Novel side-chain alternative copolymer combined FRET and DRET with large pseudo-Stokes shift and polarity-sensitive fluorescence behavior

Article abstract of DOI:10.1039/c9tc03421h

A novel alternative copolymer incorporating tetraphenylethene (TPE) and naphthalimide (NI) into styrene and maleic anhydride as side chains was designed and synthesized. Attributed to the TPE units acting both as emissive donor in the aggregation state an

Full text of DOI:10.1039/c9tc03421h

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DOI: 10.1039/C9TC03421H
Novel Side-chain Alternative Copolymer Combined FRET and DRET
with Large Pseudo-Stokes Shift and Polarity-sensitive Fluorescent
Behavior
Received 00th January 20xx,
Accepted 00th January 20xx
Yan Yu, Bohao Yang, Yongjie Yuan * and Hailiang Zhang *
DOI: 10.1039/x0xx00000x
Abstract: a novel alternative copolymer incorporating tetraphenylethene (TPE) and naphthalimide (NI) into the styrene and
maleic anhydride as side chains was designed and synthesized. Attributed to the TPE units acting both as emissive donor in
aggregation state and dark donor in solution, this copolymer possessed fluorescence resonance energy transfer (FRET) and
dark resonance energy transfer (DRET) processes from TPE to NI,so it kept emissive whether in solution or aggregation
state, and fluorescence intensity of NI was increased obviously than it was excited directly without energy transfer.
Additionally, the alternative copolymer displayed impressive pseudo-Stokes shift as large as 215 nm in aggregation state,
which was larger than all other reported systems combined FRET and DRET. Lastly, we found that this alternative copolymer
exhibited interesting polarity-sensitive fluorescent behaviors in different polarity environment, and these behaviors were
considered to be the result of the combined effects of polarity-sensitivity, energy transfer and solubility.
aggregation-caused quenching (ACQ) effect of fluorescent
molecules caused by π-π stack.[17] Typical AIE-gens, such as
Introduction
Fluorescent molecules were applied successfully in various
fields, such as chemosensor and cell-imaging for their unique
advantages like high sensitivity and easy visibility.[1-5] However,
most of them exhibited relatively small Stokes shifts inducing
serious self-quenching which limited the development of
fluorescent materials.[6-7] Over the decades, fluorescence
resonance energy transfer (FRET) strategy provided a useful
solution for the small Stokes shifts problem.[7-8] An efficient
FRET process was based on a close enough distance(1-10 nm)
between donor and acceptor , and a large overlap between the
emission spectrum of donor and the absorption spectrum of
acceptor, so that FRET-based materials can produce large
pseudo-Stokes shifts by constructing donor-acceptor pairs.[9-11]
Whereas most of reported FRET systems employed emissive
chromophore as donor, may lead to fluorescence leaking or
background influence which was harmful for the applications of
FRET materials.[12-13] Recently, Chang and co-workers developed
a novel dark resonance energy transfer (DRET) strategy using
low quantum yield fluorophores as dark donor to construct
resonance energy transfer process.[14] It was successful to avoid
fluorescence leaking or background influence induced by
emissive donor. Moreover, DRET donor-acceptor pairs had
large pseudo-Stokes shifts and showed high efficiency of energy
transfer. [15-16] The mechanism of DRET was believed as that the
quenching rate of dark donor through intramolecular-motion
channel was slower quantitatively than the rate of resonance
energy transfer between donor and acceptor.[14]
tetraphenylethene (TPE) and diphenylacrylonitrile, have a
unique feature that it was emissive in aggregation state but was
dark in solution. This unique feature is attributed to the
restriction of the intramolecular motion or other quenching
channels.[18-21] Taking advantage of this uniqueness, some
scientists employed AIE-gens as emissive donors or acceptors to
construct FRET materials which can be applied in aggregation
state.[22-24] Lately, as dark donors, AIE-gens were combined with
other fluorescent acceptors. Serdar Atılgan and co-workers
developed a significate design of molecular that TPE, an AIE-
donor, and BODIPY, a traditional fluorescent acceptor, were
linked by a long alkyl chain.[25] It was the first work to
introducing AIE-gen as a dark donor for constructing DRET pair.
Interestingly, since TPE acted as emissive donor in aggregation
state and dark donor in solution, this donor-acceptor pair which
showed both FRET and DRET features. Similarly, Yang’s group
reported a series of molecular，in which diphenylacrylonitrile
were connected with BODIPY.[26] And the author concluded that
the DRET and FRET processes probably were a normal
phenomenon for AIE-BODIPY pairs. It was worth to notice that
both above-mentioned two works reported large pseudo-
Stokes shifts over 190 nm.
Compared with small-molecule materials, the polymer
materials had excellent advantages such as good processing
performance, film-forming ability and thermal stability, so more
and more FRET pairs were introduced into copolymer materials.
[27-36]
However, FRET side-chain alternative copolymers rarely
Aggregation-induced emission (AIE) effect reported by Tang’s
group provided an excellent strategy to overcome the
were reported to date. Considering the dependency of FRET
efficiency on the distance between donor and acceptor,
theoretically, it would be a good strategy to increase energy-
transfer efficiency of FRET polymers by constructing side-chain
alternative copolymer, in which donor and acceptor were
introduced as side chain next to each other. In this work, we
designed and synthesized a fluorescent copolymer P(TPE-alt-NI)
combining DRET and FRET, in which tetraphenylethene (TPE)
Key Laboratory of Polymeric Materials and Application Technology of Hunan
Province, Key Laboratory of Advanced Functional Polymer Materials of Colleges
and Universities of Hunan Province, College of Chemistry, Xiangtan University,
Xiangtan 411105, Hunan Province, China. E-mail: hailiangzhang@xtu.edu.cn
† Electronic supplementary information (ESI) available: Experimental procedures,
characterization of new compounds, and spectra. See DOI: 10.1039/x0xx00000x
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and naphthalimide (NI) were incorporated into the styrene and
maleic anhydride as side chains (see Figure 1). This unique
construction was designed for developing a fluorescent
polymer materials with effective energy transfer whether in
solution or aggregation state, and producing a large pseudo-
Stokes shift to avoid self-quench and background influence. And
this copolymer was endowed with polarity-sensitivity by the NI
chromophores, so that its fluorescent behavior was changed
with different solution polarity. Moreover, to investigate their
own inherent fluorescence property of TPE or NI unit in polymer
and to compare with the bi-chromophore copolymer, two
copolymers P-TPE and P-NI which only contained TPE or NI
fluorophore were synthesized too. We believe that the novel
copolymer combining DRET and FRET with large pseudo-Stokes
shift and polarity-sensitivity could provide a new strategy for
the design of fluorescent polymer materials.
View Article Online
DOI: 10.1039/CO9TC03421H
O
N
O
O
N
O
O
O
N
N
Styrene
M-TPE
N-ethyl maleicimide
M-NI
n
n
n
O
O
O
O
O
O
N
N
N
O
O
O
N
O
O
N
O
N
N
P-NI
P-TPE
P(TPE-alt-NI)
Scheme 1. The chemical structures of the monomers and copolymers and polymerization
procedures
in Table 1 and the curves were depicted in Figure 3. As bi-
chromophore copolymer, the Td and Tg of P(TPE-alt-NI) were at
medium value between the corresponding value of P-TPE and
P-NI as expected. And all of them showed good thermal
stability.
UV-Vis absorption properties
Figure 1.Schematic illustration of the resonance energy transfer process and polarity-
sensitivity of P(TPE-alt-NI).
The photophysical properties of M-TPE, M-NI, P-TPE, P-NI and
P(TPE-alt-NI) in THF solution were investigated in detail, and the
concentrations of each chromophore were set as 1×10-5 M.
These results were summarized in Table 2. And the UV-Vis
absorption spectra of M-TPE, M-NI and their alternative
copolymer P(TPE-alt-NI) in THF were shown in Figure 2. The
absorb band with a maximum at 317 nm was characteristic of
the TPE chromophore, whereas absorb band with a maximum
at 410 nm was corresponded to the NI chromophore, and it was
observed that the two major absorb band of P(TPE-alt-NI) were
similar to its two monomers at 311 nm and 417 nm respectively.
This result reflected that the TPE unit and NI unit were
introduced to the alternative copolymer successfully. And as
shown in Figure S3, the absorption spectra of P-TPE and P-NI in
THF were similar with respective fluorescent monomers too.
Results and Discussion
Molecular characteristics of the monomers and copolymers
As shown in Scheme 1 ， the chemical structures of the
monomers and copolymers and corresponding polymerization
were depicted. The detailed synthetic procedures of the
monomers were summarized in supporting information. The
molecular structures of these monomers and copolymers were
confirmed by 1H NMR spectroscopy (see Figure S1 and S2). The
characteristic resonance peaks located at 5.2–6.6 ppm
corresponding to the vinyl groups of these monomers had
completely disappeared after polymerization, and other
chemical shifts of each proton agreed well with their expected
structures, indicating that these monomers were successfully
introduced to the resulting copolymers P-TPE, P-NI and P(TPE-
alt-NI). The conversion rates of monomers for polymerization,
the number-average molecular weights (Mn) and polydispersity
index (PDI) of copolymers characterized by gel permeation
chromatography (GPC) were summarized in Table 1.
Thermal properties of the copolymers
The decomposition temperature (Td) and the glass transition
temperature (Tg) of the copolymers were characterized by
thermogravimetric analysis (TGA) and differential scanning Figure 2. UV-Vis absorption spectra of M-TPE, M-NI and P(TPE-alt-NI) in THF solution
(1×10-5 M).
calorimetry (DSC) respectively. These results were summarized
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Table 1. Molecular characteristics, thermal properties of copolymers, and conversion rates of monomers.
Mn （×104）
6.47
PDI
3.82
2.21
2.50
Tg (℃)
164
216
Td (℃)
384
429
Conversion rates (%)
P-TPE
P-NI
P(TPE-alt-NI)
89
91
83
4.86
5.75
180
402
Figure 3. (a) TGA curves of copolymers at a heating rate of 20℃/min and (b) DSC curves of the copolymers during the first cooling and second heating scans at a rate
of 10℃/min.
AIE & ACQ effect of P-TPE and P-NI
To investigate the DRET process from the TPE units to NI units
in P(TPE-alt-NI), the fluorescence spectra of the P-TPE, P-NI
and P(TPE-alt-NI) excited at 320 nm and 430 nm in THF
solution were measured respectively. As shown in Figure 5a,
the fluorescence spectra of the P-TPE, P-NI and P(TPE-alt-NI)
excited at 320 nm in THF were compared. Because of the AIE
effect of TPE chromophore, the P-TPE showed no emission
when it was excited at 320 nm in solution. And because the NI
chromophore could not be excited efficiently at 320 nm, the
P-NI only demonstrated a weak emission. However, when the
alternative copolymer P(TPE-alt-NI) was excited at the
absorption wavelength of TPE (320 nm), very intensive
fluorescence corresponding to NI fluorophore at 517 nm was
observed. This result indicated distinctly there was a DRET
process, in which TPE unit acted as dark donor and NI unit
acted as an emissive acceptor, and it produced an impressive
pseudo-Stokes shift as 206 nm which was larger than all other
reported DRET systems. As depicted in Figure 5a, the
fluorescence intensity of P(TPE-alt-NI) excited at 320 nm was
higher obviously than P-NI excited at 430 nm, even though
their quantum yields were similar (see Table 2). It may be
attributed to the overlap between the emission spectrum of
TPE and the absorption spectrum of NI. This overlap made the
DRET process more effective than a single wavelength
excitation of acceptor. And the alternative construction of this
copolymer induced an easy energy transfer process, in which
every TPE unit as donor was close enough to every near-by NI
units as acceptor. Moreover, NI units were separated to each
other by TPE units, so the concentration-quenching of NI
fluorophore was reduced. Based on these unique features
including non-emissive donor, strong fluorescence intensity
and large pseudo-Stokes shift, the novel alternative copolymer
may provide a new strategy for the development of
fluorescent materials without fluorescence leaking,
background influence and self-quenching.
The fluorescence properties of copolymers which only
contained single chromophore were measured respectively by
changing fraction of water (fw) in THF solution. The results
were depicted in Figure S4. When excited at 320 nm, P-TPE
showed no emission in pure THF. However, with the increasing
fw from 10 to 90%, the emission peak at ~470 nm
corresponding to the TPE units was observed, and the
fluorescence intensity was enhanced gradually. This result
clearly comfirmed the AIE effect of P-TPE. Whereas, when
excited at 430 nm, P-NI exposed an emission peak at ~530 nm
in pure THF, but when the fw was increased, its fluorescence
was quenched obviously owing to the ACQ effect of NI
chromophores in P-NI.
Resonance energy transfer processes
Considering that an efficient overlap between the emission
spectrum of donor and the absorption spectrum of acceptor
was the necessary condition for resonance energy transfer, we
compared the emission spectrum of P-TPE with the absorption
spectrum of P-NI. As shown in Figure 4, the overlap region
from 388 nm to 590 nm meant it was suitable to employ the
TPE chromophore as donor and the NI chromophore as
acceptor in a resonance energy transfer process.
Figure 4. Overlap between the emission spectrum of P-TPE and the absorption
spectrum of P-NI.
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Figure 5. (a) Emission spectra of P(TPE-alt-NI) and P-NI excited at 320 nm and 430 nm respectively, and P-TPE excited at 320 nm, in 1×10-5 M THF solution. (b) Emission spectra of
P(TPE-alt-NI) and P-NI excited at 320 nm and 430 nm respectively, and P-TPE excited at 320 nm, in 1×10-5 M water/THF(9:1).
Table 2. Photophysical properties of monomers and copolymers as 1×10-5 M in THF solution.
λabs (nm)a
λem (nm)b
ФF (%)c
Stokes shift (nm)d
M-TPE
M-NI
P-TPE
317
410
318
--
514
--
--
--
--
--
104
--
P-NI
P(TPE-alt-NI)
424
311, 417
530
517
12.3
11.2
96
206
a. Absorption peak value of M-TPE, M-NI, and P(TPE-alt-NI) measured in THF solution. b. Emission maximum wavelength measured in THF solution. M-TPE, P-TPE and
P(TPE-alt-NI) were excited at 320 nm, and M-NI and P-NI was excited at 430 nm. c. Fluorescence absolute quantum yields measured in THF. λex = 430 nm for P-NI and
320 nm for P(TPE-alt-NI). d. Stokes shift of M-NI or P-NI and pseudo-Stokes shift of P(TPE-alt-NI).
In addition, upon 430 nm UV, P(TPE-alt-NI) in solution disappearance of emission of TPE donor with increased
demonstrated a stronger emission than P-NI too. In this case, emission of NI acceptor indicated nearly 100% efficiency of FRET
TPE chromophore in P(TPE-alt-NI) did not be excited while only in the alternative copolymer. Additionally, similar as in solution,
NI chromophore did, so energy transfer did not occur. It meant the fluorescence intensity of aggregative P(TPE-alt-NI) based on
that the emission caused by NI chromophore excited directly FRET was higher than which excited directly at 430 nm, and It
instead of energy transfer from TPE. This phenomenon could be may be attributed to alternative construction of the copolymer
explained as the before-mentioned “separated effect” of as we before-mentioned in this paper. And the red-shifted
alternative construction of this copolymer too.
emission of aggregative NI fluorophores led to a larger pseudo-
Stokes shift of P(TPE-alt-NI) as 215 nm.
Considering the AIE-active of TPE, it is feasible to act as an
emissive donor of FRET process in aggregation state. To study Moreover, when aggregative and dissolved P(TPE-alt-NI)
the fluorescent behavior of the alternative copolymer P(TPE-alt- excited both at 320 nm，their quantum yields were similar (see
NI) in aggregation state, the photophysical properties P-TPE， Table 2 and Table 3), and as shown in Figure 5a and 5b, their
P-NI and P(TPE-alt-NI) in water/THF (9:1) were investigated in fluorescence intensity were approximate and both were higher
detail, and these results were summarized in Table 3. As shown obviously than aggregative P-NI excited at 430 nm. The emission
in Figure 5b, the AIE-active P-TPE excited at 320 nm showed a of aggregative P(TPE-alt-NI) was a result of comprehensive
strong 472 nm emission in aggregation state. And P-NI, upon effect including “separated effect” and FRET process. As the
430 nm, only exhibited a red-shifted weak emission as a result precondition, the “separated effect” reduced the π-π stacking
of the close stacking of NI fluorophores. As for P(TPE-alt-NI), of NI chromophores to avoid ACQ effect, and then the FRET
when excited upon the TPE’s absorption wavelength (320 nm), process could have an emissive NI acceptor so that P(TPE-alt-NI)
this copolymer showed a 528 nm emission corresponding to NI exhibited emission in the aggregative polymer. When λex = 430
units instead of TPE units. It was indicated clearly that an nm, the ФF of P-NI in aggregation state as 7.34% was lower than
efficient FRET process occurred between emissive TPE units and the ФF of P(TPE-alt-NI) in aggregation state as 15.3%, so this
emissive NI units in the aggregative P(TPE-alt-NI). According to result indicated that the “separated effect” avoided the ACQ
the amount of quenching of the donor chromophores, the effect of NI chromophores successfully.
efficiency of FRET could be calculated.[38] The whole
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Table 3. Photophysical properties of 1×10-5 M P-TPE, P-NI and P(TPE-alt-NI) in water/THF (9:1).
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DOI: 10.1039/C9TC03421H
λab (nm)a
λem (nm)b
ФF (%)c
E (%)d
Stokes shifts (nm)e
P-TPE
P-NI
P(TPE-alt-NI)
318
431
313,421
471
540
528
9.87
7.34
10.2
--
--
≈100
153
109
215
a. Absorption peak value measured in water/THF (9:1). b. Emission maximum measured in water/THF (9:1). P-TPE and P(TPE-alt-NI) were excited at 320 nm, and P-NI
was excited at 430 nm. c. Fluorescence absolute quantum yields measured in water/THF (9:1). λex = 430 nm for P-NI and 320 nm for P-TPE, and P(TPE-alt-NI). d. Efficiency
of fluorescence resonance energy transfer. e. Stokes shifts for P-TPE and P-NI, and pseudo-Stokes shift for P(TPE-alt-NI).
Figure 6. Emission spectra of 1×10-5 M P(TPE-alt-NI) in different ratio water/THF excited at 320 nm (a) and 430 nm (c). Variation of PL intensity and wavelength maximum of 1×10-5
M P(TPE-alt-NI) in different ratio water/THF excited at 320 nm (b) and 430 nm (d).
Polarity-sensitive fluorescent behavior
(Figure S5a and S5c). The quenched fluorescence was caused by
aggregative NI chromophores in P-NI. However, comparing the
Figure S5b and S5d, it was worth noticing that the decrease of
fluorescence intensity with increasing fn was slower and less
than which with increasing fw, and the red-shift trend of
maximum emission wavelength was dramatic when water was
added into THF rather than the gradual process with increasing
fn. These phenomenon were attributed to the polarity-sensitive
NI chromophores in P-NI. As we know, water is a polar solvent
whereas n-heptane is non-polar, with the increasing fw, the
whole polarity of mixed solvent became stronger, in contrast,
the increasing fn made the polarity of mix solvent decrease. In a
word, different solvent polarity induced the changed emission
from the polarity-sensitive NI chromophores. As a result, P-NI
showed different fluorescent behaviors in water/THF and n-
heptane/THF.
4-Amino-1, 8-naphthalimide derivatives were polarity-sensitive
attributed to the intramolecular charge transfer (ICT) between
the electron-donating 4-amino substituent and the electron-
[39-41]
withdrawing imide ring.
With increasing solvent polarity,
the emission of them would became weak and red-shifted.
Pavel Kucheryavy and co-workers investigated the effect of
solvent polarity on the energy of S1 State in 4-(N, N-
[39]
dimethylaminonaphthalimide (Me2N-NI).
As shown in
Scheme S2, they found that the 4-substituent of Me2N-NI in the
ground-state was at a small angle relative to the naphthalene
ring, whereas in the optimized excited-state geometry, the 4-
substituent was at 90° angle, and this twisted excited state was
unstable in nonpolar solvent. When polarity of solvent was
increased, the twisted excited state of Me2N-NI became stable
so that the energy of S1 State of Me2N-NI became lower, which
resulted a weak and red-shifted emission.
Then we investigated fluorescent behavior of P(TPE-alt-NI)
excited at 320 nm with different fw in THF solution, and these
results were depicted in Figure 6a and 6b. In the initial stages of
adding water, the fluorescence intensity of P(TPE-alt-NI) excited
showed a dramatic decline, however, after the fw reached 10%,
the emission became enhanced gradually, and when the fw
reached 90%, its intensity even returned to the initial level as in
pure THF solution. This interesting phenomenon was
considered to be the result of the combined effects of polarity-
In previous measurement of this paper, the ACQ effect of P-NI
had been verified by investigating emission intensity of P-NI in
different ratio of water/THF (see Figure S4b). To study the effect
of solvent polarity on P-NI, further fluorescence measurement
of P-NI in different fraction of water or n-heptane in THF (fw or
fn) was investigated in detail. The fluorescence intensity both
decreased gradually as the addition of poor solvents as expect
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Figure 7. Emission spectra of 1×10-5 M P(TPE-alt-NI) in different ratio n-heptane/THF excited at 320 nm (a) and 430 nm(c). Variation of PL intensity and wavelength maximum of
1×10-5 M P(TPE-alt-NI) in different ratio n-heptane/THF excited at 320 nm (b) and 430 nm (d).
sensitivity, energy transfer and solubility. At the beginning,
P(TPE-alt-NI) was excited by 320 nm in pure THF solution, in
which happened a DRET process between TPE units as dark
donor and NI units as emissive acceptor. When water just was
added in the THF solution of P(TPE-alt-NI), the dramatically
increased polarity of solution induced the decreased
fluorescence of polarity-sensitive NI chromophores. But after
the fw reached 10%, the effect of solvent polarity against NI
chromophores did not increase anymore. Meanwhile, the
added water as a poor solvent made P(TPE-alt-NI) became more
and more aggregative, as a result, the distance between TPE
donor and NI acceptor became closer so that the emission was
enhanced by an easier energy transfer. On the other hand, the
closer aggregation of the copolymer shielded the effect of
solvent polarity. Lastly, because NI units were separated to each
other by TPE units, so that the ACQ effect of NI fluorophore was
avoided partly. Until the fw reached 90%, these above reasons
leaded to the originally quenched fluorescence gradually
returned to the initial level as in pure THF. It was worth noticing
that as the degree of aggregation increased, the AIE-active TPE
chromophore turned from a dark donor to an emissive donor,
so the DRET in P(TPE-alt-NI) switched to the FRET gradually
throughout the whole process of addition water.
Compared with addition of water ，it was different that the
fluorescence behavior of P(TPE-alt-NI) excited at 320 nm or 430
nm in different ratio of n-heptane/THF, but still was a result of
the combined effects of polarity-sensitivity, energy transfer and
solubility too. As shown in Figure 7a and 7b, initially, P(TPE-alt-
NI) excited by 320 nm in pure THF solution, in which happened
a DRET process between TPE and NI units, so this solution
exposed 517 nm emission corresponding to NI chromophores.
When fn was increased from 0 to 10%, the decreased solvent
polarity made the emission of NI chromophores enhanced
gradually. Then fn was increased from 10 to 20%, the upward
trend of fluorescence intensity became flat, this because the
effect of polarity against NI did not increase anymore. After the
fn reached 20%, the distance between TPE donor and NI
acceptor in aggregated P(TPE-alt-NI) became closer, so the
emission was enhanced continuously by the easier energy
transfer. But when fn was increased 60 to 90%, the fluorescence
intensity dropped to the initial level as in pure THF, because the
very compact aggregation of NI units caused emission
quenching. For comparison, fluorescent behavior of P(TPE-alt-
NI) excited at 430 nm in different fn was investigated. As shown
in Figure 7c and 7d, the fluorescence intensity increased
gradually as the decreased solvent polarity when n-heptane just
added. But after the fn reached 10%, the fluorescence intensity
declined continuously until the fn reached 90%. This result
indicted that without energy transfer, the emission of NI
chromophores in P(TPE-alt-NI) excited at 430 nm would be
easier to quenched by aggregation.
For comparison, fluorescent behavior of P(TPE-alt-NI) with
different fw in THF excited at 430 nm was investigated. As shown
in Figure 6c and 6d, the fluorescence intensity first declined and
then increased gradually as the water added, but until the end
it never returned to the initial level as in pure THF. And in each
water fraction, the fluorescence intensity of P(TPE-alt-NI)
excited at 430 nm was weaker than which upon 320 nm. This
phenomenon was a result of the absence of FRET or DRET when
P(TPE-alt-NI) was excited at 430 nm, and indicated that the
acceptor’s emission from resonance energy transfer process
was stronger than direct excitation upon acceptor’s absorption
wavelength.
Furthermore, as shown in Figure 7b and 7d, the curve of red-
shifted emission wavelength of P(TPE-alt-NI) with increasing fn
was different with which with increasing fn. We believe that the
red-shifted emission of P(TPE-alt-NI) with increasing fw and fn
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was a combined result of the solubility and polarity of solvent. combined effects of polarity-sensitivity, energy VtierwanAsrtficelerOanlninde
As poor solvent, both of increasing water and n-heptane solubility. We believe that this work woDuOldI: b10e.10h3e9l/pCf9uTlCf0o3r4t2h1He
contents induced a more aggregative polymer coil ，in which study of resonance energy transfer, and provide a new strategy
the π-π stacking of NI chromophores produced a red-shifted for the development of polarity-sensitive fluorescent polymer
emission of polymer. Of course, the effect of polarity of solvent materials.
also played an important role, especially in the initial stages of
process. As shown in Figure 6, when water just was added in the
THF solution of P(TPE-alt-NI), the polarity of solvent was
Experimental section
increased dramatically, and at the same time, the polymer coil
became aggregative as the added poor solvent, so the two
reasons together induced a large degree of red-shifted
emission. After the fw reached 20%, the polymer became
condense enough so that the effect of solvent polarity was
shielded and the π-π stacking of NI chromophores reached the
limit, as a result, the emission wavelength was no longer red-
shifted basically. As for the case of increasing fn (see Figure 7b
and 7d), the decreased polarity of solvent was no good for red-
shift of emission, but at the same time ，n-heptane as poor
solvent made polymer coil became more aggregative, so the
effect of solvent polarity was shielded gradually and the π-π
stacking of NI chromophores induced a continued red-shifted
emission of polymer (the process of aggregation with increasing
fn may be slower than the case with increasing fw for different
anti-solubility of the two poor solvents). In a word, solubility
and polarity of solvent induced together the red-shifted
wavelength of emission, in which polarity mainly worked in the
initial stages whereas solubility led the whole red-shift process.
Materials
N-Boc-Ethylenediamine (96%), trifluoroacetic acid (99%), 4-
bromine-1,8-naphthal anhydride (98%), maleic anhydride
(99%), 4-hydroxybenzophenone (99%), benzophenone (99%),
zinc power (99%), titanium tetrachloride (99%), and 1-
(chloromethyl)-4-vinylbenzene (99%) were purchased from
Energy Chemical Co. and were used directly. Tetrahydrofuran
(AR, Kermel Co., 99%) was refluxed over metal sodium and
distilled out before use. Azodiisobutyronitrile (AIBN, AR, 95%)
was purchased from Xilong Chemical Co., and further purified
by recrystallization from ethanol. N, N-dimethylformamide
(DMF, AR, 99%), n-heptane (AR, 99%) and Acetone (AR, 99%)
were purchased from Tianjin Fuyu Fine Chemical Co. And
copper (II) sulfate pentahydrate (AR, Guangdong Guanghua Sci-
Tech Co., 99%), acetic anhydride (AR, Kelong Co., 99%) were
used directly without further purification.
Instrumentation and measurement
Number-average molecular weights (Mn) and polydispersity
index (PDI) of the copolymers were measured by a gel
permeation chromatography (GPC, WATERS1515) instrument
calibrated with standard polystyrenes. THF was used as eluent
Lastly, an interesting phenomenon was observed that the
fluorescence intensity of P(TPE-alt-NI) in 90% water/THF and
90% n-heptane/THF were similar (see Figure S6). We implied
that when the fraction of poor solvent reached 90%, the
aggregate of P(TPE-alt-NI) had already been compact both in
water/THF and n-heptane/THF solution, so the influence of
solvent polarity was weak, and the poor solubility became the
dominant influent factor of fluorescence intensity. When P(TPE-
alt-NI) in 90% fw or 90% fn were excited, their similar compact
degree of aggregated P(TPE-alt-NI) induced similar fluorescence
intensity.
and the flow rate was 1.0 mL min-1 at 35℃. The H nuclear
1
magnetic resonance (NMR) spectra of monomers and
copolymers were measured by Bruker ARX400 MHz
spectrometer. The decomposition temperature (Td, defined as
temperature that weight percentage loss 5 wt%) of polymers
were measured by thermogravimetric analysis (TGA) Q50 with
a scanning rate of 20℃ min-1 under N2 atmosphere. The glass
transition temperature (Tg) of copolymers was measured by a
TA Q10 differential scanning calorimetry (DSC) instrument
operating with a scanning rate of 10℃ min-1 on heating and
cooling process. The transition temperatures and enthalpies
were calibrated using standard materials (indium and zinc). The
absorption spectra of monomers and copolymers in different
ratio of THF/water or THF/n-heptane were taken on an Agilent
spectrum Carry 100 spectrophotometer. The fluorescence
spectra of monomers and copolymers in different ratio of
water/THF or n-heptane/THF were taken on a PTIQm400
luminescence spectrometer. The fluorescence absolute values
were obtained using an Edinburgh FLS920 Fluorescence
Spectrometer with integrating sphere. In all measurements of
photophysical property, the concentrations of each
chromophore were set as 1×10-5 M.
Conclusion
A novel alternative copolymer incorporating tetraphenylethene
(TPE) and naphthalimide (NI) into the styrene and maleic
anhydride as side chains was designed and synthesized.
Systematic optical investigation showed that this copolymer
kept emissive whether in solution or aggregation state
associated with fluorescence resonance energy transfer (FRET)
and dark resonance energy transfer (DRET) processes from TPE
to NI, because the TPE unit can act as emissive donor in
aggregation state and dark donor in solution, and the
alternative structure increased the efficiency of energy transfer
and avoided the ACQ of NI. Particularly, this copolymer
displayed an impressive pseudo-Stokes shift as large as 215 nm
in aggregation state. Moreover, this copolymer exhibited
Polymerization
interesting polarity-sensitive fluorescent behaviors in different Equimolar amount of two monomers (M-TPE and N-ethyl
ratio water/THF and n-heptane/THF solvent, attributed to the maleicimide for P-TPE, M-NI and styrene for P-NI, M-TPE and M-
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17 J. Luo, Z. Xie, J. W. Lam, L. Cheng, H. Chen, C. QiVuie,wHA.rStic.leKOwnolinke,
X. Zhan, Y. Liu, D. Zhu, Chem. Commun. 2001, 1740-1741.
18 A. Qin, J. W. Lam, B. Z. Tang, Prog. PolDyOmI.: 1S0c.i1.02390/1C29,T3C70,3142812H-
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4562.
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Kwok, Appl. Phys. Lett. 2007, 91, 011111.
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NI for P(TPE-alt-NI), AIBN (0.01 molar equivalent of monomers),
chlorobenzene (2.3 mass equivalent of monomers) were placed
in a dry glass tube, then the tube was degassed by several
freeze–pump–thaw cycles and sealed under vacuum.
Polymerization was carried out at 75 ℃ for 4 h. After
terminated by putting the tube into an ice–water mixture, the
polymer solutions were diluted with THF and precipitated into
methanol to obtain the target polymers of P-TPE, P-NI and
P(TPE-alt-NI).
23 F. Würthner, C. Thalacker, S. Diele, C. Tschierske, Chem. Eur.
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24 S. Wang, J.-H. Ye, Z. Han, Z. Fan, C. Wang, C. Mu, W. Zhang, W.
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Conflict of interest
There are no conflicts to declare.
25 E. Şen, K. Meral, S. Atılgan, Chem. Eur. J. 2016, 22, 736-745.
26 L. Lin, X. Lin, H. Guo, F. Yang, Org. Biomol. Chem. 2017, 15,
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27 R. Abbel, C. Grenier, M. J. Pouderoijen, J. W. Stouwdam, P. E.
Leclere, R. P. Sijbesma, E. Meijer, A. P. Schenning, J. Am. Chem.
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28 Z. Wang, C. Wang, Y. Fang, H. Yuan, Y. Quan, Y. Cheng, Polym.
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29 P. Zhang, J. Chen, F. Huang, Z. Zeng, J. Hu, P. Yi, F. Zeng, S. Wu,
Polym. Chem. 2013, 4, 2325-2332.
30 M. Yu, P. Zhang, B. P. Krishnan, H. Wang, Y. Gao, S. Chen, R.
Zeng, J. Cui, J. Chen, Adv. Funct. Mater. 2018, 28, 1804759.
31 C.-T. Lo, Y. Abiko, J. Kosai, Y. Watanabe, K. Nakabayashi, H.
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Acknowledgements
This work was supported by National Natural Science
Foundation of China (No. 51573152, 51373148). Open Fund
Project of Innovation Platform in Colleges and Universities of
Hunan Province (16K085).
Keywords: aggregation-induced emission
copolymers • dark resonance energy transfer (DRET) • FRET •
solvent effects
•
alternative
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