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.