Transient resonance Raman spectroscopy of copper(II) complexes of meso-tetraphenylporphine and meso-tetraphenylchlorin

We report the transient resonance Raman spectra of five-coordinate Cu(II) complexes of meso-tetraphenylporphine (TPP) and meso-tetraphenylchlorin (TPC). With piperidine as a ligand, we observe a Raman spectrum that is consistent with either a (d,d) or a (d,pi*) excited state. With tetrahydrofuran (THF) as a ligand, two distinct excited states are observed. One of them is assigned to either a (d,pi*) or a (d,d) state by analogy with the spectrum in piperidine. The other shows spectral features that are similar to those observed for the cation radical of CuTPP. We assign the second state to a ring-to-metal (pi,d) charge-transfer excited state. The transient Raman spectrum of CuTPC in THF shows features that are consistent with a (pi,d) charge-transfer excited state only. Our results provide the first direct spectroscopic evidence for the existence of charge-transfer and, possibly, (d,d) excited states that Lie close in energy to the tripmultiplet excited states of CuTPP and CuTPC.