Effects of nonaromatic through‐bond conjugation and through‐space conjugation on the photoluminescence of nontraditional luminogens

Abstract

Photoluminescence (PL) mechanisms of nontraditional luminogens (NTLs) have attracted great interest, and they are generally explained with intra/intermolecular through‐space conjugation (TSC) of nonconventional chromophores. Here a new concept of nonaromatic through‐bond conjugation (TBC) is proposed and it is proved that it plays an important role in the PL of NTLs. The PL behaviors of the three respective isomers of cyclohexanedione and gemdimethyl‐1,3‐cyclohexanedione were studied and correlated with their chemical and aggregate structures. These compounds show different fluorescence emissions as well as different concentration, excitation and solvent‐dependent emissions. The compounds which undergo keto‐enol tautomerism and hence with a conjugated ketone‐enol structure (i.e., nonaromatic TBC) show more red‐shifted emissions. TBC effect reduces the energy gaps and facilitates the formation of stronger TSC in the aggregate state. The compounds in the ketone‐enol form are also prone to occur excited state intra/intermolecular proton transfer (ESIPT). The cooperative effect of nonaromatic TBC and TSC determines the PL behaviors of NTLs. This work provides a novel understanding of the PL mechanisms of NTLs and is of great importance for directing the design and synthesis of novel NTLs.