Our Vision: To develop efficient photocatalytic processes enabled by novel molecular or polymer supported photocatalysts, facilitated by the use of flow technology.
Conjugated Porous Polymers
Conjugated porous polymers (CPPs) are a class of fully cross-linked porous organic polymers featuring extensive π-conjugation and permanent nanoscale porosity. Through careful selection of building blocks, it is possible to tailor CPPs to various key applications including gas separation and storage, energy storage, chemosensing and (photo)catalysis.1
By incorporating dyes into the design of CPPs, they can be imparted with photocatalytic applications. For example, research within the VilelaLAB led to the development of CPPs based on boron-dypyrromethene (BODIPY) dyes.2 The singlet oxygen generating properties of these polymers were then validated under batch and continuous flow conditions, as well as in green solvents.
Photooxidation of α-terpinene by BODIPY based CPPs under flow conditions.
In collaboration with Prof. Zhengtao Xu at the City University of Hong Kong, a graphene like fused CPP was developed and applied to various applications.3 In addition to being able to act as a photocatalyst for the oxidative coupling of benzyl amines, the CPP was also able to act as a solid base catalyst for the Knoevangel condensation and as a sorbent material for toxic heavy metals. In latter case, the polymer was remove lead from water to below the limit for potable water within minutes.
Multifunctional graphene like CPP.
More recently, CPPs designed in the VilelaLAB have found applications in the regeneration of the cofactor NADH and in the degradation of dyes.4,5
In addition to developing photocatalytic applications of CPPs, research within the VilelaLAB has lead to developments in their design and synthesis. By far the most commonly utilised synthesis method involves Sonogashira-Hagihara cross-coupling: research within the group lead to a procedure to prepare monolithic CPPs via a copper-free Sonogashira coupling in water and under aerobic conditions.6 Post synthetic modification of these polymers allowed for the introduction of Ag(I) ions that were able to act as catalysts for heterocyclisation reactions.
Preparation of monolithic CPP via copper-free Sonogashira-Hagihara coupling.
Polymer Supported Photocatalysis
Although CPPs have demonstrated their versatile application, there are some limitations associated with their use outside the lab, principally the cost associated with their construction. The building blocks for photoactive CPPs are often expensive and are used to assemble the bulk of the material, not only the surface where photochemical reactions occur. Alternative approaches that have been developed in the VilelaLAB involve attachment of photoactive units to cheap and inert polymer supports.
For example, work within the research group led to the development of a benzothiadiazole based vinyl cross-linker that could be copolymerised with styrene via free radical polymerisation.7 Via this approach, heterogeneous triplet photosensitisers were prepared in three distinct forms: polymer gels (batch polymerisation), beads (suspension polymerisation) and monoliths (high internal phase emulsion polymerisation). In each case, the materials were able to efficiently generate singlet oxygen and the superoxide radical anion for the hydroxylation of arylboronic acids.
Benzothiadiazole crosslinked polystyrenes as A) gels, B) beads and C) monolithic polyHIPEs.
Flow Technology
Within the last decade, flow chemistry has emerged as a truly versatile tool for chemists from all avenues of research. In particular, photochemistry has benefited greatly from the flow revolution. In a conventional batch process irradiation is both non-uniform and inefficient. In contrast, irradiation in flow is superior due to the smaller pathlength. Within our research into both CPPs and polymer supported photocatalysts, we have demonstrated how our materials are suitable for application in flow and how operating under flow conditions enhances the photocatalytic performance of the materials.
In addition to testing our materials under flow conditions, we have also collaborated with various research groups to develop non-photocatalytic organic reactions in flow. This includes the tandem hydrogenation/oxidative heck reaction of cyclopentane-1,3-diones, 8 azide-alkyne click chemistry,9 and lithiation-substitution of 1,3,4-oxadiazoles.10
Photo-RX-Ware
References
Y.-L. Wong, J. M. Tobin, Z. Xu and F. Vilela, J. Mater. Chem. A, 2016, 4, 18677–18686.