Conjugated porous polymers for visible light-induced organic transformations
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Abstract
In this thesis, the development of novel conjugated porous polymers as efficient heterogeneous photocatalysts for visible light-driven organic transformation reactions is described. The work was conducted following three main objectives: (i) the development of metal catalyst-free synthetic routes for conjugated porous polymers with the focus on covalent triazine frameworks (CTFs); (ii) structural and morphological control of CPPs within the micro- and meso-range; and (iii) utilization of the CPPs for challenging organic photoredox reactions and processability study in continuous flow systems.
First, a novel trifluoromethanesulfonic acid vapor-assisted solid phase synthetic method to construct nanoporous covalent triazine frameworks with highly ordered hollow interconnected pores under mild reaction conditions was developed. This unique solid state synthetic route allows not only the avoidance of undesired side reactions caused by traditional high temperature synthesis but also allowed maintaining defined and precise optical and electronic properties of the nanoporous triazine frameworks. Promising photocatalytic activity of the networks was demonstrated in the photoreduction reaction of 4-nitrophenol into 4-aminophenol under visible light irradiation.
To further investigate the impact of ordered nanostructures of the covalent triazine frame works on their photocatalytic efficiency, a thiophene-containing CTF was synthesized directly onto mesoporous silica, obtaining an ordered pore structure with a diameter of ca. 3.8 nm, and a significantly high oxidation potential at +1.75 V vs. SCE. The high photocatalytic ability of the CTF was demonstrated via the selective oxidation of alcohols and saturated hydrocarbons at room temperature. The high selectivity and efficiency of the CTF were comparable with the state-of-art metal or non-metal catalytic systems reported.
Furthermore, a new concept of the structural design for covalent triazine frameworks is undertaken to enhance the photo-induced charge separation within the CTF network and increase the photocatalytic efficiency. Here, a conceptual asymmetric CTF structure was designed by introducing asymmetric donor-accepter building block into the CTF network. Four different donor-acceptor domains were obtained within the polymer backbone structure. Enhanced light-induced electron transfer within the CTF network was obtained, resulting into a high photocatalytic performance for the synthesis of benzophosphole oxides.
At last, to investigate the general processability of the CPPs in continuous flow systems as an industrial relevant application, a fix-bed photoreactor containing conjugated porous polymers-coated glass fibers was designed. A thin film of a nanoporous polymer containing benzothiadiazole with a thickness of ca. 80 nm was fabricated directly on the glass fiber with an effective catalyst content of ca. 3.2 wt%. The photocatalytic dehalogenation reaction of α-bromoacetophenones and enantioselective α-alkylation of aldehydes were carried out in the fix-bed photoreactor in a continuous flow system with comparable efficiencies as the state-of-art transition metal-based photocatalyts.