Control of Polar/Antipolar Layered Organic Semiconductors by the Odd-Even Effect of Alkyl Chain.

Some rodlike organic molecules exhibit exceptionally high layered crystallinity when composed of a link between π-conjugated backbone (head) and alkyl chain (tail). These molecules are aligned side-by-side unidirectionally to form self-organized polar monomolecular layers, providing promising 2D materials and devices. However, their interlayer stacking arrangements have never been tunable, preventing the unidirectional arrangements of molecules in whole crystals. Here, it is demonstrated that polar/antipolar interlayer stacking can be systematically controlled by the alkyl carbon number n, when the molecules are designed to involve effectively weakened head-to-head affinity. They exhibit remarkable odd-even effect in the interlayer stacking: alternating head-to-head and tail-to-tail (antipolar) arrangement in odd-n crystals, and uniform head-to-tail (polar) arrangement in even-n crystals. The films show excellent field-effect transistor characteristics presenting unique polar/antipolar dependence and considerably improved subthreshold swing in the polar films. Additionally, the polar films present enhanced second-order nonlinear optical response along normal to the film plane. These findings are key for creating polarity-controlled optoelectronic materials and devices.

Temperature induced self-assembly of amino acid-derived vinyl block copolymers via dual phase transitions.

The unique thermoresponsive phase behaviors of diblock copolymers from amino acid-derived vinyl monomers have been demonstrated in view of variation in the aggregation state in water. Amino acid-based block copolymers composed of N-acryloyl-Ala-methylester (NAAMe) and N-acryloyl-ßAla-methylester (NAßAMe) are successfully synthesized by RAFT polymerization. The resultant block copolymers poly(NAAMe48-b-NAßAMem) contain a constant degree of polymerization (DP=48) of the poly(NAAMe) block, but the DP of the poly(NAßAMe) block varies (m=80-122). The turbidimetry subjected to these copolymer aqueous solutions exhibits two LCST transitions upon heating. In the 1st LCST region, the block copolymer forms a relatively loose-molecular packing, while large aggregates due to partial dehydration of polymer molecules, which subsequently transform into a stable micelle structure in a region of 30-39°C. Finally, a tight aggregate composed of the dehydrated micelles is formed. Temperature-dependent 1H NMR spectroscopy of the diblock copolymers also supports such a postulation for the dual phase transitions and stable micelle structure formation. In addition, a typical salting-out effect is observed in the thermal behavior of the polymer, but a serious cytotoxic effect is not observed in NIH/3T3 cells, suggesting that the novel diblock copolymers are relevant for biomedical applications.