Sanjib Bagchi

Use of Fluorescence Probes for Studying Kamlet-Taft Solvatochromic Parameters of Micellar System Formed by Binary Mixture of Sodium Dodecyl Sulfate and Triton-X 100

Fluorescence probes have been used to estimate Kamlet-Taft solvatochromic parameters alpha and pi* representing hydrogen-bond donation ability and dipolarity/polarizability, respectively, of sodium dodecyl sulfate (SDS)-Triton X 100 (TX100) mixed aggregates with varying compositions. The hydrogen-bond donation ability of the mixed aggregate has been found to increase with SDS composition, whereas the dipolarity/polarizability parameter decreases. The relative contribution of electrostatic and steric effect toward the total free energy of micellization have been calculated for the mixture. The solvatochromic parameters alpha and pi* depend linearly on the total free energy of micellization, indicating a correlation between aggregational and solvatochromic properties.

UV-visible spectroscopic study of solvation in ternary solvent mixtures: ketocyanine dye in methanol + acetone + water and methanol + acetone + benzene.

Solvation characteristics of a ketocyanine dye have been studied in completely miscible ternary solvent mixtures, namely, methanol + acetone + water and methanol + acetone + benzene, by monitoring the solvatochromic absorption band of the dye. The maximum energy of absorption (E) of the solute in a ternary solvent mixture differs significantly from the mole fraction average of the E values in the component solvents. Results in the corresponding binary solvent mixtures also show a deviation of the E value from the mole fraction averaged E values. The results have been explained in terms of preferential solvation using a two phase model of solvation. The excess or deficit over the bulk composition of a solvent component in the vicinity of the solute molecule in a ternary solvent mixture has been estimated using the knowledge of solvation in the corresponding binary mixtures.

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 n discrepancy between the two values has been discussed in terms of dielectric factors. Ethanol–water and n 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.

Electronic Spectroscopic Study of Solvation of a Ketocyanine Dye in Ternary Solvent Mixtures

Solvation characteristics in ternary solvent mixtures have been studied by monitoring the solvent-sensitive electronic absorption band of a ketocyanine dye in two ternary solvent mixtures, water + ethanol + benzene and water + ethanol + cyclohexane, in which one of the pairs are partially miscible. Investigations have been done in a completely miscible region including the binodal curve. The maximum energy of absorption (E) of the solute in a ternary solvent mixture differs significantly from the mole fraction average of the E-values in the component solvents. Results in the corresponding binary solvent mixtures also show a deviation of the E-value from the mole fraction averaged E-values, indicating preferential solvation by a component solvent. The results in ternary solvent mixture have been explained in terms of a realistic model of solvation using the results on binary solvation.

UV–VIS spectroscopic study of preferential solvation in mixed binary solvents at various temperatures

Preferential solvation of N-ethyl-4-cyanopyridinium iodide and pyridinium betaine has been studied at various temperatures by monitoring the thermosolvatochromic band of the solute in mixed binary solvents. Parameters characterising preferential solvation have been calculated and the results have been interpreted in terms of existing models. A study of thermochromism of the solutes in mixed binary solvents may provide information about solvent non-ideality.

Interaction of Ketocyanine Dye with a Co2+ Ion: An Electronic Spectroscopic Study

The interaction of a ketocyanine dye with a cobalt(II) ion has been studied in solution by monitoring the electronic absorption and emission spectral characteristics of the dye. A new absorption band at a longer wavelength appears in solutions containing cobalt(II) ions. An isosbestic point is observed for systems containing a fixed dye concentration and varying Co(2+) ion concentration, pointing to the formation of a complex. The stoichiometry of the complex has been found to be 1:1. Equilibrium constant has been determined from the observed data. The nature of interaction between the dye (S(0) state) and the Co(II) ion is mostly electrostatic. Spectroscopic results have been supported by DFT/TDDFT calculation. The fluorescence band is characterized by a small blue shift. In the concentration range of 10(-3)-10(-4) M of the Co(II) ion, a quenching of the dye fluorescence is noticed. The Stern-Volmer plot points to the operation of both static and dynamic mechanisms of quenching. For a micromolar concentration of the Co(II) ion, however, an enhancement of fluorescence intensity with a slight blue shift has been observed, which has been explained in terms of formation of a different type of complex in the S(1) state at this concentration level. The value of lifetime increases at the micromolar level of concentration of the Co(II) ion, where the intensity increases and then remains practically unchanged as more salt is added to the system. Values of the decay constant for the different photophysical processes have been calculated. Complexation in the S(1) state is characterized by a slower decay of the excited dye by a nonradiative path.

Interaction of alkali, alkaline earth and transition metal ions with a ketocyanine dye: a comparative electronic spectroscopic study.

Interaction of a dye which is structurally similar to a ketocyanine dye with metal ions (alkali, alkaline earth and transition metal) has been studied by monitoring the electronic absorption, steady state and time resolved fluorescence parameters of the dye. The dye (S(0) state) forms a 1:1 complex with cations as indicated by the appearance of a new band at a longer wavelength. Equilibrium constant and other thermodynamic parameters for complexation have been determined. The interaction between the dye and the cation is mostly electrostatic in nature. Spectroscopic results have been supplemented by DFT calculation. For very low concentration of cations, where complexation is insignificant, the absorption band of the dye undergoes a slight blue shift. Enhancement of fluorescence intensity has been observed in the same concentration range. Both phenomena have been explained in terms of formation of a weak association complex where one/more cation replace equivalent solvent molecules in the cybotatic region around the dye. The binding constant of the weak association complex involving cation and the dye (S(1) state) has been determined and has been found to depend on the charge-to-size ratio of the cations. Measurement of fluorescence lifetime of the dye indicates that the association complex is slowly decaying relative to solvated dye. At higher concentration of metal ions, however, fluorescence of the dye is quenched by the metal ions. A red shift of fluorescence maximum has also been observed in this concentration range.

An electronic spectroscopic study of micellisation of surfactants and solvation of homomicelles formed by cationic or anionic surfactants using a solvatochromic electron donor acceptor dye

Solvatochromic absorption and fluorescence bands of a donor-acceptor dye have been utilised for following the micellisation and for probing the polarity of the aqueous homomicellar phase provided separately by cationic (cetyltrimethylammonimum bromide, CTAB and dodecyltrimethylammonimum bromide, DTAB) and anionic (sodium dodecyl sulphate, SDS) surfactant. Results indicate that for a low concentration of surfactant (below cmc) the dye forms a dimer in aqueous solution. In a micellar media, however, the dye exists as monomers. A strong dye-micelle interaction, as indicated by the shift of the solvatochromic intramolecular charge transfer band of the dye, has also been indicated. The absorption and fluorescence parameters of the dye have been utilised for studying the onset of aggregation of the surfactants. An iterative procedure has been developed for the estimation of cmc and the distribution coefficient (KD) of the dye between the aqueous and the micellar phase. All the parameters provide convergent values of cmc. A high value of KD indicates that the dye exists predominantly in the micellar phase. The solvatochromic parameters characterising the dipolarity-polarisability (π(*)) and H-bond donation ability (α) of modes of solvation interaction in different micellar media have been estimated. The dye is found to distribute itself between two regions in a catanionic vesicle formed by surfactants SDS and DTAB, one being relatively polar than other. The distribution coefficients have been found out using the fluorescence data.

Synthesis of a polymer bearing several coumarin dyes along the side chain and study of its fluorescence in pure and binary solvent mixtures as well as aqueous surfactant solutions.

A copolymer bearing several pendent dyes (coumarin derivatives) along the side chain has been synthesized, and its fluorescence parameters have been monitored in pure solvents and also as a function of composition of binary solvent mixtures. Fluorescence parameters (the maximum energy of fluorescence, quantum yield, and rate constant for the decay of the excited state) of the free fluorophore show significant dependence on the nature of the immediate environment around it. The value of a parameter measured in neat solvent for the fluorophore covalently bound to the polymer is different from that of the free fluorophore, indicating that the polymer chain influences the spectroscopic properties of the dye. Whereas the energy of maximum fluorescence of the free fluorophore shows a nonlinear correlation with the solvent composition of solvent mixtures, an almost linear correlation has been observed for the polymer. A significant variation of photophysical parameters of the dye dissolved in binary solvent mixtures, which is different from that of the free fluorophore, has been observed. Thus, the free fluorophore and the fluorophore bound to the polymer sense different environments in binary solvent mixtures. A dramatic variation of fluorescence intensity of the fluorophore bound to the polymer has been observed when sodium dodecyl sulfate (SDS) is added to an aqueous solution of the polymer. The results have been explained in terms of the existence of different species (polymer, polymer-SDS aggregates, micelles) in equilibrium in solution.

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.