Periodic Reporting for period 1 - PROLED (Polymer Radicals as doublet emitters for Organic Light-Emitting Diodes)
Période du rapport: 2020-10-01 au 2022-09-30
For organic emitters used in light-emitting diodes, electrical excitations generate singlet and triplet excited states in 1:3 ratio due to spin statistics. In case of fluorescent emitters, only the singlet state can decay radiatively and emit light in singlet–singlet (S1–S0) fluorescence, which means 75% of excitations are lost as dark triplets. Different strategies have been employed to harvest both singlet and triplet excitons from organic emitters, and thus increase the efficiency of light-emitting diodes. However, the challenge is that triplet–singlet (T1–S0) transition is spin forbidden. Neutral π-radicals are a completely different class of organic emitters, wherein the emission comes from spin doublet. Doublet–doublet (D1–D0) fluorescence is a totally spin allowed process enabling 100% radiative decay. Doublet fluorescence also benefits from fast emission in the nanosecond timescales, which further reduces chances for exciton quenching and improves the operational lifetime of the device.
The aim of this project is to develop polymeric π-radicals as a new class of highly luminescent emitters by chemically coupling molecular radicals into conjugated polymers. This approach combines the benefits of low-cost solution-based processing of conjugated polymers and the attainable high efficiency of doublet emission from radical materials.
The results of this project enable rational design of π-radicals with varied extent of conjugation spanning from small molecular radicals and biradicals to main-chain polyradicals, while providing practical tools for clean and quantitative synthesis of these radical materials. This project also adds to fundamental understanding of the emission process of π-radicals and reveals new approaches to the design highly emissive materials, which can be exploited in a wide range of organic optoelectronic applications where high yields of emissive excitons are needed.
The aim of this project is to develop polymeric π-radicals as a new class of highly luminescent emitters by chemically coupling molecular radicals into conjugated polymers. This approach combines the benefits of low-cost solution-based processing of conjugated polymers and the attainable high efficiency of doublet emission from radical materials.
The results of this project enable rational design of π-radicals with varied extent of conjugation spanning from small molecular radicals and biradicals to main-chain polyradicals, while providing practical tools for clean and quantitative synthesis of these radical materials. This project also adds to fundamental understanding of the emission process of π-radicals and reveals new approaches to the design highly emissive materials, which can be exploited in a wide range of organic optoelectronic applications where high yields of emissive excitons are needed.
First generation of polyradicals was synthesized to compare the performance of radicals in the polymer main chain versus radical as the pendant group. From these two systems, main chain polyradicals performed better, but the external quantum efficiencies of light-emitting diodes were low <1% for emission peaking beyond 700 nm in the near-infrared spectral region. The low performance originated from low solid-state photoluminescence yields of these polymers as well as solution processing of the devices, which commonly delivers lower efficiencies as compared to vacuum-deposited devices. Importantly, however, these experiments were the first demonstration of light-emitting diodes fabricated from conjugated polyradicals using solution-based processing. Second generation of polyradicals focused on tuning the emission wavelength. Depending on the extent of conjugation, polyradicals show either monomer-like emission below 700 nm or highly redshifted emission peaking beyond 800 nm. The latter originates from highly conjugated polyradicals, demonstrating that conjugation can be used as an effective tool to tune the wavelength of doublet emission.
After demonstrating the feasibility of polyradicals for use in solution-processed devices, the research extended to small molecular radicals and their device applications. Spectroscopic characterization aided the materials design towards mesityl-substituted TTM radicals (called MxTTM series). Mesitylation made symmetric radicals highly emissive, which contrasts with the previous understanding that symmetric radicals must be dark. Mesitylation also enhanced the emission of charge-transfer type radicals like TTM-3PCz, and the beneficiary obtained record-performing radical light-emitting diodes with an external quantum efficiency of 28% at 690 nm (internal quantum efficiency reaching 100%). Furthermore, coupling two charge-transfer radicals together boosted the photoluminescence yield to 100%, while the degree of conjugation between the two radicals affected the ground state spin multiplicity of the system and its response to an external magnetic field. Better understanding of conjugation and its relation to emission properties and spin–spin interactions will enable the access to higher spin states in small molecular and polymeric π-radicals.
The above-discussed results have been compiled into one submitted manuscript and two additional manuscripts currently being drafted. The beneficiary has also published a review article of the field in 2022. The beneficiary has presented the project results in two international conferences during 2022, and parts of the project have also been presented in conferences by other members of the host university.
After demonstrating the feasibility of polyradicals for use in solution-processed devices, the research extended to small molecular radicals and their device applications. Spectroscopic characterization aided the materials design towards mesityl-substituted TTM radicals (called MxTTM series). Mesitylation made symmetric radicals highly emissive, which contrasts with the previous understanding that symmetric radicals must be dark. Mesitylation also enhanced the emission of charge-transfer type radicals like TTM-3PCz, and the beneficiary obtained record-performing radical light-emitting diodes with an external quantum efficiency of 28% at 690 nm (internal quantum efficiency reaching 100%). Furthermore, coupling two charge-transfer radicals together boosted the photoluminescence yield to 100%, while the degree of conjugation between the two radicals affected the ground state spin multiplicity of the system and its response to an external magnetic field. Better understanding of conjugation and its relation to emission properties and spin–spin interactions will enable the access to higher spin states in small molecular and polymeric π-radicals.
The above-discussed results have been compiled into one submitted manuscript and two additional manuscripts currently being drafted. The beneficiary has also published a review article of the field in 2022. The beneficiary has presented the project results in two international conferences during 2022, and parts of the project have also been presented in conferences by other members of the host university.
During this project, the beneficiary has developed a series of novel π-radical molecules and incorporated them into conjugated structures. This work has led to three different branches of highly emissive materials: 1) monoradicals, 2) biradicals and 3) polyradicals. All three radical types are applicable in electroluminescent devices, and they reveal a number of novel optoelectronic properties.
Firstly, mesityl substitution significantly enhanced the emission of symmetric TTM radical, and the beneficiary demonstrated for the first time that light-emitting diodes fabricated from symmetric MxTTM radicals can reach external quantum efficiency as high as 14% at 590 nm in the yellow/orange region. Secondly, mesitylation also enhances the emission of charge-transfer type radicals like TTM-3PCz delivering a new record efficiency of 28% at 690 nm in the deep red spectral region. Coupling two charge-transfer radicals in a biradical configuration boosted the photoluminescence yield to the theoretical maximum of 100%, while the extent of conjugation between the radicals can be tuned to greatly affect the spin multiplicity and magnetic response of the system. Thirdly, the beneficiary has demonstrated the first functional solution-processed light-emitting diodes based on conjugated main-chain polyradicals. In the case of polyradicals, the extent of conjugation can greatly affect the emission wavelength from visible spectral region to the near-infrared without affecting the photoluminescence yield.
The beneficiary has introduced a new approach to tune the emission of radical materials by conjugation, rather than relying on charge-transfer states. The beneficiary has also developed new, widely applicable methodology for quantitative preparation and structural characterization of radical materials. This is significant because π-radicals are NMR silent, and their chemical structures cannot be characterized using conventional NMR methodologies.
The results of the project are expected to be applied widely in the organic optoelectronics field spanning from radical materials synthesis to advances in optical spectroscopy, OLED and other devices fabrication and system integration in academia and industry. Direct advantages emerge from new synthetic methodology to obtain molecular and polymeric radical materials that are clean and free of structural defects and deliver high emission efficiencies. Indirect advantages will arise from cost savings in fabrication of solution-processed and vacuum-deposited OLEDs with increased efficiency and decreased material costs.
Firstly, mesityl substitution significantly enhanced the emission of symmetric TTM radical, and the beneficiary demonstrated for the first time that light-emitting diodes fabricated from symmetric MxTTM radicals can reach external quantum efficiency as high as 14% at 590 nm in the yellow/orange region. Secondly, mesitylation also enhances the emission of charge-transfer type radicals like TTM-3PCz delivering a new record efficiency of 28% at 690 nm in the deep red spectral region. Coupling two charge-transfer radicals in a biradical configuration boosted the photoluminescence yield to the theoretical maximum of 100%, while the extent of conjugation between the radicals can be tuned to greatly affect the spin multiplicity and magnetic response of the system. Thirdly, the beneficiary has demonstrated the first functional solution-processed light-emitting diodes based on conjugated main-chain polyradicals. In the case of polyradicals, the extent of conjugation can greatly affect the emission wavelength from visible spectral region to the near-infrared without affecting the photoluminescence yield.
The beneficiary has introduced a new approach to tune the emission of radical materials by conjugation, rather than relying on charge-transfer states. The beneficiary has also developed new, widely applicable methodology for quantitative preparation and structural characterization of radical materials. This is significant because π-radicals are NMR silent, and their chemical structures cannot be characterized using conventional NMR methodologies.
The results of the project are expected to be applied widely in the organic optoelectronics field spanning from radical materials synthesis to advances in optical spectroscopy, OLED and other devices fabrication and system integration in academia and industry. Direct advantages emerge from new synthetic methodology to obtain molecular and polymeric radical materials that are clean and free of structural defects and deliver high emission efficiencies. Indirect advantages will arise from cost savings in fabrication of solution-processed and vacuum-deposited OLEDs with increased efficiency and decreased material costs.