Understanding the luminescent nature of organic radicals for efficient doublet emitters and pure-red light-emitting diodes.

The doublet-spin nature of radical emitters is advantageous for applications in organic light-emitting diodes, as it avoids the formation of triplet excitons that limit the electroluminescence efficiency of non-radical emitters. However, radicals generally show low optical absorption and photoluminescence yields. Here we explain the poor optical properties of radicals based on alternant hydrocarbons, and establish design rules to increase the absorption and luminescence yields for donor-acceptor-type radicals. We show that non-alternant systems are necessary to lift the degeneracy of the lowest energy orbital excitations; moreover, intensity borrowing from an intense high-lying transition by the low-energy charge-transfer excitation enhances the oscillator strength of the emitter. We apply these rules to design tris(2,4,6-trichlorophenyl)methyl-pyridoindolyl derivatives with a high photoluminescence quantum yield (>90%). Organic light-emitting diodes based on these molecules showed a pure-red emission with an over 12% external quantum efficiency. These insights may be beneficial for the rational design and discovery of highly luminescent doublet emitters

High stability and luminescence efficiency in donor-acceptor neutral radicals not following the Aufbau principle.

With their unusual electronic structures, organic radical molecules display luminescence properties potentially relevant to lighting applications; yet, their luminescence quantum yield and stability lag behind those of other organic emitters. Here, we designed donor-acceptor neutral radicals based on an electron-poor perchlorotriphenylmethyl or tris(2,4,6-trichlorophenyl)methyl radical moiety combined with different electron-rich groups. Experimental and quantum-chemical studies demonstrate that the molecules do not follow the Aufbau principle: the singly occupied molecular orbital is found to lie below the highest (doubly) occupied molecular orbital. These donor-acceptor radicals have a strong emission yield (up to 54%) and high photostability, with estimated half-lives reaching up to several months under pulsed ultraviolet laser irradiation. Organic light-emitting diodes based on such a radical emitter show deep-red/near-infrared emission with a maximal external quantum efficiency of 5.3%. Our results provide a simple molecular-design strategy for stable, highly luminescent radicals with non-Aufbau electronic structures.Includes EPSRC

Electron spin resonance resolves intermediate triplet states in delayed fluorescence.

Molecular organic fluorophores are currently used in organic light-emitting diodes, though non-emissive triplet excitons generated in devices incorporating conventional fluorophores limit the efficiency. This limit can be overcome in materials that have intramolecular charge-transfer excitonic states and associated small singlet-triplet energy separations; triplets can then be converted to emissive singlet excitons resulting in efficient delayed fluorescence. However, the mechanistic details of the spin interconversion have not yet been fully resolved. We report transient electron spin resonance studies that allow direct probing of the spin conversion in a series of delayed fluorescence fluorophores with varying energy gaps between local excitation and charge-transfer triplet states. The observation of distinct triplet signals, unusual in transient electron spin resonance, suggests that multiple triplet states mediate the photophysics for efficient light emission in delayed fluorescence emitters. We reveal that as the energy separation between local excitation and charge-transfer triplet states decreases, spin interconversion changes from a direct, singlet-triplet mechanism to an indirect mechanism involving intermediate states

MicroRNAs and their isomiRs function cooperatively to target common biological pathways

Background: Variants of microRNAs (miRNAs), called isomiRs, are commonly reported in deep-sequencing studies; however, the functional significance of these variants remains controversial. Observational studies show that isomiR patterns are non-random, hinting that these molecules could be regulated and therefore functional, although no conclusive biological role has been demonstrated for these molecules. Results: To assess the biological relevance of isomiRs, we have performed ultra-deep miRNA-seq on ten adult human tissues, and created an analysis pipeline called miRNA-MATE to align, annotate, and analyze miRNAs and their isomiRs. We find that isomiRs share sequence and expression characteristics with canonical miRNAs, and are generally strongly correlated with canonical miRNA expression. A large proportion of isomiRs potentially derive from AGO2 cleavage independent of Dicer. We isolated polyribosome-associated mRNA, captured the mRNA-bound miRNAs, and found that isomiRs and canonical miRNAs are equally associated with translational machinery. Finally, we transfected cells with biotinylated RNA duplexes encoding isomiRs or their canonical counterparts and directly assayed their mRNA targets. These studies allow us to experimentally determine genome-wide mRNA targets, and these experiments showed substantial overlap in functional mRNA networks suppressed by both canonical miRNAs and their isomiRs. Conclusions: Together, these results find isomiRs to be biologically relevant and functionally cooperative partners of canonical miRNAs that act coordinately to target pathways of functionally related genes. This work exposes the complexity of the miRNA-transcriptome, and helps explain a major miRNA paradox: how specific regulation of biological processes can occur when the specificity of miRNA targeting is mediated by only 6 to 11 nucleotides

Retrospective evaluation of whole exome and genome mutation calls in 746 cancer samples

Funder: NCI U24CA211006Abstract: The Cancer Genome Atlas (TCGA) and International Cancer Genome Consortium (ICGC) curated consensus somatic mutation calls using whole exome sequencing (WES) and whole genome sequencing (WGS), respectively. Here, as part of the ICGC/TCGA Pan-Cancer Analysis of Whole Genomes (PCAWG) Consortium, which aggregated whole genome sequencing data from 2,658 cancers across 38 tumour types, we compare WES and WGS side-by-side from 746 TCGA samples, finding that ~80% of mutations overlap in covered exonic regions. We estimate that low variant allele fraction (VAF < 15%) and clonal heterogeneity contribute up to 68% of private WGS mutations and 71% of private WES mutations. We observe that ~30% of private WGS mutations trace to mutations identified by a single variant caller in WES consensus efforts. WGS captures both ~50% more variation in exonic regions and un-observed mutations in loci with variable GC-content. Together, our analysis highlights technological divergences between two reproducible somatic variant detection efforts

Efficient Organic Light-emitting Diodes with Singlet, Triplet and Doublet Excitons

Organic light-emitting diodes have shown a large potential and significant research progress recently. Tightly-bound excitons play an important role in the function of energy conversion and light emission for OLEDs. Many properties of excitons are determined by the spin state including singlets, triplets and doublets. Triplet excitons are regarded as dark states due to their spin-forbidden decay process, whereas effective triplet harvesting or triplet circumvention can help to improve the OLED performance. In this thesis, methods and mechanisms to avoid loss pathways were explored from the spin management perspective, which is not only limited to the spin-flip in singlet-triplet photophysics but also extend to the energy transfer in the singlet-triplet-doublet playground. We first demonstrate a molecular design concept on carbene-metal-amide (CMA) complexes as an effective way to optimise the triplet upconversion process in the singlet-triplet manifold. Through a partially twist and tilt between the donor and acceptor, it is able to realise highly efficient CMA materials with low exchange energy while maintaining the high radiative decay rate. These superior properties can be translated to OLEDs with great electroluminescent behaviour and improved device stability. We then explore the triplet and singlet exciton harvesting for energy transfer to doublets. In the first CMA−CF3:TTM-3PCz system, ultrafast intersystem crossing process in CMA− CF3 leads to the generation of triplet excitons in high yield, enabling efficient spin-allowed Dexter type energy transfer to the doublet state in TTM-3PCz in ps−ns timescales. We also discovered other sulfone-based thermally activated delayed fluorescent (TADF) hosts which can induce efficient singlet-to-doublet energy transfer to TTM-3NCz. These mechanisms can be successfully translated to devices with maintained OLED performance and enhanced device lifetimes. Finnaly, we present the tuneability of the two novel classes of materials, CMA and doublet radicals discussed above, in solution-process near-infrared (NIR) luminescent OLEDs. The novel properties of doublet spins are maintained in both non-conjugated and conjugated radical polymers, with fast radiative decay rate and NIR emission around 700 nm. Similarly, the strong TADF and spin-orbit coupling characters are reserved in the NIR CMA complexes which exhibit high external quantum efficiency and tuneable emission colours

Research data supporting "Fast Transfer of Triplet to Doublet Excitons from Organometallic Host to Organic Radical Semiconductors"

Contains 1 zip file within which there are 20 csv or xlsx datasets as follows: Fig_1b and Fig_1c contain steady-state absorption and photoluminescence measurements (x-axis wavelength in nm, y-axis counts), Fig_1d contains normalised transient photoluminescence data (x-axis time in s, y-axis counts) of TTM-3PCz and CMA-CF3 molecular emitters. Fig_2a is the complete 2D transient absorption dataset from which exciton dynamics in 3% TTM-3PCz in CMA-CF3 blend are studied (x-axis time in s, y-axis probe wavelength in nm, z-axis deltaT/T signal). Fig_2b contains spectra extracted from the same 2D dataset (x-axis wavelength in nm, y-axis counts). Fig_2c contains populations of excited states extracted from several 2D TA datasets (x-axis time in s, y-axis counts). Fig_3 contains temperature dependent transient photoluminescence measurements within which a series is the blend and b series is pristine CMA-CF3; and Main (x-axis time in us, y-axis counts) and Inset presents the same data but integrated (x-axis time in s, y-axis normalised integrated counts). Temperatures labelled in K. Fig_4b contains the EQE curve of the OLED device (x-axis current density in mA/cm2, y-axis EQE), Fig_4c contains current density vs. voltage and radiance vs. voltage data, Fig_4d contains electroluminescence spectra at several voltages.QG is grateful to the Cambridge Trust and China Scholarship Council (GrantNo.201808060075) for the financial support. QG additionally acknowledges funding from NationalKey R&D Program of China (NO.2022ZD016101). SG acknowledges funding from the EPSRC Centrefor Doctoral Training in Integrated Functional Nano (Grant EP/S022953/1) and Christ’s College,Cambridge. FL is grateful for financial support from the National Natural Science Foundation of China(grant no. 51925303) and the programme ‘JLUSTIRT’ (grant no. 2019TD-33). FL is an academic visitorat the Cavendish Laboratory, Cambridge, and is supported by the Talents Cultivation Programme(Jilin University, China). RHF acknowledges support from the Simons Foundation (grant no. 601946)and the EPSRC (EP/M005143/1). EWE is grateful to the Royal Society for a University ResearchFellowship (grant no. URF\R1\201300) and EPSRC (EP/W018519/1) for funding. This project hasreceived funding from the ERC under the European Union’s Horizon 2020 research and innovationprogramme (grant agreement number 101020167)

Fast Transfer of Triplet to Doublet Excitons from Organometallic Host to Organic Radical Semiconductors

Spin triplet exciton formation sets limits on technologies using organic semiconductors that are confined to singlet-triplet photophysics. In contrast, excitations in the spin doublet manifold in organic radical semiconductors can show efficient luminescence. Here the dynamics of the spin allowed process of intermolecular energy transfer from triplet to doublet excitons are explored. A carbene-metal-amide (CMA-CF3) is employed as a model triplet donor host, since following photoexcitation it undergoes extremely fast intersystem crossing to generate a population of triplet excitons within 4 ps. This enables a foundational study for tracking energy transfer from triplets to a model radical semiconductor, TTM-3PCz. Over 74% of all radical luminescence originates from the triplet channel in this system under photoexcitation. It is found that intermolecular triplet-to-doublet energy transfer can occur directly and rapidly, with 12% of triplet excitons transferring already on sub-ns timescales. This enhanced triplet harvesting mechanism is utilized in efficient near-infrared organic light-emitting diodes, which can be extended to other opto-electronic and -spintronic technologies by radical-based spin control in molecular semiconductors.</p

Fast transfer of triplet to doublet excitons from organometallic host to organic radical semiconductors.

Publication status: PublishedFunder: Christ's College, CambridgeFunder: Cambridge Trust; doi: http://dx.doi.org/10.13039/501100003343Spin triplet exciton formation sets limits on technologies using organic semiconductors that are confined to singlet-triplet photophysics. In contrast, excitations in the spin doublet manifold in organic radical semiconductors can show efficient luminescence. Here we explore the dynamics of the spin allowed process of intermolecular energy transfer from triplet to doublet excitons. We employ a carbene-metal-amide (CMA-CF3) as a model triplet donor host, since following photoexcitation it undergoes extremely fast intersystem crossing to set up a population of triplet excitons within 4 ps. This enables a foundational study for tracking energy transfer from triplets to a model radical semiconductor, TTM-3PCz. Over 74% of all radical luminescence originates from the triplet channel in this system under photoexcitation. We find that intermolecular triplet-to-doublet energy transfer can occur directly and rapidly, with 12% of triplet excitons transferring already on sub-ns timescales. This enhanced triplet harvesting mechanism is utilised in efficient near-infrared organic light-emitting diodes, which can be extended to other opto-electronic and -spintronic technologies by radical-based spin control in molecular semiconductors. This article is protected by copyright. All rights reserved