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

Intersystem crossing (ISC) enhanced by a remote unpaired electron through l ong-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 manifo lds quickly due to exchange interaction between the free base TC-electrons and copper unpaired d electron. While the ISC rates in the gable dimers sho w a simple relation with number of bonds of the linkage, two linear-type di mers 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 t o the gable-type dimers. On the other hand, p-biphenyl bridged dimer, in wh ich two halves are not parallel, does not show such an effect. Parallel ori entation of the two terminals can play an important role in the long-range exchange coupling. (C) 2001 Elsevier Science B.V. All rights reserved.