water with dioxane Pure solvents (0) and acetone (~); C: benzene + acetonitrile system; D: ace- A spectroscopic transition, in general, involves tone + ethyl acetate system.] BANERJEE et al.: ABSORPTION & FLUORESCENCE OF KETOCYANINE DYE IN VARIOUS SOLVENTS 99 an equilibrium initial state and a non-equilibrium specific solute-solvent interaction is important in final state. Using the formalism developed by protic solvents. For [£(A)+ £(F)] versus Marcus 14 for treating polar media under equilibri- [2(e-I)l(2f+ 1)] plots the difference between the um and non-equilibrium conditions, one can par- lines for a particular value of the dielectric func- tition the Franck-Condon energies £ (A) and tion comes out to be around 10.5 kcal mol- 1 £(F) as which is roughly of the order of hydrogen bond energy. Similar results are obtained when other £(A)= £+(~F;~q- ~Fleq)+ £RO(O) .,. (1) dielectric functions 17 are used to represent the E(F) = E+ (Ol·;~q- ~Ffq) - £RO(1) ... (2) reaction field, or the molecule is assumed to be where £(A) and E(F) denote the energies corre- placed in an ellipsoidal cavity IX. The ratio of the sponding to maximum absorption and emission dipole moments in the S 1 and So states may be respectively. E denotes the difference in the free obtained from the ratio of the slopes of the energies of the ground and the equilibrium excit- straight lines for the case where no complexation ed states. ~f;~q and (5f;cq denote the equilibrium occurs and this does not require the value of "a", solute-solvent interaction free energy in the So the cavity radius 19. Thus, for the solute value of and S I states respectively. The third term in equ- /A-/ /A-u comes out as 1.9. Using theoretically calcu- ations (1). and (2) represents solvent reorganiza- lated vakie of /A-o = 4.l)2D, we estimate the dipole tion energy in the So and S 1 states. If 'the reorga- moment of excited state (/A- I) as about 7.60. This nization energies are equal", we obtain is consistent with the IC'F nature of the transition £(A)+ £(F)= 2£+ 2(~f;fq - ~Flq) ... (3) as suggested by theoretical calculations. A correlation of £(F) with ET (30) parameter also points £(A)- £(F)= 2E ... (4) RO to the ICT nature of the transition. Thus, the Stoke's shift is a measure of the non- The enhanced sensitivity of E (F) compared to equilibrium ERO term while £(A)+ £(F) terms £(A) towards a change in solvent polarity is intel- have only the equilibrium contribution. Plots of ligible in terms of increased solute-solvent interac- [£(A) - E(F)] and [£(A) + £(F)] versus the ap- tion in the excited state (due to an increase in the propriate dielectric functions= according to On- dipole moment upon excitation) which, for aprotie sager reaction field model" have been shown in solvents, increases with the dipolarity of the sol- Fig. 8. It appears from both the plots that the al- vent. For protic solvents, apart from the dipolar kanols form a separate class and this suggests that interaction, tighter hydrogen bonding is expected in the excited state solvate due to an increased electron density on the carbonyl oxygen. More- over, as pointed out by Kessler and Wolfbeis, an increase in the electron density on the carbonyl oxygen leads to an extension of conjugation modi- fying the energy levels". This effect would be more prominent as the hydrogen bond donating (HBD) ability of the solvent increases. Thus, in protic solvents the emitting state differs signifi- cantly from that in the aprotic solvents. This is in accordance with the double linear correlation of £(F) and ET (30) obtained in the present case. A schematic energy level diagram is given in Fig. 9. The large spectral shift (- 9 kcal mol- I) of the emission band in going from an alcohol to a non- polar solvent, e.g., benzene is thus rationalisable in terms of an increased reorganisation energy 3.5 (SfC - S~) in alcohols due to HBD interaction. 1 This is also reflected in the higher value of °11 Stoke's shift for alcohols compared to that for 0.6 0.8 '·0 '·2 aprotic solvents (Fig. 8). f(E:)= 2«(-11 In order to have an idea about the solvation ef- 2(·' fects on the fluorescence maximum due to non- Fig. 8-Plot of v(abs) ± v(fl) versus appropriate dielectric specific solvent dipolarityand specific hydrogen functions bonding ability we turned to the multiple linear 100 INDIAN J CHEM. SEC. A, FEBRUARY 1995

siC for the component solvents. Apart from the nonspecific dipolar interaction, a protic solvent SR ~iCn I interacts with the S 1 state of the solute through specific H-bonding interaction. Thus, in addition to the general phenomenon of dielectric enrich- ~c SFC 0 merit? due to a difference in the nonspecific inter- So So action, H-bonding plays a key role in determining the preferential solvation characteristics in pro- A B tic + aprotic mixture. Fig. 9-Schematic energy level diagram of the solute'S' in aprotic (A) and protic (B) environments. Subscripts 0 and 1 It may be noted that while acetone and ethyl indicate ground and first singlet excited state. Superscripts 'FC' acetate cosolvents fall on the same line (Fig. 7A), and 'R' denote Franck-Condon and relaxed states respectively the E 12 (F) values of benzene cosolvent are al- ways lower than those in the former case through- regression approach of Kamlet, Abboud and out the whole range of XEtOH' This can be ex- Taft". The following regression equation has been plained in terms of complex; formation of the so- obtained with a correlation coefficient of 0.98 us- lute with benzene. When the mole fraction of the ing fifteen solvents in the present case for which protic component is very small probably each all the three parameters are known. pro tic molecule goes to solvate solute molecules and this explains the initial steep decrease in E 12 E(F)/kcalmol~l =57.23-7.310 a-3.606 f3 (F) values (Figs 7A & B). In. the region rich in the -4.036.n* ... (5) protic component, the E 12 (F) versus mole frac- Here .n*, f3 and a represent the dipolarity, hy- tion curve for mixed aqueous solvent is different drogen bond accepting (HBA) and hydrogen in nature from that in the mixed binary solvents bond donating (HBD) ability of the solvent". The containing ethanol. In the latter case, the ,addition negative sign of the regression coefficient indi- of a small quantity of an aprotic cosolvent cates greater stabilization of the excited state rela- changes the E l2 (F) values to a negligible extent tive to the ground state. Moreover, the ratio of only, whereas in the former case a significant the regression coefficients of a and n* (viz. 7.31/ change is observed. Thus an aprotic cosolvent 4.04) indicates that the specific HBD interaction modifies the solute-water H-bonding characteris- term is more important compared to the non-spe- tics to a considerable extent. This type of behav- cific interaction term. iour has been observed in other cases also and has been explained in terms of strong self-asso- Mixed solvents ciated structure of water due to water-water hy- A considerable shift in the fluorescence maxi- drogen bonding=". The preference for acetoni- mum due to the addition of a very small quantity trile over benzene in their binary mixture may be of a protic component to an aprotic one is often explained on similar lines. Acetonitrile has a fee- taken as evidence for the formation of excited ble H-bonding ability (as evidenced by Kamlet- state cornplex+'. In the present case an enhanced Taft parameter value of 0.19 compared to 0.83 complexation of the S 1 state of the solute with a for ethanol and 1.17 for water) in addition to a protic solvent is expected in view of increased ne- stronger dipolarity. In the case of ethyl acet- gative charge on the carbonyl oxygen atom upon ate + acetone the observed data indicate no pref- excitation. In general, a non-linearity of solvatoch- erence of the solute for a particular solvent. This romic shift of the solute's UV-VIS' absorption/ can be rationalised in terms of almost similar so- emission spectra versus the mole fraction is ex- lute-solvent dipolar interaction of the two sol- plained as due to a preferential solvation of the vents. solutcv+'. The nature of deviation of E12 versus Results for mixed aqueous solvents need special mole fraction curve from linearity in the present mention. The absorption maximum in these sol- case may thus be interpreted in terms of preferen- vents varies linearly with the mole fraction (Fig..3) tial solvation of the solute in the S 1 state. indicating a lack of preference for acetone or di- As is evident from Fig. 7 a preference for the oxane over water by the solute in the ground protic cosolvent has been observed in all the pro- state. On the other hand, water is preferred by tic + aprotic solvent mixtures. The preference for the S 1 state of the solute (Fig. 7). Thus, the state one solvent component over the other in the local of solvation in the S 1 state of the solute is differ- environment of the solute may be explained in ent from that in the So state. An interesting obser- terms of a difference in solute-solvent interactions vation may be made from a plot of the Stoke's BANERJEE et al.: ABSORPTION & FLUORESCENCE OF KETOCYANINE DYE IN VARIOUS SOLVENTS 101

Department of Chemistry, Burdwan University 10 for his assistance in MO calculations. The finan- cial support from CSIR, New Delhi is also ac- o knowledged. ~8 CQ \I References ~ ... I Prendergast F G, Chem Phys Lipids, R3 (1989) 50 . !!::6 :I: 2 Buncel E & Rajagopal S, ACC chem Research, 23 (1990) V> 226. V> ·w 3 Kosower E M, JAm chem Sac, 80 (1958) 3253. ...64 4 (a) Dimroth K, Reichardt C, Shiepmann T & Bohlmann V> F, Annalen, 661. (1963) 1. Reichardt C, Solvent effects in 0.6 0.8 1.0 organic chemistry (Verlag Chemie, Weinheim, New York) XCOSOLVENT 1979. Fig. lO-Plot. ·Ol Stoke's shift in mixed aqueous solvents as a (b) Gutmann V, Electrochimica Acta, 21 (1976) 661. function of mole fraction of the dioxane (0) and acetone (.) 5 (a) Medda K, Pal M & Bagchi S, J chem Sac Faraday Trans, 84 (1988) 1501. shift versus the mole fraction (Fig. 10). The (b) Chatterjee P, Medda K & Bagchi S, J 50111 Chem, 20 Stoke's shift increases steeply when a very small (19'1)1)249. 6 Brooker L G S. Keyes G H & Heseltine D W, J Am amount of water is added to. the cosolvent, chem Soc. 73 (1951) 5350; Brooker L G S, Crieg A C, reaches a maximum and then shows a decreasing Hesltine & Jenkins P W, J Am chem Soc, X7 (1965) trend. As mentioned earlier, the Stoke's shift for 2443. the solute is a function of the hydrogen bonding 7 Abdel-Halern S T, J chem Sac Faraday Trans, 89 (1993) ability of the medium. The sharp increase in the 55. Stoke's shift in the region rich in aprotic cosol- s Kessler M A & Wollbeis 0 S, Spectrochim Acta, 47A (1991) 187. vents may thus be explained to be due to consid- 9 (a) Suppan P, J chem Sac Faraday Trans 1,83 (1987) 495. erable preferential solvation of the solute by the (b) Lerf C & Suppan P. J chem Sac Faraday Trans. 88 protic component leading to increased hydrogen (1992) 963. bonded interaction. The decreasing trend in the 10 Weissberger A, Technique of organic chemistry (Intersci- Stoke's shift in the higher mole fraction range of ence, New York), Vol. 7,1955. II Medda K, Studies all solute-solvent interaction, Ph.D. water is rationalisable in view of water-water hy- Thesis, University of Burdwan, 1991. drogen bonding interaction which lowers the 12 Dewar M J S, Zoebisch E G, Healy E F & Stewart J J P, overall hydrogen donation to the solute. JAm chem Sac, 107 (1985) 3902. Thus the present study indicates that the keto- 13 (a) Abdel-Mottalcb M S A, Sherief A M K, Ismaiel L M, cyanine dye interacts with protic solvents through Shryver F C D & Vanderauweraer M A, J chem Sac Far- aday Trans II, 85 (1989) 1779. hydrogen bonding interaction, the extent of inter- (b) Werner T C & Lyon D B, J phys Chern, 86 (1982) action being greater in the S I state. Solute-solvent 933. complexation is also indicated in aromatic hydro- 14 Marcus R A (a) J chem Phys, 38.{1963) 1858; (b) 43 carbon and cyclic ether solvents. The fluorescence (1965) 1261. maximum is sensitive towards a change in envi- 15 (a) Van der Zwan G & Hynes J T, J phys Chern, 89 0985j4181. ronment in homogeneous solvents. The large red (b) Burnschwig B S. Ehrenson S & Sutin N, J phys Chem, shift of the emission maximum in going from an 91 (1987)4714. aprotic to a protic solvent makes the solute a 16 Onsager L,J Am chem Sac, 58 (1963) 1436. good fluorescence probe for studying hydrogen 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. bonding interaction. The S I state is preferentially 19 Suppan P, J Photochem Photobiol, 50A (1990) 293. solvated by the protic cosolvent in a pro- 20 Karnlet M J, Abboud J L M & Taft R W, Prog phys org tic + aprotic mixture. The nature of preferential Chem; 13 (1980) 485. solvation in aprotic + aprotic solvent mixtures is 21 Kamlet M J, Abboud J L M, Abraham M H & Taft R W, mainly determined by the polarity effects. 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. Acknowledgement (b) Chatterjee P & Bagchi S, J chem Soc Faraday Trans, The authors thank Prof. Mihir Chowdhury of 87(1991)587;86(1990) 1785. IACS, Calcutta for helpful discussions. The au- (c) Migron Y & Marcus Y, J chern Sac Faraday Trans, 87 thors also thank Prof. S. Ghosh of Presidency (1991) 1339. College, Calcutta for extending the laboratory fac- (d) Dawber J B, Ward J & Williams R A, J. chem Soc Far- aday Trans I, 84 ( 1988) 713. ilities for measurement of the fluorescence spect- 24 Chatccrjcc P. Laha A K & Bagchi S, J chem Sac Faraday ra. Thanks are also due to Dr. Asit Kr. Chandra, TrallS,88 (1992) 1675.