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Design and Applications of Carborane-Based Luminescent Materials

Qing Jiang , Xinyi Li , Yunjun Shen , Yuzhen Zhang

Progress in Chemistry ›› 2026, Vol. 38 ›› Issue (6) : 1073 -1084.

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Progress in Chemistry ›› 2026, Vol. 38 ›› Issue (6) : 1073-1084 DOI: 10.7536/PC20251011
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Design and Applications of Carborane-Based Luminescent Materials
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Abstract

Cage-like carboranes，with their unique polyhedral spatial architecture and three-dimensional（3D）delocalized electron system，are regarded as 3D analogs of benzene rings and exhibit 3D aromaticity that is not found in classical two-dimensional（2D）aromatic systems. These compounds generally possess high chemical stability，high thermal stability，and excellent biocompatibility，endowing them with broad application prospects in the fields such as organic optoelectronic materials and biomedicine. This article systematically reviews the significant research progress achieved in recent years regarding carborane-based luminescent materials，focusing on aspects such as molecular design strategies and application field. In terms of molecular design，researchers have effectively regulated the aggregated structure，charge transfer properties，and excited-state characteristics of molecules by means of embedding carborane cages into conjugated backbones，using them as pendant groups，or employing them to construct donor-acceptor（D-A）systems. This regulation，in turn，enables the enhancement of luminescent quantum yield and the tuning of emission wavelength ranges. In the field of applications，this class of materials not only demonstrates great potential in high-performance optoelectronic materials but also exhibits prominent application value in fields such as circularly polarized luminescence（CPL），stimuli-responsive materials，and bioimaging. Finally，the article provides an outlook on the challenges and development prospects for the future development of this category of materials.

Contents

1 Introduction

2 o-Carborane based luminescent materials

2.1 o-Carborane C2B10H12 based organic luminescent materials

2.2 o-Carborane C2B10H12 based metal complex luminescent materials

3 Carborane anion luminescent materials

3.1 nido-Carborane anion [nido-C2B9H12]⁻ based luminescent materials

3.2 Monocarba-closo-dodecaborate anion [CB11H12]⁻ luminescent materials

4 Conclusion and outlook

Graphical abstract

Keywords

luminescent material / carborane / synthesis / photophysical property / functional application

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Qing Jiang, Xinyi Li, Yunjun Shen, Yuzhen Zhang. Design and Applications of Carborane-Based Luminescent Materials. Progress in Chemistry, 2026, 38(6): 1073-1084 DOI:10.7536/PC20251011

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1 Introduction

Currently, luminescent materials have become a key support for the development of optoelectronic devices, biosensing, biomedicine, and other fields. In the field of optoelectronics, there is an increasingly urgent demand for high-brightness and long-lifetime luminescent materials for OLED displays and flexible lighting, which directly affects the display performance and service life of devices; in the field of biosensing, there is a pressing need for low-toxicity and highly specific luminescent probes to achieve precise detection of intracellular metal ions, biological small molecules, etc. Luminescent materials show broad prospects in bioimaging, tumor photodynamic/photothermal therapy, disease diagnosis, and other aspects, promoting the development of targeted therapy and integrated theranostics. However, traditional luminescent materials face many bottlenecks in terms of quantum yield, biological toxicity, and environmental tolerance, making it difficult to meet the needs of multi-scenario applications. Therefore, developing new high-performance luminescent materials is of great significance for innovation and application in high-tech fields.[1-7].
However, the development of high-performance luminescent materials still faces severe challenges. Carborane compounds, with their unique electronic properties and three-dimensional cage structures, provide a new direction for addressing these challenges and advancing the research and development of luminescent materials. Carboranes are a class of closo-dodecaborane compounds featuring twelve vertices and an icosahedral structure.[8]. The extensively studied compounds of this type mainly include the electrically neutral dicarbon closo-dodecaborane C2B10H12, the nido-ortho-carborane anion [nido-C2B9H12]-, and the monocarbon closo-dodecaborane anion [closo-CB11H12]-(Figure 1). They are all derived from the substitution of one or two B—H groups in closo- or nido-boranes with C—H groups. Among them, the neutral dicarbon closo-dodecaborane C2B10H12exists in three isomers: ortho, meta, and para. In these molecules, boron and carbon atoms form an icosahedral spatial structure through electron-deficient bonding. The introduction of carbon atoms alters the electron cloud distribution of B—H active sites on the surface of the boron cage. For instance, in ortho-carborane, carbon atoms modulate the reactivity of surrounding B—H bonds via the inductive effect, providing precise sites for subsequent selective functionalization. Furthermore, based on the unique spatial structure and three-dimensional electron delocalization characteristics of cage-like boranes, their three-dimensional aromaticity can be regarded as a three-dimensional analogue of the benzene ring. These compounds exhibit excellent chemical stability, thermal stability, and biocompatibility, thus demonstrating immense application potential in fields such as organic optoelectronic materials and biomedicine.[9-11].
In this context, research on carborane-based luminescent materials has gradually become a hot direction in the field of functional materials. Current research focuses primarily on two aspects: first, the structure-property relationships between structure and luminescence performance, achieving enhanced luminescent properties and coverage of emission wavelengths from ultraviolet to near-infrared through means such as functional modification and aggregation state regulation; second, the expansion of application scenarios, promoting the transition of materials from fundamental photophysical research to practical applications in light-emitting devices, bioimaging, environmental detection, and other fields, thereby bridging the gap between basic research and practical applications. This article reviews the progress in the design and application research of carborane-based luminescent materials over the past five years and provides an outlook on the future challenges and development prospects facing these materials.

2 o-Carboranyl Luminescent Materials

2.1 o-Carboraneo-C2B10H12Organic luminescent materials

Currently, functionalization designs of o-carborane reported in the literature mainly involve introducing it into various fluorescent donor groups bearing naphthyl, anthryl, tetraphenylethylene, pyrenyl, and other moieties[12-16]. Since substitution at the cage carbon atoms of o-carborane exhibits strong electron-withdrawing characteristics, it can induce a significant intramolecular charge transfer (ICT) effect between the π-conjugated aromatic groups and the carborane cage[17-18]. Meanwhile, the high polarizability associated with its σ-aromaticity facilitates the formation of donor (aromatic part)-acceptor (o-carborane) conjugated systems. Such donor-acceptor (D-A) conjugated systems based on o-carborane derivatives typically exhibit rich and interesting luminescent behaviors. In 2020, Kim et al.[19]prepared and characterized three o-carborane-functionalized pyrenes, obtaining 1CB, 2CB, and 4CB by varying the position of the o-carborane cage (Figure 2). Photophysical studies revealed that 2CB exhibits weak locally excited (LE) emission in tetrahydrofuran (THF) at 298 K, whereas 1CB and 4CB display strong intramolecular charge transfer (ICT) emission involving the o-carborane fragment. These findings validate the relationship between o-carborane-associated ICT-based radiative decay efficiency and structural dynamics in various o-carboranes possessing aromatic luminophores.
In 2021, Smyshliaeva et al.[20]reported a convenient transition-metal-free method for C-H functionalization of the 1,3,7-triazapyrene skeleton, yielding boron-rich polynitrogen heterocyclic hydrocarbons with dual-emission behavior and aggregation-induced emission (AIE) properties (Figure 3). The revealed fluorescence characteristics indicate that these carborane derivatives can serve as potential functional units in the design of advanced organic luminescent materials. In 2022, Yang et al.[21]proposed a general photoinduced electron transfer method for constructing dual-fluorescence materials. The synthesized ortho-carborane dyads exhibited excitation wavelength-dependent fluorescence behavior independent of the aggregation state, attributed to the competition between non-radiative relaxation channels corresponding to internal conversion (IC) and photoinduced electron transfer (PET) processes. Furthermore, these dyads have been successfully applied in QR code recognition and the fabrication of multicolor fluorescent films in dark environments. This multiple excited-state strategy based on excitation wavelength-dependent photoinduced electron transfer can be further utilized for the development of novel anti-counterfeiting fluorescent materials, as well as the production of films combining multicolor and flexible properties.
In 2024, Lee et al.[22]reported a new class of o-carborane-based emitters, 2CB-BuDABNA and 3CB-BuDABNA, in which o-carborane groups were introduced into the periphery of boron- and nitrogen-doped multi-resonance thermally activated delayed fluorescence cores. Both compounds retained the inherent localized emission characteristics of their MR emission cores, exhibiting strong MR-TADF in toluene and in the rigid state, along with high photoluminescence quantum yields. Efficient blue OLEDs fabricated using 2CB-BuDABNA as the emitter achieved a maximum external quantum efficiency of up to 25%, setting a new record for the highest efficiency of OLEDs employing o-carborane-based emitters. Shao et al.[23]successfully prepared two novel DPAC derivatives (CbDPAC-Ph and bis-CbDPAC-Ph) by introducing one or two o-carborane units into theN,N'-diphenyl dihydrodibenzo[a,c]phenazine (DPAC) skeleton structure. The study revealed that the introduction of o-carborane not only enriched the packing modes of DPAC, enhanced photostability, and increased the Stokes shift, but also promoted mass transport of analytes within the fluorescent active layer due to the increase in molecular channels. Fluorescent thin-film sensors based on bis-CbDPAC-Ph demonstrated excellent cycling stability and high sensitivity towards acrylonitrile (ACN), with a detection limit as low as 22 ppb, a response time of only 2 s, and a full recovery time of less than 2 min, showcasing significant potential for application as portable ACN detectors.
Although researchers have conducted extensive and in-depth studies on the luminescent properties of carborane derivatives in recent years, the development of luminescent compounds based on chiral carboranes, as well as the systematic exploration of their unique chiroptical properties, still faces significant challenges due to synthetic limitations such as the difficulty in constructing chiral structures and poor control over stereoselectivity. In 2024, Sun et al.[24]successfully separated two chiral isomers (3aa-RPand 3aa-SP) via chiral high-performance liquid chromatography and confirmed their structures through single-crystal X-ray diffraction. Their circular dichroism spectra displayed a mirror-image relationship with alternating positive and negative Cotton effects. While no circularly polarized luminescence (CPL) signal was observed in solution, aggregation-induced enhanced CPL (AICPL) was exhibited in PMMA films, with a photoluminescence asymmetry factorgPLreaching 9.9×10⁻3, which represents an excellent level for organic small-molecule CPL materials. This work fills the research gap in boron cluster-based CPL molecules and provides a new platform for developing novel chiral luminescent materials. Recently, Nan et al.[25]developed a highly stereoselective and facile method for the asymmetric synthesis of chiral carborane molecules (Figure 4). Using readily available starting materials, this method precisely constructs optically pure chiral continuously bridged carborane derivatives via a two-step one-pot procedure, achieving scalable synthesis at the 10 g scale without the need for chiral HPLC separation. By introducing chiral carborane substituents into organic conjugated chromophores, a series of CPL-active molecules with excellent performance were obtained; their luminescence asymmetry factors (glum) reached up to the order of 10⁻3, with quantum yields (Φpl) reaching up to 0.70. Furthermore, the study found that incorporating carboranes into chiral bridged structures can significantly enhance both theΦplandglumvalues of luminescent compounds, a phenomenon of simultaneous dual-parameter enhancement that has rarely been reported.
Designing aggregation-induced emission (AIE) type carborane materials to utilize the restriction of intramolecular motion in the aggregated state, thereby suppressing non-radiative transitions and solving the aggregation-caused quenching (ACQ) problem of traditional organic materials, has been a research hotspot in recent years[26-28]. In the aggregates of such molecules, the introduction of o-carborane can effectively suppress molecular vibration and rotation in the aggregated state through its large steric hindrance and rich intermolecular interactions, reducing non-radiative energy loss, thereby significantly enhancing the solid-state luminescence efficiency and performance of the compounds. In 2020, Wu et al.[29]successfully prepared four aggregation-induced emission (AIE) luminogens with tunable fluorescence colors by introducing different types of electron-rich or electron-deficient heteroatoms into the carbon atom sites of o-carborane via an improved nickel-catalyzed cross-coupling reaction method (Figure 5). The research results indicate that the strength of the electronic effects of electron-rich or electron-deficient heteroatoms, the distance effect between the heteroatoms and the carborane core, as well as intermolecular interactions, are key factors regulating the excited-state properties of such luminogens. These factors work together not only to make the products exhibit different emission colors but also to endow them with excellent emission quantum efficiency in the solid state (ΦF>70%). This research result not only confirms an efficient strategy for constructing o-carborane functionalized AIE luminogens containing heterocyclic moieties but also indicates that such luminogens have good application prospects as advanced optoelectronic materials. In 2021, Wang et al.[6]designed and synthesized three Janus carboranes Cb-1/2/3 with different carbazole molecules (Figure 6). All Janus carboranes exhibited quenched emission in solution, with ΦPL (photoluminescence quantum yield) below 0.01. In aggregates formed in a tetrahydrofuran/water system, the PL performance was improved (ΦPL was 0.17~0.35). In crystals or solids, the PL performance was further enhanced; the ΦPL of Cb1 reached as high as 0.99, while that of Cb-2 was 0.85 and Cb-3 was 0.61, which is consistent with the characteristics of AIE enhancement. Meanwhile, organic light-emitting diode (OLED) doped devices were fabricated using these luminescent molecules, achieving a maximum external quantum efficiency of 10.1% and a maximum brightness of 5920 cd/m² at a driving voltage of 8 V.
In 2022, Sinha et al.[30]reported a series of o-carborane-modified π-conjugated fluorophores and their luminescent properties in the solid state (Figure 7). Among them, the aggregation-induced emission enhancement (AIEE) effect exhibited by fluorene derivatives paves the way for the successful preparation of o-carborane-containing organic nanoparticles that are uniformly dispersed in aqueous media while maintaining their luminescent properties. Notably, nanoparticles processed into thin films also exhibit high fluorescence efficiency, indicating their potential applications in optics and optoelectronics.
In 2023, Xu et al.[31]reported two bridged carborane-anthracene dyads exhibiting dual-emission properties. The study revealed that the carborane cage modulates the optical properties of these compounds to varying degrees. Compound 1 exhibits intramolecular charge transfer characteristics in its absorption and emission spectra and demonstrates aggregation-induced emission enhancement (AIEE) in an acetonitrile-water system. In contrast, Compound 2 displays absorption and emission properties characteristic of the anthracene moiety in solution, and exhibits excimer emission in a tetrahydrofuran-water system as well as in the crystalline state. This work provides a simple strategy for the design and synthesis of novel dual-emissive carborane-containing compounds and for regulating solid-state luminescence for potential applications. Li et al.[32]synthesized four carborane-tetraphenylethylene (TPE) dyads via a facile method and investigated their luminescent properties under different conditions (Figure 8). The carborane unit and fluorine/hydroxyl atoms exert distinct influences on the electronic structures and solid-state packing of the compounds. All compounds exhibit typical AIE properties; their solid-state emission wavelengths are basically consistent with those of the parent TPE, but their luminescence quantum yields are improved compared to the parent TPE. Therefore, by introducing three-dimensional carborane molecular structures, suitable linking groups, and substituents such as fluorine or hydroxyl groups that affect molecular packing in the aggregated state, it is expected to precisely regulate the emission properties of organic luminogens.
Wang et al.[2]designed and synthesized four o-carborane-based aggregation-induced emission molecules with ultra-large Stokes shifts. The ultra-large Stokes shift results from the synergistic effect between the diphenylphosphino group and the o-carborane group, where the o-carborane group plays a key role. This work provides a reference for the design and synthesis of o-carborane-based fluorescent molecules with ultra-large Stokes shifts. In the same year, Wang et al.[33]also designed and synthesized four o-carborane-based aggregation-induced emission molecules (Figure 9). Upon mechanical stimulation, the recrystallized solid of CB-Ph-P exhibited a red shift in its fluorescence emission wavelength from 423 nm to 608 nm. Similarly, the fluorescence emission wavelength of CB-Ph-PAn in the solid state also shifted from 393 nm to 623 nm. This polymorphism may be related to different weak interactions, packing modes, and torsion angles.
Recently, Xu et al.[34]challenged the traditional understanding that o-carborane luminescence is centered on carbon vertices. They activated charge transfer (CT) activity at boron vertices for the first time, constructing dual-emission carborane-based luminescent materials with ultra-large Stokes shifts. By introducing electron-donor carbazole groups at boron vertices, they synthesized three 9,12-boron-vertex-substituted o-carborane derivatives (1, 2a, 2b) (Figure 10). The boron-vertex-substituted compound 1 exhibits localized excitation (LE) emission, whereas 2a and 2b, which also incorporate phenyl groups at carbon vertices, display dual-emission characteristics with an emission band separation of up to 505 nm (2.1 eV). The synergistic design of "boron-vertex donor + carbon-vertex phenyl" enables a unidirectional conversion from the LE state to the charge transfer (CT) state upon photoexcitation, providing the first direct evidence of CT occurring from a functionalized boron-vertex donor to the carborane cage. This CT emission is highly sensitive to the environment, exhibiting aggregation-induced emission enhancement (AIE) characteristics, with a quantum yield reaching up to 80%. This study expands the design framework for carborane luminescent materials and offers new insights for the rational design of multi-site functionalized carboranes. It holds significant guiding value for the development of dual-band emission and near-infrared (NIR) emission materials, thereby advancing the field of carborane optics.
Regarding research on mechanisms, in 2022, Ji et al.[35]Using 1-(2-pyrenyl)-o-carborane as a model, they systematically investigated the photoinduced reversible charge transfer (CT) process and the thermodynamic equilibrium between the locally excited (LE) state and the CT state. It was found that the energy gaps and barriers among the LE state, CT state, and non-radiative "mixed" state of this compound are extremely small, allowing all three excited states to be accessible at room temperature; the rates of internal conversion and reverse internal conversion between the LE and CT states are significantly faster than the radiative decay rate, and both share the same excited-state lifetime, confirming the thermodynamic equilibrium mechanism between the LE and CT states.
In 2024, Zhao et al.[36]designed 1-(2-pyrenyl)-9,12-di(p-tolyl)-o-carborane (compound 2) (Figure 11), whose population ratio between the locally excited state (LE state) and the intramolecular charge transfer state (ICT state) is largely regulated by the dihedral angle between the C-C bond of the carborane and the plane of the pyrene ring. Temperature-dependent fluorescence tests indicated that the two emission bands in hexane solution correspond to two different excited states: at high temperatures, the ratio of the emission bands is thermodynamically controlled, while at low temperatures, it is kinetically controlled. Based on this, the researchers successfully estimated the energy barrier and energy gap between the two excited states of compound 2 through experiments.
Due to their unique three-dimensional cage-like molecular structure, high boron atom density, excellent chemical stability, and abundant functionalization sites, ortho-carborane derivatives demonstrate significant application potential in various fields, including biomedical sensing, environmental sensing, photodynamic therapy, and boron neutron capture therapy (BNCT). In 2022, Zaitsev et al.[37]developed a convenient and efficient synthetic route to efficiently synthesize a series of neutral and water-soluble fluorescent carborane-BODIPY conjugates. These conjugates can form stable complexes with serum albumin and generate singlet oxygen. Cellular experiments indicated that these compounds exhibit negligible cytotoxicity in the dark, but upon photoactivation, they potently induce lipid peroxidation and rapid cell death in HCT116 colon adenocarcinoma cells. The carborane-based BODIPY conjugates developed in this work hold potential value for photodynamic therapy. In 2024, Ma et al.[38]designed and synthesized carborane-based luminophores containing 98%10B and 95%11B, observing distinct photophysical behaviors. Compared with the 10B-enriched luminophores, the 11B-enriched analogs significantly improved luminescence efficiency, extended emission lifetimes, and reduced the full width at half maximum. On the other hand, biological evaluation of a 10B-enriched luminophore revealed low cytotoxicity, high boron uptake, and excellent fluorescence imaging capabilities, indicating its potential application value in boron neutron capture therapy (BNCT). In 2024, Shi et al.[39]developed a non-aggregation-induced emission (AIE) method that significantly enhances the fluorescence intensity of ortho-carborane derivatives. This approach enriches the photophysical modulation strategies for ortho-carborane derivatives, achieves efficient fluorescence response of ortho-carborane crystals in acidic gas environments, and provides a new solution for solid-state acidic environment sensing materials.

2.2 o-Carboraneo-C2B10H12Metal complex luminescent materials

In addition to being used for the functional modification of organic fluorophores, o-carborane has also been extensively studied in the field of metal complex chemistry[40-48]. When embedded into the ligand framework, carboranes can effectively regulate the electronic structure of the metal center by utilizing their unique electronic effects and steric hindrance, thereby significantly altering the photophysical properties of the complexes.
In 2021, Yao et al.[43]first synthesized a series of cationic cyclometalated iridium(III) complexes featuring ortho-carborane substituents on the 2-phenylbenzothiazole main ligand (Figure 12). The emission spectra of the prepared iridium(III) complexes exhibited red shifts due to the introduction of ortho-carborane at different positions on the 2-phenylbenzothiazole. Furthermore, the fluorescence intensity of the prepared complexes was significantly affected by the position of the ortho-carborane substituent. Conway-Kenny et al.[44]prepared four novel transition metal carborane photosensitizers via Sonogashira cross-coupling reactions between 1-(4-ethynylbenzyl)-2-methyl-ortho-carborane and halogenated ruthenium(II) or iridium(III) phenanthroline complexes. Among them, RuCB demonstrated the most excellent photophysical properties in solution, and the mono-carborane-phenanthroline complex achieved the highest singlet oxygen quantum yield. After incubation under photodynamic therapy conditions, none of these complexes exhibited dark toxicity in SKBR-3 cells. Notably, the mono-carborane complexes RuCB and IrCB were most readily internalized by SKBR-3 cells; upon irradiation, both killed approximately 50% of SKBR-3 cells at a concentration of 10 μM. Therefore, the high boron content and photoactivity of these photosensitizers make them promising dual anticancer agents for PDT and boron neutron capture therapy (BNCT).
Soldevila-Sanmartin et al.[40]Through lithiation and nucleophilic addition reactions, one or two o-carborane fragments were introduced onto the pyrazole backbone. Notably, pyrazolylphenyl ligands without o-carborane form 1D coordination polymers, whereas those containing o-carborane yield 0D coordination compounds; bis-substituted o-carboranylpyrazole ligands further produce 3D coordination polymers. Studies on the photoluminescence behavior of these compounds in the solid state confirm that replacing phenyl rings with carborane clusters can modulate both the molecular structure and luminescent properties of such copper complexes. This study reports, for the first time, new insights into the significant luminescent characteristics of a rare type of copper complex and further investigates the structure-property relationships between their structural features and luminescent behavior.
In 2023, Powley et al.[47]demonstrated a molecular design strategy and, through this strategy, constructed novel phosphorescent "carbene-metal-carboranyl" (CMC) Cu(I) and Au(I) complexes based on diaminocarbene ligands (Figure 13). Among them, the gold complex exhibited a radiative rate as high as 3.4×105 s-1, with thermal stability significantly enhanced to above 300 °C, making it a promising alternative comparable to platinum(II) and iridium(III) phosphorescent materials. This work points the way for future CMC material design: by using ligands that simultaneously possess stronger σ-donating ability, π-accepting ability, and more rigid structures, the radiative rate can be further increased to 107 s-1, thereby improving the performance and operational stability of future CMC-based OLED devices.
Recently, Alpatova et al.[45]designed a new class of heteroleptic carbonyl complexes containing porphyrin and carborane isocyanide ligands. In cell-free systems, these new compounds retained the photochemical characteristics of the parent porphyrin derivatives upon photoinduced activation. In cell cultures, the carborane-containing derivatives exhibited significantly greater intracellular accumulation than their non-boron-substituted analogs. As expected, irradiation at 405 nm of cells loaded with the new complexes caused photodynamic cellular damage; conversely, irradiation at 530 nm triggered carbon monoxide release, followed by cell death unrelated to photodynamic effects. These novel metal carbonyl complexes hold promise as dual-function antitumor agents combining photodynamic therapy and photoinduced CO release capabilities.

3 Carborane anion-based luminescent materials

3.1 Nido-carborane anion [nido-C2B9H12] luminescent material

Currently, most luminescent materials reported in the literature employ nido (nido) C2B9 o-carboranes, all prepared from closo-o-carboranes via deboronation reactions[49-53]. Similar to monocarborane anions, 7,8-nido-carborane anions also exhibit unique three-dimensional aromaticity, electron-rich characteristics, and significant steric hindrance effects. These properties, especially the electron-rich nature conferred by their anionic state, enable them to serve as excellent electron donors, thus they are often used to construct intramolecular charge transfer (ICT) systems. In 2021, Kim et al.[54]reported the synthesis and characterization of closo (CB1 and CB2) and nido-carboranyl (nido-CB1 and nido-CB2) compounds (Figure 14). The study showed that in tetrahydrofuran (THF) at 298 K, the closo compounds exhibited intense emission based on π-π* local excitation (LE) in the high-energy region around 380 nm, whereas the nido compounds displayed strong emission centered at approximately 450 nm, attributable to intramolecular charge transfer (ICT) transitions between the nido-o-carborane unit and the connected triazole aromatic moiety. Notably, calculations based on measured quantum efficiencies and decay lifetimes indicated that the radiative decay mechanism based on ICT transitions in nido compounds is significantly more efficient than the LE-based emission decay mechanism in closo compounds. In some novel nido-carboranyl donor-acceptor (D-A) systems, nido-carboranes facilitate thermally activated delayed fluorescence (TADF) due to their unique steric hindrance and strong electron-donating properties. In 2020, Sujith et al.[52]investigated a nido-carborane-triarylborane binary system, which exhibited strong TADF and long emission lifetimes in both tetrahydrofuran (THF) and poly(methyl methacrylate) (PMMA) films (in THF, τd = 4.8~9.7 μs), benefiting from a very small singlet-triplet energy splitting (ΔEST < 0.03 eV); furthermore, reconnecting a methyl group at the ortho position of the BMes2 group can further enhance the delayed fluorescence intensity.
In 2021, they[49]prepared and characterized a series of nido-carborane-triarylboron conjugates, in which a methyl group was introduced at the ortho position of the carborane cage within the phenylene linker (Figure 15). All nido compounds exhibited broad green emission in tetrahydrofuran with good photoluminescence quantum yields. Notably, unlike the unsubstituted para-conjugates which showed almost no TADF properties, the transient PL decay curves of the nido-carborane-triarylboron conjugates indicated the presence of weak TADF (in THF, τd = 0.9~1.4 μs). The result that the singlet-triplet energy splitting is less than 0.15 eV further supports the TADF properties.
In addition to functional modification of organic fluorophores, nido-carboranes (nido-carborane) also play an important role in metal complex chemistry. When introduced into the ligand structure of metal complexes, they can modulate the photophysical properties of the complexes from both electronic and steric aspects. In 2021, Alconchel et al.[48]utilized closo- or nido-o-carborane diphosphines to prepare cationic complexes [Cu(N^N){(PPh2)2C2B10H10}]PF6and neutral complexes [Cu(N^N){(PPh2)2C2B9H10}]. These two complexes exhibit distinctly different luminescent behaviors: the neutral complex possesses thermally activated delayed fluorescence (TADF); studies on its powder and PMMA films show that the singlet-triplet energy gap, triplet lifetime, and singlet lifetime are similar in both phases; the quantum yield of the cationic complex is far lower than that of the neutral complex and does not exhibit TADF, but it displays significant thermochromic luminescence characteristics, with its emission being highly sensitive to the medium. In the same year, Li et al.[55]prepared a series of novel neutral phosphorescent iridium(III) complexes (Ir1-Ir4) containing nido-carboranes based on three charged ligands. Notably, Ir2, Ir3, and Ir4 all exhibit obvious yellow emission, whereas Ir1 is non-emissive in both solution and solid states. Density functional theory calculations indicate that due to the strong electron-donating nature of the nido-carborane, complexes Ir2, Ir3, and Ir4 display unusual ligand-to-metal charge transfer (LMCT) excited state characteristics. Owing to its better solubility and luminescent performance, the neutral phosphorescent iridium(III) complex Ir3 was successfully applied in solution-processed organic light-emitting diodes, achieving efficient yellow emission and providing new material options for the development of organic light-emitting devices.
Compared to closo-carboranes, nido-carborane derivative anions typically possess superior water solubility and biocompatibility due to their open nido structure, thereby facilitating the development of more efficient targeted drug delivery systems, BNCT agents, and bioimaging probes. In 2024, Cao et al.[56]synthesized the fluorescent complex A-PInd-C encapsulated by the pH-sensitive polymer A. In vitro release experiments under acidic conditions demonstrated that it was released primarily in a linear manner, with a maximum release rate of 80% within 48 hours. Cell imaging experiments indicated that it could enter HeLa and HCT-116 cells, distributing mainly around the nucleus, especially in acidic environments. Furthermore, it exhibited inhibitory effects on HeLa, PC-3, and L02 cells, with an inhibition rate against PC-3 cells reaching up to 70%. This study addressed the low bioavailability of carboranes by improving boron-containing compounds, while also endowing the system with potential for boron neutron capture therapy.
Wang et al.[57]The two acrylic resin-coated nido-carborane indole fluorescent polymers, L-P-INDOLCAB and S-P-INDOLCAB, prepared by them, exhibited enhanced biocompatibility with HELA cells and could tightly envelop target cells. At a concentration of 20 μg/mL, they showed strong inhibitory effects on PC-3 and HeLa cells, providing new candidate materials for tumor therapy. Zhou et al.[58]A series of fluorescent anti-tumor drug-rhodamine pyrimidine derivatives (RP1, RP2, RP3, and RP4) were synthesized by connecting Rhodamine B with pyrimidine derivatives via an ethylenediamine bridge. Subsequently, these were introduced into nido-carboranes to form fluorescent salt derivatives. CCK8 assays revealed that RP4 and RPB4 at a concentration of 10 μg/mL exhibited strong inhibitory effects on cancer cells. The logPvalues of compounds RP4 and RPB4 range between 0 and 3, facilitating absorption and transport across cell membranes. Further cell imaging experiments indicated that compound RP4 possesses excellent imaging capabilities and tumor targeting properties, making it a potential fluorescent anticancer agent.

3.2 Monocarborane anion [CB11H12]⁻ luminescent material

o-Carborane is a neutral cage-like molecule that often serves as a strong electron-withdrawing group in the design of luminescent molecules. In contrast, the monocarbaborane anion carries a negative charge, and its high electron cloud density endows it with electron-donating characteristics.[59-63]Therefore, in the design of luminescent materials, it typically serves as an electron-donating group for constructing donor-acceptor (D-A) type luminescent molecules. Furthermore, similar to ortho-carborane, monocarborane anions effectively suppress π-π stacking of luminophores in the solid state and mitigate concentration quenching effects due to their substantial steric bulk and excellent chemical stability, thereby enhancing the solid-state emission efficiency of these materials.
Peng et al.[64]synthesized a fluorescent D-A-D′ ternary system composed of monocarborane, benzothiadiazole, and triphenylamine. The synthesized ternary system readily forms nanoparticles in tetrahydrofuran-water solutions. These in situ generated nanostructures enable highly sensitive and selective detection of silver ions, with a detection limit of 10-9 mol/L, comparable to existing literature reports, providing an efficient material choice for silver ion detection. Zhang et al.[65]utilized weakly coordinating CB11 cages as versatile building blocks for preparing luminescent monovalent copper complexes, designing and synthesizing a series of copper acetylide metal complexes (Figure 16). All products were characterized by X-ray single-crystal diffraction, revealing surprisingly complex structural diversity. In the solid state, these products exhibit room-temperature phosphorescence, with emission colors covering the entire visible spectrum, and quantum yields as high asΦ=0.99 were observed.
In 2022, our research group[62]first introduced borane anions into the system of metal platinum phosphorescent complexes. Specifically, we first grafted the closo-monocarborane anion cluster [CB11H12]⁻ onto phenylpyridine-type cyclometalated ligands via covalent C—C bond coupling; subsequently, using these functionalized ligands, we synthesized four Pt(bis-NHC)ppy complexes (Figure 17). These complexes all contain a CB11-cage suspended on the periphery of the structure, achieving overall electrical neutrality of the complexes through intramolecular charge neutralization. Notably, complex 1a exhibited a phosphorescence quantum efficiency as high as 97% in PMMA films, a significant enhancement attributed to the introduction of the CB11-cage with significant steric hindrance. Building on this, we further fabricated OLED devices based on these complexes using a solution process; when the doping concentration of the complexes reached a relatively high 20 wt%, the maximum external quantum efficiency (EQEₘₐₓ) of the devices reached 18.51%, with corresponding CIE color coordinates of (0.28, 0.55). To the best of our knowledge, thisEQEₘₐₓ value represents the highest level currently reported for solution-processed OLED devices fabricated from platinum complexes employing simple cyclometalated phenylpyridine ligands. Meanwhile, we also synthesized a series of novel platinum(II) triplet emitters containing N-pyridyl carbazole ligands; these triplet emitters exhibited deep-blue phosphorescence in both solid state and solution[61]. The study revealed that introducing monocarborane anions into bidentate ligands could significantly enhance the steric hindrance of these four platinum(II) complexes, thereby markedly weakening intermolecular interactions that typically lead to luminescence self-quenching. Therefore, carborane anions with unique steric and electronic properties can serve as multifunctional building blocks for developing new high-performance phosphorescent materials. Simultaneously, using such complexes as triplet emitting dopants, we successfully fabricated solution-processable deep-blue phosphorescent organic light-emitting diodes (PhOLEDs), achieving a maximum external quantum efficiency (EQEₘₐₓ) of 6.2% and CIE color coordinates of (0.17, 0.15). Furthermore, by varying the auxiliary ligands, we constructed a series of zwitterionic platinum complexes[59]; these complexes exhibited strong luminescence in both pure solid state and polymethyl methacrylate (PMMA) films, with lifetimes ranging from a few microseconds to tens of microseconds. Research indicates that the CB11-cage not only exerts a steric effect on the complexes (via its cage structure) but also regulates the electronic states of the complexes through its intrinsic properties. These zwitterionic complexes are electrically neutral, stable, and easy to prepare, making them promising candidates for applications in phosphorescent organic light-emitting diodes (PhOLEDs).

4 Conclusion and Outlook

The core of research on carborane-based luminescent materials lies in leveraging their unique cage structure and electronic effects to achieve efficient regulation of luminescent properties. In recent years, researchers have adopted site-selective functionalization strategies to precisely attach luminescent groups to the C—H or B—H sites of the carborane cage. By adjusting the energy level difference of the molecular frontier orbitals, they have effectively regulated the luminescent characteristics of the materials. More importantly, the three-dimensional aromaticity of carboranes endows the materials with excellent stability, while their rigid cage structure effectively prevents interactions between luminescent groups in the aggregated state, thereby significantly suppressing concentration quenching effects and enabling the materials to maintain high-efficiency luminescence even in the solid state. Currently, research has expanded from fundamental structures to the fine-tuning of aggregation behavior, laying a solid foundation for the development of high-efficiency solid-state luminescent devices and bioimaging probes.
Despite broad prospects, the development of carborane-based luminescent materials faces two major challenges: first, difficulties in synthesis and modification, where efficient synthesis of complex carborane cage structures remains a bottleneck, especially for derivatives requiring specific functional group modifications, which involve tedious synthesis and purification steps with high costs; second, the complex electronic interaction mechanisms between carborane cage structures and luminescent groups, which may involve processes such as energy transfer and charge transfer. Since these interaction rules are not yet fully understood, strategies and mechanistic explanations for regulating excited-state dynamics remain insufficient, ultimately limiting the design and development of high-performance materials.
In the face of the aforementioned challenges, the development prospects for this material remain very broad: Firstly, regarding structural innovation, regulating electronic structures through rational molecular design or constructing supramolecular assemblies to optimize aggregation-induced emission properties is expected to break through existing performance limitations, driving the material towards higher performance and broader applications; Secondly, concerning functional expansion, it is necessary to go beyond single luminescent functions and develop multifunctional smart material systems with stimulus-responsive properties (such as piezochromism and thermochromism); Finally, future research must rely on interdisciplinary collaboration to further explore applications of such materials in fields like optoelectronics and biomedicine, based on a deep understanding of photophysical processes.

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Funding

Guangxi Natural Science Foundation(2023GXNSFAA026469)

National Natural Science Foundation of China(22165002)

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