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Theoretical investigation of electronic properties and redox properties for purely inorganic and aryloxide substituted Ti-containing POM derivatives

DFT calculations have been performed to investigate the redox properties and electron transition for a series of purely inorganic and aryloxide substituted Ti-containing polyoxometalate (POM) derivatives. The modification of organic conjugated groups dramatically alters the HOMO population and decreases the energy gap. It is evident from redox analysis that Ti atoms are preferred redox centres in organic–inorganic hybrid Ti-containing POMs, and the incorporation of π-conjugated groups evokes anodic shift for reduction potential. In contrast with mono-TiW5O18 derivatives, bi-TiW5O18 derivatives exhibit narrower energy gaps and more active redox properties. It is noteworthy that the attachment of π-conjugated groups induces the low-energy electron transition to be a significant bathochromic-shift, and it has been dramatically red-shifted by 296 nm in compound 1c in contrast with that in compound 1a. When the two TiW5O18 units are covalently linked as terminals to construct bi-TiW5O18 derivatives, the low-energy electron transition presents further bathochromical shift compared with corresponding mono-TiW5O18 derivatives. In addition, the crucial charge transfer for the hybrid Ti-containing POMs are generated from π-conjugated donor to d-Ti orbital with part of p–Oc orbitals. The incorporation of the organic substituents results in attached POMs by covalent linkage to the catalyst supports, which consequently enhances the ability of catalysis.