First Example of Ultrafast Photoisomerisation-Like Photophysics for Tetrapyrrol Systems: Ethylene-Bridged Porphyrin Dimers
It is commonly accepted that porphyrin molecules are sufficiently rigid structures having relatively large (~15500 cm-1) ∆E(S1-S0) energy gap. As a result, if there is no quenching effect of central metal atom, S1→S0 radiationless decay (internal conversion) has only minor contribution, if any, to the S1 state deactivation. Sometimes, however, one can find evidences in literature that (metal-free) symmetric close-spaced porphyrin dimers, used as models of photosynthetic structures, exhibit quenched S1 state, in spite of quite normal long-lived photophysics of initial monomers. This effect usually either remained unexplained or was accounted for by a participation of some “accepting intermolecular vibration mode” in the S1 state deactivation . We believe, however, that, due to steric hindrances, close-spaced tetrapyrrol dimers can have strongly distorted macrocycles, and after excitation, structural relaxation can occur resulting in decrease of S1-S0 energy gap and corresponding increase of S1→S0 internal conversion rate. Recently synthesized  ethylene-bridged porphyrin dimers, EBPDs, of side-to-side geometry (see Fig.1) can be considered as a striking example of such unknown earlier for tetrapyrrol systems photoisomerisation-like photophysics.