Photoinduced Selective B-H Activation of nido-Carboranes.

The development of new synthetic methods for B-H bond activation has been an important research area in boron cluster chemistry, which may provide opportunities to broaden the application scope of boron clusters. Herein, we present a new reaction strategy for the direct site-selective B-H functionalization of nido-carboranes initiated by photoinduced cage activation via a noncovalent cage···π interaction. As a result, the nido-carborane cage radical is generated through a single electron transfer from the 3D nido-carborane cage to a 2D photocatalyst upon irradiation with green light. The resulting transient nido-carborane cage radical could be directly probed by an advanced time-resolved EPR technique. In air, the subsequent transformations of the active nido-carborane cage radical have led to efficient and selective B-N, B-S, and B-Se couplings in the presence of N-heterocycles, imines, thioethers, thioamides, and selenium ethers. This protocol also facilitates both the late-stage modification of drugs and the synthesis of nido-carborane-based drug candidates for boron neutron capture therapy (BNCT).

Molecular engineering of AIE-active boron clustoluminogens for enhanced boron neutron capture therapy.

The development of boron delivery agents bearing an imaging capability is crucial for boron neutron capture therapy (BNCT), yet it has been rarely explored. Here we present a new type of boron delivery agent that integrates aggregation-induced emission (AIE)-active imaging and a carborane cluster for the first time. In doing so, the new boron delivery agents have been rationally designed by incorporating a high boron content unit of a carborane cluster, an erlotinib targeting unit towards lung cancer cells, and a donor-acceptor type AIE unit bearing naphthalimide. The new boron delivery agents demonstrate both excellent AIE properties for imaging purposes and highly selective accumulation in tumors. For example, at a boron delivery agent dose of 15 mg kg-1, the boron amount reaches over 20 µg g-1, and both tumor/blood (T/B) and tumor/normal cell (T/N) ratios reach 20-30 times higher than those required by BNCT. The neutron irradiation experiments demonstrate highly efficient tumor growth suppression without any observable physical tissue damage and abnormal behavior in vivo. This study not only expands the application scopes of both AIE-active molecules and boron clusters, but also provides a new molecular engineering strategy for a deep-penetrating cancer therapeutic protocol based on BNCT.

Direct B-H Functionalization of Icosahedral Carboranes via Hydrogen Atom Transfer.

The efficient and selective functionalization of icosahedral carboranes (C2B10H12) at the boron vertexes is a long-standing challenge owing to the presence of 10 inert B-H bonds in a similar chemical environment. Herein, we report a new reaction paradigm for direct B-H functionalization of icosahedral carboranes via B-H homolysis enabled by a nitrogen-centered radical-mediated hydrogen atom transfer (HAT) strategy. Both the HAT process of the carborane B-H bond and the resulting boron-centered carboranyl radical intermediate have been confirmed experimentally. The reaction occurs at the most electron-rich boron vertex with the lowest B-H bond dissociation energy (BDE). Using this strategy, diverse carborane derivatization, including thiolation, selenation, alkynylation, alkenylation, cyanation, and halogenation, have been achieved in satisfactory yields under a photoinitiated condition in a metal-free and redox-neutral fashion. Moreover, the synthetic utility of the current protocol was also demonstrated by both the scale-up reaction and the construction of carborane-based functional molecules. Therefore, this methodology opens a radical pathway to carborane functionalization, which is distinct from the B-H heterolytic mechanism in the traditional strategies.

Facile Construction of New Hybrid Conjugation via Boron Cage Extension.

Aromatic polycyclic systems have been extensively utilized as structural subunits for the preparation of various functional molecules. Currently, aromatics-based polycyclic systems are predominantly generated from the extension of two-dimensional (2D) aromatic rings. In contrast, polycyclic compounds based on the extension of three-dimensional (3D) aromatics such as boron clusters are less studied. Here, we report three types of boron cluster-cored tricyclic molecular systems, which are constructed from a 2D aromatic ring, a 3D aromatic nido-carborane, and an alkyne. These new tricyclic compounds can be facilely accessed by Pd-catalyzed B-H activation and the subsequent cascade heteroannulation of carborane and pyridine with an alkyne in an isolated yield of up to 85% under mild conditions without any additives. Computational results indicate that the newly generated ring from the fusion of the 3D carborane, the 2D pyridyl ring, and an alkyne is non-aromatic. However, such fusion not only leads to a 1H chemical shift considerably downfield shifted owing to the strong diatropic ring current of the embedded carborane but also devotes to new/improved physicochemical properties including increased thermal stability, the emergence of a new absorption band, and a largely red-shifted emission band and enhanced emission efficiency. Besides, a number of bright, color-tunable solid emitters spanning over all visible light are obtained with absolute luminescence efficiency of up to 61%, in contrast to aggregation-caused emission quenching of, e.g., Rhodamine B containing a 2D-aromatics-fused structure. This work demonstrates that the new hybrid conjugated tricyclic systems might be promising structural scaffolds for the construction of functional molecules.

Palladium-Catalyzed Hydroboration of Alkynes with Carboranes: Facile Construction of a Library of Boron Cluster-Based AIE-Active Luminogens.

The classical aggregation-induced emission (AIE)-active luminogens (AIEgens) usually include two-dimensional aromatic systems such as tetraphenylethenes, which are synthesized in several steps by using toxic additives. Here, we proposed a new molecular design strategy for the realization of AIE properties by combining three-dimensional aromatic boron clusters of carboranes with vinyl group(s). To obtain a library of the boron cluster-based AIEgens, a Pd-catalyzed hydroboration of alkynes with carboranes is reported. This reaction protocol proceeds in one step under mild conditions with rapid reaction rate, excellent yields and regioselectivity. Photophysical property studies demonstrate that the facile molecular motions in solution can be inhibited in the solid state for these molecules, which leads to interesting AIE properties. This work provides not only a general design principle for AIEgens but also an efficient methodology to synthesize boron cluster-based photo-functional molecules.

Site-Selective Functionalization of Carboranes at the Electron-Rich Boron Vertex: Photocatalytic B-C Coupling via a Carboranyl Cage Radical.

Functionalization of carboranes in a vertex-specific manner is a perennial challenge. Here, we report a photocatalytic B-C coupling for the selective functionalization of carboranes at the boron site which is most distal to carbon. This reaction was achieved by the photo-induced decarboxylation of carborane carboxylic acids to generate boron vertex-centered carboranyl radicals. Theoretical calculations also demonstrate that the reaction more easily occurs at the boron site bearing higher electron density owing to the lower energy barrier for a single-electron transfer to generate a carboranyl radical. By using this strategy, a number of functionalized carboranes could be accessed through alkylation, alkenylation, and heteroarylation under mild conditions. Moreover, both a highly efficient blue emitter with a solid-state luminous efficiency of 42 % and a drug candidate for boron neutron capture therapy (BNCT) containing targeting and fluorine units were obtained.

Remote Effect from Boron Cluster: Tunable Photophysical Properties of o-Carborane-Based Luminogens.

We proposed a new molecular design strategy that the o-carboranyl group is attached as "an innocent unit" to the remote side of luminogens to tune photophysical properties. To verify this strategy, two o-carborane-based compounds with asymmetric molecular geometry were designed and synthesized. Photophysical properties of o-carborane-based luminogens were investigated on the basis of UV-Vis spectra, photoluminescence spectra, crystal structure analysis and theoretical calculations. The results indicate that the o-carboranyl group has a slight effect on the energy gap between the ground state (S0 ) and the first excited state (S1 ) in the solution state but a significant effect on the energy gap between S0 and S1 in the solid state. Besides, the radiative and non-radiative transition processes are modulated by the o-carboranyl unit. This leads to emission quenching in the solution state but an enhanced luminous efficiency in the aggregate state with a typical aggregation-induced emission (AIE) property.

Cage-···Cage- Interaction: Boron Cluster-Based Noncovalent Bond and Its Applications in Solid-State Materials.

Carboranes are boron-carbon clusters with important applications in the fields of materials, catalysis, pharmaceuticals, etc. However, the noncovalent interactions that could determine the solid-state structures and properties of such boron clusters have rarely been investigated. Herein, inspired by the coordinate bond in metallacarborane or ferrocene, the boron cluster-based noncovalent interaction (denoted as cage-···cage- interaction) between two nido-carborane clusters was successfully realized by using a pyridinium-based molecular barrier. The X-ray diffraction studies uncover that the cage-···cage- interaction has a contacting distance of 5.4-7.0 Å from centroid to centroid in the systems reported here. Theoretical calculations validate the formation of the noncovalent interaction and disclose its repulsive bonding nature that is overcome thanks to the positively charged pyridinium-based framework. Interestingly, such bulk crystalline materials containing the cage-···cage- interaction show relevant properties such as full-color absorption in the visible light range and important photothermal effect, which are absent for the control compound without carboranes. This study may offer fundamental insights into the boron cluster-based noncovalent interactions and open a new research avenue to rationally design boron cluster-based materials.

How Do Molecular Motions Affect Structures and Properties at Molecule and Aggregate Levels?

Molecular motions are essential natures of matter and play important roles in their structures and properties. However, owing to the diversity and complexity of structures and behaviors, the study of motion-structure-property relationships remains a challenge, especially at all levels of structural hierarchy from molecules to macro-objects. Herein, luminogens showing aggregation-induced emission (AIE), namely, 9-(pyrimidin-2-yl)-carbazole (PyCz) and 9-(5-R-pyrimidin-2-yl)-carbazole [R = Cl (ClPyCz), Br (BrPyCz), and CN (CyPyCz)], were designed and synthesized, to decipher the dependence of materials' structures and properties on molecular motions at the molecule and aggregate levels. Experimental and theoretical analysis demonstrated that the active intramolecular motions in the excited state of all molecules at the single-molecule level endowed them with more twisted structural conformations and weak emission. However, owing to the restriction of intramolecular motions in the nano/macroaggregate state, all the molecules assumed less twisted conformations with bright emission. Unexpectedly, intermolecular motions could be activated in the macrocrystals of ClPyCz, BrPyCz, and CyPyCz through the introduction of external perturbations, and synergic strong and weak intermolecular interactions allowed their crystals to undergo reversible deformation, which effectively solved the problem of the brittleness of organic crystals, while endowing them with excellent elastic performance. Thus, the present study provided insights on the motion-structure-property relationship at each level of structural hierarchy and offered a paradigm to rationally design multifunctional AIE-based materials.

The nido-Cage⋠⋠⋠π Bond: A Non-covalent Interaction between Boron Clusters and Aromatic Rings and Its Applications.

Non-covalent interactions involving multicenter multielectron skeletons such as boron clusters are rare. Now, a non-covalent interaction, the nido-cage⋅⋅⋅π bond, is discovered based on the boron cluster C2 B9 H12 - and an aromatic π system. The X-ray diffraction studies indicate that the nido-cage⋅⋅⋅π bonding presents parallel-displaced or T-shaped geometries. The contacting distance between cage and π ring varies with the type and the substituent of the aromatic ring. Theoretical calculations reveal that this nido-cage⋅⋅⋅π bond shares a similar nature to the conventional anion⋅⋅⋅π or π⋅⋅⋅π bonds found in classical aromatic ring systems. This nido-cage⋅⋅⋅π interaction induces variable photophysical properties such as aggregation-induced emission and aggregation-caused quenching in one molecule. This work offers an overall understanding towards the boron cluster-based non-covalent bond and opens a door to investigate its properties.

Boron-Cluster-Enhanced Ultralong Organic Phosphorescence.

Although carborane-based luminescent materials have been studied for years, no persistent phosphor has been reported so far. Herein, we describe boron-cluster-based persistent phosphors obtained by linking a σ-aromatic carboranyl cage to the π system of a carbazolyl group. The carboranes were found to promote intersystem crossing from a singlet to a triplet state. The rigid boron cluster was able to stabilize the ultralong triplet excitons through multiple nonclassical hydrogen bonds, such as B-H⋅⋅⋅π interactions, thus leading to a long lifetime of up to 0.666 s and an absolute phosphorescence quantum yield of 7.1 %, which is outstanding for an organic phosphor without heavy atoms. These phosphors can be excited by visible light and show dynamic emission behavior, including thermochromism and mechanochromism. This study demonstrates that non-metal/heavy-atom boron clusters can be used to develop multifunctional high-performance phosphors for potential applications.

Luminescent Mechanochromic Dinuclear Cu(I) Complexes with Macrocyclic Diamine-Tetracarbene Ligands.

Dinuclear Cu(I) complexes bearing hexadentate, macrocyclic N-heterocyclic carbene (NHC) ligands, [Cu2(L1)(CH3CN)][PF6]2 (1) and [Cu2(L2)(CH3CN)]2[Cu2(L2)(CH3CN)2][PF6]6 (2), have been synthesized by the reactions of [H4L][PF6]4 (L = L1, L2) with excess Cu2O in acetonitrile. Crystallizations of the heat-treated samples of 1 and 2 from acetone/methanol/ether or CH3NO2/ether result in [Cu2(L1)][PF6]2 (3) and [Cu2(L2)][PF6]2 (4). Complexes 1-4 are emissive with luminescent maxima at 464, 472, 540, and 488 nm in the solid state, respectively. The origin of the red shift of the emission maximum of 3 relative to the other three complexes has been studied by theoretical calculations, showing the cuprophilic interactions in the excited state of 3. The mechanochromic luminescent properties of 1-4 have been studied. After grinding in a mortar, a significant emission color change is found with a red shift of 98 nm for 1, 82 nm for 2, 20 nm for 3, and 64 nm for 4, respectively. These mechanochromic transformations are found to be a crystalline-to-amorphous conversion, which can be reverted by adding drops of the organic solvent or recrystallization. The possible correlations between the luminescent properties and structural modifications such as Cu···Cu distances are discussed.

Highly Emissive Organic Single-Molecule White Emitters by Engineering o-Carborane-Based Luminophores.

The development of organic single-molecule solid-state white emitters holds a great promise for advanced lighting and display applications. Highly emissive single-molecule white emitters were achieved by the design and synthesis of a series of o-carborane-based luminophores. These luminophores are able to induce multiple emissions to directly emit high-purity white light in solid state. By tuning both molecular and aggregate structures, a significantly improved white-light efficiency has been realized (absolute quantum yield 67 %), which is the highest value among the known organic single-molecule white emitters in the solid state. The fine-tuning of the packing modes from H- to J- and cross-stacking aggregates as well as intermolecular hydrogen bonds are successful in one molecular skeleton. These are crucial for highly emissive white-light emission in the solid state.

A carborane-incorporated mononuclear Co(ii) complex showing zero-field slow magnetic relaxation.

An air-stable mononuclear Co(ii) complex bearing an o-carborane dithiolate ligand has been synthesized which shows slow relaxation of magnetization in the absence of an applied field and a hysteresis loop at 1.8 K. This is the first example of a carborane-incorporated complex that displays SMM properties.

A carborane-triggered metastable charge transfer state leading to spontaneous recovery of mechanochromic luminescence.

An efficient strategy was designed to realize spontaneous recovery of mechanochromic luminescence by carborane-functionalized anthracene derivatives. A metastable charge-transfer emission from anthracene to o-carborane is responsible for this process.

Visible-Light-Induced Cascade Reaction of Isocyanides and N-Arylacrylamides with Diphenylphosphine Oxide via Radical C-P and C-C Bond Formation.

An effective photoredox-mediated tandem phosphorylation/cyclization reaction of diphenylphosphine oxide with three types of radical acceptors leads to P(O)Ph2-containing phenanthridines, isoquinolines, and indolin-2-ones by formation of both C-P and C-C bonds. [Ir(ppy)2(dtbpy)]PF6 (1 mol %) was used as the catalyst, CsF or Cs2CO3 as the base, and K2S2O8 as the oxidant. A series of functional groups can be tolerated at room temperature. Moderate to good yields were generated.

Carboranes tuning the phosphorescence of iridium tetrazolate complexes.

New iridium tetrazolate complexes containing o-, m-, or p-carboranyl substitution in different positions of a phenylpyridine ligand have been prepared. The carborane isomers and the effect of their substitution position in the tuning of optical properties have been examined. The neutral complexes with the carboranyl substituent on the phenyl ring in meta position relative to the metal exhibit redshifted emission bands in contrast to blueshifts for those with carboranyl in para position. All cationic complexes display evidently blueshifted dual-peak emission compared with the carborane-free complex (c-TZ) with a broad single-peak emission. Introduction of carborane leads to a blueshift over 70 nm relative to c-TZ. Carboranes also significantly improve phosphorescence efficiency (ΦP ) and lifetime (τ), that is, ΦP =0.64 versus 0.21 (c-TZ) and τ=880 ns versus 241 ns (c-TZ). The unique hydrophilic nido-carborane-based Ir(III) complex nido-o-1 shows the largest phosphorescence efficiency (abs ΦP =0.57) among known water-soluble iridium complexes, long emission lifetime (τ=4.38 µs), as well as varying emission efficiency and lifetime with O2 content in aqueous solution. Therefore, nido-o-1 has been used as an excellent oxygen-sensitive phosphor for intracellular O2 sensing and hypoxia imaging.