Ultrafast Electron Localization Dynamics Following Photo-Induced Charge Transfer

Abstract

Molecular dynamics occurring in the earliest stages following photo-induced charge transfer were investigated. Femtosecond time-resolved absorption anisotropy measurements on [Ru(bpy) 3 ] 2ϩ , where bpy is 2,2Ј-bipyridine, reveal a time dependence in nitrile solutions attributed to initial delocalization of the excited state over all three ligands followed by charge localization onto a single ligand. The localization process is proposed to be coupled to nondiffusive solvation dynamics. In contrast, measurements sampling population dynamics show spectral evolution associated with wave packet motion on the excited state surface that is independent of solvent. The results therefore reveal two important contributions to the evolution of charge transfer states in condensed phase, one that is strongly coupled to the surrounding environment and another that follows a potential internal to the molecule.Photo-induced charge transfer processes are central to a wide range of important physical and chemical phenomena. Perhaps best known is the photo-induced charge separation that occurs in photosynthetic reaction centers, yielding a transmembrane potential gradient that ultimately drives the production of adenosine triphosphate (1). In materials science, light absorption by wide band gap semiconductors results in the formation of conduction band electrons that are the basis for photoelectron conversion in photovoltaic devices (2). The surrounding medium can be an important factor in directing the course of chemical dynamics in condensed phase. When the dynamics of the chemical process in question are slow relative to the response of the medium, a quasi-steady-state condition exists between the chromophore and its environment, and the overall effect of the solvent is largely thermodynamic. Recently, there has been considerable interest both experimentally and theoretically in the dynamics of solvation, that is, when the chemistry and medium response to that chemistry occur on comparable time scales (3)(4)(5)(6)(7)(8). Under these circumstances, the role of solvent dynamics in the kinetics and mechanism of excited-state evolution can be profound. Transition metal complexes in particular can exhibit charge transfer transitions between the metal center and ligand(s) bound to that metal, a process that necessarily involves charge redistribution on the periphery of the chromophore. Because the ligands are interacting with the solvent, such compounds provide a convenient platform for examining how solvation dynamics couple to photo-induced charge transfer events.Tris-(2,2Ј-bipyridine)ruthenium(II), abbre-, is the prototype for a large class of chromophores whose study has formed the basis for most of the transition metal-based solar energy conversion schemes of the past two decades (9, 10). [Ru(bpy) 3 ] 2ϩ has been extensively studied, and much is known about the properties of its lowest energy triplet metal-to-ligand charge transfer ( 3 MLCT) excited state. The quantum yield for the formation of this state is near unity (11,12)...

Photo-Induced Spin-State Conversion in Solvated Transition Metal Complexes Probed via Time-Resolved Soft X-ray Spectroscopy

“…In the sub-picosecond domain where the formation of the 5 T 2 state occurs, 34 inertial dynamics are dominant (the inertial response time for the solvent CH 3 CN is ~60 fs). 56,57 While such sub-picosecond dynamics have been shown to influence the excited-state evolution of chargetransfer states, 58 the ~50 ps resolution of our experiment infers that there should be negligible contribution from such processes to the results we have described herein.…”

Transient Absorption Spectroscopy of Ruthenium and Osmium Polypyridyl Complexes Adsorbed onto Nanocrystalline TiO2 Photoelectrodes

“…The excitedstate relaxation processes of transition metal polypyridyl complexes include vibrational relaxation (VR), internal conversion (IC), and intersystem crossing (ISC). These dynamics occur on an ultrafast time scale 33,34 and compete with electron injection of metal polypyridyl complexes adsorbed onto TiO 2 , 35 as depicted in Scheme 2. Femtosecond transient absorption difference spectra (∆A spectra) were therefore obtained in the Figure 1.…”

Investigations of excitation energy transfer and intramolecular interactions in a nitrogen corded distrylbenzene dendrimer system

“…51,[53][54][55][56] Using fluorescence depolarization it is possible to observe this intramolecular excitation transfer because the latter process is accompanied by the reorientation of the transition dipole resulting in depolarization of the emission. Fast decay of fluorescence anisotropy ͑few picoseconds͒ in an amino-substituted triphenylbenzene derivative, ͑p-EFTP͒, was detected by femtosecond polarized transient absorption.…”