Aromatic/proaromatic donors in 2-dicyanomethylenethiazole merocyanines: from neutral to strongly zwitterionic nonlinear optical chromophores

Push–pull compounds, in which a proaromatic electron donor is conjugated to a 2‐dicyanomethylenethiazole acceptor, have been prepared, and their properties compared to those of model compounds featuring an aromatic donor. A combined experimental (X‐ray diffraction, ¹H NMR, IR, Raman, UV/Vis, nonlinear optical (NLO) measurements) and theoretical study reveals that structural and solvent effects determine the ground‐state polarisation of these merocyanines: whereas 4H‐pyran‐4‐ylidene‐ and 4‐pyridylidene‐containing compounds are zwitterionic and 1,3‐dithiol‐2‐ylidene derivatives are close to the cyanine limit, anilino‐derived merocyanines are essentially neutral. This very large range of intramolecular charge transfer (ICT) gives rise to efficient second‐order NLO chromophores with μβ values ranging from strongly negative to strongly positive. In particular, pyranylidene derivatives are unusual in that they show an increase in the degree of ICT on lengthening the π‐spacer, a feature that lies behind the very large negative μβ values they display. The linking of the formally quinoidal 2‐dicyanomethylenethiazole moiety to proaromatic donors seems a promising approach towards the optimisation of zwitterionic NLO chromophores.