A luminescent organic radical with two pyridyl groups: high photostability and dual stimuli-responsive properties, with theoretical analyses of photophysical processes

Highly Efficient Near-Infrared Luminescent Radicals with Emission Peaks over 750 nm

Purely organic molecules exhibiting near-infrared (NIR) emission possess considerable potential for applications in both biological and optoelectronic technological domains, owing to their inherent advantages such as cost-effectiveness, biocompatibility, and facile chemical modifiability. However, the repertoire of such molecules with emission peaks exceeding 750 nm and concurrently demonstrating high photoluminescence quantum efficiency (PLQE) remains relatively scarce due to the energy gap law. Herein, we report two open-shell NIR radical emitters, denoted as DMNA-Cz-BTM and DMNA-PyID-BTM, achieved through the strategic integration of a donor group (DMNA) onto the Cz-BTM and PyID-BTM frameworks, respectively. We found that the donor-acceptor molecular structure allows the two designed radical emitters to exhibit a charge-transfer excited state and spatially separated electron and hole levels with non-bonding characteristics. Thus, the high-frequency vibrations are effectively suppressed. Besides, the reduction of low-frequency vibrations is observed. Collectively, the non-radiative decay channel is significantly suppressed, leading to exceptional NIR PLQE values. Specifically, DMNA-Cz-BTM manifests an emission peak at 758 nm alongside a PLQE of 55 %, whereas DMNA-PyID-BTM exhibits an emission peak at 778 nm with a PLQE of 66 %. Notably, these represent the pinnacle of PLQE among metal-free organic NIR emitters with emission peaks surpassing 750 nm.

A Stable Chichibabin Diradicaloid with Near-Infrared Emission

Conjugated molecules with multiple radical centers such as the iconic Chichibabin diradicaloid hold promise as building blocks in materials for quantum sensing and quantum information processing. However, it is a considerable challenge to design simple analogues of the Chichibabin hydrocarbon that are chemically inert, exhibit high diradical character and emit light at a distinct wavelength that may offer an optical readout of the spin state in functional ensembles. Here we describe the serendipitous discovery of the stable TTM-TTM diradicaloid, which exhibits high diradical character, a striking sky-blue color and near-infrared (NIR) emission (in solution). This combination of properties is unique among related diradicaloids and is due to the presence of hydrogen and chlorine atoms in "just the right positions", allowing a perfectly planar, yet predominantly benzenoid bridge to connect the two sterically stabilized radical centers. In-depth studies of the optical and magnetic properties suggest that this structural motif could become a mainstay building block of organic spin materials.

Photoactivated Circularly Polarized Luminescent Organic Radicals in Doped Amorphous Polymer

Luminescent organic radicals, especially those with photoactivated circularly polarized luminescence (CPL) features, hold great significance for cutting-edge optoelectronic applications, but their development still remains a challenge. In this study, we propose a novel strategy to achieve photoactivated CPL radicals by bonding two phosphine centers within an axial chiral system, yielding a compound of R/S-5,5-bis(diphenylphosphino)-4,4'-bibenzo[d][1,3]dioxole (R/S-BDP). The photoactivated R/S-BDP molecules in polymer matrix display a robust quantum yield of 19.8 % and a dissymmetry factor (glum) of 1.2×10-4, marking this work as the first example of photoactivated CPL radicals. Furthermore, the glum is improved to 1.0×10-2 by using a liquid crystal as host. Experimental and theoretical analyses reveal that R/S-BDP molecules, endowed with double phosphine cores in axial chirality, offer a direct way for intramolecular electron transfer upon photoirradiation. This leads to the generation of radical ionic pairs, which subsequently trigger the donor-acceptor arrangement through intermolecular electron transfer, thereby resulting in stable radical emission. The extended photoactivated BDP-F exhibits a remarkably high quantum efficiency of 57.8%. Ultimately, the distinctive photo-responsive CPL radical luminescence has been successfully used for information displays and anti-counterfeiting.

Steric Control of Luminescence in Phenyl-Substituted Trityl Radicals

Triphenylmethyl (trityl) radicals have shown potential for use in organic optoelectronic applications, but the design of practical trityl structures has been limited to donor/radical charge-transfer systems due to the poor luminescence of alternant symmetry hydrocarbons. Here, we circumvent the symmetry-forbidden transition of alternant hydrocarbons via excited-state symmetry breaking in a series of phenyl-substituted tris(2,4,6-trichlorophenyl)methyl (TTM) radicals. We show that 3-fold phenyl substitution enhances the emission of the TTM radical and that steric control modulates the optical properties in these systems. Simple ortho-methylphenyl substitution boosts the photoluminescence quantum efficiency from 1% (for TTM) to 65% at a peak wavelength of 612 nm (for 2-T3TTM) in solution. In the crystalline solid state, the neat 2-T3TTM radical shows a remarkably high photoluminescence quantum efficiency of 25% for emission peaking at 706 nm. This has implications in the design of aryl-substituted radical structures where the electronic coupling of the substituents influences variables such as emission, charge transfer, and spin interaction.

Effects of halogen atom substitution on luminescent radicals: a case study on tris(2,4,6-trichlorophenyl)methyl radical-carbazole dyads

A series of halogen-substitute carbazole TTM radicals was synthesized. The effect of halogen substituents on radical luminescence was systematically evaluated. It was found that the well-known heavy atom effect does not work in the emission of radicals and that halogen substitution of the donor carbazole can change the HOMO and alter the absorption and emission wavelengths. In addition, the photostability was found to be improved with respect to TTM but not significantly different from that of closed-shell fluorescent molecules.

Luminescent Radicals

Organic radicals are attracting increasing interest as a new class of molecular emitters. They demonstrate electronic excitation and relaxation dynamics based on their doublet or higher multiplet spin states, which are different from those based on singlet-triplet manifolds of conventional closed-shell molecules. Recent studies have disclosed luminescence properties and excited state dynamics unique to radicals, such as highly efficient electron-photon conversion in OLEDs, NIR emission, magnetoluminescence, an absence of heavy atom effect, and spin-dependent and spin-selective dynamics. These are difficult or sometimes impossible to achieve with closed-shell luminophores. This review focuses on luminescent organic radicals as an emerging photofunctional molecular system, and introduces the material developments, fundamental properties including luminescence, and photofunctions. Materials covered in this review range from monoradicals, radical oligomers, and radical polymers to metal complexes with radical ligands demonstrating radical-involved emission. In addition to stable radicals, transiently formed radicals generated in situ by external stimuli are introduced. This review shows that luminescent organic radicals have great potential to expand the chemical and spin spaces of luminescent molecular materials and thus broaden their applicability to photofunctional systems.

Investigating the influence of substituent groups in TTM based radicals for the excitation process: a theoretical study

Tri-(2,4,6-trichlorophenyl)methyl (TTM) based radicals can be promising in providing relatively high fluorescence quantum efficiency. In this study, we have evaluated the photoluminescence properties of a series of TTM-based radicals by means of DFT and TD-DFT methods. The optimized structures of the ground states (D0) and the first excited states (D1) of all the radicals are calculated and the computed emission bands are comparable with previous experimental results. knr is determined from transition dipole moments (μ12) and the energy gaps between D0 and D1 (ΔE), both of which can be regulated by the conjugated structures from the substituent groups. knr was derived from the mode-averaging method and is consistent with the experimental results. Factors influencing kr and knr, including the potential energy differences (ΔG0), the vibrational reorganization energies (λ) and the electron coupling term (Hab), are discussed. By comparing kr and knr in solvents with different polarities (cyclohexane, toluene, and chloroform), TTM based radicals in cyclohexane exhibit the most promising fluorescence efficiencies. Besides, two substituted radicals, namely 2Br-TTM-3PCz and 2F-TTM-3PCz, have been fabricated. The results show that fluorine atoms are able to increase ΔG0 and a considerably small knr has been predicted. We expect that our calculation can benefit the design of light-emitting molecules in further experiments.

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

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.

Doublet-emissive materials for organic light-emitting diodes: exciton formation and emission processes

Doublet-emission is mainly discovered in stable radicals, lanthanide-metal complexes with an f¹ electron configuration and transition-metal complexes with a low-spin d⁵ electron configuration, and has a distinct radiation mechanism from closed-shell luminescent molecules and thus technology opportunities. There exists an unpaired electron in the frontier molecular orbitals which enables efficient nanosecond-scale luminescence in these materials due to the spin-allowed transitions between doublet-spin states. In this review, we summarize recent advances in these materials and their application in organic light emitting diodes (OLEDs). The photoluminescence and electroluminescence mechanisms of different doublet-emissive molecular systems are discussed, in addition to the photophysical phenomena arising from doublet states. We also outline the current challenges faced by each molecular system, and the potential outlook on the future research trends in this field.

Iridium(III) Complex Radical and Corresponding Ligand Radical Functionalized by a Tris(2,4,6-trichlorophenyl)methyl Unit: Synthesis, Structure, and Photophysical Properties

Organic radical luminescent materials with doublet excited state character based on tris(2,4,6-trichlorophenyl)methyl (TTM) have attracted extensive attention in recent years. However, how they affect the phosphorescent iridium(III) complex characterized by the triplet excited state has not been studied yet. Herein, a new iridium(III) complex radical (Ir-TTM) and corresponding ligand radical (ppy-TTM) with a TTM unit have been designed and synthesized, and their radical properties were confirmed by the single crystal structure and EPR spectra. Notably, the ligand radical ppy-TTM shows an efficient red light emission, whereas the iridium complex radical Ir-TTM emits no light, which resulted from the intramolecular quenching effect of the TTM radical unit on the iridium luminescence center. DFT calculations demonstrate that the lowest doublet (D₁) excited state of ppy-TTM shows an intramolecular charge transfer character from the 2-phenylpyridine moieties to the TTM unit, whereas the D₁ of Ir-TTM exhibits a significant charge transfer character from the iridium luminescence center moieties to the TTM unit, which further explains the luminescence quenching mechanism of the phosphorescent iridium complex radical.

Photostability of luminescent tris(2,4,6-trichlorophenyl)methyl radical enhanced by terminal modification of carbazole donor

Stable organic luminescent radicals have attracted much attention, but their stability under light irradiation is not yet satisfactory. New luminescent radicals (TTMs) based on terminal benzene ring modified carbazole donors were synthesized and evaluated. Their photostability (half-life under continuous laser irradiation) has improved by 1 order of magnitude compared to simple carbazole donors. This is a new molecular design strategy to improve the photostability of luminescent radicals without reducing other photophysical properties.

The simplest structure of a stable radical showing high fluorescence efficiency in solution: benzene donors with triarylmethyl radicals

Donor-radical acceptor systems have recently attracted much attention as efficient doublet emitters that offer significant advantages for applications such as OLEDs. We employed an alkylbenzene (mesityl group) as the simplest donor to date and added it to a diphenylpyridylmethyl radical acceptor. The (3,5-difluoro-4-pyridyl)bis[2,6-dichloro-4-(2,4,6-trimethylphenyl)phenyl]methyl radical (Mes₂F₂PyBTM) was prepared in only three steps from commercially available reagents. A stable radical composed of only one pyridine ring, four benzene rings, methyl groups, halogens, and hydrogens showed fluorescence of over 60% photoluminescence quantum yield (PLQY) in chloroform, dichloromethane, and PMMA. The key to high fluorescence efficiency was benzene rings perpendicular to the diphenylpyridylmethyl radical in the doublet ground (D₀) state. The relatively low energy of the β-HOMO and the electron-accepting character of the radical enabled the use of benzenes as electron donors. Furthermore, the structural relaxation of the doublet lowest excited (D₁) state was minimized by steric hindrance of the methyl groups. The reasons for this high efficiency include the relatively fast fluorescence transition and the slow internal conversion, both of which were explained by the overlap density between the D₁ and D₀ states.

Research Progress on Triarylmethyl Radical-Based High-Efficiency OLED

Perchlorotrityl radical (PTM), tris (2,4,6-trichlorophenyl) methyl radical (TTM), (3,5-dichloro-4-pyridyl) bis (2,4,6 trichlorophenyl) methyl radical (PyBTM), (N-carbazolyl) bis (2,4,6-trichlorophenyl) methyl radical (CzBTM), and their derivatives are stable organic radicals that exhibit light emissions at room temperature. Since these triarylmethyl radicals have an unpaired electron, their electron spins at the lowest excited state and ground state are both doublets, and the transition from the lowest excited state to the ground state does not pose the problem of a spin-forbidden reaction. When used as OLED layers, these triarylmethyl radicals exhibit unique light-emitting properties, which can increase the theoretical upper limit of the OLED's internal quantum efficiency (IQE) to 100%. In recent years, research on the luminescent properties of triarylmethyl radicals has attracted increasing attention. In this review, recent developments in these triarylmethyl radicals and their derivatives in OLED devices are introduced.

Amplification of luminescence of stable radicals by coordination to NHC-gold(I) complex

The luminescence of stable radicals can be enhanced by coordination to metal complexes. The 4% fluorescence quantum yield of (3,5-difluoro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical (F₂PyBTM) in dichloromethane was enhanced up to 36% by coordination to Au^I with N-heterocyclic carbene ligand, which is a record for metal-radical complexes.

A theoretical study on the donor ability adjustment of tris(2,4,6-trichlorophenyl)methyl-triarylamine (TTM-TPA) radicals aiming to develop better organic luminescent materials

Spin-unrestricted DFT and spin-unrestricted TDDFT calculations were performed to systematically investigate the correlation between the electron donating ability of donors and photophysical properties in D–A luminescent radicals.

The combination of skeleton-engineering and periphery-engineering: a design strategy for organic doublet emitters

A series of radicals based on tris(2,4,6-trichlorophenyl)methyl (TTM) were theoretically designed and evaluated by combining skeleton-engineering and periphery-engineering strategies.

Structural and Magnetic Studies on Nickel(II) and Cobalt(II) Complexes with Polychlorinated Diphenyl(4-pyridyl)methyl Radical Ligands

New magnetic metal complexes with organic radical ligands, [M(hfac)₂(PyBTM)₂] (M = Ni^II, Co^II; hfac = hexafluoroacetylacetonato, PyBTM = (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical), were prepared and their crystal structures, magnetic properties, and electronic structures were investigated. Metal ions in [M(hfac)₂(PyBTM)₂] constructed distorted octahedral coordination geometry, where the two PyBTM molecules ligated in the trans configuration. Magnetic investigation using a SQUID magnetometer revealed that χT increased with decreasing temperature from 300 K in the two complexes, indicating an efficient intramolecular ferromagnetic exchange interaction taking place between the spins on PyBTM and M with J/k_B of 21.8 K and 11.8 K for [Ni^II(hfac)₂(PyBTM)₂] and [Co^II(hfac)₂(PyBTM)₂]. The intramolecular ferromagnetic couplings in the two complexes could be explained by density functional theory calculations, and would be attributed to a nearly orthogonal relationship between the spin orbitals on PyBTM and the metal ions. These results demonstrate that pyridyl-containing triarylmethyl radicals are key building blocks for magnetic molecular materials with controllable/predictable magnetic interactions.

Expansion of Photostable Luminescent Radicals by Meta-Substitution

Polychlorinated pyridyldiphenylmethyl radicals having substituents meta to the position bearing the carbon-centered radical (α-carbon) are synthesized. All of them are stable in ambient conditions in solutions and fluorescent in cyclohexane. The fluorescence of the radicals with bromo, phenyl, 4-chlorophenyl, or 2-pyridyl substituents are enhanced in chloroform, while the emission of the radicals with 2-thienyl or 2-furyl substituents are quenched in chloroform. DFT and TD-DFT calculations indicate that the first doublet excited states of the former are locally excited, while the first doublet excited states of the latter are charge transfer states from the π-electron-donating substituent to the accepting radical. The latter also show much higher photostability under 370-nm light irradiation compared with the first reported photostable fluorescent radical, (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical (PyBTM), with pronounced bathochromic shifts of the fluorescence.

Radical-Based Coordination Polymers as a Platform for Magnetoluminescence

Spin-correlated electronic and magnetic properties of organic radicals have been developed, but luminescence properties, based on interplay with spins, have rarely been reported. The effect of magnetic fields on luminescence (i.e., magnetoluminescence) is a rare example of such properties, observed to date only in radicals dispersed in host matrices. We now report a novel method for achieving radical magnetoluminescence involving radical-based coordination polymers (CPs). The luminescence properties of the bis(3,5-dichloro-4-pyridyl)(2,4,6-trichlorophenyl)methyl (bisPyTM) and tris(3,5-dichloro-4-pyridyl)methyl (trisPyM) radicals and their 1D and 2D Zn^II CPs were investigated. Although solid-state emissions of bisPyTM and trisPyM were not affected significantly by external magnetic fields at 4.2 K, those of CPs were greatly modulated. Studies of the crystal structures, magnetic properties, and the temperature-dependence and time-resolved properties of the magnetoluminescence indicate that the reduction of radical-radical interactions in CPs would be a key method for achieving magnetoluminescence.

An Open-shell, Luminescent, Two-Dimensional Coordination Polymer with a Honeycomb Lattice and Triangular Organic Radical

The use of organic radicals as building blocks is an effective approach to the production of open-shell coordination polymers (CPs). Two-dimensional (2D) CPs with honeycomb spin-lattices have attracted attention because of the unique electronic structures and physical properties afforded by their structural topology. However, radical-based CPs with honeycomb spin-lattices tend to have low chemical stability or poor crystallinity, and thus novel systems with high crystallinity and persistence are in strong demand. In this study, a novel triangular organic radical possessing three pyridyl groups, tris(3,5-dichloro-4-pyridyl)methyl radical (trisPyM) was prepared. It exhibits luminescence, high photostability, and a coordination ability, allowing formation of defined and persistent 2D CPs. Optical measurements confirmed the luminescence of trisPyM both in solution and in the solid state, with emission wavelengths, λ_em, of 665 and 700 nm, respectively. trisPyM exhibits better chemical stability under photoirradiation than other luminescent radicals: the half-life of trisPyM in CH₂Cl₂ was 10 000 times that of the tris(2,4,6-trichlorophenyl)methyl radical (TTM), a conventional luminescent radical. Complexation between trisPyM and Zn^II(hfac)₂ yielded a single crystal of a 2D CP trisZn, possessing a honeycomb lattice with graphene-like spin topology. The coordination structure of trisZn is stable under evacuation at 60 °C. Moreover, trisZn exhibits luminescence at 79 K, with λ_em = 695 nm, and is a rare example of a luminescent material among 2D radical-based CPs. Our results indicate that trisPyM may be a promising building block in the construction of a new class of 2D honeycomb CPs with novel properties, including luminescence.

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.

A Pure-Red Doublet Emission with 90 % Quantum Yield: Stable, Colorless, Iodinated Triphenylmethane Solid

Red luminescence is found in off-white tris(iodoperchlorophenyl)methane (3I-PTM^H ) crystals which is characterized by a high photoluminescence quantum yield (PLQY 91 %) and color purity (CIE coordinates 0.66, 0.34). The emission originates from the doublet excited state of the neutral radical 3I-PTM^R , which is spontaneously formed and becomes embedded in the 3I-PTM^H matrix. The radical defect can also be deliberately introduced into 3I-PTM^H crystals which maintain a high PLQY with up to 4 % radical concentration. The immobilized iodinated radical demonstrates excellent photostability (estimated half-life >1 year under continuous irradiation) and intriguing luminescent lifetime (69 ns). TD-DFT calculations demonstrate that electron-donating iodine atoms accelerate the radiative transition while the rigid halogen-bonded matrix suppresses the nonradiative decay.

Spin multiplicity effects in doublet versus singlet emission: the photophysical consequences of a single electron

Ambient-stable fluorescent radicals have recently emerged as promising luminescent materials; however, tailoring their properties has been difficult due to the limited photophysical understanding of open-shell organic systems. Here we report the experimental and computational analysis of a redox pair of π-conjugated fluorescent molecules that differ by one electron. A π-dication (DC) and π-radical cation (RC) demonstrate different absorption spectra, but similar red emission (λ_emiss,max = ∼630 nm), excitation maxima (λ_exc,max = ∼530 nm), fluorescence lifetimes (1–10 ns), and even excited-state (non-emissive) lifetimes when measured by transient absorption spectroscopy. Despite their experimental similarities, time-dependent density functional theory (TDDFT) studies reveal that DC and RC emission mechanisms are distinct and rely on different electronic transitions. Excited-state reorganization occurs by hole relaxation in singlet DC, while doublet RC undergoes a Jahn-Teller distortion by bending its π-backbone in order to facilitate spin-pairing between singly occupied molecular orbitals. This relationship between the excited-state dynamics of RC and its π-backbone geometry illustrates a potential strategy for developing π-conjugated radicals with new emission properties. Additionally, by comparing TDDFT and CIS (configuration interaction singles) excitations, we show that unrestricted TDDFT accurately reproduces experimental absorption spectra and provides an opportunity to examine the relaxed excited-state properties of large open-shell molecules like RC.

Experimental and computational studies reveal mechanistic differences in the photophysics of an open- versus closed-shell π-conjugated redox pair.

Air-Stable Organic Radicals: New-Generation Materials for Flexible Electronics?

In the last few years, air-stable organic radicals and radical polymers have attracted tremendous attention due to their outstanding performance in flexible electronic devices, including transistors, batteries, light-emitting diodes, thermoelectric and photothermal conversion devices, and among many others. The main issue of radicals from laboratory studies to real-world applications is that the number of known air-stable radicals is very limited, and the radicals that have been used as materials are even less. Here, the known and newly developed air-stable organic radicals are summarized, generalizing the way of observing air-stable radicals. The special electric and photophysical properties of organic radicals and radical polymers are interpreted, which give radicals a wide scope for various of potential applications. Finally, the exciting applications of radicals that have been achieved in flexible electronic devices are summarized. The aim herein is to highlight the recent achievements in radicals in chemistry, materials science, and flexible electronics, and further bridge the gap between these three disciplines.

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.

One-dimensional magnetic chain composed of CuII and polychlorinated dipyridylphenylmethyl radical: temperature-dependent Jahn-Teller distortion correlated to π-conjugation and magnetic properties

The one-dimensional coordination polymer composed of Cu^II and an organic radical bisPyTM displayed temperature dependent Jahn-Teller distortion, which is correlated to the π-conjugation mode at the ligand as well as the magnetic properties. The ferromangetic bisPyTM-Cu^II interaction was enhanced by the reorientation of the Cu^II d_x²-y² orbital at low temperature.

Unraveling the Emission Mechanism of Radical-Based Organic Light-Emitting Diodes

Stable doublet radical molecules have recently emerged as a promising new type of emitters in organic light-emitting diodes (OLEDs), approaching 100% internal quantum efficiency. However, the detailed emission mechanism of these open-shell emitters remains elusive. Through theoretical model analysis and first-principles calculations, we unraveled the emission mechanism of a typical emitter, (4-N-carbazolyl-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)methyl (TTM-1Cz). Our study showed that the electroluminescence arises from the first doublet excited state generated by injecting one electron into the singly occupied molecule orbital (SOMO) and one hole into the highest doubly occupied molecule orbital (HDMO). Because of the distinct charge-transfer rates in charge-injection processes, the puzzle of 100% formation ratio of the emissive doublet exciton in experiments is revealed. On the basis of this understanding, we propose simple molecular designs via substitutions that can tune the HDMO-SOMO gap and hence shift the emission wavelength to the region of yellow and green light.

Fabrication and Application of Dual-Modality Polymer Nanoparticles Based on an Aggregation-Induced Emission-Active Fluorescent Molecule and Magnetic Fe₃O₄

Fluorescent magnetic nanoparticles (NPs) utilized for imaging hold great promise for biomedical applications, but it remains a challenging task. Here, we report novel dual-modality NPs using an aggregation-induced emission (AIE)-active and near-infrared (NIR) emissive dye (TPAS) and magnetic Fe₃O₄ as the core, and biocompatible polymer Pluronic F-127 as the encapsulation matrix by self-assembly procedures. The obtained fluorescent-magnetic AIE NPs have both high fluorescence quantum yield (13.8%) at 700 nm and high magnetic saturation value. With good photostability and biocompatibility, the resulting NPs show effective MRI ability, but also a stain in cytoplasm with a strong NIR fluorescent signal.

DFT/MRCI-R2018 study of the photophysics of the zinc(ii) tripyrrindione radical: non-Kasha emission?

The fluorescence of a radical-based emitter has been theoretically investigated after measurements had shown absorption bands to lie below the emission energy. The results of the all-multiplicity DFT/MRCI-R2018 study indicate D₃ emission.