Rigid Medium Stabilization of Metal-to-Ligand Charge Transfer Excited States

• Published in: The journal of physical chemistry. B 2007-06, Vol.111 (24), p.6930-6941
• Main Authors: Thompson, David W, Fleming, Cavan N, Myron, Brent D, Meyer, Thomas J
• Format: Article
• Language: English
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp068682l

The salts [Ru(bpy)3](PF6)2, cis-[Ru(bpy)2(py)2](PF6)2, trans-[Ru(bpy)2(4-Etpy)2](PF6)2, [Ru(tpy)2](PF6)2, and [Re(bpy)(CO)3(4-Etpy)](PF6) (bpy = 2,2‘-bipyridine, py = pyridine, 4-Etpy = 4-ethylpyridine, and tpy = 2,2‘:6‘,2-terpyridine) have been incorporated into poly(methyl methacrylate) (PMMA) films and their photophysical properties examined by both steady-state and time-resolved absorption and emission measurements. Excited-state lifetimes for the metal salts incorporated in PMMA are longer and emission energies enhanced due to a rigid medium effect when compared to fluid CH3CN solution. In PMMA part of the fluid medium reorganization energy, λ oo, contributes to the energy gap with λ oo ∼ 700 cm-1 for [Ru(bpy)3](PF6)2 from emission measurements. Enhanced lifetimes can be explained by the energy gap law and the influence of the excited-to-ground state energy gap, E o, on nonradiative decay. From the results of emission spectral fitting on [Ru(bpy)3](PF6)2* in PMMA, E o is temperature dependent above 200 K with ∂E o/∂T = 2.8 cm-1/deg. cis-[Ru(bpy)2(py)2](PF6)2 and trans-[Ru(bpy)2(4-Etpy)2](PF6)2 are nonemissive in CH3CN and undergo photochemical ligand loss. Both emit in PMMA and are stable toward ligand loss even for extended photolysis periods. The lifetime of cis-[Ru(bpy)2(py)2](PF6)2* in PMMA is temperature dependent, consistent with a contribution to excited-state decay from thermal population and decay through a low-lying dd state or states. At temperatures above 190 K, coinciding with the onset of the temperature dependence of E o for [Ru(bpy)3](PF6)2*, lifetimes become significantly nonexponential. The nonexponential behavior is attributed to dynamic coupling between MLCT and dd states, with the lifetime of the latter greatly enhanced in PMMA with τ ∼ 3 ns. On the basis of these data and data in 4:1 (v/v) EtOH/MeOH, the energy gap between the MLCT and dd states is decreased by ∼700 cm-1 in PMMA with the dd state at higher energy by ΔH 0 ∼ 1000 cm-1. The “rigid medium stabilization effect” for cis-[Ru(bpy)2(py)2](PF6)2* in PMMA is attributed to inhibition of metal−ligand bond breaking and a photochemical cage effect.