Z‐Shaped Pyrrolo[3,2‐b]pyrroles and Their Transformation into π‐Expanded Indolo[3,2‐b]indoles

Abstract

Sterically hindered 1,4-dihydropyrrolo[3,2-b]pyrroles possessing ortho-(arylethynyl)phenyl substituents at positions-2 and -5 were efficiently synthesized through a sila-Sonogashira reaction. These unique Z-shaped dyes showed relatively strong fluorescence in solution. Detailed optimization revealed that, in the presence of InCl3, these alkynes readily undergo an intramolecular double cyclization to give hexacyclic products bearing an indolo[3,2-b]indole skeleton in remarkable yields. Steady-state UV-visible spectroscopy revealed that upon photoexcitation, the prepared Z-shaped alkynes undergo mostly radiative relaxation leading to high fluorescence quantum yields. In the case of 7,14-dihydrobenzo[g]benzo[6,7]indolo[3,2-b]indoles, we believe that the substantial planarization of geometry in the excited state, is the underlying reason for the observed large Stokes shifts. The presence of additional electron-withdrawing groups makes it possible to further alter the photophysical properties. The two-photon absorption cross-section values of both families of dyes were found to be modest and the nature of the excited state responsible for two-photon absorption appeared to be strongly affected by the presence of peripheral groups. Serendipitous synthesis of unusual double-Z-shaped alkyne by Sonogashira and Glaser coupling is also reported.

Nonplanar Butterfly‐Shaped π‐Expanded Pyrrolopyrroles

“…For these compounds, hypsochromic shifts in emission are also apparent ( λ ≈15–40 nm). Compounds 6 a – c emit blue‐to‐cyan light in dichloromethane ( λ max =461–498 nm).The absorption properties of dyes 5 a – c and 6 a – c are comparable with those of π‐extended indolo[3,2‐ b ]indoles and 7,14‐dihydrobenzo[ g ]benzo[6,7]indolo[3,2‐ b ]indoles . The angular fusion between the rings in all these classes of heterocycles results in absorption that spans over the UV spectral region and extends into the visible region to approximately λ =450 to 470 nm.…”

“…The angular fusion between the rings in all these classes of heterocycles results in absorption that spans over the UV spectral region and extends into the visible region to approximately λ =450 to 470 nm. Unless strong electron‐withdrawing substituents are present, all of these compounds show emission maxima located at around λ =500 nm in CH 2 Cl 2 (DCM). The current study also shows that the introduction of a double‐helical motif combined with further π extension of compounds with a 1,4‐dihydropyrrolo[3,2‐ b ]pyrrole core induces dramatic decreases in the Φ fl value, as compared with previously obtained TAPPs and their π‐extended analogues .…”

Cove‐Edged Nanographenes with Localized Double Bonds

“…For the Sonogashira coupling between 3 and 8 , or between 5 and 10 , Fu's catalytic system Pd(PhCN) 2 Cl 2 /P( t ‐Bu) 3 turned out to be necessary to attain regio‐selectivity and reasonable yields at room temperature (r.t.). InCl 3 ‐mediated alkyne benzannulation reaction turned out to be most efficient method for 12 and 14 among various known cyclization conditions . Interestingly, the cyclization went to the o ‐terphenyl units in 12 , while to the pyrene moiety in 14 , which is presumably due to different electron densities at the respective reactive sites.…”

Construction of Polycyclic π‐Conjugated Systems Incorporating an Azulene Unit Following the Oxidation of 1,8‐Diphenyl‐9,10‐bis(phenylethynyl)phenanthrene

“…The approximate arrangement of the aromatic and arylethynyl groups in 1 could be a potential motif that could be used to construct a highly fused polycyclic system via alkyne benzannulation reactions . Alkyne benzannulation reactions using a variety of Lewis/Brønsted acids and some electrophiles have been powerful strategies for the direct π‐extension of aromatic compounds . During the course of our study of phenanthrenes 1 , we found that the transannulation of 1 a upon two‐electron oxidation selectively afforded the azulene‐embedded PAH 2 a , which exhibited long‐wavelength absorption and amphoteric redox events originating from the non‐alternant characteristics of the embedded azulene core.…”