Structures, Chiroptical Properties, and Unexpectedly Facile Helical Inversion of Highly Elongated Anthracene‐Fused Expanded Helicenes

Helical fused anthracenes were elongated by fusing additional aromatic units at both ends to yield novel expanded helicenes. Compounds [5]HA2N and [7]HA consisting of 19 and 21 benzene rings, respectively, were synthesized by fourfold cycloisomerization of the corresponding terminal alkyne precursors. The helical structures were confirmed by X-ray crystallographic analysis, where the aromatic frameworks stacked effectively with the helical turn numbers exceeding two. The enantiomers of the two compounds were resolved by chiral HPLC. Whereas [5]HA2N readily underwent enantiomerization at room temperature at the barrier to enantiomerization of 91 kJ mol^-1, the barrier was enhanced to 99 kJ mol^-1 for the long analog [7]HA. The enantiomers of [7]HA exhibited strong responses in the circular dichroism (CD) and circularly polarized luminescence (CPL) spectra, as scaled by dissymmetry factors |g_abs|=0.034 and |g_lum|=0.012. Theoretical calculations by the r²SCAN-3c method suggested stepwise mechanisms for the enantiomerization via helical inversion with acceptable barrier heights. The unexpectedly flexible nature of the aromatic frameworks of [5]HA2N and [7]HA was discussed on the basis of the proposed mechanism.