Abstract: The influences of the side phenyls (PHs) on the electronic energy band-structure of polydiaeetylenes have been studied by quantum-chemical EHCO-ASED method. When the backbone are changed from polybutatriene(PBT) to polydiacetylene(PDA), the band widths △E_(HO) and △E_(LU) decrease montonically from those of PBT to PDA, but the energy gap decreases from E_g(PBT)=0.43 eV to the minimum E_g(MIN)=0.11 eV firstly, and then increases to E_g(PDA)=1.43 eV (Table 2). Corresponding to the minimum, there is a symmetry inversion for HOCO and LUCO (Table 3). When PHs are coplanar with the backbone (PBT-PH1 and PDA-PH1), they will take part in the conjugation of the system to a certain extent (Table 5), and cause: (I) energy gaps E_g (PBT-PH1) and E_g(PDA-PH1) to change to 56% of E_g(PBT) and 76% of E_g(PDA) respectively (Table 4); (II) the band widths, both ΔE_(HO) and ΔE_(LU) for PBT-PH1 and PDA PH1, to decrease to 40—50% of those of PBT and PDA respectively (Table 4). When PHs are perpendicular to the plane of the backbone (PBT-PH2 and PDA-PH_2), the influences of PHs on the band structure are limitted (Table 4). According to the results and the discussions in this paper, it is shown that the intrinsic semiconductor characteristic of polydiacetylenes is hard to be improved obviously by the side group.

Key words: Polydiacetylenes, Structure of electronic energy bands, Organic semiconductor, EHCO-ASED