H. Mishra

Photoinduced proton transfer in 3-hydroxy-2-naphthoic acid

Spectral and photophysical properties of 3-hydroxy-2-naphthoic acid (3HNA) have been investigated experimentally and theoretically. In addition to its normal fluorescence, 3HNA exhibits a large Stokes-shifted emission that depends on its concentration, the nature of the solvent, pH, temperature and excitation wavelength. 3HNA seems to form different emitting species in different media. The large Stokes shift is attributed to species undergoing excited state intramolecular proton transfer (ESIPT). Ab initio calculations using configuration interaction (single excitation) reveal a single minimum in the potential energy profile corresponding to the primary form in the ground state. While semi-empirical calculations with CI (AM1/PECI = 8) predict a double well potential, single point density functional theoretic calculations (B3LYP/6-31G**) confirm the absence of a barrier in the ground state for proton transfer. Ln the first excited singlet state, however, there are two minima corresponding to the primary and tautomeric forms at both ab initio CIS and AM1/PECI = 8 levels, thus accounting for the dual emission in 3HNA. The theoretical methods also account for the observed pH dependence of the spectral characteristics qualitatively correctly.

Photoinduced proton transfer coupled with energy transfer: Mechanism of sensitized luminescence of terbium ion by salicylic acid doped in polymer

In the present work, excited state intramolecular proton transfer (ESIPT) in salicylic acid (SA) monoanion and subsequent sensitization of Tb(3+) ion in polyvinyl alcohol (PVA) have been studied. The study has been carried out both by steady state and time domain fluorescence measurement techniques at room temperature. It is found that the SA completely ionizes and exists as monoanion in PVA. It exhibits a large Stokes shifted blue emission (10 000 cm(-1)) due to ESIPT and shows a decay time of 6.85 ns. On the other hand, Tb(3+) ion shows a very weak green emission and a decay time of approximately 641 mus in PVA film. Upon incorporating Tb(3+) ion in SA doped PVA film, both intensity and decay time of SA decrease and sensitized emission from Tb(+3) ion along with 3.8 mus rise time is observed. Energy transfer is found to take place both from excited singlet as well as triplet states. A brief description of the properties of the present system from the viewpoint of luminescent solar collector material is addressed.

Photophysics and photochemistry of salicylic acid revisited

The spectral and photophysical properties of salicylic acid in various solvents have been studied by steady state and time resolved experiments. Various emitting species have been identified. In alcohols, e.g. methanol, both the zwitterion and monoanion show emission. The addition of a small amount of acid results only in zwitterionic emission, whereas in the presence of a small amount of alkali, the monoanion is produced. Only the monoanion shows emission in water. In concentrated sulfuric acid, emission from the cation is observed, whereas in concentrated KOH solution, the dianion is the emitting species. In hydrogen bonding solvents, e.g. diethyl ether, hydrogen bonded monomers are present. In polymer matrix poly(methyl methacrylate) only monomeric emission is observed. The deactivation of the cation is similar to that of the zwitterion, indicating that the presence of an extra proton in the carboxylic group increases the nonradiative process. Hydrogen bonding as well as viscosity/rigidity of the matrix have also been found to decrease the deactivation rate in the zwitterion.

An optical pH sensor based on excitation energy transfer in Nafion® film

Abstract Excitation energy transfer, between a dye pair (acriflavine and rhodamine 6G) doped in an ion exchange polymer matrix (Nafion®), has been investigated from the viewpoint of fabrication of an optical pH sensor. The study is done by steady state as well as time domain fluorescence measurements. Using fixed donor–acceptor concentration, excitation energy transfer in the pH range 2–12 has been found to follow Forster function for dipole–dipole interaction. Further, pH has been calibrated by the energy transfer parameters, e.g. overlap integral ΩDA, energy transfer efficiency ηT, rate of energy transfer KR, critical transfer distance R0A and reduced concentration γ0A in the range 2–12 and is found to show an approximately linear dependence. These results in Nafion® matrix are similar to those found by us in solution [Sens. Actuators: B Chem. 63 (2000) 18]. The system appears to be attractive for optical pH sensor with reversibility and sensitivity of 0.01 pH units.

Role of diffusion in excitation energy transfer: a time-resolved study

Abstract Excitation energy transfer between acriflavine and erythrosin B dyes in solutions of two different viscosities has been studied. For low acceptor concentrations decay is exponential whereas for high acceptor concentrations it fits with Forster function for energy transfer. The observed diffusion coefficients are higher than even the sum of diffusion cofficient for translational diffusion and energy migration for immobile donors. This has been attributed to the increased donor–donor transport in the presence of translational diffusion. It is interesting to note that our experimental results are in complete agreement with the theoretical results of one of the recent general models for energy transfer and migration (S. Jang et al., J. Chem. Phys. 102 (1995) 815).

Excitation energy transfer between acriflavine and rhodamine 6G as a pH sensor

Abstract Excitation energy transfer between acriflavine (donor) to rhodamine 6G (acceptor) molecules in water with varying pH has been studied by using steady state measurements. From absorption and fluorescence spectra of donor and acceptor, overlap integrals for donor fluorescence and donor absorption ( Ω DD ) as well as donor fluorescence and acceptor absorption ( Ω DA ) have been calculated. The corresponding critical transfer distances for dipole–dipole Forster mechanism of excitation energy transfer between donor–donor ( R 0D ) and donor–acceptor ( R 0A ), reduced concentration and efficiency of energy transfer have also been calculated. It has been found that the Ω DA change with pH whereas Ω DD remain unchanged, resulting in change in energy transfer from donor to acceptor with pH, but no change in energy migration. The efficiency of excitation energy transfer is maximum for a highly basic solution and it decreases with a decrease in pH. The energy transfer efficiency, reduced concentration, and overlap integrals show a linear dependence on pH. It has been proposed that the system may be used as a wide range (1.4 to 12) pH sensor with an accuracy of 0.01.

Edge excitation red shift and micro environmental effects on the photophysics of quinine bisulphate dication

Photophysical properties of quinine bisulphate dication (QSD) in polymer matrices viz. polymethyl methacarylate (PMMA), cellulose acetate (CA), Nafion®-117 and polyvinyl alcohol (PVA) along with a comparative study in fluid media (frozen glass) has been reported. Edge excitation red shift (EERS) is observed in all these media. The magnitude of EERS increases in the frozen glass as compared to liquid phase. The polymers also differ in the magnitude of EERS. The results suggest the presence of the molecule in different geometries in the polymeric media. The photophysical behavior has been found to be sensitive to the micro-environmental polarity and free volume of the matrix.

Effect of Polymer Microenvironment on Excitation Energy Migration and Transfer

Excitation energy transfer between the dye pair acriflavine (donor) to rhodamine-6G (acceptor) in various polymers [polyvinyl alcohol (PVA), cellulose acetate, and polymethyl methacrylate (PMMA)] was studied using steady-state and time-resolved fluorescence spectroscopy at room temperature. In all these polymers, at higher acceptor concentrations, direct energy transfer from acriflavine to rhodamine-6G followed the Förster theory, which is indicated by the agreement in the values of the observed critical transfer distance with that calculated from spectral overlap. On the other hand, at low acceptor concentrations, the excitation energy migration influences the kinetics, resulting in a significantly higher value of the observed critical transfer distance, which is explained on the basis of Loring et al. (Loring, R. F.; Anderson, H. C.; Fayer, M. D. J. Chem. Phys. 1984, 80, 5731-5744) and Huber (Huber, D. L. Phys. Rev. B: Condens. Matter Mater. Phys. 1979, 20 2307-2314) theories. It was observed that the spectral overlap for donor-donor transport (excitation migration) and donor-acceptor transfer (energy transfer) and thereby other energy transfer parameters were influenced by the microenvironment of the polymers. The efficiency of energy transfer (eta) was the highest in PMMA and the lowest in PVA. Further, the study of acceptor dynamics under energy transfer showed that the rise time of the acceptor also depends on the nature of the polymer microenvironment.

Role of diffusion in excitation energy transfer and migration

Effect of diffusion on excitation energy transfer and migration in a dye pair sodium fluorescein (donor) and Rhodamine-6G (acceptor) has been studied for different viscosities by both steady state and time domain fluorescence spectroscopic measurements. The donor-donor interaction appears to be weaker as compared to donor-acceptor interaction and thus favors direct Forster-type energy transfer. Interestingly, at low viscosity (water in this case) transfer appears to be controlled by material diffusion/energy migration. Further, acceptor dynamics reveals the fact that direct Forster transfer dominates in viscous media.