Intra- and intermolecular proton transfer reactions in 2-phenyl substituted benzazoles

Abstract

The present review describes the salient features of inter-and intramolecular proton transfer reactions of 2-(2'-aminophenyl)-', 2-(3'-aminophenyl)-, 2-(4'-aminophenyl)-, 2-(2'-hydroxyphenyl)-, 2-(3'-hydroxyphenyl)-and 2-(4'-hydroxyphenyl)-benzimidazoles, benzoxazoles and benzothiazoles. Fluorescence quantum yield of the phototautomer produced by the intramolecular hydrogen bonding decreases on going from benzimidazole to benzoxazole to benzothiazole. This indicates that the rate of internal conversion increases in the order of compounds as mentioned above. The biprotonic phototautomerism or the presence of intermolecular proton transfer has led to the formation of (i) nonfluorescent zwitterions in case of hydroxyphenyl derivatives and the ground state precursor of this species in neutral molecules, (ii) nonfluorescent monoanions from fluorescent monoanions and (iii) nonfluorescent monocations from monocations in case of aminophenyl derivatives. In the case of 2-(4'-aminophenyl)-substituted compounds, the first protonation has always led to the formation of two types of monocations; one by protonating the amino group and the other by protonating the tertiary nitrogen atom. The former is more stable in aqueous media and the latter in non-polar media.

Photoinduced intramolecular charge transfer in trans-2-[4′-(N, N-dimethylamino)styryl]imidazo[4,5- b]pyridine: effect of introducing a C=C double bond

“…The vibrational frequency observed for t-DMASIP-b (1276 cm −1 ) is close to that of DMAPIP-b and other 2-phenyl substituted benzazoles. 41 All these spectral characteristics indicate the structural similarities of t-DMASIP-b with 2-phenyl substituted benzazoles. The vibrational structure diminishes with a red shift upon increasing the polarity of the solvent.…”

Twisted Intramolecular Charge Transfer of 2-(4‘-N,N-Dimethylaminophenyl)pyrido[3,4-d]imidazole in Cyclodextrins: Effect of pH

“…Our laboratory has been active for nearly 2 decades in studying the acid−base properties of the heterocyclic molecules, containing either one or two basic centers . The acid−base chemistry carried out on 2-(4‘-aminophenyl, AP), 2-(4‘- N , N -dimethylamino phenyl, DMAP) benzimidazole , (BI), benzoxazole , (BO), and benzothiazole , (BT) has shown that the formation of the monocations (MCs) by protonating amino or dimethylamino (MC1) or the N− atom of the imidazole ring (MC2) in the S 0 and S 1 states and their relative proportions depend on the benza ring as well as on the polarity of the environments.…”

Spectral Characteristics of the Various Prototropic Species of 2-(4‘-N,N-Dimethylaminophenyl)pyrido[3,4-d]imidazole

“…Besides the fundamental interest in the area of heterocycles, these compounds can be used as intermediates, and fine products for drugs, , color industries, laser dyes, complex-forming agents, , redox systems for solar energy, and organized assemblies . Besides these applications, another area of interest is the study of hydrogen transfer and the prototropism with suitable substituents, for example, amino-, hydroxy-, − and aminophenylbenzazoles . It has been observed that usually these prototropic equilibria are different in the excited singlet (S 1 ) state than the ground (S 0 ) state and may involve intramolecular proton transfer. , …”