Accurate Regulating of Visible-Light Absorption in Polyoxotitanate-Calix[8]arene Systems by Ligand Modification.

With use of a macrocyclic polyphenol, tert-butylcalix[8]arene (TBC[8]), as ligands, a series of TBC[8]-stabilized {Ti4O2}clusters, containing penta- and hexacoordinated Ti centers, were synthesized. Such complexes are "core-shell" shaped containing a {Ti4O2} core arranged in a zigzag fashion. While outer walls of the clusters are decorated by deprotonated TBC[8], their upper and lower surfaces can be modified by various O- or N-donor ligands, and the ratio of the penta- and hexacoordinated Ti(IV) centers in the {Ti4O2} core can be precisely regulated from 4:0, to 3:1, to 2:2, to 1:3, and finally to 0:4. The combined coordination of different ligands in the axial direction shows significant influence on the adsorption of the TBC[8]-Ti4 system in the visible-light region, and their absorption edge can be precisely regulated from 600 to 700 nm. The above structural functionalization in the TBC[8]-Ti4 system also tunes their photocatalytic H2 production activities and oxidative desulfurization ability. Thus, for the first time, by confining the polyoxotitanium cluster in macrocyclic molecules, we provide an example of understanding the structure-property relationship of titanium-oxygen materials by ligand modification.