Research Article www.acsami.org
Functionality-Oriented Derivatization of Naphthalene Diimide: A Molecular Gel Strategy-Based Fluorescent Film for Aniline Vapor Detection Jiayun Fan, Xingmao Chang, Meixia He, Congdi Shang, Gang Wang, Shiwei Yin, Haonan Peng,* and Yu Fang* Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710062, People’s Republic of China S Supporting Information *
ABSTRACT: Modification of naphthalene diimide (NDI) resulted in a photochemically stable, fluorescent 3,4,5-tris(dodecyloxy)benzamide derivative of NDI (TDBNDI), and introduction of the long alkyl chains endowed the compound with good compatibility with commonly found organic solvents and in particular superior self-assembly in the solution state. Further studies revealed that TDBNDI forms gels with nine of the 18 solvents tested at a concentration of 2.0% (w/ v), and the critical gelation concentrations of five of the eight gels are lower than 1.0% (w/v), indicating the high efficiency of the compound as a low-molecular mass gelator (LMMG). Transmission electron microscopy, scanning electron microscopy, and confocal laser scanning microscopy studies revealed the networked fibrillar structure of the TDBNDI/ methylcyclohexane (MCH) gel. On the basis of these findings, a fluorescent film was developed via simple spin-coating of the TDBNDI/MCH gel on a glass substrate surface. Fluorescence behavior and sensing performance studies demonstrated that this film is photochemically stable, and sensitive and selective to the presence of aniline vapor. Notably, the response is instantaneous, and the sensing process is fully and quickly reversible. This case study demonstrates that derivatization of photochemically stable fluorophores into LMMGs is a good strategy for developing high-performance fluorescent sensing films. KEYWORDS: naphthalene diimide, aniline, molecular gels, fluorescent film, sensor response speed and sensing reversibility.18−21 This explains why molecular gels have been employed for the development of fluorescent films used for the sensing of humidity, explosives, organic amines, etc.22−26 Among the gaseous pollutants, organic amines are byproducts of cell growth and decomposition products generated by biological corruption.27 Meanwhile, as a kind of important chemical raw material, organic amines have also been widely used in various areas such as the chemical industry, cosmetic products, food additives, etc.28−32 This means that organic amines could diffuse into air from garbage incineration, wastewater, construction materials, automobile exhaust, factory emissions, etc.33−35 The excess of organic amines in air would seriously damage the ecological environment and pose severe threats to human health.36 In fact, some of the organic amines are usually considered as air quality indicators and also used as biomarkers for certain types of diseases such as uremia, hepatopathy, and lung cancer.37−40 Therefore, developing
1. INTRODUCTION Today, functionality-oriented design and synthesis of lowmolecular mass compound-based molecular gels stimulate scientific activities of chemistry and physics laboratories worldwide.1−4 Within molecular gels, the low-molecular mass gelators (LMMGs) self-assemble into three-dimensional (3D) networked structures with different morphologies through noncovalent intermolecular interactions such as hydrogen bonding, π−π stacking, van der Waals, electrostatic, coordination, charge-transfer, and additional interactions.5,6 These special morphologies and interactions make molecular gels applicable in a wide range of areas.7−13 For instance, gels with cross-linked nanoring structures exhibit breaking-repair ability as new kinds of self-healing materials.14 Furthermore, porous materials, such as xerogels and aerogels, could be produced via utilization of molecular gels and their relevant gel emulsions.15 These materials show fascinating absorbing properties when they absorb to a variety of organic liquids or vapors.16,17 Undoubtedly, functionalization of the porous materials would be a good strategy for developing promising sensing materials because the skeletons could provide rich molecular channels and more contact sites for target molecules, facilitating © XXXX American Chemical Society
Received: April 25, 2016 Accepted: June 27, 2016
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DOI: 10.1021/acsami.6b04915 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Scheme 1. Synthesis of TDBNDI and Its Intermediates a−f
and other aromatic amines.40 Utilizing a quartz crystal microbalance, even parts per million (ppm) levels of aniline could be repetitively detected. However, considering the photochemical stability, reversibility, selectivity, sensitivity, and other measures of performance of the methods reported until now, developing more pronounced methods for the detection of the volatile organic amines remains a challenge. To develop a superior and photochemically stable fluorescent sensing film of organic amines, naphthalene diimide (NDI) was chosen as a core structure because of its better solubility, ease of modification, and ideal photochemical stability.46−49 For this purpose, the HOMO and LUMO energy levels of the structure must be adjusted to adapt the sensing requirement of organic amines, such as aniline. In addition, the aforementioned molecular gel strategy would be a preferential method for fabricating the film as it could provide greater porosity, which is
effective methods for the detection of organic amines is significantly important.41,42 Recently, Zang and co-workers reported that well-defined nanofibers based on a perylene bisimide derivative showed high sensitivity to organic amines.43 Liu and co-workers presented a perylene−cyclodextrin conjugate that exhibited a pronounced improvement in both selectivity and reversibility for detecting aniline.44 Recently, Lu et al. developed an L-phenylalanine derivative (C12PhBPCP) containing a cyano group and a strong emission fluorophore of benzoxazole and realized quantitative detection of volatile acid and organic amine in the vapor phase with a fast response and a low detection limit.45 With the exception of fluorescence techniques, other techniques were also used for the detection of aromatic amines. As an example, recently, Shiratori and coworkers developed a kind of carbon nanocage-embedded nanofibrous film, which shows enormous adsorption to aniline B
DOI: 10.1021/acsami.6b04915 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) UV−vis absorption spectra of TDBNDI in different solvents at a concentration of 5.0 × 10−6 mol/L at room temperature. (b) Fluorescence emission spectra of TDBNDI recorded in different solvents at a concentration of 1 × 10−6 mol/L at room temperature (λex = 348 nm). The inset shows the normalized fluorescence emission spectra. 2.3. Synthesis of TDBNDI. The synthesis of TDBNDI is shown in Scheme 1. The details of the synthesis and the data from the characterizations are provided in the Supporting Information.
favorable for sensing. Accordingly, the degree of conjugation of NDI was intentionally extended via introduction of two phenylacetylene structures at its two bay positions. Moreover, two iso-octyl structures and two tris(dodecyloxy)-benzamide residues were also chemically linked at the proper positions of the extended conjugate because of their good compatibility with common organic solvents.50 In this way, a new NDI derivative, TDBNDI, was designed and synthesized (cf. Scheme 1). It is believed that the compound as designed would be a good candidate for sensing fluorophores of organic amines and may possess the ability to form a variety of molecular gels, laying the foundation for fabricating porous fluorescent films with superior sensing properties. This paper reports the details.
3. RESULTS AND DISCUSSION 3.1. Solution Behavior of TDBNDI. 3.1.1. UV−Vis Absorptions of TDBNDI in Different Solvents. UV−vis absorption is a simple but powerful technique for interrogating the solution behavior of a fluorophore in the solution state. Accordingly, the solution behavior of the as-prepared compound, TDBNDI, in different solvents with varying polarities was systematically studied at a concentration of 5.0 × 10−6 mol/L, and the results are shown in Figure 1a. Examining the figure reveals that the five solvents tested can be divided into two groups, of which the first includes toluene, chloroform, and tetrahydrofuran (THF) and the second nhexane and methylcyclohexane (MCH). Clearly, in the first group of the solvents, the compound shows two sets of sharper absorption bands, which are located around 340 and 528 nm. In contrast, in the second systems, the two sets of absorptions appear to be the same, but the intensities decreased, which were accompanied by broadening of the absorptions, in particular the one appearing at longer wavelengths. Theoretical calculations revealed that the absorptions located around 528 and 340 nm originate from the transition from S0 to S1 and that from to S0 to S6, respectively [cf. energy level structure of TDBNDI (Figure S1)]. The weakening and broadening of the absorptions in the second group of the solvents might be a result of aggregation of TDBNDI possibly due to intermolecular π−π stacking and hydrogen bond formation. Compared with n-hexane and MCH, toluene is an aromatic solvent, and THF and chloroform are polar solvents, which are both unfavorable for the aggregation of the molecules of the compound due to screening of the π−π stacking or hydrogen bond formation, resulting in the greater solubility of TDBNDI.51 With further interrogation of the absorption band appearing at longer wavelengths, it can also be observed that the absorptions of the compound in the two “poor” solvents are not only broadened, but also red-shifted, which is further confirmed by temperature-dependent absorption spectroscopy studies of the MCH solution of TDBNDI (cf. Figure 2), suggesting J-aggregation of the compound.52,53 3.1.2. Fluorescence of TDBNDI in Different Solvents. Via inspection of the structure of TDBNDI (Scheme 1), we
2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. 1,4,5,8-Naphthalenetetracarboxylic dianhydride (Ourchem, 98%), dibromoiso-cyanuric acid (Energy Chemical, 97%), methyl gallate (J&K, 99%), bromodecane (Aladdin, 98%), 2-ethylhexylamine (TCI, 98%), N-(3-dimethylaminopropyl)-Nethylcarbodiimide hydrochloride (J&K, 99%), Pd(PPh3)4 (Alfa Aesar, 99%), and CuI (Alfa Aesar, 98%) were used as received. Diisopropylamine and dimethylformamide were distilled from calcium hydride under argon prior to use. Toluene was distilled from sodium benzophenone ketyl under argon prior to use. All other reagents were of analytical grade and used without further purification. Water used in this work was acquired from a Milli-Q reference system except where specified otherwise. 2.2. Measurements and Characterization. 1H nuclear magnetic resonance (NMR) spectra were acquired on a Bruker AV 600 NMR spectrometer at room temperature. Pressed KBr disks for the powder samples were used for the transmission infrared spectroscopy measurements, and the measurements were conducted on a Bruker VERTEX 70v spectrometer. The MS spectra were recorded on a Bruker maxis UHR-TOF mass spectrometer in ESI positive mode. Elemental analysis of C, H, and N was performed after combustion at 950−1200 °C using IR detection and gravimetric analysis by means of a Vario EL III device. Melting point measurements were taken on an X-5 microscopic melting point meter (Beijing Tech Instrument). Scanning electron microscopy (SEM) images of the films were acquired on a Quanta 200 scanning electron microscope (Philips-FEI). Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-2100 transmission electron microscope at an acceleration voltage of 200 kV. Optical and fluorescence microscopic observations were made on a Nikon ECLIPSE Ti-U instrument. Fluorescence measurements were performed at room temperature on a timecorrelated single-photon counting Edinburgh Instruments FLS 920 fluorescence spectrometer. C
DOI: 10.1021/acsami.6b04915 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 2. UV−vis absorption spectra of TDBNDI in MCH at a concentration of 5.0 × 10−6 mol/L at various temperatures.
Figure 3. Fluorescence emission spectra of TDBNDI in MCH at various concentrations at room temperature.
anticipate that the compound could be a solvatochromic fluorophore as it consists of both a donor unit(s), the two aromatic amido-benzylethenyl structures, and an acceptor unit, the NDI residue. In other words, the fluorescence emission of the compound should be sensitive to solvent polarity. Accordingly, the fluorescence emissions of the TDBNDI solutions employed for UV−vis absorption measurements were also measured at a concentration of 1.0 × 10−6 mol/L at room temperature with 348 nm as the excitation wavelength, and the results are shown in Figure 1b. As expected, both the emission positions and the emission profiles of the compound are highly dependent on the nature of the solvent. With the exception of n-hexane, the positions of the main emissions of the compound in other solvents shift to longer wavelengths along with an increase in solvent polarity (cf. inset of Figure 1b), specifically in the following order: MCH < toluene < chloroform < THF. As for the solvent n-hexane, it is exceptional possibly because of the poor dissolving ability of the compound in it, which must promote aggregation of TDBNDI. It is the aggregate as formed that produces the broad and strong long wavelength emission around 665 nm, in support of the conjecture of J-aggregate formation as mentioned above. Further inspection of the emissions shown in the figure reveals that the most typical one is from the system with MCH as the solvent as it is more complicated and contains three parts, appearing around 530, 590, and 670 nm. Considering the possible solvatochromic and aggregation properties of the compound, it is reasonable to deduce that the first two of the three emissions should originate from two different excited states of TDBNDI, which are the S1 or the local excited (LE) state and the solvent-relaxed state or the intramolecular chargetransfer (ICT) state of the compound. The third, however, should be from the as-mentioned J-aggregate. For other solvents, the observations are similar except for the absence of one or two of the emissions. To further confirm the understanding described above, concentration-dependent fluorescence measurements were further conducted with MCH as the solvent, and the results are shown in Figure 3. As expected, the profile of the emission is highly dependent on concentration. The compound starts to aggregate at concentrations lower than 5.0 × 10−7 mol/L as the aggregate’s emission (∼650 nm) is becoming obvious at this
concentration. Moreover, the emission is increasing with a further increase in the concentration of the compound, and ultimately, it dominates the whole emission as shown in the inset of the figure, suggesting full aggregation of the compound. As for the systems with very low TDBNDI concentrations, their emissions are composed of the first two emissions appearing at shorter wavelengths, of which one belongs to the LE state and the other to the ICT state as discussed earlier, a typical behavior of solvatochromic probes. The diversity of the fluorescence emission of the compound observed in MCH at different concentrations may explain why the fluorescence emission of TDBNDI varies from one solvent to another at the concentration studied. 3.1.3. Photophysical Parameters of TDBNDI. To understand the optical properties of the as-prepared compound more thoroughly, the absorption wavelength, the molar absorption coefficient, the maximal emission wavelength, and the fluorescence quantum yield were measured, and the results are presented in Table 1. The high molar absorption coefficients in the tested solvents demonstrate the large absorption cross section of the compound as prepared. While the fluorescence quantum yield of TDBNDI varies from solvent to solvent, and specifically from
