Thanh Luan Nguyen, Dae Hee Lee, Han Young Woo, Dong Hoon Choi, Joungphil Lee, Yuxiang Li, Moon Jeong Park, Sungu Hwang
Two regioisomeric D1-A-D-A-D1 type π-conjugated molecules (1,4-bis{5-[4-(5-fluoro-7-(5-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole)]thiophen-2-yl}-2,5-bis(hexyldecyloxy)benzene (Prox-FBT) and 1,4-bis{5-[4-(6-fluoro-7-(5-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole)]thiophen-2-yl}-2,5-bis(hexyldecyloxy)benzene (Dis-FBT)) are synthesized, by controlling the fluorine topology to be proximal or distal relative to the central core. The different F geometries are confirmed by the 1H–1H nuclear Overhauer effect spectroscopy (NOESY). Clearly different optical, electrochemical, and thermal transition behaviors are obtained, i.e., stronger absorption, deeper valance band (by ≈0.2 eV), and higher melting/recrystallization temperatures (by 7–20 °C) are observed for Dis-FBT. The different intermolecular packing and unit cell structures are also calculated for the two regioisomers, based on the powder X-ray diffraction and 2D grazing-incidence wide-angle X-ray diffraction measurements. A tighter π–π packing with a preferential monoclinic face-on orientation is extracted for Dis-FBT, compared to Prox-FBT with bimodal orientations. Different topological structures significantly affect the electrical and photovoltaic properties, where Prox-FBT shows higher parallel hole mobility (2.3 × 10−3 cm2 V−1 s−1), but Dis-FBT demonstrates higher power conversion efficiency (5.47%) with a larger open-circuit voltage of 0.95 V (vs 0.79 V for Prox-FBT). The findings suggest that small changes in the topological geometry can affect the electronic structure as well as self-assembly behaviors, which can possibly be utilized for fine-adjusting the electrical properties and further optimization of optoelectronic devices.
Two regioisomeric π-conjugated molecules (1,4-bis{5-[4-(5-fluoro-7-(5-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole)]thiophen-2-yl}-2,5-bis(hexyldecyloxy)benzene and 1,4-bis{5-[4-(6-fluoro-7-(5-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole)]thiophen-2-yl}-2,5-bis(hexyldecyloxy)benzene) with different fluorine topologies (referred to as proximal or distal relative to central core) are synthesized, and the correlation between the topological geometry of fluorine atoms and optoelectronic property is examined in terms of the molecular structure, intermolecular interactions, and the resulting bulk morphology.
