Synthesis and Properties of Ladder-Type BN-Heteroacenes and Diazabenzoindoles Built on a Pyrrolopyrrole Scaffold

A simple, three-step synthesis of BN-heteroacenes and diazabenzoindoles based on the pyrrole­[3,2-<i>b</i>]­pyrrole scaffold has been developed. The incorporation of BN units has proven to be effective in modulating the electronic properties and molecular geometries of the π-conjugated backbone, creating a new type of heteroarenes. The unique planar structure and high rigidity of BN-compounds result in very high absorption coefficients and high fluorescence quantum yields, and, at the same time, very small Stokes shifts. A striking difference has been observed for a second type of derivatives: diazabenzoindoles, which remain virtually nonfluorescent, despite having a similar, rigid structure. The former class of heterocycles is characterized by a strong absorption around 400 nm and intense fluorescence observed in the 395–426 nm region, which results in very small Stokes shifts of less than 900 cm^–1

Extension of Pyrrolopyrrole π‑System: Approach to Constructing Hexacyclic Nitrogen-Containing Aromatic Systems

A facile three-step approach to synthesizing quinoline-fused pyrrolopyrroles is reported. The crucial step in this synthesis is the condensation of 2-aminophenyl substituted pyrrolopyrroles with aromatic aldehydes. The resulting hexacyclic ladder-type dyes strongly absorb UV radiation and exhibit fluorescence at 450–510 nm. The presence of pyridine-type and pyrrole-type nitrogen atoms is important for the electronic properties of this almost planar heterocycle. These heteroatoms, along with the addition of moderate electron-withdrawing and electron-accepting substituents, provide a means for fine-tuning of the emission characteristics of the polycyclic conjugates

V‑Shaped Bis-Coumarins: Synthesis and Optical Properties

A highly efficient procedure for the synthesis of bis-coumarins fused at the pyranone ring has been developed. The electron-rich phenols reacted with esters of coumarin-3-carboxylic acids, leading to substituted chromeno­[3,4-<i>c</i>]­chromene-6,7-diones. The reaction is catalyzed by both Lewis acids and 4-dimethylaminopyridine. The most probable mechanistic pathway involves Lewis acid catalyzed or DMAP catalyzed transesterification, followed by intramolecular conjugate addition of α,β-unsaturated esters to phenols and subsequent oxidation of the initially formed intermediate. The reaction is compatible with various functionalities such as NO₂, Br, and OMe. Not only benzene derivatives but also dihydroxynaphthalenes are reactive in this reaction, and the structure of the product can be controlled by adjusting the reaction conditions. Furthermore, a double addition is possible, leading to a horseshoe-shaped system comprised of seven conjugated rings. Compounds with four structurally unique skeletons have been obtained and have been shown to strongly absorb in the violet, blue, and/or green regions of the visible spectrum. Most of them display strong greenish yellow fluorescence, which can be modulated by both structural changes and the character of the solvents. Again, introduction of an electron-donating group in the chromeno­[3,4-<i>c</i>]­chromene-6,7-diones caused a significant red shift in both the absorption and emission maxima, and the effect became especially noteworthy in the case of amino substituents