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

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. [Display omitted] •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.