Optoelectronic characterization and thermoelectric figure-of-merit optimization of newly synthesized spirothiadiazole–thienothiophene-based macroheterocyclic compounds

Abstract

This work describes the synthesis of some novel organic compounds, along with the evaluation of their optoelectronic characteristics and potential for thermoelectric applications. The newly synthesized spirothiadiazole–thienothiophene-based macroheterocyclic compounds were termed as Kz1–Kz3. The synthesis of compounds followed a green approach: the one-pot condensation of thienothiophene thiohydrazide with different ketones afforded macrocyclic spiroheterocycles in good yields. Using FT-IR and NMR spectroscopies and elemental analysis, the compounds’ structures were confirmed. Thin-film optical studies revealed strong absorption and high transparency in UV-vis and NIR spectral ranges, respectively. Depending on the degree of conjugation, the energy gaps ranged from 2.99 to 3.26 eV with indirect allowed transitions. Dielectric and dispersion studies provided evidence of high refractive index and high polarizability, confirming their potential as optoelectronic devices. Thermoelectric studies demonstrated that the samples are n-type semiconductors with negative Seebeck coefficients and temperature-dependent conductivities. Among the synthesized compounds, the Kz2 compound showed the best thermoelectric performance with a power factor (PF) of 366 µW m⁻¹ K⁻² and a zT of 0.78 at 493 K after annealing at 200 °C for 5 minutes. The preliminary outcomes of this work clearly demonstrated the potential of spirothiadiazole–thienothiophene frameworks for use in next-generation flexible, eco-friendly thermoelectric and optoelectronic applications.