Water-Soluble Pyrene-Adorned Imidazolium Salts with Multicolor Solid-State Fluorescence: Synthesis, Structure,...

New water-soluble acetylpyrene-bound imidazolium salts (1-N-methyl-3-(2-oxo-2-(pyren-1-yl)ethyl)-imidazolium bromide (1), 1-N-isopropyl-3-(2-oxo-2-(pyren-1-yl)ethyl)-imidazolium bromide (2), 1-N-allyl-3-(2-oxo-2-(pyren-1-yl)ethyl)-imidazolium bromide (3), and 1-N-isopropyl-3-(2-oxo-2-(pyren-1-yl)ethyl)-imidazolium hexafluorophosphate (4)) were synthesized from the reaction between 1-bromoacetylpyrene and N-substituted imidazoles in excellent yield. The new molecules were fully characterized by elemental analysis, FT-IR, multinuclear (¹H, ¹³C, and ¹⁹F) NMR techniques, and single-crystal X-ray diffraction analysis. Investigations on the crystal packing of 1, 3, and 4 show the presence of inter/intramolecular weak interactions, including the π···π stacking interaction between the pairs of pyrene molecules. The photophysical properties were investigated in detail for the four imidazolium salts. Experiments show that the emissions observed for all the four compounds are due to the excited monomer and static excimer. Very interestingly, all the four compounds exhibit solid-state multicolor fluorescence depending on the excitation wavelength. The solid-state emissions were monitored using a fluorescence microscope. Finally, a fingerprint powder was formulated based on compound 4 and demonstrated as an efficient fluorescent fingerprint powder for forensic applications. The formulated powder revealed all the 3 level information along with peculiar individual characteristics of the fingerprints under investigation. The fingerprints were further viewed through a fluorescence microscope, and the results were discussed in detail.