Self-Assembly of Chiral-at-End Diketopyrrolopyrroles: Symmetry Dependent Solution and Film Optical Activity and Photovoltaic Performance

Article abstract of DOI:10.1002/chem.201802610

Chiral thiophene-diketopyrrolopyrrole derivatives have been synthesised to investigate the potential of stereochemistry and symmetry as a means of modulating properties by influencing self-assembly of these purely organic materials. In particular, derivatives of diketopyrrolopyrrole were employed because of their proven interest as dyes, especially for organic solar cells. The natural product myrtenal was used as the source of stereochemistry, introduced through a Kr?hnke reaction of a thiophene-bearing pyridinium salt and diketopyrrolopyrroles were prepared through Suzuki coupling with this chiral moiety at one end only as well as at both ends. Absorption spectroscopy and electrochemistry confirmed the potential suitability of the compounds for photovoltaic devices. The nanostructures formed by the compounds have been probed with circular dichroism spectroscopy in solution and in films. It is shown that a chiral C₂ symmetric molecule assembles in solution giving a strong circular dichroic signal while as a film this optical activity is nulled, whereas an asymmetric homologue is most optically active as a thin film. The X-ray crystal structure of the asymmetric compound shows a polar order of the molecules that might explain this observation. The lack of optical activity in solution is very likely a result of the high solubility of the compound. The results reaffirm the sensitivity of circular dichroism spectroscopy to inter-chromophore organisation, whereas absorption spectroscopy in the visible region reveals only slight changes to the bands. The differing order in the compounds also affects their performance in bulk heterojunction photovoltaic devices. Atomic force microscopy of the blended thin films with the fullerene derivative usually employed (PC₆₁BM) showed that smooth and well mixed films were achieved, with the conditions required during spin coating depending greatly on the derivative, because of their differing solubility. The apparently better performance of the symmetrical compound (although with very low efficiency) is probably a result of the alignment of the molecules inferred by the circular dichroism experiments, whereas the asymmetric compound presumably adopts a twisted supramolecular organisation.

Full text of DOI:10.1002/chem.201802610

A Journal of
Accepted Article
Title: Self-Assembly of Chiral-at-End Diketopyrrolopyrroles : Symmetry
Dependent Solution and Film Optical Activity and Photovoltaic
Performance
Authors: David Brian Amabilino, Paul Hume, Roderick MacKenzie,
James Monks, Flavia Pop, and Stephen Davies
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To be cited as: Chem. Eur. J. 10.1002/chem.201802610
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Self-Assembly of Chiral-at-End Diketopyrrolopyrroles : Symmetry
Dependent Solution and Film Optical Activity and Photovoltaic
Performance
Paul A. Hume, *[a, ǂ] James P. Monks,[a,b] Flavia Pop,[a, ₸], E. Stephen Davies, Roderick C.I.
[a]
[c]
[a,b]
MacKenzie and David B. Amabilino*
Abstract: Chiral thiophene-diketopyrrolopyrrole derivatives have
been synthesised to investigate the potential of stereochemistry and
symmetry as a means of modulating properties by influencing self-
assembly of these purely organic materials. In particular, derivatives
of diketopyrrolopyrrole were employed because of their proven
interest as dyes, especially for organic solar cells. The natural product
myrtenal was used as the source of stereochemistry, introduced
through a Kröhnke reaction of a thiophene-bearing pyridinium salt and
diketopyrrolopyrroles were prepared through Suzuki coupling with this
chiral moiety at one end only as well as at both ends. Absorption
spectroscopy and electrochemistry confirmed the potential suitability
of the compounds for photovoltaic devices. The nanostructures
formed by the compounds have been probed with circular dichroism
usually employed (PC61BM) showed that smooth and well mixed films
were achieved, with the conditions required during spin coating
depending greatly on the derivative, because of their differing
solubility. The apparently better performance of the symmetrical
compound (although with very low efficiency) is probably a result of
the alignment of the molecules inferred by the circular dichroism
experiments, whereas the asymmetric compound presumably adopts
a twisted supramolecular organisation.
Introduction
While chiral materials show remarkable self-assembled
nanostructures,[1,2] in the context of functional materials for
organic photovoltaic devices (OPVs) the influence of symmetry
and stereochemistry possessed by molecular or polymeric
components does not show any consistent trends.[3-8] The degree
and nature of molecular order in the active layer of OPVs is a vital
spectroscopy in solution and in films. It is shown that a chiral C
2
symmetric molecule assembles in solution giving a strong circular
dichroic signal while as a film this optical activity is nulled, whereas an
asymmetric homologue is most optically active as a thin film. The X-
ray crystal structure of the asymmetric compound shows a polar order
of the molecules that might explain this observation. The lack of
optical activity in solution is very likely a result of the high solubility of
the compound. The results reaffirm the sensitivity of circular dichroism
spectroscopy to inter-chromophore organisation, whereas absorption
spectroscopy in the visible region reveals only slight changes to the
bands. The differing order in the compounds also affects their
performance in bulk heterojunction photovoltaic devices. Atomic force
microscopy of the blended thin films with the fullerene derivative
factor influencing performance,[9-16] where supramolecular
arrangement[
17]
influences morphology, exciton splitting and
charge carrier mobility.[18] In contrast to other parameters, such as
redox and optical characteristics, the molecular order in the active
layer remains hard to control. In addition to π-π stacking of
chromophores, current strategies to improve device performance
targeting molecular arrangement utilise hydrogen bonding,[
dipolar interactions[22] and other supramolecular approaches.[23-25]
The goal of this work is to prepare small molecule chiral materials
of different symmetries that have well defined structures that can
be probed precisely spectroscopically, and with potential for use
in OPVs. It is hoped that the lessons learned from these materials
will ultimately lead to improved device performance through
symmetry controlled stereoselective self-assembly.
19-21]
[
a]
Dr. P.A. Hume, Mr. J. Monks, Dr. F. Pop, Mr. E.S. Davies and Prof.
D.B. Amabilino
School of Chemistry
University of Nottingham
University Park, NG7 2RD, UK
E-mail: david.amabilino@nottingham.ac.uk
Mr. J. Monks, Dr. F. Pop and Prof. D.B. Amabilino
The GSK Carbon Neutral Laboratories for Sustainable Chemistry
University of Nottingham,
Jubilee Campus, Triumph Road, Nottingham, NG7 2TU, UK
Dr. R.C.I. MacKenzie
[
[
b]
c]
Faculty of Engineering
University of Nottingham
University Park, NG7 2RD, UK
ǂ
₸
Present address: School of Chemical Sciences, The University of
Auckland, 23 Symonds Street, Auckland, 1010, New Zealand
E-Mail: p.hume@auckland.ac.nz
Present address: Laboratoire MOLTECH-Anjou, UMR 6200 CNRS
UFR Sciences, Bât. K, 2 bd. Lavoisier, 49045 Angers, France
Figure 1. General chemical structure of the DPP chromophore and possible
positions for chiral substituents, lateral in blue ("side" substitution) and main
chain in red ("end" substitution when the stereogenic centre is at the extreme of
the red symbol).
Supporting information for this article is given via a link at the end of
the document.
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Materials based on 1,4-diketopyrrolo[3,4-c]pyrrole (DPP,
Figure 1) have gained considerable interest as active layer
constituents in OPVs and organic field-effect transistors (OFETs)
as a result of their high charge-carrier mobility, favourable
photophysical properties, synthetic accessibility and stability.[26-29]
The DPP core exhibits a high tendency to aggregate through π-π
interactions and hydrogen bonding between adjacent lactam
moieties.[30-33] The lactam unit is therefore commonly masked with
an alkyl group, although unprotected DPP can improve device
performance through increased charge mobility compared with
the fully substituted compounds.[34] DPP based materials retaining
solubility whilst possessing a tendency to self-assemble with a
high degree of molecular order are therefore appealing.
structure on self-assembly for two novel chiral-at-end
diketopyrrolopyrrole dyes.
Results and Discussion
Synthesis. Compound 3 (Scheme 1), which could be covalently
linked with the DPP core via cross-coupling, was chosen because
of the rigidity of the aliphatic framework derived from
(
1R)-(−)-myrtenal that might minimise conformational defects in
the eventual materials (more flexible chains can adopt many
polymorphic conformations). In addition, the thiophene and
pyridine units would extend the π-conjugation length in the
molecule, decreasing the optical energy-gap. Compound 3 was
constructed by Kröhnke reaction of pyridinium salt 2 (made from
[
35]
Symmettry can also play a role in this ordering. We wished to
explore the attachment of chiral units at the end positions of bis
thiophen-2-yl-DPP. Here we report the influence of molecular
) and (1R)-(−)-myrtenal.[36,37]
1
Scheme 1. Synthesis of chiral DPPs 7 and 9.
Conversion of 3 to bromide 4 and boronic ester 5 was
achieved by N-bromosuccinimide bromination and Hartwig
borylation, respectively.[38] Clean formation of boronate 5 was
judged by analysis of the crude reaction mixture, which was
engaged directly in the subsequent reaction. Suzuki
cross-coupling of known dibromide 6 and boronate 5 gave 7 as
a blue solid. To construct mono-coupled DPP 9, 8 was reacted
with a single equivalent of bromide 4 under CH-functionalisation
conditions.[40] Although the reaction was not selective for the
formation of the mono-adduct, 9 was isolated in 18% yield as a
purple solid.
Optical Properties. The optical properties of 7 and 9 in
solution were studied by UV-visible absorption and steady-state
fluorescence spectroscopy (Figure 2, 8 is included for comparison
and key parameters are listed in Table 1). The strengths of
electronic transitions were assessed both by the extinction
coefficients at the absorption maxima and by integration of the
absorption band. The integrated absorption coefficients provided
estimates of the square modulus of the relevant transition dipole
2
moments |흁푔푒| . The optical energy-gap (E
g
) was estimated using
the onset of absorption wavelength. The absorption spectra of
compounds 7, 8 and 9 show structured low energy bands, the
positions of which are red shifted in line with increasing
chromophore conjugation.
The absorption strength associated with the HOMO-LUMO
transition also increases with conjugation. The fluorescence
spectra of 7, 8 and 9 are structured. The fluorescence quantum
yield of 9 is ca. 20% and has the largest Stokes’ shift of the series,
indicating more significant structural differences occur between
the ground and excited states. The fluorescence quantum yield
for 7 (30%) is slightly higher than that of 9 but both are significantly
lower than that of 8 (ca. 60%). The thin-film absorption spectra of
[
39]
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compounds 7 and 9 exhibited significant broadening of the
absorption bands and development of a low energy shoulder
+
Table 2. Half-wave potentials relative to Fc /Fc for the first oxidation and
reduction of compounds 7-9. [
a]
(
located at ~690 nm and ~650 nm for 7 and 9 respectively, see
Supporting Information). Interestingly, the peak absorption for
compound 9 occurs at 547 nm in the thin film, with a spectral
profile reminiscent of perylene H-aggregates where the
chromophores are rotationally offset with respect to one
another.[
red
ox
red
ox
E
1/2 (V)
E
1/2 (V)
E
onset (V)
E
onset (V)
8
9
7
−
1.60
+0.51
(0.07)
+0.42
(0.08)
+0.34
(0.12)
−1.49
−1.36
−1.32
+0.43
+0.31
+0.21
(0.07)
41]
−1.49
(
0.08)
1.44
(0.10)
−
Table 1. Optical properties of compounds 7-9 in solution.[a]
2
[
c]
[
b]
|흁푔푒|
E
g

em

fl
[a] Potentials reported as E1/2 (= (E a
+ E c
+
-1
ʎ
max
ε
max
p
p
)/2) in V vs. Fc /Fc at 0.1 Vs scan
-
1
-1
(D2)
a
(nm)
(M cm )
rate and quoted to the nearest 0.01 V. Values in parentheses are E (= E
p
E ) for the couple at 0.01 Vs . E(Fc /Fc) was 0.07 V under these
p
(
eV)
(nm)
(%)
c
-1
+
–
8
9
7
549
579
647
32900
49000
61400
46
2.18
1.94
1.81
676
559
560
62
20
30
conditions.
89
129
Electronic Structure Calculations. Singlet gas-phase geometry
[
a] Spectra recorded as solutions in dichloromethane at 20 °C. [b] First
optimisations and subsequent TD-DFT calculations with the
B3LYP hybrid functional and the 6-311G(d,p) basis set[42-44] gave
absorption maxima. [c] Determined according to 1240/ʎ(onset) (nm).
calculated orbital eigenvalues and positions of the absorption
maxima relatively close to those obtained from experiment (see
Supporting Information). Generally, the higher energy absorptions
could not be assigned to single orbital transitions due to several
contributions to the configuration-interaction (CI) expansions of
the excited states. However the peak at 381 nm in the absorption
spectra of 7 could be well approximated as arising from the
HOMO−1⟶LUMO+1 transition as this excitation possessed a CI
expansion coefficient of 0.65, with the next largest coefficient
being 0.11. In this case, both contributing orbitals are primarily
localised on thiophene units.
X-Ray Crystallography. Growth of single crystals of DPP 9
allowed a study of the position and conformation of the chiral
group in the solid state by X-ray diffractometry. Despite significant
effort. Compound 9 formed polar crystals pertaining to the triclinic
space group P1 (Figure 3). The terminal thiophene ring exists in
two distinct geometries with respect to the aromatic core and the
structure was refined accordingly. Two dimensional layers are
formed, with successive layers separated by a region populated
by the hexyl chains and the myrtenal-derived aliphatic framework.
Each molecule has both π-π stacking (3.4-3.6 Å) and lateral
contacts with approximately coplanar neighbours (2.4-2.9 Å).
Slipped packing is observed, with adjacent lactam units
interacting with thiophene rings on opposing faces of the DPP
core.
Figure 2. Absorption (solid lines) and emission (dashed lines) spectra of
compounds 7-9 as solutions in CH Cl .
2 2
Redox Properties. Cyclic voltammetry studies for 7, 8 and 9
show two oxidation processes and either one (8) or two (7 and 9)
reduction processes for each compound in the potential range
+
between ca. +1.00 and -2.10 V versus Fc /Fc (Table 2). The first
oxidation and reduction processes appear as redox couples in a
range of scan rates between 0.02 and 0.30 Vs . For 8 and 9 these
Self-Assembly. Self-assembly of the compounds was probed
using circular dichroism (CD) spectroscopy, in which molecular
solutions of 7 and 9 in THF have weak negative Cotton effects at
240 nm and 238 nm, respectively, with no other significant
features (see supporting information). Several solvents (MeOH,
MeCN, methylcyclohexane) and combinations thereof were
examined to test their ability to induce aggregation, and at
-1
reductions are essentially reversible, however the oxidations
show variation in E (= E a – E c
) with scan rate greater than that
p p
observed for ferrocene under identical conditions. Hence, these
couples appear electrochemically quasi-reversible. The CVs
show 9 to have a significantly smaller energy-gap than that of 8
due to lower potentials for both reduction and oxidation processes. concentrations suitable for CD measurements (~10-5 M).
DPP 7 follows the same trend with a further decrease in the band-
gap.
Aggregation was only detected for 7 when a solution of the
compound in THF was injected into a mixture of THF and
methanol so the final ratio of THF:MeOH was 7:3 (c = 2 × 10−5 M).
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In addition, there is a bisignate band centred at 372 nm. The
assignment of the corresponding absorption peak using TD-DFT
indicated coupling between stacked thiophene units, rotated
relative to one another. The CD signal is lost upon heating of the
solution to 60 ºC and is not recovered when the solution was
cooled to 20 ºC, indicating that the aggregates are formed under
kinetic rather than thermodynamic conditions.[45] No aggregation
in solution was observed for DPP 9 in any of the solvent systems
investigated for 7. At concentrations suitable for CD, 9 was
soluble in DCM, chloroform, toluene and THF. To induce
aggregation methanol, acetonitrile and methylcyclohexane were
tested but none was observed. We also tested latter three
solvents almost neat by diluting a tiny amount of dissolved
compound in THF with the precipitating solvent.

(nm)
Figure 3. Crystal structure of compound 9. Top = a single molecule. Below = a
view of the polar stack. A single thiophene geometry is shown.
Figure 4. Circular dichroism and associated absorbance spectra of DPP 7 in
THF/MeOH 7:3 (2 × 10−5 M) and at temperatures from 20 °C to 60 °C.
Temperature-dependent CD of 7 in solution (Figure 4) gives the
largest peak at 712 nm, corresponding to a small shoulder in the
absorption signal which evolves linearly with CD intensity over the
CD measurements were also conducted on thin-films of 7 and
prepared by casting dichloromethane solutions of the
9
compounds onto quartz substrates (Figure 5). Interestingly, 9
displayed more intense differential absorption than 7. The CD
exhibited by films of 9 is consistent with chiral order. The thin-film
spectrum of DPP 7 also displayed features indicating chiral
organisation, although with markedly lower intensity. Differential
absorption extends into the near-IR, reaching a maximum
intensity at 757 nm. Interestingly, the long wavelength CD signal
is opposite to that observed in solution.
2
range 30-60 °C (R = 0.9944). A negative signal extending into
the near infra-red region is observed, reaching maximum intensity
at approximately 880 nm. This is accompanied by a comparatively
small increase in absorbance at these wavelengths. These
features indicate coupling between delocalised π-orbitals and the
formation of ordered chiral aggregates. The positive Cotton effect
present at 712 nm overlaps with a broad peak at 616 nm, which
is located approximately halfway between the two absorption
maxima corresponding to the HOMO-LUMO transition, which
occur at 636 and 600 nm in THF/MeOH at 20 ºC. Notably, this
peak does not exhibit excitonic coupling, indicating a lack of
association between the DPP chromophores in the aggregated
state. This feature is reminiscent of the crystal structure of DPP 9.
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the effect of decreasing the donor/acceptor ratio in addition to spin
rate (for film thicknesses see Supporting Information). For blend
ratios of 1:1, 1:2 and 1:3 (by weight), PCE and Jsc increased with
increasing spin-rate, correlating with decreasing layer thickness.
In contrast, for a donor:acceptor ratio of 1:5, PCE and Jsc
decreased with increasing spin rate.
The best device performance in terms of PCE was achieved
by using a 1:3 ratio of 9/PC61BM and spin coating at 2500 rpm.
Cells prepared under these conditions exhibited Voc = 0.20V, JSC
2
=
−0.65 mA/cm , FF = 33 % and PCE of 0.04 %. The biggest
factor in the poor performance appears to be the very low Jsc. The
most obvious explanation for this effect would be that, as with
DPP 7, the blend exhibits low bulk conductivity. The J-V curve for
this cell (shown in Figure 6) shows imperfect diode behaviour,
with negative current density substantially increasing at voltages
lower than -1.5 V.
Figure 5. Circular dichroism spectra of DPPs 7 and 9 as thin-films.
Photovoltaic Performance. The performance of DPP 7 and DPP
9
as donors in OPV devices was investigated, employing PC61BM
as the acceptor. The device configuration was
ITO/PEDOT:PSS/photoactive layer/Al (ITO = indium tin oxide,
PEDOT:PSS is poly(3,4-ethylenedioxythiophene) poly-styrene
sulfonate). Bulk heterojunction cells were prepared by
spin-coating solutions containing both the donor and acceptor.
Parameters for all fabrication conditions which produced working
solar cells can be found in the Supporting Information. Initially, a
solution of DPP 7 and PC61BM 1:1 (wt.) 1% (wt.) in chlorobenzene
was deposited. However the resultant films displayed very little
visible colour, which was attributed to a large disparity between
the solubility of 7 and PC61BM in chlorobenzene (the latter is
significantly more soluble in chlorobenzene) leading to a high
degree of phase separation and particles of dye. Blend layers
prepared using a 1:1 (wt.) (1 wt.%) mixture of 7 and PC61BM in
chloroform resulted in more uniform and absorbent films. The
possible reasons for this effect are discussed below (see section
on film morphology).
Figure 6. J-V curve plotting current density (J) against Voltage (V) of best
performing devices fabricated using DPP 7:PC61BM and DPP 9:PC61BM.
Both materials were able to produce devices with Voc = ca. 0.3
V in certain conditions (see supporting information). While this is
less than the literature values for known materials (ca. 0.6 V is
typical for P3HT:PC61BM) it is high enough to strongly imply that
there is significant charge carrier generation in the blend under
Devices were prepared with a variety of active layer spin-
casting speeds (Supporting information). It was found that power
conversion efficiency (PCE) and short-circuit current density (JSC
)
increased with spin-rate, scaling inversely with layer
thickness/solvent evaporation rate. This trend suggests that DPP
illumination. However the Jsc of almost all devices was between 0
and -1 mA/cm2 (ca. 10 mA/cm2 is seen with well-performing
7
exhibits poor bulk conductivity, which could be due to a poor
charge mobility, high recombination, high density of trap states or
any combination of these factors, as the gain in extracted
photocurrent from a smaller distance travelled through the
medium outweighs the loss from a thinner layer absorbing less
light. In contrast, the open circuit voltage (VOC) scaled inversely
with film thickness (determined by spin-rate). The most efficient
device was prepared by spin-coating the photoactive layer at
P3HT:PC61BM devices) and the FF was usually less than 30
(
above 60 is often seen for P3HT:PC61BM) which strongly implies
that very little of the generated charge was extracted, leading to a
low PCE.
Film Morphology. The active layer morphology of devices
containing 7 and PC61BM was probed using atomic force
microscopy (AFM). Qualitatively it appears that clumps of material
form on the surface, in lower quantity but greater size in the
sample spun-cast from chloroform compared to the sample spun-
cast from chlorobenzene (see Figure 7).
2
000 rpm, for which PCE = 0.05%, Voc = 0.10 V, fill factor (FF) =
2
,
26% and JSC = −2.05 μA/cm (the J-V curveis shownin Figure 6).
Thermal annealing at 125 °C for 10 minutes had no effect on
device performance. These values are substantially lower than
P3HT/PC61BM devices prepared in our laboratory under identical
conditions as a reference (for which PCE = 1.03%).
In contrast to DPP 7, DPP 9 exhibited higher solubility in
chlorobenzene and films that were prepared by spin-casting from
blends prepared from it exhibited strong coloration, so all blends
of DPP 9:PC61BM employed in subsequent device measurements
were prepared from this solvent. For this compound, we explored
Furthermore it was found that samples cast from CHCl
3
exhibited smoother surface topography (RMS roughness = 0.22
nm, mean value calculated from several images) compared with
those prepared from chlorobenzene (RMS roughness = 0.47 nm).
This observation implies that phase separation of the active
components is less pronounced in the film prepared from
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chloroform as a lower surface roughness is linked to smaller
domain sizes.[46] Both the smaller disparity in the relative solubility
of the two components in chloroform and the lower evaporation
time from this more volatile solvent would contribute to this effect.
The mean RMS roughness calculated from several images of
an annealed sample of a film prepared from a 1:1 ratio blend if
DPP 9 and PC61BM was 0.45 nm compared to 0.27 nm for an
equivalent non-annealed sample. AFM images of both an
annealed and non-annealed sample show a proliferation of
nanocrystalline structures in the film, unlike those prepared using
DDP 7 and PC61BM, showing that DPP 9 is more inclined towards
ordered crystallite formation under the spin-casting conditions of
the compounds described here (see Figure 9).
Figure 7. AFM images of films spin-cast from 1:1 blends of 7 and PC61BM in
chlorobenzene (top) and chloroform (bottom).
Smaller domain sizes of DPP 7 would also be expected to
lead to more intense coloration of the blend thin film, as was
observed as samples spun-cast from chloroform had much more
intense colour than those cast from chlorobenzene. The effect of
thermal annealing on the blends containing DPP 9 and PC61BM
was investigated, however it was found that it led to the formation
of very large crystalline domains clearly visible by optical
microscopy (see Figure 8). For this reason, thermal annealing
was not used in the preparation of devices, as bulk
heterojunctions require much smaller domain sizes.
Figure 9. AFM images of films spin-cast from 1:1 blends of
in chlorobenzene with no thermal annealing (top) and annealed at 125°C
under a nitrogen atmosphere for 10 min (bottom).
9 and PC61BM
Conclusions
The novel class of chiral DPP-based molecular materials reported
herein show how the chiral at-end strategy is a way to influence
the organisation of optically active aggregates of chromophores.
In this case, chiral substitution at both “ends” is necessary to
2
create significant optical activity in solution for the C symmetric
compound, although as a thin film the asymmetric compound with
a single stereogenic group shows greatest optical activity. The
results demonstrate that CD is an extremely sensitive probe for
the organisation of these compounds, as very modest changes in
the absorption spectra are noted, making the chiral structures
useful for the understanding of related self-assembled structures
in general. The (1R)-(−)-myrtenal might at first sight seem so
large that it would encumber π-π interactions, but this was proved
not to be the case in the X-ray crystal structure of the asymmetric
molecule.
Figure 8. Optical microscope images of films spin-cast from 1:1 blends of DPP
9
and PC61BM in chlorobenzene with no thermal annealing (left) and annealed
at 125°C under a nitrogen atmosphere for 10 min (right).
OPV devices preparedemploying these DPP materials as
donors and PC61BM as an acceptor gave very low PCE and Jsc
,
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around an order of magnitude lower than devices made with
P3HT:PC61BM under similar conditions. The main reason for the
poor performance is likely to be low charge carrier mobility of the
materials. Mobility in chiral materials is known to be extremely
sensitive to supramolecualr organisation.[47] Morphological
analysis revealed that the asymmetric compound is the more
inclined to form ordered crystalline domains when spun-cast as a
blend with PC61BM, which could be of interest for larger molecules
based on similar materials with greater charge transport.[48-50]
couple used as an internal reference.46 Compensation for internal
resistance was not applied.
Circular dichroism spectra were recorded on an Applied
Photophysics Chirascan plus spectrometer using high-precision,
spectrally matched quartz cuvettes with a path length of 10 mm.
Thin film measurements were conducted on quartz substrates
and the CD spectra averaged over 4 orientations in order to
compensate for any linear dichroic effects in the plane of the film.
Current-voltage characteristics of OPV devices were
recorded with a Keithley 2401 sourcemeter.
Synthetic Procedures
(
6R,8R)-7,7-dimethyl-3-(thiophen-2-yl)-5,6,7,8-tetrahydro-6,8-
Experimental Section
methanoisoquinoline (3)
To a stirred solution of pyridinium salt 2 (2.00g, 6.04 mmol) in
formamide (7 mL) at room temperature was added ammonium
acetate (930 mg, 12.1 mmol) in one portion. The resultant solution
was heated to 50 °C and stirred for 30 min. (1R)-(−)-Myrtenal
General Methods
Commercially available reagents and solvents were used as
received. Unless otherwise noted, all reactions were performed
under an atmosphere of argon. THF used in reactions was dried
and purified by passage through activated alumina columns using
a solvent purification system. All other reagents were used as
received unless otherwise noted. Reactions were monitored by
thin-layer chromatography (TLC) carried out on E. Merck silica gel
plates using UV light and developed with potassium
permanganate where appropriate. Flash column chromatography
was carried out using either silica (Sigma-Aldrich, pore size 60 Å,
particle size 40-63 µm) or activated alumina gel (Sigma-Aldrich,
neutral, pore size 58Å, Brockman I). Size exclusion
(
0.97 mL, 6.34 mmol) was added dropwise over 5 minutes and
the solution heated to 60 °C for 12 h. The reaction mixture was
cooled to room temperature, quenched by the addition of water
(
50 mL) and then extracted with hexanes (3 × 20 mL). The
combined organic extracts were dried over MgSO and
4
concentrated in vacuo. Purification of the product by column
chromatography (hexanes/EtOAc 3:1) afforded the title
compound 3 as a white solid (1.39 g, 5.44 mmol, 90%). m.p.
2
5
1
1
16.5-117.5 °C; [훼] = −63.7 (c = 1.1 in CHCl
3
); H NMR (400
퐷
TM
MHz, CDCl ):  8.12 (1H, s, Ar-H), 7.56 (1H, dd, J = 3.7, 1.2 Hz,
chrmoatography was performed using Bio-Beads S-X1 beads
200-400 mesh).
NMR spectra were acquired on a Jeol EX270, Bruker
DPX300, Bruker AV400, Bruker DPX400, or a Bruker AV(III)500
3
Ar-H), 7.48 (1H, s, Ar-CH), 7.36 (1H, dd, J = 5.1, 1.1 Hz, Ar-CH),
(
7
(
.11 (1H, dd, J = 5.1, 3.7 Hz, Ar-CH), 3.02 (2H, dd, CH
1H, dd, J = 5.5, 5.5 Hz, CH), 2.75-2.68 (1H, m, CH ), 2.35-2.29
(1H, m, CH), 1.43 (3H, s, CH ), 1.24 (1H, d, J = 9.6 Hz, CH ); 13C
2
), 2.85
2
spectrometer. H and 13C NMR spectra were referenced to the
1
3
2
1
13
NMR (100 MHz, CDCl ):  150.5 (Ar-C), 145.7 (Ar-C), 145.2 (Ar-
residual protonated solvent ( H) or the solvent itself ( C). NMR
spectra were recorded at room temperature in CDCl solutions.
3
CH), 144.9 (Ar-C), 141.3 (Ar-CH), 127.9 (Ar-CH), 126.8 (Ar-CH),
3
1
3
2
8
23.9 (Ar-CH), 118.5 (Ar-CH), 44.4 (CH), 40.0 (CH), 39.4 (C),
2.9 (CH ), 31.9 (CH ), 26.0 (CH ), 21.4 (CH ); IR (film) νmax 2998,
945, 2908, 1600, 1551, 1537, 1476, 1388, 1227, 1217, 1055,
Chemical shifts are reported in parts per million (ppm). Coupling
constants, J, quoted to the nearest 0.1 Hz. Multiplicities are
reported as “s” (singlet), “br s” (broad singlet), “d” (doublet), “dd”
2
2
3
3
55, 814, 757, 707 cm ; HRMS (ESI+) for C16H
17NNaS [M+Na]+
-1
(
(
doublet of doublets), “ddd” (doublet of doublets of doublets), “t”
triplet) and “m” (multiplet). Infra-red spectra were recorded on a
requires 256.1154 found 256.1155.
Bruker Tensor 27 instrument equipped with a Pike GladiATR
attachment with a diamond crystal. Optical rotations were
performed on a Bellingham and Stanley ADP410 polarimeter,
using the sodium-D line (589 nm) and concentration measured in
grams per 100 mL. Melting points were determined on a
Gallenkamp melting point apparatus and are uncorrected. High
resolution mass spectra were recorded using electrospray
ionization (ESI) techniques at the School of Chemistry, University
of Nottingham. Elemental analyses (CHN) were performed by the
University of Nottingham, School of Chemistry Microanalytical
Service on an Exeter Analytical CE-440 instrument.
(
6R,8R)-3-(5-bromothiophen-2-yl)-7,7-dimethyl-5,6,7,8-
tetrahydro-6,8-methanoisoquinoline (4)
To a stirred solution of thiophene 3 (500 mg, 1.96 mmol) in CHCl
(
The resultant solution was heated to 50 °C for 24 h. The reaction
mixture was allowed to cool to room temperature, quenched with
3
20 mL) was added N-bromosuccinimide (384 mg, 2.16 mmol).
water (20 mL) and extracted with CH
combined organic extracts were dried over MgSO
concentrated in vacuo. Purification by column chromatography
CH Cl ) afforded 4 as a white solid (645 mg, 1.93 mmol, 99%).
m.p. 110.5-113.5 °C; [훼] = −46.8 (c = 0.26 in CHCl
2
Cl
2
(3 × 20 mL). The
4
and
(
2
2
2
5
1
3
); H NMR
3
400 MHz, CDCl ):  8.08 (1H, s, Ar-H), 7.39 (1H, s, Ar-H), 7.28
Absorption and fluorescence spectra were obtained using a
퐷
(
PerkinElmer Lambda 25 UV/Vis spectrometer and a FluoroMax-
®
(1H, d, J = 3.9 Hz, Ar-H), 7.05 (1H, d, J = 3.9 Hz, Ar-H), 3.01 (2H,
d, J = 3.9 Hz), 2.84 (1H, dd, J = 5.5, 5.5 Hz, CH), 2.75-2.70 (1H,
m, CH
9

3
spectrometer respectively. Cyclic voltammetric studies were
carried out using an Autolab PGSTAT20 potentiostat. Standard
cyclic voltammetry was carried out under an atmosphere of argon
using a three-electrode arrangement in a single compartment cell.
A glassy carbon working electrode, a Pt wire secondary electrode
and a saturated calomel reference electrode, chemically isolated
from the test solution via a bridge tube containing electrolyte
solution and fitted with a porous Vycor frit, were used in the cell.
Redox potentials are quoted versus the ferrocenium-ferrocene
2
), 2.35-2.30 (1H, m, CH
.6 Hz, CH ), 0.68 (3H, s, CH
149.9 (Ar-C), 146.7 (Ar-C), 145.9 (Ar-C), 145.5 (Ar-CH), 141.9
2
), 1.43 (3H, s, CH
3
), 1.23 (1H, d,
); 13C NMR (100 MHz, CDCl
):
2
3
3
(
Ar-C), 130.9 (Ar-CH), 123.8 (Ar-CH), 117.9 (Ar-CH), 114.2 (Ar-
C), 77.4 (Ar-C), 44.6 (CH), 40.1 (CH), 39.5 (C), 33.0 (CH ), 31.9
CH ), 26.1 (CH ), 21.5 (CH ); IR (film) νmax 2935, 1599, 1478,
2
(
2
3
3
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Chemistry - A European Journal
10.1002/chem.201802610
FULL PAPER
-1
1
436, 1388, 1004, 963, 865, 804, 756 cm ; HRMS (ESI+) for
2 2
vacuo. Purification by column chromatography (CH Cl containing
+
BrC16
H16NNaS [M+Na] requires 334.0260 found 334.0260.
0.1% v/v THF, 0.25% NEt
3
) afforded the title compound 9 as a
1
purple solid (39 mg, 54 µmol, 18%); m.p. 159-163 °C; H NMR
(
6R,8R)-7,7-dimethyl-3-(5-(4,4,5,5-tetramethyl-1,3,2-
3
(400 MHz, CDCl ):  9.00 (1H, d, J = 4.2 Hz, Ar-H), 8.94 (1H, dd,
dioxaborolan-2-yl)thiophen-2-yl)-5,6,7,8-tetrahydro-6,8-
methanoisoquinoline (5)
To a stirred solution of [Ir(COD)OMe] (1.3 mg, 2 µmol) and 4,4′-
2
J = 3.9, 1.2 Hz, Ar-H), 8.14 (1H, s, Ar-H), 7.65 (1H, dd, J = 5.0,
1.1 Hz, Ar-H), 7.50-7.43 (2H, m, Ar-H × 2), 7.41 (1H, d, J = 4.0
Hz, Ar-H), 7.35 (1H, d, J = 3.8 Hz, Ar-H), 7.30 (1H, dd, J = 5.0,
di-tert-butyl-2,2′-dipyridyl (1.0 mg, 4 µmol) in heptane (2 mL)
under argon was added pinacolborane (0.17 mL, 1.17 mmol)
resulting in the formation of a deep red solution. Thiophene 3 (200
mg, 780 µmol) was added in one portion and the reaction mixture
heated at 80 °C for 24 h. The reaction mixture was allowed to cool
to room temperature, filtered through Celite® and then
concentrated in vacuo. The crude product 5 thus obtained was
used directly without further purification.
4.1 Hz, Ar-H), 4.13-4.08 (4H, m, NCH
Hz, CH ), 2.87 (1H, t, J = 5.0 Hz, CH), 2.77-2.72 (1H, m, CH of
CH ), 2.36-2.33 (1H, m, CH), 1.82-1.74 (4H, m, CH × 2), 1.50-
1.25 (16H, m, CH , CH × 6, CH of CH ), 0.94-0.89 (6H, m, CH
× 2), 0.70 (3H, s, CH ):  161.6
); 13C NMR (100 MHz, CDCl
(C=O), 161.3 (C=O), 150.1 (Ar-C), 146.5 (Ar-C), 146.0 (Ar-CH),
145.5 (Ar-C), 143.4 (Ar-C), 142.1 (Ar-C), 139.8 (Ar-C), 139.5 (Ar-
C), 137.0 (Ar-CH), 137.0 (Ar-C), 135.3 (Ar-CH), 130.6 (Ar-CH),
2
× 2), 3.04 (2H, d, J = 2.9
2
2
2
3
2
2
3
3
3
1
30.0 (Ar-C), 128.8 (Ar-CH), 128.2 (Ar-C), 126.1 (Ar-CH), 125.2
(Ar-CH), 124.4 (Ar-CH), 118.3 (Ar-CH), 108.1 (Ar-C), 108.1 (Ar-C),
44.7 (CH), 42.4 (CH ), 40.2 (CH), 39.5 (C), 33.0 (CH ), 32.0 (CH ),
31.6 (CH ), 31.6 (CH ), 30.2 (CH ), 30.1 (CH ), 26.7 (CH ), 26.7
), 26.1 (CH ), 22.7 (CH × 3), 21.6 (CH ), 14.2 (CH ), 14.2
); IR (film) νmax 2924, 2856, 1793, 1599, 1553, 1424, 1407,
3
,6-bis(5'-((6R,8R)-7,7-dimethyl-5,6,7,8-tetrahydro-6,8-
methanoisoquinolin-3-yl)-[2,2'-bithiophen]-5-yl)-2,5-dihexyl-
,4-diketopyrrolo[3,4-c]pyrrole (7)
Under an atmosphere of argon, a stirred mixture of bromide 6
100 mg, 160 µmol), boronate 5 (200 mg, prepared above),
Pd(PPh (7.0 mg, 6.4 µmol) and potassium carbonate (0.8 mL,
M aq.) in THF (2 mL) was heated at 80 °C for 4 h. A further
portion of Pd(PPh (7.0 mg, 6.4 µmol) was added and heating
2
2
2
1
2
2
2
2
2
(CH
(CH
2
3
2
3
3
(
2
-1
3
)
4
1230, 1104, 1083, 1030, 732 cm ; HRMS (ESI+) Anal. Calcd for
42 47 3 2 3
C H N O S : C, 69.87; H, 6.56; N, 5.82. Found: C, 69.93; H,
2
3
)
4
6.66; N, 5.75.
continued for a further 16 h. The reaction mixture was allowed to
cool to room temperature, diluted with brine (10 mL) and extracted
General OPV device fabrication procedure
with CH
dried over MgSO
purification by column chromatography (CH
3
.1%⟶5% v/v THF) and size-exclusion chromatography (CHCl )
2
Cl
2
(3 × 10 mL). The combined organic extracts were
and concentrated in vacuo. Sequential
Cl containing
Active blend solutions were stirred overnight at 50°C. The ITO on
glass substrates (Diamond coatings) were cleaned by sonication
sequentially in Hellmanex (III) solution (1% aq.) for 5 minutes,
acetone for 5 minutes and two portions of 2-propanol for 10
4
2
2
0
afforded the title compound 7 as a blue powder (59 mg, 60.8 µmol, minutes each, rinsing with deionised water between each step.
1
3
8%); m.p. > 250 °C; H NMR (400 MHz, CDCl
3
):  8.96 (2H, d,
The substrates were then stored underwater until drying by
spinning at 5000 rpm for 30 s, followed by treatment with
UV/ozone for 20 minutes.
J = 4.2 Hz, Ar-H 2), 8.11 (2H, s, Ar-H × 2), 7.45-7.43 (2H, m,
Ar-H × 4), 7.36 (2H, d, J = 4.2 Hz, Ar-H × 2), 7.30 (2H, d, J = 3.9
Hz, Ar-H × 2), 4.09 (4H, t, J = 8.0 Hz, NCH
2
× 2), 3.00 (4H, s, CH
2
PEDOT:PSS was filtered using a PVDF syringe filter (0.22 µm)
and spun-cast dynamically at 5000 rpm for 30 seconds and the
samples heated at 150°C on a hotplate until needed for active
layer deposition. The active layer solution was filtered with a
PTFE syringe filter (0.22 µm) and dynamically spun-cast at 550
rpm for 30 seconds then 2000 rpm for 45 seconds. The samples
were then transferred to a nitrogen glovebox and 100 nm of
elemental Al deposited by thermal evaporation. J-V curves were
recorded under AM 1.5 1 sun illumination in air.
×
2), 2.84 (2H, dd, J = 5.4, 5.4 Hz CH × 2), 2.74-2.69 (2H, m, H of
CH
2
× 2), 2.33-2.30 (2H, m, CH × 2), 1.80-1.74 (4H, m, CH
× 2, CH × 6), 1.23 (2H, d, J = 8.0 Hz, H
× 2), 0.67 (6H, s, CH ):
× 2); 13C NMR (100 MHz, CDCl
161.4 (C=O), 150.1 (Ar-C), 146.3 (Ar-C), 145.9 (Ar-CH), 145.5
Ar-C), 143.2 (Ar-C), 142.0 (Ar-C), 139.1 (Ar-C), 137.1 (Ar-C),
36.9 (Ar-CH), 128.3 (Ar-C), 126.1 (Ar-CH), 125.2 (Ar-CH), 124.5
Ar-CH), 118.3 (Ar-CH), 108.4 (Ar-C), 44.7 (CH), 42.5 (CH ), 40.2
CH), 39.5 (C), 33.0 (CH ), 32.0 (CH ), 31.6 (CH ), 30.2 (CH ),
6.8 (CH ), 26.1 (CH ), 22.7 (CH ), 21.6 (CH ), 14.2 (CH ); IR
film) νmax 2923, 1467, 1597, 1549, 1427, 1212, 1082, 1023, 947,
2
× 2),
1.47-1.34 (18H, m, CH
3
2
of CH

(
1
(
(
2
2
3
3
2
2
2
2
2
2
3
2
3
3
(
Acknowledgements
-1
62 4 2 4
802, 732 cm ; HRMS (ESI+) Anal. Calcd for C58H N O S : C,
7
1.42; H, 6.41; N, 5.74. Found: C, 71.14; H, 6.61; N, 5.56.
We thank the EPSRC, GSK, and the School of Chemistry at the
University of Nottingham for funding, and Prof. Jonathan
McMaster for assistance with calculations.
3-(5'-((6R,8R)-7,7-dimethyl-5,6,7,8-tetrahydro-6,8-
methanoisoquinolin-3-yl)-[2,2'-bithiophen]-5-yl)-2,5-dihexyl-
-(thiophen-2-yl)-1,4-diketopyrrolo[3,4-c]pyrrole (9)
Under an argon atmosphere, a mixture of thiophene-DPP 8 (140
mg, 300 µmol), bromothiophene 4 (100 mg, 300 µmol), Pd(OAc)
3.4 mg, 15 µmol), pivalic acid (9.0 mg, 90 µmol) and potassium
carbonate (62 mg, 450 µmol) in dimethylacetamide (3 mL) was
heated at 100 °C for 24 h with stirring. The reaction mixture was
allowed to cool to room temperature, diluted with brine (10 mL)
6
Keywords: keyword 1 • keyword 2 • keyword 3 • keyword 4 •
keyword 5
2
(
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2012, 6, 2032.
4
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Chemistry - A European Journal
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FULL PAPER
Entry for the Table of Contents (Please choose one layout)
Layout 1:
FULL PAPER
The tendency of an asymmetric
functional organic molecule to pack
giving low symmetry structures is
proven in a solid and thin film,
Paul A. Hume, * James P. Monks, Flavia
Pop, E. Stephen Davies, Roderick C.I.
MacKenzie and David B. Amabilino*
although a more symmetrical
Page No. – Page No.
analogue is more optically active as
nanoscale aggregates in solution, and
photovoltaic characteristics also vary.
Self-Assembly of Chiral-at-End
Diketopyrrolopyrroles : Symmetry
Dependent Solution and Film Optical
Activity and Photovoltaic
Performance
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