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 interests, and they are generally explained with intra/intermolecular through-space conjugation (TSC) of nonconventional chromophores (NCCs). Here we propose a new concept of nonaromatic through-bond conjugation (TBC) and proved that it plays an important role in the PL of NTLs. The PL behaviors of cyclohexanedione (CHD), dimethyl-1,3-cyclohexanedione (DMCHD) and their three respective isomers are studied and correlated with their chemical structures and aggregate structures. These compounds show different fluorescence emissions from blue to yellow region, and they also show different concentration-dependent emission (CDE) and excitation-dependent emission (EDE) characteristics. The position of ketone groups and the steric hindrance of methyl groups in the compounds determine the occurrence of keto-enol tautomerism or not. The compounds with conjugated keto-enol structure (i.e., nonaromatic TBC) show red-shifted emissions with comparison to the compounds with only isolated diketone structures. Theoretical calculations show that TBC effect reduces the HOMO-LUMO energy gaps of single molecules, and it facilitates the formation of stronger TSC in the aggregate state. The cooperative effect of nonaromatic TBC and TSC leads to more significantly red-shifted emissions. This work provides a novel and deeper understanding of the PL mechanisms of NTLs and is of great importance for directing the design and synthesis of NTLs with enhanced and red-shifted emissions.