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

To explore structure–activity relationships with
respect
to light-harvesting behavior, a family of bis-cyclometalated iridium
complexes [Ir­(C∧N)2(Hbpdc)] 2–5 (where C∧N = 2-phenylbenzothiazole
and its functionalized derivatives, and H2bpdc =2,2′-bipyridine-4,4′-dicarboxylate)
was synthesized using a facile method. The photophysical and electrochemical
properties of these complexes were investigated and compared to those
of analogue 1 (C∧N = (4-trifluoromethyl)-2-phenylbenzothiazole);
they were also investigated theoretically using density functional
theory. The molecular structures of complexes 2–4 were determined by X-ray crystallography, which revealed
typical octahedral coordination geometry. The structural modifications
involved in the complexes were accomplished through the attributes
of electron-withdrawing CF3 and electron-donating NMe2 substituents. The UV–vis spectra of these species,
except for that of 5, displayed a broad absorption in
the low-energy region, which originated from metal-to-ligand charge-transfer
transitions. These complexes were found to exhibit visible-light-induced
hydrogen production and light-to-electricity conversion in photoelectrochemical
cells. The yield of hydrogen production from water using these complexes
was compared, which revealed substantial dependences on their structures,
particularly on the substituent of the cyclometalated ligand. Among
the systems, the highest turnover number of 1501 was achieved with
complex 2, in which the electron-withdrawing CF3 substituent was connected to a phenyl ring of the cyclometalated
ligand. The carboxylate anchoring groups made the complexes highly
suitable for grafting onto TiO2 (P25) surfaces for efficient
electron transfer and thus resulted in an enhancement of hydrogen
evolution compared to the unattached homogeneous systems. In addition,
the combined incorporation of the electron-donating NMe2 group and the electron-withdrawing CF3 substituent on
the cyclometalated ligand caused complex 5 to not work
well for hydrogen production. Their incorporation, however, enhanced
the performance of 5 in the light-harvesting application
in nanocrystalline TiO2 dye-sensitized solar cells, which
was attributed to the intense absorption in the visible region.