Preferential formation of columnar mesophases via peripheral modification of discotic π-systems with immiscible side chain pairs
When sufficient volume of dodecyl chains are attached at one imide position of a perylenediimide (PDI) or naphthalenediimide (NDI) core and triethyleneglycol (TEG) chains on the other side, the resulting molecules PDIC12/TEGG0, PDIC12/TEGG1, and NDIC12/TEGG0 self-assemble into a rectangular columnar mesophase with p2mg symmetry, forming hydrophobic/hydrophilic nano-segregation of side chains. The driving force of PDIC12/TEGG0 to form preferentially the rectangular columnar mesophase is given by the immiscibility between the side chain pairs—exclusion of other phases such as cubic, crystalline and amorphous phases, where thermodynamically unstable contacts between hydrophobic and hydrophilic chains considerably take place. In contrast, this preference is less found in the analogous molecules decorated with either dodecyl or TEG chains at both termini. PDIC12/C12G0 and PDITEG/TEGG0 form a hexagonal columnar mesophase because of the optimized chain/core volume. However, if the side chain volume grows, PDITEG/TEGG1 does not form a mesophase but undergoes a soft crystalline–isotropic phase transition, while PDIC12/C12G1 was revealed to destabilize its columnar mesophase but forms a micellar cubic phase. NDIC12/C12G0 resulted in a strong crystallization, while NDITEG/TEGG0 formed amorphous liquid. The molecular design strategy using immiscible side chain pairs potentially enables a variety of π-systems to stack up to form a columnar phase rather than other ordered phases, regardless of the chain/core volume balance.