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4H-Pyran-4-ylidenes: Strong Proaromatic Donors for Organic Nonlinear Optical Chromophores

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posted on 04.09.2009, 00:00 authored by Raquel Andreu, Laura Carrasquer, Santiago Franco, Javier Garín, Jesús Orduna, Natalia Martínez de Baroja, Raquel Alicante, Belén Villacampa, Magali Allain
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 π-spacer gives rise to rapidly increasing μβ1907 values up to 17,400 × 10−48 esu.