New Insight into an Old Problem: Analysis, Interpretation, and Theoretical Modeling of the Absorption and Magnetic Circular Dichroism Spectra of Monomeric and Dimeric Zinc Phthalocyanine Cation Radical.

The chemically or spectroelectrochemically generated formation and aggregation of zinc(II) tetra- tert -butylphthalocyanine cation radical [ZnPc ^tBu ] ^+• , which was highly soluble in common organic solvents, were investigated using UV-vis and magnetic circular dichroism (MCD) spectroscopies with an emphasis on the influence of the axial ligand on the fingerprint (∼500 nm) and NIR (720∼1000 nm) spectral envelopes. MCD spectroscopy is suggestive that the NIR band at ∼1000 nm observed for the antiferromagnetically coupled cation radical dimer, [ZnPc ^tBu ] ₂ ²⁺ , has no degeneracy, the monomer-dimeric equilibrium is temperature dependent, and higher degree aggregates can be formed at specific conditions. Sixteen different exchange-correlation functionals were tested to accurately predict the energies, intensities, and profiles of the UV-vis and MCD spectra of the phthalocyanine cation radical monomer and dimer. It was found that the M05 exchange-correlation functional (along with several other functionals that include 27-42% of Hartree-Fock exchange) provided an excellent agreement (∼0.1 eV for the degenerate excited states observed by MCD spectroscopy) between theory and experiment for the phthalocyanine cation-radical monomer and dimer. Not only did time-dependent density functional theory (TDDFT) calculations with M05 exchange-correlation functional correctly predict the nondegenerate NIR charge-transfer band at ∼1000 nm, all degenerate excited states, monomer and dimer energies, and oscillator strengths, but also they correctly described the nature of the experimentally observed at ∼500 nm MCD B -term (fingerprint band) detected for both the monomeric and dimeric phthalocyanine cation radicals. The TDDFT data explain the similarities in the UV-vis and MCD spectra of the monomeric and dimeric species observed between the UV and fingerprint spectral envelopes as well as correctly predicted the antiferromagnetic coupling between the two singly oxidized phthalocyanine macrocycles in the dimer.