PHOTOISOMERIZATION OF STERICALLY HINDERED MEROCYANINE DYES

A small series of merocyanine dyes has been synthesized in which subst ituents are incorporated into the polymethine backbone. The substituen t perturbs the local structure in the polyenic bridge, as evidenced by changes to the bond order, the valence electronic charge on certain b ridging carbon atoms, the bond length alternation, and the torsion ang le. There are corresponding variations in the rates of internal conver sion and reverse isomerization that permit identification of the doubl e bond around which rotation occurs. A good correlation exists between the rates of rotation and the torsion angle that permits a quantitati ve description of the overall photophysical properties of these compou nds in propan-1-ol at room temperature.