Intersystem Crossing in Hexahelicene

Sapir and Van der Donckt₁ have shown that, although the energies of the first excited triplet states of coroneneand hexahelicene are almost equal, their rates of intersystem crossing and of triplet decay are different, both processes being more rapid in hexahelicene. Weigang and co-workers₂ have suggested that, owing to the helical structure of hexahelicene, there is an enhanced mixing between singlet and triplet ⊼⊼ states. We report here magnetic resonance studies of the triplet states of coronene, hexahelicene, and heptahelicene which probe the crossing into the triplet manifold. We include coronene as a check of our experimental methods since its triplet state has been studied extensively by optically detected magnetic resonance.³⁴ We used the fast detection X band apparatus previously described5 for observation of magnetization following short pulses (~6 ns) of optical excitation. The method permits determination of early populations of spin sublevels in the presence of appreciable rates of spin lattice relaxation (1 /T₁≤5 X 10⁶ s^-1).