Photophysics of triphenylphosphines and their oxides: Role of dimethylamino substituents

Steady-state and time-resolved spectroscopy experiments with picosecond or subpicosecond resolution are carried out in order to clarify the anomalous fluorescence properties of solutions prepared with triphenylphosphine compounds in various non-deaerated solvents at room temperature. Evidence is given for the formation of triphenylphosphine oxides in the solutions and for the coexistence of the oxidized and non-oxidized forms at the concentrations used for time-resolved experiments. The photoinduced formation of an emissive charge-transfer state, previously reported for solutions prepared with the dimethylamino substituted triphenylphosphines, is shown to be due to the corresponding oxide which exhibits a dual fluorescence with large solvatochromism. The non-oxidized forms of both the dimethylamino substituted and unsubstituted triphenylphosphines are found to exhibit a strongly red-shifted emission band with little solvatochromism. A photoinduced geometrical change with an increase of the conjugation in the excited state is proposed to explain this observation. On the other hand, an extra fluorescence band occuring at shorter wavelengths for the non-oxidized dimethylamino-substituted compound is not completely understood.