Benzo[c][1,2,5]Thiadiazole Linkers Enhance the Effectiveness of Cavity-Containing Supramolecular Photocatalysts

A family of seven novel supramolecular photosensitizers comprising benzo[c][1,2,5]thiadiazole (BTZ) tethered calix[4]arene (C[4]) scaffolds has been synthesized using cross-coupling strategies with selectively halogenated C[4]s, affording target compounds in high yield using facile purification. Their respective abilities to produce singlet oxygen has been evaluated through the photo-oxidation of α-terpinene, and structure–activity relationships established, probing factors such as cavity:photosensitizer ratio, distance of cavity to photosensitizer linker, and steric hindrance around the upper-rim of the cavity. The presence of two C[4] cavities proximal to the photosensitizing BTZ unit provided vastly increased effectiveness in enhancing effective concentration through host:guest interactions, resulting in an eightfold increase in efficiency for the best-performing member of the compound library benchmarked against a small-molecule analogue, and reaching reaction completion in 1 h at 2.5 mol% loading. In contrast, systematically increasing the cavity:photosensitizing linker distance led to a reduction in improvement to 1.1-fold relative to the small molecule analogue, confirming that these factors are pivotal in designing a suitable system based on the guest substrate.