Multi-stimuli responsive fluorescent behaviors of a donor–π–acceptor phenothiazine modified benzothiazole derivative

Abstract

A novel donor–π–acceptor phenothiazine modified benzothiazole derivative PVBT, in which phenothiazine and phenothiazine groups act as donors and acceptors, was designed and synthesized. The PVBT showed strong yellow fluorescence in solution and the solid state. More interestingly, the PVBT exhibited multi-stimuli responsive fluorescent behaviors. First, the PVBT displayed obvious solvatochromism of fluorescence. Its emission wavelength was strongly affected by solvent polarity, indicating intramolecular change transfer (ICT) transition. Second, the PVBT exhibited reversible mechanochromic luminescence under grinding and fuming with THF vapor treatment on account of the reversible transition between the crystalline and amorphous states under external stimuli. Finally, the PVBT revealed remarkable and reversible acid/base-induced fluorescence switching properties in both solution and the solid state. In particular, the ground film of PVBT could also act as a fluorescent chemosensor for detecting volatile acid vapors of TFA, HCl and HNO₃. These results can provide us with a new idea to develop new kinds of multi-stimuli responsive fluorescent materials.

---

Introduction

In the past few years, a lot of multi-stimuli responsive fluorescent materials have been developed for both fundamental research and practical applications in the fields of sensor, data storage, detection and memory devices.^1–5 Especially, π-conjugated organic materials with solvatochromism, photochromism, electrochromism, mechanochromism, thermochromism and acidochromism showing different colors induced by solvents, light, electric fields, mechanical force, heat and acid, have gained increasing interest since they give more easily detectable signals in response to external stimuli.^6–8 However, up to now, only a few molecules have exhibited multi-stimuli responsive fluorescent behaviors in solution and in the solid state.^9–11 The challenge remains to design and synthesize a molecular structure synchronously possessing multi-stimuli responsive features. Recently, the mechanochromic fluorescence owing to displaying reversible stimuli-responsive fluorescent behaviors in the solid state has been intensively studied and developed. Mechanofluorochromic (MFC) materials can show changes in fluorescence color under mechanical stress and be restored to their original state by annealing or fuming by solvent vapor.¹² However, considering the effects of aggregation-caused quenching (ACQ) that occur in most luminescent materials, the availability of MFC materials based on changes in molecular packing mode is quite limited. Therefore, strong solid emissive materials can be used as promising candidates to develop MFC behaviors. Furthermore, there are a few reports on the application of these materials as fluorescent chemosensors for detecting volatile acid vapors. Xu's group synthesized a new mechanochromic fluorescence materials based on tetraphenylethylene, and these new dyes were found to show acid/base responsive fluorescence switching based on the piezo and protonation–deprotonation control.¹³ Response of strongly fluorescent carbazole-based benzoxazole derivatives to external force and acidic vapors were reported by our group.¹⁴

It was known that non-planar π-conjugated structure acted a key role in realizing mechanofluorochromism because of its loose molecular packing in crystal, which could be damaged easily under external stimuli. Some researches demonstrated that the D–π–A system with ICT emission might exhibit MFC behaviors.¹⁵ On the one hand, phenothiazine was selected as a building block in target molecule because it was a strong electron donor with a non-planar, butterfly conformation,¹⁶ and thus, π-conjugated molecules containing phenothiazine unit would be expected to have MFC activities. On the other hand, benzothiazole is an attractive building block in organic materials, such as organic solar cells,¹⁷ sensors,¹⁸ and data storage,¹⁹ because of its electron-withdrawing character,²⁰ high chemical/photophysical stability and multiple reaction sites compared to other heteroaromatics. In particular, molecules containing nitrogen were found to be responsive to acids because of the easy availability of the lone pair on nitrogen for proton binding.²¹ If the nitrogen atom is located in conjugated skeleton, protonation on nitrogen may change the electron distribution and therefore the color and fluorescence of the molecules. With these in mind, we designed and synthesized new D–π–A type phenothiazine-based benzothiazole derivative (PVBT, Scheme 1). Herein, PVBT emitted strong fluorescence in both solution and the solid state. Moreover, the target molecule PVBT was found to exhibit multi-stimuli responsive fluorescent behaviors. First, it was found that PVBT exhibited ICT emission, that fluorescence bands red-shifted significantly with increasing the solvent polarity, and exhibited obvious solvatochromism of fluorescence. Second, PVBT exhibited reversible mechanofluorochromic behaviors. The as-prepared crystal of PVBT exhibiting yellow fluorescence could be transformed into the powder emitting orange emission upon grinding, and the fluorescence could be recovered when the ground powder was fumed with THF vapor. The XRD patterns suggested that the reversible mechanofluorochromism was derived from the transformation between crystalline and amorphous state. Finally, the PVBT revealed remarkable and reversible acid/base-induced fluorescence switching properties in both solution and the solid state, the fluorescence emission of which could be quenched by TFA due to the formation of protonated benzothiazole and recovered by TEA. In particular, the ground film of PVBT emitted strong orange light and could act as a fluorescence sensory material to detect volatile acid vapors of TFA, HCl and HNO₃. Therefore, this work will be helpful in the design of multi-stimuli responsive fluorescent materials and the fabrication of fluorescence chemosensors with high performance.

Scheme 1 Synthetic route for PVBT.

Experimental section

Materials and measurements

¹H NMR spectra (400 MHz) and ¹³C NMR (100 MHz) spectra were recorded on a Bruker AMX-400 NMR spectrometer in CDCl₃ as the solvent at room temperature. Mass spectra were performed on Agilent 1100 MS series and AXIMA CFR MALDI/TOF (Compact) mass spectrometers. FT-IR spectra were estimated with a Nicolet-360 FT-IR spectrometer by the incorporation of samples in KBr disks. C, H and N elemental analyses were performed with a Perkin-Elmer 240C elemental analyzer. The UV-vis absorption spectra were determined on a Beijing purkinje TU-1810 Spectrophotometer. Fluorescence emission spectra were obtained using Shimadzu RF-5301 PC Spectrofluorophotometer. The fluorescence quantum yields of PVBT in different solvents were measured by comparing to a standard (9,10-diphenylanthracene in benzene, Φ_F = 0.85). The absolute fluorescence quantum yields were estimated on an Edinburgh FLS920 steady state spectrometer using an integrating sphere. The XRD patterns were obtained on an Empyrean X-ray diffraction instrument. The ground powder was prepared by grinding the as-prepared crystal with a pestle in the mortal for 30 min, the fumed sample was obtained by fuming the ground powder with THF vapor for 30 s, and the ground sample was heated at a certain temperature until the fluorescence was totally restored. Differential scanning calorimetry (DSC) curves were determined using a NETZSCH STA499F3 QMS403D/Bruker V70 at a heating rate of 10 °C min⁻¹. The frontier orbitals of PVBT were obtained by density functional theory (DFT) calculations at the B3LYP/6-31G(d) level with the Gaussian 09W program package. DMF was distilled from phosphorous pentoxide, and other chemicals and reagents were used as received without further purification.

Synthetic procedures and characterizations

(E)-3-(4-(Benzo[d]thiazol-2-yl)styryl)-10-ethyl-10H phenothiazine (PVBT). A mixture of 10-ethyl-3-vinyl-10H-phenothiazine 1 (0.32 g, 1.25 mmol), 2-(4-bromophenyl)benzo[d]thiazole 2 (0.33 g, 1.14 mmol), anhydrous potassium carbonate (0.32 g, 2.3 mmol), tetrabutylammonium bromide (0.45 g, 1.4 mmol) and Pd(OAc)₂ (20 mg, 0.089 mmol) was added into 20 mL dry DMF under N₂ atmosphere. The mixture was stirred at 110 °C overnight, and then was cooled to room temperature, followed by poured into water (200 mL) with stirring. After extraction with CH₂Cl₂ (3 × 50 mL), the organic phase was combined and washed with brine. After dried with anhydrous MgSO₄, the solvent was removed. The crude product was purified by column chromatogram (silica gel, petroleum ether/dichloromethane, v/v = 1/1) to obtain PVBT (0.43 g) as a bright yellow rod-like crystal in a yield of 81%. Mp: 217 °C (obtained from DSC); ¹H NMR (400 MHz, TMS, CDCl₃) δ = 8.06 (d, J = 8.4 Hz, 3H), 7.89 (d, J = 7.6 Hz, 1H), 7.57 (d, J = 8.4 Hz, 2H), 7.51–7.47 (m, 1H), 7.39–7.35 (m, 1H), 7.31 (s, 1H), 7.28 (d, J = 8.8 Hz, 1H), 7.17–7.12 (m, 2H), 7.08 (d, J = 16.4 Hz, 1H), 6.98 (d, J = 16.4 Hz, 1H), 6.93–6.89 (m, 1H), 6.88–6.82 (m, 2H), 3.94 (s, 2H), 1.43 (t, J = 7.2 Hz, 3H) (Fig. S1†); ¹³C NMR (100 MHz, CDCl₃) δ (ppm) = 167.74, 154.21, 140.22, 134.99, 132.26, 129.15, 127.88, 127.32, 126.78, 126.34, 126.20, 125.13, 123.11, 122.51, 121.60, 115.06, 114.98, 41.94, 13.01, 12.96 (Fig. S2†); IR (KBr, cm⁻¹): 3428, 2925, 2854, 1630, 1595, 1465, 1384, 1250, 1112, 962, 760, 747; elemental analysis calculated for C₂₉H₂₂N₂S₂: C, 75.29; H, 4.79; N, 6.06. Found: C, 75.38; H, 4.85; N, 5.91; MALDI-TOF MS: m/z: calcd: 462.6, found: 463.7 [M + H]⁺ (Fig. S3†).

Results and discussion

Synthesis

The synthetic route for the phenothiazine-based benzothiazole derivative PVBT is shown in Scheme 1. First, 10-ethyl-3-vinyl-10H-phenothiazine 1 (ref. 22) and 2-(4-bromophenyl)benzo[d]thiazole 2 (ref. 23) were synthesized according to the reported methods. Then, the target molecule PVBT was prepared easily obtained via Heck cross-coupling reaction between compounds 1 and 2 catalyzed by Pd(OAc)₂ at 110 °C for 10 h in a yield of 81%. PVBT was synthesized as a bright yellow rod-like crystalline. All the intermediates and the final product were purified by column chromatography, and the PVBT was characterized by ¹H-NMR, ¹³C-NMR, FT-IR and MALDI-TOF mass spectrometry. The peaks at 7.08 and 6.98 ppm in the ¹H-NMR spectrum from the vinyl group protons had a large coupling constant of 16.4 Hz, indicating that the vinyl group had exhibited a transformation. Meanwhile, in the FT-IR spectrum of compound PVBT, the vibration absorption bands appeared at 962 cm⁻¹, suggesting that C=C bond was in trans-form.²⁴ PVBT easily dissolved in CH₂Cl₂, CHCl₃, THF, DMF, and so on, but exhibited low solubility in hexane and cyclohexane.

Photophysical properties in solutions

Recent studies have demonstrated that a D–π–A fluorescent molecules with a large dipole is a better choice for developing MFC materials.²⁵ Thus, we initially studied its spectral characteristics in solutions to understand its donor–acceptor π-conjugated system. The normalized UV-vis absorption and fluorescence emission spectra of PVBT in different solvents (2.0 × 10⁻⁵ M) are shown in Fig. 1, and the corresponding photophysical data are summarized in Table S1.† It was clear that PVBT showed two obvious absorption bands at ca. 340 nm and ca. 400 nm in different solvents, and the former band could be ascribed to the π–π* transition, and the latter one was derived from an intramolecular charge transfer (ICT) transition, which could be confirmed by the solvent-dependent fluorescence spectra (Fig. 1b). For example, the fluorescence emission maximum of PVBT showed a remarkable red-shift of 97 nm from n-hexane (483 nm) to DMF (580 nm). We found that the Stokes shift increased from 9599 to 11 913 cm⁻¹ with the increasing solvent polarity. Moreover, the maximum of the emission bands shifted significantly to a low energy, accompanied by an obvious broadening of the emission bands in polar solvents, which indicated an intramolecular charge transfer characteristics for the excited state.^26a The fluorescence quantum yields (Φ_F) of PVBT were measured using 9,10-diphenylanthracene (Φ_F = 0.85 in benzene) as the standard. It was found that Φ_F of PVBT decreased significantly with increasing polarity of the solvent (Table S1†), indicating a positive solvatokinetic effect.^26b The Φ_F of PVBT, for example, was 0.76 in cyclohexane and reduced 0.02 in DMF. The lower Φ_F value in more polar solvents can be ascribed to the strong ICT process and dipole–dipole interaction between the molecules. When we increased the solvent polarity, PVBT showed polarity-dependent solvatochromism of fluorescence (Fig. 1c).²⁷

Fig. 1 Normalized solvents-dependent (a) UV-vis absorption and (b) fluorescence spectra (λ_ex = 400 nm) of PVBT. The concentration of the samples was 2.0 × 10⁻⁵ M. (c) Photos of solutions were upon 365 nm light irradiation. The concentration of the samples was 2.0 × 10⁻⁵ M.

Theoretical calculation

To further clarify the ICT transition, we carried out the density functional theory (DFT) calculations for PVBT by Gaussian 09W program using the B3LYP/6-31G(d) method to reveal its electronic structure. The frontier orbital plots of the HOMO and LUMO are shown in Fig. 2. We could find that the HOMO was mainly distributed over the electron donor, i.e., the phenothiazine and the vinyl units, and the LUMO was concentrated on the electron acceptor, i.e., the benzothiazole moiety. These results demonstrated that PVBT was typical D–π–A molecule. As a result, the intramolecular charge transfer would occur in the D–π–A phenothiazine modified benzothiazole derivative. Moreover, the HOMO and LUMO energy levels were also obtained by theoretical calculation. The corresponding HOMO and LUMO energy levels were located at −1.98 eV and −4.93 eV, respectively.

Fig. 2 The frontier orbital plots of the HOMO and LUMO of PVBT.

Mechanofluorochromic properties

In our previous work, we found that phenothiazine-based conjugated compounds exhibited MFC properties and the non-planar architecture of phenothiazine as well as the ICT feature of the D–π–A structures were favorable to construct MFC materials.²⁸ Herein, we predicted that phenothiazine modified benzothiazole derivative PVBT would have MFC properties. The fluorescence responses of PVBT toward grinding and fuming processes are illustrated in Fig. 3. It was clear from Fig. 3a that PVBT showed MFC behaviors. Obvious difference of the emitting colors was detected in the as-prepared crystal and ground powder of PVBT. The as-prepared crystal obtained from the mixture of n-hexane and CH₂Cl₂ was yellow crystalline and emitted bright yellow fluorescence. By simply grinding the crystal with a mortar and pestle, the orange powder with orange emission was obtained. To further reveal the MFC properties of PVBT, the normalized fluorescence intensity in different solid states are shown in Fig. 3b. It was clear that the as-synthesized crystal of PVBT emitted bright yellow luminescence centered at 542 nm. When the as-synthesized crystal was ground, it emitted orange fluorescence centered at 563 nm. Interestingly, the ground powder of PVBT showed different responses to organic solvent fuming and thermal heating. When the ground powder of PVBT was fumed with THF vapor for several seconds, the emitting color could be recovered to bright yellow one (at 542 nm), which had the same emission spectrum as that of the crystal. However, the powder with yellowish orange emission centered at 552 nm could be generated after the ground powder of PVBT was heated. When the fumed and heated powders were reground, the emitting color changed into orange, its emission band red-shifted to 563 nm again. As a result, the emitting color of PVBT could be changed between orange and bright yellow reversibly through grinding and THF fuming treatment. This process of emission color change could be repeated many times (Fig. S4†). It demonstrated that the mechanofluorochromism was reversible upon grinding and fuming treatment. In addition, we found that PVBT emitted strong yellow fluorescence in the solid state, the fluorescence quantum yields (Φ_F) of PVBT in the as-prepared crystal and the ground powder were 62.3% and 51.0%, respectively. Thus, PVBT could be used as solid emitting material.

Fig. 3 (a) Photographic images of PVBT in different solid states irradiated at 365 nm and (b) normalized fluorescence spectra of PVBT in different solid states excited at 400 nm.

It is known that the emission behaviors of solid emitters usually depended on the molecular packing.²⁹ To determine the MFC mechanism of PVBT, XRD patterns and UV-vis spectra of PVBT in different solid states were investigated. Fig. 4 showed the XRD patterns of the as-prepared crystal, the ground, and the fumed powders. The as-prepared crystal of PVBT exhibited many strong and sharp diffraction peaks, which were indicative of their regular crystalline structure. In contrast, the ground powder was amorphous because of very weak diffraction peaks.³⁰ After fuming by THF vapor, sharp and strong diffraction peaks similar to those of the crystal appeared again, implying the recovery of an ordered crystalline lattice. This result suggested that the reversibility of MFC behaviors for PVBT from the crystal to the amorphous state was based on reversible phase transformation of molecular stacking.³¹ The UV-vis absorption spectra of PVBT in different solid states are shown in Fig. S5. † It was found that the as-prepared crystal of PVBT showed two absorption peaks at 340 nm and 379 nm. After it was ground, the absorption peaks of PVBT shifted to 337 nm and 385 nm, respectively, which might be resulted from the changes of molecular packing modes.³²

Fig. 4 XRD patterns of PVBT in different solid states.

To further reveal the effect of heating on the MFC properties, the DSC curves of PVBT in different solid states are shown in Fig. 5. The as-prepared crystal of PVBT showed an evident endothermic peak at 217.0 °C, corresponding to its melting point. In addition to the endothermic peak at 216.0 °C, another peak at 65.0 °C appeared during the heating process of the ground powder, indicating the transition from the amorphous to crystalline state. The weak broad exothermic peak at 65.0 °C corresponded to recrystallization of the ground powder present in a metastable amorphous phase.

Fig. 5 DSC curves of PVBT in the as-prepared crystals (black) and ground powers (red) under nitrogen atmosphere at a heating rate of 10 °C min⁻¹.

Acid/base fluorescence switching behaviors

PVBT might be used as a sensor for detecting H⁺, because the benzothiazole unit can bind with a proton to form a cation.¹³ When trifluoroacetic acid (TFA) was gradually added to the CHCl₃ solution of PVBT, the yellow solution by degrees converted into a dark red one. To investigate the possible acidochromic properties of PVBT, the spectral response ability of PVBT towards TFA in CHCl₃ solution (5.0 × 10⁻⁵ M) was firstly measured (Fig. 6). As shown in Fig. 6a, the absorption band in the range of 380–420 nm for PVBT in CHCl₃ decreased gradually and a new absorption band located at 485 nm appeared and intensified with increasing amounts of TFA. The isometric point was observed at 435 nm, implying equilibrium between two components. We deemed that the benzothiazole unit might be protonated by TFA. Because of the increased electron-withdrawing ability of protonated benzothiazole, a new red-shifted absorption band appeared.³³ Considering the new absorption at 485 nm and the existence of one isometric point, TFA–protonated PVBT in CHCl₃ formed the cation PVBT-H⁺. The electron-withdrawing character of benzothiazole was intensified upon protonation with TFA, which strengthened the push–pull effect and further promoted the ICT process from the phenothiazine moiety to the benzothiazole-H⁺ unit, resulting in a remarkable red shift of the spectrum. The fluorescence of PVBT in CHCl₃ solution could also respond to proton. The fluorescence spectral changes of PVBT upon the addition of TFA are shown in Fig. 6b. It was clear that the maximum emission band located at ca. 552 nm. The fluorescence was quenched by 54% under additional 20 equiv. TFA, at which point the solution still emitted yellow fluorescence. A further increase in TFA amount quenched the solution emission, its fluorescence was reduced by almost 88% when exposed to additional 500 equiv. TFA, and the solution emitted a weak orange fluorescence with new peak appeared at ca. 571 nm. The strong yellow emission of PVBT could be quenched significantly by TFA. Therefore, PVBT could be used as a probe to detect TFA by the naked eye. The change in optical properties induced by TFA could be recovered by the addition of triethylamine (TEA).

Fig. 6 (a) Absorption and (b) fluorescence spectra change in CHCl₃ (5.0 × 10⁻⁵ M) from 0 equiv. to 500 equiv. with additional TFA. Excitation wavelength is 400 nm.

Then we further investigated in the solid state emission change of PVBT with response to TFA vapor. Interestingly, when the ground film was exposed to saturated TFA vapor, its color rapidly changed from orange to dark red, and its fluorescence was obviously quenched even at low concentrations (Fig. 7). When the film was exposed to saturated TEA vapors again, its emission intensity clearly returned to the original state, and its fluorescence backed to the yellow. When the quenched film was exposed to saturated ammonia vapors, its emission intensity did not return to the original state, and its fluorescence was orange emission (Fig. S6†). Herein, the ground film of PVBT was expected to detect volatile acid/base vapors. As shown in Fig. 7, the emission intensity at ca. 562 nm for the ground film of PVBT decreased significantly upon exposure to TFA vapor. The higher was the concentration of TFA vapor, the higher was the fluorescence quenching efficiency. For example, the quenching efficiency reached 97% when the concentration of TFA vapor was 1530 ppm. To demonstrate the sensitivity of the ground film of PVBT in sensing gaseous TFA, the concentration-dependent fluorescence quenching efficiency (1 − I/I₀) is shown in the inset of Fig. 7. A well linear relationship between the quenching efficiencies and the TFA vapor concentration was observed when the concentration was below 300 ppm. Accordingly, the detection limit of the ground film of PVBT could be determined to be ca. 2.3 ppm for the TFA vapor.³⁴ Therefore, such a ground film could detect TFA vapor quantitatively.

Fig. 7 Fluorescence emission spectra of the ground films of PVBT upon exposure to different amounts of TFA vapor (λ_ex = 400 nm). Inset: the concentration-dependent fluorescence quenching efficiencies of the film exposed to different amount of TFA vapor for 5 s.

The UV-vis absorption spectra of the ground film of PVBT upon exposure to different amounts of TFA vapor were investigated (Fig. S7†). We found that the absorption band in the range of 360–450 nm for the ground film of PVBT decreased gradually and a new broad absorption band at ca. 520 nm which was ascribed to protonated PVBT emerged upon exposure to TFA vapor. It was worth noting that only slightly changes in the absorption were detected when the concentration of TFA vapor was maintained at 1530 ppm. This result suggested that a small amount of PVBT molecule was protonated by TFA. These spectral changes in the ground film of PVBT on exposure to TFA clearly illustrated that the ground film of PVBT was more readily able to bind protons than the as-prepared crystal, because of the fact that there were many more naked binding sites in the ground film.

We explored the acidochromic behaviors of PVBT for making a rewritable media by coating ground film of PVBT on a filter paper (Fig. S8†). In the case of PVBT, on writing with acid, the letters were clearly visible and stable, which got erased upon fuming with TEA. This result not only implied that the protonated PVBT was stable, but also illustrated that the ground film of PVBT could be used as stable rewritable media. In addition, we found that the fluorescence in the ground film of PVBT could be quenched upon exposure to other volatile acids of HCl and HNO₃, but could not be quenched upon exposure to saturated HOAc vapors (Fig. S9–S11†). For example, upon exposure of the ground film of PVBT to saturated HCl vapor, the fluorescence intensity of the film completely quenched. When the film was treated with saturated TEA vapor, the emission intensity could be restored to their original state. Such a process can be repeated many times, indicating excellent reversibility. Thus, the ground film of PVBT has the potential application in the field of anhydrous sensing with acid/base fluorescence switching behaviors.

Conclusions

A new D–π–A type phenothiazine modified benzothiazole derivative PVBT was synthesized via Heck reaction with high yield, which displaying multi-stimuli responsive fluorescent properties. It was found that the ICT emission could be detected for PVBT, whose emission bands red-shifted significantly with increasing the solvent polarity, and exhibited strong solvatochromism with emission ranging from n-hexane (483 nm) to DMF (580 nm). Moreover, PVBT displayed significant MFC properties, and the as-prepared crystal of PVBT could emit intense yellow fluorescence under UV irradiation. After grinding, the emitting color of PVBT would change into orange light, and the emission could be recovered when the ground powder was fumed with THF vapor. The mechanochromic luminescence was reversible under the treatment of grinding and fuming with THF vapor. The XRD patterns suggested that the mechanofluorochromism was derived from the transition between the crystalline and amorphous states. Additionally, PVBT also exhibited acidochromic behaviors, TFA could lead to the changes of color and emitting color of PVBT in solution due to the formation of protonated benzothiazole. Interestingly, when the ground film of PVBT was exposed to saturated TFA vapor, its color rapidly changed from orange to dark red, and its fluorescence significantly quenched. For example, the quenching efficiency reached 97% when the concentration of TFA vapor was 1530 ppm. The detection limit of the film towards gaseous TFA was ca. 2.3 ppm. Moreover, the ground film of PVBT could also act as a fluorescence sensory material in detecting other volatile acids of HCl and HNO₃. This work will be helpful in the design of new kinds with multi-stimuli responsive fluorescent materials and the fabrication of fluorescence chemosensors with high performance.

Acknowledgements

This work is financially supported by Scientific Research Fund of Liaoning Provincial Education Department of China (No. L2015528), and Program for Innovative Research Team of the Ministry of Education and Program for Liaoning Innovative Research Team in University.

Notes and references

1. (a) Z. G. Chi, X. Q. Zhang, B. J. Xu, X. Zhou, C. P. Ma, Y. Zhang, S. W. Liu and J. R. Xu, Chem. Soc. Rev., 2012, 41, 3878; (b) X. Q. Zhang, Z. G. Chi, Y. Zhang, S. W. Liu and J. R. Xu, J. Mater. Chem. C, 2013, 1, 3376; (c) P. C. Xue, J. P. Ding, P. Chen, P. P. Wang, B. Q. Yao, J. B. Sun, J. B. Sun and R. Lu, J. Mater. Chem. C, 2016, 4, 5275.
2. (a) I. Gallardo, G. Guirado, J. Hernando, S. Morais and G. Prats, Chem. Sci., 2016, 7, 1819; (b) Y. X. Lei, Y. Z. Liu, Y. Guo, J. X. Chen, X. B. Huang, W. X. Gao, L. B. Qian, H. Y. Wu, M. C. Liu and Y. X. Cheng, J. Phys. Chem. C, 2015, 119, 23138.
3. (a) B. J. Xu, Z. G. Chi, X. Q. Zhang, H. Y. Li, C. J. Chen, S. W. Liu, Y. Zhang and J. R. Xu, Chem. Commun., 2011, 47, 11080; (b) Y. J. Zhang, C. Chen, B. Tan, L. X. Cai, X. D. Yang and J. Zhang, Chem. Commun., 2016, 52, 2835; (c) Y. X. Zhu, Z. W. Wei, M. Pan, H. P. Wang, J. Y. Zhang and C. Y. Su, Dalton Trans., 2016, 45, 943.
4. (a) M. Lee, J. H. Moon, K. M. K. Swamy, Y. Jeong, G. Kim, J. Choi, J. Y. Lee and J. Yoon, Sens. Actuators, B, 2014, 199, 369; (b) R. Zheng, X. F. Mei, Z. H. Lin, Y. Zhao, H. M. Yao, W. Lv and Q. D. Ling, J. Mater. Chem. C, 2015, 3, 10242.
5. (a) P. Alam, V. Kachwal and I. R. Laskar, Sens. Actuators, B, 2016, 228, 539; (b) A. R. Chowdhury, P. Ghosh, B. G. Roy, S. K. Mukhopadhyay, N. C. Murmu and P. Banerjee, Sens. Actuators, B, 2015, 220, 347.
6. (a) S. J. Toal, K. A. Jones, D. Magde and W. C. Trogler, J. Am. Chem. Soc., 2005, 127, 11661; (b) S. J. Lim, B. K. An, S. D. Jung, M. A. Chung and S. Y. Park, Angew. Chem., Int. Ed., 2004, 43, 6346.
7. (a) M. M. Petrov, R. D. Pichugov, M. L. Keshtov and E. E. Makhaeva, Org. Electron., 2016, 34, 1; (b) A. Carletta, X. Buol, T. Leyssens, B. Champagne and J. Wouters, J. Phys. Chem. C, 2016, 120, 10001; (c) Z. X. Fu, J. Lin, L. Wang, C. Li, W. B. Yan and T. Wu, Cryst. Growth Des., 2016, 16, 2322.
8. (a) E. Benassi, B. Carlotti, C. G. Fortuna, V. Barone, F. Elisei and A. Spalletti, J. Mater. Chem. A, 2015, 2, 323; (b) L. Xu, H. Zhu, G. K. Long, J. Zhao, D. S. Li, R. Ganguly, Y. X. Li, Q. H. Xu and Q. C. Zhang, J. Mater. Chem. C, 2015, 3, 9191.
9. (a) W. Li, P. P. Wang and H. Wang, J. Mater. Chem. C, 2016, 3, 3783; (b) S. Z. Pu, Q. Sun, C. B. Fan, R. J. Wang and G. Liu, J. Mater. Chem. C, 2016, 4, 3075; (c) C. Botta, S. Benedini, L. Carlucci, A. Forni, D. Marinotto, A. Nitti, D. Pasini, S. Gighetto and E. Cariati, J. Mater. Chem. C, 2016, 4, 2979.
10. (a) X. D. Guan, K. Q. Fan, T. Y. Gao, A. P. Ma, B. Zhang and J. Song, Chem. Commun., 2016, 52, 962; (b) S. Baddi, S. S. Madugula, D. S. Sarma, Y. Soujanya and A. Palanisamy, Langmuir, 2016, 32, 889.
11. (a) Y. X. Zhu, Z. W. Wei, M. Pan, H. P. Wang, J. Y. Zhang and C. Y. Su, Dalton Trans., 2016, 45, 943; (b) M. Wang, S. M. Sayed, L. X. Guo, B. P. Lin, X. Q. Zhang, Y. Sun and H. Yang, Macromolecules, 2016, 49, 663.
12. (a) Y. Zhang, F. Gao, X. D. Cao, Y. Q. Li, Y. Z. Xu, W. G. Weng and R. Boulatov, Angew. Chem., Int. Ed., 2016, 55, 3040; (b) B. J. Xu, Y. X. Mu, Z. Mao, Z. L. Xie, H. Z. Wu, Y. Zhang, C. J. Jin, Z. G. Chi, S. W. Liu, J. R. Xu, Y. C. Wu, P. Y. Lu, A. Lien and M. R. Bryce, Chem. Sci., 2016, 7, 2201; (c) K. Ohno, S. Yamaguchi, A. Nagasawa and T. Fujihara, Dalton Trans., 2016, 45, 5492.
13. C. P. Ma, B. J. Xu, G. Y. Xie, J. J. He, X. Zhou, B. Y. Peng, L. Jiang, B. Xu, W. J. Tian, Z. G. Chi, S. W. Liu, Y. Zhang and J. R. Xu, Chem. Commun., 2014, 50, 7374.
14. P. C. Xue, B. Q. Yao, P. P. Wang, J. B. Sun, Z. Q. Zhang and R. Lu, RSC Adv., 2014, 4, 58732.
15. (a) Y. Ooyama and Y. Harima, J. Mater. Chem., 2011, 21, 8372; (b) Y. Ooyama, Y. Kagawa, H. Fukuoka, G. Ito and Y. Harima, Eur. J. Org. Chem., 2009, 5321.
16. (a) Z. Q. Zhang, Z. Wu, J. B. Sun, B. Q. Yao, G. H. Zhang, P. C. Xue and R. Lu, J. Mater. Chem. C, 2015, 3, 4921; (b) C. P. Ma, X. Q. Zhang, Y. Yang, Z. Y. Ma, L. T. Yang, Y. J. Wu, H. L. Liu, X. R. Jia and Y. Wei, J. Mater. Chem. C, 2016, 4, 4786.
17. (a) C. Siua, C. Ho, J. He, T. Chen, X. Cui, J. Zhao and W. Wong, J. Organomet. Chem., 2013, 748, 75; (b) N. Saeed, A. Fahdan, A. M. Asiri, A. Irfan, S. A. Basaif and R. M. Shishtawy, J. Mol. Model., 2014, 20, 2517.
18. (a) F. Wanga, S. Nama, Z. Guoa, S. Parka and J. Yoon, Sens. Actuators, B, 2012, 161, 948; (b) Q. B. Mei, R. Q. Tian, Y. J. Shi, Q. F. Hua, C. Chen and B. H. Tong, New J. Chem., 2016, 40, 2333.
19. (a) Z. Su, H. Zhuang, H. F. Liu, H. Li, Q. F. Xu, J. M. Lu and L. H. Wang, J. Mater. Chem. C, 2014, 2, 5673; (b) E. Shi, J. H. He, H. Zhuang, H. Z. Liu, Y. F. Zheng, H. Li, Q. F. Xu, J. W. Zheng and J. M. Lu, J. Mater. Chem. C, 2016, 4, 2579.
20. T. Jadhav, B. Dhokale, S. M. Mobin and R. Misra, RSC Adv., 2015, 5, 29878.
21. (a) K. Wang, S. Huang, Y. Zhang, S. Zhao, H. Zhang and Y. Wang, Chem. Sci., 2013, 4, 3288; (b) J. Tolose, K. M. Solntsev, L. M. Tolbert and U. H. F. Bunz, J. Org. Chem., 2010, 75, 523; (c) J. Chen, S. Ma, J. Zhang, L. Wang, L. Ye, B. Li, B. Xu and W. Tian, J. Phys. Chem. Lett., 2014, 5, 2781.
22. (a) G. H. Zhang, J. B. Sun, P. C. Xue, Z. Q. Zhang, P. Gong, J. Peng and R. Lu, J. Mater. Chem. C, 2015, 3, 2925; (b) X. P. Qiu, R. Lu, H. P. Zhou, X. F. Zhang, T. H. Xu, X. L. Liu and Y. Y. Zhao, Tetrahedron Lett., 2007, 48, 7582.
23. (a) E. Shi, J. H. He, H. Zhuang, H. Z. Liu, Y. F. Zheng, H. Li, Q. F. Xu, J. W. Zheng and J. M. Lu, J. Mater. Chem. C, 2016, 4, 2579; (b) Z. Su, H. Zhuang, H. F. Liu, H. Li, Q. F. Xu, J. M. Lu and L. H. Wang, J. Mater. Chem. C, 2014, 2, 5673.
24. (a) Y. Zhan, J. Peng, K. Q. Ye, P. C. Xue and R. Lu, Org. Biomol. Chem., 2013, 11, 6814; (b) H. P. Zhou, P. C. Xue, Y. Zhang, X. Zhao, J. H. Jia, X. F. Zhang, X. L. Liu and R. Lu, Tetrahedron, 2011, 44, 8477.
25. (a) Y. X. Lei, Y. Z. Liu, Y. Guo, J. X. Chen, X. B. Huang, W. X. Gao, L. B. Qian, H. Y. Wu, M. C. Liu and Y. X. Cheng, J. Phys. Chem. C, 2015, 119, 23138; (b) Y. Gong, Y. Tan, J. Liu, P. Lu, C. Feng, W. Yuan, Y. Lu, J. Sun, G. He and Y. Zhang, Chem. Commun., 2013, 49, 4009; (c) Y. Sagra and T. Kato, Angew. Chem., Int. Ed., 2011, 50, 9128.
26. (a) J. S. Yang, K. L. Liau, C. M. Wang and C. Y. Hwang, J. Am. Chem. Soc., 2004, 126, 12325; (b) S. K. Wu, Progr. Chem., 2005, 17, 15.
27. (a) S. Basak, N. Nandi, A. Baral and A. Banerjee, Chem. Commun., 2015, 51, 780; (b) S. Basak, S. Bhattacharya, A. Datta and A. Banerjee, Chem.–Eur. J., 2014, 20, 5721.
28. (a) P. C. Xue, B. Q. Yao, J. B. Sun, Q. X. Xu, P. Chen, Z. Q. Zhang and R. Lu, J. Mater. Chem. C, 2014, 2, 3942; (b) P. C. Xue, B. Q. Yao, X. H. Liu, J. B. Sun, P. Gong, Z. Q. Zhang, C. Qian, Y. Zhang and R. Lu, J. Mater. Chem. C, 2015, 3, 1018; (c) G. H. Zhang, J. B. Sun, P. C. Xue, Z. Q. Zhang, P. Gong, J. Peng and R. Lu, J. Mater. Chem. C, 2015, 3, 2925.
29. W. Z. Yuan, Y. Tan, Y. Gong, P. Lu, J. W. Y. Lam, X. Y. Shen, C. Feng, H. H. Y. Sung, Y. Lu, I. D. Williams, J. Z. Sun, Y. Zhang and B. Z. Tang, Adv. Mater., 2013, 25, 2837.
30. T. Wen, D. Zhang, J. Liu, R. Lin and J. Zhang, Chem. Commun., 2013, 49, 5660.
31. (a) Y. Zhan, P. Gong, P. Yang, Z. Jin, Y. Bao, Y. Li and Y. N. Xu, RSC Adv., 2016, 6, 32697; (b) R. Misra, T. Jadhav, B. Dhokale and S. M. Mobin, Chem. Commun., 2014, 50, 9076; (c) Q. K. Qi, X. F. Fang, Y. F. Liu, P. Zhou, Y. Zhang, B. Yang, W. J. Tian and S. X.-A. Zhang, RSC Adv., 2013, 3, 16986.
32. P. Gong, H. Yang, J. B. Sun, Z. Q. Zhang, J. B. Sun, P. C. Xue and R. Lu, J. Mater. Chem. C, 2015, 3, 10302.
33. J. B. Sun, P. X. Xue, J. B. Sun, P. Gong, P. P. Wang and R. Lu, J. Mater. Chem. C, 2015, 3, 8888.
34. (a) P. C. Xue, J. B. Sun, B. Q. Yao, P. Gong, Z. Q. Zhang, C. Qian, Y. Zhang and R. Lu, Chem.–Eur. J., 2015, 21, 4712; (b) L. Shi, C. He, D. Zhu, Q. He, Y. Li, Y. Chen, Y. Sun, Y. Fu, D. Wen, H. Cao and J. Cheng, J. Mater. Chem., 2012, 22, 11629.

---

Footnote

† Electronic supplementary information (ESI) available: ¹H-NMR, ¹³C-NMR and MALDI/TOF MS; photophysical data of target molecule; reversibility of MFC processes; normalized UV-vis absorption of PVBT in different solid states; absorption spectra of the ground film of PVBT upon exposure to different amounts of TFA vapor; fluorescence spectral change of the ground film of PVBT upon exposure to saturated HCl and HNO₃. See DOI: 10.1039/c6ra19791d

---