Cheick Oumar Diarra, Carlo Massobrio, and Evelyne Martin

Heat transport in a crystalline polymer, poly(3-hexylthiophene) (P3HT), is studied using molecular dynamics. The potential energy is described by density functional theory (DFT) with an additional correction to account for van der Waals dispersion forces. The approach-to-equilibrium molecular dynamics (AEMD) method is used to create and analyse thermal transients and target transport along or between polymer chains. As expected, the approach leads to a length-dependent thermal conductivity of the system along the covalent polymer backbone. Less expectedly, we observe heat transfer between two polymers, i.e. along a non-covalent heat path, and quantify this transfer using a thermal conductance.

Jiang Jing,  Emilie Steveler,  Amira Mounya Gharbi, Sébastien Marbach, Pascal Didier, Gilles Ulrich, Ibrahim Bulut, Nicolas Leclerc, Wilfried Uhring,

Jérémie Léonard, Benoît Heinrich, Patrick Lévêque and Thomas Heiser 

Exciton dynamics play a crucial role in determining the efficiency of organic photovoltaic devices and photo-detectors. However, establishing clear correlations between molecular structure and exciton diffusion length remains a significant challenge, limiting the rational design of more efficient materials. In this study, we investigate exciton transport in thin films of a planar dumbbell-shaped electron donor composed of discotic triazatruxene end-groups and an electron-deficient central unit. These molecules self-assemble into unique bridged-columnar structures, which are known to support efficient charge transport, although their impact on exciton dynamics had not yet been explored. Using a combination of time-resolved photoluminescence (TRPL), spatially resolved TRPL, and exciton–exciton annihilation measurements, we examine how structural order influences exciton diffusion in both the columnar- nematic and crystalline phases. We show that crystallization leads to a twofold increase in exciton diffusion length, reaching values comparable to those observed in state-of-the-art non-fullerene acceptors. Although the molecules exhibit a typical Stokes shift that is not particularly favorable for Förster energy transfer (FRET), ecffiient exciton transport is nonetheless achieved—enabled by long exciton lifetimes and anisotropic energy transfer within its distinctive bridged-columnar architecture. These results, supported by FRET analysis, highlight the effectiveness of the molecule's tailored dumbbell-shaped design and its ability to self-assemble into ordered structures that support both long-range exciton diffusion and efficient charge mobility.

Cheick Oumar Diarra, Julien Taillieu, Mauro Boero, Thomas Heiser, Evelyne Martin

Organic photovoltaic (OPV) technology offers a solution to the global demand for sustainable energy production and harvesting due to its affordability, versatility, and moderate environmental impact. Recent advancements have led to an increased efficiency in bulk heterojunction (BHJ) solar cells, making practical applications feasible. However, these advancements have also raised fundamental questions regarding exciton transport mechanisms. A precise understanding of the relationship between the exciton diffusion length and material structure is crucial for improvements and applications. We formerly assessed a method to extract the exciton diffusion coefficient [Diarra, C. O.  Phys. Chem. Chem. Phys. 2023, 25, 15539–15546] via first-principles dynamical simulations in the excited state. Here, we propose a novel approach to compute the radiative lifetime using the same method. This provides an additional crucial parameter contributing to the exciton diffusion length. We benchmark the validity of the method on a specific semiconducting polymer, poly(3-hexylthiophene) (P3HT). The resulting exciton diffusion length is found to be in close agreement with the available experimental data.

This lecture will therefore recall new synthetic advances in both fields of tetrazines, and heptazines. Noticeably, a new synthetic procedure of heptazines using mechanochemistry, elaborated in the PPSM laboratory, will be presented. As well, we will describe new very low-viscosity tetrazine-based fluorescent liquids.  The original delayed fluorescence of original heptazines, which are the first species to present sometimes a singlet-triplet inversion, will be detailed, along with first results in photocatalysis. Examples featuring the molecules concerned are gathered in the Scheme 1 below.

1. G. Clavier and P. Audebert “s-Tetrazines as building blocks for new functional molecules and molecular materials”, Chem. Rev., 2010, 110, 3299.
2. P. Audebert, E. Kroke, C. Posern and S.-H. Lee, State of art in the preparation, and properties of molecular monomeric s-heptazines: syntheses, characteristics and functional applications Chem. Rev. 2021, 121, 2515.