The Characterization of the High-Frequency Vibronic Contributions to the 77 K Emission Spectra of Ruthenium−Am(m)ine−Bipyridyl Complexes, Their Attenuation with Decreasing Energy Gaps, and the Implications of Strong Electronic Coupling for Inverted-Region Electron Transfer

Abstract

The 77 K emission spectra of a series of [Ru(Am)6-2n(bpy)n]2+ complexes (n = 1-3) have been determined in order to evaluate the effects of appreciable excited state (e)/ground state (g) configurational mixing on the properties of simple electron-transfer systems. The principal focus is on the vibronic contributions, and the correlated distortions of the bipyridine ligand in the emitting MLCT excited state. To address the issues that are involved, the emission band shape at 77 K is interpreted as the sum of a fundamental component, corresponding to the {e,0'} --> {g,0} transition, and progressions in the ground-state vibrational modes that correlate with the excited-state distortion. Literature values of the vibrational parameters determined from the resonance-Raman (rR) for [Ru(NH3)4bpy]2+ and [Ru(bpy)3]2+ are used to model the emission spectra and to evaluate the spectral analysis. The Gaussian fundamental component with an energy Ef and bandwidth Deltanu1/2 is deconvoluted from the observed emission spectrum. The first-, second-, and third-order terms in the progressions of the vibrational modes that contribute to the band shape are evaluated as the sums of Gaussian-shaped contributions of width Deltanu1/2. The fundamental and the rR parameters give an excellent fit of the observed emission spectrum of [Ru(NH3)4bpy]2+, but not as good for the [Ru(bpy)3]2+ emission spectrum probably because the Franck-Condon excited state probed by the rR is different in symmetry from the emitting MLCT excited state. Variations in vibronic contributions for the series of complexes are evaluated in terms of reorganizational energy profiles (emreps, Lambdax) derived from the observed spectra, and modeled using the rR parameters. This modeling demonstrates that most of the intensity of the vibronic envelopes obtained from the frozen solution emission spectra arises from the overlapping of first-order vibronic contributions of significant bandwidth with additional convoluted contributions of higher order vibronic terms. The emrep amplitudes of these complexes have their maxima at about 1500 cm(-1) in frozen solution, and Lambdax(max) decreases systematically by approximately 2-fold as Ef decreases from 17,220 for [Ru(bpy)3]2+ to 12,040 cm(-1) for [Ru(NH3)4bpy]2+ through the series of complexes. Corrections for higher order contributions and bandwidth differences based on the modeling with rR parameters indicate that the variations in Lambdax(max) imply somewhat larger decreases in first-order bpy vibrational reorganizational energies. The large attenuation of vibrational reorganizational energies of the [Ru(Am)6-2n(bpy)n]2+ complexes contrasts with the apparent similarity of reorganizational energy amplitudes for the absorption and emission of [Ru(NH3)4bpy]2+. These observations are consistent with increasing and very substantial excited-state/ground-state configurational mixing and decreasing excited-state distortion as Ef decreases, but more severe attenuation for singlet/singlet than triplet/singlet mixing (alphage > alphaeg for ...

The influence of ligand localized excited states on the photophysics of second row and third row transition metal terpyridyl complexes: Recent examples and a case study

“…13S) is an intraligand charge transfer (ILCT) absorption, despite the fact that the wavelength and band shape are similar to those observed for the MLCT cm -1 and 12,600 cm -1 , respectively. The luminescence of these en complexes was evaluated in considerable detail and has been assigned as originating from an MLCT excited state for [Ru(Mpt)(dien)] 2+ , [27,58,59] consistent with the TD DFT results that indicate the lowest energy triplet state arises from a HOMO (d) to LUMO (*) transition (Fig. 14SA).…”

Energy Dependence of the Ruthenium(II)-Bipyridine Metal-to-Ligand-Charge-Transfer Excited State Radiative Lifetimes: Effects of ππ*(bipyridine) Mixing

“…81 As has been previously discussed, the amplitude of the dominant vibronic sideband decreases markedly as the excited state energy decreases. 20,[37][38][39] 20 The gray curves in Figure 3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 The comparison in Figure 3 is of the vibronic (or Franck-Condon) amplitudes for two different conceptual models and the resulting bandshapes should not be significantly on 3 m  , as in eq 3, results in dependent on h m . However, assuming that the calculated intensities depend vibronic progressions whose amplitudes are linearly dependent on  m .…”

“…(b) There are typically molecular distortions in a number of vibrational modes and these couple to the electronic transition (see Figure 1) as is evident from the vibronic side band features of the emission spectra. 37 The measured values of k RAD correspond to the composite of the different k RAD (vib) values contributed by the vibronic transitions at E max(vib) . This requires the use of a spectrally-weighted average energy which can be approximated 26 and is discussed below.…”

Metal-to-Ligand Charge-Transfer Emissions of Ruthenium(II) Pentaammine Complexes with Monodentate Aromatic Acceptor Ligands and Distortion Patterns of their Lowest Energy Triplet Excited States

“…The emission spectrum of the [Ru(NH 3 ) 4 bpy] 2+ complex has some resolved vibronic features, and its DFT modeling suggests that E 0′0 ( 3 MLCT) is about 200 cm −1 higher in energy than hν max (em) for component bandwidths of about 400 cm −1 . 18 When the observed spectra are broad and have no resolved vibronic components, the difference between hν max (em) and E 0′0 ( 3 MLCT) is expected to be substantially larger, 44 Since the difficulty in detecting emissions from this class of complexes is often attributed to efficient internal conversion to a similar, or lower, energy 3 MC excited state, 14 we have used DFT approaches to estimate their energies, to evaluate internal conversion barriers, and to construct more detailed models of the 3 MC excited states for some of them. The experimental spectroscopic observations and the computational modeling of the lowest energy triplet excited states in these complexes have implications for the photochemical reaction mechanisms of this class of complexes, and they point to some important considerations in the design of transition metal complex photosensitizers.…”