Time resolved FRET measurement in various heterogeneous media using merocyanine dye as a donor.

Ultrafast fluorescence resonance energy transfer (FRET) from a merocyanine dye to a Rhodamine 6G (R6G) molecule in micelles formed by the surfactants SDS and DTAB and also in a catanionic vesicle formed by SDS and DTAB has been studied by picosecond time resolved emission spectroscopy. Here the dye acts as a donor molecule and R6G acts as the acceptor molecule. Multiple timescales of FRET have been detected, namely, an ultrafast component of 100-500 ps and relatively long component (1800-3300 ps). The different time scales are attributed to different donor-acceptor distances.

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.

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.

Ketocyanine dyes as sensors for proticity and pH of a medium.

Photophysical properties of two structurally similar ketocyanine dyes have been studied in aprotic+protic (dioxane+water, tetrahydrofuran+water) and protic+protic (methanol+water) solvent mixtures. The observed photophysical properties of these dyes show different trends with an increase in solvent polarity (in the E(T)(30) scale) in aprotic and protic media. The H-bonding at the carbonyl centres of the dyes in the excited state presumably plays a role in modifying the emitting state and hence the photophysical properties show similar trend in a protic solvent or in a mixed aprotic+protic solvent at higher percentage of the later. Thus the dyes are supposed to act as useful probes for studying H-bonding ability of a medium. Absorption and fluorescence spectral characteristics of these dyes have been found to get modified in the presence of strong acid. The presence of H(+) ions persists the dyes to exist in equilibrium with the protonated forms having different absorption and fluorescence characteristics. Reversible change of the equilibrium with pH of the medium makes the dyes, particularly the symmetric one, good probes for indicating the pH of the medium.

Photophysics of representative ketocyanine dyes: dependence on molecular structure.

Spectral properties of a new fluorescent ketocyanine dye have been discussed. The energy of maximum absorption/fluorescence of the dye exhibits bathochromic shift with increasing polarity of the medium. Both dipolarity-polarisability and hydrogen bond donation interaction contribute to solvation of the dye. Study of fluorescence parameters points to existence of different emitting states of the dye for aprotic and protic solvents. While the emitting state is the (1)(π, π*) state for aprotic solvents, fluorescence supposedly take place from a different emitting state involving H-bond formation in the excited state in protic solvents. Fluorescence parameters of the dye have been compared with those for a structurally similar symmetric ketocyanine dye. The faster decay of the dye relative to its symmetric counterpart has been explained as due to an increase of nonradiative decay.

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.

Fluorimetric study of water-ethanol interaction and its effect on the micellisation of sodium dodecyl sulphate in the presence of bovine serum albumin.

The model for self-association of ethanol in water which was established earlier by compressibility and infrared absorption measurements, has been further supported in the present work by monitoring fluorimetrically the variation of the solvatochromic Kamlet-Taft pi* parameter of ethanol-water mixtures over the entire mole fraction range. This model has been used to interpret the variation of the critical micellisation concentration (cmc) of sodium dodecyl sulphate in the presence of bovine serum albumin (BSA) in aqueous ethanol mixtures (as obtained from the tryptophan fluorescence intensity of BSA) in low ethanol mole fraction range.

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.

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.

Fluorimetric study of interaction of merbromin with trypsin.

Interaction of merbromin with trypsin is of bovine origin has been studied by monitoring the absorption steady-state and time-resolved fluorescence spectral properties of the dye. Studies have been done in media of varying pH at different trypsin concentrations. It has been observed that trypsin brings about a quenching of fluorescence of the dye. The quenching is static in nature and the equilibrium constant of dye-trypsin interaction in the ground-state has been determined from quenching studies. Steady-state anisotropy of the dye increases in presence of trypsin in the medium. Values of micro-viscosity in the vicinity of the fluorophore in media containing trypsin have been determined from measurements of fluorescence anisotropy. Time-resolved fluorescence studies indicate the existence of two decaying states for the dye. The fractional contribution to the time-resolved decay changes with pH. The average lifetime, however, does not depend on the concentration of trypsin.

Ground and excited state complexation of ketocyanine dyes with alkaline earth metal ions.

Electronic absorption and emission spectral characteristics of two ketocyanine dyes have been studied in solution in the presence of alkaline earth metal ions. Absorption spectral studies indicate complex formation between the ions and the dyes in the ground state. Values of the equilibrium constant and the enthalpy change characterizing dye (S0)-metal ion interaction have been determined from the absorption spectral data. In the presence of the metal ions the fluorescence spectrum of the dyes shows two bands pointing to the existence of two emitting species, viz., the solvated and the complexed dye in solution. Time-resolved studies of the dyes in solution containing the metal ions can be explained by a two-state model and indicate the presence of two emitting species in equilibrium. Values of the equilibrium constant for the interaction of metal ion and the dyes in the S1 state have also been estimated.

Spectroscopic study of association of a hemicyanine dye in mixed aqueous binary solvents.

Electronic absorption and steady state emission properties of a hemicyanine dye [4-[4-(dimethylamino)styryl]-1-docosylpyridinium bromide], have been studied in several pure solvents and two mixed binary solvents (water+ethanol and water+acetonitrile). In pure solvents the band-width of the absorption spectrum correlates well with the Stoke's shift. In mixed aqueous solvents two different molecular forms of the solute, viz. the monomer and the dimer of the solute exists in equilibrium. Thermodynamic parameters (e.g. the Delta G degrees and Delta H degrees ) characterizing the equilibrium have been determined. While the value of Delta G degrees changes very slightly with the composition of the binary mixture, the value of Delta H degrees has been observed to depend significantly with solvent composition.

Lithium ion-ketocyanine dye interactions in the ground and excited states.

Electronic absorption and emission spectral characteristics of a ketocyanine dye have been studied in solution in the presence of lithium perchlorate. Absorption spectral studies indicate complex formation between lithium ion and the dye in the ground state. The value of the equilibrium constant along with the molar absorbance of the absorbing species has been determined for the dye-cation interaction. The energy of maximum fluorescence shifts toward the red with the addition of LiClO(4). Steady-state emission studies point to the existence of two emitting species, viz., the solvated and the complexed dye in equilibrium. The values of the equilibrium constant for the process have been determined in acetone and acetonitrile. Time-resolved studies in the picosecond domain in pure solvents reveal that the lifetime (tau) value increases as the solvation interaction increases. Time-resolved studies also indicate the presence of two emitting species in equilibrium.

UV-vis spectroscopic study of interaction of metal ions with the E(T)(30) dye involving micellar media.

Interaction between 3d-transition metal ions (Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+ and Zn2+) and the E(T)(30) dye, 2,6-diphenyl-4-(2,4,6-triphenyl 1-pyridino)phenolate in aqueous medium have been studied by distributing the dye between the aqueous and micellar phase formed by cationic surfactant cetyltrimethylammonium bromide(CTAB). Values of equilibrium constant K for the dye-metal ion interaction and the partition coefficient of the dye between the micellar and the aqueous phase have been determined. K values show a systematic correlation with ionic potential of metal ions.

Novel fluorescent probe as aggregation predictor and micro-polarity reporter for micelles and mixed micelles.

Aggregational behaviour of micelles sodium dodecyl sulphate (SDS and Triton X-100, TX-100 both in pure and mixed form) and micelle like aggregates such as polymer-surfactant system [polymer poly(vinyl pyrrolidone), PVP]-SDS have been studied by using fluorescence characteristics of a newly synthesized probe. The critical micelle concentration (CMC) values determined at various surfactant compositions are lower than the ideal values indicating a synergistic effect. The value of the interaction parameter for the surfactant mixture has been determined which agrees well with the value calculated according to molecular thermodynamic theory. The total aggregation number of surfactant in mixed micelle shows a drastic variation in the SDS mole fraction range 0 < or = alpha1 < or = 0.3 and beyond the range it remains practically constant. Molar-based partition coefficients for the dye between the micellar and aqueous phase have been determined and a non-linear variation is obtained for the mixed micellar system. Variations of micro-polarity in the mixed micellar region have been investigated as a function of surfactant composition and results have been explained in terms of a suitable realistic model.

Influence of a neutral polymer (PVP) on the solvatochromic properties of SDS micelles.

In the present work, we have studied the influence of a water-soluble neutral polymer (poly-N-vinyl pyrrolidin-2-one, PVP) on the Kamlet-Taft polarity parameters of sodium n-dodecyl sulfate (SDS) micelles. We have used pyrene as an independent dipolarity/polarizability (pi parameter) descriptor molecule. It has been found that the addition of polymer (0.1 wt % PVP) increases the pi value of SDS micelles, suggesting that PVP-SDS aggregates have a larger dipolarity/polarizability than that of SDS micelles. Linear solvation energy relationships involving the fluorescence transition energy of four structurally similar ketocyanine dyes have also been used to evaluate the polarity parameters. It is interesting to observe that the addition of polymer leads to a decrease in both the hydrogen-bond donation (alpha-parameter) and hydrogen-bond acceptance ability (beta-parameter) of the micellar aggregate formed by PVP-SDS, whereas the dipolarity/polarizability value shows an increasing trend. Moreover, convergent results have been obtained for the pi value for PVP-SDS aggregates, using pyrene as a pi descriptor and ketocyanine dyes as molecular probes.

Use of a solvatochromic probe for study of solvation in ternary solvent mixture.

Solvation characteristics of 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridino)phenolate in completely miscible ternary solvent mixtures (viz., methanol + acetone + water, methanol + acetone + benzene, and methanol + chloroform + benzene) have been studied by using an electronic spectroscopic procedure. The transition energy (E) corresponding to the charge-transfer band maximum of the solute in a ternary solvent mixture differs significantly from the average E-values in the component solvents weighted by the mole fraction of the solvents. A two-phase model of solvation has been invoked to explain the results. The excess or deficit of solvent components in the local region of the solute molecule over that in the bulk has been estimated using the knowledge of solvation in binary solvent mixtures.

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.