[Paper Review] Collective Polaritonic Effects on Chemical Dynamics Suppressed by Disorder

Juan B. Pérez-Sánchez, Federico Mellini+2 more|arXiv (Cornell University)|Aug 7, 2023

Strong Light-Matter InteractionsPhysics and Astronomy60 references3 citations

TL;DR

This paper introduces d-CUT-E, a computationally efficient method to simulate ultrafast quantum dynamics in disordered molecular polariton systems under collective strong coupling. It shows that strong coupling inferred from linear spectra is a poor predictor of reactivity: broadband excitation yields no net change in reaction yield due to cavity leakage, while narrowband excitation selectively prepares reactive high-frequency vibrational states via cavity-mediated Rabi oscillations, revealing a non-chemical, optical initial-state preparation effect.

ABSTRACT

We present a powerful formalism, disordered collective dynamics using truncated equations (d-CUT-E), to simulate the ultrafast quantum dynamics of molecular polaritons in the collective strong coupling regime, where a disordered ensemble of $N\gg10^{6}$ molecules couples to a cavity mode. Notably, we can capture this dynamics with a cavity hosting a single extit{effective} molecule with $\sim N_{bins}$ electronic states, where $N_{bins}\ll N$ is the number of bins discretizing the disorder distribution. Using d-CUT-E we conclude that strong coupling, as evaluated from linear optical spectra, can be a poor proxy for polariton chemistry. For highly disordered ensembles, total reaction yield upon broadband excitation is identical to that outside of the cavity, while narrowband excitation produces distinct reaction yields solely due to differences in the initial states prepared prior to the reaction.

Motivation & Objective

• To address the lack of scalable, accurate methods for simulating ultrafast quantum dynamics in disordered molecular ensembles under collective strong coupling.
• To resolve the discrepancy between strong coupling observed in linear optical spectra and actual chemical reactivity in disordered systems.
• To investigate how disorder and cavity-mediated dynamics influence photoreactivity in molecular polaritons.
• To develop a method that captures collective effects in large ensembles ($N \gg 10^6$) with minimal computational cost by exploiting permutational symmetries.

Proposed method

• The d-CUT-E method generalizes CUT-E by coarse-graining disorder into $N_{\text{bins}}$ bins, where molecules in the same bin are treated as identical, preserving permutational symmetry.
• Each disorder bin is mapped to a single effective molecule with enhanced coupling strength $g\sqrt{N}\sqrt{P_i}$, reducing the full $N$-molecule problem to a single effective system with $N_{\text{bins}}$ electronic states.
• The effective Hamiltonian (Eq. 1) includes kinetic energy, potential energy surfaces (PESs), cavity coupling, and diabatic coupling between excited states, with $P_i$ as the fraction of molecules in bin $i$.
• The method is numerically exact for short times, with convergence achieved at $N_{\text{bins}} \ll N$, enabling efficient simulation of large disordered ensembles.
• Dynamics are simulated under broadband and narrowband excitation, with initial states prepared as superpositions of cavity and molecular states to probe selective reactivity.
• The approach is applied to Gaussian exciton-frequency disorder in a two-excited-state model with one cavity-coupled state, focusing on the first excitation manifold.

Experimental results

Research questions

• RQ1 Does strong coupling, as indicated by Rabi splitting in linear absorption, reliably predict changes in chemical reactivity in disordered molecular polariton systems?
• RQ2 How does disorder impact the relationship between optical response and chemical reactivity in collective strong coupling regimes?
• RQ3 Can narrowband excitation selectively enhance reactivity by preparing specific high-energy vibrational states via cavity-mediated dynamics?
• RQ4 To what extent do observed reactivity changes stem from polariton formation versus initial state preparation effects?
• RQ5 Is collective strong coupling in large ensembles ($N \gg 10^6$) truly advantageous for chemical control compared to conventional narrowband lasers?

Key findings

• Broadband excitation produces no net change in total reaction yield in the large disorder regime, despite visible polariton bands in absorption spectra, because changes are dominated by cavity leakage rather than chemical modification.
• Narrowband excitation of the upper polariton (UP) band leads to significantly higher reaction yields than excitation of the lower polariton (LP) band, due to selective preparation of high-frequency, reactive vibrational states.
• The enhanced reactivity from UP excitation arises not from polariton-mediated chemistry, but from cavity-mediated Rabi oscillations that mix vibronic states across disorder bins, preparing highly reactive configurations.
• The reaction yield difference between UP and LP excitation cannot be explained by absorption differences alone, as absorption slightly favors the LP band.
• The observed reactivity enhancement is an optical effect: the cavity enables selective initial state preparation of reactive configurations that could also be accessed with tailored linear lasers.
• In the large disorder limit, reaction yields under narrowband excitation converge to those outside the cavity, indicating that the cavity's role is not catalytic but preparative.