3RD-ORDER NONLINEAR-OPTICAL PROPERTIES OF ORGANOBORON COMPOUNDS - MOLECULAR-STRUCTURES AND 2ND HYPERPOLARIZABILITIES

The third-order nonlinear optical properties (second molecular hyperpo larizabilities, gamma), of a series of organoboron compounds containin g dinesitylboron moieties have been investigated by third harmonic gen eration (THG) measurements at 1.907 mu m. Symmetric systems with B(mes )(2) groups (mes=2,4,-Me(3)C(6)H(2)) at both ends, e.g. (mes)(2)B-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, Me(2)N, H2N, etc., and Y=C6H4, C6H4-C=C-, C6H4-C H=CH-, C6H4-CH=CH-C6H4. The B(mes)(2) group compares favorably with th e more commonly used NO2 acceptor function for enhancing gamma. Values of gamma increase significantly with increasing length of pi-conjugat ion for both symmetric and unsymmetric molecules. The MeS group is muc h more efficient than MeO for enhancing gamma. Values as high as 229 x 10(-36) esu are reported for ns-trans-(mes)(2)B-CH=CH-(C6H4)(2)-CH=CH -B(mes)(2) which has a lambda(max) for absorption of 370 mn. Crystal a nd molecular structures of the bis(dimesitylboryl) benzene and anthrac ene compounds 1,4-C6H4-{B(mes)(2)}(2) and 9,10-C14H8{(mes)(2)}(2), are reported, as are fluorescence maxima for the symmetric molecules.