Synthesis Of Delayed Fluorescent Photocatalysts And Its Application In Visible-Light-Driven ATRP

Atom transfer radical polymerization(ATRP)has emerged as a powerful“living”/controlled polymerization method.However,conventional ATRP commonly use transition-metal complex as the catalyst.The metal-catalyst contamination leads to coloring,degradation,and potential biotoxicity of polymer products,which restricts the practical application of ATRP.Recently,the visible-light-driven ATRP(photo-ATRP)that use organic photoredox catalysts has drawn immense intention.The design and synthesis highly efficient catalysts is the key issue of this field.Thermally activated delayed fluorescence(TADF)materials that own small energy band gap(<0.30 e V)between their single(S₁)and triple(T₁)excited states exhibit large intersystem crossing(ISC)and reverse intersystem crossing(RISC)rate constants.Therefore,TADF strategy enables long-lived excited states and high T₁energy levels of molecules.Herein,we intend to employ a class of TADF-type photoredox catalyst to increase the excited-state concentration,so as to reduce catalyst loading while maintaining the controllability of photo-ATRP.In this paper,a series of organic photocatalysts with TADF properties was designed and synthesized.Their photophysical and electrochemical properties have been systemically investigated and have been applied to catalyze visible-light-driven atom transfer radical polymerization.The main studies are as follows:I.Two D-A type TADF molecules X₁and Z₁that contain a phenoxazine group were designed and synthesized.Their photophysical and electrochemical properties,as well as catalytic performance were systemically studied.Their TADF properties were demonstrated by theoretical calculations and spectroscopic studies.Cyclic voltammetry(CV)study reveals that the oxidation potentials E₀(PC^·+/PC)of Z₁and X₁were 0.34 V and 0.33 V,respectively;the excited-state reduction potentials E₀(PC^·+/¹PC^*)of X₁and Z₁,in combination with the optical band gaps,were calculated to be-1.51 V and-1.35 V,respectively.By using Z₁or X₁as photoredox catalysts,we conducted visible-light-induced ATRP of methacrylate monomers.Z₁could obtain polymers with a molecular weight distribution(?)<1.5 at a catalyst loading of 50 ppm(relate to the monomer),and a conversion of 71.7%at a catalyst dosage of 1000ppm.X₁could obtain a conversion of 50.2%,as well as polymers with?=1.76 at a catalyst loading of 50 ppm.The structure of polymers and the chain-ended initiator groups were confirmed by ¹H-NMR spectra.The chain-end fidelity of the bromine atoms was demonstrated by MALDI-TOF mass spectrometry.To further demonstrate the activity at polymer chain ends,a synthesized PMMA-Br(M_n,GPC=6060 Da,?=1.26)was used as macroinitiators to perform chain-extending reactions,and benzyl methacrylate(Bn MA)was used as monomers.The block polymer PMMA-b-PBn MA(M_n,GPC=18600 Da,?=1.60)was successfully synthesized.II.The chemical structures of X₁and Z₁were modified by bromination and Suzuki coupling reaction.Z₁-Br,X₁-Br,Z_1-1,Z_1-2,X_1-1and X_1-2that with bromine,biphenyl,and thiophene groups at the 3,7-position of phenoxazine were synthesized.These compounds have red-shifted and enhanced absorption in visible light.The excited-state reduction potentials of Z_1-1and Z_1-2are comparable to that of Z₁while the excited-state reduction potentials of the other photocatalysts enhanced.These compounds were employed as photoredox catalysts for photo-ATRP.Z_1-1could obtain polymers with?=1.43 at a catalyst loading of 500 ppm,and a conversion of 85.8%.The conversion of 43.7%can be obtained at the loading of 5 ppm catalyst.Z_1-2can obtain high conversion and initiation efficiency when the loading of catalyst is 500ppm and 100ppm.X_1-1and X_1-2can also obtain good results of visible-light-induced ATRP.