Photochemistry of (η6 -Arene)Cr(CO)3 (Arene = Methylbenzoate, Naphthalene, or Phenanthrene) in n-Heptane Solution: Population of Two Excited States Following 400 nm Excitation As Detected by Picosecond Time-Resolved Infrared Spectroscopy

The photochemistry of (η6-methylbenzoate)Cr(CO)3, (η6 - naphthalene)Cr(CO)3, and (η6-phenanthrene)Cr(CO)3 in n-heptane solution was investigated by picosecond time-resolved infrared spectroscopy (TRIR). The observation of two transient IR features in the organic carbonyl region at 1681 and 1724 cm-1 following 400 nm excitation of (η6-methylbenzoate)Cr(CO)3 confirms formation of two excited states which are classified as metal-to-arene charge transfer (MACT) and metal-to-CO charge transfer (MCCT), respectively. Time-dependent density functional theory calculations have been used to support these assignments. Population of the MCCT excited state results in a slow (150 ps) expulsion of one CO ligand. Excitation of (η6-naphthalene)Cr(CO)3 or (η6-phenanthrene)Cr(CO)3 at either 400 or 345 nm produced two excited states: the MCCT state results in CO loss, while the MACT excited state results in a change to the coordination mode of the polyaromatic ligands before relaxing to the parent complex. A comparison of the infrared absorptions observed following the population of the MACT excited state with those calculated for nonplanar polyaromatic intermediates provides a model for the reduced hapticity species. [ABSTRACT FROM AUTHOR]