Band Gap Modulation in Fluorescein-Based Isostructural Coordination Polymers for Enhanced Photocatalytic Hydrogen Evolution under Visible Light

Coordination polymers (CPs) and metal–organic frameworks (MOFs) have emerged as promising materials for the photocatalytic hydrogen evolution reaction (HER). The flexibility to choose the organic linkers or the metal center provides the scope to alter their various properties like light absorption capability, optical band gap, etc. In this context, the dye molecules are the best choice to be utilized as a linker due to their ability to absorb visible light. However, the CPs/MOFs containing the dye molecules as a backbone are rarely explored. Herein, two isostructural fluorescein-based CPs were reported and shown to be efficient photocatalysts for the HER under white light without using a cocatalyst or a sensitizer. Here, the dye molecule was strategically incorporated into the backbone of the CPs, [Cd­(FC)­(BPY)­(H2O)] n (Cd-FCB) and [Mn­(FC)­(BPY)­(H2O)] n (Mn-FCB), and their efficiency as photocatalysts were explored. The band gap of the isostructural CPs was found to have a huge effect on their photocatalytic efficiency. The rate of hydrogen production was 71.6 μmol g–1 h–1 for Cd-FCB, and for Mn-FCB, it was three times higher, i.e., 224.1 μmol g–1 h–1. The higher efficiency can be attributed to the extended visible light absorption as well as the lower band gap of Mn-FCB than that of Cd-FCB. Moreover, the longer lifetime of the photogenerated electrons in Mn-FCB as indicated by the time-resolved photoluminescence spectra also validates such higher activity.