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.

Rational design of a room temperature molecular spin switch. The light-driven coordination induced spin state switch (LD-CISSS) approach

“…However, racemisation can only proceed via a (short-lived) decoordinated (diamagnetic) state. 19 F signals of meta and para fluorine are in agreement with this hypothesis.…”

“…10 and 13) can be explained by the fast conversion of the two respective enantiomers. The 19 F NMR spectra add more evidence for this process. cis-2 exhibits four signals for ortho fluorine (1 : 2 : 2 : 1), two signals for para fluorine (1 : 2) and three signals for meta fluorine (1 : 2 : 3) atoms (Fig.…”

“…There is a reliable spin state switch between low-spin (S = 0) and high-spin (S = 1) if the coordination is changed from square planar (coordination number CN = 4) to square pyramidal (CN = 5) or square bipyramidal (CN = 6). [19][20][21] (see Fig. 1) 2.…”

Excited-state proton-coupled electron transfer within ion pairs

“…The low energy absorption bands centered around 460 nm were assigned as metal-to-ligand charge-transfer (MLCT) transitions. 62 Absorption features in the UV were due to ligand centered transitions. The addition of the R-SA− derivatives to CH 3 CN solutions of Ru-LL induced changes in the UV-vis absorption spectra.…”

Enhanced Photocatalytic Performance of Porphyrin/Phthalocyanine and Bis(4‐pyridyl)pyrrolidinofullerene modified Titania

“…Efficient attachment can be obtained by the incorporation of suitable anchoring groups capable of connecting to the semiconductor's surface. Among others, aminoacid, anhydride, carboxylate,, cyanoacrylate,, ester,, hydroxamate,, pyridyl, and phosphonate groups have been used for that purpose. It is also possible to introduce a linkage molecule that is able to interact with both the chromophore and the semiconductor surface.…”