Identifying Vibrations that Control Non-adiabatic Relaxation of Polaritons in Strongly Coupled Molecule–Cavity Systems

Abstract

The strong light–matter coupling regime, in which excitations of materials hybridize with excitations of confined light modes into polaritons, holds great promise in various areas of science and technology. A key aspect for all applications of polaritonic chemistry is the relaxation into the lower polaritonic states. Polariton relaxation is speculated to involve two separate processes: vibrationally assisted scattering (VAS) and radiative pumping (RP), but the driving forces underlying these two mechanisms are not fully understood. To provide mechanistic insights, we performed multiscale molecular dynamics simulations of tetracene molecules strongly coupled to the confined light modes of an optical cavity. The results suggest that both mechanisms are driven by the same molecular vibrations that induce relaxation through nonadiabatic coupling between dark states and polaritonic states. Identifying these vibrational modes provides a rationale for enhanced relaxation into the lower polariton when the cavity detuning is resonant with specific vibrational transitions.

Enhanced Excitation Energy Transfer under Strong Light-Matter Coupling: Insights from Multi-Scale Molecular Dynamics Simulations

“…Since dark states lack group velocity, and are therefore stationary, while excitonic couplings between molecules are neglected in our model (see SI), propagation in the diffusive regime must still involve bright polariton states. Our simulations therefore suggest that while, initially, molecular vibrations drive population transfer from the propagating bright states into the stationary dark states [62], this process is reversible, causing new wavepackets to form continuously within the full range of LP group velocities. Likewise, the propagation of transiently occupied bright states is continuously interrupted by transfers into dark states, and re-started with different group velocities.…”

“…Initially the propagation of the wavepacket is dominated by ballistic motion of the population in the bright polaritonic states moving at the maximum group velocity of the LP branch. However, due to non-adiabatic coupling [62], some of that population is transferred into dark states that are stationary. Because non-adiabatic population transfer is reversible, the wavepacket propagation undergoes a transition into a diffusion regime, which is significantly slower, as also observed in the ideal cavity (Figure 3c).…”

Ab Initio Molecular Cavity Quantum Electrodynamics Simulations Using Machine Learning Models

“…[1][2][3][4][5][6][7] These new created hybrid states, which are commonly referred to as polaritons, have been shown to facilitate new chemical reactivities and selectivities. 1,6,[8][9][10] Theoretical investigations play a crucial role in understanding the fundamental limit and basic principles in this emerging field, 5,6,[11][12][13][14][15][16][17] as these polariton chemical reactions often involve a rich dynamical interplay among the electronic, nuclear, and photonic degrees of freedom (DOFs).…”

“…Thus, the MQC methods, including the Ehrenfest and trajectory surface hopping (TSH) methods, have been widely used in the nonadiabatic polariton dynamics recently. [11][12][13][14][15][16][17][23][24][25][26][27][28] In the propagation of the polariton dynamics with the MQC methods, besides the energies of the electron-photon hybrid states, we need to derive the nuclear gradients and the couplings between these states, where the derivatives of molecular dipoles (including permanent dipoles and transition dipoles) are the key ingredients. 25,26 For some model systems with well-defined diabatic electronic states, the dipoles of/between these diabatic states can be set to constants.…”

“…13,15,27 Groenhof et al approximated transition dipoles by a first-order Taylor expansion around the ground state equilibrium and obtained the derivatives through least-squares fitting with a large number geometrical conformations. 12,14,16,17 The lack of accurate derivatives on dipoles (at the correlated wavefunction level of theory) had become the major theoretical bottleneck in performing ab initio on-the-fly simulations of polariton chemistry. To the best of our knowledge, there is no previous work on using correlated wavefunction level of theory to perform ab initio on-the-fly quantum dynamics simulations of polariton chemistry with the rigorous nuclear gradient.…”