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 centra l bis(terpyridine) ruthenium(II) complex, PH2-Ru-PAu, have been studied in low-temperature glassy media by steady state and time-resolved methods. Com parison of the photophysical properties of this triad with those of the ref erence molecular models and of the related dyads (Ru-PH2, Ru-PAu) allows th e photoinduced processes occurring in PH2-Ru-PAu to be elucidated. The phot oinduced processes in glassy media in the dyads Ru-PH2 and Ru-PAu are basic ally similar to those occurring at room temperature where the absorbed ener gy is transferred with 100% efficiency to the pertinent porphyrin lowest tr iplet state. For Ru-(PAu)-P-3 the decay to the ground state is similar to t he model (PAu)-P-3 and displays double-exponential behavior, while in the c ase of Ru-(PH2)-P-3 some perturbing effect of the ruthenium center in accel erating the inter-system crossing to the ground state by the heavy atom eff ect 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 ro om-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 p robed by transient absorption spectroscopy. This process, which occurs by a n exchange mechanism, is mediated by the interposed Ru(II) bisterpyridine c omplex, which acts as an electron relay.