2,2'-Diborylazobenzenes with double N-B coordination: Control of fluorescent properties by substituents and redox reactions

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.