Defense of Kevin Mall Haidaraly’s thesis (E-POM and ECP group)


Tuesday 9th April 2024
at 2 pm

Pierre et Marie Curie campus
Esclangon building
Astier amphitheater

Kevin Mall Haidaraly, a doctoral student in the E-POM and ECP group, will present his thesis entitled

« Self-assembled donor-acceptor polyoxometalate-based materials from molecular engineering to
photoelectrical performances »

In this thesis work, we focused on the development of nano-segregated organic-inorganic donor-acceptor (D-A) hybrid materials, using mesogenic organic derivatives as electron donors and polyoxometalate (POM) derivatives as electron acceptors to develop photoactive liquid crystals. The nano-segregation between donors and acceptors in these materials aimed to create distinct pathways for electron and hole percolation to improve their charge transport properties. These compounds were fully characterized both in solution and as thin films. In addition, preliminary measurements of their photoelectrical performances were carried out on these materials.

The first chapter establishes the theoretical background and state of the art necessary for the understanding and development of this thesis work. First, the concept of donor-acceptor electron materials for optoelectronics is discussed in detail. Their intrinsic properties (optical, electronic and photophysical) and their characterization are described. Then, the different architectures of optoelectronic devices, as well as their characteristic parameters, are presented. The following section focuses on liquid crystals and their characterization. Examples of LCs used as semiconductor materials in optoelectronic devices are also presented. The last section reviews the state of the art of polyoxometalates (POMs) and their functionalization by anchoring covalent bonds with one or more organic functions. The remarkable properties of these POM-based hybrids, their synthesis routes and their uses in the field of molecular electronics are also described.

The second chapter presents the multi-step synthesis of calamitic compounds based on diaryl_benzothiadiazole (DBT) and discotic compounds based on alcynyl_pyrene. This part focuses particularly on the issue of dissymmetrization of photoactive organic units, with a view to developing donor-acceptor systems. Three organic units and one hybrid were synthesized: DBT-2, TiPS-RamPYR, tNHPYR and TBA-KSi[DBT-2]/POM-DBT. The objective of this chapter is to present the design of calamitic and discotic photoactive mesogenic compounds for the development of lamellar and multilayered columnar mesomorphic materials with nano-segregated donor and acceptor domains.

The last chapter focuses on the in-depth characterization of the developed materials, both in solution and in the solid state. DBT-2exhibits a mesogenic behavior at room temperature, strong luminescence (PLQY of 73% and 56% in solution and thin film), and redox properties allowing charge separation by photoinduced electron transfer after covalent coupling with a POM. The study of the POM-DBT hybrid reveals a luminescence quenching (PLQY of 41% and 4% in solution and thin film), suggesting a charge transfer process. The multilayered D-A lamellar organization of both materials is evidenced by X-ray studies. Thermal annealing significantly improves the long-range organization. The morphological study of the films shows a high homogeneity over large areas with micrometer-sized domains with step-like topology. POM-DBT films were integrated into optoelectronic devices, demonstrating photoconduction properties superior to those of DBT-2. Long-range organization plays a crucial role, with photophysical performances doubled in annealed films. Preliminary studies on pyrene derivatives have shown redox and photophysical properties similar to those of DBT-2, making them promising candidates for the creation of functionalized columnar hybrid materials with POM units.

Find out more about the E-POM and ECP team