Synthesis and Optical Properties of Diboron Complexes of 1,4‐Anthraquinone Derivatives

N^O‐type 1,4‐diboron complexes ( 5 : R = H, 6 : R = CF 3 , 7 : R = OMe, and 8 : R = NMe 2 ) 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 S 0 –S 2 transition to longer wavelengths, and in 8 this transition overlaps with S 0 –S 1 , 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.