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

The organometallic compounds [4-O2NC6H3(2-CH3)N=NC6H3(5'-CH3)-2'-(eta(5)-C5H4)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-methoxyphenyl) pyrazol-1-yl]borate} have been synthesized, characterized and their second-order non-linear optical (NLO) properties studied. In both cases the location 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 structure of 1 has been determined to be non-centrosymmetric, by contrast to the centrosymmetric related 'linear' compound [4-O2NC6H3(2-CH3)N = NC6H4-4'-(eta(5)-C5H4)Fe(eta(5)C(5)H(5))] having the ferrocenyl group at the para position of the aryl group. The three non-zero components of the second-harmonic susceptibility chi(ij)((2)) (2 omega;omega,omega) tensor have been determined for poled poly(methyl methacrylate) films containing compounds 1 and 2; chi(31)((2)) > chi(33)((2)), suggesting a tilted (npn-parallel) optical transition moment in relation to the permanent dipole moment. This also may account for the breaking of Kleinman's symmetry under near-resonant conditions, i.e. chi(15)((2)) not equal chi(31)((2)). Moreover, by comparing the results with those of related compounds having the ferrocenyl group in the para position of the aryl group, it has been concluded that molecular 'bending' strongly reduces the second-order NLO response, but it maintains strong off-diagonal (chi(31)((2))) components of the chi(ij)((2)) tensor.