Molecular dynamics and electronic structure simulations of photoexcited chromophores in the context of molecular heaters.
Mariana T. do Casal1,2, Josene M. Toldo1, Mario Barbatti1
1Aix-Marseille University, CNRS, ICR, Marseille, France
2Physical Chemistry Section, Department of Chemistry, KU Leuven, Celestijnenlaan 200f, 3001 Leuven, Belgium
One of the main challenges of the XXI century is food security. One strategy to overcome it is to extend locations suitable for agriculture at higher altitudes, protecting the crops from the cold. Photochemistry is at the core of the solution by enabling a new class of molecular heaters specially designed for plant applications that absorb UV-vis radiation and convert it into heat. Computational photochemistry is a key tool in this regard.
In order to understand the potential of candidates as molecular heaters, we investigated the photophysics of molecules inspired by nature's efficient photoreactive mechanism, the sinapoyl malate derivatives. Besides identifying its main deactivation channel to the ground state mediated by a twisted conformation and excited-state lifetimes (J. Phys. Chem. Lett., 2021, 12, 337; J. Phys. Chem. A, 2021, 125, 5499; Comm. Chem., 2022, 5, 1, 1-9), we were also able to assess the most suitable candidates. We also proposed a model for the intricate surface of leaves and its interaction with a chromophore, which is not trivial from a molecular point of view.
Second, I worked with the diketopyrrolopyrroles (DPP) derivatives, which are well-known for their stability and good UV-vis absorber properties. Besides their potential application as molecular heaters, DPPs are also widely used in organic photomaterials as a jack-of-all-trades. I shine a light on the importance of doubly excited states in the photophysics of such molecules (Phys. Chem. Chem. Phys., 2022, 24, 23279-23288; Phys. Chem. Chem. Phys., 2022,24, 20138-20151). I show that by controlling the energy of this doubly excited state through the increase of the π-scaffold, we can control internal conversion and fluorescence. This work opened up an important conceptual question regarding the description of doubly excited states. The discussion of what is considered a doubly excited state is still present in the literature, even for simpler systems such as, for example, butadiene. One of my main contributions was to propose a general definition for such a class of states (Chem. Sci., 2023, 14, 4012). This definition is based on the norm of the transition density matrix and, thus, is connected to a physical observable. With this more general definition based on the system's electronic density, independent of the electronic structure method used, we contribute to unifying the language used in the community to discuss experimental results.