Resonance Raman spectroscopy reveals a1u vs. a2u character and pseudo-Jahn-Teller distortion in radical cations of nickel(II), copper(II), and chloroiron(III) octaethyl- and tetraphenylporphyrins

Resonance Raman (RR) spectra have been analyzed for porphyrin radical cations prepared electrochemically from the Ni2+, Cu2+, VO2+, and (Cl-)Fe3+complexes of octaethylporphyrin (OEP) and meso-tetraphenylporphyrin (TPP). Band assignments have been secured via polarization measurements and isotope frequency shifts associated with 15N and meso-d4 (OEP) or pyrrole-d8and phenyl-d20(TPP) substitution. These isotope shifts are similar to those of the neutral metalloporphyrins, establishing that the normal mode compositions for high-frequency skeletal modes are not significantly altered by radical formation. There are, however, appreciable changes in the mode frequencies between neutral porphyrin and radical cation, which are different for the OEP's and TPP's. In particular, the mode v2, which is largely pyrrole CβCβstretching in character, shifts up and down upon radical formation in OEP's and TPP's, respectively. These are the directions expected if the OEP and TPP radicals have a1uand a2ucharacter, respectively, since the alu and a2uorbitals are antibonding and bonding, respectively, with respect to the CβCβbonds. The pyrrole half-ring stretch, v4, shifts down on OEP radical formation, consistent with the CαCβ bonding character of the a1uorbital. The a2uorbital is slightly antibonding with respect to the CαN and CαCβbonds, and the TPP radical v4shifts are smaller and variable. A novel feature of the radical cation RR spectra is the appearance of anomalously polarized bands in the-1000-cm-1region with excitation near the Soret electronic transitions. These bands are sensitive to meso-d4 (OEP) or pyrrole-d8 (TPP) substitution, but neither their frequencies nor their isotope shifts correlate with A2g modes of the neutral porphyrins. These bands are suggested to reflect pseudo-Jahn--Teller mixing between the Aluand A2u electronic states via the A2g vibrational modes (A1u x A2u = A2g). The resulting decrease in the A2g force constants greatly lowers the frequencies and remixes the mode compositions. It is further suggested that the saddle distortion observed in several porphyrin radical cation crystal structures so far reported might also be a pseudo-Jahn—Teller manifestation. The normal mode associated with the saddle distortion is B2u in D4h symmetry, but A2 in the D2d symmetry induced by the saddle distortion itself. The dynamic and/or static pseudo-Jahn--Teller distortions can account for the Alu/A2umixing which is evident from EPR spectra; they may also play a role in the electron-transfer dynamics of porphyrins and related aromatic macrocycles. Striking departures for the radical cations from the usual resonance enhancement pattern of metalloporphyrins (B-band resonant A2g mode enhancement, Q-as well as B-band resonant A1gmode domination) are discussed in terms of the altered electronic character of the excited states and their effects on A, B, and C term RR scattering mechanisms.