Theoretical study on the two-dimensional second-order nonlinear optical properties: a series of charge-transfer covalently bonded organoimido derived hexamolybdate complexes

To probe the cooperativity of charge transfer between organoimido and hexamolybdate, and enhance the second-order nonlinear optical (NLO) response of organoimido derivatives of hexamolybdates, electronic structures and second-order NLO properties of a series of charge-transfer covalently bonded organoimido derived hexamolybdate complexes with donor-(pi conjugated bridge)- acceptor-(pi conjugated bridge)-donor or acceptor( p conjugated bridge)- donor-( p conjugated bridge)- acceptor structures were studied by density functional theory. Studies show that different combinations of the donor, acceptor, heterocycle, -C C- and -N= N- moieties, and orientation of heterocycle remarkably affect the second-order NLO responses. The complexes containing electronic acceptor matched with the direction of charge transfer possess remarkable large molecular second-order polarizabilities. Electronic transitions to crucial excited states show that x-polarized transition, contributed to the off-diagonal second-order polarizabiliy tensor (beta(zxx)), possesses lower excited energy compared with z-polarized transition which accounted for the diagonal second-order polarizabiliy tensor (beta(zzz)) and thus led to the large in-plane nonlinear anisotropy (u = beta(zxx)/beta(zzz)) value, as well as good two-dimensional ( 2-D) second-order NLO properties. These complexes can be used as excellent 2-D second-order NLO materials from the standpoint of both large beta and u values.