Variation of the Backbone Conjugation in NLO Model Compounds: Torsion‐Angle‐Restricted, Biphenyl‐Based Push‐Pull‐Systems

Abstract

AbstractTerminal piperidinyl‐ and nitro‐functionalized biphenyls, bridged between the 2 and 2′ positions by a variable number of methylene groups, are synthesized and fully characterized. These push‐pull systems with defined and restricted torsion angles between their phenyl rings are ideal model compounds to investigate the influence of the chromophore's conjugation in nonlinear optic (NLO) responses. A general synthetic route that can be implemented to access these model compounds is reported, starting from dibromo or ditriflate biphenyls. Hartwig–Buchwald cross‐coupling, a selective azacycloalkylation of diaminobiphenyls and a mild oxidation of primary amines to nitro groups in the presence of a tertiary amine summarizes the synthetic pathway towards the desired model compounds. NLO properties of the series of torsionally constrained push‐pull biphenyls are collected by electric‐field‐induced second‐harmonic generation (EFISH) experiments. The results agree qualitatively with semi‐empirical simulations based on the AM1 Hamiltonian. A linear dependence of the quadratic response on the cos2(Φ) of the inter‐aryl dihedral angle is observed, which points to oscillator strength loss as the dominant effect of increasing backbone twist.