Synthesis of 2,5,8-Tris(1-phenyl-1<i>H</i>-benzo[<i>d</i>]imidazol-2-yl)benzo[1,2-<i>b</i>:3,4-<i>b</i>′:5,6-<i>b</i>″] Trithiophenes and Their Spontaneous Orientation Polarization in Thin Films
Wei‐Chih Wang, Kyohei Nakano, Chain‐Shu Hsu, Keisuke Tajima
Abstract
To investigate the relationship between molecular structures and spontaneous orientation polarization (SOP) in organic thin films, 2,5,8-tris(1-phenyl-1 H -benzo[ d ]imidazol-2-yl)benzo[1,2- b:3,4- b ′:5,6- b ″] trithiophene (TPBTT) and its ethyl derivative ( m -ethyl-TPBTT) were synthesized. Variable angle spectroscopic ellipsometry and two-dimensional grazing-incidence wide-angle X-ray scattering showed that the vacuum-deposited films of TPBTT and m -ethyl-TPBTT had a higher degree of molecular orientation parallel to the substrate compared with that of prototypical 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1- H -benzimidazole) (TPBi) due to the larger π-conjugated benzotrithiophene core. However, TPBTT films showed a lower SOP of +54.4 mV/nm than did the TPBi film (+77.3 mV/nm), indicating that the molecular orientation alone did not determine the SOP. In contrast, m -ethyl-TPBTT showed a larger SOP of +104.0 mV/nm in the film. Quantum chemical calculations based on density functional theory suggested that the differences in the stable molecular conformation and the permanent dipole moments between TPBTT and m -ethyl-TPBTT caused the differences in SOP. These results suggest that the simultaneous control of the orientational order and conformation of the molecules is important to achieving a large SOP in films.