Challenging the Auxiliary Donor Effect on Molecular Hyperpolarizability in Thiophene-Containing Nonlinear Chromophores:  X-ray Crystallographic and Optical Measurements on Two New Isomeric Chromophores

To reexamine the established “auxiliary donor” effect of thiophene in nonlinear optical (NLO) chromophores, we have prepared two isomeric donor−acceptor azo dyes, differing only in the position of the thiophene. Experimental analysis of these chromophores, including electric field-induced second harmonic generation (EFISH) and X-ray crystallography, contradicts previous experimental findings on similar chromophores but is consistent with the majority of computational precedents. We have found that the thiophene on the donor side produces a compound with a larger dipole moment; however, the isomer with the thiophene on the acceptor side is more nonlinear and has a higher figure of merit for NLO device applications.