Solution processable polydiacetylenes (PDAs) through acyclic enediyne metathesis polymerization

Article abstract of DOI:10.1039/c3sc51264a

Novel polydiacetylenes (PDAs) bearing alkyl and phenyl substituents have been synthesized, for the first time, by solution polymerization using acyclic enediyne metathesis. The resulting polymers are soluble in common organic solvents and show distinct ph

Full text of DOI:10.1039/c3sc51264a

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Solution processable polydiacetylenes (PDAs) through
acyclic enediyne metathesis polymerization†
Cite this: Chem. Sci., 2013, 4, 3649
a
b
b
a
*
*
Keda Hu, Haishen Yang, Wei Zhang and Yang Qin
Novel polydiacetylenes (PDAs) bearing alkyl and phenyl substituents have been synthesized, for the first
time, by solution polymerization using acyclic enediyne metathesis. The resulting polymers are soluble in
common organic solvents and show distinct physical and photophysical properties both in solution and
as thin films, caused by different steric and electronic effects from the side-groups. Bulk heterojunction
solar cells employing these PDAs have been fabricated and evaluated.
Received 7th May 2013
Accepted 2nd July 2013
DOI: 10.1039/c3sc51264a
www.rsc.org/chemicalscience
Notably, there have been efforts in building short-chain PDA
analogs having a few eneyne repeating units through iterative
Introduction
Since the rst report on the solid-state preparation of poly-
diacetylene (PDA) in 1969,1 extensive research efforts have been
devoted to this classic type of conjugated polymers.2 PDAs are
considered quasi one-dimensional semiconductors having
macroscopic long-range coherence and anisotropy.3 These
materials undergo intriguing optical changes among blue, red
and yellow phases upon exposure to external stimuli including
temperature,4 solvents,5 pressure,6 light exposure7 and specic
biological analytes.8 PDAs have found widespread applications
in non-linear optics,9 organic conductors10 and sensory mate-
rials.11 Despite a large number of examples, PDAs reported to
date have been synthesized exclusively from a single type of
reaction, i.e. irradiation induced topochemical polymerization
of diacetylene (DA) monomers in crystals, lms, gels and
micellar structures.2a Such reactions occur efficiently only if the
DA monomers are able to pack into periodic structures within
very strict geometrical boundaries.12 Flexibility around diac-
etylene moieties is required to accommodate spatial rear-
rangement when two sp hybridized carbon atoms in every DA
monomer change to sp2 hybridization in resulting PDA
chains.13 As a result, few types of DA monomers, typically having
alkyl spacers between DA cores and functional side-groups, can
be successfully polymerized.2 Electronic properties of resulting
PDAs are thus difficult to modify by substituent variation.14
Moreover, the obtained PDAs usually have limited solubility,
making integration of these materials into solution-fabricated
electronic devices challenging.
condensation reactions and the resulting oligomers serve as
discrete models for studying corresponding long-chain polymer
properties.15 Although these synthetic methods have led to well-
dened oligomers with precise structural control, high molecular
weight polymers are difficult to achieve using these step-by-step
approaches.
We report here the synthesis of solution processable PDAs
through acyclic enediyne metathesis polymerization16 of methyl
terminated trans-enediyne17 monomers bearing different substit-
uents. Physical and electronic properties of the resulting PDAs are
shown both experimentally and theoretically to depend on the
nature of the side-groups. To the best of our knowledge, our
method represents the rst example of PDA synthesis by solution
polymerization and of alkyne metathesis on enediyne substrates.
Results and discussion
The synthesis of enediyne monomers bearing n-undecyl and
4-tert-butylphenyl substituents, and their metathesis polymeri-
zation are summarized in Scheme 1; detailed experimental
procedures and characterization data are included in the ESI.†
Notably, compounds 3a and 3b, obtained, respectively, from
McMurry coupling reactions of 2a and 2b, have trans congura-
tions in the central double bonds due to the bulky triisopro-
pylsilyl moieties.18 Solution polymerization of M-a and M-b was
initially attempted using Mo(CO)6 and various phenolic ligands,19
as well as the well-dened Schrock alkylidyne complex
(tBuO)3WCCMe3,20 as catalysts under conditions that have been
successfully applied in alkyne metathesis and preparation of
poly(aryleneethynylene)s. No polymerization was detected,
however, as no color change was observed and only starting
materials were recovered aer prolonged reaction times. This
lack of reactivity may be attributed to enhanced conjugation and
thus higher stability of triple bonds in these trans-enediyne
molecules relative to those in isolated and aromatic alkynes.
aDepartment of Chemistry
& Chemical Biology, University of New Mexico,
MSC03-2060, 1 UNM, Albuquerque, NM 87131, USA. E-mail: yangqin@unm.edu
bDepartment of Chemistry & Biochemistry, University of Colorado Boulder, 215 UCB,
Boulder, CO 80309, USA. E-mail: wei.zhang@colorado.edu
† Electronic supplementary information (ESI) available: Synthetic details, SEC
proles, thermochromism studies, DFT calculations, DSC curves, XRD proles,
device data and NMR spectra. See DOI: 10.1039/c3sc51264a
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Fig. 1 13C NMR spectra (aromatic region) of M-a, M-b, PDA-CH and PDA-Ph.
Scheme 1 Synthesis of monomers and polymers.
synthesis. However, no polymeric ions could be observed
presumably due to the non-polar nature of these polymers.
UV-Vis absorption and uorescence measurements are per-
formed on dilute chloroform solutions of the polymers (2.5 ꢀ
10ꢁ4 M r.p. units) with gradual additions of methanol up to 58%
by volume, beyond which both polymers start to precipitate. As
shown in Fig. 2, PDA-CH has a lmax of 430 nm in chloroform,
blue-shied by ca. 30 nm from typical values for solutions of
yellow-chain PDAs bearing side-chains capable of non-covalent
interactions, which have been shown to adopt “Porod–Kratky”
worm-like conformations in solution.25 The blue-shied absorp-
tion of PDA-CH thus suggests a more exible backbone and
shorter persistence and effective conjugation lengths, presum-
ably due to the absence of intramolecular non-covalent
We have recently developed a well-dened and highly active
alkyne metathesis catalyst [Mo] obtained by treating the pre-cata-
lyst [Mo-1] with triphenolsilane ligand L in situ, as shown in
Scheme 1.21 By using such podand ligand design,22 one of the two
open substrate-binding sites is blocked and small alkyne poly-
merization side reactions are completely suppressed. Both M-a
and M-b were successfully polymerized in CCl4 using ca. 7–8 mol%
˚
in situ generated [Mo] in the presence of 5 A molecular sieve
powders (scavenger for the 2-butyne by-product)23 to give respec-
tive PDA-CH and PDA-Ph in high yields as red solids. Molecular
weights of these polymers were estimated against polystyrene
standards from size exclusion chromatography (SEC, Fig. S1†) to
be Mn ¼ 15 600, PDI ¼ 1.9 and DP (degree of polymerization) ¼ 47
for PDA-CH; and Mn ¼ 10 200, PDI ¼ 1.4 and DP ¼ 32 for PDA-Ph.
Chemical structures of both polymers were conrmed by 1H and
13C NMR spectroscopy24 as shown in Fig. 1 and ESI.† As shown in
Fig. 1, two acetylenic 13C signals are observed at 79.2 and 93.6 ppm
for M-a and 81.0 and 94.5 ppm for M-b. Upon polymerization,
each set of signals downeld shis and merges into a single signal
at 99.3 ppm for PDA-CH and 99.9 ppm for PDA-Ph, respectively.
Such observation conrms the successful polymerization leading
to symmetrically substituted triple bonds along polymer back-
bones. The fact that the acetylenic chemical shi values for both
polymers are close to 100 ppm, a value previously predicted by
extrapolating oligomer data toward innite chain length,15d
conrms high molecular weights for both polymers. 13C signals of
double bonds in both polymers are also slightly downeld shied
from those in corresponding monomers due to enhanced conju-
gation. Additionally, in the 1H NMR spectra of both polymers
(ESI†), signals corresponding to terminal methyl groups as
observed in the spectra of respective monomers disappear, sug-
gesting relatively high molecular weights for the polymers. We
have also performed matrix-assisted laser desorption–ionization
time-of-ight (MALDI-TOF) mass spectrometry analysis in order to
Fig. 2 UV-Vis absorption (normalized, A and C) and fluorescence (B and D)
spectra of PDA-CH (A and B) and PDA-Ph (C and D), respectively, in chloroform
solutions (2.5 ꢀ 10ꢁ4 M r.p. units) with gradual additions of methanol. Black
arrows indicate increasing methanol volume fractions at 0, 17, 29, 40, 49 and
probe the possibilities of ring-formation during the polymer 58%.
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interactions.26 On the other hand, PDA-Ph shows a lmax at
Similar trends are observed in thin lms of PDA-CH and
500 nm, a value close to those of red-chain PDAs. Upon gradual PDA-Ph as shown in Fig. 3.28 Compared with solution data, lmax
additions of methanol, lmax's of PDA-CH slightly red shi and a of as-cast PDA-CH thin lm red shis to ca. 475 nm and the
shoulder at ca. 525 nm becomes apparent, indicating the shoulder peak at 525 nm becomes more pronounced. Three
formation of red-chain species.4b Fluorescence signals (lexc
¼
emission peaks (lexc ¼ 460 nm) are observed at 482, 568 and 610
430 nm, lem ¼ 515 nm, chloroform) also red shi and become nm. The 482 nm peak blue shis and decreases in relative
more structured (lem ¼ 495, 555 and 605 nm, 58% methanol). intensity, when compared with solution emission proles, and
This yellow to red transition in PDA-CH is attributed to aggre- the other two both red shi and increase in intensities. Aer
gation and planarization of the polymer main-chain, leading to annealing the same lm at 155 ꢂC for 10 min, the 525 nm
enhanced delocalization and longer conjugation lengths.
absorption shoulder peak becomes more resolved, accompa-
On the other hand, only a slight blue shi of lmax up to 10 nm nied by a decrease in relative intensity of the 482 nm peak as
and no apparent change in uorescence (lem ¼ 580 nm, lexc ¼ 500 well as slight red shi and increase in relative intensities of the
nm) are observed for PDA-Ph (Fig. 2C and D). This is presumably peaks at 570 and 615 nm. Raman scattering measurements
caused by the presence of bulky tert-butylphenyl substituents that show identical signals at 1525 and 2121 cmꢁ1 for as-cast and
prevent close packing and planarization of PDA chains. Aggrega- annealed lms. These frequencies are due to respective double
tion may even force the main-chain to be more twisted, leading to and triple bond stretches and correspond to red–yellow-chain
the observed blue shi in absorption. Such aggregation behavior PDA species.29 Thus, the red-shi in emission spectra of
is also observed in thermochromism studies as shown in Fig. S3.† annealed lm is likely due to conformational change and
While the THF solution of PDA-CH shows red-shied lmax's and ordering of the side-groups without perturbing signicantly the
pronounced absorption shoulders at ca. 530 nm at low tempera- main-chain structures and bond orders.30 Exact nature of the
tures, blue-shied and completely featureless absorption prole is yellow- and red-chain species requires further investigation.
observed at 70 ꢂC. On the contrary, only a slight red-shi in lmax
(ca. 10 nm) is observed for PDA-Ph in THF by going from 70 to tophysical properties as in solutions (Fig. 3B). lmax blue shis
ꢂC, and no shoulder peaks can be observed. Thus, the red slightly to 485 nm in as-cast lm and further to 480 nm in thermal
As expected, thin lms of PDA-Ph show almost identical pho-
0
appearance of PDA-Ph is most likely due to modication of the annealed lm. Fluorescence proles in both as-cast and annealed
main-chain electronic states through direct conjugation of the lms show no difference with those in solutions and identical
aromatic substituents, even though the main-chain conformation Raman scattering is observed for both lms. The observed blue
may still be in a “yellow form”. Fluorescence quantum yields in
dilute chloroform solutions are estimated to be 0.1% for PDA-CH
and 0.09% for PDA-Ph using quinine bisulfate (in 0.1 M H2SO4) as
a standard.15k Such low quantum efficiencies are consistent with
literature reported values for yellow-chain PDAs.27
The observed side-group effects are elucidated by density
functional theory (DFT) calculations (B3LYP, 6-31G(d)) per-
formed on trimeric model compounds bearing methyl (TriDA-
Me) and phenyl (TriDA-Ph) groups (Fig. S4†). TriDA-Ph has a
bandgap of 2.6 eV, 0.5 eV smaller than that of TriDA-Me,
consistent with the longer wavelength absorptions by PDA-Ph.
TriDA-Me shows a completely planar optimized geometry while
TriDA-Ph has a slightly bent backbone with twisted phenyl
substituents, indicating difficulties in close packing of polymer
chains in the case of PDA-Ph. Indeed, differential scanning
calorimetry (DSC) measurements on both polymers show
distinct thermal behaviors (Fig. S5†). Multiple melting transi-
tions are observed for PDA-CH, indicating high crystallinity in
the solid state. The transitions at ca. 15 ꢂC and 40 ꢂC are
ꢂ
assigned to side group melting and that at ca. 140 C to main-
chain melting. On the contrary, PDA-Ph shows no melting
behavior up to 250 ꢂC with only a glass transition observed at ca.
155 ꢂC (onset), indicating its amorphous nature. The crystalline
nature of PDA-CH is further conrmed by powder X-ray
diffraction (XRD) as multiple sharp scattering peaks are
observed (Fig. S6†). The lamellar distance is calculated to be
ꢂ
˚
30.6 A (2q ¼ 2.84 ), corresponding to stacking of fully stretched
C
11H23 side-chains without interdigitation. On the other hand,
Fig. 3 UV-Vis absorption, fluorescence and Raman spectra of thin films of PDA-
CH (A) and PDA-Ph (B) drop-cast from chloroform solutions. Data for both as-cast
films and the same films annealed at 155 ꢂC for 10 min are presented.
only two broad peaks are observed in the XRD prole of PDA-Ph
˚
(Fig. S7†), giving a lamellar packing distance of ca. 16 A.
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shi in absorption is likely due to ordering of the bulky side-
groups that leads to conformations favoring slightly more twisted
main-chain geometries. The double bond stretch of PDA-Ph at
1478 cmꢁ1 is surprisingly in the range of typical “blue-chain”
PDAs.29 This is likely due to electron delocalization from double
bonds to the phenyl side-groups, thus reducing electron density
and bond strengths, causing reduction in vibrational frequencies.
There have been increasing interests in using conjugated
polymers for organic electronic devices through low-cost high-
throughput solution processes.31 Existing PDAs have rarely been
applied in electronic devices due to limited solubility and
tunability in electronic properties. Our methodology provides a
facile way to produce soluble PDAs having different side-groups
directly attached to the main-chains. For a proof of concept, we
have fabricated bulk heterojunction (BHJ) organic solar cells
using PDA-CH and PDA-Ph, respectively, blended with equal
weight of phenyl-C61-butyric acid methyl ester (PCBM) as active
layers. A conventional device structure of ITO/MoO3/active
layer/Al is applied and device performances are summarized in
Fig. S9.† The best devices are made from PDA-CH, giving on
average a short circuit current (JSC) of 0.44 mA cmꢁ2, an open
circuit voltage (VOC) of 0.6 V, a ll factor (FF) of 35% and a power
conversion efficiency (PCE) of 0.09%. Devices made from PDA-
Ph showed reduced JSC, thus lower PCEs, likely caused by the
polymer's amorphous nature. Both polymers out-performed, by
more than 100-fold, the only previous example of solar cells
employing PDAs from solid-state polymerization.32 We are
currently optimizing cell performance and studying charge
generation mechanisms in these PDA devices in more detail.
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Acknowledgements
The authors would like to acknowledge University of New
Mexico and NSF (DMR-1055705) for nancial support for this
research. National Science Foundation is acknowledged for
supporting the NMR facility at UNM through grants CHE-
0840523 and 0946690.
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