Coordination-Directed Assembly of Luminescent Semiconducting Oligomers and Weak Interaction-Induced Morphology Transformation.

Luminescent semiconducting oligomers (LSOs) have been one of the most popular molecular materials that can be applied in various fields because of their distinctive optical properties. The study of molecular packing and morphological change of oligomers is essential for the rational design of materials and regulation functions. Herein, we report two novel LSOs (OFBB and OFBT) with a slight difference in chemical structures but show a distinct difference in self-assembly behaviors in the coordination-driven process. OFBB forms spherical particles with Zn(II). Compared with OFBB, OFBT has an additional thiazole moiety, which forms spherical particles with Zn(II) and then transforms to a crystalline nanobelt in 2 h. The process and mechanism of the nanosphere and nanobelt formation were investigated in detail. The double S···N interaction between two benzothiazoles in adjacent oligomers played a significant contribution in this dynamic morphology transformation. In addition, the as-prepared two products showed excellent sensing toward nitrobenzene with good selectivity over other nitro-aromatic explosives.

Introduction

In recent years, the functionalized organic luminescent polymers and oligomers, especially luminescent semiconducting oligomers (LSOs) have drawn increasing attention as they have presented great potentials in opto-electronics.[1−4] These kinds of evolving materials always have known chemical structures and modifiable molecular units, they can be modified by a variety of methods depending on the demand for functions. The modified oligomers can significantly improve the performance of various applications, many researchers introduce functional groups or special atoms which could stimulate the response to modify the functionalized oligomers, thus widening the application filed.[5−7] For example, some of the modified oligomers were introduced to the luminescent or halogen-containing functional groups and applied in light-harvesting, organic field effect transistors, and sensors.[8−13]

The arrangement of these oligomers and intermolecular interactions have critical influences on the morphology and final functions. The interactions employed in oligomer assembly also present diversely, such as hydrogen bond, metal coordination, and other weak interactions.[14−16] Coordination-driven self-assembly has great advantages as its force strength is between van der Waals force and covalent bonds, and coordination mode is also diverse which could realize some dynamic changes.[17] The different coordination numbers and geometries of central metal cations lead to diverse packing models.[18] Many excellent works have reported the morphological change of the coordination polymers and oligomers can be induced by the pH and concentration of reactants, which indicated that the morphology could be finely regulated by controlling the coordination and the weak interactions during the assembly process.[19−22] Great efforts had been made on the preparation of dimers, oligomers, and even polymers by the synergies between the coordination and subtle interactions, and numerous stimuli-response structures or dynamically transformed structures were developed and applied in various fields.[23−25]

Our previous work revealed that the modification of ligands with long-alkyl chains can significantly improve the colloidal stability of coordination polymers. By proper selection of metal ions, nanowires, nanospheres, and nanodisks could be facilely prepared, which were applied in light harvesting and sensors.[2,5,13] In this work, we focus on the coordination-directed assembly and investigate how weak interactions affect the self-assembly behavior. We introduced two novel fluorescent ligands OFBB and OFBT bearing two carboxylic acid groups (Scheme a,b), which can coordinate with metal ions to form polymeric products. In our previous study, Zn(II) with a relatively low coordination number could induce a one-dimensional (1D) structure.[26] Here, we introduced a benzothiazole unit into ligand OFBT because it can form a dimeric structure via double S···N interactions, which is expected to affect the 1D assembly. OFBB formed blue-emitting nanospheres by coordination with zinc acetate. It is interesting that OFBT formed green fluorescent nanospheres very quickly and then transformed to crystalline nanobelts in 2 h. In addition, the as-prepared two products could sense nitrobenzene (NB) with very good selectivity over other nitro-aromatic compounds, such as picric acid (PA) and trinitrotoluene (TNT). Compared to the majority of the reported nitro-aromatic sensors, the as-prepared zinc-based nanospheres and nanobelts can identify NB from its counterparts.[27−30]

Scheme 1

Chemical Structures of (a) OFBB and (b) OFBT

Results and Discussion

Synthesis and Characterization

In this study, a general solvent reaction was introduced to prepare Zn-OFBB and Zn-OFBT simultaneously, we preset different assembly times. The morphologies of the two studied MOFs presented are obviously different.

Generally, Zn-OFBB samples presented spherical shapes. The mixed solution turned out to be muddy during the process of titration, and then a partial sample was taken out after titration was completed. When the assembly time was 1 min, the Zn-OFBB sample presented a glued ball with a diameter of approximately 200 nm (shown in Figure a), there was an umbilical substance connecting one ball to another. As the assembly time increased, the nanospheres became more independent and the diameter gradually becomes larger (as shown in Figure b,c). Especially, when the assembly time increased to 60 min, the diameter of the spheres grew to about 300 nm (as shown in Figure d), at this time, the particle size of the sample is not particularly uniform. When the reaction time increased to 120 min, the sample appeared more independent and more uniform in particle size (as shown in Figure e). The transmission electron microscopy (TEM) image revealed the nanospheres were solid (as shown in Figure f).

Figure 1

SEM and TEM images of different assembly times of Zn-OFBB samples: 1 (a), 5 (b), 30 (c), 60 (d), 120 min (e), and TEM image (f).

Meanwhile, the assembly of Zn-OFBT was also conducted. Likewise, we also preset different reaction times. Similar to above, the solution turned cloudy during the titration process, and the morphology of the samples appeared to be greatly different from Zn-OFBB. When setting the reaction time as 1 and 5 min, the samples both presented spherical shape like Zn-OFBB (shown in Figure a,b), interestingly, as the assembly time increased to 30 min, there was a belt-like substance appeared rarely (as shown in Figure c). In addition, with the assembly time continually increased, there were more and more belts in the samples, and the number of spherical particles become less and less (as shown in Figure d), simultaneously, the size and the thickness of the belts became bigger. Until the assembly time increased to 120 min, the sample showed absolutely belts and almost no spheres were left (as shown in Figure e). The TEM image showed excellent light transmission (as shown in Figure f). It is really interesting that the two ligands have similar chemical structures but can form distinct morphologies upon coordination with the same metal ion.

Figure 2

SEM images of different assembly times of Zn-OFBT samples: 1 min (a) (inset is magnification of certain area), 5 (b), 30 (c), 60 (d), 120 min (e), and TEM images of Zn-OFBT (f).

X-ray diffraction (XRD) was conducted to further investigate the assembled products. The XRD patterns shown in Figure a revealed that the spherical products (both Zn-OFBB and Zn-OFBT) were amorphous. It is noted that the nanobelt showed several distinct diffraction peaks indicative of high crystallinity. These results also implied that the precipitation caused by the rapid reaction of zinc and carboxyl groups cannot crystallize the sample, and slow growth can largely strength the crystallinity.[31,32]

Figure 3

(a) XRD pattern of Zn-OFBB and Zn-OFBT samples collected at different times; (b) FTIR spectrums of OFBB (blue), OFBT (green), Zn-OFBB (black), and Zn-OFBT (red).

The Fourier transform infrared spectrometer (FTIR) spectrum of the ligands, Zn-OFBB and Zn-OFBT were collected to investigate the coordination mode of the carboxylic acid group (as shown in Figure b), as carboxylate has different coordination modes, which will make a difference in the corresponding spectrum.[18] The FTIR spectrums of Zn-OFBB and Zn-OFBT samples revealed two dominant peaks located at around 1609 and 1406 cm–1 with Δ > 200 cm–1 (νas – νs = 203 cm–1). This indicated that the carboxylate ligands adopt a monodentate coordination mode in both Zn-OFBB and Zn-OFBT.[18,26]

Self-Assembly and Mechanism Study

The structure of OFBT was similar to OFBB besides the central benzene ring being replaced by a 2,1,3-benzothiadiazole. Two carboxyl groups OFBB and OFBT can react rapidly with Zn(II) to form 1D polymeric chains (as shown in Scheme a) via the strong metal–ligand interactions. Each Zn(II) was coordinated with two carboxyl groups from two ligands, and N,N-dimethylformamide (DMF) molecules were also attached to Zn(II) to saturate the coordination of the Zn atom (as shown in Scheme d). Because of the low solubility of these long chains, they folded and aggregated to form nanospheres (as shown in Scheme b). With the time increase, the spheres became slight bigger followed by keeping stable (as shown in Scheme c).

Scheme 2

Schematic Illustration of Coordination Modes and Growth Processes and Mechanisms of Zn-OFBB and Zn-OFBT: (a) 1D Zn-OFBB Polymeric Chain, (b) Zn-OFBB Nanospheres at 1 min, (c) Zn-OFBB at 120 min, (d) Coordination Mode between Zn(II) and Two Ligands, (e) 1D Zn-OFBT Polymeric Chain, (f), Zn-OFBT Nanospheres at 1 min, and (g) Zn-OFBT Nanobelts at 120 min (the Enlarged Area Showed the “2S–2N” Interaction)

Meanwhile, for the assembly of Zn-OFBT, the Zn(II) coordinate with carboxyl groups in the same way as Zn-OFBB initially (as shown in Scheme e), which led to the spherical product of Zn-OFBT at the beginning (as shown in Scheme f). In addition, as time increased, the sulfur and nitrogen atoms on 2,1,3-benzothiadiazole began to work, two thiadiazol groups from the two nearby ligands attracted each other to form “2N–2S” squares (as shown in Scheme g, the enlarged area) due to the complicated forces.[33] The “2N–2S” square interactions in benzothiadiazole not only pulled several rigid long chains and regular tetrahedral units growing along the same direction, but also connect several chains assembled into belt-like shapes (as shown in Scheme g), which is more stable than the random packing in amorphous spheres.

To further verify the double S···N interactions, another comparison experiment was conducted by the addition of palladium nitrate. It is well known that the palladium ion has high affinity to sulfur, which is expected to interfere with the weak S···N interactions and thus affect the self-assembly process. After the addition of Zn(OAc)2 into a solution of OFBT, we added a solution of palladium nitrate after 1 min. After 2 h, we collected the precipitate and washed by the above mentioned mixed solution 3 times. Scanning electron microscopy (SEM) data showed that there was no nanobelt, but only spherical particles formed (as shown in Figure S1). Because Pd(II) had a strong affinity toward sulfur atoms,[34−36] it occupied coordination sites of the sulfur atom rapidly, which results in the sulfur atom not being able to form “2N–2S” squares with nitrogen atoms, thus leading to the samples presented spherical shape. The verification experiment strongly demonstrated the view that the form of Zn-OFBT belts is wholly because of “2S–2N” square interactions.

Luminescent Properties and Sensing of Nitroaromatic Explosives

The fluorescence properties of OFBB, OFBT, Zn-OFBB, and Zn-OFBT were investigated at room temperature. As was depicted in Figure a, for the OFBB and Zn-OFBB, the luminescent spectrum exhibits characteristic emission peaks at 416 nm with a shoulder at about 432 nm, which was attributed to the two fluorenes located on the symmetrical sides of the central benzene ring.[37,38] Meanwhile, for the OFBT and Zn-OFBT, both had strong peaks at about 532 nm. In the OFBT ligand, the two fluorene functional groups acted as an electron donor and the central benzothiadiazole group performed as an acceptor, the transfer of electrons from the donor to the acceptor resulted in a decrease in luminescence energy and a longer wavelength. Interestingly, Zn(II) did not change the characteristic peak on the spectrum of the ligand after the assembly process.

Figure 4

(a) Luminescent properties of OFBB, Zn-OFBB, OFBT, and Zn-OFBT; (b) quenching efficiency of Zn-OFBB and Zn-OFBT dispersed in water with the addition of different nitro-aromatic explosives at the same concentration; (c) detect performance of Zn-OFBB toward NB; (d) detect performance of Zn-OFBT toward NB.

To investigate the potential recognition performances of Zn-OFBB and Zn-OFBT in sensing nitro-aromatic explosives, the fully ground products were dispersed in ethanol, subjected to ultrasonic treatment for 10 min, and then tested the quenching properties of these two oligomers by various nitro-aromatic explosives. NB, 2,4-dinitrotoluene (DNT), 2,4-dinitrophenol (DNP), 2,4,6-trinitrotoluene (TNT), and 2,4,6-trinitrophenol (PA) were dissolved in mixed water at different concentrations.

First, we tested the selectivity of the two luminescent coordination polymers. In this regard, the response of the fluorescence of Zn-OFBB and Zn-OFBT toward the mentioned 5 nitroaromatic explosives was evaluated (as shown in Figure b). From these compounds emerged a varied degree of quenching efficiencies on the luminescent intensity. Among the various compounds, only NB committed high quenching efficiency toward Zn-OFBB and Zn-OFBT, which reached 94.04 and 62.93%, respectively. In addition for the others, the quenching efficiencies appeared relatively low, especially Zn-OFBB toward DNT and TNT, which presented almost no annihilation effect. According to this feature, we can effectively distinguish NB from various nitro-aromatic explosives, and this feature was quite different from most of the nitro-aromatic probes that have been reported.

Consecutively, we also conducted batch experiments to investigate the performance of the two as-prepared products on NB. As was shown in Figure c, the NB solution can quench Zn-OFBB to almost no obvious peaks compared to the blank Zn-OFBB emission when the NB concentration was 1.25 × 10–5 mol/L, and the quenching efficiency can up to 94.04%. For the Zn-OFBT emission (as shown in Figure d), the NB solution also can weaken the fluorescence intensity. Subsequently, we calculated the fluorescence intensities at different NB concentrations by means of integration, fitting the correlation between the two values, and calculated the detect limitations. The results demonstrated that Zn-OFBB was more sensitive than Zn-OFBT toward NB, and the detect limitations can reach to 1.1 × 10–6 and 1.6 × 10–6 mol/L, respectively.

Conclusions

In conclusion, we have demonstrated an interesting coordination-driven self-assembly process in which two similar fluorescent oligomers showed distinct assembly behaviors. Oligomer OFBT bearing benzothiazole groups form a spherical product, which then transformed to a crystalline nanobelt in 2 h. The mechanism study revealed that the double S···N interactions played a vital role in the dynamic morphology transformation. Their special luminescent function and stable chemical properties strongly supported their further application in NB detection and showed the detect limitations were 1.1 × 10–6 and 1.6 × 10–6 mol/L, respectively. In addition, they can also effectively distinguish the NB from other nitro-aromatic explosives. This work provided a possibility to regulate the morphology of oligomers and polymers by designing the structure of the ligand.

Experimental Section

Chemicals

As an additional note, all the agents were used as received without any further purification. Zinc acetate dihydride was purchased from Sigma-Aldrich (St. Louis, USA). DMF, methanol, ethanol, and tetrahydrofuran were also used as solvents in this experiment, which were purchased from Anaqua Global International Inc. Limited, (USA). Palladium nitrate (Pd(NO3)2), NB, DNT, DNP, 2,4,6-trinitrotoluene (TNT) standard solution, and 2,4,6-trinitrophenol (PA) were purchased from Macklin Reagent (Shanghai, China). Oligomers OFBB and OFBT were synthesized by the method reported in our recent work.[39]

Synthesis of Zn-OFBB and Zn-OFBT

A solution of Zn(OAc)2 in DMF (1 mL, 1 × 10–3 mol/L) was added into the solution of OFBB in DMF (1 mL, 1 × 10–3 mol/L) dropwise under stirring. The precipitates were collected at different preset times (1, 5, 10, 30, 60, and 120 min) for characterization. The whole assembly process was under ambient conditions. The collected samples were centrifuged at a speed of 8000 rpm for 10 min and washed by the mixed solution (ethanol/DMF = 1:1) three times. Zn-OFBT was synthesized by the same method as Zn-OFBB but OFBT was used instead of OFBB.

Characterizations

A polycrystalline diffractometer (Rigaku D/max-2550 VB) with Cu Kα radiation (λ = 0.154 nm) was employed to measure the powder XRD of the prepared samples. The morphology was characterized by a field emission SEM (Zeiss Merlin). TEM images were observed by a Tecnai F30 electron microscope with an acceleration voltage of 300 kV. FTIR were recorded on Nicolet iS50 spectrometers with KBr pellets. The luminescent properties and sensing performance were measured by a HORIBA iHR320 fluorescence spectrometer.

Experiments of Aromatic Explosives Sensing

First, batch experiments were conducted to test the selectivity of Zn-OFBB and Zn-OFBT. The as-prepared Zn-OFBB and Zn-OFBT were dispersed in ethanol with a concentration at 1000 ppm. Then, the two suspensions were ultrasonicated for 10 s to make sure the particles dispersed homogeneously in the solution. 5 μL of Zn-OFBB and Zn-OFBT was evenly dispersed in 4 mL of mixed solution (ethanol/water = 1:1) and 50 μL of 1 mmol/L nitro-aromatic explosives (NB, DNT, DNP, TNT, PA) that dissolved in mixed solution (ethanol/water = 1:1). After shaking for 1 min, measure the fluorescence intensities of the samples.

Further, we investigated the sensing performance of Zn-OFBB and Zn-OFBT toward NB by the same method, but different volumes of 1 mmol/L NB were used instead of solutions of various nitro aromatic explosives. We set the volumes of NB from 0 to 50 μL, and 5 μL as an interval. After that, ethanol was added in every bottle till the total volume of the liquid reached 4.100 mL, after reacting for 15 min, the fluorescence spectrums of the samples were collected.