Towards deep NIR emissive simple D–A–D dyes: a novel acceptor block providing anti-Kasha's rule emission

Abstract

In this work, we have developed 1,2,5-selenadiazolo[3,4-d]pyridazine as an electron-withdrawing unit for donor–acceptor dyes achieving bright room-temperature phosphorescence in the near infrared (NIR) spectral region. Novel symmetrical and asymmetrical fluorescent D–A–D compounds synthesized based on this unit demonstrate intensive emission in the NIR spectral region of 900–1100 nm. To obtain these molecules, the conditions for selective aromatic nucleophilic substitution of one or two bromine atoms by nitrogen nucleophiles are found. Calculations performed using the TD-DFT approach reveal the phosphorescence nature of the experimentally observed emission band. Moreover, the Stokes shift for this band reaches up to 9500 cm⁻¹. Thus, introduction of the selenium atom in the acceptor fragment leads to the Kasha rule violation followed by a large red-shift of the emission spectra. Besides, some molecules exhibit additional luminescence bands associated with intramolecular charge transfer (ICT) emission nature in the orange-red spectral region. A cell toxicity test showed that the obtained compounds do not have a significant detrimental effect on cells at concentrations at which the investigated dyes are capable of emitting brightly in the NIR region of the spectrum.