Third-order Nonlinear Optical Properties of Organoboron Compounds: Molecular Structures and Second Hyperpolarizabilities

The third-order nonlinear optical properties (second molecular hyperpolarizabilities, γ), of a series of organoboron compounds containing dimesitylboron moieties have been investigated by third harmonic generation (THG) measurements at 1.907 μm. Symmetric systems with B(mes)2 groups (mes=2,4,6–Me3C6H2) at both ends, e.g. (mes)2B–Y–B(mes)2 where Y=(C6H4)n (n=1,2) or trans-trans-CH=CH–(C6H4)n–CH=CH– (n=1,2), were prepared as well as push– pull systems of the form D–Y–B(mes)2 where D=MeS, Me2N, H2N, etc., and Y=C6H4, C6H4–C≅C–, C6H4–CH=CH–, C6H4–CH=CH–C6H4. The B(mes)2 group compares favorably with the more commonly used NO2 acceptor function for enhancing γ. Values of γ increase significantly with increasing length of π-conjugation for both symmetric and unsymmetric molecules. The MeS group is much more efficient than MeO for enhancing γ. Values as high as 22910–36 esu are reported for trans-trans-(mes)2-B–CH=CH–(C6H4)2–CH=CH–B(mes)2 which has a lmax for absorption of 370 nm. Crystal and molecular structures of the bis(dimesitylboryl) benzene and anthracene compounds 1,4-C6H4-{B(mes)2}2 and 9,10-C14H8{B(mes)2}2 are reported, as are fluorescence maxima for the symmetric molecules.