Ground‐ and Excited‐State Symmetry Breaking and Solvatofluorochromism in Centrosymmetric Pyrrolo[3,2‐b]pyrroles Possessing two Nitro Groups

Abstract Six centrosymmetric pyrrolo[3,2‐ b ]pyrroles possessing either two nitro or two CF 3 SO 2 strong electron‐withdrawing groups have been synthesized in a straightforward manner from simple building blocks and their one and two‐photon properties investigated. Although two‐photon absorption maxima clearly correspond to the S 0 →S 2 transition, there is always a considerable shoulder at the region related to the S 0 →S 1 transition. This fact clearly points to a small degree of symmetry breaking in the ground state which becomes more prevalent in the excited state. The systems showing the strongest excited‐state symmetry breaking have also been identified by first principles calculations. Both experimental data and theoretical calculations (performed with time‐dependent density functional theory) have revealed that changing the degree of conjugation between the nitro−aryl substituents and the core enables fine‐tuning of not only emission wavelengths but also the solvent dependent fluorescence intensity. Although there is no charge‐transfer between electron‐withdrawing 4‐cyanophenyl substituents located on nitrogen atoms and the core, nevertheless, their presence modifies the absorption and emission maxima. These groups also enable a record high fluorescence quantum yield in toluene (0.97) to be reached. By probing ground‐state symmetry by two‐photon absorption and excited‐state symmetry by solvatofluorochromism, we are able to obtain insight regarding the pathway of the molecule during and after the electronic transition.