Synthesis of organic materials for electrooptical applications

Abstract

In Chapter 1, we provide an overview of organic materials for light-emitting diodes (OLEDs). We review fundamental photophysical limitations on device efficiencies and discuss how different strategies use highly efficient photophysical pathways to overcome these limitations. The history of delayed emission strategies, particularly thermally activated delayed fluorescence, is described, along with organic radicals as a competing approach for high-efficiency OLEDs. In Chapter 2, we synthesize a donor-acceptor iptycene scaffold with thermally activated delayed fluorescence (TADF). The scaffold bears two carbazole substituents that can be equipped with solubilizing side chains, and a thiadiazoloquinoxaline core with lateral aryl bromides which allows further modification through cross-coupling reactions. Photophysical studies on a model compound suggest that polymeric material based on this scaffold may be highly emissive and TADF-active.

In Chapter 3, we report a twisted donor-acceptor approach as an alternative strategy to achieve TADF. A new class of electron-deficient pyrazinoquinoxaline core acceptors with a variety of donor substituents was prepared. The reaction cascade yields an iptycene-capped p-dibromo-quinoxalinophenazine, to which a variety of aryl and heteroaryl substituents can be cross-coupled. The products' luminescence can be tuned across the visible spectrum. We find that the induced torsion angle between donor and acceptor moieties is insufficient in producing TADF-active compounds. In Chapter 4, we explore a combination of synthetic supramolecular chemistry and materials science to develop exciplexes for TADF. We designed a bowl-shaped acceptor molecule for which we synthesized shape complementary donors that bind in a lock-andkey fashion.

The investigation of three independent donor families, guided by density functional theory calculations, allows coverage over a wide range of the visible spectrum and derive empirical relationships for the prediction of the exciplex emission color. In Chapter 5, we describe a transition-metal-free methodology for the synthesis of extended aromatic structures through dehydrative C-C coupling of readily accessible 1,4-diols with (hetero)arenes in high to quantitative yield. These reactions proceed under mild, open flask conditions and offer high atom economy, while providing an attractive alternative approach to metal-catalyzed cross-coupling reactions. In Chapter 6, we sought to expand the small molecule coupling methodology to dehydrative polymerizations. We synthesized a range of 1,4-diols in order to address reactivity and stability considerations required for the diols to serve as effective monomers.

Titanium(IV) chloride is found to efficiently couple these diols in high yield (up to 93% yield), producing oligomers with molecular weights up to 10 kDa. In future research, an appropriate dehydrating agent with attenuated reactivity will likely allow access to polymers.