A versatile and efficient strategy to discrete conjugated oligomers

Article abstract of DOI:10.1021/jacs.7b05299

An efficient and scalable strategy to prepare libraries of discrete conjugated oligomers (D = 1.0) using the combination of controlled polymerization and automated flash chromatography is reported. From this two-step process, a series of discrete conjugated materials from dimers to tetradecamers could be isolated in high yield with excellent structural control. Facile and scalable access to monodisperse libraries of different conjugated oligomers opens pathways to designer mixtures with precise composition and monomer sequence, allowing exquisite control over their physical, optical, and electronic properties.

Full text of DOI:10.1021/jacs.7b05299

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Article
A Versatile and Efficient Strategy to Discrete Conjugated Oligomers
Jimmy Lawrence, Eisuke Goto, Jing M. Ren, Brenden McDearmon, Dong Sub Kim,
Yuto Ochiai, Paul G Clark, David S Laitar, Tomoya Higashihara, and Craig J. Hawker
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b05299 • Publication Date (Web): 05 Sep 2017
Downloaded from http://pubs.acs.org on September 5, 2017
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A Versatile and Efficient Strategy to Discrete Conjugated Oligomers
Jimmy Lawrence†‡, Eisuke Goto†§‡, Jing M. Ren†, Brenden McDearmon†, Dong Sub Kim†, Yuto
Ochiai§, Paul G. Clark#, David Laitar#, Tomoya Higashihara§, and Craig J. Hawker†*
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†Materials Research Laboratory and Departments of Materials, Chemistry and Biochemistry, University of Califor-
nia, Santa Barbara, California, United States. §Department of Organic Materials Engineering, Graduate School of
Science and Engineering, Yamagata University, 4-3-16, Jonan, Yonezawa 992-8510, Japan. #The Dow Chemical Com-
pany, Midland, Michigan 48674, United States.
ABSTRACT: An efficient and scalable strategy to prepare libraries of discrete conjugated oligomers (Đ = 1.0)
using the combination of controlled polymerization and automated flash chromatography is reported. From
this 2-step process, a series of discrete conjugated materials from dimers to tetradecamers could be isolated in
high yield with excellent structural control. Facile and scalable access to monodisperse libraries of different
conjugated oligomers opens pathways to designer mixtures with precise composition and monomer sequence,
allowing exquisite control over their physical, optical and electronic properties.
been developed to access discrete conjugated oligo-
mers, including the linear iterative method involving
INTRODUCTION
successive chain-end functionalization and mono-
mer coupling steps,22 pioneered by Meijer and
Tour.23-25 Similarly, the divergent/convergent cou-
pling strategy involves multiple steps and uses chain
length doubling through selective functionalization
and coupling of bifunctional building blocks to typi-
cally grow dimer, tetramer, octamer, etc. repeat se-
ries.24, 26-28 While such step-wise synthetic methods
have been successful in producing oligomeric mate-
rials with precise length, they remain inefficient for
preparing large scale libraries of discrete oligomers
and are challenging targets for nonexperts (Scheme
1). In contrast, a separation-based strategy employing
reversed-phase HPLC has been reported for a sub-set
of conjugated oligomers, but requires specialized
column and is limited to mg-scales.29
Conjugated polymers are an attractive class of ma-
terials owing to their unique optoelectronic proper-
ties and promising application in organic electronics
and nanoscale assemblies.1-6 While conjugated pol-
ymers are generally synthesized as broadly disperse
mixtures (Đ > 2.0),7 synthetic methods to access con-
jugated polymers with narrow dispersity (Đ < 1.2)
have been developed.8-11 However, narrow dispersity
polymers remain statistical mixture of molecular
weights, with small differences in the degree of
polymerization and/or dispersity having a significant
impact on properties and optical behavior.12-14 This
impact is more pronounced for conjugated materials
with a degree of polymerization (DP) less than 10
repeat units (oligomers), which is comparable to
their effective conjugation length. Below the effective
conjugation length, conjugated oligomers exhibit
strong length-dependent properties with cascade
energy transfer between different species leading to
an ensemble average of optical properties.15-19 As a
result, the performance of conjugated polymers re-
flects a broad distribution of species and not the in-
dividual, discrete components.
Scheme 1. Comparison of Traditional Iterative
Coupling and Oligomer Separation Strategy
For a detailed understanding of the structure-
property relationship of conjugated polymers and to
enable the design of new optoelectronic systems,
discrete oligomers (Đ = 1.0) are important synthetic
targets.20,21 Several step-wise synthetic strategies have
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In addressing this challenge, we report a scalable MALDI-MS analysis of the resulting fractions con-
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and versatile strategy for the multi-gram synthesis of
libraries of discrete conjugated oligomers. This ap-
proach involves the initial synthesis of low dispersity
oligomer mixtures using controlled polymerization
methods, followed by automated separation to afford
multiple discrete oligomers in a single step.30
firmed a stepwise elution of discrete oligothiophenes
from trimer to dodecamer. Significantly, this allows a
full spectrum of discrete o3HT oligomers to be iso-
lated in a single step. These oligomers were fully
characterized using a combination of spectroscopic
and chromatographic techniques (see the Support-
ing Information) with MALDI-MS analysis of the
isolated discrete o3HT samples showing a single mo-
lecular ion that correlates with the expected oligomer
molecular weights and the expected single bromo-
and H-chain ends (Figure 1). As expected, each dis-
crete species is separated by 166 a.m.u., correspond-
ing to the 3-hexylthiophene repeating unit, and iso-
topic analysis of the discrete molecular ions further
reinforced their structural purity. Significantly, from
a 1.4 grams of crude sample, ~500 mg of discrete spe-
cies corresponding to trimer to dodecamer were ob-
tained after a single separation with the remaining
compounds being traces of dimers, mixed samples
and o3HT species with DP > 12.
RESULTS AND DISCUSSION
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To examine the feasibility of this strategy for the
large-scale isolation of discrete conjugated oligo-
mers, 3-hexylthiophene oligomers were initially syn-
thesized using Kumada catalyst-transfer polycon-
densation of
2-bromo-3-hexyl-5-iodothiophene
(o3HT, DP ≈ 8, Figure 1).9 A broad distribution of
species was observed by MALDI-MS analysis of the
crude mixture, despite its low dispersity value ac-
cording to SEC analysis (ĐSEC = 1.3). As the elution of
oligothiophenes hinges on the selection of the ap-
propriate mobile phase, initial solvent mixture
screening was performed using thin-layer chroma-
tography, where an optimized pentane/toluene gra-
dient was found to offer well-resolved separation of
the constituent species (see Supporting Infor-
mation). The crude sample (ca. 1–2 grams) was then
purified on a standard normal-phase silica gel car-
tridge (Biotage®) using pentane/toluene gradient as
eluent.
A major advantage of discrete conjugated oligo-
mers is their length-dependent optoelectronic prop-
erties which allows structure/property relationships
to be accurately determined. This is illustrated for
the o3HT series of oligomers from n = 3 to n = 12,
through analysis of the change in UV-Vis and photo-
luminescence spectra (UV illumination at 365 nm)
with degree of polymerization. As shown in Figure 2,
the absorbance and emission maxima for discrete
oligothiophenes red-shifts with increasing oligomer
length and converge at dodecamer, with the effective
conjugation length determined to be tetradecamer
(Figure S11).31
Figure 2. (a) Discrete o3HT series, tetramer to do-
decamer and the UV/vis spectra of the 4-, 6-, 8-, 10- and
12-mer. (b) Discrete o3HT series, tetramer to do-
decamer under 365 nm UV light and the PL emission
spectra of the 4-, 6-, 8-, 10- and 12-mer.
Figure 1. (Top) Synthesis of oligo(3-hexylthiophene)
(o3HT). (Bottom) MALDI-MS spectra of discrete
o3HTs (n = 4 to 10) isolated after chromatographic sep-
aration.
To illustrate the broad utility of this approach to
other conjugated oligomers, an oligo(fluorene) and
an alternative sequence of discrete oligo(thiophene)
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derivatives were prepared and separated into discrete bone protons also broadens as the oligomer series
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libraries. In the case of oligo(fluorene) (oF), 9,9-
dioctylfluorene was polymerized by Pd/PtBu3-
progresses to the decamer, with the higher molecular
weight oligomers (dodecamer and tetradecamer)
resembling the characteristic spectrum for high mo-
lecular weight poly(3-hexylthiophene). We noted
that while the adopted synthesis protocol gives func-
tional chain-ends, it introduces a tail-tail linkage at
the end or inside the oligomer chain. Nevertheless,
this overall strategy should be applicable to other
synthesis of conjugated material.34
catalyzed
Negishi
catalyst-transfer
polycondensation32 and in a similar fashion to the
thiophene libraries, the oligo(fluorene) mixture
could be separated into a series of discrete species
(each separated by 388.7 a.m.u.), as shown by
MALDI-MS analysis (see Supporting Information,
Figure S5). It is noteworthy that this separation could
also be scaled (~1–2 g) to give the corresponding dis-
crete oligomers in synthetically useful quantities.
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For the bithiophene derivatives, the oligomer series
is tetramer, hexamer, octamer, etc. with a different
arrangement of hexyl side chains along the backbone
when compared to the o3HT series described above.
To access this library, 3,4’-dihexyl-2,2’-bithiophene
was initially prepared by coupling 2-chloromagnesio-
4-hexylthiophene with 2-bromo-3-hexylthiophene
(Figure 3).8 The bithiophene product was then bro-
minated
at
the
2-position
using
N-
bromosuccinimide followed by polymerization using
Ni(dppp)Cl2 catalyst to give crude oligo(bithiophene)
(o3HT2, ~1.4 g, ĐSEC = 1.3–1.4). The relatively narrow
dispersity of o3HT2 is attributed to lower monomer
concentration (ca. 0.04 M) to suppress intramolecu-
lar catalyst transfer.33 Purification using the proce-
dure above affords a library of discrete oligothio-
phene derivatives. MALDI-MS analysis of the frac-
tions showed the isolation of discrete species (DP =
4, 6, 8…) up to the tetradecamer (calculated: MW =
2,408.88, found: m/z = 2,408.80) with excellent iso-
lated yields (see Supporting Information, Figure S6).
As expected, the molecular ion for each discrete oli-
gomer was also separated by 332 a.m.u. which corre-
spond to the 3,4’-dihexyl-2,2’-bithiophene repeat
unit. Isotope analysis of these discrete molecular
ions also confirmed the structural purity of the iso-
lated discrete o3HT2 samples with each oligomer
showing a single set of molecular ions that correlates
with the expected molecular structure (Br- and H-
chain ends).
Figure 3. (a) Synthesis of oligofluorene (oF) and oli-
go(3,4’-dihexyl-2,2’-bithiophene) (o3HT2). (b) H NMR
spectra for the discrete tetramer, hexamer and decamer
o3HT2. All spectra are normalized.
1
In addition to the controlled number of repeat
units, a key feature of these discrete conjugated oli-
gomers is the presence of well-defined chain-ends.
This leads to increased versatility and makes these
materials synthetically powerful building blocks. As
an illustrative example, Stille coupling of the hex-
americ o3HT2 with vinyl tri-n-butyltin affords the
vinyl terminated oligothiophene hexamer, opening
further chemistry (e.g., thiol-ene functionalization)
while retaining the discrete nature of the oligomer.
MS analysis confirmed the molecular weight change
corresponding to the mass difference between the
Br- and vinyl chain ends (53 a.m.u) with NMR spec-
troscopy also showing the expected appearance of
The structural purity of the discrete o3HT and
1
o3HT2 samples was further reinforced by H NMR
analysis, which reveals spectra changes that corre-
spond to the structural evolution from tetramer to
decamer. In all cases, as the oligomer size increases,
the relative intensity of aromatic and aliphatic back-
bone protons increases in a linear fashion when
compared with the chain-end protons (Figure 3b).
For example, the methylene signal adjacent to the
thiophene ring (2.63–2.82 ppm) gradually shifts
downfield and the overall peak shape for the back-
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resonances for the vinyl group (Figure 4, see the
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Supporting Information).
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Figure 4. (Top) Synthesis of vinyl terminated o3HT2
hexamer. (Bottom) MS spectra of o3HT2 hexamer
(black) and vinyl-o3HT2 hexamer (blue).
Figure 5. Comparison of photoluminescence for dis-
In addition to oligomer lengths having a significant
impact on optical performance, molecular weight
dispersity can also play a role in defining physical
properties. This can be easily observed in the photo-
luminescence (PL) color difference between hexamer
samples that have discrete (green) and disperse (yel-
low) molecular weights. In this case, the redshifted
appearance of the disperse sample is attributed to
ensemble distribution of species and cascade energy
transfer between the different oligomer lengths pre-
sent in the mixture. Access to a library of discrete
oligomers then opens the intriguing possibility of
preparing artificial oligomer blends with precise con-
trol of both DP and dispersity leading to unique and
tunable optical properties. Using a library of discrete
oligothiophenes with PL color progressing from blue
to orange-red, a wide range of artificial mixtures can
be simply prepared by direct mixing to obtain mate-
rials with unique optical properties. For example,
mixing a 3:1 molar ratio of the discrete oligothio-
phene tetramer (DP = 4) and dodecamer (DP = 12)
affords a blend that is nominally a hexamer, DP = 6.
More importantly, this artificial blend leads to near-
white emission which is in striking contrast to the
bright green emission of the discrete hexamer and
yellow emission of the as-prepared, disperse hex-
amer. Of equal significance is that a series of ‘artifi-
cial’ oligothiophene hexamer samples can be pre-
pared by mixing the tetramer with varying amounts
of octamer, decamer and dodecamer (Figure 5).
From these isomeric materials, different emissions
and variable tuning of the optical properties can be
obtained. This ability to use mixtures of discrete oli-
gomers to control properties underlines the simplici-
ty of this strategy and the influence of dispersity on
the properties of conjugated oligomers.
crete, disperse and artificial mixtures of oligothiophene
with the same average molecular weight (DP = 6) but
varying dispersity (*: ĐSEC).
Besides providing an efficient access to precision
blends, a library of discrete oligomers can be used to
construct a set of novel materials with precise struc-
tures, for example isomers with specific monomer
sequence. To demonstrate this, we examined the ef-
fect of positioning acceptor-type benzothiadiazole
(BT) at different locations within the backbone of a
donor-type 3-hexylthiophene (3HT) octamer. Three
isomers were prepared: (3HT)8-BT (D8A), (3HT)6-
BT-(3HT)2
(D6AD2),
and
(3HT)4-BT-(3HT)4
(D4AD4). Purification of these asymmetric and
symmetric donor-acceptor materials using a combi-
nation of normal-phase and reversed-phase chroma-
tography afforded pure materials with distinct opti-
cal properties (Figure 6). Specifically, as the series
progresses from D8A to D4AD4, absorbance peak
splitting occurs (unimodal to bimodal), and the on-
set of absorption redshifts while the absorbance
maxima blue-shifts, in agreement with time-
dependent DFT calculations (Figure 6, see the Sup-
porting Information). Interestingly, the small change
in the location of the BT unit from D8A to D6AD2,
results in a large emission difference (yellow, 583 nm
to red, 658 nm), which not only reinforces the signif-
icance of structural purity and monomer sequence
for tuning material properties, but also highlights
the potential utility of asymmetric materials for tar-
geted applications.
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Author Contributions
1
2
‡ J.L. and E.G contributed equally.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
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9
The authors thank the MRSEC program of the National
Science Foundation (DMR 1121053, C.J.H.), the Dow
Chemical Company through the Dow Materials Insti-
tute at UCSB (J.L., C.J.H.), and the Institute for Collabo-
rative Biotechnologies through grant W911NF-09-0001
from the U.S. Army Research Office (C.J.H.). The con-
tent of the information does not necessarily reflect the
position or the policy of the U.S. government, and no
official endorsement should be inferred. E.G. thanks
the support by Grant-in-Aid from JSPS, Research Fel-
lowship for Young Scientists (15J00430) and Innovative
Flex Course for Frontier Organic Material Systems
(iFront) at Yamagata University. J.M.R. thanks the Vic-
torian Endowment for Science, Knowledge and Innova-
tion (VESKI) for a postdoctoral fellowship.
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Figure 6. Top: asymmetric and symmetric donor-
acceptor materials, D8A, D6AD2 and D4AD4 (D=3-
hexylthiophene, A=benzothiadiazole). Bottom: UV-vis
and PL spectra of D8A (black), D6AD2 (blue) and
D4AD4 (red).
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