One-pot synthesis of star-shaped conjugated oligomers based on 3-hexylthiophene, pyrene and triphenylamine as TNT chemosensors

Highlights

• A star-shaped conjugated oligomer based on pyrene, 3-hexylthiophene and triphenylamine moieties has been synthesized.

• The molar absorption coefficient of T3HTP was 80,890 M⁻¹ cm⁻¹, indicating high fluorescence efficiency.

• The use of T3HTP for tracing explosive TNT compounds through FRET mechanism has been investigated.

Abstract

Star-shaped conjugated oligomer T3HTP containing pyrene, 3-hexylthiophene and triphenylamine, has been successfully synthesized via a simple one-pot sequential direct arylation process. The chemical structure and optical properties of T3HTP oligomer were characterized via proton nuclear magnetic resonance (¹H-NMR), UV–vis and fluorescence spectroscopies. The results indicated that T3HTP oligomer showed good fluorescence quenching toward TNT, which is useful for identifying explosive nitro aromatic compounds.

Introduction

In the last few years, the detection of explosive traces such as 2,4,6-trinitrotoluene (TNT), is an issue of global concern which is crucial for homeland security, anti-terrorism, military issues and environment cleaning [1,2]. In addition, the detection of explosives is also a foundation for forensic examination, land mine detection and in the study of environment safety. Explosives have caused enormous property damage and great public health threat, killed tens of thousands of people per year and also commonly used by terrorists.

The methods for explosive detection include trained canine teams [3], surface-enhanced Raman spectroscopy [4], mass spectroscopy [5], X-ray imaging [6], ion mobility spectroscopy [7,8], gas chromatography coupled with mass spectrometry [9] and electrochemical technique [10]. These methods have been used for the advantages of detection of low contamination of explosive compounds. However, certain features of these methods are lack of portability. Moreover, the spectroscopy methods are complicated and much expensive due to complicated equipments as well as requirement of well trained technicians for equipment operation.

Recently, optical detection approach has attracted a great deal of attention due to low-cost, good portability, high sensitivity and selectivity. These promising techniques are based on colorimetric and fluorimetric responses which are mainly focused on by vast optical sensors for explosive detection. Several reviews have extensively described fluorescence-based explosive sensors from different perspectives including polymer sensors, optical chemosensors, and various fluorescent materials such as conjugated oligomers/polymers, luminescent metal-organic frameworks and nanomaterials [[11], [12], [13], [14], [15]]. In particular, recent research progress aims at the methodology of fluorescence-based explosive sensors [[16], [17], [18], [19], [20]]. In generally, fluorescence-based explosive sensors have been designed based on the Forster resonance energy transfer (FRET) mechanism which can dramatically enhance the fluorescence-quenching efficiency and improve sensitivity. In this mechanism, an initially excited molecule (donor) returns to the ground state orbital, while the transferred energy simultaneously provides an electron on the acceptor to the excited state [17,21]. Poly(p-phenylene vinylene)s (PPVs) is one of the excellent conjugated polymers exhibiting photo-and electro-luminescence which can act as fluorescent sensory materials [14,19,[22], [23], [24], [25], [26], [27]]. Chang et al [22] synthesized dialkoxy and diphenyl substituted PPVs towards fluorescence sensing of TNT and 2,6-DNT vapors. Swager and coworkers prepared alternating conjugated polymers based on pentiptycene molecule with dialkoxy phenylene moieties which exhibited fluorescence quenching in the presence of TNT and 2,4-DNT vapors [14,19]. Accordingly, the intensity of emission quenching was influenced by the exposure time and by thin film thickness. The best results were obtained for thin films of 25 Å and exposure time of 60 s with quenching efficiencies of 100 % and 75 % for 2,4-DNT and TNT, respectively. The group of Swager also reported conjugated polymers containing polyacetylene and pentiptycene, which exhibited a moderate quenching of emission intensities by 16 %–28 % in the presence of 2,4-DNT vapor [28]. Feng et al [29] has designed and synthesized PPV-based mesoporous silica hybrid nanoparticles (PPV@MSNs-NH₂), which could selectively detect TNT through FRET with a detection limit of TNT concentration of 1 × 10⁻⁷ M. Liu and coworkers reported the synthesis of dibenz[a,h]-anthracene-containing polymer exhibiting efficient quenching in the presence of 2,4-DNT at the bands of 427 and 453 nm in chloroform [30]. Water soluble poly(2,5-methoxy-propyloxysulfonate phenylenevinylene) was also reported, with its fluorescence quenching modifiable by surfactants [31]. Wu et al also reported water‐dispersible hyperbranched conjugated polymer nanoparticles (HCPN‐S) based on 9,9‐dihexyl‐2,7‐dibromofluorene and 1,3,5‐tris‐[4‐(4,4,5,5‐tetramethyl‐ [1,3,2]dioxaborolane)phenyl]‐benzene bearing the sulfonate terminal groups which was obtained via Suzuki coupling polymerization in an oil‐in‐water miniemulsion followed by post‐functionalization. HCPN‐S can sense TNT or other nitro compounds with a detection limit at the nanomolar level [32]. Niamnont et al [33] has reported the synthesis of star-shaped conjugated molecules based on triphenyl amine linked with pyrene or corannulene for visual detection of TNT. Lu and co-worker reported branched benzothiadiazole-cored conjugated oligomers with terminal carbazoles [34]. Films based on these oligomers could emit strong red light and their fluorescence emission intensities decreased obviously when exposed to gaseous TNT and DNT [34]. More recently, Das and co-workers have reported conjugated triptycene-based azobenzene-functionalized polymers (TBAFPs) towards chemsensors for picric acid detection, with the azo linkage capable of switching from trans to cis isomerization resulting in enhancement of the fluorescent emission intensity [35]. However, most of conjugated oligomers and polymers used as explosive sensors have usually been synthesized via multiple complicated steps, leading to decrease in the quality of the products.

It should be noted that conjugated molecules tend to aggregate in the solid state, causing quenching in fluorescence. In order to prevent the aggregation of chromophores, star-shaped or branched architecture structures have been proved to be useful for suppression of aggregation-caused-quenching phenomenon and obtaining high solid fluorescence quantum yields [[36], [37], [38]]. Due to branched topological structures and numerous terminal groups in their, hyperbranched and star-shaped structures are expected to enhance host–guest interactions, as, well as the possibility of capturing and sensing.

It is well-known that pyrene with a large delocalized π system and well-matched orbital energy levels shows good selectivity and sensitivity for TNT detection [39]. On the other hand, sensors derived from electron-rich triphenyl amine have been developed for detection of nitroaromatic explosives [[40], [41], [42]]. In addition, while both pyrene and triphenylamine have good fluorescent properties, 3-hexylthiophene acts as a strong donor moiety. Inspired by these fascinating characteristics, we hypothesized that a fluorophore molecule involving triphenylamine, pyrene and 3-hexylthiophene maybe act as a promising candidate for visual detection of TNT. Due to the incorporation of multiple fluorescent units, amplification of fluorescence quenching could be achieved.

To the best our knowledge, there have not been any reports on the synthesis of sensors based on these molecules for explosive detection. In this research, we focus on the synthesis of star-shaped molecules containing triphenyl amine as a core with branches containing 3-hexylthiophene and pyrene moieties via one-pot direct arylation coupling reaction. The obtained conjugated molecules have been characterized via FTIR, ¹H NMR, UV–vis and photoluminescence (PL) spectroscopies for TNT sensoring application.