Figure 1

Chemical structures and isomerism of BBTE.

(a) Schematic illustration of conversion relationship of five thermally stable isomers in BBTE. Three ring-open conformers are thermally isolated (red dashed arrows) due to extremely high rotational barrier (pink arrows). While the parallel ring-open isomer is photochemically inert (magenta dash arrows), the anti-parallel ring-open enantiomers are able to reversibly transform to the corresponding ring-closed enantiomers (olive arrows), according to Woodward-Hoffmann rule on photochemical conrotatory reactions of 4n + 2 π-electron system. (b) ORTEP representation of X-ray single crystal structures for p-, M-ap-, P-ap-, (S,S)-c- and (R,R)-c-BBTE drawn with 50% probability from top and front views (S: yellow; N: blue; C: gray). The solvent molecule (dioxane) in p-BBTE and all the hydrogen atoms are removed for clarity. Atoms with lighter colours indicate their relatively back location. Detailed crystal data including packing diagrams can be found in Supplementary Information.