Application of time-resolved infrared spectroscopy has had an important impact on transition metal photochemistry. 1 The emphasis has been on metal carbonyl and metal cyano complexes because the oscillator strengths of ν j(CO) and ν j(CN) are high, and tunable lasers are available in the relevant spectral region. 2 Until recently, time-resolved infrared spectroscopy using Fourier transform interferometry has been limited to a time resolution of g5 µs. However, application of step-scan FT-IR has greatly expanded the time window. 3,4 It is now possible to acquire spectra with high resolution and sensitivity on a time scale of tens of nanoseconds over the entire mid-IR region. 5 In this communication, we report the application of step-scan FT-IR absorbance difference time-resolved spectroscopy (S 2 FT-IR ∆A TRS) with spectra acquired on the 10 ns time scale to the study of electronic structure in the metal-toligand charge transfer (MLCT) excited states of two related complexes of ruthenium(II) containing only the ligands 2,2′bipyridine (bpy), 4-(carboxyethyl)-4′-methyl-2,2′-bipyridine (4-COOEt-4′-CH 3 bpy) and 4,4′-(dicarboxyethyl)-2,2′-bipyridine (4,4′-(COOEt) 2 bpy):Comparison of the relative vibrational energies of the MLCT states leads to specific and significant conclusions regarding the distribution of electron density in these states.Spectra were obtained on a step-scan-modified Bruker IFS 88 FT-IR spectrometer. Samples were dissolved in acetonitrile in sufficient concentration (∼5 mM) to give an absorbance between 0.2 and 0.6 for the ester CdO stretch at 1731 cm -1 in a 0.25 mm path length. The solutions were sparged with argon before loading by syringe into the CaF 2 -windowed IR cell. For the S 2 FT-IR ∆A TRS, or ∆A, measurements, samples were excited by third-harmonic pulses (355 nm) from a Quanta Ray DCR-1 Nd:YAG laser (10 ns at 10 Hz; 3 mJ/pulse). The data acquisition sequence was controlled by a Stanford Research Model 455 pulse generator. Data were collected at 2-6 cm -1 spectral resolution. The liquid-N 2 -cooled Kolmar Technologies MCT detector was operated in the AC/DC-coupled mode and had an effective rise time of ∼20 ns. Spectra before and after the laser pulse were sampled at 10 ns intervals. The effects of a total of 100-300 laser flashes were averaged for each of the interferogram points. The transient absorption difference spectra ∆A (after-minus-before) were calculated from the single-beam ∆I transforms by the relation ∆A(ν j,t) ) -log[1 + ∆I(ν j,t)/I(ν j)], where I(ν j) is the intensity before laser excitation and ∆I(ν j,t) is the change in intensity at time t.In Figure 1 are shown (A) the ground state FT-IR spectrum of [Ru II (bpy) 2 (4-COOEt-4′-CH 3 bpy)] 2+ (1) and (B-D) its ∆A † Duke University. Figure 1. FT-IR spectra in CH3CN: (A) ground state spectrum of [Ru(bpy)2(4-COOEt-4′-CH3bpy)] 2+ ; (1) (B) laser-induced ∆A spectrum of 1 in the absence of any quencher; (C) ∆A spectrum of 1 in the presence of the reductive quencher 10-methylphenothiazine; (D) ∆A spectrum of 1 in the presence o...

Step-scan Fourier transform infrared absorption difference time-resolved (S(2)FTIR DeltaA TRS) and time-resolved resonance Raman (TR(3)) spectroscopies have been applied to a series of questions related to excited-state structure in the metal-to-ligand charge transfer (MLCT) excited states of [Ru(bpy)(2)(4,4'-(CO(2)Et)(2)bpy)](2+), [Ru(bpy)(2)(4-CO(2)Et-4'-CH(3)bpy)](2+), [Ru(bpy)(4,4'-(CO(2)Et)(2)bpy)(2)](2+), [Ru(4,4'-(CO(2)Et)(2)bpy)(3)](2+), [Ru(bpy)(2)(4,4'-(CONEt(2))(2)bpy)](2+), [Ru(bpy)(2)(4-CONEt(2)-4'-CH(3)bpy)](2+), and [Ru(4-CONEt(2)-4'-CH(3)bpy)(3)](2+) (bpy is 2,2'-bipyridine). These complexes contain bpy ligands which are either symmetrically or unsymmetrically derivatized with electron-withdrawing ester or amide substituents. Analysis of the vibrational data, largely based on the magnitudes of the nu(CO) shifts of the amide and ester substituents (Deltanu(CO)), reveals that the ester- or amide-derivatized ligands are the ultimate acceptors and that the excited electron is localized on one acceptor ligand on the nanosecond time scale. In the unsymmetrically substituted acceptor ligands, the excited electron is largely polarized toward the ester- or amide-derivatized pyridine rings. In the MLCT excited states of [Ru(bpy)(2)(4,4'-(CO(2)Et)(2)bpy)](2+) and [Ru(bpy)(2)(4,4'-(CONEt(2))(2)bpy)](2+), Deltanu(CO) is only 60-70% of that observed upon complete ligand reduction due to a strong polarization interaction in the excited state between the dpi(5) Ru(III) core and the excited electron.

Step-scan Fourier transform absorption difference time-resolved spectroscopy (S2FTIR ∆A TRS) has been used to collect mid-IR time-resolved infrared spectra of the transient electronic excited states of polypyridine transition metal complexes with 10 ns time resolution. The time-resolved data can be used for kinetic analysis or to generate “snapshots” of the lowest lying excited state. Shifts of vibrational bands in the excited state relative to the ground state can be used to infer significant details of the electronic structure of the excited state. The multiplex advantage of the FTIR technique allows a wide variety of vibrational bands to be analyzed for this purpose. In the example illustrated, the shift of the ester ν(CO) band in {Ru(bpy)[4,4′-(COOEt)2bpy]2}2+ compared to those in related complexes has been used to address the question of electron delocalization in the excited state.