Role of rotamerisation and excited state intramolecular proton transfer in the photophysics of 2-(2'-hydroxyphenyl)benzoxazole, 2-(2'-hydroxyphenyl)benzimidazole and 2-(2'-hydroxyphenyl)benzothiazole: a theoretical study

Abstract

Semiempirical (AM1-SCI) calculations have been performed to rationalise the experimental findings in relation to the photophysics of 2-(2'-hydroxyphenyl)benzoxazole (HBO), 2-(2'-hydroxyphenyl)benzimidazole (HBI) and 2-(2'-hydroxyphenyl)benzothiazole (HBT). The calculations reveal that, while for HBO and HBI, two rotameric isomers are present in the ground state, there is only one stable species in the S₀ state of HBT. Excited state intramolecular proton transfer (ESIPT) reaction is, however, operative in the lowest excited singlet (S₁) and triplet (T₁) states for all the three molecular systems; resulting altogether three fluorescence bands for HBO and HBI and two for HBT. The excitation, fluorescence and phosphorescence bands have been assigned theoretically. The calculated results agree well with the existing experimental reports. The potential energy surfaces (PES) have been generated for the intramolecular proton transfer (IPT) reactions. The PES reflect that although the IPT process is not favourable in the ground state, the ESIPT process is feasible, both thermodynamically as well as kinetically, for all the three molecular systems in the S₁ as well as T₁ states.