Tuning of the mesogenic, electronic, and optical properties of new conjugated 3,3′-bipyridine derivatives

We present two series of new 6,6'-distyryl-3,3'-bipyridine derivatives synthesized via a Knoevenagel condensation reaction of 6,6'-dimethyl-3,3'-bipyridine in the first series, and 5,5',6,6'-tetramethyl-3,3'-bipyridine in the second one, with aromatic aldehydes para-substituted with electron donor or acceptor groups. These molecules were characterized by spectroscopic methods (NMR, UV-vis, photoluminescence), and cyclic voltammetry. Some compounds were found to exhibit thermotropic liquid crystalline (LC) phases, whose structures were analyzed by DSC and optical microscopy. The effects of molecular design on the mesogenic behavior, redox potentials, and fluorescence (nature and lifetimes of the excited states) were investigated. The high degree of conjugation and the mesogenic character of these molecules can lead to nonlinear optical (NLO) applications for the "push-pull" compounds. Absorption in the UV range and high fluorescence are other characteristics of some members of this group. The latter property, associated with high electron affinity, opens up light-emitting diode (LEDs) applications.