Chromophore Environmental Effects in Saltlike Nonlinear Optical Materials. A Computational Study of Architecture/Anion Second-Order Response Relationships in High-β Stilbazolium Self-Assembled Films

The very large cation-anion environmental effects on second-order nonlinear optical response observed in self-assembled stilbazolium monolayer ion-exchange experiments are investigated theoretically using the ZINDO/SOS quantum chemical formalism. Ion pairs consisting of the 4'-(dimethylamino)-A-methyl-4-stilbazolium cation with chloride, iodide, and p-aminobenzenesulfonate counteranions in differing mutual orientations, are examined as models to relate calculated ion pair hyperpolarizabilities to anion-exchange-induced changes in macroscopic linear optical and nonlinear optical response. Calculated hyperpolarizabilities depend strongly on the relative cation-anion packing configuration. In accord with anion exchange experiments, enhancements of the second-order nonlinear optical response on passing from Cl^- to I^- and p-aminobenzenesulfonate can be related to likely displacement of the bulkier I^- and p-aminobenzenesulfonate anions from more closely packed structural arrangements originally produced as a consequence of the self-assembly process.