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

摘要

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 form 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.