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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.


■ INTRODUCTION
Carboranes are a class of polyhedral boron−carbon molecular clusters. 1 As one of the famous families of boron clusters, carboranes have been extensively studied because of the important applications of their derivatives in the fields of functional materials, catalysis, pharmaceuticals, etc. 2−10 For example, through tailoring the structures, carboranes can undergo efficient photo/electroluminescence, thus showing excellent performances in luminescent materials and fluorescent sensors. 11−14 In addition, carboranes as pharmacophores have also been developed for medicinal applications such as in boron neutron capture therapy (BNCT). 15−19 Despite these known important investigations, a comprehensive understanding of the structure−property relationship has not been fully established yet 20,21 because of the lack of research concerning noncovalent interactions for these polyhedral boranes.
As three-dimensional analogs of aromatic rings, carboranes have unique structural features like bulk-size-cage geometry, delocalization of the skeletal electrons through multicenter two-electron bonds, and three-dimensional global aromaticity. 22−28 All these render carboranes with different attributes compared to planar aromatic rings (Figure 1a). Even though the noncovalent interactions involved in two-dimensional aromatic species are ubiquitous and play vital roles in many important chemical and biological processes, 29−37 those based on carborane moieties have been seldom studied, except B− H···π, B−H···H−B, and other related interactions. 38 −47 In particular, the noncovalent interaction generated by two threedimensional and negatively charged aromatic carborane moieties is unknown to date.
Herein, inspired by the coordinate bond in transition metal complexes 48 (Figure 1b, left), the cationic pyridinium-based molecular barrier is used to obtain the boron cluster-based cage − ···cage − interaction ( Figure 2). This unique noncovalent interaction could be validated in carborane derivatives with a contacting distance of 5.4−7.0 Å from centroid to centroid. Theoretical calculations also clarified the formation of such cage − ···cage − interaction, and disclosed its repulsive bonding nature, which is not the same as that of the known π···π bond. Moreover, such cage − ···cage − interaction-constructed bulk crystalline materials showed interesting photophysical and electrical properties such as the red-shift charge transfer absorption in the crystalline state, the enhanced electrical conductivity in bulk crystals, and the efficient photothermal conversion (vide infra). Therefore, this study not only reports a boron cluster-based noncovalent interaction but also explores its bonding nature and potential applications.

Molecular Design and Synthesis
It is well-known that the three-dimensional nido-carborane possesses a cage structure and partially negatively charged B− H bonds. These structural features may give rise to an easy packing arrangement between the nido-carborane and different counterions. 47 Thus, the cage − ···cage − interaction is hardly realized because of both strong Coulomb repulsion and large steric hindrance. This is also proven by a search of all the crystal structures containing the nido-carborane unit in the Cambridge Crystallographic Data Centre (CCDC) database. No such cage − ···cage − interaction with close contact of B/C··· B/C (Figure 3b) could be found, except cage···π, 47 B− H···π, 38,39 or B−H···H−B 41 interactions ( Figure S1). In this regard, it remains a challenge to obtain a noncovalent bond between two large-sized and aromatic boron cluster anions with close proximity.
On the other hand, chemical bonds are present when the bonding atoms are in thermodynamically stable or metastable assemblies. 49,50 As revealed in coordination chemistry, a coordinate bond is generated if an electron-deficient metal center meets an electron-rich ligand. Such a binding interaction enables it to overcome the repulsive interactions from other anionic ligands in the coordinative sphere, which ultimately leads to the formation of stable complexes such as ferrocene and metallacarboranes (Figure 1b, left). 51 This inspired us to design such an arrangement where two negatively charged nido-carboranes are placed together (see Figure 1b, right) by the use of a positively charged but larger organic counterion, instead of a metallic atom, as demonstrated in Figure 2 to balance the electronic repulsion, driving to the formation of an intermolecular interaction involving two boron clusters. We are aware of the existence of many examples of supramolecular systems that stabilize two or more like-charged species such as F − , Cl − , Br − , SO 4 2− , HCO 3 − , or H 2 PO 4 − , among others. 52−61 However, if stabilized, the proposed cage − ···cage − interaction would represent a unique type of anion···anion interaction.
To validate the above hypothesis, we designed positively charged structural units of π-based frameworks 52 (Figure 2, left). Because of the attractive interaction between a cationic molecular barrier and nido-carborane anion, the electrostatic repulsion between the two nido-carboranes could be counterbalanced. Besides, the π-based framework could provide a tailorable molecular topology, which may devote to the formation of a matched molecular barrier with a specific confined space to stabilize such a repulsive noncovalent interaction. Herein, the rational design of the template of molecular barrier to form a cage − ···cage − interaction on the basis of nido-carborane is presented for the first time.
The nido-carborane-based compounds were synthesized by ion exchange in high yields (Schemes S1−S4). The success of the ion exchange reaction also supports the attractive interaction between the bipyridinium-based cation and the nido-carborane anion. These complexes were carefully characterized by 1 H-, 13 C-, 11 B-NMR, and X-ray diffraction (Table S1, Figure S2).

Structure Analysis of Cage
The crystal packing structures of these compounds are shown in Figures 3 and 4 and Figures S3−S10. As expected, the pyridinium systems determine the crystal packing structures, particularly for the interaction between the two nidocarboranes. For compound carb-1, the two nido-carboranes are surrounded by multiple pyridinium cations ( Figure S3a). With careful inspection, a sandwich-type packing conformation between two nido-carboranes and two pyridiniums was observed (Figure 3a and Figure S3, note that some of the nido-carborane anions in Figures 3 and 4 were omitted in the Noncovalent interaction is formed between two nido-carboranes (right). nido-C 2 B 9 H 10 2− was used as a ligand to form stable complexes (left). Hydrogen atoms of the molecules are omitted for clarity. The pink ball represents the BH unit, whereas the gray ball represents the CH unit. Figure 2. Cage − ···cage − interaction in boron clusters. This noncovalent bond is generated when nido-carborane units are surrounded by pyridinium units, which are used as molecular barriers to hold the two negatively charged nido-carboranes. Hydrogen atoms of the molecules are omitted for clarity. The pink ball represents the BH unit, whereas the gray ball represents the CH unit. crystal packing structures for clarity), which is reminiscent of the coordinative bond in metallacarboranes. As a consequence, the formed multiple intermolecular contacts such as C−H···B and B−H···C interactions should devote to stabilizing the repulsive interaction between the two nido-carboranes ( Figure  2 and Figure S3a). The cation-based framework seems to create a molecular barrier ( Figure 2 and Figure S3) to provide a confined space, thus holding nido-carboranes together and leading to the cage − ···cage − interaction with a distance of 5.523 Å (centroid to centroid, Figure 3b). Interestingly, the two C 2 B 3 rings from two nido-carboranes adopt an antisymmetric and parallel displaced geometry with a B···B/C contact of 3.579− 3.999 Å (Figure 3b) below the sum of van der Waals radii (r B = 1.92 Å, r C = 1.70 Å). 62 This further demonstrates the close contact of two boron clusters, similar to the packing mode of a π···π bond as found in two-dimensional aromatic systems. 63 Next, effect of the variation in pyridinium on the supramolecular assembly was examined. If the molecular barrier is lengthened by adding one double bond or one triple bond or even replacing the methyl of pyridinium with an ethyl as shown in compounds carb-2−4 (Figures 3c−f and Figure   S4−S6), the longer distances of the cage − ···cage − interaction (6.464−7.035 Å) were observed. The close contact of paralleldisplaced C 2 B 3 ···C 2 B 3 units in carb-1 disappears, although the confined space triggered by the molecular barrier and the formed multiple intermolecular interactions like B−H···C or B−H···H−C interactions are still present in carb-2−4. This suggests that holding two nido-carboranes in close contact is not easy to achieve because of the repulsive interaction and the bulky cage structure. Namely, if the nido-carborane is attached to a nonmatched molecular barrier, the attractive interactions between the nido-caborane moiety and the π-based molecular barrier based on multiple B−H···C and/or C−H···B noncovalent bonds and others cannot overcome the liability of the repulsive cage − ···cage − interaction in the crystalline state ( Figure 3). This directly leads to a longer contacting distance of cage − ···cage − interaction. Obviously, a matched molecular barrier is crucial for the realization of the interaction between two nido-carboranes.
To further demonstrate that the parallel-displaced cage − ··· cage − interaction with close proximity can be tuned by a cationic molecular barrier, we designed and synthesized more  Figure S7), which contribute to the construction of the restricted lattice environment. The crystallizing water reduces the electron density of the pyridine nitrogen atom through the formation of a hydrogen bond. This further benefits the formation of the cage − ···cage − interaction. On the contrary, if a longer molecular barrier is chosen as used in compound carb-6, multiple intermolecular interactions like C−H···B (2.816− 3.186 Å, Figure S8) are observed. Such a packing shape shows a relatively loose packing arrangement between nido-carborane and molecular barrier, thus leading to a longer bonding distance of 5.889 Å for the cage − ···cage − interaction ( Figure  4d).
Additionally, the influence of substituent at the nidocarborane cage on the cage − ···cage − interaction was explored as well. In doing so, we designed methyl-substituted nidocarboranes at both boron (carb-7) and carbon sites (carb-8), respectively, but using the same pyridinium-based molecular barrier as in carb-1. As expected, the confined space for holding two nido-carboranes is also present in carb-7 ( Figure  S9). Moreover, the parallel-displaced bonding arrangement still remains but shows longer cage − ···cage − interaction (6.483 Å, Figure 4e). Consequently, no close C/B···C/B contact of nidocarboranes could be observed in this example because of the additional steric hindrance introduced by the methyl group. In the case of carb-8, multiple intermolecular interactions such as C−H···B and C−H···H−B were identified ( Figure S10). However, because of the larger steric hindrance from the methyl-substituted nido-carborane and the orientation, a longer cage − ···cage − distance of 6.668 Å is generated (Figure 4f). Correspondingly, this leads to an inverse packing orientation of the C 2 B 3 planes of the nido-carborane units.
Note that carb-5, carb-6, and carb-7 contain paralleldisplaced cage − ···cage − interactions and show the common feature of the antisymmetric packing style of two C 2 B 3 planes facing each other. Both close and specific contact of the two . Crystal packing structures of nido-carborane-based compounds with different cationic frameworks or nido-carboranes with different methyl substitution (blue, nitrogen; dark gray, carbon; pink, boron; light gray, hydrogen; red, oxygen). The contacting distances of the noncovalent bonds are indicated with light blue/black dashed lines. Some hydrogen atoms have been omitted for clarity. C 2 B 3 planes also suggests effective electronic coupling of the two nido-carborane cages. As a whole, noncovalent interactions have been synthetically obtained in boron clusters, which can be tuned by a positively charged molecular barrier.

Energy Decomposition Analysis
The above eight nido-carboranes with different cations (carb-1 to carb-8) have been quantum chemically modeled at the ZORA-BLYP-D3BJ/TZ2P level of theory (see the Supporting Information for full computational details). There is a good agreement between computed and crystal packing structures, which have been taken as the initial structures for the molecular structure optimization, as it can be seen from the comparison of the depicted most relevant distances concerning either cage − ···cage − , cage···π, and B−H···π interactions or H··· H contacts ( Figure S11).
The referred interactions presented in this set of structures have been further analyzed by means of a quantitative Kohn− Sham molecular orbital and energy decomposition analysis Scheme 1. Fragmentation Schemes Considered in the Energy Decomposition Analyses a a Fragments depicted are based on carb-1, although same fragmentation scheme has been applied to the rest of systems. Interactions analyzed for each fragmentation scheme are (1) the interaction between the two three dimensional nido-carborane units and the two π-based cations (cage 2 ···π 2 ); (2) the cage − ···cage − interaction between the two isolated three dimensional nido-C 2 B 9 H 12 units (cage − ···cage − ) and in the presence of two π-based cations (π-cage···π-cage); (3) the cage···π interaction between the three-dimensional nido-carborane and the π-based cation (cage···π); (4) the B−H···π interaction between one B−H bond of the carborane and the π-system of one heterocycle (B−H···π); and (5) the interaction between the two three-dimensional nido-carborane units and one of the two π-based cations (cage 2 ···π). Colored in green the C 2 B 3 ring and in blue the rest of the carborane cluster.
(EDA) with several fragmentation schemes (Scheme 1). In this EDA, the interaction energy (ΔE int ) is decomposed into Pauli (ΔE Pauli ), electrostatic (ΔV elstat ), orbital interaction (ΔE oi ), and dispersion terms (ΔE disp ). 64−67 First, for carb-1 (Table 1), we found that the cage − ···cage − interaction between the two nido-carborane units alone, i.e., without the molecular barrier, is destabilized by 40.3 kcal mol −1 , mainly due to the repulsive electrostatic interaction caused by the two anionic systems. However, if the cage − ··· cage − interaction is analyzed by enclosing one cation per each carborane, it becomes attractive by −17.2 kcal mol −1 (π-cage···π-cage). Thus, the cage − ···cage − interaction becomes "stable" thanks to the presence of the cationic molecular barrier. The main reason is the drastic decrease of the repulsive ΔV elstat term together with more favorable orbital and dispersion interactions due to the formation of B−H···π, C− H···B, or B−H···C noncovalent interactions with the cage and the rings of the cationic units. These interactions (B−H···π or B−H···π′) between the cationic ring and the carborane are attractive by −136.4 and −137.8 kcal mol −1 , basically due to attractive ΔV elstat between the interacting cation and the anionic cage. The sandwich interaction (cage 2 ···π 2 ) between the two nido-carboranes together (charge = −2) with the two cationic units together (charge = +4) is attractive by −450.1 kcal mol −1 , an interaction fully governed by ΔV elstat (−431.0 kcal mol −1 ). And finally, the cage 2 ···π interaction amounts to −255.0 kcal mol −1 , somehow larger than half of the sandwich one. From the EDA terms, one can see that the Pauli, the electrostatic, and the dispersion interactions in cage 2 ···π are halved compared to the sandwich (cage 2 ···π 2 ), whereas the orbital interaction remains almost the same. The presence of the second cationic bipyridinium increases, in absolute value, the Pauli, the electrostatic, and the dispersion interactions but does not change the orbital interaction term. Indeed, a similar ΔE oi concurs with similar charge transfer from the cages to the π-based cation(s). Overall, the electrostatic interaction is the most determinant term in all of the above-commented interactions.
Systems carb-3 and carb-4 (Table S2) present very similar values and trends to carb-1. Both carb-1 and carb-4 have the two C 2 B 3 faces in a parallel but reverse conformation, with face to face distance of 3.83 and 4.13 Å, respectively; whereas in carb-3 the C 2 B 3 rings do not face one to each other. This difference translates into a somewhat less repulsive cage − ··· cage − interaction for carb-3 (37.7 kcal mol −1 , Table 1). Finally, carb-2 (Table S2) presents a completely different conforma-tion, with two more separated carborane units, and just one cationic unit. This translates into a less repulsive cage − ···cage − interaction, due to the longer distance between the two carborane units. And the cage···π interactions appear to be similar to previous B−H···π ones. In a very recent study, the same nido-carborane has been analyzed interacting with cationic bipyridiniums, and we have proven the existence of a new nido-cage···π noncovalent interaction with similar components of the interaction energy as those of the cage···π interaction in carb-2. 47 For completeness, the EDA scheme corresponding to the total dissociation of each system into its fragments, i.e., separate nido-carborane clusters and cationic frameworks, has also been undertaken (Table S3), which shows that both initial carborane and pyridinium units experience minor geometry changes when forming the whole compound, with deformation energies (ΔE strain ) being in the range of 2.1−7.5 kcal mol −1 . The second set of systems, i.e., carb-5 to carb-8 (Table S4) provide similar trends and are discussed in the Supporting Information.
It must be also pointed out that the orientation of one nidocarborane versus the other one is not determinant, i.e., those with face-to-face orientation like carb-1 present a similar behavior to those that do not present it, like carb-3. Thus, the carborane acts as a whole anionic unit not much influenced by the orientation of the two C 2 B 3 rings. This is further supported if we analyze how the bonding energy is affected by changing the packing orientation of the C 2 B 3 faces of carb-1 ( Figure  S12), with a maximum difference of 8.8 kcal mol −1 . In addition, in the case of the same carb-1, the increase of distance between the two C 2 B 3 faces from the equilibrium distance to that found in other systems causes a relatively small change in the interaction energy ( Figure S12). Besides, the methyl group of nido-carborane (carb-7 and carb-8) or that in the cationic bipyridinium itself has a minor effect on the electronic properties of these compounds ( Figure S13 and Table S5).

Charge Transfer and Molecular Orbital (MO) Interactions
According to Table 1, a charge transfer from the carborane to the cationic aromatic rings of about 0.5 e takes place in carb-1 (also in carb-3 and carb-4, Table S2). Comparison of the molecular electrostatic potential and VDD charges for the pyridinium and nido-carborane systems alone to that of the whole complex clearly shows how the negative charge of the latter is reduced, i.e., transferred to the cationic units, which become less positive ( Figure 5 and Figure S14). The reduction of the negative charge in the nido-carborane units is particularly important in the C 2 B 3 ring (computed QTAIM, Hirshfeld, or Mulliken charges give the same trend, Figures S15 and S16). Thus, the clearly repulsive interaction between the two nidocarboranes in the cage − ···cage − fragmentation scheme is drastically reduced by the presence of the cationic framework that acts as a counterion but at the same time efficiently disperses the negative charge of the nido-carboranes, as clearly seen in Figure 5. Importantly, from the charge analysis, we can see how the interaction mainly comes through the partially negative hydrogen atoms, i.e., B/C−H δ− ···H−C and B− H δ− ···π interactions between the cages and the molecular barrier.
Stabilization due to the charge transfer from the anionic nido-carborane to the cationic framework is included in the orbital interaction term (ΔE oi ). On the basis of the EDA cage 2 ···π 2 scheme in Table 1 for carb-1, the ΔE oi amounts to  Figure S17). The overlap between these HOMO of the carborane and LUMO of the aromatic ring amounts to 0.017.
As a whole, the cage − ···cage − interaction alone is always repulsive. However, such a repulsive interaction can be stabilized by boosting a donor−acceptor interaction with an important charge transfer between the anionic carborane to the cationic aromatic structure. The generated cage − ···cage − bonding interaction as denoted by the π-cage···π-cage fragmentation scheme (Scheme 1 and Table 1), together with the cage···π interaction, is also proven by noncovalent interaction (NCI) plots ( Figure S18). Finally, regarding the aromaticity of the C 2 B 3 ring, NICS values indicated that it is quite constant along the series (Table S6). Some of us have recently reported 27 that the C 2 B 3 ring is not the most aromatic ring in the nido-carborane. Indeed, the strongest diatropic ring currents are found in a parallel plane located between the two main five-membered rings of the cluster, i.e., the referred C 2 B 3 ring and the B 5 ring below.

Other Aromatic Systems
Would the proposed strategy work for other aromatic anionic species like cyclopentadienyl anion or deprotonated pyrrole? Both systems ( Figure S19) have also been analyzed by means of the same procedure as the above series of carboranes, but only using the cationic molecular barrier of carb-1. The frontier molecular orbitals of the cyclopentadienyl anion (Cp − ) compared to those of the nido-carborane ( Figure S20) support the similar electronic structure of the two anionic aromatic units. When such a noncovalent interaction is analyzed by means of EDA (Table S7), it is found that the closer distance between the two cyclopentadienyl units as compared to the nido-carboranes causes a higher repulsion (ΔE int = 109.5 kcal mol −1 for Cp − ···Cp − ). All terms in absolute value are higher for the two cyclopentadienyl units because of the shorter distance. However, the weight of the different contributions to the final ΔE int remains more or less the same indicating the similarity between the cage − ···cage − and Cp − ···Cp − interactions. The VDD charges also prove the charge transfer from the cyclopentadienyl anion to the cation ( Figure S21). Would it be possible to find similar intermolecular interactions in organometallic species? To answer this question, we have considered the dimer of the cyclopentadienylcobalt dicarbonyl complex, CpCo(CO) 2 . We have found that the dimer with the two Cp units facing each other at 3.560 Å is the most stable. The noncovalent interaction between the two Cp − units is weak (only 2.9 kcal mol −1 ), and it is stabilized by the presence of the two units of the cationic Co I (CO) 2 complex (Table S9). These results show that it could be possible to synthesize organic and organometallic aromatic systems with repulsive π − ···π − noncovalent interactions.

Applications in Solid-State Materials
To explore the properties and potential applications of this new type of cage − ···cage − interaction, a set of experiments were conducted on the selected compound carb-1 and the model compound MV with a bulk-size counteranion of trifluoromethane sulfonimide (Figure 6a). The UV/vis spectra in acetonitrile solution were first investigated. carb-1 and MV showed similar absorption spectra ( Figure S24), suggesting no obvious differences in their electronic structures of bipyridinium in the ground state. However, a broad red-shifted absorption was observed for carb-1 in the crystalline state in comparison to MV (Figure 6b). Particularly, it covers the ultraviolet light and the whole visible light ranges, thus might have potential applications in the light-absorbent materials. In sharp contrast, compound MV with poor electron donor ability of the trifluoromethane sulfonimide anion showed the shortwavelength absorption band in the solid state (Figure 6b), which is similar to that observed in solution ( Figure S24). Thus, the long-wavelength absorption for compound carb-1 might be attributed to the charge transfer absorption from the donor of nido-carborane to the acceptor of bipyridinium. To confirm this, we analyzed the crystal packing structures of the two compounds. Compound MV solely shows the O···C and O···N contacts (Figure 6c) between the trifluoromethane sulfonimide anion and bipyridinium cation. In sharp contrast,  the compound carb-1 demonstrates strong interactions between nido-carboranes and bipyridinium cation ( Figure  3a). Thus, the charge transfer would easily occur, further leading to the long-wavelength absorption in the crystalline state. This also accords with the above theoretical results and is similar to a reported example. 47 Apart from the above studies, the excited state properties were also investigated. Unexpectedly, compound carb-1 showed luminescence quenching in the solid state, whereas compound MV showed ultraviolet light emission ( Figure S25). According to the crystal packing structure, there is no π···π interaction for compound carb-1, which could exclude the excimer-induced quenching effect from the two bipyridinium units. In addition, the ion titration experiment, in which nidocarborane-based derivative with a cation of tetramethyl quaternary ammonium was added to the solution of model compound MV, displayed a gradually reduced luminous intensity (Figure 6d and Figure S26). Clearly, the charge transfer between nido-carborane and pyridinium in the crystal packing structures causes such a luminescence quench in the solid state. As such, the cage − ···cage − interaction-induced packing structure leads to different emission behavior in comparison to the model compound MV. Obviously, the boron cluster-based noncovalent bond could be a new supramolecular assembly tool to fine-tune photophysical properties.
Considering the nonemissive nature of carb-1, it might devote to maximizing the conversion of light to heat ( Figure  7a). Thus, the photothermal conversion of carb-1 and MV crystals was investigated (Figure 7b). The temperature increased very quickly and reached 83 degrees in 1 min of laser irradiation. The efficient photothermal conversion should be attributed to the large absorptivity of carb-1 molecule as well as the nearly unity efficiency of nonradiative relaxation from the excitons in the excited state (Figure 7a). Such a promising photothermal behavior shows great potential for practical applications. 68 In contrast, control compound MV showed a weaker photothermal effect as the temperature increased only to 31°, which may be attributed to the weaker absorption ability and the competing radiation transition of MV. Besides, the amorphous carb-1 sample, having similar ultraviolet absorption as in acetonitrile solution ( Figure S24), also shows a negligible photothermal effect (Figures S27−29) compared with that in the crystalline state (Figure 7b). These data demonstrate that the unique packing structures in carb-1 enable it to show high performance of the photothermal effect. The photobleaching resistance property of carb-1 was further evaluated by the alternative heating and cooling process, that is, the crystal of carb-1 was irradiated by laser for 2 min, and then naturally cooled to ambient temperature (Figure 7c). After five cycles, the maximized elevated temperature of carb-1 showed negligible changes, indicating no degradation under continuous laser irradiation.
Furthermore, the electrical conductivity of carb-1 and MV was measured using an insulation resistance tester on a crystal with a thickness of 1.0 mm (Figure 7d and Figure S30). The conductivity of carb-1 crystal was determined to be 5.25 × 10 −8 S/m at 298 K, which is nearly five times higher than that of control compound MV. The enhanced conductivity might be related to the unique packing structures of carb-1 in the bulk materials. However, such a phenomenon is rarely investigated in carborane-based functional materials. 22−28 According to these results, the boron cluster-based noncovalent interactions endow the bulk crystalline materials with distinct properties, which imparts carborane derivatives with great potential to develop advanced materials.

■ CONCLUSIONS
In this study, we reported a boron cluster-based noncovalent interaction between two three-dimensional and aromatic nidocarborane units, i.e., cage − ···cage − interaction. The nature of this new noncovalent cage − ···cage − bond is electrostatic dominant, mainly supported by B/C−H δ− ···H−C and B− H δ− ···π interactions between the cages and the cationic pyridinium-based molecular barriers, accompanied by a certain amount of orbital and dispersion interactions. The repulsive interaction between the two nido-carboranes is drastically reduced by the presence of the cationic framework that acts as a counterion, while at the same time efficiently dispersing the negative charge of the nido-carboranes. The designed carborane derivatives also show interesting properties such as full-color absorption in the crystalline state and efficient photothermal conversion. This work not only provides a new research prospect for carborane chemistry but also offers a paradigm to explore new applications based on boron clusters.
Schemes S1−S4 are the chemical structures and the synthetic routes for target compounds in Figures 2 and  3, respectively; Figures S1−S10 and Tables S1 display crystal structures and crystallographic data in this study, respectively; Figures S11−S22 and Tables S2−S8 list