The Synthesis And Characterization Of Novel Oligothienoacenes With Seven Fused Thiophene Rings Based On Dithieno [3,2-b:3',4'-d]thiophene

Fused thiophenes as an important organic material has been widely used as the active layer in organic field effect transistor and organic solar cells. In this thesis, we focused on the efficient synthesis of dithieno[3,2-b:3',4'-d]thiophene and based on which, two new structural heptathienoacenes were designed and succefully prepared.Part-I: Improved synthesis of dithieno[3,2-b:3',4'-d]thiopheneThe synthesis of dithieno[3,2-b:3',4'-d]thiophene(4) is challenging in literature, and in this thesis,we improved the preparation of 4 by utilizing TMS groups as the solubilizing and protecting groups in the synthetic strategy, resulting in a significantly elevated yield from 13 % to 51%. The synthetic routes are as following: starting from 3,4-dibromo-2,5-bis-(trimethylsilanyl)thiophene, via the Br/Li exchange, addition of S8 and oxidation by K3Fe(CN)6, the disulfide compound(2) could be obtained. Subsequently, the fresh made 3-thiophene lithium attacked the disulfide precursor 2 and generated 3-Bromo-4-(thiophen-3-ylsulfanyl)-2,5-bis-trimethylsilanyl-thiophene(3). Then, n-BuLi is employed for both Br/Li exchange and deprotonation to obtain dicarbanions, and then CuCl2 was used for oxidative coupling to generate 2,7-di(TMS)-dithieno[3,2-b:3',4'-d]thiophene(4) in good yield of 75%. By utilizing TMS groups as the solubilizing and protecting groups in the synthetic strategy, which reduced the non-target dicarbanion intermediates formation and improve the solubility of both reactant and the final DTT unit. In additon, the molecular structures of compound 2, 3 and 4 were confirmed by crystal structures.Part-II: The Synthesis of HeptathienoacenesThere are a lot of isomers for heptathienoacenes, but only three types have been reported in literature. Based on the efficient synthesis of 4, two new structure heptathienoacenes(HT-1and HT-2) with linear and bull's horn shapes were designed and synthetized.The detailed synthetic routes are as following: Firstly, LDA-mediated lithiation is used to remove the protons at the unprotected ? positions in 4 and the ?-brominated(5) could be generated by quenching the reaction with C2Cl4Br2. The ?-brominated(6) could be afforded via the bromine dance reaction of 5. Then, the corresponding dimer could be produced from the brominated DTT(5 or 6) and heptathienoacenes(HT-1 and HT-2) is generated via the cyclization reaction in the presence of(PhSO2)2S. In addition, HT-2 has been confirmed by crystal structure, its crystal belongs to orthorhombicspace group Pnma and all seven thiophene rings are fused together to form a bull's horn-shape with a coplanar molecular structure. In the packing of HT-2, the intermolecular interactions, such as S···S contact is observed, indicating that HT-2 might be a good candidate in organic semiconductors.Part-III: Spectroscopic properties and electrochemical behaviors of the title compoundsWe have investigated the spectroscopic properties of compound 4, HT-1 and HT-2. Compound 4 as the basic DTT unit presents two peaks at 270 nm, 300 nm. HT-1 has peaks at 337 nm, 386 nm, and 406 nm, HT-2 shows peaks at 263 nm, 336 nm. The bathochromic shift of the absorption onset for HT-2 is rather substantial as 10 nm, compared with 4. However, through a different connection manner, HT-1 presents an obvious bathochromic onset to 428 nm and elevated absorbance intensity. HT-1 has a larger molecular configuration than that of HT-2 due to different molecular structure. The photo luminicence activities of HT-1 and HT-2 are characterized and HT-1shows a stronger fluorescence emission than that of HT-2. Consequently, the quantum yield(?F) of HT-1 is 0.44. The visualized highest occupied molecular orbital(HOMO) of HT-1 is-5.36 eV,energy gap is 2.94 eV and lowest unoccupied molecular orbital(LUMO)-2.42 eV. HOMO of HT-2 is-5.5eV,LUMO is-2.03 eV and energy gap is 3.47 eV.