Determination of the T₁ state energy levels of metalloporphyrins using temperature-dependent time resolved spectroscopy

The determination of the energy of the lowest triplet excited state (<i>E</i>_T1) for metalloporphyrin photosensitizers, a key parameter for rationalizing energy transfer processes in metalloporphyrins, facilitates a better understanding and optimization of their functionality. In this study, a strategy was proposed for determining the <i>E</i>_T1 values in metalloporphyrin systems by analyzing the temperature dependence of the oxygen quenching rate constant using time-resolved spectroscopy, with Platinum(II) octaethylporphyrin (PtOEP) and Palladium(II) octaethylporphyrin (PdOEP) as representative examples. Through a thermal equilibrium approximation of the excited triplet state (T₁) and the excited state of oxygen, a theoretical model was introduced in which the oxygen quenching rate exhibits a nonlinear dependence on temperature, enabling the determination of <i>E</i>_T1 values. Additionally, the validity of this approach was demonstrated through excitation spectra of metalloporphyrins. Our work provides an effective pathway and offers new insights into the energy-loss mechanisms involved in related photoconversion processes.