Pyrene-Based AIE System: A Bridge Model to Regulate Aggregate Structures

The bridging role of aggregate between single-molecule structures and macroscopic photophysical properties has attracted intense attention. Pyrene (Py), as the simplest dimer candidate, is a proper model for studying accurate structure-property relationships of aggregates. Herein, a series of Py and derivatives were systematically investigated. With a largely planar and conjugated conformation, Py shows anomalous aggregation-induced emission (AIE) characteristics due to the oxygen quenching at the molecular level but turn-on fluorescence in the aggregate state because of the oxygen isolation. Although introducing substituents induces molecular motion and weakened luminescence in the molecular state, the impact of substituents on the aggregate-state photophysical properties enormously differs. A small and electron-conjugated substituent (-CHO) can generate long-range ordered dimer stacking, leading to redshifted and weakened emission. Meanwhile, the larger substituent (-CyA) can disrupt long-range stacking and result in discrete dimers, leading to blue-shifted but enhanced emission. Further, incorporating a natural and bulky alicyclic structure (-DAA) inhibits dimer formation and activates strong molecular motions in the crystalline state, resulting in the shortest wavelength and weakest emission. Interestingly, crystalline Py-DAA exhibits both mechanochromic and acidichromic properties, which can be synergistically applied in dynamic encryption-decryption. These results indicate that substituents significantly affect their fluorescent behaviors due to the complexity of aggregate structures. This work not only elucidates the unique AIE performance of Py for the first time but also applies it as an accurate model for regulating the aggregate structures, offering precise molecular design strategies for aggregate-state luminescent materials.