The Tetraarylpyrrolo[3,2-b]pyrroles—From Serendipitous Discovery to Promising Heterocyclic Optoelectronic Materials

Conspectus Progress in organic optoelectronics requires compounds possessing a suitable combination of photophysical and electronic properties. Another key constraint encompasses the availability of feasible, and hopefully scalable, synthetic procedures for preparing the molecular scaffolds of interest. A multicomponent reaction of aromatic aldehydes, aromatic amines, and butane-2,3-dione that was discovered in 2013 gives straightforward access to previously unavailable 1,2,4,5-tetraarylpyrrolo[3,2- b ]pyrroles. These dyes are examples of heteropentalenes—a class of 10-π-electron aromatic compounds. The unsurpassed variety of aromatic aldehydes and primary aromatic amines, which are commercially available or easy to prepare, allows for potential routes to thousands of 1,2,4,5-tetraarylpyrrolo[3,2- b ]pyrroles that are currently unknown. This synthetic procedure offers a means for preparing the pyrrolopyroles in gram quantities and isolating them by simple filtration. Typically, the construction of an aromatic core is merely the first phase in a long procedure toward multistep functionalization. Conversely, the synthesis of 1,2,4,5-tetraarylpyrrolo[3,2- b ]pyrroles leads to preinstalled substituents in frames with C 2 symmetry, which “opens Sesame” to a wealth of structural possibilities. In addition, steric hindrance of the aldehyde components, rather than presenting a problem, is beneficial for increasing the yields of the products. This feature provides invaluable routes for the synthesis of a broad range of π-extended systems possessing the pyrrolo[3,2- b ]pyrrole core in just a few steps. Indeed, this approach has enabled the preparation of a large number of previously unknown ladder-type heteroacenes possessing additional rings based on carbon–carbon, carbon–nitrogen, and nitrogen–nitrogen double bonds as well as nitrogen–boron single bonds. This set of chromophores includes planar and curved structures bearing up to 14 conjugated rings. 1,2,4,5-Tetraarylpyrrolo[3,2- b ]pyrroles manifest broad absorption bands between about 300 and 450 nm, strong violet-blue or blue fluorescence with typical quantum yields of ∼60%, significant Stokes shifts ranging between 3000 and 5800 cm –1, and emission while in the solid state. Should the two peripheral aryl groups have an electron-deficient character, the two-photon absorption cross section also becomes pronounced, i.e., ∼400 GM. Perhaps the most important feature of these dyes is their strong solvatofluorochromism, which predestines their value as environment-sensitive probes. Extension of the π-conjugation of 1,2,4,5-tetraarylpyrrolo[3,2- b ]pyrroles enables further modifications of their photophysical properties, such as shifting the emission bathochromically, increasing the Stokes shift beyond 10 000 cm –1, and attaining solvatofluorochromism for curved, butterfly-shaped analogues without a decrease in emission intensity when the solvent polarity is increased. Common features of these chromophores include a significant difference between the geometries of their relaxed ground and relaxed excited states as well as strong electronic coupling through their aromatic cores. Past and future intense exploration of the wide chemical space built around the pyrrolo[3,2- b ]pyrrole skeleton offers unprecedented opportunities for comprehensive elucidation of how photoexcitation increases the electronic coupling through biaryl linkages.