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

摘要

Six centrosymmetric pyrrolo[3,2-b]pyrroles possessing either two nitro or two CF3SO2 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 S0→S2 transition,there is always a considerable shoulder at the region related to the S0→S1 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 finetuning of not only emission wavelengths but also the solvent dependent fluorescence intensity.Although there is no chargetransfer 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.