: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Organic light-emitting diodes (OLEDs)1–5, quantum-dot-based LEDs6–10, perovskite-based LEDs11–13 and micro-LEDs14,15 have been championed to fabricate lightweight and flexible units for next-generation displays and active lighting. Although there are already some high-end commercial products based on OLEDs, costs must decrease whilst maintaining high operational efficiencies for the technology to realise wider impact.  Here we demonstrate efficient action of radical-based OLEDs16, whose emission originates from a spin doublet, rather than a singlet or triplet exciton. While the emission process is still spin-allowed in these OLEDs, the efficiency limitations imposed by triplet excitons are circumvented for doublets. Using a luminescent radical emitter, we demonstrate an OLED with maximum external quantum efficiency of 27 per cent at a wavelength of 710 nanometres—the highest reported value for deep-red and infrared LEDs. For a standard closed-shell organic semiconductor, holes and electrons occupy the highest occupied and lowest unoccupied molecular orbitals (HOMOs and LUMOs), respectively, and recombine to form singlet or triplet excitons. Radical emitters have a singly occupied molecular orbital (SOMO) in the ground state, giving an overall spin-1/2 doublet. If—as expected on energetic grounds—both electrons and holes occupy this SOMO level, recombination returns the system to the ground state, giving no light emission. However, in our very efficient OLEDs, we achieve selective hole injection into the HOMO and electron injection to the SOMO to form the fluorescent doublet excited state with near-unity internal quantum efficiency. Organic light-emitting devices containing radical emitters can achieve an efficiency of 27 per cent at deep-red and infrared wavelengths based on the excitation of spin doublets, rather than singlet or triplet states.

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Efficient radical-based light-emitting diodes with doublet emission

Near-infrared organic light-emitting diodes and semiconductor lasers could benefit a variety of applications including night-vision displays, sensors and information-secured displays. Organic dyes can generate electroluminescence efficiently at visible wavelengths, but organic light-emitting diodes are still underperforming in the near-infrared region. Here, we report thermally activated delayed fluorescent organic light-emitting diodes that operate at near-infrared wavelengths with a maximum external quantum efficiency of nearly 10% using a boron difluoride curcuminoid derivative. As well as an effective upconversion from triplet to singlet excited states due to the non-adiabatic coupling effect, this donor–acceptor–donor compound also exhibits efficient amplified spontaneous emission. By controlling the polarity of the active medium, the maximum emission wavelength of the electroluminescence spectrum can be tuned from 700 to 780 nm. This study represents an important advance in near-infrared organic light-emitting diodes and the design of alternative molecular architectures for photonic applications based on thermally activated delayed fluorescence. Near-infrared organic light-emitting diodes that operate in the 700 nm region with an external quantum efficiency of almost 10% are reported.

High-efficiency electroluminescence and amplified spontaneous emission from a thermally activated delayed fluorescent near-infrared emitter

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Near-Infrared (NIR) Organic Light-Emitting Diodes (OLEDs): Challenges and Opportunities

Light in the second near-infrared window, especially beyond 1500 nm, shows enhanced tissue transparency for high-resolution in vivo optical bioimaging due to decreased tissue scattering, absorption, and autofluorescence. Despite some inorganic luminescent nanoparticles have been developed to improve the bioimaging around 1500 nm, it is still a great challenge to synthesize organic molecules with the absorption and emission toward this region. Here, we present J-aggregates with 1360 nm absorption and 1370 nm emission formed by self-assembly of amphiphilic cyanine dye FD-1080 and 1,2-dimyristoyl-sn-glycero-3-phosphocholine. Molecular dynamics simulations were further employed to illustrate the self-assembly process. Superior spatial resolution and high signal-to-background ratio of J-aggregates were demonstrated for noninvasive brain and hindlimb vasculature bioimaging beyond 1500 nm. The efficacy evaluation of the clinically used hypotensor is successfully achieved by high-resolution in vivo dynamic vascular imaging with J-aggregates.

J-Aggregates of Cyanine Dye for NIR-II in Vivo Dynamic Vascular Imaging beyond 1500 nm

The development of high-efficiency and low-cost organic emissive materials and devices is intrinsically limited by the energy-gap law and spin statistics, especially in the near-infrared (NIR) region. A novel design strategy is reported for realizing highly efficient thermally activated delayed fluorescence (TADF) materials via J-aggregates with strong intermolecular charge transfer (CT). Two organic donor-acceptor molecules with strong and planar acceptor are designed and synthesized, which can readily form J-aggregates with strong intermolecular CT in solid states and exhibit wide-tuning emissions from yellow to NIR. Experimental and theoretical investigations expose that the formation of such J-aggregates mixes Frenkel excitons and CT excitons, which not only contributes to a fast radiative decay rate and a slow nonradiative decay rate for achieving nearly unity photoluminescence efficiency in solid films, but significantly decreases the energy gap between the lowest singlet and triplet excited states (≈0.3 eV) to induce high-efficiency TADF even in the NIR region. These organic light-emitting diodes exhibit external quantum efficiencies of 15.8% for red emission and 14.1% for NIR emission, which represent the best result for NIR organic light-emitting diodes (OLEDs) based on TADF materials. These findings open a new avenue for the development of high-efficiency organic emissive materials and devices based on molecular aggregates.

Highly Efficient Thermally Activated Delayed Fluorescence via J-Aggregates with Strong Intermolecular Charge Transfer.

Despite the great potential for applications spanning from military night-vision displays and information-secured devices to civilian medical diagnostics and phototherapy, the development of highly efficient, stable, and low-cost near-infrared (NIR) emitting lumophores is still a formidable challenge Herein, we report two novel NIR-emitting homoleptic facial Ir(III) complexes based on extended π-conjugated benzo[g]phthalazine ligands, namely, tris[1,4-di(thiophen-2-yl)benzo[g]phthalazine] iridium(III) (Ir(dtbpa)3, 1) and tris[1-(2,4-bis(trifluoromethyl)phenyl)-4-(thiophen-2-yl)benzo[g]phthalazine] iridium(III) (Ir(Ftbpa)3, 2) Actually, these two ligands not only enable simple one-pot synthesis of homoleptic Ir(III) complexes without any catalyst under mild conditions, but also contribute to intense NIR-emission with high photoluminescence quantum yield up to 52% at 824 nm for 1 and 173% at 765 nm for 2, respectively Single-crystal structure of 1 demonstrates desired facial form with short Ir–N and Ir

Homoleptic Facial Ir(III) Complexes via Facile Synthesis for High-Efficiency and Low-Roll-Off Near-Infrared Organic Light-Emitting Diodes over 750 nm

Though urgently needed, high-performance near-infrared organic light-emitting diodes (NIR-OLEDs) are still rare. NIR-OLEDs based on conventional NIR fluorescent materials usually suffer from low external quantum efficiencies (EQEs) because of the intrinsic obstacles according to the spin-statistics limit and energy-gap law. Herein, we realized high-efficiency and low efficiency roll-off fluorescent NIR-OLEDs through efficient triplet fusion of a bipolar host doped with a special naphthoselenadiazole emitter (4,9-bis(4-(2,2-diphenylvinyl)phenyl)-naphtho[2,3-c][1,2,5]selenadiazole, NSeD). Unlike typical NIR organic donor-acceptor (D-A) chromophores, NSeD features a non-D-A structure and a very large HOMO/LUMO overlap and displays a strong deep-red to NIR fluorescence and unique ambipolar character. The corresponding photoluminescence quantum efficiency of NSeD reaches 52% in solution and retains 17% in the blend film. The optimized NIR-OLEDs demonstrated a strong emission at 700 nm, a high maximum EQE of 2.1% (vs. the predicted theoretical maximum efficiency of 1.3%) and the EQE remained at around 2% over a wide range of current densities from 18 to 200 mA cm-2, which is amongst the highest performance for NIR-OLEDs based on organic fluorescent materials.

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High-efficiency and low efficiency roll-off near-infrared fluorescent OLEDs through triplet fusion

Designing probes for real-time imaging of dynamic processes in living cells is a continuous challenge. Herein, a novel near-infrared (NIR) photoluminescence probe having a long lifetime was exploited for photoluminescence lifetime imaging (PLIM) using an iridium-alkyne complex. This probe offers the benefits of deep-red to NIR emission, a long Stokes shift, excellent cell penetration, low cytotoxicity, and good resistance to photobleaching. This example is the first PLIM probe applicable to the click reaction of copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) with remarkable lifetime shifts of 414 ns, before and after click reaction. The approach fully eliminates the background interference and distinguishes the reacted probes from the unreacted probes, thus enabling the wash-free imaging of the newly synthesized proteins within single living cells. Based on the unique properties of the iridium complexes, it is anticipated to have applications for imaging other processes within living cells.

Photoluminescence Lifetime Imaging of Synthesized Proteins in Living Cells Using an Iridium–Alkyne Probe

The simultaneous realization of high quantum yield and exciton utilizing efficiency (ηr) is still a formidable challenge in near-infrared (NIR) fluorescent organic light-emitting diodes (FOLEDs). Here, to achieve a high quantum yield, a novel NIR dye, 4,9-bis(4-(diphenylamino)phenyl)-naphtho[2,3-c][1,2,5]selenadiazole, is designed and synthesized with a large highest occupied molecular orbital/lowest unoccupied molecular orbital overlap and an aggregation-induced emission property, which demonstrates a high photoluminescence quantum yield of 27% at 743 nm in toluene and 29% at 723 nm in a blend film. For a high ηr, an orange-emitting thermally activated delayed fluorescent material, 1,2-bis(9,9-dimethyl-9,10-dihydroacridine)-4,5-dicyanobenzene, is chosen as the sensitizing host to harvest triplet excitons in devices. The optimized devices achieve a good ηr of 45.7% and a high external quantum efficiency up to 2.65% at 730 nm, with a very small efficiency roll-off of 2.41% at 200 mA cm−2, which are among the most efficient values for NIR-FOLEDs over 700 nm. The effective utilization of triplet excitons via the thermally activated delayed fluorescence-sensitizing host will pave a way to realize high-efficiency NIR-FOLEDs with small efficiency roll-off.

Jie Xue

Papers

Highly Efficient Thermally Activated Delayed Fluorescence via J-Aggregates with Strong Intermolecular Charge Transfer.

Homoleptic Facial Ir(III) Complexes via Facile Synthesis for High-Efficiency and Low-Roll-Off Near-Infrared Organic Light-Emitting Diodes over 750 nm

High-efficiency and low efficiency roll-off near-infrared fluorescent OLEDs through triplet fusion

Photoluminescence Lifetime Imaging of Synthesized Proteins in Living Cells Using an Iridium–Alkyne Probe

High-Efficiency Near-Infrared Fluorescent Organic Light-Emitting Diodes with Small Efficiency Roll-Off: A Combined Design from Emitters to Devices