Thesis defense:
Molecular dynamics and electronic structure simulations of photoexcited chromophores in the gas-phase and complex environments

Mariana Casal

Institut de Chimie Radicalaire, Aix-Marseille Université/CNRS, Marseille

09:00 am – Friday 16th September 2022 (Salle de Thèse, Campus St. Jérôme)


Developing a new class of molecular heaters to be applied in plants is a new strategy to extend locations suitable for agriculture to higher altitudes, preventing damages caused by the cold. The underlining idea is that molecular heaters are able to absorb UV-vis radiation and convert it into heat, in this case, to the leaves’ surface.
Inspired by nature and its efficient photoprotective features, we study derivatives of sinapoyl malate (SM). These derivatives exhibit a fast and efficient nonradiative decay. Their relaxation mechanisms involve a twist around their ethylenic-like double bond to generate a dark twisted charge transfer state, reaching a twisted conical intersection between S1/S0 states. These molecules were also investigated in a different medium to understand the effect of the environment in their photophysics.
Another class of potential candidates as molecular heaters are the diketopyrrolopyrroles due to their photostability and strong visible light absorption. Moreover, it was recently shown that nonradiative pathways dominate the photophysics of some dimer derivatives. Here, we show that the dimer has a low-lying doubly excited state that is not energetically accessible to the monomer, and this delays the fluorescence allowing internal conversion to occur first. We characterize the doubly excited state wavefunction by systematically changing the derivatives to tune the p-scaffold size and the acceptor and donor characters. This analysis opens new ways to control the balance between luminescence and internal conversion in such systems.
However, a proper characterization of doubly excited states is still a challenge, and no rigorous and transferable classification scheme between methods exists. Then, we propose classifying doubly excited states according to two limiting cases: the open- and closed-shell doubly excited states (DOS and DCS, respectively). We propose a classification scheme based on descriptors extracted from density matrices. As an example of DOS, we discuss formaldehyde dimer. On the other hand, DCS was more elusive. An investigation of butadiene and longer polyenes shows that none yields the idealized case. Additionally, we test the consistency of our classification scheme within ADC(3), MRCI and DFT/MRCI descriptions, in which most variation is due to the amount of static correlation.