Conjugated Molecules, Macromolecules, and Nanoparticles with Ladder-Type Backbones

Conjugated ladder polymers consist of π-conjugated, fused-ring repeating units, linked together by multiple strands of bonds. Such structural feature rigidifies the backbone and imparts unique chemical, optical, and electronic properties into ladder polymers. These valuable properties create an urgent incentive for solving the challenges associated with these materials: synthetic and processing challenges arise from the rigid, multiple-stranded backbones, and characterization of the solid-state morphology of these materials is limited. To overcome the synthetic challenges, ring-closing olefin metathesis (RCM) was applied to two model synthetic targets. RCM annulation can construct extended fused-ring structures in a highly efficient manner due to the strong thermodynamic driving force towards aromatization. We therefore applied RCM to create the fused-ring backbone in previously challenging systems: a donor–acceptor ladder polymer and a crosslinked porous ladder polymer network. Historically, the former had been limited by inefficient annulation reactions with electron-deficient backbone units, and the latter requires an annulation which can take place within the pores of the network. The employment of RCM led to successful synthesis in both cases due to its lessened dependence on backbone electronics and the catalyst’s ability to diffuse through the network pores, respectively. The network was made dispersible in organic solvents using a miniemulsion polymerization, affording nanoparticles that remained highly porous. Chemical, optical, and physical properties of these polymers were studied and reported, with emphasis on the importance of the rigidified backbone for various processes, including optical properties, gas adsorption, and fabrication into composite materials. In order to accelerate the use of conjugated ladder polymers in practical applications, a stronger understanding of solid-state morphology is required, especially due to the strong effect of molecular orientation on their properties. However, such morphological studies are lacking for ladder-type macromolecules compared to non-ladder analogues. To this end, ladder-type molecules and molecules were studied using grazing incidence X-ray scattering (GIXS), demonstrating controllable morphology using thermal annealing and solvent vapor exposure. By studying a variety of molecular sizes and structures, size-dependent and structure-dependent effects were identified and correlated to morphological changes. Overall, using RCM as a synthetic method and GIXS as an analytical technique both contributed to the advance of the research field of conjugated ladder polymers. These results provide valuable strategies for design, synthesis, and characterization of conjugated ladder polymers in order to improve the understanding of these materials in the pursuit of next-generation functional organic materials