Response of strongly fluorescent carbazole-based benzoxazole derivatives to external force and acidic vapors

Abstract

A carbazole-based D–π–A benzoxazole derivative was found to self-assemble into long fibers, and its emission increased because of the presence of J-aggregates. These fibers exhibited a distinctive response to volatile acid vapors. The vapor of strong acids such as hydrochloric acid or trifluoroacetic acid destroyed the molecular packing within the fiber and produced a fluorescent color change. However, the fibers did not react to the vapor of weak acids such as acetic acid (HOAc). In addition, the fibrous film exhibited isothermally reversible piezo-fluorochromism. Its blue fluorescence was converted to blue-green under a mechanical force and was spontaneously restored at room temperature. More importantly, the response of the fibrous film to HOAc vapor could be controlled by a mechanical stimulus. The colorless ground film readily absorbed HOAc vapor and emitted an orange fluorescence. Furthermore, such colored films were not self-healing, but returned to colorless upon heating, with a blue emission. Acetic acid vapor, may thus, selectively act as a stabilizer and developer to retain the information imparted by a mechanical force. These results show that the response of organic nanofibers to stimuli may be adjusted and controlled by a mechanical stimulus, and vice versa.

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Introduction

Stimulus-active organic functional materials have recently attracted considerable attention.¹ These materials change their physical or chemical characteristics upon exposure to a particular external stimulus, such as heat, light, mechanical force, electricity, magnetism or chemical stimuli. Among these materials, organic molecules with a strong light emission in the solid-state have attracted attention as functional materials in electronics, photographic devices, and solid lasers.² These strongly emissive fluorophores may also used in selectively detecting chemicals such as explosives,³ organic amines,⁴ acidic gases,⁵ metal ions,⁶ and oxygen.⁷

The response of fluorophores to these chemical agents is strongly affected by the morphology of the fluorophore. For example, self-assembled fluorescent fibers used as sensing materials for the detection of explosives and organic amines are required to be small in diameter in order to maximize their surface area. Only ultrathin polymer films have a sufficiently rapid response and low detection limit for explosive vapors, because vapor molecules are the most readily absorbed and diffused in thin films.⁸

Solid-state emission reflects the molecular arrangement and structure in the solid-state.⁹ Molecular orientation and intermolecular interaction are perturbed by mechanical forces such as shearing, grinding, tension or hydrostatic pressure.¹⁰ The presence of a dramatic emission color change indicates piezofluorochromism (PFC). A number of types of organic molecules exhibiting PFC have been designed and synthesized. For example, studies have shown that tetraphenylethene, triphenylethene, and 9,10-bis-vinyl anthracene derivatives or D–π–A molecules produce a change in fluorescence when subjected to pressure.¹¹

Because external forces can stimulate a change in the morphology and stacking of organic molecules, we attempted to determine whether materials exhibiting PFC showed a different response to other stimuli before and after force was applied, or whether such an additional stimulus regulated the PFC behavior of these materials.

In the present study a D–π–A benzoxazole derivative, C1CVB,¹² was used as well as related compounds exhibiting isothermally reversible PFC, to study the manner in which additional stimuli influenced PFC (Scheme 1).

Scheme 1 Molecular structure of C1CVB, C1CVB-H and C1CVBH.

It was found that certain molecules in solvents became self-assembled into long fibrous aggregates and emitted aggregation induced radiation. The nanofibres produced were shown to respond selectively to volatile acid vapors, depending on their level of acidity. The vapors of strong acids such as hydrochloric acid (HCl) or trifluoroacetic acid (TFA) induced a change in the color of the fluorescence of the nanofibres, but acetic acid (HOAc) vapor did not produce this response. More importantly, the ground film changed the color emitted from blue-green to orange when exposed to HOAc vapor, and the film retained its colored state. This difference in sensing behavior has now been explored, and the results suggest that the sensing properties of functional materials towards external stimuli can be significantly modified by secondary stimuli, and vice versa.

Results and discussion

C1CVB was synthesized as a colorless, needle-like crystalline material by a one-step Knoevenagel reaction (ESI Scheme S1†)¹³ and was characterized by elemental analysis, nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy, and mass spectrometry. The peaks at 8.08 and 7.15 ppm in the ¹H-NMR spectrum from the vinyl group protons had a large coupling constant of 16.2 Hz, indicating that the vinyl group had exhibited a transformation. This finding was confirmed by its single-crystal structure (see below). C1CVB dissolves readily in methylene chloride (CH₂Cl₂), benzene, toluene, tetrahydrofuran or dimethylformamide, but at room temperature it has low solubility in cyclohexane, hexane, or polar alcohols.

Under excitation, solutions of C1CVB are weak emitters. Its fluorescence quantum yield (Φ_F) in toluene was 0.01, and in other solvents the Φ_F was also low (ESI Table S1†). The average lifetime in toluene was 0.95 ns (ESI Fig. S1†), and the radiative (K_r) and non-radiative (K_nr) rates were 0.012 and 1.4 ns⁻¹, respectively. Such small values of K_r and large values of K_nr indicate that the excited molecules are mainly deactivated through non-radiative processes. The intramolecular rotation of single bonds and the cis–trans isomerisation of the double bond are responsible for a low value of Φ_F.¹⁴

C1CVB has been used as a sensor for detecting H⁺, because the benzoxazole moiety can bind with a proton to form a cation.¹⁵ Fig. 1(a) shows that when TFA was added, the absorption band of C1CVB in chloroform (CHCl₃) exhibited a continuous bathochromic shift, and a new peak appeared at 445 nm. This peak resulted in a color change in the solution from pale yellowish to strongly yellow. The isobestic point was observed at 415 nm, implying a reactive equilibrium between two components. The blue emission band at 447 nm upon excitation at 370 nm gradually decreased when TFA was added, and an enhanced green emission band emerged at 511 nm (Fig. 1(b)). In addition, the fluorescence spectra of C1CVB were more sensitive than the absorption spectra. For example, in a two-fold excess of TFA, the fluorescence of a CHCl₃ solution was reduced by almost 70%. A 10-fold excess of TFA almost quenched the emission peak at 447 nm, but the original absorption from free C1CVB was still evident at this TFA concentration (ESI Fig. S2†). The increase in solvent polarity induced by the addition of TFA was responsible for the high sensitivity of the fluorescence spectrum to TFA, because the polarity of the solvent can quench the emission of an intramolecular charge-transfer molecule. Considering the new absorption at 445 nm and the existence of one isobestic point, TFA-protonated C1CVB in CHCl₃ forms the cation, C1CVBH (ESI Fig. S3†).

Fig. 1 Absorption and emission spectra of C1CVB in CHCl₃ (2 × 10⁻⁴ M) with additional TFA (a) and (b) and HOAc (c) and (d); λ_em = 370 nm.

The ¹H-NMR spectra before and after the addition of TFA were measured and compared in order to further understand the interaction between C1CVB and TFA. Clear downfield shifts were observed in all protons, particularly at the benzoxazole ring and vinyl group, when TFA was added. For example, the resonance peaks for H1 and H2 at 8.01 and 7.11 ppm were shifted to 8.38 and 7.22 ppm, respectively, i.e., shifts of 0.11 and 0.37 ppm. This result suggests that the proton binding of C1CVB may be ascribed to protonation of the nitrogen atom in the benzoxazole unit, thus reducing the electron density around all protons and causing downfield shifts in the NMR spectra.^13,16 The effect of protonation also accounts for the strong acidity of TFA.

What happens when a weak acid such as HOAc is added? The color of the solution of C1CVB in CHCl₃ was still yellowish, even after the addition of 50 equiv. of HOAc. Only a slight decrease in absorbance was observed when HOAc was added (Fig. 1(c)), and no shift was observed in the absorption band, nor did the addition of HOAc change the color of the fluorescence. Fig. 1(d) shows that the emissive intensity only continued to decrease if the HOAc was added gradually.

This spectral change illustrates that HOAc does not protonate the nitrogen atom of C1CVB to form a cation. Furthermore, adding 5 equiv. of HOAc to a solution of C1CVB in deuterated chloroform induced a downshift in all protons, but the shifts were smaller than those when TFA was used (ESI Fig. S3†). For example, the signals for H1 and H2 were shifted to 8.04 and 7.14 ppm, respectively, meaning a down-field shift of 0.03 ppm in the two protons. These spectral changes suggest that because of the weak acidity of HOAc a hydrogen bond complex (C1CVB-H) of C1CVB and HOAc had been formed. These results clearly illustrate that C1CVB in solution showed a different response to TFA and HOAc, because of their distinct ability to protonate C1CVB. C1CVB in the solid-state can, therefore, be expected to respond distinctively to TFA and HOAc vapors.

It is known that fluorescent nanofibers have been used as sensing materials for the detection of explosives and organic amines. Thin fibers of C1CVB are easily prepared by ultrasound treatment and aging in cyclohexane. A fibrous film was selected to study the response of C1CVB to volatile acid vapors, and was prepared as follows.

A clear solution of C1CVB in hot cyclohexane (3.1 mM) was initially treated in an ultrasonic bath until an ivory suspension was obtained. A portion (50 μl) of the suspension was then dropped on to a silica plate, and a watch glass used to delay evaporation of the solvent. After the solvent had been completely removed, a film of C1CVB was obtained. A scanning electron microscopy (SEM) image of the film showed that the C1CVB had self-assembled into long fibers with a diameter between 50 and 500 nm (Fig. 2, inset). The fibrous film emitted strong fluorescence, with an absolute fluorescence quantum yield of 0.71, which is 26 times greater than in solution. The time-resolved fluorescence spectrum showed that the fiber had a long fluorescence lifetime (2.3 ns), a large K_r (0.32 s⁻¹), and small K_nr (0.13 s⁻¹). The obvious red shift in the absorption spectrum of the fibrous film relative to that in the cyclohexane solution suggested the presence of J-aggregation in the nanofiber (ESI Fig. S4†).¹⁷ The increased emission could therefore be ascribed to the formation of J-aggregation and the restriction of intramolecular rotation.¹⁸

Fig. 2 Fluorescence spectra of the C1CVB film after exposure to the saturated vapor of (a) TFA or (b) HOAc. Inset is an SEM image of the film.

The behavior of the fibrous film in response to a volatile acid was then studied. On exposure to saturated TFA vapor the colorless film rapidly became red, the color of the fluorescence changed from bright blue to red and the intensity of the emission decreased. As shown in Fig. 2(a), the fibrous film had maximum emission at 452 nm, with some shoulder peaks ascribed to vibrational bands. The fluorescence was quenched by 48% under TFA vapor at 337 ppm, at which point the film still emitted blue fluorescence. A further increase in TFA concentration quenched the film emission, its fluorescence was reduced by almost 92% when exposed to TFA vapor at 2359 ppm, and the film emitted a weak red fluorescence. A weak new peak at approximately 470 nm in the absorption spectrum was observed (ESI Fig. S5(a)†), but the original absorption band was still present and was reasonably strong. This spectral change indicated that C1CVB could bind TFA even in the solid-state, although not all molecules formed C1CVBH at this TFA concentration. However, the colorless film maintained a blue fluorescence when exposed to saturated HOAc vapor. Fig. 3 shows that HOAc vapor produced a slight decrease in emissive intensity, but no obvious change in the absorption spectra of the film was observed after exposure (ESI Fig. S5(b)†). In addition, it was observed that the strongly acidic HCl vapor gave the same response as with TFA (ESI Fig. S6(a)†). The different response of C1CVB film to TFA and HOAc can therefore be attributed to the difference between the acidity of the two and this result is similar to the response observed in solution.

Fig. 3 Fluorescence spectra of ground C1CVB films before and after exposure to saturated (a) TFA or (b) HOAc vapor.

In order to explain the difference in response of the film to TFA and HOAc, the molecular arrangement of the C1CVB crystal was investigated using single-crystal X-ray diffraction (XRD) analysis. The crystals and fibers had similar XRD patterns, indicating the same packing model.¹² In the rod-like crystals, one dimensional (1D) π–π molecular packing occurred, in which the distance between two adjacent C1CVB molecules was 3.56 Å and the sliding angle was 34.4° (ESI Fig. S7(a)†). This result clearly indicates the formation of J-aggregation in the crystals, verifying the result obtained by ultraviolet-visible spectroscopy.¹⁹ Many 1D J-aggregates were stacked together by weak intermolecular interaction (ESI Fig. S7(b)†). This close intermolecular stacking in the crystal did not provide a large enough space to allow HOAc molecules to diffuse into the interior of the fiber and induce protonation. Furthermore, a few polar benzoxazole groups were exposed on the fiber surface, because of the fact that cyclohexane is a nonpolar solvent and the N atom of the benzoxazole moiety is located in the center of the entire molecule. Thus, only a small proportion of HOAc molecules can be absorbed on sites on the surface of the fiber at which the nitrogen atoms of the benzoxazole units are located, forming hydrogen bond complexes (C1CVB-H). Consequently, the fluorescence of the fiber film was only slightly quenched by HOAc vapor, even at high concentrations. When TFA and HCl made contact with the N atoms of benzoxazole, the strong acidity of TFA and HCl caused the nitrogen atom to become protonated and transformed into a cation, which then destroyed close packing in the nanofibers. As a result, other TFA or HCl molecules could then diffuse into the interior of the fiber to form C1CVBH, and this accounts for the new absorption peak at 470 nm when the film was exposed to TFA vapor.

This hypothesis was confirmed by observation of the XRD spectra. HOAc vapor did not induce a change in the XRD spectrum of the fibrous film (ESI Fig. S8†), but relatively weak diffraction peaks in XRD pattern were observed when the fibrous film was exposed to TFA, because of the fact that C1CVBH weakened the stacking order of molecules in the nanofibers. The fibrous film of C1CVB thus responded selectively to volatile acids according to their acidity.

The effect of HOAc vapor on the fibrous C1CVB film is not easy to assess, because HOAc is unable to diffuse freely into the fibers. Mechanical stimuli such as shearing, grinding, rubbing, or static pressure, can, however, convert the crystalline phase to an amorphous solid or another crystal phase, which is accompanied by a color change and possibly a change in the fluorescence color. This is a feature of either loose or close stacking of the molecules. Therefore the influence of mechanical stimuli on the response of C1CVB to volatile acids was examined.²⁰

The ground film used as a sensor was prepared as follows. Firstly, the fibrous thin film was formed as described previously and shear stress was then applied using a spatula to form a colorless thin film. The fibrous film became yellowish, even though the absorption spectra of the ground and the fibrous films were similar (ESI Fig. S9(a)†). Application of mechanical shearing to the fibrous film caused a change in the fluorescence from blue to blue-green. Fig. S9(b) (ESI†) shows that the emission band of the ground film was broad and red-shifted, with a maximum at 484 nm. Shearing also caused a decrease in the intensity of the fluorescence.

The spectral change implies that C1CVB is a PFC organic molecule.²¹ The XRD patterns of the fibrous film showed a number of sharp peaks, indicating that the fiber was crystalline in nature. In contrast, the ground film became amorphous, with a lack of XRD peaks (ESI Fig. S10†).²² No obvious shift in the absorption spectra was observed when shear was applied, suggesting that π–π interactions still existed in the ground sample and that mechanical shearing had simply generated disordered small pieces, in which the π–π interaction was maintained, and a large number of naked N atoms of benzoxazole units appeared. When the ground powder was exposed to CH₂Cl₂ vapor for several seconds the weak blue-green fluorescence changed rapidly to a strong blue color, and the process of fluorescence color change could be repeated a number of times. In addition, the absorption spectrum and XRD of the ground film recovered to the original version on fuming, suggesting that the change in fluorescence could be ascribed to the recovery of the crystalline condition.

These spectral changes clearly indicated a reversible microbial fuel cell behavior. More importantly, it was found that the emission of the ground samples recovered spontaneously at room temperature within 20 min (ESI Fig. S11†), in the manner of a self-healing material.²³ This intriguing property was explained using differential scanning calorimetry (DSC; ESI Fig. S12†). A fibrous sample gave an endothermic peak at 211 °C, corresponding to its melting temperature, and the DSC curve of the ground powder showed two transition peaks, one the melting peak and the other a weak broad exothermic peak at 64 °C. The latter corresponded to recrystallization of the ground powder present in a metastable amorphous phase.²⁴ A low transition temperature was to be expected if the ground film was able to recover its fluorescence spontaneously.

Interestingly, when the ground film was exposed to TFA or HCl vapor, its color rapidly changed from colorless to orange, and its fluorescence from strongly blue to weakly red, even at low concentrations (Fig. 3(a) and ESI Fig. S6(b)†). When the TFA concentration was 337 ppm, the fluorescence of the ground film was converted from blue-green to green, and the emission peak maximum shifted to 532 nm. It should be noted that the peak shoulders below 525 nm may be the result of molecular emission in the crystalline state. A further increase in TFA concentration quenched the fluorescence of the film, and the emission peak gradually shifted to red. When the concentration reached 1348 ppm a red band with a maximum at 603 nm was observed. When HCl vapor was used, the color of the fluorescence changed from green to orange at low concentrations (228 ppm), and a red emission (λ_em = 606 nm) was observed at higher concentrations (ESI Fig. S6(b)†). Furthermore, there was an obvious color change in the ground film on exposure to TFA or HCl vapor. A new absorption band with a maximum at 427 nm appeared, and the original absorption band disappeared (ESI Fig. S13(a)†), indicating the transformation of C1CVB into C1CVBH. These spectral changes in the ground film on exposure to TFA and HCl clearly illustrated that C1CVB was more readily able to bind protons than the fibrous film, because of the fact that there were many more naked binding sites in the ground film.

When the ground film was selected to assess its response to HOAc, a rapid and obvious color change from blue-green to yellow occurred in its fluorescence. The maximum emissive peak occurred at 547 nm (Fig. 3(b)). On exposure to saturated HOAc vapor the yellowish ground material darkened in color, and the absorption spectrum of the ground film became similar to that of the fibrous film (ESI Fig. S13(b)†). Some diffraction peaks appeared (ESI Fig. S10†), suggesting that HOAc did not itself promote the formation of C1CVBH but that HOAc vapor caused a number of molecules in the ground film to recrystallize and form aggregates.²⁵ At the same time, a significant number of C1CVB-H were present because of the availability of naked binding sites in the ground film. This confirmed that the distinctive response of C1CVB films to HOAc vapor was caused by different stacking between the sensing molecules. This result also showed that the response of a sensing material to an external stimulus can be modified by alternative stimuli.

The fibrous film exhibited a different response to volatile acid vapor under the stimulus of an external mechanical force, for example volatile acid vapor can adjust the piezochromism of fibrous films. Fig. 4 illustrates the easy formation of a blue-green fluorescent pattern by writing with a stainless steel bar. The pattern showed a poor contrast to the blue fluorescent background because of a small spectral shift after grinding. An orange fluorescent pattern was observed when the patterned film was exposed to a saturated HOAc vapor. The fluorescent background remained blue, because HOAc as a weak acid was absorbed preferentially on to the C1CVB molecules in the patterned region (Fig. 4). HOAc vapor can thus provide an alternative approach to realizing high contrast PFC in the fluorescent color. Additionally, it was found that the color of a HOAc-dyed pattern did not disappear spontaneously at room temperature, even after several days, or on fuming with organic solvents. HOAc vapor may, therefore, be regarded as a stabilizer and developer in retaining the information pattern.

Fig. 4 Photographic images of fibrous film mounted on a piece of weighing paper and subjected to shearing, HOAc vapor, or heating under UV light (365 nm).

It was significant that the HOAc-dyed pattern was erased by heating at 100 °C for 30 min, and the recovered film could be re-used repeatedly (Fig. 4). When saturated TFA vapor was used as a second stimulus, the whole fibrous film became red (ESI Fig. S14†) because of the formation of a large amount of C1CVBH. On the other hand, exposing the patterned film briefly to TFA vapor (1000 ppm for 2 s) caused a yellow pattern, and orange fluorescence, and the unpatterned background remained colorless with blue emission. This phenomenon occurred because the naked C1CVB molecules bound TFA molecules more quickly than those in the fibers (Fig. 5).

Fig. 5 Schematic illustration of films patterned in response to HOAc and TFA vapor at low concentrations.

Conclusions

A carbazole-based benzoxazole derivative was able to self-assemble into long fibers with augmented emission. The fibrous film exhibited PFC, and its fluorescence changed from blue to blue-green when it was ground. The ground film was able to self-heal, and its blue fluorescence was spontaneously restored at room temperature as a result of its low recrystallization temperature. The nanofibers did not respond to HOAc vapor, but the ground film changed its emission color from blue-green to orange when exposed to HOAc vapor because of the large number of naked binding sites present after grinding. The ground HOAc-treated film maintained its colored condition until it was heated to a high temperature. These results showed that the response of the material to a stimulus can be modified by other stimuli, and that features of the material can be manipulated using multiple stimuli.

Acknowledgements

This study was supported financially by the National Natural Science Foundation of China (21103067, and 21374041), the Youth Science Foundation of Jilin Province (20130522134JH), the Open Project of the State Key Laboratory of Supramolecular Structure and Materials (SKLSSM201407), and the Open Project of the State Laboratory of Theoretical and Computational Chemistry (K2013-02).

References

1. (a) M. K. Beyer and H. Clausen-Schaumann, Chem. Rev., 2005, 105, 2921–2948; (b) L. Xia, R. Xie, X. Ju, W. Wang, Q. Chen and L. Chu, Nat. Commun., 2013, 4, 2226–2237; (c) H. Li, D. X. Chen, Y. L. Sun, Y. B. Zheng, L. L. Tan, P. S. Weiss and Y. W. Yang, J. Am. Chem. Soc., 2013, 135, 1570–1576; (d) Q. Benito, X. F. Le Goff, S. Maron, A. Fargues, A. Garcia, C. Martineau, F. Taulelle, S. Kahlal, T. Gacoin, J. Boilot and S. Perruchas, J. Am. Chem. Soc., 2014, 136, 11311–11320; (e) W. R. Browne and B. L. Feringa, Nat. Nanotechnol., 2006, 1, 25–35; (f) H. Jintoku, M. Yamaguchi, M. Takafuji and H. Ihara, Adv. Funct. Mater., 2014, 24, 4105–4112.
2. (a) A. Z. Ruiz, H. Li and G. Calzaferri, Angew. Chem., Int. Ed., 2006, 45, 5282–5287; (b) S. S. Babu, V. K. Praveen and A. Ajayaghosh, Chem. Rev., 2014, 114, 1973–2129; (c) J. Ye, C. Zhang, C. L. Zou, Y. Yan, J. Gu, Y. S. Zhao and J. Yao, Adv. Mater., 2014, 26, 620–624; (d) M. Matsui, M. Fukushima, Y. Kubota, K. Funabiki and M. Shiro, Tetrahedron, 2012, 68, 1931–1935; (e) D. Li, H. Zhang and Y. Wang, Chem. Soc. Rev., 2013, 42, 8416–8433.
3. (a) Y. Che, D. E. Gross, H. Huang, D. Yang, X. Yang, E. Discekici, Z. Xue, H. Zhao, J. S. Moore and L. Zang, J. Am. Chem. Soc., 2012, 134, 4978–4982; (b) M. E. Germain and M. J. Knapp, J. Am. Chem. Soc., 2008, 130, 5422–5423; (c) K. K. Kartha, S. S. Babu, S. Srinivasan and A. Ajayaghosh, J. Am. Chem. Soc., 2012, 134, 4834–4841; (d) B. Xu, X. Wu, H. Li, H. Tong and L. Wang, Macromolecules, 2011, 44, 5089–5092; (e) Y. Wang and Y. Ni, Anal. Chem., 2014, 86, 7463–7470; (f) H. Luo, S. Chen, Z. Liu, C. Zhang, Z. Cai, X. Chen, G. Zhang, Y. Zhao, S. Decurtins, S. Liu and D. Zhang, Adv. Funct. Mater., 2014, 24, 4250–4258.
4. (a) P. Xue, Q. Xu, P. Gong, C. Qian, A. Ren, Y. Zhang and R. Lu, Chem. Commun., 2013, 49, 5838–5840; (b) Y. Che and L. Zang, Chem. Commun., 2009, 5106–5108; (c) B. Jiang, D. Guo and Y. Liu, J. Org. Chem., 2010, 75, 7258–7264; (d) B. Jiang, D. Guo and Y. Liu, J. Org. Chem., 2011, 76, 6101–6107; (e) H. Peng, L. Ding, T. Liu, X. Chen, L. Li, S. Yin and Y. Fang, Chem.–Asian J., 2012, 7, 1576–1582; (f) X. Liu, X. Zhang, R. Lu, P. Xue, D. Xu and H. Zhou, J. Mater. Chem., 2011, 21, 8756–8765; (g) X. Zhang, X. Liu, R. Lu, H. Zhang and P. Gong, J. Mater. Chem., 2012, 22, 1167–1172; (h) L. Shi, Y. Fu, C. He, D. Zhu, Y. Gao, Y. Wang, Q. He, H. Cao and J. Cheng, Chem. Commun., 2014, 50, 872–874; (i) Y. Fu, Q. He, D. Zhu, Y. Wang, Y. Gao, H. Cao and J. Cheng, Chem. Commun., 2013, 49, 11266–11268.
5. (a) P. Xue, R. Lu, P. Zhang, J. Jia, Q. Xu, T. Zhang, M. Takafuji and H. Ihara, Langmuir, 2013, 29, 417–425; (b) Y. Li, K. Liu, J. Liu, J. Peng, X. Feng and Y. Fang, Langmuir, 2006, 22, 7016–7020; (c) F. Robert-Peillard, E. Palacio-Barco, B. Coulomb and J. L. Boudenne, Talanta, 2012, 88, 230–236.
6. (a) Y. Cao, L. Ding, W. Hu, L. Wang and Y. Fang, Appl. Surf. Sci., 2013, 273, 542–548; (b) W. Wang, X. Wang, Q. Yang, X. Fei, M. Sun and Y. Song, Chem. Commun., 2013, 49, 4833–4835; (c) W. Wang, Y. Li, M. Sun, C. Zhou, Y. Zhang, Y. Li and Q. Yang, Chem. Commun., 2012, 48, 6040–6042; (d) P. Li, C. Ji, H. Ma, M. Zhang and Y. Cheng, Chem.–Eur. J., 2014, 20, 5741–5745.
7. (a) Y. Tian, B. R. Shumway and D. R. Meldrum, Chem. Mater., 2010, 22, 2069–2078; (b) D. O. Mártire, W. Massad, H. Montejano, M. C. Gonzalez, P. Caregnato, L. S. Villata and N. A. García, Chem. Pap., 2014, 68, 1137–1140; (c) C. S. Smith and K. R. Mann, J. Am. Chem. Soc., 2012, 134, 8786–8789; (d) C. A. Kelly, C. Toncelli, J. P. Kerry and D. B. Papkovsky, J. Mater. Chem. C, 2014, 2, 2169–2174; (e) X. Wanga and O. S. Wolfbeis, Chem. Soc. Rev., 2014, 43, 3666–3761.
8. Y. Salinas, R. Martínez-Máñez, M. D. Marcos, F. Sancenón, A. M. Costero, M. Parraad and S. Gil, Chem. Soc. Rev., 2012, 41, 1261–1296.
9. (a) H. Y. Zhang, Z. L. Zhang, K. Q. Ye, J. Y. Zhang and Y. Wang, Adv. Mater., 2006, 18, 2369–2372; (b) Y. Dong, B. Xu, J. Zhang, X. Tan, L. Wang, J. Chen, H. Lv, S. Wen, B. Li, L. Ye, B. Zou and W. Tian, Angew. Chem., Int. Ed., 2012, 51, 10782–10785.
10. (a) Z. Chi, X. Zhang, B. Xu, X. Zhou, C. Ma, Y. Zhang, S. Liu and J. Xu, Chem. Soc. Rev., 2012, 41, 3878–3896; (b) X. Zhang, Z. Chi, Y. Zhang, S. Liu and J. Xu, J. Mater. Chem. C, 2013, 1, 3376–3390; (c) A. Pucci and G. Ruggeri, J. Mater. Chem., 2011, 21, 8282–8291; (d) M. M. Caruso, D. A. Davis, Q. Shen, S. A. Odom, N. R. Sottos, S. R. White and J. S. Moore, Chem. Rev., 2009, 109, 5755–5798; (e) K. Ariga, T. Mori and J. P. Hill, Adv. Mater., 2012, 24, 158–176; (f) F. Ciardelli, G. Ruggeri and A. Pucci, Chem. Soc. Rev., 2013, 42, 857–870.
11. (a) Q. Qi, J. Zhang, B. Xu, B. Li, S. X. Zhang and W. Tian, J. Phys. Chem. C, 2013, 117, 24997–25003; (b) T. Han, Y. Zhang, X. Feng, Z. Lin, B. Tong, J. Shi, J. Zhi and Y. Dong, Chem. Commun., 2013, 49, 7049–7051; (c) X. Zhang, Z. Chi, B. Xu, B. Chen, X. Zhou, Y. Zhang, S. Liu and J. Xu, J. Mater. Chem., 2012, 22, 18505; (d) C. Li, X. Luo, W. Zhao, C. Li, Z. Liu, Z. Bo, Y. Dong, Y. Dong and B. Z. Tang, New J. Chem., 2013, 37, 1696–1699; (e) X. Luo, W. Zhao, J. Shi, C. Li, Z. Liu, Z. Bo, Y. Q. Dong and B. Z. Tang, J. Phys. Chem. C, 2012, 116, 21967–21972; (f) X. Luo, J. Li, C. Li, L. Heng, Y. Q. Dong, Z. Liu, Z. Bo and B. Z. Tang, Adv. Mater., 2011, 23, 3261–3265; (g) X. Zhang, Z. Chi, H. Li, B. Xu, X. Li, W. Zhou, S. Liu, Y. Zhang and J. Xu, Chem.–Asian J., 2011, 6, 1470–1478; (h) R. H. Pawle, T. E. Haas, P. Müller and S. W. Thomas III, Chem. Sci., 2014, 5, 4184–4188; (i) R. Li, S. Xiao, Y. Li, Q. Lin, R. Zhang, J. Zhao, C. Yang, K. Zou, D. Liam and T. Yi, Chem. Sci., 2014, 5, 3922–3928.
12. P. Xue, B. Yao, J. Sun, Z. Zhang and R. Lu, Chem. Commun., 2014, 50, 10284–10286.
13. P. Xue, B. Yao, J. Sun, Z. Zhang, K. Li, B. Liu and R. Lu, Dyes Pigm., 2015, 112, 255–261.
14. (a) Z. Song, Y. Hong, R. T. K. Kwok, J. W. Y. La, B. Liu and B. Z. Tang, J. Mater. Chem. B, 2014, 2, 1717–1723; (b) J. Huang, R. Tang, T. Zhang, Q. Li, G. Yu, S. Xie, Y. Liu, S. Ye, J. Qin and Z. Li, Chem.–Eur. J., 2014, 20, 5317–5326; (c) Y. Liao, K. Li, M. Wu, T. Wu and X. Yu, Org. Biomol. Chem., 2014, 12, 3004–3008.
15. (a) P. Xue, P. Chen, J. Jia, Q. Xu, J. Sun, B. Yao, Z. Zhang and R. Lu, Chem. Commun., 2014, 50, 2569–2571; (b) A. Mishra, S. Chaterjee and G. Krishnamoorthy, J. Photochem. Photobiol., A, 2013, 260, 50–58; (c) T. A. Fayed, J. Photochem. Photobiol., A, 1999, 121, 17–25.
16. (a) P. Xue, R. Lu, J. Jia, M. Takafuji and H. Ihara, Chem.–Eur. J., 2012, 18, 3549–3558; (b) C. F. G. C. Geraldes, A. D. Sherry, M. P. M. Marques, M. C. Alpoim and S. Cortes, J. Chem. Soc., Perkin Trans. 1, 1991, 2, 137–146; (c) K. Wang, S. Huang, Y. Zhang, S. Zhao, H. Zhang and Y. Wang, Chem. Sci., 2013, 4, 3288–3293; (d) D. Liu, Z. Zhang, H. Zhang and Y. Wang, Chem. Commun., 2013, 49, 10001–10003.
17. (a) F. Würthner, T. E. Kaiser and C. R. Saha-Möller, Angew. Chem., Int. Ed., 2011, 50, 3376–3410; (b) H. Jintoku, M. Yamaguchi, M. Takafuji and H. Ihara, Adv. Funct. Mater., 2014, 24, 4105–4112; (c) H. Jintokua and H. Ihara, Chem. Commun., 2012, 48, 1144–1146.
18. (a) R. Hu, J. W. Y. Lam, H. Deng, Z. Song, C. Zheng and B. Z. Tang, J. Mater. Chem. C, 2014, 2, 6326–6332; (b) P. Xue, R. Lu, G. Chen, Y. Zhang, H. Nomoto, M. Takafuji and H. Ihara, Chem.–Eur. J., 2007, 13, 8231–8239; (c) R. Hu, N. L. C. Leung and B. Z. Tang, Chem. Soc. Rev., 2014, 43, 4494–4562.
19. (a) S. Yoon, S. Varghese, S. K. Park, R. Wannemacher, J. Gierschner and S. Y. Park, Adv. Opt. Mater., 2013, 1, 232–237; (b) J. Gierschner and S. Y. Park, J. Mater. Chem. C, 2013, 1, 5818–5832.
20. (a) C. Ma, B. Xu, G. Xie, J. He, X. Zhou, B. Peng, L. Jiang, B. Xu, W. Tian, Z. Chi, S. Liu, Y. Zhang and J. Xu, Chem. Commun., 2014, 50, 7374–7377; (b) J. Zhang, J. Chen, B. Xu, L. Wang, S. Ma, Y. Dong, B. Li, L. Ye and W. Tian, Chem. Commun., 2013, 49, 3878–3880.
21. (a) C. Wang, S. Chen, K. Wang, S. Zhao, J. Zhang and Y. Wang, J. Phys. Chem. C, 2012, 116, 17796–17806; (b) G. Zhang, J. Lu, M. Sabat and C. L. Fraser, J. Am. Chem. Soc., 2010, 132, 2160–2162; (c) K. Nagura, S. Saito, H. Yusa, H. Yamawaki, H. Fujihisa, H. Sato, Y. Shimoikeda and S. Yamaguchi, J. Am. Chem. Soc., 2013, 135, 10322–10325; (d) A. E. Metz, E. E. Podlesny, P. J. Carroll, A. N. Klinghoffer and M. C. Kozlowski, J. Am. Chem. Soc., 2014, 136, 10601–10604; (e) W. Ni, Y. Qiu, M. Li, J. Zheng, R. W. Sun, S. Zhan, S. W. Ng and D. Li, J. Am. Chem. Soc., 2014, 136, 9532–9535; (f) Y. Gong, Y. Zhang, W. Z. Yuan, J. Z. Sun and Y. Zhang, J. Phys. Chem. C, 2014, 118, 10998–11005; (g) M. Yuan, D. Wang, P. Xue, W. Wang, J. Wang, Q. Tu, Z. Liu, Y. Liu, Y. Zhang and J. Wang, Chem. Mater., 2014, 26, 2467–2477; (h) M. Krikorian, S. Liu and T. M. Swager, J. Am. Chem. Soc., 2014, 136, 2952–2955.
22. (a) Z. Ma, M. Teng, Z. Wang, S. Yang and X. Jia, Angew. Chem., Int. Ed., 2013, 52, 12268–12272; (b) P. Xue, B. Yao, J. Sun, Q. Xu, P. Chen, Z. Zhang and R. Lu, J. Mater. Chem. C, 2014, 2, 3942–3950; (c) Y. Sagara, T. Mutai, I. Yoshikawa and K. Araki, J. Am. Chem. Soc., 2007, 129, 1520–1521.
23. N. Mizoshita, T. Tani and S. Inagaki, Adv. Mater., 2012, 24, 3350–3355.
24. X. Luo, J. Li, C. Li, L. Heng, Y. Q. Dong, Z. Liu, Z. Bo and B. Z. Tang, Adv. Mater., 2011, 23, 3261–3265.
25. (a) G. Zhang, H. Wang, M. P. Aldred, T. Chen, Z. Chen, X. Meng and M. Zhu, Chem. Mater., 2014, 26, 4433–4446; (b) Y. Zhang, T. Han, S. Gu, T. Zhou, C. Zhao, Y. Guo, X. Feng, B. Tong, J. Bing, J. Shi, J. Zhi and Y. Dong, Chem.–Eur. J., 2014, 20, 8856–8861.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra10330k

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