Tunable Photocatalytic Singlet Oxygen Generation by Metal–Organic Frameworks via Functionalization of Pyrene-Containing Linkers

Abstract

Metal–organic frameworks (MOFs) are highly versatile materials that have shown great promise in chemical warfare agent (CWA) adsorption and decontamination. Sulfur mustard has been one of the most prominently used CWAs over the last century; therefore, the development of effective detoxification strategies is of utmost importance. However, typical routes of detoxification are slow and/or result in the production of harmful byproducts. NU-1000 has previously shown promise as a “soft” oxidizer that can readily detoxify sulfur mustard and its simulant 2-chloroethyl ethyl sulfide (2-CEES) through the generation of singlet oxygen in the presence of either UV (396 nm) or blue (465 nm) light. Several variants of NU-1000 were synthesized (MOF-R, R = −Cl, −NO₂, −CH₃) with functional groups positioned either ortho or meta to the carboxylic acid on the linker. NU-1000-o-(Cl)₄ and NU-1000-m-(Cl)₄ showed significant enhancement of photooxidation of 2-CEES due to spin–orbit coupling, enhancing the intersystem crossing into the MOF triplet (T₁) state. Furthermore, substitution of MOF linkers led to pyrene–phenyl rotation. Linkers with substituents in the ortho-position were shown to have smaller pyrene–phenyl torsion angles, leading to enhanced conjugation between the rings and a subsequent red shift in the absorption spectra. This red shift leads to enhanced reactivity of NU-1000-o-(Cl)₄ under blue light conditions and gives perspective on making materials with enhanced reactivity utilizing visible light.