Charge transfer in 2D Covalent Organic Frameworks for optoelectronics

Abstract

Currently, the development of novel sustainable nanomaterials that offer high performance in optoelectronics is of great interest not only to the scientific community but also to industry. With the aim of developing state-of-the-art optoelectronics devices, research is being carried out on the properties offered by different materials to be used in either light-to-electric or electric-to-light applications. Covalent Organic Frameworks (COFs) are a type of nanomaterial with excellent potential to the future of optoelectronics. COFs are a class of organic polymers with permanent porosity and highly ordered crystalline structures. Specifically, COFs have interesting properties for the development not only of solar cells but also for Organic Light-Emitting Diodes (OLEDs). In the case of solar cells, materials are sought that absorb as much solar electromagnetic radiation as possible and convert it into electricity. In the case of OLEDs, materials are sought that generate visible light in the presence of electric current. Establishing the structure-property relationship of COFs from computational calculations is essential to reduce the time, cost and effort of the synthesis process by bringing to it only the candidates with the most promising properties. In this project, three different families of experimentally reported 2D-COFs have been studied by means of computational chemistry, using Density Functional Theory (DFT) and its Time-Dependent extension (TD-DFT) methods. More specifically, we use the ωB97XD range-corrected functional with the def2-SVP basis set. First, for each system, it has been necessary to determine the most appropriate 0D cluster model (MAM) able to characterize the excited state properties of the periodic 2D-COF monolayer, by cost-effective calculations. Therefore, for each COF, the singlet excited state with the lowest energy has been evaluated for increasing size 0D models of the periodic 2D-COF structure until convergence. This research has been carried out by characterizing the molecular orbitals (MOs) involved in the excitation, the excited state energy and the Charge Transfer (CT) character. It is concluded that, in order to correctly determine the MAM, it is necessary that the MOs are well surrounded by an environment sufficiently close to that of the periodic monolayer. It has been determined that, a small change in the structure of 2D-COF, which is repeated periodically, give rise to monolayers with different properties. At the level of study carried out, of all systems under study, systems 1 and 3 have the most promising properties for light-to-electric optoelectronics. Both systems have well-defined CT from the linker (the donor) to the ligand (the acceptor).