Increase in the coordination number of a cobalt porphyrin after photo-induced interfacial electron transfer into nanocrystalline TiO2.

Spectroscopic, electrochemical, and kinetic data provide compelling evidence for a coordination number increase initiated by interfacial electron transfer. Light excitation of Co(I)(meso-5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin) anchored to a nanocrystalline TiO(2) thin film, abbreviated Co(I)P/TiO(2), immersed in an acetonitrile:pyridine electrolyte resulted in rapid excited state injection, k(inj) > 10(8) s(-1), to yield Co(II)P/TiO(2)(e(-)), followed by axial coordination of pyridine to the Co(II)P and hence an increase in coordination number from four to five. The formal oxidation state and coordination environment of the Co metalloporphyrin on TiO(2) were assigned through comparative studies in fluid solution as well as by comparisons to previously reported data. The kinetics for pyridine coordination were successfully modeled with a pseudo-first order kinetic model that yielded a second-order rate constant of k(+py) = 2 × 10(8) M(-1) s(-1). Spectro-electrochemical measurements showed that pyridine coordination resulted in a ∼200 mV negative shift in the Co(II/I) reduction potential, E°(Co(II/I)/TiO(2)) = -0.72 V and E°(Co(II/I)(py)/TiO(2)) = -0.85 V vs NHE. With some assumptions, this indicated an equilibrium formation constant K(f) = 400 M(-1) for the Co(II)P(py)/TiO(2) compound. The kinetics for charge recombination were non-exponential under all conditions studied, but were successfully modeled by the Kohlrausch-Williams-Watts (KWW) function with observed rate constants that decreased by about a factor of 100 when pyridine was present. The possible mechanisms for charge recombination are discussed.