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

Abstract

In the course of photoisomerization, polymethine cyanines as well as stilbene and its derivates decay from the S 1 potential energy minimum, corresponding to the perpendicular geometry, to yield either cis or trans ground-state molecules. The fraction of cis isomers obtained, α, 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 S 1 perp species. Making use of radiationless transition theory results, it is shown the relative location of the S 1 minimum and S 0 maximum along the internal rotation coordinate is crucial to the evaluation of α: 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 (α=0.5).