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 emission. Applications of light generation in this range span from bioimaging to surveillance. Although the unpaired electron arrangements of radicals enable efficient radiative transitions within the doublet-spin manifold in organic light-emitting diodes, their performance is limited by non-radiative pathways introduced in electroluminescence. Here we present a host–guest design for organic light-emitting diodes that exploits energy transfer with up to 9.6% external quantum efficiency for 800 nm emission. The tris(2,4,6-trichlorophenyl)methyl-triphenyl-amine 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.


Main text Introduction
Advances in efficient near-infrared (NIR) organic light-emitting diodes (OLEDs) can enable light generation in the biological window for healthcare diagnosis and treatment.The requirement for long-wavelength light generation beyond the visible range is also motivated by communications and security applications.Whilst > 20% external quantum efficiency (EQE)   in electroluminescence (EL) has been demonstrated for visible-light OLEDs, and commercial displays are commonplace, the performance of NIR OLEDs is generally limited to 5% EQE using fully-organic emitters with emission peak wavelengths at 800 nm and longer. 1 The materials approach and mechanisms for efficient visible-light OLEDs based on maximising luminescence from singlet and triplet excitons have not translated to efficient NIR OLEDs.][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Luminescent organic radicals enable high photoluminescence quantum yield (PLQY) in the NIR range, where immunity from normal 'energy gap law' considerations is linked to their unique electronic structure. 18Almost 100% internal quantum efficiency (IQE)   for EL was demonstrated in radical OLEDs exploiting tris (2,4,6-trichlorophenyl)methyl (TTM)-based radicals. 4This performance shows that using the doublet-spin manifold in radicals for luminescence can circumvent typical efficiency limits (25% IQE) arising from the formation of singlet and triplet excitons in standard closed-shell molecule-based devices. 16,17 recently reported efficient NIR OLEDs from triphenyl amine-substituted (2-chloro-3pyridyl)bis (2,4,6-trichlorophenyl)methyl (TPA-PyBTM') with a maximum EQE of 6.4% for 800 nm peak emission. 19However, these devices showed efficiency roll-off at high current densities and were limited by unbalanced electron and hole currents.Energy transfer mechanisms using thermally-activated delayed fldrvuorescence (TADF) materials for charge recombination and sensitization of radical emitters for EL showed promise for moving the exciton generation event away from radicals to combat the performance shortfall. 20re, we use an anthracene derivative, 2-methyl-9,10-bis(naphthalene-2-yl)anthracene (MADN), as a host component that enables efficient charge transport to generate excitons that transfer to a triphenyl amine-substituted tris(2,4,6-trichlorophenyl)methyl (TTM-TPA) NIR radical emitter (see ref. 18 for synthetic details).The high-energy singlet state (near 3 eV) mitigates losses from the 'energy gap law' in MADN, whilst spin-allowed transfer between long-lived, low-energy host triplet and emissive radical doublet exciton states leads to efficient delayed emission.A maximum EQE for OLEDs of 9.6% is obtained at ⁓800 nm with reduced efficiency roll-off, enhanced radiance and device stability.

Near-infrared radical design of intersystem energy transfer
Figure 1a shows the available energy transfer pathways between singlet (S1) and triplet (T1) excitons of MADN and doublet (D1) excitons of TTM-TPA in the MADN:TTM-TPA system, and their chemical structures are depicted in Fig. 1b.The scheme shows the opportunity for energy harvesting of both singlet and triplet excitations in the non-radical host to form radical dopant states.This strategy exploits efficient spin-conserving transfer processes in the two pathways: singlet-doublet Förster resonance energy transfer (FRET) (S1 + D0 → S0 + D1) and triplet-doublet Dexter energy transfer (DET) (T1 + D0 → S0 + D1).MADN triplet emission extends between 700 nm and 900 nm, 21 which is energy-resonant with TTM-TPA doublet emission (Fig. 1c).Accordingly, the MADN:TTM-TPA system enables the study of exciton harvesting in the limit of small energy difference (|∆ETD| < 0.1 eV) between MADN T1 and TTM-TPA D1, and where substantial host non-radiative losses due to the 'energy gap law' are minimised.
The steady-state photophysical properties of TTM-TPA, MADN, and 4,4'bis(carbazol-9-yl)biphenyl (CBP) are depicted in Fig. 1c.Steady-state photophysical characteristics of TTM-TPA in different solutions are shown in Supplementary Fig. 1.The variation of photoluminescence (PL) peak wavelength between 730 nm and 890 nm with an increase in solvent polarity suggests the formation of intramolecular charge-transfer (CT) excited states, in line with our previous reports. 4,19The CBP:TTM-TPA system is used as a reference for studies of energy transfer mechanisms in the MADN:TTM-TPA system.The PL spectra of MADN and CBP neat films overlap with the absorption of TTM-TPA so that photoexcited singlet excitons generated in the host non-radical components in MADN:TTM-TPA and CBP:TTM-TPA systems (at 330 nm for CBP and 370 nm for MADN: see Supplementary Fig. 2 for absorption spectra for CBP and MADN neat films) undergo efficient singlet-doublet transfer to the radical guest, with NIR fluorescence observed at ~800 nm (Fig. 1c).Small contributions of host emission to the total PL are observed and provide characteristic signatures for the singlet-doublet energy transfer channels in these systems.The PLQY of TTM-TPA in toluene is 24% (excited at 370 nm), while CBP:TTM-TPA 3% and MADN:TTM-TPA 3% films have 19% and 27% PLQY, respectively, at the same host excitation wavelength.This shows that high-energy singlet materials with efficient singlet-doublet transfer can be used to host NIR radical emitters.
2b-e and summarised in Table 1.The resulting MADN:TTM-TPA OLED gives NIR EL at a peak wavelength of 800 nm with a maximum EQE of 9.6% (Fig. 2b), which is much higher than previous performance limits for reported NIR OLEDs beyond 780 nm peak emission, 1,19 and approximately 60% higher than a maximum EQE of 6.1% obtained in the reference device with CBP:TTM-TPA.Whereas the CBP:TTM-TPA device suffers a large efficiency drop beyond 10 mA cm -2 , the MADN:TTM-TPA device sustains a relatively high efficiency of 4.2% up to 100 mA cm -2 .Consequently, the maximum radiance of the MADN:TTM-TPA device reaches 68,000 mW sr -1 m -2 , which is nearly an order of magnitude higher than the 8,100 mW sr -1 m -2 obtained in the CBP:TTM-TPA device (Fig. 2c).Fig. 2d shows the NIR doublet EL emission spectra for the devices.Interestingly, in contrast to the PL (Fig. 1c) of CBP:TTM-TPA and MADN:TTM-TPA, no host emission is observed.This indicates that singlet-doublet energy transfer is not the main mechanism at play for EL.Current density-voltage (J-V) plots for devices with and without radical doping are shown in Fig. 2e.Firstly, we observe a steeper J-V gradient for MADN:TTM-TPA versus CBP:TTM-TPA devices.This is consistent with MADN having better electron and hole transporting properties than CBP, as demonstrated using single-carrier device analysis (Supplementary Information S2).Secondly, we find that TTM-TPA doping causes negligible differences between J-V curves for MADN:TTM-TPA and MADN-only devices, whereas a substantially shallower curve is seen in CBP:TTM-TPA versus CBP-only devices.We consider that this indicates radical energy transfer through singlet and triplet channels following exciton formation at host MADN sites in the MADN:TTM-TPA device (Fig. 1a), whereas the J-V characteristics for the CBP-based device suggest the involvement of radical charge trapping. 22 have tested these devices for stability under constant drive.These devices are exposed to the nitrogen atmosphere between sublimation steps and are operated without encapsulation.
Under these conditions, we do find the MADN:TTM-TPA device shows nearly a factor of 10 improved lifetime (to 50% EL) of 58 hr (at 0.1 mA cm -2 ) compared to 7 hr in the CBP:TTM-TPA device (Supplementary Fig. 3), which also presents a substantial increase over previous results. 12,19The MADN:TTM-TPA device performance sets a new benchmark for stability, which is not generally reported for NIR OLEDs, and higher maximum efficiency and radiance than other reported 780-900 nm devices as summarised in Fig. 2f and Supplementary Table 1. 1,23,24ble 1 | Summary of device performance.
CBP:TTM-TPA 3%  (TTA) (blue dotted line, Fig. 3a). 25,26The CBP-only device shows no delayed emission in transient EL but only prompt decay (τ = 44 ± 1 ns), which suggests that triplets formed in CBP do not contribute to the overall emission process (purple dotted line, Fig. 3a).As expected from the full device and single-carrier device characteristics (Fig. 2b-e, Supporting Information S2), 8][29][30] In contrast, the MADN:TTM-TPA device exhibits a delayed EL decay of τ = 370 ± 5 ns (blue solid line, Fig. 3a), which is different from the transient EL profile of the MADN-only device.This supports that the EL mechanism in the MADN:TTM-TPA device involves energy transfer from triplet excitations to emissive radical doublet states without an intermediate TTA process.

Exciton dynamics and energy transfer
We performed transient optical spectroscopy studies to investigate the exciton dynamics and the available radical energy transfer pathways depending on T1-D1 energy alignment in these host:radical systems.Picosecond transient absorption (TA) studies were carried out at low fluences to exclude exciton-exciton annihilation.TA under 400 nm pump for host-selective excitation (Fig. 4a) reveals faster decay of MADN excited-state features assigned to S1 excitons in 3% TTM-TPA in MADN films compared to pristine MADN (Supplementary Fig. 7).The S1 decay for MADN:TTM-TPA mirrors a rise in D1 photoinduced absorption from the TTM-TPA component where the timescale for singlet-doublet transfer is rapid (SD = 8 ps).TA studies under radical-selective photoexcitation were performed on CBP:TTM-TPA and MADN:TTM-TPA films with 532 nm excitation (Fig. 4b).Faster decay of D1 excitons is observed in MADN:TTM-TPA, where |∆ETD| < 0.1 eV, versus CBP:TTM-TPA, where |∆ETD| > 0.8 eV (CBP T1: 2.6 eV). 37This suggests that radical D1 excitons formed via S1 → D1 FRET can transfer energy to closely lying excited triplet T1 states on MADN.
The TA and PL dynamics in MADN:TTM-TPA film with low ∆ETD allow us to conclude that triplet-doublet and doublet-triplet energy transfer pathways are present in this system (Supplementary Information S3).This results in excited-state (re)cycling of triplet and doublet states in a delayed emission mechanism.The temperature dependence of transient PL in MADN:TTM-TPA film shows thermal activation of delayed radical emission under selective radical excitation (Fig. 4e), where the only available processes are doublet luminescence, doublet-triplet and triplet-doublet energy transfer and triplet diffusion.Doublet luminescence is temperature-independent in donor-acceptor TTM radicals. 38Arrhenius analysis reveals an activation energy of 26.0 ± 1.4 meV (Fig. 4f).This small energy gap is comparable to thermal energy kBT at room temperature and, therefore, can be efficiently overcome in OLEDs.We assign its origin to diffusion-limited reformation of triplet-radical encounter pairs, as described below.

Modelling of energy transfer
An amorphous sample made of MADN as the host doped with radical TTM-TPA molecules at a 3.1% m/m concentration was prepared using classical force-field Molecular Dynamics (MD) simulations.After equilibration, a few interacting MADN:TTM-TPA pairs were extracted from the sample, and their ground-state equilibrium geometries were subsequently relaxed at the density functional theory (DFT) level (ωB97X-D/6-31G(d,p)). Vertical excitation energies were computed by resorting to an optimally-tuned screened range-separated hybrid (OT-SRSH) approach (LC-ωhPBE/6-311G(d,p)) within the time-dependent (TD) DFT in the Tamm-Dancoff approximation (TDA) (Supplementary Information S4). 39,40 the most stable pair (dim1), these calculations yield the first singlet, S1, and triplet, T1, excited states localised on the anthracene core of MADN at 3.17 eV and 2.00 eV, respectively, while the two lowest doublet excited states on TTM-TPA are 1.84 eV (D1) and 2.81 eV (D2) above the ground state.The analysis of the natural transition orbitals (NTOs) for monomers in Supplementary Fig. 11 shows that D1 is an intramolecular charge-transfer (intra-2 CT) excitation, while D2 has a dominant locally excited ( 2 LE) character on the TTM moiety.
The computed excited state energies for all pairs are shown in Fig. 5a.Excitations localised on each fragment show relatively narrow energy distributions, with a slightly larger standard deviation for D1, as expected from its intra-2 CT character.The calculations also suggest the presence of a much broader distribution of inter-2 CT excitations, mostly involving transitions from the anthracene core of MADN to the TTM moiety (Supplementary Fig. 13), that are energy-resonant with D1 and T1 and could thus potentially act as mediating states in tripletdoublet energy transfer.The large energy range spanned by these inter-2 CT states originates from the heterogeneous conformational and electrostatic landscape in amorphous solids. 41,42ese results, i.e., nearly degeneracy of inter-2 CT with D1 and T1, should be taken with caution since the optimisation of isolated molecular pairs might facilitate the formation of strongly interacting dimers difficult to encounter in the real system.Hence, optimised dimeric models are expected to exhibit shorter intermolecular distances than those in the amorphous material, triggering an overstabilisation of charge-separated states.Indeed, when inter-2 CT states are directly computed on MD molecular pairs, transition energies are considerably higher (Supplementary Table 7).
Excitation energy transfer (EET) rates for the S1-D1 and S1-D2 processes were computed with the Marcus-Levich-Jortner equation, where all the key parameters (i.e., reorganisation energies, Huang-Rhys factors, electronic couplings and energy differences) were obtained by quantum-chemical calculations.For dim1, we calculate an EET time constant from S1 to D1 of 3.0 ps (excluding outliers, an average value of 9 ps is obtained for the investigated pairs, Supplementary Table 9).Despite the smaller energy offset between the states, the corresponding EET time constant from S1 to D2 is significantly longer at 20 ps (average 23 ps) because of reduced Coulomb coupling and smaller oscillator strength associated with D2 compared to D1.We conclude that singlet-doublet energy transfer occurs primarily through the S1-D1 pathway on timescales of a few ps, in excellent agreement with experiment.
We now turn to triplet-doublet energy transfer.A triplet state interacting with a radical can form either an overall doublet or quartet encounter pair in a 1:2 statistical ratio (Fig. 5b). 20r overall doublet pairs, triplet-doublet energy transfer can occur with spin conservation.The quantum-mechanical coupling between states can take the form of a two-electron exchange integral, as in DET.However, the presence of nearby inter-2 CT excitations additionally supports a superexchange-mediated mechanism, where the effective coupling is proportional to the product of two, typically much larger, one-electron matrix elements. 38Building on the pure spin-states of individual fragments, both the direct two-electron and the indirect oneelectron electronic couplings were computed for the same pairs as above (Supplementary Information S4).Our calculations show that a direct exchange mechanism provides very slow T1-D1 energy transfer times, with values approaching tens of ns in some pairs.However, superexchange couplings are extremely sensitive to wave function overlap and, therefore, to the dimer geometry and, for some pairs, bring the energy transfer timescales down to tens of ps (which is in the same range as CT-mediated triplet-doublet energy transfer in related covalently-linked radical-chromophore molecules). 38It is likely that the conversion from the host triplet to the emissive doublet states is limited by diffusion of the triplet excitations within the MADN host.As a first step towards the modelling of triplet diffusion, we computed T1 hopping rates to all nearest neighbours of three randomly selected MADN molecules (Supplementary Information S4).While the values vary over multiple orders of magnitude, the fastest event for the three cases approaches a few tens of ns (Supplementary Table 12), which is typically orders of magnitude slower than the T1-D1 energy transfer.We thus conclude that the thermally activated delayed radical emission is controlled by triplet diffusion within the host, which limits the rate of (re)formation of overall doublet encounter pairs.

Conclusion
Electrical excitation with a fast charge-transporting host leads to the generation of singlet and triplet exciton states that can be harvested by doublet radicals towards highly efficient NIR EL in OLEDs.Here, the handling of excitations mitigates the energy gap law for non-radiative decay by a design that combines high-energy singlet (S1) and low-energy triplet (T1) excitons of the host with matching to low-energy doublet (D1) excitons of the radical emitter.The principle is demonstrated using the MADN:TTM-TPA combination, which shows rapid singlet-doublet transfer (τ = 8 ps) upon photoexcitation and reversible doublet-triplet cycling with efficient delayed emission (τ > 0.16 µs).The luminescent NIR radical system is implemented in high-performing OLEDs with a maximum EQE of 9.6% for EL at 800 nm that operate to the high maximum radiance of ⁓68,000 mW sr -1 m -2 , with low efficiency roll-off and enhanced stability.Our design boosts performance in radical-based OLEDs and has broad implications for reducing non-radiative losses in devices beyond light-emitting applications with NIR light.

Fig. 1 |
Fig. 1 | Radical energy harvesting design for high efficiency near-infrared emission.a, Schematic illustration of intersystem dual energy transfer between host MADN and radical TTM-TPA in doublet EL devices.b, Chemical structures of TTM-TPA, CBP, and MADN.c, Absorption coefficient for TTM-TPA in toluene; film PL spectra of CBP neat, MADN neat, and TTM-TPA 3% doped in CBP and MADN.The spectral overlap between TTM-TPA absorption (green squares) with CBP (blue circles) and MADN (red triangles) PL allows singlet-doublet energy transfer.

Fig. 2 |
Fig. 2 | Radical OLED device structure and optoelectronic characterisation.a, Device structure with energy levels.b,c, EQE-current density plots and radiance-current density plots for the devices.d, EL spectra at 1 mA cm -2 .e, J-V characteristics for the devices with and without TTM-TPA doping.f, Comparison of NIR OLEDs with peak wavelengths between 780 nm and 900 nm regarding maximum EQE and radiance.

,
where EL(B) and EL(0) are the EL intensity in the presence and absence of a magnetic field, B, respectively.The CBP-based devices show almost negligible magnetic field dependence of the EL regardless of TTM-TPA doping in CBP-only (purple diamonds) and CBP:TTM-TPA (purple squares) devices in Fig. 3b.The MADN-based devices with and without TTM-TPA doping are distinguished from the CBP-based devices by positive MEL profiles.The net positive MEL in the MADN-onlyOLED is attributed to magnetosensitivity of the polaron-pair hyperfine mechanism (positive MEL) that dominates over the dependence from TTA (negative MEL).25,31,36The non-identical MEL profiles for the MADN:TTM-TPA device compared to the MADN-only device also imply an EL mechanism without indirect radical energy harvesting by TTA.The broader magnetic field dependence in the MEL profile for the MADN:TTM-TPA OLED is assigned to triplet-doublet energy transfer, where magnetosensitivity originates from larger triplet zerofield splitting interactions (> 10 mT) compared to smaller hyperfine interactions (~1-10 mT) in the polaron-pair mechanism.31,34

Fig. 3 |
Fig. 3 | Transient EL and MEL studies of radical OLEDs.a, Transient EL profiles for CBP and MADN devices with and without TTM-TPA doping.The voltage pulse corresponds to 1 mA cm - 2 , and the off-voltage was -5V for de-trapping the charge carriers after turn-off.In the CBPbased devices (purple), no delayed emission is observed.MADN-based devices (blue) show strong delayed emission features.b, MEL for devices studied at 1 mA cm -2 .The CBP-based devices show negligible MEL while MADN-based devices show positive signatures that reflect energy transfer contributions to the EL mechanism.

Fig. 4 |
Fig. 4 | Time-resolved spectroscopy.a, Excited state singlet (S1) and doublet (D1) population kinetics extracted from transient absorption of neat MADN and MADN:TTM-TPA 3% films under 400 nm excitation.The decay of S1 in the blend and the matching rise of D1 are due to rapid singlet-doublet FRET.b, Comparison of D1 population kinetics for CBP:TTM-TPA 3% and MADN:TTM-TPA 3% films under radical-only 532 nm excitation.Faster decay observed in the MADN blend is indicative of doublet-triplet energy transfer.c,d, Transient PL profiles averaged over 720-880 nm for radical emission following host-selective (400 nm, 330 nm) and radical-selective (532 nm) excitation.Delayed radical emission is observed in the MADN blend under both host and radical excitation.e, Temperature-dependent transient PL profiles of MADN:TTM-TPA 3% excited at 532 nm.Delayed radical emission is faster at elevated temperatures.f, Arrhenius plot for the MADN:TTM-TPA 3% system revealing a small activation energy for delayed radical emission.

Fig. 5 |
Fig. 5 | Excited state pathways.a, Calculated energetic landscape in dimers of MADN and TTM-TPA.The full black circle represents the average adiabatic excitation energies and vertical bars quantify the standard deviation: T1 = 2.02 ± 0.02 eV; S1 = 3.25 ± 0.04 eV; D1 = 1.93 ± 0.08 eV; D2 = 2.79 ± 0.03 eV; inter-2 CT = 1.89 ± 0.12 eV.Inset shows the molecular conformation of dim1.The computed average lifetime of SD is 23 ps and 9 ps to the D2 and D1 state, respectively.Energy transfer from T1 to D1 occurs in a superexchange-like mechanism mediated by the presence of low-lying inter-2 CT states, with a computed overall lifetime TD spanning from tens of ns to tens of ps.b, Scheme of exciton pathways and their approximate rates.Triplet excitons form either an overall doublet ( 2 [D0-T1]) or quartet ( 4 [D0-T1]) encounter pair when adjacent to a radical site.Reversible energy transfer occurs in the doublet configuration, while quartet pairs separate during triplet diffusion.