Conjugated donor-acceptor compounds are investigated by electronic absorption and Raman spectroscopy with and without the electric field of a nonconjugated positive charge. The shifts observed can be rationalized by considering these compounds as unsymmetrical cyanine dyes. The cyanine model allows to rationalize the observation that the vibrational and optical absorption energy are linked to each other. This model can also be applied to the visual chromophore inside the protein pocket which is placed in the unsymmetric electrostatic field of the surrounding charged amino acids. (C) 2013 Elsevier B.V. All rights reserved.