Synthesis of Porphyrin-based Large Covalent Organic Cages and Macrocycles and Their Applications

DoctorThis thesis describes the studies on (1) largest porphyrinic organic cages (P12L24) and their applications by performing a photocatalytic reaction with different sized substrates, encapsulating a guest molecule and constructing metal-organic polyhedra inside the cavity of ZnP12L24 (MOP⸦ZnP12L24 complex), and (2) a truncated cone-shaped porphyrinic macrocycle (P3L3), constructed by one-pot synthesis and its self-assembly into a permanent porous material. Chapter 1 provides a broad overview of these research topics. Covalent organic cages are one of the emerging supramolecular architectures due to their chemical stability, solubility, low toxicity, and so on. In particular, they have an intrinsic pore that can accommodate guest molecules of an appropriate size. Among the cage-like architectures, porphyrin-based organic cages, which possess an intrinsic void surrounded by multiporphyrin units, have received great attention in various applications including light-harvesting, photocatalysis and guest encapsulation. However, such applications are usually limited to small-sized porphyrin cages because of the synthetic difficulty of large-sized cages. We rationally designed P12L24 from a combination of 12 porphyrin monomers and 24 suitable bent linkers. Through numerous synthetic efforts, we successfully constructed a gigantic porphyrinic cage (P12L24) with an inner cavity of ~4.3 nm in diameter. To validate the solid-state applications of P12L24, heterogeneous photocatalytic studies were carried out which are presented in Chapter 2. The Photooxidation of 1,5-dihydroxynaphthalene (DHN) derivatives of different sizes was investigated in the presence of a catalytic amount of crystalline P12L24. These studies confirmed that the large pore of the cage facilitated the mass transport of the substrates. In the context of an exploration of the inner cavity of synthetic cages, studies on the encapsulation of guest molecules into the host cages have been widely investigated in supramolecular chemistry. Chapter 3 describes our investigation on the host-guest chemistry of P12L24 in the solution state. To explore the scope of guest encapsulation of P12L24, a solution-processable Zn2+-metallated P12L24 (ZnP12L24) was prepared. The guest encapsulation experiment was performed by using a pyridine terminated linear pillar linker as a molecular ruler that has the proper length to coordinate with the porphyrinatozinc(II) moieties at the opposite ends of ZnP12L24. These results illustrate P12L24 which possesses large intrinsic cavity can be potentially useful for the encapsulation of intriguing large guest molecules such as proteins and homogeneous catalysis. In Chapter 4, beyond the encapsulation of a simple pillar linker, a study on the construction of metal-organic polyhedra (MOP) inside the cavity of ZnP12L24 (MOP⸦ZnP12L24 complex) that requires more exquisite synthetic effort is presented. To prepare the MOP⸦ZnP12L24 complex reminiscent of the core-shell structures, we proposed well-designed synthetic approaches, such as “bottle-around-a-ship” and “ship-in-a-bottle” strategies. Even though the formation of core-shell structures has not been fully characterized, the mass spectrometric analysis indicated the possible construction of MOP⸦ZnP12L24. Macrocycles are one of the most fascinating molecular systems that have played an important role in establishing and developing supramolecular chemistry. In particular, the truncated cone-shaped macrocycles have a tendency to self-assemble into higher-order nanostructures such as channels and columns, which have potential applications in gas separation, storage, ion transport, and so on. To make better use of the diverse properties of macrocyclic self-assembly, macrocycles must be prepared from easily accessible building blocks by a single-step reaction and have shape-persistency to enhance non-covalent interactions. In Chapter 5, a study on the construction of a truncated cone-shaped porphyrin macrocycle (P3L3) via an one-pot imine condensation reaction is presented. Thanks to the truncated cone shape and shape persistency of P3L3 macrocycle, the solid-state packing of P3L3 molecules forms 1D hollow channels, which was confirmed by the single crystal X-ray analysis. The permanent porosity of the channels was demonstrated through gas sorption and PXRD experiments. We envision that the approach presented here applies to the synthesis of other macrocycles of various sizes by tuning the angle of the bent linker