METAL DEPENDENCE OF THE HIGHEST OCCUPIED MOLECULAR-ORBITAL IN STERICALLY HINDERED OCTAETHYLTETRAPHENYLPORPHYRINS

Resonance Raman (RR) spectra are reported for nickel(II) and copper(II ) 13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin (MOETPP) cation r adicals in order to examine the influence of the substituent pattern a nd of out-of-plane distortions on the character of the porphyrin front ier orbitals. Isotopic frequency shifts (C-13(III), N-15, d(20)) were used to secure assignments of the RR bands. The highest occupied molec ular orbital was found to switch from a(2u) to a(1u) between CuOETPP() and NiOETPP(+) as evidenced by (1) opposite shifts in the CbetaCbeta stretching mode, upsilon(2), and (2) selective enhancement of phenyl modes in CuOETPP(+) vs ethyl modes in NiOETPP(+). However, the upsilon (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 great er in the OETPP(+) radicals. A greater amount of mixing in OETPP(+) is consistent with a small energy gap between the two states and with th e out-of-plane distortion. The dominant mixing mechanism is a bond alt ernant (A(2g)) distortion, which is evident in the CuOETPP crystal str ucture and in the appearance of anomalously polarized bands in the Sor er-enhanced radical cation RR spectra. Intensity analysis of the absor ption spectra supports the inference of orbital switching between NiOE TPP and CuOETPP but also indicates an increased energy gap between the two metals, relative to OEP and TPP. This increase in metal sensitivi ty is attributable to the out-of-plane distortions observed in Ni- and CuOETPP.