Switching of electron- to energy-transfer by selective excitation of different chromophores in arrays based on porphyrins and a polypyridyl iridium complex

Excitation at 350-355 nm of the dyads PAu-Ir, PH2-Ir and of the triads PH2-Ir-PAu and PZn-Ir-PAu where Ir, iridium(III) bis-terpyridine, is covalently linked to gold (111), free-base and zinc(II) tetraaryl-porphyrins (PAu, PH2, PZn), produces to a predominant extent the ligand centered triplet excited state of the iridium complex unit, 3 In The processes occurring in the arrays upon UV excitation have been characterized by steady state and time-resolved spectroscopic methods. Energy transfer to the porphyrin triplets dominates the deactivation of Ir-3 in PAu-Ir, PH2-Ir, and PH2-Ir-PAu, with rates of 2.9 x 10(10) s(-1) (Ir-3 --> (PAu)-P-3) and ca. 10(11) s(-1) (Ir-3 --> (PH2)-P-3), in contrast to what has been shown to occur upon selective excitation of the PH2 unit, which yields electron transfer leading to charge separation. The different outcome is discussed on the basis of the overlap of the HOMO and LUMO orbitals involved in the electron-transfer reaction for the Ir acceptor unit and the PH2 donor unit, with the aid of semiempirical calculations. Remarkably, the PZn based array PZn-Ir-PAu displays efficient electron transfer with the formation of a charge separated state with unitary yield, irrespective of the component which is excited. A very high driving force, DeltaGdegrees = -1.1 eV, could explain the prevalence of the electron-transfer reaction in the latter case.