EPR Spectra, Luminescence Data, and Radiationless Decay Processes of Copper(II) Porphyrins

Abstract

For a series of copper(II) porphyrins, we report EPR data from solid solutions as well as E0 values for the first ring oxidation, emission spectra, and luminescence lifetimes in methylene chloride. Although the EPR parameters are fairly constant, the potentials vary by almost 700 mV, and the room-temperature lifetimes range from 300 ns for Cu(TCl2PP) to 15 ns for Cu(TMeOPP), where TCl2PP denotes 5,10,15,20-tetra(2‘,6‘-dichlorophenyl)porphyrin and TMeOPP denotes 5,10,15,20−tetra(4‘-methoxyphenyl)porphyrin. The data show that the variation in the lifetime of the emitting π−π* state is not due to the thermal population of another excited state of either d−d or charge-transfer parentage. However, the results are consistent with a model originally introduced by Asano et al. who proposed that an important vibronic distortion occurs in the emitting trip-doublet and trip-quartet states when the excitation involves the a2u orbital of the porphyrin (Asano, M.; Kaizu, Y.; Kobayashi, H. J. Chem. Phys. 1988, 89, 6567−6576). In view of the fact that the distortion is unique to the copper systems, we suggest that it involves movement toward a sitting-atop structure, consistent with the role the d10 configuration is likely to have in the excited-state wave function.