Second-order non-linear optical properties of ‘bent’ ferrocenyl derivatives

Abstract

The organometallic compounds [4-O₂NC₆H₃(2-CH₃)N=NC₆H₃(5′-CH₃)-2′-(η⁵-C₅H₄)Fe(η⁵-C₅H₅)] 1 and [Mo(Cl)(Tp^An)(NO){NHC₆H₃(3-CH₃)[N=NC₆H₃(5-CH₃)-2-(η⁵-C₅H₄)Fe(η⁵-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}}n2N 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}}n2NC₆H₄-4′-(η⁵-C₅H₄)Fe(η⁵-C₅H₅)] having the ferrocenyl group at the para position of the aryl group. The three non-zero components of the second-harmonic susceptibility χ_ij⁽²⁾ (2ω;ω,ω) tensor have been determined for poled poly(methyl methacrylate) films containing compounds 1 and 2; χ₃₁⁽²⁾>χ₃₃⁽²⁾, suggesting a tilted (non-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. χ₁₅⁽²⁾≠χ₃₁⁽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 (χ₃₁⁽²⁾) components of the χ_ij⁽²⁾ tensor.

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