Merocyanines where a polyenic spacer separates a 4H-pyran-4-ylidene moiety and different strong organic acceptors have been synthesized. According to NMR studies and X-ray diffraction data, these compounds have weakly alternated structures and remarkably zwitterionic ground states, with a partial aromatic character that is compared to those of other pyran derivatives. The proaromaticity of the 4H-pyran-4-ylidene donor lies behind the cyanine-like behavior and low (positive or negative) second-order optical nonlinearities of the shorter derivatives. On the other hand, lengthening the pi-spacer gives rise to rapidly increasing mubeta(1907) values up to 17,400 x 10(-48) esu.

In this paper, we describe the second-order nonlinear optical properties of a series of 1,3-dithiole-based electron donor-acceptor systems incorporating proaromatic donor and spacer groups. Modification of the proaromaticity of the quinoid spacer gives rise to NLO-phores with mubeta values ranging from -2000 x 10(-)(48) esu to +3000 x 10(-)(48) esu. Quite surprisingly, compounds with a p-benzoquinoid spacer and a strong acceptor group show negative mubeta values, usually associated to zwitterionic ground states, and yet they are largely quinoid, as evidenced by crystallographic data and theoretical calculations. Progressive benzoannulation of the spacer and introduction of alkylsulfanyl substituents on the dithiole donor unit result in a shift to more positive mubeta values. DFT and ab initio calculations verify these empirical trends.

4‐[2‐tetrathiafulvalenyl‐ethenyl]pyridine (1) has been prepared from a Wittig reaction between formyltetrathiafulvalene and 4‐picolyltriphenylphosphonium chloride hydrochloride. Conversion of the pyridine moiety of 1 by reaction with methyl iodide leads to 4‐[2‐tetrathiafulvalenyl‐ethenyl]‐1‐methylpyridinium iodide (2a). Neutralization of 1 with a large excess of L‐tartaric acid affords 4‐[2‐tetrathiafulvalenyl‐ethenyl]‐1‐methylpyridinium hydrogen tartrate (3). These TTF‐π‐spacer‐acceptor compounds have been characterized by elemental analysis, and IR and 1H NMR spectroscopy. The crystal structure of 2a has been determined by X‐ray diffraction. The cation is essentially planar. Examination of the bond lengths in 2a, UV/Vis spectra and CV data, and calculations indicate that an intramolecular charge transfer occurs in the studied compounds, although it is rather limited, and larger in 2a and 3 than in 1.