Tetraaryl‑, Pentaaryl‑, and Hexaaryl-1,4-dihydropyrrolo[3,2‑<i>b</i>]pyrroles: Synthesis and Optical Properties

Abstract

Efficient conditions for the synthesis of tetra-, penta-, and hexasubstituted derivatives of 1,4-dihydropyrrolo­[3,2-<i>b</i>]­pyrrole were developed. The tetraaryl derivatives were obtained in a novel one-pot reaction among aromatic aldehydes, aromatic amines, and butane-2,3-dione. After a thorough examination of various reaction parameters (solvent, acid, temperature) <i>p</i>-toluenesulfonic acid was identified as the crucial catalyst. As a result, 1,4-dihydropyrrolo­[3,2-<i>b</i>]­pyrroles were obtained in the highest yields reported to date. The scope and limitation studies showed that this new method was particularly efficient for sterically hindered aldehydes (yields 45–49%). Pentaaryl- and hexaaryl-1,4-dihydropyrrolo­[3,2-<i>b</i>]­pyrroles were prepared from tetraaryl-1,4-dihydropyrrolo­[3,2-<i>b</i>]­pyrroles via direct arylation by employing both electron-poor and electron-rich aromatic and heteroaromatic haloarenes. Strategic placement of electron-withdrawing substituents at the 2-, 3-, 5-, and 6-positions produced an acceptor–donor–acceptor type fluorophore. The resulting multiply substituted heteropentalenes displayed intriguing optical properties. The relationship between the structure and photophysical properties for all compounds were directly compared and thoroughly elucidated. All synthesized products displayed strong blue fluorescence and exhibited moderate to large Stokes shifts (3000–7300 cm^–1) as well as high quantum yields of fluorescence up to 88%. Two-photon absorption cross-section values measured in the near-IR region were surprisingly high (hundreds of GM), given the limited conjugation in these propeller-shaped dyes