Triplet-triplet energy transfer between porphyrins linked via a ruthenium(II) bisterpyridine complex

The photophysical properties of a multicomponent array consisting of a free-base aryl-porphyrin and a gold(III) aryl-porphyrin, assembled via a central bis(terpyridine) ruthenium(II) complex, PH2-Ru-PAu, have been studied in low-temperature glassy media by steady state and time-resolved methods. Comparison of the photophysical properties of this triad with those of the reference molecular models and of the related dyads (Ru-PH2, Ru-PAu) allows the photoinduced processes occurring in PH2-Ru-PAu to be elucidated. The photoinduced processes in glassy media in the dyads Ru-PH2 and Ru-PAu are basically similar to those occurring at room temperature where the absorbed energy is transferred with 100% efficiency to the pertinent porphyrin lowest triplet state. For Ru-(PAu)-P-3 the decay to the ground state is similar to the model (PAu)-P-3 and displays double-exponential behavior, while in the case of Ru-(PH2)-P-3 some perturbing effect of the ruthenium center in accelerating the inter-system crossing to the ground state by the heavy atom effect can be noticed. In the triad PH2-Ru-PAu the primary photoinduced steps are similar to those occurring at room temperature and, by energy-transfer steps, populate both triplets of peripheral porphyrins. In contrast with room-temperature events, a further energy-transfer step from PH2-Ru-(PAu)-P-3 to the spatially opposed (PH2)-P-3-Ru-PAu (center to center distance = 2.1 nm) occurs in glassy media with a rate constant of 2.5 x 10(7) s(-1), as probed by transient absorption spectroscopy. This process, which occurs by an exchange mechanism, is mediated by the interposed Ru(II) bisterpyridine complex, which acts as an electron relay.