Modelling of the cis-trans partitioning in the photoisomerizations of cyanines and stilbene derivatives

In the course of photoisomerization, polymethine cyanines as well as stilbene and its derivates decay from the S1 potential energy minimum, corresponding to the perpendicular geometry, to yield either cis or trans ground-state molecules. The fraction of cis isomers obtained, alpha, spans a larger range of values for symmetric cyanines than for stilbene derivatives. It is argued that such different behaviour for the two classes of compounds should be traceable to the electronically different nature of their S1 perp species. Making use of radiationless transition theory results, it is shown that the relative location of the S1 minimum and S0 maximum along the internal rotation coordinate is crucial to the evaluation of alpha: even small differences between these critical twisting angles, which are more reasonably expected for polymethine cyanines than for stilbene-like compounds, may cause strong deviations from equipartitioning (alpha = 0. 5 ).