Effects of polar substituents on photoreduction and quenching of aromatic ketones by amines. Fluorenone and substituted dimethylanilines

Photoreduction of fluorenone by dimethylaniline in benzene leads to the 9-hydroxy-9-fluorenyl radical I and the N-methylanilinomethyl radical II, addition of II to the oxygen of ground state fluorenone forming radical III, and combination and cross combination of radicals I and III. Hydrolysis of these products leads to fluorenone pinacol, 97 percent yield, and to high yields of N-methylaniline and formaldehyde. Quantum yields for the photoreduction are at a maximum, approximately 0.8, with the unsubstituted dimethylaniline and with weak electron donating or withdrawing substituents. Quantum yields decrease with strong electron donating or withdrawing substituents. Very strong electron donation may lead to no photoreduction but to efficient quenching, presumably due to excessive stability of the initially formed charge-transfer complex. Values of k/sub ir/, rate constant for charge-transfer interaction of excited fluorenone triplet with the substituted dimethylanilines, have been measured in quenching studies. They vary from 3.2 x 10/sup 6/ M/sup -1/ sec/sup -1/ for the p-CN compound to approximately 10/sup 10/ M/sup -1/ sec/sup -1/ for the p-OEt and p-N(CH/sub 3/)/sub 2/ compounds and lead to a linear plot of log k/sub ir/ vs. sigma values, sigma/sup +/ for the strong electron donating substituents, sigma/sup -/ for p-CN, with rho = -1.96. Photoreductions of Methylene Blue by substituted phenylglycines, of benzophenone by substituted dimethylanilines and benzylamines, and of p-aminobenzophenone by dimethylanilines are discussed.