Orthogonal Geometry Enhancing the Intersystem Crossing and Photosensitive Efficiency of <i>Spiro</i> Organoboron Compounds

Based on the reported spiro organoboron compounds (PS1 and PS2 as potent ¹O₂ sensitizers), several new organoboron molecules (PS4-PS9) were constructed through structural modification, and their low-lying excited states and photophysical properties have been explored by density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. The predicted effective intersystem crossing (ISC) processes arise from the S₁→T₂ transition for PS4-PS6 and the S₁→T₄ transition for PS1, and corresponding K_ISC rate constants reach the order of magnitude of 10⁹ (s^-1). The organoboron compounds with a (N, N) chelate acceptor are predicted to exhibit relatively higher ISC efficiency than those bearing a (N, O) acceptor, and the planar C₃NBN ring and the orthogonal configuration between the donor and acceptor moieties are responsible for the ISC rate enhancement. Importantly, the geometric features of the lowest singlet excited state (S₁) for these compounds play a decisive role in their photosensitive efficiency. The present results provide a basis for better understanding of the photosensitivity of these spiro organoboron compounds and the structural modification effect.