Axial ligand binding and release processes in nickel(II)-tetraphenylporphyrin revisited: a resonance Raman study

Processes of axial ligand(s) binding and release for nickel meso-tetraphenylporphyrin (NiTPP) dissolved in mixed benzene/piperidine solvents have been quantitatively studied by resonance Raman spectroscopy in using both continuous-wave and pulsed low-power nanosecond excitations at 441.6 mn. Global-fit analysis of intensity changes in the Raman spectra has been performed, along with a mathematical modeling, by a rate equations approach, of the photoinduced ligand binding/release processes at the central Ni atom. The existence of a five-coordinate high-spin ground-state NiTPP(Pip) species has been confirmed, whose concentration amounts to not more than a few percent of the total porphyrin concentration at any benzene/piperidine ratio. We also found that this five-coordinate species does not substantially contribute to the photoinduced processes of axial ligand(s) binding and release. Four-coordinate NiTPP and six-coordinate NiTPP(Pip)(2) are the major species which can directly convert into each other. On photoexcitation of the NiTPP(Pip)(2) species, from one quarter to one half of bis-ligated molecules reversibly loose their axial ligands, while on NiTPP photoexcitation, less than 10% of non-ligated molecules reversibly attach axial ligands. (C) 2002 Published by Elsevier Science B.V.