Porphyrinic dyads and triads assembled around iridium(III) bis-terpyridine: Photoinduced electron transfer processes

Multicomponent arrays based, on a central iridium(III) bis-terpyridine complex (Ir) used as assembling metal and free-base, zinc(II) or gold(III) tetraaryl-porphyrins (PH2, PZn, PAu) have been designed to generate intramolecular photoinduced charge separation. The rigid dyads PH2-Ir, PZn-Ir, PAu-Ir, and the rigid and linear triads PH2-Ir-PAu, PZn-Ir-PAu, as well as the individual components Ir, PH2, PZn, PAu have been synthesized and characterized by various techniques including electrochemistry. Their photophysical properties either in acetonitrile or in dichloromethane and toluene have been determined by steady-state and time-resolved methods. In acetonitrile, excitation of the triad PH2-Ir-PAu leads to a charge separation with an efficiency of 0.5 and a resulting charge-separated (CS) state with a lifetime of 3.5 ns. A low-lying triplet localized on PH2 and the presence of the heavy Ir(III) ion offer the CS state an alternative deactivation path through the triplet state. The behavior of the triad PZn-Ir-PAu in dichloromethane is rather different from that of PH2-Ir-PAu in acetonitrile since the primary electron transfer to yield PZn+-Ir--PAu is not followed by a secondary electron transfer. In this solvent, both unfavorable thermodynamic and electronic parameters contribute to the inefficiency of the second electron-transfer reaction. In contrast, in toluene solutions, the triad PZn-Ir-PAu attains a CS state with a unitary yield and a lifetime of 450 ns. These differences can be understood in terms of ground-state charge-transfer interactions as well as different stabilization of the intermediate and final CS states by solvent.