THESIS
2020
xxix, 181 pages : color illustrations ; 30 cm
Abstract
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...[
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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.
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