Role of flexible bulky groups and weak interactions involving halogens in the vapoluminescence of a metal-free dye

Abstract

A metal-free pyrazine dye exhibited vapoluminescence when exposed to benzene vapour. Single-crystal X-ray analysis revealed that its flexible bulky groups and weak interactions involving halogens play an essential role in the occurrence of the observed vapoluminescence.

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Materials showing a change in their optical properties in response to external stimuli have attracted attention due to their potential application as sensor, switching, and memory devices.¹ Among the various external stimuli, the change of colour upon exposure to solvent vapours, called vapochromism, can be used to detect volatile organic compounds (VOCs) in the environment and develop novel functional devices. Moreover, several VOC components of human breath are considered to be biomarkers for diseases such as cancer,² and thus vapochromic materials might also enable easier disease diagnosis. Many of the reported vapochromic materials contain metal atom(s) and can be termed organometallic compounds, whereas few metal-free organic compounds have been reported.³ To improve the versatility of these materials, it is important to develop organic vapochromic compounds. Effective strategies for the molecular design of organic vapochromic materials were proposed in the scarce reports containing complete structural data for the vapour reaction. For example, alteration of intermolecular interactions, such as the π–π interaction,⁴ and the interactions between donor and acceptor substituents by inclusion of solvent molecules,⁵ in addition to host–guest interactions,⁶ plays an essential role in the occurrence of vapochromism in non-fluorescent organic compounds. These reports suggest that the formation of solvates by exposure to solvent vapours contributes to the improvement of vapour sensing selectivity. Several organic vapoluminescent compounds containing an anthracene unit with bulky substituents were reported, exhibiting fluorescent properties that are tuned by inclusion of vapour molecules.⁷ However, this strategy is difficult to apply to the various well-known fluorophores due to the general occurrence of aggregation-caused quenching (ACQ) in crystalline organic dyes.⁸ In contrast, aggregation-induced emission (AIE) and crystallization-induced emission (CIE), which prevent the occurrence of ACQ, were also found to be an effective strategy for vapoluminescent materials, despite the fact that no inclusion of solvent molecules into the crystal structure upon exposure to solvent vapours was observed for the reported compounds exhibiting AIE and CIE.⁹ Additionally, many publications on the vapoluminescence were not included the detailed crystal structure data. Herein, we report the vapoluminescence of a purely organic 2,5-diamino-3,6-dicyanopyrazine dye, where the fluorophore-independent factors, i.e. bulky groups with high molecular flexibility and halogen substituents are essential for the occurrence of vapoluminescence. These two factors were found to contribute to the inhibition of fluorophore aggregation and favour the formation of solvate upon exposure to benzene vapour.

Compound 1 (2,5-bis[N,N-di-(4-chlorophenyl)methylamino]-3,6-dicyanopyrazine, Fig. 1) has five crystal forms with different colours. Among them, crystal structures of the three non-solvate forms (1Y, 1YO, and 1R) have been previously reported,¹⁰ although the relationship between crystal structures and measured optical properties were not found. These forms were crystallized from several solvent combinations, whereas the crystals of 1O were only obtained from benzene solution.^10b The results of thermal analysis and powder X-ray diffraction (XRD) suggest that 1O can be regarded as a novel solvate form (Fig. S1 and S2†). Interestingly, powdered 1O could be immediately obtained by adding a small amount of benzene to powdered 1Y (Fig. S3†). In addition, 1O was converted to 1Y by heating at 35 °C for ca. 1 h. It is also possible to obtain 1O by treating 1Y with benzene vapour at room temperature (Fig. 2a and S4†). The absorption and emission maxima of 1Y in the visible region were observed at 484 and 558 nm, respectively (Fig. 2b). The broad single peak was shifted to longer wavelengths upon exposure of 1Y (λ_max = 508 nm, F_max = 581 nm) to benzene vapour. We have previously reported that the crystal colour of 1Y was correlated with molecular deformation, particularly with changes of the amino group geometry, rather than with intermolecular interactions, such as exciton interaction.^10a The exciton interaction of 1O calculated by the expanded dipole model¹¹ brought a negligible bathochromic shift of ca. 26 cm⁻¹, since the distance between the fluorophores was large due to the bulky groups. In addition, both the absorption and emission spectra peaks and the high fluorescence quantum yields of 1O and 1Y were similar to those of 1 in CHCl₃ solution (Fig. S5†). These results strongly suggest that the electronic states of the monomer dominate the optical properties of 1O and 1Y. The optical properties of the monomer in this crystalline dye are expected to correlate with the inhibition of molecular aggregation by the bulky groups.

Fig. 1 Chemical structure of compound 1 and its five crystal forms.

Fig. 2 (a) Colour change of 1Y induced by exposure to benzene vapour. (b) Absorption and emission spectra of 1O and 1Y.

Single-crystal XRD analysis revealed that the crystal structure of 1O exhibited a 1 : 2 ratio of compound 1 and benzene molecules (Table S1†). Conformational analysis indicated that the chromophore of 1O, which contains a pyrazine ring, cyano groups, and amino groups, was relatively planar, similarly to 1Y, whereas the amino group geometry of 1O was different from that of 1Y (Fig. 3a and b). The sum of the three angles around the amino nitrogen in 1O was 359.6°, thus the hybridisation of the nitrogen was regarded as sp²-like. In contrast, the hybridisation of the amino nitrogen of 1Y was considered as sp³-like.^10b The dihedral angles between the amino group and the pyrazine ring in 1O and 1Y were 28.8° and 43.8°, respectively. The amino groups of 1O are expected to be better electron donors for the chromophore than those of 1Y. The absorption spectra calculated by TD-DFT (B3LYP/6-31G*+LanL2DZ) based on each molecular conformation in the crystal structures of 1O and 1Y were consistent with the observed absorption spectra (Table S2†). The calculated absorption in the visible region for the two forms was assigned to the HOMO–LUMO transition. In 1O, the level of the HOMO, which has contributions from the π-electrons of the pyrazine ring and the lone pairs of amino groups, was destabilized compared to that of 1Y, whereas the LUMOs of both forms had essentially the same energy (Fig. 3c and d). Particularly, the HOMOs of the two forms had differing contributions of the amino groups, revealing that the different colour of 1O and 1Y originated from their deformation.

Fig. 3 Conformational difference between (a) 1O and (b) 1Y in the observed crystal structures. The red dashed line represents the amino group. (c) Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of 1O and (d) 1Y calculated by TD-DFT.

To understand the mechanism of benzene inclusion, we compared the crystal structures of 1O and 1Y. In the former, the dye molecules formed 1-D chromophore stacking columns, with benzene molecules positioned between these columns, more specifically, between benzyl groups (Fig. 4a). The spaces occupied benzene molecules in the crystal structure of 1O were not seen for 1Y, although the dye molecules in 1O and 1Y were found to form the same 1-D chromophore stacking columns.^10b The presence or absence of the spaces in the crystal structure resulted from the subtle conformational difference between the two forms in the position of the benzyl group (Fig. 4b). In other words, this difference generated the spaces between the 1-D chromophore stacking columns. The benzyl group in 1Y can move to the position corresponding to 1O upon exposure to benzene vapour (Fig. 4c), which contributes to the generation of suitable spaces for the inclusion of benzene molecules in the crystal structure of 1O (Fig. 4d). One of the possible driving forces of the conformational change was the difference in the thermodynamical stability of these two conformations. The calculation of conformational energy in the gas phase using B3LYP/6-31G* resulted that the conformation of 1O was more stable than that of 1Y. The energy difference was 4.9 kcal mol⁻¹. In contrast, the lattice energy calculated by the atom–atom Coulomb–London–Pauli model¹² revealed that the crystal structure of 1O was less stable than that of 1Y. The energy difference was 31.3 kcal mol⁻¹. These results indicated that the more stable crystal form was 1Y. However, when the intermolecular interactions of the crystal packing of 1Y become weaker by external stimuli such as exposure to solvent vapour, the conformation of 1Y can convert to that of 1O by the conformational energy difference. Furthermore, weak interactions played an important role in the stabilization of the conformational deformation by exposure to benzene vapour. In the intermolecular interactions between the 1-D chromophore stacking columns (Fig. S6†), there found the intermolecular C–H⋯π/C interaction and weak interactions such as C–H⋯Cl and C–Cl⋯π/C interactions based on the short contacts in 1O, whereas such intermolecular interactions were not observed in 1Y (Table S3†). The interaction energy of a molecular pair between the 1-D columns of 1O (−10.0 kcal mol⁻¹) was lower than that of 1Y (−7.3 kcal mol⁻¹), although the distance between the molecular pair of 1O (11.670 Å) was almost the same as that of 1Y (11.684 Å). In addition, the proportion of coulombic term in the interaction energy in 1O (10.8%) was larger than that of 1Y (6.9%). These results suggested that the electrostatic attraction by weak interactions contributed to the stabilization of the conformational deformation by benzene vapour in the crystalline state. Particularly, weak interactions involving the terminal halogen substituents of the benzyl groups can play a significant role in the inclusion of benzene molecules via the conformational deformation. This hypothesis was experimentally supported by examining an analogous crystalline dye 2Y, which is the yellow crystal form of a compound 1 derivative with methyl groups in place of Cl atoms. 2Y did not exhibit vapochromism upon exposure to benzene vapour (Fig. S7†), despite its crystal structure being isomorphous with that of 1Y.^10b Moreover, no vapochromism upon exposure to benzene vapour was also observed for 3Y, which is the yellow crystal form of a compound 1 derivative with Br atoms in place of Cl atoms (Fig. S7†). The molecular conformations of 3Y and 1Y were the same, whereas their crystal structures exhibited a subtle difference in benzyl group packing.^10b These results suggest that both the weak interactions involving halogens and the packing of benzyl groups have a great impact on the formation of spaces for benzene molecules in the crystal structures. As a result, the ability to exhibit vapochromism is mainly influenced by the terminal substituents of the benzyl groups as well as by conformational flexibility of the pyrazine dye.

Fig. 4 (a) Crystal structure of 1O. The benzene molecules (shown in blue) are packed between the 1-D chromophore stacking columns (shown in green squares). (b) Conformational difference between the benzyl groups of 1O (shown in orange) and 1Y (shown in green). The red squares represent a significantly different shape of the benzyl group in the two motifs. (c) Crystal structures of 1Y and (d) 1O viewed down the b-axis. The red squares in (c) and (d) indicate a significantly different shape of the benzyl group compared to (b). The benzyl groups of 1Y presumably moved in the direction of the red arrows in (c) upon exposure to benzene vapour.

Exposure of 1Y to vapours of solvents other than benzene did not induce a reversible change of colour (Fig. S8a†). Pyridine, cyclohexane, and chloroform vapours induced an irreversible colour change of 1Y from yellow to orange (Fig. S8b†). The crystals of 1Y presumably dissolved in these vapours due to the high solubility of 1Y and the high vapour pressure of these solvents. This observation suggested that the spaces for solvent molecules of the crystal structure could not be varied by solvent size. In addition, the strength of the interactions between the pyrazine dye and solvent molecules influenced the formation of solvates upon exposure to solvent vapours.

Conclusions

In summary, exposure to benzene vapour led to a reversible change in the optical properties of the yellow form of the pyrazine dye. The change of crystal colour resulted from the deformation of the molecular conformation. The flexibility of the benzyl groups and the interactions involving their terminal substituents played an important role in the formation of the benzene solvate. These results indicated that the flexible bulky benzyl groups with halogen substituents can contribute to the development of novel organic vapoluminescent materials without metal atoms.

Acknowledgements

This research was partially supported by the Sasakawa Scientific Research Grant from The Japan Science Society.

Notes and references

1. (a) J. R. Askim, M. Mahmoudi and K. S. Suslick, Chem. Soc. Rev., 2013, 42, 8649; (b) Z. Chi, X. Zhang, B. Xu, X. Zhou, C. Ma, Y. Zhang, S. Liu and J. Xu, Chem. Soc. Rev., 2012, 41, 3878; (c) M. Irie, Chem. Rev., 2000, 100, 1685; (d) A. Fihey, A. Perrier, W. R. Browne and D. Jacquemin, Chem. Soc. Rev., 2015, 44, 3719; (e) C. Reus and T. Baumgartner, Dalton Trans., 2016, 45, 1850.
2. (a) M. Hakim, Y. Y. Broza, O. Barash, N. Peled, M. Philips, A. Amann and H. Haick, Chem. Rev., 2012, 112, 5949; (b) M. Phillips, K. Gleeson, J. M. B. Hughes, J. Greenberg, R. N. Cataneo, L. Baker and W. P. McVay, Lancet, 1999, 353, 1930; (c) W. Miekisch, J. K. Schubert and G. F. E. Noeldge-Schomburg, Clin. Chim. Acta, 2004, 347, 25.
3. (a) X. Zhang, B. Li, Z. H. Chen and Z. N. Chen, J. Mater. Chem., 2012, 22, 11427; (b) O. S. Wenger, Chem. Rev., 2013, 113, 3686.
4. H. Xia, D. Liu, K. Song and Q. Miao, Chem. Sci., 2011, 2, 2402.
5. E. Takahashi, H. Takaya and T. Naota, Chem.–Eur. J., 2010, 16, 4793.
6. (a) M. Kaftory, H. Taycher and M. Botoshansky, J. Chem. Soc., Perkin Trans. 2, 1998, 407; (b) K. Fujii, A. Sakon, A. Sekine and H. Uekusa, Cryst. Growth Des., 2011, 11, 4305; (c) J. L. Scott, A. Almesaker, Y. Sumi and K. Tanaka, Cryst. Growth Des., 2007, 7, 1049.
7. (a) H. Naito, Y. Morisaki and Y. Chujo, Angew. Chem., Int. Ed., 2015, 54, 5084; (b) P. Rajamali, P. Gandeepan, M. J. Huang and C. H. Cheng, J. Mater. Chem. C, 2015, 3, 3329; (c) T. H. Brehmer, P. P. Korkas and E. Weber, Sens. Actuators, 1997, B44, 595.
8. (a) J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer, 3rd edn, 2006; (b) Y. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Commun., 2009, 4332.
9. (a) M. Tanioka, S. Kamino, A. Muranaka, Y. Ooyama, H. Ota, Y. Shirasaki, J. Horigome, M. Ueda, M. Uchiyama, D. Sawada and S. Enomoto, J. Am. Chem. Soc., 2015, 137, 6436; (b) X. Luo, W. Zhao, J. Shi, C. Li, Z. Liu, Z. Bo, Y. Q. Dong and Z. Tang, J. Phys. Chem. C, 2012, 116, 21967; (c) Z. He, L. Zhang, J. Mei, T. Zhang, J. W. Y. Lam, Z. Shuai, Y. Q. Dong and B. Z. Tang, Chem. Mater., 2015, 27, 6601; (d) B. K. An, S. K. Kwon, S. D. Jung and S. Y. Park, J. Am. Chem. Soc., 2002, 124, 14410; (e) B. K. An, S. K. Kwon and S. Y. Park, Bull. Korean Chem. Soc., 2005, 26, 1555; (f) S. J. Yoon, J. W. Chung, J. Gierschner, K. S. Kim, M. G. Choi, D. Kim and S. Y. Park, J. Am. Chem. Soc., 2010, 132, 13675; (g) M. S. Yuan, D. E. Wang, P. Xue, W. Wang, J. C. Wang, Q. Tu, Z. Liu, Y. Zhang and J. Wang, Chem. Mater., 2014, 26, 2467; (h) P. S. Hariharan, D. Moon and S. P. Anthony, J. Mater. Chem. C, 2015, 3, 8381.
10. (a) S. Matsumoto, Y. Uchida and M. Yanagita, Chem. Lett., 2006, 35, 654; (b) Y. Akune, H. Gontani, R. Hirosawa, A. Koseki and S. Matsumoto, CrystEngComm, 2015, 17, 5789 . CSD refcode: 1R (KELFOX), 1Y (KELFOX01), 1YO (KELFOX02), 2Y (HUMFIG), 3Y (KELGAK).
11. T. Kobayashi, J-Aggregates, World Scientific Publishing, 1996, pp. 1–40.
12. A. Gavezzotti, New J. Chem., 2011, 35, 1360.

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Footnote

† Electronic supplementary information (ESI) available: experimental procedures and crystallographic data. CCDC 1479193. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra15966d

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