Long-Lived Photoinduced Charge Separation in Ru(Bpy)32+/Viologen System at Nafion Membrane−Solution Interface

The photoinduced electron transfer from the excited state of tris(2,2‘-bipyridine)ruthenium(II) (Ru(bpy) 3 2+ ) incorporated into Nafion membranes to propylviologen sulfonate (PVS°) in the surrounding solution has been examined both by photochemical and photoelectrochemical measurements. N, N ‘-Tetramethylene-2,2‘-bipyridinium (DQ 2+ ) entrapped in the Nafion membranes is used as an electron relay. Luminescence quenching studies indicate that the quenching reaction of Ru(bpy) 3 2+ with DQ 2+ is both of dynamic and static nature. Ru(bpy) 3 3+ generated from the luminescence quenching remains in the Nafion matrix, while DQ +• migrates to the Nafion−water interface by an electron hopping mechanism, which transfers an electron to PVS° to produce PVS -• . The negatively charged PVS -• is repelled into the bulk solution by the anionic Nafion surface. The isolation of the photoinduced oxidized species Ru(bpy) 3 3+ in Nafion from the ultimate reduced species PVS -• in solution prevents them from undergoing back electron transfer, and a long-lived (up to a few hours) charge separation state is achieved. The low quantum yield for the charge separation was demonstrated to be mainly originated from the back electron transfer in the initial Ru(bpy) 3 3+ /DQ +• pair. An electrode was fabricated by coating Ru(bpy) 3 2+ -DQ 2+ -incorporated Nafion film on an ITO glass. The photoinduced voltage of this electrode was measured with a saturated calomel reference electrode in PVS° solution to be ca. 350 mV when the light intensity was ca. 60 mW cm -2 . This electrode was also used as the light electrode to construct a photogalvanic cell with a platinum electrode as the dark electrode. Irradiation of the light electrode with visible light results in cathodic photocurrent, and there is no net chemical change associated with the functioning of the cell which converts light to electricity.