Enantioselective γ-Alkylation of α,β-Unsaturated Aldehydes Using New Cinchona-Based Primary Amine Catalyst

Article abstract of DOI:10.1002/ejoc.201901219

New cinchona-based primary amine catalysts were prepared and screened as organocatalysts for the γ-alkylation of α,β-unsaturated aldehydes with bis(4-dimethylaminophenyl)methanol. Catalyst C3 containing acetic acid group yielded γ-alkylated products in good yields (up to 94 %) with up to 90 % ee. This new primary aminocatalyst provide new opportunities to explore novel asymmetric transformations.

Full text of DOI:10.1002/ejoc.201901219

Article
pubs.acs.org/cm
Cite This: Chem. Mater. 2019, 31, 2507−2515
Semiconducting Properties and Geometry-Directed Self-Assembly
of Heptacyclic Anthradithiophene Diimide-Based Polymers
Jianyao Huang,†,∥ Zhihui Chen,†,‡,∥ Jie Yang,† Huanxin Ju,§ Weifeng Zhang,† and Gui Yu*,†,‡
†Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
‡School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
§National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, China
S
* Supporting Information
ABSTRACT: Backbone geometry and side-chain self-assem-
bly are key factors that govern the aggregation of conjugated
semiconducting polymers and affect charge transport. Tuning
of charge transport properties hinges on being able to strike a
balance between electronic structures, self-assembly, and
robustness to disorder. This work describes a top-down
design strategy focusing on the geometry of polymer
aggregates. A heptacyclic acceptor anthradithiophene diimide
was designed to build self-assembled polymers. The
corresponding polymers possess a pseudolinear rodlike
backbone geometry with interdigitated alkyl chains. Using a fluorinated comonomer suppresses the rotational disorder and
preserves the backbone symmetry at a relatively long length scale, leading to improved planarity and facilitating intrachain
charge transport. These polymers exhibit ambipolar transistor performance with the maximum hole mobility of 4.21 cm2 V−1
s−1. The experimental microstructures are in good agreement with the simulated unit cells, indicating an extended
dimensionality of charge transport pathways. These promising results may help to complement the design rationales of
semiconducting polymers.
To close the understanding gap, we aimed at adapting a
design approach toward controllable self-assembled aggregates
in a predictable manner. Rather than simple installation of
chemical functionalities to the poly(arylene-vinylene) con-
jugation pathway, we first formulated the overview of polymer
geometry using a top-down design strategy and then added
suitable groups to fill in the frameworks. Using computational
tools, the supramolecular organization can be quantitatively, or
at least semiquantitatively, predicted by an ideal packing
model, which reflects, in an extreme form, the general
structure−property relationship. As shown in Figure 1, a
densely packed aggregate of a semiconducting polymer often
manifests itself in interdigitation.24 Molecular mechanics
(MM)/molecular dynamics (MD) simulations can realize
this atomistic understanding. Most of the state-of-the-art high-
mobility materials pack in fully interdigitated motifs, whereas
those with moderate to low performances usually possess
partly interdigitated or noninterdigitated assembly. Herein, we
report the design and synthesis of a new heptacyclic molecule
to build conjugated semiconductors with fully interdigitated
distribution of the side chains and discuss the importance of
molecular geometry in designing high-mobility materials.
INTRODUCTION
■
Central to the area of semiconducting polymers has been the
exploration of structure−function maps. To optimize candidate
materials for flexible electronics, material scientists have
developed diverse π-conjugated chromophores.1−10 The
promise of high-mobility flexible field-effect transistors
(FETs) and high-efficiency organic solar cells based on low-
band-gap semiconductors has driven the rapid advance in
donor−acceptor (D−A)-type semiconducting oligomers and
polymers over the past few years.11−15 Although many
structural parameters affecting charge transport properties
have been intensively evaluated (such as coplanarity,
conformational preference, molecular symmetry, and structural
isomerism),16−23 those relevant to mesoscale (submicrometer)
ordering remain elusive. In this context, a conventional
materials design involves a bottom-up strategy by which
diverse building blocks are compiled to achieve function-
oriented local structural features. It focuses on the molecule’s
electronic structure, an important feature that realizes the
tunability of frontier orbital energetics but ignores the
mesoscale assembly. Such overlooked assembly behavior at
various length scales is important to reveal the molecular origin
of semiconducting properties but has yet to be manipulated.
Moreover, the increasing molecular complexity poses a
daunting challenge to expand the chemical space of these
materials.
Received: December 29, 2018
Revised: March 9, 2019
Published: March 11, 2019
© 2019 American Chemical Society
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Figure 1. Design strategies toward high-performance polymer semiconductors: (a) different interdigitated packing motifs for aggregates, (b)
representation of changes in self-assembly via three different modifications of molecular geometries, and (c) determinants of packing motifs.
20 Å, as described in many reports.31,32 Similar situations are
found for different substitution positions or tilting angles
between the side chain and the backbone. A modification of
the alkylated monomer’s geometry exerts a significant effect on
RESULTS AND DISCUSSION
■
Design, Synthesis, and Structural Characterization. A
quick survey of recent state-of-the-art semiconducting
polymers bearing lactam or imide reveals that linear backbone
aids the π-stacking, whereas a wavelike backbone geometry is
packing. Owing to the cavity or steric crowding, a large c- or a-
sensitive to site disorders or conformational traps.25,26 In
axis projection of the connection between the substitution
positions of the side chains in a unit cell also leads to the open-
armed packing. As illustrated in the schematic picture, the
branched chain lying adjacent to the backbone interacts only
with one chain, thus providing a less stable structure than the
fully interdigitated assembly. This open-armed geometry is
likely sensitive to thermal fluctuations of the lattice that cause
displacement of the main chain, accompanying an uncertain
transfer integral. The absence of fully staggered conformations
of alkyl chains further indicates the instability of the lattice.
Orientational disorders, stacking faults, and dislocations may
occur,33 thereby leading to a deleterious effect on charge
hopping.
Our design commenced with the choice of the 2-
decyltetradecyl side chain to construct a highly interdigitated
polymer aggregate (type A or B) because it is operationally
simple and commercially available. Other structural consid-
erations include (1) a conjugated backbone with natom of ca.
20; (2) suitable protruding sites for side-chain attachment so
that the steric interactions between the side chain and
backbone can be minimized to enhance π-stacking; (3) a
pseudolinear backbone motif that has robustness to conforma-
tional rotation and propensity for aggregation; and (4) a
minimized conformational ambiguity that enables long-range
ordering. We chose maleimides as the substituent sites and an
angular-shaped anthradithiophene as the core structure,
namely, anthradithiophene diimide (ADTDI). This new planar
heptacyclic electron acceptor can form an interdigitated
assembly when coupled with conventional donors such as
bithiophene. Moreover, the steric hindrance between exocyclic
carbonyl groups in the maleimides and spatially adjacent atoms
principle, most high-mobility semiconducting polymers possess
collinear (or, in other terms, pseudostraight) conjugated units.
On the premise that a linear backbone is indispensable, the
backbone geometry and the substitution position of side chains
correlate the incoherent molecular attributes with the
mesoscale self-assembly. If the side chain is selected, the
number of carbon atoms across the conjugation pathway is
likely the determinant of the self-assembled structure. We note
that commercially available 2-decyltetradecyl or 2-octyldodecyl
alkyl chains favor a backbone with approximately 14
(P(NDI2OD-T2)27) to 22 (PDVT-1028) atoms across the
conjugation pathway of a co-repeating unit (natom) (Figure 1c).
In these cases, branched alkyl chains form interdigitated
aggregates that exhibit preferable molecular order than
noninterdigitated polymers. Take the example of polymers
bearing the 2-decyltetradecyl side chain. Figure 1a shows the
schematic view of four typical types of polymer geometry,
where the alignment of 2-decyltetradecyl chains was extracted
from snapshots of MM/MD-simulated state-of-the-art poly-
mers (for detailed data, see the Supporting Informa-
tion).24,29,30 Structural modification of the molecular geometry
leads to a significant change in packing mode. For natom < 12,
overcrowded side chains with a possible formation of a
distorted globule disable the interdigitation. For natom > 24, the
presence of cavities can be predicted when the side chain is
interdigitated, generating a thermodynamically disfavored
structure; consequently, the branched chains deviate from
the a-axis direction and pack into a partially interdigitated
“open-armed” pattern that can fill in the cavities to afford a
denser structure. This MM-simulated packing mode results in
limited solubility as well as a lamellar d-spacing of shorter than
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Scheme 1. Synthetic Route to PADTDI-Based Polymers
is minimized. These features represent the manifestation of our
design strategy.
Such fluorination strategy serves as a common approach to
reduce the dihedral angle distribution of the polymer chain and
to allow the delocalization of extended π-orbitals (Figure
S4).38 The resulting polymers, PADTDI-BT and PADTDI-
FBT, have moderate number-averaged molecular weights of
21.2 and 25.6 kg mol−1, respectively. This is due to the
moderate coupling reactivity in the presence of imide
functional groups.39 Thermogravimetric analyses show that
the 5% weight loss temperatures are 388 and 399 °C for
PADTDI-BT and PADTDI-FBT, respectively, indicative of
good thermal stabilities. Differential scanning calorimetry
traces of the two polymers exhibited no phase transitions in
the range of 25−300 °C (Figure S7).
Optical and Electrochemical Properties. Experimental
UV−vis absorption spectra of ADTDI-based monomers and
polymers were recorded in chloroform solution and in thin
films (Figure 2 and Table 1). We also performed time-
dependent density functional theory (TD-DFT) calculations at
the B3LYP/6-31G(d) level in chloroform solution to illustrate
the electronic transitions of ADTDI. The spectrum of
monomers in chloroform solution reveals well-matched
multiple absorption bands. The low-energy absorption bands
in the 440−510 nm range are attributed to the transitions from
the ground state to the first and second excited states. The
highest occupied molecular orbital−lowest unoccupied molec-
ular orbital (HOMO−LUMO) excitation (98.2%) is the major
contributor of the first excited state. The excitation energy is
2.7450 eV with an oscillator strength of 0.0451. The second
The synthetic route to ADTDI involves two cyclization
reactions (Scheme 1).34−36 2-Oxo-2-(thiophen-3-yl)acetic acid
was sequentially treated with thionyl chloride and ammonia
water to afford 2-oxo-2-(thiophen-3-yl)acetamide (1). This
compound was mixed with dimethyl 2,2′-(2,5-dibromo-1,4-
phenylene)diacetate (2) and sodium tert-butoxide. The
resulting imide (3) was alkylated in the presence of alkyl
iodide and potassium carbonate to obtain diastereoisomeric
mixtures (4) (Figure S2). These diastereoisomers were
converted to the same product in the next cyclization step.
The key intramolecular cyclization proceeded under mild
conditions in the presence of KOAc as the base and Pd(PPh3)4
as the catalyst. The next bromination step encountered a
synthetic difficulty due to the electron-withdrawing nature of
the ADTDI greatly retarding the reaction. We used a strong
acid system, trifluoroacetic acid, to increase the reactivity. The
reaction mixture was stirred for a day to give dibromo ADTDI
in a yield of ca. 25%.
Bithiophene (BT) and 3,3′-difluoro-2,2′-bithiophene (FBT)
were selected to copolymerize with ADTDI. Both monomers
have topological linkages similar to that of ADTDI, preserving
the backbone linearity to maximize long-range ordering,
persistence length of the chain, and robustness to positional
lattice disorder.37 For FBT, the F atom tends to approach the
adjacent S atom in the σS* direction, forming a nonbonded S···
F interaction that can further improve the backbone rigidity.
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hexafluorophosphate in anhydrous acetonitrile was used as the
electrolyte. The HOMO and LUMO energy levels of
PADTDI-BT are estimated to be −5.41 and −3.47 eV from
the onset potentials of 1.01 and −0.93 V, respectively, with a
band gap of 1.94 eV, using the equation E = −(Eonset
−
E
1/2(Fc+/Fc) + 4.80 eV). In comparison, fluorinated PADTDI-
FBT exhibits deep-lying HOMO (−5.89 eV, onset potential:
1.49 V) and LUMO (−3.54 eV, onset potential: −0.86 V)
energy levels because of the stabilization effect of fluorine
atoms. The HOMO energy levels are corroborated by
ultraviolet photoelectron spectroscopy measurements. Using
the equation IP = hν − (Ecutoff − EH,onset), the ionization
potentials are 5.49 and 5.86 eV for PADTDI-BT and
PADTDI-FBT, respectively. These energy levels indicate
possible ambipolar semiconducting properties. Fluorination
also leads to backbone planarization that suppresses the
rotational disorder, as demonstrated by calculated geometries
of trimers (Figure S4) and resonance Raman spectroscopy
(Figures S11).
Figure 2. UV−vis absorption spectra of ADTDI-based monomers and
polymers: (a) ADTDI in dilute chloroform solution (concentration:
ca. 3 × 10−5 mol L−1); TD-DFT-calculated absorption spectrum of
(b) ADTDI, (c) PADTDI-BT, and (d) PADTDI-FBT. The spectrum
was calculated at the B3LYP/6-31G(d) level in chloroform.
Simulated and Experimental Microstructures. We
performed MM simulations with periodic boundary conditions
to simulate the infinite system. As previously reported,40 the
chirality of side chains is not a determinant factor to the
calculation of d-spacing; therefore, we used (R,S)-alkylated
ADTDI to perform the calculation, in which the two alkyl
chains are centrosymmetrically distributed so as to simplify the
modeling. The simulation of PADTDI-based polymers
involves the following steps. First, a simplified model of a
PADTDI-BT aggregate containing four dimers was built
(Figure S6). This model confirms that ADTDI-based polymers
favor side-chain interdigitation to afford a dense packing with
an interlayer distance of less than 30 Å and a π-stacking
distance of approximately 3.5 Å. A single co-repeating unit was
extracted from the model and used as the input for MM
calculations. We built many starting cell structures by slightly
varying the lattice parameters and the orientation of the alkyl
chains. We performed 100 ps-quenched dynamics on each
structure at 300 and 600 K to afford the most stable structures.
The simulated structure shows that alkyl chains self-assemble
into ordered lamellar packing; the branched decyl chains lie
parallel to the a-axis of the unit cell, whereas the dodecyl
chains bend to adopt a supramolecular organization with
adjacent chains. Such a structure extends the backbone order
to the spatially adjacent chains and improves the dimension-
ality of charge transport. Introducing fluorine atoms into the
backbone has a minute effect on the lattice parameters,
providing similar interdigitated packing modes. The simulated
crystal of PADTDI-FBT exhibits weak CH···F interactions
with adjacent alkyl chains (2.77−2.78 Å), as well as enhanced
backbone planarity, leading to the variance of d-spacing. The
calculated d100 distances are 26.6 and 26.8 Å for PADTDI-BT
and PADTDI-FBT, respectively.
Table 1. Optical and Electrochemical Properties
λmax (nm)
Eogpt
(eV)
ELUMO
(eV)
EHOMO
(eV)
Ecgv
(eV)
polymer
soln
film
PADTDI-BT
PADTDI-FBT
506
508
507
507
2.05
2.06
−3.47
−3.54
−5.41
−5.89
1.94
2.35
excited state is composed mainly of two transitions, HOMO −
1 to LUMO (94.2%) and HOMO to LUMO + 2 (3.8%), with
an oscillator strength of 0.2050. By comparing the molecular
orbital distributions, both low-energy absorption bands
indicate push−pull interactions between the internal anthra-
dithiophene (electron-donating) and peripheral imide (elec-
tron-accepting) units. Because rigid π-conjugated frameworks
and donor−acceptor structures can typically lead to
fluorescence, we also measured the emission spectrum of
ADTDI. The solution of ADTDI displays green-yellow
fluorescence with a moderate Stokes shift of 27 nm, indicative
of the relatively rigid backbone (Figure S8). Figure 2c,d
illustrates the UV−vis absorption spectra of polymers in dilute
chloroform solution and in thin films. All spectra exhibit broad
absorption bands. The absorption maxima are 506 and 508 nm
in solution for PADTDI-BT and PADTDI-FBT, respectively.
For thin films, vibronic fine structures are shown in the
absorption spectra, revealing slightly different aggregate
tendencies. The optical gaps are estimated to be 2.05 eV for
PADTDI-BT and 2.06 eV for PADTDI-FBT. This result
implies the minute effect of the backbone fluorination on
optical gaps. After thermal annealing, increased relative
strengths of 0−0 to 0−1 vibronic peaks, together with
approximately 5 nm bathochromic shifts of absorption maxima,
correspond to enhanced aggregation for both polymers (Figure
S9).
To evaluate the geometric design strategy, we obtained the
microstructural information using two-dimensional grazing
incidence X-ray diffraction (GIXRD) measurements (Figures 3
and S11, Table S2). The GIXRD patterns exhibit typical arc
(h00) peaks up to the fourth order along the out-of-plane
direction. The (h00) peaks of the as-spun and annealed
PADTDI-BT thin-film diffraction patterns correspond to
interlayer d-spacings of 26.3 and 26.5 Å, respectively, in
good agreement with the simulated values. The (001)
diffraction peaks are also observed in the patterns, demonstrat-
ing a moderate ordering along the c-axis that extends the
We measured the electrochemical characteristics (Figure
S10) by cyclic voltammetry (CV) measurements using
ferrocene+/ferrocene as internal calibration, a Ag/AgCl (Ag
in 0.01 mol L−1 KCl) electrode as the reference electrode, a
platinum wire as the counter electrode, and a glassy carbon
stick electrode coated with a dip-coated polymer film as the
working electrode. A solution of 0.1 M tetrabutylammonium
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Figure 3. Simulated unit cells and experimental grazing incidence X-
ray diffraction (GIXRD) patterns of PADTDI-BT and PADTDI-FBT.
Figure 4. Typical transfer and output characteristics of the TGBC
FETs based on (a) PADTDI-BT and (b) PADTDI-FBT thin films.
dimensionality of charge transport pathways. The correspond-
ing d(001) distance is approximately 17.5 Å. This value is smaller
than the calculated distance of a co-repeating D−A unit (ca.
19.7 Å), indicating the tilt along the c-axis of the polymer
polymers exhibit ambipolar charge transport behaviors with
predominant p-channel characteristics. Annealing treatment at
a mild temperature of 100 °C afforded the highest carrier
mobilities (Figure S13). The electron mobilities of the two
polymers are in the range of 10−3−10−2 cm2 V−1 s−1. By
contrast, PADTDI-BT-based devices exhibit the maximum
hole mobility of 1.47 cm2 V−1 s−1 with an on/off current ratio
of 103 and a threshold voltage of −12 V. By contrast,
PADTDI-FBT-based devices show higher mobilities of up to
4.21 cm2 V−1 s−1 with an on/off current ratio of 103 and a
threshold voltage of −18 V. The average mobilities of at least
five devices for PADTDI-BT- and PADTDI-FBT-based thin-
film transistors are 1.19 and 3.84 cm2 V−1 s−1, respectively
(Figure S14). The high charge-carrier mobility is among the
highest mobility values reported for new-core polymer-based
FET devices in recent years (Table S1). To evaluate the
reliability of the devices, we measured the mobilities at the
linear regimes using the equation IDS = (W/L)Ciμ(VGS − Vth)
VDS. The maximum linear hole mobilities for PADTDI-BT-
and PADTDI-FBT-based devices are 0.99 and 3.53 cm2 V−1
s−1 with the average values of 0.80 and 3.10 cm2 V−1 s−1,
respectively. Moreover, the relatively small gate-voltage
dependence of mobility, sweep-rate dependence, and hysteresis
were observed during the test (Figures S15−S18). This result
may pave the way for the potential in organic electronics.
Evidently, such high mobilities highlight that the underlying
geometry-directed assembly facilitates the charge transport.
The effect of annealing treatment was characterized by
atomic force microscopy (AFM), GIXRD, and carbon K-edge
near-edge X-ray absorption fine structure (NEXAFS) spec-
troscopy. First, we studied the surface morphological changes
using tapping-mode AFM. The topological AFM images of the
PADTDI-BT and PADTDI-FBT thin films are shown in
Figure 5, exhibiting interconnected textures in both thin films.
PADTDI-BT films exhibit granular morphologies, whereas
fibrillar-like features are observed in PADTDI-FBT thin films.
Compared with the as-spun thin film, annealing tends to afford
a homogeneous film that fills in the energetic traps and
semicrystal. In the in-plane direction, the (010) peak at qxy
=
ca. 1.80 Å−1 corresponds to a cohesive π-stacking of 3.49 Å.
According to the Scherrer equation, the average grain size Dhkl
= 2πK/Δqhkl, where K is the Scherrer constant (0.89) and
Δqhkl is the full width at half-maximum of the diffraction peak
with regard to the scattering vector q.41 The average grain sizes
of the as-spun PADTDI-BT thin film were 248 and 40.4 Å for
d−d and π−π stacking, respectively, corresponding to average
numbers of Bragg planes of 9 and 11 with respect to the (100)
and (010) Bragg reflections. After annealing, such average
grain sizes increased to 272 and 46.7 Å, respectively. The d−d
(26.6 Å) and π−π (3.49 Å) distances of PADTDI-FBT films
are almost identical to those of PADTDI-BT films. The
average grain sizes of each Bragg reflections are slightly smaller
than those of the nonfluorinated analogues. Such micro-
structural information validates the well-packed feature of
these polymers.
Field-Effect Transistors and Device Optimization. The
semiconducting characteristics of PADTDI-BT- and PADTDI-
FBT-based devices were measured using the top-gate/bottom-
contact (TGBC) FET configuration (channel length/width =
50/4500 μm). We used poly(ethylene terephthalate) (PET)
s u b s t r a t e s a n d h y b r i d b i l a y e r p o l y -
(perfluorobutenylvinylether)/poly(vinyl alcohol) (Cytop/
PVA) dielectrics (40 nm thick Cytop, 300 nm thick PVA, Ci
= 15.6 nF cm−2).42 The mobilities were extracted from the
saturation regimes using the following equation IDS = (W/2L)
Ciμ(VGS − Vth)2, where IDS is source−drain saturation current,
W/L is the channel width/length, Ci is the capacitance per unit
area of the insulator, μ is the mobility, VGS is the gate voltage,
and Vth is the threshold voltage. Figure 4 shows the transfer
and output characteristics of the PADTDI-BT- and PADTDI-
FBT-based devices with approximately ideal I−V curves, and
the corresponding FET parameters are listed in Table 2. Both
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a
Table 2. FET Characteristics of ADTDI-Based Copolymers
p-channel
n-channel
b
c
b
c
polymer
μh (cm2 V−1 s−1
)
Ion/Ioff
Vth (V)
μe (cm2 V−1 s−1
)
Ion/Ioff
Vth (V)
PADTDI-BT
PADTDI-FBT
1.19 0.28 (0.80 0.19)
3.84 0.37 (3.10 0.43)
102−103 (104−105)
−11
−16
2
2
0.010 0.004 (0.009 0.003)
0.005 0.001 (0.002 0.001)
100−101 (103−104)
15
19
5
6
103−104 (103−104)
100−101 (103−104)
a
b
TGBC devices with L = 50 μm and W = 4500 μm fabricated on PET substrates. Mobilities were extracted from the saturation and linear (in
parentheses) regime. The average mobilities were obtained from at least five devices. Estimated from the saturation and linear (in parentheses)
c
regime.
the corresponding values are 6.35, 5.95, and 5.76%. These
results demonstrate that the enhanced crystallinity of grains is
beneficial to the charge transport.
We performed carbon K-edge near-edge X-ray absorption
fine structure (NEXAFS) spectroscopy in total electron yield
(TEY) mode to further study the surface microstructures
before and after annealing (Figure 6).47−49 TEY detection is
Figure 5. AFM height images of as-spun and annealed polymer thin
films of (a, b) PADTDI-BT and (c, d) PADTDI-FBT on a PET
substrate. From left to right: as-spun and annealed films at 100 °C.
morphological defects, leading to fewer grain boundaries and
facilitating charge transport.
The orientation preferences of the polymer chains were
qualified by Herman’s orientation parameter (S) (Figure S19).
All of the thin films exhibit weak edge-on packings. The
calculated S(200) values of as-spun and annealed PADTDI-BT
thin films are in the range of 0.24−0.25, demonstrating minute
orientation changes toward edge-on packing during the
annealing treatment. In contrast, decreased edge-on fractions
with the decreased S values are evident in the fluorinated
polymer films. These fluorinated polymer-based thin films
showed a slight tendency toward isotropic alignment after
thermal treatment as the S value changes from 0.314 for the as-
spun thin film to 0.290 for the film after annealing at 100 °C.
Figure 6. Angle-resolved NEXAFS spectra of polymer thin films in
To further reveal the variation of crystallinity during the
total electron yield mode: (a) as-spun PADTDI-BT, (b) annealed
annealing treatment, we performed paracrystalline disorder
analysis. The paracrystalline disorder due to numerous degrees
of conformational freedom results in the broadening of
diffraction peaks because of the variance of interplanar
spacing.43−46 For paracrystals, Δb = l/d[l/N + (πgh)2],
where Δb is the integral breadth (b = 2 sin θ/λ = q/2π), N
is the average number of Bragg planes, h is the order of the
diffraction peak, and g is the paracrystalline distortion
parameter. The g value can be readily obtained from the
slope of the Δb−h2 plot (Figure S21). An ideal crystal is
described by g = 0, whereas g values of semicrystalline
conjugated polymers are in the range of 1−10%. Given the
significant overlap between the (400) diffraction and the alkyl
chain stacking peaks, only the first three peaks were used to
calculate the g value. The g value for as-spun PADTDI-BT is
5.71% and decreases to 5.05 and 4.81% after thermal treatment
at 60 and 100 °C, respectively. In the case of PADTDI-FBT,
PADTDI-BT, (c) as-spun PADTDI-FBT, (d) annealed PADTDI-
FBT, and (e) schematic picture of the geometry of NEXAFS
spectroscopy. The inset values are the dichroic ratio (R) of the 1s →
π* intensity and the average tilt angle (⟨α⟩) of a polymer backbone.
surface sensitive with the information depth of a few
nanometers for a polymer sample.50 This measurement reflects
the molecular orientation at the dielectric/polymer interface
where charge transport occurs for a top-gate transistor. The
lower energy features at approximately 284.8 eV correspond to
the transition of an electron-withdrawing aromatic core, and
those in the 285−287 eV range are associated with the 1s →
π* transition of bithiophene and substituted unsaturated
carbon atoms. These spectra also exhibit significant C 1s → σ*
features due to the presence of alkyl side chains. The resonance
intensity is dependent on the relative molecular orientation to
the electric field vector of the synchrotron beam. For the C 1s
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→ π* transition, the transition dipole moment is oriented
perpendicular to the aromatic backbone. The variation in
resonance intensity as a function of incidence angle can be
used to determine the backbone orientation. The dichroic ratio
(R), defined as the difference of the intensities at 90 and 0°
incidence divided by their sum, and the corresponding average
tilt angle (⟨α⟩) are two typical parameters to describe the
orientation. We fitted the π* intensities of each curve (details
are given in the Supporting Information); the results show that
the R values are 0.58, 0.70, 0.70, and 0.53 for as-spun
PADTDI-BT, annealed PADTDI-BT, as-spun PADTDI-FBT,
and annealed PADTDI-FBT thin films, respectively. These
positive values indicate large average tilt angles of the transition
dipole with respect to the substrate normal. Consequently,
polymer strains at the top surface pack in a highly edge-on
manner. Thermal treatment leads to different propensities for
backbone alignment; after annealing, PADTDI-BT prefers an
enhanced edge-on packing, whereas PADTDI-FBT gives a
reduced fraction of edge-on crystallites. This phenomenon is
likely due to fluorination altering the surface energy of the
polymer. Such tendencies are consistent with GIXRD analyses
described above.
spectra; CV traces and UPS profiles; Raman analyses;
additional information for FETs; GIXRD patterns of
polymer thin films before and after annealing; NMR
spectra of new compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: yugui@iccas.ac.cn.
ORCID
Jianyao Huang: 0000-0003-4177-6393
Gui Yu: 0000-0001-8324-397X
■
Author Contributions
∥J.H. and Z.C. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Key
Research and Development Program of China
(2017YFA0204703 and 2016YFB0401100), the National
Natural Science Foundation of China (Grants 21673258,
21774134, and 21474116), and the Strategic Priority Research
Program of the Chinese Academy of Sciences
(XDB12030100). The authors are grateful to the assistance
of researchers from the BL14B1 Station of Shanghai
Synchrotron Radiation Facility and the Catalysis and Surface
Science Endstation in National Synchrotron Radiation
Laboratory in Hefei during the GIXRD and NEXAFS tests.
CONCLUSIONS
■
The design of new semiconducting polymers is still difficult to
rationalize because many parameters are involved. Recent
design strategies have focused mainly on electronic structures,
molecular planarity, torsional barriers, conformational locks,
robustness to structural disorder, and side-chain engineering.
Besides tuning of electronic structures, other factors are all
related to geometric control at various length scales. In this
contribution, we develop a new heptacyclic acceptor,
anthradithiophene diimide, and discuss in detail the
correlations between backbone geometry and semiconducting
properties. The high mobility of up to 4.21 cm2 V−1 s−1 is likely
ascribed to the geometry-directed self-assembled nature of the
polymer. The ideal packing modes show sufficient evidence
that geometric factors, including the side-chain length,
backbone linearity, positions of substituents, and tilting angle
between the side chain and the backbone, contribute to the
formation of self-assembly at short range. Interdigitation of
side chains is supramolecularly preferable for high-mobility
semiconducting polymers. These insights may help to probe
into the underlying of the self-assembled structure beyond the
molecular formula and the design of high-mobility materials
with bulk dimensionalities of charge percolation pathways. A
modification of the D−A polymers is not a simple permutation
and combination. Geometric control is equally important to
electronic structures. A good candidate should be both
electronically preferable and geometrically optimized. Ruling
out the unfavorable candidates in the first stage accelerates the
study in this field that holds promise for the future evolvement
of polymeric semiconductors.
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