2,2′‐Diborylazobenzenes with Double N–B Coordination: Control of Fluorescent Properties by Substituents and Redox Reactions

Abstract

Abstract 2,2′‐Bis[bis(pentafluorophenyl)boryl]azobenzenes were synthesized. X‐ray crystallographic analysis exhibits a planar core structure of the azobenzene moiety, double intramolecular N–B coordination, and two tetracoordinate boron atoms. The 2,2 ′ ‐diborylazobenzenes show fluorescence emission with orange and red colors upon irradiation. The double N–B coordination causes redshifts in both the absorption and emission maxima and a decrease in the Stokes shifts relative to those of the 2‐borylazobenzene derivative. On the basis of the density functional theory calculations of the molecular orbitals of 2,2′‐diborylazobenzene, the π* orbital (LUMO) was found at a much lower energy level (–4.67 eV) than that in 2‐borylazobenzene (–3.70 eV). A reversible reduction wave was observed at a low reduction potential in the cyclic voltammograms of the 2,2′‐diborylazobenzenes. Single‐electron reduction of one of the 2,2′‐diborylazobenzenes generates an azobenzene radical anion, which was confirmed by an active ESR signal. The fluorescence was quenched by the reduction and recovered by air oxidation in this azobenzene.