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 <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