Regioselective three-component synthesis of highly fluorescent 1,3,5-trisubstituted pyrazoles

3,5-Disubstituted and 1,3,5-trisubstituted pyrazoles are readily synthesized from acyl chlorides, terminal alkynes, and hydrazines by a consecutive one-pot three-component Sonogashira coupling/Michael addition/cyclocondensation sequence in good to excellent yields. These pyrazoles are highly fluorescent, both in solution and in the solid state. Investigation of the electronic properties by UV/Vis and fluorescence spectroscopy and by DFT and ZINDO Cl computations reveal that the excited state is highly polar and allows fine-tuning of the absorption and emission properties. X-ray structure analyses of 3,5-disubstituted pyrazoles reveal self-organization by hydrogen bonding and pi-stacking. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).