Effect of alkyl side chains on properties and organic transistor performance of 2,6-bis(2,2′-bithiophen-5-yl)naphthalene

Herein we report the synthesis and characterization of C-2-symmetric 2,6-bis(5'-alkyl-2,2'-bithiophen-5-yl) naphthalene derivatives with alkyl side chains of various length and shape, including even the less commonly examined cyclohexyl and cyclohexylalkyl groups. By investigation of how these side chains influence solubility, self-assembly and charge mobility, we aimed to extend the knowledge of structure-function property relationship of the oligothiophene compounds. Charge mobilities observed in this study ranged from 10(-3) to 10(-1) cm(2) V-1 s(-1). The highest values referred to thin layers of derivatives end-capped with linear alkyl chains. XRD methods uncovered OFET-favorable layering of tilted molecular arrangement with the molecular long axes in the out of plane direction. Although the charge mobilities showed variation of as much as two orders of magnitude, XRD measurements of these derivatives revealed similar packing and self-assembly behavior. Hence, the charge mobility differences are more a result of the naturally diverse spatial demands of the tested alkyl groups, with the linear alkyl chains offering the tightest close packing of the pi-conjugated backbone. On the other hand, the bulky 2-hexyldecyl group turned out to be the only group that made it possible to avoid insolubility of these derivatives and provided excellent solubility (23 mg/mL of chloroform), while the charge mobility remained acceptable even of the spin-coated thin layers.