Efficient near-infrared organic light-emitting diodes with emission from spin doublet excitons

The development of luminescent organic radicals has resulted in materials with excellent optical properties for near-infrared (NIR) emission. Applications of light generation in this range span from bioimaging to surveillance. Whilst the unpaired electron arrangements of radicals enable efficient radiative transitions within the doublet-spin manifold in organic light-emitting diodes (OLEDs), their performance is limited by non-radiative pathways introduced in electroluminescence. Here, we present a host:guest design for OLEDs that exploits energy transfer with demonstration of up to 9.6% external quantum efficiency (EQE) for 800 nm emission. The tris(2,4,6-trichlorophenyl)methyl-triphenylamine (TTM-TPA) radical guest is energy-matched to the triplet state in a charge-transporting anthracene-derivative host. We show from optical spectroscopy and quantum-chemical modelling that reversible host-guest triplet-doublet energy transfer allows efficient harvesting of host triplet excitons

Reversible spin-optical interface in luminescent organic radicals

Molecules present a versatile platform for quantum information science, and are candidates for sensing and computation applications. Robust spin-optical interfaces are key to harnessing the quantum resources of materials. To date, carbon-based candidates have been non-luminescent, which prevents optical read-out. Here we report the first organic molecules displaying both efficient luminescence and near-unity generation yield of high-spin multiplicity excited states. This is achieved by designing an energy resonance between emissive doublet and triplet levels, here on covalently coupled tris(2,4,6-trichlorophenyl) methyl-carbazole radicals (TTM-1Cz) and anthracene. We observe the doublet photoexcitation delocalise onto the linked acene within a few picoseconds and subsequently evolve to a pure high spin state (quartet for monoradicals, quintet for biradical) of mixed radical-triplet character near 1.8 eV. These high-spin states are coherently addressable with microwaves even at 295 K, with optical read-out enabled by intersystem crossing to emissive states. Furthermore, for the biradical, on return to the ground state the previously uncorrelated radical spins either side of the anthracene show strong spin correlation. Our approach simultaneously supports a high efficiency of initialisation, spin manipulations and light-based read-out at room temperature. The integration of luminescence and high-spin states creates an organic materials platform for emerging quantum technologies

Direct linearly polarized electroluminescence from perovskite nanoplatelet superlattices

Polarized light is critical for a wide range of applications, but is usually generated by filtering unpolarized light, which leads to substantial energy losses and requires additional optics. Here we demonstrate the direct emission of linearly polarized light from light-emitting diodes made of CsPbI3 perovskite nanoplatelet superlattices. The use of solvents with different vapour pressures enables the self-assembly of the nanoplatelets with fine control over their orientation (either face-up or edge-up) and therefore their transition dipole moment. As a result of the highly uniform alignment of the nanoplatelets, as well as their strong quantum and dielectric confinement, large exciton fine-structure splitting is achieved at the film level, leading to pure red light-emitting diodes with linearly polarized electroluminescence exhibiting a high degree of polarization of 74.4% without any photonic structures. This work demonstrates the potential of perovskite nanoplatelets as a promising source of linearly polarized light, opening up the development of next-generation three-dimensional displays and optical communications from a highly versatile, solution-processable system

Interplay of Spin and Photophysics in Luminescent Open-Shell Molecular Semiconductors

Luminescent organic radicals are an emerging class of molecular semiconductors which exhibit many unique properties attractive for optoelectronic and spintronic devices. In this thesis, we employ optical and spin-based probes to reveal the dynamics of photogenerated excitons in a selection of novel tris(2,4,6-trichlorophenyl)-methyl (TTM)-based radicals. The first three chapters present a motivation, relevant theory and methodology. In Chapter 4, we focus on the intrinsic properties of luminescent doublet (*S*=1/2) states. We find evidence of intermolecular charge transfer excitations which drastically alter the emission spectrum and lifetime in thin films. In Chapter 5, we investigate solid state intermolecular interactions between radicals and triplet (*S*=1) states on closed-shell materials, and show their management can lead to improvements in Organic Light Emitting Diode (OLED) performance. For the first time we observe cycling between the triplet and doublet manifolds, and direct energy transfer on sub-nanosecond timescales. In Chapter 6, we present the first organic molecules which can reversibly access a quartet (*S*=3/2) excited state. This is achieved by engineering strong exchange coupling between resonant radical and triplet manifolds in covalently linked structures. The resulting high-spin states are coherently addressable with microwaves even at 295 K, with optical read-out enabled by intersystem crossing to the energetically accessible radical state. In Chapter 7, we extend these results to a luminescent biradical structure which supports a quintet (*S*=2) excited state. The light-induced cycling through this state drastically increases the strength of the exchange coupling between the two radical spins, and leads to a long-lived ground-state polarisation. The findings and models developed in this thesis open a path to few functionalities for open-shell semiconductors, as outlined in Chapter 8, ranging from improved light emission to molecular quantum information science.Engineering and Physical Sciences Research Council (NanoDTC, no. EP/S022953/1) Christ's College, Cambridge European Research Council (SCORS, no. 101020167

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.

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

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

Research data supporting "Mesitylated trityl radicals, a platform for doublet emission: symmetry breaking, charge-transfer states and conjugated polymers"

Compressed (.zip) folder containing data from normalized photoluminescence spectra of M3TTM and M2TTM-3PCz radicals and PFMTTM polyradical in 0.1 mM toluene solutions (Fig1c.xlsx); normalized steady-state emission spectra of MxTTM radicals in 0.1 mM toluene solutions (Fig3a.xlsx) and emission kinetics in the 580-610 nm region showing rapid emission following 520 nm excitation (Fig3b.xlsx); normalized steady-state emission spectra of MxTTM radicals in 8 wt% evaporated films in CBP showing increasing emission red-shift and linewidth broadening with decreasing mesitylation (Fig.3c.xlsx) and total emission kinetics in the range of 550-880 nm following 520 nm excitation using 100 fs pulses with fluence 5 μJ cm-2 (Fig3d.xlsx), inset shows early time kinetics (Fig3dInset.xlsx); time-gated photoluminescence spectra of 8 wt% M2TTM radical in CBP film showing monomer emission at nanosecond times and exciplex emission at microsecond times (Fig3e.xlsx); dynamics of emission of MxTTM radicals in CBP films showing time dependence of peak emission wavelength (Fig3f1.xlsx) and integrated photoluminescence counts in the range of 550-880 nm (Fig3f2.xlsx); external quantum efficiency versus current density (Fig4b.xlsx) and current density-voltage-luminance characteristics of organic light-emitting diodes with M2TTM-3PCz radical as the emitter (Fig4c.xlsx); normalized photoluminescence spectrum of the light-emitting layer of 5 wt% M2TTM-3PCz doped CBP film following 405 nm excitation and normalized electroluminescence spectrum of the device at 0.2 mA cm-2 (Fig4d.xlsx); computational atomic coordinates of optimized MxTTM structures in the ground and excited states calculated at the UB3LYP(D3)/def2-SVP and UCAM-B3LYP(D3)/def2-SVP levels of theory, respectively (MxTTM_coordinates.docx).This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreements No. 891167, No. 859752 and No. 886066, from the European Research Council under the European Union’s Horizon 2020 research and innovation programme grant agreement No. 101020167 and from the Engineering and Physical Sciences Research Council NanoDTC, EP/S003126/1, EP/S022953/1

Mesitylated trityl radicals, a platform for doublet emission: symmetry breaking, charge-transfer states and conjugated polymers.

Acknowledgements: We thank Dr Andrew Bond for carrying out the X-ray crystallography measurements and data analysis at the Yusuf Hamied Department of Chemistry, University of Cambridge. P.M., R.C. and W.Z. have received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreements No. 891167, No. 859752 and No. 886066. We acknowledge funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme grant agreement No. 101020167 (R.H.F., P.M., E.G., S.G.) and the Engineering and Physical Sciences Research Council NanoDTC, EP/S003126/1, EP/S022953/1 (S.G.).Neutral π-radicals have potential for use as light emitters in optoelectronic devices due to the absence of energetically low-lying non-emissive states. Here, we report a defect-free synthetic methodology via mesityl substitution at the para-positions of tris(2,4,6-trichlorophenyl)methyl radical. These materials reveal a number of novel optoelectronic properties. Firstly, mesityl substituted radicals show strongly enhanced photoluminescence arising from symmetry breaking in the excited state. Secondly, photoexcitation of thin films of 8 wt% radical in 4,4'-bis(carbazol-9-yl)-1,1'-biphenyl host matrix produces long lived (in the order of microseconds) intermolecular charge transfer states, following hole transfer to the host, that can show unexpectedly efficient red-shifted emission. Thirdly, covalent attachment of carbazole into the mesitylated radical gives very high photoluminescence yield of 93% in 4,4'-bis(carbazol-9-yl)-1,1'-biphenyl films and light-emitting diodes with maximum external quantum efficiency of 28% at a wavelength of 689 nm. Fourthly, a main-chain copolymer of the mesitylated radical and 9,9-dioctyl-9H-fluorene shows red-shifted emission beyond 800 nm