Non-adiabatic quantum dynamics of polycyclic aromatic hydrocarbons exhibiting anti-Kasha emission from the S2 state

Abstract

The non-adiabatic quantum dynamics of four prototypical polycyclic aromatic hydrocarbons (PAHs) exhibiting anti-Kasha emission from the S₂ state, i.e. 3,4-benzopyrene, 3,4-benzotetraphene, 1,12-benzoperylene and 2′-methyl-1,2-benzanthracene, has been investigated using the multiconfiguration time-dependent Hartree (MCTDH) method. While setting up a model linear vibronic coupling Hamiltonian, we introduce a novel parameter , which quantifies the energy gap between the S_m state at the Franck–Condon geometry and the S_n/S_m minimum energy crossing point of two diabatic states. This parameter serves as a versatile and easy-to-use tool for selecting the most relevant intrastate vibronic couplings and tuning normal modes Q_i. The decay dynamics of the studied PAHs has been simulated following photoexcitation to either the first dark excited state (S₁) or the first bright excited state (S₂). The simulations reveal the dynamic equilibrium between the S₁ and S₂ states in the early time regime (up to 3 ps). Due to the exceptionally small S₁–S₂ energy gap and efficient electron–vibrational coupling between these states, molecules in the S₁ state can transiently access the S₂ state, sustaining a steady population in S₂ that can fluoresce spontaneously in violation of the Kasha rule. These findings highlight the critical role of the interplay of the electronic energy gap and vibronic coupling in the excited-state dynamics of molecular systems exhibiting non-trivial anti-Kasha emission.