Mechanism of the Non‐Kasha Fluorescence in Pyrene

ABSTRACT The high‐energy shoulder in the gas‐phase fluorescence emission spectrum of pyrene is a well‐known example of non‐Kasha emission. We comparatively assess two approaches, vibronic perturbation theory and nonadiabatic dynamics, in their ability to predict and explain the gas‐phase fluorescence spectrum of pyrene. While both methods qualitatively capture the non‐Kasha emission, they differ in their computational requirements, accuracy, and physical interpretation. Vibronic perturbation theory and nonadiabatic dynamics are complementary and can be combined in a two‐step approach to non‐Kasha fluorescence.