Charge-Transfer Photochemistry of Aromatic ?-Complexes. Hexamethylbenzene and Mercuric Trifluoroacetate

Article abstract of DOI:10.1021/jo00360a029

The ?-complex of hexamethylbenzene (HMB) and mercuric trifluoroacetate (HgT2) is converted with high quantum yields to pentamethylbenzyl trifluoroacetate and either mercurous trifluoroacetate or metallic mercury upon irradiation of the charge-transfer absorption band.X-ray crystallography establishes the charge-transfer (CT) excitation to derive from the η²-bonding of the mercuric atom to only two of the aromatic carbons of HMB.The CT photochemistry proceeds via the radical-ion pair in accord with Mulliken theory.The high quantum efficiency is ascribed to the rapid ligand dissociation of HgT2 anion radical which minimizes the energy wastage due to back electron transfer.The thermal reactions following photoactivation are monitored by following the changes in the CT absorption band and the ESR spectrum of HMB cation radical.The charge-transfer photochemistry of the complex shows an unusually high dependency on the solvent-the nature of the mercury-containing products and the quantum yield Φ_P for pentamethylbenzyl trifluoroacetate in dichloromethane being distinctively different from that obtained in trifluoroacetic acid.The formation of metallic mercury with quantum yields in trifluoroacetic acid which can be as high as 3 arises from a radical-chain process which sustains itself after the light is turned off.The critical role played by the mercurous trifluoroacetate radical HgO2CCF3 in the CT photochemistry is described.