The Effect of the N-Oxide Oxygen Atom on the Crystalline and Photophysical Properties of [1,2,5]Oxadiazolo[3,4-d]pyridazines

A series of novel fluorescent donor–acceptor–donor (D-A-D) dyes containing [1,2,5]oxadiazolo[3,4-d]pyridazine and its 1-oxide as electron-withdrawing groups has been synthesized and thoroughly investigated using X-ray diffraction and molecular spectroscopy methods. This study showed that the introduction of N-oxide into the 1,2,5-oxadiazole ring in the acceptor fragment leads to a significant decrease in the luminescence intensity and quantum yield of the dyes. A comprehensive comparison of the photophysical properties of the obtained compounds containing the 1,2,5-oxadiazole ring with the previously studied [1,2,5]thia- and 1,2,5-selenadiazolo[3,4-d]pyridazine analogs showed that the oxygen substitution in the acceptor fragment shifts the phosphorescence maximum from the NIR region of 980–1100 nm to the red region of 690–770 nm. In contrast, for oxygen- and sulfur-containing dyes, purely red fluorescence with a maximum in the spectral range of 620–900 nm is observed. The crystal structures of furoxan-containing 3d·½CHCl3 and furazan-containing 4d exhibit a non-planar [1,2,5]oxadiazolo[3,4-d]pyridazine fragment. We have found that short non-covalent interactions of the furoxan system with a lattice chloroform molecule in 3d lead to luminescence quenching. Meanwhile, in the 4d dye, the intermolecular π-π interactions of pyridazine nitrogen atoms with the N-carbazolyl group of the adjacent molecule should facilitate intermolecular charge transfer (ICT) emission. Thus, the luminescence maxima for these dyes can be tuned across a broad range of 700–1100 nm by varying the number of chalcogen atoms, highlighting the potential for tailoring optical properties in optoelectronic applications.