Internal acylation-induced AIE/AIEE switch of pyrimido[2,1-b][1,3]benzothiazoles (PBTs): Restriction of access to dark state caused by distortion of 4H-pyrimidine ring

As atypical propeller-shaped AIEgens, pyrimido[2,1-b][1,3]benzothiazoles (PBTs) exhibit good to excellent solid-state fluorescence quantum yields and full-color tunability. However, the working mechanism of PBT AIEgens still remains elusive. In this article, three internally acylated PBTs (PBT-B7−B9) were synthesized. In agreement with our previous report, locking the exocyclic ester resulted in the emergence of solution-phase fluorescence, i.e. AIE/AIEE switch. Theoretical calculation results indicated that the 4H-pyrimidine ring experiences the most significant electronic structure alterations upon excitation. Comparative analysis of potential energy curves (PECs) and optimized excited-state geometries of both locked and unlocked PBTs revealed that as a cumulative result of bond length/angle changes in 4H-pyrimidine, the distortion of the ring is the direct cause for decoupling of π-conjugated MOs and major driving force for access to the resulting dark state. On the other hand, the restriction of rotation of exocyclic ester or carbonyl in locked PBTs maintains the conformation of 4H-pyrimidine, thereby avoiding the generation of dark state and making them emissive in solution. This AIE mechanism well explained the drastically decreased fluorescence quantum yields of unlocked PBTs in PMMA films, and indicated that locking the exocyclic ester or carbonyl could be an effective strategy to improve their PL performance in polymer matrix.