Direct observation of reduced bond-length alternation in donor/acceptor polyenes

There has been tremendous interest in asymmetric cyanine and merocyanine compounds because of their applications as photographic sensitizers, membrane potential probe, and photochromic dyes for all-optical memory. On the basis of UV-visible spectroscopic data, Brooker suggested that merocyanine molecules could be described by a superposition of neutral and charge-separated canonical resonance forms and that, by changing the basicity of the endgroups and/or the solvent polarity, one could tune the molecular structure from neutral and polyene-like through polar and cyanine-like (with equal contributions from neutral and charge-separated resonance forms) to highly polar, charge-separated polyene-like.