4390
J. Phys. Chem. 1996, 100, 4390-4394
Vibrational Sidelines in the Localized 3MLCT Emission of [Ru(bpy)2(bpz)]2+ in [Zn(bpy)3](ClO4)2 (bpy ) 2,2′-Bipyridine, bpz ) 2,2′-Bipyrazine) H. Riesen,* L. Wallace, and E. Krausz Research School of Chemistry, The Australian National UniVersity, Canberra, ACT 0200, Australia ReceiVed: October 13, 1995; In Final Form: December 1, 1995X
It is accepted that the transferred electron in the lowest-excited 3MLCT states of [Ru(bpy)2(bpz)]2+ is localized on the bpz ligand. We have investigated the luminescence spectrum of the complexes [Ru(bpy)2(bpz)]2+ and [Ru(bpy-d8)2(bpz)]2+ in the [Zn(bpy)3](ClO4)2 host with great precision, and we are able to unambiguously identify sidelines associated with the spectator bpy/bpy-d8 ligands over the entire vibrational frequency range. Thus, the mere presence or absence of high-frequency vibrational sidelines in 3MLCT emission cannot be used to argue for delocalization or localization of the transferred electron. Dominant sidelines reported previously by another group for [Ru(bpy)2(bpz)]2+ in [Zn(bpy)3](ClO4)2 are due to a contamination of the sample with the chromophore [Os(bpy)2(bpy-d8)]2+.
1. Introduction Extensive studies by excited state resonance Raman spectroscopy have established that the transferred electron in the lowest-excited triplet metal-to-ligand charge transfer (3MLCT) states of [M(bpy)3]2+ (M ) Ru, Os; bpy ) 2,2′-bipyridine) in solutions and solid amorphous hosts is essentially localized on a single ligand; that is, these states are best formulated as [M3+(bpy)2(bpy-)]2+.1-7 Thus, the excitation exchange interaction between metal-ligand subunits in the 3MLCT states must be less than ≈200 cm-1, as the inhomogeneous broadening of charge transfer transitions in solutions and glasses is on the order of magnitude of ≈1000 cm-1. If the coupling was comparable to or larger than the inhomogeneous distribution, it would overcome the inequivalence of the MLCT energies of the three M-bpy subunits and the 3MLCT states would be delocalized. We have established via a wide range of incisive experiments such as Zeeman,8 Stark,8 luminescence and excitation linenarrowing, and hole-burning measurements,9-12 studies of deuteration effects,9,13 and the spectroscopy of the mixed chelates14 [Ru(bpy)3-x(phen)x]2+ (x ) 0-3) that the three lowestexcited 3MLCT states of [Ru(bpy)3]2+ are indeed localized in the crystalline [Zn(bpy)3](ClO4)2 host. For example, independent 3MLCT transitions involving the bpy or the bpy-d8 ligands are observed in the complexes [Ru(bpy)3-x(bpy-d8)x]2+ (x ) 1, 2) in [Zn(bpy)3](ClO4)2.9,13 By using time-resolved luminescence line-narrowing experiments, we have shown that stochastic excitation energy transfer ()interligand “electron” transfer) between equivalent ligands occurs as slowly as ≈1 × 108 s-1 in level II at 1.8 K. In contrast, the lowest-excited states of the [Os(bpy)3]2+ chromophore in well-defined crystalline hosts with narrow inhomogeneous distributions (
