The development of rechargeable solar thermal batteries containing photochromic molecules capable of reversible photoisomerization is receiving a great deal of attention. One of such molecules is a fulvalene diruthenium complex, FvRu₂(CO)₄, developed at Prof. P. Vollhardt’s group in UC Berkeley, which could capture ~20.8 kcal/mol of photon energy through photo-isomerization and reversibly release it in the form of thermal energy when needed.

To understand the transition intermediate state during such process is particularly important in facilitating the design and synthesis of more efficient molecular systems.

Through a collaboration between research groups at Argonne, Northwestern University, Berkeley, MIT, University of North Carolina and Chalmers University, a team of scientists successfully elucidated the mechanism of the photostorage step in the thermally reversible photoisomerization of FvRu₂(CO)₄ through a combined ultrafast X-ray transient and IR spectroscopic, DFT computational and laboratory time scale experimental study. The complex captures sunlight, causing initial Ru–Ru bond rupture to furnish a long-lived triplet biradical of syn configuration. This species requires thermal activation to reach a crossing point (middle) into the singlet manifold on route to its thermal storage isomer (right) through the anti biradical route.

Argonne’s participants include Michael Harpham, Andy Stickrath, D.-J. Liu and Lin Chen (lead). The work is published in the most recent issue of Angewandte Chemie International Edition.

• Relevant study: ​“X-ray Transient Absorption and Picosecond IR Spectroscopy of Fulvalene(tetracarbonyl)diruthenium on Photoexcitation” Michael R. Harpham, Son C. Nguyen, Zongrui Hou, Jeffrey C. Grossman, Charles B. Harris,* Michael W. Mara, Andrew B. Stickrath, Yosuke Kanai,* Alexie M. Kolpak, Donghwa Lee, Di-Jia Liu, Justin P. Lomont, Kasper Moth-Poulsen, Nikolai Vinokurov, Lin X. Chen,* and K. Peter C. Vollhardt*  Angew. Chem. Int. Ed. 2012, 51, 1 – 6