Assignment of the optical spectrum of metal porphyrin and phthalocyanine radical anions

The approach developed previously to assign the major π → π* and π* → π* bands in the optical spectra of metallophthalocyanine radical species [MPc(-n)]^(N-2)- (n = 1-6), which was based on spectral band deconvolution analyses of the UV-vis absorption and magnetic circular dichroism (MCD) spectra and ZINDO calculations, is reviewed in this paper and extended to the major bands observed in the spectrum of the zinc tetraphenylporphyrin radical anion [ZnTPP(-3)]^-. The magnetic circular dichroism spectrum is dominated by an intense pseudo A term associated with the B (or Soret) transition with bands at 445 and 465 nm. Weaker bands associated with the π* → π* transition out of the partially filled LUMO give rise to two widely separated, oppositely signed, coupled B terms in the MCD spectrum at 538 and 910 nm. The Q transition is assigned to a weak absorption band at 750 nm and a shoulder slightly to the blue at 728 nm.