Enhancing Insulated Conjugated Polymer Fluorescence Quenching by Incorporating Dithia[3.3]paracyclophanes

Article abstract of DOI:10.1021/acs.macromol.1c00136

Insulated π-conjugated polymers exhibit enhanced chemical stability, photostability, fluorescence quantum yield, electroluminescence, solubility, and intrachain charge transport. However, insulated polymer fluorescence quenching by acceptor molecules is significantly hampered as the π-face is insulated. Photoinduced charge transfer is one of the key steps in amplified fluorescence quenching sensors and organic solar cells for charge generation. Inspired by the myelin sheath gaps in nerve cell axons, herein, we synthesized a series of insulated random copolymers of adamantanocyclophane with an increasing percentage of dithia[3.3]paracyclophane (PCP) from 5 to 30% to enhance the insulated polymer fluorescence quenching with acceptor molecules. As the percentage of the dithia[3.3]paracyclophane monomer increases, the copolymers showed an increase in absorption in the red region of the spectrum and also the copolymers' photoluminescence quantum yield reduced. The Stern-Volmer quenching constant of the 30% copolymer is ca. 4.5 times higher than that of the adamantanocyclophane homopolymer. A comparison with the control polymers indicated that the through-space-coupled interactions in PCP could be a plausible reason for the enhanced fluorescence quenching in copolymers in addition to the reduced steric hindrance by PCP. The developed copolymers combine the advantages of polymer insulation without significantly sacrificing the photoinduced charge transfer, which will help further their applicability as amplified fluorescence quenching sensors and in organic solar cells.

Full text of DOI:10.1021/acs.macromol.1c00136

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Article
Enhancing Insulated Conjugated Polymer Fluorescence Quenching
by Incorporating Dithia[3.3]paracyclophanes
Ryan Lillis,† Maximillian R. Thomas,† Manikandan Mohanan, and Nagarjuna Gavvalapalli*
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ABSTRACT: Insulated π-conjugated polymers exhibit enhanced chemical
stability, photostability, fluorescence quantum yield, electroluminescence,
solubility, and intrachain charge transport. However, insulated polymer
fluorescence quenching by acceptor molecules is significantly hampered as the
π-face is insulated. Photoinduced charge transfer is one of the key steps in
amplified fluorescence quenching sensors and organic solar cells for charge
generation. Inspired by the myelin sheath gaps in nerve cell axons, herein, we
synthesized a series of insulated random copolymers of adamantanocyclophane
with an increasing percentage of dithia[3.3]paracyclophane (PCP) from 5 to 30%
to enhance the insulated polymer fluorescence quenching with acceptor molecules.
As the percentage of the dithia[3.3]paracyclophane monomer increases, the copolymers showed an increase in absorption in the red
region of the spectrum and also the copolymers’ photoluminescence quantum yield reduced. The Stern−Volmer quenching constant
of the 30% copolymer is ca. 4.5 times higher than that of the adamantanocyclophane homopolymer. A comparison with the control
polymers indicated that the through-space-coupled interactions in PCP could be a plausible reason for the enhanced fluorescence
quenching in copolymers in addition to the reduced steric hindrance by PCP. The developed copolymers combine the advantages of
polymer insulation without significantly sacrificing the photoinduced charge transfer, which will help further their applicability as
amplified fluorescence quenching sensors and in organic solar cells.
enhances the diffusivity of analytes in the thin films and
therefore the sensory response of polymers toward electron-
deficient analytes.19,20 Thus, the straps help to overcome the
limitations of conjugated polymers and render new properties
in few cases. Because of these reasons, the research in this
direction, although limited for now, has been increasing
recently. Therefore, multiple strategies have been devised to
insulate π-conjugated polymer backbones, including molecular
sheathing by cyclodextrins and the use of alkyl chains and
cycloalkyl straps.2,6−8,10,11,15,16
Recently, we have shown that the cycloalkyl straps efficiently
block the π-face of the repeat unit and render soluble polymers
with higher solubility limits than the conventional π-
conjugated polymers that have pendant solubilizing chains.6,18
We have also shown that the size of the strap is useful to
control the polymer solubility by demonstrating that solubility
increased as the height of the cycloalkyl group increased.6 In
addition to imparting solubility, the strapped polymers also
render the advantage of selectively blocking the π-face of the
INTRODUCTION
■
Sheathing a π-conjugated polymer backbone insulates the
conjugated backbone and enhances its chemical stability,
photostability, fluorescence quantum yield, electrolumines-
cence, and intrachain charge transport.1−18 In addition, these
insulated conjugated polymers also enable nonlinear optical
properties.2,4 For example, conjugated polymers do not
typically form self-free-standing films like conventional plastics;
however, Takeuchi and Sugiyasu have shown that doubly
strapped conjugated polymers are thermoformable like
conventional plastics.15 Recently, Bronstein and co-workers
have shown that doubly strapped π-conjugated polymers
hinder interchain interactions and reduce the gap between the
solution and solid-state quantum yields.10,13,14 Instead of alkyl
straps, chemically diverse straps can be used to generate novel
functional polymers; for example, Giovannitti and co-workers
have used ethylene oxide straps instead of alkyl straps to
selectively detect sodium and potassium ions in both solution
and solid states.15 The Smith group has used aryl straps based
on sterically encumbered m-terphenyl instead of alkyl straps to
semi-insulate π-conjugated polymers.11,12 The aryl straps also
reduce interchain interactions in the solid state and enhance
the sensory response toward nitroaromatic vapors similar to
pentiptycene polymers derived by the Swager19,20 group.
Swager’s pentiptycene polymers, although not strapped
polymers, reduce the interchain interactions between the
polymers in thin films and generate a porous structure, which
Received: January 19, 2021
Revised: March 9, 2021
Published: March 25, 2021
https://doi.org/10.1021/acs.macromol.1c00136
© 2021 American Chemical Society
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repeat unit unlike the cyclodextrin-encapsulated insulated
polymers (Scheme 1).6−8 This allows the strapped monomers
methane (TCNQ) acceptor. The photoluminescence quench-
ing of a donor polymer by an acceptor is one of the key steps in
charge generation in organic solar cells and amplified
fluorescence quenching sensors. Therefore, even though the
insulated polymers have several advantages, charge transfer to
acceptor molecules and other polymer chains is significantly
hampered as the π-face is insulated.2,4 Insulated polymers that
combine the advantages of insulation without significantly
sacrificing the photoinduced charge transfer will help further
their applicability as amplified fluorescence quenching sensors
and in organic solar cells.
Scheme 1. Paracyclophane Units Incorporated along the
Insulated Polymer Backbone Enhance the Insulated
Polymer’s Photoinduced Charge Transfer to Acceptor
Molecules and Hence the Polymer’s Fluorescence
Quenching
Insulated conjugated polymers are not unique to the
synthetic material world as this structural design is observed
in nature in nerve cells as well. Nerve cell axons are coaxially
covered by myelin, an insulating sheath, to increase the
transport rate of electrical impulses along the axon.21−23
Nature cleverly does not insulate the axon continuously but
leaves periodic gaps known as myelin sheath gaps or nodes of
Ranvier. These gaps mostly contain ion channels and are
exposed to extracellular space, which are useful to regenerate
the action potential. The insulated axon structure with myelin
sheath gaps serves as a good inspiration for overcoming the
challenges mentioned above in the case of synthetic insulated
conjugated polymers and generating the next generation of
insulated π-conjugated polymers.
Inspired by this, herein, to enhance the insulated π-
conjugated polymer fluorescence quenching by acceptor
molecules, dithia[3.3] paracyclophanes (PCPs) are incorpo-
rated along an insulated adamantanocyclophane polymer
(Scheme 1). Unlike an adamantanocyclophane (ACP),
[m.n]cyclophanes contain two phenyl rings, and both the
phenyl rings interact via through-space π−π interactions called
transannular π−π interactions.24−28 It has been shown that the
to fuse along the polymer backbone to generate soluble
nanoribbon polymers without pendant solubilizing chains.7,8
However, insulation of the π-face of the conjugated polymer
reduces the fluorescence quenching by a tetracyanoquinodi-
Scheme 2. Synthesis of Copolymers (CP-1−4)
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transannular π−π interactions enable the charge-transfer
complexes of bilayer and multilayer [3.3]cyclophanes with
the tetracyanoethylene acceptor as well as interchain charge
and energy transfer.24−27,29−32
the polymer backbone, as a racemic mixture of monomers was
used to synthesize the copolymers. The monomer feed ratio
was adjusted for the desired amount of PCP in the copolymer
backbone (5, 10, 20, and 30%) (Table 1). The copolymers
The PCP units are randomly incorporated along the
adamantanocyclophane polymer, resulting in random cyclo-
phane copolymers (CPs). We hypothesize that PCPs will act
similar to the myelin sheath gap node in the axons and
enhance the photoinduced charge transfer with acceptor
molecules. In order to understand the role of the PCP units,
four different random copolymers of ACP and PCP are
synthesized with the systematically increasing fraction of PCP
from 5, 10, 20, to 30% (CP-1 to CP-4). UV−vis absorption,
emission, and fluorescence quenching properties of the
copolymers are compared with the control polymers. The
through-space-coupled phenyl rings in the PCP units have
significantly altered the UV−vis absorption and emission
properties of the copolymers and indicate that PCP acts as an
acceptor in the copolymers. As the percentage of PCP
increases, the fluorescence of the copolymers is quenched
more effectively by the TCNQ acceptor. The Ksv value of CP-4
with 30% PCP units is ca. 4.5 times higher than that of the
ACP homopolymer. The reduced steric hindrance due to the
PCP unit compared to the ACP unit allows TCNQ to easily
access the polymer backbone and quench the emission. A
comparison with the copolymers’ Ksv value indicated that, in
addition to sterics, the through-space interactions in PCP
could be a plausible reason for enhanced quenching.
Table 1. Structural Details of Copolymers
percent PCP
copolymer PCP feed ratio Mn (kDa) PDI
incorporated
CP-1
CP-2
CP-3
CP-4
5
10
20
30
13.5
12.1
11.2
11.0
2.58
2.56
1.77
2.49
4.7
9.9
19.7
29.2
with more than 30% of the PCP units are not soluble enough
to grow into the desired molecular weight. The polymerization
time was adjusted to achieve similar molecular weights for all
the copolymers. Having copolymers of similar molecular
weight eliminates the effect of molecular weight when
comparing the copolymers’ properties. The polymerization
reaction mixture was added to methanol to stop the
polymerization, and the resultant polymer precipitate was
purified by soxhlation using methanol and chloroform. The
chloroform solution was concentrated under a vacuum and
reprecipitated in ether and filtered. The resultant polymer was
dried and used for further characterization. The copolymer
molecular weights were determined using gel permeation
chromatography (tetrahydrofuran as the eluent) against
polystyrene standards. The number average molecular weights
of the copolymers are in the range of 11−14 kDa (Table 1 and
Figure S1). All the copolymers are soluble in chloroform and
tetrahydrofuran. However, only the copolymers having a lower
percentage of PCP (CP-1−2) are soluble in dichloromethane,
indicating that the PCP units promote interchain interactions.
The percent inclusion of PCP units into the copolymers was
RESULTS AND DISCUSSION
■
( )-Diethynyl adamantanocyclophane monomer (( )-8) was
synthesized from 1,3-adamantane dicarboxylic acid (1)
following our previously reported protocol (Scheme 2).6
Compound 1 was esterified and reduced to generate 1,3-
adamantane dimethanol (3). Compound 3 was converted into
a thioester and then reduced to generate 1,3-adamantane
dimethanethiol (5). Compound 5 was reacted with 1,4-
dibromo-2,5-bis(bromomethyl)benzene under high-dilution
conditions to yield a racemic mixture of ( )-dibromo
adamantanocyclophane (6). Compound 6 is obtained as a
racemic mixture due to planar chirality. Sonogashira coupling
of compound ( )-6 with trimethylsilyl (TMS)acetylene
followed by desilylation resulted in the ACP monomer
( )-8. ( )-Diethynyl dithia[3.3] paracyclophane monomer
(13) was synthesized from 1,4-bis(bromomethyl)benzene (9),
following the reported synthetic protocols.33 Compound 9 was
reacted with thioacetic acid in the presence of potassium
carbonate in methanol to generate 1,4-bis(thiomethanol)-
benzene (10). Compound 10 and 1,4-dibromo-2,5-bis-
(bromomethyl)benzene were reacted under high-dilution
conditions for 3 days to generate ( )-dibromo dithia[3.3]-
paracyclophane (( )-11). Sonogashira coupling of compound
( )-11 with TMS acetylene followed by desilylation resulted
in the PCP monomer ( )-13.
1
determined using H NMR analysis (Figure 1 and Table 1).
6
1
Based on the H NMR data of the ACP homopolymer, the
two peaks downfield from the CDCl3 residual solvent peak (δ
= 7.26 ppm) in the copolymer correspond to the aryl peaks
from ACP. A good (1:1) correlation between the aryl peaks
and highly shielded adamantane methylene protons further
confirms that these two aryl peaks are from ACP. The aryl peak
upfield from the CDCl3 residual solvent peak corresponds to
the four-aryl protons of the through-space-coupled phenyl ring
of PCP. The two-aryl protons corresponding to the diethynyl
phenyl ring of PCP are buried inside the CDCl3 solvent
1
residual peak. To confirm this, the H NMR data of CP-2
(10% PCP) was recorded in CD2Cl2 and is shown in Figure 1.
Indeed, there is an aryl peak around 7.24 ppm corresponding
to the protons of the diethynyl phenyl ring of PCP. The aryl
protons corresponding to the through-space-coupled phenyl
ring of PCP are observed at 7.13 and 7.08 ppm. The ratio of
integration of the protons corresponding to the diethynyl
phenyl ring of PCP and the through-space-coupled phenyl
peaks of PCP matches well with the structure. As CP-3 and
CP-4 are sparingly soluble in CD2Cl2, the 1H NMR data of all
the copolymers are recorded in CDCl3 and compared. The
stacked 1H NMR spectra of the copolymers in Figure 1
demonstrate the growth of the PCP peak at 7.1 ppm with an
increasing feed ratio of the PCP monomer in the polymer-
ization. The percentage of PCP units in the copolymers (Table
1) is determined by the ratio of the integration of the through-
space-coupled phenyl peaks in PCP to the aryl peaks in ACP.
Random copolymers of the ACP monomer (8) and PCP
monomer (13) were synthesized, following the similar Glaser−
Hay polymerization protocol used for the ACP homopolymer
synthesis.6 In a typical procedure, both the monomers (( )-8
and ( )-13) were reacted in the presence of copper (I)
chloride and tetramethylethylenediamine (TMEDA) in
toluene in the presence of air at 50 °C (Scheme 2). All of
the generated copolymers are atactic in nature, that is, there is
no control over the orientation of the cyclophane units along
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1
1
Figure 1. (Left) H NMR spectra of copolymers in CDCl3; (right) H NMR spectra of CP-2 in CD2Cl2; the sharp peak at 7.3 ppm (*) is from
chloroform.
Based on this analysis, there is a one-to-one correlation
between the feed ratio of PCP (f1) to its incorporation (F1)
into the copolymer (Figure 2). Based on the Mayo−Lewis
unit molecular weight. The absorption spectra of the
copolymers are significantly similar to each other, with the
absorption maximum around ca. 420 nm. The absorption
spectra (Figures 3 and S2) of copolymers differ in the higher
wavelength (>450 nm) and lower wavelength (<300 nm)
regions. The absorbance of peaks below 300 nm and above
450 nm increases as the percentage of the PCP monomer
increases. The optical properties of CP-4 are compared with
the optical properties of two control polymers, adamantano-
cyclophane homopolymer (ACP homopolymer) and control-
CP, to further gain insights into the role of the through-space-
conjugated phenyl ring in the PCP repeat unit (Figure 4). The
ACP homopolymer contains only ACP repeat units, whereas
the control-CP contains an equimolar mixture of both
adamantocyclophane and cyclohexanocyclophane repeat
units. Cyclohexanocyclophane contains the cyclohexyl group
instead of the phenyl ring as part of the cyclophane.
Cyclohexane is the saturated version of the phenyl ring, so
the difference in the optical properties between the control-CP
and CP-4 can be attributed to the through-space-conjugated
phenyl ring in CP-4, that is, transannular interactions. The
synthesis and optical properties of the ACP homopolymer and
the control-CP have been reported by us previously.6 For easy
comparison between these polymers, the optical data of the
ACP homopolymer and the control-CP polymer are plotted
along with those of CP-4 in Figure 4. The absorption spectrum
(Figure 4) of CP-4 is similar to that of the ACP homopolymer
except for the regions below 300 nm and above 450 nm. The
intensity of the peaks below 300 nm is higher for CP-4
compared to the control polymers. The additional phenyl ring,
that is, the through-space-conjugated phenyl ring, might be a
Figure 2. Mayo−Lewis plot of the PCP copolymer composition.
model for copolymer composition (eq 1),34−36 it is indicated
that the reactivities of both the monomers are equal to unity
and the resultant copolymers are random copolymers.
r f 2 + f1f2
1
1
F =
1
r f 2 + 2f f + r2f
2
1
(1)
1
1 2
2
The equal reactivities of both the ACP (r2) and PCP (r1)
monomers indicate that the transannular π−π interactions in
PCP have no effect on the reactivity of acetylenes in the PCP
monomer under Glaser−Hay polymerization conditions.
To understand the impact of incorporating the PCP units
along the polymer backbone, the UV−vis absorption and
emission spectra of the copolymers were recorded in
chloroform. The samples are prepared based on the repeat
Figure 3. (a) UV−vis absorption spectra of copolymers in chloroform; (b) zoomed tail-end region of the UV−vis absorption spectra of the
copolymers; and (c) fluorescence spectra of the copolymers in chloroform.
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Figure 4. (a) Comparison of control polymer structures; comparison of (b) UV−vis absorption spectra and (c) emission spectra of CP-4 with
control polymers (ACP homopolymer and control-CP).
Table 2. Comprehensive Optical Properties of Copolymers
molar extinction
coefficient (×103)
(ε, M−1 cm−1
)
a
Ksv (×103) (M−1
)
abs
max
abs
max
abs
max
em
polymer %PCP
λ
(nm) in CHCl3
λ
(nm) in thin film @λ
@472 nm
λ (nm) in CHCl3 I480/I448 quantum yield
max
CP-1
CP-2
CP-3
CP-4
5%
10%
20%
30%
422
421
421
420
421
424
420
418
28.3
29.5
28.7
23.8
1.3
448, 480
449, 480
448, 478
446, 478
46%
53%
62%
67%
0.11
0.10
0.08
0.06
35
42
44
58
4
3
3
6
1.45
1.95
2.4
a
Relative to perylene in ethanol.
possible reason for the higher intensity of peaks below 300 nm
in CP-4. CP-4 has a higher absorbance near the tail end of the
spectrum (between 455 and 500 nm) compared to the ACP
homopolymer and the control-CP. This indicates that the
appearance of a new peak near the tail end of the absorption
spectrum is due to the PCP monomers. The increase in the
absorbance of the peak between 455 and 500 nm as the
percentage of the PCP monomer increases could be due to
either the aggregation of polymers in solution and/or
intramolecular charge transfer between the ACP and PCP
units. In order to determine the root cause of this, the
concentration-dependent UV−vis absorption spectra of the
copolymers are recorded at low concentrations (13−50 μM) in
chloroform. The concentrations of the copolymers are
determined using the corrected repeat unit molecular weight
that includes the percentage of the ACP and the PCP repeat
units. The intensity of the red region of the spectrum increased
linearly with concentration for all the copolymers (Figures S3−
S6). Molar absorptivities of the copolymers at 472 nm
(corresponding to the red region of the spectrum) and at
the absorption maximum are determined (Table 2) using the
Beer−Lamberts law. The molar absorptivities of copolymers at
472 nm increases from 1300 to 2400 M−1 cm−1 as the
percentage of the PCP repeat units increases from 5 to 30%.
However, the molar absorptivities at the absorption maximum
showed no correlation with the percentage of the PCP repeat
units. Dynamic light scattering measurements on the
copolymer samples close to the UV−vis sample concentrations
showed no clear indications of aggregates in the solution.
Thus, the presence of an absorption peak at 472 nm even at a
low copolymer concentration, the linear correlation between
the absorbance at 472 nm and the copolymer concentration,
and the increase in the molar absorptivity of the copolymers
with an increase in the PCP repeat units indicate that the
absorption near the tail end of the spectrum is due to the
intramolecular charge transfer between the ACP and the PCP
units. In the case of conventional π-conjugated alternating
copolymers of PCP, it has been shown that the PCP unit acts
as an acceptor and quenches the emission.37,38 In the
copolymers reported here also, the PCP unit acts as an
acceptor and the ACP unit acts as a donor. Frontier molecular
orbitals (FMOs) of a model trimer (ACP−PCP−ACP) are
calculated using DFT-B3LYP/6-31G* calculations and are
shown in Figure 5 (Table S1). There is some electron density
Figure 5. Frontier molecular orbitals of a model trimer: ACP−PCP−
ACP; xylene is used instead of adamantanocyclophane.
located on the through-space-conjugated phenyl ring of the
PCP units in both the FMOs, suggesting that it has a role in
the optical properties of the copolymers. The absorption
spectra of copolymer thin films are recorded and are shown in
Figure S7. The thin-film absorption spectra of copolymers are
similar to that of the solution state spectra. A negligible shift in
the absorption maximum from the solution to thin films is also
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observed in PCP containing alternating copolymers37,38 and is
not unique to the CPs synthesized in this work.
(SV) plot. In general, the nonlinear Stern−Volmer plot is due
to simultaneous dynamic and static quenching.39,40 The data
were fitted to a reported nonlinear SV equation41 to obtain the
SV constant (Ksv) (Figures S14−S17). Ksv increased as the
percentage of the PCP units increased in the CPs (Table 2).
Moreover, Ksv of CP-4 with 30% PCP units is 4.5 times that of
the ACP homopolymer.
The emission spectra of the random CPs in solution are
shown in Figures 3 and S8. All the CPs show an emission
maximum at ca. 448 nm and a shoulder at 480 nm. The
emission capability of the copolymers, that is, the intensity of
the emission maximum, reduces with the increasing percentage
of the PCP monomer (Figure S8). Indeed, the photo-
luminescence quantum yield reduces from 14 to 6% with the
increasing percentage of the PCP monomer from 0 to 30%
(Table 2 and Figures S9−S13). Interestingly, the ratio of the
intensity of the shoulder (480 nm) peak to the emission
maximum (448 nm) increases from 35 to 67% as the
percentage of the PCP units increases from 0 to 30% (Table
2). This indicates that the PCP units show more impact on the
emission process corresponding to the emission maximum (ca.
448 nm peak) compared to the shoulder peak (ca. 480 nm).
The emission spectrum of CP-4 is compared with that of the
ACP homopolymer and the control-CP (Figure 4). The ratio
of the intensity of the shoulder (480 nm) peak to the emission
maximum (448 nm) is 34 and 40% for the ACP homopolymer
and the control-CP respectively. Even though the control-CP
contains 50% of the cyclohexanocyclophane comonomer, the
intensity ratio (40%) is significantly lower than that of CP-4
(67%) and much closer to that of the ACP homopolymer
(34%). This clearly indicates that the through-space-con-
jugated phenyl ring has an impact on the emission properties
of the copolymers. As mentioned above, the PCP units act as
acceptors and contribute to the partial quenching of the
copolymers’ fluorescence, resulting in a reduction of the
copolymers’ emission intensity with an increasing percentage
of the PCP units.
In order to determine the role of the through-space-
conjugated phenyl ring and hence the PCP monomer on the
increase in Ksv, the Ksv values of the ACP homopolymer (Ksv:
13 × 103 M−1) and the control-CP (Ksv: 17 × 103 M−1) are
compared with that of CP-4.6 The increase in the Ksv value
from the ACP homopolymer to the control-CP was attributed
to the smaller height/size of the cyclohexanocyclophane unit.6
The butadiyne linkers provide enough space between the
repeat units and allow the repeat units to rotate freely around
the polymer axis. Hence, the polymer structure resembles a
cylinder where the cycloalkyl groups or the through-space-
coupled phenyl ring form a sheath around the polymer
backbone (Scheme 1). The diameter of the cylinder is
determined by the height of the repeat unit. The height of
cyclohexanocyclophane is lower than that of adamantanocy-
clophane by 1.3 Å (reduction in height of 18%).6 This allows
the TCNQ molecules to approach the control-CP polymer
backbone relatively closely compared to the ACP homopol-
ymer backbone, resulting in enhanced fluorescence quenching
by 30%. Based on a similar analysis, the PCP unit is smaller in
height compared to the ACP unit by 3.52 Å (reduction in
height of 48%). The Ksv value of CP-4 is enhanced by 346%.
Hence, the reduction in the repeat unit height has a significant
contribution in increasing the Ksv value. However, the increase
in the percentage of Ksv with the drop in the repeat unit height
is dramatic in the case of CP-4 compared to the control-CP.
Thus, in addition to steric reasons, the through-space-
conjugated phenyl ring also might play a significant role in
increasing the sensing capability of CP-4. The through-space
interactions in PCP could be a plausible reason for the
enhanced photoinduced charge transfer from the polymer
backbone to the TCNQ molecule.
Fluorescence quenching studies of the copolymers with a
TCNQ acceptor are done to determine the role of PCP in the
copolymers. The fluorescence quenching studies of the CPs
with a TCNQ acceptor were carried out and compared with
that of the ACP homopolymer and the control-CP (Figure 6).
The fluorescence intensity of all the copolymers reduced as the
TCNQ concentration increased, and the reduction in intensity
followed a concave-upward curvature of the Stern−Volmer
CONCLUSIONS
■
To summarize, a series of π-face-insulated polymers with an
increasing amount of dithia[3.3]paracyclophane have been
synthesized, and their optical properties have been studied.
The similar reactivity of adamantanocyclophane (insulated
monomer) and dithia[3.3]paracyclophane resulted in the
random incorporation of PCP units along the polymer
backbone. The PCP unit acts as an acceptor, resulting in an
intramolecular charge-transfer peak in the UV−vis absorption
spectra of copolymers and a reduction in the photo-
luminescence quantum yield of the copolymers. The smaller
size of the PCP units allowed the TCNQ acceptor molecules
to approach the polymer backbone closely, resulting in an
enhanced fluorescence emission quenching of the insulated
polymer backbone. The Ksv value of random CPs increased as
the amount of PCP units increased. The Ksv value of the 30%
CP is ca. 4.5 times that of the adamantanocyclophane
homopolymer. A comparison of the Ksv values of the control
polymers with that of the copolymers indicated that the
through-space interactions of PCP could also play a role, in
addition to the reduced steric hindrance, for the enhancement
of Ksv. Thus, the incorporation of the PCP units along the π-
face-insulated polymer backbone enhances the efficiency of
Figure 6. Fluorescence emission quenching of copolymers upon the
addition of TCNQ. The mole ratio of TCNQ to the polymer repeat
unit is shown in the legend.
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(6) Chaudhuri, S.; Mohanan, M.; Willems, A. V.; Bertke, J. A.;
Gavvalapalli, N. ″beta-Strand inspired bifacial pi-conjugated poly-
mers″. Chem. Sci. 2019, 10, 5976−5982.
photoinduced electron transfer to acceptor molecules. The
through-space interactions in the PCP units are also expected
to facilitate interchain charge transfer/transport. Thus, the
developed strategy will help to generate insulated polymers
that combine the advantages of insulation without significantly
sacrificing the charge and energy transport across the bulk
material, which furthers their applicability in many electronic
and optoelectronic devices.
(7) Fiesel, R.; Huber, J.; Apel, U.; Enkelmann, V.; Hentschke, R.;
Scherf, U.; Cabrera, K. ″Novel chiral poly(para-phenylene) derivatives
containing cyclophane-type moieties″. Macromol. Chem. Phys. 1997,
198, 2623−2650.
(8) Schluter, A.-D.; Löffler, M.; Enkelmann, V. ″Synthesis of A Fully
̈
Unsaturated All-Carbon Ladder Polymer″. Nature 1994, 368, 831−
834.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.macromol.1c00136.
(9) Masai, H.; Terao, J. ″Stimuli-responsive functionalized insulated
conjugated polymers″. Polym. J. 2017, 49, 805−814.
(10) Royakkers, J.; Bronstein, H. ″Macrocyclic Encapsulated
Conjugated Polymers″. Macromolecules 2021, 54, 1083−1094.
■
sı
̈
(11) Smith, R. C. ″Covalently Scaffolded Inter-I€-System
Synthesis, characterization details, and characterization
of polymers (PDF)
̈
Orientations in I€-Conjugated Polymers and Small Molecule
Models″. Macromol. Rapid Commun. 2009, 30, 2067−2078.
(12) Morgan, B. P.; Gilliard, R. J.; Loungani, R. S.; Smith, R. C.
″Poly(p-phenylene ethynylene) Incorporating Sterically Enshrouding
m-Terphenyl Oxacyclophane Canopies″. Macromol. Rapid Commun.
2009, 30, 1399−1405.
AUTHOR INFORMATION
Corresponding Author
■
Nagarjuna Gavvalapalli − Department of Chemistry, Institute
for Soft Matter Synthesis and Metrology, Georgetown
University, Washington, D.C. 20057, United States;
orcid.org/0000-0002-2812-1694; Email: ng554@
georgetown.edu
(13) Leventis, A.; Royakkers, J.; Rapidis, A. G.; Goodeal, N.;
̌
Corpinot, M. K.; Frost, J. M.; Bucar, D.-K.; Blunt, M. O.; Cacialli, F.;
Bronstein, H. ″Highly Luminescent Encapsulated Narrow Bandgap
Polymers Based on Diketopyrrolopyrrole″. J. Am. Chem. Soc. 2018,
140, 1622−1626.
(14) Royakkers, J.; Minotto, A.; Congrave, D. G.; Zeng, W.; Hassan,
A.; Leventis, A.; Cacialli, F.; Bronstein, H. ″Suppressing Solid-State
Quenching in Red-Emitting Conjugated Polymers″. Chem. Mater.
2020, 32, 10140−10145.
(15) Pan, C.; Sugiyasu, K.; Wakayama, Y.; Sato, A.; Takeuchi, M.
″Thermoplastic Fluorescent Conjugated Polymers: Benefits of
Preventing π-π Stacking″. Angew. Chem., Int. Ed. 2013, 52, 10775−
10779.
(16) Giovannitti, A.; Nielsen, C. B.; Rivnay, J.; Kirkus, M.; Harkin,
D. J.; White, A. J. P.; Sirringhaus, H.; Malliaras, G. G.; McCulloch, I.
″Sodium and Potassium Ion Selective Conjugated Polymers for
Optical Ion Detection in Solution and Solid State″. Adv. Funct. Mater.
2015, 26, 514−523.
(17) Lo, W.-Y.; Bi, W.; Li, L.; Jung, I. H.; Yu, L. ″Edge-on Gating
Effect in Molecular Wires″. Nano Lett. 2015, 15, 958−962.
(18) Das, M. K.; Hameed, F.; Lillis, R.; Gavvalapalli, N. ″A twist in
the non-slanted H-mers to control π-conjugation in 2-dimensions and
optical properties″. Mater. Adv. 2020, 1, 2917−2925.
(19) Yang, J.-S.; Swager, T. M. ″Porous Shape Persistent Fluorescent
Polymer Films: An Approach to TNT Sensory Materials″. J. Am.
Chem. Soc. 1998, 120, 5321−5322.
(20) Yang, J.-S.; Swager, T. M. ″Fluorescent Porous Polymer Films
as TNT Chemosensors: Electronic and Structural Effects″. J. Am.
Chem. Soc. 1998, 120, 11864−11873.
Authors
Ryan Lillis − Department of Chemistry, Institute for Soft
Matter Synthesis and Metrology, Georgetown University,
Washington, D.C. 20057, United States
Maximillian R. Thomas − Department of Chemistry, Institute
for Soft Matter Synthesis and Metrology, Georgetown
University, Washington, D.C. 20057, United States
Manikandan Mohanan − Department of Chemistry, Institute
for Soft Matter Synthesis and Metrology, Georgetown
University, Washington, D.C. 20057, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.macromol.1c00136
Author Contributions
†R.L. and M.R.T. contributed equally.
Funding
This work was supported by a National Science Foundation
CAREER grant (NSF-1944184) and Georgetown University.
Notes
The authors declare no competing financial interest.
(21) Susuki, K.; Rasband, M. N. ″Molecular mechanisms of node of
Ranvier formation″. Curr. Opin. Cell Biol. 2008, 20, 616−623.
(22) Susuki, K. ″Myelin: A Specialized Membrane for Cell
Communication″. Nat. Educ. 2010, 3, 59.
REFERENCES
■
(1) Cacialli, F.; Wilson, J. S.; Michels, J. J.; Daniel, C.; Silva, C.;
Friend, R. H.; Severin, N.; Samorì, P.; Rabe, J. P.; O’Connell, M. J.;
Taylor, P. N.; Anderson, H. L. ″Cyclodextrin-threaded conjugated
polyrotaxanes as insulated molecular wires with reduced interstrand
interactions″. Nat. Mater. 2002, 1, 160−164.
(23) Waxman, S. G.; Kocsis, J. D.; Stys, P. K. The Axon: Structure,
Function and Pathophysiology; Oxford university Press: New York,
1995.
(2) Frampton, M. J.; Anderson, H. L. ″Insulated molecular wires″.
Angew. Chem., Int. Ed. 2007, 46, 1028−1064.
(24) Morisaki, Y.; Chujo, Y. ″Through-space conjugated polymers
based on cyclophanes″. Angew. Chem., Int. Ed. 2006, 45, 6430−6437.
(25) Morisaki, Y.; Chujo, Y. ″Cyclophane-containing polymers″.
Prog. Polym. Sci. 2008, 33, 346−364.
(26) Morisaki, Y.; Chujo, Y. ″Through-space conjugated polymers
consisting of [2.2]paracyclophane″. Polym. Chem. 2011, 2, 1249.
(27) Jagtap, S. P.; Collard, D. M. ″2D Multilayered π-stacked
conjugated polymers based on a U-turn pseudo-geminal [2.2]-
paracyclophane scaffold″. Polym. Chem. 2012, 3, 463−471.
(28) Batra, A.; Kladnik, G.; Vazquez, H.; Meisner, J. S.; Floreano, L.;
Nuckolls, C.; Cvetko, D.; Morgante, A.; Venkataraman, L.
(3) Terao, J.; Ikai, K.; Kambe, N.; Seki, S.; Saeki, A.; Ohkoshi, K.;
Fujihara, T.; Tsuji, Y. ″Synthesis of a head-to-tail-type cyclodextrin-
based insulated molecular wire″. Chem. Commun. 2011, 47, 6816−
6818.
(4) Brovelli, S.; Cacialli, F. ″Optical and electroluminescent
properties of conjugated polyrotaxanes″. Small 2010, 6, 2796−2820.
(5) Terao, J.; Wadahama, A.; Matono, A.; Tada, T.; Watanabe, S.;
Seki, S.; Fujihara, T.; Tsuji, Y. ″Design principle for increasing charge
mobility of pi-conjugated polymers using regularly localized molecular
orbitals″. Nat. Commun. 2013, 4, 1691.
3118
https://doi.org/10.1021/acs.macromol.1c00136
Macromolecules 2021, 54, 3112−3119

Macromolecules
pubs.acs.org/Macromolecules
Article
″Quantifying through-space charge transfer dynamics in pi-coupled
molecular systems″. Nat. Commun. 2012, 3, 1086.
(29) Shibahara, M.; Watanabe, M.; Yuan, C.; Goto, K.; Shinmyozu,
T. ″Structural properties of charge-transfer complexes of multilayered
[3.3]paracyclophanes″. Tetrahedron Lett. 2011, 52, 3371−3375.
(30) Shibahara, M.; Watanabe, M.; Yuan, C.; Shinmyozu, T.
″Structural properties of charge-transfer complexes of four- and five-
layered [3.3]metacyclophanes″. Tetrahedron Lett. 2011, 52, 5012−
5015.
(31) Shibahara, M.; Watanabe, M.; Iwanaga, T.; Matsumoto, T.;
Ideta, K.; Shinmyozu, T. ″Synthesis, structure, and transannular pi-pi
interaction of three- and four-layered 3.3 paracyclophanes″. J. Org.
Chem. 2008, 73, 4433−4442.
(32) Nakano, T. ″Synthesis, structure and function of π-stacked
polymers″. Polym. J. 2010, 42, 103−123.
(33) Xia, J.-L.; Zhang, C.; Zhu, X.; Ou, Y.; Jin, G.-J.; Yu, G.-a.; Liu, S.
H. ″Substituted diethynyldithia[3.3]paracyclophanessynthetically
more accessible new building blocks for molecular scaffolding″. New
J. Chem. 2011, 35, 97−102.
(34) Mayo, F. R.; Walling, C. ″Copolymerization″. Chem. Rev. 1950,
46, 191−287.
(35) Mayo, F. R.; Lewis, F. M. ″Copolymerization. I. A Basis for
Comparing the Behavior of Monomers in Copolymerization; The
Copolymerization of Styrene and Methyl Methacrylate″. J. Am. Chem.
Soc. 1944, 66, 1594−1601.
(36) Painter, P. C.; Coleman, M. M. Essentials of Polymer Science and
Engineering; DEStech Publications, 2008.
(37) Wang, W.; Xu, J.; Lai, Y.-H.; Wang, F. ″Alternating aromatic
and transannular chromophores with and without linker: Effect of
transannular pi-pi interaction on the optical property of dithiapar-
acyclophane-based copolymers″. Macromolecules 2004, 37, 3546−
3553.
(38) Wang, W.-L.; Xu, J.; Sun, Z.; Zhang, X.; Lu, Y.; Lai, Y.-H.
″Effect of transannular pi-pi interaction on emission spectral shift and
fluorescence quenching in dithia 3.3 paracyclophane-fluorene
copolymers″. Macromolecules 2006, 39, 7277−7285.
(39) Yu, C.-Y.; Hsu, C.-C. ″Synthesis, characterization and
aggregation-induced emission of alternating copolymers containing
cyclophanes and tetraphenylethenes″. Polymer 2018, 137, 30−37.
(40) Fraiji, L. K.; Hayes, D. M.; Werner, T. C. ″Static And Dynamic
Fluorescence Quenching Experiments For The Physical-Chemistry
Laboratory″. J. Chem. Educ. 1992, 69, 424−428.
(41) Campbell, K.; Zappas, A.; Bunz, U.; Thio, Y. S.; Bucknall, D. G.
″Fluorescence quenching of a poly(para-phenylene ethynylenes) by
C-60 fullerenes″. J. Photochem. Photobiol. 2012, 249, 41−46.
3119
https://doi.org/10.1021/acs.macromol.1c00136
Macromolecules 2021, 54, 3112−3119