The solid-state structures of 5T and 6T were determined via X-ray diffraction analysis of single crystals obtained from a hexane/chloroform and a pentane/THF solution, respectively (Figs. and). Compound 5T adopts a C2-symmetric structure in which five thiophene rings are fused at the 3,4-positions to form a 10-membered ring. Compound 6T adopts a screw (or helical) structure with D2 symmetry, similar to hexaphenylene 2b,2e-2h and hexa(2,3-thienylene)s. The distance between the centroids of the central thiophene rings (av. 3.4 Å) is typical for intramolecular π-π interactions. In both crystals, the enantiomers are aligned alternatingly reflecting the centric space group (I2/a). The average dihedral angles (θ) of the α-β-β-α and β-β-β-β positions of the 3,3ʹ-bithiophene moieties are shown in Figs. and. The dihedral angles in 5T and 6T are 46.0° (θa), 90.8° (θb), and 87.8° (θc), and 41.8° (θd), 46.0° (θe), and 124.5° (θf), respectively (see Fig. in ESI for details). Considering the dihedral angle in 4T (54.3°), determined using X-ray crystallography, 4e the smallest dihedral angle in 4T, 5T, and 6T decreases with increasing number of thiophene rings. In order to gain insight into the size-dependent structural characteristics, DFT calculations were conducted. The structures of 5T and 6T, optimized at the B3LYP-D3/6-31G(d) level nicely reproduced the structures observed by X-ray crystallography, including the dihedral angles (Fig. ; see Fig. in ESI for details). Compound 6T has two possible stable conformations, i.e., a screw and a crown conformation. § The Gibbs free energy of 6T with the screw conformation is 3.5 kcal•mol -1 lower than that of the crown conformation, which was not observed by NMR spectroscopy in this study (see Fig. in ESI for details). Strain energies of 5T and 6T were calculated to be 19.9 and 1.1 kcal•mol -1 , respectively, using hypothetical homodesmotic reactions. This result indicates that 5T has a strained structure, and the decrease in energy of 6T might be due to the stabilization by intramolecular π-π interaction (see Fig. in ESI for details). The enantiomerization between C2-symmetric 5T and 5Tʹ as well as between D2symmetric 6T and 6Tʹ is illustrated in Figs. and. Our calculations suggest Cs symmetry for the transition states (TSs) of 5T and 6T, and activation energies for the interconversion of 5T and 6T of 5.0 and 26.5 kcal•mol -1 , respectively. The highlighted thiophene ring resides initially on the C2 axis in 5T; after interconversion via the enantiomerization pathway, this thiophene ring does not reside anymore on the C2 axis in 5Tʹ. By repeating these enantiomerization steps, all α-protons become magnetically equivalent. This dynamic motion, together with the low activation energy, leads to a rapid exchange of the αprotons even at low temperatures. In the enantiomerization process of 6T, the highlighted thiophene rings on the C2 axis becomes the central thiophene rings, and four thiophene rings are symmetrically identical. Although this process leads to the magnetic equivalence of the α-protons, the reaction rate should be slow because of the high activation energy (26.5 kcal•mol -1 ). Taken together, the NMR spectrum of 4T shows a singlet because of its highly symmetric structure (D2d). Although 5T has low symmetry (C2), the fast dynamic motion of the molecule enables the α-protons of the thiophene rings to reach magnetic equivalence. Compound 6T also has low symmetry (D2), albeit that its unsymmetrical NMR signals indicate a slow exchange of the α-protons on the NMR timescale. Thus, the size-dependent structural characteristics of 4T, 5T, and 6T were determined using a combination of NMR measurements and DFT calculations.