Intriguing Facets of Solution Processable Cross-Linked Porous Organic Polymers

Abstract

Porous organic polymers (POPs) are organic networks distinguished by their highly cross-linked structures and their intrinsic porosity. The growing emphasis on POPs is driven by their exceptional hydrothermal stability and diverse application prospects. However, traditional metal-catalyzed high-temperature reactions using rigid building units yield POPs in insoluble powder form, posing challenges for processing into different shapes for device integration and optoelectronic applications. The successful fabrication of a soluble porous organic polymer relies on the employment of specific design strategies and reaction conditions to restrict the molecular weight and extensive cross-linking. In recent decades, researchers have been actively exploring various design strategies, such as limiting molecular weight through hyperbranching and employing controlled polymer growth strategies, to produce solution processable amorphous porous organic polymers. However, targeted synthesis for specific applications remains underdeveloped, justifying the need for an in-depth deliberation of currently available strategies and possible future avenues. In this context, this Account highlights the advancements in the field of solution processable amorphous cross-linked porous organic polymers (SCPOPs), describing diverse design strategies and function-led applications. In order to address the challenges associated with the solution processing of amorphous cross-linked POPs, our research group has focused on fine-tuning the noncovalent interactions among the molecular building blocks, the key to achieving both porosity and solubility in the resultant porous polymer. Following this principle, we introduce long alkyl chains as flexible groups in the monomer and comonomer units that offer a high degree of rotational freedom and a substantial twist angle. This approach facilitates alleviation of the pronounced π–π stacking interaction and extensive cross-linking, thereby enhancing the solubility of the porous polymer. As a result, the facile interaction between the analytes and inefficiently packed polymer chains with aromatic building units in SCPOPs opens the scope for fluorescence-based nitroaromatic sensing in solution. Further, a stable dispersion of fluorescent porous polymer nanoparticles could be an attractive platform for analyte detection in water with enhanced sensitivity. The porous nature of the fluorescent SCPOPs enables the encapsulation of diverse dye molecules, and controlling the energy transfer efficiency from polymer to dyes results in fluorescence tuning, leading to the emission of white light in solution, nanoparticles, gel, and a thin transparent film. Furthermore, we demonstrate that incorporating alternate donor–acceptor units into the cross-linked polymer leads to the optimum band positions for light-driven redox reactions, such as photooxidation of benzylamine and hydrogen evolution. We investigated a biphasic catalysis route employing solution-processable POPs to enhance the efficiency while facilitating the recovery and reusability of the photocatalysts. As detailed in this review, the comprehensive inspection of the design strategies and potential applications of SCPOPs paves the way to delve into designing new processable functional porous materials, catering to the need for current sustainable development goals.