On the nature of relaxation among degenerate electronic states of square molecules

D-4h-symmetric molecules, typified by metalloporphyrins and their symmetric derivatives, generally have allowed optical transitions to a pair of degen erate electronic states. Should emission from these states display little o r no splitting, one may postulate that static Jahn-Teller effects are unimp ortant and that the states remain essentially degenerate during relaxation. We show that the quantum mechanics and stochastic kinetics of such 'rigoro usly square' systems require degeneracy between two relaxation rates that h ave previously been assumed distinct from one another. We discuss the need for reinterpretation of certain time-resolved anisotropy data and explore t he resolution of some dilemmas arising. (C) 2000 Elsevier Science B.V. All rights reserved.