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 bi- nary 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 corre- lation 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 flu- orescence 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 merocya- nine 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 so- lute 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 sug- gests 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-prop- anol (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