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

Multicomponent arrays based, on a central iridium(III) bis-terpyridine comp lex (Ir) used as assembling metal and free-base, zinc(II) or gold(III) tetr aaryl-porphyrins (PH2, PZn, PAu) have been designed to generate intramolecu lar 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 ind ividual components Ir, PH2, PZn, PAu have been synthesized and characterize d by various techniques including electrochemistry. Their photophysical pro perties either in acetonitrile or in dichloromethane and toluene have been determined by steady-state and time-resolved methods. In acetonitrile, exci tation of the triad PH2-Ir-PAu leads to a charge separation with an efficie ncy 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 I r(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 r ather different from that of PH2-Ir-PAu in acetonitrile since the primary e lectron transfer to yield PZn+-Ir--PAu is not followed by a secondary elect ron transfer. In this solvent, both unfavorable thermodynamic and electroni c 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.