The optical properties of NH 2 -UiO-66 as a visible light-active metal organic framework was further enhanced through the diazotization reaction with π-conjugated 1-naphthol reagent. Diffuse re ectance UV-Vis spectrum of diazotized MOF, named as Azo-UiO-66, exhibited a signi cant red shift compared to unfunctionalized NH 2 -UiO-66 due to the formation of diazo compound. Also, Tauc calculations indicated considerable decrease in band gap energy from 2.68 to 1.7 eV, resulted in improvement of visible light harvesting. Furthermore, other physicochemical techniques e.g. X-ray diffraction (XRD), N 2 adsorption-desorption analysis, thermogravimetric analysis (TGA), energy dispersive X-ray (EDX), and CHN elemental analyses demonstrated the successful MOF diazotization with 1-naphthol and preservation of NH 2 -UiO-66 framework upon post modi cation process. The reduction of hexavalent chromium, Cr(VI), as a serious contaminant in wastewater to less toxic Cr(III) was performed over prepared photocatalyst, which demonstrated the positive role of ligand functionalization and enhancement of visible light absorption on overall photocatalytic performance of Azo-UiO-66.