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

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.

Excited‐State Intramolecular Proton Transfer in 2‐(2′‐Hydroxyphenyl)benzoxazole Derivatives

“…In the benzoxazole derivatives 3 and 4 [9], the methoxy substituent is placed at 3-and 4-positions respectively of the phenyl moiety. In a non-polar solvent, such as benzene, HBO 3 revealed two emission peaks, at about 363 and 525 nm ( Figure 32.3), which have been attributed to the enol and keto tautomers, respectively.…”

“…Copyright 2007 American Chemical Society observed from other HBO derivatives such as 3 and 4 in methylcyclohexane[9]. It is likely that the content of rotamer 1a is increased at the lower temperature, as the intramolecular O--H Á Á Á N hydrogen bond is energetically more favorable than the O--H Á Á Á O bond in the rotatomer 1b(Figure 32.7).…”

Electronic and photophysical properties of 2-(2′-hydroxyphenyl)benzoxazole and its derivatives enhancing in the excited-state intramolecular proton transfer processes: A TD-DFT study on substitution effect

“…The photophysics of HBO as the parent compound in our present work has been widely studied experimentally [1,2,5,7,9,18,[21][22][23][24] and theoretically [25][26][27][28][29][30][31][32][33][34][35] because of its structural simplicity and easily chemical modification. Thus, its derivatives [14,25,31,[36][37][38] have also been increasingly investigated both in experiment and theoretical studies.…”

“…Thus, its derivatives [14,25,31,[36][37][38] have also been increasingly investigated both in experiment and theoretical studies. The photophysical behaviors driven by ESIPT of HBO and its derivatives depend not only on solvents but also on their nature and position of various substituents on the phenol fragment as a substituent can influence the hydrogen bonding strength and consequence PT capability [8].…”

“…The photophysical behaviors driven by ESIPT of HBO and its derivatives depend not only on solvents but also on their nature and position of various substituents on the phenol fragment as a substituent can influence the hydrogen bonding strength and consequence PT capability [8]. Recently, Ohshima et al [31] reported a comprehensive comparison of electron donating substituents of methoxy (-OCH 3 ) in 3′-, 4′-and 5′-positions of the phenol moiety. They summarized that in non-polar solvent, the substitution of methoxy on 3′-position gave two emission peaks for the enol (365 nm) and the keto (525 nm) because of the competitive O-HO bonding between the methoxy group and the OH group of the phenol moiety.…”