An Element‐Substituted Cyclobutadiene Exhibiting High‐Energy Blue Phosphorescence
Yoshiaki Shoji, Yasuhiro Ikabata, Ivan Ryzhii, Rabia Ayub, Ouissam El Bakouri, Taiga Sato, Qi Wang, Tomoaki Miura, Buddhika S. B. Karunathilaka, Youichi Tsuchiya, Chihaya Adachi, Henrik Ottosson, Hiromi Nakai, Tadaaki Ikoma, Takanori Fukushima
Abstract
Abstract 1,3,2,4‐Diazadiboretidine, an isoelectronic heteroanalogue of cyclobutadiene, is an interesting chemical species in terms of comparison with the carbon system, whereas its properties have never been investigated experimentally. According to Baird's rule, Hückel antiaromatic cyclobutadiene acquires aromaticity in the lowest triplet state. Here we report experimental and theoretical studies on the ground‐ and excited‐state antiaromaticity/aromaticity as well as the photophysical properties of an isolable 1,3,2,4‐diazadiboretidine derivative. The crystal structure of the diazadiboretidine derivative revealed that the B 2 N 2 ring adopts a planar rhombic geometry in the ground state. Yet, theoretical calculations showed that the B 2 N 2 ring turns to a square geometry with a nonaromatic character in the lowest triplet state. Notably, the diazadiboretidine derivative has the lowest singlet and triplet states lying at close energy levels and displays blue phosphorescence.