Linear and nonlinear optical properties of three-coordinate organoboron compounds

Three-coordinate boron, isoelectronic with a trigonal planar carbonium ion, possesses a vacant p orbital which can conjugate with an organic pi system and thus serve as a pi acceptor. We and others have prepared a series of s ymmetric compounds of the general form (mes)(2)B double left arrowX double right arrowB(mes)(2) [mes = mesityl = 2,4,6 Me3C6H2; double left arrowX dou ble right arrow = conjugated organic pi system such as -(p-C6H4)(n)- or tra ns-transCH=CH-(p-C6H4)(n)-CH=CH-] as well as donor-acceptor compounds of th e general form D double right arrowX double right arrowB(mes)(2) [D=pi dono r such as MeO, MeS, H2N, Me2N, Ph2P, ferrocenyl; double right arrowX double right arrow = conjugated organic pi system such as -C6H4-, -C6H4-CH=CH-, - C6H4-C=C-, -C6H4-CH=CH-C6H4-] and examined representative single-crystal st ructures as well as linear (UV-vis absorption and fluorescence) and second- and third-order nonlinear optical properties (by electric field-induced se cond 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 lim ited solvatochromic behavior in absorption spectra but significant solvatoc hromic effects in their emission spectra consistent with small dipole momen ts in the ground state and large dipole moments in the excited states. Resu lts from other groups on the use of related boron compounds as either elect ron-transport materials or emitting agents in electroluminescent devices, a nd on the preparation and optical properties of three-coordinate boron-cont aining conjugated polymers are presented. Preliminary results of extended H uckel (EH) and density functional (DF) molecular-orbital calculations on re presentative symmetric diboron compounds and extended Huckel tight-binding (EHTB) calculations on a model boron-containing polymer are reported. The B (mes)(2) group is a good pi acceptor, and three-coordinate boron moieties s how significant promise for use in molecular and polymeric optical material s. (C) 2000 Academic Press.