Synthesis, characterization, and field-effect transistor properties of carbazolenevinylene oligomers: From linear to cyclic architectures

Two cyclic carbazolenevinylene dimers 1 and 2 were synthesized by McMurry coupling reactions. A linear compound 3 was also prepared for comparison. Compounds 1-3 were fully characterized by means of NMR spectroscopy, HRMS, elemental analysis, and UV/Vis absorption spectroscopy. Quantum chemical simulations showed that the cyclic compounds possessed smaller HOMO-LUMO gaps and more extended conjugation. The UV/Vis absorption spectra of the cyclic compounds showed blueshifts compared with that of the linear compound 3. Time-dependent DFT (TD-DFT) analysis revealed that this was due to the different selection rules for molecules with cyclic and linear architectures. The cyclic conformation also significantly affected the molecular ordering in the solid state. The X-ray crystal structure of 1. showed partial pi-pi overlapping between the adjacent molecules. Thin films of 1-3 were fabricated by the vacuum-deposition method on Si/SiO2 substrates. Multi crystalline thin films were obtained from compounds 1 and 2, but only amorphous thin films could be obtained for the linear compound 3. Another important difference between the cyclic and linear compounds was the reduced reorganization energy for the cyclic compounds. These two facts have resulted in improved field-effect transistor (FET) mobilities for the cyclic compounds compared with the linear compound. In addition, as the substrate temperature has a significant influence on the morphology and the degree of crystallinity of the thin films deposited, the device performance could be optimized by varying the substrate temperature. The FET devices based on 2 gave the highest mobility of 0.013 cm(2)V(-1)s(-1). The results showed that carbazole derivatives with cyclic structures might make better FETs.