End Groups Of Oligothiophene / Benzothiophene Compound Electronic Structure And Transmission Performance

Phenyl-thiophene oligomers and their derivatives were widely used in organic electroluminescence devices (OLEDs) and organic field-effect transistors(OFETs) due to their properties of modified structure easily and controlled photoelectric. It is showed that conjugated bridge with the introduction of acetylenic spacers or varying terminal group can improve antioxidant capacity and properties of the charge carrier transport significantly. Unfortunately, it is since that the related report about the conjugated bridge and changing of terminal group for the influence on the material performance is still absent systematacially. Therefore, the novel organic semiconductor materials which took the oligothiophenes, benzo[2,1-b:3,4-b']- dithiophene dibenzo[b,d]thiophene as conjugated bridges and benzene and oligothiophenes as terminal group have been designed, which was based on the charge carrier transporting materials of phenyl-thiophene types synthesized in the experiments.Thirty two compounds, taking styrene, phenylacetylene or oligothiophenes as terminal group, bithiophene (2T), benzo[2,1-b:3,4-b']dithiophene (TPT) dibenzo[b,d] thiophene (PTP) and dithieno[2,3-b:3',2'-d]thiophenes (DTTs) as conjugated bridges, were systematically studied by density functional theory (DFT, TDDFT) at the B3LYP/6-31G(d) and PBEO/6-31G(d) level. Based on optimized geometries in the neutral state and ionic state, frontier orbital energies, ionization potentials (IPs), electron affinities (EAs), reorganization energy (λh/λe) and electronic absorption spectrum were obtained. Terminal groups and conjugated bridges for influence on electronic structure and photophysical properties were discussed.By systematacial theoretical research, the conclusion may be drawn as following:(1) when the compounds having the same conjugated bridge(bithiophene (2T), benzo[2,1-b:3,4-b']dithiophene (TPT) and dibenzo[b,d]thiophene (PTP)), styrene replaced phenyl- acetylene as terminal group, the lowest unoccupied molecular orbital (LUMO) energy level was little affected, while the highest occupied molecular orbital (HOMO) energy level was obviously lowered. Consequently, the HOMO-LUMO energy gap(△E) was increased and absorption spectrum was blue-shifted about 10～30 nm. In addition, reorganization energies (λh/λe) of majority phenylethynyl compound were smaller than styrene compound. (2) As for different conjugated bridge (2T, TPT and PTP) with the introduction of same terminal group, the HOMO energy level was lowered, the LUMO energy level was heightened, so the HOMO-LUMO energy gap (△E) was increased with the absorption spectrum blue-shift of 30～45 nm, reorganization energies was decreased, respectively. Although theλh/λe of compounds including PTP conjugated bridge was smaller than the TPT compounds,λh andλe were different so as not to transport mobility easily. Moreover, the compounds with TPT conjugated bridge are easier to balance carrier transport and improve carrier transport rate, due to small and close reorganization energies of the hole and the electron.(3) The reorganization energies (λh andλe were respectively 0.21 and 0.22 eV) of diphenylacetylene-benzodithiophene (DPATPT) were smaller than distyrene-benzodithiophene (0.22 and 0.24 eV) which has high charge carrier mobility. At the same time, theλh and Ae of DPATPT was closer than the DSTPT. So the diphenylacetylene-benzodithiophene (DPATPT) is expected to become a potential carrier transport material with high transport efficiency and strong antioxidant ability.(4) Benzo[2,1-b:3,4-b']dithiophene (TPT) or dithieno[2,3-b:3',2'-d]thiophenes (DTTs) as conjugated bridge, increasing the number of thiophene, the HOMO energy level was evidently increased, the LUMO energy level was evidently decreased. For same conjugated bridge with the introduction of different terminal group that was changed from oligothiophene to fused-thiophenes, the HOMO energy level was lowered, the LUMO energy level was heightened. Replaced DTTs conjugated by TPT, the HOMO energy level was elevated, the LUMO energy level was degraded. The acetylenic spacer between the conjugated bridge and the terminal group, the HOMO and LUMO energy level have been reduced. It is more obvious to regulate the LUMO energy level than the HOMO's by increasing the number of thiophene, inserting the acetylenic in conjugated bridge and terminal group and fused-thiophenes. However, it is contrary to regulate the energy level by changing benzo[2,1-b:3,4-b']dithiophene (TPT) conjugated to dithieno[2,3-b:3',2'-d]thiophenes (DTTs).