Infrared data in the nu(CO) region (1800-2150 cm(-1), in acetonitrile at 298 K) are reported for the ground (nu(gs)) and polypyridyl-based, metal-to-ligand charge-transfer (MLCT) excited (nu(es)) states of cis-[Os(pp)2(CO)(L)](n)(+) (pp = 1,10-phenanthroline (phen) or 2,2'-bipyridine (bpy); L = PPh3, CH(3)CN, pyridine, Cl, or H) and fac-[Re(pp)(CO)3(4-Etpy)](+) (pp = phen, bpy, 4,4'-(CH3)2bpy, 4,4'-(CH3O)2bpy, or 4,4'-(CO2Et)2bpy; 4-Etpy = 4-ethylpyridine). Systematic variations in nu(gs), nu(es), and Delta(nu) (Delta(nu) = nu(es) - nu(gs)) are observed with the excited-to-ground-state energy gap (E(0)) derived by a Franck-Condon analysis of emission spectra. These variations can be explained qualitatively by invoking a series of electronic interactions. Variations in dpi(M)-pi(CO) back-bonding are important in the ground state. In the excited state, the important interactions are (1) loss of back-bonding and sigma(M-CO) bond polarization, (2) pi(pp*-)-pi(CO) mixing, which provides the orbital basis for mixing pi(CO)- and pi(4,4'-X(2)bpy)-based MLCT excited states, and (3) dpi(M)-pi(pp) mixing, which provides the orbital basis for mixing pipi- and pi(4,4'-X(2)bpy*-)-based MLCT states. The results of density functional theory (DFT) calculations on the ground and excited states of fac-[Re(I)(bpy)(CO)3(4-Etpy)](+) provide assignments for the nu(CO) modes in the MLCT excited state. They also support the importance of pi(4,4'-X2bpy*-)-pi(CO) mixing, provide an explanation for the relative intensities of the A'(2) and A' ' excited-state bands, and provide an explanation for the large excited-to-ground-state nu(CO) shift for the A'(2) mode and its relative insensitivity to variations in X.

Characteristic patterns of infrared bands in the ν(CO) region have been observed in the time-resolved infrared (TRIR) spectra of fac-rhenium tricarbonyl complexes that allow for identification of transient states that result following laser flash excitation. These patterns can be interpreted by combining experimental TRIR data with density functional theory (DFT) calculations. The DFT calculations are particularly valuable as they provide vibrational energy shifts between the ground and excited states and an analysis of the electronic interactions in terms of the orbitals involved in the excitation. TRIR and DFT results for four different transient excited states, intraligand π f π*, metal-to-ligand charge transfer (MLCT), intramolecular (dπ-Οπ) f π* excited states, and a redox-separated (RS state), are presented here. A unique example of competing excited states studied by TRIR is also presented. The complexes studied include fac-[Re I (CO) 3 (Me 2 dppz)(4-Etpy)] + , fac-[Re I (CO) 3 (bpy)(4-Etpy)] + , fac-[Re I (CO) 3 (4,4′-(CH 3 ) 2 bpy)(OQD)] , fac-[Re I (CO) 3 (Me 2 dppz)(py-PTZ)] + , and fac-[Re I (CO) 3 (dppz)(py-PTZ)] + (Me 2 dppz is dimethyl dipyrido[3,2-a:2′,3′-c]phenazine; dppz is dipyrido-[3,2-a:2′,3′-c]phenazine; 4Etpy is 4-ethylpyridine; bpy is 2,2′-bipyridine; 4,4′-(CH 3 ) 2 bpy is 4,4′-(CH 3 )-2,2′bipyridine; OQD is 1-methyl-6-oxyquinone; py-PTZ is 10-(4-picolyl)phenothiazine). In addition to the DFT studies on the lowest triplet states probed by TRIR spectroscopy, time-dependent DFT (TD-DFT) calculations were also performed to analyze several of the lowest singlet and triplet excited states for each of the complexes.

The first time-resolved infrared difference spectra for metal-to-ligand charge transfer (MLCT) excited states in the fingerprint region from 1400 to 1625 cm-1 are reported for [Ru(bpy)3]2+* and [Re(bpy)(CO)3(4-Etpy)]+* in CD3CN at 298 K (bpy is 2,2‘-bipyridine; 4-Etpy is 4-ethylpyridine). The spectra are assigned by comparison to ground-state spectra and electrochemically generated [RuIII(bpy)3]3+ and [RuII(bpy•-)(bpy)2]+. The data provide clear evidence for the localized description [RuIII(bpy•-)(bpy)2]2+* on the ∼100 ns time scale. They also give insight into electronic distribution in the excited state based on the magnitudes and directions of the infrared shifts.

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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...