Visible light mediated photocatalysis by lanthanide metal–organic frameworks: enhanced specificity and mechanistic insights

Abstract

The utilization of earth-abundant photosensitizers with visible light absorption to enable sustainable photocatalysis is a long-standing challenge. Overcoming such a challenge, in this work, two lanthanide (Ln³⁺ = Tb, Eu) based metal–organic frameworks (Ln-MOFs) have been synthesized utilizing a Co³⁺-based metalloligand. Both Ln-MOFs function as remarkable photocatalysts for the selective oxidation of assorted alcohols and sulfides to their corresponding aldehydes/ketones and sulfoxides using visible light. The photophysical behavior of both Ln-MOFs and mechanism of photocatalysis is comprehensively investigated using time-resolved transient absorption spectroscopy, electrochemical impedance spectroscopy, electron paramagnetic resonance spectroscopy, photoluminescence and phosphorescence studies. In both Ln-MOFs, a metalloligand acts as a light-harvester, being excited by visible light, while Ln³⁺ ions endow the resulting MOFs with long-lived triplet excited states. Ultrafast transient absorption spectroscopy, further supported by electron paramagnetic resonance spectra, revealed excited-state electron transfer from metalloligands to the Ln³⁺ ions and transient generation of Ln²⁺ sites alongside the facilitation of intersystem crossing. The excited Ln²⁺ ions transfer energy to the ground-state triplet oxygen (³O₂) to generate singlet oxygen (¹O₂). The HOMO–LUMO positions of both Ln-MOFs support the generation of ˙O₂⁻ and ¹O₂ but inhibit strongly-oxidizing yet non-selective ˙OH radicals. Scavenger experiments, ¹O₂ traps and electron paramagnetic resonance spectra confirmed the generation of singlet oxygen. The heavy-metal effect of a lanthanide ion in Ln-MOFs for the generation of triplet excitons is confirmed by the synthesis of a non-heavy-metal analogue involving a zinc ion via a single-crystal-to-single-crystal transformation strategy. The present results are noteworthy and may aid in the development of other earth-abundant metalloligand-based photocatalysts for challenging yet sustainable catalysis.