Synthesis of thienothiophene based materials and investigation of their electronic and optoelectronic properties

Abstract

Conjugated organic materials have emerged as key components in the development of next-generation optoelectronic devices, including organic photovoltaics (OPVs), organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), supercapacitors, and various sensor systems. Among π-conjugated building blocks, thiophene and its derivatives have shown outstanding performance due to their structural tunability, high electron density, excellent thermal stability, and ease of functionalization. Thienothiophenes (TTs), particularly the thieno[3,2-b]thiophene isomer, represent one of the most rigid and planar π-extended fused thiophene systems. Their flat molecular structure enhances π–π stacking interactions, reduces band gaps, and improves charge transport characteristics, making them ideal scaffolds for organic semiconductor design. In this study, two novel TT-based π-conjugated molecules were designed and synthesized: TT-Th3-TPA3 and TT-EDOT3-TPA3. These molecules contain triphenylamine (TPA) and 3,4-ethylenedioxythiophene (EDOT) moieties as electron-donating groups, which are introduced by Suzuki and Stille cross-coupling processes, respectively. To further enhance their processability and electrochemical performance, TT-based π-systems were hybridized with single-walled carbon nanotubes (SWCNTs) through non-covalent interactions, yielding binder-free, flexible hybrid films. Additionally, TT's polymeric analogs were synthesized via Stille coupling and electropolymerization strategies. All synthesized materials were thoroughly characterized using various techniques. Monomer structural characterization was achieved by ¹H NMR spectroscopy, while optic properties were studied using UV-Vis and fluorescence spectroscopy. Cyclic voltammetry (CV) was used to analyze electrochemical behaviors, while SEM and AFM were used to investigate morphological characteristics. The TT-based molecules, TT-Th3-TPA3 and TT-EDOT3-TPA3, exhibited strong light absorption in the visible region, significant bathochromic shifts, and relatively low optical band gaps (2.33 and 2.36 eV, respectively), along with large Stokes shifts. Electrochemical polymerization of TT-Th in an optimal potential window (0.0–1.6 V) successfully yielded stable polymer films with significantly enhanced electrochemical activity. Subsequent CV analysis revealed that although the redox process involves a measurable diffusion-controlled contribution, the strong linearity of log(current) vs. log(scan rate) plots, with slopes close to unity, confirms that the charge storage mechanism is predominantly surface-controlled and capacitive in nature. These findings highlight the polymer film's potential for high-rate energy storage applications. Furthermore, P(TT-EDOT) exhibited a high BET surface area of 152 m²/g and uniform microporosity, supported by its rough, porous morphology observed in SEM images, indicating its suitability for applications requiring efficient ion transport, such as supercapacitors or sensing systems. Hybrid films based on TT and SWCNTs were evaluated for supercapacitor applications. The electrochemical studies showed high specific capacitance, low internal resistance, and excellent charge-discharge cycling stability, confirming their suitability for energy storage devices. In particular, TT-EDOT3-TPA3–SWCNT films demonstrated superior performance, attributed to the enhanced π-electron delocalization and strong interfacial interactions with the nanotubes. Furthermore, TT-EDOT3-TPA3–SWCNT film thermal analysis revealed high stability, with decomposition temperatures above 400°C. This research highlights the promising role of TT-based π-conjugated systems in the field of organic electronics. The successful synthesis of functionalized TT-derivatives and their integration into hybrid nanostructures and porous organic polymers provides a versatile platform for applications in optoelectronics and energy storage technologies. These findings contribute to the further development of high-performance, solution-processable organic semiconductors.