Ruthenium(II) MLCT Excited States. Stabilization toward Ligand Loss in Rigid Media

Abstract

Photosubstitution and/or photoisomerization in cis-[Ru(bpy)2(py)2](PF6)2 and trans-[Ru(bpy)2(4-Etpy)2](PF6)2 or the molecular assemblies cis-[Ru(bpy)2(MQ+)2](PF6)4 and trans-[Ru(bpy)2(MQ+)2](PF6)4 are greatly inhibited in the rigid matrixes poly(methyl methacrylate) (PMMA) films and silica sol−gel monoliths. There can also be profound changes in photophysical properties as illustrated below for emission from cis-[Ru(bpy)2(py)2](PF6)2 in CH3CN and in the silica sol−gel.

X-ray transient absorption structural characterization of the 3MLCT triplet excited state of cis-[Ru(bpy)2(py)2]2+

“…1). [27][28][29]33 Consistently, previous computational studies highlight that the 3 MC state is likely to be the lowest-lying triplet excited state in cis-[Ru(bpy) 2 ( py) 2 ] 2+ and its population occurs through a 3 MLCT-3 MC potential energy surface crossing. Density Functional Theory (DFT) shows that 3 MC states have dissociative character due to the population of σ-antibonding orbitals with a high metal d-orbital component.…”

“…Moreover k −2 << k 2 , resulting in irreversible population of the 3 MC state from the 3 MLCT state. [27][28][29]33 According to such a scenario and considering the OTA time components together with a 20% photochemical yield φ, the populations of the 3 MLCT ES and PHP species, that mainly contribute to the XTA signal at the investigated time-points, can be calculated (see ESI SIC6.1 †) and are represented in Fig. 3.…”

“…It is commonly believed that short-lived 3 MC states display significantly distorted geometries and are associated with the quenching of optically-active 3 MLCT states via nonradiative decay to the ground state. [27][28][29][30][31] This latter aspect is also fundamental in many of the aforementioned applications where the lifetime of the redox-active 3 MLCT state is crucial. Temperature dependence of the 3 MLCT luminescence lifetime has been used to investigate the interconversion kinetics of the 3 MLCT and 3 MC states in several [Ru(bpy) 2 L 2 ] n+ complexes.…”

Photo-Induced Pyridine Substitution in cis-[Ru(bpy)2(py)2]Cl2: A Snapshot by Time-Resolved X-ray Solution Scattering

“…11,47 (For more details regarding the most relevant paths contributing to the EXAFS signal, see Supporting Information). 59 The lack of emission in aqueous solution of the complex is due to a low-lying 3 MC (metal-centered) or 3 LF (ligand-field) state, which deactivates the 3 MLCT state and is also responsible for the py dissociation. 10 The relatively long laser pulse was used to enhance the fraction of excited molecules thus increasing the difference signal.…”

Long-range photoinduced electron transfer dynamics in rigid media

“…The medium and the transition between fluid and rigid/semi-rigid states in polymer films, [1][2][3][4][5] sol-gels, [6][7][8][9][10] glasses [11][12][13][14][15][16] and other media, 17,18 on excited-state properties and energy and electron transfer dynamics is a key element in potential device applications at interfaces and in films. Rigid medium effects have been documented for electron and energy transfer in biological membranes [19][20][21] and in thin films 22,23 and these media have been used to evaluate the distance dependence of electron/ energy transfer in frozen solutions and glasses.…”