Efficient non-radiative deactivation and conformational flexibility of meso-diaryloctaalkylporphyrins in the excited triplet state

The excited triplet state deactivation of zinc(II) meso-diaryloctaalkylporp hyrins (ZnDAOAP) has been studied over a wide temperature range using trans ient triplet-triplet absorption spectroscopy together with steady-state and time-resolved phosphorescence techniques, The results from transient absor ption measurements show that the depopulation of the initially formed tripl et state (T-1A state) is unusually fast at temperatures above 150 K. The ef ficiency of the deactivation originates from a spin allowed transition to a second tripler 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 negligi ble contribution. The subsequent ground-state recovery T-1B-->S-0 is also v ery efficient in comparison to many other porphyrins. Due to the substantia l activation energy found for the transformation process, it most likely in volves a conformational distortion of the porphyrin macrocycle. At low temp erature, however, the relaxation of the T-1A State occurs by direct intersy stem crossing to the ground state.