Application of transient infrared and near infrared spectroscopy to transition metal complex excited states and intermediates

Transient infrared spectroscopy (or time-resolved infrared spectroscopy, TRIR) on the nanosecond and faster timescales has continued to evolve as a routine and, sometimes, definitive tool both for elucidation of electronic and molecular structures in metal complex excited-states. This review examines examples from the literature since 1998 and discusses experimental methods for performing transient infrared experiments and recent novel applications of TRIR to the excited states of transition metal complexes. While the interrogation of "reporter" ligands such as ν(CO) and ν(CN) modes in metal carbonyl and cyanide complexes and ν(C{double bond, long}O) ligand modes, has grown extensively toward the identification of excited states and important features of their bonding, there have been many exciting extensions of the transient infrared technique in recent years. TRIR has been increasingly applied to many types of excited states, resulting in a well-established methodology for assigning excited-state identities. The usefulness of this method has been demonstrated in the unraveling of the sometimes complicated photophysical behavior associated with the complex interplay of multiple excited states, such as closely-spaced MLCT, intra- (IL) and interligand, and dd (ligand-field (LF)) excited states. In recent years, efforts to relate ground-to-excited state vibrational band shifts with other excited state properties (such as the ground-to-excited state energy gap), and medium effects have brought new insights to the understanding of electronic structure in excited states. Application of electronic structure calculations, such as density functional theory approaches, has proven to be a very powerful tool when combined with TRIR in this regard. Relatively new developments, such as non-linear 2D infrared (T2D-IR) spectroscopy, spectroscopic extension into the near infrared, and time-resolved dynamic imaging methods offer exciting possibilities for future applications, and have already presented new capabilities for providing additional insight into the excited states of transition metal complexes.