Photochemical Formation of the Anion Radical of Zinc Phthalocyanine and Analysis of the Absorption and Magnetic Circular Dichroism Spectral Data. Assignment of the Optical Spectrum of [ZnPc(-3)]^-

An extensive analysis of the optical absorption and magnetic circular dichroism (MCD) spectra of the anion radical of zinc phthalocyanine ([ZnPc-(-3)]^-) is described. Novel photochemical formation of the ring-reduced [ZnPc-(-3)]^- from (hydrazine)Zn^IIPc(-2) is reported for reactions carried out at room temperature using visible-wavelength light and hydrazine as the electron donor. Absorption and MCD spectra of the radical anion species have been obtained at both room and cryogenic temperatures. Phosphorescence and fluorescence lifetime studies of ZnPc(-2) show that the photoexcited (hydrazine) ZnPc(-2) complex reacts via the triplet state to form the ring reduced anion radical, [ZnPc(-3)]^-. The complete lack of temperature dependence assignable to orbital degeneracies in the low-temperature MCD spectrum shows conclusively that the ²E_g ground state of [ZnPc(-3)] is split into nondegenerate components at least 800 cm^-1 apart. The ground and excited states are completely nondegenerate. It is proposed that the coupled effects of the loss of aromaticity with the addition of the 19th π-electron, Jahn-Teller distortion, and nonsymmetric solvation of the ring lead to a change in molecular geometry from D_4h of ZnPc(-2) to C_2υ for [ZnPc(-3)]^-1. The first complete assignment of the optical spectrum of any porphyrin or phthalocyanine anion radical is proposed on the basis of a ²B₁ ground state and supported by results from extensive deconvolution calculations. Comparison between the absorption and MCD spectral data indicated that a significant fraction of the spectral intensity observed at room temperature can be assigned as “hot” bands. The hot bands, which are much more pronounced in the spectral data of [ZaPc(-3)]^- than in the spectral data of the parent ZnPc(-2), are associated with interactions between the solvent, the Pc(-3) ring, and vibronic bands associated with the split ground state. A detailed study of the temperature dependence of the absorption and MCD spectra showed that meaningful spectral deconvolution calculations could only be carried out on spectra obtained from vitrified solutions of [ZnPc(-3)]^- at cryogenic temperatures. Bandwidths calculated to fit the absorption spectrum increase in magnitude as a function of the transition energy from 10 000 to 33 000 cm^-1, which allows the classification of sets of bands to one of five major electronic transitions, namely, Q between 750 and 1000 nm, n → π* between 580 and 750 nm, π* → π* between 430 and 650 nra, B1 and B2 between 300 and 450 nm.