Design and synthesis of 4,4′-π-conjugated[2,2′]-bipyridines: a versatile class of tunable chromophores and fluorophores

A series of 4,4′-disubstituted[2,2′]-bipyridines, featuring π-conjugated substituents such as donor-(acceptor-) substituted styryl, thienylvinyl, phenylimino and phenylazo groups, have been synthesized. X-Ray structures are provided for three ligands containing –CC–, –CN– and –NN– linkers, respectively. These chromophores display good to excellent thermal stabilities with decomposition temperatures of up to 350 °C. The strong influence of the nature of the endgroups and π-linkers on the optical properties is discussed. The stepwise protonation of 4,4′-dibutylaminostyryl-[2,2′]-bipyridine and its coordination behavior to different metallic moieties [Zn(II), Hg(II), Pd(II), Re(I), Re(VII)] have also been investigated. It is found that the absorption and emission maxima can be easily tuned by these exogenous additives over a wide range of wavelengths (360 < λ_abs < 560 nm ; 482 < λ_em < 646 nm).