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 β0, 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 modifi...