Abstract
Nonconventional luminescent materials have been rising stars in organic
luminophores due to their intrinsic characteristics, including
water-solubility, biocompatibility and environmental friendliness, and
have shown potential applications in diverse fields. As an indispensable
branch of nonconventional luminescent materials, polysiloxanes which
consist of electron-rich auxochromic groups, have exhibited outstanding
photophysical properties due to the unique silicon atom. The flexible
Si-O bonds benefit the aggregation, and the empty 3d orbitals of Si atom
can generate coordination bonds like N → Si and O → Si, altering the
electron delocalization of the material and improving the luminescent
purity. Herein, we review the recent progress in luminescent
polysiloxanes with different topologies and discuss the challenges and
perspectives. With an emphasis on the driving force for the aggregation
and the mechanism of tuned emissions, the role of Si atoms played in the
nonconventional luminophores is highlighted. This review may provide new
insights into the design of nonconventional luminescent materials and
expand their further applications in sensing, biomedicine, lighting
devices, etc.
Introduction
Aggregation-induced emission (AIE) photoluminescent (PL) materials are
of vital importance in organic luminescent materials due to their
antithetic emission behaviors to aggregation-caused quenching (ACQ)
luminophores and wide applications in organic light-emitting diode
(OLED),[1] bioassay,[2,3]bioimaging,[4,5] circularly polarized
luminescence,[6,7] liquid
crystal,[8,9]anti-couterfeiting[10-12] and
therapy.[13,14] Coined by Tang and coworkers in
2001, AIE is a unique phenomenon that nonemissive luminophores are
induced to emit upon aggregation, which settled the ACQ puzzle and
experiences booming development since then.[15,16]
Traditional AIE luminophores generally contains π conjugates, for
example tetraphenylethene (TPE),[17]hexaphenylsilole (HPS),[18]10,10′,11,11′-tetrahydro-5,5′-bidibenzo[a,d][7]-annulenylidene
(THBA),[19]etc. They are highly emissive in aggregated state owing to the activated
radiative transition caused by the restriction of intramolecular motions
(RIM).[20.21]
As an indispensable branch of AIE luminophores, nonconventional
luminophores, such as poly(amido amine)
(PAMAM),[22] polyethylenimine
(PEI),[23] poly(amino esters)
(PAE),[24] polyureas,[25]polysiloxanes[26] and nonaromatic small
molecules,[27]are nonaromatic or small aromatic
systems, and have aroused increasing
attention.[28] In the absence of large π
conjugates, these nonconventional AIE luminophores are consisted of
electron-rich auxochromic groups, leading to potential biocompatibility,
water-solubility and easy preparation. According to clustering-triggered
emission (CTE) mechanism, the emission of the nonconventional AIE
luminophores comes from the electron delocalization generated by the
lone-pare electrons in electron-rich groups.[29]Guided by the CTE mechanism, a growing number of synthetic
nonconventional AIE luminophores are
developed.[30] Among these emerging luminophores,
AIE polymers with intrinsic emissions have attracted great interests.
Silicon is an important abundant element of group Ⅳ with no reported
connate toxicity, which provides application potential in biological and
medical. The first reported organosilica luminescent material dated back
to 1998, when Lianos et al. synthesized a nonconventional sol-gel silica
network.[31] After that, various polysiloxanes
with different topologies have been developed.[32]As an integral part of organic-inorganic hybrid polymers, hyperbranched
polysiloxanes combine the advantage of polysiloxanes and hyperbranched
structures, thereby process wide temperature resistance,
multifunctionality, low viscosity, etc.[32] As for
now, the most popular methods of synthesize hyperbranched polysiloxanes
are hydrosilylation and hydrolysis polycondensation. However, the former
needs expensive catalyst, and the later suffer from poor control of the
hydrolysis process.
In 2015, our group developed a new way to synthesis hyperbranched
polysiloxanes, that is transesterification polycondensation
reaction.[26] Using this method, various
hyperbranched polysiloxanes (abbreviated as HBPSi) that contains Si-O-C
chain segment and employing Si as branching point were synthesized from
various siloxanes, dihydric alcohols or dicarboxylic acids. Different
from traditional polysiloxanes consisted of Si-O-Si chain segment, the
bond angle of Si-O-C chain segment is 120 º, which is less than
conventional Si-O-Si segment (130 º) and greater than C-O-C (110 º) and
C-C-C (109 º) segments, thus renders HBPSis good flexibility and
rigidity simultaneously. The good flexibility of organic chains
contributes to the aggregation of HBPSis, and the rigidity of inorganic
chains suppresses the rotation of chain segment, leading to AIE-active
fluorescence. Due to the unique bond angle of Si-O-C chain segment,
HBPSis exhibit extraordinary photophysical features. For example, most
HBPSis can emit bright monochromatic or even multicolour fluorescence
with high purity, due to the presence of abundant heteroatom-bearing
groups and N → Si coordination bonds.
The unique silicon atom endows
polysiloxanes outstanding photophysical properties. Thus, the
understanding of polysiloxanes, especially the role of Si atoms played
in these luminophores, would benefit in unveiling the emission mechanism
of nonconventional luminophores and expanding their applications. In
this review, we summarize recent progress in luminescent polysiloxanes
with different topologies, including hyperbranched polysiloxanes, linear
polysiloxanes, POSS-based polymers, etc (Scheme 1), with a focus on the
driving force for the aggregation and the mechanism of tuned emissions.
1. Aggregation of polysiloxanes
1.1 Photophysical properties of hyperbranched polysiloxanes
The observation of fluorescence from HBPSi can date back to 2016, when
our group synthesized aliphatic tertiary amine-containing hyperbranched
polysiloxanes (TAHPSis) through the polycondensation reaction of
tetraethoxysilane, triethanolamine, N-methyldiethanolamine or diethylene
glycol.[26,33] Contrary to the generally believed
oxidation mechanism, TAHPSis can emit bright blue fluorescence under 365
nm UV light without further oxidation or acidification, and their
fluorescence intensities were enhanced with increasing concentrations,
showing typical AIE characteristics (Figure 1A). Investigations of
TAHPSis reveal that the aggregation of hydroxyl groups is responsible
for their luminescence.
The results of TAHPSis revealed that
oxidation of aliphatic amines may not be the intrinsic interpretation of
luminescence mechanism, and we infer that the PL behavior of HBPSi
strongly depends on the aggregation of functional groups. With the
appearance of terminal hydroxyl groups, strong inter/intramolecular
hydrogen bonds are formed, leading to aggregated electron-rich clusters.
These aggregated electron-rich clusters could serve as the luminescent
center and generate strong PL under ambient conditions. To further
verify our hypothesis, HBPSis with terminal hydroxyl, amine and epoxide
groups were synthesized and their PL behaviors were systematically
studied (Figure 1B).[34,35] We found that HBPSis
with terminal hydroxyl groups are highly fluorescent, while almost no
fluorescence is observed when the hydroxyl groups are blocked byt -butyl acetoacetate. The connection with t -butyl
acetoacetate disturbed the hydrogen bonding of hydroxyl groups, leading
to limited aggregation of HBPSi and the formation of luminescent center.
Epoxide group could also promote the aggregation of HBPSi and enhances
their luminescence. It works with hydroxyl group synergistically in
HBPSi and make a quantum yield (QY) of 4.61%.[35]
Another critical factor that influences the molecular aggregation is the
steric hindrance. To clarify the effect of steric hindrance, we
synthesized two HBPSis with similar structures. With the same terminal
hydroxyl and amine groups,[34,36] the fluorescence
intensity, lifetime and QY of HBPSi carrying neopentyl glycol moiety is
higher than those of another HBPSi containing 2-methyl-1,3-propanediol
moiety. This could be attributed to the larger steric hindrance of
neopentyl glycol. On the one hand, its larger steric hindrance would
restrict the aggregation of HBPSi, going against the formation of
luminescent center. On the other hand, the larger steric hindrance may
restrict the molecular motion, favoring the fluorescence. With the
hydrogen bonds promoted aggregation of terminal hydroxyl and amine
groups, the larger steric hindrance of neopentyl glycol benefits more
from the restricted molecular motion than the restricted aggregation,
leading to enhanced PL properties.
With the empty 3d orbital of Si atoms, coordination bonds can be formed
between electron-rich atoms and Si atoms. In 2015, Feng et al. reported
the N → Si coordination bonds in Si-assisted
polymers.[37] Similarly, the N → Si coordination
bonds can also promote the aggregation of HBPSi. XPS revealed the
existence of N → Si bonds in HBPSis, and density functional theory (DFT)
further conformed the molecular aggregation enhanced by N → Si
coordination bonds.[38] Besides N atoms,
electron-rich atoms, such as S and O atoms, can form coordination bonds
with empty 3d orbitals of silicon atoms, generating d-d orbital
splitting. The electron in split d orbitals can also absorb UV energy
and generate radiative decay, which is beneficial to the fluorescence of
HBPSis.
Theoretical calculations reveal that the bond angle of Si-O-C is about
120 º, which is between Si-O-Si (130 º) and C-O-C (110 º) (Figure 1C).
As a consequence, the Si-O-C chain segment integrates the good
flexibility of organic chains and the rigidity of inorganic chains,
which further promotes the aggregation of molecules. Our group reported
the long carbon chain integrated HBPSi which used 1,6-hexanediol as the
reactant.[39] As shown in Figure 1D, the carbon
chain in 1,6-hexanediol is more flexible than previously reported
dihydric alcohols and it facilitates the formation of compact aggregates
together with the Si-O-C chain segment. DFT results confirm that more
hydrogen bonds and intra/intermolecular O···O and O···N interactions
contribute to the overlap of electron clouds of electron-rich atoms,
thus leading to the generation of through-space conjugation (TSC). With
the satisfying flexibility and rigidity generated by Si-O-C chain
segment and the long carbon chain in 1,6-hexanediol, the QY of the
synthesized HBPSi reached 17.88%.
The above results reveal that the electron-rich atoms could accelerate
the aggregation of HBPSis, leading to unique AIE emissions. DFT
predictions suggest that not only hydrogen bonds, the interaction of O
and N atoms and N → Si coordination bonds are also be found in the HBPSi
aggregates. These inter/intramolecular interactions drive the
aggregation of electron-rich atoms, making them clustered in close
proximity. The electron clouds of these electron-rich atoms overlap with
each other, generating “clustered chromophores” via TSC. With enriched
energy levels, the energy gaps between HOMO and LUMO orbitals are
reduced, thus benefiting excitation. The abundant hydrogen bonds, O···O
and O···N interactions, and N → Si coordination bonds are also conducive
to rigidify the molecular conformation, thereby favoring radiative
decay. Guided by this idea, more attentions were given to the
exploration of structure-promoted aggregation at the molecular level,
and multicolor HBPSis were synthesized afterward.
1.1.1 Hyperbranched polysiloxanes
with local conjugations
The absence of aromatic structures generated unique advantages of
HBPSis, but also results in relatively low QY and short lifetime, and
the emission bands are mainly centered in the blue region. Inspired by
the classic luminescence theories, carbonyl groups were brought in to
fabricate local conjugated HBPSis. By adjusting the aggregation and
relative positions of carbonyl groups, the QY of HBPSis can be enhanced
to 47.8% and multicolor fluorescence was
observed.[40-42]
One simple way to bring in carbonyl groups is to replace the dihydric
alcohols with dicarboxylic acids. Together with the Si-O bond that
exhibit partial double bond features, local conjugations were formed in
HBPSi containing conjugated O=C-O-Si-C=C segment (marked as
P1).[43] In comparison, HBPSis contain
O=C-O-Si-O-C=O segment (P2) and C-C-O-Si-C=C segment (P3) as reference.
The three HBPSis all exhibit strong AIE characteristics, with a QY of up
to 43.9% for P1. The largest conjugated segment O=C-O-Si-C=C renders P1
the highest QY among reported non-conjugated fluorescent polymers then.
The optimized structural parameters and topology results suggest that P1
molecules are aggregated with TSC due to the strong intermolecular H···O
interactions, O → Si coordination
bonds and overlapping between carbonyl and C=C groups. Thus, as depicted
in Figure 2A, the energy levels are enriched and energy gaps are lowered
with the increasing of molecular numbers, favoring the excitation of
HBPSis. In the meantime, the strong H···O interactions and abundant O →
Si coordination bonds also help to rigidify the molecular conformation,
thereby promoting the radiative decay. DFT prediction also revealed that
the C=O, C=C, and Si-O groups promote the formation of supramolecular
TSC in P1, and coplanar TSC is generated when these groups are located
in local conjugated positions (Figure 2B and 2C). The above factors lead
to the highest QY of P1 jointly.[43]
Recently, disulfide was integrated with O=C-O-Si-C=C segment to further
improve the QY of HBPSi. The electron-rich S atoms favor the formation
of space electronic communication and the dipole moment of the disulfide
bond intensifies the distortion of the chain segment, which contribute
to the molecular aggregation and restricted non-radiative decay (Figure
2D). With the increase in disulfide content, compact aggregates are
formed and a QY of 47.8% is achieved.[40]
Another way to bring in carbonyl groups is to employ carbonyl containing
silanes. The methacryloxypropyltriethoxysilane and 1,3-propanediol were
used to prepare a new HBPSi that contains conjugated C=C-C=O
segment.[41] In comparison, a linear structured
polysiloxane was synthesized. Experiments suggest that the local
conjugated HBPSi has a higher QY than the linear polysiloxane, which are
7.71% and 1.12% respectively. Under varying excitations, the HBPSi can
emit distinct colors (Figure 2F). However, the reference linear
polysiloxane can only emit weak blur fluorescence. DFT calculations
reveal that HBPSis possess more compact aggregation and stronger
oscillator strength than the linear polysiloxane due to the large number
of terminal hydroxyl groups. As shown in Figure 2E, the multiring TSC is
found in the ground state geometry of HBPSi, which leads to different
electron delocalization systems in the supramolecular aggregates. Under
different excitation wavelengths, the long-wavelength emission may be
generated by the big TSC rings consist of the HBPSi main chain, and
short-wavelength emission may be caused by the small rings that exist in
their side chains. This conjecture was concluded as multiring-induced
multicolour emission (MIE), and further supported by other
non-conjugated AIE polymers. Conversely, it is more difficult for the
linear polysiloxane to form strong multiring TSC, thus render it
relatively low QY and weak blue emission.[41]