Photoisomerization of sterically hindered merocyanine dyes

A small series of merocyanine dyes has been synthesized in which substituents are incorporated into the polymethine backbone. The substituent perturbs the local structure in the polyenic bridge, as evidenced by changes to the bond order, the valence electronic charge on certain bridging carbon atoms, the bond length alternation, and the torsion angle. There are corresponding variations in the rates of internal conversion and reverse isomerization that permit identification of the double bond around which rotation occurs. A good correlation exists between the rates of rotation and the torsion angle that permits a quantitative description of the overall photophysical properties of these compounds in propan-1-ol at room temperature.