Second-Order Nonlinear Optical Properties of Polyoxometalate Salts of a Chiral Stilbazolium Derivative

Abstract

The synthesis of nonlinear optical (NLO) active salts with stilbazolium derivatives and polyoxometalate (POM) counterions has been investigated. With known nonchiral stilbazolium derivatives, such as MOMS(+), compounds with centrosymmetric structures have been isolated, like for instance the centrosymmetric salt (MOMS)(4)[Mo(8)O(26)] (1), synthesized under hydrothermal conditions. A new chiral derivative of the known DAMS(+) molecules, named here CHIDAMS(+), has therefore been synthesized in order to force the crystallization of the hybrid ionic salts in noncentrosymmetric space groups. The CHIDAMS(+) cation has been crystallized under two polymorphic PF(6)(-) salts, (CHIDAMS)PF(6) (2a and 2b), and its reactivity with various POMs has been investigated. The ionic salt (CHIDAMS)(2)[Mo(5)O(13)(OEt)(4)(NO){Na(H(2)O)(0.5)(DMF)(0.5)}] (4) crystallizes in the noncentrosymmetric P2(1) group, but the push-pull axis of the CHIDAMS(+) cations adopts a quasi-antiparallel alignment. The ionic salt (CHIDAMS)(3)[PW(12)O(40)].2DMF (5) associating three CHIDAMS(+) cations and a PW(12)O(40)(3-) Keggin anion crystallizes also in the P2(1) space group, but the disposition of the cations in the solid state is far more favorable. Diffuse reflectance experiments have evidenced a charge transfer between the organic and inorganic components in 5, and Kurtz-Perry experiments show that this salt exhibits a second harmonic generation efficiency more than 10 times higher than those of the PF(6)(-) salts 2a and 2b, the hybrid salt 4, and all of the other NLO active POM molecular materials reported in the literature.