The Polaritonic Index: A TD-DFT Approach for Polariton Chemistry

Light-matter interactions at the nanoscale are crucial for fields like quantum chemistry and nanophotonics. This research is grounded in the field of polaritonic chemistry, that focuses on controlling chemical reactions through the creation of light-matter interactions known as polaritons or plexcitons. Inspired by the groundbreaking work of Ebbesen et al. [1], we employed TD-DFT to achieve a comprehensive understanding of the coupling mechanisms between a photoswitch and a silver nanocluster [2-3]. In line with recent findings of strong coupling between surface plasmon-polaritons and excitons leading to hybrid states (polaritons) [4], this work explores the emergence of these states and their impact on the photoswitch potential energy landscape. We introduced a novel figure of merit, the Polaritonic Index (PI), to identify all hybrid excitations and differentiate between polaritonic states (Figure 1) and charge-transfer states. Polaritonic states are confined to the barrier region between the two stable configurations of the thermal isomerization of the analyzed molecule, while charge-transfer excitations appear along the entire isomerization pathway. This suggests potential new pathways for the investigated isomerization process. This work highlights two key points. First, it demonstrates the need for advanced theoretical models to capture the complex interactions between metal and molecule in this system. Second, it paves the way for utilizing polaritonic chemistry to manipulate light-sensitive molecules opening doors for future applications.