Assignment of the optical spectra of metal phthalocyanine anions

Specially designed spectroelectrochemical cells were used to record W-vis absorption and magnetic circular dichroism (MCD) spectral data of the phthalocyanine ring-reduced dianion, [ZnPc(-4)](2-), in situ at room temperature. [ZnPc(-4)](2-) was generated at cryogenic temperatures through photoreduction of a vitrified dimethyl formamide/hydrazine hydrate solution using hydrazine as the electron donor. Molecular orbital calculations for [ZnPc(-(2 + n))](-n) (n = -1-4) species were performed through the use of the ZINDO program. The approach that was developed previously to assign the major pi-->pi* and pi*-->pi* transitions in the optical spectrum of [ZnPc(-3)](-), which was based on analysis of the relative signs and intensities of close lying Faraday B terms in the MCD spectrum, is used here to assign the major bands seen in the spectrum of [ZnPc(-4)](2-). The spectrum is dominated by a pi*-->pi* transition out of the partially filled lowest unoccupied molecular orbital (LUMO) which gives rise to a pair of oppositely-signed, coupled B terms in the MCD spectrum at 506 and 620 nm. The Q transition of the phthalocyanine ring is assigned to a weak absorption band at 865 nm, while the second pi-->pi* transition is assigned to a band at 370 nm. The band assignment scheme is extended to data that have been reported previously for more highly ring-reduced main group [MPc(-5)](3-) and [MPc(-6)](4-) species (M = Mg, Zn). The Q band of [MgPc(-5)](3-) is assigned to a band at 1125 nm. Bands at 589 and 825 nm are assigned to pi*-->pi* transitions out of the partially occupied Jahn-Teller split e(g)* LUMO. The second pi-->pi* transition is assigned to a band at 338 nn. As the LUMO is fully occupied, the spectrum of [MgPc(-6)](4-) is dominated by pi*-->pi* bands at 625 and 840 nm. An additional band at 305 nm is assigned to the second pi-->pi* transition. A general band assignment scheme is proposed for both main group and transition metal phthalocyanine anion species.