Synthesis of Oligosaccharide-Based Block Copolymers with Pendent π-Conjugated Oligofluorene Moieties and Their Electrical Device Applications

Abstract

We report the synthesis and electric device applications of oligosaccharide-based diblock copolymers consisting of a maltoheptaose (MH) block and a poly(4-oligofluorenylstyrene) block (PStFln, n = 1 or 2), referred to as MH-b-PStFln. MH-b-PStFln was prepared by the Cu(I)-catalyzed click reaction of azido-terminated PStFln (PStFln-N3), which was obtained from the azidation reaction of the bromo-terminated PStFln (PStFln-Br), with excess ethynyl-terminated MH in the THF/DMF mixture solvent. The resulting diblock copolymers self-assembled to spherical microdomains with sub-10 nm sizes in both bulk and thin film state after annealing process. Thereafter, the MH-b-PStFln thin film (∼50 nm) with the self-assembled nanoscale spherical aggregates was used as the charge storage layer for the pentacene-based field-effect transistor type memory devices. The MH-b-PStFln-based devices had the excellent hole mobility (0.25–0.52 cm2 V–1 s–1) and the high ON/OFF current (ION/IOFF) ratio of 107–108, of which the MH-b-PStFl1-based one had the higher mobility than that of the MH-b-PStFl2-based one because the pentacene crystal in the former device possessed the larger grain size and fewer boundaries. On the other hand, the MH-b-PStFl2-based device showed a larger memory window than the MH-b-PStFl1-based one because the stronger electron-donating effect of the difluorenyl group in MH-b-PStFl2 increased the charge storage capability of its related device. All the memory devices showed a long-term retention time over 104 s with the high ION/IOFF ratio of 106–108. Among these devices, the MH-b-PStFl1-based device showed a good WRER endurance over 180 cycles. This work not only demonstrates the tunable electrical memory characteristics by adjusting the π-conjugation length of the oligofluorenyl side chain in the polymer electret but also provides a promising approach for developing the next-generation “green electronics” using natural materials.