Molecular Engineering of Benzothiazolium Salts with Large Quadratic Hyperpolarizabilities: Can Auxiliary Electron-Withdrawing Groups Enhance Nonlinear Optical Responses?

A series of push-pull chromophores comprising a dimethylamino or diphenylamino electron-donating functionality and a cationic benzothiazolium acceptor with an additional electron-withdrawing group (EWG = NO2 or CN) at various positions of the heterocyclic benzene ring have been synthesized and comprehensively investigated for their linear and quadratic nonlinear optical (NLO) properties by means of UV-visible spectroscopy and hyper-Rayleigh scattering, as well as by quantum-chemical calculations at different levels of theory (B3LYP, CAM-B3LYP, MP2, and RI-CC2). In general, all chromophores under study display large static quadratic hyperpolarizabilities beta(0), comparable to or in most cases even larger than their conventional stilbazolium-containing analogues, which makes these systems attractive for practical NLO applications. The introduction of an auxiliary EWG into the heterocyclic benzene ring causes a substantial red shift of the intramolecular charge-transfer band. Still, at the same time, this modification of the structure reduces the beta(0) values in systems with longer pi-conjugated spacers. The unexpected negative impact of the EWG substitution pattern on the quadratic NLO activity is rationalized by quantum-chemical calculations as well as by experimentally determined one-photon absorption characteristics and is discussed in detail. Furthermore, computational studies revealed that push-pull benzothiazolium dyes with a "reverse" polarity with respect to the commonly used one would be a more worthwhile target for synthesis, because of their enhanced NLO response due to the positive effect of an auxiliary electron-withdrawing group.