Tetracyanoethylene\nas a Building Block in the π‑Expansion\nof 1,4-Dihydropyrrolo[3,2‑<i>b</i>]pyrroles

The outcome of the reaction of tetracyanoethylene with\n1,4-dihydropyrrolo[3,2-<i>b</i>]pyrroles (DHPPs) strongly\ndepends on the character of\nthe substituents present at positions 2 and 5. With electron-withdrawing\nsubstituents, the reaction does not occur at all, while, in contrast,\nthe presence of electron-donating substituents yields addition–elimination\nproducts. When thiazol-2-yl substituents are located at positions\n2 and 5, addition occurs at the thiazole ring, rather than of the\nDHPP core. In cases where very electron-rich heterocycles are present\nat positions 2 and 5, a second addition occurs followed by aromatization,\nleading to the formation of an additional benzene ring bridging two\nheterocyclic scaffolds. The reaction occurs only at one site since\nthe presence of the strongly electron-withdrawing tricyanoethylene\ngroup has a profound impact on electron density at the remaining free\nposition 6. The DHPPs possessing a tricyanoethylene group are strongly\npolarized and thus enable a push–pull system showing red-shifted\nabsorption and negligible fluorescence. In contrast, dyes possessing\na 1,2-dicyanobenzene moiety exhibit strong emission bathochromically\nshifted by over 100 nm compared to parent 1,4-dihydrotetraarylpyrroles[3,2-<i>b</i>]pyrroles (TAPPs). Computational studies shed light on\nthe evolution of the photophysical properties as a function of the\nsubstitution pattern of the final systems.