Energy Transfer between Ru(II) and Os(II) Polypyridyl Complexes Linked to Polystyrene

Abstract

Energy transfer between RuII and OsII polypyridyl complexes covalently attached to polystyrene has been in studied in CH3CN. The polymer is a 1:1 styrene-p-aminomethylstyrene copolymer derivatized by amide coupling with the acid-functionalized metal complexes [MII(bpy)2(bpy-COOH)](PF6)2 (MII = RuII, OsII; bpy is 2,2‘-bipyridine and bpy-COOH is 4‘-methyl-2,2‘-bipyridine-4-carboxylic acid). In the resulting polymer [co-PS−CH2NHCO−(RuII11OsII5)](PF6)32, 11 of, on the average, 16 polymer sites are derivatized by RuII and five by OsII. Photophysical properties compared to the homopolymers [co-PS−CH2NHCO−(RuII16)](PF6)32 and [co-PS−CH2NHCO−(OsII16)](PF6)32 reveal that excitation at RuII is followed by efficient energy transfer to the lower energy OsII sites with near unit efficiency (95%). Time-correlated single photon counting measurements with picosecond time resolution reveal that quenching of RuII* produced adjacent to an OsII trap site is quenched with an average rate constant 〈ken〉 = 4.2 × 108 s-1. RuII* decay and OsII* sensitization kinetics are complex because the polymer sample consists of a distribution of individual strands varying in chain length, loading pattern, and number of styryl spacers. The kinetics are further complicated by a contribution from random walk energy migration. An average energy transfer matrix element of 〈Ven〉 ∼ 2 cm-1 for RuII* → OsII energy transfer has been estimated by using emission spectral fitting parameters to calculate the energy transfer barrier.