Carborane Photochemistry Triggered by Aryl Substitution: Carborane-Based Dyads with Phenyl Carbazoles

Energy Barriers and Gap Between Two Excited States in a Dual-Emissive Carborane

A thorough understanding of the internal conversion process between excited states is important for the designing of ideal multiple-emissive materials. However, it is hard to experimentally measure both the energy barriers and gaps between the excited states of a compound. For a long time, it is dubious if what was measured is the energy gap or barrier between two excited states. In this paper, we designed 1-(pyren-2'-yl)-9,12-di(p-tolyl)-o-carborane (2), which shows dual-emission in solution. Temperature-dependent fluorescence measurements show that the two emission bands in hexane are corresponding to two different excited states. The ratio of the emission bands is controlled by thermodynamics at higher temperatures and by kinetics at lower temperatures. Thus, the energy barrier and energy gaps between the two excited states of 2 can be experimentally estimated.

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.

Improvement in Radiative Efficiency Via Intramolecular Charge Transfer in ortho-Carboranyl Luminophores Modified with Functionalized Biphenyls

In this study, we found that the electronic effects of the functional groups on aromatic units attached to o-carboranyl species can enhance the efficiency of intramolecular charge transfer (ICT)-based radiative decay processes. Six o-carboranyl-based luminophores having attached functionalized biphenyl groups with CF₃, F, H, CH₃, C(CH₃)₃, and OCH₃ substituents were prepared and fully characterized by multinuclear magnetic resonance spectroscopy. In addition, their molecular structures were determined by single-crystal X-ray diffractometry, which revealed that the distortion of the biphenyl rings and the geometries around the o-carborane cages were similar. All compounds exhibited ICT-based emissions in the rigid state (solution at 77 K and film). Intriguingly, the quantum efficiencies (Φ_em) of five compounds (that of the group with CF₃ could not be measured because of its extremely weak emissions) in the film state increased gradually as the electron-donating power of the terminal functional group modifying the biphenyl moiety increased. Furthermore, the nonradiative decay constants (k_nr) for the group with OCH₃ were estimated to be one-tenth of those for the group with F, whereas the radiative decay constants (k_r) for the five compounds were similar. The dipole moments (μ) calculated for the optimized first excited state (S₁) structures gradually increased, from that of the group with CF₃ to that of the group with OCH₃, implying that the inhomogeneity of the molecular charge distribution was enhanced by electron donation. The electron-rich environment formed as a result of electron donation led to efficient charge transfer to the excited state. Both experimental and theoretical findings revealed that the electronic environment of the aromatic moiety in o-carboranyl luminophores can be controlled to accelerate or interrupt the ICT process in the radiative decay of excited states.

Electrochemical Cage Activation of Carboranes

Carboranes are boron-carbon molecular clusters that possess unique properties, such as their icosahedron geometry, high boron content, and delocalized three-dimensional aromaticity. These features render carboranes valuable building blocks for applications in supramolecular design, nanomaterials, optoelectronics, organometallic coordination chemistry, and as boron neutron capture therapy (BNCT) agents. Despite tremendous progress in this field, stoichiometric chemical redox reagents are largely required for the oxidative activation of carborane cages. In this context, electrosyntheses represent an alternative strategy for more sustainable molecular syntheses. It is only in recent few years that considerable progress has been made in electrochemical cage functionalization of carboranes, which are summarized in this Minireview. We anticipate that electrocatalysis will serve as an increasingly powerful stimulus within the current renaissance of carborane electrochemistry.

Highly Deformed o-Carborane Functionalised Non-linear Polycyclic Aromatics with Exceptionally Long C-C Bonds

The effect of substituting o-carborane into the most sterically hindered positions of phenanthrene and benzo(k)tetraphene is reported. Synthesised via a Bull-Hutchings-Quayle benzannulation, the crystal structures of these non-linear acenes exhibited the highest aromatic deformation parameters observed for any reported carborane compound to date, and among the largest carboranyl C-C bond length of all organo-substituted o-carboranes. Photoluminescence studies of these compounds demonstrated efficient intramolecular charge-transfer, leading to aggregation induced emission properties. Additionally, an unusual low-energy excimer was observed for the phenanthryl compound. These are two new members of the family of carborane-functionalised non-linear acenes, notable for their peculiar structures and multi-luminescent properties.

Spirobifluorene-Based o-Carboranyl Compounds: Insights into the Rotational Effect of Carborane Cages on Photoluminescence

9,9'-Spirobifluorene-based closo-o-carboranyl (SFC1 and SFC2) compounds and their nido-derivatives (nido-SFC1 and nido-SFC2) were prepared and characterized. The two closo-compounds displayed major absorption bands assignable to π-π* transitions involving the spirobifluorene group, as well as weak intramolecular charge-transfer (ICT) transitions between the o-carboranes and their spirobifluorene moieties. The nido-compounds exhibited slightly blueshifted absorption bands resulting from the absence of the ICT transitions corresponding to the o-carborane moieties due to the anionic character of the nido-o-carboranes. While SFC1 exhibited only high-energy emissions in THF at 298 K (only from locally excited (LE) states assignable to π-π* transitions on the spirobifluorene group), remarkable emissions in the low-energy region were observed in the rigid state such as in THF at 77 K and in the film state. SFC2 displayed intense emissions in the low-energy region in all states. The fact that neither of the nido-derivatives of SFC1 and SFC2 exhibited low-energy emissions and the TD-DFT calculation results of each closo-compound clearly verified that the low-energy emission was based on ICT-based radiative decay. The conformational barriers from each relative energy calculation upon changing the dihedral angles around the o-carborane cages for both compounds confirmed that the rotation of the o-carborane cages and terminal phenyl rings for SFC1 is freer than that for SFC2.

Multicenter-Bond-Based Quantum Interference in Charge Transport Through Single-Molecule Carborane Junctions

Molecular components are vital to introduce and manipulate quantum interference (QI) in charge transport through molecular electronic devices. Up to now, the functional molecular units that show QI are mostly found in conventional π- and σ-bond-based systems; it is thus intriguing to study QI in multicenter bonding systems without both π- and σ-conjugations. Now the presence of QI in multicenter-bond-based systems is demonstrated for the first time, through the single-molecule conductance investigation of carborane junctions. We find that all the three connectivities in carborane frameworks show different levels of destructive QI, which leads to highly suppressed single-molecule conductance in para- and meta-connected carboranes. The investigation of QI into carboranes provides a promising platform to fabricate molecular electronic devices based on multicenter bonds.

Transition-Metal-Catalyzed Selective Cage B-H Functionalization of o-Carboranes

Carboranes are a class of carbon-boron molecular clusters with unusual thermal and chemical stabilities. They have been proved as very useful building blocks in supramolecular design, optoelectronics, nanomaterials, boron neutron capture therapy agents and organometallic/coordination chemistry. Thus, the functionalization of o-carboranes has received growing interests. Over the past decades, most of the works in this area have been focused on cage carbon functionalization as the weakly acidic cage C-H proton can be readily deprotonated by strong bases. In sharp contrast, selective cage B-H activation/functionalization among chemically very similar ten B-H vertices is very challenging. Considering the differences in electron density of ten cage B-H bonds in o-carborane and the nature of transition metal complexes, we have tackled this selectivity issue by means of organometallic chemistry. Our strategy is as follows: using electron-rich transition metal catalysts for the functionalization of the most electron-deficient B(3,6)-H vertices (bonded to both cage CH vertices); using electron-deficient transition-metal catalysts for the functionalization of relatively electron-rich B(8,9,10,12)-H vertices (with no bonding to both cage CH vertices); and using the combination of directing groups and electrophilic transition metal catalysts for the functionalization of B(4,5,7,11)-H vertices (bonded to only one cage CH vertex). Successful applications of such a strategy result in the preparation of a large variety of cage B-functionalized carboranes in a regioselective and catalytic manner, which are inaccessible by other means. It is believed that as this field progresses, other cage B-functionalized carboranes are expected to be synthesized, and the results detailed in this concept article will further these efforts.

Substitution of the laser borane anti-B18H22 with pyridine: a structural and photophysical study of some unusually structured macropolyhedral boron hydrides

One-step derivatization of the laser borane, anti-B₁₈H₂₂, with pyridine gives three uniquely structured macropolyhedral boranes with interesting photophysical properties.

Photoarylation of Iodocarboranes with Unactivated (Hetero)Arenes: Facile Synthesis of 1,2-[(Hetero)Aryl]n -o-Carboranes (n=1,2) and o-Carborane-Fused Cyclics

Photoarylation of iodocarboranes with unactivated arenes/heteroarenes at room temperature has been achieved, for the first time, thus leading to the facile synthesis of a large variety of cage carbon mono(hetero)arylated and di(hetero)arylated o-carboranes. This work represents a clean, efficient, transition-metal-free, and cheap synthesis of functionalized carboranes, which has significant advantages over the known methods.

Solid-State Emission of the Anthracene-o-Carborane Dyad from the Twisted-Intramolecular Charge Transfer in the Crystalline State

The emission process of the o-carborane dyad with anthracene originating from the twisted intramolecular charge transfer (TICT) state in the crystalline state is described. The anthracene-o-carborane dyad was synthesized and its optical properties were investigated. Initially, the dyad had aggregation- and crystallization-induced emission enhancement (AIEE and CIEE) properties via the intramolecular charge transfer (ICT) state. Interestingly, the dyad presented the dual-emissions assigned to both locally excited (LE) and ICT states in solution. From the mechanistic studies and computer calculations, it was indicated that the emission band from the ICT should be attributable to the TICT emission. Surprisingly, even in the crystalline state, the TICT emission was observed. It was proposed from that the compact sphere shape of o-carborane would allow for rotation even in the condensed state.

Aggregation-Induced Delayed Fluorescence Based on Donor/Acceptor-Tethered Janus Carborane Triads: Unique Photophysical Properties of Nondoped OLEDs

Luminescent materials consisting of boron clusters, such as carboranes, have attracted immense interest in recent years. In this study, luminescent organic-inorganic conjugated systems based on o-carboranes directly bonded to electron-donating and electron-accepting π-conjugated units were elaborated as novel optoelectronic materials. These o-carborane derivatives simultaneously possessed aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) capabilities, and showed strong yellow-to-red emissions with high photoluminescence quantum efficiencies of up to 97 % in their aggregated states or in solid neat films. Organic light-emitting diodes utilizing these o-carborane derivatives as a nondoped emission layer exhibited maximum external electroluminescence quantum efficiencies as high as 11 %, originating from TADF.

Palladium-Catalyzed Regioselective Diarylation of o-Carboranes By Direct Cage B-H Activation

Palladium-catalyzed intermolecular coupling of o-carborane with aromatics by direct cage B-H bond activation has been achieved, leading to the synthesis of a series of cage B(4,5)-diarylated-o-carboranes in high yields with excellent regioselectivity. Traceless directing group -COOH plays a crucial role for site- and di-selectivity of such intermolecular coupling reaction. A Pd(II)-Pd(IV)-Pd(II) catalytic cycle is proposed to be responsible for the stepwise arylation.

Carborane dyads for photoinduced electron transfer: photophysical studies on carbazole and phenyl-o-carborane molecular assemblies

o-Carborane-based donor-acceptor dyads comprising an o-carboranyl phenyl unit combined with N-carbazole (1) or 4-phenyl-N-carbazole (2) were prepared, and their dyad characters were confirmed by steady-state photochemistry and photodynamic experiments as well as electrochemical studies. The absorption and electrochemical properties of the dyads were essentially the sum of those of the carbazole and o-carboranyl phenyl units; this indicates negligible interaction between the carbazole and o-carborane units in the ground state. However, the emission spectra of 1 and 2 indicated that carbazole fluorescence was effectively quenched and a new charge-transfer (CT) emission was observed in solvents, varying from hexane to acetonitrile, which exhibited large Stoke shifts. The CT emission properties of o-carborane-based dyads were further analyzed by using Lippert-Mataga plots to show that unit charge separation occurred to form a charge-separated species in the excited state, namely, 1⋅2. This excited-state species was confirmed by nanosecond transient absorption spectra and spectroelectrochemical measurements; the photoexcitation of carbazole generated the CT state in which a radical cation and anion were formed at the carbazole and o-carborane units, respectively, within a few nanoseconds. DFT calculations corroborated the presence of this CT species and showed localized populations of the highest singly occupied molecular orbital on 2 in the reduced anionic state. As a result, molecular assemblies formed by linking the carbazole group with the o-carborane cage through a phenylene or multi-phenylene spacer revealed that the photoinduced electron-transfer process occurred intramolecularly.

o-Carborane-based anthracene: a variety of emission behaviors

An o-carborane-based anthracene was synthesized, and single crystals, with incorporated solvent molecules, were obtained from the CHCl3 , CH2 Cl2 , and C6 H6 solutions. The anthracene ring in the crystal is highly distorted by the formation of a π-stacked dimer between the anthracene units. The crystals exhibited a variety of emission behaviors such as aggregation-induced emission (AIE), crystallization-induced emission (CIE), aggregation-caused quenching (ACQ), and multichromism.

Syntheses and reductions of C-dimesitylboryl-1,2-dicarba-closo-dodecaboranes

An investigation ofC-dimesitylboryl-ortho-carboranes, 1-(BMes₂)-2-R-1,2-C₂B₁₀H₁₀(1and2), reveals that the carborane is the electron-acceptor and the mesityl group is the electron-donor in these dyads.