Regioregular dithienosilole- and dithienogermole-based small molecules with symmetric distal/distal orientation of F atoms

This study describes the synthesis and characterization of a series of regioregular dithieno(3,2-b;2′,3′-d)-silole (DTSi)- and dithieno(3,2-b;2′,3′-d)-germole (DTGe)-based small molecules-7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(5-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (DTSi(BTFTh2)2), 7,7′-(4,4-bis(2-ethylhexyl)-4H-germolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(5-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (DTGe(BTFTh2)2), 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bi-s(4-(benzofuran-2-yl)-5-fluorobenzo[c][1,2,5]thiadiazole) (DTSi(BTFBFu)2), and 7,7′-(4,4-bis(2-ethylhexyl)-4H-germolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(4-(benzofuran-2-yl)-5-fluorobenzo[c][1,2,5]thiadiazole) (DTGe(BTFBFu)2) (DTSi(BTFTh2)2, DTGe(BTFTh2)2, DTSi(BTFBFu)2, and DTGe(BTFBFu)2 in which two F atoms are in a symmetric distal/distal configuration relative to the central core units. The optical and electrochemical properties are strongly influenced owing to changes of the terminal units (BFu and Th2) rather than by the tuning of the core units (DTSi and DTGe). In addition to a high fraction of the edge-on orientation in the neat films of all of the molecules, the presence of hexyl side chains in the Th2 terminal units are found to play a critical role in forming high-quality films, leading to homogeneous and smooth surfaces in the films of Th2-containing molecules. When used as donor materials in small-molecule:fullerene solar cells, the best power-conversion efficiency of 3.26% was achieved with the DTSi(BTFTh2)2-based system. Our study herein provides a relevant step toward understanding such isomorphic small molecules.