Synthesis of fluorescent 2,3,5,6-tetraalkynylpyridines by site-selective Sonogashira-reactions of 2,3,5,6-tetrachloropyridines
4-Substituted 2,3,5,6-tetraalkynylpyridines were prepared by tetra-fold Sonogashira reactions of the corresponding 2,3,5,6-tetrachloropyridines. 2,6-Dialkynyl-3,5-dichloropyridines were prepared by site-selective Sonogashira reactions from various 4-unsubstituted and 4-substituted tetrachloropyridines. Subsequent two-fold Sonogashira reactions of the products allowed for the synthesis of various 2,3,5,6-tetraalkynylpyridines containing different alkynyl groups. The products exhibit interesting UV/Vis and fluorescence properties. The position of absorption and emission bands can be tuned by systematic variation of the type of alkynyl substituent and by the type of substituent located at position 4 of the pyridine moiety. The presence of electron withdrawing substituents or of an alkynyl group at position 4 as well as the presence of donor substituted alkynyl groups at positions 2, 3, 5 and 6 resulted in high fluorescence quantum yields of up to 0.6, presumably due to the push–pull substitution pattern of the molecules.