Syntheses, structures and photoelectrochemical properties of three water-stable, visible light absorbing mental-organic frameworks based on tetrakis(4-carboxyphenyl)silane and 1,4-bis(pyridyl)benzene mixed ligands

Photovoltaics (PV), which directly convert solar energy into electricity generally using semiconductors, offer a practical and sustainable solution to the current energy shortage and environmental pollution crisis. Photovoltaic applications of metal-organic frameworks (MOFs) belong to a relatively new area of research. Given that UV light accounts for only 4% while visible light contributes 43% of solar energy, it is rather imperative to develop semiconductors with narrow band gaps so that they could absorb visible light. In this work, three water-stable, narrow band semiconducting MOFs of [Cu(H₂TCS)(H₂O)] (1), [Co(H₂TCS)(BPB)] (2) and [Ni(H₂TCS)(BPB)] (3) were synthesized using tetrakis(4-carboxyphenyl)silane (H₄TCS) and 1,4-bis (pyridyl)benzene (BPB) in water, and structurally characterized by single-crystal X-ray diffractions. MOF 1 has a 2D structure. MOF 2 and 3 are isostructrual and have 3D frameworks formed by interwoven 2D layers. All three MOFs are stable in acidic water solutions and can be stable in water for 7 days. MOFs 1–3 absorb UV and visible light and have band gaps of 0.50, 1.77 and 1.49 eV, respectively. Rapid and stable photocurrent responses of MOFs 1–3 under UV and visible light illuminations are observed. This work demonstrates that using electron rich Cu²⁺, Co²⁺, or Ni²⁺ as metal nodes can effectively decrease the band gaps of MOFs to make them absorbing visible light. To increase the conjugation in the linker is generally considered to be the method to decrease the band gap of MOFs. The conjugation in H₄TCS is not significant and this ligand basically only absorbs UV light. However, by using electron rich Cu²⁺ ions as metal nodes, the prepared [Cu(H₂TCS)(H₂O)]·H₂O (1) absorbs broadly in the visible light region. Thus, this work suggests that by using electron rich Cu²⁺, many narrow-band semiconductor MOFs can be prepared even by using ligands which only absorbs UV light.