In the crystal, four crystallographically nonequivalent molecules (1-H-a-d in Figure ) were found. Two of the four (1-H-a and 1-H-b) have the same helical chirality around the sevenmembered-ring moieties (PMP), whereas the other two ( and) have the opposite chirality (MPM). Continuous π-π stacking of alternating 1-H-a and 1-H-b units (Figure ) results in the formation of homochiral nanofibers that mesh in the needle-shaped microcrystals with adjacent nanofibers composed entirely of molecules having the opposite chirality (1-H-c and 1-Hd, Figure ). Figures and provide a closer look at the molecular alignment structures within a single nanofiber. Each nanofiber comprises a double helix structure wherein 1-H molecules of the same chirality are stacked with 45° twist angles. Judging from the result that the diameters of a single nanofiber (2.8 nm, Figure ) matches the height of the thin nanofiber found in the AFM measurement (2.8 nm, Figure ), this supramolecular double helix alignment beautifully accounts for the structure of the nanofibers found in the gel state. The pitch length of the helix is 3.1 nm, and 8 molecules constitute a helical repeat unit (Figure ). These findings clearly demonstrate the power of 3D electron crystallography for the structural determination of nanometer-sized molecular alignments, even for cases in which single crystals suitable for x-ray crystallography cannot be obtained. (G) A double-helix nanofiber with its diameter and twist angle. (H) A double-helix nanofiber with its helix pitch; a unit cell is shown in gray.