Donor-acceptor-substituted alkynes, endowed with 11,11,12,12-tetracyano-9,10-anthraquinodimethane (TCAQ) and N,N-dimethylaniline (DMA) units, have been further functionalized by a [2+2] cycloaddition with tetracyanoethylene (TCNE) followed by a subsequent retro-electrocyclization to form distorted nonplanar structures with bridging 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) units. Comprehensive spectroscopic, electrochemical, and computational studies have been carried out to compare the electronic communication in planar (alkyne bridges) and nonplanar (TCBD bridges) TCAQ-based push-pull chromophores. Cyclic voltammetry and UV/Vis absorption measurements confirm the electronic communication between the TCAQ and DMA units despite the nonplanarity of the TCBD group. The experimental trends are strongly supported by density functional theory calculations, which further support the active electron-withdrawing role of the TCBD bridges. The novel push-pull TCAQ-based derivatives incorporating the TCBD bridge show a broad absorption in the whole visible range while the structures are highly distorted from planarity.
Keywords: cycloaddition/electro-cyclization reactions; density functional calculations; donor-acceptor systems; tetracyanobutadiene.
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