1.
Introduction
1.1 AIE and its incorporation into polymers
Fluorescent molecules have shown their great potential in a diverse
range of applications since their first discovery, such as for digital
technologies, fluorescence molecular tagging, staining biological cells,
and probes for detecting environmental
variations.[1-5] Conventionally, fluorescent
molecules emit strongly in the molecular or solution state, but
experience appreciable effects of photoluminescence (PL) quenching in
the aggregated state, which is a well-known concept discovered and
termed by Förster as Aggregation-Caused Quenching (ACQ) in
1954.[6, 7]
In stark contrast to this phenomenon, in 2001, Tang and co-workers
discovered a type of special fluorescent molecule that emits poorly in
the molecular or diluted state but emits strongly upon radiative
excitation in the aggregated state. They coined this phenomenon as
Aggregation-Induced Emission (AIE).[8-12] At that
time, only a peculiar class of silole compounds where a molecule,
1-methyl-1,2,3,4,5-pentaphenylsilole (Figure 1A ), fluoresces
strongly only upon aggregation was discovered.[8]Subsequently, this new phenomenon attracted a significant amount of
research attention, leading to the discovery of a series of new
molecules with AIE property in the next few years. Meanwhile, the
mechanistic understanding of this new phenomenon also became a hot topic
in this field.
The competing effect of ACQ and AIE for any given luminophores depends
on multiple factors including (but not limited to) molecular structure
and composition, molecular behavior when isolated and when in close
proximity to other molecules (i.e. aggregated state). Researchers have
attempted to rationalize such observations and among the many proposed
explanations, the restriction of intramolecular motion (RIM) theory took
the throne.[13-15] This mechanism comprises two
parts: restriction of intramolecular rotation (RIR) (Figure 1B )
and restriction of intramolecular vibration (RIV) (Figure 1C ).
This theory assumes most molecules that underwent ACQ instead of AIE,
possess highly coplanar aromatic rings in them, while AIE molecules
adopt a “propeller-shaped” structure where the aromatic rings
represent the “rotors”, able to rotate freely in the molecular state
and promote energy transfer among molecules, hence generating a new path
for non-radiative decay.[16] In the aggregated
form, RIM imposed onto the molecules forces energy dissipation to occur
via the radiative pathway instead of the standard mechanical energy
dissipation pathway, with fluorescence emission. For example as shown inFigure 2A , a classic ACQ luminophoreN,N -dicyclohexyl-1,7-dibromo-3,4,9,10-perylenetetracarboxylic
diimide (DDPD) shows an intense color when dissolved in tetrahydrofuran
(THF) solution, but forms insoluble aggregates when water was added due
to the solubility (free volume) effect,[3] thus
quenching the PL.[17] Contrary to this
observation, AIE luminogens (AIEgens) reverses the effect of ACQ
(Figure 2B ).[18] A solution of
hexaphenylsilole (HPS) in THF displayed extremely low PL owing to the
freely rotatable peripheral rings, but shows an intense color when water
was added, by forming insoluble aggregates.
Since its first discovery, AIE molecules were believed to have many new
applications that cannot be achieved by conventional fluorescent
molecules. However, AIE molecules alone have only limited applications
due to their poor mechanical and film-forming properties. Therefore, the
need for incorporating AIE components into polymers is considered
necessary in many applications, such as in optoelectronic and biomedical
applications where luminescent materials are commonly employed as films
and aggregates, with properties vastly different from single isolated
molecules.[19] In addition, these AIE molecules
can be used as probes when incorporated into aggregates by monitoring
their PL intensities, which is especially useful in the field of
material science and engineering, where information on reaction
mechanisms and processes are of paramount importance.
In 2003, Tang et. al . reported the world’s first AIE-active
polymer and set the stage for many researchers globally to follow this
research pathway in understanding the mysteries of the AIE
phenomenon.[20] AIE polymers overall, provides
more benefits than small AIE molecules, such as ease of processing, good
ability to form films, and structural diversity. Since then AIE polymers
have found various applications such as AIE-active polytriazole-based
explosive chemosensors synthesized via click
polymerization,[21] high performance polymeric
light-emitting diodes with low-cost wet fabrication, high fluorescence
quantum nanoparticles with excellent thermal and film-forming
stability,[22] and fluorescent polymeric
nanoparticles (FPNs) synthesized from a “one-pot” multicomponent
Mannich reaction as bio-imaging agents for L929
cells.[23] Some reviews have already explored the
structure, design, reaction pathways, and applications of AIE
polymers,[16, 18, 24-26] while other reviews
explored the area of AIE polymers for biomedical-related
applications,[27]chirality,[28] supramolecular AIE
polymers,[29] AIE click
polymerization,[30, 31] one-component AIE
polymerization, two-component AIE polymerization, and multi-component
polymerization.[30, 32] However, the AIE polymers
that were synthesized till date with pre-determined molecular weights,
low dispersity values and well-defined structures via
Reversible-Deactivation Radical Polymerization (RDRP) specifically has
not been systematically summarized. Moreover, these AIE polymers
synthesized via RDRP with well-designed structure, chain length,
well-controlled molecular weights and molecular weight distributions are
of great importance in certain applications such as theranostics, FPNs,
and environmental variation detection. Therefore, this review is
dedicated to highlight and summarize some of these recent works that
utilized RDRP to design and produce AIE polymers, including the
different types of RDRP methods, synthetical strategies, and their
potential applications.
1.2 RDRP
Since the early 1980s, researchers from around the world have realised
that the addition of certain chemical compounds into a polymerization
mixture allows reversible reaction with chain carrier
molecules.[33] Many terms have been used to
describe these polymerization reactions including (but not limited to):
controlled/living radical polymerization, ‘controlled’ and ‘living’
polymerization, and radical polymerization with minimal
termination’.[34] In 2010, the International Union
of Pure and Applied Chemistry (IUPAC) stepped in to generalize all such
polymerization reactions by coining the term: Reversible-Deactivation
Radical Polymerization (RDRP).[33, 34] RDRP can be
defined as a polymerization reaction where side reactions such as chain
transfer reactions and termination reactions, are considered trivial or
negligible throughout the polymerization process and the molecular
weight of the growing polymer increases linearly with monomer
conversion. This revolutionary polymerization method sparked
possibilities in synthesizing complex, well-defined polymer
architectures and morphologies with multi-functionalities, which
otherwise will not be possible by conventional
methods.[33, 34]
RDRP polymerization techniques include,
Nitroxide-Mediated Polymerization
(NMP),[35-37] Atom Transfer Radical Polymerization
(ATRP),[38-45] Reversible Addition-Fragmentation
Chain Transfer (RAFT),[46-49] Iodine-Transfer
Polymerization,[50] Reverse Iodine-Transfer
Polymerization (RITP),[51] Reversible Chain
Transfer Catalyzed Polymerization (RTCP),[52]Reversible Complexation Mediated Polymerization
(RCMP),[53] Organotellurium-Mediated Radical
Polymerization (TERP),[54] Cobalt Mediated Radical
Polymerization and Catalytic Chain Transfer
(CCT),[55, 56] Iniferter
Polymerization,[57, 58] Selenium-Centred
Radical-Mediated Polymerization,[59] and
Organostibine-Mediated Radical Polymerization
(SBRP).[60] Even though a range of different RDRP
technique have been developed over the past decades, the most popular
methods for designing AIE polymers via RDRP are RAFT and ATRP owing to
their applicability to a wide range of monomers and reaction conditions,
including the robustness of both techniques.
The use of RDRP as a polymerization technique stems from the fact that
it is fundamentally more versatile and powerful compared to conventional
radial polymerization: (1) the ability to synthesize polymer chains with
predetermined molar masses and narrow molar mass distribution
(dispersity, Ð ); (2) the ability to continue polymerization by
adding more monomers owing to the better stability of the dormant
propagating chain; (3) high-chain end fidelity and ease of attaching
functional groups to polymer chain ends; (4) lower probability of side
reactions such as termination reactions occurring; and (5) ease of
fabricating various polymer shapes and
morphologies.[61] All these benefits of using RDRP
over other types of polymerization led many researchers to search for
and invent unique ways to synthesize polymers with AIE properties in a
controlled manner, resulting in a plethora of morphologies discovered
and produced over time.
Given the success and advantages of RDRP, this polymerization technique
is capable of producing a multitude of polymer morphologies such as
single block and block co-polymers spanning a huge range of topological
morphologies such as homopolymer, di-/tri-/multi-block, star-shaped,
sequence-defined, (hyper)branched, dendritic, graft and brush type,
cyclic (ring), network, single-chain nanoparticles
(NPs),[62] bearing unique properties such as
stimuli responsiveness to mechanical stress,[63,
64] temperature and pH changes,[65, 66] and
light irradiation.[67] Such polymers found
potential applications in the field of therapeutics such as
nanomedicine, nanotechnology and materials
science,[68-73] energy production and efficiency
optimisation, and electronics.[74-76] Ever since
the discovery of the AIE phenomenon in 2001, there are over 10,000
publications till date detailing the different aspects of AIE
(Figure 3 ). Specifically, to AIE polymers synthesized using the
RDRP techniques aforementioned, there exists more than 140 publications
and the number is projected to increase given the multitude of benefits
in using RDRP to synthesize AIE polymers. In this review, we describe
firstly a brief introduction to RDRP, the AIE phenomenon, and AIE
polymers. We will then elaborate on the design of AIE monomers and
provide a list of some polymers synthesized via RDRP with the
incorporation of AIE moieties. Next, we explore how RAFT can be used to
design AIE polymers, including the design, the different types of
process and polymerization mechanisms involved. Afterwards, similar to
RAFT polymerization, we explore ATRP polymerization. Then, some
elaborations on the other types of RDRP for AIE polymers, and potential
applications of these AIE polymers. Finally, we present a summary and
our perspective on the current progress of the AIE-active polymers.