Stable spontaneous orientation polarization by widening the optical band gap with 1,3,5,7-tetrakis(1-phenyl-1H-benzo[d]imidazol-2-yl)adamantane

Abstract

The spontaneous orientation polarization (SOP) of 1,3,5,7-tetrakis(1-phenyl-1H-benzo[d]imidazol-2-yl)adamantane (TPBAD) and its photosensitivity were investigated and compared with those of 2,2′,2′′-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), a prototypical molecule exhibiting SOP. Despite the large difference in the molecular shapes of TPBAD (tetragonal) and TPBi (disc-shaped), their vacuum-deposited films exhibited similarly high SOPs of +74.7 and +77.2 mV nm⁻¹, respectively. Due to the non-conjugated adamantane core, TPBAD had a wider optical band gap (4.22 eV) than TPBi (3.50 eV). This reduced the sensitivity of SOP in TPBAD films to light irradiation, and the decay time constant was 10 times longer than that of TPBi films. Variable angle spectroscopic ellipsometry and two-dimensional grazing-incidence wide-angle X-ray scattering revealed that TPBAD films are optically isotropic with a lower packing order than TPBi films, suggesting that the subtle directional selectivity of the TPBAD molecules in the amorphous film generates large SOP.