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 (Phi(SF)) of racemic 1-4 could increase to 93, 48, 80, and 100%, respectively, via a suitable heteroenantiomeric self-assembly, but the Phi(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 Phi(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.