Charge-Transfer Photochemistry from the Donor-Acceptor Complexes of Anthracenes with Tetranitromethane

Article abstract of DOI:10.1021/jo00225a050

The formation of electron donor-acceptor or EDA complexes of a series of 9-substituted and 9,10-disubstituted anthracenes with tetranitromethane (TNM) can be visually observed in dichloromethane solutions.The colors arise from charge-transfer (CT) transitions from the aromatic donor (An) to the tetranitromethane acceptor.Specific irradiation of the CT absorption band at λ > 500 nm leads to the rapid bleaching of the solution, and high yields of photoadducts 1 can be isolated especially from those anthracenes bearing electron-withdrawing substituents.X-ray crystallography establishes the photoadducts 1 to derive from an overall anti addition of the TNM fragments, C(NO2)3 and NO2, across the meso positions of 9-bromo- and 9-phenylanthracene.The high quantum yields are associated with the efficient dissociative electron transfer accompanying the charge-transfer excitation of the 1:1 EDA complex.The subsequent multistep pathway by which the geminate species -, NO2> combine regiospecifically and stereospecifically to form the photoadduct 1 is described.The structural elucidation of the byproducts underscores the efficiency of ion-pair and radical-pair interactions following the CT excitation of EDA complexes.