Ultrafast Solvent-Induced Charge Localization in Tris-(2,2′ -Bipyridine) Ruthenium(II)

Abstract

Abstract. Two distinct excited state processes are resolved in tris- [2,2'-bipyridine] ruthenium(II). Time resolved anisotropy measurements reveal a solvent dependent depolarization indicative of charge localization. Transient absorption measurements resolve wavepacket motion on the excited state surface(s) independent of solvent.Tris-(2,2'-bipyridine)ruthenium (II) or [Ru(bpy) 3 ] 2+ is the prototype to study the photoinduced properties of transition metal charge transfer complexes. [Ru(bpy) 3 ] 2+ has been extensively studied and much is known about the properties of its lowest-energy metal-to-ligand charge transfer (MLCT) state [1]. The electronic properties of this MLCT state have been found to depend on the molecule's environment. For example, evidence has been found in solid matrices for the photoexcited electron to be delocalized among the ligands in the MLCT state [2,3]. In solution, however, Raman experiments [4,5] observe an excited state with charge localized on a single ligand. Previous work from this lab focused on time-resolving the evolution of the Franck-Condon state to the 3 MLCT excited state using transient absorption pump-probe measurements [6]. The excited state absorption spectrum for ∆t ≥ 300 fs matched spectra acquired on the nanosecond time scale. This was an indication that the formation of the 3 MLCT state occurs with a time constant of τ ~ 100 fs. Although this study defined the overall time scale for excited-state formation, it left unanswered a number of important, fundamental questions concerning the nature of the initial photoexcited state, as well as the role of the solvent in the evolution of the charge-transfer state. We have addressed both of these issues by examining the time dependence of the anisotropic absorption of the molecule.The point group of [Ru(bpy) 3 ] 2+ is D 3 . The principal axis of symmetry is three-fold orthonormal to the plane bisecting all three bipyridine ligands. The ligands themselves are each bisected by a C 2 axis thereby rendering all three bipyridine ligands indistinguishable in the ground state. Upon photoexcitation, an electron is promoted from the metal to the ligand(s). In the case of initial delocalization of the electron among the ligands, the transition moment will be doubly degenerate with (x,y) polarization and each ligand will remain indistinguishable from each other. However, if the electron is localized to a single bipyridine ligand, the degeneracy of the transition moment is reduced with the reduction in molecular symmetry from D 3 to C 2 . This situation can be realized upon photoexcitation or may occur as the result of excited-state

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