Nitrogen-inversion-based racemate aggregation and interenantiomer -stacking-caused solid-state fluorescence enhancement

Abstract

The solid-state fluorescence of fluorophores is usually weakened or quenched by strong π-π interactions. Here, the solid-state fluorescence quantum yields (Φ_SF) of tetrasubstituted tetrahydro-pyrimidines (TTHPs) are enhanced rather than decreased/or quenched by strong π-π interactions. All investigated TTHPs (5a−k) possess with aggregation-induced emission (AIE) characteristics: completely no emission in solutions owing to their soft, stero and low-conjugated molecular structures, but become emission upon aggregation, with Φ_SF equal to 3%−87.7%, because of the formation of excellent through-bond/space/hyperconjugation-mixed electron cojugation system and radiative--transition-favored molecular packing modes as well as the restriction of molecular motions. Unexpectedly, achiral TTHPs aggregate via meso-enantiomers for 5k (both N1 and N3 atoms show pyramidal configuration) and via rac-enantiomers for other TTHPs (N1 shows racemic pyramidal inversion configurations and N3 shows planar configuration). Even more surprisingly, unlike conventional AIE fluorophores that can efficiently prevent cofacial π−π interaction, strong cofacial π−π interaction exists between the N1-inversion-based rac-enantiomers and can significantly enhance rather than decrease Φ_SF values by efficiently decreasing k_nr values, which is first reported. Designing compounds with pyramidal inverstion might be an efficient stratgy to obtain highly emissive π−π stacking aggregates.