Rational Construction of a Pyrimido[2,1-b] Benzothiazole-Based Photoswitchable Smart AIE Material: A Theoretical Insight into Fluorescence Switching Mechanism of the Chimeric Dyad.

Currently, AIEgen-photochromics conjugates and intrinsic photochromic AIEgens are the two major molecular design strategies for photoresponsive AIE materials. However, these two strategies still have their own limitations. In our previous research, we discovered pyrimido[2,1-b][1,3]benzothiazole (PBT) as a novel core structure of AIEgens. We herein explored a chimeric strategy to rationally integrate photochromic bisthienylethene (BTE) as the orthogonal head group into a new PBT molecule (PBTE). Compared to the conventional design strategies, the compact chimeric design of PBTE not only well maintains the AIE and photochromic properties of the parent fragments but also leads to excellent AIE-photoswitching capability in both films and single crystals. Theoretical calculations revealed that MO energy level arrangement of PBTE reorganized upon photoisomerization and the fast vibrational relaxation (VR) along with internal conversion (IC) from S to S state may serve as a competitive channel for the fluorescence quenching of PBTE-c. Comparison of the kinetics of the nonradiative decay with those of the excited-state energy transfer (EET) processes clearly showed that the ultrafast intramolecular Förster resonance energy transfer (FRET) is the dominant cause of fluorescence-off state. The applications of PBTE in erasable optical memory material and multi-dimensional anti-counterfeiting have also been demonstrated.