SQUARYLIUM DYES - STRUCTURAL FACTORS PERTAINING TO THE NEGATIVE 3RD-ORDER NONLINEAR-OPTICAL RESPONSE

Off-resonant electric field induced second harmonic generation (EFISH) and resonant third harmonic generation (THG) measurements of two squa rylium dyes are reported. The magnitude of gamma is large for both pro cesses. The EFISH measurement clearly shows that the off-resonant thir d-order polarizability is negative, a consequence of the dominance of a non-two-photon mechanism. This unusual result is in agreement with o ur earlier reports on two other squarylium dyes. Resonant THG measurem ents strongly suggest that the two-photon contribution, while smaller than the one-photon contribution, is not negligible and cannot be enti rely ignored when defining the overall third-order optical susceptibil ity. Consequently, the squaryliums are best qualitatively described by a three-level model that truncates to two dominant perturbation terms . Also reported is the isomeric distribution of squaryliums. Proton NM R results are interpreted by way of molecular mechanics to show that t he squaryliums can readily exist as a mixture of isomers. The possible effects on nonlinear optical (NLO) measurements via an orientational contribution to the EFISH gamma are discussed. An X-ray structure for the squarylium dye TSQ is presented and interpreted. The X-ray structu re clearly suggests that the squarylium dyes are extensively bond delo calized with relatively little bond length alternation.