Photoinduced electron transfer in paraquat inclusion complexes of porphyrin-based receptors

A series of porphyrins, strapped with aryloxy-substituted polyether chains of various lengths and different substitution patterns, has been photophysically characterised. These molecules act as receptors for bipyridinium dications and their association constants with paraquat in acetonitrile solutions have been determined by absorption and emission spectroscopy; they have been found to be in the range 10²–10⁵. The photophysical characterisation of the complexes has been made in solutions containing more than 95% of associated porphyrin component. A photoinduced electron transfer from the porphyrin excited state to the paraquat guest, responsible for the porphyrin luminescence quenching, takes place with τ « 20 ps and is followed by a very fast recombination of the resulting charge-separated state. In only a few cases could the charge-separated state be detected and a lifetime of 20 ps was measured, whereas in most complexes the recombination is faster than 20 ps. In none of the examined complexes was escape from the geminate recombination of the charge-separated state observed.