Multistimuli-Responsive Fluorescent Organogelator Based on Triphenylamine-Substituted Acylhydrazone Derivative.

A new triphenylamine-based acylhydrazone derivative (TPAH-B8) was synthesized. TPAH-B8 could form organogels in cyclohexane through ultrasonic treatment. A typical gelation-induced fluorescence enhancement property was observed, which was attributed to the formation of J-aggregate in the gel state. More interestingly, TPAH-B8 exhibited multistimuli responsive behaviors. First, TPAH-B8 showed a solvatochromic effect, with the emission color changing from blue to cyan with the change in solvent from nonpolar cyclohexane to polar dimethyl sulfoxide (DMSO). Second, TPAH-B8 showed a reversible mechanofluorochromism. The xerogel of TPAH-B8 emitted a blue fluorescence, while the fluorescence color changed to cyan after grinding. The cyan and blue colors could be repeated with the treatment of grinding and annealing, which was explored and ascribed to the transformation between crystalline and amorphous states. Third, TPAH-B8 revealed acidochromic property. The fluorescence color of TPAH-B8 in organogel and solid states could be switched by trifluoroacetic acid (TFA)/triethylamine (TEA). This work not only demonstrated the multistimuli-responsive fluorescent properties of TPAH-B8 but also offered an easy way to develop new kinds of multistimuli-responsive fluorescent materials.

In recent years, stimuli-responsive luminescent materials have attracted widespread attention due to their potential applications in sensors, data storage, optical devices, drug delivery, cell imaging, etc.[1−6] Among these materials, the fluorescent low-molecular-weight organogelators with the stimuli-responsive property have attracted the special interest of researchers.[7−14] This is because the supramolecular structures formed by the fluorescent low-molecular-weight organogelators through noncovalent bonds (such as hydrogen bonds, π–π interactions, n class="Disease">van der Waals forces, etc.) can produce obvious responses to external stimuli and have multichannel response characteristics including fluorescence change, color variation, phase transition, etc.[15−18]

Recently, luminescent materials that possess mechanofluorochromic property have attracted substantial attention due to their potential applications in the fields of mechanosensors, security papers, optical recording, and data storage.[19−22] As a kind of luminescent material, there are some reports about the fluorescent low-molecular-weight organogelators with mechanofluorochromism. For example, Lu’s group synthesized a series of carbazole-modified n class="Chemical">pyrazole derivatives. Some of the compounds exhibited gelation-induced fluorescence enhancement properties, and these organogelators exhibited reversible mechanofluorochromic behavior under the treatment of grinding/fuming with CH2Cl2.[23] Yi’s group studied a series of organogelators based on naphthalimide which possess mechanofluorochromic behaviors, and these organogelators were used to sense low pressures in the range of 2–40 MPa with a fluorescent signal output.[24] Furthermore, the reports on the detection of acid by the multichannel response characteristics of the orgnaogelators have attracted much attention in recent years. For example, Lu’s group studied two organogelators derived from carbazol derivatives, which can be used to detect volatile acid in different states, such as gel, solution, and film.[25] Cao’s group synthesized a new fluorescent organogelator and achieved continuous, instant, and visual multichannel sensing of volatile acid and organic amine gases.[26] However, the examples of fluorescent low-molecular-weight organogelators with stimuli-responsive properties are mainly focused on single stimulus-response property.[23−26] Integrating multiple responsive properties into a single organogelator to achieve multistimuli-responsive properties of a single molecule has always been of interest to the researchers.[27,28] However, reports on multistimuli response of a fluorescent low-molecular-weight organogelator are relatively few,[26,28−30] especially those with both mechanofluorochromic and acidochromic properties. As far as we know, there are only two such examples so far. For example, Xue’s group designed a galunamide derivative. Due to the introduction of an amide group, a large sterically hindered group (cyanoethylene group), and a benzoxazole group capable of accepting protons, the synthesized organogelator has multistimuli-responsive properties, including mechanofluorochromic and acidochromic properties.[31] Recently, our group has reported an anthracene-substituted acylhydrazone derivative organogelator, which showed mechanofluorochromic behavior upon grinding. Furthermore, the compound exhibited remarkable and reversible acid/base stimulated fluorescence switching properties in both gel and solid states, which was attributed to the protonation of the molecule.[32]

To design a multistimuli-responsive fluorescent organogelator containing mechanofluorochromic and acidochromic properties, we introduced two functional groups. First, we introduced a triphenylamine group into the molecule. As we know, the n class="Chemical">triphenylamine group was used in the preparation of mechanofluorochromic materials in recent years due to its good fluorescent property and its nonplanar arrangement.[33,34] Second, an acylhydrazone group was introduced into the molecule because studies have shown that acylhydrazone group can be protonated and used to detect acid or acid vapors.[32,35] Meanwhile, triphenylamine group as a strong electron donor group and acylhydrazone group with certain electron-accepting properties can form a donor–acceptor (D–A) structure, giving the molecule a certain solvatochromic effect.[36] In addition, the introduced aclhydrazone group and long alkyl chain have the ability to induce molecular self-assembly to form a gel according to prior experience.[37,38] Fortunately, as expected, the synthesized triphenylamine-based acylhydrazone derivative (TPAH-B8) (Scheme ) could gelate certain solvents (e.g., cyclohexane) by the treatment of ultrasound, and the gel exhibited a strong blue fluorescence emission due to the gelation-induced enhanced fluorescence emission property. Moreover, TPAH-B8 exhibited multistimuli-responsive fluorescence properties. A typical solvatochromic effect of TPAH-B8 was observed by changing the solvent from nonpolar cyclohexane to polar dimethyl sulfoxide (DMSO). The TPAH-B8 xerogel obtained from cyclohexane showed a reversible mechanofluorochromic property, and the fluorescent color switched between blue and cyan. More interestingly, TPAH-B8 showed remarkable and reversible fluorescence switching properties in both organogel and solid states. This work may provide some help for the future design and synthesis of new organogelator with multiple stimuli-responsive fluorescent properties.

Scheme 1

Molecular Structure of TPAH-Bn

Results and Discussion

Photophysical Properties of TPAH-B8 in Solutions

As the molecule (TPAH-B8) possesses D and A groups, the intramolecular charge transfer (ICT) transition is expected. The normalized ultraviolet–visible (UV–vis) absorption and fluorescence emission spectra of n class="Chemical">TPAH-B8 in different solvents (1.0 × 10–5 mol L–1) are shown in Figure , and the detailed spectral parameters are listed in Table S1.

Figure 1

Normalized (a) UV–vis absorption and (b) fluorescence emission spectra (λex = 360 nm) of compound TPAH-B8 in different solvents (1.0 × 10–5 mol L–1).

As shown in Figure a, the maximum absorption band of TPAH-B8 was centered at ca. 300 and ca. 365 nm. Theoretical calculations suggested that the absorption band at ca. 365 nm could be assigned to the S0–S1 transition and the band at ca. 300 nm could be assigned to S0–S3 and S0–S4 transitions, which will be discussed in detail below. From the nonpolar solvent (n class="Chemical">cyclohexane) to the strong polar solvent (DMSO), the maximum absorption peak of the compound only showed a slight red shift (∼8 nm), which indicated that the electronic and structural nature of the ground state and Franck Condon (FC) excited state do not show obvious change.

However, obvious differences are found in its emission spectra. TPAH-B8 showed an intense emission at 401 nm with a fine vibrational structure (417 nm) in nonpolar n class="Chemical">cyclohexane (Figure b). When the polarity of solvent increased, the maximum emission peak red-shifted with the increase of solvent polarity and exhibited a large red-shift of about 59:420 nm in toluene, 429 nm in tetrahydrofuran (THF), 453 nm in dimethylformamide (DMF), and 460 nm in DMSO (Table S1, Supporting Information). Such large red shifts indicated that the dipole moment in the excited state is much larger than that in the ground state due to a charge-transfer process.[39,40] In addition, the fluorescence quantum yields (Φf) of TPAH-B8 in solutions were measured (Table S1, Supporting Information). For example, it was only 3.73% in cyclohexane and 23.28% in DMSO, showing a negative solvatokinetic effect.[23,41]

Study on Electronic-State Transitions and Frontier Molecular Orbitals

To get a deep understanding of the ground and electronic excited state properties and the mechanism of the intramolecular charge transfer, quantum chemical calculations were performed at the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) levels (CAM-B3LYP/6-31G(d,p)).[42] Multiwfn and n class="Disease">VMD were employed for visualizing the molecular orbital, electron density variation, and charge transfer.[43,44] To save the computational source, the long octyloxy group (−OC8H17) of TPAH-B8 was replaced by a short methoxy group (−OCH3, TPAH-B1; Scheme ). The reliability of this replacement has been proved in many computational studies. The optimized structure of TPAH-B1 showed a nonplanar structure in the ground state (Figure S1, Supporting Information). The physical picture of the electronic excitation of TPAH-B1 was revealed by the theoretical calculations at a time-dependent density functional theory (TD-DFT) level. Table S2 (Supporting Information) lists the excitation energy and oscillator strength (f) of the five lowest electronic transitions. The theoretical calculations revealed that the three highest allowed transitions in the TPAH-B1 were S0–S1 (4.04 eV, f = 0.97), S0–S3 (4.68 eV, f = 0.22), and S0–S4 (4.74 eV, f = 0.14), and the S0–S1 corresponded to one absorption band and S0–S3 and S0–S4 to the other absorption band in the absorption spectra. Figures a and S2 (Supporting Information) present the frontier molecular orbitals involved in the highest allowed two transitions of TPAH-B1. As can be seen, these occupied frontier molecular orbitals all mainly consist of π-bonding orbital, while the unoccupied orbitals were composed of the π-antibonding orbital. It made absorption bands to show the π–π* transition features. We found that the highest occupied molecular orbital (HOMO) density was mainly localized on the electron-donor triphenylamine moiety, whereas the lowest unoccupied molecular orbital (LUMO) density was mainly distributed in the electron–acceptor aclhydrazone group (Figure a). Moreover, the HOMO and LUMO energy levels of TPAH-B1 were also obtained by theoretical calculation. The corresponding HOMO and LUMO energy levels were located at −6.43 and −0.01 eV, respectively.

Figure 2

(a) Frontier orbitals plots of the HOMO and LUMO and (b) plot of electron density difference between the ground and the first excited states of TPAH-B1 calculated with the CAM-B3LYP/6-31G(d,p) method.

(a) Frontier orbitals plots of the HOMO and LUMO and (b) plot of electron density difference between the ground and the first excited states of TPAH-B1 calculated with the n class="Chemical">CAM-B3LYP/6-31G(d,p) method.

To get a direct view of the intramolecular charge transfer, the electron density differences between the ground state and the first excited state in the gas phase (Figure b) have been calculated.[45] The yellow and cyan regions stand for positive and negative of the electron density differences, which indicate the increase and decrease of electron density in the first excited state as compared with the ground state. In TPAH-B1, the electron density differences over the terminal n class="Chemical">diphenylamine were almost negative; in the adjacent benzene ring, the area of the negative parts and the positive parts were nearly the same; and around −CH=N– group, the electron density differences exhibited an obvious positive value, indicating that the electrons have been transferred from the triphenylamine group (donor) to the aclhydrazone group (acceptor) in TPAH-B1. These above results demonstrated that ICT occurred from the electron donor to the acceptor unit in the molecule, which was in accordance with spectroscopic results.[46]

Gelation and Gelation-Induced Fluorescence Enhancement Properties of TPAH-B8

The gelation ability of TPAH-B8 was evaluated in different solvents. Gels could not be formed by the classic heating–cooling process. However, by the treatment of ultrasound, n class="Chemical">TPAH-B8 gels could be formed in cyclohexane and ethanol with the critical gelation concentrations (CGCs) of 8 and 14 mg mL–1, respectively (Table ). Figure S3 (Supporting Information) shows the gel–sol transition temperature (Tgel) of TPAH-B8 in cyclohexane and ethanol. The sol–gel transition could be repeated without fatigue by heat and ultrasound stimuli (Figure S4, Supporting Information). Figure S5 (Supporting Information) displays the rheological properties of the TPAH-B8 gels at room temperature. The frequency sweep experiment of the TPAH-B8 gels (Figure S5a) reveals that G′ > G″, indicating the gel nature of the samples. In contrast, TPAH-B8 gels could not be obtained in other solvents by the same treatment. TPAH-B8 can be dissolved in all these solvents after heating. After ultrasonic treatment of the obtained hot solutions, these samples finally showed different states: precipitation in ethyl acetate, DMSO, and DMF; partial gel in n-hexane, acetone, petroleum ether, methanol, and n-octanol; and clear and transparent solution in toluene, chloroform, and THF.

Table 1

Gelation Abilities of Compound TPAH-B8 in Different Solvents with Ultrasound Treatment at Room Temperaturea

solvent	TPAH-B8	solvent	TPAH-B8
n-hexane	PG	toluene	S
cyclohexane	G(8)	acetone	PG
petroleum ether	PG	ethyl acetate	P
methanol	PG	THF	S
ethanol	G(14)	DMSO	P
n-octanol	PG	DMF	P
chloroform	S

S = solution, P = precipitate, PG = partial gelation, G = gelation. Numbers in parentheses represent their critical gelation concentrations (CGC, mg mL–1).

As a typical example, the spectroscopic study was performed in cyclohexane because n class="Chemical">TPAH-B8 cyclohexane gel possesses good gelation property (relatively low CGCs and high Tg). As shown in Figure , the fluorescence emission spectra of TPAH-B8 in hot solution and gel states (8 mg mL–1) were measured. The results showed that the fluorescence emission intensity of TPAH-B8 was weak, and the maximum emission peak was at 436 nm. However, in the gel phase, the fluorescence emission intensity at 436 nm was increased by about six times, showing a typical gelation-induced fluorescence enhancement property.[37,47] The UV–vis spectra of the ultrasound gel, which had bands at 300 and 381 nm, displayed a 19 nm red shift from the solution, indicating the formation of J-aggregate in the gel state (Figure S6, Supporting Information).[48,49] The formation of the J-aggregate could also be confirmed by the temperature-dependent UV–vis absorbance spectra (Figure S7, Supporting Information), where the bands of TPAH-B8 are red-shifted (2 and 7 nm, respectively) during the cooling from 120 °C to room temperature.

Figure 3

Fluorescence emission spectra of the TPAH-B8 hot solution and organogel in cyclohexane (8 mg mL–1). The insets are photographs of TPAH-B8 organogel and hot solution under UV light.

Fluorescence emission spectra of the TPAH-B8 hot solution and organogel in n class="Chemical">cyclohexane (8 mg mL–1). The insets are photographs of TPAH-B8 organogel and hot solution under UV light.

To understand whether ultrasound affects the molecular self-assembly and molecular alignment of TPAH-B8, we compared the precipitate (not treated by ultrasound) and the n class="Chemical">xerogel from cyclohexane by means of Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and field emission scanning electron microscopy (FE-SEM). The precipitate and the xerogel had very similar FT-IR spectra, both displaying −NH vibrations at 3217 cm–1 and C=O vibrations at 1635 and 1641 cm–1, as shown in Figure S8 (Supporting Information), which meant that the intermolecular hydrogen bonds in the two kinds of samples were consistent. In addition, the XRD patterns of the precipitation and xerogel were similar too, indicating the same molecular packing mode (Figure S9, Supporting Information). The aggregation morphology of TPAH-B8 in the state of xerogel and precipitation was observed by FE-SEM (Figures a and S10, Supporting Information), where a three-dimensional (3D) network pattern entangled by long fibers was obtained from the xerogel, while short bars were obtained from precipitation. All of these results indicated that ultrasound just induced rapid nucleation but did not change the packing mode of the molecules.[50,51]

Figure 5

FE-SEM images of TPAH-B8 (a) xerogel from cyclohexane and (b) ground xerogel and (c) after annealing treatment for (b).

Mechanofluorochromic Property of TPAH-B8

As discussed above, the simplified compound TPAH-B1 is nonplanar (Figure S1, Supporting Information). Therefore the mechanofluorochromic behavior of n class="Chemical">TPAH-B8 can be expected.[52,53] As shown in Figure , the xerogel of TPAH-B8 from cyclohexane emitted a bright blue fluorescence with an emission peak at ca. 434 nm. Upon grinding, the fluorescence color of the xerogel converted to cyan and its emission peak red-shifted to 466 nm. When the ground powder was annealed at 70 °C for 10 min, the fluorescence emission of the ground powder restored to its original state. This mechanofluorochromic behavior conversion could be repeated for at least three times without fatigue (Figure S11, Supporting Information). These observations demonstrated that the mechanofluorochromism of TPAH-B8 was reversible upon grinding and annealing treatments. Similar results could also be observed in ethanol xerogel (Figure S12, Supporting Information). Since the cyclohexane xerogel and ethanol xerogel of TPAH-B8 exhibit similar mechanofluorochromic properties, we selected cyclohexane xerogel of TPAH-B8 and explored its mechanofluorochromic mechanism.

Figure 4

(a) Photographic images of TPAH-B8 in different solid states irradiated at 365 nm and (b) normalized fluorescence emission spectra of TPAH-B8 in different solid states excited at 360 nm.

(a) Photographic images of TPAH-B8 in different solid states irradiated at 365 nm and (b) normalized fluorescence emission spectra of n class="Chemical">TPAH-B8 in different solid states excited at 360 nm.

The aggregation morphology of TPAH-B8 was examined by FE-SEM. As shown in Figure a, the n class="Chemical">TPAH-B8 cyclohexane xerogel initially exhibited a fibrous entangled network structure, whereas amorphous powder with random distribution was observed after grinding (Figure b). After annealing the ground sample at 70 °C for 10 min, the rodlike aggregates could be observed (Figure c).

FE-SEM images of TPAH-B8 (a) n class="Chemical">xerogel from cyclohexane and (b) ground xerogel and (c) after annealing treatment for (b).

The hydrogen-bonding interactions of n class="Chemical">TPAH-B8 were observed by FT-IR spectra (Figure S13, Supporting Information). The characteristic stretching vibration band of the amide N–H group was observed at 3217 cm–1, which was slightly red-shifted to 3218 cm–1, and the intensity of the band decreased; moreover, the C=O stretching vibrations at around 1640 cm–1 partly shifted to higher frequencies upon grinding, which indicated that the intermolecular hydrogen bonding weakened after grinding.[54]

To gain an insight into the mechanofluorochromic mechanism of TPAH-B8, X-ray diffraction experiments were investigated in different states. As shown in Figure , the as-prepared n class="Chemical">xerogel from cyclohexane exhibited many sharp and intense peaks, indicating an ordered crystalline arrangement. In contrast, the ground sample showed very weak diffraction peaks, which demonstrated that it was amorphous.[55,56] After annealing, sharp and strong diffraction peaks reappeared, implying the recovery of an ordered crystalline state. This result indicated that the reversibility of the mechanofluorochromic behavior was due to the reversible phase transition between crystalline and amorphous states.[57]

Figure 6

XRD patterns of TPAH-B8 in different solid-state (a) xerogel as prepared, (b) after grinding, and (c) after annealing.

XRD patterns of TPAH-B8 in different solid-state (a) n class="Chemical">xerogel as prepared, (b) after grinding, and (c) after annealing.

The formation of an amorphous state upon grinding could be also confirmed by differential scanning calorimetry (DSC) experiments. As shown in Figure S14a (Supporting Information), the as-prepared xerogel of n class="Chemical">TPAH-B8 showed an evident endothermic peak at 157 °C, corresponding to its melting point. Different from the result of the as-prepared xerogel, in addition to the obvious endothermic peak at 157 °C, another weak broad exothermic peak at 54 °C appeared during the heating process of the ground powder, indicating the transition from the amorphous to crystalline state (Figure S14b, Supporting Information). The weak broad exothermic peak at 54 °C corresponded to the recrystallization of the ground powder present in a metastable amorphous phase.[58]

Acidochromic Property of TPAH-B8

Because the acylhydrazine group can bind with a proton to form a cation,[32,35] n class="Chemical">TPAH-B8 might be used as a sensor for detecting H+. UV–vis absorption and fluorescence spectra of TPAH-B8 in chloroform with the addition of different acids were obtained, and the results indicated that the changes were most obvious only upon the addition of trifluoroacetic acid (TFA) (Figure S15, Supporting Information). Here, to test the response behavior of TPAH-B8 to acid, trifluoroacetic acid (TFA) was selected and the response behavior of TPAH-B8 to TFA under different states (gel, solid, and solution) was studied. As shown in Figure , with the addition of TFA (20 equiv) onto the top of the cyclohexane organogel, the pale-yellow gel gradually collapsed and turned into a brown-red solution, and the whole process was completed within 3 min. Simultaneously, the blue emission band at 433 nm of the organogel gradually quenched with the organogel turning into a solution. These results showed that TPAH-B8 gel was sensitive to TFA. With addition of TFA, it can make obvious changes in UV−vis absorption and emission spectra, or even visible changes in phase and color. In addition, with the addition of triethylamine (TEA) and ultrasound treatment (this process was completed within 3 min after the gel was collapsed by the addition of TFA), the solution could be restored to organogel with the fluorescent emission recovered (Figure ).

Figure 7

Fluorescence emission spectra of TPAH-B8 cyclohexane organogel (8 mg mL–1) treated with TFA–TEA (20 equiv). The insets are photographs of TPAH-B8 gel–sol transition by treatment with TFA–TEA (20 equiv) under daylight and UV light.

Fluorescence emission spectra of TPAH-B8 cyclohexane organogel (8 mg mL–1) treated with n class="Chemical">TFA–TEA (20 equiv). The insets are photographs of TPAH-B8 gel–sol transition by treatment with TFA–TEA (20 equiv) under daylight and UV light.

Additionally, the acidochromic property of TPAH-B8 in the solid state was also investigated. As shown in Figure S16 (Supporting Information), when the cast film was fumigated with n class="Chemical">TFA, the naked-eye color of TPAH-B8 film changed from pale-yellow to deep yellow, while its fluorescence color changed from bright blue to dark yellow. The fluorescence emission intensity of the TPAH-B8 film decreased with the increase of TFA concentration (ppm), and its maximum emission peak shifted from 447 to 519 nm (Figure ). When the TFA concentration reached 1315.2 ppm, the fluorescence quenching efficiency of TPAH-B8 film reached 81.6%. To demonstrate the sensitivity of the film in sensing gaseous TFA, the concentration-dependent fluorescence quenching efficiency (1 – I/I0) is shown in the inset of Figure . Accordingly, the detection limit was determined to be 1.1 ppm for the TFA vapor.[25,59] Therefore, the film based on TPAH-B8 could be quenched by TFA vapor and used as fluorescent sensory materials. Interestingly, upon fuming this film with TEA, the intensive blue fluorescence emission band at 447 nm can be recovered, and upon fuming this film by TFA again, the blue fluorescence is quenched immediately again. This process can be repeated more than three times (Figure S17, Supporting Information).

Figure 8

Fluorescence emission spectra of TPAH-B8 film upon exposure to different amounts of TFA vapor (λex = 360 nm). Inset: the concentration-dependent fluorescence quenching efficiencies of the film exposed to different amounts of TFA vapor for 10 s.

Fluorescence emission spectra of TPAH-B8 film upon exposure to different amounts of n class="Chemical">TFA vapor (λex = 360 nm). Inset: the concentration-dependent fluorescence quenching efficiencies of the film exposed to different amounts of TFA vapor for 10 s.

The interaction of TPAH-B8 with n class="Chemical">TFA was investigated by UV–vis spectra titration experiment. The UV–vis absorption spectra of TPAH-B8 toward TFA in chloroform (tested within about 1 min) are shown in Figure S18 (Supporting Information). Upon titration of TPAH-B8 solutions with TFA, the bands at ca. 300 and 375 nm disappeared gradually and a new broad shoulder band at ca. 445 nm appeared and intensified, and the color of the solution changed from colorless to yellow gradually. The new absorption band implied that TPAH-B8 was protonated by TFA.[32,35] However, the UV–vis spectrum of the TPAH-B8 dilute solution containing a certain amount of TFA changed after being left for different times. As shown in Figure S19a (Supporting Information), when TFA was not added, the UV–vis spectrum of the dilute solution of TPAH-B8 remained essentially unchanged over time (0–54 min). However, when 15 equiv of TFA was added, the UV–vis spectrum of the dilute solution of TPAH-B8 changed significantly over time (0–40 min). The peak that appears at ca. 445 nm gradually decreases, while the peak at ca. 375 nm gradually increases (Figure S19b, Supporting Information). Similar results were obtained after adding 50 equiv of TFA to a dilute solution of TPAH-B8 (Figure S19c, Supporting Information). The disappearance of the peak at ca. 445 nm and the enhancement of the peak at ca. 375 nm might be the hint that the TPAH-B8 is hydrolyzed with the addition of TFA. The hydrolysis of TPAH-B8 caused the disappearance of the peak at ca. 445 nm, which means that acidochromism was not caused by the hydrolysis of TPAH-B8. To demonstrate the above finding, a further spectroscopic study was carried out with the chloroform solution of aldehyde, hydrazide, and their mixture. We know that the hydrolysis of acylhydrazone yields aldehydes and hydrazides.[60,61] For the TPAH-B8 molecule mentioned in this paper, if the molecule undergoes hydrolysis, it will be hydrolyzed into compound 1 (Figure S20a, Supporting Information) and compound 2 (Figure S20b, Supporting Information). We used compound 1 (purchased from Innochem, 98%), compound 2,[62] and mixtures of the two to study the UV–vis spectrum before and after the addition of TFA. As shown in Figure S21a (Supporting Information), when TFA is not added, the maximum absorption peaks of compound 1, compound 2, and their equivalent mixtures are at 363, 262, and 363 nm, respectively. However, when a large equivalent (200 equiv or more) of TFA was added, the maximum UV–vis absorption peaks showed red shifts to varying degrees; however, no large absorption peaks appeared around 445 nm (Figure S21b–d, Supporting Information). This shows that after adding TFA, the maximum absorption peak at ca. 445 nm in the TPAH-B8 solution (Figure S18, Supporting Information) is caused by protonation rather than hydrolysis.

The protonation of TPAH-B8 and then the hydrolysis of n class="Chemical">TPAH-B8 after adding TFA can also be proved by 1H NMR spectral analysis (Figure S22, Supporting Information). With the addition of TFA (15 equiv), the peak of CH=N shifted a lot, which means that the TPAH-B8 molecule is protonated,[32,35] and the protonation of TPAH-B8 can also be proved by the 1H NMR titration experiment (Figure S23, Supporting Information). The small peak at 9.8 ppm before adding TFA is assigned to the H atom of the aldehyde group in compound 1 (Figure S24, Supporting Information). The appearance of this peak means that TPAH-B8 was also hydrolyzed in CDCl3 solution to some extent (this may be due to slightly acidic solution and the presence of certain of water in the solution). After adding TFA, the peak at 9.8 ppm moved to 9.67 ppm and the integrated area of the peak increased from 0.01 to 0.06 (both based on −OCH2– near 4.0 ppm, the integrated area of −OCH2– peak was set as 4.0), which means that the degree of hydrolysis of TPAH-B8 was enhanced (Figure S22, Supporting Information). The peak area at 9.67 ppm increased with the storage time (0–1470 min), which means that the degree of hydrolysis increased gradually (Figure S22, Supporting Information).

Conclusions

A new triphenylamine-substituted acylhydrazone derivative (n class="Chemical">TPAH-B8) was synthesized. TPAH-B8 could gelate cyclohexane and ethanol with the ultrasonic treatment. A typical gelation-induced fluorescence enhancement property was observed in cyclohexane, which was attributed to the formation of J-aggregate in the gel form. With ICT and nonplanar properties, TPAH-B8 showed fluoresponsive properties such as solvatochromism and mechanofluorochromism. A 59 nm red shift of fluorescence in a dilute solution from cyclohexane to DMSO was observed, which suggested the ICT characteristic of this D–A molecule. Further DFT calculation revealed that the electron was transferred from the triphenylamine group (D) to the acylhydrazone group (A) during the ICT process. The TPAH-B8 molecule possessed mechanofluorochromic property. The color of the fluorescence of TPAH-B8 xerogel from cyclohexane could reversibly change between cyan and blue with grinding and annealing treatments. SEM, XRD, FT-IR, and DSC studies revealed that the mechanofluorochromic mechanism was the transition between the crystalline and amorphous states upon external stimuli. In addition, TPAH-B8 possessed acidochromic property. After the addition of TFA (20 equiv), the cyclohexane gel of TPAH-B8 collapsed and the color of the system changed from pale-yellow to brownish red, while the fluorescence of the system was significantly quenched. Interestingly, when the TPAH-B8 film was exposed to saturated TFA vapor, its color rapidly changed from pale-yellow to deep yellow and its fluorescence significantly quenched. The quenching efficiency reached 81.6% when the concentration of TFA vapor reached 1315.2 ppm. The detection limit of the TPAH-B8 film toward gaseous TFA was ca. 1.1 ppm. Moreover, the fluorescence color of TPAH-B8 in organogel and solid states could also be switched by TFA/TEA. The acidochromism of TPAH-B8 was due to the protonation of TPAH-B8 caused by TFA. This work will be helpful for designing new kinds of multistimuli responsive fluorescent materials in the future.

Experimental Section

Characterization

1H NMR spectra were recorded with a Mercury-300BB 300 MHz spectrometer, using n class="Chemical">tetramethylsilane (TMS) as an internal chemical shift reference. Field emission scanning electron microscopy (FE-SEM) observations were taken with a JSM-6700F apparatus. X-ray diffraction (XRD) data were collected on a Bruker Avance D8 X-ray diffractometer. The FT-IR spectra were recorded with a Perkin-Elmer spectrometer (Spectrum one B). Sonication was performed on a KQ-2200V ultrasound cleaner (maximum power, 100 W, 40 kHz, Kunshan Meimei Ultrasonic Instrument Co, Ltd., China). The UV–vis absorption spectra were determined on a Shimadzu UV-2550 spectrometer, and photoluminescence was measured on a Perkin-Elmer LS 55 spectrometer. The room-temperature luminescence quantum yields in solutions were determined relative to quinine sulfate in sulfuric acid aqueous solution (0.546), and calculated according to the following equation: Φunk = Φstd(Iunk/Aunk)(Astd/Istd)(ηunk/ηstd)2, where Φunk is the radiative quantum yield of the sample; Φstd is the radiative quantum yield of the standard; Iunk and Istd are the integrated emission intensities of the sample and the standard, respectively; Aunk and Astd are the absorptions of the sample and the standard at the excitation wavelength, respectively; and ηunk and ηstd are the indexes of the refraction of the sample and standard solutions (pure solvents were assumed), respectively. The thermal properties of the samples were investigated with a TA Q20 DSC instrument. The rate of heating and cooling was 10 °C min–1.

Gelation test: the gelator and solvents were put into a septum-capped test tube and heated until the solid was completely dissolved into the solvent. The resulting solution was left to cool to room temperature for over 1 h or sonicated using an ultrasonic cleaner (100 W, 40 kHz) for several minutes. The gel was considered successfully formed by the “inverse flow” method.[63]

Synthesis of TPAH-B8

TPAH-B8 was synthesized by having n class="Chemical">3,4-bis(octyloxy)benzhydrazide[64] (785.2 mg, 0.002 mol) reacting with 4-(N,N-diphenylamino)benzaldehyde (546.7 mg, 0.002 mol) in ethanol (100 mL) under reflux condition for 10 h (Scheme ).[14] The crude product was further purified by recrystallization from ethyl acetate–cyclohexane (v/v = 3:1) mixed solvents to get pure product as a pale-yellow powder (1052.2 mg, 81.2%). The melting point is 157 °C confirmed by DSC.

Scheme 2

Synthetic Route of TPAH-B8

1H NMR (300 MHz, n class="Chemical">DMSO-d6): δ ppm 11.53 (s, 1H), 8.37 (s, 1H), 7.59 (d, J = 8.4 Hz, 2H), 7.51 (td, J = 18.9, 8.1 Hz, 2H), 7.34 (td, J = 15.6, 7.8 Hz, 4H), 7.16–7.02 (m, 7H), 7.00–6.93 (d, J = 8.7 Hz, 2H), 4.07–3.95 (m, 4H), 1.79–1.65 (m, 4H), 1.52–1.18 (m, 20H), 0.92–0.78 (m, 6H); 13C NMR (75 MHz, DMSO-d6): δ ppm 162.4, 151.6, 148.9, 148.1, 147.0, 146.6, 129.8, 128.3, 127.8, 125.6, 125.0, 124.0, 121.6, 121.3, 112.9, 112.5, 68.7, 68.4, 31.3, 28.9, 28.8, 25.6, 22.2, 14.0; FT-IR (silicon wafer, cm–1): 3217, 3066, 3040, 2952, 2923, 2852, 1641, 1635, 1595, 1581, 1540, 1506, 1493, 1468, 1427, 1391, 1358, 1336, 1298, 1277, 1230, 1221, 1150, 1129, 1060, 963, 864, 838, 820, 813, 750, 728, 693, 650, 617, 611, 567, 536, 521, 487; Elemental analysis: calculated for C42H53N3O3 (%): C, 77.86; H, 8.25; N, 6.49. Found: C, 78.22; H, 8.01; N, 6.47.