Fluorescence from the Second Excited Singlet State and Radiationless Processes in Pyrene Vapor

The fluorescence spectrum originating from the second excited singlet state S2 of pyrene in the vapor phase at 170°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 state S1 has a comparatively constant quantum yield Φ1F (between 0.14 and 0.08), regardless of the excitation energy. In contrast to this, the quantum yield Φ2F of the fluorescence from S2 increases rapidly with excitation into successively higher singlet states. The ratio Φ2F/Φ1F is of the order of 0.001 for excitation into S2, but it is larger than 0.1 for excitation into the fourth excited singlet state S4. Addition of cyclohexane as an inert foreign gas to the pyrene vapor results in a marked decrease in Φ2F/Φ1F, which seems to reach a constant value at high pressures of cyclohexane (〉60 mm Hg). From an analysis of the experimental data and a consideration of the kinetics of radiative and radiationless processes in a collision-free molecule, the rate constant of the internal conversion S2*→S1*, k21 (the asterisk refers to excited vibrational states), and that of the reverse process S2*←S1*, k12, are estimated to be 5.8×1011sec−1 and 3.2×108 sec−1, respectively, for excitation into the S4 state. The ratio k12/k21 is shown to be related to the ratio between the densities of vibrational states for the vibronic states S2* and S1*. Most of the anomalous S2-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 transition S0←S2 from S2 to the ground state is much greater than that for the radiative transition S0←S1; (2) the S2 state is located close to the S1.