The photophysics of the binuclear complexes [(phen)2M(tatpp)M(phen)2]4+, where M = Ru or Os, phen = 1,10-phenanthroline, and tatpp = 9,11,20,22-tetraazatetrapyrido[3,2-a:2'3'-c:3'',2''-l:2''',3''']pentacene, has been studied in acetonitrile and dichloromethane by femtosecond and nanosecond time-resolved techniques. The results demonstrate that complexes of different metals have different types of lowest excited state: a tatpp ligand-centered (LC) triplet in the case of Ru(II); a metal-to-ligand charge-transfer (MLCT) triplet state in the case of Os(II). The excited-state kinetics is strongly solvent-dependent. In the Ru(II) system, the formation and decay of the LC state take place, respectively, in 25 ps and ca. 5 ns in CH3CN and in 0.5 ps and 1.3 micros in CH2Cl2. These solvent effects can be rationalized on the basis of a thermally activated decay of the LC state through the upper MLCT state. In the Os(II) system, the formation and decay of the MLCT state take place, respectively, in 3.8 and 60 ps in CH3CN and in 0.5 and 4 ps in CH2Cl2. These effects are consistent with the solvent sensitivity of the MLCT energy, in terms of driving force and energy-gap law arguments. The relevance of these results for the use of ladder-type aromatic bridges as potential molecular wires is discussed.
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