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

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• 作者：Darren Achey ; Gerald J. Meyer
• 刊名：Inorganic Chemistry
• 出版年：2013
• 出版时间：August 19, 2013
• 年：2013
• 卷：52
• 期：16
• 页码：9574-9582
• 全文大小：481K
• 年卷期：v.52,no.16(August 19, 2013)
• ISSN：1520-510X

文摘

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 (¹A_1g) Ni^II compound and a high-spin S = 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 螖H and 螖S. 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 鈭?0 to 鈭?0 掳C, revealed activation energies for ligand dissociation that were the same for the compound in fluid solution and anchored to TiO₂, E_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.