Benzotriquinoxaline-based discotic liquid crystals stabilizing room temperature columnar self-assembly with high charge carrier mobility

Abstract

Novel large benzotriquinoxaline-based discotic liquid crystals stabilizing room temperature columnar phases are reported herein, where the type of the mesophase, mesophase width and charge carrier mobility along the columns can be modulated by subtle variations in the molecular structure.

---

The development of liquid crystalline materials has garnered remarkable interest because of their unique combination of fluidity and molecular order and also their synthetic tunability, which allows the tuning of their self-assembly and bulk properties. In columnar (Col) phases formed by the discotic liquid crystals (DLCs), the charge transport properties depend on the delicate balance between the molecular structure and their organization within the columnar stacks.¹ The molecular design of large disc-like planar polyaromatic cores decorated with flexible peripheral chains as in the case of triphenylene (TP),² hexaazatriphenylene (HAT),³ hexaazatrinaphthylene (HATN),⁴ and hexabenzocoronene (HBC)⁵ promotes spontaneous self-assembly into Col phases driven by the π–π interactions and nanophase segregation of immiscible molecular units like central aromatic cores and peripheral flexible chains. 2D polymers,⁶ hydrogen-bonded organic frameworks,⁷ and covalent organic frameworks⁸ based on highly conjugated planar cores also exhibit promising charge carrier mobilities, further underscoring the importance of core planarity and stacking in charge transport. This work focuses on the novel benzo[1,2-g:3,4-g′:5,6-g″]triquinoxaline core (BTQ), which has not been explored in stabilizing columnar self-assembly. There are reports on hydrogen-bonded organic frameworks based on BTQ derivatives exhibiting crystalline properties.⁷ The electron-deficient, nitrogen-rich, and planar nature of the pyrazine ring offers several advantages, including the modulation of electronic properties,⁹ enhancement of charge transport,¹⁰ and the ability of the nitrogen atoms to engage in hydrogen bonding,¹¹ providing BTQ with added functionality compared to TP-based systems. Quinoxaline and pyrazino[2,3-g] quinoxaline-based derivatives, having a heteroaromatic molecular structure analogous to BTQ, have been utilized in the fabrication of organic electronic devices such as OLEDs,¹² OFETs,¹³ and OSCs,¹⁴ and in the biological field.¹⁵ Enhanced electron delocalization was observed in the bigger, more planar core when HAT and tri-HAT derivatives were compared.¹⁶ However, with the increase in the core size, there is a trade-off between solubility and processability, which is vital for practical applications. Keeping these factors in mind, we have prepared large planar discotics, BQE and BQA, with the introduction of multiple flexible chains at the periphery, while the connecting groups (ester and amide) were varied to understand the effects on self-assembly. Amide groups provide additional hydrogen bonding interactions, which may increase the order within the column; however, this increases the core–core interaction and hence the clearing point. As discussed below, these derivatives form Col phases at room temperature, and the type of the Col phase and clearing temperature (mesophase width and stability) are dependent on the linking group through which the peripheral phenyl group is connected to the central core.

The synthetic routes of BQE and BQA are illustrated in Schemes S1 and S2. BQE synthesis involved Friedel–Crafts acylation of anisole to 1,2-bis(4-methoxyphenyl)ethane-1,2-dione (2), followed by demethylation to diol 3. Methyl 3,4,5-trihydroxybenzoate (4) underwent Williamson etherification to form 5, then hydrolysed to acid 6, further converted to acid chloride 7, and then coupled with 3 to give 8. BQA synthesis involved benzoin condensation of methyl 4-formylbenzoate to afford 10, followed by oxidation to diketone 11, then hydrolysed to 12, later converted to acid chloride 13, and finally coupled with amine 17. Intermediates 8 and 17 condensed with triphenylene-2,3,6,7,10,11-hexaamine hexahydrochloride to afford BQE (78%) and BQA (48%). All compounds were characterized using ¹H NMR, ¹³C NMR, IR spectroscopy, and MALDI-TOF mass spectrometry. The characteristic shift in the broad amide N–H stretch to ∼3281 cm⁻¹ alongside the C=O amide band at 1649 cm⁻¹ in BQA supports the hydrogen bonding in BQA (Fig. S31).¹⁷ The ¹H NMR studies conducted at three temperatures, 30 °C, 35 °C and 45 °C, showed that the amide protons moved upfield as the temperature was increased, suggesting the weakening of hydrogen bonds at higher temperature (Fig. S25).

The thermal behavior of BQE and BQA was investigated by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and X-ray diffraction (XRD). The TGA analyses show that BQE and BQA are stable with a 5% weight loss at 345 °C and 281 °C, respectively (Fig. 1c and Fig. S32). The mesomorphic behavior of the compounds is presented in Fig. 1b. POM and DSC showed that BQE and BQA have a clearing temperature of 185 °C (ΔH = 3.79 kJ mol⁻¹) and 317 °C (ΔH = 0.76 kJ mol⁻¹), respectively (Table S2). Upon cooling from their isotropic liquids, both compounds show texture (Fig. S33 and S34). The DSC cooling curve of BQE shows transition peaks at ∼178 °C and ∼109 °C, while that of BQA shows a single transition peak at ∼309 °C (Fig. S35). It is interesting to note that both these compounds show no crystallization down to room temperature, while a mesophase-to-mesophase transition was observed in DSC. It was hard to differentiate between the textures at different temperatures (Fig. S33 and S34), and thus, to understand the symmetry of the Col phases, XRD analysis of BQE at 140 °C, 70 °C, and 28 °C, and of BQA at 230 °C and 30 °C, was carried out. The XRD patterns of BQE at various temperatures were similar. The XRD pattern at 28 °C (Fig. 2c) shows three strong reflections in the small-angle region (2θ ≈ 2°–3°) and several weak reflections in the mid-angle region (2θ ≈ 3°–10°). The reflections can be indexed to Miller indices (10), (01), (01), (11), (20), (12), (22), (32), and (42) of a columnar rectangular lattice with lattice parameters a = 41.7 Å and b = 38.5 Å. Furthermore, a diffuse peak was present in the wide-angle region (2θ ≈ 21°). At 28 °C, the lattice parameter a (41.7 Å) exceeds the molecular diameter, whereas the lattice parameter b (38.5 Å) is 7% shorter than the molecular diameter. The lattice area (S) and volume (V) of the rectangular unit cell, determined from XRD at 28 °C, were found to be 1602.7 Ų and 6923.6 ų, respectively. The number of molecules forming a rectangular unit cell (Z) was ∼1 across the different temperature intervals of the mesophase range (Table S3).

Fig. 1 Structures of DLCs (a); bar graphs representing their mesomorphic behavior considering the first cooling of the DSC thermogram (b); and thermal stability obtained from the TGA plots (c).

Fig. 2 POM images of BQE (a) and BQA (b); plot of the intensity against 2θ obtained from the powder XRD pattern of the Col_r phase of BQE (c) at 28 °C, and the same for the Col_h phase of BQA (d) at 30 °C; schematics showing the self-assembly of BQE into the Col_r phase (at 28 °C) (e) and self-assembly of BQA into the Col_h phase (f) (at 30 °C).

The intensity vs. 2θ plots at 230 °C and 30 °C of BQA also reveal a similar pattern. At 30 °C (Fig. 2d), a prominent sharp peak in the small-angle region (2θ ≈ 2°) was observed with a d-spacing of 39.6 Å, which can be indexed to the (10) Miller index. In the mid-angle region (2θ ≈ 4°–6°), three reflections are present, which can be indexed to the (11), (20), and (21) indices of a columnar hexagonal lattice. A diffuse peak with a d-spacing of 4.4 Å was observed in the wide-angle region (2θ ≈ 20°) corresponding to the packing of flexible alkyl chains. The hexagonal lattice parameter (a), surface area (S), and volume (V) of the hexagonal unit cell at 30 °C were determined to be 45.8 Å, 1814.5 Ų, and 7946.3 ų, respectively. The number of molecules forming a hexagonal unit cell (Z) was calculated to be ∼1 at both temperatures (Table S3). The hexagonal lattice parameter a obtained at 30 °C (45.8 Å) was 19% lower than the estimated molecular diameter of 56.8 Å, suggesting that the peripheral flexible chains interdigitate within neighbouring columns.

Next, the photophysical properties of the derivatives were studied by measuring absorption and emission in micromolar chloroform solution and in thin film (Fig. 3). Both derivatives exhibit a similar absorption pattern, with differing peak intensities. BQE and BQA have absorption maxima at 290–295 nm and 347–348 nm, respectively, with high molar extinction coefficients (Table S4). The optical band gaps of the compounds calculated from the red edge of the absorption spectra were both 2.77 eV. Their absorption in the thin-film state shows a pattern similar to that in solution (Fig. 3). The emission spectra of the BQE and BQA thin films show maxima at 485 nm and 573 nm, respectively. The difference in the connectivity of the derivatives possibly resulted in different photophysical behavior. The Stokes shift for BQE in the thin-film state is 15 229 cm⁻¹, which is higher than that in the solution state (12 011 cm⁻¹) (Table S4). For BQA, the Stokes shift in the thin-film state (12 042 cm⁻¹) is much greater than that in solution (7368 cm⁻¹), while the emission intensity was low in comparison to BQE (Table S4). The absolute quantum yield in solution was found to be ∼55% for BQE, while BQA showed only 0.4%. In the solid state, both had low values of ∼2% and 0.4%, respectively. Concentration-dependent studies of the more emissive BQE in chloroform solutions showed a decrease in emission intensity with increasing concentration (Fig. S38). Cyclic voltammetry (CV) was performed on BQE and BQA in anhydrous dichloromethane. It was observed that BQA showed slightly higher LUMO and HOMO levels (Fig. 4 and Fig. S39, Table S5).

Fig. 3 Normalized absorption and emission spectra of BQE (a) and BQA (b) in chloroform (solid line) and in the thin film state (dotted line). Insets: Digital photographs of the compounds drop-casted on quartz substrates and micromolar chloroform solutions of the compounds under long-wavelength UV light.

Fig. 4 Cyclic voltammograms of BQE and BQA in millimolar dichloromethane solutions (a). Energy band level diagram showing the HOMO and LUMO energy levels (b).

Density functional theory (DFT) studies reveal that both derivatives have similar LUMO contours, which are localized primarily on the benzotriquinoxaline core. In BQE, the HOMO is localized on the benzotriquinoxaline core and to a little extent on the dangling phenyl groups. But, in BQA, it was localized on one of the terminal phenyl groups (Fig. S41). The electrostatic potential map of the compounds shows that electron density is concentrated on heteroatoms, while in other regions the electron density varies from electron rich to neutral (Fig. S40). This visualizes the electron-rich and electron-deficient regions in the molecule and hence the possibility of ambipolar charge carrier mobility.

These structural features and their propensity to self-assemble into one dimensional (1D) columnar structures motivated us to explore their charge transport properties using the space-charge-limited current (SCLC) technique. The hole and electron mobilities in the BQE compound have been investigated by fabricating hole-only (ITO/PEDOT:PSS/BQE/Au) and electron-only (ITO/ZnO/BQE/LiF/Ag) SCLC devices (Fig. S42). The thickness and surface area of the BQE compound were controlled at approximately 5 µm and 25 mm², respectively. The current density vs. voltage (J–V) curves were realised after verifying that the BQE molecules in the device were aligned optimally (Fig. S43). The J–V curves are shown in Fig. 5.

Fig. 5 Current density versus voltage (J–V) curves of (a) hole-only and (b) electron-only SCLC devices at 30 °C. The insets show the distribution of the energy levels of different layers of the SCLC devices. The second inset of (a) presents the molecular alignment of the BQE compound in the SCLC device. The thickness and surface area of the active material (i.e., BQE) were ∼5 µm and 25 mm², respectively, and the relative permittivity of BQE was 2.33.

Assuming a minimum influence of traps, the Mott–Gurney equation J = 9ε₀εµ_(h,e)V²/8d³ was used to determine the hole and electron mobility. In the hole-only device (Fig. 5a), the charges are injected by the Au electrodes, which further migrate through the material. At low applied voltage, the current density follows Ohm's law (J ∝ V); however, on increasing the voltage, the SCLC region appears in which the current density follows quadratic behaviour (J ∝ V²).^18–20 These two regions are clearly visible in Fig. 5a, resulting in a room temperature hole mobility of 3.74 ± 0.37 × 10⁻² cm² V⁻¹ s⁻¹ for the BQE compound. This high hole mobility is associated with the highly ordered Col_r phase, demonstrated by the consistently oriented columns, as shown in the optical texture (inset of Fig. 5a). This high value of hole mobility is superior to many prior reports and is one of the highest values recorded so far.^18,21,22 Conversely, the room temperature electron mobility of the BQE compound is around 4.57 ± 0.45 × 10⁻³ cm² V⁻¹ s⁻¹, which is an order of magnitude lower than its hole mobility (i.e., µ_h > µ_e). The charge carrier mobilities of the BQE and BQA compounds are tabulated in Table 1.

Table 1 Comparison of charge carrier mobility at 30 °C

Compounds	Hole mobility (cm^2 V^−1 s^−1)	Electron mobility (cm^2 V^−1 s^−1)
BQE	3.74 ± 0.37 × 10^−2	4.57 ± 0.45 × 10^−3
BQA	Non-extractable	1.57 ± 0.15 × 10^−2

---

The molecular structure of compound BQA slightly differs from that of BQE, with the variation of the linking group from ester to amide. The presence of amide units directs the columns to organize in a hexagonal fashion, while leading to a higher clearing temperature (above 300 °C), due to their ability to form intermolecular hydrogen bonding. The exceptionally elevated isotropic temperature enhances the mesophase width; yet, it introduces a difficulty in thermal annealing during device fabrication. The electron-only device of the BQA compound exhibited an electron mobility of 1.57 ± 0.15 × 10⁻² cm² V⁻¹ s⁻¹ (Fig. S44). Regrettably, the hole-only device failed to produce consistent J–V curves; hence, we could not determine the hole mobility of the BQA compound. Analogous liquid crystalline electron-deficient hexaazatriphenylene derivatives showed a higher electron mobility of <0.01–0.59 cm² V⁻¹ s⁻¹ at room temperature when measured using the pulse-radiolysis time-resolved microwave-conductivity technique.²³

In conclusion, we report the first examples of benzotriquinoxaline-based molecules that exhibit room temperature columnar self-assembly and high charge carrier mobility. Compound BQE is an exceptional ambipolar organic semiconductor with high room temperature hole and electron mobilities of 3.74 ± 0.37 × 10⁻² and 4.57 ± 0.45 × 10⁻³ cm² V⁻¹ s⁻¹, respectively. Conversely, the BQA compound demonstrated an electron mobility of 1.57 ± 0.15 × 10⁻² cm² V⁻¹ s⁻¹; however, we were unable to ascertain the hole mobility of the BQA compound due to inconsistencies. Further structural engineering can be done with the introduction of branched chains to reduce the clearing point, making it useful for further exploration. We have also shown that such polyaromatic molecules can be made solution processable and can exhibit room temperature columnar phases by proper molecular design. The enhanced charge carrier mobilities exhibited by these molecules have great potential in organic electronics.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: experimental details, NMR, mass spectrometry, thermal behavior, XRD studies, photophysical studies, electrochemical studies, DFT studies and charge carrier mobility studies. See DOI: https://doi.org/10.1039/d6cc00678g.

Acknowledgements

ASA sincerely thanks the Science and Engineering Board (SERB) DST, Govt. of India and BRNS-DAE for funding this work through project no. CRG/2018/000362 and no. 2012/34/31/BRNS/1039, respectively. We thank the Ministry of Human Resource and Development for the Centre of Excellence in FAST (F. no. 5-7/2014-TS-VII). We are thankful to Dorothée Dewaele for the CHNSO experiments. DPS is thankful to ULCO for BQR 2026 funding.

References

1. V. Lemaur, D. A. da Silva Filho, V. Coropceanu, M. Lehmann, Y. Geerts, J. Piris, M. G. Debije, A. M. van de Craats, K. Senthilkumar, L. D. A. Siebbeles, J. M. Warman, J.-L. Brédas and J. Cornil, J. Am. Chem. Soc., 2004, 126, 3271–3279.
2. (a) M. Vadivel, I. S. Kumar, K. Swamynathan, V. A. Raghunathan and S. Kumar, ChemistrySelect, 2018, 3, 8763–8769; (b) A. Gowda, L. Jacob, N. Joy, R. Philip and S. Kumar, New J. Chem., 2018, 42, 19034–19042; (c) T. R. Zhang, P. G. Hu, W. H. Yu, Y. Shi, S.-K. Xiang, P. Hu, K. Q. Zhao, B. Wang, Y.-Y. Gui and C. Feng, Cryst. Growth Des., 2023, 23, 4424–4434; (d) T. R. Zhang, P. G. Hu, W. H. Yu, Q. G. Li, Y. Shi, S. K. Xiang, K. Q. Zhao, B. Q. Wang and C. Feng, J. Mol. Liq., 2024, 408, 125399.
3. (a) B. Gao, L. Zhang, Q. Bai, Y. Li, J. Yang and L. Wang, New J. Chem., 2010, 34, 2735–2738; (b) A. Hinokimoto, T. Ono, M. Fujiwara, H. Mori, R. Akiyoshi, S. Nakamura, O. Tsutsumi, A. Saeki, Y. Kitagawa, S. Horike and D. Tanaka, Chem. – Asian J., 2022, 17, e202200225.
4. M. C. Yeh, S. C. Liao, S. H. Chao and C. W. Ong, Tetrahedron, 2010, 66, 8888–8892.
5. (a) W. Wang, X. Liu and J. Pu, Molecules, 2011, 16, 9101–9108; (b) W. W. J. Wong, J. Subbiah, S. R. Puniredd, B. Purushothaman, W. Pisula, N. Kirby, K. Müllen, D. J. Jones and A. B. Holmes, J. Mater. Chem., 2012, 22, 21131–21137; (c) N. Hu, R. Shao, Y. Shen, D. Chen, N. A. Clark and D. M. Walba, Adv. Mater., 2014, 26, 2066–2071.
6. S. Jhulki, J. Kim, I. C. Hwang, G. Haider, J. Park, J. Y. Park, Y. Lee, W. Hwang, A. A. Dar, B. Dhara, S. H. Lee, J. Kim, J. Y. Koo, M. H. Jo, C. C. Hwang, Y. H. Jung, Y. Park, M. Kataria, Y. F. Chen, S. H. Jhi, M. H. Baik, K. Baek and K. Kim, Chem, 2020, 6, 2035–2045.
7. I. Hisaki, Q. Ji, K. Takahashi, N. Tohnai and T. Nakamura, Cryst. Growth Des., 2020, 20, 3190–3198.
8. (a) J. Guo, Y. Xu, S. Jin, L. Chen, T. Kaji, Y. Honsho, M. A. Addicoat, J. Kim, A. Saeki, H. Ihee, S. Seki, S. Irle, M. Hiramoto, J. Gao and D. Jiang, Nat. Commun., 2013, 4, 2736; (b) X. Cao, Q. Shi, Y. Feng, X. Zhang, E. Zhou, L. Zhu and Y. Wang, Mater. Today Chem., 2022, 26, 101082; (c) Y. Jiang, H. Jung, S. H. Joo, Q. K. Sun, C. Li, H. J. Noh, I. Oh, Y. J. Kim, S. K. Kwak, J. W. Yoo and J. B. Baek, Angew. Chem., Int. Ed., 2021, 60, 17191–17197.
9. (a) H. Wang, Y. Wen, X. Yang, Y. Wang, W. Zhou, S. Zhang, X. Zhan, Y. Liu, Z. Shuai and D. Zhu, ACS Appl. Mater. Interfaces, 2009, 1, 1122–1129; (b) S. Ma, J. Wang, K. Feng, H. Zhang, Z. Wu, Y. Wang, B. Liu, Y. Li, M. An, R. Gonzalez-Nuñez, R. P. Ortiz, H. Y. Woo and X. Guo, ACS Appl. Mater. Interfaces, 2023, 15, 1639–1651.
10. (a) Y. Shi, Y. Zhang and X. Cai, Chem. Phys. Lett., 2021, 772, 138595; (b) C. P. Yu, S. Kumagai, T. Kushida, M. Mitani, C. Mitsui, H. Ishii, J. Takeya and T. Okamoto, J. Am. Chem. Soc., 2022, 144, 11159–11167.
11. (a) A. Jana, P. Mishra and N. Das, Beilstein J. Nanotechnol., 2019, 10, 494–499; (b) Q. Ji, K. Takahashi, S. Noro, Y. Ishigaki, K. Kokado, T. Nakamura and I. Hisaki, Cryst. Growth Des., 2021, 21, 4656–4664.
12. (a) K. R. J. Thomas, M. Velusamy, J. T. Lin, C. H. Chuen and Y. T. Tao, Chem. Mater., 2005, 17, 1860–1866; (b) S. Chen, X. Xu, Y. Liu, W. Qiu, G. Yu, H. Wang and D. Zhu, J. Phys. Chem. C, 2007, 111, 1029–1034; (c) V. K. Vishwakarma, S. Nath, M. Gupta, D. K. Dubey, S. S. Swayamprabha, J. H. Jou, S. K. Pal and A. A. Sudhakar, ACS Appl. Electron. Mater., 2019, 1, 1959–1969; (d) V. K. Vishwakarma, M. R. Nagar, N. Lhouvum, J. H. Jou and A. A. Sudhakar, Adv. Opt. Mater., 2022, 10, 2200241.
13. (a) H. You, H. Kang, D. Kim, J. S. Park, J. W. Lee, S. Lee, F. S. Kim and B. J. Kim, ChemSusChem, 2021, 14, 3520–3527; (b) Z. Dai, D. Zhang and H. Zhang, Front. Chem., 2022, 10, 934203.
14. (a) J. Yang, M. A. Uddin, Y. Tang, Y. Wang, Y. Wang, H. Su, R. Gao, Z. K. Chen, J. Dai, H. Y. Woo and X. Guo, ACS Appl. Mater. Interfaces, 2018, 10, 23235–23246; (b) H. Liu, Y. Geng, Z. Xiao, L. Ding, J. Du, A. Tang and E. Zhou, Adv. Mater., 2024, 36, 2404660; (c) L. P. Zhang, K. J. Jiang, G. Li, Q. Q. Zhang and L. M. Yang, J. Mater. Chem. A, 2014, 2, 14852–14857.
15. (a) A. K. Parhi, Y. Zhang, K. W. Saionz, P. Pradhan, M. Kaul, K. Trivedi, D. S. Pilch and E. J. LaVoie, Bioorg. Med. Chem. Lett., 2013, 23, 4968–4974; (b) C. Ma, M. S. Taghour, A. Belal, A. B. M. Mehany, N. Mostafa, A. Nabeeh, I. H. Eissa and A. A. Al-Karmalawy, Front. Chem., 2021, 9, 725135; (c) Y. Hong, W. Geng, T. Zhang, G. Gong, C. Li, C. Zheng, F. Liu, J. Qian, M. Chen and B. Z. Tang, Angew. Chem., Int. Ed., 2022, 61, e202209590; (d) D. Chen, J. Shao, T. Zhang, K. Xu, C. Liang, Y. Cai, Y. Guo, P. Chen, X. Z. Mou and X. Dong, Nano Lett., 2024, 24, 7524–7533.
16. R. Juárez, M. M. Oliva, M. Ramos, J. L. Segura, C. Alemán, F. Rodríguez-Ropero, D. Curcó, F. Montilla, V. Coropceanu, J. L. Brédas, Y. Qi, A. Kahn, M. C. R. Delgado, J. Casado and J. T. L. Navarrete, Chem. – Eur. J., 2011, 17, 10312–10322.
17. (a) R. I. Gearba, M. Lehmann, J. Levin, D. A. Ivanov, M. H. J. Koch, J. Barbera, M. G. Debije, J. Piris and Y. H. Geerts, Adv. Mater., 2003, 15, 1614–1618; (b) K. Hanabusa, C. Koto, M. Kimura, H. Shirai and A. Kakehi, Chem. Lett., 1997, 429.
18. A. Patra, P. K. Behera, A. Shah, D. P. Singh, V. A. Raghunathana, A. S. Achalkumar and S. Kumar, ACS Appl. Electron. Mater., 2024, 6(7), 4891–4902.
19. Y. Cao, T. Nirgude, F. Dubois, D. P. Singh, F. Xi, F. Liu and M. Alaasar, J. Mater. Chem. C, 2026, 14, 1799–1804.
20. S. Vanishree Bhat, D. P. Singh, F. Dubois, V. A. Raghunathan, A. Roy and S. Kumar, ACS Appl. Electron. Mater., 2024, 6(12), 8828–8837.
21. A. Shah, V. K. Vishwakarma, N. Lhouvum, A. S. Achalkumar, P. Kumar, A. K. Srivastava, F. Dubois, T. Chomchok, N. Chattham and D. P. Singh, J. Mol. Liq., 2024, 393, 123535.
22. M. Lehmann, N. Scheuring, L. Mager, D. P. Singh, R. Mandle and A. Eremin, Adv. Sci., 2025, 12(38), e10009.
23. (a) G. Kestemont, V. de Halleux, M. Lehmann, D. A. Ivanov, M. Watson and Y. H. Geerts, Chem. Commun., 2001, 2074–2075; (b) M. Lehmann, G. Kestemont, R. G. Aspe, C. Buess-Herman, M. H. J. Koch, M. G. Debije, J. Piris, M. P. de Haas, J. M. Warman, M. D. Watson, V. Lemaur, J. Cornil, Y. H. Geerts, R. Gearba and D. A. Ivanov, Chem. Eur. J., 2005, 11, 3349–3362.

---