The decay mechanisms of the excited triplet state of porphyrin in porphyrin (metal-free) cytochrome c, zinc cytochrome c, and tin cytochrome c have been characterized by time-resolved emission spectra and decay kinetics. Delayed fluorescence provides a significant radiative pathway for triplet decay at room temperature for the cytochrome c derivatives and permits a sensitive measure of the triplet state. The lifetime of delayed fluorescence is identical with that of the phosphorescence, and the decay follows first-order kinetics. The intensity of delayed fluorescence is directly proportional to the laser flash intensity. These results indicate that the cytochrome derivatives exhibit E-type delayed fluorescence, which results from the repopulation of the excited singlet state by thermal activation of the triplet molecules. The activation energies of the delayed fluorescence are 2573 cm-1 or 7.35 kcal/mol for porphyrin, 2560 cm-1 or 7.32 kcal/mol for zinc, and 2213 cm-1 or 6.33 kcal/mol for tin cytochrome c derivatives. These values are less than those predicted by the observed energy differences between the singlet and the triplet states from the spectra, and this discrepancy may be due to the inability to estimate the true 0-0 energy gap. Exchange of hydrogens in porphyrin cytochrome c by deuterium resulted in an increase in its triplet lifetime from 9 to 14 ms at 77 K. This result has been interpreted to be due to the existence of vibrational coupling between the T and the S0 states, which provides a route for deactivation of the porphyrin triplet state.
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