INTRASTRAND ELECTRON AND ENERGY-TRANSFER BETWEEN POLYPYRIDYL COMPLEXES ON A SOLUBLE POLYMER

Intrastrand electron and energy transfer between pendant sites in a derivatized 1:1 copolymer of styrene and p-(chloromethyl)styrene have been studied by laser flash photolysis. The polymer [p-PS-RuII22OsII5](PF6)54, in which 22 of the approximately 27 repeat units of an average strand were RuII and 5OsII, was prepared by nucleophilic displacement of chloride by [M(bpy)2(bpyCH2OH)]2+ (M is Ru or Os; bpy is 2,2'-bipyridine; bpyCH2OH is 4-Me-4'-CH2OH-2,2'-bipyridine) under basic conditions. Following Ru(II) --> bpy laser flash excitation there was evidence for a rapid (tau < 5 ns) Ru(II*) --> Os(II) energy transfer component with the remaining Ru(II*) emission relatively unperturbed compared to [p-PS-Ru(II)27](PF6)54. On the basis of these observations it was inferred that Ru(II*) --> Os(II) energy transfer (which is spontaneous by 0.36 eV) does occur with k(295 +/- 2K, CH3CN) > 2 X 10(8) s-1, but only to Os(II) sites that are adjacent to Ru(II*). Energy transfer self-exchange from Ru(II*) to Ru(II) (DELTAG-degrees = 0.0 eV) is slow with k < 1 x 10(6) s-1. In the presence of 35 mM of the irreversible, oxidative quencher [p-CH3OC6H4N2](BF4) in CH3CN approximately 95% of the emission from [p-PS-RuII22OsII5](PF6)54 is quenched. Transient absorption measurements under these conditions show an instantaneous loss of Os(II) --> bpy, Ru(II) --> bpy absorption (bleaching) in the visible, consistent with excitation and oxidative quenching during the laser pulse. Careful measurements at lambda > 500 nm reveal a slower change that follows first-order kinetics consistent with intramolecular oxidation of Os(II) by Ru(III) for which DELTAG-degrees = -0.42 eV. The kinetics are first order with k(295 +/- 2 K, mu = 0.035 M) = 5.3 +/- 0.9 X 10(6) s-1 in solutions 10(-3) mM in polymer. This rate constant is slower by approximately 6 than the estimated rate constant for self-exchange within an association complex of [Ru(bpy)3]2+ and [Ru(bpy)3]3+ under the same conditions. At higher concentrations (> 1 X 10(-3) mM) there is evidence for intermolecular oxidation of Os(II) by Ru(III).