Excited States and Intermediates by Time-Resolved Infrared Spectroscopy

Abstract

For coordination compounds containing CO or CN groups, fast time-resolved infrared spectroscopy (TRIR) provides a convenient method of probing excited states and intermediates. TRIR has proved particularly powerful for probing the structure and kinetics of organometallic intermediates. The interpretation is particularly straightforward when combined with IR data from matrix isolation experiments, although there can be some subtle differences. In excited state studies, shifts in ν(CO) and ν(CN) frequencies, from ground to excited state, are sensitive to the changes in electron distribution on excitation, thus allowing the distinction between charge-transfer and non-charge-transfer transitions. Subtle effects on excited state ν(CO) band positions occur with change from fluid to rigid solvent-“infrared rigidochromism”. There is often a change in ν(CO) band width on excitation; this can be interpreted in terms of specific interactions between the excited species and the solvent. This paper presents some of our recent work in this area.

Metal-to-Ligand Charge Transfer Excited-State ν(CO) Shifts in Rigid Media

“…MLCT Excited-State ν(CO) Spectra. The PMMA TRIR spectra in Figure and Figure B in the Supporting Information have features in common with solution spectra. ,− In solution two bands appear in the ground state, arising from an A 1 mode and two unresolved E modes, as expected for pseudo- C 3 v symmetry (Figure ). The same ground-state pattern is observed in PMMA but there is evidence of an effective lowering of symmetry with separate components appearing at ∼1922 and ∼1930 cm -1 for the low-energy band.…”

“…In the study of Turner and co-workers, it was noted that the fluid-to-glass transition in a 5:4 (v/v) butyronitrile/propionitrile mixture at 77 K had a profound effect on the magnitude of the excited-to-ground-state ν(CO) shifts in fac -[Re I (bpy)(CO) 3 Cl]. As noted in the Introduction, there is a general tendency for ν(CO) band energies to increase in MLCT excited states because partial oxidation at the metal decreases dπ(Re)−π*(CO) back-bonding. ,− σ(M−CO) bond polarization may also play a role…”

“…All were used as received. The salts fac-[Re I (4,4′-X 2 bpy)(CO) 3 (4-Etpy)](PF 6 ) were prepared in several steps from Re(CO) 5 Cl according to literature procedures. [26][27][28] IR and TRIR spectra were recorded in a gastight liquid cell consisting of two BaF 2 plates separated by a 0.75 mm Teflon spacer purchased from Harrick.…”

“…A characteristic feature in the infrared spectra of CO-containing metal-to-ligand charge transfer (MLCT) excited states is an increase in excited-state ν(CO) band energies compared to the ground state. − For cis -[Os(bpy) 2 (CO)(Cl)] + * (eq 1), ν̄(CO) = 2022 cm -1 in the excited state (ν̄ es ) and 1948 cm -1 in the ground state (ν̄ gs ) with a difference, Δν̄(CO) ( =ν̄ es − ν̄ gs ), of 75 cm -1 in CH 3 CN at 298 K . In the pseudo- C 3 v symmetry of fac -[Re(bpy)(CO) 3 (4-Etpy)] + there are two ground-state ν(CO) bands arising from an A 1 mode and a degenerate pair of E modes.…”

Derivatives of the [Ru(bipy)(CN)4]2– chromophore with pendant pyridyl-based binding sites: synthesis, pH dependent-luminescence, and time-resolved infrared spectroscopic studies

“…11, 12 The syntheses of the two mononuclear complexes and the pH dependence of their optical absorption and luminescence properties are described in this paper, and show some interesting results related to competing effects on the optical properties of protonation at pendant pyridyl sites, or at the cyanide sites. which possess an obvious infrared 'reporter group' which is sensitive to the electron density on the metal centre, 13 notably Re()-carbonyl and Ru()-cyanide complexes, and have also been used to follow the inter-component energy-transfer processes in multinuclear complexes which contain an IR active chromophore. 6, 14 The cyanide ligands of the [Ru(bipy)(CN) 4 ] 2Ϫ unit are an ideal target for such a study.…”