Synthesis and characterization of push–pull bithienylpyrrole NLOphores with enhanced hyperpolarizabilities

•Synthesis of (bi)thienylpyrrole derivatives through simple experimental procedures.•The optical properties (linear and nonlinear) of the new compounds were investigated.•Enhancement of the SHG achieved through the introduction of a second thiophene.•Dicyanovinyl–bithienylpyrrole exhibits the larger 1st hyperpolarizability reported to date.•(Bi)thienylpyrrole push–pull heterocycles as efficient SHG chromophores.

Novel push–pull second harmonic generation (SHG) chromophores 3-6 were synthesized in order to study how their optical properties are affected by an increase in the size of the π-bridge and also by connecting different electron accepting moieties (dicyanovinyl- and tricyanovinyl) to the (bi)thienylpyrrole system at position 5 of the pyrrole ring or at position 5 of the thiophene heterocycle. The precursor aldehydes were prepared through two different synthetic methodologies: Suzuki coupling (5a-b) or Suzuki coupling followed by Vilsmeier formylation (2a-b). The final donor-acceptor systems 4-6 were prepared by tricyanovinylation reaction of precursors 1a-b or by Knoevenagel reaction of the precursor aldehydes 2b or 5b with malononitrile. Hyper-Rayleigh scattering in dioxane solutions using a fundamental wavelength of 1064 nm was employed to evaluate their second-order nonlinear optical properties. The experimental results showed that the optical properties are influenced by the electron-acceptor strength of the groups, the length of the π-spacer/donor group as well as by the relative position of the pyrrole and thiophene heterocycles to the acceptor groups. The dicyanovinyl derivative 6b, which embodies the longer π-conjugated bridge functionalized in the thiophene ring, exhibited the largest first hyperpolarizability (β = 10,900 × 10−30 esu, using the T convention) thus indicating its potential application as a SHG chromophore. As far as we are aware, this represents the largest hyperpolarizability reported to date for a bithienylpyrrole derivative.

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