Insight into structural influences on the optical properties and heteroenantiomeric self-assembly of racemic C6-unsubstituted tetrahydropyrimidines with strong aggregation-induced emission

Abstract

Racemic C-6 unsubstituted tetrahydropyrimidines (THPs), the products of an efficient five-component reaction (5CR), are novel fluorophores with strong aggregation-induced emission (AIE). Here, the influences of molecular structures on the optical properties and heteroenantiomeric self-assembly of THPs have been investigated in details via the design and synthesis of 66 THPs with different substituents R1−R4, the analysis of 22 X-ray diffraction single crystals of 16 THPs and the theoretically calculated HOMOs/LUMOs based on the conformations in single crystals. All THPs with different R1−R4 in n-hexane solutions showed no emission and had similar absorbance wavelengths (λab) (315–320 nm) except R2 = R4 = alkyl (λab = 293 nm). However, upon aggregation, 64 THPs with aromatic R2 and R4 were emissive and had very different solid-state fluorescence quantum yields (ΦF) (3–100%), excitation and emission wavelengths (340–409 nm and 436–492 nm, respectively), except two non-emissive THPs with aliphatic R2 and R4. It was also found that the influences of all R1−R4 depended on their combinations, and the solid-state ΦF values of THPs were greatly enhanced by simply changing the combinations of R1−R4 with larger R1/smaller R3 or smaller R1/larger R3 combining suitable R2 and R4 optimal. 20 THPs with ΦF > 80% were mainly from various aryls except 4-OHPh and rigid naphthalenyl as R3 combining with suitable R1 (Et/Me), R2 and R4 (Ph/4-Cl/4-CF3/3-CF3/3-BrPh). The R- and S-enantiomer alignments of THPs in the 22 crystals were classified into four molecular packing modes (MPMs), whose formation mainly depended on R3 on chiral carbon. The through-space conjugation existing in the HOMOs and LUMOs of THPs with aryl R2 and R4 also correlated with R3: aromatic R3 and R4 when R3 = 4-CNPh, or only R4 when R3 = other aryls participating in conjugation through space. The mechanisms of structural effects on the solid-state fluorescence properties and heteroenantiomeric self-assembly of THPs are discussed based on the obtained experimental results and our previous work on AIE mechanism. The simple and efficient 5CR built a large library of diverse compounds, the relationship between substituent on chiral carbon and heteroenantiomeric self-assembly, the strong and useful AIE characteristics, and moreover, the firstly reported efficient method of enhancing solid-state fluorescence via simply changing the combinations of substituents will be expected to make racemic THPs potential materials and attract great interest in the development and applications of new interesting racemic fluorophores.