Sneh K. Dogra

Ground and excited state prototropic reactions in 2-(o-hydroxyphenyl)benzimidazole

Abstract A solvent dependence study of the steady state absorption, excitation and fluorescence spectra of 2-( o -hydroxyphenyl) benzimidazole has indicated the presence of different tautomers in the S 1 state which have the same precursor in the S 0 state. A similar study as a function of hydrogen ion concentration has identified the presence of dication, monocation, zwitterion, phototautomer and monoanion species. The respective pK a and pK * a values have been determined with fluorimetric titrations and Forster cycle methods and compared.

Excited-state intramolecular proton transfer in 2-(2′-aminophenyl) benzimidazole

Abstract An excited-state intramolecular proton transfer process has been studied in 2-(2′-aminophenyl)benzimidazole (2-APBI) in different solvents using steady state and time resolved fluorescence spectroscopy. Semi-empirical quantum mechanical calculations (AM1 and CNDO/S-CI) have also been carried out. Dual fluorescence (normal and tautomer fluorescence) is observed in non-polar solvents. Based on the fluorescence excitation spectra recorded at different wavelengths, it is concluded that the normal fluorescence originates from rotamer II or IV and tautomer fluorescence from rotamer V. The tautomer fluorescence intensity decreases with increase in the polarity and hydrogen bonding capacity of the solvents. Different fluorescence lifetimes of the normal and tautomer bands indicate that rotamer I and II are not in equilibrium in the excited state. Semi-empirical quantum mechanical calculations have shown that the rotamer II is slightly more stable than the rotamer I in the ground state whereas in polar solvents, the stability of rotamer II increases due to extra solvation. For isolated molecule the activation energy for the interconversion of rotamers is 2.71 kJ mol−1 in the ground state, whereas it is 36 kJ mol−1 in the first excited singlet state.

Proton transfer of 2-(2′-hydroxyphenyl)benzoxazole in the excited singlet state

Abstract The solvent and pH dependence of the absorption and fluorescence of 2-(2′-hydroxyphenyl)benzoxazole was studied. Four different species were observed in both ground and excited states: the anion, the neutral molecule with intramolecular hydrogen bonding, the monocation and the dication. The PKa and PKa* were calculated by absorptiometric and fluorometic titrations respectively.

Photoluminescence of 2-(o-aminophenyl)benzimidazole

Abstract The absorption and fluorescence spectra of 2-( o -aminophenyl)benzimidazole in different solvents have shown the existence of two structures formed by intramolecular hydrogen bonding. Both are equally favourable in non-polar media whereas one is more stable in polar solvents. pH effects have also been discussed with the evaluation of the ground and excited state equilibrium constants of the various equilibria.

Spectral characteristics of 2-(2′-aminophenyl)benzimidazole in β-cyclodextrin

Abstract The spectral characteristics of 2-(2′-aminophenyl)benzimidazole (2-APBI) were studied as a function of the β-cyclodextrin (β-CDx) concentration in aqueous medium and the time. The fluorescence quantum yields and fluorescence decay times were also determined. The results indicate that 2-APBI forms a 1 : 1 inclusion complex with β-CDx. The absorption and fluorescence spectra are shifted to the red. The increase in the fluorescence quantum yield of the normal Stokes-shifted band is attributed to the decrease in the non-radiative decay rate constant. The decrease in the fluorescence quantum yield of the large red-shifted fluorescence band is due to back proton transfer from the β-CDx hydroxyl groups to the imine group to yield the amine group in the S1 state. It is also concluded that the pyridine nitrogen atom and the amino nitrogen are present near the larger rim of β-CDx.

Spectral characteristics of three different isomeric 2-(aminophenyl)benzoxazoles: Effect of solvents and acid concentrations

Abstract Spectral characteristics of 2-(2′-aminophenyl)-, 2-(3′-aminophenyl )- and 2-(4′-aminophenyl) benzoxazoles (o-APBO, m-APBO, p-APBO respectively) have been studied in solvents of different polarity and hydrogen bond forming ability and at various acid concentrations. The infrared, ultraviolet and fluorescence spectra and low pKa value for the monocation-neutral equilibrium indicate the presence of intramolecular hydrogen bonding in o-APBO. Stokes shifts in different solvents have indicated that m-APBO is more polar than p-APBO in the S1 state compared to the S0 state. Only three prototropic species (dication, monocation and neutral) are observed in the ground state, whereas five prototropic species (dication, non-fluorescent monocation (2′), monocation (2), neutral and non-fluorescent monoanion) are present in the S1 state. Biprotonic phototautomerism is observed in the monocations of all the molecules. MO calculations (PPP) have also been used to aid in the interpretation of certain experimental results.

EXCITED STATE INTRAMOLECULAR PROTON TRANSFER IN 2-(2'-BENZAMIDOPHENYL)BENZIMIDAZOLE : EFFECT OF SOLVENTS

Abstract Excited state intramolecular proton transfer (ESIPT) process in 2-(2′-benzamidophenyl)benzimidazole (2-BAPBI) has been studied in different solvents by means of absorption, fluorescence, fluorescence excitation and time resolved spectroscopy. Dual fluorescence (normal and tautomer emissions) is observed in all the solvents. The quantum yield of the tautomer emission is always larger than that of the normal emission. This is because of the enhanced rate of ESIPT process which arises due to the presence of the electron withdrawing benzoyl group as a substituent to one of the hydrogen atom of the amino group in 2-(2′-aminophenyl)benzimidazole (2-APBI).

Solvatochromic effects in the fluorescence of a few diamino aromatic compounds

Abstract Solvatochromic effects of 14 solvents and various dioxane-water mixtures on the fluorescence spectra of three isomeric phenylenediamines, 2, 3-diaminonaphthalene (2,3-DAN) and 2,7-diaminofluorene (2, 7-DAF) have revealed the following: (i) the Bilot-Kawski expression of polarizability provides a better specific analysis of solvatochromic effects than the Lippert expression or the Dimroth parameter; (ii) no expression correlating the solvent polarity and Stokes shift expresses the effect of charge transfer interactions in exciplex formation; (iii) the two amino groups in 2,3-DAN are twisted with respect to the plane of the naphthalene ring.

The fluorescence spectra of dianions of α- and β-naphthylamines

Abstract The fluorescence spectra of the dianions of α- and β-naphthylamines were observed at high OH - concentrations and the fluorescence quantum yields were calculated. The p K a * values were determined for the respective equilibria.

Excited state prototropism of 6-aminochrysene

Abstract Absorption and fluorescence spectra obtained in various solvents indicate that the amino group acts as a hydrogen acceptor in the S 0 state and a hydrogen donor in the S 1 state. The lack of correspondence between the disappearance of 6-aminochrysene (CNH 2 ) and the appearance of CNH 3 + in the fluorometric titration is due to proton-induced quenching of the 6-aminochrysene. The value of the quenching constant is about 10 9 dm 3 mol −1 s −1 .