DFT study of the excited state structure and relaxation dynamics of open-shell metalloporphyrins as energy conversion materials

Metal Metalloporphyrins incorporating 3d transition-metal ions with closed- or open-shell d-electron configurations are key reactants in biochemical energy cycles, and understanding their photoconversion processes is of great importance. The key to elucidating the early stages of photoexcited-state dynamics lies in the relaxation pathways of the characteristic near-ultraviolet B band (300–400 nm) and the lower-energy visible Q band (500–600 nm). In open-shell systems, the decay of the Q state has been reported to occur on an ultrafast timescale (tens of femtoseconds). Recently, Naumova et al. combined transient absorption spectroscopy with time-resolved X-ray emission spectroscopy and directly tracked the electronic dynamics within photoexcited Ni porphyrins in solution, revealing the presence of a “hot” excited triplet state with charge-transfer character. In this study, we investigate the excited states of d⁸ Ni(II) porphyrins and their relaxation pathways using density functional theory (DFT). By applying excited-state normal-mode analysis based on linear-response time-dependent density functional theory (TDDFT), we clarify the relaxation and internal conversion dynamics through analysis of the dimensionless Huang–Rhys (HR) factors, which quantify the strength of the coupling between electronic and vibrational states. The influence of substituents and solvent polarity on the relaxation processes is also discussed.