The photochemical properties of a series of newly synthesized dendrimers, 4-6, having a 2-(2'-hydroxyphenyl)benzoxazole (HBO) core, were studied in benzene. The fluorescence quantum yields (Phi(f)) were determined to be 0.022, 0.030, and 0.038 for 4, 5, and 6, respectively, increasing in higher generation dendrimers. With transient absorption spectroscopy, the quantum yields of the isomerization from the (E)-keto form ((1)K(E)*) to the (Z)-keto form ((1)K(Z)) (Phi(E)(-->)(Z)) and those of intersystem crossing (Phi(isc)) can be estimated. Whereas Phi(E)(-->)(Z) values decreased in higher generation dendrimers, Phi(isc) values were almost the same among 4-6. The quantum yields of nonradiative decay (Phi(nr)) increased in higher generation dendrimers. The dendrimer structure also affected the reverse tautomerization process.
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.
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