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(II)Pc(-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 life:time 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 (2)Eg, 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 pi-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-2D, for [ZnPc(-3)](-). The first complete assignment of the optical spectrum of any porphyrin or phthalocyanine a nion radical is proposed on the basis of a B-2(1) 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 [ZnPc(-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,n calculations could only he carried out on spectra obtained from vitrified solutions of [ZnPc(-3)](-) at cryogenic temperatures eratures. 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 tran;sitions, namely, Q between 750 and 1000 nm, n-->pi(*) between 580 and 750 nm pi(*) --> pi(*) between 430 and 650 nm, B1 and B2 between 300 and 450 nm.