PHOTOINDUCED ELECTRON AND ENERGY-TRANSFER IN RIGIDLY BRIDGED RU(II)-RH(III) BINUCLEAR COMPLEXES

A series of binuclear Ru(II)-Rh(III) complexes of general formula (ttp y)Ru-tpy-(ph)(n)-tpy-Rh(ttpy)(5+) (n = 0-2) have been synthesized, whe re ttpy = 4'-p-tolyl-2,2':6,2 ''-terpyridine and tpy-(ph)(n)-tpy repre sents a bridging ligand where two 2,2':6',2 ''-terpyridine units are e ither directly linked together (n = 0) or connected through one (n = 1 ) or two (n = 2) phenyl spacers in the 4'-position. This series of com plexes is characterized by (i) rigid bridge structures and (ii) variab le metal-metal distances (11 Angstrom for n = 0, 15.5 Angstrom for n = 1, 20 Angstrom for n = 2). The photophysics of these binuclear comple xes has been investigated in 4:1 methano/ethanol at 77 K (rigid glass) and 150 K (fluid solution) and compared with that of mononuclear [Ru( ttpy)(2)(2+) and Rh(ttpy)(2)(3+)] or binuclear [(ttpy)Ru-tpy-tpy-Ru(tt py)(4+)] model compounds. At 77 K, no quenching of the Ru(II)-based ex cited state is observed, whereas energy transfer from excited Rh(III) to Ru(II) is observed for all complexes. At 150 K, energy transfer fro m excited Rh(III) to Ru(II) is again observed for all complexes, while quenching of excited Ru(II) by electron transfer to Rh(III) is observ ed, but only in the complex with n = 0. The reasons for the observed b ehavior can be qualitatively understood in terms of standard electron and energy transfer theory. The different behavior between n = 0 and n = 1, 2 can be rationalized in terms of better electronic factors and smaller reorganizational energies for the former species. The freezing of electron transfer quenching but not of energy results arising from multiphotonic and conformational effects have also been observed with these systems.