Synthesis And Their Fluorescent Properties Of Polyimide-COFs

Covalent organic frameworks?COFs?compound is a new class of ordered porous crystalline materials covalently bonded by some light elements such as C,O,N,B,prepared through thermodynamically controlled reversible polymerization reaction of organic building units.Due to some outstanding advantages including regular network structure,large specific surface area(up to 4000 m².g^-1),high thermal stability?400-500 ??,low density(down to 0.17 g·cm^-3),multiple open sites,excellent crystallinity,porosity,wide pore size range from micropores to mesopores and diversified building units,COFs materials have showed good application potential in gas storage,energy storage,photoelectricity,luminescene,chromatogyaphic separation,catalysis,and so on.Especially,2D COFs with p-conjugated structure have displayed excellent photoelectric,fluorescent and electrochemical sensing properties.In this thesis,four novel polyimide-based COFs?PI-COFs?have been designed and synthesized by employing primary amines and anhydrides as basic structural units.Because of their big conjugated systems and rigid planar structure,the prepared PI-COFs showed excellent fluorescence properties.Taking advantage of the specific influence of the metal ions on their luminescent properties,several fluorescent sensing platforms based on turn-on or turn-off modes were successfully developed for quick and sensitive determination of some metal ions.This thesis mainly consists of three parts as follows:Two kinds of novel polyimide-based COFs?PI-COF 101 and PI-COF 102?with high thermal stability were prepared by exploiting melanine and pyromellitic anhydride or 1,4,5,8-naphthalenetetracarboxylic dianhydride as raw materials,respectively,with a simple solvent-free direct heating procedure and their structures and properties were characterized in detail.Due to their particular structural characteristics,both of the obtained COFs materials could emit strong fluorescence.And among common metal ions,Fe³⁺ with high electron affinity could strongly quench the luminescence of these two COFs through an intramolecular nonradiative energy transfer between the excitation state of Fe?III?coordinated COFs to 3d orbitals of Fe³⁺,which could be successfully employed to detect Fe³⁺ sensitively and quickly.Porphyrin molecules,a kind planar moleculres with bigger delocalized structure,are effective building blocks for preparing 2D strongly fluorescenct COFs materials.However,traditional synthesis methods for porphyrin compounds mostly possessed some shortcomings,such as harsh experimental condition,low yield and difficult product separation.Tetraaminophenyl porphyrin was synthesized at high yield through improving the experimental process and optimizing experimental conditions.The proposed method possessed some obvious advantages including convenient operation,high yield?70 %?,easy separation and high product purity.By using the prepared tetraaminophenyl porphyrin and pyromellitic anhydride or 1,4,5,8-naphthalenetetracarboxylic dianhydride as raw materials respectively,another two novel polyimide-based COFs?PI-COF 201 and PI-COF 202?with high-intensity fluorescence were synthesizedby a simple hydrothermal method.The results showed the obtained COFs materials had fine crystallinity,uniform channel distribution and relatively big specific surface area.Based on strong quenching effect of Fe³⁺ on the luminescence of these two COFs,two new fluorescence analytical methods for quick and sensitive determination of Fe³⁺ were developed.At the same time,based on strong fluorescence quenching effect on PI-COF 201 and fluorescence enhancement effect on PI-COF 202 of Cr³⁺,two new fluorescent analytical methods for Cr³⁺ detection with high sensitivities were established.