Unassisted Uranyl Photoreduction and Separation in a Donor–Acceptor Covalent Organic Framework

The donor-acceptor covalent organic framework (COF) TTT-DTDA (TTT = thieno[3,2-b]thiophene-2,5-dicarbaldehyde and DTDA = 4,4',4 ''-(1,3,5-triazine-2,4,6-triyl)trianiline) was prepared and found to have long-lived excited states (>100 ms) characterized by transient absorption spectroscopy. These excited-state lifetimes were sufficient to perform the direct photoreduction of uranium at ppm concentration levels. The photoreduction of soluble uranyl species to insoluble reduced uranium products is an attractive separation for uranium, typically accomplished with sacrificial reagents and protective gases. In the case of TTT-DTDA, illumination in aqueous solutions containing only uranyl ions produced crystalline uranyl peroxide species ([UO2 (O-2)]) at the COF that were characterized by powder X-ray diffraction, X-ray photoelectron spectroscopy, and infrared spectroscopy. The maximum absorption capacity of TTT-DTDA was found to be 123 mg U/g COF at pH 5 after 10 h of illumination in solutions devoid of sacrificial reagents or protective gases. The TTT-DTDA COF was recyclable and maintained high selectivity for uranium in competing ion experiments, which are necessary requirements for a practical uranium extraction strategy based on photochemical uranium reduction.