Metal-organic frameworks (MOFs) containing d0 metals such as NH2-MIL-125(Ti), NH2-UiO-66(Zr) and NH2-UiO-66(Hf) are among the most studied MOFs for photocatalytic applications. Despite structural similarities, we demonstrate that the electronic properties of these MOFs are markedly different. As revealed by quantum chemistry, EPR measurements and transient absorption spectroscopy, the highest occupied and lowest unoccupied orbitals of NH2-MIL-125(Ti) promote a long lived ligandto-metal charge transfer upon photoexcitation, making this material suitable for photocatalytic applications. In contrast, in case of UiO materials, the d-orbitals of Zr and Hf, are too low in binding energy and thus cannot overlap with the π orbital of the ligand, making both frontier orbitals localized at the organic linker. This electronic reconfiguration results in short exciton lifetimes and diminishes photocatalytic performance. These results highlight the importance of orbital contributions at the band edges and delineate future directions in the development of photo-active hybrid solids.
Bibliographical noteFunding Information:
Dr. Sonia Castellanos Ortega is gratefully acknowledged for scanning electron microscopy. The Dutch National Research School Combination Catalysis Controlled by Chemical Design (NRSC-Catalysis) is acknowledged for funding the work at Delft. The work at Bath was supported by the European Research Council (Starting Grant 277757), and access to ARCHER through membership in the UK's HPC Materials Chemistry Consortium, which is funded by EPSRC (Grant No. EP/L00202). S.L.V. and M.V.F. thank the Russian Science Foundation (no. 14-13-00826).