Time-Resolved Vibrational Spectroscopy of Electronically Excited Inorganic Complexes in Solution.

Abstract

Time-resolved vibrational spectroscopic studies of electronic excited states in solution were reviewed by Morris and Woodruff in 1987.1 This review focused on time-resolved resonance Raman (TR3) experiments. While these efforts continue to be vital, transient Raman spectroscopy, and more recently transient infrared spectroscopy, are being increasingly applied to molecular assemblies to study the intermediate excited states formed following photoinduced electron or energy transfer. The ability to obtain structural information following the absorption of a photon is an important component of photochemical and photophysical research, which includes research related to artificial photosynthesis, photoinitiated reactions, energy transduction, energy conversion, photocatalysis, and photoremediation. Time-resolved infrared (TRIR) spectroscopy complements the Raman method, and with the advances made recently in TRIR spectroscopy, infrared spectra can now be measured on the time scales previously obtained only by the transient Raman method. This technique is particularly valuable in, but not limited to, the study of metal complexes containing CO or CN-. These ligands are useful because the ν(CO) and ν(CN) bands have large oscillator strengths and the sensitivities of their frequencies and bandwidths to electronic and molecular structure are well established. Electronic excitation generally produces transient changes in the infrared absorption that are characteristic of the changes in electron density within the complex. A potentially valuable approach to transient infrared studies of excited states on the nanosecond time scale uses step-scan interferometry. This approach has the ability to supply vibrational information throughout the middle-infrared region.2 Some studies utilizing time-resolved infrared spectroscopy of excited states of transition-metal complexes have been reviewed by Turner and Ford.3