Simple zwitterionic merocyanines as potential NLO chromophores

Abstract

A suite of zwitterionic pyridylidene-based merocyanines that contain no interconnecting π-bridge between the donor and acceptor rings has been synthesised and their second-order NLO properties evaluated largely by semi-empirical computational methods (MOPAC 97/AM1). Contrary to expectation, increasing the degree of inter-ring twist (θ), at least up to 55°, in these new pyridylideneazolone chromophores is found to have little or no effect on the figure of merit [μβ(0)]. An X-ray crystallographic appraisal of one of these chromophores, 14, reveals however that the twist angle (albeit in the solid state) is greater than that predicted by computation and that all other features are consistent with the highly zwitterionic nature of these systems. In spite of this, a combination of factors—insufficient acceptor strength, insufficient extent of conjugation and perhaps insufficient twist angle, in particular—clearly leads to the low values of the quadratic hyperpolarisabilities. The trade-off between targeting a more modest β from a minimum of π-conjugating framework between D and A (and therefore synthetic expediency) and seeking a moderate-to-high dipole moment has therefore resulted in only low figures of merit for these systems.