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Title: 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.
Author: Xie P, Chen YJ, Uddin MJ, Endicott JF.
Journal: J Phys Chem A; 2005 Jun 02; 109(21):4671-89. PubMed ID: 16833808.
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 configurational mixing coefficients at the ground-state and excited-state potential energy minima, respectively); it is inferred that 0.18 > or = alphage2 > or = 0.09 for [Ru(bpy)3]2+ and 0.37 > or = alphage2 > or = 0.18 for [Ru(NH3)4bpy]2+ in DMSO/water glasses, where the ranges are based on models that there is or is not a spin restriction on configurational mixing (alphage > alphaeg and alphage = alphaeg), respectively, for these complexes.

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