Optical Properties of Neutral and Charged Low Band Gap Alternating Copolyfluorenes： TD-DFT Investigation

Electronic structure and optical properties of neutral and charged low band gap alternating copolyfluorenes （Green 1, which is based on alternating repeating units consisting of alkyl-substituted fluorene and a thiophene-[1,2,5]thiadiazolo-[3,4]quinoxaline-thiophene （T-TDQ-T） unit were investigated theoretically with time-dependent density functional theory （TD-DFT） method, and their excited state properties were further analyzed with 2D site and 3D cube representations. For neutral Green 1, the band gap, binding energy, exciton binding energy, and nuclear relaxation energy were obtained. The transition dipole moments of neutral and charged Green 1 are compared using 3D transition density, which reveals the orientation and strength of transition dipole moments. The charge redistribution of neutral and charged Green 1 upon excitation are displayed and compared with 3D charge difference density. The electron-hole coherences of neutral and charged Green 1 upon excitation are investigated with 2D site representation （transition density matrix）. The excited state properties of neutral Green 1 calculated with TD-DFT method are compared with that calculated with ZINDO method, which reveals the importance of electron-electron interaction （in TD-DFT） in the excited state properties.

Fluorinated p-n Type Copolyfluorene as Polymer Electret for Stable Nonvolatile Organic Transistor Memory Device

Abstract In this study, a kind of fluorinated copolyfluorene, named poly[（4-（octyloxy）-9,9-diphenylfluorene-2,7-diyl）-alt- （2,3,5,6-tetrafluoro-1,4-phenylene）] （PODPF-TFP）, is synthesized by facile palladium-based direct aromatization. Compared to the non-fluorinated counterpart, poly[（4-（octyloxy）-9,9-diphenylfluorene-2,7-diyl）-alt-（p-phenylene）] （PODPF-P）, deeper HOMO/LUMO energy level combined with steric hindrance effect endow PODPF-TFP with excellent spectra and morphology stability. Finally, organic field-effect transistor （OFET） memory devices are fabricated with PODPF-P/PODPF- TFP as the dielectric layers, and they both exhibit flash type storage characteristic. Owing to the electronegativity of fluorine atom, the device based on PODPF-TFP exhibits larger memory window and more stable Ion/Ioff ratio during a retention time of 10^4 s as well as a better aging stability. The present study suggests that fluorinated p-n copolyfluorene electrets could enhance the capabilities of charge trapping and storage, which are promising for OFET memory devices.