Time-Resolved Resonance Raman Scattering Study of Cu-Porphyrins in the Excited Triplet State: Is the Porphyrin Structure Nonplanar?

摘要

Benzene solutions of Cu(II)-octaethylporphyrin and Cu(II)-tetraphenylporphyrin complexes are studied at room temperature by the technique of time-resolved resonance Raman scattering (RRS) excited by 10-ns laser pulses. The nonstationary RRS spectra from the lowest excited trip–doublet–quartet (, *) 2,4T1states are obtained and discussed. Specific features of the triplet RRS spectra are (1) the low intensity and significant broadening of the spectral lines related to skeletal modes of the porphyrin macrocycle; (2) the absence of the unique mutual correlation between Raman lines scattering from the excited triplet and ground electronic states; and (3) the absence of the broadening of Raman lines related to peripheral substituent groups, which are thus enhanced relative to the Raman lines of skeletal vibrational modes of porphyrin. A partial vibrational analysis is performed, and the mechanisms of processes occurring in the excited 2,4T1 state are discussed. It is assumed that the above features of triplet RRS spectra of Cu-porphyrins can be explained by the existence of multiple structure conformers caused by both nonplanar perturbations and perturbations related to the Jahn–Teller effect. The presence of nonplanar conformers in the excited triplet state is assumed to be quite common for the metal-free and Zn- and Cu-porphyrin complexes investigated to date by the time-resolved RRS method. At present, studies aimed at substantiating this hypothesis are being continued by our group.