The influence of small molecule adsorption on the spectral characteristics of B₁₂N₁₂ superatoms

Previous research has identified several superatoms with distinct spectral properties, including Au₂₀,¹ Na₄₀,² Al₁₃⁻,³ and C₆₀.⁴ The spectral characteristics of B₄₀ fullerene are investigated, and these characteristics distinguish its hollow cage structure from other quasi-planar configurations.⁵ Subsequent theoretical studies confirmed it as a superatom.⁶ Recently, investigations into the luminescence properties of small molecule-excited B₄₀ have demonstrated that the adsorbed pyrazine intensifies both the absorption and emission spectra of B₄₀ and lead to a redshift into the visible light range.⁷ The non-metallic nature of boron–nitrogen (BN) cages, such as B₁₂N₁₂, B₁₆N₁₆, and B₂₈N₂₈, which are also superatoms,^8,9 prompts inquiry into whether the adsorbed small molecules can similarly augment or alter the spectral properties of BN superatoms.

In recent years, significant attention has been devoted to the exploration of several BN-based molecules and materials for hydrogen storage.^10–13 Studies have delved into the adsorption properties of BN fullerene, revealing its potential utility as drug delivery carriers^14–17 and collector of gases.^18,19 Moreover, investigations into the CO oxidation capabilities of BN fullerene have highlighted its promise for CO and CO₂ capture, attributed to the presence of B–B bonds.²⁰ In addition, theoretical analyses have demonstrated the efficacy of B₁₂N₁₂ in adsorbing and decomposing methanol molecules at room temperature.²¹ Furthermore, doping B₁₂N₁₂ with aluminum has been shown to enhance the adsorption energy and thermodynamic stability of aspirin molecules.²² Thus, the adsorption of small molecules onto B₁₂N₁₂ fullerene presents an intriguing avenue for potentially enhancing its spectral properties.

This study employs density functional theory (DFT)²³ to explore the adsorption of small molecules (pyrazine, pyridine, and benzene) on BN fullerenes. Results indicate that the adsorbed benzene enhances the absorption spectrum of B₁₂N₁₂, whereas the adsorption of pyrazine and pyridine enhances its emission spectrum. Furthermore, the number of adsorbed small molecules influences the spectrum. The primary objective of this research is to elucidate the influence of small molecules on the spectra of B₁₂N₁₂ superatoms.

The model is constructed by adsorbing small molecules (pyrazine, pyridine, and benzene) onto the B₁₂N₁₂ superatom. Given that B₁₂N₁₂ fullerene comprises eight hexagonal and six tetragonal BN rings,²⁴ the pyrazine, pyridine, and benzene adsorb at the hexagon, tetragon, and B atom sites of the B₁₂N₁₂ fullerene to form the studied structures. Following theoretical simulation and structural optimization, three stable geometric configurations are obtained, as depicted in Fig. 1. In Figs. 1(a) and 1(b), the N atoms of pyrazine and pyridine are bonded with the B atom of B₁₂N₁₂, forming chemisorption structures denoted as pyrazine-B₁₂N₁₂ and pyridine-B₁₂N₁₂, respectively. Figure 1(c) illustrates benzene adsorption at a tetragonal BN ring of B₁₂N₁₂, identified as the physisorption structure benzene-B₁₂N₁₂. The relative energies are listed in Table S1 of the supplementary material.

The empirical dispersion-corrected density functional theory (DFT-D3)²⁵ is employed to fully optimize the geometric structures using hybrid functionals B3LYP^26,27 with 6-31G* basis sets.^28–30 All optimized structures are verified to be local minima. Subsequently, the time-dependent-DFT (TD-DFT) method^31,32 is utilized to compute electronic transitions and absorption and emission spectral properties based on the optimized structures and first excited states. For this purpose, range-separated hybrid functionals CAM-B3LYP^33,34 are chosen to calculate absorption and emission spectra with 6-31+G* basis sets,³⁵ as previous studies have shown that the CAM-B3LYP functional can improve spectroscopic properties for materials.^36–38 In addition, 30 and 10 electronic states are selected for calculation in the absorption and emission spectra. All calculations are conducted using the Gaussian 16 package.³⁹ 

The ground states of the three investigated structures are singlet states, and their corresponding molecular orbitals (MOs) diagrams are depicted in Fig. 2. In the case of pyrazine-B₁₂N₁₂, the lowest unoccupied molecular orbital (LUMO) and LUMO+1 are predominantly localized on pyrazine, whereas the highest occupied molecular orbital (HOMO), HOMO−1, HOMO−2, HOMO−3, HOMO−4, HOMO−5, HOMO−6, HOMO−8, HOMO−12, LUMO+10, LUMO+8, LUMO+7, LUMO+6, LUMO+5, LUMO+4, LUMO+3, and LUMO+2 mainly occupy the B₁₂N₁₂ superatom. In addition, HOMO−10 exhibits delocalization across the entire structure [refer to Fig. 2(a)]. Similarly, for pyridine-B₁₂N₁₂, LUMO+1, LUMO, and HOMO−10 are predominantly localized on pyridine, while LUMO+8, LUMO+7, LUMO+6, LUMO+5, LUMO+4, LUMO+3, LUMO+2, HOMO, HOMO−1, HOMO−2, HOMO−3, HOMO−4, HOMO−5, HOMO−6, HOMO−8, and HOMO−9 mainly occupy the B₁₂N₁₂ superatom. HOMO−11 is distributed over both pyridine and B₁₂N₁₂ [see Fig. 2(b)]. In contrast to pyrazine-B₁₂N₁₂ and pyridine-B₁₂N₁₂, the HOMO and HOMO−1 of benzene-B₁₂N₁₂ are localized on benzene, while LUMO+14, LUMO+13, LUMO+12, LUMO+11, LUMO+9, LUMO+8, LUMO+4, LUMO+3, LUMO+2, LUMO+1, LUMO, HOMO−2, HOMO−3, HOMO−4, HOMO−5, HOMO−6, and HOMO−7 primarily occupy the B₁₂N₁₂ superatom. Furthermore, LUMO+6 is situated on both benzene and B₁₂N₁₂ [as depicted in Fig. 2(c)]. Consequently, in the pyrazine-B₁₂N₁₂ and pyridine-B₁₂N₁₂ structures, electron transitions occur from B₁₂N₁₂ to small molecules, while in benzene-B₁₂N₁₂, electron transitions occur from benzene to the B₁₂N₁₂ cage.

To explore the impact of small molecule excitation on B₁₂N₁₂ superatoms, we analyzed the UV–Vis absorption spectra, as depicted in Fig. 3. The absorption spectrum of B₁₂N₁₂ (black curve) in Fig. 3 exhibits typical absorption peaks near 167 and 180 nm, which have also been observed in previous studies.⁴⁰ These peaks primarily arise from transitions from HOMO, HOMO−1, HOMO−2, HOMO−3, HOMO−4, and HOMO−5 to LUMO, LUMO+1, LUMO+2, LUMO+4, and LUMO+5. In addition, Table I presents the data for five typical absorption peaks of pyrazine-B₁₂N₁₂. The first prominent absorption peak of this structure (blue curve) is situated at around 234 nm, primarily stemming from the transition from HOMO−10 to LUMO. The second absorption peak, near 183 nm, originates from transitions involving HOMO−4 to LUMO+6, and HOMO to LUMO+2 and LUMO+10. The remaining three typical absorption peaks, located approximately at 186, 216, and 217 nm, result from transitions including HOMO−12 to LUMO, HOMO−10 to LUMO+1, HOMO−5 to LUMO, HOMO−4 to LUMO, HOMO−1 to LUMO+1, and HOMO to LUMO+2. In addition, referring to the frontier MOs diagram [Fig. 2(a)], it becomes evident that the primary absorption peaks of pyrazine-B₁₂N₁₂ stem from transitions involving B₁₂N₁₂ to B₁₂N₁₂ and pyrazine.

Moreover, the data pertaining to the four typical absorption peaks of pyridine-B₁₂N₁₂ are provided in Table I. The first strong absorption peak is positioned at around 202 nm, primarily originating from transitions including HOMO−4, HOMO−5, HOMO−6, HOMO−8, and HOMO−11 to LUMO. The subsequent absorption peak is observed near 225 nm, originating from the transition from HOMO−9 to LUMO. The remaining two typical absorption peaks, located approximately at 182.51 and 183.4 nm, are relatively weak and stem from transitions such as HOMO−3 and HOMO−10 to LUMO and LUMO+1. Consequently, the primary absorption peaks of pyridine-B₁₂N₁₂ predominantly arise from transitions involving B₁₂N₁₂ and pyridine to pyridine.

In addition, we analyzed the absorption spectrum of the benzene-B₁₂N₁₂ structure, represented by the green curve in Fig. 3. The data regarding four typical absorption peaks are provided in Table I. The first prominent absorption peak is situated near 174 nm, stemming from transitions including HOMO and HOMO−1 to LUMO+9, LUMO+11, LUMO+12, and LUMO+13. The second strong absorption peak, positioned at around 172 nm, originates from transitions from HOMO and HOMO−1 to LUMO, LUMO+1, LUMO+11, LUMO+12, and LUMO+13. Furthermore, the remaining two typical absorption peaks, located near 172 and 182 nm, result from transitions such as HOMO and HOMO−1 to LUMO, LUMO+1, LUMO+9, LUMO+11, LUMO+12, and LUMO+13. Hence, the primary absorption peaks of benzene-B₁₂N₁₂ arise from the transition from benzene to B₁₂N₁₂.

From the absorption spectra, it is evident that the adsorption of pyrazine and pyridine results in a reduction in the intensity of the absorption spectrum of B₁₂N₁₂ and induces a redshift in its absorption spectrum, albeit remaining within the ultraviolet range. In contrast, the adsorption of benzene notably amplifies the intensity of the absorption spectrum of B₁₂N₁₂. This phenomenon arises due to the fact that the absorption peaks observed in pyrazine-B₁₂N₁₂ and pyridine-B₁₂N₁₂ primarily originate from transitions from B₁₂N₁₂ to small molecules, whereas the absorption peaks in benzene-B₁₂N₁₂ primarily arise from transitions from benzene to B₁₂N₁₂. For accuracy, we also calculated the absorption spectra of pyrazine-B₁₂N₁₂, pyridine-B₁₂N₁₂, and benzene-B₁₂N₁₂ using CAM-B3LYP/def2-TZVP,⁴¹ based on the optimized structures. As shown in Fig. S1 of the supplementary material, the intensity of absorption spectra is lower than that calculated by CAM-B3LYP/6-31+G*. In addition, the absorption spectral peaks calculated by the two basis sets are slightly shifted.

To elucidate the fluorescence emission characteristics of small molecules adsorbed on B₁₂N₁₂, we conducted an investigation into the single excited states of B₁₂N₁₂, pyrazine-B₁₂N₁₂, pyridine-B₁₂N₁₂, and benzene-B₁₂N₁₂, as depicted in Fig. 4. The fluorescence emission peak of B₁₂N₁₂ appears at around 214 nm (depicted by the black curve), with a relatively low intensity in its emission spectrum. In contrast, the emission spectrum of pyrazine-B₁₂N₁₂ exhibits a higher intensity compared to that of B₁₂N₁₂. Furthermore, the spectrum displays four distinctive emission peaks situated near 251, 262, 271, and 277 nm, as illustrated by the blue curve in Fig. 4. These fluorescence emission peaks of pyrazine-B₁₂N₁₂ originate from transitions between LUMO and LUMO+1 to HOMO, HOMO−3, HOMO−5, HOMO−6, and HOMO−13. In essence, the fluorescence emission peaks of pyrazine-B₁₂N₁₂ predominantly stem from the transition from pyrazine to B₁₂N₁₂. The fluorescence emission wavelengths and the corresponding transition properties are detailed in Table II.

Furthermore, the fluorescence emission spectrum of pyridine-B₁₂N₁₂ (indicated by the red curve in Fig. 4) exhibits a significantly greater intensity compared to that of B₁₂N₁₂ and pyrazine-B₁₂N₁₂. In particular, the emission peaks of pyridine-B₁₂N₁₂ are characterized by three distinct peaks, and the emission data for the three typical peaks are presented in Table II. The first strong emission peak, observed at ∼251 nm, arises from transitions from LUMO to HOMO−3, HOMO−5, HOMO−6, and HOMO−7 orbitals. The remaining two typical emission peaks occur at ∼235 and 238 nm, respectively, originating from transitions from LUMO to HOMO−3, HOMO−5, HOMO−6, and HOMO−11 orbitals. Consequently, the primary emission peaks of pyridine-B₁₂N₁₂ predominantly stem from the transition from pyridine to B₁₂N₁₂. In contrast, the fluorescence emission peaks of benzene-B₁₂N₁₂ are observed at ∼190 and 207 nm, resulting from the transition from B₁₂N₁₂ to benzene. The intensity of its emission spectrum is notably weaker than that of B₁₂N₁₂, pyrazine-B₁₂N₁₂, and pyridine-B₁₂N₁₂, as depicted by the green curve in Fig. 4 and detailed in Table II. Therefore, the adsorption of pyridine and pyrazine not only enhances the emission spectrum of B₁₂N₁₂ but also induces a redshift in the emission spectrum of B₁₂N₁₂, whereas the adsorption of benzene weakens its intensity. This disparity arises from the fact that the emission peaks of pyrazine-B₁₂N₁₂ and pyridine-B₁₂N₁₂ structures transition from small molecules to B₁₂N₁₂, while the emission peaks of benzene-B₁₂N₁₂ transition from B₁₂N₁₂ to small molecules. We also calculated the emission spectra of pyrazine-B₁₂N₁₂, pyridine-B₁₂N₁₂, and benzene-B₁₂N₁₂ using CAM-B3LYP/def2-TZVP (Fig. S1 of the supplementary material). Compared to the emission spectra calculated by CAM-B3LYP/6-31+G*, the spectra calculated using CAM-B3LYP/def2-TZVP exhibit a lower intensity and relative redshift, but it is not significant.

Finally, we analyzed the influence of the number of adsorbed small molecules on the absorption and emission spectra of B₁₂N₁₂ superatoms. The absorption spectra for 2pyrazine-B₁₂N₁₂, 4pyrazine-B₁₂N₁₂, 2pyridine-B₁₂N₁₂, 4pyridine-B₁₂N₁₂, 2benzene-B₁₂N₁₂, and 4benzene-B₁₂N₁₂ are depicted in Fig. 5. Their corresponding structural diagrams are presented in Fig. S2 of the supplementary material. In particular, the absorption spectra of pyrazine-B₁₂N₁₂, 2pyrazine-B₁₂N₁₂, and 4pyrazine-B₁₂N₁₂ are represented by blue, purple, and grayish-blue curves, respectively, in Fig. 5(a). As the number of adsorbed pyrazine molecules increases, the absorption spectra exhibit a redshift, with the spectrum of 2pyrazine-B₁₂N₁₂ showing a notably higher intensity. Furthermore, the absorption spectra of pyridine-B₁₂N₁₂, 2pyridine-B₁₂N₁₂, and 4pyridine-B₁₂N₁₂ are indicated by red, brown, and pink curves in Fig. 5(b). Among these, 2pyridine-B₁₂N₁₂ has the most intense absorption spectrum. A similar redshift occurs as the number of adsorbed pyridine molecules increases, although the extent of this shift varies. For benzene adsorption, the absorption spectra are shown in Fig. 5(c). The spectrum of 2benzene-B₁₂N₁₂ again demonstrates the greatest intensity. While the absorption spectra also redshift with increasing number of adsorbed benzene molecules, the magnitude of this shift is smaller compared to that observed with pyrazine and pyridine adsorption. Notably, benzene adsorption results in the most pronounced enhancement of the absorption spectrum of B₁₂N₁₂.

In addition, Fig. 6 displays the emission spectra of B₁₂N₁₂ with variation in the number of adsorbed small molecules. In particular, the emission spectra of pyrazine-B₁₂N₁₂, 2pyrazine-B₁₂N₁₂, and 4pyrazine-B₁₂N₁₂ are represented by blue, purple, and grayish-blue curves, respectively, in Fig. 6(a). It is evident that the emission spectra intensity decreases progressively as the number of adsorbed pyrazine molecules increases, accompanied by a redshift in the emission peaks. Similarly, an increase in the number of adsorbed pyridine molecules leads to a gradual decrease in emission spectra intensity and a redshift in the emission peaks, as depicted in Fig. 6(b). Previous discussions have indicated that the emission peak of benzene-B₁₂N₁₂ is notably weak. In contrast, the emission peaks of 2benzene-B₁₂N₁₂ and 4benzene-B₁₂N₁₂ are significantly stronger than that of benzene-B₁₂N₁₂, with the emission spectra intensity of 2benzene-B₁₂N₁₂ being the most pronounced, as shown in Fig. 6(c). Moreover, the emission peaks of 4benzene-B₁₂N₁₂ exhibit a redshift relative to those of 2benzene-B₁₂N₁₂.

In summary, the absorption and fluorescence emission spectra of pyrazine-B₁₂N₁₂, pyridine-B₁₂N₁₂, and benzene-B₁₂N₁₂ were investigated using first-principles. Analysis of the frontier MOs revealed that the LUMO and LUMO+1 of pyrazine-B₁₂N₁₂ and pyridine-B₁₂N₁₂ are predominantly localized on the pyrazine and pyridine molecules, respectively. In contrast, the HOMO and HOMO−1 of benzene-B₁₂N₁₂ are primarily located on the benzene molecule. These findings indicate that benzene adsorption notably enhances the absorption spectrum of B₁₂N₁₂, due to the absorption peaks of benzene-B₁₂N₁₂ mainly arising from transitions from benzene to B₁₂N₁₂. Conversely, the adsorption of pyrazine and pyridine significantly enhances the emission spectrum of B₁₂N₁₂, with pyridine showing a stronger effect. This enhancement is attributed to the emission peak transitions from the small molecules to B₁₂N₁₂ for pyrazine-B₁₂N₁₂ and pyridine-B₁₂N₁₂, while those of benzene-B₁₂N₁₂ result from transitions from B₁₂N₁₂ to the benzene molecules. Furthermore, the impact of the number of adsorbed small molecules on the spectra was also analyzed. The results showed that the emission spectra decrease as the number of adsorbed pyrazine or pyridine molecules increases. However, the absorption spectra of 2pyrazine-B₁₂N₁₂ and 2pyridine-B₁₂N₁₂ are more intense. In addition, both the absorption and emission spectra of 2benzene-B₁₂N₁₂ are stronger than those of benzene-B₁₂N₁₂ and 4benzene-B₁₂N₁₂. Therefore, small molecule adsorption enhances the absorption and emission spectra of B₁₂N₁₂, but the effects vary depending on the category and number of adsorbed molecules. Our work offers new insights into the design of luminescent materials.

See the supplementary material for the relative energies, the absorption and emission spectra calculated by CAM-B3LYP/def2-TZVP, and structural diagrams of 2pyrazine-B12N12, 4pyrazine-B12N12, 2pyridine-B12N12, 4pyridine-B12N12, 2benzene-B12N12, and 4benzene-B12N12.

This work was financially supported by the Education Department of Jilin Province (Grant Nos. JJKH20230510KJ and JJKH20180426KJ), the Natural Science Foundation Project of Jilin Province (Grant Nos. YDZJ202101ZYTS075 and 20220101039JC), and the National Natural Science Foundation of China (Grant No. 11947039).

The authors have no conflicts to disclose.

Jia Wang contributed to the initial conception; participated in the calculation work, theoretical analysis, and results presentation and interpretation; and drafted the manuscript. Meiqi Wang participated in drafting. Ming-Xing Song participated in spectral calculation and analysis. Bo Wang contributed to the initial conception and manuscript revisions. Zhengkun Qin supervised the research and contributed to manuscript writing. All authors gave final approval for publication.

Jia Wang: Investigation (lead); Writing – original draft (lead). Meiqi Wang: Methodology (equal). Ming-Xing Song: Formal analysis (equal). Bo Wang: Formal analysis (equal). Zhengkun Qin: Conceptualization (equal).

The data that support the findings of this study are available within the article and its supplementary material.