Ligand Coordination and Spin Crossover in a Nickel Porphyrin Anchored to Mesoporous TiO₂ Thin Films

The coordination and spin equilibrium of a Ni^II <i>meso</i>-tetra­(4-carboxy­phenyl)­porphyrin compound, NiP, was quantified both in fluid solution and when anchored to mesoporous, nanocrystalline TiO₂ thin films. This comparison provides insights into the relative rate constants for excited-state injection and ligand field population. In the presence of pyridine, the spectroscopic data were consistent with the presence of equilibrium concentrations of a 4-coordinate low-spin <i>S</i> = 0 (¹A_1g) Ni^II compound and a high-spin <i>S</i> = 1 (³B_1g) 6-coordinate compound. Temperature-dependent equilibrium constants were consistently smaller for the surface-anchored NiP/TiO₂, as were the absolute values of Δ<i>H</i> and Δ<i>S</i>. In the presence of diethylamine (DEA), the ground-state 6-coordinate compound was absent, but evidence for it was present after pulsed light excitation of NiP. Arrhenius analysis of data, measured from −40 to −10 °C, revealed activation energies for ligand dissociation that were the same for the compound in fluid solution and anchored to TiO₂, <i>E</i>_a = 6.6 kcal/mol, within experimental error. At higher temperatures, a significantly smaller activation energy of 3.5 kcal/mol was found for NiP­(DEA)₂/TiO₂. A model is proposed wherein the TiO₂ surface sterically hinders ligand coordination to NiP. The lack of excited-state electron transfer from Ni^IIP*/TiO₂ indicates that internal conversion to ligand field states was at least 10 times greater than that of excited-state injection into TiO₂

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(<i>meso-</i>5,10,15,20-tetrakis­(4-carboxyphenyl)­porphyrin) anchored to a nanocrystalline TiO₂ thin film, abbreviated Co^IP/TiO₂, immersed in an acetonitrile:pyridine electrolyte resulted in rapid excited state injection, <i>k</i>_inj > 10⁸ s^–1, to yield Co^IIP/TiO₂(e^–), followed by axial coordination of pyridine to the Co^IIP and hence an increase in coordination number from four to five. The formal oxidation state and coordination environment of the Co metalloporphyrin on TiO₂ 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 <i>k</i>_+py = 2 × 10⁸ 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, <i>E</i>°(Co^II/I/TiO₂) = −0.72 V and <i>E</i>°(Co^II/I(py)/TiO₂) = −0.85 V vs NHE. With some assumptions, this indicated an equilibrium formation constant <i>K</i>_f = 400 M^–1 for the Co^IIP­(py)/TiO₂ 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

Non-Nernstian Two-Electron Transfer Photocatalysis at Metalloporphyrin–TiO₂ Interfaces

A long-standing question in the photochemical sciences concerns how to integrate single-electron transfers to catalytic multielectron transfer reactions that produce useful chemical fuels. Here we provide a strategy for the two-electron formation of C–C bonds with molecular catalysts anchored to semiconductor nanocrystallites. The blue portion of the solar spectrum provides band gap excitation of the semiconductor while longer wavelengths of light initiate homolytic cleavage of metal–carbon bonds that, after interfacial charge transfer, restore the catalyst. The semiconductor utilized was the anatase polymorph of TiO2 present as a nanocrystalline, mesoporous thin film. The catalyst was cobalt meso-5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin chloride, Co(TCPP)Cl. For this catalyst and iron protoporphyrin IX chloride, Fe(PPIX)Cl, two distinct and sequential metal-based MIII/II and MII/I reductions were observed under band gap illumination. Spectroelectrochemical characterization indicated that both reductions were non-Nernstian, behavior attributed to an environmentally dependent potential drop across the molecule–semiconductor interface. Reaction of CoI(TCPP)/TiO2 with organobromides (RBr = 1-Br-hexane or benzyl bromide) resulted in the formation of CoIII–R(TCPP)/TiO2. Visible light excitation induced homolytic cleavage of the Co–C bond and the formation of C–C-bonded products. The reactions were catalytic when band gap excitation or an electrochemical bias provided TiO2 electrons to the oxidized catalyst. Sustained photocurrents were quantified in photoelectrosynthetic solar cells under forward bias