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

Abstract

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 π-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 μm). For the mesogenic chromophores, first hyperpolarizabilities extrapolated to infinite wavelength, β0, between 16 and 85 × 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 n2 and the two-photon absorption (TPA) coefficient α2 were simultaneously measured at λ = 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 n2 values between 1 × 10−16 and 4 × 10−16 m2 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.