Ultrafast relaxation from 1La to 1Lb in pyrene: a theoretical study

Pyrene and its photophysics have been extensively studied, especially experimentally. Yet to this day there is no conclusive theoretical description of the relaxation from the photo-accessible L-1(a) state to the fluorescent L-1(b) state, which is partly due to the challenge of adequately modeling both states without over-stabilizing one or the other. We first compare several quantum chemical methods to find the best model that offers a good trade-off between a balanced description of both states and computational effort On the basis of the selected electronic structure method, we simulate this relaxation process with two state-of-the-art dynamical techniques. We perform wave packet propagations in reduced dimensions and full-dimensional on-the-fly surface hopping to corroborate our choice of coordinates and obtain a more complete picture of the relaxation mechanism. Our results confirm the important role of a conical intersection between the two excited states that has been postulated in the literature.