Orientation effect of enhanced intersystem crossing in copper(II) porphyrin-free base porphyrin dimers

Intersystem crossing (ISC) enhanced by a remote unpaired electron through long-range interaction was studied in linear-type copper(II) porphyrin-free base porphyrin dimers in comparison with the gable-type dimers. The excited singlet state of the free base moiety is deactivated to its triplet manifolds quickly due to exchange interaction between the free base π-electrons and copper unpaired d electron. While the ISC rates in the gable dimers show a simple relation with number of bonds of the linkage, two linear-type dimers behave differently. In the case of a p-phenyl bridged dimer, where the two porphyrin halves are almost parallel, the ISC rate is much larger than that expected from the simple through-bond coupling, which is applicable to the gable-type dimers. On the other hand, p-biphenyl bridged dimer, in which two halves are not parallel, does not show such an effect. Parallel orientation of the two terminals can play an important role in the long-range exchange coupling.