Laser-induced excited-state ligation changes for nickel tetraphenylporphine monitored by Raman spectroscopy

Abstract

Metalloporphyrins offer the interesting possibility of undergoing changes in axial ligation associated with the pumping of excited ligand field states via intersystems crossing from initially populated porphyrin ..pi..-..pi..* states. One example is the extremely rapid and efficient photodissociation of the CO adduct of heme proteins, which has been widely exploited in studying the heme-linked protein dynamics of these O/sub 2/-carrying and -activating macromolecules. Nickel porphyrins offer an attractive system for investigation since they are 4-coordinate and low spin in noncoordinating and weakly coordinating solvents but 6-coordinate and high spin in strongly coordinating solvents. Holten and co-workers have shown with transient optical spectroscopy that the accessible excited state (low spin) of the 6-coordinate species rapidly loses its axial ligands, while the excited state of 4-coordinate nickel porphyrin (high spin) becomes ligated, thanks to the hole created in d/sub z /sup 2// orbital. It should therefore be possible to shift the equilibrium between 4- and 6-coordinate species with photons. In this report the authors demonstrate such pumping with CW laser excitation of stationary samples of NiTPP (nickel tetraphenylporphine) in pyridine and piperidine, using resonance Raman (RR) spectroscopy to monitor the ligation changes. The RR spectra also allow assessment of the structural changemore » attendant upon ligation, and they permit identification of a 5-coordinate species in piperidine solution, with a ground state that is probably high spin.« less