Structural studies of photoinduced intramolecular electron transfer in cyclopentadienylnitrosylnickel

A structural study based on EXAFS, FTIR, and optical absorption spectroscopies has been conducted on a photogenerated, metastable state of cyclopentadienylnitrosylnickel (CpNiNO) produced by a reversible photochemical reaction. The photogenerated, metastable state with distinctively different EXAFS, FTIR, and optical absorption spectra from those of the ground state was created by irradiating the sample at 20 K with the 365-nm line of a mercury lamp. At the same temperature, the reverse reaction was induced by irradiation with the 313-nm line from a mercury lamp. On the basis of the analysis of the EXAFS data, the photogenerated, metastable state of CpNiNO has undergone considerable nuclear rearrangements cmopared to its ground state. The nuclear movement is characterized by a 0.12-Å elongation of the Ni-N bond and by a bending of the Ni-N-O. A shift of the N-O stretching frequency from 1824 to 1387 cm^-1 was observed in the photoinduced reaction with 365-nm light, consistent with previous studies. This implies that a NO^- like species results from intramolecular electron transfer from Ni to NO. The absorption spectrum of the long-lived, metastable, charge-transfer state exhibited reduced absorption of the 385-nm band and an additional broad band in the near-IR region, which is likely a consequence of the intramolecular electron transfer and the Ni-N-O bending. On the basis of the structures obtained from EXAFS, ZINDO calculations for the ground state of CpNiNO reproduced the general features of the observed absorption spectrum and were qualitatively consistent with the complicated dependence of the charge-transfer photoreaction on wavelength.