Enhancing Light-Absorption and Luminescent Properties of Non-Emissive 1,3,4,6,9b-Pentaazaphenalene through Perturbation of Forbidden Electronic Transition by Boron Complexation

Abstract

We report an improvement of optical properties of the non‐emissive molecule through shuffling the molecular orbitals by boron complexation. The 1, 3, 4, 6, 9b‐pentaazaphenelene (5AP) derivative having a 2‐(dimesitylboryl)phenyl group was synthesized through a substitution reaction from a 2‐bromophenyl compound. The formation of the B−N dative bond was confirmed by ¹¹B{¹H} NMR spectroscopy and a single‐crystal X‐ray structure analysis. Most of the conventional 5AP derivatives hardly showed emission despite their planar and rigid molecular structures. The previous reports on 5AP derivatives ascribed the non‐luminescent behavior to the forbidden transition between frontier orbitals. On the other hand, the 5AP‐based boron complex in this work showed enhanced light absorption and luminescence from the HOMO−LUMO transition. Theoretical calculations suggested that the boron complexation should play a critical role in perturbing the forbidden character of the HOMO−LUMO transition. Because the nitrogen atom on the 5AP moiety formed the Lewis acid−base pair with the boron atom, the energy level of the HOMO of 5AP was downshifted. The character of the HOMO−LUMO transition of the boron complex was changed to afford the improved optical properties of the boron complex.