Abstract

It is commonly accepted that tetrapyrrol systems are sufficiently rigid structures for which, if quenching effect of central metal ion is absent, Born-Oppenheimer approximation is fulfilled well for the pair of electronic states S₁-S₀- It means that S₁-->S₀ radiationless decay channel (internal conversion) has only minor contribution, if any, to the S₁ state deactivation. Sometimes, however, evidences are met in literature that some (metal-free) symmetrical close-spaced tetrapyrrol dimers exhibit quenched S₁ state in a comparison with monomers. This effect usually either remained unexplained or was accounted for by some "accepting intermolecular vibration mode" participation [1]. We believe, however, that photoisomerisation-like conformational relaxation in an excited state can be responsible for this quenching effect for such strongly distorted structures which are close-spaced tetrapyrrol dimers. Recently synthesized [2] 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.

© 1996 Optical Society of America