Sequence matters: Modulating electronic and optical properties of conjugated oligomers via tailored sequence

Article abstract of DOI:10.1021/ma400123r

Although sequence must necessarily affect the photophysical properties of oligomers and copolymers prepared from donor and acceptor monomers, little is known about this effect, as nearly all the donor/acceptor materials have an alternating structure. A series of sequenced p-phenylene-vinylene (PV) oligomers was synthesized and investigated both experimentally and computationally. Using Horner-Wadsworth-Emmons (HWE) chemistry, a series of dimers, trimers, tetramers, pentamers, and hexamers were prepared from two building block monomers, a relatively electron-poor unsubstituted p-phenylene-vinylene (A) and an electron-rich dialkoxy-substituted p-phenylene-vinylene (B). UV-vis absorption/emission spectra and cyclic voltammetry demonstrated that the optoelectronic properties of these oligomers depended significantly on sequence. Calculations predicting the HOMO-LUMO gap of the sequenced oligomers correlated well with the experimental properties for the 2- to 4-mers, and the consensus model developed was used to design hexameric sequences with targeted characteristics. Despite the weak acceptor qualities of the A monomer employed in the study, HOMO-LUMO gap differences of ～0.25 eV were found for isomeric, sequenced oligomers. In no case did the alternating structure give the largest or smallest gap. The use of sequence as a strategy represents a new dimension in tailoring properties of π-conjugated polymers.

Full text of DOI:10.1021/ma400123r

Article
pubs.acs.org/Macromolecules
Sequence Matters: Modulating Electronic and Optical Properties of
Conjugated Oligomers via Tailored Sequence
Benjamin N. Norris,† Shaopeng Zhang,‡ Casey M. Campbell,§ Jeffrey T. Auletta,‡ Percy Calvo-Marzal,‡
Geoffrey R. Hutchison,‡,* and Tara Y. Meyer‡,*
†Department of Chemistry, Frostburg State University, 101 Braddock Road, Frostburg, Maryland 21532, United States
‡Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, United States
§School of Chemistry and Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive, Atlanta, Georgia 30332-0400, United
States
S
* Supporting Information
ABSTRACT: Although sequence must necessarily affect the
photophysical properties of oligomers and copolymers
prepared from donor and acceptor monomers, little is
known about this effect, as nearly all the donor/acceptor
materials have an alternating structure. A series of sequenced
p-phenylene−vinylene (PV) oligomers was synthesized and
investigated both experimentally and computationally. Using
Horner−Wadsworth−Emmons (HWE) chemistry, a series of
dimers, trimers, tetramers, pentamers, and hexamers were
prepared from two building block monomers, a relatively
electron-poor unsubstituted p-phenylene−vinylene (A) and an
electron-rich dialkoxy-substituted p-phenylene−vinylene (B). UV−vis absorption/emission spectra and cyclic voltammetry
demonstrated that the optoelectronic properties of these oligomers depended significantly on sequence. Calculations predicting
the HOMO−LUMO gap of the sequenced oligomers correlated well with the experimental properties for the 2- to 4-mers, and
the consensus model developed was used to design hexameric sequences with targeted characteristics. Despite the weak acceptor
qualities of the “A” monomer employed in the study, HOMO−LUMO gap differences of ∼0.25 eV were found for isomeric,
sequenced oligomers. In no case did the alternating structure give the largest or smallest gap. The use of sequence as a strategy
represents a new dimension in tailoring properties of π-conjugated polymers.
to engineer the desired optoelectronic properties as a hybrid of
the properties of the respective homopolymers.2−9 Although
alternating and random copolymers/oligomers containing a
variety of donor and acceptor monomers have been
prepared,2,10−13 no systematic effort has been made to
determine the effect of the donor−acceptor sequence on the
optoelectronic properties. For example, units that encode
sequence in some form are nearly always symmetric14−20 and
there are only a few studies that include more than two
examples of materials that have complex sequences21 or are
isomeric but sequentially diverse.22,23
Recent results from our groups suggest that monomer
sequence can have as much influence on properties of relevance
to photovoltaics as the identities and ratios of the monomers.
We developed a genetic algorithm for surveying the structure
space of conjugated oligomers assembled from various donor−
acceptor dimers and computationally predicted the power-
conversion efficiencies of photovoltaic cells.24 Oligomers with
complex sequences of dimers exhibited surprisingly large
INTRODUCTION
■
Nature refines the properties of biopolymers, not just by
composition, but also by orchestration of monomer sequence.
For example, the photosynthetic pathway exhibits optical
absorption, energy transfer, electron transfer, and chemical
transformation motifs all within a self-assembled package.1 In
stark contrast, efforts to synthesize organic solar cells focus
almost solely on chemical variation of monomer structure,
seeking to derive optimal optical, energetic, and charge transfer
properties using a very limited number of patterns.2−4 Organic
photovoltaics promise to significantly reduce the cost of solar
electrical generation, so optimization of the material should be
driven by sequence as well as composition. Here we
demonstrate by combined synthesis, computational design,
and optical and electrochemical characterization that altering
the sequence of widely studied conjugated phenylene−vinylene
oligomers can significantly modulate both optical and redox
properties. We show that neither long block nor alternating
sequences will likely yield optimal properties for photovoltaics.
Third generation photovoltaic polymers rely on the donor−
acceptor approach in which electron-poor acceptor and
electron-rich donor monomers are copolymerized in an effort
Received: January 18, 2013
Published: February 11, 2013
© 2013 American Chemical Society
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Figure 1. Synthetic approach to sequenced oligomers.
differences in optoelectronic properties and photovoltaic
efficiencies.
group; (4) oligomers are labeled dimer, trimer, etc. based on
the number of phenyl units (rather than complete phenylene−
vinylene units) to avoid the use of the more exact but
cumbersome n.5-mer terminology.
The power of sequence to control oligomer and polymer
properties in applications other than photovoltaics is
increasingly being investigated.25−27 We have, for example,
examined the effects of sequence on the properties of
poly(lactic-co-glycolic acid)s and poly(fluorene-co-methylene)-
s.28−33 The power of sequence to control the properties of
oligomers and polymers can also be seen in the metal-catalyzed
control of stereochemistry in polyolefins,34 polylactides,11,17
and other monomers35 and convergent/divergent assembly of
precise oligomers,36 sequential polycondensation,37 acyclic
diene metathesis,38 controlled free-radical polymeriza-
tions,39−42 and template synthesis of sequences.43
We report herein a model study, combining synthesis and
characterization of sequenced oligomers and computational
design, that demonstrates the power of sequence to control
optoelectronic properties. The interplay between the exper-
imental and theoretical work is synergistic throughout.
Experimental results from the synthesis of a library of easily
prepared shorter oligomers were used to verify our computa-
tional approach. The experimental trends and calculations were
then exploited to design targeted hexamers. Computational
screening of the sequences proved critical as the longer
oligomers are synthetically complex and difficult to survey
experimentally due to the exponential increase in possible
combinations with oligomer length.
In our initial investigations we encountered low stereo-
selectivity when using acetal-protected monomersa perplex-
ing result given the lack of discussion of stereoselectivity in
reports of previous syntheses using the HWE reaction. Our
model reactions involved the HWE reaction between alkoxy-
substituted bromobenzaldehyde Br-B-CHO and either alkoxy-
substituted phosphonate monomer P-B’-CN (4) or the
derivative in which the terminal cyano is replaced with an
acetal, which we expected to be the most difficult HWE
combinations for steric reasons, to give the alkoxy-substituted
dimers. The reaction with the acetal-protected monomer
(Table 1, entry 1) proceeded in quantitative yield and without
a
Table 1. Stereoselectivity of HWE Reactions
RESULTS
■
a
1
E:Z determined by H NMR spectroscopy.
Synthesis of Sequenced Oligomers. Oligo(phenylene−
vinylene)s (OPVs) were targeted for this sequence study
because these oligomers are well-known to have varied,
substituent-dependent optoelectronic properties.9,23 A variety
of methods for preparation of OPVs have been reported by our
group and other researchers.21,23,44−46 The approach selected
for this study was a modification of the synthesis by Jørgensen
and Krebs47 featuring alternating Horner−Wadsworth−Em-
mons (HWE) olefinations of a p-cyanobenzyl phosphonate
monomer with an oligomer aldehyde followed by DIBAL-H
reduction to yield a new reactive aldehyde (Figure 1). The key
differences in our approach are the iterative coupling of single
phenylene units, rather than dimeric units, which allows for the
synthesis of oligomers of any length and pattern, and the use of
nitrile-terminated units rather than acetals, which increased
both the E-selectivity of the HWE reaction and the ease of
purification of the oligomers.
loss of the bromine atom, as reported by Jørgensen and
Krebs.47 This reaction, however, gave the dimer as a 2:1
mixture of E and Z isomers. As these isomers are difficult to
separate by chromatography, we pursued further modifications.
Addition of LiCl, which has been shown to increase E-
selectivity in HWE reactions,48,49 increased the selectivity to 4:1
E:Z at the expense of conversion and yield (Table 1, entry 2).
Nitrile monomer 2 (Table 1, entry 3) gave Br-BB′-CN in a
70% yield with higher E-selectivity (5:1). Addition of LiCl to
this reaction increased both the yield to 100% and the E-
selectivity to >9:1. Additionally, the CN-terminated oligomers
were more easily purified by chromatography.
Four dimers, Br-AA′-CN, Br-AB′-CN, Br-BA′-CN, and Br-
BB′-CN were prepared by this procedure in high yields and E-
selectivities (Table 2). The nitrile group of each dimer could
then be reduced with DIBAL-H to produce an aldehyde end
group that allowed for subsequent HWE reactions to increase
chain length. By repeating successive cycles of nitrile reduction
and HWE coupling a total of 22 sequenced oligomers were
The following naming conventions are employed through-
out: (1) unsubstituted p-phenylene units are designated A; (2)
dialkoxy-substituted units are designated B; (3) A′ and B′ are
used for p-phenylene units bearing the conjugated cyano end
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Table 2. Sequenced Oligophenylene Vinylenes
a
b
oligomer
R1
R2
R3
R4
R5
R6
% yield
E:Z
c
d
Br-AA′-CN
Br-AB′-CN
Br-BA′-CN
Br-BB′-CN
H
H
H
















85
>20:1
c
OnC6H13
92
20:1
20:1
9:1
c
OnC6H13
OnC6H13
H
96
c
OnC6H13
96
Br-AAB′-CN
Br-BAA′-CN
Br-BAB′-CN
Br-ABA′-CN
Br-ABB′-CN
Br-BBA′-CN
H
H
OnC6H13
H


















78
93
89
87
82
79
20:1
20:1
20:1
9:1
OnC6H13
OnC6H13
H
H
H
OnC6H13
OnC6H13
OnC6H13
OnC6H13
H
H
OnC6H13
H
8:1
OnC6H13
8:1
d
Br-BAAB′-CN
Br-ABAB′-CN
Br-BABA′-CN
Br-BBAA′-CN
Br-AABB′-CN
Br-ABBA′-CN
OnC6H13
H
OnC6H13
OnC6H13
H
H
H
OnC6H13
OnC6H13
H












83
71
85
98
80
73
>20:1
d
OnC6H13
H
>20:1
d
H
OnC6H13
>20:1
d
OnC6H13
H
H
H
>20:1
OnC6H13
OnC6H13
OnC6H13
20:1
d
H
OnC6H13
H
>20:1
Br-AABBB′-CN
Br-BABAB′-CN
Br-BBAAA′-CN
H
H
OnC6H13
OnC6H13
H
OnC6H13
OnC6H13
OnC6H13
H



59
90
88
10:1
d
OnC6H13
OnC6H13
H
H
H
>20:1
d
OnC6H13
>20:1
Br-AABBBA′-CN
Br-BABABA′-CN
Br-BBAAAB′-CN
H
H
OnC6H13
OnC6H13
H
OnC6H13
OnC6H13
OnC6H13
H
H
H
66
74
9:1
d
OnC6H13
OnC6H13
H
H
H
>20:1
d
OnC6H13
OnC6H13
60
b
>20:1
a
1
Yield over two steps from relevant previous oligomer (e.g., Br-BBAA′-CN was prepared from Br-BBA′-CN), unless noted. Estimated from H
c
d
NMR spectra. Yield over one step from Br-A-CHO (1) or Br-B-CHO (2). No peaks for Z isomers were observed.
Table 3. Optoelectronic Properties of Sequenced OPVs
a
b
a
c
em
max
d
e
ox
peak
e
f
ec
/10‑3 cm‑1 M‑1
λ
/nm
λ
/nm
ΔE /eV
E
/V
E
/V
peak
ΔE /eV
abs
max
em
max
opt
red
oligomer
λ
/ nm
ε
gap
gap
Br-AA′-CN
Br-BA′-CN
Br-AB′-CN
Br-BB′-CN
327
54.5
379
450
418
450
463
460
443
519
3.44
2.97
2.99
2.89
1.45
1.06
1.23
1.06
−1.93
−1.96
−1.94
−2.17
3.38
3.02
3.17
3.23
309, 362
316, 364
303, 380
28.1, 29.2
29.4, 24.9
16.9, 26.6
Br-AAB′-CN
Br-BAA′-CN
Br-BAB′-CN
Br-ABA′-CN
Br-ABB′-CN
Br-BBA′-CN
385
93.2
433
476
474
477
478
488
507
497
504
514
524
522
2.86
2.84
2.77
2.65
2.63
2.62
1.06
0.95
0.97
0.84
0.81
0.78
−1.94
−1.93
−1.97
−1.92
−1.93
−1.94
3.00
2.88
2.94
2.76
2.74
2.72
383
73.2
396
72.4
334, 406
329, 412
333, 412
37.9, 50.2
34.0, 53.8
29.4, 53.9
Br-BAAB′-CN
Br-ABAB′-CN
Br-BABA′-CN
Br-BBAA′-CN
Br-AABB′-CN
Br-ABBA′-CN
408
93.3
485
499
492
515
492
511
512
549
534
553
547
541
2.72
2.58
2.56
2.56
2.55
2.47
0.87
0.69
0.69
0.65
0.70
0.67
−1.94
−2.02
−1.98
−1.99
−1.99
−1.95
2.81
2.71
2.67
2.64
2.69
2.62
422
73.9
425
89.9
366, 424
360, 425
337, 437
41.4, 86.2
47.9, 83.7
35.3, 78.3
Br-AABBB′-CN
Br-BABAB′-CN
Br-BBAAA′-CN
351 449
435
35.5, 90.7
97.7
522
508
500
578
557
583
2.43
2.52
2.56
0.63
0.70
0.69
−1.97
−1.96
−1.96
2.60
2.66
2.65
427
78.1
Br-AABBBA′-CN
Br-BABABA′-CN
Br-BBAAAB′-CN
342 462
448
40.8, 94.0
112.2
538
509
494
616
580
566
2.29
2.46
2.53
0.54
0.64
0.67
−1.92
−1.94
−1.99
2.46
2.58
2.66
430
108
a
b
c
d
Measured in ∼10−6 M chloroform solution. Calculated at λmabasx. Thin film, cast from chloroform solution. Determined at the onset of the
e
f
ec
ox
red
absorption spectrum. Potential vs Ag/Ag+, 240 μM in 0.1 M Bu4NPF6 in THF. Determined as ΔE = e(E
− E ).
gap
peak
peak
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prepared: four dimers, six trimers, six tetramers, three
pentamers, and three hexamers. All HWE reactions and
DIBAL-H reductions used to prepare the trimers and tetramers
proceeded in good to excellent yields. The yields of the
pentamers and hexamers were lower in some cases due to the
decreased solubility of the longer oligomers. All oligomers were
prepared with high (>8:1) E:Z-selectivity, with the lowest
selectivity observed for those oligomers with two or more
adjacent B units. In many cases, the Z-isomers were not
observable by NMR after purification which suggests an upper
limit of 3−5% contamination. In order to facilitate further
elaboration of the oligomers, including the possibility of
incorporating them into polymeric materials in the future,
each OPV was prepared with a bromide group on one terminus
and a nitrile on the other.
Computational Approach. Computational methods offer
an easy mechanism to screen optoelectronic properties of π-
conjugated materials.50−55 While density functional theory
(DFT) computed orbital eigenvalues are nonphysical,56,57
numerous studies have found a high degree of correlation
between these energies and vertical ionization potentials and
electron affinities58,59 as well as accurate predictions of optical
band gaps.50 For solution electrochemistry, the redox potentials
can be determined based on the free energy change,60,61 such as
the adiabatic difference in total energy between the neutral and
charged systems (ΔSCF). In many cases, systematic deviations
reflect a linear free energy relationship62 between computed
and experimental properties, which can be captured simply by
linear regression. This regression also corrects for other errors,
such as differences in computed and experimental conforma-
tions.
Since our objective was to reliably and accurately screen for
targeted properties of sequenced oligomers, we sought to
extend these regression techniques by use of a “consensus
model” to minimize both systematic and random errors, i.e., to
improve accuracy and correlation. The consensus model
employed here combines two different computational pre-
dictions of an experimental property using multivariate
regression, e.g., oxidation potential. For redox potentials,
DFT eigenvalues and adiabatic total energy differences
(ΔSCF) were used, and for optical absorption energies and
oscillator strengths, ZINDO and time-dependent DFT
(TDDFT) methods were combined.
The computational method was originally developed and
calibrated using optical and electrochemical data from
sequenced 2-, 3-, and 4-mers. The method was then used to
predict the properties of all possible hexamer sequences with a
1:1 A:B ratio (Supporting Information, Table S5). Using these
data, three hexamers with specifically targeted behavior, Br-
AABBBA′-CN, Br-BBAAAB′-CN and Br-BABABA′-CN, were
selected for synthesis. Prior to discussing the computational
results/predictions in more detail, however, the characterization
data for all oligomers are presented.
Optical Spectroscopy. The optical spectra of the
oligomers vary significantly with sequence (Table 3, Figure 2,
and Supporting Information for all spectra). The absorption
maxima of both the trimer and tetramer series vary over a range
of ∼30 nm. The HOMO−LUMO gaps, estimated at the onset
of absorption, vary over a range ∼0.25 eV.
Figure 2. Absorption and emission spectra in CHCl3 (10−6 M): (a)
absorption spectra for selected tetramers; (b) absorption spectra for
hexamers; (c) emission spectra for selected tetramers; (d) emission
spectra for hexamers.
units. The emission maximum in both solution and thin film
follow a similar trend to λambasx. There is, however, a noticeable
red shift in the solid state that is consistent with
aggregation.63,64
The absorption spectra of Br-ABBA′-CN, Br-BAAB′-CN,
and a representative alternating (Br-BABA′-CN) and blocky
(Br-AABB′-CN) sequence are presented in Figure 2a. Among
the tetramers, the sequence with the smallest band gap is Br-
ABBA′-CN, while the complementary sequence, Br-BAAB′-
CN, exhibits the largest. The alternating sequences (Br-ABAB′-
CN and Br-BABA′-CN) and the blocky sequences (Br-AABB′-
CN and Br-BBAA′-CN) are intermediate. The effect of Z-
isomer contamination on the optical spectra is negligible, as the
materials were prepared with high E-selectivity (vide infra).
Sequence also impacts the absorption profile of the
oligomers. Several sequences exhibit a well-separated higher
energy absorption band. These tended to be sequences with AA
or BB blocks, for example Br-BBAA′-CN or Br-ABBA′-CN,
among the tetramers. While the longer wavelength absorptions
are primarily π−π* transitions delocalized across the entire
oligomer, the higher energy, weaker absorptions likely derive
from excitations between BB and AA blocks. These peaks,
which were also found in the computational results, arise from
shorter geometric distances and, thus, exhibit smaller transition
dipole moments.
Although the range of band gaps for the pentamer series is
small (0.13 eV), the range for the hexamer series is large (0.24
eV), similar to the tetramer series. The sequence Br-AABBBA′-
CN exhibits the smallest band gap while the complementary
sequence Br-BBAAAB′-CN has the largest. The alternating
hexamer Br-BABABA′-CN exhibits intermediate properties, as
predicted (vide supra).
Electrochemistry. The electrochemistry of the oligomers is
also strongly dependent on sequence (Figure 3, Table 3 and
Supporting Information for the complete data set). All
oligomers with sequences containing multiple B units exhibit
multiple oxidation peaks in their differential pulse voltammo-
grams (DPVs). The first oxidation potentials of the oligomers
Among the trimers, band gap decreases with decreasing A
content, which is consistent with previous reports.20 Sequence
is still an important factor, however; Br-ABA′-CN exhibits a
smaller gap than Br-BAB′-CN, despite the latter having more B
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and largest ΔEegcap. The alternating hexamer Br-BABABA′-CN
exhibits intermediate properties. The range of gaps is 0.2 eV, in
close agreement with the spectroscopic range of 0.24 eV.
Thermal Properties. Although not targeted for computa-
tional prediction in this investigation, the thermal properties of
the prepared oligomers were also acquired and found to
depend on sequence (Figure 4, Table 4). All oligomers were
Figure 4. DSC thermograms of all six sequenced tetramers and all
three sequenced hexamers.
Table 4. Thermal Properties of the Sequenced OPVs
a
a
b
oligomer
Tiso /°C
TLC /°C
TC /°C
151, 167
Br-AA′-CN
Br-BA′-CN
Br-AB′-CN
Br-BB′-CN
197




83.2
30.6

c
74.7
Figure 3. Cyclic voltammograms and differential pulse voltammo-
grams of Br-BAAB′-CN and Br-ABBA′-CN in THF.
96.3
68.8
Br-AAB′-CN
Br-BAA′-CN
Br-BAB′-CN
Br-ABA′-CN
Br-ABB′-CN
Br-BBA′-CN
105



41.6
demonstrate clear dependence on sequence and follow similar
trends to the absorption maxima.
c
125

104
185
114
109
77.8
c
The first oxidation potentials of the trimers depend partly on
composition, with Br-AAB′-CN and Br-BAA′-CN exhibiting
higher first oxidations than Br-ABB′-CN and Br-BBA′-CN,
due to higher A unit concentration. This effect reverses for the
other two trimers. The number of oxidation peaks and the
shapes of the oxidation profiles clearly demonstrate sequence
dependence as well, although there is not an obvious trend. The
reduction potentials show little dependence on sequence,
composition, or conjugation length. With few exceptions, the
first reduction potential is at ca. −1.90 V vs Ag/Ag+, likely due
to reduction of the cyano group.
64.8
79.5, 85.3, 92.7

43.2


Br-BAAB′-CN
Br-ABAB′-CN
Br-BABA′-CN
Br-BBAA′-CN
Br-AABB′-CN
Br-ABBA′-CN
119
123
94
81.2
85.0
75.4
111

41.7

116
144
191
57.5
114, 121
124
c
66.3

For the tetramer series, the first oxidation potentials vary
over a range of ∼200 mV in THF, with Br-BAAB′-CN
exhibiting the highest first oxidation potential, and the
complementary Br-ABBA′-CN exhibiting a much lower
oxidation potential (0.87 and 0.67 V vs Ag/Ag+, respectively).
The alternating and blocky sequenced tetramers fall in between,
with one exception; Br-BBAA′-CN exhibits the least positive
first oxidation potential.
Br-AABBB′-CN
Br-BABAB′-CN
Br-BBAAA′-CN
169
130
171
160
110
156
106
153
c
86.2
Br-AABBBA′-CN
Br-BABABA′-CN
Br-BBAAAB′-CN
184
134
122
178, 161
92, 105

165, 169
55.3

a
b
Exothermic transition observed on second heating scan. Exothermic
transition observed on second cooling scan. Transition observed in
first scan only.
The electrochemical HOMO−LUMO gaps, ΔEegcap, follow
similar patterns to the first oxidation potentials since the first
reduction potentials exhibit minimal variation. In contrast to
the optical band gaps, the electrochemical gaps exhibit greater
variation with sequence. In four out of five pairs of reverse
sequences (for example, Br-ABB′-CN and Br-BBA′-CN), the
B-first sequence exhibits a lower gap than the A-first sequence.
A high degree of correlation was otherwise observed between
electrochemical and spectroscopic gaps (R2 = 0.92).
We find a similar trend in the redox potentials of the
hexamers as was found in the optical spectroscopy. The
sequence Br-AABBBA′-CN exhibits the lowest oxidation
potential and smallest ΔEgeacp, while the complementary
sequence Br-BBAAAB′-CN has the highest oxidation potential
c
crystalline with melting points (Tiso) ranging from 80 to 170 °C
and most exhibited clear crystallization exotherms (Tc) during
differential scanning calorimetry (DSC). Oligomers with two
unsubstituted terminal A monomers, Br-AA′-CN, Br-ABA′-
CN, Br-ABBA′-CN, etc. exhibited higher temperature melting
transitions than other sequences of the same length. Multiple
melting transitions observed for several of the longer oligomers
are consistent with the existence of liquid crystalline phases
with narrow ranges of stability.
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Comparison of Computed and Experimental Data. As
stated above, the computational consensus models were
calibrated by the experimental results on the 2-, 3-, and 4-
mers. In general, computed properties (Table 5) show only
Table 5. Computed First Oxidation and Reduction Peak
comp
gap
Potentials and Optical Excitation Energies ΔE
of
Sequenced OPVs from Consensus Models
predicted Eox/eV predicted Ered/eV ΔE /eV
comp
oligomer
gap
Br-AA′-CN
Br-BA′-CN
Br-AB′-CN
Br-BB′-CN
1.43
1.13
1.21
1.03
−1.97
−1.99
−1.99
−2.00
3.41
3.12
3.20
3.03
Br-AAB′-CN
Br-BAA′-CN
Br-BAB′-CN
Br-ABA′-CN
Br-ABB′-CN
Br-BBA′-CN
1.01
0.96
0.88
0.89
0.84
0.81
−1.97
−1.97
−1.98
−1.96
−1.99
−1.97
2.97
2.93
2.85
2.85
2.82
2.78
Figure 5. Correlations between computed (a) first oxidation potential,
(b) first reduction potential, (c) optical excitation energies ΔEcgoapmp, and
(d) extinction coefficients with their experimental counterparts.Note
that for all predicted properties, a consensus model of two predictors
was used.
Br-BAAB′-CN
Br-ABAB′-CN
Br-BABA′-CN
Br-BBAA′-CN
Br-AABB′-CN
Br-ABBA′-CN
0.80
0.75
0.72
0.73
0.76
0.69
−1.96
−1.95
−1.95
−1.95
−1.96
−1.96
2.76
2.70
2.67
2.67
2.72
2.65
conventional alternating sequences Br-ABABAB′-CN and Br-
BABABA′-CN fell in between. As observed experimentally,
these predictions proved relatively correct, although the
difference in the experimental band gaps between hexamers
was larger than that predicted. The calculated difference in
predicted gaps spanned a range of only 0.05 eV, while the
experimental gaps spanned 0.2 and 0.24 eV for optical and
electrochemical data, respectively.
Br-AABBB′-CN
Br-BABAB′-CN
Br-BBAAA′-CN
0.62
0.65
0.66
−1.97
−1.96
−1.94
2.58
2.61
2.61
Br-AABBBA′-CN
Br-BABABA′-CN
Br-BBAAAB′-CN
0.54
0.61
0.62
−1.97
−1.95
−1.96
2.51
2.56
2.59
There are several possible explanations for this difference. It
is well-known that using TDDFT with conventional functionals
underestimates band gaps in longer oligomers due to incorrect
asymptotic behavior. For this reason, when screening the
hexamers, we solely used ZINDO calculations. Also, such
behavior has been observed previously and attributed to
differences in computed and experimental conformations.65,66
The calculations were performed on a low energy conforma-
tion, tending toward planarity in longer oligomers, not a
solution ensemble of different conformations with shorter
effective conjugation lengths.66 This effect likely explains the
smaller range in predicted band gaps in the hexamers,
compared with experiment. Still, sequence determines both
the orbital overlap and partial charge transfer between B and A
monomers involved in the electronic excitations, and also
dictates conformation in solution.31
small residual errors compared to their experimental counter-
parts. The main exception is the predicted LUMO energies or
ΔSCF(-) values, compared to the electrochemical first
reduction potentials, which are largely dominated by the
localized cyano reduction. DFT calculations predict, incorrectly,
that the LUMOs are strongly delocalized across the entire
oligomer (Supporting Information, Figure S72 for the
tetramers).
We find mean unsigned errors (MUE) between computed
and experimental parameters after the linear regression analysis
to be very low, as illustrated in Figure 5, with ∼0.04 eV MUE
for oxidation potentials (R2 = 0.96), ∼0.04 eV MUE for
reduction potentials, ∼0.07 eV MUE for optical excitation
energies (R2 = 0.89), and ∼10% MUE for optical absorption
extinction coefficients (R2 = 0.94). The high degree of
agreement is not surprising because the sequenced oligomers
define a closely analogous series, and the consensus technique
minimizes systematic and random errors.
On the basis of these consensus models, we predicted
properties of all 20 sequenced hexamers prior to their synthesis
(Supporting Information, Table S5), and found distinct
differences, despite the subtle variation in electronic structure
of the B and A monomers. Br-AABBBA′-CN was predicted to
exhibit the lowest HOMO energy and one of the smallest
optical band gaps, while the complementary sequence Br-
BBAAAB′-CN had a higher HOMO energy and gap. The
DISCUSSION
■
Using an iterative strategy, we prepared multiple unsymmetric
p-phenylene−vinylene oligomers that differ only in sequence.
E:Z selectivity in the key HWE coupling was improved by using
a nitrile precursor for the aldehyde, in lieu of the more
commonly exploited acetal. The product oligomers bear
functional end groups that allow for further elaboration
including potential inclusion as units in a repeating sequence
copolymer. Characterization of these oligomers establishes that
the optical absorption and emission energies and intensities,
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first oxidation potentials, and thermal properties were all
modulated by sequence.
Although the differences in the monomers were modest,
relative to true donor−acceptor pairs, we find a measurable
difference in the sequenced oligomers across a wide range of
characteristics. We predict that much stronger sequence effects
will be found in systems with greater variation between donor
and acceptor monomers, e.g. thiophenes, pyrroles, etc., and are
presently pursuing this direction.
Control of sequence provides an entirely new dimension for
optimization of conjugated materials. In parallel with the
extremely productive strategy of creating novel monomers,
sequence engineering offers a pathway to tailor targeted
properties using existing, synthetically accessible monomers.
Finally, the future correlation of sequence with other properties
of interest, e.g., hole mobility, film morphology, and interfacial
organization, should allow for the rational design of materials
from known monomers that can satisfy the multiplicity of
criteria that are necessary for the performance of these materials
in real-world photovoltaic applications.
Our computational approach predicted the sequence-based
optoelectronic properties of the conjugated oligomers with
outstanding agreement. We were able, as a result, to selectively
prepare longer oligomers with targeted characteristics. In
particular, we both predicted and confirmed by synthesis that
the oxidation potentials and optical excitation energies would
exhibit the following trend: Br-AABBBA′-CN < Br-BABABA′-
CN < Br-BBAAAB′-CN.
As discussed above, to facilitate synthesis and for future
incorporation into polymers, we used Br− and −CN end
groups. One might suppose, that given the subtle difference in
electronic structure between A and B monomers, the variation
in optoelectronic properties is due solely to end group effects
(e.g., the electron-withdrawing ability of CN on A′ and B′) and
not to sequence effects. Instead, sequence generally dominated
over end group effects except with the first reduction potential,
which was dictated by the terminal cyano group. To further
elucidate these effects, calculations were performed on
tetramers and hexamers, both with, and without Br- and -CN
end groups. While some variations in the exact pattern of
sequence effects are found, suggesting both sequence and end
groups have influence, the range of computed HOMO energies
and gaps was retained (i.e., a span of 0.15 and 0.23 eV for
tetramers, with and without end groups, respectively).
Evidence that sequence effects generally dominate over end
groups can also be seen through the comparison of the optical
spectra of specific compounds. The trimers Br-ABA′-CN and
Br-BBA′-CN have almost identical optical band gaps (2.65 and
2.62 eV, respectively). When these trimers are extended with B
and A monomers to form Br-ABAB′-CN and Br-BBAA′-CN,
however, their band gaps narrow but remain nearly identical
despite the addition of different end groups. The tetramers Br-
ABAB′-CN and Br-AABB′-CN have similar spectra (band gaps
of 2.58 and 2.55 eV, respectively) while Br-BAAB′-CN shows a
much higher optical band gap (2.72 eV) despite the fact that all
three tetramers have the same B′-CN end group. In another
example, the pentamers Br-AABBB′-CN and Br-BABAB′-CN
have the same B′-CN end group and exhibit an optical gap
difference of 0.09 eV. When these pentamers are extended to
hexamers by adding an additional A′-CN unit to give Br-
AABBBA′-CN and Br-BABABA′-CN the optical gap differ-
ence almost doubles to 0.17 eV, a clear indication that retaining
identical end groups is not sufficient to determine gaps.
The primary trend observed across all properties and the
computational results, is that the alternating sequences e.g.,
ABABAB or BABABA, are generally neither the highest nor
lowest in any category. A secondary widespread trend is that
the oligomers that bear A monomers in both the first and last
positions tend to exhibit smaller HOMO−LUMO gaps, more
positive first oxidation potentials, and higher melting points.
We also find that among absorption intensities (ε), sequences
that bear B monomers in both the first and last positions (e.g.,
Br-BAAB′-CN or Br-BABAB′-CN) exhibit larger extinction
coefficients than their counterparts. Although we find these
trends, it is important to acknowledge that the use of complex
sequences can lead to synergistic effects, and thus unique
“outliers”. The use of accurate, reliable computational screening
methods makes the identification of these unique sequences
practical.
METHODS
■
Materials. Unless otherwise noted all compounds were purchased
from Aldrich. Anhydrous DMF, nBuLi (1.6 M in hexanes), and
DIBAL-H (1.0 M in hexanes) were dispensed using air-sensitive
techniques. NBS was purchased from Alfa Aesar. Benzoyl peroxide and
NBS were stored at −20 °C. KOtBu was stored in a desiccator over
anhydrous CaSO4. LiCl was purchased from Fisher Scientific and dried
at 120 °C for at least 24 h. Anhydrous diethyl ether for lithiation
reactions was opened immediately prior to use. Reagent grade THF
was used for most reactions; notably the HWE reactions used reagent
grade THF. DCM for reactions was purified by distillation from CaH2
or by passing through a column of alumina. All other reagents and
solvents were used as received. Column chromatography was carried
out on standard grade silica gel (60 Å pore size, 40−63 μm particle
size), which was purchased and used as received. Hexanes,
dichloromethane, and ethyl acetate used for column chromatography
were purchased and used as received. Melting points for all
compounds were determined by DSC and are found in the main
text in Table 4, listed as Tiso.
Synthesis. General HWE Procedure. Aldehyde (Br-A-CHO, Br-B-
CHO, or OPV-CHO) (1 equiv), 4-cyanobenzylphosphonate (P-A-
CN or P-B-CN) (1.5 equiv), and LiCl (2.3 equiv) were dissolved in
THF (12 mL per mmol aldehyde) and cooled to 0 °C under N2.
KOtBu (2.3 equiv) was added portion-wise over 5 min, and the
reactions were allowed to come to room temperature overnight with
stirring. The reaction mixtures were poured into saturated aqueous
NH4Cl (2.5 mL per mL of THF). The aqueous layers were extracted
thrice with EtOAc or CH2Cl2 (equal volume). The combined organic
layers were dried over MgSO4, and the solvent was removed in vacuo.
The residues were purified by column chromatography. Yields and
spectroscopic data for specific oligomers can be found in the
Supporting Information.
General DIBAL-H Reduction Procedure. OPV nitriles (1 equiv)
were dissolved in dry dichloromethane (5 mL per mmol nitrile) and
cooled to 0 °C. DIBAL-H (1.0 M in hexanes, 1.1 equiv) was added
dropwise. The reaction mixtures were stirred at 0 °C for 1 h. Wet silica
(0.4 mL of H2O and 1.3 g of SiO2 per mmol of nitrile) was added and
the mixture was stirred at 0 °C for 1 h. Then, K2CO3 (0.5 g per mmol
of nitrile) and MgSO4 (0.5 g per mmol of nitrile) were added. The
mixtures were filtered and the solids washed with dichloromethane.
The combined filtrate and washes were reduced in volume in vacuo,
and the residues were purified by column chromatography, except as
noted. Yields and spectroscopic data for specific oligomers can be
found in the Supporting Information.
1
Spectroscopy. NMR Spectroscopy. H (300 and 400 MHz) and
13C (75, 100, and 150 MHz) NMR spectra were recorded on Bruker
This initial study is particularly promising despite the highly
similar electronic characteristics of the two monomers.
1
spectrometers. Chemical shifts were referenced to residual H or 13C
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signals in deuterated solvents (7.27 and 77.0 ppm, respectively, for
CHCl3 and 5.32 and 54.0 ppm, respectively, for CH2Cl2).
Mass Spectrometry. HRMS were recorded on EI-quadrupole or
ESI-TOF instruments in the Mass Spectrometry Facility of the
University of Pittsburgh.
ACKNOWLEDGMENTS
■
Funding from the NSF (CHE-0809289) and the University of
Pittsburgh is acknowledged.
Optical Spectroscopy. UV/vis absorption spectra were recorded in
CHCl3 on a Perkin-Elmer Lambda 9 UV/vis/NIR spectrometer.
Solution (CHCl3) and film emission spectra were recorded on a
Varian Cary Eclipse fluorimeter. Films were drop cast on quartz slides
from CHCl3.
Thermal Analysis. DSC was performed on a Perkin-Elmer Pyris 6
with a heating and cooling rate of 10 °C/min.
Electrochemistry. Cyclic voltammetry (CV) and differential pulse
voltammetry (DPV) were recorded on a CHI Electrochemical
Workstation Model 430a (Austin, TX). Data were collected using a
three electrode system consisting of a glassy carbon disk (3 mm dia.)
as working electrode, a nonaqueous Ag/Ag+ reference electrode (1
mM AgNO3 in acetonitrile), and a Pt-wire as auxiliary electrode in 0.1
M Bu4NPF6 in THF freshly distilled from sodium. CV were recorded
at 100 mV/s. DPV parameters were as follows: scan rate of 25 mV/s,
pulse amplitude 0.05 V and pulse period 0.16 s.
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ASSOCIATED CONTENT
■
S
* Supporting Information
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AUTHOR INFORMATION
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Corresponding Author
*E-mail: (G.R.H.) geoffh@pitt.edu; (T.Y.M.) tmeyer@pitt.edu.
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