H.B. Tripathi

Dual emission and double proton transfer in salicylic acid

Abstract The photophysics of salicylic acid (SA) monomer and dimer has been studied by using steady-state and time-resolved spectroscopic techniques. Dilute solution in alkanes emits at 450 nm, which as in methyl salicylate is due to intramolecular proton transfer. In concentrated solutions and in solid state, the SA dimer shows two emissions, at 370 nm and 450 nm, with some unusual behaviour in both the steady state and the time domain fluorescence. The concept of double proton transfer and the tunneling mechanism in the excited state can rationalize the observed photophysical behaviour.

Photophysics of doubly-charged quinine: Steady state and time-dependent fluorescence

Abstract The quinine dication in aqueous solution (1 N H2SO4) gives two fluorescence lifetimes (τ1 = 2.80 ns and τ2 = 19.36 ns) at ambient temperature. τ2 shows a small increase with an increase in acid concentration between 0.1 N and 15 N. Quenching by Cl− shows that τ1 and τ2 are differentially quenched. The Stern—Volmer quenching constant KSV for τ1 is 10 M−1 and for τ2 is 75 M−1. In addition, KSV is dependent on emission wavelength. In acidified solution, τ2 increases with an increase in emission wavelength, whereas τ1 exhibits a behaviour which resembles a two-state mechanism with a negative amplitude in the region of longer emission wavelength. However, the two-state theory does not give an entirely satisfactory mechanism for the time-dependent emission. Time-resolved emission spectroscopy (TRES) shows a spectral relaxation which partially explains the dependence of τ2 on emission wavelength in accordance with Bakhshiev formulation. Transient and steady state fluorescence studies from 80 to 290 K show that at 160 K there is a rapid relaxation process resulting in an increase in τ2 and a sudden spectral shift. We propose that the complex behaviour of quinine decay consists of two major relaxation processes: a charge-transfer process which occurs around 160 K and a solvent reorientation process which occurs in the fluid medium.

Time resolved fluorescence spectroscopy of quinine sulphate, quinidine and 6-methoxyquinoline: pH dependence

Abstract The excited state dynamics of quinine sulphate (QS), quinidine (Qd) and 6-methoxyquinoline (6MQ) has been studied as a function of pH in steady state and nanosecond time resolved fluorescence experiments. The solvent relaxation process is a dominant process for all the molecules studied, irrespective of pH. Moreover, 6MQ undergoes a proton transfer reaction in the excited state at pH 7 whereas QS and Qd do not exhibit excited state protonation.

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.

EXCITED STATE SOLVATION DYNAMICS OF 6-METHOXYQUINOLINE

Abstract Steady state and transient studies of 6-methoxyquinoline (6MQ) were undertaken. 6MQ undergoes a large change of dipole moment on excitation. The low energy absorption band L b does not change in position with solvent polarity whereas the emission maxima shift towards lower frequencies with broadening of the spectra. The edge excitation red shift, which is associated with the time-dependent red shift of emission, is observed in all polar solvents. The fluorescence decay is monoexponential and is dependent on emission wavelength. The data are explained with the help of the Bakhshiev model of solvent relaxation. The solvent relaxation time τ r and the fluorescence lifetime τ f increase with the polarity of the solvent. In aqueous solution, 6MQ undergoes a protolytic reaction in the excited state. The rate constant for the proton transfer is 1.2×10 8 s −1 .

Excited-state intermolecular proton transfer reaction of 6-hydroxyquinoline in protic polar medium

Steady state and time resolved characteristics of 6-hydroxyquinoline (6-HQ) in alcoholic solvents and in a mixture of solvent (methanol:water) are studied. In neat alcoholic solvents, 6-hydroxyquinoline (6-HQ) shows emission around 376 nm. The emission corresponds to normal species and shows a single exponential decay (1.45 ns). However, in methanol containing water, dual emission i.e., normal one at ∼376 nm and a large Stokes shifted emission at ∼570 nm is observed. In presence of small percentage of water the decay time shows a decreasing trend and at higher percentage of water (>10% in v) a clear departure from the single exponential is observed. The results are interpreted in terms of multiple proton transfer through methanol and water chain.

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).

Photophysics of quinidine dication in relation to quinine dication and 6-methoxyquinoline monocation

Nanosecond time resolved emission spectroscopy was used to investigate the excited state solute-solvent interaction in quinidine dication. The emission spectrum is susceptible to the wavelength of excitation and the viscosity of the medium. The fluorescence lifetime is dependent on the emission wavelength. Spectral relaxation is observed on a nanosecond time scale. The room temperature data have been explained using Bakshievs formulation of solvent relaxation. However, transient and steady state fluorescence studies from 80 to 290 K reveal that at 160 K, a rapid relaxation process other than the solvent relaxation occurs. A comparison of the photophysical data of protonated quinidine, quinine and 6-methoxyquinoline shows close similarities among these three molecules. The major two relaxation processes in these molecules are solvent relaxation and charge transfer.

Hydrogen-bonding effect on the dual emission of salicylic acid

Abstract The effect of hydrogen bonding, taking diethyl ether (Et 2 O) as the hydrogen-bonding partner, on the dual emission of salicylic acid (SA) has been investigated with both steady-state and time-domain experiments. It has been observed that in place of dual emission, only a single fluorescence band is observed int he presence of Et 2 O. This has been attributed to the fact that all SA molecules are converted into hydrogen-bonded complexes. Further, two configurations for the SA-Et 2 O complexes are suggested to explain the dynamical behaviour at low temperatures.

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.