On The Nature Of Excited States In Ruthenium Complexes: Towards Renewable Energy

The 77 K radiative properties (spectra, quantum yields and lifetimes) of ruthenium-polypyridyl complexes are investigated to better understand the effects of electronic mixing on metal-to-ligand-charge-transfer (3MLCT) excited state properties and how metal-centered (3MC) excited states affect the properties of potential ruthenium photosensitizers.The radiative rate of relaxation (kRAD) determines the maximum possible excited state lifetime when all other relaxation pathways are blocked (kn = 0 for all n RAD). Thus, the excited state will relax only by means of an emission characteristic of the polypyridyl chromophore. kRAD is expected to increase as the excited state energy increases while the value of the non-radiative decay (kNRD) should decrease. Given this relationship in the decay kinetics, the radiative rate constant can be an important factor in determining the lifetimes of high energy photosensitizers. Additionally, the formalisms used in discussions are based on Einsteinian rate constants for atomic fluorescence spectra and not that of phosphorescent donor-acceptor complexes. Other factors should be considered such as difference in spin multiplicity and molecular distortions in vibrational modes that are coupled to electronic transitions (evident from vibronic side band features in emission spectra). Density functional theory (DFT) has indicated that the excited state distortions for these systems are due to electronic mixing of Ru-bpy 3MLCT excited state with the bpy ligand and * orbitals, or alternatively from the electronic mixing between the 3MLCT and * excited states. The spectroscopic and computational results suggests that a pure diabatic 3MLCT excited state is not greatly distorted and that its emission has weak vibronic contributions in the region of bpy-ligand vibrational modes in addition to a very small radiative rate constant. Furthermore, there is no evidence that a pure 3MLCT emission has ever been observed. Additionally, some of the observed spectroscopic properties will depend on the differences in excited state molecular geometries based on the reorganizational energy for crossing between the 3MLCT and 3MC states

Origin and Detection of Microstructural Clustering in Fluids with Spatial-Range Competitive Interactions

Fluids with competing short-range attractions and long-range repulsions mimic dispersions of charge-stabilized colloids that can display equilibrium structures with intermediate range order (IRO), including particle clusters. Using simulations and analytical theory, we demonstrate how to detect cluster formation in such systems from the static structure factor and elucidate links to macrophase separation in purely attractive reference fluids. We find that clusters emerge when the thermal correlation length encoded in the IRO peak of the structure factor exceeds the characteristic lengthscale of interparticle repulsions. We also identify qualitative differences between the dynamics of systems that form amorphous versus micro-crystalline clusters.Comment: 6 pages, 5 figure