All three Ru(II) complexes in Fig. 7 have strong absorption bands (e ~ 15,000 - 20,000 M-1cm-1) in the visible region from 450 to 480 nm which are assigned to Ru dn ^ bpy (n*) MLCT transitions. As described previously, [Ru(bpy)3]2+* refers to the thermally equilibrated 3MLCT excited-state which is the state responsible for luminescence. This 3MLCT state is converted from the initial 1MLCT state (often denoted [Ru(bpy)3]2+**) with near unit efficiency in less than 300 fs.99,100 The 3MLCT state decays by both luminescent and non-radiative decay pathways. As shown in Table 1, emission quantum yields (^em) in MeCN at room temperature are on the order of 0.03 to and 0.07 for [Ru(phen)3]2+ and [Ru(bpy)3]2+, respectively,54 showing that non-radiative decay processes dominate this temperature. A survey of tris-diimine Ru(II) compounds reveals that such yields for ^em are commonly between 0.10 and 0.001 however exceptions as high as 0.40 and as low as 5x10-6 are known.54 One major non-radiative decay pathway for [Ru(bpy)3]2+ is via thermal population of ligand field (d-d) states which are approximately 3000 cm-1 (0.37 eV) higher in energy that the 3MLCT state.101 At low temperature, this pathway is attenuated and the complexes often show dramatically longer luminescent life times. Nonetheless, these d-d states are sufficiently high in energy that at room temperature that the lifetimes (t) of the 3MLCT state range from 400 to 1200 ns for many simple Rubpy analogues. Furthermore, because the decay pathways are well understood, several strategies can be employed to considerably lengthen the excited-state lifetimes.
For example, the excited-state lifetime of [Ru(terpy)2]2+ is less than 250 ps at room temperature102 however, synthetic modifications of the terpy ligand have lead to bis-terpy complexes with lifetimes as long as 200 ns.103,104 As with the trisdiimine complexes, the non-radiative decay for [Ru(terpy)2]2+ is attributed thermal population of low lying d-d states. However, in this case the tridentate terpy ligand causes a larger distortion of the Oh ligand field which further lowers the energy of some d-d states to energies close to or below those of the 3MLCT state. At room temperature, thermal population of these d-d states provides a rapid, non-radiative decay pathway to the ground state. At low temperature (77 K), thermal population of the d-d states is lessened and vibronic coupling between the 3MLCT and d-d states can be frozen out to an appreciable degree, leading to excited state lifetimes on the order of 10 |s.105
As might be expected, simply lowering the energy of the 3MLCT state relative to the d-d states results in longer excited state lifetimes. Balzani and coworkers found that modifying the terpy ligands with peripheral electron withdrawing methylsulfo-nate groups, lengthened the room temperature lifetime of the complex to 25 ns.105 Similarly, the cyano-derivatized terpy complex 1 has a t = 75 ns at 298 K, however, there is a limit to this approach.106 The lifetime for the homoleptic complex 2 is only 50 ns indicating that if the 3MLCT is lowered too much, direct non-radiative decay to the GS from the 3MLCT state can become an important factor, as expected from the energy gap law.107 Hanan and coworkers showed that stabilizing the LUMO of the terpy ligand by extending the planar aromatic system also extends the luminescent lifetime.103,104 For the heteroleptic [Ru(terpy)(terpy-cyano pyrimidine)]2+ complex 3, shown in Fig. 7, t is 200 ns at 298 K which is attributed to a combination of the extended planar aromatic system and the EWG cyano substituent.
In general, when the excited-state is sufficiently long-lived (typically > 1 ns), bi-molecular reactions in which the excited state acts as a reactant become possible. Typical reactions for quenching the excited state of Rubpy occur via one of the three following mechanisms. Schemes showing reductive and oxidative electron-transfer were shown in Fig. 6. Reaction 18 shows a third quenching mechanism known as energy transfer whereby relaxation of the 3MLCT to the ground state is coupled to
formation of an excited state in the quencher. While other quenching mechanisms are known these three are the dominant ones observed with Rubpy complexes.
Literally, hundreds of inorganic and organic compounds have been examined as quenchers for Ru2+* and its analogues. An extensive list of quenchers for excited state metal complexes (including Rubpy complexes), their rate constants (kq) and mechanism of quenching (reductive, oxidative or energy transfer) was compiled by Hoffman and coworkers.108 Quenching rate constants (kq) are typically obtained via a Stern-Volmer analysis.109 Typical rate constants for reductive and oxidative quenching of photoexcited Rubpy complexes usually range from 105 Lmol-1s-1 to the diffusion limit of ~1010 Lmol-1s-1. Energy transfer rate constants typically fall between 107 L mol-1s-1 and the diffusion limit. Some of the more commonly used reductive quenchers are organic amines (triethylamine (TEA), triethanolamine (TEOA), dime-thylaniline (DMA)), organics (phenathiazine, phenols, hydro-quinones), coordination complexes ([Fe(CN)6]4-, [Co(bpy)3]2+), and simple inorganic ions (I-, Eu2+, S2O62-). Commonly oxidative quenchers include organics, such as methylviologen (MV2+), quinones, and nitrobenzene, coordination complexes ([Co(NH3)5Cl]2+, Cr(acac)3) and simple inorganic ions (S2O82-, Cu2+) and molecules (O2). A number of donors (e.g., trialkylamines) and acceptors (e.g., [Co(NH3)5Cl]2+ and S2O82-) are known to rapidly decompose upon electron-transfer which thereby prevents any back-reaction between quencher and Rubpy photoproduct.110-112 These donors and acceptors are considered sacrificial and are useful when one desires to trap the reduced [Ru(bpy)3]1+ or oxidized [Ru(bpy)3]3+ photoproduct.
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