10.1021/jo901142f.s001 Raquel Andreu Raquel Andreu Laura Carrasquer Laura Carrasquer Santiago Franco Santiago Franco Javier Garín Javier Garín Jesús Orduna Jesús Orduna Natalia Martínez de Baroja Natalia Martínez de Baroja Raquel Alicante Raquel Alicante Belén Villacampa Belén Villacampa Magali Allain Magali Allain 4<i>H</i>-Pyran-4-ylidenes: Strong Proaromatic Donors for Organic Nonlinear Optical Chromophores American Chemical Society 2009 nonlinearitie polyenic spacer moiety zwitterionic ground states NMR studies Proaromatic Donors esu donor compound μβ1907 values pyran derivatives data acceptor proaromaticity diffraction Organic Nonlinear Optical ChromophoresMerocyanines 2009-09-04 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/4_i_H_i_Pyran_4_ylidenes_Strong_Proaromatic_Donors_for_Organic_Nonlinear_Optical_Chromophores/2830243 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.