Second-order non-linear optical properties of 'bent' ferrocenyl derivatives

The organometallic compounds [4-O2NC6H3(2-CH3)N=NC6H3(5'-CH3)-2'-(eta(5)-C5 H4)Fe(eta(5)-C5H5)] 1 and [Mo(Cl)(Tp(An))(NO){NHC6H3(3-CH3)[N=NC6H3(5-CH3)- 2-(eta(5)-C5H4)Fe(eta(5)-C5H5)]-4}] 2 {Tp(An) = hydrotris[3-(p-methoxypheny l) pyrazol-1-yl]borate} have been synthesized, characterized and their seco nd-order non-linear optical (NLO) properties studied. In both cases the loc ation of the ferrocenyl donor group at the ortho position of the aryl group with respect to the N=N group breaks the 'molecular linearity' defined by the bridging axis between the donor and acceptor groups. The crystal struct ure of 1 has been determined to be non-centrosymmetric, by contrast to the centrosymmetric related 'linear' compound [4-O2NC6H3(2-CH3)N = NC6H4-4'-(et a(5)-C5H4)Fe(eta(5)C(5)H(5))] having the ferrocenyl group at the para posit ion of the aryl group. The three non-zero components of the second-harmonic susceptibility chi(ij)((2)) (2 omega;omega,omega) tensor have been determi ned for poled poly(methyl methacrylate) films containing compounds 1 and 2; chi(31)((2)) > chi(33)((2)), suggesting a tilted (npn-parallel) optical tr ansition moment in relation to the permanent dipole moment. This also may a ccount for the breaking of Kleinman's symmetry under near-resonant conditio ns, i.e. chi(15)((2)) not equal chi(31)((2)). Moreover, by comparing the re sults with those of related compounds having the ferrocenyl group in the pa ra position of the aryl group, it has been concluded that molecular 'bendin g' strongly reduces the second-order NLO response, but it maintains strong off-diagonal (chi(31)((2))) components of the chi(ij)((2)) tensor.