Five compounds, based on diketopyrrolopyrrole (DPP) and phenylene thiophene (PT) moieties, are synthesized to investigate the effect of structural variations on material’s solid state properties. The structural variations are focused on (a) backbone length by changing the number of thiophenes on both sides of DPP, (b) alkyl substitution (n-hexyl or ethylhexyl) on DPP, and (c) the presence of n-hexyl group at the end of the molecular backbone.

1. Influence of Structural Variation on Solid-State Properties of
Diketopyrrolopyrrole-Based Oligophenylenethiophenes
Jianhua Liu, Chunki Kim, Jason Lin and Thuc-Quyen Nguyen
Center for Polymers and Organic Solids and Department of Chemistry & Biochemistry, University of California,
Santa Barbara, California, 93106 USA
Abstract Optical Absorption and Solubility
Five compounds, based on diketopyrrolopyrrole (DPP) and phenylene Solution state Film state
thiophene (PT) moieties, are synthesized to investigate the effect of structural 1.0 (a) 1.0 (b) C6PT1C6
variations on material’s solid state properties. The structural variations are C6PT2C6
C6PT3C6
0.8 0.8
Absorbance (a.u.)
Absorbance (a.u.)
focused on (a) backbone length by changing the number of thiophenes on both EHPT2C6
C6PT2
sides of DPP, (b) alkyl substitution (n-hexyl or ethylhexyl) on DPP, and (c) the 0.6 0.6
presence of n-hexyl group at the end of the molecular backbone. 0.4 0.4
O
N O N
S
0.2 0.2
S S
S S
N S
0.0 0.0
N O
C6PT1C6 O
Conjugation length EHPT2C6
Alkyl chain on DPP
300 400 500 600 300 400 500 600 700
C6PT2C6 O N
Wavelength (nm) Wavelength (nm)
S S
S S
Summary of Optical and Thermal Properties
N O
Conjugation length End alkyl chain
O
O
N Solution Film
N S Tm Tc Solubility
S S
S
S S
S S
Name λmax λonset Eg (opt) ɛ λmax λonset Eg (opt) HOMO LUMO
S
S
N
N
O (º (º (mg/mL)
C) C)
C6PT3C6
O C6PT3C6 (nm) (nm) (eV) (M-1cm-1) (nm) (nm) (eV) (eV) (eV)
C6PT1C6 505 571 2.17 3.2104 582 629 1.97 -5.63 -3.66 181 143 228
C6PT2C6 521 591 2.09 4.6104 614 674 1.84 -5.16 -3.32 231 194 11
Single Crystal Structure and Film X-ray Diffraction C6PT3C6 528 602 2.06 5.6104 608 667 1.86 -5.24 -3.38 263 229 1.5
EHPT2C6 514 584 2.12 4.8104 532 626 1.98 -5.49 -3.51 164 77 53
Two molecular packing motif observed Thin film XRD
C6PT2 516 587 2.11 4.4104 554 671 1.85 -5.37 -3.52 249 184 6.3
(c)
C6PT1C6 EHPT2C6
(a) (b)
C6PT2C6 C6PT2 Absorption redshift saturates as increasing conjugate length.
C6PT3C6
a Tm and solubility have opposite sequence.
b b Ethylhexyl substitution significantly affects solid state absorption.
c a c
d
(d) e Film Morphology and Diode Mobility
f
g
(e) C6PT1C6 C6PT2C6 C6PT3C6 EHPT2C6 C6PT2
h
i
(f) j
10
2 ° 20
 30
C6PT1C6, C6PT2C6, C6PT3C6, and C6PT2 Experimental measured patterns
(a-c) Vs. EHPT2C6 (d-f). Unit cells viewed (a, c, e, g, and i) Vs. Theoretical
from side (a, d), a axis (b), c axis (c, f), and b patterns (b, d, f, h, and j)
axis (e).
φ2 O
R1
Intermolecular Packing and Intramolecular Conformation
φ1
R2
S N
-5
AFM height images of as-cast (top) 10
Hole mobility (cm /Vs)
Intermolecular Overlapping Area and Distance N S and annealed (bottom) films.
Conjugated Backbone ConformationR2
R1
Annealing condition: 100 º for 10
C
2
O
min. Scan size: 2 µm  2 µm.
Intermolecular backbone Interplane
Name
overlappinga distanceb (Å)
Name φ1 (o) φ2 (o)
-6
C6PT1C6 3.28 C6PT1C6 26.2 22.8 10
C6PT2C6 23.7 10.2
Hole mobility of as-cast (25 º
C)
C6PT1C6
C6PT2C6 3.26
and thermal annealing (80 º C C6PT2C6 EHPT2C6
C6PT3C6 25.1 16.5 C6PT2
and 100 º films.
C) C6PT3C6
EHPT2C6 37.6 17.4
40 80
C6PT3C6 3.37
C6PT2 26.2 14.2 Temperature (C)
aφ1 (C-C-C-C) and φ2 (X-C-C-C) as
3.27c
EHPT3C6
illustrated by the general molecular Summary
structure at bottom.
C6PT2 3.23
 Structural variation has remarkable effects on the materials’ fundamental
φ2 O
R1
properties including molecular packing, thermal transitions, crystallinity,
R2
aViewed orthogonal to the molecular long axis with red S
φ1 N
rectangles highlighting the overlapping area. bThe interplanar optical absorption, energy levels, film morphology, and hole mobility.
distance is calculated from phenyl-phenyl rings. cCalculated N S
R2
 Single crystal structures of these DPP-based compounds were compared
R1
O
from overlapped DPP rings.
Name φ1 (o) φ2 (o) in terms of molecular packing style, intermolecular overlapping (areas and
Thermal Property and Energy Level C6PT1C6 26.2 22.8 distance), and intramolecular conformation.
C6PT2C6 23.7 10.2
 Differences in material properties are found to arise from changes in intra-
DSC thermograms (10 oC/min) Ionization potential (IP)
C6PT3C6 25.1
and
16.5
electron and intermolecular interactions in the solid state caused by the structural
EHPT2C6 37.6 17.4
affinity (EA) C6PT2 26.2 14.2
variations.
C6PT1C6 LUMO
EA
-3.32
-3.32 -3.38
-3.38 -3.51
 This study demonstrates a systematic investigation of structure-property
-3.66 -3.52
-3.43
-3.51
C6PT2C6
-3.66
relationships in conjugated small molecules.
Exothermic
1.841.84 1.86
opt 1.86 1.85
C6PT3C6 Eg E opt 1.971.97
1.98
1.98
1.94
References
g
EHPT2C6 -5.16 Liu, J.; Kim, C.; Lin, J.; Tamayo, A.; Walker, B.; Wu, G.; Nguyen, T. -Q. Chemistry of Materials 2012, 24, 1699.
-5.16 -5.24
-5.24 -5.37
-5.49 -5.37
C6PT2 HOMO
IP -5.63 -5.49
-5.63 Acknowledgments: This work is supported by the NSF-CAREER/SPECIAL CREATIVITY AWARD and NSF-
C6PT1C6 C6PT2C6 C6PT3C6 EHPT2C6 C6PT2
C6PT1C6 C6PT2C6 C6PT3C6 EHPT2C6 C6PT2 SOLAR. C. K. thanks the Department of Energy, Office of Basic Research for financial support. T.-Q.N thanks
the Camille Dreyfus Teacher Scholar Award and the Alfred Sloan Fellowship.
0 50 100 150 200 250 300 IP determined by UPS and EA obtained by EA = Egopt - |IP|.
Temperature (C)