English

• In recent two decades, aggregation induced emission (AIE) [1] has received more and more attention in many fields due to its unique properties and wide range of applications. It provides an excellent platform for the development of useful luminescent materials [2]. At the same time, due to its unique luminescence behavior and strong aggregation state fluorescence emission, AIE has shown a wide range of applications in fluorescence sensors [3], organic light-emitting diodes (OLED) [1,4], biological imaging [1] and functional materials [5]. So far, many AIEgens with adjustable color and high quantum yield have been reported such as hexaphenylsilole [6], tetraphenylethylene [7], and so on [8-10]. Although there are various AIEgens had been reported, the development of simple, high efficiency and easy to synthesis AIEgen is always an attractive task. While, we all know that the structure of AIEgens plays an important role in the AIE properties. Therefore, it is very important to deeply understand the AIE mechanism and structure-property relationship of AIEgens, which very helpful to design efficiency AIEgens.

  Azine derivatives are easy to synthesize [11] and some of them have good AIE properties and can be used in chemical sensors [12] and biological imaging [13]. Moreover, Azine derivatives with different structure showed different AIE properties [14,15]. Therefore, systematic study of AIE mechanism and structure-property relationship [16,17] of these compounds can help us deeply understanding the AIE mechanism of this kind of AIEgens and provide ideas for the design, synthesis and application of new simple and efficient AIEgens.

  In view of these, as part of our research interest in AIE-based supramolecular materials, herein, the AIE mechanism and structure-property relationship of simple azine-based AIEgens have been carefully investigated. As shown in Fig. 1a, firstly, a series of azine derivatives ADs including AD1~AD10 which containing various functional substitute groups such as electrondrawing, electrondonor as well as hydrogen bonding groups have been synthesized. Secondly, the AIE mechanism as well as the influence of substitute groups and structure on AIE property of these azine derivatives have been carefully studied by experiments, single crystal structure and theoretical calculations. Interestingly, the o-hydroxyl aryl substituted azine compounds AD2 and AD10 show good AIE performance, and the o-hydroxyl groups play key role to restrict the intramolecular rotation of the aryl groups which induced the AIE. Meanwhile, based on the AIE, the AD2 and AD10 can be used to selectively fluorescent detection Al³⁺ and Cu²⁺ in aqueous solution.

  Figure 1

  

  Figure 1.  (a) The synthetic route of ADs. (b) Fluorescence intensity of ADs (1 × 10⁻⁵ mol/L, λ_ex = 365 nm, Slit = 5/10 nm) in DMSO and DMSO/H₂O = 3:7 binary solution. (c) Fluorescence photos of AD2 and AD10 in DMSO/H₂O binary solution with different volumetric fractions of water.

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  In order to study the structure-property relationship of azine derivatives on the performance of AIE, the fluorescence properties of ADs were primarily investigated in a series of organic-water (organic solvents are DMSO, acetone or THF) binary solutions with different water contents (Figs. S7-S15 in Supporting information). According to these experiments, as shown in Figs. S7-S15, Figs. 1b and c, with the increasing of the water volume fraction, the fluorescence emission intensity of the AD2 DMSO-H₂O solution showed a gradual increase at λ_em ≈ 530 nm and reached the strongest state when the water volume fraction is 90%. While, in this process, the UV absorption of AD2 showed gradually decreases (Fig. S16a in Supporting information). The fluorescence quantum yield of AD2 in DMSO: H₂O = 1:9 was calculated to be 0.27 (Figs. S17 and S18 in Supporting information) [18]. Similarly, the fluorescence emission intensity of AD10 DMSO-H₂O solution showed a gradual increase at λ_em ≈ 530 nm and reached the strongest state when the volume fraction of water is 70%. In this process, the UV absorption of AD10 showed gradually decreases under the 30%~60% water volume fractions (Fig. S16b in Supporting information) and slight increase at 70%~90% water volume fractions. The fluorescence quantum yield of AD10 in DMSO: H₂O = 3:7 was calculated to be 0.77 (Figs. S17 and S19 in Supporting information) [18]. These results indicated that AD2 and AD10 possess AIE property in DMSO-H₂O solution. While, in acetone-H₂O or THF-H₂O system, the AD2 and AD10 also have similar AIE property (Figs. S7-S15). However, according to Figs. S7-S15, other eight compounds AD1 and AD3~AD9 have no obvious AIE properties in these organic-water systems. According to these results, we can find that the o-hydroxyl aryl substituted azine compounds AD2 and AD10 show good AIE performance, and the o-hydroxyl aryl groups play key role in the AIE process.

  Secondly, it is noteworthy that the HOMO-LUMO gap of AD9 is smaller than that of AD2, but AD9 does not exhibit AIE properties. Among AD1~AD10, AD2 and AD10 are o-hydroxyl ary substituted azines (Figs. 2a and b). This result indicated that the o-hydroxyl substituting for aryl group in the azine is the key factor for the AIE of this kind of compounds. The single crystal structure and theoretical calculations can help us better understand the role of o-hydroxyl group in the AIE properties of the azines. As shown in Fig. 2e, the o-hydroxyl group could form intramolecular hydrogen bonds, which could restrict the intramolecular rotation of the aryl groups, this is the foundation of the AIE for AD2 and AD10. Meanwhile, according to theoretical calculations, as shown in Fig. 2c and Fig. S21 (Supporting information), in ESP diagrams of AD2 and AD10, hydroxyl oxygen atoms have a negative charge distribution, and N of C=N unit has a positive charge distribution. Therefore, the o-hydroxyl group could conduct unique excited state intramolecular proton transfer (ESIPT) process with C=N group, which also lead AD2 and AD10 producing strong AIE properties. While, other ADs could not form stable intramolecular hydrogen bonds and could not show AIE property.

  Figure 2

  

  Figure 2.  (a) The HOMO and LUMO orbital gaps of single molecule of AD1, AD2, AD9 and AD10. (b) The HOMO and LUMO orbital gaps of single molecule of AD1~AD10. (c) ESP of AD2, AD2+Al, AD2+Cu and AD10, AD10+Al, AD10+Cu. (d) The colored scatter plots isosurfaces of IGM of AD2 and AD10. (e) The various intermolecular interactions in single crystal of AD2 and AD10.

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  In order to gain a deeper understanding of aggregating process of AD2 and AD10, the non-covalent interactions in adjacent AD2 and AD10 molecules were carefully investigated by independent gradient model (IGM) and single crystal structure analysis. As shown in Fig. 2d, in the IGM, the strength of the interaction can be characterized by the volume and color of the adjacent AD2 and AD10 molecular interaction region, so the green region appears as a weak interaction, such as the formation of the van der Waals interaction. The blue areas indicate strong interactions, such as hydrogen bonds that form. The IGM diagram shows that AD2 and AD10 have strong intramolecular hydrogen bonds and there are intermolecular hydrogen bonds, π-π and C-H···π interaction existing in adjacent molecules.

  Moreover, the single crystal structure analysis also supporting the calculated aggregating process, as shown in Fig. 2e, in the crystal, each AD2 and AD10 molecules adopt a completely planar conformation. In addition, there are strong intramolecular hydrogen bonds between the nitrogen atoms of the AD2 and AD10 azine units and the o-hydroxyl on the aryl groups, and the distances are 1.89 Å and 1.85 Å respectively. The strong intramolecular hydrogen bonds greatly enhance the rigidity of the molecule and contribute to the planarization of the molecule. Meanwhile, as shown in Fig. 2e, among the adjacent molecules of AD2, there are intermolecular C-H···O hydrogen bonds (d = 2.43 Å). Moreover, there are several kinds of C-H···π (d = 2.43~2.98 Å, Fig. 2e) interactions existing among the adjacent molecules of AD2 and AD10. In addition, several kinds of π-π interactions also exists between ary ring (d = 4.74 Å, Fig. S22 in Supporting information) and C=N double bond (d =3.38 Å, Fig. S22) in AD2. Furthermore, in AD10, the binary ring (d = 6.11 Å > 6.0 Å, Fig. S23 in Supporting information), there is no π-π interaction [19], but there is also π-π interaction between naphthalene ring and C=N double bond (d = 4.28 Å, Fig. S23). These intramolecular and intermolecular supramolecular interactions including hydrogen bonds, C-H···π and π-π interactions not only lead the aggregate of AD2 and AD10 but also restricted the intramolecular rotation and induced the AIE of these two compounds.

  In light of the nice AIE properties of AD2 and AD10, the application property of these two AIE compounds have been investigated as metal ions sensor by separately adding 20 equiv. different cations including Mg²⁺, Ca²⁺, Cr³⁺, Fe³⁺, Co²⁺, Ni²⁺, Al³⁺, Cu²⁺, Zn²⁺, Ba²⁺, Ag⁺, Cd²⁺, Hg²⁺, Pb²⁺, Tb³⁺, Eu⁴⁺ and La³⁺ (aqueous solution, 0.1 mol/L) in to the DMSO/H₂O (v: v = 8:2) solution of AD2 and AD10, respectively. As shown in Fig. 3a and Fig. S27a (Supporting information), the free AD2 and AD10 showing very weak fluorescence in DMSO/H₂O (v: v = 8:2) solution, after the addition of Al³⁺, the fluorescence of AD2 at λ_em ≈ 450 nm and AD10 at λ_em ≈ 530 nm show "turn on" response, and other ions could not lead similar response, which indicated that AD2 and AD10 can selectively recognize Al³⁺ through fluorescence "turn on" model in DMSO/H₂O (v: v = 8:2) solution. According to the fluorescence titrations (Fig. 3d and Fig. S27d in Supporting information), the lowest detection limit (LOD) of AD2 and AD10 for Al³⁺ recognitions were calculated as 1.94 × 10⁻⁷ mol/L and 8.47 × 10⁻⁶ mol/L, respectively. Meanwhile, based on the competitive experiments (Fig. 3c and Fig. S27c in Supporting information), other ions could not interfere in Al³⁺ recognition processes. As shown in Fig. S28 (Supporting information), the detection rate also has been investigated, after the Al³⁺ (3 × 10⁻⁴ mol/L) was added to AD10 in (1 × 10⁻⁵ mol/L) DMSO: H₂O = 8:2 solution, the fluorescence of the solution increased rapidly and achieved to the maximum after within 50 min.

  Figure 3

  

  Figure 3.  Fluorescence photo of AD10 (1 × 10⁻⁵ mol/L) in the presence of 20 equiv. of various metal ions in (a) DMSO/H₂O (v/v = 8:2) and (b) DMSO/H₂O (v/v = 3:7) binary solution (λ_ex = 365 nm). (c) competitive experiments of the sensor AD10 (1 × 10⁻⁵ mol/L) with addition of 20 equiv. of Al³⁺ and 20 equiv. of other common ions. (d) Fluorescence titrations spectra of AD10 (1 × 10⁻⁵ mol/L) after the addition of 0-32 equiv. of Al³⁺ into the DMSO/H₂O (v/v = 8:2) binary solution. (e) Fluorescence changes of AD10-based test strips after the addition of different concentration (1 × 10⁻¹ ~ 1 × 10⁻⁷ mol/L) of Cu²⁺.

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  In addition, as shown in Fig. 3b and Fig. S27, in DMSO/H₂O (v: v = 1:9) and DMSO/H₂O (v: v = 3:7) solutions, AD2 and AD10 show good AIE phenomenon, after the addition of Cu²⁺ into these solutions, the AIE fluorescence of AD2 and AD10 quenched, and other ions could not induce similar response. Therefore, the AD2 and AD10 can selectively recognize Cu²⁺ in DMSO/H₂O (v: v = 1:9) and DMSO/H₂O (v: v = 3:7) solution, respectively. Through fluorescence titrations (Figs. S29-S31 in Supporting information), the LOD of AD2 and AD10 for Cu²⁺ were measured as 8.76 × 10⁻⁸ mol/L and 1.73 × 10⁻⁷ mol/L, respectively. As shown in Fig. S29, other ions could not interfere in the Cu²⁺ sensing process.

  In order to further develop the practical application of the ADs, test strips based on AD10 were prepared to detect Cu²⁺. As shown in Fig. 3e, the test strips could show fluorometric response for different concentrations of Cu²⁺ under a 365 nm UV lump. Interestingly, the test strips could be applied to detect in real sample of Cu²⁺ containing wastewater from the industry.

  In order to verify the structure property relationships of the azine derivatives for metal ions recognitions, the similar experiments were carried out on AD1 and AD3~AD9. However, as shown in Figs. S24-S26 (Supporting information), AD1 and AD3~AD9 could not show any selective response for metal ions. Therefore, the o-hydroxyl substituted aryl groups in AD2 and AD10 play important role not only in the AIE but also in ions sensing processes of the azine derivatives.

  Additionally, the ¹H NMR, FT-IR and ESI-MS experiments as well as theoretical calculation were applied to investigated the Al³⁺ and Cu²⁺ recognition mechanism of AD2 and AD10. According to the host-guest ¹H NMR spectra (Fig. S32 in Supporting information), after adding Al³⁺, the proton signals of H_a (-OH) and H_b (CH=N) on AD2 showed distinct downfield shifts, which could attribute to the Al³⁺ coordinated with O (-OH) and N (CH=N) on AD2. Meanwhile, the proton signals of H_c-H_f on benzene groups also showed a slightly downfield shifts (Fig. S32), which could be attributed to the coordination of Al³⁺ with AD2 induced deshielding effect. Meanwhile, in FT-IR spectra (Fig. S34 in Supporting information), after complexing with Al³⁺, the stretching vibration peaks of CH=N and -OH on AD2 change from 1622 cm⁻¹, 3438 cm⁻¹ to 1614 cm⁻¹, 3427 cm⁻¹, respectively, which also indicated that Al³⁺ coordinated with O (-OH) and N (CH=N) on AD2. This proposed coordination mechanism of AD2+Al also supported by theoretical calculation, as shown in Fig. 4a, the IGM diagram shows that AD2 has strong coordination bonds with Al³⁺. Meanwhile, after the AD2 coordinated with Al³⁺, the HOMO-LUMO gap decreased (Fig. 4b), which also indicated the formation of stable complex. Moreover, in the HR-MS (Fig. S36 in Supporting information), the peak at 329.1076 corresponding to [AD2·Al·H₂O·CH₃CH₂OH-2H]⁺, which revealed a 1:1 complex ratio between AD2 and Al³⁺.

  Figure 4

  

  Figure 4.  (a) The colored scatter plots isosurfaces of IGM of AD2+Al, AD2+Cu, AD10+Al and AD10+Cu. (b) The HOMO-LUMO of AD2, AD2+Al, AD2+Cu and AD10, AD10+Al, AD10+Cu by DFT optimized.

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  Then, the fluorescent response mechanism of the AD2 for Al³⁺ has been investigated by theoretical calculation [20]. According to the ESP (Fig. 2c) and HOMO-LUMO (Fig. 4b) of AD2, with the coordination of AD2 with Al³⁺, a clear electron transition from O (-OH) and N (CH=N) on AD2 to Al³⁺ could be observed. This result revealed that the coordination of Al³⁺ with AD2 not only restricted the intramolecular rotation of AD2, but also caused the photoinduced electron transfer (PET) process of compound AD2 is forbidden [21], making the fluorescence of the solution "turn on". Similarly, as shown in Figs. 2c and 4a, Figs. S33-S38 (Supporting information), the AD10 has a similar recognition mechanism for Al³⁺.

  Meanwhile, Cu²⁺ recognition mechanism of AD2 and AD10 also has been investigated. In FT-IR spectra (Fig. S34), after complexing with Cu²⁺, the stretching vibration peaks of CH=N and -OH on AD2 change from 1622 cm⁻¹, 3438 cm⁻¹ to 1592 cm⁻¹, 3422 cm⁻¹, respectively, indicating that Cu²⁺ coordinated with O (-OH) and N (CH=N) on AD2. This proposed coordination mechanism of AD2+Cu also supportd by theoretical calculation, as shown in Fig. 4a, the IGM diagram shows that AD2 has strong coordination bonds with Cu²⁺. Meanwhile, after the AD2 coordinated with Cu²⁺, the HOMO-LUMO gap decreased (Fig. 4b), which also indicated the formation of stable complex. Meanwhile, in the HR-MS (Fig. S37), the peak at 302.0226 corresponding to [AD2·Cu-H]⁺, which revealed a 1:1 complex ratio between AD2 and Cu²⁺ (Fig. 4a).

  Then, the fluorescent response mechanism of the AD2 for Cu²⁺ has been investigated by theoretical calculation. According to the ESP (Fig. 2c) and HOMO-LUMO (Fig. 4b) of AD2, with the coordination of AD2 with Cu²⁺, a clear electron transition from O (-OH) and N (CH=N) on AD2 to Cu²⁺ could be observed. The formation of complex will bend the rigid structure of AD2, leading to the rearrangement of AD2, and strong chelation enhances fluorescence quenching [22]. In addition, the coordination also destroys the OH···N hydrogen bond within the molecule, which seriously interferes with the formation of AIE system. Moreover, the paramagnetic of Cu²⁺ also is a reason for fluorescent quenching [21]. All these factors lead the addition of Cu²⁺ quenching the fluorescence of AD2. Similarly, as shown in Fig. 4, Figs. S35-S39 (Supporting information), AD10 also has a similar Cu²⁺ recognition mechanism.

  In this paper, the AIE mechanism and structure-property relationship of simple azine-based AIEgens have been carefully investigated. Because the AD2 and AD10 have ortho-hydroxyl groups, they can form intramolecular hydrogen bonds and prevent intramolecular rotation, so they show good AIE properties. However, the AD1 and AD3~AD9 could not form intramolecular hydrogen bond, therefore they have no AIE properties. In addition, AD2 and AD10 can selectively recognize Al³⁺ and Cu²⁺ in different DMSO/H₂O systems (under low water ratio condition, they can selective detection Al³⁺ by fluorescence "turn-on" response. While, under high water ratio and aggregate condition, they can selective detection Cu²⁺ trough AIE "turn off" pathway), while other ADs could not show any response to metal ions. In addition, test strips based on AD10 has been prepared, which can conveniently detect Cu²⁺ in industrial wastewater. In summary, the structure-property relationships of simple azine derivatives on AIE and ions recognition performance were studied, which provided experience for future design of high efficiency and easy to make azine based fluorescence sensors.

  Declaration of competing interest

  The authors declare that there are no conflicts of interest.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (NSFC, No. 22065031), the Key R & D Program of Gansu Province (No. 21YF5GA066), Gansu Province College Industry Support Plan Project (No. 2022CYZC-18), Natural Science Foundation of Gansu Province (Nos. 2020-0405-JCC-630, 20JR10RA088), Fundamental Research Funds for the Central Universities (Nos. 31920190041, 31920200002, 31920190018, 31920190013), Young Doctor Foundation of Gansu Province (No. 2021QB-148) and The Science and Technology Project Funded by Social Capital in Longnan City of Gansu Province (No. 2021-SZ-01).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.107792.

  
  
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  Figure 1  (a) The synthetic route of ADs. (b) Fluorescence intensity of ADs (1 × 10⁻⁵ mol/L, λ_ex = 365 nm, Slit = 5/10 nm) in DMSO and DMSO/H₂O = 3:7 binary solution. (c) Fluorescence photos of AD2 and AD10 in DMSO/H₂O binary solution with different volumetric fractions of water.

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  Figure 2  (a) The HOMO and LUMO orbital gaps of single molecule of AD1, AD2, AD9 and AD10. (b) The HOMO and LUMO orbital gaps of single molecule of AD1~AD10. (c) ESP of AD2, AD2+Al, AD2+Cu and AD10, AD10+Al, AD10+Cu. (d) The colored scatter plots isosurfaces of IGM of AD2 and AD10. (e) The various intermolecular interactions in single crystal of AD2 and AD10.

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  Figure 3  Fluorescence photo of AD10 (1 × 10⁻⁵ mol/L) in the presence of 20 equiv. of various metal ions in (a) DMSO/H₂O (v/v = 8:2) and (b) DMSO/H₂O (v/v = 3:7) binary solution (λ_ex = 365 nm). (c) competitive experiments of the sensor AD10 (1 × 10⁻⁵ mol/L) with addition of 20 equiv. of Al³⁺ and 20 equiv. of other common ions. (d) Fluorescence titrations spectra of AD10 (1 × 10⁻⁵ mol/L) after the addition of 0-32 equiv. of Al³⁺ into the DMSO/H₂O (v/v = 8:2) binary solution. (e) Fluorescence changes of AD10-based test strips after the addition of different concentration (1 × 10⁻¹ ~ 1 × 10⁻⁷ mol/L) of Cu²⁺.

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  Figure 4  (a) The colored scatter plots isosurfaces of IGM of AD2+Al, AD2+Cu, AD10+Al and AD10+Cu. (b) The HOMO-LUMO of AD2, AD2+Al, AD2+Cu and AD10, AD10+Al, AD10+Cu by DFT optimized.

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