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 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, 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, 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 ...