Engineering Band Gap and Photoconduction in Semiconducting Metal Organic Frameworks: Metal Node Effect.

We report a systematic study on the correlation of the metal nodes in M-THQ conducting MOFs (M = Fe, Ni, Cu, and Zn; THQ = tetra-hydroxybenzoquinone) with their structure, photophysical property, and photoconductivity. We found that the structural preference in these MOFs is controlled by metal node identity where Cu prefers a square planar coordination which leads to a 2D Kagome-type structure. Fe, Ni, and Zn prefer an octahedral sphere which leads to a 3D structure. Fe-THQ has the smallest band gap and highest photoconduction as well as a long-lived ligand-to-metal charge transfer state due to the mixed valence state revealed by time-resolved optical and X-ray absorption and terahertz spectroscopy. These results demonstrate the importance of the metal node in tuning the photophysical and photocatalytic properties of MOFs.