Syntheses and quadratic nonlinear optical properties of salts containing benzothiazolium electron-acceptor groups

A series of chromophoric salts has been prepared in which electron-rich 4-(dimethylamino)phenyl groups are connected via polyenyl chains to electron-accepting N-methylpyridinium or 3-methylbenzothiazolium units. These compounds have been characterized by using various techniques, including electronic absorption spectroscopy and cyclic voltammetry. Single-crystal X-ray structures have been determined for several salts, all of which crystallize centrosymmetrically. Molecular quadratic nonlinear optical (NLO) responses have been determined using femtosecond hyper-Rayleigh scattering (HRS) at 1300 and 800 nm and via Stark (electroabsorption) spectroscopic studies on the intense, visible π - → π* intramolecular charge-transfer (ICT) bands. Large red shifts in the ICT transitions on replacing a pyridinium with a benzothiazolium unit indicate that the latter acts as a more effective electron acceptor. Both HRS and Stark measurements show that the static first hyperpolarizability β0 increases with polyene chain extension in both types of chromophore, and the benzothiazolium salts have larger NLO responses than their pyridinium analogues. The results of time-dependent density functional theory calculations using a polarizable solvent continuum model agree with the observation that β0 increases with chain lengthening, but the observed superiority of the benzothiazolium acceptor is not predicted either in the ICT energies or β0 values. Coupled perturbed Hartree - Fock and semiempirical INDO/S calculations similarly fail to reproduce this principal conclusion from the experimental studies. © 2006 American Chemical Society.