Design and synthesis of polymers for chiral photonics

Article abstract of DOI:10.1021/ma401731x

Chiral photonics deals with enantioselective polarization control of linear and nonlinear optical functions and holds a great promise for a wide range of applications including optical signal processing, biosensing, and chiral bioimaging. Development of c

Full text of DOI:10.1021/ma401731x

Article
pubs.acs.org/Macromolecules
Design and Synthesis of Polymers for Chiral Photonics
Bruno Nowacki,†,‡ Heongsub Oh,† Cristiano Zanlorenzi,‡ Hongsub Jee,† Alexander Baev,†
Paras N. Prasad,*,† and Leni Akcelrud*,‡
†Institute for Lasers, Photonics, and Biophotonics, University at Buffalo, The State University of New York, Buffalo, New York 14260,
United States
‡Laborator
́ ́ ́
io de Polímeros Paulo Scarpa (LaPPS), Universidade Federal do Parana, P.O. Box 19081, Curitiba 81531-990, Parana,
Brazil
S
* Supporting Information
ABSTRACT: Chiral photonics deals with enantioselective
polarization control of linear and nonlinear optical functions
and holds a great promise for a wide range of applications
including optical signal processing, biosensing, and chiral
bioimaging. Development of chiral materials with optical
activity exceeding that of natural materials therefore becomes a
prerequisite to realizing the full potential of this field. Herein,
we report on a study of structure−property relations of two
chiral polymers with ester functional groups in lateral branch.
To enhance rotational strength, the macroscopic measure of
chirality, we employ a fluorene−quinoxaline motif in the monomer unit. To the best of our knowledge, we find the optical
activity of one of the studied polymers to be the highest reported so far. Theoretical calculations reveal a correlation between the
topological conformations and the simulated CD spectra in agreement with the experimental measurements and help clarify the
mechanism of optical activity enhancement that could create insight for further enhancement of chirality.
involves, among others, chiral control of linear and nonlinear
optical functions, biosensing, and chiral imaging.
INTRODUCTION
■
The optical properties of anisotropic polymer films comprising
aligned strands have been utilized for a wide variety of
applications including the fabrication of anisotropic linear and
nonlinear optical devices such as transistors, lasers and
molecular strain sensors for polymer films, polarized-light
emitting diodes, optical filters, infrared polarizers, and liquid
crystal displays, to name a few examples.1−12 An efficient means
to realize anisotropy is unidirectional alignment of conjugated
species in a polymeric host by tensile drawing, as done
previously with inorganic crystals.13,14 Recently, development
of polymeric media mimicking the key properties of artificially
nano-engineered nonpolymeric materials usually referred to as
metamaterials, such as isotropic 3D bulk negative refractive
index, and possessing low absorption loss at the same time, has
emerged as an alternative avenue for a new class of flexible, low-
cost materials.15−17 In this context chirality is the mostly sought
after property with a great potential to realize negative or near-
zero refractive index in true 3D bulk materials.18−20 Thermal
realignment and self-organization of polymer fibrils in annealed
films enhance optical activity in this case.18,19 Generally
speaking, the whole area of chiral photonics is getting redefined
with the advent of novel classes of flexible, low-cost chiral
materials with optical activity non-accessible in naturally
occurring materials. These materials hold great promise for
use in metaphotonics (manipulation of electromagnetic fields in
nano-engineered materials to control propagation of light) as
well as for a much broader field of chiral photonics, which
Enhancement of chirality and associated optical activity can
be achieved in several ways such as the preparation of self-
assembling structures and cholesteric liquid crystals, whereas in
polymer systems this can be done by means of enantioselective
interactions with small chiral molecules, doping polymeric films
with plasmonic or excitonic nanoparticles, preparing nano-
composites, or initiating helical assembly of the polymeric
fibrils.18,19,21−25 The latter could be sensitively controlled by
solvent, temperature, pH, and the presence of solvent
vapor.26−28 In solid state, changes in the chiroptical properties
of large macromolecules can be induced by annealing.29
In this contribution, chirality was introduced into polymeric
strands by building helicity into their backbone via incorporat-
ing asymmetric chiral centers (side chains) to the monomer
units. This resulted in large circular dichroismthe main
feature associated with the macromolecular configuration and
conformation. Two polymers with a novel chiral side chain
were synthesized. One is a polyfluorene homopolymer
derivative (poly[9,9-bis(3-((S)-2-methylbutylpropanoate)-
fluorene]) (LaPPS61), and the other is an alternated
copolymer of fluorene and quinoxaline, (poly[9,9-bis(3-((S)-
2-methylbutylpropanoate))fluorene-alt-5,8-(2,3-bis(phenyl)-
Received: August 19, 2013
Revised: August 22, 2013
© XXXX American Chemical Society
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Scheme 1. Synthesis of Monomers M1, M2, and M3
quinoxaline)]) (LaPPS62), which is also a donor−acceptor
structure (DA). The chiral site is located at the ester
ramification at C9 in the fluorene moiety; this side chain also
served to improve solubility. Chirality results in optical activity
such as circular dichroism, circular birefringence, and circularly
polarized luminescence. In chiral media the effective refractive
index for circularly polarized light is given by
range helical ordering occurring for instance in cholesteric
phases.50−52
The optical properties of LaPPS61 and LaPPS62 were
studied using circular dichroism, UV−vis, and photolumines-
cence spectroscopy along with density functional theory (DFT)
calculations of electronic properties of interest.
EXPERIMENTAL SECTION
■
neff
=
ε′μ′
k
(1)
Materials and Methods. The chemicals 2,7-dibromofluorene, tert-
butyl acrylate, tetrabutylammonium bromide, sodium borohydride,
tetrakis(triphenylphosphine)palladium(0), [1,1′-bis-
(diphenylphosphino)ferrocene]dichloropalladium(II), 4,7-
dibromobenzo[c][1,2,5]thiadiazole), cobalt dichloride hexahydrate,
benzil, bis(pinacolato)diboron, potassium acetate, trifluoroacetic acid,
and the solvents water, toluene, N,N′-dimethylformamide, dichloro-
methane, ethanol, and chloroform were purchased from Aldrich.
Magnesium sulfate was purchase from Fisher. Silica gel (60−200 μm)
was purchased from BDH. Sodium hydroxide and potassium carbonate
were purchase from J.T. Baker. Hydrochloric acid, sulfuric acid, and
acetic acid were purchased from EMD. Deuterated chloroform and
deuterated dimethyl sulfoxide were purchased from Cambridge
Isotope Laboratories. All reagents and solvents were used as received
without further purifications.
where μ′ and ε′ stand for the real parts of the electric
permittivity and magnetic permeability, respectively, and κ is
the chirality parameter, which is proportional to optical rotation
(circular birefringence).30−32 This implies that the effective
refractive index could be made negative, zero, or near-zero for
one sense of circular polarization without achieving negative
values of μ′ and ε′, if the chirality parameter is sufficiently large.
In this context polyfluorene derivatives are an interesting
group of conjugated polymers due to their optical activity in the
visible and near-visible regions, good solubility, good
processability, and high efficiencies of light absorption and
emission combined with a variety of supramolecular
organizations such as crystalline and noncrystalline phases
that can increase the optical activity as already reported.33−39
Quinoxaline is a electron-acceptor unit usually applied in
photovoltaic structures with a variety of possibilities of
substitution at the 2,3-positions through the reaction of
diamine and diketone groups.40,41 The poly(quinoxaline-2,3-
diyl) can assume a helical conformation that once formed is
irreversible.42,43 The insertion of the ester functionality has
been reported in similar cases, as for example in the fabrication
of anionic polyfluorene focusing on its use in optoelectronic
devices such as LEDs and photovoltaic cells as well as in
biochemical sensors.44−47 Circular dichroism (CD) spectros-
copy has been used to study the macromolecular organization
of chiral π-conjugated polymers, including polyfluorenes.48,49
The magnitude and the sign of CD have been shown to be very
sensitive to interchain electronic interactions as well as to long-
1H and 13C NMR spectra were recorded on a Varian Inova-400
1
Instrument (399.65 MHz for H, 100.40 MHz for 13C). UV−vis and
CD spectra were measured on a Shimadzu UV-3101PC spectrometer
and a JASCO J-715 CD spectropolarimeter (scanning rate of 200 nm
min−1, bandwidth of 1 nm, and response time of 1 s, using a single
accumulation). For the temperature dependence studies all films were
prepared by spin-coating at 1100 rpm from chloroform/chlorobenzene
(10:1) 10 mg mL−1 solutions and annealed from room temperature
(RT) to 200 °C in an argon atmosphere for 2 h. For the thickness
dependence studies five different solutions were prepared: 5, 10, 20,
30, and 40 mg mL−1 using chloroform/chlorobenzene (10:1) as
solvent. All films were prepared using these solutions by spin-coating
at 1100 rpm and annealed at 150 °C in an argon atmosphere for 2 h.
The thickness of films was measured from scanning electron
microscopy (SEM) images. The equipment used was a Hitachi
S4000 field emission microscope at an acceleration voltage of 25 kV.
DSC analyses were performed with a TA Instruments Q200 system
with an RCS-90 cooling device. All samples were heated from 20 to
200 °C at a rate of 10 °C min−1 in a nitrogen atmosphere and then
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Scheme 2. Synthesis of Polymers
cooled to 20 at 10 °C min−1. The glass transition temperatures, Tg, of
the samples were determined from the midpoints of the transitions.
Chemical Procedures. The chemical procedures for the
preparation of the monomers and final polymers are illustrated in
Schemes 1 and 2.
the reaction was stirred overnight at 60 °C. After this time, the product
was extracted three times with ethyl ether. The organic layers were
combined and washed three times with water and then dried with
MgSO4. The solvent was removed, and the crude product was purified
in a chromatographic column to give 650 mg of white solid with a
1
2,7-Dibromo-9,9-bis(3-(tert-butylpropanoate))fluorene (1). To a
three-necked round-bottom flask, 2,7-dibromofluorene (5.00 g, 15.5
mmol), tetrabutylammonium bromide (125 mg, 0.445 mmol), and
toluene (30 mL) were added under an Ar atmosphere. After 60 min,
12 mL of solution 50 wt % aqueous NaOH was added dropwise, and
the solution was stirred for 60 min. After that, tert-butyl acrylate (7.9 g,
61.6 mmol) was added dropwise, and the mixture was stirred at 25 °C
overnight. After this time, the reaction product was extracted with
dichloromethane, washed three times with water, and dried with
anhydrous MgSO4. After solvent removal the product was purified in a
chromatographic column to give 6.55 g of white solid with a 73% yield.
1H NMR (CDCl3, δ): 7.55−7.42 (m, 6H); 2.30 (t, 4H), 1.47 (t, 4H),
56.5% yield. H NMR (CDCl3, δ): 7.84−7.72 (m, 6H); 3.78−3.65
(ddd, 4H); 2.45 (t, 4H); 1.59−1.47 (m, 6H); 1.38 (s, 24H); 1.32−
1.02 (m, 4H); 0.85−0.79 (m, 12H). 13C NMR (CDCl3, δ): 173.52;
147.43; 143.80; 134.42; 128.92; 128.83; 119.78; 83.83; 68.88; 53.47;
34.48; 33.89; 28.88; 25.88; 25.00; 24.90; 16.30; 11.13.
5,8-Dibromo-2,3-bis(phenyl)quinoxaline (M3). To a one-necked
round-bottom flask, 4,7-dibromobenzo[c][1,2,5]thiadiazole (700 mg,
2.38 mmol), cobalt dichloride hexahydrate (35 mg, 0.14 mmol), and
dry ethanol (20 mL) were added under argon at 0 °C. After 20 min,
sodium borohydride (2 g, 52.9 mmol) was added in four equal
portions with interval of 20 min. The solution was heated to room
temperature and stirred overnight without light. After that, the solvent
was removed under reduced pressure, and the product was neutralized
with 10% HCl (50 mL) and then extracted three times with
dichloromethane. The organic layers were combined, and the solvent
was removed under reduced pressure. The product 3,6-dibromoben-
zene-1,2-diamine (3) is unstable at light and air, and then it was used
without purification. A mixture of 3,6-dibromobenzene-1,2-diamine
(3) (600 mg, 2.2 mmol), benzil (4) (450 mg, 2.2 mmol), ethanol (50
mL), and acetic acid (15 mL) was refluxed overnight. The mixture was
cooled to room temperature and dropped into 300 mL of water. The
precipitated formed was filtrated and purified by recrystallization
giving yellow crystals (650 mg, 61% yield). 1H NMR (CDCl3, δ): 7.93
(s, 2H); 7.69−7.64 (m, 4H); 7.45−7.34 (m, 6H). 13C NMR (CDCl3,
δ): 154.13; 139.34; 137.93; 133.08; 130.26; 129.57; 128.36; 123.72.
Poly(9,9-bis(3-((S)-2-methylbutylpropanoate))fluorene)
(LaPPS61). To a three-necked round-bottom flask, 2,7-dibromo-9,9-
bis(3-((S)-2-methylbutylpropanoate))fluorene (M1) (176 mg, 0.29
mmol), 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-bis(3-
((S)-2-methylbutyl propanoate))fluorene (M2) (205 mg, 0.29 mmol),
2 M K2CO3 solution (12 mL), and toluene (36 mL) were added under
an Ar atmosphere. After 30 min, tetrakis(triphenylphosphine)-
palladium(0) (20 mg, 0.017 mmol) was added quickly, and the
mixture was stirred at 90 °C for 60 h. Excess of 1-bromobenzene (end-
cap) was added, and the mixture was continuously stirred for 12 h.
The product was extracted three times with chloroform, and the
organic layers were combined and dried with MgSO4. After partial
evaporation, the product was purified by chromatographic column to
1.33 (s, 18H). 13C NMR (CDCl3, δ): 172.16; 149.89; 139.02; 130.00;
126.41; 122.01; 121.39; 80.41; 53.99; 34.35; 29.78; 27.96.
2,7-Dibromo-9,9-bis(3-(propionic acid))fluorene (2). To a three-
necked round-bottom flask, 2,7-dibromo-9,9-bis(3-(tert-butyl
propanoate))fluorene (1) (3.00 g, 5.19 mmol) and dichloromethane
(10 mL) were added under an Ar atmosphere at 0 °C. After 40 min,
trifluoroacetic acid (1.2 mL, 15.6 mmol) was added dropwise, and the
mixture was stirred at 25 °C overnight. After this time, the mixture was
dropped on basified water and impurities were extracted with ethyl
ether. The aqueous layer was acidified, forming a white solid
precipitate that was solubilized in ethyl acetate. The organic layer
was washed with water three times and dried with MgSO4. The solvent
was rotoevaporated and purified in a chromatographic column to give
2.1 g of white solid with a 87% yield. 1H NMR (DMSO-d, δ): 12.43 (s,
2H); 8.15−8.09 (m, 4H); 7.87−7.82 (m, 2H); 2.70 (t, 4H), 1.71 (t,
4H). 13C NMR (DMSO-d, δ): 173.64; 150.29; 138.82; 130.74; 126.48;
122.24; 121.33; 54.15; 33.36; 28.72.
2,7-Dibromo-9,9-bis(3-((S)-2-methylbutylpropanoate))fluorene
(M1). To a two-necked round-bottom flask, 2,7-dibromo-9,9-bis(3-
(propionic acid))fluorene (2) (2.00 g, 4.28 mmol), (S)-2-methyl-
butanol (3.27 g, 37.1 mmol), and 2 drops of sulfuric acid concentrated
were added under an Ar atmosphere and refluxed overnight. After that,
the product was extracted with ethyl acetate and washed three times
with cold water and one time with NaHCO3 aqueous solution and
then dried with MgSO4. After, the solvent was evaporated under
reduced pressure. The crude product was purified in a chromato-
1
1
graphic column to give 2.16 g of white solid with a 83.7% yield. H
give 130 mg of pale gray-yellow solid with 51.2% yield. H NMR
NMR (CDCl3, δ): 7.57−7.47 (m, 6H); 3.81−3.66 (m, 4H); 2.38 (t,
4H); 1.60−1.54 (m, 6H); 1.38−1.03 (m, 4H); 0.86−0.81 (m, 12H).
13C NMR (CDCl3, δ): 172.89; 149.49; 139.16; 131.18; 126.41;
(CDCl3, δ): 7.91−7.61 (m, 6H); 3.70(m, 4H); 2.60(br, 4H); 1.73 (br,
4H); 1.53 (m, 2H); 1.35−0.99 (m, 4H); 0.79 (m, 12H).
Poly(9,9-bis(3-((S)-2-methylbutylpropanoate))fluorene-alt-5,8-
(2,3-bis(phenyl)quinoxaline)) (LaPPS62). To a three-necked round-
bottom flask, 5,8-dibromo-2,3-bis(phenyl)quinoxaline (M3) (198 mg,
0.45 mmol), 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-
bis(3-((S)-2-methylbutyl propanoate))fluorene (M2) (315 mg, 0.45
mmol), 2 M K2CO3 solution (12 mL), and toluene (36 mL) were
added under an Ar atmosphere. After 30 min, tetrakis-
(triphenylphosphine)palladium(0) (20 mg, 0.017 mmol) was added
quickly, and the mixture was stirred at 90 °C for 60 h. Excess of 1-
bromobenzene (end-cap) was added, and the mixture was
122.00; 121.48; 69.20; 54.19; 34.42; 33.91; 28.82; 25.91; 16.36; 11.10.
2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-bis(3-
((S)-2-methylbutylpropanoate))fluorene (M2). To a three-necked
round-bottom fl ask , 2, 7-dibromo-9, 9-b is(3-(( S)-2-
methylbutylpropanoate))fluorene (M1) (1.00 g, 1.65 mmol),
potassium acetate (0.97 g, 9.9 mmol), bis(pinacolato)diboron (1.46
g, 5.77 mmol), and dry DMF (25 mL) were added under an Ar
atmosphere. After 30 min, [1,1-bis(diphenylphosphino)ferrocene]-
dichloropalladium(II) (72.4 mg, 0.099 mmol) was added quickly, and
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continuously stirred for 12 h. The product was extracted three times
with chloroform, and the organic layers were combined and dried with
MgSO4. After partial evaporation, the product was purified by
chromatographic column to give 100 mg of green-yellow solid with
results have demonstrated that polyfluorene chains undergo
conformational changes with annealing.36 Typically for the
octyl-substituted homopolymer (PFO) an ordered conforma-
tion called β phase predominates up to 90 °C. This phase is
characterized by fluorene units placed in an alternating up and
down fashion, giving rise to a planar zigzag morphology with an
extended conjugation length of about 30 repeating units, and its
spectrum is red-shifted as compared to the amorphous
PFO.37,55 The absorption at 425 nm was assigned to this
type of morphology, which gradually disappears due to the
interruption of the planar conformation induced by thermal
displacements of chain segments.
Accordingly, a blue-shift is observed due to the shortening of
the effective conjugation length. With further raising the
temperature, the free volume increases, and there is room for
the formation of the more stable helical conformations. The
DSC traces (Supporting Information) supports the assumption
of disruption of the β phase in the region of the glass transition
temperature (132 °C). No other transitions were detectable in
the thermogram.
1
47.9% yield. . H NMR (CDCl3, δ): 8.10−7.88 (m, 6H); 7.76−7.51
(m, 6H); 7.42−7.27 (m, 6H); 3.79−3.57 (br, 4H); 2.78−2.52 (br,
4H); 1.99−1.77 (br, 4H); 1.05−0.86 (br, 6H); 0.80−0.67 (m, 12H).
Theoretical Methods. To determine the topology of LaPPS61
and LaPPS62 helical oligomers, a sequence of steps was followed
involving quantum chemical calculations. Initially, the geometry of
each monomer was optimized with density functional theory (DFT),
using B3LYP functional and 6-311G(d,p) split-valence basis set as
implemented in the Gaussian 09 package.53,54 Based on the previous
step and using the same level of theory, the geometry of LaPPS61 and
LaPPS62 tetramers was optimized for two different conformations: R
and S helix forms, related to clockwise or counterclockwise rotational
sense. The electronic absorption and circular dichroism spectra were
then calculated by using time-dependent DFT.54 The relative
populations of each conformer were calculated based on the ratios
of their statistic Boltzmann factors.
RESULTS AND DISCUSSION
■
Dependence of Chiroptical Properties on Film Thickness:
LaPPS61. Figure 2 shows that absorption and the CD spectra
Synthesis of Polymers. The molar masses of LaPPS61
and LaPPS62 as determined by GPC using polystyrene as
standards and DMF as eluent were Mw = 18 700 g mol−1 with
polydispersity (PDI) of 2.47 and Mw = 8200 g mol−1 with
polydispersity (PDI) of 1.34, respectively. LaPPA62 shows a
low molar mass due to the low solubility in DMF. Only the
soluble fraction was used for further characterization. Also, as
shown by Robert Abbel et al.49 molar masses around 8−10 kg
mol−1 provide higher CD signals.49 The polymers showed good
solubility in organic solvents such as CHCl3, THF, and
dichloromethane. DSC showed a glass transition (Tg) of 132
°C for LaPPS61 and 167 °C for LaPPS62. The DSC traces are
included in the Supporting Information. The thermal stability
of similar structures containing the same ester pendant group
polymer was studied using TGA, showing that this group is
thermally stable until 200 °C under a nitrogen atmosphere.45
Spectroscopy (Absorption and CD). Dependence of
Chiroptical Properties on Annealing Temperature: LaPPS61.
Figure 1 displays the absorption and the CD spectra of
Figure 2. Film thickness dependence of CD and absorption spectra in
thin films of LaPPS61.
turn gradually more intense with the increase in thickness, in a
Lambert−Beer linear fashion (see Supporting Information),
with no discernible deviation of such behavior up to 0.410 μm.
This is a promising finding since it points out to the possibility
of the preparation of self-supporting films without losing the
high CD signals.
Dependence of Chiroptical Properties on Annealing
Temperature: LaPPS62. Figure 3 displays the absorption and
the CD spectra of LaPPS62. The absorption profile has the
typical “camel back” dual bands of a DA copolymer: the donor
(fluorene which contains the chiral center) is placed in an
alternate fashion with the acceptor (quinoxaline).
As seen before with the homopolymer (LaPPS61), the CD
signal increases with the temperature of annealing. It is
noteworthy that the two signals of the Cotton effect of the
copolymer (LaPPS62) are in the positive sense: the more
intense at 425 nm and the less pronounced at 340 nm. On
other hand, in the homopolymer (LaPPS61) spectrum which
also consists of two bands, one is in the negative sense
(stronger band at 390 nm) and the other in the positive sense
(weaker band at 350 nm). A careful observation of the
spectrum in the temperature range of 125−150 °C reveals a
slight increase in absorption at around 475 nm. This finding
Figure 1. CD and UV−vis absorption spectra of annealed films of
LaPPS61.
LaPPS61 annealed films at various temperatures for 2 h,
showing a bisignate negative Cotton effect, with the intensity
gradually increasing between room temperature and 100 °C,
stabilizing at 150 °C, and then decreasing drastically between
150 and 200 °C. The negative peak at RT is centered at 395 nm
and gradually blue-shifts upon heating to 200 °C. Reported
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could be ascribed to some degree of chain aggregation brought
about by annealing, which could be playing a role in the
inversion effect observed. The DSC trace of the pristine
polymer (see Supporting Information) shows a glass transition
temperature of 167 °C and an endotherm at 65−70 °C that
could be ascribed to crystalline domains temperature (RT) up
to 200 °C.
discussed. The new morphology developed with these changes
as H aggregates, twisted or helical conformations are
responsible for the increase in the CD signal observed at
100−150 °C.49 A lack of dependence of gabs on the thickness is
observed at ∼400 nm (Figure 5b). This is the signature of true
mesoscopic chirality, developed after annealing through
formation of chiral aggregates.19 Concerning LaPPS62, no
variation was observed in the spectral profile of gabs, whereas the
intensity increased up to 150 °C, decaying afterward (Figure
5c). The exponential growth of CD with the thickness versus
the linear one of the absorbance makes for a prominent
dependence of the dissymmetry ratio on the film thickness
(Figure 5d). The reason for this behavior is not completely
clear and will be a subject of future study. One can speculate
about possible propagation effects (pseudo-chiral dichroism)56
or contribution of linear dichroism due to perpendicular rather
than parallel alignment of the polymer strands with respect to
the incident light beam or even conformational changes. The
findings described so far were discussed in terms of
morphological changes. Conceptual comments on the subject
have been made in the literature.56
Chirality Parameter. To obtain the dispersion of the
chirality parameter, κ, we applied Kramers−Kronig trans-
formation to the measured ellipticity (CD) spectrum with a
subsequent algebraic transformation of the resulting optical
rotation. The variation of the chirality parameter (κ) of the
polymers with wavelength is shown in Figure 6. It was observed
that the measured value for LaPPS62 at 455 nm was almost 3-
fold higher (1.02 × 10−2) than that of LaPPS61 (−3.5 × 10−3)
at 400 nm. These results are among the highest observed so far
for chiral polymers.18,19
Figure 3. CD and UV−vis absorption spectra of LaPPS62 films that
were annealed at room temperature (RT) up to 200 °C.
Dependence of Chiroptical Properties on Film Thickness:
LaPPS62. The main conclusion that can be withdrawn from
Figure 4 is that whereas the absorption intensity increases
Theoretical Analysis. In order to provide more insight into
the possible conformations that could be adopted by the
polymer chains, a theoretical study was undertaken. The study
was initiated by performing a conformational search starting
from the optimized geometry of dimers, varying the dihedral
angle at the interconnect of one monomer unit to the other,
and monitoring the energy, in order to find the minima
associated with each conformation. The purpose of this step
was to find the precursor geometries that originated the
direction of the coiled conformations. It should be noted that
the conformations in which the side groups attached to
adjacent carbon 9 in the fluorene unit were placed in the syn
conformation were not considered. A difference in energy of
1.84 and 4.10 kJ mol−1 was found between the R- and S-form
conformers for LaPPS61 and LaPPS62, respectively. Based on
these findings, the geometries of tetramers were optimized for
both oligomers. The optimized geometries of the helical
oligomers are depicted in Figure 7 for both R form (clockwise
sense) and S form (counterclockwise sense). It was found that
in all oligomers a large dihedral angle is acquired between the
fluorene units (LaPPS61) and between fluorene and quinoxa-
line units (LaPPS62) due to steric hindrance between the
hydrogen atoms of neighboring units forcing the polymeric
structure to adopt a coil conformation to minimize the
electrostatic repulsion.
Figure 4. Dependence of absorption and of the CD signal on film
thickness of LaPPS62.
linearly with thickness, the CD signal grows exponentially (see
Supporting Information), reaching the value of 2330 mdeg at
0.200 μm thickness. As compared to the homopolymer,
LaPPS61, half of thickness of LaPPS62 yields a CD signal 3
times higher than that of its parent homopolymer, LaPPS61.
Dissymmetry Ratio (gabs). The absorption dissymmetry
ratio, gabs, is defined as 2(AL − AR)/(AL + AR), where AL and AR
are the absorbances of left and right circularly polarized light,
respectively. The dependence of the dissymmetry ratio gabs on
annealing temperature and film thickness is shown in Figure 5.
In the case of LaPPS61, the transition at 430 nm in Figure 5a is
probably associated with the β phase which as noted increases
up to 50 °C where it becomes the most prominent band in the
spectrum.
It is fair to assume that this angle which systematically
appears among the units is the main reason for the formation of
the helical structures.
Figure 8 shows the calculated (as described in the
Theoretical Methods section) circular dichroism (CD) spectra
of each conformer of the oligomers corresponding to LaPPS61
and LaPPS62 as well as their topological projections (screw
With further temperature increase the transition progres-
sively decreases to a shoulder until its complete disappearance.
This is accompanied by a blue-shift as before noted and
E
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Figure 5. Dissymmetry ratio spectra, gabs, of LaPPS61 films that were annealed from room temperature (RT) up to 200 °C (a) and film thickness
dependence (b). LaPPS62 films that were annealed at room temperature (RT) up to 200 °C (c) and film thickness dependence (d).
Figure 6. Chirality parameter spectra of LaPPS61 (blue line) and of
LaPPS62 (red line).
Figure 7. Optimized geometries of the helical conformers of the
tetramers of (a) LaPPS61-R, (b) LaPPS61-S, (c) LaPPS62-R, and (d)
LaPPS62-S (hydrogen atoms omitted).
sense) figured out based on the molar weight as determined by
GPC and corresponding number of coil turns based on
calculated radius and pitch. Table 1 displays all the results
obtained by the calculations performed.
LaPPS61 and LaPPS62 using the TDDFT methodology. A
reasonable correlation was found between the calculated and
the experimental results for the circular dichroism spectra of
LaPPS61-R. This finding indicates that the polymeric chain
stays mostly in its right-handed form. Moreover, taking into
account the relative free Gibbs energy, this form represents
80% of the total population of both conformers, based on the
Boltzmann distribution. This correlation does not hold well for
LaPPS62, since the sign of the rotational strength at 450 nm
for the R-conformer, representing 96% of the overall
population according to our calculations, is opposite to the
experimentally observed at 450 nm (the sign matches the
experimentally observed at the shorter wavelength of ∼430 nm
though). It is however known that the sign of the rotational
Figure 8 and Table 1 data show that conformers (S and R
forms) related to each oligomer present great similarity in
electronic aspects, as the low excitation energies, in very good
agreement with the experimental results. Regarding the
optimized geometries, LaPPS62(R,S) presents a higher
number of turns per unit length: 75% higher for R form and
94% higher for S form in comparison with LaPPS61. This
means that the pitch of the LaPPS61 helix is 2 times that of the
LaPPS62 one. This fact helps understand the larger optical
activity observed experimentally for LaPPS62 over LaPPS61
since the tighter the helix − the larger the optical activity. We
have also calculated the electronic and CD spectra of the
F
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ASSOCIATED CONTENT
* Supporting Information
■
S
Scanning electron microscopy (SEM) images of thin films,
DSC analyses of polymers, absorbance and CD peak intensities
vs thickness. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: pnprasad@buffalo.edu (P.N.P.).
*E-mail: leni@leniak.net (L.A.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Figure 8. Projected coiled structures of the polymers, based on the
optimized geometries; electronic circular dichroism (ECD): exper-
imental (solid line) and calculated (colored bars); dimensions: (a)
width, (b) depth, (c) pitch size, and (d) length based on molar weight.
■
This work was supported in part by a grant from the Air Force
Office of Scientific Research (grant no. FA95500610398). B.N.,
C.Z., and L.A. are thankful to CNPq and CAPES.
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To conclude, we studied both experimentally and by means of
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a
a
LaPPPS61-R
38.33
LaPPPS61-S
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LaPPPS62-R
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LaPPPS62-S
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dihedral angle (deg)
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80
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20
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381.37 (450 nm)
)
Boltzmann average
96
4
a
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Dihedral angle between fluorene−fluorene. Based on dimensions. Relative to ground state.
G
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