ИСТИНА

The absorption and emission spectra of pyrene are simulated with their vibronic structure.i Pyrene has two low-lying singlet excited states: dark (symmetry-forbidden) 1Lb and bright (symmetry-allowed) 1La. Light absorption leads to 1La, but the emission proceeds from 1Lb with relatively high quantum yield. The absorption spectra calculated within the Franck–Condon double harmonic parallel-mode approximation perfectly agree with the experiment, which indicates the validity of these approximations for light absorption in pyrene. However, the emission spectra calculated either within the Franck–Condon or Herzberg–Teller approximations show poor agreement with the experiment. This indicates the importance of nonadiabatic effects and intensity borrowing in the pyrene fluorescence. Nonadiabatic coupling matrix elements between the 1Lb and 1La are calculated. The emission spectra taking into account nonadiabatic effects are constructed.