Synthesis of Bis(arylethynyl)pyrrolo[3,2-b]pyrroles and an Effect of Intramolecular Charge-transfer on Their Photophysical Behavior.

3,6-Bis(arylethynyl)pyrrolo[3,2-b]pyrroles have been synthesized via a two-step procedure involving double direct alkynylation of the electron-rich core followed by Sonogashira coupling. In comparison with the parent tetraarylo-pyrrolo[3,2-b]pyrroles and benzo-fused pyrrolopyrroles, these new dyes show moderately redshifted absorption. Almost all derivatives show positive fluorescence solvatochromism, and for the first time red-emitting pyrrolopyrroles have been obtained. Computational studies revealed that, in most cases, there is negligible change in the geometry between ground and excited states. Interestingly, there is a fundamental difference between pyrrolopyrroles possessing electron-withdrawing substituents at positions 2 and 5 and their analogs lacking these substituents. The former dyes behave like dipolar chromophores with the lowest excited state both one-photon and two-photon allowed, which corresponds to intramolecular charge-transfer occurring along the branches perpendicular to the pyrrolopyrrole long axis. In the latter compounds, intramolecular charge transfer takes place along the long axis of pyrrolopyrrole and consequently the one-photon transitions are not two-photon allowed. Despite displaying quadrupolar core-to-peripheral intramolecular charge transfer, these derivatives show two-photon absorption cross-sections in the NIR1 region comparable to tetraaryl-pyrrolo[3,2-b]pyrroles lacking π-expansion (up to about 500 GM).