Identifying charge-transfer and trip–multiplet states in Co(i), Co(ii), and Co(iii) phthalocyanines using (magneto)optical spectroscopy and (TD)DFT calculations

Abstract

Herein we compare the electronic structures of the Co(I), Co(II), and Co(III) phthalocyanines, which were elucidated using UV-vis-NIR and magnetic circular dichroism (MCD) spectroscopy as well as density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. The NIR triplet–multiplet bands in Pc^R4(2-)Co^IIL₂ (L = nil, Py, or nBuNH₂; R = H or tert-Bu) complexes were studied by MCD spectroscopy for the first time and compared to those reported earlier by us in Pc^R4(2-)Cu (R = tert-Bu or SO₃Na) compounds (J. Porphyrins Phthalocyanines 2025, 29, 110–122). In all cases, a Faraday MCD pseudo A-term was observed for this transition. DFT and TDDFT calculations successfully explained a systematic blue-shift in the metal-to-ligand charge-transfer (MLCT) and B1-band transitions going from [Pc^R4(2-)Co^I]⁻ to Pc^R4(2-)Co^IIL₂ to [Pc^R4(2-)Co^IIIX₂]⁻ (X = CN⁻ or Br⁻) complexes. Additionally, absorption bands observed in the 370–530 nm spectral envelope in [Pc^R4(2-)Co^IIIX₂]⁻ complexes were assigned with a high level of confidence for the first time. This work provides the first combined systematic experimental and theoretical study that highlights similarities and differences in (magneto)optical spectroscopy of cobalt phthalocyanines spanning three oxidation states at the central metal ion.