Pronounced Side Chain Effects in Triple Bond-Conjugated Polymers Containing Naphthalene Diimides for n-Channel Organic Field-Effect Transistors

Abstract

Three triple bond-conjugated naphthalene diimide (NDI) copolymers, poly{[N,N'-bis(2-R-1)-naphtha-lene-1,4,5,8-bis(dicarboxlinid e)-2,6- diyl] [(2,5-bis (2-R-2) 1,4-phenylene)bis(ethyn-2,1-diyl)1} (PNDIR1-R-2), were synthesized via Sonogashira coupling polymerization with varying alkyl side chains at the nitrogen atoms of the imide ring and 2,5-positions of the 1,4-diethynylbenzene moiety. Considering their identical polymer backbone structures, the side chains were found to have a strong influence on the surface morphology/nanostructure, thus playing a critical role in charge-transporting properties of the three NDI-based copolymers. Among the polymers, the one with an octyldodecyl (OD) chain at the nitrogen atoms of imide ring and a hexadecyloxy (HO) chain at the 2,5-positions of 1,4-diethynylbenzene, P(NDIOD-HO), exhibited the highest electron mobility of 0.016 cm(2)V(-1) s(-1) as compared to NDI-based copolymers with an ethylhexyl chain at the 2,5-positions of 1,4-diethynylbenzene. The enhanced charge mobility in the P(NDIOD-HO) layers is attributed to the well-aligned nano-fiber-like surface morphology and highly ordered packing structure with a dominant edge-on orientation, thus enabling efficient in-plane charge transport. Our results on the molecular structure-charge transport property relationship in these materials may provide an insight into novel design of n type conjugated polymers for applications in the organic electronics of the future.