Doublet Emission Research Articles

Synthesis and Structural Characterization of Carbazole‐Tailored Luminescent Triarylmethyl Radical and its Stable Cation

AbstractThe development of novel luminescent radicals, characterized by their unique doublet emission, endows a significant challenge. In this study, we reported the synthesis of a luminescent neutral radical, BCzAnM‐R, tailored by two carbazolyl groups and an anthryl group to achieve a nonalternant structure. It exhibited near‐infrared emission with a peak at 1020 nm in toluene. Interestingly, its corresponding cation, BCzAnM‐C, was synthesized through an unconventional SnCl2‐mediated reduction‐aromatization‐oxidation reaction in one‐pot and gram‐scale. The cation demonstrated remarkable stability for up to weeks in ambient conditions and facilitated the silica‐gel chromatography isolation as an organic salt with SnCl3− as the counter ion. The carbazolyl groups effectively modulate molecular structures, photophysical properties, and stabilities. Notably, BCzAnM‐R represents the first luminescent triarylmethyl radical with two carbazolyl groups directly attached to the central carbon.

Stable Xanthene Radicals and Their Heavy Chalcogen Analogues Showing Tunable Doublet Emission from Green to Near-infrared.

Organic luminescent radicals, unlike traditional closed-shell fluorescent emitters, exhibit distinct luminescence mechanisms, offering promising potential for optoelectronic devices. To date, stable luminescent radicals have predominantly been confined to polychlorinated triphenylmethyl radicals, underscoring the need for new platforms to expand their emission spectra. In this study, we report the synthesis of stable 9-aryl-substituted xanthene radicals and their heavy chalcogen analogues (1a-c and 2a-c), which exhibited excellent chemical stability and emission ranging from green to near-infrared (527~714 nm). Notably, the selenium-substituted radical (1c) demonstrates a significantly enhanced photoluminescence quantum yield of 41% when doped into its precursor solid. Additionally, the introduction of methoxyphenyl groups has largely enhanced the stability of the radical, showcasing an excellent photostability with the longest half-life of around 1792 h. The high internal quantum efficiency of up to 81% was further validated in organic light-emitting diode. This study introduces a novel class of stable carbon-centered radicals with high tunability and functionality for photoelectric applications.

Stable Xanthene Radicals and Their Heavy Chalcogen Analogues Showing Tunable Doublet Emission from Green to Near‐infrared

Organic luminescent radicals, unlike traditional closed‐shell fluorescent emitters, exhibit distinct luminescence mechanisms, offering promising potential for optoelectronic devices. To date, stable luminescent radicals have predominantly been confined to polychlorinated triphenylmethyl radicals, underscoring the need for new platforms to expand their emission spectra. In this study, we report the synthesis of stable 9‐aryl‐substituted xanthene radicals and their heavy chalcogen analogues (1a‐c and 2a‐c), which exhibited excellent chemical stability and emission ranging from green to near‐infrared (527~714 nm). Notably, the selenium‐substituted radical (1c) demonstrates a significantly enhanced photoluminescence quantum yield of 41% when doped into its precursor solid. Additionally, the introduction of methoxyphenyl groups has largely enhanced the stability of the radical, showcasing an excellent photostability with the longest half‐life of around 1792 h. The high internal quantum efficiency of up to 81% was further validated in organic light‐emitting diode. This study introduces a novel class of stable carbon‐centered radicals with high tunability and functionality for photoelectric applications.

Dual Channel Emissions of Kasha and Anti-Kasha from a Single Radical Molecule.

Stable open-shell luminescent radicals have recently attracted much attention due to their unique luminescence properties. However, a radical molecule with both Kasha and anti-Kasha doublet emission properties has not been reported. Herein, we have successfully synthesized a stable chlorine-substituted Chichibabin's hydrocarbon, TTM-TTM, along with its mono-radical counterpart, TTM-HTTM. The emission of TTM-TTM follows Kasha's rule in the near infrared region. However, TTM-HTTM shows dual channel doublet emissions of Kasha and anti-Kasha. Remarkably, these two types of emission compete dynamically in both solution and condensed states. Our findings provide valuable insights into the rational design and discovery of stable radicals that possess distinctive luminescent properties, thus broadening the horizons of luminescent materials research.

Synthesis and Structural Characterization of Carbazole-Tailored Luminescent Triarylmethyl Radical and its Stable Cation.

The development of novel luminescent radicals, characterized by their unique doublet emission, endows a significant challenge. In this study, we reported the synthesis of a luminescent neutral radical, BCzAnM-R, tailored by two carbazolyl groups and an anthryl group to achieve a nonalternant structure. It exhibited near-infrared emission with a peak at 1020 nm in toluene. Interestingly, its corresponding cation, BCzAnM-C, was synthesized through an unconventional SnCl2-mediated reduction-aromatization-oxidation reaction in one-pot and gram-scale. The cation demonstrated remarkable stability for up to weeks in ambient conditions and facilitated the silica-gel chromatography isolation as an organic salt with SnCl3-as the counter ion. The carbazolyl groups effectively modulate molecular structures, photophysical properties, and stabilities. Notably, BCzAnM-R represents the first luminescent triarylmethyl radical with two carbazolyl groups directly attached to the central carbon.

Dual Channel Emissions of Kasha and Anti‐Kasha from a Single Radical Molecule

AbstractStable open‐shell luminescent radicals have recently attracted much attention due to their unique luminescence properties. However, a radical molecule with both Kasha and anti‐Kasha doublet emission properties has not been reported. Herein, we have successfully synthesized a stable chlorine‐substituted Chichibabin's hydrocarbon, TTM‐TTM, along with its mono‐radical counterpart, TTM‐HTTM. The emission of TTM‐TTM follows Kasha's rule in the near infrared region. However, TTM‐HTTM shows dual channel doublet emissions of Kasha and anti‐Kasha. Remarkably, these two types of emission compete dynamically in both solution and condensed states. Our findings provide valuable insights into the rational design and discovery of stable radicals that possess distinctive luminescent properties, thus broadening the horizons of luminescent materials research.

Solar Atmospheric Oscillations as Measured by the GOES-R Series EXIS EUVS-C Instrument

This Letter presents first observations of distinct solar atmospheric oscillations from signals collected by the Extreme Ultraviolet and X-ray Irradiance Sensors EUVS-C instrument, which is part of each Geostationary Observational Environmental Satellite R-Series instrument payload. The EUVS-C instrument is a full-disk, normal-incidence spectrograph that covers a narrow band in the mid-ultraviolet between 276 and 284 nm, where it can measure the magnesium ii emission doublet at ∼280 nm and the photospheric continuum. The primary goal of EUVS-C data is to construct the well-known Mg ii index, which is often used as a proxy for chromospheric activity. Because of the high temporal and spectral resolution of EUVS-C measurements, the data provide a unique opportunity to observe discernible solar atmospheric waves that have definite signatures of 3 and 5 minute oscillation periods, where the frequency response of these signals is dependent on what part of the spectrum is analyzed (e.g., Mg emission lines). Furthermore, both photospheric and chromospheric waves can simultaneously be examined. With the recent increase in solar activity for Solar Cycle 25, these waves exhibit enhanced amplitudes and phase shifts during the impulsive shock of strong solar flares. This Letter will discuss the analysis for deriving these waves, and results from both a quiescent Sun and an X-class solar flare event will be presented.

On the Doublet Flux Ratio of Mg ii Resonance Lines in and Around Galaxies

Observations of metallic doublet emission lines, particularly Mg ii λ λ2796, 2803, provide crucial information for understanding galaxies and their circumgalactic medium. This study explores the effects of resonant scattering on the Mg ii doublet lines and the stellar continuum in spherical and cylindrical geometries. Our findings show that under certain circumstances, resonance scattering can cause an increase in the doublet flux ratio and the escaping flux of the lines beyond what is expected in optically thin spherical media. As expected, the doublet ratio is consistently lower than the intrinsic ratio when the scattering medium is spherically symmetric and dusty. However, if the scattering medium has a disk shape, such as face-on disk galaxies, and is viewed face-on, the doublet ratio is predicted to be higher than 2. It is also shown that doublet ratios as low as those observed in compact star-forming galaxies cannot be explained solely by pure dust attenuation of intrinsic Mg ii emission lines in spherical models unless dust opacity deviates markedly from that expected based on the dust-to-Mg+ gas ratio of our Galaxy. The importance of the continuum-pumped emission lines and expanding media is discussed to understand observational aspects, including doublet flux ratios, which can be lower than 1.5 or higher than 2, as well as symmetric or asymmetric line profiles. It is also discussed that the diffuse warm neutral medium may be an important source of Mg ii emission. These results provide insight into the complexity of the shape and orientation of distant, spatially unresolved galaxies.

Exploring the Charge-Transport and Optical Characteristics of Organic Doublet Radicals: A Theoretical and Experimental Study with Photovoltaic Applications.

Herein, we present a series of stable radicals containing a trityl carbon-centered radical moiety exhibiting interesting properties. The radicals demonstrate the most blue-shifted anti-Kasha doublet emission reported so far with high color purity (full width at half-maximum of 46 nm) and relatively high photoluminescence quantum yields of deoxygenated toluene solutions reaching 31%. The stable radicals demonstrate equilibrated bipolar charge transport with charge mobility values reaching 10-4 cm2/V·s at high electric fields. The experimental results in combination with the results of TD-DFT calculations confirm that the blue emission of radicals violates the Kasha rule and originates from higher excited states, whereas the bipolar charge transport properties are found to stem from the particularity of radicals to involve the same molecular orbital(s) in electron and hole transport. The radicals act as the efficient materials for interlayers, passivating interfacial defects and enhancing charge extraction in PSCs. Consequently, this leads to outstanding performance of PSC, with power conversion efficiency surpassing 21%, accompanied by a remarkable increase in open-circuit voltage and exceptional stability.

Full correction of the self-absorption of laser-induced plasma beryllium emissions via sample preparation

Self-absorption effect is always encountered in laser-induced breakdown spectroscopy (LIBS) and immediately distorts the calibration curves especially for the elemental determination of complex materials. With the aim to provide an expeditious approach for LIBS measurements susceptible to self-absorption effect, an exploratory study for sample preparation by powder mixing is implemented, and the self-absorption of laser-induced plasma beryllium emissions is investigated as a function of the dilution factor. For comparison, boric acid and wax powder are separately used as typical binding agent to mix with beryl powder to produce two sets of pressed pellets with sequential gradient of beryllium content variation. For the resonant lines most prone to self-absorption of Be II emission doublet at 313.042 nm and 313.107 nm, the beryllium spectral line shapes can be directly regulated and improved in the case of both sets of pellets as the dilution factor increases. In addition, the self-absorption effect for the strongly emitting beryllium spectral lines is further assessed by calculating the self-absorption coefficients based on the radiation transport equation. With regard to the both types of binder as diluent, when the dilution factor for beryl increases to 10 with a weight percent of 0.386% for beryllium in the pellet, the SA values may exponentially increase in general to around 0.8 and eventually asymptotically approaches to 1 without exception. Thus the self-absorption effect of laser-induced beryl plasma emissions can be readily overcome with sample preparation by powder mixing. By contrast with laser pulse irradiance in the present work, the dilution factor for powder mixing plays a dominant role in eliminating self-absorption effect. This research proposes a potentially effective approach to reduce self-absorption in laser-induced plasma emissions.

Quality index for Martian in-situ laser-induced breakdown spectroscopy data

Laser-induced breakdown spectroscopy (LIBS) has been widely implemented in Mars explorations as a versatile technique for in-situ chemical analysis. Determination of elemental composition from Martian LIBS typically relies on uni/multi-variate models trained on specific laboratory datasets, which in turn requires similarities between this training dataset and Martian data. However, plasma excitation can affect this similarity. The inability to identify insufficiently excited data may ignore poor focusing for some observations or lead to unawareness of compositional uncertainty. This study establishes the LIBS Quality Index (LQI) for Martian LIBS data quality by evaluating the carbon ionic doublet emission around 658 nm for the Martian atmosphere, assessing its signal-to-noise ratio as well as its deviation from the line profile standard of the instrument-specific laboratory dataset. This method allows the LQI to be used as a confidence level for assessing LIBS excitation quality and measurement for similarity with instrument-specific laboratory datasets. The LQI evaluation routines were established for ChemCam, MarSCoDe, and SuperCam. Validation of LQI was carried out on the laboratory datasets and Mars data with known quality assessments of the three instruments. The index was verified to be sensitive to input irradiance, applicable with three current Mars LIBS instruments, robust against various material types (except carbonate), and moderate interference from H, Fe, and Ti. Applying the LQI, the study was able to identify the remote micro-imager (RMI) autofocus as an optimal mechanism, to discover the influence of environmental conditions on data quality, to discuss the impact of target material consolidation and morphology on data quality, and to investigate the contribution of data quality to quantification uncertainty. These application examples demonstrated the feasibility of LQI as a useful and sensitive tool to assess data quality and its potential to aid current and future Martian LIBS explorations.

Water-Soluble Trityl Radicals for Fluorescence Imaging.

Stable tris(trichlorophenyl)methyl radicals have gained interest as all-organic bioimaging agents combining fluorescent and paramagnetic properties. However, cellular uptake has so far only been reported for nanoparticles, because molecular hydrophobic trityl radicals are not soluble in aqueous media. Here, we report the synthesis and characterization of new water-soluble tris(trichlorophenyl)methyl radical derivatives exhibiting red doublet emission. Solubility in water is achieved through functionalization with oligoethylene glycol (OEG) chains. The emission behavior of OEG functionalized trityl radicals is studied in polar environments. Donor-functionalization with carbazole evokes a charge-transfer excited state that is efficiently quenched in polar solvents. In contrast, click-reaction mediated attachment of OEG-azide and trityl acetylene furnishes a triazole functionalized radical with locally excited states and emission in water. Confocal fluorescence microscopy proves successful uptake of the material by macrophages in cell culture, showing the potential of our water soluble trityl radical for fluorescence bioimaging.

Aza[7]helicene Functionalized Triphenylmethyl Radicals with Circularly Polarized Doublet Emission (Advanced Optical Materials 4/2024)

Aza[7]helicene Functionalized Triphenylmethyl Radicals with Circularly Polarized Doublet Emission (Advanced Optical Materials 4/2024)

Discovery of optical emission from the supernova remnant G108.2−0.6 and its atomic environment

ABSTRACT We report on the first detection of optical emission from the shell-type Galactic supernova remnant (SNR) G108.2−0.6. We obtained Hα images and long-slit spectra using the 1.5-m RTT150 telescope to examine the morphological and spectral characteristics of the SNR. We detected several filaments along its north and south regions, which is consistent with its SNR nature. The spectra exhibit [S ii]/Hα ratios in the range of 0.4–1.1, indicating emission from shock-heated gas. The oxygen doublet emission lines [O i] λλ6300, 6363 detected in the south region also support the indicator of the presence of shocks. We estimate the electron density using the [S ii] 6716/6731 ratio ranging from 15 to 1800 cm−3. The spectra show a relatively low shock velocity of Vs ∼ 80 km s−1 with the pre-shock cloud density of nc ∼18–57 cm−3. The Hα/Hβ ratios show significant variation across the observed regions with extinction E(B − V) ranging from 0.22 to 1.65. We also analysed the archival H i data and estimated the kinematic distance to G108.2−0.6 of ∼0.8 kpc and its dynamical age as ∼70 ± 10 kyr.

Highly Efficient Stable Luminescent Radical-Based X-ray Scintillator.

Stable luminescent radicals are open-shell emitters with unique doublet emission characteristics. This feature makes stable luminescent radicals exhibit widespread application prospects in constructing optical, electrical, and magnetic materials. In this work, a stable luminescent radical-based X-ray scintillator of AuPP-1.0 was prepared, which exhibited a high X-ray excited luminescence (XEL) efficiency as well as excellent stability. A mechanism study showed that the heavy atom of Au in AuPP-1.0 endowed it with effective absorption of X-rays, and the doublet emission characteristics of AuPP-1.0 significantly increased its exciton utilization rate in the radioluminescence process. Moreover, AuPP-1.0 has good processability to fabricate a flexible screen for high-quality X-ray imaging, whose resolution can reach 20 LP mm-1. This work demonstrates that the doublet emission is beneficial for improving the exciton utilization rate of radioluminescence, providing a brand-new strategy for the construction of high-performance X-ray scintillators.

Photoluminescence properties of Sb3+-activated Ca2YTaO6 dual color emitting phosphors for plant growth LEDs applications

Developing highly efficient phosphors-converted white LEDs with excellent emissions characteristics in the blue and deep-red spectral regions, is essential for improving artificial white light for plant growth. Due to the characteristic 3Pn−1S0 transitions (n = 0, 1, 2), Sb3+ is considering as an efficient activator emitting light in the desired deep red and far-red spectral regions. In this paper, we successfully developed Sb3+-doped Ca2YTaO6 double perovskite phosphors, showing broadband doublet emission peaks, in the visible spectral ranges. Computational and experimental investigations were used to confirm the phase purity and structural analysis. The photoluminescence excitation and emission spectra, fluorescence decay curves, thermal stability and CIE chromaticity coordinates have been measured and determined to demonstrate the application potential of the Ca2YTaO6:Sb3+ double perovskites, as suitable phosphors for plant growth acceleration. The blue (420 nm) and deep red (683 nm) emission peaks were observed under 313 nm UV excitation. Efficient energy transfer was observed from the blue emission of the host phosphor to the doped Sb3+ ions, which resulted in an enhancement of the deep-red emission. The appeared dual color emission makes this phosphor a potential candidate for the generation of artificial white light for plant growth.

Aza[7]helicene Functionalized Triphenylmethyl Radicals with Circularly Polarized Doublet Emission

AbstractSynthesis and characterization of configurationally stable chiral aza[7]helicene‐functionalized trityl radicals are reported. To overcome the problem of incomplete radical formation after coupling the trityl radical to the chiral aza[7]helicenes in a nucleophilic aromatic substitution, this study instead employs Pd‐catalyzed C‐N cross‐coupling. In contrast to the conventional approach, the open‐shell character is retained allowing quantitative yield of the radical. The obtained dinaphthocarbazole and diphenanthrocarbazol‐functionalized tris(trichlorophenyl)methyl (TTM) radicals can be separated into the respective enantiomers. The resulting chiral radicals exhibit photoluminescence quantum yields as high as 43% and circularly polarized luminescence. Time‐dependent density functional theory (TD‐DFT) calculations support the experimental findings. The newly reported aza[7]helicene‐based radicals hold promise for advanced optoelectronic as well as quantum technological applications.

A Simple Molecular Design Strategy for Luminescent Radicals to Achieve Near-Infrared Emission.

The spin-allowed doublet emission of luminescent radicals has recently attracted significant attention. However, the spectral range of most reported luminescent radical emitters and their corresponding organic light-emitting diodes (OLEDs) is confined to the red and deep red regions, with only a few extending to the near-infrared region, specifically in the context of an emission peak exceeding 800 nm. Herein, a luminescent radical, 2-(4-(bis(2,4,6-trichlorophenyl)methyl radical)-3,5-dichlorophenyl)-4-phenyl-4H-thieno[3,2-b]indole (TTM-2PTI), with NIR emission peaking at 830 nm in toluene, was obtained through attaching a 4-phenyl-4H-thieno[3,2-b]indole group to the TTM radical core. An organic light-emitting diode (OLED) utilizing TTM-2PTI as the emitter exhibits electroluminescence (EL) emission peaking at 870 nm, which is the longest EL wavelength among the doublet-emissive near-infrared (NIR) OLEDs. This work provides a simple molecular design strategy to achieve NIR emission of radicals by leveraging the lower steric hindrance and electron-donating ability of thiophene.

Variations in the intermediate wind region of the blue supergiant 55 Cygni

Context. The quantitative near-infrared (NIR) spectroscopic synthesis is an important technique for determining wind properties of massive stars. The Brα line is an excellent mass-loss tracer and provides valuable information on the physical conditions of intermediate-wind regions. The knowledge of the wind properties gained by studying the NIR lines could provide extra ingredients to the theory of line-driven winds, mainly because the standard theory does not predict observed properties of blue supergiants, such as high values for the β parameter (β> 2), low terminal velocities, and mass-loss variability. Aims. We seek to enhance our understanding of the wind properties of B supergiants. To this end, we propose analysing their NIR spectra over different epochs to study wind variability and its connection with phenomena arising from regions close to the photosphere. Methods. We present the first sets of multi-epoch high-resolution K- and L-band spectra of 55 Cyg acquired with the Gemini Near-InfraRed Spectrograph (GNIRS). We measured line equivalent widths and modelled the Brα line to derive (unclumped) mass-loss rates. Synthetic line profiles were computed for a homogeneous spherical wind by solving the radiative transfer equations in the co-moving frame for a multi-level atom in non-local thermodynamic equilibrium (NLTE). Results. We observe variations in the spectral lines originating in the upper photosphere and the wind. The perturbations, on average, have periods of ~13 and ~23 days; the latter is similar to that found previously from optical data (22.5 days). The NIR lines observed in 2013 are described with the same wind structure used to model a quasi-simultaneous observation in Hα. By contrast, from observations taken in 2015, we derived a higher mean mass-loss rate. Variations in the mass-loss rate are also detected within a few weeks. Interestingly, we find that the profile shape of the Hu14 line sets constraints on the mass loss. Moreover, we find the Mg II doublet in emission, which suggests a tenuous circumstellar gas ring or shell. Conclusions. The variability detected in the NIR H emission lines of 55 Cyg is related to changes in the mass-loss rate, which doubled its value between 2013 and 2015. Furthermore, the short-term variability (within three weeks) in the spectral lines and mass loss supports the hypothesis of strange-mode oscillations. This pilot project demonstrates the importance of comprehensive monitoring of blue supergiants’ variability to deeply understand the physical properties of their stellar winds and the role of pulsations in recurrently enhancing mass loss.

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.