Pyrenylpyrazole-based donor/acceptor fluorescent dyes: Synthesis and photophysical properties

The reaction of 1-acetylpyrene phenylhydrazone with Vilsmeier reagent (DMF-POCl3) afforded 1-phenyl-3-(pyren-1-yl)-1H-pyrazole-4-carbaldehyde in good yield. Condensation of this compound with a series of active methylene compounds (methyl ethyl ketone, diethyl ketone, malononitrile, 1,3-indandione and N,N-diethythiolbarbituric acid) gave a series of donor/acceptor dyes exhibiting fluorescence in the almost whole range of the visible spectrum. The electronic structure of the dyes was studied theoretically by the (TD) DFT method. Calculations elucidated the nature of intramolecular charge transfer (ICT) processes leading to emissive excited states of the synthesised compounds. It turned out that the electron acceptor ability influences the direction of ICT. For compounds with relatively weak acceptor groups, ICT from the pyrene moiety toward the remaining part of the molecule prevailed, whereas for those having stronger acceptor groups the main ICT pathway became that from the phenylpyrazole group to the acceptor group.