| 著者 |
Kubota Y, Ogasawara A, Hakamada A, Suzuki N, Taki M, Mizuno S, Inuzuka T, Funabiki K, Yamada-Kubota C, Kuwahara M, Hara T.
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| Abstract |
N^O-type 1,4-diboron complexes (5: R = H, 6: R = CF3, 7: R = OMe, and 8: R = NMe2) were synthesized via a simple two-step reaction and their photophysical properties were systematically investigated. Single-crystal x-ray diffraction analysis of 5 revealed a planar diboron chromophore with two β-iminoenolate structures, while the N-phenyl substituents are significantly twisted from the chromophore plane. Boron complexation rigidified the chromophore, affording vibronically resolved and spectrally narrowed absorption bands for 5-7, whereas 8 exhibited pronounced spectral broadening accompanied by a bathochromic shift. TDDFT calculations indicated that increasing electron-donating ability shifts the S0-S2 transition to longer wavelengths, and in 8 this transition overlaps with S0-S1, accounting for the spectral broadening. Complexes 5-7 exhibited near-infrared fluorescence, while 8 was nonfluorescent. Dynamic quenching of 5 by N,N-dimethylaniline via intermolecular photoinduced electron transfer (PeT) was confirmed by Stern-Volmer analysis with lifetime shortening. The decrease in fluorescence quantum yield with increasing electron-donating ability was attributed to intramolecular PeT, and protonation restored fluorescence. The nonfluorescent nature of the 1,5-diboron complex was attributed to enhanced nonradiative decay in accordance with the energy gap law, together with possible intramolecular PeT.
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