Ramkrishna Pramanik

Studies of solvation of ketocyanine dyes in homogeneous and heterogeneous media by UV/Vis spectroscopic method.

Solvation characteristics of ketocyanine dyes (I-VI) have been investigated in pure solvents and heterogeneous media by absorption and fluorescence studies. The dyes are good reporters of solvent polarity. In protic solvents they exist as equilibrium mixtures of bare and hydrogen-bonded form in the ground state (S0), the latter being the emitting species. In aprotic solvents of low polarity association of the S1 state of the dye takes place. In aqueous micellar media the dye resides at the micelle water interface. The binding constant for dye-micelle interaction has been determined. Fluorescence data in beta-cyclodextrine solution resemble that for that neutral micellar solution indicating that the interaction between the -OH group of the heterogeneous part (micelle/cyclodextrine cavity) and the carbonyl oxygen of the dye is important in both the cases.

Studies of solvation in homogeneous and heterogeneous media by electronic spectroscopic method

Solvation characteristics in homogenous (pure and mixed binary solvents) and heterogeneous media (aqueous micelles, beta- and gamma-cyclodextrine solutions) have been studied by monitoring the emission characteristics of a newly synthesised dye. The longest wavelength absorption and emission band of the dye arise due to transition between S(0) and S(1) state. The maximum energy of electronic transition involving intramolecular charge transfer is found to be dependent on both the hydrogen-bond donating ability and the polarity-polarisability in pure solvent. The dipole moment in the S(1) state, as determined by solvatochromic procedure, agrees well with the value obtained by theoretical calculation at the AM1 level. Preferential solvation of the dye by alcohols has been found to occur in ethanol+water, propan-1-ol+water, propan-2-ol+water binary mixtures. In aqueous micellar media the dye molecule is located at the water-micelle interface. The binding constant for the dye-micelle interaction has also been determined. The results have been compared with those for a structurally related symmetrical ketocyanine dye.

Fluorescence anisotropy of ketocyanine dyes in homogeneous and heterogeneous media. Estimation of micellar microviscosity

Fluorescence anisotropies (r) of two ketocyanine dyes have been determined in homogeneous solvents (pure and mixed binary) and in micellar media. For pure solvents the rotational reorientation times have been calculated and compared with the values calculated using the modified Stokes–Einstein–Debye equation. The discrepancy between the two values has been discussed in terms of dielectric factors. Ethanol–water and ethanol–octan-1-ol binary mixtures exhibit ideal solvation behaviour and the anisotropy values are well represented by Perrins equation. For a protic–aprotic solvent mixture, non-ideal solvation behavior is observed which has been explained as due to solute–solvent as well as solvent–solvent interaction for the two component solvents. The microviscosity around a micellar region and the related activation energy has been measured from the r values in these media at various temperatures.

Fluorescence of a ketocyanine dye in pure and mixed binary solvents at 77 K

Emission and excitation spectrum of a ketocyanine dye has been studied at 77 K in pure solvents (methanol, ethanol, 2-methyltetrahydrofuran (2MTHF)) and in several mixed solvents. In a pure solvent at 77 K a single fluorescence band appears. In protic solvents the band gets reduced in intensity on melting and a red shifted emission band is obtained. On the other hand a blue shifted band appears on melting for aprotic solvents. This has been explained as due to a geometry change on excitation. Two emitting species have been identified in a glass containing mixed protic + aprotic solvents at 77 K. Preferential solvation of the solute by ethanol over 2MTHF has been indicated in a mixed binary solvent containing these two solvents.

Absorption spectral band width of charge transfer transition of ET(30) dye in homogeneous and heterogeneous media

Solvation characteristics in homogeneous and heterogeneous media have been probed by monitoring the band width of ICT band of 2,6-di-phenyl-4(2,4,6-triphenyl-1-pyridino) phenolate, the indicator solute for E(T)(30) scale, in pure, mixed binary solvents and aqueous micellar solution. Non-ideal solvation behaviour is observed in all the binary solvent mixtures. Index of preferential solvation has been calculated as a function of solvent composition. Study in micellar media indicates that the dye is located at the micelle-water interface. The effects of variation of micelle concentration, temperature and electrolyte concentration have also been studies.

UV visible spectroscopic study of solvation of 2,6-diphenyl-4(2,4,6-triphenyl-1-pyridino)phenolate in ternary solvent mixtures

A new method for analyzing the cybotactic zone in a ternary solvent system by means of electronic absorption spectroscopy has been presented. The position of the intramolecular charge transfer absorption band of 2,6-diphenyl-4(2,4,6-triphenyl-1-pyridino) phenolate has been monitored in four ternary solvent mixtures in which one of the pairs are partially miscible. Investigations have been done in the completely miscible region of the ternary mixtures including the binodal curve. Results have been explained by extending the existing models of solvation in binary solvent mixtures to ternary systems.

Fluorescence anisotropy of ketocyanine dyes in pure and mixed binary solvents

Abstract Fluorescence anisotropies ( r ) of two ketocyanine dyes have been determined in neat alcohols, nitriles and their binary mixtures. For a particular class of solvents the values of r is given by Perrins equation. The lifetimes ( τ f ) of the solutes in the S 1 state have been determined from a study of variation of r with the viscosity of the solvent. The calculated τ f values for the alcohols are higher than that of the nitriles and the results agree well with those obtained by independent measurements in picosecond domain. In alcohol+nitrile binary mixture the solute exhibits preferential solvation by alcohol molecules. The limiting fluorescence anisotropies of the solutes in all the solvents come as ∼0.38 indicating that absorption and emission dipole moments are parallel.

A study of excited-state dipole moments by monitoring the band width of the charge-transfer transition of ion pairs or dipolar molecules

A suitable analysis of the band-width parameters of the charge-transfer (CT) transition of ion pairs or dipolar molecules has been presented that provides information regarding the change in the ground-state dipole moment on excitation (Δµ) or the extent of charge transfer (Δe) for a dipolar molecule. The method has been demonstrated by determining Δµ or Δe for N-ethyl-4-cyanopyridinium iodide and betaine-26 [2,6-di-tert-butyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate]. The relative advantage of the method for these molecules over the solvatochromic procedure has also been discussed.