The three crystallographically independent nanorings in P6-II differ in their molecular geometries. The blue and green ones have substantial curving of the butadiyne linkers and porphyrinic walls are tilted to the side, similarly to polymorph P6-I. The porphyrin units in the blue and green nanorings are planar (root-mean-square deviation, RMSD, of the 24-atom mean porphyrin plane of 0.046 and 0.062 Å, respectively), with zinc(II) ions only slightly displaced from the plane (0.23 and 0.22 Å) and the distance between carbon atoms in the butadiyne linkers and the mean six-Zn plane is clink = 0.75-1.03 Å, as shown in Fig. . The linkers in the red rings lie approximately in the Zn plane (largest clink = 0.09) Å) and the porphyrins bend outwards (RMSD 0.204 Å), exposing the zinc(II) centers 0.44 Å above the porphyrin mean plane (ca. twice as much as in the other two conformers in this crystal structure), similar to the gas-phase calculated geometry. Structure P6-III. In the second trigonal polymorph P6-III, the length of the unit cell edge a is close to that of P6-II, but the c parameter is roughly twice as long. In contrast to P6-II, and reflected by in the unit cell dimensions, there are three distinct layers (A, B and C; Fig. ) occupied in total by four crystallographically independent nanorings. The main difference with respect to P6-II is that in each layer P6-III, six rings (six blue or purple in layers A/C and three red and three green in layer B) surround nanoring-sized voids. Thus, the nanorings do not form infinite tubular stacks in the c-direction. Instead, the blue, red and purple rings for trimeric stacks (Fig. ), separated by the vacancies surrounded by red and green rings. Three distinct molecules adopt wavy conformations (clink = 0.61-1.06 Å), whereas in the fourth (red) the butadiyne linkers lie approximately in the Zn atom plane, similar to the gas-phase geometry (clink = 0.04-0.15 Å), as illustrated in Fig. . In parallel with P6-II, the only intermolecular butadiyne-porphyrin contacts are between the red (linkers in the Zn plane) and green rings (wavy linkers) and the contacts between blue rings and also between the violet rings are butadiyne-to-butadiyne (Fig. ).