Racemates Have Much Higher Solid-State Fluorescence Efficiency than Their Levo- and Dextrorotary Enantiomers

C6-unsubstituted tetrahydropyrimidines (THPs) are compounds with a chiral carbon and strong aggregation-induced emission. The fluorescence properties of their racemates have been studied in detail, but those of their enantiomers have not. The solid-state fluorescence properties of the racemates and enantiomers of four chiral tetrahydropyrimidines (THPs <b>1</b>–<b>4</b>) have been investigated by the steady-state and time-resolved fluorescence, single-crystal X-ray structures, and HOMOs and LUMOs of their seven racemic (three of them are polymorphs), four <i>R</i>- and three <i>S</i>-enantiomeric crystals. It was found that the <i>R</i>- and <i>S</i>-enantiomers of <b>1</b>–<b>4</b> can self-assemble as <i>RS</i>-paired, <i>RS</i>-, or <i>RR</i>/<i>SS</i>-overlapped mode in their racemates and as the same <i>RR</i>/<i>SS</i>-overlapped mode in their <i>R</i>- and <i>S</i>-enantiomers. Unexpectedly, the solid-state fluorescence quantum yields (Φ_SF) of racemic <b>1</b>–<b>4</b> could increase to 93, 48, 80, and 100%, respectively, via a suitable heteroenantiomeric self-assembly, but the Φ_SF values of their seven enantiomers are only 25–46%, owing to much larger nonradiative rate constants than those of their racemates. This means that heteroenantiomeric self-assembly can be used as a new efficient method enhancing Φ_SF values. The advantage of racemates is first reported and expected to encourage the development and application of racemates as a new kind of fluorescent materials