Synthesis and characterization of solution-processable ladderized n-type naphthalene bisimide copolymers for OFET applications

Article abstract of DOI:10.1021/ma2004822

Solution-processable n-type ladder-based polymers are highly desirable due to their potential capability to form strong π-π interactions. A series of 5 highly soluble naphthalene diimide (NDI) polymers are presented, differing in the degree to which they

Full text of DOI:10.1021/ma2004822

ARTICLE
pubs.acs.org/Macromolecules
Synthesis and Characterization of Solution-Processable Ladderized
n-Type Naphthalene Bisimide Copolymers for OFET Applications
Matthew M. Durban,† Peter D. Kazarinoff,‡ Yukari Segawa,‡,§ and Christine K. Luscombe*,‡
†Department of Chemistry, University of Washington, Seattle, Washington 98195-1750, United States
‡Department of Material Science and Engineering, University of Washington, Seattle, Washington 98195-2120, United States , and
§Department of Organic and Polymeric Materials, Graduate School of Engineering, Tokyo Institute of Technology, 2-12-1-H120,
O-okayama, Meguro-ku, Tokyo, 152-8552, Japan
S Supporting Information
b
ABSTRACT: Solution-processable n-type ladder-based poly-
mers are highly desirable due to their potential capability to
form strong πꢀπ interactions. A series of 5 highly soluble
naphthalene diimide (NDI) polymers are presented, differing in
the degree to which they are able to form imine-bridged ladder
polymer structures. Average electron mobilities as high as
0.0026 cm2 Vꢀ1 sꢀ1, which show an electron-mobility improve-
ment of 4 orders of magnitude following ladderization, and on/off current ratios on the order of 104 are reported for the novel
material PNDI-2BocL, an alkyl-substituted poly(benzoquinolinophenanthrolinedione). The structureꢀproperty relationship of
the aforementioned series of copolymers is presented and discussed as it pertains to organic field-effect transistor (OFET)
performance.
’ INTRODUCTION
heteroaromatic ladder polymer materials.63 Novel solution-pro-
cessable ladder polymers are thus desirable in addressing a goal
toward improving organic electronic device performance.
Gradual development within the field of organic electronics
has resulted in a progressive enrichment of the variety of novel
materials that are available for use in a wide range of applications.
Electron-transporting n-type materials are slowly beginning to
gain ground with regard to their representation within the field
compared to their corresponding hole-transport p-type analo-
gues. n-Type materials have typically been hindered by issues
associated with material solubility,1,2 applicable processability,
and as pertaining to photovoltaic applications, limited light absorp-
tion relative to the solar spectrum.3 However, a variety of solution
processable n-type materials have recently been synthesized and
reported including; fullerenes,4ꢀ9 acenes,10ꢀ16 siloles,17ꢀ19
azoles,20 perylene,21ꢀ28 and naphthalene diimides.29ꢀ35 Of
these materials, naphthalene diimide (NDI) copolymers36ꢀ43 have
shown exceptional promise in low-cost optoelectronic applications
due to their relatively high electron mobilities,42 solution-
processability,40,42 air-stability,41 good light absorption
characteristics,43 and ambipolar capabilities.44,45
The high structural-planarity and regularity of π-conjugated
ladder polymer materials can allow for desirable intermolecular
πꢀπ stacking interactions as a result of improved π-electron
delocalization.46ꢀ61 Such an example has been demonstrated with
the ladder material poly(benzobisimidazobenzophenanthroline)
(BBL) which has shown high organic field-effect transistor
(OFET) electron mobilities amid possessing poor solubility in
conventional organic solvents due to its exceptional long-range
packing order and crystallinity.62 Meanwhile, there remains a
limited amount of available information pertaining to soluble
Additional insight into the structureꢀproperty relationships
associated with naphthalene-diimide copolymers is presented
through the synthesis and characterization of a series of solution
processable naphthalene diimide-based, di-tert-butyl dicarbonate
(Boc) substituted benzene copolymers (PNDI-0Boc [an alkyl-
substituted poly(benzophenanthrolinetetraone)], PNDI-1Boc
[an alkyl-substituted poly(benzophenanthrolinetetraone-phenyl-
carbamate)], and PNDI-2Boc [an alkyl-substituted poly(benzo-
phenanthrolinetetraone-phenyldicarbamate)], see Scheme 1
for the monomers and Scheme 2 for the polymers) which are
capable of undergoing a solution-based intramolecular cycliza-
tion process64ꢀ67 in yielding soluble n-type ladder polymer
materials (PNDI-1BocL, which is an alkyl-substituted poly-
(benzoquinolinophenanthrolinetrione) and PNDI-2BocL, an
alkyl-substituted poly(benzoquinolinophenanthrolinedione),
see Scheme 2). To our knowledge, these are the first n-type
ladder polymers, which are soluble in common organic solvents,
to be reported. Recently, a series of soluble high-performance
naphthalene diimide core donorꢀacceptor based polymers
(using the same naphthalene diimide core as the materials
presented in this report) containing a varying number of thio-
phene subunits within the polymer backbone was reported by our
Received: March 2, 2011
Revised:
May 21, 2011
Published: June 03, 2011
r
2011 American Chemical Society
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Scheme 1. Characteristic Stannylated-BOC-Monomer Synthesisa
a Key: i, oxalyl chloride, cat. DMF; ii, NaN3, tBuNH4Br, tBuOH; iii, MeLi, tBuLi, trimethyltin chloride.
Scheme 2. NDI Polymer Synthesis and Ladderizationa
precursor thin films were thermally treated to remove the Boc
groups and promote cyclization. However, naphthalene diimide’s
branched octyl-dodecyl solubilizing chains proved to be suffi-
ciently capable of bestowing solubility upon the final ladderized
polymer products (PNDI-1BocL and PNDI-2BocL). There-
fore, solution based cyclization routes were used to obtain the
final ladderized structures.
’ EXPERIMENTAL SECTION
Instrumentation and Materials. 1H and 13C NMR spectra were
obtained using a Bruker AV-300 or AV-500 spectrometer. Polymer
molecular weights were obtained using a Waters-1515 gel permeation
chromatograph (GPC) coupled with UV and RI detectors. Polystyrene
GPC standards in THF were used as a reference. Cyclic voltammetry for
all polymers was obtained under nitrogen using a BAS CV-50 W
voltammetric analyzer with 0.1 M tetra-n-butylammonium hexafluor-
ophosphate in anhydrous acetonitrile as supporting electrolyte. A
platinum wire working and counter electrode as well as an Ag/AgNO3
reference electrode was used and ferrocene (Fc/Fcþ) was designated as
an internal standard for all measurements. The scan rate was 50 mV/s.
The reference energy level used for ferrocene as compared to vacuum
was 4.8 eV. A HP4145B semiconductor parameter analyzer controlled
by locally written LabView codes through a GPIB interface was used for
OFET device characterization. All device fabrication and electrical
characterization were performed in a nitrogen atmosphere. Samples
for UV/vis, FTIR, and X-ray diffraction analysis were cleaned prior to
use. Absorption spectra were obtained using a Varian Cary 5000 UVꢀ
visꢀNIR spectrophotometer. FTIR spectra were obtained using a Bruker
Vector 33 Fourier transform infrared spectrophotometer equipped to
analyze NaCl sample plates. Mass spectrometer data (HR/LR) was
obtained using a JEOL HX-110 mass spectrometer and a Bruker Esquire
LC-Ion Trap mass spectrometer. Density functional theory (DFT)
calculations were performed using the Gaussian 03 software package
using the B3LYP calculation method and 6-31G basis set.68ꢀ70 Geo-
metry optimization followed by frequency calculation was performed to
obtain the lowest energy conformations. All reagents (including 2a)
used for synthesis were obtained from Sigma-Aldrich and were used as-is
without further purification with the exception of the catalyst, Pd-
(PPh3)2Cl2, which was obtained from Strem Chemicals.
Synthetic Procedures. The monomers used to synthesize the
NDI-polymers were synthesized using standard literature procedures—
specifically, NDIꢀBr2,40,42,71,72 and the stannylated phenyl monomers;
a The asterisk denotes that Stille coupling reaction NDI-polymers were
end-capped with phenyl groups using 1-bromobenzene.
1
73 and 3.74
Synthesis of 2c. To a 100 mL air-free Schlenk flask, 2,5-dibromo-
benzoic acid (2a) (2.5 g, 8.9 mmol) was added to 30 mL of toluene
followed by oxalyl chloride (1.7 g, 13.4 mmol) and a catalytic amount of
DMF (1 drop). The solution was heated at reflux for 2 h. The gold-
colored clear reaction solution was cooled to room temperature to
provide a yellow residue (2b) after the solvent was removed under
reduced pressure, which was not isolated. The residue was dissolved in
dry dichloromethane (14 mL) and added dropwise to a solution of
group.43 OFET electron mobilities as high as 0.076 cm2 Vꢀ1 sꢀ1
were achieved using the novel material PNDI-3Th (an alkyl-
substituted poly[terthiophenebenzophenanthrolinetetraone])
and provided incentive into developing a soluble NDI-based
ladder polymer. A soluble-precursor polymer (PNDI-1Boc
and PNDI-2Boc) route was initially investigated whereby the
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Table 1. Polymer Properties and Yields
yield (%) Mn (kDa) [PDI]a soln λmaxb abs (nm) film λmaxc abs (nm) Egopt d (eV) E1/2rede (V) LUMOf (eV) HOMOg (eV)
PNDI-0Boc
PNDI-1Boc
PNDI-2Boc
PNDI-1BocL
PNDI-2BocL
87
74
76
74
93
8.8 [2.5]
4.0 [1.8]
7.2 [1.6]
4.3 [1.9]
14 [1.8]
453
380
381
467
578
471
384
382
476
576
2.31
2.19
1.94
2.37
1.96
ꢀ0.96
ꢀ0.90
ꢀ0.84
ꢀ1.20
ꢀ1.18
ꢀ3.79
ꢀ3.83
ꢀ3.90
ꢀ3.54
ꢀ3.54
ꢀ6.10
ꢀ6.02
ꢀ5.84
ꢀ5.90
ꢀ5.49
a Number-average molecular weight and polydispersity (SEC vs polystyrene standards in THF). b Solution absorption spectra (∼5 ꢁ 10ꢀ6 M CHCl3).
c Thin film absorption spectra from spin-cast CHCl3 solutions. d Optical energy gap estimated from the absorption-edge or onset of organic thin films.
e CV measurements of thin films vs Fc/Fcþ. f Estimated from LUMO = ꢀ(normalized Fc/Fcþ) ꢀ E1/2red. g Estimated from HOMO = LUMO ꢀ Eg
.
opt
t
saturated (6.4 M) sodium azide (7 mL), BuNH4Br (4.1 mg), and
dichloromethane (3.5 mL) at ꢀ5 ꢀC over 1.5 h. The biphasic mixture
was stirred for 10 min after the addition was complete. The organic
phase was separated and the aqueous phase was extracted with cold
dichloromethane (2 ꢁ 20 mL). The organic phases were combined and
washed with cold water (2 ꢁ 30 mL). The solution was then dried over
MgSO4 at 0 ꢀC and filtered over a mixture of Celite and MgSO4. The
filtrate was subsequently dried over CaH2 at 0 ꢀC for 40 min. The
mixture was filtered again through a pad of Celite and MgSO4. The light
(2 ꢁ 60 mL). The combined organics were washed with water (2 ꢁ
50 mL) and dried over Na2SO4. The solution was concentrated by
vacuum and precipitated into methanol. The precipitated polymer was
subjected to a Soxhlet extraction with acetone and methanol for 48 h to
remove low-molecular weight polymer fragments. The polymer was
redissolved in chloroform, precipitated into methanol, and dried under
reduced pressure. See Table 1 for further polymer data. PNDI-0Boc:
1
orange solid (87%). H (300 MHz, CDCl3): 8.89 (br, 2H), 7.58 (br,
4H), 4.11 (br, 4H), 2.01 (m, 2H), 1.03ꢀ1.49 (m, 64H), 0.84 (m, 12H).
13C (300 MHz, CDCl3): 163ꢀ162, 147, 140, 136, 129ꢀ128, 128ꢀ125,
124ꢀ123, 45, 37ꢀ36, 32, 31, 30ꢀ29, 26, 23ꢀ22, 14. PNDI-1Boc: red
t
yellow filtrate was added to BuOH (15 mL) and heated at reflux
overnight. The cloudy-white solution was cooled to room temperature
and the solvent removed under reduced pressure to provide an off-
white solid. The residue was purified via flash chromatography using a
mixture of ethyl acetate and hexane (1:4) as eluent. 2c was isolated as
a white solid (2.4 g, 77%). 1H (300 MHz, CDCl3): δ 8.32 (d, 1H), 7.26
(d, 1H), 6.94 (dd, 1H), 6.92 (s, 1H), 1.46 (s, 9H). 13C (75 MHz,
CDCl3): δ 151.9, 137.4, 133.1, 126.6, 122.5, 122.0, 110.5, 81.6, 28.2 (3).
HRMS-FABþ (m/z): calcd for C11H13O2NBr2, 350.92947; found,
350.92859; m/z 351 (14%), 296 (39%), 251 (16%), 165 (8%), 154
(100%), 107 (32%).
Synthesis of 2. A 50 mL air-free Schlenk flask was charged with 2c
(1.1 g, 3.1 mmol) in diethyl ether (20 mL) at ꢀ78 ꢀC. 1.6 M MeLi
(2.05 mL, 3.3 mmol) was added to the solution dropwise and allowed to
stir for 10 min at ꢀ78 ꢀC. The solution was allowed to warm to 0 ꢀC.
The yellow solution was slowly transferred via cannula to a 100 mL air-
1
solid (74%). H (500 MHz, CDCl3): 8.90 (br, 2H), 8.26 (br, 1H),
7.92ꢀ7.51 (br, 1H), 7.30 (br, 1H), 6.12 (br, 1H), 4.15 (br, 4H), 2.04 (m,
2H), 0.97ꢀ1.73 (m, 73H), 0.86 (m, 12H). 13C (125 MHz, CDCl3):
163ꢀ162, 153ꢀ152, 147, 144, 141, 137ꢀ134, 129ꢀ123, 81ꢀ80, 45,
37ꢀ36, 32ꢀ31, 31ꢀ29, 29ꢀ28, 27ꢀ26, 23ꢀ22, 14. PNDI-2Boc:
1
brown-red solid (76%). H (500 MHz, CDCl3): 8.91 (br, 2H), 7.99
(br, 2H), 6.03 (br, 2H), 4.16 (br, 4H), 2.03 (m, 2H), 0.99ꢀ1.61 (m,
82H), 0.85 (m, 12H). 13C (125 MHz, CDCl3): 164ꢀ160, 154ꢀ152,
144ꢀ143, 141, 139ꢀ136, 134ꢀ130, 129ꢀ123, 122ꢀ119, 81ꢀ80, 45,
37ꢀ36, 32ꢀ31, 31ꢀ29, 29ꢀ27, 27ꢀ26, 23ꢀ22, 14.
General Ladderization Treatment Generating PNDI-xBocL. Litera-
ture procedures were adapted to form the ladder polymers.74 To a
single-neck round-bottom flask, PNDI-xBoc (0.1 mmol) was dissolved
in dichloromethane (5ꢀ10 mL). Trifluoroacetic acid (3ꢀ4 mL) and
anisole (0.5 mL) were added and the mixture was heated at reflux
overnight. The reaction solution was subsequently cooled to room
temperature and the solution was carefully and slowly added to a mixed
solvent of triethylamine (10 mL) and acetone (100 mL). The precipitate
was collected by filtration and then dissolved in triethylamine (5 mL).
The solution was heated in a screw-cap vial at 80 ꢀC overnight. The
solution was then cooled to room temperature, precipitated in acetone,
and filtered. The solids were washed with acetone and dried to yield the
t
free Schlenk flask containing 1.7 M BuLi (6.08 mL, 10.3 mmol) in
diethyl ether (7 mL) at ꢀ78 ꢀC. The mixture was stirred for 4 h at
ꢀ78 ꢀC before allowing the reaction to slowly warm to 0 ꢀC. The dark
orange slurry was recooled to ꢀ78 ꢀC before adding 1 M trimethyltin
chloride (8.33 mL, 8.33 mmol). The light clear orange solution was
allowed to warm to 0 ꢀC before quenching with water (20 mL). The
organics were separated and the aqueous phase was extracted with
diethyl ether (2 ꢁ 20 mL). The organics were combined and washed
with water (2 ꢁ 30 mL) and brine (30 mL). The isolated organics were
dried over Na2SO4, filtered, and the solvent removed under reduced
pressure to produce a red-orange oil. The residue was purified via flash
chromatography by eluting through basic silica (treated with 9:1 hexane:
triethylamine) using hexane as eluent. 2 was isolated as a white crystal-
line solid (0.31 g, 19%). 1H (300 MHz, CDCl3): 7.56 (s, 1H), 7.39 (d,
1H), 7.25 (d, 1H), 6.24 (s, 1H), 1.50 (s, 9H), 0.32 (s, 9H), 0.28 (s, 9H).
13C (75 MHz, CDCl3): δ 153.93, 143.68, 142.82, 136.42, 136.03,
132.23, 130.53, 80.28, 28.44 (3), ꢀ8.55 (3), ꢀ9.49 (3). MS/EI: m/z
540 (Naþ), 504 (17%), 448 (100%), 400 (10%), 286 (15%), 165 (46%),
135 (10%).
1
final ladderized polymer product. PNDI-1BocL: red solid (74%). H
(500 MHz, CDCl3): 9.74ꢀ10.50 (br, 2H), 7.83ꢀ9.53 (br, 3H),
3.79ꢀ4.52 (br, 4H), 1.88ꢀ2.52 (m, 2H), 0.95ꢀ1.79 (m, 64H), 0.84
(m, 12H). 13C (125 MHz, CDCl3): 165ꢀ160, 150ꢀ140, 138ꢀ134,
130ꢀ120, 110, 45, 37ꢀ36, 32ꢀ31, 31ꢀ29, 27ꢀ26, 23ꢀ22, 14. PNDI-
1
2BocL: violet-black solid (93%). H (500 MHz, CDCl3): 9.63ꢀ10.50
(br, 1H), 9.24 (br, 2H), 7.04ꢀ8.76 (br, 1H), 5.23 (br, 4H), 2.37 (m,
2H), 1.04ꢀ2.25 (m, 64H), 0.87 (m, 12H). 13C (125 MHz, CDCl3):
165ꢀ158, 151ꢀ146, 145ꢀ137, 135ꢀ117, 115ꢀ106, 45, 37ꢀ35,
32ꢀ31, 31ꢀ29, 29ꢀ27, 27ꢀ26, 23ꢀ22, 14.
OFET Device Fabrication. Thin film transistors were fabricated as
typical top-contact bottom-gate devices on silicon substrates. Heavily
doped p-type silicon Æ100æ substrates from Montco Silicon Technolo-
gies Inc. with a 300 nm ((5 nm) thermal oxide layer acted as common
gate, dielectric layer, and substrate. The substrates were cleaned by
sequential ultrasonication in acetone, methanol, and isopropyl alcohol
for 10 min followed by an air plasma treatment. The optimized
conditions used for obtaining the electron mobility measurements are
General Synthesis for PNDI-xBoc. To an air-free Schlenk flask,
Pd(PPh3)2Cl2 (0.005 mmol), NDIꢀBr2 (0.10 mmol), and 1,73 2, or
374 (0.10 mmol) were added within a dry glovebox and dissolved with
toluene (5 mL). The capped and sealed vessel was heated at 90 ꢀC for 4
days prior to injecting bromobenzene (0.2 mL) and stirring for 12 h. The
reaction was cooled to ambient and KF (1 g) in water (2 mL) was
injected and stirred for 2 h. The solution was extracted with chloroform
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Figure 1. (A) Solution UV/vis absorption spectra (∼5 ꢁ 10ꢀ6 M CHCl3) and (B) thin-film UV/vis absorption spectra.
Figure 2. (A) Spectra of PNDI-1Boc before and after ladderization and (B) respective spectra of PNDI-2Boc.
as follows: polymer thin films were deposited from a filtered 5 mg/mL
chlorobenzene solution by spin-coating at 1000 rpm for 60 s
(octadecyltrichlorosilane (OTS) treated substrates were also used, but
yielded poor performance compared to devices fabricated on bare oxide
layer substrates). The devices were dried on a temperature-controlled
hot-plate at 150 ꢀC for 10 min under nitrogen atmosphere. Interdigi-
tated source and drain electrodes (W = 9000 μm, L = 90 μm) made of
gold (50 nm thick) were deposited through shadow masks on top of the
polymer active layer by thermo evaporation at 1.0 Å/s by a resistively
heated Mo boat under high vacuum (5.0 ꢁ 10ꢀ7 Torr). At least four
devices were fabricated to obtain average electron mobilities.
adjacent carbonyl (imide) groups in forming intramolecular
imine-bridged planar structures.74 The di-Boc functionalized stannyl
monomer, 3, is capable of forming imine-bridged structures with
both ends of the adjacent naphthalene diimide comonomer in
generating a fully ladderized copolymer. Whereas, the monoboc
functionalized stannyl monomer, 2, is capable of only reacting with
one of the two adjacent naphthalene comonomers in thereby
forming a copolymer with localized ladder polymer structures. The
stannyl monomer with no Boc functionalities, 1, was thus used to
synthesize a polymer with no imine-bridges for comparative refer-
ence purposes. Monomer 2 was synthesized using a procedure
analgous to the method referenced for 3 in which a commercially
available functionalized benzoic acid compound was reactively
converted to an isocyanate which spontaneously underwent a
Curtius rearrangement (Scheme 1).74 Hydrolysis by tert-butyl
alcohol yields the final target as a Boc-protected amine, 2c. Stannyla-
tion was performed by deprotonation of the Bocꢀamine by MeLi
followed by bromineꢀlithium exchange using tBuLi and quenching
with trimethyltin chloride.
’ RESULTS AND DISCUSSION
The highly electron-deficient naphthalene diimide acceptor
unit with branched-alkane, octyldodecyl-, solubilizing groups
was copolymerized with p-bis(trimethylstannyl)benzene (1),73
N-(tert-butoxycarbonyl)-1-amino-2,5-bis(trimethylstannyl)benzene
(2),andN,N0-bis(tert-butoxycarbonyl)-1,4-diamino-2,5-bis(tri-n-bu-
tylstannyl)benzene (3),74 using Stille conditions and Pd(PPh3)2Cl2
as catalyst (see Scheme 2). The mono- and di-Boc-functionalized
stannyl comonomers were chosen due to their acid-sensitivity in
yielding free amine groups which are capable of reacting with
Solutions of the dissolved Boc-containing copolymers, PNDI-
1Boc and PNDI-2Boc, were treated with trifluoroacetic acid at
elevated temperatures to cleave the electronically insulating Boc
groups and form intramolecular imine-bridges to generate
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Figure 3. (A) Transfer curve of PNDI-2BocL and (B) PNDI-2Boc at a constant source-drain voltage of þ100 V plotted with the square root of current
as a function of gate voltage.
Table 2. Electrical Characterization of NDI Co-Polymers
Table 3. DFT Computed PNDI Sub-Unit Dihedral (Torsion)
Angles in Deg
μea (cm2 Vꢀ1 sꢀ1
)
Ion/off
Vt (V)
0Boc
1BocL
2BocL
1Th
PNDI-0Boc
PNDI-1Boc
PNDI-2Boc
PNDI-1BocL
PNDI-2BocL
6 ꢁ 10ꢀ6 ((1 ꢁ 10ꢀ6
4 ꢁ 10ꢀ6 ((1 ꢁ 10ꢀ6
)
)
104
102
102
102
104
9
10
50
7
a
C1ꢀC2ꢀC3ꢀX4
55.06
ꢀ61.87
0
ꢀ41.21
a X = C for PNDI-0Boc, PNDI-1BocL, and PNDI-2BocL, X = S for
1.9 ꢁ 10ꢀ6 ((4 ꢁ 10ꢀ7
)
4 ꢁ 10ꢀ7 ((1 ꢁ 10ꢀ7
)
PNDI-1Th.
2.6 ꢁ 10ꢀ3 ((4 ꢁ 10ꢀ4
)
5
a Average value of at least four devices with standard deviation. Calcu-
lated from the Ids1/2 vs Vg plot using the equation Ids = (μWCo /2L)-
(Vg ꢀ Vt)2.
improved absorption characteristics due to its lack of any Boc
functional groups. Furthermore, the absorption spectrum of
PNDI-1BocL closely resembles that of PNDI-0Boc as a result
of Boc-removal. The reduced conjugation length of PNDI-
1BocL (Eg = 2.37 eV) compared to PNDI-2BocL (Eg = 1.96 eV)
is likely due to the greater number of rotational degrees of
freedom (polymer backbone twists) in PNDI-1BocL which
allow for potential breaks in π-conjugation between individual
ladderized subunits. While there is not a significant band gap
difference between PNDI-2Boc and PNDI-2BocL, the large
observed λmax shift appears to be a result of the extended imine-
bridge ladder structure functioning as an improved absorptive
chromophore. Additionally, the large red-shift in the λmax for the
charge-transfer band (531 nm) of the UVꢀvis absorption
spectrum of PNDI-2BocL shows further evidence of the ex-
istence of extensive conjugation in imine-bridged ladder polymer
systems.64
Imine-bridge formation was further confirmed by FTIR
analysis (Figure 2) before and after the ladderization reaction
step. The most notable confirmation is evident in the loss of the
two ꢀNꢀH stretch peaks at 3360 and 3450 cmꢀ1 which
correspond to the loss of the Boc functionality in both the
PNDI-1BocL and PNDI-2BocL spectra. Further evidence of
imine-bridge formation can be seen in the loss of a -CdO stretch
peak at 1740 cmꢀ1 in the PNDI-1Boc spectrum and a loss of two
corresponding peaks at 1715 and 1735 cmꢀ1 in the PNDI-2Boc
spectrum coupled with the significant gain of a single imine
stretch peak at 1600 cmꢀ1 for PNDI-2BocL and the gain of
several in the same location for the PNDI-1BocL spectrum due
to the existence of several isomeric polymer structures. The
relatively low-intensity of the ꢀCdO stretch for the BOC-group
in the PNDI-1Boc spectrum can be attributed to the reduced
abundance of the BOC-functional group compared to the
PNDI-1BocL and PNDI-2BocL. The molecular weights of all
polymers synthesized were determined using size-exclusion
chromatography (SEC) relative to polystyrene standards in
THF and are listed in Table 1. Determination of polymer
molecular weights via integrative NMR calculations was not
possible since the peaks for the terminal phenyl groups over-
lapped with the polymer peaks. The polymer molecular weights
were recognized as being overestimations of the actual polymer
mass due to NDI’s rigid-rod-like polymeric character and the
additional rigidity incurred by ladderization.75 Increasing the
planarity of the copolymers by this means will effectively increase
the hydrodynamic radius of the polymer when characterized by
SEC. This phenomenon was observed in that the respective
polymer molecular weights increased to a degree following
ladderization. In the case of PNDI-2Boc, the molecular weight
nearly doubled from 7.2 to 14 kDa.
Optical absorption spectra (Figure 1) were taken for all five
polymer films spin coated from CHCl3 onto glass revealing a
marked red-shift of the absorption band for PNDI-1Boc and
PNDI-2Boc when compared to the polymer absorption spectra
after ladderization. The PNDI-1Boc spectrum reveals a shift
from λmax = 384 nm to λmax = 476 nm for PNDI-1BocL as a
result of this structural transformation. Likewise, a similar trend is
observed for PNDI-2Boc with a higher magnitude shift from
λmax = 382 nm to λmax = 576 nm for PNDI-2BocL. The
absorption maximum similarity between PNDI-1Boc and
PNDI-2Boc may be caused by the relatively bulky Boc-func-
tional groups which inhibit the absorption of each polymer to
a similar degree. PNDI-0Boc thus exhibits comparatively
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Figure 4. DFT computed reduced-geometry structures of PNDI-1BocL (left) and PNDI-2BocL (right).
PNDI-2Boc polymer material and other carbonyl-containing
organic molecules.
ladderized-subunit structure of PNDI-1BocL showed negligible
performance gains following imine-bridge formation. Further-
more, the 5 polymers were analyzed by X-ray diffraction (XRD).
However, the XRD characterization performed on the polymer
series yielded no distinct peaks—indicating that the 5 NDI
polymers have very little, if any, long-range ordering in the solid
state. This is further supported by the lack of a significant shift
between the solution and solid state UV/vis spectra of the
polymer series. Varying OFET device annealing temperatures
between 80 and 200 ꢀC had little effect on the electron charge
mobility which may also coincide with the lack of a phase
transition in this temperature range. In addition, surface mor-
phology of the polymer films as observed by AFM showed no
distinct features, which led us to believe the morphology in the
thin films was amorphous in nature. While this insight may lead
one to believe that the ladder polymers were amorphously cross-
linked—solid-state ladderization experiments were performed
on polymer thin-films of PNDI-2Boc by heating and annealing
the polymers to ∼250 ꢀC. The characterized PNDI-2BocL films
made by this process showed the same indicative FTIR and
UVꢀvis spectra coinciding with imine-bridge formation. How-
ever, the solid-state generated ladder polymer films were com-
pletely insoluble amid exposure to all common organic solvents,
exhibited entirely amorphous structures, and performed poorly
in OFET devices as a result of the formed cross-linked polymer
structures.
The LUMO energy levels were calculated from the onset of
the first polymer reduction peak using cyclic voltammetry. Cyclic
voltammetry was carried out in a standard three-electrode
electrochemical cell employed with a platinum working electrode
and a Ag/AgNO3 reference electrode while using ferrocene as an
internal standard. The energy level referenced for ferrocene
compared to vacuum was 4.8 eV.76 As observed in our previous
study of NDIꢀthiophene copolymers, the LUMO values for all
polymers in our current study that possessed unmodified NDI
cores (PNDI-0Boc, PNDI-1Boc, PNDI-2Boc) were found to
be approximated to ∼-3.8 eV due to the intrinsic properties of
NDI.42,43,71,77 However, modification of the NDI backbone due
to imine-bridge formation after ladderization was found to raise
the LUMO to ꢀ3.54 eV for both PNDI-1BocL and PNDI-
2BocL due to a slight reduction in electron-deficiency as a result
of forming an imine-bridge in place of a more electron-with-
drawing carbonyl oxygen. The HOMO values were calculated
using the optical band gap (Egopt) from the calculated LUMO.
The compiled data is presented in Table 1. No oxidation waves
were observed for any of the five NDI polymers.
OFET devices using the five polymers were fabricated using
typical top-contact, bottom-gate geometry on doped-Si/SiO2
wafers. The active layers of the devices were spin-cast from a
5 mg/mL solution of chlorobenzene. All fabrication and testing
was performed in an inert atmosphere.
DFT calculations were conducted to determine the potential
twist geometry present within PNDI-0Boc, PNDI-1BocL, and
PNDI-2BocL as related to OFET device performance. The
geometry of PNDI-1Th, a high-performing (μe = 3.1 ꢁ
Each of the five polymers showed n-type behavior at positive
gate-source biases (Vg = þ100 V). The polymers did not display
ambipolar or p-type response; exhibiting low current flow, and
on/off ratios below 10 when a negative gate-source bias was
applied (Vg = ꢀ100 V). A linear fit was applied in the saturation
region of the Ids1/2 vs VG curve to calculate the saturated charge
carrier mobility of each material according to the saturation
current equation: Ids = (μWC0/2L)(Vg ꢀ Vt)2.78
10ꢀ3 cm2 Vꢀ1 ꢀ1) polymer analogue to PNDI-0Boc (with a
s
thiophene replacing the benzene comonomer component) pre-
viously synthesized and reported by our lab,43 was also calculated
to determine if additional geometry-performance correlations
could be made. The geometries and electronic structures of
methyl-substituent analogues of all calculated polymers were
analyzed with Gaussian 03 software.70 Becke’s three-parameter
gradient-corrected function (B3LYP) with a polarized 6-31G
basis set was used for full geometry optimization.68,69 Methyl
groups were used in approximation of the branched octylꢀ
dodecyl alkyl chains and polymers were reduced to three
monomeric subunits focused around the benzene component
in order to limit calculation time. DFT computed dihedral
(torsion) angles between NDI and adjacent aromatic rings
(phenyl or thiophene) of the listed polymers are summarized
in Table 3 while structures for PNDI-1BocL and PNDI-2BocL
are shown in Figure 4. As predicted, there is a sufficiently large
55.06ꢀ out-of-plane twist of the phenyl component of PNDI-
0Boc as well as within PNDI-1BocL (ꢀ61.87ꢀ) which appears to
An electron mobility (μe) as high as 2.6 ꢁ 10ꢀ3 cm2 Vꢀ1 sꢀ1
was measured from OFETs yielded by PNDI-2BocL (Figure 3).
As shown in Table 2, PNDI-2BocL was found to possess the
highest charge carrier mobility of the series—an increase of over
3 orders of magnitude compared to the nonladderized PNDI-
2Boc (1.9 ꢁ 10ꢀ6 cm2 Vꢀ1 sꢀ1). Very little change in electron
mobility was observed between PNDI-1Boc and its ladderized
constituent PNDI-1BocL. PNDI-0Boc also performed similarly
compared to the polymers PNDI-1Boc and PNDI-1BocL. As
can be seen in Figure 3, although the on/off ratio of PNDI-0Boc
was the same as the on/off ratio for PNDI-2BocL, both the Ion
and Ioff for PNDI-0Boc was greatly reduced compared to PNDI-
2BocL. Complete ladderization in the case of PNDI-2BocL
led to an increase in electron mobility, whereas the locally
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exhibit a backbone consisting of alternating in-plane and out-of-
plane subunits which would severely hinder πꢀπ stacking and
device performance as seen in Table 2. Likewise, PNDI-2BocL
which exhibited the highest OFET device performance was
predicted to possess a planar polymer architecture lacking in
the torsional strain seen with PNDI-1BocL which would aid in
facilitating charge transport. The thiophene analogue, PNDI-
1Th, possessed a calculated dihedral angle of ꢀ41.21ꢀ which
renders the polymer with slightly improved planarity compared
to PNDI-0Boc and PNDI-1BocL. However, PNDI-2BocL, with
a theoretically fully planar geometry shows similar performance
with respect to its charge mobility by PNDI-1Th (3.1 ꢁ
10ꢀ3 cm2 Vꢀ1 sꢀ1).43 Further structural studies of the thin film
are currently being performed to elucidate why, despite the
increased planarity, PNDI-2BocL and PNDI-1Th show similar
mobilities: the two polymers may orient differently with respect
to the surface, which will affect OFET mobilities.
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The authors would like to acknowledge the NSF (CAREER
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