EXCITED-STATE INTERACTIONS IN LIGAND-BRIDGED CHROMOPHORE QUENCHER COMPLEXES CONTAINING RHODIUM(III) AND RUTHENIUM(II) POLYPYRIDYL UNITS

This work consists of two parts: (i) photophysical studies on the mononuclear Rh(III)-polypyridyl complexes ([Rh(dpp)2Cl2]+, [Rh(bpy)2(dpp)]3+, and [Rh(dpp)2(bpy)]3+ and (ii) and examination of the intramolecular excited-state interactions in the ligand-bridged complex, [(bpy)2Ru(II)-dpp-Rh(III)(bpy)2]5+ using luminescence and transient absorption spectral studies. Over the temperature range 77-293 K, the lowest excited state of [Rh(dpp)2Cl2]+ is metal-centered (MC or d-d). At 77 K, mixed ligand complexes [Rh(bpy)2(dpp)]3+ and [Rh(dpp)2(bpy)]3+ show strong emission from ligand-centered (LC or pi-pi*) and a very weak one from metal-centered excited states. Lifetime studies indicate the two low-lying excited states to be nonequilibrated in rigid alcoholic glasses. Only very weak (pi, pi*) emission is observed in fluid solutions (293 K). Distinct transient absorption following short laser pulse excitation allows establishment of spectra and lifetimes of these excited states in fluid solutions at ambient temperature. Visible light excitation of the mixed metal Rh-dpp-Ru complex leads to formation of the luminescent charge-transfer (CT) excited state of Ru(II)-polypyridyl based chromophore. The very short lifetime of this excited state species in fluid solutions as compared to model compounds can be caused by enhanced nonradiative decay (mechanism I) or by intramolecular electron-transfer or energy-transfer quenching (mechanisms II and III, respectively) involving an adjacent Rh(III)-polypyridyl unit. Analysis of the quenching pathways using the electrochemical and photophysical data on the mixed metal and relevant mononuclear complexes leads to the conclusion that the quenching is primarily by electron transfer (mechanism II).