Abstract
Steady-state and time-resolved emission spectroscopy are valuable tools to probe photochemical processes of metal-ligand, coordination complexes. Ru(II) polyazine light absorbers are efficient light harvesters absorbing in the UV and visible with emissive 3MLCT excited states known to undergo excited state energy and electron transfer. Changes in emission intensity, energy or band-shape, as well as excited state lifetime, provide insight into excited state dynamics. Photophysical processes such as intramolecular electron transfer between electron donor and electron acceptor sub-units may be investigated using these methods. This review investigates the use of steady-state and time-resolved emission spectroscopy to measure excited state intramolecular electron transfer in polyazine bridged Ru(II),Rh(III) supramolecular complexes. Intramolecular electron transfer in these systems provides for conversion of the emissive 3MLCT (metal-to-ligand charge transfer) excited state to a non-emissive, but potentially photoreactive, 3MMCT (metal-to-metal charge transfer) excited state. The details of the photophysics of Ru(II),Rh(III) and Ru(II),Rh(III),Ru(II) systems as probed by steady-state and time-resolved emission spectroscopy will be highlighted.
Highlights
Supramolecular complexes composed of multiple metal centers capable of light and/or redox induced processes are of interest in designing molecular machines [1]
Steady-state emission spectroscopy shows a substantial decrease in the quantum yield of emission (Φem in the supramolecular assembly compared with the respective model complexes (Φ0em ) at room temperature when intramolecular electron transfer occurs, equations 11 and 12
Room temperature emission spectroscopic studies display significant quenching of the emissive 3MLCT excited states with respect to the corresponding model systems, allowing determination of rates of intramolecular electron transfer. This requires that the rate of intramolecular electron transfer is competitive with the rate of the radiative and nonradiative decay pathways of these emissive 3MLCT excited states
Summary
Supramolecular complexes composed of multiple metal centers capable of light and/or redox induced processes are of interest in designing molecular machines [1] In this sense, supramolecular complexes which couple multiple molecular components whose individual properties provide a unique function to the supramolecule are of wide interest [2]. Transitions that are emissive in the solid state and/or solution at room temperature have been widely explored [1,2,4,6,7] The coupling of these MLCT light absorbers to other units provides a means of deactivating the emissive 3MLCT excited states harvesting the stored energy which include intermolecular (i.e., bimolecular deactivation) or intramolecular (i.e., unimolecular decay) pathways. The coordination environment of each metal can modulate orbital energetics impacting the driving force for intramolecular electron transfer
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