Ultrafast excited state dynamics of tris-(2,2'-bipyridine) ruthnium(II).

摘要

Time resolved anisotropy measurements and time dependent transient absorption measurements are used to study the evolution of the photoexcited Franck-Condon state to the formation of the long-lived triplet metal-to-ligand charge-transfer (3MLCT) state in tris-(2,2′-bipyridine) ruthenium(II). [Ru(bpy)₃]2+ represents a large class of inorganic compounds with interesting and potentially applicable photophysical properties. These compounds have generated much interest in the inorganic chemistry community because their photophysical properties are easily manipulated by synthetic chemistry methods. However, little remains known about the processes which govern the evolution from initial photoexcitation to the formation of the long-lived excited state.; Metal to ligand charge transfer, when used to describe inorganic compounds, is a description of how the compound reacts to the absorption of light. Typically, these inorganic compounds are made of a transition metal with organic ligands, consisting of carbon, nitrogen and oxygen, coordinated in a highly symmetrical manner to the metal center. The highest occupied molecular orbitals (HOMO) in these compounds are isoenergetic with the frontier atomic orbitals of the metal and are localized to the metal center. The lowest unoccupied molecular orbitals (LUMO) are isoenergetic with the coordinating ligand orbitals and are localized to the ligands. Upon absorption of light in which an electron is promoted from the HOMO to the LUMO, the electron is excited from the metal to the ligands.; Time dependent transient absorption measurements resolve the evolution of the initially excited state to the formation of the 3MLCT state. These measurements also reveal ultrafast dynamics which are characterized as intramolecular in nature and are associated with non-radiative relaxation processes. Symmetry argues the excited state should be delocalized among the bipyridine ligands. Rapid, solvent dependent depolarization of femtosecond anisotropy measurements indicate a change in symmetry of the excited state from a doubly degenerate delocalized state to a singly degenerate localized state. The anisotropy measurements also reveal that localization is facilitated through interactions with the solvent environment.; Two distinct processes have been resolved in the evolution of the [Ru(bpy) ₃]2+ Franck-Condon state to the formation of the 3MLCT state. Charge localization and ultrafast dynamics associated with non-radiative relaxation pathways are distinguished by their susceptibility to intermolecular interactions. These experiments represent the first measurements resolving these processes in the [Ru(bpy)₃]2+ metal to ligand charge transfer complex.