Radiationless transitions between excited singlet states of biacetyl

Abstract

AbstractEmission processes from lower excited states S1 (fluorescence) and T1 (phosphorescence) have been studied in the gas and liquid phases when biacetyl is excited into the second singlet state S2. (In agreement with Kasha's rule no fluorescence is observed from the S2 state.)In the liquid phase, when biacetyl is excited into the singlet states S1 and S2, no difference is observed between these emission processes. This phenomenon certainly results from an efficient nonradiative transition between the second excited singlet state S2 and the first excited state S1 with practically no excess vibrational energy. The quantum yield of this transition is almost unity and does not depend on the nature of the solvent.In the gas phase no emission processes are observed when biacetyl is excited into the S2 state at low pressure (less than 10 mm Hg). High pressure of inert gas is necessary in order to observe these processes. As for excitation into the S1 state with vibrational energy, loss of vibrational energy through collisions occurs from the S2 state. The quantum yield of the S2 → S1 transition by excitation at 290 nm is estimated around 0.5–0.6 at 6 atm of inert gas (ethane, ethylene, or carbon dioxide).