Dipanwita Guha

Proton transfer reaction of a new orthohydroxy Schiff base in protic solvents at room temperature.

Ground and excited state inter- and intramolecular proton transfer reactions of a new o-hydroxy Schiff base, 7-ethylsalicylidenebenzylamine (ESBA) have been investigated by means of absorption, emission and nanosecond spectroscopy in different protic solvents at room temperature and 77 K. The excited state intramolecular proton transfer (ESIPT) is evidenced by a large Stokes shifted emission (approximately 11000 cm(-1)) at a selected excited energy in alcoholic solvents. Spectral characteristics obtained reveal that ESBA exists in more than one structural form in most of the protic solvents, both in the ground and excited states. From the nanosecond measurements and quantum yield of fluorescence we have estimated the decay rate constants, which are mainly represented by nonradiative decay rates. At 77 K the fluorescence spectra are found to be contaminated with phosphorescence spectra in glycerol and ethylene glycol. It is shown that the fluorescence intensity and nature of the species present are dependent upon the excitation energy.

Proton transfer reaction of 4-methyl-2,6-diamidophenol in some non-polar solvents at room temperature and 77 K

Abstract The fluorescence properties of 4-methyl-2,6-diamidophenol (MDOH) are examined in some non-polar solvents in relation to 4-methyl-2,6-diformyl phenol (MFOH) and quantum yield measurements are made at room temperature and 77 K. The excited state intramolecular proton transfer (ESIPT) is evidenced by a large Stokes-shifted emission (∼9000 cm −1 ) which is likely to originate from the enol tautomer. From nanosecond measurements and quantum yields of fluorescence, we have determined proton transfer rates. It is observed that the non-radiative channels are always dominant in the decay processes of the excited state of MDOH. At 77 K, MDOH exhibits, like MFOH, phosphorescence in all the non-polar solvents studied here and the conversion of fluorescence into phosphorescence in the case of MDOH is relatively slow compared to that of MFOH.

Proton transfer reaction of 4-methyl-2,6-diacetylphenol and an analysis with AM1 potential-energy surfaces

The ground and excited state proton transfer processes of 4-methyl-2,6-diacetylphenol (MAOH) have been studied by means of steady-state absorption, emission and time resolved spectroscopy in different protic and aprotic solvents at room temperature and 77 K. The relative fluorescence quantum yield measurements are made at different excitation wavelength and both in presence and absence of added base. The emission properties of MAOH at 77 K have been examined in relation to those of 4-methyl-2,6-diformylphenol (MFOH). At this temperature, unlike MFOH, MAOH show phosphorescence only in presence of base like triethylamine in all the solvents studied here. From nanosecond measurements and fluorescence quantum yield we have estimated the decay rate constants. The nonradiative decays are always dominant in the decay processes of the excited states. The energetics of the ground- and excited-state proton transfer in MAOH molecule has been studied by the configuration interaction method at AM1 level of approximati...

Proton transfer reaction of 4-methyl-2,6-diamidophenol in aqueous medium at room temperature and 77 K

The spectral properties and proton transfer reaction of 4-methyl-2,6-diamidophenol (MDOH) in relation to 4-methyl-2,6-diformylphenol (MFOH) and 4-methyl-2,6-diacetyl phenol (MAOH), have been investigated by steady-state and transient-fluorescence experiments in aqueous medium both in the presence and absence of base and acid at room temperature and 77 K. The absorption, emission and excitation spectra are recorded and fluorescence quantum yields are calculated. In the case of MDOH, similar to MFOH and MAOH, intramolecularly hydrogen bonded closed conformer was detected predominantly in pure water in the ground state. It is shown that the Stokes-shifted fluorescence is likely to originate from the anion conformers for all the compounds. It is proposed that owing to the structural difference between the three compounds, the proton transfer to the solvent is relatively easier in the case of MDOH and MFOH, in comparison to MAOH. At 77 K the fluorescence due to anion is markedly suppressed, showing that at this temperature anion conformer is less stable. At 77 K both MFOH and MDOH show phosphorescence in aqueous medium. The measured decay rates of MDOH and MFOH are relatively slower than those of MAOH.

Study of proton transfer reactions in binary solvent mixtures by steady state and nanosecond spectroscopy

Abstract The proton transfer reaction of 4-methyl-2,6-diformyl phenol (MFOH) in different solvent mixtures has been studied by steady state and nanosecond transient emission spectroscopic technique. The nature of the product formed are shown to be dependent on the dielectric properties and proton accepting ability of the solvent, both in the ground and excited states. Ethanol is found to be more basic (‘higher proton accepting’) than both water and butanol. In alcoholic mixtures, proton transfer reaction can be described by a dielectric continuum model whereas in water/organic solvent mixtures, the molecular aspect of the solvent needs to be considered. Solvation of the dissociated proton is also shown to be very important in the proton transfer reaction. The fluorescence quantum yield, spectral position, decay rate and the nature of the conformer formed are found to vary with solvent composition.

Interaction of 3-aminophthalhydrazide with 5-hydroxytetracycline and chloramphenicol:: a fluorescence quenching study

Abstract The fluorescence quenching of 3-aminophthalhydrazide (luminol) due to the interaction with 5-hydroxytetracycline (teramycin, HTC) and chloramphenicol (chloromycetin, CLM) has been studied employing steady-state and time-correlated single photon counting techniques. In the case of luminol-HTC system, complex formation in the ground state is assumed to explain the higher value of the quenching rate constant and is consistent with positive curvatures in the Stern–Volmer (S-V) plots. The equilibrium constant for the complex formation by hydrogen bonding interaction has been calculated from absorption spectral changes. On the other hand, linear S-V plots are obtained in the case of luminol-CLM system and the bimolecular rate constants obtained in this case are found to be similar to the rate constant for diffusion controlled process. A plausible explanation of the quenching mechanism has been discussed on the basis of hydrogen bonding interaction between the colliding species.