Detection and Distinction of DNT and TNT with a Fluorescent Conjugated Polymer Using the Microwave Conductivity Technique

We report the detection and distinction of dinitrotoluene (DNT) and trinitrotoluene (TNT) by the microwave conductivity technique using a cyclopentadithiophene–bithiazole-based polymer (CPDT-BT) as sensor. Although the conventional fluorescence quenching experiments showed just “turn OFF” of the polymer fluorescence for both DNT and TNT, time-resolved microwave conductivity (TRMC) revealed that the photoconductivity of the polymer, which is “turned OFF” in the pristine state became “ON” in the presence of DNT but remained “OFF” with TNT, allowing easy distinction between them. Moreover, the decay rate of the transient kinetics was found to be sensitive to the DNT concentration, implementing a unique method for the determination of unknown DNT concentration. The observations are discussed in viewpoint of charge separation (CS) and formation of charge transfer (CT) complex by considering deeper LUMO of TNT than DNT calculated from the DFT method. This study brings out a novel technique of speedy detection and distinction of environmentally important analytes, an alternative to the fluorescence quenching

p/n Switching of Ambipolar Bithiazole–Benzothiadiazole-Based Polymers in Photovoltaic Cells

Two new alternating copolymers <b>P1</b> and <b>P2</b>, of bithiazole (BT) and benzothiadiazoles (BTZ), differing in their side chain positioning at the thiophene units which sandwich the BT unit, were designed and synthesized. Both polymers exhibited broad absorption ranging from 300 to 700 nm with a narrow optical bandgap in the film state. Control over structural ordering of polymer chains was achieved in <b>P1</b> by treating with a small amount of additive (1,8-octanedithiol, ODT) as evident by a large red shift of absorption peak and also from the XRD measurements. In contrast, no such effects were observed in the case of <b>P2</b> in the presence of additive. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) experiments revealed that the transient photoconductivity of <b>P1</b> is far superior to that of <b>P2</b>, which is further increased when processed with ODT. The charge carrier mobility, as determined by the space-charge-limited current (SCLC) technique, indicates that <b>P1</b> exhibits both electron and hole mobilities with a clear dominance of the latter. The charge carrier mobilities become higher and more balanced for ODT-modified <b>P1</b> films compared to that of <b>P1</b> alone. TRMC analysis revealed that the photoconductivity of <b>P1</b> reduced when blended with PCBM in the absence of additive, whereas significant enhancement was obtained in presence of additive. The blend with P3HT exhibited an increase in photoconductivity in both the presence and absence of additive. In complete accordance with the TRMC results, in the absence of additive, <b>P1</b> acted as an n-type material (P3HT as donor), whereas in presence of additive, it exhibited ambipolar nature acting as both n-type and p-type (P3HT as donor and PCBM as acceptor, respectively) material. Switching of the major charge carrier species was demonstrated simply by the presence of additive for <b>P1</b> in the present paper

Gold Nanoparticle Assisted Self-Assembly and Enhancement of Charge Carrier Mobilities of a Conjugated Polymer

A composite of bithiazole–benzothiadiazole-based semiconducting conjugated copolymer and gold nanoparticles (AuNPs) was prepared in situ and characterized by transmission electron microscopy, thermogravimetry, and UV–vis absorption spectroscopy. The polymer interacts with the nanoparticle surface through the nonbonding electrons of the nitrogen and sulfur atoms, which provides stability to the nanoparticles as well as planarity and rigidity to the polymer backbone. As a result, the effective conjugation length and delocalization of π-electrons of the polymer improved as evident from 130 nm red-shift in the UV–vis absorption spectrum. The nanoparticle along with the chemisorbed layer of polymer acts as a template for the self-assembly of the remaining polymer which is dispersed in the solution through π–π-stacking and van der Waals interactions. The self-assembly process enhances the polymer packing as well as ordering as seen from the shorter <i>d</i> spacing and from the more than threefold increase in the intensity of X-ray diffraction of the composite film. The charge carrier mobilities in the short and long ranges were measured by flash-photolysis time-resolved microwave conductivity and space-charge-limited current methods, respectively, which showed enhancement for the composite material compared to the pristine polymer. A more significant increase was observed in the hole mobilities (more than 12-fold), and hence the p-type nature of the composite was further studied by preparing blend films with typical acceptors such as phenyl-C₆₁-butyric acid methyl ester (PCBM) and <i>N</i>,<i>N′</i>-bis­(1-hexylheptyl)­perylene-3,4–9,10-tetracarboxylbisimide (PBI). Due to its spherical geometry, PCBM was found to disturb the ordering of polymer chains in the composite, resulting in the lowering of photoconductivity signals. On the other hand, planar PBI molecules coassemble with the composite leading to significant enhancement of photoconductivity. Thus, we demonstrated a versatile approach of controlling planarization, π-stacking, and ordering of a conjugated polymer leading to the improvement of optoelectronic properties using AuNPs as a template