Apex hydrogen bonds in dendron assemblies modulate close-packed mesocrystal structures

Impact of peripheral alkyl chain length on mesocrystal assemblies of G2 dendrons

Unique sphere-packing mesophases such as Frank-Kasper (FK) phases have emerged from the viable design of intermolecular interactions in supramolecular assemblies. Herein, a series of C_n-G2-CONH₂ dendrons possessing an identical core wedge are investigated to elucidate the impact of peripheral alkyl chain lengths (C_n) on the formation of the close-packed structures. The C₁₈ and C₁₄ dendrons, of which the contour lengths of the periphery L_p are longer than the wedge length L_w, assemble into a uniform sphere-packing phase such as body-centred cubic (BCC), whereas the C₈ dendron with short (L_p < L_w) corona environment forms the FK A15 phase. Particularly in the intermediate C₁₂ and C₁₀ dendrons (L_p ≈ L_w), cooling the samples from an isotropic state leads to cooling-rate-dependent phase behaviours. The C₁₂ dendron produces two structures of hexagonal columnar and sphere-packing phases (BCC and A15), while the C₁₀ dendron generates the A15 and σ phases by the fast- and slow-cooling processes, respectively. Our results show the impact of peripheral alkyl chain lengths on the formation of mesocrystal phases, where the energy landscape of the dendrons at L_p/L_w ≈ 1 must be more complex and delicate than those with either longer or shorter peripheral alkyl chains.