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