Synergistic design strategies for the improvement of thermally activated delayed fluorescence properties in near-infrared emissive molecules

Abstract

In the last decade, organic light-emitting diode (OLED) technology has risen from novelty to dominance due to its appealing advantages in terms of reduced energy consumption, compatibility with flexible substrates, large-scale processability, and tuneability of material properties. However, several challenges must still be overcome in the development of efficient OLED emitters, especially at the long wavelength edge of the visible spectrum. More specifically, the expansion into the deep-red and near-infrared region (DR/NIR, 650–1400 nm) represents a significant development that addresses particular needs within phototherapy, bio-imaging, spectroscopy, sensing, cryptography, and optical communication. Despite these benefits, the commercialization of DR/NIR OLEDs has been constrained by challenges tied to their modest internal and external quantum efficiency (IQE and EQE, respectively). In parallel with a high intrinsic photoluminescence quantum yield (PLQY), it is vital for new materials to be able to generate emissive excitons from both singlet and (typically inaccessible) triplet states, thereby achieving a higher IQE (up to 100%) than their classical fluorescent dye counterparts. Different mechanistic approaches (denoted as different OLED generations) have been developed over the last decades, which include thermally activated delayed fluorescence (TADF), transition metal complex-based phosphorescence, hybridized local and charge transfer (HLCT), hyperfluorescence (HF), and radical doublet emission. Among these, TADF (and in extension HF) has been identified as a viable candidate toward the further development of sustainable, efficient NIR-OLED materials. TADF is supported by the available thermal energy of the environment, and becomes active when the energy difference between singlet and triplet states (or singlet-triplet energy gap, ΔEST) is minimized. The design of charge-transfer (CT) TADF emitters that achieve such small ΔEST typically relies on the use of electron-donating (D) and electron-accepting (A) molecular subunits, which maintain a twisted conformation along their connecting bond(s). As such, careful and balanced molecular design and structural finetuning are both crucial for the development of novel high-performance TADF materials. Unfortunately, pushing the emission toward the longer wavelengths (lower energy range) of the NIR region is further complicated by the infamous ‘energy gap law’. This principle refers to the increasing interaction between the excited emissive state and the vibrational levels of the ground state as the energy gap between the two becomes smaller. This results in more non-radiative vibrational losses which compete and therefore reduce the emission efficiency. Naturally, improving the rigidity of the emitter can help to counteract these losses but this also makes the active material more vulnerable to undesired aggregation, potentially leading to aggregation-caused quenching of the NIR emission and solubility issues during material preparation. A careful balance must therefore be maintained. In this thesis, three (synergistic) design strategies were selectively employed to probe the influence of structural templates on the most important emission parameters (i.e. TADF performance, luminescence efficiency, and emission wavelength) and to subsequently improve them through the development of novel NIR-TADF emitters. A first strategy, known as ‘isomeric modulation’, entailed the investigation of regio-isomers of existing, well-performing NIR-TADF emitters to compare the performance between reported and unexplored isomeric motifs. In most cases (as evidenced by Chapter 2, 3, and 5) this led to the discovery of molecular templates which showed an improved TADF performance. Meanwhile, numerous efforts were focused on the creation of small, but nonetheless strongly electron-deficient acceptor moieties in an attempt to enlarge the attainable emission red-shift without the material falling victim to aggregation-induced luminescence quenching. Additionally, this strategy helped to limit the overall size of the emitter, which could potentially improve its overall solubility and processability, making them more suitable for both solution-processed and thermally evaporated OLED devices. Various novel acceptor units with deep LUMO (lowest unoccupied molecular orbital) energies were eventually designed, synthesized, and subsequently employed, showing varying degrees of success in terms of their emission red-shift and/or luminescence efficiency (Chapter 3, 4, and 5). Nonetheless, other potential candidates remain of interest, provided that a suitable synthesis pathway can be found. Finally, both D-A-D and D-A type structures were often obtained – either by choice or by chance – which allowed to examine the influence of the number of donor units on their respective emission properties. Since the presence of a second donor unit notably affected the aggregation behavior, molecular geometry, and excited state distribution of a certain emitter template, its advantageous role varied within each series of chromophores. In the end, a variety of novel DR/NIR (TADF) emitters were successfully developed, some of which were even incorporated into (preliminary) solution-processed OLED devices. Despite their admittedly limited performance in terms of the attainable emission red-shift, PLQY, and EQE, deeper understanding of structure-property relationships was gained, providing valuable insights into the possibilities and shortcomings of NIR-TADF emitters and expanding my laboratory expertise and investigative skills as a young academic researcher.