Metal Dependence of the Highest Occupied Molecular Orbital in Sterically Hindered Octaethyltetraphenylporphyrins

Resonance Raman (RR) spectra are reported for nickel(II) and copper(II) 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin (MOETPP) cation radicals in order to examine the influence of the substituent pattern and of out-of-plane distortions on the character of the porphyrin frontier orbitals. Isotopic frequency shifts ((13)C(m), (15)N, d(20)) were used to secure assignments of the RR bands. The highest occupied molecular orbital was found to switch from a(2u) to a(1u) between CuOETPP(+) and NiOETPP(+) as evidenced by (1) opposite shifts in the C(beta)C(beta) stretching mode, nu(2), and (2) selective enhancement of phenyl modes in CuOETPP(+) vs ethyl modes in NiOETPP(+). However, the nu(2) shifts are smaller than those seen in NiOEP(+) (A(1u)) or CuTPP(+) (A(2u)), indicating that mixing of the A(1u) and A(2u) states is greater in the OETPP(+) radicals. A greater amount of mixing in OETPP(+) is consistent with a small energy gap between the two states and with the out-of-plane distortion. The dominant mixing mechanism is a bond alternant (A(2g)) distortion, which is evident in the CuOETPP crystal structure and in the appearance of anomalously polarized bands in the Soret-enhanced radical cation RR spectra. Intensity analysis of the absorption spectra supports the inference of orbital switching between NiOETPP and CuOETPP but also indicates an increased energy gap between the two metals, relative to OEP and TPP. This increase in metal sensitivity is attributable to the out-of-plane distortions observed in Ni- and CuOETPP.