Excited-State Distortions and Electron Delocalization in Mixed-Valence Dimers: Vibronic Analysis of the Near-IR Absorption and Resonance Raman Profiles of [Fe2(OH)3(tmtacn)2]2+

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Abstract
The near-IR transition associated with valence delocalization in the class III mixed-valence dimer [Fe2(OH)3(tmtacn)2]2+ is studied using variable-temperature (VT) electronic absorption and resonance Raman (RR) spectroscopies to gain insight into the properties of electron delocalization in this dimer. Laser excitation into this absorption band leads to dominant resonance Raman enhancement of totally-symmetric [Fe2(OH)3]2+ core vibrational modes at 316 and 124 cm-1, descriptions of which are calculated from a normal coordinate analysis. Vibronic analysis of the near-IR resonance Raman excitation profiles and VT-absorption bandshapes using an anharmonic excited-state model provides a description of the geometric distortions accompanying this excitation. The excited-state distortion is dominated by expansion of the [Fe2(OH)3]2+ core along the Fe···Fe axis, reflecting the significant Fe−Fe σ → σ* character of this transition. The ground-state σ-interaction between the two metals has been identified as the orbital pathway for valence delocalization, and the σ → σ* distortion analysis is used to quantify the structural dependence of the electronic-coupling matrix element, HAB, associated with this pathway. The dominant role of totally-symmetric nuclear coordinates in the absorption and RR spectroscopies of [Fe2(OH)3(tmtacn)2]2+ is also discussed in relation to the Q- vibrational coordinate and the vibronic spectroscopies of other class II and class III mixed-valence dimers. It is shown that intensity contributions from the Q- coordinate to the absorption and RR spectra of [Fe2(OH)3(tmtacn)2]2+ are small relative to those of the totally-symmetric coordinates due to the inefficient change-in-curvature mechanism by which the Q- coordinate gains intensity, compared to the efficient excited-state displacement mechanism allowed for totally-symmetric coordinates. This is in contrast with the dominance of the Q- coordinate over other totally-symmetric coordinates observed in intervalence transfer (IT) absorption and RR spectroscopies of class II mixed-valence complexes.

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