Chromones Functionalized With Nitroxide Radicals: Synthesis, Structural Characterization, and Photoinduced Luminescence Switching

ABSTRACT Chromones represent a class of molecular switches that convert into fluorescent products upon UV irradiation. To investigate their switching properties, we synthesized new 2‑furyl‑3‑acylchromones bearing different paramagnetic groups—2‑imidazoline‑3‑oxide‑1‑oxyl, 2‑imidazoline‑1‑oxyl, or piperidine‑ N ‑oxyl—in the acyl moiety of the molecules. The structures of the paramagnetic derivatives were determined by x‑ray diffraction. This analysis revealed that the benzoyl moiety adopts a conformation nearly perpendicular to the chromone plane. None of the studied spin‑labeled chromone derivatives exhibited luminescence in their native state. However, upon UV irradiation, they underwent a transformation accompanied by the appearance of a luminescence band with a maximum in the 491–514 nm range. Theoretical studies showed that the weak long‑wavelength bands in the electronic spectra of the spin‑labeled chromones correspond to combinations of α‑electron transitions: SOMO→LUMO and HOMO→SUMO. In contrast, the most intense band primarily arises from HOMO→LUMO electron transfers within the chromone moiety. This excitation leads to a transfer of electron density from the furan ring to the chromone moiety, facilitating a nucleophilic attack of the carbonyl oxygen on the furan C atom. Subsequent rearrangement yields furo[3,4‑ b ]chromones. Notably, this process is quenched by paramagnetic substituents located in the furan ring, but not by those in the acyl moiety.