Structure-Emission Relationships in Carbazole-Based Multichromophoric Systems

Abstract

One of the major challenges in the development of functional materials is understanding molecular design principles to synthesise systems with desirable properties in a predictive manner. Hence, a lot of focus is given to establishing structure-property relationships to achieve this ambitious goal in the long term. We focus on organic emitters with applications in display and imaging. Using a multidisciplinary approach that combines synthesis, spectroscopy, crystallography, and computation, we investigate 2 pairs of carbazole multimers in the 2 parts of this thesis. Two trimers (2,7-Cz-T and 3,6-Cz-T) demonstrate how modification in the substitution pattern allows differential population and utilisation of triplet excitons. With 3,6-Cz-T demonstrating superior triplet-harvesting ability, we extend the system to a hexamer, Cz-H. The 3,6 trimer and Cz-H both exhibit the rare phenomenon of dual delayed emission. While 3,6-Cz-T displays a roughly equal thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP), Cz-H shows a preference for RTP. The crystal structure of Cz-H, solved by three-dimensional electron diffraction (3D ED), revealed a dense network of C-H···π interactions, suggesting increased rigidity in the solid state. Further, energetics and spin-orbit coupling information obtained from quantum chemical calculations shed light on competing intersystem and reverse intersystem crossing pathways, thereby justifying a greater RTP contribution in the hexamer.Together, these heavy-atom-free, donor-only systems offer efficient triplet harvesting pathways for potential application in light-emitting devices.