Samaresh Mukherjee

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.

Emission spectra of 4-methyl-2,6-diacetylphenol in 3-methylpentane at room temperature and 77 K

Abstract The fluorescence properties of 4-methyl-2,6-diacetylphenol (MAOH) are examined in 3-methylpentane and quantum yield measurements are made at room temperature and 77 K. The fluorescence quantum yield is temperature and excitation wavelength dependent. 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 does not show any phosphorescence in 3-methylpentane. From this observation it is confirmed that phosphorescence appears due to the rotation of the formyl group in MFOH. The fluorescence decay rate of MAOH is relatively faster than that of MFOH and non-radiative decays are dominant in the decay processes of the excited states of both compounds.

Excitation wavelength and solvent dependent emission spectra in weakly polar aprotic solvents at room temperature and 77 K

Abstract Excitation wavelength-dependent emission spectra of 4-methyl-2,6-diformylphenol (MFOH) and o -hydroxybenzaldehyde (OHBA) have been examined both in pure weakly polar aprotic solvents and in the presence of a base at room temperature and 77 K. It is shown that fluorescence quantum yield shape, position of the spectra, and number of conformers are dependent upon the excitation energy and also on the proton-accepting ability of the solvents. Fluorescence spectra cannot be correlated with the solvent dielectric properties. At 77 K, deactivation occurs via phosphorescence only at a particular experimental condition in all the solvents studied here. The decay rates are relatively slower in an environment where the probability of hydrogen bonding interaction is stronger.

Nanopools Governing Proton Transfer in Diametrical Ways in the Ground and Excited State

We present here the effects of geometrically constrained environments on the proton transfer reaction of 4-methyl 2,6-diformyl phenol (MFOH) both in the ground and excited states by employing steady-state and time-resolved fluorescence spectroscopy having picosecond and femtosecond resolutions. The nanometer-sized water pools formed in the ternary microemulsion of n-heptane-aerosol OT-water promote reprotonation of the probe. As we go on increasing the water content up to a certain value in the ground state whereas deprotonation is favored in the excited state. The emission intensity has a complex behavior as the water content is changed in the system. The lower fluidity of confined water within the reverse micelle with respect to the normal bulk water alters the related dynamics of the H-bonded network. These observations are rationalized on the basis of altered ionic water activity in the confined surroundings, i.e., on dielectric constant, ionic mobility, pH, and the favorable orientation of dipoles in the medium. Our observations might be helpful to infer about the characteristics of nanoreactors, which often mimic many biological hydrophilic pockets.

Complex formation and photophysical properties of luminol: solvent effects

Abstract The photophysical properties of the lowest excited singlet state of luminol were studied in various protic and aprotic solvents with different dielectric constants using steady state and nanosecond emission spectroscopy at room temperature and 77 K. A red shift is observed in the emission spectra on going from aprotic to protic solvents. We show that this red shift is due to the formation of a relatively long- lived complex in the excited state between luminol and protic solvents. The decay rates correlate well with the solvent shifts. The dependence of the quantum yields of fluorescence on the excitation energy and solvent characteristics was studied.

Strengthening of the intramolecular hydrogen bond in 7-ethylsalicylidene aniline due to steric repulsion

Abstract In a newly synthesized orthohydroxy Schiff base, 7-ethylsalicylidene aniline (ESA), the hydrogen atom in C–C(H) N group is replaced by an ethyl group. The crystal structures of ESA determined by X-ray crystallography and ab initio calculations at the level of B3LYP/6-31G∗∗ are performed. The results obtained indicate that the steric effects lead to the strengthening of intramolecular hydrogen bond (O–H⋯N) of ESA. We have compared the results of ESA with the results of another two previously studied Schiff bases, 2-(N-methyl-α-iminoethyl)-phenol (I) and 2-(N-benzyl-α-iminoethyl)-phenol (II). We have also constructed the potential energy surfaces on which the proton is supposed to move and analyzed the reaction path.

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 a new orthohydroxy schiff base at room temperature and 77 K

One new orthohydroxy Schiff base, 2-(N-benzyl-alpha-iminoethyl)naphthol (BEIN) has been synthesized. The proton transfer reaction of BEIN has been investigated by means of absorption, steady state and time resolved fluorescence spectroscopy in different solvents at room temperature and 77K. The behavior of BEIN in ethanol and water, has been studied in neutral, acidic and basic conditions. Excited state intramolecular proton transfer (ESIPT) is evidenced by a large Stokes shifted ( approximately 11,000 cm-1) fluorescence in solid crystalline media at room temperature. We present the observation of phosphorescence both in non-polar and protic solvents at 77K. The observed decay dynamics of the phosphorescence and delayed fluorescence indicates that the triplet state can be attributed to the cis-keto form. The molecular structures are determined by B3LYP/6-31G** calculation. From theoretical study it is suggested that the strengthening of hydrogen bond result from the steric repulsion of the phenyl ring. The presence of benzene ring increases the proton transfer barrier in case of BEIN compared to previously studied 7-ethylsalicylidenebenzylamine (ESBA).

Excited state proton transfer reaction of two new intramolecularly hydrogen bonded Schiff bases at room temperature and 77K

Two new orthohydroxy Schiff bases, 7-phenylsalicylidene benzylamine (PSBA) and 7-ethylsalicylideneaniline (ESA) have been synthesized. The excited state intramolecular proton transfer (ESIPT) and the structure of PSBA and ESA in its crystalline form and in the solvents n-hexane, n-heptane and 1,4-dioxane have been investigated by means of absorption, emission and nanosecond spectroscopy at room temperature and 77K. One ground state species has been detected both in neutral and basic solutions of both PSBA and ESA: the cis-enol form with an intramolecular hydrogen bond. The ESIPT and formation of keto tautomer are evidenced by a large Stokes shifted emission (approximately 12000 cm(-1)) at room temperature only in the case of ESA. On the other hand the keto tautomer is the predominant species at 77K in a solid matrix and as a solid sample at room temperature both in the case of ESA and PSBA. In the case of both ESA and PSBA the more intense, higher energy emission is due to the species which has not undergone ESIPT and attributed mainly due to cis-enol form. The trans-enol form is also observed by changing the excitation wavelength. Both the compounds are found to undergo a structural change to a zwitterionic and intermolecular hydrogen bonded form in the presence of a strong base like triethylamine. From the nanosecond measurements and quantum yield of fluorescence we have estimated the decay rates of proton transfer reaction in the case of PSBA. Our theoretical calculation at the AM1 level of approximation shows that the ground singlet state has a rather large activation barrier both in the case of PSBA and ESA. The barrier height is much lower on the corresponding excited singlet surface only in the case of ESA. The process is predicted to be endothermic in the ground state and exotherrmic in the excited singlet state.

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.