6,6′-distyryl-3,3′-bipyridine derivatives:: a novel class of tunable chromophores for second- and third-order nonlinear optical applications

We report on the design, synthesis, structural characterization and quadratic as well as cubic nonlinear optical (NLO) properties of a series of organic chromophores (6, 6'-distyryl-3, 3'-bipyridine derivatives disubstituted with donor (D) and/or acceptor (A) end-groups). The compounds are either dipolar (push-pull molecules) or apolar (symmetric D-A-A-D molecules). By lateral substitutions of the pi-conjugated bridge and by varying the nature of the donor/acceptor (D/A) pair, we were able to tune both the mesogenic and NLO properties of the chromophores. Their second-order NLO properties were evaluated by the electric-field-induced second-harmonic technique. All the compounds are transparent at the second-harmonic wavelength of typical laser sources (1.32 and 1.55 mum). For the mesogenic chromophores, first hyperpolarizabilities extrapolated to infinite wavelength, beta(0) between 16 and 85 x 10(-30) esu were measured. These compounds exhibit enhanced hyperpolarizabilities compared to typical NLO liquid crystal chromophores reported in the literature. Both the nonlinear refractive index n(2) and the two-photon absorption (TPA) coefficient alpha(2) were simultaneously measured lambda = 1064 nm, using a single-shot (50 ps, 10 Hz) multi-channel Z-scan method. For all molecular compounds, Z-scan measurements in dichloromethane solutions show a large nonlinear refractive index, close to that of pure carbon disulfide, without detectable TPA in most cases. These measurements lead to extrapolated nonresonant n(2) values between 1 x 10(-16) and 4 x 10(-16) m(2) W-1, whatever the electronic structure of the compounds. Hence, apolar symmetric D-A-A-D molecules appear to be an interesting novel family of NLO dyes.