Solution photochemistry of derivatives of Michler's Ketone

Abstract

A series of analogues of 4,4-bisdimethylaniinobenzophenone, Michler's ketone, in which the terminal dimethylamino-groups have been replaced by diethylamino, pyrrolidino, piperidino, N-methylpiperazino and morpholino moieties has been prepared utilising the reaction between an aryl-lithium and a substituted benzonitrile. The aromatic ketones have been reduced to their hydrols and pinacols with sodium borohydride and aluminium amalgam, respectively.
The UV-visible spectra of the substituted benzophenones in cyclohexane and ethanol are similar to those of Michler's ketone in the same solvents. All the compounds fluoresce in solution at room temperature with quantum yields of the order of 0.001 and decay times shorter than 100 psec. At 77 K both fluorescence and phosphorescence are observed. The phosphorescence spectra usually exhibited a shoulder to the red of the emission maximum, although a second peak is occasionally observed. In methylcyclohexane, phosphorescence lifetimes of the order of 10 - 20 msec were found, which increased by a factor of ten when ethanol was used as solvent.
Photolysis of the ketones in cyclohexane resulted in their decomposition with high efficiency. The hydrol and the pinacol
were identified amongst the photolysis products in most cases. No reaction was observed when 2-propanol was used as the solvent. Microsecond flash photolysis revealed two transient species which were assigned to the excited triplet state and to the ketyl radical. The transient absorption spectra of the former were recorded. The decay kinetics of both species were found to be strongly dependent on the age of the ketone sample; different kinetics were observed for new samples which had not been flashed and for samples which had been flashed at least once. The results have been rationalised in terms of abstraction of hydrogen from both the solvent and any photolysis product by the excited triplet state. The decay of the ketyl radical was also dependent on the presence or absence of previously formed products.
A series of some twenty pyridyl analogues of Michlers ketone has also been synthesised and their UV-visible, fluorescence and phosphorescence spectra have been measured in ethanol or methylcyclohexane. Fluorescence decay times in the region of 0.5 - 2.5 nsec were recorded and 3-(4-dimethylaminopyridyl) 2-pyridyl ketone was found to fluoresce very efficiently.