Influence of structural variation on the solid-state properties of diketopyrrolopyrrole-based oligophenylenethiophenes: Single-crystal structures, thermal properties, optical bandgaps, energy levels, film morphology, and hole mobility

Article abstract of DOI:10.1021/cm202852f

Five new compounds, based on diketopyrrolopyrrole (DPP) and phenylene thiophene (PT) moieties, were synthesized to investigate the effect of structural variations on solid state properties, such as single-crystal structures, optical absorption, energy lev

Full text of DOI:10.1021/cm202852f

Article
pubs.acs.org/cm
Influence of Structural Variation on the Solid-State Properties
of Diketopyrrolopyrrole-Based Oligophenylenethiophenes:
Single-Crystal Structures, Thermal Properties, Optical Bandgaps,
Energy Levels, Film Morphology, and Hole Mobility
Chunki Kim,†,‡,∥ Jianhua Liu,†,‡,∥ Jason Lin,†,‡ Arnold B. Tamayo,§ Bright Walker,†,‡ Guang Wu,‡
and Thuc-Quyen Nguyen*,†,‡
†Center for Polymers and Organic Solids and ‡Department of Chemistry and Biochemistry, University of California at Santa Barbara,
Santa Barbara, California 93106, United States
§Next Energy Technologies, 5385 Hollister Avenue, Santa Barbara, California 93111, United States
S
* Supporting Information
ABSTRACT: Five new compounds, based on diketopyrrolopyrrole (DPP) and phenylene
thiophene (PT) moieties, were synthesized to investigate the effect of structural variations
on solid state properties, such as single-crystal structures, optical absorption, energy levels,
thermal phase transitions, film morphology, and hole mobility. The molecular structures
were modified by means of (i) backbone length by changing the number of thiophenes on
both sides of DPP, (ii) alkyl substitution (n-hexyl or ethylhexyl) on DPP, and (iii) the
presence of an n-hexyl group at the end of the molecular backbone. These DPP-based
oligophenylenethiophenes were systematically characterized by UV−visible spectroscopy,
differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), cyclic
voltammetry (CV), ultraviolet photoelectron spectroscopy (UPS), atomic force microscopy
(AFM), and hole-only diodes. Single-crystal structures were provided to probe insight into
structure−property relationships at a molecule level resolution. This work demonstrates the
significance of alkyl substitution as well as backbone length in tuning material’s solid-state properties.
KEYWORDS: diketopyrrolopyrrole, oligophenylenethiophene, structure−property relationships, single-crystal structure,
molecular packing, conjugated molecule
intermolecular packing. For example, solar cell characteristics
may change considerably when the utilized conjugated polymer
has inconsistent molecular weights and polydispersity.21,22 In
addition, the end group effects of conjugated polymers should
be considered,23,24 which makes the investigation of structural
variations complicated.
Solution processable small molecule donor materials have
advantages over polymers in terms of well-defined structure,
monodisperse molecular weight, and convenient purification
methods. These assets allow for a clear understanding of
structure−property relationships. BHJs comprising small
molecule/fullerene derivatives have been developed rapidly
over past few years, and small molecular donor materials with
numerous structures have achieved comparable device perform-
ance with PCEs up to 7%.25−32 Diketopyrrolopyrrole (DPP)-
based materials have demonstrated great potential as donor
materials due to their desirable properties such as broad optical
absorption, chemical and thermal stability, and ease of synthesis
and modification.25,26,33−36 Solution processed BHJ solar cells
INTRODUCTION
■
Bulk heterojunction (BHJ) organic solar cells, using conjugated
polymers1−10 or small molecules11−13 as electron donors and
fullerene derivatives as electron acceptors, have drawn much
attention due to their potential as low-cost, flexible, and large-
area renewable energy sources. To function as effective electron
donors for high-performance BHJ organic solar cells, materials
should exhibit broad absorption extending to the near-infrared
region of the solar spectrum for light harvesting, high hole
mobility for facile charge transport, energy levels well matched
to those of electron acceptors for efficient charge separation
and large open circuit voltage, and desirable phase separation to
form continuous percolation pathways for charge transport and
collection. A thorough understanding of structure−property
relationships in the solid state is essential for developing
materials to fulfill these requirements.
By a judicious design of molecular structures combined with
material processing and device engineering, BHJ solar cells
using a blend of a conjugated polymer and fullerene derivative,
have reached power conversion efficiency (PCE) over 8%.14−20
However, systematically elucidating structure−property rela-
tionships is challenging for conjugated polymers due to
inherent polydispersity, batch-to-batch variation, and indistinct
Received: September 22, 2011
Revised: April 22, 2012
Published: May 4, 2012
© 2012 American Chemical Society
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Chemistry of Materials
Article
was added n-BuLi (2.6 mL, 6.5 mmol, 2.5 M in hexane) dropwise
at −78 °C. After stirring for 30 min at −78 °C, the solution was warmed
to room temperature and stirred for 1 h. Then, the mixture was cooled
to −78 °C and 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dixoaborolane
(1.5 g, 8.1 mmol) was added at once. The mixture was warmed to
room temperature and stirred overnight. After reaction was quenched
with water, the organic layer was extracted by ethyl acetate and dried
over magnesium sulfate. After solvent was removed under reduced
pressure, the crude product was purified by column chromatography
based on DPP-containing small molecules have achieved high
PCEs of up to 4.4%;35 however, a systematic study of molecular
donors especially DPP-based materials to establish structure−
property relationships has not been investigated. This inves-
tigation of structure effects on the material’s basic properties is
critical not only for further developing DPP-based donor
materials but also for the rational design of other small molecular
donor materials.
In this work, we report the synthesis and characterizations of
a series of DPP-based oligophenylenethiophenes focusing on
structural effects on materials' properties such as molecular
packing, thermal transitions, optical bandgaps, energy levels,
film morphology, and hole mobility. The chemical structure of
these DPP-based oligophenylenethiophenes is successfully
varied by tuning (i) conjugated backbone length by changing
the number of thiophene rings on both sides of DPP, (ii) alkyl
substitution (n-hexyl or ethylhexyl) on DPP, and (iii) the
presence of n-hexyl groups at the end of the rigid molecular
backbone. In the solid state, both conjugated backbone and
alkyl substitution significantly affect materials’ various physical
properties. Single-crystal structures of these compounds are
provided as an efficient platform to understand these structural
effects at a molecular level.
1
with 7% acetone in hexane to obtain 5 (1.9 g, 77%). H NMR (400
MHz, CDCl3): 7.53 (d, 1H, J = 2.8 Hz), 7.22 (d, 1H, J = 3.6 Hz), 7.12
(d, 1H, J = 4.0 Hz), 6.99 (t, 2H, J = 4.0 Hz), 6.69 (d, 1H, J = 2.8 Hz),
2.79 (t, 2H, J = 7.2 Hz), 1.68 (quintet, 2H), 1.29−1.41 (m, 18H), 0.90
(t, 3H, J = 6.8 Hz).
2,5-Dihexyl-3,6-bis[4-(5-hexylthiophene-2-yl)phenyl]-
pyrrolo[3,4-c]-pyrrole-1,4-dione, C6PT1C6. To a mixture of 2a
(0.50 g, 0.81 mmol), 5-hexyl-2-thiophene boronic acid pinacol ester
(0.60 g, 2.0 mmol), Pd2(dba)3 (0.037 g, 0.041 mmol), tri-tert-
butylphosphonium tetrafluoroborate (0.071 g, 0.24 mmol), and
potassium phosphate (2.8 g, 13 mmol) was added into degassed THF/
water (27 mL/2.7 mL). After stirring under argon at 80 °C overnight, the
reaction mixture was poured into methanol. The crude product was
collected by filtration and purified by column chromatography on a silica
gel with gradient chloroform/hexane from 2/1 to 5/1 (v/v) to obtain
C6PT1C6 (0.46 g, 72%). 1H NMR (400 MHz, CDCl3): 7.86 (d, 4H, J =
8.8 Hz), 7.72 (d, 4H, J = 7.6 Hz), 7.26 (d, 2H, J = 4.0 Hz), 6.80 (d, 2H,
J = 3.6 Hz), 3.79 (t, 4H, J = 7.6 Hz), 2.84 (t, 4H, J = 7.6 Hz), 1.60−1.77
(m, 8H), 1.18−1.45 (m, 24H), 0.90 (t, 6H, J = 6.8 Hz), 0.84 (t, 6H, J =
6.8 Hz). Anal. Calcd for C50H64N2O2S2 (%): C, 76.10; H, 8.17; N, 3.55.
Found: C, 76.20; H, 8.20; N, 3.67.
2,5-Dihexyl-3,6-bis[4-(5-hexyl-2,2′-bithiophene-5 yl)-
phenyl]pyrrolo[3,4-c]-pyrrole-1,4-dione, C6PT2C6. The proce-
dure for the synthesis of C6PT1C6 was followed using 5-hexyl-2,2′-
bithiophene-5′-boronic acid pinacol ester (0.92 g, 2.4 mmol), instead
of 5-hexyl thiophene boronic acid pinacol ester, to yield in 91%
(0.85 g). 1H NMR (400 MHz, CDCl3): 7.88 (d, 4H, J = 8.4 Hz), 7.74
(d, 4H, J = 7.2 Hz), 7.33 (d, 2H, J = 3.6 Hz), 7.10 (d, 2H, J = 4.0 Hz),
7.05 (d, 2H, J = 3.2 Hz), 6.72 (d, 2H, J = 3.6 Hz), 3.80 (t, 4H, J = 7.6
Hz), 2.81 (t, 4H, J = 7.6 Hz), 1.60−1.75 (m, 8H), 1.18−1.45 (m,
24H), 0.90 (t, 6H, J = 6.8 Hz), 0.84 (t, 6H, J = 6.4 Hz). Anal. Calcd for
C58H68N2O2S4 (%): C, 73.06; H, 7.19; N, 2.94. Found: C, 73.10; H,
7.13; N, 3.04.
2,5-Dihexyl-3,6-bis[4-(5-hexyl-2,2′:5′,2″-terthiophene-5″-
yl)phenyl]pyrrolo[3,4-c]-pyrrole-1,4-dione, C6PT3C6. The pro-
cedure for the synthesis of C6PT1C6 was followed using 5-hexyl-
2,2′:5′,2″-terthiophene-5″-boronic acid pinacol ester (0.90 g, 2.0 mmol),
instead of 5-hexyl thiophene boronic acid pinacol ester, to yield in 78%
(0.68 g). 1H NMR (400 MHz, CDCl3): 7.90 (d, 4H, J = 8.4 Hz), 7.75
(d, 4H, J = 8.4 Hz), 7.36 (d, 2H, J = 4.0 Hz), 7.17 (d, 2H, J = 3.6 Hz),
7.13 (d, 2H, J = 3.6 Hz), 7.03 (quartet, 4H), 6.71 (d, 2H, J = 3.6 Hz),
3.81 (t, 4H, J = 7.6 Hz), 2.81 (t, 4H, J = 7.6 Hz), 1.61−1.74 (m, 8H),
1.20−1.42 (m, 24H), 0.90 (t, 6H, J = 6.4 Hz), 0.84 (t, 6H, J = 7.0 Hz).
Anal. Calcd for C66H72N2O2S6 (%): C, 70.92; H, 6.49; N, 2.51. Found:
C, 71.10; H, 6.45; N, 2.70.
EXPERIMENTAL SECTION
■
Material Synthesis. 5-Hexyl-2-thiophene boronic acid pinacol
ester, 2,2′-bithiophene-5-boronic acid pinacol ester, and 5-hexyl-2,2′-
bithiophene-5′-boronic acid pinacol ester were purchased from Sigma-
Aldrich Chemical Co. and used as received. Other chemicals and
solvents were used as received from commercial sources without
further purification. Tetrahydrofuran (THF) was distilled over
sodium/benzophenone. 3,6-Bis(4-bromophenyl)-2,5-dihydropyrrolo-
[3,4-c]pyrrolo-1,4-dione, 1, was synthesized following the previously
published procedure.33
2,5-Dihexyl-3,6-bis(4-bromophenyl)pyrrolo[3,4-c]pyrrolo-
1,4-dione (2a). To a solution of 1 (5.0 g, 11 mmol) in N,N
dimethylformamide (DMF) (50 mL) were added 1-bromohexane
(7.4 g, 45 mmol) and cesium carbonate (11 g, 34 mmol) at 40 °C.
After stirring for 24 h, the reaction mixture was filtered to remove
solid. The filtrate was extracted with chloroform and recrystallized
1
from methanol to yield red needle-like crystal (4.1 g, 60%). H NMR
(400 MHz, CDCl3): 7.66 (s, 8H), 3.70 (t, 4H, J = 7.6 Hz), 1.58 (m,
4H), 1.19 (m, 12H), 0.83 (t, 6H, J = 6.8 Hz).
2,5-Dihexyl-3,6-bis(4-bromophenyl)pyrrolo[3,4-c]-pyrrole-
1,4-dione (2b). The procedure for the synthesis of 2a was followed to
prepare 2b using 2-ethylhexyl bromide (3.5 g, 18 mmol) instead of
1-bromohexane. The crude product was purified by column
chromatography on a silica gel using gradient solvent with dichloro-
methane/hexane from 1/1 to 2.5/1 (v/v) to yield in 22% (0.66 g). 1H
NMR (400 MHz, CDCl3): 7.65 (s, 8H), 3.72 (d, 4H, J = 4.4 Hz), 1.45
(m, 2H), 1.08−1.20 (m, 16H), 0.81 (t, 6H, J = 6.4 Hz), 0.73 (t, 6H,
J = 7.2 Hz).
2,5-Dihexyl-3,6-bis[4-(5-hexyl-2,2′-bithiophene-5-yl)-
phenyl]pyrrolo[3,4-c]-pyrrole-1,4-dione, EHPT2C6. The proce-
dure for the synthesis of C6PT1C6 was followed, using 2b (0.58 g,
0.87 mmol) and 5-hexyl-2,2′-bithiophene-5′-boronic acid pinacol ester
(0.81 g, 2.2 mmol) instead of 2a and 5-hexyl thiophene boronic acid
5-Hexyl-2,2′:5′,2″-terthiophene (4). To a mixture of 2-bro-
mothiophene (1.56 g, 9.57 mmol), 5-hexyl-2,2′-bithiophene-5′-boronic
acid pinacol ester (3.0 g, 8.0 mmol), tri(dibenzylidene-acetone)
palladium (0) (Pd2(dba)3) (0.15 g, 0.17 mmol), tri-tert-butylphos-
phonium tetrafluoroborate (0.18 g, 0.65 mmol), and potassium
phosphate (14 g, 64 mmol) was added degassed THF/water (30 mL/
3 mL). After stirring under argon at 80 °C overnight, the reaction
mixture was poured into methanol. The crude product was collected
by filtration and purified by column chromatography on a silica gel
1
pinacol ester to yield in 86% (0.75 g). H NMR (400 MHz, CDCl3):
7.83 (d, 4H, J = 8.4 Hz), 7.71 (d, 4H, J = 8.4 Hz), 7.30 (d, 2H, J = 3.6
Hz), 7.08 (d, 2H, J = 4.0 Hz), 7.04 (d, 2H, J = 3.6 Hz), 6.71 (d, 2H,
J = 3.6 Hz), 3.80 (d, 4H, J = 7.6 Hz), 2.81 (t, 4H, J = 7.6 Hz), 1.70
(quintet, 4H), 1.30−1.43 (m, 12H), 1.03−1.28 (m, 16H), 0.91 (t, 6H,
J = 6.8 Hz), 0.70−0.81 (m, 12H). Anal. Calcd for C62H76N2O2S4 (%):
C, 73.76; H, 7.59; N, 2.77. Found: C, 74.00; H, 7.40; N, 2.89.
2,5-Dihexyl-3,6-bis[4-(2,2′-bithiophene-5-yl)phenyl]pyrrolo-
[3,4-c]-pyrrole-1,4-dione, C6PT2. The procedure for the synthesis
of C6PT1C6 was followed using 2,2′-bithiophene-5′-boronic acid
pinacol ester (0.89 g, 3.1 mmol), instead of 5-hexyl thiophene boronic
1
with hexane to obtain 4 (2.1 g, 81%). H NMR (400 MHz, CDCl3):
7.21 (d, 1H, J = 4.0 Hz), 7.16 (d, 1H, J = 2.4 Hz), 7.06 (d, 1H, J = 3.6
Hz), 7.03 (m, 3H), 6.69 (d, 1H, J = 3.2 Hz), 2.79 (t, 2H, J = 8.0 Hz),
1.68 (quintet, 2H), 1.28−1.44 (m, 6H), 0.90 (t, 3H, J = 7.2 Hz).
5-Hexyl-2,2′:5′,2″-terthiophene-5″-boronic Acid Pinacol
Ester (5). To a solution of 4 (1.8 g, 5.4 mmol) in 45 mL of THF
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Chart 1. Molecular Structures of All DPP-Based Oligophenylenethiophenes
1
acid pinacol ester to yield in 78% (0.75 g). H NMR (400 MHz,
CDCl3): 7.88 (d, 4H, J = 8.4 Hz), 7.74 (d, 4H, J = 8.4 Hz), 7.34 (d,
2H, J = 4.0 Hz), 7.24 (quartet, 4H), 7.18 (d, 2H, J = 3.6 Hz), 7.06
(quartet, 2H), 3.80 (t, 4H, J = 7.6 Hz), 1.66 (quintet, 4H), 1.18−1.32
(m, 12H), 0.84 (t, 6H, J = 6.4 Hz). Anal. Calcd for C46H44N2O2S4
(%): C, 70.37; H, 5.65; N, 3.57. Found: C, 70.40; H, 5.54; N, 3.66.
Characterization. 1H NMR spectra were recorded with a 400
MHz spectrometer (Varian, ASM-100). Elemental analyses were
performed by the UC Santa Barbara Marine Science Institute
Analytical Laboratory. Thermal properties of the compounds were
determined by differential scanning calorimetry (DSC) at 10 °C/min
under N2 purge (TA Instrument, Q20). Thermogravimetric analyses
(TGA) were carried out at a heating rate of 10 °C/min under N2
purge (Mettler Toledo, TGA/SDTA851e). UV−vis absorption spectra
were measured at a concentration at 0.9−2.2 × 10−5 M in chloroform
(CHCl3). UV−vis absorption spectra were collected with a UV−vis
spectrophotometer (Beckman Coulter, DU 800). Thin films for
UV−vis absorption spectra were prepared by spin coating on quartz
substrate. Atomic force microscopy (AFM) images were collected in
air under ambient conditions using the MultiMode scanning probe
microscope and the controller IIIa (BrukerNano). Silicon probes with
resonant frequencies of 75 kHz (Budget Sensors) were used for
tapping mode AFM measurements. The hole mobility for pristine
films was measured by fabricating a hole-only diode with a device
architecture of ITO/PEDOT:PSS/DPP-based oligophenylenethio-
phene/Au. PEDOT:PSS is poly(3,4-ethylenedioxythiophene) poly-
(styrenesulfonate). Current density as a function of voltage was
recorded on a Keithly 4200 in a nitrogen atmosphere. The value of
mobility was extracted by fitting the current density−voltage curve
with a Mott−Gurney relationship (space charge limited current,
SCLC). Cyclic voltammetry (CV) was performed using an electro-
chemical analyzer (CH Instrument, CHI730B) to study the electro-
chemical properties of the materials in dilute solution. The experiment
was carried out in a chloroform solution of 0.1 M tertra-n-
butylammonium hexafluorophosphate (Bu4NPF6) at a scan rate of
50 mV/s using a glassy carbon electrode as a working electrode,
platinum wire as a counter electrode, and Ag/Ag+ electrode as a
reference electrode. The electrochemical potential was calibrated
against Fc/Fc+, whose oxidation potential was located at 0.24 V to the
Ag/Ag+. The energy levels of HOMO and LUMO were calculated
according to the equations HOMO = −(Eox + 4.56) (eV) and
LUMO = −(Ered + 4.56) (eV), where Eox and Ered are oxidation potential
onset and reduction potential onset versus Ag/Ag+, respectively. For
ultraviolet photoelectron spectroscopy (UPS), 75 nm thick Au films
were thermally deposited onto precleaned Si substrates with a thin
native oxide, followed by spin coating of the 2 mg/mL of small
molecules in chloroform at 3000 rpm in the glovebox under nitrogen.
The UPS measurement was carried out, using a He I (hν = 21.2 eV)
source, equipped with a hemispherical electron energy analyzer
(Kratos Ultra Spectrometer).
Single-crystal X-ray diffraction was collected on a Bruker Kappa
Apex II diffractometer using a graphite monochromator with a Mo Kα
X-ray source (λ = 0.71073 Å). The crystals were mounted on a nylon
loop under Paratone-N oil, and all data were collected at 100 K using
an Oxford nitrogen gas cryostream system. Data collection, cell
parameter determination, data integration, and parameter refinement
were performed using Apex2 software (V2011). Absorption correction
of the data was applied using SADABS software. Structure
determination was done using direct method and difference Fourier
techniques. All hydrogen atom positions were idealized and rode on
the atom of attachment. Structure solution and refinement were
performed using SHELXTL. Powder X-ray diffraction was recorded on
a Bruker D8 diffractometer with Cu Kα radiation (λ = 1.54178 Å).
The powder samples were prepared by scratching thin films that were
casted from chloroform solutions.
RESULTS AND DISCUSSION
■
Material Design and Synthesis. The molecular structure
of each compound is displayed in Chart 1. The compounds
were assigned names with a form of APTBC based on their
topologies, where A refers to the alkyl substitution on DPP, PT
represents the phenylene thiophene linkage, B refers to the
number of thiophene rings, and C denotes the presence or
absence of an alkyl chain at the end of the terminal thiophene
rings. The synthetic routes for these compounds are illustrated
in Scheme 1. Diphenyl-DPP precursor (1) was prepared
according to a reported literature.33 Precursors 2a and 2b were
synthesized via N-alkylation of 1. This process has been
reported to suffer a lower yield (less than 50%),37,38 compared
to the highly efficient N-alkylation of dithienyl-DPP.39 An
enhanced yield (up to 60%) was achieved by carrying out the
reaction with Cs2CO3 as a base catalyst at 40 °C when using
1-bromohexane for the N-alkylation; however, the yield still
remains less than 30% when using 2-ethylhexyl bromide. The
final products were achieved by reacting precursor 2a or 2b
with oligothiophene boronates via a palladium-catalyzed Suzuki
coupling. The yield of this reaction was found to be insensitive
to the base used, such as K2CO3 and K3PO4, but significantly
affected by solvents. A THF/water (10/1, v/v) mixing solvent
can give optimal yields up to 90%.
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a
Scheme 1. Synthesis Routes for DPP-Based Oligophenylenethiophenes
a
Conditions: (i) cesium carbonate, alkylbromide, DMF, 40 °C; (ii) appropriate thiophene boronic acid pinacol ester, Pd2(dba)3, HP(tBu)3BF4,
K3PO4, THF; (iii) 2-bromothiophene, Pd2(dba)3, HP(tBu)3BF4, K3PO4, THF; (iv) n-BuLi, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane.
Table 1. Crystallographic Data for DPP-Based Oligophenylenethiophenes
crystal
C6PT1C6
C6PT2C6
C6PT3C6
EHPT2C6
C6PT2
empirical formula
crystal habit, color
crystal size (mm3)
crystal system
space group
a (Å)
C50H64 N2O2S2
plate, red
1.0 × 1.0 × 0.3
monoclinic
P21/c
C58H68N2O2S4
plate, red
0.4 × 0.3 × 0.1
monoclinic
P21/c
C66H72N2O2S6
plate, dark red
0.4 × 0.1 × 0.05
monoclinic
P21/c
C62H76N2O2S4
needle, light red
0.5 × 0.4 × 0.2
monoclinic
C2/c
C46H44N2O2S4
plate, red
0.4 × 0.2 × 0.05
monoclinic
P21/c
15.215(3)
14.797(3)
10.103(2)
90.00
18.236(1)
14.338(1)
9.876 (1)
90.00
19.462(2)
15.099(1)
9.847 (1)
90.00
57.201(8)
10.523(2)
9.102(1)
90.00
13.414(2)
14.427(2)
10.079(2)
90.00
b (Å)
c (Å)
α (°)
β (°)
γ (°)
101.369(4)
90.00
96.891(5)
90.00
96.944(6)
90.00
97.505(3)
90.00
92.804(9)
90.00
Z
2
2
2
4
2
R factor
5.85
5.40
5.49
6.3
8.5
density
1.18
1.24
1.29
1.23
1.34
Crystallography. To gain insight into how molecular
structures affect the material’s properties in the solid state,
single-crystal structures of these DPP-based oligophenylthio-
phenes were investigated. The single crystals of these compounds
were successfully grown by an interface diffusion method, using
chloroform and acetone as the good solvent and nonsolvent,
respectively. Table 1 summarizes the basic information of these
single-crystal structures. The details can be viewed from the
crystallographic information files (CIFs) (Supporting Informa-
tion). As shown in Table 1, all DPP-based oligophenylth-
iophenes have the monoclinic crystal system regardless of
different unit cell parameters (axis length and angle). Moreover,
all compounds with n-hexyl side chains attached on DPP have
the same space group (P21/c) and a similar molecular packing
style in their unit cells as shown in Figure 1(a)−(c). The
compound with a branched ethylhexyl side chains (EHPT2C6)
has a C2/c space group and a difference packing style as shown
in Figure 1(d)−(f). Increasing the backbone length (i.e., from
C6PT1C6 to C6PT3C6) and removing the end-capping n-hexyl
groups (C6PT2C6 vs C6PT2) do not change their packing
style; however, using branched side chains leads to a big
difference in the intermolecular packing (Table 2).
oligophenylthiophenes. The overlapping areas are highlighted
by red rectangles. Due to the nonplanar conformation of these
compounds (vide infra), it is difficult to get the interplanar
overlapping distances for entire molecules. For reference, the
interplanar distances calculated from phenyl rings, which are
parallel to each other and have the shortest overlapping
distance, are listed in Table 2. For compound EHPT2C6, the
interplanar distance is calculated from DPP rings because the
overlapping only exists on the DPP moieties.
As shown in Table 2, when increasing the conjugated backbone
length from C6PT1C6 to C6PT2C6, the overlapping area
increases concomitantly; however, further increasing the back-
bone length from C6PT2C6 to C6PT3C6 does not lead to the
corresponding enlargement in the overlapping area. A possible
reason is the nonplanar conformations of these compounds.
Compared to C6PT2C6, EHPT2C6 shows a significant effect
from the side chains. The ethylhexyl groups substituted on the
DPP ring dramatically reduce the intermolecular overlapping.
In contrast, removing the end-capping n-hexyl groups (from
C6PT2C6 to C6PT2) almost does not change the overlapping
location but leads to a slight offset in vertical direction of
overlapping. Comparing to the overlapping areas, the overlapping
distances as listed in Table 2 are not sensitive to the molecular
structures possibly because these overlapping distances are mainly
calculated from stacked phenyl rings.
Besides these packing styles, the intermolecular backbone
overlapping can be revealed from their single-crystal structures.
Table 2 provides a simple summary of the intermolecular
backbone overlapping (area and distance) of all DPP-based
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Figure 1. Two typical molecular packing styles observed in single crystals as illustrated by C6PT1C6 ((a)−(c); a unit cell viewed from side (a), a
axis (b), and c axis (c)) and EHPT2C6 ((d)−(f); a unit cell viewed from side (d), b axis (e), and c axis (f)). Due to the chirality of the ethylhexyl
side chains, only the mesomer of EHPT2C6 was shown in the unit cell. All hydrogen atoms were omitted for clarity. Atoms of C, O, N, and S are
shown in gray, red, cyan, and yellow, respectively.
Table 2. Intermolecular Backbone Overlapping Area and
Distance of All DPP-Based Compounds Derived from Their
Single-Crystal Structures
Figure 2. Primary backbone structure (top) indicating two dihedral
angles of DPP−phenyl (φ1) and phenyl−thiophene (φ2) linkages and
a table (bottom) summary of φ1 and φ2 from single-crystal structures.
a
The backbone overlapping is determined from their single-crystal
planarity is primarily dominated by φ1. All n-hexyl-substituted
compounds have very similar φ1 values (23−27°), while the
ethylhexyl-substituted compound (EHPT2C6) has a much
larger φ1 (37.6°) indicating the least planarity among these
compounds. The single-crystal structures clearly indicate effects
of structural moieties in these DPP-based oligophenylenethio-
phenes. The phenyl groups incorporated in conjugated
backbones lead to nonplanar conformation due to steric
hindrance. Bulky alkyl side chains (2-ethylhexyl) attached on
DPP further twist this nonplanar conformation, leading to a
weak intermolecular π−π interaction. Therefore, EHPT2C6
should have a large optical bandgap and low charge carrier
mobility in the solid state (vide infra).
structures. Images here show the stacked molecules as viewed
orthogonally to the molecular backbones with red rectangles high-
lighting the overlapping area. All alkyl chains are simplified into methyl
b
groups for clarity. The interplanar distance is calculated from
c
phenyl−phenyl rings. Calculated from overlapped DPP rings.
Intramolecular conformation can also be obtained from the
single-crystal structures of these DPP-based oligophenylthio-
phenes. Figure 2 shows a simplified molecular backbone
structure of these DPP compounds with two primary σ bond
linkages (DPP−phenyl linkage and phenyl−thiophene linkage).
The dihedral angles of these two linkages (φ1 and φ2)
obtained from single-crystal structures were listed in a table in
Figure 2. Both φ1 and φ2 are not coplanar; φ1 has a larger
value than φ2 for all compounds. Therefore, the molecular
Due to the polymorphism of crystals, the crystal structures
solved from solution-grown single crystals can be different from
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the crystal structures existing in the polycrystalline films. To
evaluate this possible difference, the thin film XRD of these
compounds was compared with theoretical XRD patterns
simulated from their single-crystal structures. As shown in
Figure 3, the experimental peaks in a region of 3−20° match
the same linear alkyl substitution both on DPP and at the end
of the molecular backbone. As the conjugation length increases
from C6PT1C6 to C6PT3C6, the melting temperature (Tm) is
elevated from 181 to 263 °C, concomitantly with a strong
tendency to crystallize during the cooling scan. The highest
melting temperature of C6PT3C6 is mainly due to its highest
molecular weight among the three compounds. While
C6PT2C6 and C6PT3C6 show only one transition upon
heating, C6PT1C6 shows two transitions at 125 and 181 °C,
respectively. This double transition indicates a possible liquid
crystal state of C6PT1C6. The smaller transition that occurs at
low temperature is due to a liquid crystal phase transition. The
liquid crystal phase disappears on other compounds probably
because the ratio of flexible alkyl chain to rigid backbone that is
required for the liquid crystal phase is reduced.
Changing the alkyl substitution on the compounds with the
same conjugated backbone drastically affects the thermal
properties, as clearly evidenced by C6PT2C6, EHPT2C6,
and C6PT2. Compared to n-hexyl substitution on DPP
(C6PT2C6), 2-ethylhexyl substitution (EHPT2C6) effectively
reduces both Tm (from 231 to 164 °C) and Tc (from 194 to
77 °C). Unlike C6PT2C6 which has sharp and large melting
and crystallization peaks, EHPT2C6 exhibits wide and small
melting and crystallization peaks. This difference in phase
transition is especially significant in the cooling scan in
which EHPT2C6 has only a very tiny crystallization peak at
77 °C. Moreover, EHPT2C6 shows a cold crystallization peak
at 70 °C upon heating. All these characteristics observed in
EHPT2C6’s thermogram indicate a low degree of crystallinity
in the solid state, which is due to its inferior intermolecular
π−π interaction as revealed from the single-crystal structure
(Figure 1d−f and Table 2). Removing the alkyl chains at the
end of the conjugated backbone (C6PT2C6 versus C6PT2)
causes similar effects including the reduced Tm, a broad cry-
stallization peak, and cold crystallization upon heating. However,
such effects are not as significant as the ones observed when
replacing the n-hexyl substituent with 2-ethylhexyl (C6PT2C6
versus EHPT2C6) on DPP rings. In addition, compared to that
of C6PT2C6, the Tm of C6PT2 has not decreased but increases
from 231 to 249 °C because both reduced flexible alkyl chains
and intermolecular packing remained the same as shown in
single-crystal structures.
Figure 3. Measured thin film XRD (a, c, e, g, and i) and corresponding
simulated patterns (b, d, f, h, and j) of DPP-based oligophenylene-
thiophenes.
well with simulated ones for these compounds. EHPT2C6 and
C6PT2 films show very weak diffraction peaks in the region of
3−20°, indicating a low degree of crystallinity in the film state.
In a wider angle region (>20°), a deviation on peak positions
can also be observed. This indicates a slight difference between
molecular packing in the film state and single crystals. However,
given this qualitative comparison, it can be concluded that
the molecular packing in thin films is similar to that in single
crystals. Surface morphology and charge carrier mobility of
these films will be discussed later.
Thermal Properties. Generally, thermal properties of a
material depend on its molecular weight and the strength of the
intermolecular interactions. The thermal properties of these
DPP-based oligophenylenethiophenes were evaluated by DSC.
To remove the effect of thermal history, the third heating and
cooling scan loop of each compound was employed to determine
thermal transition temperatures. The DSC thermograms as
shown in Figure 4 exhibit clear melting and crystallization phase
The thermal stability of all the compounds was investigated
by TGA at a heating rate of 10 °C/min under N2. The table
lists 5% weight loss temperatures (Td) of these compounds
determined from TGA curves (Figure S1, Supporting
Information). Each compound exhibits high thermal stability
with a Td of greater than 410 °C.
Solubility and Optical Properties. The conjugated
backbone length has a large impact on the solubility. As listed
in Table 3, C6PT1C6 is soluble in CHCl3 up to 228 mg/mL;
however, adding thiophene rings (C6PT2C6 and C6PT3C6)
reduces the solubility substantially to 11 and 1.5 mg/mL,
respectively. Replacing the linear alkyl chain (C6PT2C6) with
the ethylhexyl chain (EHPT2C6) increases the solubility to
53 mg/mL. Eliminating the terminal alkyl chain (C6PT2) also
affects the solubility (6.3 mg/mL).
Figure 4. DSC thermograms of DPP-based materials with a scanning
rate of 10 °C/min under N2.
UV−vis absorption spectra in CHCl3 solution were obtained
at 0.9−2.2 × 10−5 M. Films were prepared by spin coating
1 mg/mL CHCl3 solutions on quartz substrates. All absorption
spectra are presented in Figure 5; the related optical data are
summarized in Table 3.
transitions. The corresponding melting temperatures are listed in
Table 3.
The effect of the conjugated backbone length is observed by
comparing C6PT1C6, C6PT2C6, and C6PT3C6, which have
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Table 3. Optical and Thermal Properties of DPP−Based Oligophenylenethiophenes
a
b
solution
film
solubility
f
c
c
d
d
e
name
λmax (nm) λonset (nm) Eogpt (eV) ε (M−1 cm−1
)
λmax (nm) λonset (nm) Eogpt (eV) Tm (°C) Tc (°C) Td (°C)
(mg/mL)
C6PT1C6
C6PT2C6
C6PT3C6
EHPT2C6
C6PT2
505
521
528
514
516
571
591
602
584
587
2.17
2.09
2.06
2.12
2.11
3.2 × 104
4.6 × 104
5.6 × 104
4.8 × 104
4.4 × 104
582
614
608
532
554
629
674
667
626
671
1.97
1.84
1.86
1.98
1.85
181
231
263
164
249
143
194
229
77
411
415
425
413
422
228
11
1.5
53
184
6.3
a
b
c
Measured in chloroform solution. Cast from chloroform solution. The optical bandgap calculated from Egopt = 1240/λonset in the absorption
d
e
f
spectrum. Melting point on heating scan by DSC under N2. Temperature with 5% weight loss by TGA under N2. In chloroform at room
temperature determined by the method in ref 40.
Figure 5. Absorption spectra of all DPP-based oligophenylenethiophenes (C6PT1C6: black solid line; C6PT2C6: red dashed line; C6PT3C6: blue
dash-dotted line; EHPT2C6: green dash-dot-dotted line; C6PT2: olive short dashed line) in (a) dilute CHCl3 solution at a concentration of 10−5
and (b) as-cast films on a quartz plate.
M
Figure 5(a) shows the solution absorption spectra of all
compounds in CHCl3. One absorption maximum is apparent
in the range of 300−450 nm for the π−π* transition of the
phenylenethiophene moiety, and the other absorption max-
imum in the 450−600 nm range is for the intramolecular
charge transfer (ICT) transition. As the incorporated thiophene
units increase from C6PT1C6 to C6PT2C6 and C6PT3C6,
the wavelength of maxima absorption (λmax) shifts from 505 to
521 and 528 nm, respectively. However, the alkyl substitution
on conjugated backbones has much less impact on spectral shift,
which is demonstrated by the similar absorption of C6PT2C6,
EHPT2C6, and C6PT2 in solution. Thus, the optical absorp-
tions of dilute solutions of these compounds are determined
mainly by the conjugated backbone.
Figure 5(b) displays the absorption spectra of all compounds
in the film state. Compared with their absorption in solution,
C6PT1C6, C6PT2C6, and C6PT3C6 exhibit a strong red shift
(77−97 nm) on λmax. In addition, unlike the absorption spectra
of dilute solutions which exhibit broad featureless absorption
in the long-wavelength region, the film absorption of each
compound shows a shoulder peak in the long-wavelength
region. The red shift on λmax is due to strong intermolecular
interactions and more planar conformation of the conjugated
backbone in the solid state, which results in a longer effective
conjugation length than in the solution state. The accompanied
shoulder peaks result from vibronic coupling related to the
solid-state packing.41 Compared to C6PT2C6, EHPT2C6
shows a much smaller red shift (18 nm) on λmax (532 nm)
without a shoulder peak. The twisted conformation due to the
presence of the ethylhexyl groups as revealed by its single-
crystal structure and the low crystallinity demonstrated by thin-
film XRD and DSC, which significantly reduce the effective
conjugation length and intermolecular interactions, is the main
reason for the weakest red shift, featureless absorption in the
solid state, and a slightly larger optical bandgap. Similar to
EHPT2C6, C6PT2 also shows a weak red shift (38 nm) on
λmax from the solution to film state. The absence of straight
alkyl substitution at the end of the conjugated backbone in
C6PT2 reduces intermolecular interactions in films leading to a
loss of vibronic shoulder in the absorption. This observation
agrees with DSC results that the material’s crystallinity is
reduced for C6PT2 as compared to C6PT2C6.
For the compounds with n-hexyl substitutions on DPPs, an
effective conjugation between DPP and thiophene moieties
still exists when the dihedral angle is about 25−27° as revealed
from their single crystals. A similar result was observed in
polyfluorene derivatives in which the dihedral angle is about
20°.42 The conjugation seems to be retarded when the dihedral
angle is 37°, as demonstrated by EHPT2C6 which has a much
larger optical bandgap in the solid state, although the less
intermolecular overlapping may also attribute to the large optical
bandgap. In addition, compared with C6PT3C6, C6PT2C6 has
a larger optical bandgap in solution but a slightly narrower
optical bandgap in the solid state probably due to its more
planar conformation (smallest dihedral angle) and better inter-
molecular overlapping (shortest interplanar distance).
Annealing these films at 100 °C for 10 min reveals another
remarkable effect of alkyl substitution, as shown in Figure S2
(Supporting Information); while no obvious change was
observed with C6PT2C6, EHPT2C6 exhibits an enhanced
absorption in the short-wavelength region (300−450 nm) and a
decreased absorption in the long-wavelength region (450−
600 nm). C6PT2 shows a red shift on λmax as well as a shoulder
peak, similar to the optical absorption of C6PT2C6. Such
different annealing effects between C6PT2 and EHPT2C6
imply that the branched alkyl chains (2-ethylhexyl) on DPP
have different impact on the conjugation length and molecular
packing in solid state from terminal linear alkyl chains (n-hexyl).
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In addition to the thermal annealing, the effect of concentra-
tion on absorption spectra of as-cast films was investigated
by comparison of films prepared from 0.5, 1.0, and 5.0 mg/mL
solutions in chloroform. The related absorption spectra are
provided in Figure S3 (Supporting Information). The
C6PT2C6 films prepared from higher concentrations show a
slight increase of the aggregation (shoulder) peak at 613 nm
and similar absorption onsets. EHPT2C6 films prepared from
higher concentrations show very similar spectral change as that
observed in the annealed films: (1) the peak at 365 nm was
shifted by 10 nm, and absorption intensity was increased, and
(2) absorption at 550 nm was decreased (Figure S3, Supporting
Information). In contrast to C6PT2C6 and EHPT2C6,
C6PT2 exhibits weak concentration dependence of absorption
spectra with nearly the same absorption maximum and onset.
Energy Levels. To determine the HOMO and LUMO
levels of all compounds under different environments, cyclic
voltammetry (CV, see Figure 6) and UPS measurements (see
copolymers, in which the LUMO was determined by the
DPP and the HOMO was dependent on the donor moiety.43
The effect of alkyl substitution on energy levels in solution is
negligible, as evidenced from C6PT2C6, EHPT2C6, and
C6PT2. The energy levels in solution are largely determined by
the conjugation length, which is consistent with the solution
UV−vis absorptions.
To obtain energy levels in the solid state, UPS was used to
determine the ionization potential (IP) of each compound. The
optical band gap (Egopt) was determined by the absorption
onset and used to calculate the electron affinity (EA) from
EA = Egopt − |IP|. Figure 7 shows the Egopt, IP, and EA of each
Figure 7. Optical band gap and energy levels of ionization potential
(IP) and electron affinity (EA) of all DPP-based oligophenylenethio-
phenes. IP determined by UPS and EA obtained by EA = Egopt − |IP|.
compound. As the number of thiophenes increases from
C6PT1C6 to C6PT2C6, the IP and EA are elevated, and the
band gap decreases. Further addition of thiophenes (from
C6PT2C6 to C6PT3C6) causes a much smaller change in
energy levels. UPS measurements of C6PT1C6, C6PT2C6,
and C6PT3C6 indicate that the electronic properties such as
band gap, IP, and EA are saturated after the addition of two
thiophenes on each side of the DPP (C6PT2C6). This observa-
tion is consistent with the film UV−vis spectra, where similar
absorption onsets are observed for C6PT2C6 and C6PT3C6.
Comparison of the solid-state energy levels among
C6PT2C6, EHPT2C6, and C6PT2 reveals remarkable effects
of alkyl substitution. EHPT2C6 and C6PT2 possess deeper IPs
and EAs than those of C6PT2C6. The absence of a straight
alkyl chain at the end of the conjugated backbone almost does
not change the bandgaps (1.84 vs 1.85 eV) but lowers the IP
and EA levels, as shown for C6PT2C6 and C6PT2 (Table 4).
In addition, replacing linear alkyl chains on the DPP with
2-ethylhexyl chains (EHPT2C6) leads to much deeper IP (5.49
versus 5.16 eV) and EA (3.51 versus 3.32 eV) than those of
C6PT2C6. Considering the same conjugated backbone of
C6PT2C6, EHPT2C6, and C6PT2, these variations in energy
Figure 6. Cyclic voltammograms of all DPP-based oligophenylene-
thiophenes in 0.1 M Bu4NPF6 chloroform solution at a scan rate of
50 mV/s.
Figure S4, Supporting Information) were performed on the
chloroform solutions and the films, respectively. The results are
summarized in Table 4.
From the values of oxidation potential onset and reduction
potential onset obtained by CV, HOMO and LUMO as well as
electrochemical band gap (Egec) were calculated. As the number
of thiophenes increases from C6PT1C6 to C6PT2C6 and
C6PT3C6, the LUMO changes very slightly from −3.30
to −3.39 eV. C6PT3C6 has a higher-lying HOMO (−5.19 eV)
than C6PT1C6 (−5.48 eV) and C6PT2C6 (−5.46 eV). Similar
results have been observed in DPP-based donor−acceptor
Table 4. Energy Levels and Band Gaps of DPP-Based Oligophenylenethiophenes
a
b
solution
film
c
c
d
d
e
f
g
h
f
ox
onset
red
E
(V)
E
(V)
onset
HOMO (eV)
LUMO (eV)
Eegc (eV)
Eogpt (eV)
HOMO (eV)
LUMO (eV)
Eogpt (eV)
C6PT1C6
C6PT2C6
C6PT3C6
EHPT2C6
C6PT2
0.92
−1.26
−1.17
−1.26
−1.27
−1.24
−5.48
−5.46
−5.19
−5.42
−5.46
−3.30
−3.39
−3.30
−3.29
2.18
2.07
1.89
2.13
2.14
2.17
2.09
2.06
2.12
2.11
−5.63
−5.16
−5.24
−5.49
−5.37
−3.66
−3.32
−3.38
−3.51
1.97
1.84
1.86
1.98
1.85
0.90
0.63
0.86
0.90
−3.32
−3.52
a
b
c
d
Measured in chloroform solution. Cast from chloroform solution. Determined by cyclic voltammetry to Ag/Ag+. Calibrated based on the
e
f
oxidation potential of Fc/Fc+. Egec = LUMO − HOMO. The optical band gap calculated from Egopt = 1240/λonset in the absorption spectrum.
g
h
Measured by UPS. EA obtained by EA = Egopt − |IP|.
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levels suggest that the molecular packing in the solid state,
which is strongly influenced by alkyl substituents, is one of the
critical factors that determine the resulting energy levels.
Such results are consistent with a study on pentacene and
6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene)
in which the solid state electronic properties were reported to
be significantly affected by the substitutions on the conjugated
backbone.44 While pentacene and TIPS pentacene showed
similar HOMO levels by CV measurements in solutions, UPS
measurements performed on TIPS pentacene thin films
exhibited a much (ca. 1.0 eV) larger IP than that of pentacene.
Film Morphology and Hole Mobility. Figure 8 displays
the AFM images of as-cast and thermal annealed (100 °C) films
of all compounds. The AFM images of the annealed films at
80 °C are shown in Figure S5 (Supporting Information).
C6PT1C6 and C6PT2C6 and C6PT3C6 have plate-like
structures, which are fused together. The surface roughnesses
of the as-cast C6PT1C6 and C6PT2C6 and C6PT3C6 films
are 0.87, 1.29, and 0.79 nm, respectively. The plate-like
structures grow larger and show clear domain boundaries after
thermal annealing. Upon thermal annealing, the surface
roughness reduces to 0.73 and 1.06 nm for C6PT1C6 and
C6PT2C6, respectively. In contrast, the surface roughness of
the annealed C6PT3C6 film increases to 1.11 nm. One expects
that the molecular packing in the plate-like domains is similar to
that of their single crystals as confirmed by having similar XRD
patterns (Figure 3). The crystalline domain size in these films
is influenced by the molecular structure of the compounds.
A larger conjugated backbone results in smaller crystalline
domains in both as-cast and annealed films, which is
demonstrated by comparing the surface morphology of
C6PT1C6, C6PT2C6, and C6PT3C6. The domain sizes for
C6PT1C6 and C6PT2C6 are 162−270 nm and 128−167 nm,
respectively. Besides the molecular structures, thermal annealing
can increase the domain size effectively. In particular, C6PTC6
shows a large domain size (380−500 nm) in the annealed films,
whereas for C6PT2C6 the domain size increases to 183−
218 nm. For C6PT3C6, the domain size is very small for the
as-cast film but grows up to 100 nm. In contrast, the as-cast
and annealed films of C6PT2 show featureless with surface
roughness of 0.31 and 0.75 nm, respectively, which can be
ascribed to the low crystallinity as demonstrated by thin-film
XRD and DSC. The as-cast EHPT2C6 surface is comprised of
wavy structures with high surface roughness of 3.86 nm, which
reduces to 2.99 nm upon thermal annealing at 100 °C. Overall,
the surface morphology is dependent on the molecular structure
and thermal annealing and is correlated well with the thin film
XRD and DSC results.
The hole mobility of each material was determined from hole-
only diodes using the space charge limited current (SCLC)
model. Figure 9 shows the hole mobility of as-cast and annealed
Figure 9. Hole mobility measured using hole-only diodes with as-cast
(25 °C) and thermal annealing (80 and 100 °C) films.
films at different temperatures for the five DPP compounds. The
as-cast film of C6PT2 does not exhibit SCLC dependence, thus
the mobility cannot be extracted. Among the as-cast films,
C6PT2C6 has the best hole mobility of 7.8 × 10−6 cm2/(V s),
while EHPT2C6 has the lowest mobility (8.9 × 10−7 cm2/(V
s)), as a result of adding branched alkyl chains. This observation
agrees well with the XRD, DSC, and AFM results. The hole
Figure 8. Tapping mode AFM height images of C6PT1C6 (a and b),
C6PT2C6 (c and d), C6PT3C6 (e and f), EHPT2C6 (g and h), and
C6PT2C6 (i and j) films at conditions of as-cast (left) and after
thermal annealing (right). Annealing conditions: 100 °C for 10 min.
Scan size: 2 μm × 2 μm.
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mobility of the as-cast C6PT1C6 film is slightly higher than
that of the as-cast C6PT3C6 film (4.6 × 10−6 vs 2.4 × 10−6
cm2/(V s)). Thermal annealing of C6PT1C6, C6PT3C6, and
C6PT2 at 80 and 100 °C leads to a slight increase of hole
mobility possibly due to a higher degree of crystallinity. How-
ever, EHPT2C6 and C6PT2C6 exhibit a slight decrease in hole
mobility upon thermal annealing at 100 °C. Overall, the hole
mobilities among these compounds do not change significantly
upon thermal annealing. Due to the 2-ethylhexyl substitution,
EHPT2C6 has the smallest intermolecular π−π overlapping
area in the crystal structure and amorphous-like as-cast film,
therefore resulting in the lowest hole mobility. Comparing the
linear alkyl chain series, C6PT2C6 has a balanced intermolecular
overlapping and crystalline film morphology, resulting in highest
hole mobility. These observations indicate a strong correlation
among molecular structure, crystal structure, film morphology, and
hole mobility.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: quyen@chem.ucsb.edu.
Author Contributions
∥These authors equally contributed.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
C.K. thanks the Department of Energy, Office of Basic
Research, for financial support. J. Liu and J. Lin are supported
by the NSF-CAREER/SPECIAL CREATIVITY and NSF-
SOLAR, respectively. B.W. is supported by the Office of Naval
Research. T.-Q.N. thanks the Camille Dreyfus Teacher Scholar
and the Alfred Sloan programs. We thank J. Sherman and Dr. S. R.
Parkin for helping with initial single-crystal structures of
C6PT2C6 and C6PT3C6.
CONCLUSION
■
DEDICATION
■
To understand the effect of structural variation on solid state
properties, a series of DPP-based oligophenylenethiophenes were
synthesized and characterized. Structural variation is found to
have remarkable effects on the materials’ fundamental properties
including molecular packing, thermal transitions, crystallinity,
optical absorption, energy levels, film morphology, and hole
mobility. Single-crystal structures of these DPP-based oligophe-
nylenethiophenes were resolved and compared in terms of
molecular packing style, intermolecular overlapping (areas and
distance), and intramolecular conformation. The polycrystalline
films have molecular packing similar to their single crystals. The
observed structural effects on properties can be explained
according to the single-crystal structures. Branched alkyl chain
substitution can dramatically decrease melting temperature and
crystallinity. While the molecular backbone determines the optical
absorption in solution, the optical absorption in the solid state is
affected by both conjugated backbone and alkyl substitution. CV
conducted in solution confirms that alkyl substitution does not
affect energy levels in the solution state. In contrast, UPS
measurements indicate that the position of energy levels in given
materials with the same conjugation length can be tuned by
changing alkyl substitution on the DPP unit as well as at the end
of the conjugation backbone. The film morphology and hole
mobilities measured using hole-only diodes show strong
correlations with their molecular structures and single-crystal
structures. This systematic investigation of structural effect on the
evolution of materials’ fundamental properties provides an
effective approach to establish structure−property relationships
and guidelines to design new molecules for use in BHJ solar cells.
The alkyl chains attached on the conjugated backbone affect not
only the solubility but also the molecular packing in the solid state
and hence film absorption and energy levels. Effects of chemical
structure on solar cell performance will be published elsewhere.
This paper is dedicated to the memory of Mananya Tantiwiwat.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Absorption spectra of as-cast and annealed film of C6PT2C6,
EHPT2C6, and C6PT2; TGA profiles, UPS spectra, AFM
images (PDF), and crystallographic information files (CIFs) of
all compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
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dx.doi.org/10.1021/cm202852f | Chem. Mater. 2012, 24, 1699−1709

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