Impact of Ligand Modification on Hydrogen Photogeneration and Light-Harvesting Applications Using Cyclometalated Iridium Complexes

摘要

To explore structure−activity relationships with respect to light-harvesting behavior, a family of bis-cyclometalatedniridium complexes [Ir(C∧N)2(Hbpdc)] 2−5 (where C∧N = 2-phenylbenzothiazole and its functionalized derivatives, and H2bpdcn=2,2′-bipyridine-4,4′-dicarboxylate) was synthesized using a facile method. The photophysical and electrochemical properties ofnthese complexes were investigated and compared to those of analogue 1 (C∧N = (4-trifluoromethyl)-2-phenylbenzothiazole);nthey were also investigated theoretically using density functional theory. The molecular structures of complexes 2−4 werendetermined by X-ray crystallography, which revealed typical octahedral coordination geometry. The structural modificationsninvolved in the complexes were accomplished through the attributes of electron-withdrawing CF3 and electron-donating NMe2nsubstituents. The UV−vis spectra of these species, except for that of 5, displayed a broad absorption in the low-energy region,nwhich originated from metal-to-ligand charge-transfer transitions. These complexes were found to exhibit visible-light-inducednhydrogen production and light-to-electricity conversion in photoelectrochemical cells. The yield of hydrogen production fromnwater using these complexes was compared, which revealed substantial dependences on their structures, particularly on thensubstituent of the cyclometalated ligand. Among the systems, the highest turnover number of 1501 was achieved with complex 2,nin which the electron-withdrawing CF3 substituent was connected to a phenyl ring of the cyclometalated ligand. The carboxylatenanchoring groups made the complexes highly suitable for grafting onto TiO2 (P25) surfaces for efficient electron transfer andnthus resulted in an enhancement of hydrogen evolution compared to the unattached homogeneous systems. In addition, thencombined incorporation of the electron-donating NMe2 group and the electron-withdrawing CF3 substituent on thencyclometalated ligand caused complex 5 to not work well for hydrogen production. Their incorporation, however, enhanced thenperformance of 5 in the light-harvesting application in nanocrystalline TiO2 dye-sensitized solar cells, which was attributed to thenintense absorption in the visible region.