Takeda Hiroyuki, Monma Yu, Sugiyama Haruki, Uekusa Hidehiro, Ishitani Osamu
Department of Chemistry, School of Science, Tokyo Institute of Technology, Tokyo, Japan.
Front Chem. 2019 Jun 6;7:418. doi: 10.3389/fchem.2019.00418. eCollection 2019.
The visible-light responsive Cu(I)-complex photosensitizers were developed by introducing various aromatic substituents at the 4,7-positions of a 2,9-dimethyl-1,10-phenanthroline (dmp) ligand in a heteroleptic Cu(dmp)(DPEphos)-type complexes (DPEphos = [2-(diphenylphosphino)phenyl]ether) for photocatalytic CO reduction. Introducing biphenyl groups (Bp-) on the dmp ligand enhanced the molar extinction coefficient (ε) of the metal-to-ligand charge transfer (MLCT) band in the visible region (ε = 7,500 Mcm) compared to that of the phenyl (Ph-)-containing analog (ε = 5,700 Mcm at λ = 388 nm). However, introducing 4-R-Ph- groups (R = the electron-withdrawing groups NC-, or NO-) led to a red shift in the band to λ = 390, 400, and 401 nm, respectively. Single-crystal X-ray analysis showed the Ph- groups were twisted because of the steric repulsion between the 2,6-protons of the Ph- groups and 5,6-protons of the dmp ligand. The result strongly indicated that the π-conjugation effect of the 4-R-Ph- groups is so weak that the lowering of the energy of the dmp π orbitals is small. However, when 4-R-ph- was substituted by a 5-membered heterorings, there was a larger red shift, leading to an increase in the ε value of the MLCT absorption band. Thus, the substitution to 2-benzofuranyl- groups resulted in visible-light absorption up to 500 nm and a shoulder peak at around 420 nm (ε = 12,300 Mcm) due to the expansion of π-conjugation over the substituted dmp ligand. The photocatalytic reaction for CO reduction was tested using the obtained Cu complexes as photosensitizers in the presence of a Fe(dmp)(NCS) catalyst and 1,3-dimethyl-2-phenyl-2,3-dihydro-1-benzo[d]imidazole as a sacrificial reductant, which showed improved CO generation.
通过在异质配位Cu(dmp)(DPEphos)型配合物（DPEphos = [2-(二苯基膦基)苯基]醚）的2,9-二甲基-1,10-菲咯啉（dmp）配体的4,7位引入各种芳基取代基，开发了可见光响应的Cu(I)配合物光敏剂用于光催化CO还原。与含苯基（Ph-）的类似物相比，在dmp配体上引入联苯基（Bp-）增强了可见光区域中金属-配体电荷转移（MLCT）带的摩尔消光系数（ε）（ε = 7500 M cm⁻¹）（在λ = 388 nm时，含苯基类似物的ε = 5700 M cm⁻¹）。然而，引入4-R-Ph-基团（R = 吸电子基团NC-或NO-）分别导致该带红移至λ = 390、400和401 nm。单晶X射线分析表明，由于Ph-基团的2,6-质子与dmp配体的5,6-质子之间的空间排斥，Ph-基团发生了扭曲。结果强烈表明，4-R-Ph-基团的π共轭效应非常弱，以至于dmp π轨道的能量降低很小。然而，当4-R-ph-被五元杂环取代时，红移更大，导致MLCT吸收带的ε值增加。因此，由于取代的dmp配体上π共轭的扩展，被2-苯并呋喃基取代导致可见光吸收高达500 nm，并在约420 nm处出现一个肩峰（ε = 12300 M cm⁻¹）。使用所获得的Cu配合物作为光敏剂，在Fe(dmp)(NCS)催化剂和1,3-二甲基-2-苯基-2,3-二氢-1-苯并[d]咪唑作为牺牲还原剂的存在下，测试了用于CO还原的光催化反应，结果表明CO生成有所改善。
