Periodic Reporting for period 1 - TADFNIR (High performance OLEDs using deep red (DR) and near-infrared (NIR) TADF emitters)
Reporting period: 2020-11-02 to 2022-11-01
Organic light-emitting diodes (OLEDs) are widely used in niche lighting, smartphones, televisions/displays, watches and automotive displays, smart phones, wearable electronics, and solid-state lighting applications, owing to their phenomenal performances, including fast response time, wide viewing angles, thinner display, better contrast ratio, low power consumption, lightweight and exciting optical and electrical properties. Commercial OLEDs still rely on expensive and rare metal iridium (cost – US$5400/Oz, global production 3 tonnes per annum). Therefore, the development of economical and sustainable emitters for the replacement of expensive metal-based emitters is indispensable. Recent years have witnessed that thermally activated fluorescence (TADF) materials could be an alternative solution to expensive metal-based emitters. The remarkable features of TADF materials are that they can achieve 100% IQE, while maintaining a facile color-tuning ability. To date, numerous TADF materials have been reported and exploited in fabricating sky blue, green, and red OLEDs.
Deep red (DR) and near-infrared (NIR) emissive materials (650 nm to 750 nm) have occupied special attention given their potential in applications such as night vision devices, optical communication etc. Though osmium, iridium, or platinum complexes are known to exhibit long wavelength emission, they suffer from large efficiency roll-offs at high current densities, making them largely unsuitable for such commercial applications. The developments of TADF-based DR/NIR materials are not in pace with other color TADF emitters because of their low photoluminescence quantum yields and OLED device performances. We have addressed the low photoluminescence quantum yields of emitters of DR/NIR TADF materials by adopting (i) a rigid and strong acceptor strategy, (ii) aggregation-induced emission active unit incorporation into the deep red TADF emitter. We obtained moderate PLQY values and then employed these emitters in OLEDs. In the course of our efforts, we discovered and assessed the potential of a weak electron donor for use in blue TADF emitter design.
Deep red (DR) and near-infrared (NIR) emissive materials (650 nm to 750 nm) have occupied special attention given their potential in applications such as night vision devices, optical communication etc. Though osmium, iridium, or platinum complexes are known to exhibit long wavelength emission, they suffer from large efficiency roll-offs at high current densities, making them largely unsuitable for such commercial applications. The developments of TADF-based DR/NIR materials are not in pace with other color TADF emitters because of their low photoluminescence quantum yields and OLED device performances. We have addressed the low photoluminescence quantum yields of emitters of DR/NIR TADF materials by adopting (i) a rigid and strong acceptor strategy, (ii) aggregation-induced emission active unit incorporation into the deep red TADF emitter. We obtained moderate PLQY values and then employed these emitters in OLEDs. In the course of our efforts, we discovered and assessed the potential of a weak electron donor for use in blue TADF emitter design.
Each WP consists of designing materials, computational screening, designing synthetic routes, successful synthesis with high purity, optoelectronic characterization and finally investigating their EL performance.
Work Package 1
Modelling of TADF emitters containing a 1,9-pyrazoloanthrone: We modelled the proposed three target emitters containing 1,9-pyrazoloanthrone acceptor using DFT/TD-DFT calculations at PBE0/6-311G (d, p) level of theory revealing small singlet-triplet energy gaps in the range of 10-150 meV; indicating that these can be potential TADF emitters.
Work Package 2
Synthesis and molecular characterization of 1,9-pyrazoloanthrone containing emitters: We synthesized the proposed three emitters containing 1,9-pyrazoloanthrone acceptor and donors such as carbazole, phenoxazine, and phenothiazine successfully. The emitters were characterized by NMR (1H & 13C) and high-resolution mass spectrometry (HRMS) and high-performance liquid chromatography (HPLC).
Work Package 3
Physical and optoelectronic characterization of 1,9-pyrazoloanthrone based emitters: We have investigated photophysical and electrochemical properties of three emitters containing 1,9-pyrazoloanthrone. Interestingly, of the three emitters, the carbazole-substituted analog exhibited mechanochromism. Disappointingly, the photoluminescence quantum yields (PLQYs) of the emitters were very low (< 2%) in the solid state and hence these compounds are unsuitable as emitters for OLEDs and devices were not fabricated. (Manuscript under preparation)
Work Package 4
Deep red TADF emitters containing a benzo[g]quinoxaline-5,10-dione acceptor: We designed three emitters that showed thermally activated delayed fluorescence covering from orange to the deep red region with moderate PLQYs ranging from 31% to 34%. Interestingly, they also exhibited remarkable mechanical pressure-induced spectral shifts (∆λ > 150 nm) caused by a crystalline-to-amorphous transformation. To the best of our knowledge, this is the first TADF emitter that showed such a large shift of the emission upon grinding. We have fabricated the deep red OLEDs using one of these emitters. (Manuscript under preparation)
Work package 5
AIE-active deep red TADF emitters: We have proposed a strategy to boost the PLQY of the DR/NIR TADF emitters by incorporating of AIE active unit, tetraphenylethene. We designed two emitters with AIE units that showed AIE activity, TADF, yet modest PLQYs in the range of 11% to 19. Particularly, aggregates (90% THF/water mixture) showed prominent near-infrared emission > 700 nm. Two control emitters were also synthesized without the TPE group to explore what impact if any the TPE group has on the photophysical properties. (Manuscript under preparation)
Work package 6
Anthraquinone-based deep red/near-infrared emitters: In this project, four emitters were designed in which donors are decorated in the cis and trans manner on the acceptor core. The compounds were modelled that have a small singlet and triplet energy gap (∆EST) in the range of 10- 160 meV. We have synthesized and structurally characterized them. Their photophysical and electrochemical properties were investigated. Four emitters exhibited the emission ranging from 700 nm to 820 nm in the solid state and PLQYs are 0.1% to 5% range.(Manuscript under preparation)
Work package 7
Azaborine-based deep blue TADF emitters: We have introduced into the TADF toolbox azaborine as a very weak donor moiety for the construction of donor-acceptor TADF emitters. Using this donor, two emitters were designed and investigated their optoelectronic properties. Further, these compounds were used as emitters in OLEDs that exhibit decent maximum external quantum efficiencies of 2.1% and 10.5% in the blue region. This work has been placed on a preprint server (DOI: 10.26434/chemrxiv-2022-jb4v0) and has been submitted for peer review.
Work Package 1
Modelling of TADF emitters containing a 1,9-pyrazoloanthrone: We modelled the proposed three target emitters containing 1,9-pyrazoloanthrone acceptor using DFT/TD-DFT calculations at PBE0/6-311G (d, p) level of theory revealing small singlet-triplet energy gaps in the range of 10-150 meV; indicating that these can be potential TADF emitters.
Work Package 2
Synthesis and molecular characterization of 1,9-pyrazoloanthrone containing emitters: We synthesized the proposed three emitters containing 1,9-pyrazoloanthrone acceptor and donors such as carbazole, phenoxazine, and phenothiazine successfully. The emitters were characterized by NMR (1H & 13C) and high-resolution mass spectrometry (HRMS) and high-performance liquid chromatography (HPLC).
Work Package 3
Physical and optoelectronic characterization of 1,9-pyrazoloanthrone based emitters: We have investigated photophysical and electrochemical properties of three emitters containing 1,9-pyrazoloanthrone. Interestingly, of the three emitters, the carbazole-substituted analog exhibited mechanochromism. Disappointingly, the photoluminescence quantum yields (PLQYs) of the emitters were very low (< 2%) in the solid state and hence these compounds are unsuitable as emitters for OLEDs and devices were not fabricated. (Manuscript under preparation)
Work Package 4
Deep red TADF emitters containing a benzo[g]quinoxaline-5,10-dione acceptor: We designed three emitters that showed thermally activated delayed fluorescence covering from orange to the deep red region with moderate PLQYs ranging from 31% to 34%. Interestingly, they also exhibited remarkable mechanical pressure-induced spectral shifts (∆λ > 150 nm) caused by a crystalline-to-amorphous transformation. To the best of our knowledge, this is the first TADF emitter that showed such a large shift of the emission upon grinding. We have fabricated the deep red OLEDs using one of these emitters. (Manuscript under preparation)
Work package 5
AIE-active deep red TADF emitters: We have proposed a strategy to boost the PLQY of the DR/NIR TADF emitters by incorporating of AIE active unit, tetraphenylethene. We designed two emitters with AIE units that showed AIE activity, TADF, yet modest PLQYs in the range of 11% to 19. Particularly, aggregates (90% THF/water mixture) showed prominent near-infrared emission > 700 nm. Two control emitters were also synthesized without the TPE group to explore what impact if any the TPE group has on the photophysical properties. (Manuscript under preparation)
Work package 6
Anthraquinone-based deep red/near-infrared emitters: In this project, four emitters were designed in which donors are decorated in the cis and trans manner on the acceptor core. The compounds were modelled that have a small singlet and triplet energy gap (∆EST) in the range of 10- 160 meV. We have synthesized and structurally characterized them. Their photophysical and electrochemical properties were investigated. Four emitters exhibited the emission ranging from 700 nm to 820 nm in the solid state and PLQYs are 0.1% to 5% range.(Manuscript under preparation)
Work package 7
Azaborine-based deep blue TADF emitters: We have introduced into the TADF toolbox azaborine as a very weak donor moiety for the construction of donor-acceptor TADF emitters. Using this donor, two emitters were designed and investigated their optoelectronic properties. Further, these compounds were used as emitters in OLEDs that exhibit decent maximum external quantum efficiencies of 2.1% and 10.5% in the blue region. This work has been placed on a preprint server (DOI: 10.26434/chemrxiv-2022-jb4v0) and has been submitted for peer review.
We developed a series of DR/NIR emitters by following two effective strategies. Strategy 1 is introducing a rigid and strong acceptor to suppress the vibrational quenching. Based on this strategy, we have developed promising DR TADF emitters with moderate PLQYs and OLED performance. Astoundingly, two emitters exhibit mechanochromism with a large magnitude of spectral shift. Strategy 2 is combining an aggregation-induced emission (AIE) active unit with a red TADF emitter to suppress the aggregation-caused quenching (ACQ). Utilizing this strategy, a series of molecules were designed and investigated. These emitters exhibited aggregation-induced near-infrared emission (>700 nm) with moderate PLQY values. During these efforts, we also discovered an efficient and powerful weak electron acceptor that we demonstrated could be used in the design of blue TADF emitters.