Periodic Reporting for period 1 - TADF-DLC-OLEDs (Development of highly efficient solution-processable blue organic light-emitting diodes based on liquid-crystalline thermally activated delayed fluorescent emitter materials)
Reporting period: 2021-09-02 to 2023-09-01
This project aimed to develop efficient, stable, and inexpensive blue thermally activated delayed fluorescence (TADF) emitters based on discotic liquid crystalline (DLC) molecules, which will facilitate horizontal alignment of the transition dipole moment in solution-processed films in order to improve the light-outcoupling efficiency and hence the external quantum efficiency (EQE) in solution-processable organic light-emitting diodes (SP-OLEDs). To achieve this horizon, we first explored how to regulate the columnar self-assembly in the liquid crystalline state of the discotic TADF emitters, which had not been investigated in the previous reports. In this project, we proposed the following different design strategies to obtain horizontal alignment in DLCs with TADF emission: i) Strategy 1 is through implication of the central multi-resonant TADF (MR-TADF) moiety attached with peripheral mesogenic unit maintaining the discotic shape of the designed molecule; ii) Strategy 2 regulates the electronic coupling between a donor-acceptor (D-A) TADF, which consists of a small molecular acceptor and a donor-containing mesogenic unit, through molecular conjugation.
Objectives
Overall assessment: The project has achieved one of its two major objectives and milestones for the period, with relatively minor deviations.
Over the period of the fellowship, JD has successfully achieved specific scientific and training objectives as specified in the DoA:
Computational calculations: JD has carried out numerous theoretical calculations for TADF emitters and model compounds using density functional theory (DFT) protocols in the group.
Organic synthesis: JD has received the opportunity to strengthen his organic synthetic skills by continuing to work on air and moisture-sensitive reactions such as lithiation reactions, C-C, and C-N couplings.
Advanced purification techniques: JD received hands-on experience in using purification techniques such as combiflash, gel permeation chromatography (GPC), and gradient temperature sublimation techniques.
Analytical skills: JD strengthened his analytical skills through hands-on training on GC-MS, HPLC, and 1D/2D NMR (1H, 13C, 11B, 19F) spectroscopy.
Optoelectronic characterization skill: JD expanded his knowledge of spectroscopic instruments such as UV-vis absorption spectrophotometers, electrochemical workstations, fluorimeters, time-resolved fluorescence instruments, and temperature-dependent PL.
Device fabrication skill: JD reinforced device fabrication techniques and expanded his device fabrication skills to solution-processed OLEDs.
Data management: JD has received training and learned better ways to resource and data management.
Communication skills: JD has strengthened his communication and presentation skills by presenting his projects and progress to group meetings and subgroup meetings. JD has received multiple opportunities to discuss his projects with visiting group leaders from other universities and collaborators.
Objectives
Overall assessment: The project has achieved one of its two major objectives and milestones for the period, with relatively minor deviations.
Over the period of the fellowship, JD has successfully achieved specific scientific and training objectives as specified in the DoA:
Computational calculations: JD has carried out numerous theoretical calculations for TADF emitters and model compounds using density functional theory (DFT) protocols in the group.
Organic synthesis: JD has received the opportunity to strengthen his organic synthetic skills by continuing to work on air and moisture-sensitive reactions such as lithiation reactions, C-C, and C-N couplings.
Advanced purification techniques: JD received hands-on experience in using purification techniques such as combiflash, gel permeation chromatography (GPC), and gradient temperature sublimation techniques.
Analytical skills: JD strengthened his analytical skills through hands-on training on GC-MS, HPLC, and 1D/2D NMR (1H, 13C, 11B, 19F) spectroscopy.
Optoelectronic characterization skill: JD expanded his knowledge of spectroscopic instruments such as UV-vis absorption spectrophotometers, electrochemical workstations, fluorimeters, time-resolved fluorescence instruments, and temperature-dependent PL.
Device fabrication skill: JD reinforced device fabrication techniques and expanded his device fabrication skills to solution-processed OLEDs.
Data management: JD has received training and learned better ways to resource and data management.
Communication skills: JD has strengthened his communication and presentation skills by presenting his projects and progress to group meetings and subgroup meetings. JD has received multiple opportunities to discuss his projects with visiting group leaders from other universities and collaborators.
Work Package 1:
A series of TADF active DLC emitters were modelled computationally: We modelled the excited state optoelectronic properties of all the studied DLCs by spin-component scaling second-order approximate coupled-cluster (SCS-CC2) calculations. Besides, the ground state electronic structure was modelled at the PBE0/6-31G(d,p) level of density functional theory (DFT) for all the molecules. The results of the theoretical calculation demonstrated that all the molecules can show potential TADF characteristics with small ΔEST values in the range of 10-300 meV. We successfully synthesized the five final TADF DLC emitters. The emitters were characterized by NMR (1H & 13C) and high-resolution mass spectrometry (HRMS) and high-performance liquid chromatography (HPLC).
Work Package 2:
Liquid crystalline nature of the target compounds was established by polarized optical microscopy (POM). The careful textural observation provided an indication of the formation of columnar (Col) mesophases. Exact phase transition temperatures corresponding to the ob-served transitions were confirmed by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). For further determination of the self-assembling behavior of the Col mesophases into different type of lattices, small and wide angle X-ray scattering (SAXS/WAXS) studies were carried out.
Work Package 3:
Physical and optoelectronic characterization of the emitters: We have investigated the photophysical and electrochemical properties of five DLC emitters in solution and solid state. In all the doped and neat films of these emitters, we observed the characteristic delayed emission in temperature-dependent time-resolved PL decay. All the emitters exhibited moderate to high PLQYs in the range of 55% to 81.2% in the doped as well as neat films of the emitters. Consequently, these compounds are used as emitters in OLEDs.
Work Package 4:
In order to study the molecular orientation of the DLC emitters in solution-processed OLEDs, angle-dependent photoluminescence spectroscopy studies was carried out.
A series of TADF active DLC emitters were modelled computationally: We modelled the excited state optoelectronic properties of all the studied DLCs by spin-component scaling second-order approximate coupled-cluster (SCS-CC2) calculations. Besides, the ground state electronic structure was modelled at the PBE0/6-31G(d,p) level of density functional theory (DFT) for all the molecules. The results of the theoretical calculation demonstrated that all the molecules can show potential TADF characteristics with small ΔEST values in the range of 10-300 meV. We successfully synthesized the five final TADF DLC emitters. The emitters were characterized by NMR (1H & 13C) and high-resolution mass spectrometry (HRMS) and high-performance liquid chromatography (HPLC).
Work Package 2:
Liquid crystalline nature of the target compounds was established by polarized optical microscopy (POM). The careful textural observation provided an indication of the formation of columnar (Col) mesophases. Exact phase transition temperatures corresponding to the ob-served transitions were confirmed by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). For further determination of the self-assembling behavior of the Col mesophases into different type of lattices, small and wide angle X-ray scattering (SAXS/WAXS) studies were carried out.
Work Package 3:
Physical and optoelectronic characterization of the emitters: We have investigated the photophysical and electrochemical properties of five DLC emitters in solution and solid state. In all the doped and neat films of these emitters, we observed the characteristic delayed emission in temperature-dependent time-resolved PL decay. All the emitters exhibited moderate to high PLQYs in the range of 55% to 81.2% in the doped as well as neat films of the emitters. Consequently, these compounds are used as emitters in OLEDs.
Work Package 4:
In order to study the molecular orientation of the DLC emitters in solution-processed OLEDs, angle-dependent photoluminescence spectroscopy studies was carried out.
During the period of the fellowship, I have gained a significant amount of knowledge and experience for reaching the next level of my career. I have gained expertise in organic syn-thesis, photophysics, molecular design and simulation, and solution-processed devices, and received continuous training in advanced techniques to deepen my understanding of organic semiconductors, i.e. training related to circular dichroism spectroscopy, CPL-spectroscopy, ambient photoemission spectroscopy, gated iCCD system, etc. I have improved my leader-ship skills by mentoring visiting students from outside, collaborating with team members and involving in day-to-day lab management. Through utilizing existing contacts of EZ-C, I have acquired an opportunity to build up a very healthy collaborative network with other groups at USTAN and with renowned groups all over the globe. I have improved my oral and presentation skills by presenting my research on numerous occasions to experts in the field and to non-experts. During the fellowship period, I benefitted from courses offered through USTAN’s training program for researchers via the CEED https://www.st-andrews.ac.uk/ceed/(opens in new window) provided many additional profes-sional development courses, resources, and consultancy, all of which will ensure the possi-bility of reaching a position of professional maturity in the near future. I received an oppor-tunity to participate in the 71st Lindau Nobel Laureate Meeting. This meeting provided me with a unique chance to spend an inspiring week with Nobel Laureates to exchange experiences and establish valuable contacts in science and beyond for doing world-leading research. The design strategies I developed during this project have provided mechanistic insight into devising emitters that show enhanced out-coupling efficiency and demonstrated the feasibility of fab-ricating OLEDs with the TADF active DLC emitter I designed.