Two Regioisomeric pi-Conjugated Small Molecules: Synthesis, Photophysical, Packing, and Optoelectronic Properties

Abstract

Two regioisomeric D-1-A-D-A-D-1 type pi-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 H-1-H-1 nuclear Over-hauer effect spectroscopy (NOESY). Clearly different optical, electrochemical, and thermal transition behaviors are obtained, i.e., stronger absorption, deeper valance band (by approximate to 0.2 eV), and higher melting/recrystallization temperatures (by 7-20 degrees 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 pi-pi 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 x 10(-3) cm(2) 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.