Brightening up Circularly Polarized Luminescence of Monosubstituted Polyacetylene by Conformation Control: Mechanism, Switching, and Sensing

Article abstract of DOI:10.1002/anie.202108010

The first example of luminescent monosubstituted polyacetylenes (mono-PAs) is presented, based on a contracted cis-cisoid polyene backbone. It has an excellent circularly polarized luminescence (CPL) performance with a high dissymmetric factor (up to the order of 10^?1). The luminescence stems from the helical cis-cisoid PA backbone, which is tightly fixed by the strong intramolecular hydrogen bonds, thereby reversing the energy order of excited states and enabling an emissive energy dissipation. CPL switches are facilely achieved by the solvent and temperature through reversible conformational transition. By taking advantages of fast response and high sensitivity, the thin film of mono-PAs could be used as a CPL-based probe for quantitative detection of trifluoroacetic acid with a wider linear dynamic range than those of photoluminescence and circular dichroism. This work opens a new avenue to develop novel smart CPL materials through modulating conformational transition.

Full text of DOI:10.1002/anie.202108010

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Brightening up Circularly Polarized Luminescence of
Monosubstituted Polyacetylene by Conformation Control:
Mechanism, Switching, and Sensing
Authors: Sheng Wang, Deping Hu, Xiaoyan Guan, Siliang Cai, Ge Shi,
Zhigang Shuai, Jie Zhang, Qian Peng, and Xinhua Wan
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10.1002/anie.202108010
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RESEARCH ARTICLE
Brightening up Circularly Polarized Luminescence of
Monosubstituted Polyacetylene by Conformation Control:
Mechanism, Switching, and Sensing
Sheng Wang†,[a] Deping Hu†,[b] Xiaoyan Guan,[a] Siliang Cai,[a] Ge Shi,[a] Zhigang Shuai,[b] Jie Zhang,*[a]
Qian Peng,*[c] and Xinhua Wan*[a]
[a]
Dr. S. Wang, Dr. X. Guan, S. Cai, Dr. G. Shi, Prof. J. Zhang, Prof. X. Wan
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and
Molecular Engineering, Peking University, Beijing 100871, China.
E-mail: jz10@pku.edu.cn, xhwan@pku.edu.cn
[b]
[c]
[†]
Dr. D. Hu, Prof. Z. Shuai
Key Laboratory of Organic OptoElectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing
100084, China.
Prof. Q. Peng
School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing, 100049, China.
E-mail: qpeng@iccas.ac.cn
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: The conjugated polymer monosubstituted polyacetylenes
(mono-PAs) have been generally considered to be non-emissive.
Herein we report the first example of luminescent mono-PAs based
on contracted cis-cisoid polyene backbone, displaying an excellent
circularly polarized luminescence (CPL) performance with a high
dissymmetric factor (up to the order of 10-1). The luminescence stems
from the helical cis-cisoid PA backbone, which is tightly fixed by the
strong intramolecular hydrogen bonds, thereby reversing the energy
order of excited states and enabling an emissive energy dissipation.
CPL switches are facilely achieved by the solvent and temperature
through reversible conformational transition. By taking advantages of
fast response and high sensitivity, the thin film of mono-PAs could be
used as a CPL-based probe for quantitative detection of trifluoroacetic
for quantitative detection, which commonly requires regularly
linear dynamic ranges of glum value, high glum value, and good
switchability. [2a,8] Most of responsive CPL probes reported so far
only present the relatively lower glum (mainly located between 10−5
and 10−2), and limited linear relationship between glum and the
analyte concentration, far away from the practical requirements.
Examples of CPL probes satisfying all of the above features are
even scarce.
Conjugated polymers, which commonly possess easy
manufacturing processes, facile structural modification, and finely
tunable emission wavelengths, have become one of promising
CPL sensing materials.[9] Polyacetylene (PA) is an important
archetypal conjugated polymer with unique photoelectric
properties.[10] Optically active monosubstituted PAs (mono-PAs)
are one type of the dynamic helical polymers featuring a highly
stereoregular conjugated backbone, easily tunable helical
conformations under external stimuli, and mild polymerization
condition.[11] These characteristics make them promising
candidates for stimuli-responsive CPL materials. However, mono-
PAs are generally considered to be nonemissive due to the
unfavorable energy levels of the ground and low-lying excited
states (E(1Bu)>E(2Ag), Figure 1a).[12] The 1Bu state decays into
the 2Ag state via fast internal conversion, but the 2Ag to 1Ag
transition is electronically dipole-forbidden according to symmetry
selection rule.[12a] In comparison, the steric effect on the polyene
backbone of disubstituted PAs (di-PAs) makes the energy level of
the 1Bu state become lower than that of the 2Ag state, enabling
an emissive energy dissipation from 1Bu to 1Ag state with
electronically dipole-allowed transition.[13] But di-PAs cannot be
applied as stimuli-responsive CPL materials due to their stable
trans-transoid (tt) structure. Given the fact that most reported
mono-PAs take stretched cis-transoid (ct) conformations,[14] we
assume that the mono-PAs taking contracted cis-cisoid (cc)
acid with
a
wider linear dynamic range than those of
photoluminescence and circular dichroism. This work opens a new
avenue to develop novel smart CPL materials through modulating
conformational transition.
Introduction
Circularly polarized luminescence (CPL), which indicates the
molecular chirality in the excited state, is attracting enormous
attention owing to its extensive applications in 3D displays,
information technologies, photoelectric devices, biosensors, etc.[1]
Compared to circular dichroism (CD) or nonpolarized
photoluminescence (PL), CPL spectroscopy could eliminate the
interferences of other achiral luminophores and chromophores,
thereby displays higher sensitivity and resolution when applied in
molecular sensors or detectors.[2] Responsive CPL materials
5]
under external stimuli, such as light,[3] pH,[4] metal ions,[2b,
force,[6] etc.[7] have been achieved. However, it still remains a
formidable challenge for the use of a CPL-based ratiometric probe
1
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wavelength region, strong mono-signate Cotton effects are
observed with perfect mirror images, evincing the formation of
preferred one-handed helices with opposite screw senses.[15]
Photoluminescence. In THF, sP presents a broad emission
band with an emission maximum at 530 nm upon photoexcitation
at 365 nm, whereas the corresponding monomer sM is non-
luminescent (Figure 1d). Similar emission spectra are observed
in CHCl3, powders, or in drop-cast film (Figures S5-S9). The
absolute PL quantum yields (Փ) are about 7% and 4% in THF and
CHCl3, respectively, and reach around 12% in films. Moreover,
their PLs decay single exponentially at 530 nm (Figures 2a and
S10, and Table S2). PL lifetime (τ) in solution is ~0.5 ns, nearly
half of that in solids or film, similar to that of di-PAs.[13e] It has been
known that hydrogen bond donating solvent CH3OH can destroy
the hydrogen bonds and trigger the cc-to-ct transition of sP in
CHCl3.[15] Expectedly, the fluorescence is gradually quenched and
vanishes completely when 11 vol% CH3OH is added (Figures 2b
and S11). Moreover, the cis-trans isomerization of -C=C- caused
by grinding[17] also makes the fluorescence disappear (Figures
S12 and S13), indicating again the importance of cc polyene
structure in this unprecedented fluorescence.
Figure 1. Molecular design to obtain fluorescent mono-PA. Strategy to
obtain switchable fluorescence in mono-PA in our work and the energy levels of
ground and low-lying excited states of ct helix (a) and cc helix (b). The
contracted cc helix is stabilized by strong intramolecular hydrogen bonding. (c)
UV-vis absorption and CD spectra of sP and rP in THF and CHCl3. (d) Emission
spectra of sM and sP in THF (5.0×10-5 M). Excitation wavelength: 365 nm.
Insert: Photographs of sM and sP in dilute THF solution, and their drop-cast film
taken under UV illumination (365 nm).
It is worth noting that the aggregation-enhanced emission
(AEE) is also observed in this system. Addition of nonsolvents
ethyl acetate (EtOAc) (Figure 2c) or acetone (Figures S15-S18)
results in the distinct intensification of emission. In THF/EtOAc
(10/90, v/v), the intensity is about twice stronger than that in THF
(Փ = 14%). Decreasing temperature has a similar effect. As shown
in Figure 2d, the PL intensity of sP in THF gradually increases
o
helical conformations possess the tight polyene backbone, in
which the molecular motions would be restricted, thereby
reversing the energy levels of 1Bu and 2Ag, similar to the steric
effect in di-PAs. We expect that optically active mono-PAs taking
contracted cc helical conformations could be photoluminescent
and CPL-active even if no fluorophore pendant is attached (Figure
1b).
Herein, we report the first example of luminescent mono-PAs
based on contracted cc polyene backbone, subverting the
traditional cognition that mono-PA is non-emissive. Our molecular
design strategy is built on poly(3,5-diamide substituted
phenylacetylene)s, where strong intramolecular hydrogen
bindings between the vicinal amide groups stabilize the
contracted cc helix and restrict the possible molecular motions
(Figure 1b). Even without pendant fluorophore, these mono-PAs
can emit strong greenish yellow fluorescence. Experiments
combined with theoretical calculations verify that this
luminescence stems from the tight cc PA backbone. By taking
advantage of the solvent- and thermo-responsiveness of
hydrogen bonds, reversible fluorescent and CPL switch is
realized in solution. Their cholesteric films exhibit excellent CPL
with glum values up to the order of 10-1, and can be reversibly
turned ON/OFF via vapor-responsive conformational switch.
upon cooling and reaches a maximum at -75 C. This emission
enhancement could be attributed to the increasing restriction of
intramolecular motions caused by lowering temperature or adding
nonsolvent. At elevated temperatures, the emission intensity is
obviously reduced (Figure S20), ascribed to the accelerated
molecular motions.
To investigate the origin of luminescence, PL spectra are traced
at different excitation wavelengths. Similar profiles of emission
spectra peaked at 530 nm are observed when excited at different
wavelengths from 240 to 420 nm (Figure 2e). No excitation-
dependent PL behavior occurs, probably ascribed to the highly
ordered main-chain stereostructure of sP. The optimal excitation
wavelength is 365 nm, consistent with the absorption of polyene
backbone (Figure 2f). The achiral poly(3,5-diamide substituted
phenylacetylene)s (aP-2C12) with cc conformation in THF (Figure
S21) also display PL properties in both solution and solid states,
certifying that the chiroptical activity of the side groups have no
effect on the luminescence. For a better comparison, as the
conjugated polyene backbone is changed to unconjugated
polystyrene, V-sP, only very weak ultraviolet fluorescence is
observed (Figure S22). It further substantiates that the polyene
backbones in sP and rP contribute to this strong long-wavelength
emission.
Theoretical calculations. To gain a deeper insight into the
emitting mechanism of cc mono-PA, theoretical calculations were
performed to unveil the excited state characters (Figure 3 and
Table S3) by using the OM2/MRCI[18] (orthogonalization model 2
and multireference configuration interaction) methods. To simplify
the calculation, only the PA backbone was kept in our model at
first. The calculation results for tt PA were also presented for
comparison. For S1 minimum of tt PA, the S1 is a dark state with
a vanishing-oscillator strength, suggesting that no fluorescence
Results and Discussion
The target polymers sP and rP were prepared from the
corresponding diamide monomers sM and rM with high yields and
large molar masses (Scheme S1 and Table S1), following our
previously reported procedures.[15] sP and rP exhibit absorptions
peaked at 355 nm in THF, ascribed to the π-π* transition of
contracted cc polyene backbone (Figure 1c).[15-16] In the same
2
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Figure 2. PL properties of mono-PA. (a) PL decay of sP in CHCl3, THF, solids, and drop-cast film at wavelength of 530 nm. Excitation wavelength: 378 nm. The
film of sP was obtained from THF solutions (6.0 mg/mL). Emission spectra of sP in CHCl3/CH3OH mixtures with different vol% of CH3OH (b) and in THF/EtOAc
mixtures with different vol% of EtOAc (c) at 298 K. (d) Emission spectra of sP in THF at various temperatures. Excitation wavelength: 365 nm. (e) Emission spectra
of sP in THF at different excitation wavelengths. (f) Relationship between emission intensity at 530 nm (Iem, 530 nm) and the excitation wavelength. Concentration:
5.0×10-5 M.
can be observed after excitation according to Kasha rule (Figure
3a).[19] The S1 mainly consists of one double excitation from
HOMO to LUMO (34.1%), and two single excitations from HOMO-
1 to LUMO (22.6%) and from HOMO to LUMO+1 (23.5%) (Table
S3). The HOMO and HOMO-1 are Ag symmetry, and the LUMO
and LUMO+1 belong to Bu symmetry. According to the principle
of symmetry combination, S1 is an Ag state. Since the ground state
is still Ag one, the electric dipole transition from S1 to S0 is
symmetry-forbidden. On the contrary, the main transition of S2 is
one single excitation from HOMO to LUMO, resulting in S2 to be
a Bu state. Thus, the electric dipole transition from S2 to S0 is
symmetry-allowed. The excitation energy of the Bu state (E(Bu)) is
largely decreased from 3.36 eV to 2.24 eV, when tt PA is changed
to cc one. At this time, E(Ag) slightly increases from 2.22 eV to
2.61 eV. Thus, the energy order of Ag and Bu is exchanged for cc
PA and the S1 becomes a bright state, which explains the
fluorescence of cc conformer (Figure 3c,e). Moreover, the
calculated results for S1 minimum of ct PA are very close to that
of tt PA (Figure 3b). The symmetry of S1 and S2 is Ag and Bu,
respectively. Thus, the S1 state is also a dark state with a very
small oscillator strength (0.067), and no emission can be
observed for ct conformer.
backbone. Therefore, we conclude that the side chains do not
participate in the formation of excited state, even though they play
an important role in stabilizing cc helix. In addition, the electronic
characters do not change much from ct PA to ct PPA (Figure
S23d), which further confirms our supposition.
Experimental results combined with theoretical calculations
convince well that the emission behavior of cc poly(3,5-diamide
substituted phenylacetylene)s originates from the tight polyene
backbone, in which the molecular motions are terrifically restricted,
thereby reversing the energy levels of 1Bu and 2Ag similar to the
13e]
steric effect in di-PAs.[12a,
Considering some reported cc
mono-PAs are non-luminescent,[15-16, 20] it suggests that the cc
structure is a necessary but insufficient prerequisite. Restriction is
also indispensable to suppress the possible nonradiative channel
thereby populating the radiative pathway. For those non-
luminescent cc mono-PAs, their intramolecular interactions are
presumably not strong enough to efficaciously restrict the
molecular motions. By comparison, two amide substituents in our
system can provide stronger intramolecular hydrogen bonds to fix
the whole polyene backbone, and then brings out fluorescence.
Aggregation and lowering temperature are beneficial to further
enhance restriction and increase the PL efficiency. Moreover, it is
worth noting that intense emission still can be excited under 270
nm, which correlates with the absorption of pendant groups
(Figure 2e,f). It suggests that adjacent phenyl pendants and
amide groups are conjugated with polyene backbone, they can
absorb the energy and transfer to the cc main chain, albeit with a
lower PL efficiency.
To understand the role of side chains in the emission of cc
mono-PA, the model system was expanded and the phenyl
substituted PA (PPA) was involved in our calculations (Figure 3d
and Table S3). Comparing the results of PA with and without
phenyl group, we can see that the excited state properties of
these polymers are very close (Figure 3e). Both of the orbitals
involved in the electronic transition for S1 and S2 are from the PA
3
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Figure 3. Theoretical calculations. The partial molecular mode and frontier molecular orbitals of (a) tt PA, (b) ct PA, (c) cc PA, and (d) cc PPA of S1 minima
calculated using the OM2/MRCI method. The hydrogens are hidden for convenience. (e) Energy levels of ground and low-lying excited states of tt PA, tt PPA, tt
poly(di-phenylacetylene) (PDPA), ct PA, ct PPA, cc PA, and cc PPA. The unit of energy levels is eV.
Reversible thermo fluorescent switch. The thermoresponsive
helix-helix transition of sP can be achieved in the mixture of CHCl3
and CH3OH. In pure CHCl3 or in a mixed solvent of CHCl3 and
CH3OH containing less than 5 vol% CH3OH (VCH3OH < 5 vol%), sP
retains cc helix in the temperature ranging from 2 to 30 oC (Figure
S24). While at VCH3OH = 5 vol%, reversible temperature-
dependent conformational variation occurs. During the cooling
process, the strong absorption centered at 355 nm gradually
decreases accompanied with the appearance of intense
absorption at 455 nm, which is ascribed to the stretched ct
polyene backbone (Figure 4a). Meanwhile, the negative Cotton
effects of cc main chain at 355 nm vanish, and a weak positive
one appears at around 455 nm accordingly, consistent with the
cc-to-ct transition.[15-16] The transition completes at 15 oC, and an
inverse process can be triggered upon heating (Figure S25). This
behavior can be rationalized by the competition of intramolecular
hydrogen bonding and intermolecular hydrogen bonding between
the polymer and hydrogen bond donating solvent, i.e. CHCl3. At
high temperature, intramolecular hydrogen bonding is stronger
than the hydrogen bonding between the polymer and solvents,
and favors the existence of cc helix. Whereas, the latter becomes
dominant at a lower temperature since the solvent molecules
interact more strongly with the macromolecules, which trigger cc-
to-ct transition. Interestingly, the transition temperature can be
tuned by altering CH3OH content. Increasing the volume fraction
of CH3OH raises the critical temperature (Tct) at which cc helix
completely transfers to the ct one (Figures S26 and S27). In the
case of VCH3OH = 8 vol% and 15 vol%, Tct increases to 22 and 33
oC (Figure 4d), respectively. As the hydrogen bond donating
ability of CH3OH is stronger than CHCl3, its addition can result in
the formation of more defects, such like helical reversals, which
increases the flexibility of polymer chains. So higher temperature
is needed to extrude out solvent molecules and reconstruct
intramolecular hydrogen bonding. According to the inverse
Arrhenius rate equation: τ* = τ0 exp(Ea/RT) (Figures 4f and S32),
activation energy (Ea) is estimated to be 76.3, 121.5, and 147.3
kJ∙mol-1, respectively, for VCH3OH = 5, 8, and 15 vol%. Ea increases
with VCH3OH, indicating an increased conformational transition
barrier with increasing VCH3OH. Similar conclusions can be
obtained from PL spectra. The fluorescence gradually vanishes
with temperature decreasing, and slowly recovers to the initial
spectrum again during the heating process in agreement with the
conformational changes (Figures 4b and S28-S30). This cc-to-ct
transition and fluorescence change exhibit a good reversibility.
The on/off switching of fluorescence was repeated up to five
cycles without visible fatigue (Figures 4c and S31).
It is noteworthy that obvious hysteresis exists between the
heating and cooling transition curves (Figure 4d,e). During the
cooling process, once a small amount of intramolecular hydrogen
bonds between amide groups are destroyed, cc conformer will
quickly transfer to ct conformer (Figure 4g), showing a fast
transition process. But for ct conformer, the polymer is surrounded
4
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Figure 4. Thermoresponsive conformation and fluorescent switch. (a) UV-vis and CD spectra of sP in CHCl3/CH3OH (v/v, 95/5) mixtures during cooling process.
Insert: The partial amplification of CD spectra from 350 nm to 500 nm. (b) Reversible PL spectra of sP in CHCl3/CH3OH (v/v, 95/5) mixtures changing with
temperature. (c) Xct (Xct = (εabs - εcc)/(εct - εcc) is used to quantitatively evaluate the content of ct helix) and emissiom intensity at 545 nm (I545nm) reversibly switch with
alternative temperature change in CHCl3/CH3OH (v/v, 95/5) mixtures. Xct (d) and I545nm (e) change with temperature during the cooling (solid line and closed symbol)
and heating process (dash line and open symbol) in different solvent mixtures. (f) Characteristic conformation transition time (τ*) exhibits an Arrhenius temperature
dependence for different CH3OH/CHCl3 mixtures. Solid lines are fits to the inverse Arrhenius rate equation: τ* = τ0exp(Ea/RT). (g) Schematic representation of
conformational transition of sP during the cooling and heating processes.
by large amounts of solvent molecules and controlled by the
intermolecular hydrogen bonding. Only most of the solvent
molecules are extruded out at elevated temperature, can the
partial ct polymer chain transfer to cc conformation. The
reorganization process of intramolecular hydrogen bonding is
stepwise and much slower than its destroying process, leading to
the occurrence of hysteresis.
CHCl3/CH3OH mixtures (Figure 5b), and o-dichlorobenzene
(Figures S35-S36). It exhibits very good stimuli-responsive
behavior via varying the solvent polarity and temperature. At high
contents of CH3OH (20 vol%), the strong CPL disappears (Figure
5b), coinciding with the cc-to-ct transition. Moreover, the above
temperature-dependent conformational transition can be well
applied to turn ON/OFF the CPL performance of rP and sP. With
the temperature decreasing to 10 oC, the cc helix transfers to the
ct one, accompanied with the disappearance of CPL (Figure 5c).
Upon the temperature increasing to 40 oC, the initial CPL is
recovered (Figure S37), which presents an outstanding
reversibility without the apparent decline even after five cycles
(Figure 5d). Brightness is another important parameter of CPL
(BCPL).[21] It is defined as BCPL = ελ × ϕ × |glum|/2, where ελ and ϕ is
the molar extinction coefficient measured at the excitation
Switchable circularly polarized luminescence. Because of the
presence of both helicity and photoluminescence, CPL is
expected for sP and rP. In THF, strong CPL signals are observed
at ~530 nm (Figure 5a), which corresponds to the emission peak
of the polyene backbone, and the glum values are +0.0037 and –
0.0028 for rP and sP (Figure S33), respectively. Similar CPL
performance is obtained in other solvents like CHCl3,
5
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remarkable complementary CPL signals centered at around the
PL emission bands (Figure 6a). The glum values of sP and rP in
films are +0.17 and −0.18, close to the largest value achieved in
the di-PAs.[13e] No obvious optical anisotropy is observed in film
(Figure S39). Moreover, their films exhibit the characteristic
bisignate Cotton effects in CD spectra at the region of the
absorption of polyene backbone (Figure 6b), further evincing the
formation of chiral supramolecular structure.
The helical structure and CPL property of sP and rP in N*-LC
film also can be reversibly switched ON/OFF with the acidic vapor.
Upon exposure to trifluoroacetic acid (TFA) vapor, the color of N*-
LC film quickly changed from light yellow to orange in 1 min,
accompanied with the red shift of the absorption of polyene
backbone from 360 nm to 450 nm (Figure 6b). The corresponding
main-chain Cotton effects present the coincident shift, evincing
the conformational transition from cc to ct helices. At this time, the
fluorescence and CPL disappeared (Figure 6a). Considering the
film of rP-TFA maintains the N*-LC texture (Figure S45), it
suggests that TFA vapor just switches the helical conformation
instead of its assembly structure (Figure 6e). When exposed to
THF vapor for 48 h, the cc helix and CPL intensity is recovered
(Figure S40). An alternative treatment with TFA and THF vapors
can reversibly tune the CPL property. Its glum value does not show
obvious fatigue even after five cycles (Figure 6d), exhibiting an
excellent switching performance.
Figure 5. Stimuli-responsive CPL in solution. CPL spectra of sP and rP in
THF (a), CHCl3, and CHCl3/CH3OH (80/20, v/v) (b) at 25 oC. (c) CPL spectra of
sP and rP in CHCl3/CH3OH (95/5, v/v) at 10 and 40 oC. (d) glum of rP in
CHCl3/CH3OH (95/5, v/v) switched by alternatively altering the temperature
between 40 and 10 oC. c = 0.5 mM.
wavelength (λ) and the emission quantum yield, respectively. BCPL
in THF and CHCl3 is evaluated as 0.62 M-1∙cm-1 and 0.36 M-1∙cm-
1, respectively. In CHCl3/CH3OH (95/5, v/v), BCPL also presents
the similar thermo-responsiveness (Figure S38).
Thanks to the large aspect ratio of contracted cc helices, both
sP and rP are able to form cholesteric mesophases as evidenced
by the fingerprint textures observed under polarizing optical
microscope (Figures 6c and S43). The cholesteric films display
CPL sensor. TFA has been widely used in the production of
pharmaceuticals, pesticides, and dyes, and becomes one of the
tricky industrial pollutant. Its detection is of great significance to
the human health and environmental protection. Based on the fast
response and high glum value, our CPL switch was then applied to
quantitively detect TFA. Different amounts of TFA were dissolved
in CHCl3. The CPL intensity can quickly respond to TFA vapor
Figure 6. Stimuli-responsive CPL in N*-LC film. CPL, glum (a), UV-vis, and CD (b) spectra of sP, rP, sP-TFA, and rP-TFA in drop-cast N*-LC film before and after
treatment of TFA vapor for 1 min. The measurements were carried out with excitation using unpolarized light at 365 nm. (c) The partial amplification of POM image
(Supplementary Fig. 37) of rP in the drop-cast film showing a fingerprint texture with a half helical pitch of 1.4 μm. (d) glum of sP and rP in in drop-cast N*-LC film
switched by the alternative treatment of TFA and THF vapor. (e) Schematic representation of N*-LC phase and the conformational switch for rP. Red and blue
helices represent cc and ct PPA backbones, respectively.
6
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complicated CD spectral change with the TFA content seriously
hinders its application in quantitatively detecting TFA (Figure S41),
further indicating the unique superiority of CPL in molecular
sensor. Moreover, for other common organic acid like formic acid
(FA), acetic acid (AA), propionic acid (PA), and butyric acid (BA),
they are unable to quench CPL at the same condition (1 vol% for
1 min, Figures 7c and S42), exhibiting an outstanding specific
identification to TFA.
Conclusion
We have demonstrated the first examples of circularly polarized
luminescent mono-PAs bearing no pendant fluorophore. The key
of molecular design is the introduction of two amide substituents
at 3,5-positions of poly(phenylacetylene), which helps stabilize
the contracted cc helix of polyene backbone through the
intramolecular hydrogen bondings between the vicinal amide
groups. Such a compacted macromolecular architecture not only
exchanges the energy level order of Ag and Bu but also restricts
intramolecular motions that usually suppress radiative energy
dissipation, thereby enabling main-chain fluorescence.
Reversible fluorescent switch can be on-demand achieved by
changing temperature through cc-to-ct helical transition. The films
cast from chiral nematic solutions exhibit excellent CPL property
with glum values as large as 0.18. By taking advantage of the
stimuli-responsive conformational transition, the CPL property
can be used to specific identification and quantitative detection of
TFA. Our work highlights the significance of conformational
transition on exchanging the electronic energy level of polyene
backbone, the concept of which may be appliable to other
conjugated polymers and enriches molecular design toolkit for
novel stimuli responsive CPL materials.
Acknowledgements
The financial support from the National Natural Science
Foundation of China (No. 51833001 and 52073001) is greatly
appreciated.
Keywords: polyacetylene • circularly polarized luminescence •
conformational transition • stimuli-responsive • CPL sensor
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Figure 7. CPL sensor. (a) CPL and DC spectra of rP in drop-cast N*-LC film
changed with different TFA contents in CHCl3. The responsive time to the mixed
solvent vapor was set as 1 min. (b) The quantitative relationship between CD
intensity at 503 nm, PL intensity, glum and the TFA content in CHCl3. (c) glum of
rP film after exposed to different organic acids (FA, AA, TFA, PA, and BA) in
CHCl3 (1 vol%) for 1 min.
even in the low fraction of 0.2 vol% of TFA in CHCl3, showing a
high sensitivity (Figure 7a). CPL intensity diminishes fast with the
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of TFA (Figure 7a,b). The glum value is linearly proportional to the
content of TFA in the range of 0.2 ~ 1.0 vol% (R2 = 0.9994),
evincing the feasibility of quantitative detection. By contrast, CD
and PL spectroscopy exhibit the relatively lower sensitivity, worse
linear relationship, and narrower detection range (Figure 7b). The
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Entry for the Table of Contents
We describe a novel strategy that enables the conventionally non-luminescent monosubstituted polyacetylenes bearing no pendant
fluorophore to light up and show a switchable circularly polarized luminescence (CPL) with a large dissymetric factor (glum, 10-1), multiple
reversible stimili-responsiveness, and quantitative detection by conformational transition of polyene backbone.
9
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