Bridging the Gap Between Traditional and Nontraditional Luminogens With Strong Non-Aromatic Through-Bond Conjugation and Through-Space Conjugation

Abstract

The development of nontraditional luminogens (NTLs) with superior photoluminescence (PL) properties is of great scientific and practical significance and has drawn rapidly growing interest in recent years. An extremely important but unresolved question is that if there are any distinct differences in the structures and PL mechanisms between traditional luminogens (TLs) and NTLs. In this work, four dihydropyridine derivatives with strong non-aromatic through-bond conjugation (TBC) were designed and synthesized, and the influence of strong non-aromatic TBC and through-space conjugation (TSC) effects on their PL behaviors was studied. These compounds in solutions show significant concentration-dependent emission (CDE) and excitation-dependent emission (EDE), which are typical PL behaviors of NTLs. In solid state, the compounds show wide excitation spectra while narrow emission spectra, with high quantum yields up to 57.4%, but they do not show significant EDE, similar to TLs. And very impressively, two kinds of crystals also exhibit optical waveguide property, which is the first report in NTLs. The UV–vis spectra, crystal structures, and theoretical calculations prove the presence of large non-aromatic TBC interactions in these NTLs, and strong non-aromatic TSC can be formed among the molecules that are in a planar conformation and stacked into layers through intermolecular hydrogen bonding and π⋅⋅⋅π interactions. The combined effect of strong non-aromatic TBC and TSC endows the compounds unique PL behaviors that are between those of TLs and NTLs, thus bridging the gap between TLs and NTLs.