Design and synthesis of red thermally activated delayed fluorescence emitters for OLEDs, sensors and bioimaging

Abstract

Developing efficient red thermally activated delayed fluorescence (TADF) emitters remains a formidable challenge due to their low photoluminescence quantum yield governed by the energy-gap law. This thesis concerns the study of orange-to-red TADF materials, focusing on their molecular design, synthesis and photophysics. In addition to OLEDs, these materials were employed as sensors and bioimaging reagents. Chapter 1 introduces fundamental principles of photoluminescence, electroluminescence, and provides a review of red TADF emitters for OLEDs and TADF emitters for bioimaging. Chapter 2 documents the relationship between different N-doped polycyclic aromatic hydrocarbons (PAH)-acceptor compounds (DMACBP, DMACPyBP, DMACBPN and DMACPyBPN), their photophysics and OLED performance. Chapter 3 focuses on a molecular design strategy that combines a p-conjugated acceptors with a TPA donor. By modulating the solid-state solvatochromism and adjusting the p-p stacking interactions, a deep red emission was obtained from both the TPAPyBPN-doped films and its OLED. Furthermore, we demonstrated the first TADF optical sensor for ZnCl₂ based on TPAPyBP. Chapter 4 explores four new yellow-to-red TADF dendrimer emitters, which were designed to be solution processable. We systematically investigated the effect of substitution position and the strength of the donors on the optoelectronic properties of these four dendrimers. Finally, we employed these emitters in solution-processed OLEDs. Chapter 5 continues the work on TADF dendrimer emitters by investigating the effect of extending the p-conjugation of the acceptor on the photophsics of the compound. 2GCzBPPZ, with a much larger planarity and p-conjugation acceptor, shows unusual dual emission that is both concentration-dependent and temperature-dependent in solution. 2GCzBPPZ is the first small molecule TADF compound used for colorimetric temperature sensing. Chapter 6 presents three tetra(donor)-acceptor compounds TCzPhCor, TDMACPhCor, and TPXZPhCor employing corannulene, a curved PAH, as the acceptor. These three emitters exhibit room-temperature phosphorescence from different excited triplet states. While TPXZPhCor doped film can act as an optical temperature sensor in the range from 77 K to 298 K. We also fabricated solution-processed afterglow OLEDs using TPXZPhCor. Chapter 7 explores the use of D-A TADF emitters in bioimaging. We discussed two types of water-soluble materials: nanoparticles and water-soluble sodium-salt emitters. We also document our first attempt to study these compounds in vitro.