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a Theoretical chemistry Approach to tiME-resolved molecular Plasmonics

Periodic Reporting for period 4 - TAME-Plasmons (a Theoretical chemistry Approach to tiME-resolved molecular Plasmonics)

Reporting period: 2020-02-01 to 2021-09-30

Ultrafast spectroscopy is a powerful tool able to disclose the atomistic real-time motion picture of the basic chemical events behind technology and Life, such as catalytic reactions or photosynthetic light harvesting.
Nowadays, by cleverly harnessing the interaction of the studied molecules with plasmons (collective electron excitations supported, e.g. by metal nanoparticles) it is becoming possible to focus these investigations on specific nanoscopic regions, such as a portion of a catalytic surface or of a photosynthetic membrane. This coupling can also produce new quantum effects such as molecule-plasmon hybrid excitations.
On the other hand, it makes the real-time molecular evolution and its perturbation by light more complex, and thus calls for new theoretical treatments. The ones available before TAME-Plasmons were unable to tackle this complexity, because they consisted of phenomenological models focused on field enhancements or on generic features of the various plasmon-molecule coupling regimes.

The goal of TAME-Plasmons was to develop a theoretical chemistry approach to directly simulate the real time evolution of molecules interacting with plasmons and light. This goal has been achieved: the approach has been developed and the related scientific software, that builds on existing quantum chemistry codes, has been implemented and released to the scientific community as open source software. Based on these theoretical and software developments, we could clarify how photochemical reactions work when molecule-plasmon hybrid excitations are possible, what is the role of quantum decoherence in molecular plasmonics and in photocatalysis by hot carrier injection (a timely subject as it is related to new way to convert sun light into chemical energy) and we could provide a comprehensive theoretical understanding of spectroscopic experiments at the frontier of the field.
The project activity has been organized in different research lines, that are here summarized together with the main results obtained for each line.
1. Development of a computational approach and the related software to simulate in real time the behavior of plasmons simulated within classical electromagnetic theory. During this period the basic approach to perform this task has been elaborated and implemented in a specific software code called TDPlas. The latter is currently private, will be made public by the end of the project.
Main results achieved along this line: open source, public version of the code TDPlas, that is able to simulate, within the approximations intrinsic in the model, the time evolutions of plasmons supported by metal nanoparticle, taking into account the chemical nature of the metal, the shape of the metal nanoparticle and the dielectric nature of the environment.

2. Interfacing the modeling of plasmons with that of molecules, based on an atomistic quantum mechanics descriptions. The interface has been created for one of the most common quantum mechanical approaches to describe electronic states of molecule, and has been expanded to treat exquisitely quantum mechanical effects such as decoherence and relaxation. The theory has been implemented in a scientific software (WaveT) that has been also made suitable to run on the supercomputers available in EU national supercomputer centers, thanks to a specific EU high-performance computer grant called PRACE preparatory access. Main results achieved along this line: open source, public version of the scientific code WaveT and its interface with TDPlas, parallelized to make possible the use of high performance supercomputers.

3. Development of theory and implementation to study light-triggered chemical reactions when both the molecule and the plasmons behave quantum mechanically. This has been performed so far with a model description of plasmons and a dynamical, atomistic description of molecule. Main results achieved along this line: extension of an existing quantum chemistry code to treat molecule coupled to quantum plasmons; first ever simulation of the photochemistry of a realistic molecule (azobenzene) in strong coupling conditions, and characterization of an unexpected quantum yield enhanchement mechanism.

4. Applications of the models and software developed to specific case studies. Main results achieved along this line: Design of a nanoplasmonic setup to reveal quantum mechanical effects in a protein involved in the first stages of photosynthesis (i.e. in the harvesting of sun light); Simulation of existing ultrafast spectroscopy experiments using an atomistic, quantum mechanical description of the investigated molecules; Characterization at the atomistic level of how the mechanism of reactions induced by light are modified when molecules and plasmons both behave quantum mechanically and are hybridized.
In terms of model development, we have set up a multiscale model able to simulate in real time the evolution of the electronic state of a molecule, described by atomistic quantum mechanical approaches, interacting with plasmonic nanostructures, and accounting for ubiquitous quantum mechanical effects such as decoherence. These models and the corresponding scientific software go beyond the state of the art, and allow the investigation of a specific class of ultrafast spectroscopy, one of the original target of the project. Moreover, we could investigate the effects of a nanoplasmonic setup on the properties of a protein involved in the light-harvesting step of photosynthesis, and found how this setup can be used to manipulated the excited states of this protein. The models that we developed were also used to characterize, for the first time, the atomistic mechanism of a photochemical reaction in the regime of strong coupling with plasmons.This has been extended to a realistic setup including all the element of an experiment of photochemistry for cage-encapsulated molecules in a plasmonic nanocavity. We have also developed a time-dependent description of the dynamics of hybrid molecule-plasmons (i.e. plexcitons) states that was not available before the starting of the ERC project. More than 30 articles related to the TAME-Plasmons achievements have been published in international peer reviewed scientific journals. All of them are openly accessible on dedicated repositories such as arxiv and zenodo.

Specific collaborations with experimental groups have been setup. The software developed in the project, made user-friendly, have been publicly released as free, open-source code at .
Summary of the molecular plasmonic applications tackled by TAME-Plasmons