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

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 (PStFl n, n = 1 or 2), referred to as MH- b -PStFl n . MH- b -PStFl n was prepared by the Cu(I)-catalyzed click reaction of azido-terminated PStFl n (PStFl n -N 3 ), which was obtained from the azidation reaction of the bromo-terminated PStFl n (PStFl n -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 -PStFl n 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 -PStFl n -based devices had the excellent hole mobility (0.25–0.52 cm 2 V –1 s –1 ) and the high ON/OFF current ( I ON / I OFF ) ratio of 10 7 –10 8, of which the MH- b -PStFl 1 -based one had the higher mobility than that of the MH- b -PStFl 2 -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 -PStFl 2 -based device showed a larger memory window than the MH- b -PStFl 1 -based one because the stronger electron-donating effect of the difluorenyl group in MH- b -PStFl 2 increased the charge storage capability of its related device. All the memory devices showed a long-term retention time over 10 4 s with the high I ON / I OFF ratio of 10 6 –10 8 . Among these devices, the MH- b -PStFl 1 -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.