The Study of Thermally Activated Delayed Fluorescence Mechanism in Mono and Bimolecular Systems

After 30 years since the first organic light emitting diode (OLED) was reported by Tang and VanSlyke, devices based on thermally activated delayed fluorescence (TADF) emitters have shown to be the most promising and efficient approach to convert dark triplet states into emissive singlet states. The TADF mechanism relies on thermal energy to raise the triplet state to a vibronic sub level that is isoenergetic with the singlet state, thus enabling reverse intersystem crossing (rISC) and allowing internal quantum efficiency values up to 100%. Major challenges faced by TADF studies persist, concerning the full understanding of the mechanism, as it is strongly affected by the environment in which the emitter is dispersed and the different conformations that the molecules can access. Throughout the course of this thesis, the photophysical and chemical properties of the TADF mechanism were investigated in various organic molecules including novel D-A-D and D-A3 molecules, bimolecular (exciplex) blends and excited state intramolecular proton transfer (ESIPT). Important new contributions towards the full elucidation of the TADF mechanism are presented, mainly regarding the current TADF vibronic coupling mechanism model, which highlights the need for three excited states (singlet charge transfer state; triplet charge transfer states; and triplet local excited state) to come into resonance to achieve high TADF efficiency. In addition, a solution to the dilemma that a TADF emitter cannot have both unity photoluminescence quantum yield and fast rISC rates is presented and high efficient OLEDs are shown. Moreover, it is discussed how different molecular conformers affect the efficiency of the TADF mechanism by studying molecules that show dual charge transfer emission. Furthermore, it is shown that the emitter and host combination must be optimized to minimize the rISC barrier and maximize the TADF in blue OLEDs. Additionally, the use of ESIPT emitters to generate TADF is discussed

Improving processability and efficiency of Resonant TADF emitters : a design strategy

This work is funded by the EC through the Horizon 2020 Marie Sklodowska-Curie ITN project TADFlife. The St Andrews team would also like to thank the Leverhulme Trust (RPG-2016- 047) and EPSRC (EP/P010482/1) for financial support. Computational resources have been provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifiques de Belgique (F.R.S.-FNRS) under Grant No. 2.5020.11, as well as the Tier-1 supercomputer of the Fédération Wallonie-Bruxelles, infrastructure funded by the Walloon Region under the grant agreement n1117545. AP acknowledges the financial support from the Marie Curie Fellowship (MILORD project, N°. 748042). DB is a FNRS Research Director. We thank Franck-Julian Kahle for support with data analysis.A new design strategy is introduced to address a persistent weakness with resonance thermally activated delayed fluorescence (R-TADF) emitters to reduce aggregation-caused quenching effects, which we identify as one of the key limiting factors. The emitter Mes3DiKTa shows an improved photoluminescence quantum yield of 80% compared to 75% for the reference DiKTa in 3.5 wt% mCP. Importantly, emission from aggregates, even at high doping concentrations, is eliminated and aggregation-caused quenching is strongly curtailed. For both molecules, triplets are almost quantitatively upconverted into singlets in electroluminescence, despite a significant (~0.21 eV) singlet-triplet energy gap (ΔEST), in line with correlated quantum-chemical calculations, and a slow reverse intersystem crossing. We speculate that the lattice stiffness responsible for the narrow fluorescence and phosphorescence emission spectra also protects the triplets against non-radiative decay. An improved EQEmax of 21.1% for Mes3DIKTa compared to the parent DiKTa (14.7%) and, importantly, reduced efficiency roll- off compared to literature resonance TADF OLEDs, shows the promise of this design strategy for future design of R-TADF emitters for OLED applications.Publisher PDFPeer reviewe

Ansiedade cognitiva de provas e procrastinação acadêmica: um estudo com universitários do Brasil:

The aim of this study was to investigate the predictive power of cognitive test anxiety on academic procrastination in university students from the interior of northeastern Brazil, as well as to determine if the levels of these constructs differ based on gender. A total of 204 university students from the northeastern region participated in the study, with a majority of female participants. The Cognitive Test Anxiety Scale, Tuckman Procrastination Scale, and sociodemographic questions were used in the data collection. The results, through Pearson correlation and multiple regression, demonstrated that cognitive test anxiety was positively related to academic procrastination. Finally, the independent samples t-test showed that women had higher levels of cognitive test anxiety compared to men. It can be concluded that cognitive test anxiety explains academic procrastination and that women are more vulnerable to anxiety in evaluative contexts.Objetivou-se verificar o poder preditivo da ansiedade cognitiva de provas na procrastinação acadêmica de universitários do interior do nordeste brasileiro, além de verificar se os níveis dos construtos abordados se diferem em função do gênero. Participaram 204 universitários do interior do nordeste brasileiro, que eram em maioria mulheres. Foram utilizadas Escala de Ansiedade Cognitiva de Provas, a Escala de Procrastinação de Tuckman e questões sociodemográficas. Os resultados, por meio da correlação de Pearson e regressão múltipla, demonstraram que a ansiedade cognitiva de provas estava relacionada positivamente com a procrastinação acadêmica. Por fim, o teste-t de Student, para amostras independentes, demonstrou que as mulheres apresentaram maiores níveis de ansiedade cognitiva frente a provas em comparação aos homens. Conclui-se que a ansiedade cognitiva de provas explica a procrastinação acadêmica e que as mulheres que se apresentam como mais vulneráveis para a ansiedade em contextos avaliativos

Cognitive test anxiety and academic procrastination: a study with university students in Brasil

Objetivou-se verificar o poder preditivo da ansiedade cognitiva de provas na procrastinação acadêmica de universitários do interior do nordeste brasileiro, além de verificar se os níveis dos construtos abordados se diferem em função do gênero. Participaram 204 universitários do interior do nordeste brasileiro, que eram em maioria mulheres. Foram utilizadas Escala de Ansiedade Cognitiva de Provas, a Escala de Procrastinação de Tuckman e questões sociodemográficas. Os resultados, por meio da correlação de Pearson e regressão múltipla, demonstraram que a ansiedade cognitiva de provas estava relacionada positivamente com a procrastinação acadêmica. Por fim, o teste-t de Student, para amostras independentes, demonstrou que as mulheres apresentaram maiores níveis de ansiedade cognitiva frente a provas em comparação aos homens. Conclui-se que a ansiedade cognitiva de provas explica a procrastinação acadêmica e que as mulheres que se apresentam como mais vulneráveis para a ansiedade em contextos avaliativos.The aim of this study was to investigate the predictive power of cognitive test anxiety on academic procrastination in university students from the interior of northeastern Brazil, as well as to determine if the levels of these constructs differ based on gender. A total of 204 university students from the northeastern region participated in the study, with a majority of female participants. The Cognitive Test Anxiety Scale, Tuckman Procrastination Scale, and sociodemographic questions were used in the data collection. The results, through Pearson correlation and multiple regression, demonstrated that cognitive test anxiety was positively related to academic procrastination. Finally, the independent samples t-test showed that women had higher levels of cognitive test anxiety compared to men. It can be concluded that cognitive test anxiety explains academic procrastination and that women are more vulnerable to anxiety in evaluative contexts.Grupo de Investigación HUM-672 AREA (Análisis de la Realidad EducativA

Mecanismos de fluorescência atrasada em polímeros conjugados

Exportado OPUSMade available in DSpace on 2019-08-14T01:05:42Z (GMT). No. of bitstreams: 1 disserta__o_palomalays.pdf: 3636773 bytes, checksum: bf30e932fe785091fa35c821a44815c8 (MD5) Previous issue date: 4Neste trabalho estudamos as propriedades ópticas em dois conjuntos de amostras, amostras fabricadas com o copolímero F8T2 (Poly(9,9-dioctylfluorene-alt-bithiophene)) e amostras fabricadas com o polímero P3HT (Poly(3-hexylthiophene-2,5-diyl)). As amostras foram produzidas em dois arranjos distintos, em solução e em filmes com Zeonex®. A caracterização óptica foi realizada através de técnicas padrões de absorção óptica, fotoluminescência e eficiência quântica de emissão. Foram utilizadas também técnicas mais sofisticadas que apresentam resolução temporal, são elas, fluorescência resolvida no tempo e medidas de fluorescência atrasada, utilizando, respectivamente, um sistema baseado no método de contagem de fótons únicos correlacionados temporalmente e um sistema dipondo de uma iCCD (intensified Charge-Coupled Device) capaz de detectar a emissão de fluorescência atrasada e a emissão fosforescente. As medidas ópticas foram realizadas sob diferentes temperaturas e diferentes potências do laser de excitação. Mostramos, com base em resultados experimentais, que o mecanismo termicamente ativado (TADF) é o responsável pela geração de emissão fluorescente atrasada no conjunto de amostras fabricadas com o copolímero F8T2. Para o conjunto de amostras fabricadas com o polímero P3HT, mostramos que o mecanismo gerador de fluorescência atrasada é o mecanismo de aniquilação tripleto-tripleto (ATT). Além da caracterização do efeito de fluorescência atrasada apresentamos ainda os espectros de emissão fosforescente dos dois materiais, o que nos permitiu um estudo mais completo dos estados tripletos.In this work, we studied optical properties of two different groups of samples, which were made with the F8T2 copolymer (Poly(9,9-dioctylfluorene-alt-bithiophene)) and the P3HT polymer (Poly(3-hexylthiophene-2,5-diyl)) belonging to the first and second group, respectively. The samples were produced in two different ways: in diluted solutions and in films containing Zeonex®. The optical characterizations of the samples were performed by standard Optical Absorption, Photoluminescence and Photoluminescence Quantum Efficiency techniques. Other sophisticated techniques with temporal resolution namely Time Correlation Single Photon Counting Technique and measurements of Delayed Emissions and Phosphorescence using a sensitive gated iCCD camera (intensified Charge-Coupled Device) were also used. The measurements were performed as a function of the excitation laser intensity and/or as a function of temperature. We showed that the delayed fluorescence (DF) recorded for F8T2 samples originate from Thermally Activated Delayed Fluorescence mechanism (TADF) whereas it was origineted from Triplet-Triplet Annihilation mechanism for P3HT samples. Beyond the delayed fluorescence characterization, the phosphorescent emission spectra were also recorded for F8T2 and P3HT, which allowed us to make a complete study of the triplet states for these materials

Excited State Dynamics of Thermally Activated Delayed Fluorescence from an Excited State Intramolecular Proton Transfer System

We describe the photophysical processes that give rise to thermally activated delayed fluorescence in the excited state intramolecular proton transfer (ESIPT) molecule, triquinolonobenzene (TQB). Using transient absorption and time-resolved photoluminescence spectroscopy, we fully characterize prompt and delayed emission, phosphorescence, and oxygen quenching to reveal the reverse intersystem crossing mechanism (rISC). After photoexcitation and rapid ESIPT to the TQB-TB tautomer, emission from S1 is found to compete with thermally activated ISC to an upper triplet state, T2, very close in energy to S1 and limiting photoluminescence quantum yield. T2 slowly decays to the lowest triplet state, T1, via internal conversion. In the presence of oxygen, T2 is quenched to the ground state of the double proton transferred TQB-TC tautomer. Our measurements demonstrate that rISC in TQB occurs from T2 to S1 driven by thermally activated reverse internal conversion from T1 to T2 and support recent calculations by Cao et al. (Cao, Y.; Eng, J.; Penfold, T. J. Excited State Intramolecular Proton Transfer Dynamics for Triplet Harvesting in Organic Molecules

Use of pyrimidine and pyrazine bridges as a design strategy to improve the performance of thermally activated delayed fluorescence organic light emitting diodes

Authors are grateful to the Engineering and Physical Sciences Research Council (EPSRC) for support from grants EP/P010482/1 and EP/R035164/1. P. Rajamalli acknowledges support from a Marie Skłodowska-Curie Individual Fellowship (MCIF; No. 749557). Dongyang Chen thanks the China Scholarship Council (grant numbers 201603780001).We present a study of two isomeric thermally activated delayed fluorescence (TADF) emitters 9,9'- (sulfonylbis(pyrimidine-5,2-diyl))bis(3,6-di-tert-butyl-9H-carbazole) ( pDTCz-DPmS ) and 9,9'- (sulfonylbis(pyrazine-5,2-diyl))bis(3,6-di-tert-butyl-9H-carbazole) ( pDTCz-DPzS ). The use of pyrimidine and pyrazine as bridging units between the electron donor and acceptor moieties is found to be advantageous compared to the phenyl- (pDTCz-DPS) and pyridine-based analogues ( pDTCz-3DPyS and pDTCz-2DPyS ). Conformational modulation of the donor groups as a function of the bridge results in high photoluminescence quantum yields (ΦPL > 68%) and small energy gaps between singlet and triplet excited states (ΔEST < 160 meV). OLEDs using pDTCz-DPmS and pDTCz-DPzS as emitters exhibit blue and green electroluminescence, respectively, with higher maximum external quantum efficiencies (EQEmax of 14% and 18%, respectively) and reduced efficiency roll-off as compared to the reference devices using pDTCz-DPS , pDTCz-3DPyS , and pDTCz-2DPyS as the emitters. Our results provide a more complete understanding on the impact of the bridge structure in D-A-D TADF systems on the optoelectronic properties of the emitter, and how the balance between color purity and EQE in the devices can be controlled, advancing the design strategies for TADF emitters.PostprintPeer reviewe

Dynamics of aggregated states resolved by gated fluorescence in films of room temperature phosphorescent emitters

Phenazine derivative molecules were studied using steady state and time resolved fluorescence techniques and demonstrated to lead to strong formation of aggregated species, identified as dimers by time dependent density functional theory calculations. Blended films in a matrix of Zeonex®, produced at different concentrations, showed different contributions of dimer and monomer emissions in a prompt time frame, e.g. less than 50 ns. In contrast, the phosphorescence (e.g. emission from the triplet state) shows no significant effect on dimer formation, although strong dependence of the phosphorescence intensity on concentration is observed, leading to phosphorescence being quenched at higher concentration

Use of pyrimidine and pyrazine bridges as a design strategy to improve the performance of thermally activated delayed fluorescence organic light emitting diodes

We present a study of two isomeric thermally activated delayed fluorescence (TADF) emitters 9,9'- (sulfonylbis(pyrimidine-5,2-diyl))bis(3,6-di-tert-butyl-9H-carbazole) (pDTCz-DPmS) and 9,9'- (sulfonylbis(pyrazine-5,2-diyl))bis(3,6-di-tert-butyl-9H-carbazole) (pDTCz-DPzS). The use of pyrimidine and pyrazine as bridging units between the electron donor and acceptor moieties is found to be advantageous compared to the phenyl- (pDTCz-DPS) and pyridine-based analogues (pDTCz-3DPyS and pDTCz-2DPyS). Conformational modulation of the donor groups as a function of the bridge results in high photoluminescence quantum yields (ΦPL &gt; 68%) and small energy gaps between singlet and triplet excited states (ΔEST &lt; 160 meV). OLEDs using pDTCz-DPmS and pDTCz-DPzS as emitters exhibit blue and green electroluminescence, respectively, with higher maximum external quantum efficiencies (EQEmax of 14% and 18%, respectively) and reduced efficiency roll-off as compared to the reference devices using pDTCz-DPS, pDTCz-3DPyS, and pDTCz-2DPyS as the emitters. Our results provide a more complete understanding on the impact of the bridge structure in D-A-D TADF systems on the optoelectronic properties of the emitter, and how the balance between color purity and EQE in the devices can be controlled, advancing the design strategies for TADF emitters

Efficient sky-blue organic light-emitting diodes using a highly horizontally oriented thermally activated delayed fluorescence emitter

Organic thermally activated delayed fluorescent (TADF) materials can harvest 100% of the electrically generated excitons as a result of their small singlet-triplet energy difference. However, maximizing the External Quantum Efficiency (EQE) of a device also requires enhancing the light-outcoupling efficiency. In this work, we present a new Acceptor-Donor-Acceptor (ADA) emitter employing an indolocarbazole donor and diphenyltriazine acceptors that shows nearly-completely horizontal orientation regardless of the host matrix, leading to a sky-blue OLED (λEL=483nm, CIE coordinates of 0.17, 0.32) with EQEMAX of 22.1%, a maximum luminance of 7800 cd/m2 and blue emission