Proximity Enabled Heterogeneous Photocatalysis Using Covalent Organic Frameworks

Majumder, Poulami (2024) Proximity Enabled Heterogeneous Photocatalysis Using Covalent Organic Frameworks. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Nowadays, heterogeneous catalysts play a crucial role to produce more than 80% of all products through out the world. Therefore, scientist has thoroughly studied to evolve stable heterogeneous catalysts with high porosity compared to homogeneous ones. The reactants can get access to the catalytically active sites due to the arranged pore and the higher surface porosity of the heterogeneous backbone which also facilitate mass transfer process and increases the catalyst’s turn over number. Still now, many porous materials like ZIF, Carbon (porous), silica and Metal Organic Framework have already been designed as heterogeneous catalysts. The newly, crystalline materials that are thermally stable, and lightweight has flourished as a heterogeneous catalyst backbone or stable support called Covalent Organic Frameworks (COFs). High surface area with long-range order and uniform pore size with tenability make them suitable for uptake of gas, separation of molecules, sensing, and storage of energy. Researchers have successfully utilized this COF material for heterogeneous catalysis after the discovery of a ketamine-based stable backbone. At the heart of catalysis science, in the heterogeneous catalysis field, porous COFs may introduce a new, research field. Chapter-1 of this thesis depicts the heterogeneous catalysis of various porous nanostructured materials for example carbon materials, MOFs, porous silica and COFs. The primacy of using COFs as heterogeneous backbones due to their high crystallinity, long-range ordered structure, tunable porosity, electron-hole separation properties, etc. The factors and parameters that control the heterogeneous catalysis process have also been discussed with various examples already reported in the literature. Here, also showcased chemical, electrical and photocatalysis in presence of COFs moiety. Chapter-2 of the thesis addresses C–H Functionalization by photochemically using a COF Confined Fe₂IV–μ–oxo Species in H₂O. This work introduces a novel photochemical method that utilizes high-valent iron oxo species encapsulated within COFs to facilitate the C─H functionalization of the un-activated carbon-hydrogen bonds of the alkane and alkenes in water. We have introduced TpDPP COF nanospheres and films as photosensitizers and porous hosts to develop a catalytically active FeIV–μ–oxo intermediate. For the first time we have introduced high stability and excellent activity of a high valent μ–oxo–(FeIV)₂ radical species within the covalent organic framework. The synthesizing of the μ–oxo–(FeIV)₂ radical species was confirmed by using several spectroscopy methods. To overcome the problem of aggregation and recyclability of TpDPP COF nanospheres, we converted COF nanospheres into thin films via covalent self-assembly, resulting in very good yield (64%) and very good selectivity (cis:trans 74:1). We have also got outstanding activity and selectivity for the activation of strong C–H bonds with high bond energy (96 kcal/mol to 98 kcal/mol) in water. The extraordinary activity of the μ–oxo–(FeIV)2 species towards the functionalization of C–H bonds could pave the way for the immobilization and stabilization of transient species in water media. Chapter-3 of the thesis describes alkene oxidative cleavage via photoexcited FeIV species within covalent organic framework thin films in water. In this chapter we have presented a study in which an iron macrocycle is encapsulated within a covalent organic framework (COF) as a heterogeneous support. We use a stable β-ketoenamine 2D COF thin film, and it can also act as a photosensitizer. Upon excitation (λ = 440 nm) the COF can transfer charge to the iron macrocycle, and this allows for the system to oxidatively cleave alkenes without H₂O₂. Furthermore, we present a very thorough and complete substrate scope, as well as mechanistic studies and a proposed reaction rate. Structural integrity and composition analysis for the COF and the support on COF system are presented as well. Showing studies on before and after catalysis, as well as showing cycling of the catalyst. Chapter-4 of the thesis demonstrates metal free β-keto-enamine-based COFs for heterogeneous catalysis. for the light induced metal-free oxo azidation and oxo trifluoromethylation in unactivated alkenes. The protocol can demonstrate high selectivity towards both electron-rich and deficient substrates. We have chosen 3 different type of COFs, with unlike building units in order to get an idea of the role of various physical and photophysical properties (e.g., surface area, absorbance, and band gap) in controlling photocatalytic performances. TpAzo showcased the highest yields among all of 3 COFs due to its highest surface area (1725 m²g⁻¹), broader absorbance (400−600nm), and low band gap (1.78eV). Furthermore, PL and other electrochemical experiments were done to examine the charge separation capability of the COFs. It shows up to five cycles, and the COFs’ backbone remains intact with high crystallinity and long-range order. Chapter-5 of the thesis illustrates the using of TpAzo COF as metal-free backbone for heterogeneous catalysis for the visible-light-driven metal-free carbon-sulphur cross coupling. We reported an efficient metal free visible-light photoredox arylation of thiols with aryl halides at r.t using COF backbone. This metal-free reaction under photocatalytic conditio is compatible with both electron rich and electron deficient functional groups.

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