Tetrazine bridged osmium dimers: Electrochemical vs photoinduced electron transfer

Monolayers of the dimeric complex [p0p Os(bpy)(2) 4-tet Os(bpy)(2) Cl](3+), where p0p is 4,4 ' -bipyridyl, bpy is 2,2 ' -bipyridyl, and 4-tet is 3,6-b is(4-pyridyl)-1,2,4,5-tetrazine, have been formed by spontaneous adsorption onto platinum microelectrodes. These monolayers are stable, and both metal centers exhibit well-defined voltammetric responses for the Os2+/3+ redox reaction. Adsorption isotherms reveal that the areas of occupation for the dimer and a model monomer, [p0p Os(bpy)(2) 4-tet](2+), are identical at 158 +/- 14 Angstrom2. This result is consistent with the dimer adopting an ext ended configuration rather than lying coplanar with the electrode surface. High-speed chronoamperometry reveals that the standard heterogeneous electr on-transfer rate constants, k, for the "inner" [p0p Os(bpy)(2) 4-tet](2+) a nd "outer" [4-tet Os(bpy)(2)Cl](+) moieties are (1.3 +/- 0.2) x 10(6) and ( 1.1 +/- 0.1) x 10(4) s(-1), respectively. The reorganization energy is at l east 0.6 +/- 0.1 eV. The relatively small decrease in the heterogeneous ele ctron-transfer rate constant across the 14 Angstrom 4-tet bridge is interpr eted in terms of electron superexchange. Solution phase transient emission measurements reveal that the rate of photoinduced electron transfer (PET) b etween the two metal centers is (1.6 +/- 0.1) x 10(7) s(-1). This rate cons tant is a factor of approximately 400 smaller than the ground-state electro n-transfer rate constant for monomeric [4-tet Os(bpy)(2) Cl](+) monolayers when the driving forces are identical. This significant difference is inter preted in terms of the energy separation between the ground or excited stat es and the bridge. These data also reveal that the strength of electronic c oupling across the tetrazine bridge is significantly greater for two metal centers than for a metal electrode and a remote redox moiety.