Relationships between main-chain chirality and photophysical properties in chiral conjugated polymers

Article abstract of DOI:10.1039/c4tc01160k

A series of R- and S-binaphthyl-containing polyfluorenes, bearing different contents and types of axial chirality in the main chain, have been synthesized through Suzuki polycondensation to investigate the influence of the covalently incorporated chirality on the photophysical properties of chiral conjugated polymers. The experimental measurements obtained by UV-Vis and photoluminescence (PL) spectroscopy, cyclic voltammetry, and circular dichroism spectroscopy reveal that the chiral copolymers possess high thermostability, high luminescence efficiency, reversible electrochemical properties, and intrachain transferred dichroism. Surprisingly, the R-chiral polymers exhibit better spectral thermostability, stronger suppressing ability upon the β-phase formation of the main-chain, and higher PL quantum efficiency than S-chiral polymers in solid films. The theoretical insights obtained by either ab initio density functional theory (DFT) calculations or molecular dynamics (MD) simulations suggest that these differences probably resulted from the more planar chain conformation of S-chiral polymers, which leads to stronger interchain interaction and an increased tendency to form inefficient and unstable excimers or quenchers. The different effects of enantiomers on the photophysical properties of chiral conjugated polymers may provide an import update in the understanding of chiro-optics. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4tc01160k

Journal of
Materials Chemistry C
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PAPER
Relationships between main-chain chirality and
photophysical properties in chiral conjugated
polymers†
Cite this: J. Mater. Chem. C, 2014, 2,
7336
a
a
b
bc
Yang Wang,a
*
Chao Zheng, Zhongfu An, Yosuke Nakai, Taiju Tsuboi,
Huifang Shi,ac Runfeng Chen, Huanhuan Li, Yimu Ji, Junfeng Li and Wei Huang
a
a
a
a
ac
*
*
A series of R- and S-binaphthyl-containing polyfluorenes, bearing different contents and types of axial
chirality in the main chain, have been synthesized through Suzuki polycondensation to investigate the
influence of the covalently incorporated chirality on the photophysical properties of chiral conjugated
polymers. The experimental measurements obtained by UV-Vis and photoluminescence (PL)
spectroscopy, cyclic voltammetry, and circular dichroism spectroscopy reveal that the chiral copolymers
possess high thermostability, high luminescence efficiency, reversible electrochemical properties, and
intrachain transferred dichroism. Surprisingly, the R-chiral polymers exhibit better spectral
thermostability, stronger suppressing ability upon the b-phase formation of the main-chain, and higher
PL quantum efficiency than S-chiral polymers in solid films. The theoretical insights obtained by either ab
initio density functional theory (DFT) calculations or molecular dynamics (MD) simulations suggest that
these differences probably resulted from the more planar chain conformation of S-chiral polymers,
which leads to stronger interchain interaction and an increased tendency to form inefficient and
unstable excimers or quenchers. The different effects of enantiomers on the photophysical properties of
chiral conjugated polymers may provide an import update in the understanding of chiro-optics.
Received 3rd June 2014
Accepted 25th June 2014
DOI: 10.1039/c4tc01160k
www.rsc.org/MaterialsC
Taking the widely available chiral 1,10-binaphthyls as an
example, since the 1990s, when Pu's laboratory started
systematic investigations on these axial chiral compounds,11
Introduction
Chirality plays a fundamental part in the activities of biological
molecules and broad classes of chemical reactions and inter-
actions,1 providing a wide range of potential applications in
pharmacology,2 chiral separation,3 enantioselective sensors,4
asymmetrical catalysis and reaction,5 and nonlinear optics.6 The
addition of a chiral compound into an achiral polymer is one of
the most promising means to introduce chirality to a polymer.7
Successful attempts to covalently plant chirality into the back-
bone of the widely used and optically and electronically active
p-conjugated polymers have produced exciting features for both
sides of chiral molecules and optoelectronic polymers, resulting
in a new generation optoelectronic material of chiral conju-
gated polymers.8–10
various main-chain chiral conjugated polymers have been
prepared by covalently incorporating the biaryl-conjugated and
highly stable chiral 1,10-binaphthyl into the main-chain of a
variety of conjugated polymers,7,12–14 including arylenvinylenes,
arylenes, aryleneethylnylenes, thiophenes, and uorenes. The
chirality of 1,10-binaphthyl is generally well preserved, trans-
ferred, or even amplied in the corresponding chiral conju-
gated polymers,15 which are attractive for advanced applications
in chiral uorescent sensors,16,17 polymeric enantioselective
catalysts,18 polarized or anisotropic light emissions,19 and
helical macromolecules.20 In addition to the axial chirality,
the highly soluble and twisted molecular structure of
1,10-binaphthyl also benets the conjugated polymers with good
solution processability and high luminescent efficiency due to
the alleviated self-quenching between the luminophores in
amorphous phase.21,22 In addition, the changeable dihedral
angle from 60ꢀ to 120ꢀ between the two bulky naphthyl groups
of the non-planar binaphthyl offers a feasible way to control the
effective conjugation length, and thus the emission color of the
conjugated polymer chain without the need to insert an addi-
tional bandgap adjusting unit.23 As a result, binaphthyl-based
conjugated polymers have found impressive applications in
optoelectronics, such as in organic light emitting diodes
aInstitute of Advanced Materials, Key Laboratory for Organic Electronics & Information
Displays (KLOEID), Nanjing University of Posts and Telecommunications (NJUPT),
Wenyuan Road 9, Nanjing 210023, China. E-mail: iamrfchen@njupt.edu.cn
bFaculty of Engineering, Kyoto Sangyo University, Kamigamo, Kyoto 603-8555, Japan
cJiangsu-Singapore Joint Research Center for Organic/Bio-Electronics & Information
Displays and Institute of Advanced Materials, Nanjing Tech University, Nanjing
211816, China
† Electronic supplementary information (ESI) available: The detailed
characterization of the chiral monomers and conjugated polymers, and the
temperature-dependent PL spectra of poly(9,90-octyluorene) and the chiral
conjugated copolymers. See DOI: 10.1039/c4tc01160k
7336 | J. Mater. Chem. C, 2014, 2, 7336–7347
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(OLEDs),24 transistors,25 non-linear optics,26,27 and solar cells.28 and R/S-binaphthol-uorene copolymers were synthesized
In the above research elds of conjugated polymers, however, according to methods reported in the literature.15,16 Both start-
the effects of chirality on the optoelectronic properties have ing chiral compounds (R-binaphthol and S-binaphthol) for
been less addressed and R-binaphthyls were used in most synthesis of chiral (R/S)-6,60-dibromo-2,20-dihydroxy-1,10-
reports because it is widely believed that the stereo conforma- dinaphthyl have high stereoregularity (ee > 99%). Aer careful
tions of the R- and S-enantiomers have identical or symmetric purication, the purity of the prepared R- and S-monomers was
optical and electronic properties in symmetric environments. In also identied to be higher than 99.5% by high-performance
our previous work,29 (R)-6,60-dibromo-2,20-bis(octyloxy)-1,10- liquid chromatography (HPLC). A detailed synthesis and char-
binaphthyl, however, was found to have a higher photo- acterization of the R-type binaphthol monomer and its chiral
luminescence (PL) quantum efficiency than its S-enantiomer in copolymers can be found in our previous report.14 The S-type
the condensed solid state, possibly due to its more twisted monomer and copolymers were prepared via the same proce-
molecular structure at both ground and excited states, which dure used for the R-type.
suppresses intermolecular interaction and the formation of
excimers and quenchers. This nding provides a new clue for
exploring the inherent chiral effects on the photophysical
properties of optoelectronic materials.
Copolymerization
To a mixture of 2,20-(9,9-dioctyl-9H-uorene-2,7-diyl)bis(1,3,2-
dioxaborinane) (1 equiv.), dibromo compounds of (R/S)-6,60-
dibromo-2,20-bis(octyloxy)-1,10-binaphthyl and 9,9-dioctyl-2,7-
dibromouorene (1 equiv.), Aliquat 336, and 2.0 mol%
Pd(PPh3)4 was added a degassed mixture of toluene ([mono-
mer] ¼ 0.25 M) and aqueous 2 M potassium carbonate (3 : 2 in
In this paper, we incorporated the two chiral 1,10-binaphthol
enantiomers into the backbone of polyuorene to further
investigate the inuence of the different types of main-chain
chirality on the photophysical properties of conjugated poly-
mers. The rich optical and electronic properties of poly-
uorenes,30,31 such as high PL efficiency, high charge-carrier
mobility, good processability, as well as the ease in excimer and/
or uorenone defect formation, glassy phase (a-phase) and
ordered phase (b-phase) transition,31 and bandgap and colour
tuning, provide a satisfactory platform for the in-depth inves-
tigation of structure–property relationships. Experimentally, by
UV-Vis absorption and PL spectroscopy, circular dichroism (CD)
spectroscopy, cyclic voltammetry, and quantum efficiency
measurements, as well as theoretically by ab initio density
functional theory (DFT) calculations and molecular dynamics
(MD) simulations, the chirality–photophysical property rela-
tionships were systematically investigated. We found obvious
differences between the R- and S-conjugated polymers. The
R-polymers have a higher PL quantum efficiency with higher
spectral thermostability, while S-polymers exhibit a stronger
ability to suppress the b-phase formation for the construction of
a color stable amorphous a-phase in the solid state, probably
ꢀ
volume). The mixture was vigorously stirred at 85–90 C for 72
h. Aer the mixture was cooled to room temperature, the
solution was add dropwise to a mixture of methanol and
deionized water (220 mL, 10 : 1 v/v). A brous solid was
observed and collected by ltration. The solid was dissolved in
THF for purication using ash column chromatography
(neutral Al2O3 gel, THF) to remove the residual catalyst. The
concentrated solution obtained was dropped slowly into
methanol (250 mL) again. The brous solid was collected and
further puried by continuous washing with hot acetone in a
Soxhlet apparatus for 2 days to remove oligomers and catalyst
residues (yields: 55–70%). 1H NMR spectra (Fig. S5†) were
obtained to identify the actual composition in the copolymer
according to the characteristic signals of uorene (2.11 ppm)
and binaphthol (4.00 ppm).
Measurements
owing to the stronger interchain interaction of S-polymers Nuclear magnetic resonance (NMR) spectra were collected on a
resulting from their more planar chain conformation. The easy Bruker Ultra Shield Plus 400 MHz instrument with chloroform-
introduction and transition of chirality into the backbone of D as the solvent and tetramethylsilane (TMS) as the internal
polyuorene to control the conjugation length, intrachain standard. Mass spectra of monomers were obtained a Shimadzu
order, PL efficiency, and dichroism may enhance the current GCMS-QP2010. The number-average molecular weight (Mn) and
understanding of the effects of enantiomers on photophysical weight-average molecular weight (Mw) of the polymers were
properties of main-chain chiral conjugated polymers.
measured by gel permeation chromatography (GPC) with Shim-
pack GPC-80X columns using polystyrene as the standard and
THF as the eluent. The UV-vis absorption spectra were recorded
Experimental
Materials
on
a UV-3600 Shimadzu UV-vis spectrophotometer. The
concentrations of the polymer solution (in THF) were adjusted
All reagents, unless otherwise specied, were purchased from to be less than 0.01 mg mLꢁ1. The thin solid lms were
Aldrich, Across, or Alfa, and used without further treatments. prepared by spin-coating on quartz substrates from THF solu-
All manipulations involving air-sensitive reagents were per- tions at a spin rate of 3500 rpm. Photoluminescence spectra at
formed in an atmosphere of dry N2. Tetrahydrofuran (THF) and various temperatures were measured with Spex Fluorolog-3
toluene were dried and puried by routine procedures. Shimadzu spectrophotometers. Circular dichroism (CD) spec-
9,9-Dioctyluorene-2,7-dibromide, 9,9-dioctyluorene-2,7-bis- troscopy measurements of the chiral polymers in THF solution
(trimethylene boronate), (R/S)-6,60-dibromo-2,20-dihydroxy-1,10- were carried out on JASCO J-810 spectropolarimeter at 25 ꢀC,
binaphthyl, (R/S)-6,60-dibromo-2,20-bis(octyloxy)-1,10-binaphthyl while their CD spectroscopy measurements in thin solid lms
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were measured using a JASCO J-40A spectropolarimeter with a
450 W Xe-lamp as the light source. The optical rotation of
copolymers in THF solution was measured using a WZZ-2S (2SS)
digital automatic polarimeter, and the wavelength of the
sodium lamp was 589.44 nm. Thermogravimetric analyses
(TGA) were conducted on a DTG-60 Shimadzu thermal analyst
system under a heating rate of 10 ꢀC minꢁ1 and a nitrogen ow
rate of 50 cm3 minꢁ1. Differential scanning calorimetry (DSC)
was run on a Pyris 1 DSC (Perkin Elmer, Co.) Shimadzu thermal
analyst system. The glass transition temperature (Tg) was
measured on the second curve aer heating to 230 ꢀC and
cooling to room temperature, and the decomposition temper-
ature (Td) was measured from room temperature to 600 ꢀC
Results and discussion
Synthesis and physical properties
To eliminate the impurity effects, the chiral monomers were
prepared from high-standard commercial R- and S-binaphthols
(ee > 99%) and carefully puried to remove chemical and chiro-
optical impurities (purity > 99.5%). The chiral conjugated
copolymers were then synthesized via Suzuki coupling poly-
condensation between uorene and chiral (R or S) binaphthols
comonomers under various feed contents of 5, 10, 20, 30, and
50 mol% (Scheme 1), resulting in corresponding copolymers
(R-polymers or S-polymers) of R5, R10, R20, R30, R50 and S5,
S10, S20, S30, S50. The polymers were also carefully puried.
ꢁ1
1
ꢀ
under a nitrogen atmosphere at a heating rate of 10 C min
.
From the characteristic H NMR signals of the repeating units
The highest occupied molecular orbital (HOMO), the lowest
unoccupied molecular orbital (LUMO) and the energy gap
between them (Eg) were measured by cyclic voltammetry (CV).
The CV measurements were performed at room temperature
under argon protection on a CHI660E system in a typical three-
electrode cell with a working electrode (glass carbon), a refer-
ence electrode [Ag/Ag+, referenced against ferrocene/ferroce-
nium (FOC)], and a counter electrode (Pt wire) in an acetonitrile
of uorene (2.11 ppm) and binaphthol (4.00 ppm), the actual
compositions of the copolymers were calculated and are listed
in Table 1. The slightly higher R-binaphthol content in copoly-
mer than in feed (especially obvious at a low feed ratio) suggests
the copolymerization reactivity of dibromo-(R)-binaphthol is
higher than that of dibromouorene.33 In contrast, the
dibromo-(S)-binaphthol shows a little lower copolymerization
activity than dibromouorene under the same Suzuki copoly-
merization conditions. This difference between reactivity of the
two enantiomers observed experimentally may be the main
reason for the higher number-average molecular weights (Mn) of
R-polymers based on the more reactive R-binaphthol. Despite
the slight differences in both chain composition and molecular
weight, the enantiomorphic R- and S-copolymers have similar
thermal properties (Table 1) with a slightly increased glass
transition temperature (Tg) and decreased decomposition
temperature (Td) aer the incorporation of an increased content
of bulky and twisted binaphthol units. In comparison with
polyuorene (Tg ¼ 51 ꢀC, Td ¼ 469 ꢀC),33 the increased Tg of the
binaphthol containing conjugated functional polymers along
solution of Bu4NPF6 (0.10 M) at a sweeping rate of 100 mV sꢁ1
.
The solutions were bubbled with a constant nitrogen ow for
15 min before measurements. The HOMO/LUMO energy levels
of the material were estimated based on the reference energy
level of ferrocene (4.8 eV below the vacuum): HOMO/LUMO ¼
ꢁ[Eonset ꢁ (0.015)] ꢁ 4.8 eV, where the value of FOC vs. Ag/Ag+ is
0.015 V and Eonset is the onset potential.
Calculations
Theoretical ab initio calculations of the model R- and S-mole-
cules were performed with the Gaussian 09 program using
6-31G(d) basis set. In the single molecular states, the ground-
state geometries were fully optimized by Becke's three-param-
eter exchange functional along with Lee–Yang–Parr's correla-
tion function (B3LYP), followed by harmonic vibration
frequency analysis to check that real local minima had been
found without imaginary vibrational frequency. The simulated
circular dichroism (CD) spectra in vacuum were obtained by the
time-dependent density functional theory (TDDFT) method of
B3LYP based on the optimized ground state geometries
adjusted by changing the rotation angles of the two biphenyl
rings from 40ꢀ to 140ꢀ. The corresponding HOMO and LUMO
energy levels, energy variation, and oscillator strength were
collected according to literature reports.32 Molecular dynamics
(MD) simulations were performed using NAMD (version 2.6)
with periodic boundary conditions in an NPT ensemble at
atmospheric pressure and 300 K. The general amber force eld
(GAFF) was chosen, because it was developed to cover a wide
range of organic species and has good performance for the
ꢀ
with high Td (>400 C) offers stable amorphous states for their
optoelectronic applications, since microcrystallization generally
shows detrimental effects on luminescence efficiencies.34
However, the slightly lower liquid crystalline temperature
ꢀ
(TLC ¼ 130 C) of R5 compared to that of polyuorene (TLC
¼
175 ꢀC)35 can be easily found, but there is no TLC in S5, sug-
gesting that S-binaphthol has a stronger impact on the packing
behaviors of the poly(9,90-octyluorene) chains. Excellent solu-
bility of the chiral R/S-polymers in common solutions such as
chloroform, tetrahydrofuran (THF), toluene, and chlorobenzene
was also observed, attributed to both the twisted chiral
binaphthol in the backbone and to the long alkyl chains of octyl
substituents on binaphthol and uorene. Good thin solid lms
of the chiral copolymers can be easily constructed by spin-
coating on quartz substrates through their organic solutions,
which is feasible for potential optoelectronic device
fabrications.36
From the literature reports37 there are certain relationships
between the synthesis method and the solid lm polymer
phase. The aggregated structure is intrinsically determined by
its chain structure. However, there are many more factors37c,d
that have profound effects on the formation of the polymer
˚
structure prediction of polymers. A cutoff of 11 A for non-
bonded interactions and a 1 ps equilibration with a time-step of
5 ns were adopted to obtain the energy-minimized polymer
conformation. The molecular graphics soware VMD was used
to visualize the trajectories.
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Scheme 1 Synthesis of R/S-type chiral conjugated polymers with corresponding R/S-binaphthol feed contents of 5, 10, 20, 30, and 50 mol%.
Table 1 Structure and thermal properties of the chiral conjugated polymers
Polymer
Feed (%)
Compos. (%)
Mn
Mw/Mn
Tg (ꢀC)
Td (ꢀC)
R5/S5
5
10
20
30
50
7.5/4.9
11/9.4
20/19.9
30/29.3
50/50
21 500/21 100
19 200/19 900
18 700/15 900
16 600/13 400
9280/8500
1.8/1.6
1.7/1.7
1.6/1.6
1.5/1.7
1.3/1.2
51/51
53/54
57/56
62/59
68/67
420/423
418/420
417/418
413/415
411/408
R10/S10
R20/S20
R30/S30
R50/S50
aggregation phases, such as concentration, solvent, and the R- and S-binaphthol enantiomers have identical effects on
temperature. Moreover, we have noticed that the Suzuki tuning the conjugation length of polyuorene. The signicant
condensation method has an advantage for the preparation of conjugation length reduction happens when the binaphthol
binaphthol-based chiral copolymers,14 since it can avoid the content is higher than 30 mol%, accompanied by not only the
racemization during the polymerization. We can further inves- signicantly blue-shied absorption peak but also the signi-
tigate the effects of chiral polymer structure on solid lm phase cantly enlarged optical bandgap (EOg pt) judged from the onset of
formation under the same aggregation conditions.
the absorption spectra. This was further conrmed by their PL
spectra in THF (Fig. 1a). R50 and S50 show a new luminescence
peak around 395 nm, while the other chiral copolymers exhibit
similar 0–0, 0–1, and 0–2 vibronic emissive transitions of poly-
Photophysical properties
The inuence of main-chain chirality on the photophysical uorene with corresponding emission peaks around 415, 440,
properties of optoelectronically active conjugated polymers and 470 nm, respectively.30 The single molecular photophysical
were investigated in terms of ultraviolet-visible (UV-Vis) properties of the binaphthol copolymers measured in THF
absorption and photoluminescence (PL) spectra, luminescent solution indicate that the conjugation length of polyuorene
quantum efficiency, and PL thermostability in both organic can tolerate a large incorporation ratio of binaphthol up to 30
solvent and thin solid lms (see Fig. 1 and Table 2). The R- and mol% without obvious effects on both emission peaks and
S-chiral copolymers in dilute THF solution show very similar luminescent quantum yields (Table 1), suggesting that
absorption spectra with an intense band around 384–347 nm binaphthol is a good building block for the construction of
associated with a p–p* transition of the p-conjugated back- optoelectronic conjugated polymers.
bone.14 The gradual blue shi of the absorption peak from 384
In the solid thin lm state (Fig. 1b), however, R- and
to 347 nm with the increasing content of chiral binaphthol S-polymers behave quite differently in terms of both absorption
units in the copolymers from 4.9% to 50%, suggests that (1) the and emission properties. Despite the gradual blue-shi of
twisted binaphthol unit partly breaks and decreases the effec- absorption peaks due to the increased binaphthol incorpora-
tive conjugation length of the polyuorene backbone, and (2) tion ratio, a new peak (or shoulder) associated with the b-phase
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Fig. 1 Normalized absorption and photoluminescent spectra of chiral conjugated polymers in dilute THF (a) and in thin solid films (b).
absorption appears at 434 nm in R5 lm. The characteristic absorption spectra of the copolymers. Only in the alternative
emission bands of the b-phase were also observed in the PL copolymers of R50 and S50 at the high loading content of
spectra of R5 and R10, located at 440, 465, and 497 nm, which binaphthol units were the emission peaks only slightly blue-
are very close to that of pure poly(9,90-dioctyluorene) lm.33 In shied due to the restricted main-chain conjugation of uorene
sharp contrast, the S-polymers do not show visible b-phase copolymers, indicating again the large tolerance of the PL
signals even when the incorporation ratio of chiral binaphthol properties to binaphthol incorporation. The identical PL
is low, suggesting that the S-enantiomer is a strong suppressor spectra with identical Commission International de L'Eclairage
of the formation of an intrachain ordered b-phase in poly- (CIE) in solution and of S-polymer lms (Table 2) when the
uorenes even when its content is lower than 5%. When the composition of binaphthol is changing over a wide range
content of the R-enantiomer of binaphthol is higher than 10%, (30 mol%) provide a good platform for independent tuning38a of
the b-phase is completely eliminated and the copolymers show the interesting properties of the chiral units without inuencing
identical emission of a-phase polyuorene lm around 425 and the photophysical properties of the chiral conjugated
447 nm, indicating that the luminophore of the copolymers has copolymers.
identical emission properties. This phenomenon is more
In order to obtain further understanding of the different
apparent in S5, S10, S20, and S30 as they have very similar PL effects of R- and S-enantiomers on the intrachain ordered
spectra regardless of the changing composition and shied b-phase emission in the condensed solid lm, their
Table 2 Optical and electrochemical properties of the chiral conjugated polymers at room temperature
UVl (nm)
PLl (nm)
FF (%)
CIE (x, y)
max
max
Polym. THF Film
EOg pt (eV) THF
Film
Sol.a Film THF
Film
HOMO/LUMO (eV/eV) EEgL (eV)
R5
S5
386
384
380
379
373
372
363
361
347
347
389, 434 2.95
394 2.88
383(434) 2.96
418, 441 440, 465, 497
417, 442 425, 447
418, 442 438, 464, 493 109
417, 442 424, 446
417, 440 423, 446
416, 440 423, 446
415, 439 420, 445
415, 438 420, 444
394, 416 400, 420
397, 415 413(431)
93
89
24.3 (0.157, 0.034) (0.155, 0.095) ꢁ5.77/ꢁ2.17
17.0 (0.157, 0.034) (0.154, 0.064) ꢁ5.77/ꢁ2.19
23.6 (0.157, 0.034) (0.150, 0.058) ꢁ5.76/ꢁ2.16
11.8 (0.157, 0.035) (0.154, 0.064) ꢁ5.76/ꢁ2.16
22.5 (0.157, 0.035) (0.153, 0.063) ꢁ5.75/ꢁ2.16
22.9 (0.157, 0.035) (0.155, 0.053) ꢁ5.77/ꢁ2.15
20.8 (0.157, 0.036) (0.154, 0.055) ꢁ5.74/ꢁ2.14
16.2 (0.157, 0.036) (0.156, 0.051) ꢁ5.74/ꢁ2.10
23.6 (0.159, 0.030) (0.155, 0.062) ꢁ5.72/ꢁ2.10
15.0 (0.158, 0.032) (0.155, 0.063) ꢁ5.71/ꢁ2.01
3.60
3.58
3.60
3.60
3.59
3.62
3.60
3.64
3.62
3.70
R10
S10
R20
S20
R30
S30
R50
S50
385
374
372
364
362
352
353
2.92
2.98
2.94
3.00
2.97
3.17
3.10
110
110
99
109
70
87
86
a
FF in cyclohexane was estimated using 9,10-diphenylanthracene (FF ¼ 0.9) as the standard.
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temperature-dependent photoluminescence behaviors (Fig. 2) different temperature-dependent properties in comparison with
were investigated. As we reported previously38b, when the that of poly(9,90-dioxyluorene).38b Only the 0–0 transition of the
temperature drops from 290 to 12 K (Fig. S6†), poly(9,90-diox- a-phase (424 nm) is relatively unchanged upon temperature
yluorene) lm shows gradually enhanced emissions with xed variation, and the emission wavelengths of the b-phase are very
0–0 vibronic transition bands of the a-phase (424 nm) and sensitive to the chiral binaphthol incorporation even when the
b-phase (442 nm) state, while the 0–1 and 0–2 transition- composition is as low as 5%. Consequently, the b-phase emis-
induced emission peaks either related to the b-phase or a-phase sion disappears in the copolymers when the binaphthol content
gradually red-shi from 447 nm (a-phase), 465 and 497 nm is higher than 10%, in accordance with emission found at room
(b-phase) at room temperature to 454 (a-phase), 470 and 505 nm temperature. A weak b-phase emission of S5, which was over-
(b-phase) at 12 K, respectively. The emission enhancement due looked at room temperature, was observed at 12 K. However, the
to reduced non-irradiative deactivation processes at low difference between the b-phase emission of R- and S-polymers is
temperatures was also observed in both types of the chiral still signicant at low temperature (Fig. 2c), which further
conjugated polymers. However, the emission intensities of R30, conrms that the S-binaphthol is a stronger suppressor of the
S30, R50, and S50 decrease at very low temperature (<60 K), formation of b-phase in polyuorenes compared to
suggesting that a new non-irradiative species has been formed, R-binaphthol.
which overwhelms the benets achieved for irradiative emis-
To investigate the inuence of chirality on the photophysical
sion at low temperature. More interestingly, the emission stability, PL spectra of the spin-coated chiral polymer lms
decrease of S-polymers is more severe than that of enantiomeric annealed at different temperatures were obtained. There was no
R-polymers (Fig. S7†), indicating that the formation of the non- change in the PL spectra of all the chiral copolymers when
irradiative species is chirality dependent. The a-phase and they were annealed below 200 ꢀC in nitrogen, indicating
b-phase emissions of the chiral conjugated polymers also show increased
photophysical
stability
aer
binaphthol
Fig. 2 PL spectra of R5 (a) and S5 (b) films with temperature changes from 299 K to 12 K and PL spectra of all of the R- (c) and S- (d) conjugated
polymers at 12 K when excited at 350 nm.
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incorporation compared to that in polyuorene.14 However, selective performance optimization of the materials without
aer the lms were annealed at 200 ꢀC in air for 10 h (Fig. 3), an disturbance of their electronic band structures.
obvious green band around 520 nm that is typically observed in
The luminescent quantum efficiencies (FF) of the chiral
poly(9,90-octyluorene) appeared in copolymers with 5% and conjugated polymers in both solution and thin solid lms were
10% content of chiral binaphthol. Excellent spectral thermal measured using 9,10-diphenylanthracene (F360 ¼ 0.9) as refer-
stability was found for R20 and R30, which showed almost no ence standard in dilute cyclohexane solutions (Table 1) and by
change in the PL spectra before and aer annealing at 200 ꢀC in integral sphere, respectively, to investigate the inuence of the
air for 10 h. The signicantly stabilized PL spectra aer both chirality. High and similar FFs in solution for both R- and
R- and S-binaphthol incorporation may be due to the presence S-polymers were obtained, while in the condensed lm state,
of the twisted conformation of binaphthol units, which restrict R-polymers generally show higher FF than the corresponding
the close packing of the polymer chains and reduce the prob- S-polymers. In our previous study,29 S-binaphthyl was experi-
ability of interchain interactions by suppressing the formation mentally identied to have lower PL efficiency due to its
of aggregates and excimers in polymers. Nevertheless, R-poly- stronger tendency to form dimers and excimers with its more
mers generally have higher stability than the corresponding planar molecular structure, especially at excited states
S-polymers, demonstrating the different effects of the chirality compared to R-binaphthyl. In this study, it seems that the R-
of enantiomers given that their copolymers have similar and S-polymers can inherit this feature of their monomers,
compositions and molecular weights.
which is distinct from the generally accepted concept that
The electrochemical properties of the chiral copolymers were enantiomers should have the same physical properties in a
measured by cyclic voltammetry (CV). The chiral conjugated symmetrical environment. It seems that the inuence of
polymers show reversible electrochemical redox CV curves, and different chirality from the enantiomers cannot be ignored
from the onset potential of electrochemical reduction and when they are selected as building blocks for the construction of
oxidation, the LUMO and HOMO can be calculated, respectively optoelectronic materials.
(Table 2), on the basis of the reference energy level of ferro-
cene.39 The R- and S-binaphthol copolymers show very close
HOMO and LUMO energy levels and electronic bandgaps (EgEL
)
Circular dichroism properties
with low variation (<0.15 eV) even when the content of To clarify the chiro-optical properties of the chiral conjugated
binaphthol is as high as 50% in alternating copolymers of R50 polymers, specic rotations ([a2D5]) and the circular dichroism
and S50. This interesting feature provides binaphthol-contain- (CD) spectra in both solution and thin solid lms were
ing optoelectronic materials with intriguing potential in measured. From Table 3, the absolute values of [a2D5] in THF
increase with increasing content of binaphthol in both R- and
S-polymers due to a propagating helical chain conformation of
the copolymers.40 However, the negative [a2D5]s of R-copolymers
are opposite to their monomer with positive [a2D5], which was
also found in S-copolymers. The absolute values of [a2D5] of
S-polymers are relatively larger than those of the corresponding
R-polymers. The relationship between [a2D5] (Y) and content of
binaphthol (X) in copolymers can be described by the following
line-equations: Y ¼ ꢁ16.84 ꢁ 4.084X and Y ¼ 32.02 + 4.433X for
R- and S-polymers, respectively. Both the intercept and slope of
the equation of S-polymers are larger than those of R-polymers,
indicating a relatively more sensitive change in [a2D5] with a
change in the content of S-binaphthol.
R-Binaphthol comonomers show positive CD signals from
280 to 340 nm, while S-binaphthol has negative signals in a
mirror image, as typically observed in enantiomers.14 In stark
contrast, CD spectra in polymers (Fig. 4) show only negative and
newly emerged CD signals around 255, 290, and 375 nm in
R-polymers, while in S-polymers only positive signals in mirror
images of those of R-polymers were observed in both THF
solution and lms, providing direct evidence for the successful
introduction and transfer of the chirality into the conjugated
polymers. This transition between monomer and polymer
conrmed by both CD spectra and [a2D5] indicates the formation
a secondary structure41 probably due to the transfer of the
chirality in the polymer backbone. Due to the high enantio-
merization barrier of binaphthols,42 they are stable enough to
survive the polymerization conditions and main-chain
Fig. 3 Normalized PL spectra of R-polymers (a) and S-polymers (b)
annealed at 200 ꢀC in air for 10 h in thin films.
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Table 3 Specific rotation ([a2D5]) in THF and peak wavelength of CD spectra (lmax) of the chiral copolymers in both THF and thin films
Specic rotation
C (g per 100 mL)
lmax (nm)
Copolymer
[a2D5] (deg.)
Solution
Film
R5/S5
0.031/0.035
0.032/0.027
0.025/0.022
0.025/0.029
0.036/0.021
ꢁ35.8/47.9
280, 391/387
282, 387/383
289, 379/291, 378
289, 373/293, 374
295, 365/299, 367
—
—
R10/S10
R20/S20
R30/S30
R50/S50
ꢁ52.0/61.1
ꢁ87.6/146.4
ꢁ170.4/171.1
ꢁ208.1/243.4
257, 291/259, 293
253, 299, 384/257, 297, 381
265, 305, 382/259, 304, 378
the dihedral angle between the two naphthalene moieties in the
R-oligomer is 91.9ꢀ, while that in the S-oligomer is ꢁ92.5ꢀ. This
slightly larger dihedral angle was also observed in S-binaph-
thols in our previous study,29 which suggests that the molecular
conformation of S-binaphthols is more planar and apt to form
dimers at the ground states and excimers at excited states upon
photoexcitation. The calculated dihedral angles are both close
to 90ꢀ, which may prevent the effective p-electron interaction
and lead to the breakage of the main-chain conjugation as
discussed above. However, in polymers this angle may be varied
due to spatial hindrance of the repeating units and the entan-
glement of polymer chains. The inuence of this dihedral angle
change on the molecular energy and CD spectra was investi-
gated theoretically (Fig. 5). With a low energy barrier of 0.3 eV,
the dihedral angle can be varied from 60ꢀ to 120ꢀ, suggesting
the high exibility of the binaphthol units and thus the polymer
chain. However, the calculation results indicate that this
feasible change of the dihedral angle could result in signi-
cantly different CD spectra. Note that the shorter wavelength CD
Fig. 4 CD spectra of the chiral conjugated polymers in thin solid films peak around 290 nm becomes red-shied and weakened, while
at room temperature.
the longer wavelength CD peak around 330 nm is blue-shied
and enhanced, when the dihedral angle is increased from 90ꢀ to
120ꢀ or decreased from 90ꢀ to 60ꢀ. Comparing these theoretical
relaxation to maintain their intrinsic chirality and induce new
ones aer interactions with the other units in the conjugated
backbone.10 With an increase in the chiral binaphthol content,
the CD peak at short wavelength (around 290 nm) is red-shied
and slightly decreased, whereas the peak at the longer wave-
length (around 375 nm) is blue-shied but signicantly
enhanced. However, the enhancement requires a high content
of chiral binaphthol units in copolymers (more than 20%) to
establish the newly emerged CD signals of the chiral conjugated
polymers in thin lms.
results for the intensity ratios of the longer and the shorter
wavelengths (IL/IS) with the experimentally obtained results
(about 0.7 and 1.0 for R30 and S30, respectively, see Fig. 4), it
can be proposed that the real dihedral angles in R30 and S30 are
in the range from 80ꢀ to 110ꢀ and the angle of R30 should have
smaller variation around its most stable conformation of 91.9ꢀ
compared to that of S30 due to its lower IL/IS observed experi-
mentally. Since the low dihedral angle will lead to a crowded
and unstable chain conformation, the actual dihedral angle in
the copolymers is supposed to be larger than 100ꢀ instead of
lower than 90ꢀ, and S-polymers have larger dihedral angles than
R-polymers.
Considering that the CD spectrum is sensitive to the polymer
conformation43 and the model compounds of R- and S-trimers
do not have appropriate structures and sufficient chain length
to represent R30 and S30, respectively. MD simulations were
performed to investigate the polymer chain structures of these
chiral conjugated copolymers at 300 K based on R- and
S-21mers with similar backbone compositions and substitu-
tions of the corresponding copolymers (Fig. 6). The dihedral
angle between the two naphthalene moieties controls the main-
chain chirality and conjugation length, while the n-alkyl
Theoretical investigations and discussions
For an in-depth understanding of the chiro-optical properties of
the binaphthol-containing chiral conjugated polymers, the
TDDFT method was used to predict CD spectra based on the
pre-optimized geometries of the model compounds (R- and
S-trimers in Fig. 5a) of R30 and S30 at the B3LYP/6-31G(d) level,
since R30 and S30 have the typical molecular structures of the
copolymers and relatively higher CD intensities with stable PL
properties. The DFT optimized R- and S-trimer structures at the
ground state are not perfect enantiomers as expected; however,
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Fig. 5 The model compounds (R- and S-trimers, (a)) of R30 and S30 with the theoretically predicted relationships (b and c) between the dihedral
angles (from 40ꢀ to 140ꢀ) and the CD spectra, the intensity ratio (IL/IS) of the peak in longer wavelength (IL) and that in short wavelength (IS), and
molecular energy variation (DE).
substituents on both binaphthol and uorene guide the
However, the inherent reason for the different planarity of
packing behaviors such as the formation of a-phase or b-phase R- and S-monomers and polymers revealed by DFT and MD
in the condensed solid state. The MD-predicted equivalent calculations still needs to be investigated experimentally. In
trajectories of the single polymer chains at 300 K give good addition, the effects of chemical and enantiomeric impurities,
theoretical support for our above assumptions that dihedral although controlled to be lower than 0.5% and 1.0%, respec-
angles are generally larger than 100ꢀ (except for R50) and S- tively, may still lead to the observed difference in the solid state.
polymers have larger dihedral angles than R-polymers. The Nevertheless, these factors might not be strong enough to
larger dihedral angle of S-polymers suggested by both DFT disrupt the relationships between the chirality and the photo-
calculations and MD simulations, makes the conjugated back- physical properties of the conjugated polymers observed from
bone more planar and the chain structure more open. The more this systematic study on R- and S-conjugated polymers with
planar backbone conformation results in longer effective similar chain compositions and molecular weights. In a
conjugation lengths and lower optical band-gaps of the S-poly- controlled experiment, we deliberately mixed R- and S-binaph-
mers (Table 2), while the more open chain structure leads to thols to prepare a partially racemic conjugated polymer with
stronger interchain interactions in S-polymers especially in the high enantiomeric impurities. The prepared R4S1, in which the
condensed phase. These stronger interactions will disturb the R-binaphthol content was 4% and the S-binaphthol content was
intrachain ordering process induced by the n-alkyl substituents 1%, showed an obviously higher a-phase emission than R5 but
to form b-phase and facilitate the formation of inefficient and still a b-phase emission at the identical conditions of R5 and S5
unstable excimers, resulting in the lower content of b-phase (Fig. S8a†), suggesting the articially incorporated 1%
(Fig. 2), luminescent quantum efficiency (Table 2) and spectrum S-binaphthol partially disturbed b-phase formation due to its
thermal stability (Fig. 3) of the S-polymers, respectively.
stronger b-phase suppressibility. Considering the possible
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Fig. 6 Front view and side view of the MD simulated chiral main-chain conformations based on the R- and S-21mers with similar compositions
of the experimental copolymers.
effects of the different interactions between the chiral polymer chain composition, thermal properties, UV-vis absorption and PL
and the quartz surface on the photophysical properties, the PL spectra at various temperatures, PL efficiency, electrochemical
spectra were further collected from chiral polymer lms (R5, S5, properties, and circular dichroism spectra in both solution and
and R4S1) spin-coated on the surfaces of silicon wafers and lms were experimentally investigated. The results reveal that the
mica sheets (Fig. S8b and S8c†). The same trend in their PL R- and S-chirality can be preserved and transferred to the conju-
variations was observed, suggesting that the PL difference in the gated backbone, and the chirality has its effects on various
solid state is originally due to their different molecular chirality. aspects of the photophysical properties of the chiral conjugated
The thermostability of R4S1 is also between that of R5 and S5 polymers as follows: (1) the thermostability was signicantly
(i.e., stronger than that of S5 but lower than that of R5), as improved in comparison with that of polyuorene even when the
indicated by their PL spectra aer annealing at 200 ꢀC in air for content of binaphthol was as low as 5 mol%, and R-polymers have
10 h (Fig. S9†). The PL quantum efficiency of the R4S1 lm relatively better spectral thermostability than S-polymers; (2) the
(18.4%) was higher than that of S5 lm (17.0%) and lower than incorporated binaphthol units greatly suppress the formation of
that of R5 lm (24.3%), which is consistent with the ndings in the b-phase, and S-chirality shows stronger suppressing ability
enantiomeric pure copolymers that R-chiral luminescent poly- than R-chirality; and (3) the chiral conjugated polymers have very
mers generally have higher PL quantum efficiencies than their high luminescent efficiencies, and R-polymers possess higher PL
S-counterparts.
quantum efficiency than S-polymers in the condensed state. The
theoretical studies based on both DFT calculations and MD
simulations show that the main possible reason for the different
photophysical properties induced by the main-chain chirality
may be that the S-binaphthol has a more planar molecular
structure that favors interchain interaction for the formation of
inefficient and unstable excimers or quenchers. These ndings,
although quite different from the current general understanding
of enantiomers, may provide new clues for the fundamental study
and application of chiral conjugated polymers.
Conclusions
A series of chiral binaphthol-uorene conjugated copolymers
were successfully synthesized by Pd-catalyzed Suzuki poly-
condensation to covalently incorporate the different axially chiral
R-andS-binaphtholintothepolyuorene backbone. Basedonthe
careful control of the chemical and chiro-optical impurities, the
inuencesofthemain-chainchiralconjugatedcopolymersonthe
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20 E. Yashima, K. Maeda, H. Iida, Y. Furusho and K. Nagai,
Chem. Rev., 2009, 109(11), 6102–6211.
Acknowledgements
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The present work was supported by the National Natural
Science Foundation of China (Projects no. 21274065, 21304049,
61136003, and 51173081), the Natural Science Foundation of
Jiangsu Province (BK2011751), the Ministry of Education of
China (no. IRT1148), a project funded by the priority academic
program development of Jiangsu higher education institutions
(PAPD), the Program for Postgraduate Research Innovations in
University of Jiangsu Province (CXLX11_0419, CXZZ11_0412,
and CXZZ12_0455), the Scientic Research Foundation of
Nanjing University of Posts and Telecommunications
(NY210017, NY210046), and by a Grant-in-Aid for Scientic
Research from the Japan Society for the Promotion of Science
(Project no. 22560013).
27 Y. Zeng, C. S. Wang, X. B. Huang and Y. X. Cheng, J. Appl.
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