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 ethynylterminated 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 (similar to 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 pi-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.