Fig. 2

Procedures for the preparation of mirror symmetry-broken helical PBTA^BA nanoribbons. a Photograph of a DMF (0.7 mL) solution of BTA^BA (3 mg, 5.3 × 10⁻⁶ mol). b Photograph of a CD-silent gel of PBTA^BA ([BTA^BA] = 5.3 mM) formed in DMF/water (7/3, v/v; 1 mL) and SEM image of its air-dried specimen. The gel can be prepared via an antisolvent method through direct addition of water (poor solvent) into a DMF (good solvent) solution of BTA^BA without magnetic rotary stirring. c Photograph of a DMF/water (7/3, v/v; 1 mL) solution of BTA^BA (5.3 mM) after heating at 100 °C for 3 min. d, e Typical photographs of CD-active DMF/water (7/3, v/v; 1 mL) suspensions of PBTA^BA ([BTA^BA] = 5.3 mM) and SEM images of their air-dried specimens. The suspensions of (−)-PBTA^BA and (+)-PBTA^BA display mirror-image CD spectra of one another (Fig. 4b). They can be prepared by natural cooling of a clear, hot DMF/water (7/3, v/v; 1 mL) solution of BTA^BA (5.3 mM) with magnetic rotary stirring at 1200 rpm for 40 min. f Photograph of a CD-silent precipitate of PBTA^BA ([BTA^BA] = 5.3 mM) in DMF/water (7/3, v/v; 1 mL) and SEM image of its air-dried specimen. This precipitate can be prepared by natural cooling of a clear, hot DMF/water (7/3, v/v; 1 mL) solution of BTA^BA (5.3 mM) without magnetic rotary stirring