Thienothiophene-hydrazide biginelli based organic thin film with optical and electrical tunability optimized for gaseous environment sensing

Four thienothiophene-hydrazide-Biginelli derivatives with hydroxyl, methyl, fluoro and dimethylamino were synthesized and fabricated into organic thin films for study of the correlation between the molecular structure, the optical and electrical properties, and the ammonia sensing performance. The SEM and XRD analyses revealed that the surface morphology and film structure are mostly amorphous or poorly ordered and are dependent on the substituents. The films showed a very wide UV absorption and tunable optical band gap ranging between 2.89 and 3.33 eV which showed that peripheral molecular substitution has a significant effect on the electronic structure. The Urbach-energy analysis suggested different structural disorders indicating that the fluoro-substituted derivative (compound 7) has the lowest energetic disorder. The films were semiconducting with activation energies of 0.63-0.78 eV. Compound 7 showed the best electrical and sensing properties such as low activation energy (≈0.63 eV), and high response to NH₃ gas at room temperature. It exhibited very good response and recovery times of around 7 s and 14 s, respectively, and a good linear relationship (R 2 = 0.994) with concentration ranging from 20 to 100 ppm. This improved sensing ability is explained by the synergistic effect of electronic polarization due to the presence of the fluorine atom, decrease in energetic disorder, charge transport and uniform granular surface morphology. The results obtained herein have proved that the molecular substitution in the structure of thienothiophene-hydrazide-Biginelli is an effective way to obtain tunable organic thin films for ammonia sensing and other optoelectronic applications at room temperature.