Experimental and theoretical comparison of the charge-transfer and ligand-centered excited states in dicyano-substituted Cr(III), Fe(II), and Co(III) phthalocyanines

UV-vis-NIR spectroscopy and magnetic circular dichroism (MCD) were used to characterize electronic transitions in the dicyano-substituted phthalocyanines. [Pc tBu (2-)Cr(III)(CN) 2 ] - , [Pc(2-)Fe(II)(CN) 2 ] 2- , and [Pc(2-)Co(III)(CN) 2 ] - . Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were used to validate transition assignments, corresponding energies, and orbital contributions. Specifically, calculations were employed to provide insight into the multiple overlapping transitions present in the 250-500 nm region and to propose assignments for all transitions. In addition, energies corresponding to the charge-transfer transitions were analyzed to study the impact of closed-shell (Fe(II) and Co(III)) and open-shell (Cr(III)) configurations on ligand-centered, charge-transfer, and triplet-multiplet transitions. For dicyano-substituted Cr(III) phthalocyanine, we provide convincing evidence for reassignment of the lowest-energy transitions in the NIR to triplet-quartet transitions and suggest that the lowest-energy ligand-to-metal charge transfer transition is located near 500 nm. This work presents a comprehensive experimental and theoretical investigation of the electronic behavior of three dicyano-substituted metal phthalocyanines (MPcs). It provides a broader context for general trends in ligand-centered and CT transitions in metalated macrocycles.