Nonlinear optical properties of diaromatic stilbene, butadiene and thiophene derivatives

Abstract

Series of highly polar stilbene (1a–e), diphenylbutadiene (2a–c) and phenylethenylthiophene (3a–c) derivatives were prepared via Horner–Wadsworth–Emmons method with a view to produce new and efficient materials for second harmonic generation (SHG) in the solid-state. The single-crystal X-ray structures of compounds 1–3 reveal extensive polymorphism and a peculiar photodimerization of the 2-chloro-3,4-dimethoxy-4′-nitrostilbene derivative 1a to afford two polymorphs of tetra-aryl cyclobutane 4. The stilbene congeners 2-chloro-3,4-dimethoxy-4′-nitrostilbene (1a·non-centro), 5-bromo-2-hydroxy-3-nitro-4′-nitrostilbene (1b) and 4-dimethylamino-4′-nitrostilbene (1e), as well as 4′-fluoro-4′′-nitro-1,4-diphenyl-1,3-butadiene (2a) present the ideal, non-centrosymmetric arrangement of the chromophores for nonlinear optical (NLO) activity. Compounds 1b and 2a exhibit only relatively low intensity for second harmonic generation (0.04 and 0.18 times that of urea reference, respectively), while the stilbene polymorph 1a·non-centro shows NLO activity of over 32 times that of urea. In addition, the conjugated diaromatic compounds 1–3 display fluorescence behaviour in CH₂Cl₂ solutions with the exception of stilbene derivative 1b.