Intersystem crossing in the helicenes

The rate constants of radiationless S1 → Tn and T1 → S0 intersystem crossing of six helicenes have been evaluated by combining emission lifetimes and quantum yields of fluorescence and triplet state formation. The data are compared to those of linear acenes. In contrast to the linear series, the photophysical properties of the helicenes evolve steadily as a function of the number of ortho-fused benzene rings.