Efficient Non-Radiative Deactivation and Conformational Flexibility of meso-Diaryloctaalkylporphyrins in the Excited Triplet State

The excited triplet state deactivation of zinc(II) meso -diaryloctaalkylporphyrins (ZnDAOAP) has been studied over a wide temperature range using transient triplet−triplet absorption spectroscopy together with steady-state and time-resolved phosphorescence techniques. The results from transient absorption measurements show that the depopulation of the initially formed triplet state (T 1A state) is unusually fast at temperatures above 150 K. The efficiency of the deactivation originates from a spin allowed transition to a second triplet state (T 1B state). The transformation process T 1A →T 1B is therefore the dominating deactivation channel of the T 1A state in this temperature range, and direct intersystem crossing T 1A →S 0 makes negligible contribution. The subsequent ground-state recovery T 1B →S 0 is also very efficient in comparison to many other porphyrins. Due to the substantial activation energy found for the transformation process, it most likely involves a conformational distortion of the porphyrin macrocycle. At low temperature, however, the relaxation of the T 1A state occurs by direct intersystem crossing to the ground state.