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Escaping from the Franck-Condon region : a theoretical approach to describe molecular STructural ReorganIzation for reversible EnerGy and information storage at the Excited State

Periodic Reporting for period 4 - STRIGES (Escaping from the Franck-Condon region : a theoretical approach to describe molecular STructural ReorganIzation for reversible EnerGy and information storage at the Excited State)

Reporting period: 2020-05-01 to 2021-04-30

STRIGES is a theoretical project aimed at developing new and non parametrized computational approaches and descriptors essentially rooted on Density Functional Theory (DFT) and Time Dependent DFT (TD-DFT) enabling to design new single molecule architectures able to undergo to significant light induced electronic and structural reorganization. To this end specific tools and theoretical approaches will be developed in order to be able to explore the Potential Energy Surfaces of real-life molecular systems in order to rationalize and predict their excited state behavior.
The present project has therefore a dual aim. From a fundamental point of view we aim at the development of new theoretical approaches for the description of photochemical and photophysical processes in molecular systems, and with this tool in hands, we aim to apply them for the description and prediction of photoinduced phenomena which is indeed of fundamental importance also in many research fields of technological relevance, ranging from artificial photosynthesis to molecular electronics.
To this end, we develop, implement and apply suitable theoretical tools enabling the accurate description of potential energy surfaces of the lowest lying excited states not exclusively within the Franck-Condon region which are those more often addressed by current theoretical approaches. From the application point of view, the end point of this project is the in-silico design and optimization of new classes of light activated molecular systems.
The project has focused on the development and validation of new computational approaches for the description of excited state potential energy surfaces of molecular systems as well to their application to real systems. To this aim, during the project descriptors based on density were derived (essentially but not exclusively) in the framework of Density Functional Theory and Time Dependent Density Functional Theory and applied to design new systems for different applications.
A set of adapted theoretical approaches were developed, implemented and critically applied including :
1) Density based descriptors enabling the identification of the nature of the excited states (independently of the symmetry of the system considered)
2) Density based descriptors enabling to describe Excited state evolution
3) Density based indexes enabling to spot pathologic description of excited states (for example of Charge Transfer type) using Density Functional Approximation (DFA)
5) Methods of embedding enabling to better describe photophysical properties in condensed phases
6) Application to light activated molecular systems

A series of programs and scripts were made available for the community.
We propose a new way of surfing on the ES potential energy surface based on the density based indexes which enables (in the TD-DFT approach) also to more easily follow a specific excited state. This approach enable to more easily interpret and predict the photophysics/reactivity of molecular systems. We developped indexes based on density and applicable with any QM methods enabling to assess the nature of the excited states and also -in the case of TD-DFT applications- to spot their possible spurious or ghost nature.
We proposed a new method to describe absorption and emission properties in molecular crystal enabling the correct recovering of the effect of the crystal environment. This method can have multiple direct applications and it is foreseen to be of interest especially in the design of new molecule based materials with fine-tuned optical properties. In this sense we have already applying this method to the study of mechanochromic molecular material and of low dimensional systems
All the tools developed are freely distributed under request.
logo of the STRIGES project