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a Theoretical chemistry Approach to tiME-resolved molecular Plasmonics

Periodic Reporting for period 3 - TAME-Plasmons (a Theoretical chemistry Approach to tiME-resolved molecular Plasmonics)

Reporting period: 2018-08-01 to 2020-01-31

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 available ones are unable to tackle this complexity, because they consist of phenomenological models focused on field enhancements or on generic features of the various plasmon-molecule coupling regimes.

The goal of TAME-Plasmons is to develop a theoretical chemistry approach to directly simulate the real time evolution of molecules interacting with plasmons and light.
The project activity is organized in different research lines
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: first version of 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: first 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.

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.

By the end of the project we expect to extend the model developed so far to a larger class of ultrafast spectroscopy and light-triggered phenomena, such as bidimensional electronic spectroscopies, generation of high-order harmonics of an incident light, photochemical reactions of cage-encapsulated molecules. Specific collaborations with experimental groups have been setup. Moreover, we expect to reach an even greater level of realism and of details in the description of the system, leading to a further improvement of the accuracy and the predictive power of our simulations. The software developed so far, made user-friendly, will be publicly released as free, open-source code by the end of the project.