AbstractOrganic light-emitting diodes (OLEDs) are nowadays part of our everyday life, since they can be found in display applications such as modern television or smartphone screens and in solid-state lighting. OLEDs have become the most successful branch of organic electronics owing to their ability to compete with other technologies such as inorganic LEDs, which is related to their ability to achieve high efficiencies (i.e. the light output versus the power input), high color purity and color tunability. Especially for display applications, their contrast and color gamut are unparalleled.
OLED technology has come a long way since Pope et al. discovered electroluminescence in an anthracene crystal.1 Several classes of compounds have been used, moving from fluorescent molecules (1st generation OLEDs) to heavy-metal containing organometallic complexes (2nd generation OLEDs) and, ultimately, the use of emitters displaying thermally activated delayed fluorescence (TADF, 3rd generation OLEDs). The 1st generation fluorescent emitters suffered from rather low device efficiencies as the triplet state acts as a loss pathway for 75% of the excitons formed in a charge recombination process. Heavy-metal organometallic complexes allowed to overcome this issue by using the triplet state as the main radiative pathway. This emission, known as phosphorescence, enabled up to 100% exciton conversion as the heavy-metal complexes possess significant spin-orbit coupling (SOC) for the remaining 25% singlet excitons to cross over to the triplet state via intersystem crossing (ISC). These phosphorescent emitters make up most of the current commercial OLEDs. However, the use of heavy-metal atoms, which are scarce and expensive, is discouraged by the ongoing search for a more sustainable and eco-friendly society. Here, TADF emitters might provide an answer. By tuning of the molecular structure of donor-acceptor type compounds, the singlet and triplet excited state energy levels can be brought close enough for reverse intersystem crossing (rISC) to occur. The remaining energy barrier can be overcome by thermal energy and the main emission pathway is fluorescence. Due to the thermally activated rISC, the secondary emission is delayed with a lifetime in between that of the prompt fluorescence and phosphorescence for organic compounds.
Nowadays, the main focus of OLED research, is on the emissive material, as many advances have already been made for the other parts of the OLED device stack and these are generally applicable to all generations. Most attention has been given to blue emitters as these are the most difficult material to obtain due to stability issues. Investigations on red emitters have lagged behind. However, an increasing interest for the red and near-infrared (NIR) range of the electromagnetic spectrum is apparent in recent literature.
In this PhD thesis, the main goal was to expand on the known pool of donor and acceptor moieties that can be used to construct D-A and D-A-D type emissive materials for 3rd generation OLEDs. To achieve this, different fields of organic electronics, most notably the field of organic photovoltaics (OPV), served as an inspiration for new building blocks that have not been applied before in the field of OLEDs. The use of quantum chemistry methods such as density functional theory (DFT) allowed me to rationally design new emitters based on the findings from the calculations.
The second chapter therefore dealt with the quest to find the optimal exchange-correlation functional (XCF), a crucial part of the DFT calculations as it determines the accuracy of the obtained properties to a large extent. A series of 10 prototypical donor-acceptor compounds were subjected to thorough investigation with DFT and time-dependent DFT (TDDFT) and their excited state properties such as the excitation energies and oscillator strengths were calculated using 19 different XCFs with various levels of complexity. These values were benchmarked against a high level wavefunction method called resolution-of-the-identity second order approximation coupled-cluster (riCC2). While some XCFs performed well for the singlet-triplet energy splitting, the individual singlet and triplet energies were highly over- or underestimated, rendering these functionals untrustworthy. Finally, we opted for range-separated XCFs, which include a given percentage of exact Hartree-Fock (HF) exchange that increases with the interelectronic distance. Proper tuning of the range-separating parameter (ω) leads to a correct balance between a small amount of HF exchange at small distances and a large amount of HF exchange at large distances and gave the best results. The most notable functionals were LC-BLYP and LC-ωPBE with a value for ω of 0.17 bohr-1. Additionally, a so-called global hybrid XCF M06-2X, with a fixed percentage of HF exchange of 54% at all distances, gave comparable results. Application of the Tamm-Dancoff approximation (often used to overcome triplet instabilities) was found to improve the error on the triplet excitation energies with respect to those obtained using riCC2, and resulted in even smaller errors for the singlet-triplet energy splittings.
With the best-performing XCF, I engaged in the design of novel TADF emitters. Benzo[1,2-b:4,5-b']dithiophene (BDT) is a well-known donor unit in organic photovoltaics as it possesses a high electron-donating strength, high planarity and often affords a high charge carrier mobility. Unfortunately, conventional coupling via the α positions would lead to planar D-A molecules, not-likely to show TADF properties. As such, a synthetic pathway to couple the donor and acceptor units via the benzene core of the BDT had to be developed. In chapter 3, the BDT unit was coupled to 2 different acceptors: 9,9-dimethyl-9H-thioxanthene-10,10-dioxide (TXO2) and dibenzo[a,c]phenazine-11,12-dicarbonitrile (CNQxP). Moreover, to compare their properties, two 9,9-dimethyl-9,10-dihydroacridine (DMAC) analogues were designed, which previously showed TADF behavior. Although the TDDFT calculations predicted a rather large gap for the novel BDT compounds, the large HOMO/LUMO separation seemed promising and the compounds were synthesized in order to investigate their photophysical properties. Photophysical characterization in zeonex films showed prompt and delayed emission for all 4 compounds. However, their nature differed. The DMAC-containing compounds showed TADF, whereas the BDT-based compounds showed room temperature phosphorescence (RTP) when TXO2 was chosen as the acceptor and TADF when CNQxP was chosen as the acceptor. The RTP behavior of TXO2-BDT-TIPS is unusual and was attributed to the presence of multiple sulfur atoms in the BDT unit. Therefore, we investigated the BDT-TIPS donor unit by itself and found similar RTP behavior in zeonex film. CNQxP-BDT-TIPS and CNQxP-DMAC showed long-lived TADF with some triplet-triplet annihilation (TTA) at very long emission times. The main difference between TXO2-BDT-TIPS and CNQxP-BDT-TIPS is the acceptor strength. While the localized triplet state of the BDT-TIPS group, responsible for the phosphorescent behavior, is below the CT states for TXO2-BDT-TIPS, this is not the case for CNQxP-BDT-TIPS. The smaller experimental singlet-triplet energy splitting also resulted in the possibility of rISC to occur for CNQxP-BDT-TIPS, whereas this is not possible for TXO2-BDT-TIPS.
In Chapter 4, the difluorodithieno[3,2-a:2',3'-c]phenazine (DTPz) scaffold, again known from the OPV field, was used as an acceptor unit to construct (TADF) emissive materials. DTPz possesses a significant electron-accepting strength allowing the development of red-emitting materials. Two donor materials, BDT-TIPs (introduced in chapter 3) and DMAC, were coupled to the DTPz acceptor, resulting in D-A type compounds with CT emission. The calculated singlet-triplet energy splitting was small for DTPz-DMAC (0.03 eV), whereas for DTPz-BDT-TIPS (0.43 eV) it was of the same order as for CNQxP-BDT-TIPS (chapter 3). Time-resolved emission decays in zeonex film revealed a different behavior for the DMAC and BDT-TIPS containing compounds. While DTPz-DMAC showed TADF behavior with long-lived emission and a peak maximum around 630 nm, DTPz-BDT-TIPS showed RTP behavior with an onset similar to the phosphorescence of DTPz-DMAC. This suggests emission from the DTPz core instead of the BDT-TIPS unit, as found in chapter 3. Subsequent analysis of DTPz in zeonex showed that DTPz by itself indeed shows similar RTP behavior.
One of the most well-known TADF emitter materials, is 4CzIPN, originally reported by Adachi and coworkers.2 It was further studied by Etherington et al.3 and was found to show extensive dimer formation in doped OLED films. Dimer emission is undesired in OLED devices as it compromises the color purity. The dimers are formed by interactions between the carbazole units of the 4CzIPN molecules. To overcome these interactions, 4H dithieno[3,2 b:2',3' d]pyrrole (DTP) was chosen as the donor unit to replace 9H-carbazole in chapter 5. DTP is a well-known donor unit in the field of OPV, where it is typically coupled via the α-positions and the central nitrogen atom is alkylated for improved solubility of the resulting polymers. Synthesis via a carbamate intermediate allows the free-base DTP to be obtained after which it can be used in nucleophilic aromatic substitution or Buchwald-Hartwig type reactions. The former was applied in this thesis to construct 4DTP-IPN, using similar reaction conditions as for the synthesis of 4CzIPN. 4DTP-IPN showed red-shifted emission with respect to that of 4CzIPN and exhibited TADF properties in a variety of films. Due to the very small theoretical and experimental singlet-triplet splittings, it was difficult to distinguish between its delayed emission and phosphorescence, even at 80 K. While its aggregation behavior is certainly different to that of 4CzIPN, the photophysics of 4DTP-IPN at various concentrations in solution and thin film give rise to contradicting observations and further experiments are needed to solidify whether it is less likely or rather more likely to form dimers.
In the final chapter (Chapter 6), four additional publications were discussed in which quantum-chemical calculations were applied to gain insights into the experimental properties of small molecule or polymeric compounds. The first publication dealt with the design of boron dipyrromethene (BODIPY) photosensitizers, developed to efficiently form singlet oxygen for photodynamic therapy. (TD)DFT calculations lead to insights in the photophysics of the BODIPY dyes and indirectly pointed toward the presence of exciplex emission, which was confirmed experimentally. The incorporation of a DMAC unit was found to be crucial for the singlet oxygen generation and the exciplex formation and the latter is expected to be responsible for the efficient ISC. The remaining publications all dealt with structure-property relationships in which oligomeric species were used to mimic the behavior of polymer chains. By looking at their optimized geometries, we were able to explain the interplay between the electronic properties and molecular orbital delocalizations in series of similar polymers.
|Date of Award||2020|
|Sponsors||UHasselt - UNamur special research fund|
|Supervisor||Benoit CHAMPAGNE (Supervisor), Wouter Maes (Supervisor), Dirk Vanderzande (Co-Supervisor), Guillaume Berionni (President), Anitha Ethirajan (Jury), Vincent Liegeois (Jury), Andrew Monkman (Jury) & Juan Carlos Sancho-Garcia (Jury)|
- thermally activated delayed fluorescence
- Computational Chemistry
- organic light emitting diodes
- Organic chemistry
- fluorescent materials