Redox properties of ground and electronically excited states: [Ru(bpy)(2)Qbpy](2+) monolayers

Dense monolayers of [Ru(bpy)(2)Qbpy](2+), where bpy is 2,2'-bipyridyl and Q hpy is 2,2':4,4 ":4'4 "-quarterpyridyl, have been formed by spontaneous ads orption onto clean platinum microelectrodes. Cyclic voltammetry of these mo nolayers is nearly ideal, and five redox states are accessible over the pot ential range from +1.3 to -2.0 V. Chronoamperometry conducted on a microsec ond time scale has been used to measure the heterogeneous electron-transfer rate constant, k, for both metal- and ligand-based redox reactions. Hetero geneous electron transfer is characterized by a single unimolecular rate co nstant (k/s(-1)). Standard heterogeneous electron-transfer late constants, k degrees, have been evaluated by extrapolating Tafel plots of In k vs over potential, eta, to zero driving force to yield values of (5.1 +/- 0.3) x 10 (5) s(-1), (3.0 +/- 0.1) x 10(6) s(-1), and (3.4 +/- 0.2) x 10(6) s(-1) for k degrees(3+/2+), k degrees(2+/1+), and k degrees(l+/0), respectively. Tem perature-resolved measurements of k reveal that the electrochemical activat ion enthalpy, Delta H-double dagger, decreases from 12.1 +/- 1.7 kJ mol(-1) for the 3+/2+ reaction to 7.5 +/- 0.8 kJ mol(-1) for the 2+/1+ process. Pr obing the temperature dependence of the formal potential gives the reaction entropy, Delta S(rc)degrees. Significantly, the free energy of activation is constant at 6.9 +/- 0.6 kJ mol(-1) for all three redox couples investiga ted. The electronic transmission coefficient, K-cl, describing the probabil ity of electron transfer once the transition state has been reached, is con siderably less than unity for all three redox processes. Following photoexi tation using a laser pulse at 355 nm, emission is observed from the monolay ers with an excited-state lifetime (6.2 mu s) that exceeds that of the comp lex in solution (1.4 mu s). It appears that weak electronic coupling betwee n the adsorbates and the electrode means that the excited states are not co mpletely deactivated by radiationless energy transfer to the metal. For the first time, we have used voltammetry conducted at megavolt per second scan rates to directly probe the rodox potentials and electron-transfer charact eristics of electronically excited species.