Engineering the optical response of the titanium-MIL-125 metal-organic framework through ligand functionalization

Herein we discuss band gap modification of MIL-125, a TiO ₂/1,4-benzenedicarboxylate (bdc) metal-organic framework (MOF). Through a combination of synthesis and computation, we elucidated the electronic structure of MIL-125 with aminated linkers. The band gap decrease observed when the monoaminated bdc-NH₂ linker was used arises from donation of the N 2p electrons to the aromatic linking unit, resulting in a red-shifted band above the valence-band edge of MIL-125. We further explored in silico MIL-125 with the diaminated linker bdc-(NH₂)₂ and other functional groups (-OH, -CH₃, -Cl) as alternative substitutions to control the optical response. The bdc-(NH₂)₂ linking unit was predicted to lower the band gap of MIL-125 to 1.28 eV, and this was confirmed through the targeted synthesis of the bdc-(NH₂)₂-based MIL-125. This study illustrates the possibility of tuning the optical response of MOFs through rational functionalization of the linking unit, and the strength of combined synthetic/computational approaches for targeting functionalized hybrid materials.