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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 3 - 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: 2018-11-01 to 2020-04-30

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 two new classes of photomolecular devices.
In the first period –ie 30 months- the project has focused on the development and validation of new computational approaches for the description of both charge transfer excitations and potential energy surfaces of systems taken as models for photochemical and photophysical processes occurring in molecular systems. The work perfomed basically corresponds to the first two methodological tasks (out of the four) scheduled in the project. To this end the STRIGES team of researchers (composed by the PI, three PhD students and two postdoctoral researches) has developed a new index to diagnostic the performance of Time Dependent DFT approaches for the description of Charge Transfer (CT) excitations. This index is important for the theoretical chemists community since it allows to spot erratic behaviour of TD-DFT that can determine an erroneuous interpretation or prediction of the absorption/emission properties of the systems analysed.
Furthermore, we validated the use of ad-hoc developed density based indexes for the determination of minima and regions of decay of Excited State potential energy surface. Software enabling the optimization at the Excited State based on Density Indexes has been developed and tested.
To better describe environmental effects, techniques allowing to include the effect of the molecular crystal and amorpohus environment on photphysical properties have been also developed and validated. These new tools are now ready to be applied for the accurate description of potential energy surfaces of the lowest lying excited states not exclusively within the Franck-Condon region thus for the design of new molecular architectures.
In this first period, the STRIGES project has been focused on the development of new theoretical and computational tools which shall have a direct impact in allowing to trustfully modeling the Excited States of molecular systems out of the Franck Condon region.
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, in publication, constitutes a significant step further with respect to the current TD-DFT methodology since it can enable to more easily interpret and predict the photophysics/reactivity of molecular systems.
Furthermore, we proposed a new method to describe 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 materials.
All the tools developed are freely distributed under request.
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