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

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.