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

Control of Intramolecular Isomerization Reactivity of an Azobenzene Derivative Anchored to ZrO₂ Nanoparticle Thin Films

Chemical adsorption of molecules to nanomaterials imparts thermodynamic and kinetic variations to molecular reactivity. However, these differences are challenging to measure or predict and thus are frequently overlooked in nanomaterial/molecular interfacial systems. In this study, interfacial attachment of an azobenzene derivative to ZrO₂ nanoparticle thin films more than doubles the azobenzene thermal isomerization rate and decreases the extent of photoisomerization by up to a factor of 3 compared to that of fluid solution. The magnitude of these changes can be controlled by selectively anchoring either the <i>cis</i> or <i>trans</i> isomer of azobenzene to the ZrO₂ film. Furthermore, the coadsorption of the sterically hindering molecule chenodeoxycholic acid to a ZrO₂ thin film previously treated with <i>cis</i>-azobenzene results in a decreased extent of isomerization that mimics the <i>trans</i>-azobenzene anchored ZrO₂ film. Accordingly, steric hindrance of the azobenzene isomerization at the interface is implicated as an explanation for the observed variations in the reaction dynamics