Electronic-Relaxation Processes in Acetone

Abstract

The emission properties of acetone and acetone‐d₆ have been studied in order to investigate the mechanisms for relaxation of electronic excitation. The results are summarized in Fig. 2 and Table I. The phosphorescence lifetime of acetone at 77°K in ether—IPA glass was 4×10⁻⁴ sec and the quantum yield for phosphorescence was Φ_P=0.03±0.01. Deuteration of the acetone increased the phosphorescence lifetime to 1×10⁻³ sec and the quantum yield to Φ_P=0.08±0.02. The phosphorescence of acetone was found to be quenched by the addition of pentene−2 to the rigid solution, but the fluorescence was unaffected. From the quenching data it is concluded that the excited‐singlet→excited‐triplet intersystem‐crossing quantum yield is Φ_IC=1.0±0.1, independent of temperature between 298° and 77°K. The intersystem‐crossing rate constant is k_IC=4×10⁷ sec⁻¹ and is not affected by deuteration. The excited triplet state is populated with nearly unit efficiency upon excitation of acetone to its lowest singlet state, and therefore the low phosphorescence yield, even in acetone‐d₆, indicates that radiationless transitions from the triplet state to the ground state are primarily responsible for determining the lifetime of the lowest triplet state. A medium effect on the acetone phosphorescence lifetime at 77°K was observed; it was found to vary from 3×10⁻⁴ in glycerine solutions to 1×10⁻³ sec in crystalline acetone. The triplet‐state lifetime in crystalline acetone decreased only slightly upon increasing the temperature to just below the melting point of crystalline acetone, but in the case of glycerine solutions, the triplet‐state lifetime decreased by at least a factor of 10 in raising the temperature from 77° to 200°K, even though the glycerine remained quite solid. The fluorescence quantum yield in liquid solutions was found to be independent of solvent at 25°C. The lifetime of the lowest (n, π^*) singlet state of acetone is primarily determined by intersystem crossing to the lower‐lying excited triplet state and is therefore τ_S=2.5×10⁻⁸ sec. The quantum yield for acetone fluorescence at room temperature was found to be Φ_F=0.01±0.003. No deuterium effect on Φ_F was observed and therefore it may be concluded that either there is no deuterium effect on the rate of internal conversion from the excited singlet state to the ground state, or, what is more likely, that internal conversion is much slower than intersystem crossing which is not affected by deuteration. The fluorescence spectra of acetone and acetone‐d₆ were found to be identical under all conditions, as were the phosphorescence spectra of acetone and acetone‐d₆. The phosphorescence was found to be polarized perpendicular to the lowest singlet—singlet absorption, consistent with theoretical predictions.