Neighbors move electrons jointly: An ultrafast molecular movie on metal complexes in a crystal

Sticks and balls model of the transition metal complex iron(II)-tris-bipyridine [Fe(bpy)3]2+. Iron-atoms (Fe) are brown, nitrogen (N) blue, carbon grey, and hydrogen (H) white. The six nitrogen atoms are at the corners of an octahedron around the Fe atom. The planes of the 3 bipyridine subunits (N2C10H8) are mutually perpendicular. Credit: MBI

Applying femtosecond X-ray methods, researchers at the Max-Born-Institute in Berlin (Germany) and the Ecole Polytechnique Federale de Lausanne (Switzerland) observed an extremely fast, collective electron transfer of ~100 molecular ions after excitation of a single electron in a crystal of transition metal complexes.

Photochemistry and molecular photovoltaics make frequent use of so-called transition metal complexes which consist of a central metal ion bonded to a group of surrounding ligands. Such materials display a strong absorption of ultraviolet or visible light, making them attractive as primary light absorbers in molecular solar cells and other devices of molecular optoelectronics.

The counterions in our crystal are two hexa-fluoro-phosphate (PF6-) molecular subunits [phosphorus (P), fluorine (F)]. Again, the six F atoms are at the corners of an octahedron around the P atom. We show here a 3-dimensional surface of constant electron density. The electron density was chosen in such a way that we are most sensitive to the motion of electronic charge located at the PF6- anion. In the movie (www.mbi-berlin.de) we observe upon photo excitation a pronounced reduction of the electron density on that PF6- anion, i.e. a shrinkage of the iso-electron density surface. Credit: MBI

Absorption of light is followed by an extremely fast shift of electrons from the metal ion to the ligands, a mechanism that is essential for generating an electric voltage. All applications rely on solid state materials in which transition metal complexes are densely packed and can interact with each other. So far, the influence of this interaction on the very fast electron motions following the absorption of light has remained unclear.