Structural Studies of Photoinduced Intramolecular Electron Transfer in Cyclopentadienylnitrosylnickel

Abstract

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-A elongation of the Ni-N bond and by a bending of the Ni-N-0.A shift of the N -0 stretching frequency from 1824 to 1387 cm-' 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-0 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.

Capturing a Photoexcited Molecular Structure Through Time-Domain X-ray Absorption Fine Structure

“…Because many light-driven processes occur in disordered media, techniques that do not rely on the longrange order of a system must be developed. The approach of laser pump/x-ray probe x-ray absorption fine structure (XAFS) was proposed earlier (8)(9)(10), and transient molecular structures, generated in light-induced processes, were captured without using the temporal resolution of the source (11)(12)(13)(14)(15)(16)(17)(18)(19).…”

Taking Snapshots of Photoexcited Molecules in Disordered Media by Using Pulsed Synchrotron X‐rays

“…Early studies by using older generation synchrotron sources were carried out mainly by one of three approaches: 1) gating detectors or chopping an X-ray pulse to gain the time resolution, [22][23][24][25][26][27][28][29][30] 2) cryogenic trapping of excited-state species for a long period of time so that they can be examined by the steady-state methods, [31][32][33][34][35] and 3) by using energy dispersive X-ray absorption, which captures the excited-state structure within the detector readout time. The birth and development of synchrotron facilities provided X-ray photon fluxes that were several orders of magnitude higher than X-rays generated by a rotating anode.…”

“…However, the probing of excited state molecules in disordered media remained elusive until the third generation of synchrotron facilities were available, which provided an unprecedented X-ray photon flux within a single X-ray pulse. Early studies by using older generation synchrotron sources were carried out mainly by one of three approaches: 1) gating detectors or chopping an X-ray pulse to gain the time resolution, [22][23][24][25][26][27][28][29][30] 2) cryogenic trapping of excited-state species for a long period of time so that they can be examined by the steady-state methods, [31][32][33][34][35] and 3) by using energy dispersive X-ray absorption, which captures the excited-state structure within the detector readout time. [36][37][38][39][40][41] However, these early works were carried out on microsecond time resolution, which excludes many excited states, especially singlet excited states, with shorter lifetimes.…”

Molecular Structure Determination for Photogenerated Intermediates in Photoinduced Electron Transfer Reactions Using Steady-State and Transient XAFS

“…In the past several years, we have been working on structural determination of photoinduced charge-separated meta-stable states with relatively long lifetimes on second generation synchrotron beamlines [3][4][5]. The experiments were conducted at Cayogenc temperatures at which the metastable species can be trapped for a relatively long time.…”