title: Ligand coordination and spin crossover in a nickel porphyrin anchored to mesoporous TiO2 thin films.
abstract: The coordination and spin equilibrium of a Ni(II) meso-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 S = 0 ((1)A1g) Ni(II) compound and a high-spin S = 1 ((3)B1g) 6-coordinate compound. Temperature-dependent equilibrium constants were consistently smaller for the surface-anchored NiP/TiO2, as were the absolute values of ΔH and ΔS. 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, Ea = 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(II)P*/TiO2 indicates that internal conversion to ligand field states was at least 10 times greater than that of excited-state injection into TiO2.
authors.affiliation: Department of Chemistry , Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States.
authors.forename: Darren
authors.initials: D
authors.lastname: Achey
authors.forename: Gerald J
authors.initials: GJ
authors.lastname: Meyer
mesh_terms: D008024:Ligands; D008665:Metalloporphyrins / Q000138:chemical synthesis / Q000737:chemistry*; D015394:Molecular Structure; D009532:Nickel / Q000737:chemistry*; D010316:Particle Size; D016062:Porosity; D013499:Surface Properties; D013816:Thermodynamics; D014025:Titanium / Q000737:chemistry*
chemical_list: D008024:Ligands; D008665:Metalloporphyrins; C009495:titanium dioxide; D009532:Nickel; D014025:Titanium