Nitrogen-inversion-based racemate aggregation and interenantiomer π-stacking-induced solid-state fluorescence enhancement

Abstract

The solid-state fluorescence of fluorophores is usually weakened or quenched by cofacial π-stacking. Here, the solid-state fluorescence quantum yields (Φ_SF) of tetrasubstituted tetrahydro-pyrimidines (TTHPs) are enhanced rather than decreased or quenched by cofacial π-stacking. All investigated TTHPs (5a–k) exhibit aggregation-induced emission (AIE) characteristics: completely non-emissive in solutions owing to their soft, stereo and low-conjugated molecular structures, but become emissive upon aggregation, with Φ_SF equal to 3%–88%, because of the formation of an excellent through-bond/space/hyperconjugation mixed electron conjugation system, radiative-transition-favored molecular packing modes, and the restriction of molecular motion. Unexpectedly, achiral TTHPs aggregate via meso-enantiomers for 5k (both N1 and N3 atoms show a pyramidal configuration) and via rac-enantiomers for the other TTHPs (N1 shows racemic pyramidal inversion configurations and N3 shows a planar configuration). Even more surprisingly, unlike conventional AIE fluorophores that efficiently prevent cofacial π-stacking, there is tight cofacial π-stacking between the N1-inversion-based rac-enantiomers, and this stacking can significantly enhance rather than decrease Φ_SF values by efficiently decreasing k_nr values, which is reported for the first time. Designing compounds with pyramidal inversion might be an efficient strategy to obtain highly emissive π-stacking aggregates.