Cucurbit[7]uril-threaded fluorene-thiophene-based conjugated polyrotaxanes

Article abstract of DOI:10.1039/c6ra21622f

Here we investigate the effect of cucurbit[7]uril (CB7) on the thermal and optical properties of fluorene-thiophene based conjugated polyelectrolytes. For this purpose, poly(9,9′-bis(6′′-(N,N,N-trimethylammonium)hexyl)fluorene-alt-co-thiophenelene) P1 and

Full text of DOI:10.1039/c6ra21622f

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Cucurbit[7]uril-threaded fluorene–thiophene-
based conjugated polyrotaxanes†
Cite this: RSC Adv., 2016, 6, 98109
ac
M. Idris,a M. Bazzar,ac B. Guzelturk,bc H. V. Demirbc and D. Tuncel
*
Here we investigate the effect of cucurbit[7]uril (CB7) on the thermal and optical properties of fluorene–
thiophene based conjugated polyelectrolytes. For this purpose, poly(9,90-bis(600-(N,N,N-trimethylammonium)-
hexyl)fluorene-alt-co-thiophenelene) P1 and poly(9,90-bis(600-(N,N,N-trimethylammonium)propyl)fluorene-
alt-co-thiophenelene) P2 and their CB7-based polyrotaxane counterparts, P1CB7 and P2CB7, are
synthesized by threading the part of the conjugated backbone of these polymers with CB7 during their
synthesis. Threading efficiency in the P1CB7 containing hexyl pendant of as high as 50% is achieved, but in
the case of P2, with the propyl pendant, only around 15% is achieved. We observed significant changes in
the optical properties of both P1CB7 and P2CB7 with respect to their polymers P1 and P2. Fluorescent
quantum yields of P1 and P2 which are 0.11 and 0.35 have increased to 0.46 and 0.55 for P1CB7 (>4 fold)
and P2CB7, respectively. Moreover, polyrotaxanes compared to their polymers exhibit longer fluorescence
lifetimes in the solution and the solid state thanks to the suppressed overall nonradiative recombination via
encapsulation of the conjugated polymer backbone. Thermal analysis also indicates that polyrotaxanes have
higher thermal stabilities than their polymer counterparts. In order to demonstrate the applicability of the
synthesized materials, we also fabricated proof-of-concept light emitting diodes from P1 and its CB7-based
polyrotaxane counterpart P1CB7. The CB7-integrating polymer showed lower turn-on voltages with high
electroluminescence colour purity due to balanced charge injection in P1CB7 as compared to the P1 polymer.
Received 29th August 2016
Accepted 10th October 2016
DOI: 10.1039/c6ra21622f
www.rsc.org/advances
orthogonal solvents).7–12 In order to render these polymers
water-soluble, hydrophilic groups are used as pendant groups,
Introduction
however, due to the large aromatic hydrophobic backbone the
water solubility could be still limited. The attachment of the
ionic groups is another possibility but this can cause
a quenching in their emission properties. Luminescence
quenching is especially an important drawback when the
polymer solutions are casted as a lm for device fabrication.
The morphology of polymers is considerably affected by both
inter and intra-chain interactions; there are several approaches
for controlling these. One of the reasons of the quenching is the
p–p stacking between the polymer chains.13 A number of
different methods have been used to decrease the p–p stacking;
for instance, attaching bulky pendant groups such as dendrons
to polymer backbone, embedding polymers in silica nano-
particles or zeolites.14–16
Another promising approach is the encapsulation or insu-
lation of conjugated polymer backbone by rotaxanation.17,18
Rotaxanation is basically the threading of a polymer chain by
a macrocyclic ring. The properties of polymers can be altered
dramatically by the chemical nature of the macrocycle and the
degree of threading. There are many examples on the use of
cyclodextrins to insulate the conjugated polymer back-
bones,19–21 however, examples are very scarce on the use of
cucurbit[n]urils (CB[n]).13,22–25 Cucurbiturils (CBs) are macro-
cycles with a rigid symmetrical structure that are composed of n
Conjugated polymers are very adaptable materials and as
a result, can nd many important applications including in
light emitting diodes (LED), photovoltaic cells, solid state
lighting, actuators and articial muscles.1–6 They are particu-
larly appealing in the fabrication of LEDs for exible and roll-
able display applications owing to their tunable optical
properties (e.g. absorption, emission wavelengths and band
gap) through judicious chemical modications in their struc-
tures. Some of the important parameters regarding these poly-
mers are uorescent quantum yields, photo and thermal
stabilities as well as the solubility. Their solubility properties
can be tuned by attaching appropriate solubilizing groups to the
backbone of the polymers by not harming the p-conjugation.
Water solubility is also important to eliminate the use of
harmful organic solvents and also for multilayer lm formation
to achieve different solubilities of the lms (e.g., using
aDepartment of Chemistry, Bilkent University, 06800 Ankara, Turkey. E-mail:
dtuncel@fen.bilkent.edu.tr
bDepartments of Electrical and Electronics Engineering and Physics, Bilkent University,
06800 Ankara, Turkey
cUNAM–National Nanotechnology Research Center, Institute of Materials Science and
Nanotechnology, Bilkent University, Ankara 06800, Turkey
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra21622f
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glycoluril units (where n ¼ 5, 6, 7, 8, 10, and 14) connected by
The structure of P1, P2, P1CB7 and P2CB7 were veried by 1H
1
methylene bridges and possess two identical hydrophilic NMR spectroscopies (Fig. 1 and 2). The broad peaks in the H-
portals with a hydrophobic cavity.26–29 The diameter of the cavity NMR spectra of P1 and P1CB7 can be attributed to the chain
varies depending on the number of glycoluril units. A variety of entanglement due to the long hexyl side chain creating different
nanostructured materials including nanoparticles, nano- environment of the protons in the structure. Comparing P1 and
composites, vesicles, rods have been prepared by taking P1CB7 1H-NMR spectra, peaks at around 6.0–6.5 ppm in P1CB7
advantage of the varying cavity size of the CB homologues, their spectrum can be assigned to the protons of thiophene which are
ability to accommodate more than one guests in their cavities, encapsulated by CB7. The integration of these peaks reveals that
their rigid symmetrical structures.30–32 Moreover, they have been approximately for every two repeating units in the polymer
widely used to encapsulate and solubilize dyes.33,34
Herein, by taking advantage of the ability of CB7 to encap- sponds to about a 50% threading efficiency.
backbone, one thiophene is encapsulated by CB7 which corre-
sulate the properly sized aromatic rings and water solubility,
1H-NMR spectra of P2 and P2CB7 show less aggregation
poly(9,90-bis(600-(N,N,N-trimethylammonium)hexyl)uorene-alt- between the polymer chains. This can be explained by the
co-thiophenelene) P1 and poly(9,90-bis(600-(N,N,N-trimethy- shorter side chain of these polymers. Although the presence of
1
lammonium)propyl)uorene-alt-co-thiophenelene) P2 and their CB7 is evident in the H-NMR spectrum of P2CB7, no peak is
CB7-based polyrotaxane counterparts, P1CB7 and P2CB7 are observed around 6.0–6.5 ppm probably due to low contents of
synthesized by threading the part of conjugated backbone of CB7. From the integration of signals of CB7 protons, the
these polymers with CB7 during their synthesis and their threading efficiency was calculated to be around 15% which
optical and thermal properties have been compared.
corresponds to about every six repeating unit containing one
CB7.
In order to further support the presence of CB7 due to the
encapsulation of conjugated backbone rather than complexa-
tion with pendant, polymers P1 and P2 were separately mixed
with CB7 at certain ratios and dissolved in dilute aqueous
Results and discussion
Synthesis and structural characterization
All polymers and polyrotaxanes were synthesized according to solution of HCl (around 0.01 M) and subsequently the solu-
reaction Scheme 1 in good yields. First, 2,7-dibromo-9,9-bis(6- tions were subjected to the ultraltration as used in the puri-
bromohexyl)-9H-uorene and 2,7-dibromo-9,9-bis(3-bromo- cation of the aforementioned polymers and polyrotaxanes.
propyl)-9H-uorene were synthesized following the literature 1H-NMR spectrum of retentate recorded aer the ultraltra-
preparation,35–37 and subsequently, they were converted into the tion showed the presence of only trace amount of CB7. This
monomers M1 and M2 respectively by treating with excess tri- experiment indirectly proves that the presence of CB7 is due to
methylamine solution in tetrahydrofuran (THF) (Fig. S1–S10, the encapsulation of the part of the polymer main chain as
ESI†). Polymers, P1 and P2 were synthesized by Pd-catalyzed aimed.
1
Suzuki coupling of M1 and M2 with 2,5-thiophenediboronic
We have also recorded H-NMR spectrum of the mixture of
ester. For the synthesis of polyrotaxanes, 2,5-thiophenedibor- 2,7-dibromo-9,9-bis(6-bromohexyl)-9H-uorene with CB7 in
onic ester and excess CB7 in aqueous solution were rst stirred order to reveal whether CB is threaded onto hexyl pendant
at 50 ꢀC for 2 h to allow complexation of CB7 with 2,5-thio- group. In the spectrum, no changes were observed in the
phenediboronic ester followed by the coupling reaction. The chemical shis of the methylene protons of the hexyl group. We
OH groups in 2,5-thiophenediboronic acid were protected using have also reported this in our previous publication.25 Although
1,3-propanediol from forming hydrogen bond with the carbonyl we can expect an ion–dipole interaction between ammonium
oxygens of CB7 so that inclusion of the thiophene into CB7 will ions and the carbonyl portals of the CB, there are a lot of
not be hindered. Fig. S11† shows the spectra of 2,5-thio- potassium ions (coming from K2CO3 used as a base in the
phenediboronic ester in the presence (3 folds excess of 2,5-thi- polymerization) which can compete with ammonium ions and
ophenediboronic ester) and in the absence of CB7. In order to complex with excess, free CB7.
remove excess, free CB7 and unreacted monomers and oligo-
Thus, the resulting complexes are removed during ultral-
mers, the purication was carried out by ultraltration using a 5 tration/dialysis.
kDa membrane against distilled water. The ultraltration was
The reason of the higher threading efficiency in P1CB7
kept until the dialysates contain no free CB7 and other small compared to P2CB7 can be explained by the side chain lengths
molecules. Retentate solution was concentrated to a possible of the uorene monomers used. Monomer M2 contains a short
minimum volume and precipitated into excess acetone in order propyl pendant group and during Suzuki coupling, propyl
to remove the remaining catalyst residues. Dark green or dark group acts as a rigid moiety by preventing the CB7-thiophene
yellowish precipitates were collected and dried under vacuum. diboronic ester being in close proximity due to steric effect.
We have also tried to use CB6 in the polyrotaxane synthesis but Whereas, monomer M1 contains a exible hexyl group that can
apparently due to low solubility of CB6 in water we could not ex itself during the coupling and not preventing CB7-
obtain the desired polyrotaxanes.
thiophene diboronic ester to be in a closer contact.
Both polymers and the polyrotaxanes are soluble in water. P1
These compounds were further characterized by FTIR spec-
and P2 are also soluble in methanol however the solubility of troscopy (Fig. S14, ESI†). The peak at 1740 cmꢁ1 for P1CB7 and
the polyrotaxanes in methanol is very limited.
P2CB7 suggests the presence of carbonyl group due to CB7
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Scheme 1 Synthetic schemes for poly[9,9-bis{6(N,N,N-trimethylamino)hexyl}fluorene-co-2,5-thienylene (P1), poly[9,9-bis{3(N,N,N-trimethy-
lamino)propyl}fluorene-co-2,5-thienylene (P2) and their CB7-based polyrotaxane counter parts P1CB7 and P2CB7. Reaction conditions: (i)
TBAB, 50% wt NaOH (aq), DMSO; (ii) Me3N, THF, 25 ꢀC, 24 h; (iii) Pd(OAc)2, K2CO3, H2O, 80 ꢀC, 48 h.
Fig. 1 1H-NMR (400 MHz, 298 K) spectra of (a) P1CB7 in D2O and (b) P1 in D2O. * denotes acetone residue.
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Fig. 2 1H-NMR (400 MHz, 298 K, D2O) spectra of (a) P2CB7 (b) P2.
which is absent in P1 and P2. Although we have tried various original curves and listed in Table 1. Conversely, TGA prole of
conditions and solvents for molecular weight determination of polymers shows that decomposition occurs in several steps, in
all the polymers and polyrotaxanes using size exclusion chro- which involves the decomposition of alkyl chains, ammonium
matography (SEC), we were not able to obtain satisfactory groups and CB7. Polyrotaxanes P1CB7 and P2CB7 showed
results probably due to the irreversible adsorption of the poly- higher thermal stability than the corresponding polymers (P1
ꢀ
mer onto the column packing material due to the ionic charge and P2). The T20 value has increased from 205 to 240 C for
ꢀ
ꢀ
on the polymers. However, ESI suggest a molecular weight up to P2CB7 and from 240 C to 380 C for P1CB7. Although both
5000 Da with multiple charges of both polymers and poly- polyrotaxanes exhibit higher thermal stabilities than their
rotaxanes. The estimate number of repeating units for polymers polymer counterparts, polyrotaxane P1CB7 with a higher
and polyrotaxanes would be around 7 and 2–3, respectively. threading efficiency than P2CB7 starts to decompose even at
However, ESI-MS is probably under estimating the real MW of
the polymers and the polyrotaxanes owing to difficulty of the
ionizing the large chains.
Table 1 Thermal properties of P1, P2 and polyrotaxanes P1CB7,
P2CB7
C. Y.b (%)
a
T20 (ꢀC)
Thermal properties
Polymers
The thermal stability of the polymers P1, P2 and polyrotaxanes
P1CB7 and P2CB7 were evaluated by thermal gravimetric
analysis (TGA) under nitrogen atmosphere at a heating rate of
P1
P1CB7
P2
240
380
205
240
47
40
50
35
10 C minꢁ1. Fig. 3 presents their TGA curves. Corresponding
ꢀ
P2CB7
a
b
weight loss temperatures of 20% (T20) and char yield-weight of
polymers remained at 650 ꢀC were all determined from
T20: temperature for 20% weight loss. C. Y.: char yield-weight of
polymer remained at 650 ꢀC.
Fig. 3 TGA curves of polymers P1, P2 and polyrotaxanes P1CB7, P2CB7 under N2 at 10 ꢀC minꢁ1
.
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Fig. 4 UV-vis absorption and emission spectra of P1, P2, P1CB7 and P2CB7 in aqueous media.
higher temperatures. Thus, this clearly conrms the effect of
CB on the thermal stability of polymers.
We checked the uorescence kinetics of the polymers
through time resolved uorescence spectroscopy. Fluorescence
lifetime of P1 and P2 are 0.86 and 1.145 ns in solution state.
When these are drop casted on quartz substrates, the uores-
cence lifetimes decrease down to 0.036 and 0.161 ns for P1 and
P2, respectively. On the other hand, polyrotaxanes P1CB7 and
P2CB7 show much longer uorescence lifetime in their solid
lms (0.559 and 0.681 ns, respectively) as compared to their
solutions (1.112 and 1.385 ns, respectively). The slow-down of
the uorescence decay in P1CB7 and P2CB7 as compared to
their pristine versions (i.e., P1 and P2) indicates that the
quenching of the threaded polymers is suppressed at the solid-
state by blocking the formation of inter-chain quenching/
trapping sites. Thus, this makes CB7-incorporated polymers
appealing for potential applications including organic light
emitting diodes (OLEDs). From Fig. 4–6 and Table 2, P1CB7
showed increased of the uorescence lifetime in both aqueous
media and solid state compared to P1. Albeit the difference in
life time of P2CB7 and P2 is not as large as the difference
observed in the case of between P1CB7 and P1. This can be
explained by the previous argument of the low threading effi-
ciency of P2CB7.
Photophysical properties
Blue shi was observed in the absorption and uorescence
maxima of P1CB7 and P2CB7 compared to P1 and P2, respec-
tively (Fig. 4) in aqueous media. This spectral shi was also
observed in the solid state uorescence spectra (Fig. 5). In both
P1CB7 and P2CB7, a great enhancement in both quantum yield
efficiency and molar absorptivity was observed relative to P1
and P2, respectively. The quantum yield efficiency and peak
molar absorptivity of P1CB7 in water was found to be 0.46 and
86 294 Mꢁ1 cmꢁ1 compared to 0.11 and 26 563 Mꢁ1 cmꢁ1 for P1
(Table 2). Similarly the quantum yield efficiency and peak molar
absorptivity of P2CB7 in water was found to be 0.55 and 42 611
Mꢁ1 cmꢁ1 compared to 0.35 and 28 376 Mꢁ1 cmꢁ1 for P2. This
enhancement could be attributed to the reduction of p–p
interaction between the polymer chains. Moreover, while the
solubility of the polymers in water is increased, the aggregate
formation is decreased upon rotaxanation. The enhancement of
quantum yield and molar absorptivity of P2CB7 is less
compared to P1CB7 because of the lower threading efficiency in
P2CB7.
In order to further conrm that spectral changes mentioned
above are because of the encapsulation of conjugated backbone
Fig. 5 Photoluminescence spectra of P1, P2, P1CB7 and P2CB7 in solid state.
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Table 2 Photophysical properties of P1, P2, P1CB7 and P2CB7
a
lmax,solution
lmax,lmb (nm)
(nm)
3
d (cm M)ꢁ1
me (cm M)ꢁ1
ssolution (ns)
slmg (ns)
c
f
UV
PL
PL
FPL
P1
P1CB7
P2
427
407
413
400
501
474
465
463
541
532
528
510
0.11
0.46
0.35
0.55
26 563
86 294
28 376
42 611
2921
39 696
9932
0.860
1.112
1.145
1.385
0.036
0.559
0.161
0.681
P2CB7
23 436
a
In aqueous media. b On quartz. c Fluorescent quantum yields (FPL) were measured relative to quinine sulfate 0.1 M H2SO4 (FPL ¼ 0.55). d Molar
absorptivity (per repeat unit). e Brightness (m) ¼ FPL ꢃ 3. Life time (s) in aqueous media. g Life time (s) on quartz.
f
Fig. 6 Biexponentially fitted fluorescence lifetime decay curves of P1, P2, P1CB7 and P2CB7 in aqueous media and in solid state.
rather than complexation of CB7 with pendant or ammonium PEDOT:PSS/poly-TPD acts as hole injection-transport and elec-
groups, optical properties of the aqueous solutions of physical tron blocking layers, and ZnO nanoparticles acts as electron
mixtures of polymers P1 and P2 with CB7 at certain ratios (1 : 1, injection and hole-blocking layer. The OLEDs are fabricated as
one equivalent per repeating unit of polymer/one equivalent described in the Experimental section. Fig. 7a shows the elec-
CB7) were investigated by UV-vis absorption, steady-state and troluminescence spectra of the OLED with P1 polymer, which
time-resolved uorescent spectroscopies (see spectra in exhibited spectrally broad emission arising from both P1 and
Fig. S15–S17, ESI†).
poly-TPD hole-transport layer (peak at ꢂ425 nm). This suggests
No spectral changes or shis were observed in the absorp- that exciton formation is not complete in the OLED with P1
tion and emission wavelengths as well as in their uorescent emissive polymer layer. This could be either due to impeded
quantum yields of the samples of the physical mixtures in hole injection into the P1 or signicant leakage of electrons into
aqueous media with respect to polymers P1 and P2 (Fig. S15 and the poly-TPD layer through P1. In polyuorene-based OLEDs,
S16, ESI†).
hole injection has been well-known to be quite limited due to
However, the lifetimes of P1 (0.860 ns) and P2 (1.145 ns) low-lying HOMO levels in these polymers. Therefore, exciton
decreased to 0.481 ns and 0.569 ns when they are physically formation is not efficient in the emissive polymer layer when
mixed with CB7 (Fig. S17, ESI†); this is in marked contrast to the using P1. In the case of OLED with P1CB7 active emissive layer,
long lifetimes of polyrotaxanes which are 1.112 ns and 1.385 for the electroluminescence spectrum shows dominant P1CB7
P1CB7 and P2CB7, respectively. These ndings also conrm that emission (see Fig. 7b). This strongly suggests that exciton
the part of the conjugated backbone is encapsulated by CB7.
formation is efficient in the P1CB7 layer. This could be due to
modication of the HOMO level in the P1CB7 polymer. More-
over, better solid-state packing of the P1CB7 polymer, which has
shown signicantly higher PL quantum yields as compared to
its pristine counterpart, could also allow for the enhanced
carrier injection and exciton formation in the active layer.
Another observation is the lower turn-on voltages of the P1CB7
OLEDs (<6 V) as compared to the one with P1 (ꢂ9 V). This also
highlights that overall charge injection is more favorable when
using P1CB7. These observations make a point that introducing
Electroluminescence properties
Enhanced mechanical and photophysical properties in the
P1CB7 and P2CB7 polymers suggest that their electrolumines-
cence performance in OLEDs could surpass those of the pristine
polymers P1 and P2. To test this hypothesis, we produced proof-
of-concept solution-processed OLEDs having in the structure
of ITO/PEDOT:PSS/poly-TPD/polymer/ZnO NPs/Al, where
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Fig. 7 The normalized electroluminescence spectra of the OLEDs that employ (a) P1, (b) P1CB7 at different driving voltages.
CB7 group into the uorene–thiophene based conjugated Talha Erdem for his help in the measurement of some of the
polyelectrolytes improves their electroluminescence perfor- time resolved uorescence spectra, reading our manuscript and
mance as well.
providing insightful comments.
Conclusions
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Here, we have shown that the optical and thermal properties of
uorene–thiophene based conjugated polyelectrolytes could be
altered by threading with CB7. Threading efficiency in P1CB7
containing hexyl pendant was estimated to be as high as 50%
but in the case of P2 with propyl pendant was only around 15%.
We attributed the threading efficiency differences to the steric
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voltages as compared to that of the pristine polymers thanks
to the improved nanoscale morphology and photophysical
properties in the CB7 threaded polymers.
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