Energy transfer pathways in pyridylporphyrin metal adducts and side-to-face arrays

Side-to-facearrays in which a free-base porphyrin is connected by axial coordination of its meso-pyridyl groups to one or more ruthenium porphyrin units exhibit interesting photophysical properties. The photophysics is characterized by efficient energy transfer, at the triplet level, from the peripheral ruthenium units to the central free-base. When the central unit is zinc-metalated the triplet energy transfer is reversible, leading to an excited-state equilibrium. In the arrays, the singlet state of the central unit is always quenched relative to the isolated free-base (or zinc) porphyrin, as a consequence of the heavy-atom effect of ruthenium. This general behavior is confirmed by the behavior of a number of adducts between pyridylporphyrins and simple Werner-type ruthenium complexes. Some insight into the mechanisms of the heavy-atom quenching is obtained from the time-resolved spectroscopy of these systems: besides enhanced intersystem crossing within the porphyrin chromophore, singlet-triplet energy transfer to the ruthenium center(s) acts as an additional efficient channel for deactivation of the pyridylporphyrin chromophore. (C) 2002 Elsevier Science B.V. All rights reserved.