(E)-5,5′-Di(thiophen-2-yl)-3,3′-bi[thiophen-3(2H)-ylidene]-2,2′-diones—from conspicuous blue impurities to “quasi-metallic” golden-bronze crystals

(E)-5,5'-Di(thiophen-2-yl)-3,3'-bi[thiophen-3(2H)-ylidene]-2,2'-dione () has caught the eye of several chemists who have encountered it as a trace by-product in a variety of reactions. We report here the first deliberate synthesis of this intense blue dye (λ(max) 635 nm; ε(max) 32,400 L cm(-1) mol(-1)). We prove on the basis of predictions made with TDDFT that 1a has previously been misassigned as a structural isomer and we correct the visible absorption data that was reported. Derivatives of 1a equipped with pentyl (1b) and butylsulfanyl (1c) groups are readily prepared with our divergent synthetic route (3-7% yield over five steps). Crystals of 1b and 1c exhibit an attractive "quasi-metallic" golden-bronze lustre. This effect is rare in molecular crystals and is observed only with the most intense dyes.