Single crystals of H1, H2, H3, and H4 were grown by slow solvent diffusion of methanol into their racemic solutions in DCM, DCM, toluene, and benzene, respectively. X-ray crystallographic analysis revealed that both (P) and (M) enantiomers co-existed in the crystal structure (Figure ). In all cases, the expanded helicene architecture constitutes a supramolecular chiral column along the axis with an intermolecular π-π stacking. For H1, the two enantiomers are packed extremely compact via strong π-π interactions (d = 3.27-3.33 Å) into a one-dimensional (1D) polymer chain, and the chains are further packed into a three-dimensional (3D) structure with additional [CH…π] and van der Waals interactions (Figure ). The angle between the two planes of the terminal rings A and K (θAK = 5.7°) (Figure ) is significantly smaller than that of [7]helicene (θ = 32.8°), indicating less strain in the expanded helicenes with a large helical diameter (dh = 9.15 Å) (Figure in SI). The crystallographic analysis of H2 showed that only one of the enantiomers was packed into a long polymer chain (P chain or M chain) through close π-π contacts with distances of 3.32-3.40 Å, and each chiral chain exists independently (Figure ). The close intermolecular packing along the chain also results in a very small distortion angle θAK close to 0°. On the other hand, the angle (θEO) between the two spatially overlapped rings O and E is measured as about 22.27° due to larger strain (Figure ). H3 displays a similar packing structure to that of H1, with close π-π interactions (d = 3.39 Å) between the neighboring P/M enantiomers in the individual column (Figure ). The angles θAK and θEO are measured as 0° and 14.42°, respectively (Figure ). For H4, P/M enantiomer pairs are identified in the molecular packing, where enantiomers with the same chirality (P or M) are packed in a columnar fashion via inter-molecular π-π interactions (d1 = 3.36 Å, d2 = 3.35 Å, and d3 = 3.36 Å) (Figure ). The molecule makes about 3.5 turns, with angles θAK and θEO measured as 0° and 16.77°, respectively (Figure ). The structures of H1-H4 were further characterized by 1 H NMR and C NMR, and all the 1 H NMR peaks were assigned by 2D NOESY NMR technique (Figures S1-S4 in SI). Most of the protons on the π-conjugated skeleton appear at low field (δ = 8.0~13.0 ppm), except that the protons on the two terminal benzene rings show up-field shift due to spatial overlap (shielding effect) with the benzenoid rings in the neighboring layer, indicating that these expanded helicenes have localized aromatic character like kekulenes and cycloarenes. This is further supported by nucleus-independent chemical shift, anisotropy of the induced current density, and 3D isochemical shielding surface calculations (Figures S21-S23 in SI). Accordingly, compounds H1-H4 show similar UV-vis absorption spectra in DCM, with the absorption onset slightly red shifted with the extension of the molecular size (Figure ). Time-dependent density functional theory (TD DFT) calculations suggest that the low-energy absorption band at about 440-500 nm can be correlated to the multiple HOMO-n→LUMO+m (n, m = 0-2) electronic transitions (Tables S5-S8 in SI). These four compounds also show similar fluorescence spectra, with a slight red shift of the emission maximum (487, 505, 514 and 519 nm for H1, H2, H3 and H4, respectively) with the extension of length (Figure ). The absolute photoluminescence quantum yields (PLQYs) of H1, H2, H3, and H4 in DCM were determined to be 21.0%, 19.1%, 18.9%, and 16.6%, respectively, by using integrating sphere technique. The relatively low PLQYs can be explained by the nearly forbidden HOMO→LUMO electronic transitions (f = 0, 0.030, 0.025, 0.030, respectively, according to TD DFT calculations). Furthermore, their corresponding fluorescence lifetimes (τ) were measured as 10.2, 12.8, 13.1 and 13.0 ns, respectively (Figure The racemic mixture of H2, H3, and H4 can be resolved by chiral HPLC at room temperature. However, no separation was detected under any condition tested for H1 due to its small racemization barrier (12.65 kcal/mol according to DFT calculations, Figure in SI). The enantiomers of H2 were separated by using a steroid-loaded silica gel column under HPLC conditions (eluent: 40% MeCN/DCM) to give (-)400-H2 and (+)400-H2 (Figure in SI). The enantiomers, (-)400-H3/(+)400-H3 and (-)400-H4/(+)400-H4, were separated under similar conditions (H3: 50% MeCN/DCM, H4: 52% MeCN/DCM). All the circular dichroism (CD) spectra of enantiomers are complete mirror images (Figure ), and the absolute configurations are assigned as (+)400 = (P)/(-)400 = (M), based on experimental and theoretical CD spectra (Figure in SI). Positive Cotton effects (CEs) between 354-500 nm for (P)-H2, 347-500 nm for (P)-H3 and 344-500 nm for (P)-H4 are observed, while strong negative CEs are found in the ranges of 254-354 nm for (P)-H2, 253-347 nm for (P)-H3 and 250-344 nm for (P)-H4. It is worth noting that H2, H3, and H4 show |gabs| maxima of up to 0.020 at 420 nm, 0.021 at 421 nm and 0.024 at 451 nm, respectively, which are among the highest values for helicenes and helicene-like structures in the visible range. To understand the origin of the high |gabs| values, transition dipole moments and chiroptical properties were also investigated. According to theory, the dissymmetry factors can be calculated by the equation g = 4cosθ|m||μ|/(|m| 2 +|μ| 2 ), wherein μ, m, and θ represent the electric and magnetic transition dipole moments, as well as the angle between μ and m, respectively. 17 For organic materials, the |m| value is normally much lower than the |μ| value. The above equation can thus be simplified as g = 4cosθ|m|/|μ|. Calculation suggests that with the extension of the molecular size, the |m| values for the S0-S1 transition increase from H1 to H3, presumably due to more effective π-electron delocalization (Figure ). However, the value slightly drops at H4, indicating the saturation of effective conjugation, which is in agreement with the frontier orbital profile analysis (Figures S11, S13, S15, S17 in SI). It is also found that the θ angle strongly depends on the molecular symmetry, that is, for H1 and H2, the μ and m are almost aligned along one line, while for H3 and H4, large θ angles (nearly 80 o ) are calculated. The calculated absorption dissymmetry factors gcal (H1: 0.015, H2: 0.017, H3: 0.006, and H4: 0.045) are consistent with the trend observed experimentally.