Characterization of the Lowest Excited States of [Rh(bpy-h8)n(bpy-d8)3-n]3+by Highly Resolved Emission and Excitation Spectra

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Abstract
Highly resolved emission, excitation, and resonantly line-narrowed spectra, as well as emission decay properties of [Rh(bpy-h8)n(bpy-d8)3-n]3+ (n = 0, 2, 3; bpy = 2,2‘-bipyridine) doped into [Zn(bpy-h8)3](ClO4)2 are presented for the first time. [Rh(bpy-h8)3]3+ and [Rh(bpy-d8)3]3+ exhibit one low-lying triplet T1 at 22 757 ± 1 and 22 818 ± 1 cm-1, respectively (blue shift 61 cm-1), while [Rh(bpy-h8)2(bpy-d8)]3+ has two low-lying triplets at 22 757 ± 1 and 22 818 ± 1 cm-1. The well-resolved vibrational satellite structures show, that the equilibrium positions of the triplet and the singlet ground S0 state are not very different and that the force constants in T1 are mostly slightly smaller than in S0. Moreover, the vibrational satellite structure is strongly dominated by vibrational ligand modes, which demonstrates the ππ* character of the corresponding transition. However, the occurrence of several very weak vibrational modes of metal−ligand character displays a small influence of the metal ion. This is supported by the emission decay behavior. [Rh(bpy-h8)2(bpy-d8)]3+ exhibits an emission which is clearly assignable to the protonated ligand(s), even when the deuterated ligand is selectively excited. Obviously, an efficient intramolecular energy transfer from the deuterated to the protonated ligand(s) occurs, presumably mediated by the small Rh3+ d-admixture. A so-called “dual emission” is not observed. Moreover, a series of spectroscopic properties of the lowest excited state of [Rh(bpy)3]3+ (energies of electronic origins, emission decay times, zero-field splittings, structures of vibrational satellites, etc.) is compared to properties of bpy, [Pt(bpy)2]2+, [Ru(bpy)3]2+, and [Os(bpy)3]2+. This comparison displays in a systematic way the increasing importance of the metal d and/or MLCT character for the lowest excited states and thus provides guidelines for an experimentally based classification. In particular, the lowest excited states of [Rh(bpy)3]3+ may be ascribed as being mainly of 3ππ* character confined to one ligand in contrast to the situation found for [Ru(bpy)3]2+ where these states are covalently delocalized over the whole complex.