Azobenzene Photoswitch in Asymmetric Polydiacetylenes Dictates Molecular Ordering and Soft-Crystalline Transitions.

Design of stimuli-responsive chiral polydiacetylenes (PDAs) with an innate photoswitch offers exciting opportunities toward tunable chromogenesis and chiroptical behavior as a result of E-Z photoisomerization. We designed a series of asymmetric photopolymerizable diacetylene bisamides bearing variable alkyl chains, DAn (n = 2, 5, 10), to connect the DA with a chiral azobenzene motif. Topochemical polymerization in thin films of DA forms orange colored PDAs to furnish long-ranged fibrillar nanostructures with increasing alkyl chain lengths. The polymer nanostructures exhibit axial or lateral growth after thermal annealing due to improved ordering in the polymeric domains. The PDA self-assembly exhibits crystalline or soft-crystalline packing based on alkyl chain stacking, aromatic interactions, and hydrogen bonding. Differential molecular packing in the PDAs dictates the E to Z photoisomerization percentages, eventually resulting in diminishing chiroptical signatures and anisotropic textures. Photoisomerization-mediated dynamic disassembly in 1D nanofibers are observed in the PDAs bearing decyl chain spacers, which show prominent room temperature soft-crystalline switching behavior. Reverse photoisomerization of Z to E mediated structural reordering result recovery of the chiral packing, morphology, and birefringent patterns. Such tunable soft-crystalline PDAs provide a convenient access to soft adaptable chiroptical materials toward solid-state actuation applications.