A triphenylamine derivative and its Cd(II) complex with high-contrast mechanochromic luminescence and vapochromism†

Qiong-Fang Liang , Han-Wen Zheng , Dong-Dong Yang and Xiang-Jun Zheng *
Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China. E-mail: xjzheng@bnu.edu.cn

First published on 26th November 2021

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Introduction

Stimulus responsive luminescent materials can reversibly change their luminescence in response to external stimuli (such as mechanical stress, chemical vapor, light, and heat).^1–10 The high-contrast luminescence color or intensity change can be easily observed by the naked eye, making them good candidate materials in the fields of chemical sensors, environmental monitoring, data recording, optoelectronic devices, and security inks.^11–22 Among them, mechanical force is one of the most common stimuli in nature. Therefore, mechanochromic luminescent (MCL) materials that can change their luminescence characteristics when exposed to external mechanical forces (such as shearing, scratching, grinding, and crushing) have attracted extensive attention. The original emission states can be restored by applying another external stimulus (such as chemical vapor or heat) to achieve reversible mechanochromic luminescence.^23–30 In recent years, there have been many reports on MCL materials including organic molecules,^31,32 metal–organic complexes,^30,33,34 and organic polymers.^35,36 Currently, the mechanism of the mechanochromic behavior is mainly attributed to the phase transformation from a crystalline state to an amorphous state or single-crystal-to-single-crystal transformation.^37–40 Hydrogen bonding, π–π stacking, metal–metal interaction and other intermolecular interactions as the driving forces for molecular arrangement and packing can be changed when exposed to external stimuli, so that the molecular packing mode and conformation are changed and the HOMO–LUMO energy levels are perturbed, consequently resulting in the MCL properties.^28,41–44 Therefore, designing a distorted molecule with loose molecular packing modes, making the intermolecular interactions easily damaged or modified by external mechanical stimuli, is a convenient and effective strategy to achieve MCL. It has been proved workable by adopting D–A structure, introducing rotatable aromatic ring and flexible group.^{1,6,12,13,23,37,38,40,44–46} For vapochromic luminescence materials, their color or emission can change reversibly when exposed to the vapor atmosphere of volatile compounds.⁴⁷ Generally, conformational changes, proton transfer, changes in intermolecular interactions, and the adsorption and desorption of vapor molecules are referred as the mechanism of this process.^{4,5,16–18,48,49} Among them, quite a few materials have been exploited for demonstrating fluorescence switching by treating with HCl and NH₃ vapor.^50–52

A triphenylamine (TPA) moiety is a typical electron donor with a propeller-shape non-coplanar structure, and is widely used in the fabrication of efficient luminescent solid materials due to its high photoelectric energy conversion efficiency and hole-transporting capability.^53–55 On the other hand, the highly distorted structure of the triphenylamine moiety prevents the formation of close molecular packing and produces loose aggregation patterns, which has also been applied in the design of stimulus-responsive smart luminescent materials.^56–60 Here, we designed and synthesized the Schiff base molecule HL and its Cd(II) complex with 4-(diphenylamino)benzaldehyde as the electron donor and 4-hydroxybenzohydrazide as the acceptor. The molecule HL contains rich hydrogen bond donors and acceptors, such as OH, NH, CN and carbonyl groups, which could affect the molecular conformation through hydrogen bonding. HL could adopt a highly distorted structure and loose molecular packing mode. After the coordination with a Cd²⁺ ion, the conformation of the Schiff base molecule changes, and the emission position of the Cd complex is different from that of HL·2CH₃CN, indicating that the formation of coordination bonds and hydrogen bonds regulates the luminescence properties of the Schiff base molecule successfully. Both Schiff base ligands and their complexes exhibit high-contrast mechanochromic luminescence due to the phase transformation from the crystalline state to the amorphous state. More interestingly, color and emission changes were also observed after being exposed to hydrochloric acid vapor, indicating that HL and its Cd complex also have acid–base responsive properties.

Results and discussion

Crystal structure of HL·2CH₃CN and complex 1

As shown in Scheme 1, both Schiff base HL and complex 1 are synthesized by a one-pot method through the aldimine condensation reaction.

Both HL·2CH₃CN and complex 1 have the same crystal system (triclinic) and space group (P), but different stacking modes. As shown in Fig. 1a, there are two Schiff base molecules with different orientations and four acetonitrile molecules in the unit cell of HL·2CH₃CN, and the Schiff base molecules are highly staggered. There are a variety of intermolecular hydrogen bonds such as N–H⋯N (3.045–3.051 Å), O–H⋯O (2.670–2.737 Å), C–H⋯N (3.332–3.434 Å) and C–H⋯O (3.374 Å) in the crystal, as well as weak C–H⋯π (2.61–2.96 Å) interactions, which hold the Schiff base molecules and acetonitrile molecules together (Fig. S1†). It is worth noting that the Schiff base molecules are arranged in a staggered arrangement through the O–H⋯O hydrogen bonds formed by the carbonyl O atom and the phenolic hydroxyl group, and the Schiff base molecules and the acetonitrile molecules interact through the N–H⋯N hydrogen bonds formed by the N atom of the acetonitrile molecule and the –NH– group of the Schiff base molecule (Fig. 2a). For every HL molecule, there are four aromatic rings. For convenience, they are defined as rings A, B, C and D, respectively. These rings do not adopt a coplanar conformation, and there are large dihedral angles between them as expected (Table S1†). The dihedral angles of the different rings are in the range of 47.1–80.41°, indicating that the molecules are highly twisted.

For complex 1, one cadmium ion is coordinated with two Schiff base ligands arranged head-to-tail and two chloride ions to form a six-coordination octahedral geometry. Due to the participation of chloride ions, the Schiff base ligands are not deprotonated but in neutral form. There are also a variety of intermolecular hydrogen bonds O–H⋯Cl (3.168 Å) (Fig. 2b) and intramolecular hydrogen bonds, such as N–H⋯Cl (3.307 Å), C–H⋯Cl (3.521–3.569 Å) and C–H⋯O (3.254–3.266 Å), and intermolecular C–H⋯π (2.800 Å) interactions in complex 1. Due to the coordination with a Cd²⁺ ion, on the one hand, there is O–H⋯Cl hydrogen bonding between two adjacent coordination compounds, which is completely different from HL·2CH₃CN. On the other hand, there are π–π interactions (3.831–3.936 Å) between the aromatic rings of the triphenylamine moiety in complex 1, which makes the molecule extend into a one-dimensional chain along the a-axis (Fig. S1†). In addition, the dihedral angle between rings A and B of the Schiff base molecule in complex 1 is only 12.05°, much smaller than that in HL·2CH₃CN, indicating that the Schiff base molecule in complex 1 has better planarity than that in HL·2CH₃CN. The dihedral angles between other rings are also different as shown in Table S1.† It is the different interactions, especially hydrogen bonding between the molecules, that result in the different molecular conformation for HL.

Mechanochromic luminescence properties

The crystals of HL·2CH₃CN showed blue fluorescence with an emission peak at 464 nm under the irradiation of a 365 nm UV lamp (Fig. 3a), with the fluorescence lifetime τ = 1.03 ns and Φ_F = 4.80% (Fig. S2†). Maybe due to the existence of acetonitrile molecules in the lattice, the crystals of HL·2CH₃CN are apt to effloresce with the emission red-shifted to 483 nm as shown in Fig. 3c. After grinding with a pestle in a mortar, the fluorescence emission position red-shifted to 520 nm (τ = 1.36 ns, Φ_F = 4.60%), accompanied by the appearance of green fluorescence. When the ground sample was exposed to acetonitrile vapor, the emission peak returned to the original emission of 464 nm with the blue fluorescence observed again. But when the fumed sample was placed under ambient conditions for 10 minutes, the emission shifted to 473 nm. The MCL process was also observed in complex 1. Under 365 nm UV lamp irradiation, complex 1 exhibited cyan emission with a peak at 490 nm, and with the fluorescence lifetime τ = 1.31 ns and Φ_F = 20.1% (Fig. S3†), indicating that the excited state energy level of the ligand was changed after coordination. After grinding, the emission peak red-shifted to 572 nm (τ = 3.27 ns, Φ_F = 14.8%), and the emission color changed to yellow. After fumigating the ground sample with ether, it can be observed that the luminous color returns to green, and the emission position returns to 538 nm (Fig. 3b and d). The changes of the emission colors of HL·2CH₃CN and complex 1 under different external mechanical stimuli were quantified with the CIE diagrams as shown in Fig. S4.† Therefore, both HL·2CH₃CN and complex 1 exhibited high-contrast MCL properties.

In order to investigate the MCL mechanisms of HL·2CH₃CN and complex 1, PXRD experiments on the samples in different states were carried out. It can be seen from Fig. 4a and b that the experimental spectrum of the original sample of HL·2CH₃CN is consistent with the PXRD pattern simulated by the single crystal data, indicating that the original sample has good crystallinity and purity. The disappearance of the diffraction peaks upon grinding indicated that grinding caused a phase transformation from a crystalline state to an amorphous state. In addition, comparing the thermogravimetric analysis (TGA) curve (Fig. S5†) of the original crystals with that of the ground sample of HL·2CH₃CN, the original crystal began to lose weight at 60 °C, which was attributed to the loss of acetonitrile molecules in the crystal lattice. Meanwhile, there is no weight loss for the ground sample at this temperature, indicating that grinding also led to the departure of acetonitrile molecules. In the PXRD pattern of the fumed sample, the diffraction peaks consistent with the original crystal did not all recover, which may be due to the efflorescence of the sample after leaving the acetonitrile vapor atmosphere. The disappearance and recovery of the diffraction peaks can also be seen in the PXRD patterns of complex 1 in different states. Moreover, the diffraction pattern of the fumed sample showed that the sample is a mixture of the amorphous state and the original crystalline state, which is consistent with the redshift of the emission peak of the sample compared to that of the original sample. The transformation from the crystalline state to the amorphous state was further confirmed by differential scanning calorimetry (DSC) experiments. The ground sample of HL·2CH₃CN exhibited an exothermic peak at 200 °C upon heating, which originated from the cold recrystallization (Fig. S6†). Similar experimental results were also obtained with the ground sample of complex 1. These experimental results showed that the high-contrast MCL processes of complex 1 and HL·2CH₃CN were due to the phase transformation from the crystalline state to the amorphous state, while the high-contrast process of HL·2CH₃CN was also accompanied by the departure and recovery of the acetonitrile molecules. As mentioned above, the existence of weak interactions and the dihedral angles between the rotatable aromatic rings makes the molecules adopt a loose and easily-destructible packing mode. Upon grinding, these weak interactions can be destroyed and the aromatic rings twist, giving rise to the MCL properties of HL·2CH₃CN and complex 1. Since the conformational twist between TPA phenyl and acceptor groups controlled the solid state fluorescence,⁶⁰ the emission of complex 1 is more red-shifted than that of HL·2CH₃CN, which may be attributed to the difference in the conformation of the Schiff base molecules and molecular interactions caused by chelation.

Acidochromic properties

Under ambient light, it can be observed that the HL·2CH₃CN crystal was pale yellow, and the fluorescence emission position was centered at 464 nm as shown in Fig. 5. When exposed to hydrochloric acid vapor (36% concentrated hydrochloric acid vapor), an obvious color change from pale yellow to orange-red can be easily observed by the naked eye. The orange-red sample was named HL-HCl. The emission peak of HL-HCl was red-shifted to 594 nm (τ = 0.56 ns), accompanied by the luminescence color change to orange-red. Excitingly, when HL-HCl was continued to be fumigated with NH₃ vapor (25% concentrated ammonium hydroxide vapor), named HL-HCl-NH₃, the light yellow color under ambient light was restored, with the maximum emission peak blue-shifted to 494 nm (τ = 0.98 ns), and the fluorescence returned to light cyan, which was similar to that of the ground sample. Therefore, HL·2CH₃CN exhibited a high-contrast tricolor acidochromic behavior. For complex 1, after being treated with HCl vapor (36% concentrated hydrochloric acid vapor), the fluorescence emission color changed from cyan to orange, the maximum emission position was red-shifted from 490 to 587 nm (τ = 0.91 ns), and the sample turned orange, which was named 1-HCl. When 1-HCl was further fumigated with NH₃ vapor (25% concentrated ammonium hydroxide vapor), the emission peak of the sample (named 1-HCl-NH₃) reverted to 503 nm (τ = 1.17 ns). Simultaneously, the color under ambient light and the emission color are consistent with the original sample. The above results indicated that complex 1 has reversible and high-contrast acidochromic properties.

PXRD, solid-state UV-vis absorption spectroscopy and X-ray photoelectron spectroscopy (XPS) were carried out to explore the mechanism of the acidochromism of HL·2CH₃CN and complex 1. As shown in Fig. S7,†HL·2CH₃CN has strong absorption at 411 nm and weak absorption at 589 nm. The strong absorption peak position of HL-HCl is at a longer wavelength than that of HL·2CH₃CN (480 nm). Its weak absorption appears at 674 nm. After treatment with NH₃, a similar absorption peak to the original one appeared again. These results can be the reason why the color of HL-HCl is different from those of HL·2CH₃CN and HL-HCl-NH₃. Similarly, complex 1 and 1-HCl both have strong absorption at 407 nm, while 1-HCl has new weak absorption peaks at 522 nm and 672 nm, which also leads to the difference in their colors under ambient light.

In order to investigate whether the composition of the surface of the materials has changed, XPS experiments on the original sample and the sample fumigated with HCl vapor were performed (Fig. 6). By comparing the XPS results of HL·2CH₃CN with those of HL-HCl (Fig. S8†), it can be found that the binding energy of Cl 2p appears at 198.10 eV in HL-HCl, but not in HL·2CH₃CN, indicating that the elemental composition of HL-HCl also contains Cl. In addition, we found that both HL·2CH₃CN and HL-HCl have a binding energy belonging to N 1s at 400.10 eV, while HL-HCl also has a shoulder peak at 402.26 eV belonging to N⁺ (Fig. 6d). Meanwhile, there is no difference in the binding energy of O 1s in HL·2CH₃CN and HL-HCl. Based on the above results, it is speculated that the fumigation with HCl vapor caused the –NH– group of the Schiff base molecules to accept protons from HCl, protonizing into quaternary ammonium salt. The protonation process may cause the HOMO–LUMO energy level of the whole molecule to be perturbed, which causes the color and emission position of the sample to change, making HL·2CH₃CN show acidochromic properties. As the structure of the Schiff base molecules in complex 1 was consistent with that in HL·2CH₃CN, we can infer that the vapor discoloration of complex 1 is also due to the protonation process of the Schiff base molecules. Undoubtedly, our inference is confirmed by the XPS results of 1 and 1-HCl. Compared with complex 1, the binding energy of N⁺ also appeared at 402.03 eV in sample 1-HCl, while the binding energy of O 1s, Cd 3d and Cl 2p showed no significant difference (Fig. S9†). This also showed that the complex is not decomposed under the influence of the HCl vapor.

In order to explore whether the states of the sample have changed after being treated with different vapors, the PXRD results were carefully compared. As shown in Fig. 7a and b, the PXRD pattern of HL-HCl showed different diffraction peaks from that of the original sample of HL·2CH₃CN indicating that the crystal structure of the original crystal was destroyed, and another crystal structure appeared after protonation. The PXRD pattern of the HL-HCl-NH₃ sample shows a peak attributed to the amorphous state and the characteristic diffraction peak of NH₄Cl at 2θ = 32.76°. Therefore, when the HL-HCl sample was further treated with NH₃ vapor, the Schiff base molecules were deprotonated and NH₄Cl was generated, but the original crystalline state was not recovered, which was also confirmed by the consistency of the fluorescence emission of HL-HCl-NH₃ and the ground sample. As shown in Fig. S5,† there is no weight loss for HL-HCl-NH₃ at 60 °C, and its weight loss takes place at 220°, which is lower than that of the ground sample. This may be due to the decomposition of NH₄Cl generated on the surface. Similar to HL·2CH₃CN, 1-HCl also has a different diffraction pattern from the original sample 1, and the characteristic diffraction peak of NH₄Cl also existed in the PXRD pattern of 1-HCl-NH₃. The emission peak of 1-HCl can only be restored to 504 nm, which may be related to the incomplete recovery of the original characteristic diffraction peak.

To further understand the acidochromic mechanism of HL·2CH₃CN and complex 1, ¹H NMR spectra of the HL·2CH₃CN, HL-HCl, HL-HCl-NH₃, 1 and 1-HCl samples were recorded as shown in Fig. 8. Compared with the ¹H NMR spectrum of HL·2CH₃CN, there are some new peaks with low intensity which could be ascribed to the proton of reactants 4-(diphenylamino)benzaldehyde and 4-hydroxybenzohydrazide.^61,62 This indicates that the adsorbed HCl leads to the decomposition of HL in DMSO. In addition, HL-HCl shows a significant upfield shift of the NH proton, indicative of the protonation of the nitrogen on the hydrazide unit. When HL-HCl was treated with NH₃, the ¹H NMR spectrum resembles that of HL·2CH₃CN except the proton signal of NH, indicating the reversible acidochromic behavior. The ¹H NMR spectrum of the Cd complex is similar to that of HL, confirming that a similar change occurred for complex 1. Based on the above results, protonation and phase transformation are the reasons for the acid–base responsive behavior of HL·2CH₃CN and complex 1.

Writable and acid stimulus responsive test paper

Based on the high contrast MCL and vapochromic properties of HL·2CH₃CN and complex 1, a simple test paper that can be written on and responds to acid vapor was produced (Fig. 9). The specific process is as follows. The HL·2CH₃CN crystals were ultrasonically crushed and dispersed in acetonitrile, then they were added dropwise on a filter paper, and air dried. Due to the small amount of the sample loaded, the test paper was white, but showed strong blue fluorescence under 365 nm irradiation. After that, an “L” character was written on the test paper by using a glass rod. There was no significant change on the test paper under ambient light, but a green-emitting “L” character was seen under UV irradiation. When the test paper was exposed to HCl vapor (36% concentrated hydrochloric acid vapor), its color changed to red and the fluorescence emission became dark red.

Further treatment with NH₃ vapor (25% concentrated ammonium hydroxide vapor) restored it to yellow and the fluorescence color changed to green. Similarly, a test paper of complex 1 was fabricated. After writing the character “L” on the white test paper with cyan fluorescence using a glass rod, the yellow color with a yellow-emitting character “L” appeared. When treated with HCl vapor, the test paper turned yellow and the fluorescence color turned orange-yellow. The original cyan fluorescence was restored after treatment with NH₃ vapor. Therefore, HL·2CH₃CN and complex 1 can be used as writable and acid stimulus responsive materials.

Conclusions

In summary, a triphenylamine derivative and its Cd(II) complex were successfully synthesized by a one-pot reaction. Both compound HL·2CH₃CN and complex 1 exhibited high-contrast mechanochromic luminescence and acidochromism. The experimental results showed that the molecules of both HL·2CH₃CN and complex 1 possess rich non-covalent interactions, adopting loose packing modes. Therefore, the phase transformation from a crystalline state to an amorphous state can occur effectively, which leads to the mechanochromic luminescence. After treating with organic vapor, the original emission can be restored. In addition, the color and luminescence color of HL·2CH₃CN and complex 1 both changed after being exposed to HCl vapor, showing acid–base responsive properties. The results of XPS showed that the protonation of the –NH– group leads to the acidochromic properties. After further treatment with NH₃ vapor, 1-HCl can be restored to the original state, while HL-HCl can only be restored to the same state as the ground sample due to the serious collapse of the original crystal structure and the departure of acetonitrile molecules in the protonation and deprotonation process, thus showing a tricolor acid–base responsive behavior. The research results provide a new strategy for the development of multi-stimulus responsive luminescent materials.

Experimental

Materials and methods

All solvents and reagents in the present work were obtained from commercial sources and used without further purification. The Fourier transform infrared (FT-IR) spectra were recorded on an Avatar 360 FT-IR spectrometer using KBr pellets as an internal standard in 4000–400 cm⁻¹. The ¹H NMR spectra were recorded on a Bruker Avance III 600 MHz spectrometer. The fluorescence quantum yield was measured on a HAMAMATSU Quantaurus-QY instrument. The single crystal X-ray diffraction data were collected with an XtaLAB Synergy-DW and an Oxford Diffraction SuperNova area-detector diffractometer using mirror optics monochromated Cu-Kα radiation (λ = 1.54184 Å) at low temperature (100 K). The X-ray powder diffraction (PXRD) patterns were obtained using a SHIMADZU XRD-7000 diffractometer with Cu-Kα radiation (λ = 1.5418 Å) at 25 °C. The thermogravimetric (TG) data were collected using a Mettler TGA instrument in the range of 30–500 °C under a constant nitrogen flow with a heating rate of 10 K min⁻¹. Differential scanning calorimetry (DSC) was carried out using a Mettler DSC 1 instrument. The solid-state UV-vis absorption spectra were recorded on a TU-1901 spectrophotometer with an integrating sphere at room temperature in the range of 250–800 nm. The X-ray photoelectron spectra (XPS) were obtained by using an ESCALAB 250 Xi spectrometer.

Synthesis

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the financial support from the National Natural Science Foundation of China (21671022).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Tables S1 and S2 and Fig. S1–S9 (PDF) showing the structures, spectra, TGA curves, DSC curves, dihedral angles, and X-ray crystallographic data for HL·2CH3CN and complex 1 (CIF). CCDC 2099606 and 2099607. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1ce01319j

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