Photochemistry of <i>fac</i>‐[Re(bpy)(CO)₃Cl]

<jats:title>Abstract</jats:title><jats:p>The photochemistry of <jats:italic>fac</jats:italic>‐[Re(bpy)(CO)<jats:sub>3</jats:sub>Cl] (<jats:bold>1 a</jats:bold>; bpy=2,2′‐bipyridine) initiated by irradiation using <330 nm light has been investigated. Isomerization proceeded in THF to give the corresponding <jats:italic>mer‐</jats:italic>isomer <jats:bold>1 b</jats:bold>. However, in the presence of a small amount of MeCN, the main product was the CO‐ligand‐substituted complex (<jats:italic>OC</jats:italic>‐6‐24)‐[Re(bpy)(CO)<jats:sub>2</jats:sub>Cl(MeCN)] (<jats:bold>2 c</jats:bold>; bpy=2,2′‐bipyridine). In MeCN, two isomers, <jats:bold>2 c</jats:bold> and its (<jats:italic>OC</jats:italic>‐6‐34) form (<jats:bold>2 a</jats:bold>), were produced. Only <jats:bold>2 c</jats:bold> thermally isomerized to produce the (<jats:italic>OC</jats:italic>‐6‐44) form <jats:bold>2 b</jats:bold>. A detailed investigation led to the conclusion that both <jats:bold>1 b</jats:bold> and <jats:bold>2 c</jats:bold> are produced by a dissociative mechanism, whereas <jats:bold>2 a</jats:bold> forms by an associative mechanism. A comparison of the ultrafast transient UV‐visible absorption, emission, and IR spectra of <jats:bold>1 a</jats:bold> acquired by excitation using higher‐energy light (e.g., 270 nm) and lower‐energy light (e.g., 400 nm) gave detailed information about the excited states, intermediates, and kinetics of the photochemical reactions and photophysical processes of <jats:bold>1 a</jats:bold>. Irradiation of <jats:bold>1 a</jats:bold> using the higher‐energy light resulted in the generation of the higher singlet excited state with <jats:italic>τ</jats:italic>≤25 fs, from which intersystem crossing proceeded to give the higher triplet state (<jats:sup>3</jats:sup>HES(<jats:bold>1</jats:bold>)). In THF, <jats:sup>3</jats:sup>HES(<jats:bold>1</jats:bold>) was competitively converted to both the triplet ligand field (<jats:sup>3</jats:sup>LF) and metal‐to‐ligand charge transfer (<jats:sup>3</jats:sup><jats:sc>M</jats:sc>LCT) with lifetimes of 200 fs, in which the former is a reactive state that converts to [Re(bpy)(CO)<jats:sub>2</jats:sub>Cl(thf)]<jats:sup>+</jats:sup> (<jats:bold>1 c</jats:bold>) within 10 ps by means of a dissociative mechanism. Re‐coordination of CO to <jats:bold>1 c</jats:bold> gives both <jats:bold>1 a</jats:bold> and <jats:bold>1 b</jats:bold>. In MeCN, irradiation of <jats:bold>1 a</jats:bold> by using high‐energy light gives the coordinatively unsaturated complex, which rapidly converted to <jats:bold>2 c</jats:bold>. A seven‐coordinate complex is also produced within several hundred femtoseconds, which is converted to <jats:bold>2 a</jats:bold> within several hundred picoseconds.</jats:p>