Linear and Nonlinear Optical Properties of Three-Coordinate Organoboron Compounds

Abstract

Three-coordinate boron, isoelectronic with a trigonal planar carbonium ion, possesses a vacant p orbital which can conjugate with an organic π system and thus serve as a π acceptor. We and others have prepared a series of symmetric compounds of the general form (mes)2 B⇐X⇒B(mes)2 [mes=mesityl=2,4,6 Me3C6H2; ⇐X⇒=conjugated organic π system such as –(p-C6H4)n– or trans–trans–CH=CH–(p-C6H4)n–CH=CH–] as well as donor–acceptor compounds of the general form D⇒X⇒B(mes)2 [D=π donor such as MeO, MeS, H2N, Me2N, Ph2P, ferrocenyl; ⇒X⇒=conjugated organic π system such as –C6H4–, –C6H4–CH=CH–, –C6H4–C≡C–, –C6H4–CH=CH–C6H4–] and examined representative single-crystal structures as well as linear (UV–vis absorption and fluorescence) and second- and third-order nonlinear optical properties (by electric field-induced second harmonic generation (EFISH) and third harmonic generation (THG)). The bulky mesityl groups protect the boron centers from hydrolysis. Many of the compounds are highly fluorescent, with the unsymmetric ones displaying limited solvatochromic behavior in absorption spectra but significant solvatochromic effects in their emission spectra consistent with small dipole moments in the ground state and large dipole moments in the excited states. Results from other groups on the use of related boron compounds as either electron-transport materials or emitting agents in electroluminescent devices, and on the preparation and optical properties of three-coordinate boron-containing conjugated polymers are presented. Preliminary results of extended Hückel (EH) and density functional (DF) molecular-orbital calculations on representative symmetric diboron compounds and extended Hückel tight-binding (EHTB) calculations on a model boron-containing polymer are reported. The B(mes)2 group is a good π acceptor, and three-coordinate boron moieties show significant promise for use in molecular and polymeric optical materials.