Inter- and Intra-Hydrogen Bonding Strategy to Control the Fluorescence of Acylhydrazone-Based Conjugated Microporous Polymers and Their Application to Nitroaromatics Detection

Abstract

Acylhydrazone-based fluorescent conjugated microporous polymers (CMPs) with inter-and intra-hydrogen bonding-controlled emissive properties were prepared. The synthesized CMPs (BH-CMP and ABH-CMP) were characterized by Fourier-transform infrared spectroscopy, X-ray diffraction, solid-state ¹³C cross polarization/magic angle spinning nuclear magnetic resonance spectroscopy, and photoluminescence spectroscopy. Interestingly, BH-CMP exhibited emission enhancement via adsorption of water molecules, whereas the emission of ABH-CMP, which possesses free amine groups, decreased upon the addition of water molecules. The differences in the emission trends of BH-CMP and ABH-CMP in the presence of water molecules originate from the formation of different hydrogen-bonding networks in each CMP. The acylhydrazone-based CMPs were applied to the detection of nitroaromatic compounds. As a result, ABH-CMP in DMF exhibited high selectivity for 1,3,5-trinitrotoluene (TNT) over other nitroaromatic compounds nitrobenzene, 1-chloro-4-nitrobenzene, 2,3-dichloronitrobenzene, and 2,4-dinitrotoluene.

1. Introduction

Aggregation-induced emission luminogens (AIEgens) have been intensively applied in optoelectronics such as organic light-emitting diodes (OLEDs) [1,2,3,4,5,6,7], sensors [8,9], and bio-imaging [10,11,12]. Most organic luminogens exhibit lower fluorescence in the solid-state than in the liquid state via a nonradiative decay pathway due to the occurrence of strong π-π interactions in the solid state, whereas AIEgens emit strongly in the solid-state. The possible mechanisms of emission of AIEgens mainly include J-aggregate formation [13,14], twisted intramolecular charge transfer [15,16], and restriction of intramolecular motions (RIM) [17,18], among which the latter has been most commonly proposed. In a previous study, we discovered that the solid-state emission of bis-arylacylhydrazone (BAH)-based luminogens can be turned on and off by reversible adsorption of water vapors, which enabled the fabrication of rewritable papers with “water-ink” [19]. Specifically, we found that the formation of interlayer hydrogen-bonding networks in the presence of water results in parallel J-type aggregates with planarization, thereby enhancing the fluorescence. In other words, we proposed that the fluorescence in BAH is caused by water-specific polymorphism-induced emission enhancement (PIEE) instead of by RIM. However, to confirm this hypothesis, an analysis in network polymer systems with significantly limited intramolecular motion rather than in a single molecule is required.

Conjugated microporous polymers (CMPs) are a class of porous network polymers that have nanopores and π-conjugated skeletons [20,21]. CMPs are attractive candidates as chemosensory materials because of their large surface area, which provides numerous interaction sites for analyte recognition [22,23]. For example, Geng et al. reported that benzoquinone-based CMPs showed a highly sensitive fluorescence quench by picronitric acid [24]. Liu et al. developed a highly luminescent carbazole-based CMP that works as a fluorescent chemosensor toward nitroaromatic compounds (NACs) such as nitrobenzene (NB), 2-nitrotoluene, and 2,4-dinitrotoluene (DNT) [25]. Notably, the detection of NACs is essential for the environment and national security because of their explosiveness and toxicity [26,27].

To construct CMPs, various linkages have been intensively used, including boroxine [28,29], boronic ester [30], imine [31], azine [32], imide [33], and hydrazone [34]. Among them, hydrazone linkages are effective candidates for the construction of CMP building blocks with synthetic accessibility [35] and stimuli responsiveness [36]. Loh et al. reported acylhydrazone-based 2-dimensional covalent organic frameworks (COFs) with tunable solid-state photoluminescence (PL) via interlayer and intralayer hydrogen bonds [37]. They attributed the origin of emission of the acylhydrazone-based COFs to the restriction of intramolecular rotation (RIR) mechanism. However, we previously showed that the emission of BAH moieties occurs not by RIR but by planarization and formation of J-aggregates [19]. Thus, to gain more insight into the origin of the emission in BAH, we were interested in investigating network polymer systems by introducing acylhydrazone into CMPs with limited intramolecular motion.

Herein, we report novel acylhydrazone-based fluorescent CMPs with and without intramolecular hydrogen-bond donors (Scheme 1). We investigated the origin of emissive behaviors of the acylhydrazone-based CMPs, which are in a state of minimal intramolecular motions. In particular, we studied the effect of the water-induced hydrogen-bonding pattern on the luminescence properties of the CMPs. Furthermore, the synthesized fluorescent CMPs were utilized for the detection of NACs, including 1,3,5-trinitrotoluene (TNT).

2. Experimental Section

2.1. Materials

2-Aminoterephthalic-1,4-terephthaloylhydrazide [38] and TNT [39] were synthesized following previously reported procedures. 1,3,5-Triformylbenzene was purchased from Acros Organics (Geel, Belgium). Terephthalic dihydrazide was purchased from Alfa Aesar (Wand Hill, MA, USA). Nitrobenzene (NB) was purchased from Junsei Chemical Co., Ltd. (Tokyo, Japan). 2,3-Dichloronitrobenzene (DCNB) was purchased from Tokyo Chemical Industry CO., Ltd. (Tokyo, Japan). 1-Chloro-4-nitrobenzene (CNB) and DNT were purchased from Sigma-Aldrich (St. Louis, MO, USA). Dimethylformamide (DMF), tetrahydrofuran (THF), N-methyl-2-pyrrolidone (NMP), dioxane, mesitylene, acetic acid (AcOH) were purchased from Samchun Pure Chemical Co., Ltd. (Seoul, Korea). All the chemicals were used as received without further purification.

2.2. Instruments

PL spectra were recorded on a FluoroMate FS-2 fluorescence spectrophotometer (SCINCO, Seoul, Korea) under constant measurement conditions (500 V PMT voltage, 10 nm slit width, and λ_ex = 330 nm, room temperature). Fourier transform infrared (FTIR) spectra were recorded using a VERTEX 70 spectrometer equipped with a diamond-attenuated total reflection (ATR) unit (Bruker Optics Inc., Ettlingen, Germany). Powder X-ray diffraction (PXRD) patterns were obtained using a Rigaku Ultima IV device with a Cu K_α X-ray source (Rigaku Co., Ltd., Tokyo, Japan). Solid-state ¹³C cross-polarization/magic-angle spinning nuclear magnetic resonance (CP/MAS NMR) spectra were recorded on a 500 MHz solid-state NMR spectrometer (AVANCE III HD, Bruker, Germany) at the National Center for Inter-University Research Facilities (NCIRF) at Seoul National University.

2.3. Synthesis

Synthesis of BH-CMP. BH-CMP was synthesized via the AcOH-catalyzed reversible condensation reaction. Briefly, a 10 mL Pyrex tube was charged with 1,3,5-triformylbenzene (0.1 g, 0.62 mmol), terephthalic dihydrazide (0.18 g, 0.93 mmol), dioxane (1.5 mL), mesitylene (3.0 mL), and AcOH (0.5 mL, 6 M). The mixture was sonicated for 5 min, degassed under vacuum, and then heated at 120 °C for 3 days to afford white powder. To remove the starting material and solvent, the product was centrifuged, and washed with THF and NMP. Yield: 0.23 g (82%).

Synthesis of ABH-CMP. ABH-CMP was obtained as a yellow powder following the same method as described above for the synthesis of BH-CMP using 1,3,5-triformylbenzene (0.1 g, 0.62 mmol) and 2-aminoterephthalic-1,4-terephthaloylhydrazide (0.19 g, 0.93 mmol). Yield: 0.25 g (85%).

2.4. PL Measurements

A 1.0 mg portion of the CMPs was added into 1 mL of a H₂O/DMF solvent mixture. The resulting mixture was sonicated for 10 min for dispersion. Then, the PL of the CMP suspension was measured by adding a concentrated solution of NACs until the intensity of the PL signal decreased by at least 70% of the initial PL intensity at maximum emission wavelength (BH-CMP 60: 470 mm, ABH-CMP 0: 478 nm, ABH-CMP 60: 486 nm) upon excitation at 330 nm with exposure for 50 ms at room temperature.

3. Results and Discussion

We found in our previous work that the emission of the BAH moiety is enhanced by planarization and J-aggregation induced by water-specific polymorphism rather than by the RIM mechanistic pathway exclusively [19], which led us to introduce the arylacylhydrazone into a CMP to obtain minimal intramolecular motion. We designed two types of acylhydrazone-based CMPs, i.e., with and without intramolecular hydrogen-bond donors (Scheme 1).

Thus, 2-aminoterephthalic-1,4-terephthaloylhydrazide was chosen as the building block for introducing intramolecular hydrogen bonding into the CMP, and terephthalic dihydrazide was selected to prepare a CMP without intramolecular hydrogen bonding. The acylhydrazone-based CMPs were easily synthesized via the AcOH-catalyzed hydrazone formation reaction between 1,3,5-triformylbenzene and terephthalic dihydrazide (BH-CMP) or 2-aminoterephthalic-1,4-terephthaloylhydrazide (ABH-CMP). In the FTIR spectra, the imine (C=N) stretching bands of BH-CMP and ABH-CMP were observed at 1624 and 1616 cm⁻¹, respectively (Figure S1). In addition, the free amine contained in ABH-CMP showed an N–H stretching mode at around 3300 cm⁻¹, whereas a similar peak was not observed for BH-CMP. The solid-state ¹³C CP/MAS NMR spectra of BH-CMP and ABH-CMP revealed the presence of the carbonyl group (C=O), which gave rise to signals in the 192–199 ppm range (Figure S2), and the signal from the C=N group at 167 ppm supports the formation of an acylhydrazone bond. The PXRD patterns of BH-CMP and ABH-CMP exhibited both sharp and broad features, which indicates the coexistence of amorphous and crystalline components (Figure S3).

To investigate the emission properties of BH-CMP and ABH-CMP, we measured the water fraction-dependent PL emission spectra for BH-CMP and ABH-CMP (Figure 1). Initially, a BH-CMP dispersion in DMF (1.0 mg/mL) was non-emissive, whereas ABH-CMP dispersed in DMF exhibited bright blue emission at 478 nm. Upon increasing the water fraction in the water/DMF cosolvent system containing BH-CMP (from 0% to 60%, Figure 1a), a central emission band appeared at approximately 470 nm with full width at half-maximum (FWHM) of 108 nm and increased gradually. However, the addition of water beyond the water fraction of 60% led to a slight decrease in the emission intensity due to the poor dispersibility of the water/DMF cosolvent causing precipitation (Figure 1b). Notably, this emission enhancement occurred in a network polymer system, which is already “aggregated”, indicating that the emission of the acylhydrazone moiety is induced by water-specific polymorphism and not by RIM.

Alternatively, the water fraction-dependent PL emission spectra of ABH-CMP showed a different pattern from that of BH-CMP. Upon increasing the water fraction in the water/DMF mixture containing ABH-CMP from 0% to 90%, the maximum emission band that initially appeared at 478 nm with FWHM of 63 nm showed a bathochromic shift (~10 nm), and the emission intensity gradually decreased. This red-shift in the PL emission spectra possibly originated from the change in the microenvironment of the chromophore in ABH-CMP. It is noteworthy that with the increase of the water fraction, the fluorescence emission was enhanced in the case of BH-CMP, whereas the fluorescence emission gradually decreased for ABH-CMP.

Figure 2 depicts a possible mechanism for the two different phenomena. The intramolecular rotation of the central benzene of BH-CMP can occur weakly even in the crystalline phase (Figure 2a), causing a non-radiative decay. After adding water molecules to the BH-CMP dispersion, the carbonyl oxygen and amide proton in BH-CMP are locked via intermolecular hydrogen bonding. Furthermore, this hydrogen bonding induces the planarization and formation of J-aggregates, resulting in water-specific PIEE [19]. Unlike BH-CMP, ABH-CMP exhibits a planar conformation due to intramolecular hydrogen bonding between the amine group on the central benzene and the carbonyl oxygen (Figure 2b). This intramolecular hydrogen bonding can block the non-radiative decay pathway induced by RIM, resulting in ABH-CMP emitting fluorescence even in the absence of water molecules. In contrast, after adsorption of water molecules, the planarized and fluorescent ABH-CMP is distorted and disaggregated due to disruption of the intramolecular hydrogen bonding, resulting in fluorescence quenching. This result demonstrates that planarization and J-aggregate formation play a decisive role in the emission of acylhydrazone-based CMPs.

Generally, NACs have an electron-deficient planar benzene ring due to the presence of a nitro (NO₂) group and can easily form a donor–acceptor complex with an electron-rich aromatic substance [40]. During complexation, the PL in electron-donor aromatic groups is quenched because an electron in the excited state moves to the π*-orbital of the NACs instead of to the ground state of the donor molecules [41]. BH-CMP and ABH-CMP, which contain electron-rich and photo-luminous acylhydrazone, seem suitable for the detection of NACs. Therefore, we investigated the fluorescence detection ability of ABH-CMP and BH-CMP for various NACs, including the well-known explosive material TNT.

After dispersing BH-CMP and ABH-CMP at 1 mg/mL in the 60% H₂O/DMF mixture in which BH-CMP shows the highest PL intensity, a TNT concentration-dependent quenching effect was observed. Thus, the emission of a BH-CMP suspension in 60% H₂O/DMF (BH-CMP 60) was almost quenched at a TNT concentration of 1.7 mM, but an ABH-CMP suspension in 60% H₂O/DMF (ABH-CMP 60) was quenched at a TNT concentration 2.8 times higher (4.8 mM) (Figure 3a,b). In contrast, the quenching concentration of TNT in an ABH-CMP suspension in DMF solvent (ABH-CMP 0) was 1.5 mM, which is similar to that of BH-CMP (Figure 3c). This result indicates that ABH-CMP does not form donor–acceptor complexes with TNT in an H₂O/DMF co-solvent system.

The Stern–Volmer plot of the relative PL intensity of BH-CMP 60 with different TNT concentrations showed an upward-bent curve, which indicates the occurrence of amplified quenching (Figure 3a, inset) [42]. Moreover, the limit of detection (LOD) of BH-CMP 60 for TNT was 3.5 μM. The plot of the relative PL quenching intensity of ABH-CMP 60 versus the TNT concentration was linear, and the LOD for TNT was 29.1 μM, which is higher than that of BH-CMP 60 (Figure 3b, inset). Unlike BH-CMP 60, ABH-CMP 0, in which intramolecular hydrogen bonding can be formed, exhibited a downward-bent curve in the Stern–Volmer plot (Figure 3c, inset). At low TNT concentrations, the relative PL intensity of ABH-CMP 0 for TNT was higher than that of BH-CMP 60. However, at higher TNT concentrations, above 1.1 mM, the relative PL intensity of ABH-CMP 0 for TNT was lower than that of BH-CMP 60. The LOD of ABH-CMP 0 for TNT was 8.8 μM, which was much lower than that of ABH-CMP 60 but higher than that of BH-CMP 60.

We further analyzed the PL quenching concentrations of NACs such as NB, CNB, DCNB, and DNT in BH-CMP and ABH-CMP suspensions (Figures S4–S6). BH-CMP 60 exhibited PL quenching with a 70% reduction of the initial PL intensity with NAC concentrations ranging from 1.7 mM (TNT) to 5.7 mM (NB), and the LODs of BH-CMP 60 for NACs were in the range of 3.48 μM (TNT) to 213 μM (DCNB) (Figure S4 and Table S1). ABH-CMP 60 showed the same PL reduction in the range of 4.8 mM (TNT) to 15.2 mM (NB), and the LODs of ABH-CMP 60 for NACs were in the range of 15.3 μM (NB) to 201 μM (DNT) (Figure S5 and Table S1). Interestingly, according to the Stern–Volmer constants obtained for the NACs (Figure 3f), ABH-CMP 0 exhibited the highest selectivity for TNT over other NACs. ABH-CMP 0 exhibited 70% quenching of the initial PL intensity at concentrations between 1.5 mM (TNT) and 17.3 mM (CNB), and the LODs of ABH-CMP 0 for the NACs were in the range of 2.24 μM (DNT) to 408 μM (DCNB) (Figure S6 and Table S1). Meanwhile, BH-CMP 60 showed the highest TNT sensitivity estimated from the LOD; however, its selectivity for NACs was relatively insufficient (Figure 3d). ABH-CMP 60 showed low sensitivity and poor selectivity for NACs compared with other CMP suspensions (Figure 3e).

4. Conclusions

We synthesized two types of acylhydrazone-based fluorescent CMPs, i.e., BH-CMP and ABH-CMP, and found that different mechanistic pathways of emission occurred upon adsorption of water molecules, namely, intermolecular hydrogen bonding-induced emission enhancement and emission quenching caused by disruption of intramolecular hydrogen bonding, respectively. The fluorescent BH-CMP and ABH-CMP were utilized for the sensing of nitroaromatic compounds. The fluorescence emissions of BH-CMP and ABH-CMP were quenched by photoinduced electron transfer from an excited state of a CMP to the π* orbitals of NACs including NB, CNB, DCNB, DNT, and TNT. We confirmed the selectivity and sensitivity of BH-CMP and ABH-CMP suspensions in different solvent systems for NACs by analyzing the LODs and Stern–Volmer constants. An ABH-CMP suspension in DMF exhibited high selectivity for TNT over other NACs. We believe that the insight provided in this work offers a new platform for the development of novel fluorescence sensory materials.