Fig. 1: Structural characteristics of chiral- and racemic-NPB.
a–c Schematic X-ray single-crystal structures of a R-NPB, b S-NPB, and c racemic-NPB. d, e In-plane views of [PbBr4]2− layers in d R-NPB and e S-NPB show two different equatorial Pb–Br–Pb bond angles of 143° and 157° associated with two different Br atoms, denoted as purple and red spheres, respectively. The axial Br atoms are omitted for clarity. Opposite and dissimilar out-of-plane helical distortions (indicated by small and big curved arrows) of red and purple Br atoms can be seen from the spiraling of corresponding Pb–Br–Pb bonds (marked by thicker black and gray lines in d and e) around the 21-screw axes (red and purple arrows). f Racemic-NPB shows a single equatorial Pb–Br–Pb bond angle of 152° with no out-of-plane distortions of Br atoms. Also shown in d–f are the organic terminal -CH-NH3+ groups, represented as solid and shaded dumbbells for the upper and lower organic layers, respectively. g–i, Hydrogen-bonding interactions between the equatorial Br atoms and NEA+ cations in g R-NPB, h S-NPB, and i racemic-NPB. The axial Br atoms and associated interactions are omitted for clarity. The out-of-plane distortions of equatorial Br atoms in d and e are clearly seen in g and h for R-NPB and S-NPB, respectively. Green, red/purple, black, blue, and pink spheres denote Pb, Br, C, N, and H atoms, respectively.
