The Effect of meta-Substitution on the Photochemical Properties of Benzoxazole Derivatives

Abstract

Abstract The photochemistry of 2-(2-hydroxy-3-methoxyphenyl)benzoxazole (3-MHBO) and 2-(2-hydroxy-4-methoxyphenyl)benzoxazole (4-MHBO) have been investigated. The excited-state properties of 4-MHBO were similar to those for the parent compound 2-(2-hydroxyphenyl)benzoxazole (HBO), whereas different properties were observed for 3-MHBO. 4-MHBO in benzene exhibited large Stokes-shifted fluorescence from the keto-form in the excited singlet state (KE*) after excited-state intramolecular proton transfer (ESIPT), whereas 3-MHBO emitted dual fluorescence from E* and KE* species. Fluorescence due to KE* for 3-MHBO (λmax = 525 nm) occurred at a considerably longer wavelength than that of 4-MHBO (λmax = 480 nm). The fluorescence efficiency of 3-MHBO (Φf = 0.002) was much smaller than those of 4-MHBO and HBO (Φf = 0.018 and 0.02, respectively). These results are consistent with suppression of ESIPT, stabilization of KE* species, and acceleration of non-radiative decay, probably because of the formation of a hydrogen bond between phenol oxygen and methoxy hydrogen at 3-position in 3-MHBO. Thus, 3-MHBO displayed a meta-substituent effect involving the relaxation pathway of excited state HBO derivatives.