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

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.