Experimental Studies on A New Fluorescent Ensemble of Calix[4

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Experimental Studies on A New Fluorescent Ensemble of Calix[4]pyrrole and Its Sensing Performance in the Film State Qingqing Sun, Yanchao Lü, Lingling Liu, Kaiqiang Liu, Rong Miao,* and Yu Fang* Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710119, P. R. China S Supporting Information *

ABSTRACT: The supramolecular approach plays a pivotal role in the construction of smart and functional materials due to the reversible nature of noncovalent interactions. In the present work, two compounds, cholesterol-functionalized calix[4]pyrrole (CCP) and perylene bisimide diacid (PDA), were synthesized. Little fluorescence is observed in the ethanol solution of the mixture of CCP and PDA, while the solution turns fluorescent upon introduction of ammonia, which is attributed to the formation of a supramolecular ensemble, PDA/(CCP)2/NH3. The fluorescence emission of the as-formed ensemble is sensitive to the presence of phenol, an electron-rich analyte. Interestingly, the sensing can also be observed in the film state, and the relevant detection limit (DL) is lower than 1 ppb. Moreover, the sensing could also be performed in a visualized manner. Upon the basis of the findings, a sensor device with instant response and good reversibility was developed. Further studies revealed that the as-developed fluorescent ensemble is also sensitive to the presence of TNT, an electron-poor compound. The DL for this sensing is ∼80 nM. To our knowledge, this is the first report that a fluorescent sensor could be used for phenol sensing in the vapor state, and for sensing of both electron-rich and electron-poor analytes in solution state. It is believed that the present study presents a distinctive example that demonstrates how smart sensing is realized via combination of the host−guest chemistry of calix[4]pyrrole and the aggregation and disaggregation property of PBI derivatives. KEYWORDS: calix[4]pyrrole, perylene bisimide, supramolecular ensemble, host−guest interaction, fluorescent sensing

1. INTRODUCTION A supramolecular self-assembled system constructed via noncovalent interactions has attracted increasing interest due to its stimuli-responsive and reversible nature.1 To date, a number of noncovalent interactions, including hydrogen bonding, π−π stacking, donor−acceptor interaction, hydrophobic effect, and metal−ligand coordination, etc., have been explored to produce supramolecular structures with excellent properties and specific functionalities.2 In addition, such systems are also inherently rich in “molecular information”,3,4 which makes them responsive to subtle changes in the environment, such as light,5,6 heat,7,8 electricity,9 ions, and small molecules,10−14 etc. These features endow them with potential applications in the area of self-healing materials, integrated motion systems, and adaptive smart materials, etc.15−20 While there are quite a few supramolecular systems, macrocyclic compounds have aroused great interest. Calix[4]pyrrole, which consists of four pyrrole units connected to one another via four fully substituted sp3 hybridized carbon atoms, is one of the most attractive macrocyclic compounds. Its derivatives have unique advantages in supramolecular structure formation and have garnered considerable interest in various application areas, including anion recognition, separation and © 2016 American Chemical Society

extraction, multitopic ion pair receptors, and as a host for neutral molecules that can accept pyrrole NH-anion hydrogen bonds.21−23 Usually, calix[4]pyrrole is prone to exist in a 1,3alternate conformation in the absence of a corresponding anion, but it converts to anion-bound cone-like conformer when a corresponding anion is introduced.24 Besides, the −NH groups in calix[4]pyrrole can bind to carboxyl in some cases, and this process can be controlled by pH adjustment. These characteristics lay the foundation for the formation of calix[4]pyrrole-based supramolecular ensembles and make it possible to control their functions at the monomer level via conformational switching. Recently, through combination of electrostatic and donor−acceptor interactions between three monomers (porphyrin carboxylate anion, tetrathiafulvalene calix[4]pyrrole and Li+-encapsulated C60), Sessler et al. developed a supramolecular triad with favorable photoactive electron transfer property.3 On the basis of a similar principle, a redox- and pH-responsive orthogonal supramolecular ensemble with preferable molecular switching characteristics was also realized.5 Received: July 14, 2016 Accepted: October 5, 2016 Published: October 5, 2016 29128

DOI: 10.1021/acsami.6b08642 ACS Appl. Mater. Interfaces 2016, 8, 29128−29135

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ACS Applied Materials & Interfaces

Figure 1. Molecular structures of the two compounds of CCP and PDA synthesized and used in this study and the schematic representation of the ammonia-induced formation of a supramolecular ensemble, PDA/(CCP)2, of CCP and PDA.

It is well-known that fluorescence is a dominant technique used extensively in many fields and shows many advantages in sensing.28 Considering the characteristics of the calix[4]pyrrole and fluorescence technique, it is reasonable to predict that supramolecular association between some typically designed calix[4]pyrrole derivatives and fluorescent molecules would result in systems with remarkable properties. Moreover, when such systems are designed to be responsive to a specific stimulus, they would probably offer favorable candidates for sensing. On the basis of these considerations, we synthesized two compounds (c.f. Figure 1), of which one is cholesterolfunctionalized calix[4]pyrrole (CCP) and the other perylene bisimide diacid (PDA). On the one hand, introduction of the cholesteryl unit would facilitate further application of the asprepared supramolecular ensemble via film fabrication, a prerequirement for sensor device development.27 On the other hand, the derivatives of perylene bisimide (PBI) have high fluorescence quantum yields, good thermal and photochemical stability, and they are widely used in organic semiconductors and fluorescence sensing.25−28 Meanwhile, fluorescence of the PBI derivatives are highly dependent on their existence form: they are highly emissive in the monomer state, but the emission often diminishes or shifts to longer wavelengths when they are aggregated (depending on what kinds of aggregates were formed).27 It is anticipated that the carboxylic acid groups of PDA could offer hydrogen-bonding sites for supramolecular association between PDA and CCP. Besides, reversible assembly and disassembly between the two compounds would be easily achieved by deprotonation or protonation of the carboxylic acid moiety. In addition, association of CCP with PDA could probably promote disaggregation of PDA, and result in enhanced and observable fluorescence, an indication of disaggregation of PDA. Furthermore, the as-obtained supramolecular fluorescent ensemble may show potential applications in sensing due to its unique constitution and structure. This paper reports the details.

2. EXPERIMENTAL SECTION 2.1. Materials. Diethanolamine (Alfa aesar, >99.0%), cholesterol chloroformate (Alfa aesar, >98.0%), pyrrole (Aladdin, > 99.7%), ethyl levulinate (Alfa aesar, >98.0%), N,N′-dicyclo-hexylcarbodiimide (Aladdin, >99.0%), 4-dimethylaminopyridine (Aladdin, >99.0%), 3,4,9,10-perylene tetracarboxylic dianhydride (TCI, >98.0%), N-methyl-2-pyrrolidone (Aladdin, >99.0%), 12-aminododecanoic acid, (Alfa aesar, >98.0%), 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU) (Aladdin, >99.7%), Phenol (TCI, >99%) were used directly without further purification. Dichloromethane and chloroform were distilled from calcium hydride under nitrogen before use. 2,4,6Trinitrotoluene (TNT) and other reagents were of analytical grade and used without further purification or treatment. Water used in this work was acquired from a Milli-Q reference system throughout. N2 used in the work was a product of the nitrogen cylinder, which is also of analytical grade (purity: 99.2%). The vapor in a bottle of concentrated ammonia solution was used as the ammonia source, and to promote formation of the supramolecular ensemble, the ammonia gas was introduced into the solution of CCP and PDA using a syringe. As for the disassembly of the ensemble, the solution was continuously bubbled with nitrogen gas. For sensing tests in the solution state, standard solution of TNT (1 mM) and that of phenol (5 mM) were prepared in pure ethanol. Whereas for the tests in the vapor state, saturated phenol vapor (0.93 ppm, 20 °C) was employed as the vapor source. The concentrations of the analytes were changed by varying the addition volume of the standard solutions, or by using a self-made stationary gas supply system, of which the details will be described later. 2.2. Measurements. 1H NMR measurements were acquired on a Fourier Digital NMR spectrometer (Bruker Avance 600 MHz) at room temperature. Pressed KBr disks for the powder samples were used for the transmission infrared (FTIR) spectroscopy measurements, and their FTIR spectra were obtained with a Bruker VERTEX 70 V infrared 29129


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Figure 2. UV−vis absorption spectra (a) and fluorescence emission spectra (b) of the ethanol solutions of the systems under study: CCP (black line), PDA (red line), Mix (purple line), Mix +1 eq. of CCP (green line), Mix +2 eq. of CCP (pink line), Mix +3 eq. of CCP (yellow line), and Mix +3 eq. of CCP + N2 (blue line), respectively. Concentration of PDA is 5 μM (ethanol solution). (c) The photographs of the ethanol solutions of PDA, PDA/NH3, PDA/(CCP)2/NH3, and PDA/(CCP)2 /NH3/N2 under (1, 2, 3, 4) sunlight and (1′, 2′, 3′, 4′) UV light (365 nm).

spectrometer in an attenuated total reflectance (ATR) mode. The MS were collected on a Bruker maxis UHR-TOF mass spectrometer in ESI positive mode. UV−vis absorption spectra were recorded on a spectrophotometer (Lambda 950, PerkinElmer, U.S.A.) at room temperature. Fluorescence emission spectra were conducted on a time-correlated single photon counting Edinburgh FLS 920 fluorescence spectrometer at room temperature, using an excitation wavelength of 486 nm.

3. RESULTS AND DISCUSSION 3.1. Supramolecular Ensemble Formation. As mentioned in the Introduction, PDA shows different absorption and fluorescent properties in its monomer state and aggregated

Figure 4. Partial 1H NMR spectra of the CD3OD solution of PDA and ammonia with sequential addition of CCP. Highlighted are the signals for the pyrrole NH protons.

state. Therefore, the absorption and fluorescence emission spectra of the ethanol solution of PDA in the absence and presence of other components were recorded, and the results are shown in Figure 2a,b, respectively. It is seen that PDA shows very weak absorption and fluorescence itself, which should be a result of low concentration of monomeric PDA in the solvent.29 Besides, in accordance with the results reported by others, CCP shows no observable absorption and emission in the wavelength region concerned.30 However, the ethanol solution of PDA exhibits much enhanced absorption (blue line

Figure 3. Reversibility of the formation of ensemble of PDA/(CCP)2 in ethanol upon injection and evaporation of ammonia, respectively (λex = 486 nm, λem = 535 nm). The insets are the images of the solutions of PDA/(CCP)2 (a), PDA/(CCP)2/NH3 (b), and PDA/ (CCP)2/NH3/N2 (c), respectively, under UV light (365 nm). 29130


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Figure 5. Fluorescence emission spectra of the ethanol solution of PDA/(CCP)2/NH3 (PDA = 5 μM) in the presence of different concentrations of TNT. Inset is the quenching efficient plot of the quenching data and photographs of PDA/(CCP)2/NH3 (left) and PDA/(CCP)2/NH3 + TNT.

Figure 7. Response of the fluorescent film-based sensor device for phenol detection in vapor state (10 ppb, 20 ppb, 30 ppb, 40 ppb, and 50 ppb, respectively). Detection was repeated five times for each concentration. The inset is the self-made device for monitoring “phenol leaking” in air, which contains a sampling structure, an optical component, a signal amplifying and processing unit, and a result displaying unit, the unit within the blue box is a stationary gas supply system, and the unit within the red box is the system controller.

PDA/NH3, addition of CCP also induced a blue shift of the absorption and emission of PDA. Specifically, for the absorption, the absorption bands shifted from 459, 491, and 525 nm to 453, 486, and 521 nm, respectively, but for the emission, the emissions shifted from 540 and 579 to 535 and 573 nm, respectively, implying that as a component of the 2:1 ensemble (CCP: PDA), PDA exists mainly in a monomer state.32 Interestingly, to the mixture solution, purging of excessive nitrogen resulted in a dramatic reduction in the absorption and emission, an indication of breaking of the possible ensemble. More interestingly, all of the changes were accompanied by obvious color changes as shown in Figure 2c. With reference to the pictures, it is observed that the ethanol solution of PDA looks red in color and shows very weak fluorescence (1 and 1′ in Figure 2c); the PDA/NH3 system looks light yellow−green and blue−green when it is illuminated by sunlight and UV light, respectively (2 and 2′ in Figure 2c); the system turned to green (in sunlight and UV light) when 2 equimolar of CCP was added (3 and 3′ in Figure 2c); the color disappeared or turned to blue upon introduction of nitrogen (4 and 4′ in Figure 2c). Clearly, the formation of the supramolecular ensemble could be modulated by ammonia, and the process could be repeated at least 5 times (c.f. Figure 3). On the basis of the studies described, it is reasonable to deduce that a supramolecular ensemble is formed between CCP and PDA in the presence of ammonia, and the ensemble breaks upon removing the introduced ammonia. To reveal the nature of the interaction between CCP and PDA in the presence of ammonia, 1H NMR measurements were conducted. It was found that with introduction of an excessive amount of ammonia into the CD3OD solution of PDA and CCP, several changes were observed in the spectrum of CCP (c.f. Figure 4). For instance, the signal of NH protons split into two peaks and shifted from 8.12 to 8.36 ppm in the system of 1:1 molar ratio of CCP to PDA. In addition, with increasing the concentration of CCP, the signal of the NH protons shifted further. However, the shift reached a maximum

Figure 6. Fluorescence emission spectra of the ensemble-based film in the presence of different concentrations of phenol vapor (λex = 486 nm). Inset is the fluorescence quenching efficiency plot of the film against the concentration of the phenol vapor. The inset photographs show the ensemble-based fluorescent film before (left) and after (right) exposure to phenol vapor.

in Figure 2a) when excess ammonia was introduced, and at the same time, fluorescence with peaks around 540 and 570 nm, which are typical features of PBI monomer emission,31 appears. This could be understood by considering the possibility that addition of ammonia may cause deprotonation of the carboxyl groups of PDA and thus facilitate dissolution of PDA, leading to enhanced UV−vis absorption and fluorescence emission. Moreover, with further addition of CCP (1−2 equiv), the absorption and emission intensities of the system increase significantly, which may be a result of further dissolution of PDA owing to, possibly, supramolecular ensemble formation between CCP and the deprotonated PDA. However, further increasing the ratio of CCP produced little effect upon the absorption and emission, suggesting that the stoichiometry of the ensemble is 2:1 (CCP: PDA). Further inspection of the spectra, it can also be seen that compared to the system of 29131


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Figure 8. Schematic representation of the formation and disassembly of a fluorescent supramolecular ensemble of CCP with PDA, and its interaction with TNT and phenol in two different modes. Note: the former is unreversible, but the later could be reversed by purging of N2 or air.

3.1.1. Chemo-Responsive Performance Studied. The supramolecular ensemble of PDA/(CCP)2/NH3 as discussed is dynamic in nature, and decomposition of it will result in decrease in fluorescence emission. Therefore, it may find applications in sensing. As reported by Sessler and co-workers, calix[4]-pyrrole and its derivatives are good hosts of some electron-deficient compounds such as trinitrotoluene (TNT).35,36 Accordingly, response of the ensemble to TNT and some other nitro-aromatics was studied. As is known, TNT is a widely used explosive in industry and military operations. It contaminates the environment and groundwater, imposing great threat to the ecological system and to human health. Therefore, sensitive and fast detection of TNT has been given great attention during the last few decades.37−40 However, supramolecular interaction-based fluorescence methods for the detection of TNT have not been reported until now. In the study, we found that the as-prepared supramolecular ensemble, PDA/(CCP)2/NH3, could serve as a favorable candidate for TNT sensing because the fluorescence of the ensemble could be quenched remarkably upon addition of TNT (c.f. Figure 5). Quantitative calculation demonstrates that the detection limit (DL) is ∼80 nM (the details of calculation are provided in the SI). To study the interaction between the ensemble and TNT, the fluorescence response of the ensemble to different dinitrotoluenes (2,6-dinitrotoluene, 2,6-DNT; 2,4-dinitrotoluene, 2,4-DNT; 3,4-dinitrotoluene, 3,4DNT; 2,5-dinitroluene, 2,5-DNT) and nitrobenzene (NB) were recorded (c.f. Figure S7). It was found that NB and 2,5DNT showed almost no effect upon the fluorescence emission of the ensemble, but the others, in particular 2,4-DNT, showed some quenching effect, suggesting that two nitro-groups positioned in proper positions on a benzene ring are needed to produce a significant effect upon the emission of the supramolecular ensemble. It is also demonstrated that the binding of TNT with the supramolecular ensemble was more efficient compared with other nitroaromatic compounds. This is a result basically consistent with that reported by others.17,36 In another control experiment, the effect of TNT upon the

when the ratio of CCP to PDA reached 2:1, confirming that the ratio stands for the composition of the ensemble. Further introduction of nitrogen into the system resulted reversed shift and fusion of the two peaks of the signal, an indication of a breakage of the ensemble and confirmation of the importance of hydrogen bonding to the formation of the supramolecular associate.4,20 As for why the NH signals split into two peaks upon formation of the supramolecular ensemble, it will be discussed later. FTIR measurements also provide evidence for the reversible assembly process (c.f. Figure S5 of the Supporting Information, SI). It is seen that the spectrum of PDA/CCP shows a typical pyrrole NH absorption band at 3328 cm−1, and the spectrum of PDA shows the characteristic COOH absorption at 1696 and 1593 cm−1, respectively.33 The as-mentioned three absorptions appear in the spectrum of the mixture of CCP and PDA, indicating that the compounds are independent from each other. However, with introduction of ammonia into the PDA/CCP solution, the pyrrole NH absorption shifted to 3236 cm−1 and the COOH absorption shifted to 1627 and 1521 cm−1, respectively, an indication of hydrogen-bonds formation between CCP and PDA. With increasing quantities of CCP, the pyrrole NH absorption continued to shift until the molar ratio of CCP/PDA reached 2:1, which is an additional evidence for the formation of the ensemble of PDA/(CCP)2/NH3.34 With further injection of nitrogen, the original pyrrole NH and  COOH signals (3328, 1696, and 1593 cm−1) appeared, an indication of breaking of the ensemble. To further confirm if the supramolecular ensemble was formed, 2D 1H DOSY NMR experiments were carried out in a mixture solvent of CDCl3 and d6-DMSO (v/v = 7/3), and the results are shown in Figure S6. It can be seen that in the absence of DBU, the diffusion coefficient of proton signals corresponding to CCP is about 1.057 × 10−9 m2/s. The value, however, decreased to 1.005× 10−9 m2/s upon introduction of the organic base, which must be a result of an increase in the size of the CCP-related structure, a direct evidence for the formation of the supramolecular ensemble of PDA/(CCP)2. 29132


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ACS Applied Materials & Interfaces fluorescence emission of PDA/NH3 was investigated, and it was shown that addition of TNT shows little effect on the emission of the system, inferring the importance of CCP in the sensing of the as-discussed ensemble. As is known, fluorescence quenching could be a result of formation of a nonemissive complex between a fluorophore and a quencher, or a result of excited state energy transfer from a fluorophore to a quencher during collision of them, which is a diffusion controlled process or dynamic quenching in nature.27 Fluorescence lifetime measurement is a powerful technique to reveal the nature of the quenching process. Accordingly, the fluorescence lifetime of the as-discussed ensemble in ethanol solution was measured in the presence of different concentrations of TNT, and the results are presented in a Stern− Volmer plot (c.f. Figure S8). It was found that the fluorescence intensity of the solution is related to the concentration of TNT, which is attributed to the fact that both PDA and TNT can associate with CCP and there exists competition between them. Further inspection of the plot reveals that it is a straight line at low TNT concentrations, but the plot starts to deviate the line when TNT concentration exceeds a critical value (∼30 μM), which could be an indication of the complexity of the microenvironment of the supramolecular ensemble under study. The corresponding fluorescence lifetime plot is almost a straight line with a slope of nearly zero, indicating that the quenching process is dominated by static quenching. On the basis of the results as presented, it is reasonable to deduce that the host−guest interaction between TNT and the ensemble caused the breakup of the supramolecular ensemble, resulting in reaggregation of PDA. Overall, it is obvious that the asprepared ensemble offers a good candidate for discrimination of TNT, relevant DNT, and NB. To further investigate the nature of the fluorescence quenching induced by TNT, the chemical shift of pyrrole NH signals in the system of PDA/(CCP)2/NH3 was monitored before and after introduction of TNT. The test was carried out in an 8:2 (v/v) mixture of CD3OD and CDCl3. The results are shown in Figure S8. With reference to the spectra, it can be seen that the signals of pyrrole NH moved upfield (from 8.52 to 8.43 ppm) upon addition of TNT, and another change is the appearance of free CCP, as indicated by its unique and fused pyrrole NH signals appearing at 8.12 ppm. These discoveries suggest the possible breaking of the supramolecular ensemble. The interaction between TNT and CCP was further confirmed by another 1H NMR test, at which TNT was introduced in CCP/NH3, and it was found that the NH signal of CCP shifted from 8.12 to 8.22 ppm, an indication of direct interaction between CCP and TNT (c.f. Figure S9). Upon the basis of the results described, a probable sensing mechanism is proposed: the supramolecular fluorescent ensemble transferred from the cone conformation to 1,3-alternative conformation when TNT was added and led to the breakup of the ensemble. The ammonium salt of PDA as released would aggregate or precipitate, and result in diminished fluorescence due to π−π stacking of the PBI cores or decrease in the concentration of the PDA molecules. It is to be noted that the 1H NMR spectrum of one of the control systems, CCP/NH3, also reveals the reason why the NH signal of CCP is split into two in the systems under study. As reported in literatures, PBI and its derivatives are electronpoor compounds, and they could form complexes with some electron-rich aromatic derivatives, showing diminished fluorescence. Accordingly, the sensing performance of the ensemble

as prepared to phenol was investigated. Unlike other electronrich aromatic derivatives, phenol is volatile and listed as a highpriority pollutant by the U.S. Environmental Protection Agency and other organizations due to its high toxicity and persistence.41,42 Pollution of phenol is commonly found in both air and water, and thereby various techniques have been developed to detect phenol. However, most of the reported methods are conducted in solution and rarely can they be used in the vapor phase.43−46 Thus, methods for vapor phase sensing of phenol are highly desired. To test the ability of the asprepared ensemble in phenol vapor sensing, a solution-phase study was initially carried out (c.f. Figure S10). As we expected, the ensemble shows sensitive response to phenol in ethanol solution, and the process is irreversible. So we continue to study the phenol sensing in the vapor state and an ensemblebased film was fabricated (to avoid the influence from ammonia evaporation, another base, DBU, was chosen instead; details of the fabrication are provided in the SI). As is well-known, the sensing performance of a film is closely related to its micro/ nano-structures of the sensor materials on the substrate surface, therefore, the microstructure of the film was characterized by fluorescent micrographs (c.f. Figure S11). It presents a regular structure which laid the foundation for the phenol vapor sensing. It was found that the fluorescence of the film was quenched gradually with the introduction of phenol vapor. A significant decrease of the fluorescence intensity of the film was occurred upon the introduction of 9 ppb of phenol vapor and the fluorescence decreased to the lowest when concentration of phenol vapor reached 7.92 ppm (c.f. Figure 6). Quantitative calculation revealed that the detection limit (DL) of the film to phenol vapor is lower than 1 ppb, which is the only result for phenol vapor detection using a fluorescence technique reported up to now, and are comparable with the sensitivity of a recently reported electric sensor.46 However, the response of our system to the presence of phenol is instantaneous and could be repeatedly used as will be demonstrated later. Further inspection of the quenching efficient plot suggests that the fluorescence quenching of the film is almost linear in the concentration range of 9−90 ppb (c.f. inset of Figure 6), suggesting the potential for quantitative analysis of phenol in air. Interestingly, other phenolic compounds, such as pnitrophenol and p-tert-butylphenol, and common organic solvents (acetone, benzene, and acetonitrile) showed much lower quenching efficiency compared to phenol vapor (c.f. Figure S12), which could be a result of difference in the electron densities and energy levels of the compounds and solvents.25 It can be seen that the binding of phenol vapor with the sensing system was efficient and reversible. Upon the basis of the as-described supramolecular ensemble film, a sensor device was developed for monitoring phenol leaking in air (c.f. Figure 7). It was found that the performance of the sensor device is excellent as evidenced by its fast, sensitive, and fully reversible response to the presence of phenol vapor. These results demonstrate that the device as developed possesses a great potential for real-life uses. More interestingly, detection of phenol vapor in air can also be visualized (Video S1). It was found that the fluorescence emission of the ensemble-based film was quenched rapidly when phenol vapor was introduced. However, the control, the PDA-based film, showed little change, indicating again the importance of ensemble formation for sensing. Moreover, the quenching in the film state is also found to be static in nature, as revealed by fluorescence lifetime measurements (c.f. Figure 29133


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ACS Applied Materials & Interfaces S13). However, with further examination of the fluorescence intensity plot shown in the figure, it is seen that the plot is straight at a low pressure of phenol vapor, but the plot deviated from the line with further increasing vapor pressure. This is a not a very surprising result because the film as developed shows complicated structures (c.f. Figure S11), which may make some of the supra-molecules of the ensemble difficult to reach by the analyte, resulting in decreased quenching efficiencies. Upon the basis of all the results and discussions as presented, understanding of the formation of the as-discussed ensemble and its interactions with TNT and phenol is schematically depicted in Figure 8.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (21273141, 21527802), the 111 project (B14041), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT-14R33).

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4. CONCLUSIONS Cholesterol-functionalized calix[4]pyrrole (CCP) was synthesized and used to construct a fluorescent supramolecular ensemble with the ammonia salt of a perylene bisimide derivative (PDA). It was found that formation and disruption of the supramolecular ensemble could be easily controlled by injecting and evaporating ammonia. Meanwhile, the reversible process is accompanied by remarkable color change in both absorbance and fluorescence. Photophysical and 1H NMR studies revealed that the composition of the supramolecular ensemble is that two molecules of CCP combine one molecule of the ammonia salt of PDA. Interestingly, the fluorescence emission of the supramolecular ensemble in solution state is sensitive to the presence of trinitrotoluene (TNT) (DL < 80 nM), but partially to dinitrotoluenes (DNTs) depending on the specific structure of them. For nitrobenzene (NB), however, it shows no effect upon the fluorescence emission of the ensemble. In addition, phenol in the vapor state is also an efficient quencher of the fluorescence emission of the ensemble. On the basis of the findings, a fluorescent film based on the supramolecular ensemble and the film-based sensor device was fabricated, and successfully used for monitoring “phenol leaking” with excellent performances. It is believed that the present study represents an extraordinary example that shows that smart sensing could be realized via combination of host− guest chemistry of calix[4]pyrrole and aggregation and disaggregation of PBI derivatives.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08642. Synthesis methods; determination of detection limit; fabrication of the supramolecular ensembel-based sensing film; NMR spectra; FTIR spectra; 2D 1H DOSY NMR spectra; interference test−TNT; quenching mechanism− TNT; titration and dilution NMR spectra; the sensing behavior of the ensemble to phenol in solution; microstructure of the film; interference test -phenol vapor; and quenching mechanism−phenol vapor (PDF) Conceptual device for monitoring phenol in vapor phase (AVI)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Tel: 0086-29-81530786. Fax: 0086-29-81530787. E-mail: [email protected] (R.M.). *Tel: 0086-29-81530786. Fax: 0086-29-81530787. E-mail: [email protected] (Y.F.). 29134


Research Article


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Experimental Studies on A New Fluorescent Ensemble of Calix[4

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