Design And Photocatalytic Properties Of Aluminum(Ⅲ) Complexes With Pyridylpyrrole Ligand

Using the green design concept of photocatalytic system to convert solar energy into high-density chemical energy has important theoretical and practical significance,in which photosensitizers play an important role as a medium for photon absorption and energy transfer.Traditional noble metal photosensitizers such as Ru,Ir,and Pt have received extensive attention due to their excellent photophysical and chemical properties.However,the low abundance and high cost of these precious metals on earth limit their large-scale use.Aluminum（Ⅲ）is the most abundant metal element in the earth’s crust,and its aluminum（Ⅲ）-based complexes have been widely used as optoelectronic materials.However,due to poor stability and short excited state lifetime,only a few are used as photosensitizers.Therefore,the design and preparation of inexpensive metal Al（Ⅲ）photosensitizers to replace noble metal photosensitizers has become particularly urgent.This paper focuses on the preparation of luminescent aluminum（Ⅲ）complexes,the regulation of photophysical properties,and the construction of inexpensive photocatalytic systems.The specific research contents mainly include the following aspects:（1）Preparation and characterization of aluminum（Ⅲ）complexes.The pyridylpyrrole ligands HL1-HL8 modified with electron-donating group at different sites were prepared by Suzuki coupling reaction and cyclization reaction;Al-1～Al-6 complexes were successfully synthesized by the reaction of HL1-HL8 and trimethylaluminum（Ⅲ）.These ligands and complexes were characterized by means of NMR,IR,ESI-MS,UV-Vis,etc.The solid structures of the complexes Al-1～Al-5 were obtained by X-ray single crystal diffraction.（2）Study on the photophysical properties of the complexes Al-1～Al-6.The relationship between the structure and photophysical properties of the complexes Al-1～Al-6 was systematically studied by various steady-state/time-resolved spectroscopy and TD-DFT calculations.The redox properties of the complexes Al-1～Al-4 were studied by cyclic voltammetry.The photophysical information of the complexes Al-1～Al-4 at nanosecond and picosecond time scales were obtained by transient absorption spectroscopy.（3）Application of aluminum（Ⅲ）complexes in photocatalytic organic reactions.The photocatalytic activity of photosensitizers Al-1 and Al-3 in the alkylation of ethyl difluorobromoacetate,the reduction of diethyl maleate and the isomerization of trans-diphenylethylene were investigated at room temperature under visible light.Among them,the yields of electron transfer and energy transfer catalyzed reactions involving Al-3 complexes are all>80%.The mechanism of photocatalytic redox reactions involving Al（Ⅲ）complexes was investigated by the quenching experiments of excited states.（4）Application of complexes Al-1～Al-4 in photocatalytic CO₂reduction.The effects of catalyst and light source on photocatalytic performance were studied.With an Fe（II）-quaterpyridine catalyst,the most methylated Al-4 complex as the photosensitizer achieved quantum efficiency of 2.81%at 450 nm for selective（>99%）CO₂-to-CO conversion,which is nearly 35 times of the unmethylated one（0.08%）under identical conditions.A maximum TON value of 10250 was achieved under optimal reaction conditions.The mechanism of photoinduced electron transfer in the Al（Ⅲ）sensitized system was investigated by quenching experiments of fluorescence spectra.