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-<i>d</i>]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 <i>N</i>-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-<i>d</i>]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 <b>3d</b>·½CHCl₃ and furazan-containing <b>4d</b> exhibit a non-planar [1,2,5]oxadiazolo[3,4-<i>d</i>]pyridazine fragment. We have found that short non-covalent interactions of the furoxan system with a lattice chloroform molecule in <b>3d</b> lead to luminescence quenching. Meanwhile, in the <b>4d</b> dye, the intermolecular π-π interactions of pyridazine nitrogen atoms with the <i>N</i>-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.