Structurally rigid 2,6-distyrylpyridines - A new class of fluorescent dyes. 1. Synthesis, steric constitution and spectral properties

A group of structurally rigid analogues of 2,6- distyrylpyridine was synthesised. Molecular geometry of the synthesised dyes in solutions was studied by H-1- NMR, electronic absorption and fluorescence spectrometry. The spectral data testify all the compounds exist in E- configuration of their styryl residues. The most planar molecular conformation is typical for the compounds with five- membered side aromatic moieties. In the case of pyridines with six- membered aromatic residues steric hindrance results in turning the above mentioned cyclic groups out of the plane of the central pyridine moiety. The violation of planarity in this case is not significant and saves the possibility of pi- electronic conjugation in the molecules. The synthesised compounds are characterized by high fluorescence quantum yields in solutions. The electronic absorption spectra of titled pyridines demonstrate low sensitivity to the nature of the substituents introduced into the side aromatic rings. In contrast to this, the fluorescence bands considerably change their position under the influence of electron donor substituents. The fluorescence spectra display substantial positive solvatofluorochromism only in the cases of the dialkylamino- derivatives, especially on going from aprotic solvents to proton donor ones. Generally, the synthesised structurally rigid distyrylpyridine derivatives have prospects for their application as multi- purposes fluorescent probes.