The Design And Synthesis Of Copper Photosensitizers For Water Splitting

Energy is the lifeblood of national economy,the essential power of industrial economy.However,the energy crisis and petrochemical energy pollution have been the world’s"the sword of Damocles”.So it’s an extremely urgent topic to seek sustainable and clean energy.Decomposition of water directly by solar energy,which makes the solar energy into chemical energy at hydrogen,is the research frontiers of solar energy utilization.Highly efficient,stable,inexpensive and non-toxic material of light conversion（photosensitizer）is the core material of water splitting system,also is the key problem which restricts the efficiency of solar energy utilization.In this article we used heteroleptic copper photosensitizers which have nitrogen ligand and phosphine ligand as the structure model,designed and synthesized a series of 1,10-phenanthroline derivatives as nitrogen ligands and two phosphonic compounds which has a more rigidity structure as the phosphine ligands.The light energy conversion ability of these copper photosensitizers in the water splitting system were studied by in-situ method.First reference the synthesis method of Xantphos（I）,we synthesized the phosphine ligand 2,8-dimethyl-4,6-bis（2,8-dimethylphenoxaphosphinine-10-yl）phenoxathiine（II）which has greater rigidity in the structure.Starting from 1-bromo-4-methylbenzene and p-cresol,4,4’-oxybis（methylbenzene）was synthesized by the Williamson reaction.Then aluminium trichloride was used as the catalyst,4,4’-oxybis（methylbenzene）was reacted with elementalsulfurandphosphorustrichloriderespectively,2,8-dimethylphenoxathiine and 10-chlorine-2,8-dimethylphenoxaphosphinine was obtained;Finally 4,6-dilithium-2,8-dimethylphenoxathiine was formed as a intermediate by the lithium hyrdogen exchange of2,8-dimethylphenoxathiine and alkyl lithium,then quenched by10-chlorine-2,8-dimethylphenoxaphosphinine.The expected phosphine ligand（II）was obtained with 34%yield.Starting from 4,7-dibromoaryl-1,10-phenanthroline,organic lithium was used as the nucleophilic reagent and lithium halogen exchange regent,quenching of the reaction mixture with CO_2.After the one-pot reaction conditions（the dosage of alkyl lithium reagent and the reaction time）were optimized,the desired 2,9-dialkyl-4,7-bis（carboxy acid）aryl-1,10-phenanthroline derivatives（nitrogen ligands A-B）was obtained with 59%yield.Using selenium dioxide as the oxidant,2,9-diformyl-4,7-diphenyl-1,10-phenanthroline（C）was synthesized from the oxidation of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline.Then 2,9-diethyl acetal-4,7-diphenyl-1,10-phenanthroline（D）was gotten by the condensation reaction of C with 87%yield.At last,the copper photosensitizers were synthesized by in situ method in water reduction system,using the synthesized 2,9-dialkyl-4,7-diphenyl-1,10-phenanthroline derivatives（A-D）and 2,9-dialkyl-4,7-diphenyl-5,6-disulfonic acid-1,10-phenanthroline（E-F）as the nitrogen ligands,Xantphos（I）and2,8-dimethyl-4,6-bis（2,8-Dimethylphenoxaphosphinine-10-yl）phenoxathiine（II）as the phosphine ligands,together with Cu（MeCN）₄PF₆.Their photocatalytic activity of hydrogen generation from water splitting and their photophysical properties were explored and discussed.Compared with CuPS IA and CuPSIIA,CuPSIB and CuPSIIB have stronger ultraviolet absorption and a more wide absorption band,but they have no hydrogen production activity.When2,8-dimethyl-4,6-bis（2,8-dimethylphenoxaphosphinine-10-yl）phenoxathiine as the phosphine ligand,2,9-dibutyl-4,7-diphenyl-5,6-disulfonic acid-1,10-phenanthroline as the nitrogen ligand,CuPSIIF displayed the highest turnover number up to 1530.