The fluorescence spectrum originating from the second excited singlet stateS2of pyrene in the vapor phase at 170deg;C and 0.21 mm Hg has been studied with particular attention to changes of emission characteristics with the excitation energy. The fluorescence from the first excited singlet stateS1has a comparatively constant quantum yield PHgr;1F(between 0.14 and 0.08), regardless of the excitation energy. In contrast to this, the quantum yield PHgr;2Fof the fluorescence fromS2increases rapidly with excitation into successively higher singlet states. The ratio PHgr;2F/PHgr;1Fis of the order of 0.001 for excitation intoS2, but it is larger than 0.1 for excitation into the fourth excited singlet stateS4. Addition of cyclohexane as an inert foreign gas to the pyrene vapor results in a marked decrease in PHgr;2F/PHgr;1F, which seems to reach a constant value at high pressures of cyclohexane (rang;60mm Hg). From an analysis of the experimental data and a consideration of the kinetics of radiative and radiationless processes in a collisionhyphen;free molecule, the rate constant of the internal conversionS2*rarr;S1*, k21(the asterisk refers to excited vibrational states), and that of the reverse processS2*larr;S1*, k12, are estimated to be5.8times;1011secminus;1and3.2times;108thinsp;secminus;1, respectively, for excitation into theS4state. The ratiok12/k21is shown to be related to the ratio between the densities of vibrational states for the vibronic statesS2*andS1*. Most of the anomalousS2hyphen;fluorescence emission occurs through the mechanism involving the reverse internal conversion process. This mechanism is expected to be particularly effective in the case of the pyrene molecule, because it possesses the following favorable properties: (1) the rate constant for the radiative transitionS0larr;S2fromS2to the ground state is much greater than that for the radiative transitionS0larr;S1; (2) theS2state is located close to theS1.
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