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 1 – 4 ) 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 R - and three S -enantiomeric crystals. It was found that the R - and S -enantiomers of 1 – 4 can self-assemble as RS -paired, RS -, or RR / SS -overlapped mode in their racemates and as the same RR / SS -overlapped mode in their R - and S -enantiomers. Unexpectedly, the solid-state fluorescence quantum yields ( SF ) of racemic 1 – 4 could increase to 93, 48, 80, and 100%, respectively, via a suitable heteroenantiomeric self-assembly, but the 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 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.