Mechanistic studies and applications of heteroatom-containing luminogens with aggregation-induced emission characteristics

Traditional research in luminogenic materials usually concerns the emission behavior of isolated molecules in dilute solution as an ideal condition. However, the emission of many conventional dyes turns out to be quenched from solution to the solid state due to the aggregation-caused quenching (ACQ) effect. Thus, the photophysical behaviors of aggregates are often ignored until the discovery of the “aggregation-induced emission (AIE)” phenomenon in 2001, which triggers the photophysical study in the aggregate/solid state. Luminogens with AIE characteristics (AIEgens) often show weak or no emission in the isolated state but become highly emissive in an environment with constraints. AIE researches not only lead us to gain deeper insights into aggregate/solid state photophysics, but also provide a simple approach to modulate luminescence by controlling the state of aggregation, which is quite useful in analytical and biological applications such as sensing and imaging. To aid the design of new AIEgens, elucidating the working mechanism of AIE is a prerequisite. Many AIEgens, especially for heteroatom-containing AIEgens, are non-emissive in the solution state because of the quenching effect of dark states which have small transition probabilities. In fact, some nonradiative processes such as photo-induced electron transfer (PET), twisted intramolecular charge transfer (TICT), intersystem crossing (ISC) are all related to dark states such as (n,π*) state, charge transfer state, and triplet state. The excited state molecule can undergo intramolecular motions and relax to the dark-state geometry. However, in the aggregate/solid state, the restriction of intramolecular motion (RIM) will lead to the restriction of access to dark state (RADS), thus recovering the bright state emission. Therefore, the RADS mechanism is established to decipher the connotation of RIM for many heteroatom-containing AIE systems. For each chapter in this thesis, AIEgens that undergo different nonradiative processes are chosen as model compounds to illustrate the RADS mechanism. For each model compound, multiple excited states are concerned (e.g. three-state model, four-state model, five-state model). Excited-state molecular motions leading to dark states are identified (e.g. twisting of the heteroatom-bearing group). Their structure-property relationships are discussed, and the resulting design strategies to achieve unique photophysical properties (e.g. high affinity to albumin protein, room temperature phosphorescence, nonmonotonic fluorescence response to stimuli) are concluded. Besides the mechanistic study, the potential applications of these model compounds are explored including fluorescence sensing of metal ion, volatile gas, pH, protein, etc., making these AIEgens fundamentally important and practically appliable.