New second-order NLO chromophores based on 3,3 '-bipyridine: Tuning of liquid crystal and NLO properties

We report on the synthesis via a Knoevenagel condensation reaction of a. ne w family of push-pull molecules based on 6,6'-distyryl-3,3'-bipyidine. Thes e chromophores were characterized by spectroscopic methods (NMR, UV-vis, ph otoluminescence), their mesogenic behavior was investigated by DSC and opti cal microscopy, and their second-order NLO propel ties were evaluated by th e electric field second harmonic (EFISH) technique. All the compounds are t ransparent at the second harmonic wavelength of typical laser sources (1.32 and 1.55 mum), and some of them exhibit a strong emission. By lateral subs titutions of the pi -conjugated bridge and by varying the nature of the acc eptor/donor pair, we were able to tune both the mesogenic and NLO propertie s of the chromophores. For the mesogenic chromophores, first hyperpolarizab ilities extrapolated to infinite wavelength, Po, between 16 x 10(-30) and 8 5 x 10(-30) esu were measured. These compounds exhibit enhanced hyperpolari zabilities compared to typical NLO liquid crystal chromophores reported in the literature.