CHEMI-LUMINESCENCE
ENERGY-TRANSFER
BUILDING BLOCK
COMPLEXES
ELECTROCHEMISTRY
ELECTRODES
MOLECULES
SURFACE
2,2'-4,4''-4',4'''-QUATERPYRIDYL
FLUORESCENCE
Dense monolayers of [Ru(bpy)(2)Qbpy](2+), where bpy is 2,2'-bipyridyl and Qhpy is 2,2':4,4 ":4'4 "-quarterpyridyl, have been formed by spontaneous adsorption onto clean platinum microelectrodes. Cyclic voltammetry of these monolayers is nearly ideal, and five redox states are accessible over the potential range from +1.3 to -2.0 V. Chronoamperometry conducted on a microsecond time scale has been used to measure the heterogeneous electron-transfer rate constant, k, for both metal- and ligand-based redox reactions. Heterogeneous electron transfer is characterized by a single unimolecular rate constant (k/s(-1)). Standard heterogeneous electron-transfer late constants, k degrees, have been evaluated by extrapolating Tafel plots of In k vs overpotential, 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. Temperature-resolved measurements of k reveal that the electrochemical activation 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. Probing 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 investigated. The electronic transmission coefficient, K-cl, describing the probability of electron transfer once the transition state has been reached, is considerably less than unity for all three redox processes. Following photoexitation using a laser pulse at 355 nm, emission is observed from the monolayers with an excited-state lifetime (6.2 mu s) that exceeds that of the complex in solution (1.4 mu s). It appears that weak electronic coupling between the adsorbates and the electrode means that the excited states are not completely 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 characteristics of electronically excited species.