4<i>H</i>-Pyran-4-ylidenes: Strong Proaromatic Donors for Organic Nonlinear Optical Chromophores

Merocyanines where a polyenic spacer separates a 4<i>H</i>-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 4<i>H</i>-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 π-spacer gives rise to rapidly increasing μβ<sub>1907</sub> values up to 17,400 × 10<sup>−48</sup> esu.