a PdCl2(dppf) (5.0 mol%) or XPhos Pd G2 (5.0 mol%), K3PO4 (excess), 1,4-dioxane/H2O or THF/H2O, RT, 12 h or 24 h. b 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-pyrrole (1.5 eq.), SPhos Pd G2 (m = 1; 2.5 mol%) or XPhos Pd G2 (m = 2; 5.0 mol%), K3PO4(excess), THF/H2O, RT, 12 h. c [Ir(cod)OMe]2 (1.0 mol%), DTBPY (3.0 mol%), B2pin2 (0.5 eq.), THF, 80 ˚C, 12 h. d 4m-Bpin (m = 0: 3.0 eq.; m = 1, 2: 2.5 eq.), XPhos Pd G2 (5.0 mol%), K3PO4 (excess), THF/H2O, RT, 24 h. e 1) PIFA (n = 9: 6.0 eq.; n = 11: 8.0 eq.; n = 13, 15: 10 eq.; n = 17, 19: 12 eq.), CH2Cl2, -78 ˚C to RT, 3 h, 2) NaBH4 (excess), MeOH, RT, 10 min. f 3m (m = 0: 2.2 eq.; m = 1: 2.6 eq.; m = 2: 2.1 eq.), XPhos Pd G2 (5.0 mol%), K3PO4 (excess), THF/H2O, RT, 24 h. X-ray crystal structures. Fortunately, single crystals suitable for X-ray diffraction (XRD) analysis were obtained by slow vapor diffusion of n-hexane into solutions of [n]AHs in THF, acetone, or 1,4-dioxane. The structures are shown in Figure in which the solvent molecules coordinated to the NH moiety via hydrogen bonds are omitted. Because of the sophisticated molecular design, intermolecular interactions between [n]AHs were not observed in any of the structures, thereby contributing good solubility. Indeed, five and seven acetone molecules were included in the asymmetric units of [13]AH and [15]AH, respectively, (Figures , 5-5) whereas seven 1,4-dioxane molecules were included in the asymmetric unit of [19]AH (Figure ). For [9]AH, the vertical distance (interplanar distance) between the centroid of terminal benzene ring A and the mean plain of the opposite terminal benzene ring I is 3.807 Å. As the typical distance of π-stacking is approximately 3.4 Å, interlayer interaction between the terminal benzene rings may be minimal, similarly to that of [7]carbohelicene (3.8-3.9 Å). The interplanar angle between the stacked two benzene rings is approximately 40°, which also suggests that the interlayer π-π interaction is weak. By contrast, for [11]AH, the longest and shortest vertical distances are 3.567 Å and 3.202 Å, respectively, suggesting a distinct interlayer π-π interaction between the stacked aromatic rings. The interplanar angles are 16.3°, 11.6°, and 20.4°, being smaller than that of [9]AH in accordance with the effective interlayer π-π interactions. The crystal structures of [13]AH and [15]AH are also shown in Figure . As the space group is P32 for the former and the lattice contains only one enantiomer, spontaneous chiral resolution took place for [13]AH upon crystallization. Chiral separation using HPLC apparatus is discussed later. In For all the parameters, the trend of the experimentally determined values agreed with those obtained from the theoretical calculations, which reproduced the notable changes from [9]AH to [11]AH. This result reflects the weak intramolecular π-π interactions at the terminal aromatic ring of helicenes. A decrease in the interplanar distances or angles results in an increase in the dihedral angles, indicating that the intramolecular interactions become stronger and the aromatic rings become more densely stacked with helicene elongation, while the strain energies of the [n]AHs increase with helicene elongation. To estimate the inherent strain energy of [n]AHs, we attempted to adopt a homodesmotic reaction model. Accordingly, the strain energy (ΔH) of [n]AHs increased continuously as the number 'n' increased (Figure ). These estimations are in accordance with several cases of expanded helicenes previously reported. 6d) Finally, the interlayer π-π interactions of [n]AHs between the two neighboring layers were visualized using the noncovalent interaction (NCI) plot analysis (Figures 2A and S7-11). Dispersion interactions between the two stacked layers of [n]AHs are obvious (green surface shows intramolecular ππ interactions). In particular, intramolecular π-π interactions across three layers was confirmed for The negative shift in the first oxidation potentials can be attributed to an increase in the number of electron-rich pyrrole units in the helicene core. In particular, [19]AH was confirmed to be very easily oxidized. Nevertheless, the as-synthesized [19]AH was purified using flash chromatography on silica at room temperature and recrystallized from THF/n-hexane, allowing the growth of single crystals under ambient conditions (vide supra). Molecular orbital calculations were performed for the optimized structures of [n]AH in the ground state (Figure ). The HOMO energy level gradually increased as the helical length increased. By contrast, the LUMO energy level was not markedly affected by the elongation, resulting in a gradual decrease in the HOMO-LUMO gaps. To gain further insight into the structure-property relationship in these benzannulated azahelicenes, the corresponding aza[n]helicenes without benzannulated structures (i.e., [n]AH') were also calculated (Figure ). The HOMO energy levels of [n]AH's are generally higher than those of Aromaticity. To evaluate the aromaticity of [n]AHs, the NICS(0) value at the center of each rings was calculated using the GIAO method at the B3LYP/6-311G (d,p) level based on the optimized structures. The NICS(0) values of [9]AH and [11]AH are in the range from -6.6 to -11.6 ppm and from -6.3 to -12.3 ppm, respectively (Figure ). Rings 3 and 5 were relatively shielded whereas the local aromaticity in the pyrrole units (Rings 2, 4, ( )) and the terminal benzene moiety (ring 1) were dominant. The HOMA values are consistent with the local aromaticity of these rings. Notably, the benzo segments (rings 6 and 7 for [9]AH and rings 7 and 8 for [11]AH) exhibit relatively deshielded features compared with the neighboring six-membered ring. The anisotropy of the induced current density (ACID) calculations revealed the localized aromatic ring-current flows of the pyrrole and the annulated benzene rings in [9]AH and [11]AH as the dominant contributions (Figure ). These results represented Clar's sextet rule as shown in Figure . Such local aromaticity suppresses the increase in the HOMO energy levels to the extent that the isolation of these molecules without any kinetically stabilizing substituents is allowed under air. The same calculations were performed for longer [n]AH and the same trend in their aromaticity was observed (Figures , S7-10). (Chir)optical properties. The UV/vis absorption and fluorescence spectra of a series of aza[n]helicenes were measured in THF at room temperature (Figures and). The edges of the absorption spectra are red-shifted, and the molar extinction coefficient (ε) increases with helicene π-extension. In particular, the absorption spectral edge of the longest [19]AH reaches approximately 500 nm, suggesting that the effective conjugation length (ECL) in this system was not saturated. 29) This result was supported by the HOMO-LUMO energy gaps obtained from theoretical calculations (Figure ). All the aza[n]helicenes exhibited blueto-yellow luminescence with some vibrational bands. The edges of the fluorescence spectra also show gradual redshifts along with the helicene π-extension. The absolute fluorescence quantum yields (ΦF) tended to decrease with increasing helical length (e.g. ΦF = 0.21 for [9]AH and ΦF = 0.08 for [19]AH) (Figure ). However, even [19]AH exhibits distinct blue-green fluorescence with ΦF = 0.08 as a characteristic feature of heterohelicenes. By contrast, [17]AH exhibits a high fluorescence quantum yield with ΦF = 0.18, which may be attributed to its rigid structure and suppressed nonradiative deactivation as a result of the large steric constraints caused by the triple-layered structure. The chiral resolution was examined using HPLC equipped with a chiral stationary phase to investigate the chiroptical properties of [n]AHs. Despite the adoption of numerous conditions, the enantiomers could not be separated, except for [9]AH. The inability to separate the enantiomers may be partly ascribed to the limitations of the solvent, which was eluted as a gradient. Thus, we focused on N-alkylation of [n]AHs to improve their solubility in various organic solvents. Recently, we reported that N-alkylation of aza [8]circulenes improves their solubilities and stabilities owing to the lowered HOMO energy levels. 30) Thus, we performed Nalkylation of [n]AHs with an excess of 1-iodobutane in the presence of t BuOK in DMSO at room temperature (Scheme 2). Completely N-butylated aza[n]helicenes (n = 9, 11, 13), namely 46). The XRD measurements also confirmed that the pyrrole units in the azahelicene core remained undreacted in the middle layer (Figures S5-17, S5-18). Presumably, for [15]AH and [17]AH, the central NH moieties are less reactive owing to the steric hindrance caused by the upper and lower layers. Unfortunately, [19]AH was not cleanly derivatized under the same conditions, which resulted in the formation of a complex mixture, probably because of its highly electron-rich core. With these N-butyl aza[n]helicenes, enantiomeric separation could be accomplished by using chiral stationary phases with [9]AH-Bu4 and [11]AH-Bu5 (Fig. ). The CD spectra of both the first and second eluents were measured to display mirror-imaged spectra (Figure ). The spectra showing a positive-to-negative Cotton effect around 350 nm were those of the first eluted solutions in all cases, which was assigned as (M)-enantiomers, as determined by TD-DFT calculations (Figure ). The intensities of the main bands of the molar circular dichroism (Δε) with the maxima located at 328 nm and the dissymmetry factors (gCD) measured for ). Finally, we examined the dissymmetry factors of the azahelicenes using TD-DFT calculations. A clear trend of improved chiroptical response as a function of the helicene length was revealed (Figure ). The behavior of [n]AH in racemization is clarified using theoretical calculations at the B3LYP-D3(BJ)/def2-SVP level. The results showed that the most stable structures had C2 symmetry and the transition states had Cs symmetry. As indicated in our previous study, the inversion barrier at 298 K was relatively smaller (+24.3 kcal/mol) for