Solvatochromism in diketopyrrolopyrrole derivatives: Experimental and computational studies

Article abstract of DOI:10.1071/CH14057

A series of seven thiophen-substituted diketopyrrolopyrrole derivatives were synthesised and their solution absorption spectra characterised in a range of solvents of varying polarity. The absorption spectra of these diketopyrrolopyrrole derivatives exhibited significant negative solvatochromism. The behaviour is consistent with results of time-dependent density-functional theory calculations of excitation energies. Calculated electronic structures suggest that there is significant charge transfer between the electron-donating thiophen substituents and the diketopyrrolopyrrole core but that the magnitude of this charge shift is reduced in the excited state compared with the ground state. The resulting reduction in polarity of the excited state accounts for the negative solvatochromism observed. The implications of the results for the potential application of diketopyrrolopyrrole compounds as photovoltaic materials are considered.

Full text of DOI:10.1071/CH14057

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2014, 67, 1330–1337
Full Paper
http://dx.doi.org/10.1071/CH14057
Solvatochromism in Diketopyrrolopyrrole Derivatives:
Experimental and Computational Studies
A
A
A C
,
Rebecca S. Szabadai, Jesse Roth-Barton, Kenneth P. Ghiggino,
Jonathan M. White,
A C
,
B
and David J. D. Wilson
A
School of Chemistry and BIO-21 Institute, The University of Melbourne,
Melbourne, Vic. 3010, Australia.
B
Department of Chemistry, La Trobe Institute for Molecular Science,
La Trobe University, Melbourne, Vic. 3086, Australia.
C
Corresponding authors. Email: ghiggino@unimelb.edu.au; whitejm@unimelb.edu.au
A series of seven thiophen-substituted diketopyrrolopyrrole derivatives were synthesised and their solution absorption
spectra characterised in a range of solvents of varying polarity. The absorption spectra of these diketopyrrolopyrrole
derivatives exhibited significant negative solvatochromism. The behaviour is consistent with results of time-dependent
density-functional theory calculations of excitation energies. Calculated electronic structures suggest that there is
significant charge transfer between the electron-donating thiophen substituents and the diketopyrrolopyrrole core but
that the magnitude of this charge shift is reduced in the excited state compared with the ground state. The resulting
reduction in polarity of the excited state accounts for the negative solvatochromism observed. The implications of the
results for the potential application of diketopyrrolopyrrole compounds as photovoltaic materials are considered.
Manuscript received: 6 February 2014.
Manuscript accepted: 8 May 2014.
Published online: 5 June 2014.
Introduction
A typical absorption spectrum of DPP derivatives consists
of a short-wavelength band (typically in the region 300–
400 nm), which has been attributed to p–p* transitions within
the aromatic substituents, and a long-wavelength band at
400–600 nm, which has been assigned to intramolecular
charge transfer (ICT) between the donor aryl substituents
and the central lactam core as the acceptor moiety. In order
to fully characterise the photophysical properties of DPP
and its derivatives, we measured the absorption spectra of a
range of DPP compounds in solvents of varying polarity.
We were particularly interested in determining whether the
long-wavelength charge-transfer band in the thiophen deri-
vatives exhibits significant solvatochromism. Moderate posi-
tive and negative solvatochromism has been noted previously
for a variety of aryl-substituted DPP derivatives but has not
been investigated for thiophen derivatives.[3e] The DPP deri-
vatives 2a–2g (Fig. 2) were synthesised and used for the
present study.
Diketopyrrolopyrrole (DPP, Fig. 1) and its derivatives are
pigments that are typically used in inks, plastics and dyes,[1]
with colours ranging from orange-yellow to red to purple.[2]
Many DPP derivatives exhibit a deep colour and luminescence
with a large Stokes shift as well as weather-fastness and high
photostability in the solid state. Such features render these
compounds promising candidates for a wide variety of appli-
cations, including in organic photovoltaic (OPV) devices.[3] In
particular, DPP derivatives with thiophen substituents have
small band gaps, which enable them to absorb a large portion
of the solar spectrum, while they can also be readily functio-
nalised to further tune the electronic energy levels. These
attributes give them the potential to realise high efficiencies
in photovoltaic devices because larger open-circuit voltages
can be achieved.
The lactam units within DPP are electron-deficient, which
causes the chromophore to have a high electron affinity. The
DPP core participates in strong p–p interactions and has polar
carbonyl groups, which are capable of acting as hydrogen-bond
acceptors. This is dueto theplanar, conjugated, bicyclic structure
and explains why the unsubstituted core is highly insoluble.[4]
Results and Discussion
The dithienyl-DPP derivative 2a was prepared by condensation
of 2-thiophencarbonitrile with diisopropyl succinate in the
presence of strong base (Scheme 1) according to the literature
procedure;[3a] subsequent bromination of 2a gave the inter-
mediate dibromide 3a, which was then derivatised by Suzuki
coupling with the appropriate arylpinacolboronates 4b–4g to
give derivatives 2b–2g. Although most derivatives were readily
handled and stable, the bi-tris-thiophen-substituted DPP deri-
vative 2e appeared to be readily oxidised. When freshly purified,
1
Fig. 1. Diketopyrrolopyrrole core (1).
Journal compilation Ó CSIRO 2014
the H NMR spectrum of 2e exhibited sharp readily assigned
www.publish.csiro.au/journals/ajc

Solvatochromism in Diketopyrrolopyrrole Derivatives
1331
2a,
2b,
ϭ
ϭ
ϭ
2f,
2c,
ϭ
ϭ
2g,
2d,
2e,
ϭ
ϭ
Fig. 2. Small-molecule diketopyrrolopyrrole (DPP) derivatives.
determined from the fluorescence quantum yield (ff) and
fluorescence lifetime (tf) using the relationships:
B(OR)2
OHC
2b
4b
kr ¼ ff =tf
O
B(OR)2
knr ¼ ð1 ꢀ ff Þ=tf
4c
2c
In addition to shifts in the absorption and emission spectra,
the solution photophysical rate data indicate that addition of
substituents on the thiophen generally results in a reduction of
the fluorescence quantum yield, primarily due to increases in the
non-radiative rate constant with minor contributions from radi-
ative rate changes.
The polarity of the solvents used in this study were represen-
ted by the normalised Dimroth–Reichardt (ENT ) solvent polarity
parameter,[5] which is a measure ofthe ionisingpower of a solvent
based on the maximum wavenumber of the longest-wavelength
electronic absorption band of a zwitterion in the given solvent.
The absorption spectra of 2a and 2b in solvents of varying
polarity are presented in Fig. 4.y
Ph
R
B(OR)2
S
N
N
O
4d
2d
2a, R ϭ H
3a, R ϭ Br
O
S
B(OR)2
C6H13
S
S
R
4e
2e
O
S
B(OR)2 ϭ Pinacolboronate
Conditions: Pd[(C6H5)3P]4, K2CO3, THF, 75ЊC
O
B(OR)2
4f
2f
Ph
A plot of the lowest-energy absorption maxima as a function
of the Dimroth–Reichardt parameter for each solvent is pre-
sented in Fig. 5 for 2a–2g. A linear fit for each dataset is
provided to emphasise the trend of the data only.
NC
4g
B(OR)2
From these data, it is clear that 2a–2g exhibit negative
solvatochromism, indicating that the absorption maxima
shift to higher energy with increasing solvent polarity.
Time-dependent density-functional theory (TD-DFT) calcula-
tions were carried out on a model N-methyl substituted DPP
compound (cf. Experimental section) to probe the origin of
the observed negative solvatochromism. Singlet excitations
(S0 - S1) were calculated with six solvents of varying polarity:
water (e ¼ 78.4), acetonitrile (35.7), acetone (20.5), THF (7.4),
chloroform (4.7), and toluene (2.4). Results are plotted in Fig. 6.
There is a consistent increase in energy of absorption (and
decrease in wavelength) with increase in solvent polarity, which
is reproduced by a variety of DFT methods (B3LYP, vB97XD,
PBE, PBE0) and basis sets. The theoretical results provide
further confirmation of the negative solvatochromism observed
in the experimental study.
2g
Scheme 1. Synthesis of diketopyrrolopyrrole (DPP) derivatives 2a–2g.
1
signals; however, on standing in air, the H NMR spectrum
exhibited broadened signals, particularly those on the thiophen
ring immediately attached to the DPP core, suggesting the for-
mation of a radical cation species.
Selected photophysical data for solutions of 2a–2g are
summarised in Table 1. The solutions for each derivative
show similar absorption features to one another, with an int-
ense band in both the ultraviolet (300–400 nm) and the visible
(400–600 nm) regions (Fig. 3). The solution emission spectra
maxima of 2a–2g range from 564 nm for 2a to 695 nm for 2e.
The Stokes shift (defined as the shift between absorption
and emission maxima) was largest for 2g at 1878 cmꢀ1. The
radiative (kr) and non-radiative (knr) rate constants were also
To gain further insight into the origin of the negative
solvatochromism displayed by 2a–2g, the ground (S0) and
excited (S1) singlet-state structures of the N-methylated
yCorresponding spectra for 2c–2g are provided in the Supplementary Material.

1332
R. S. Szabadai et al.
Table 1. Selected photophysical data for small-molecule diketopyrrolopyrrole (DPP) derivatives 2a–2g in chloroform
Fluorescence quantum yield (Ff) and lifetime (tf) data for 2a from Ref. [3a]; radiative (kr) and non-radiative (knr) rate constants determined as described in text
Compound
Absorption
Emission
Stokes shift
[cmꢀ1
FfA
tf [ns]
(ꢁ0.1)
kr
knr
[108 sꢀ1
e
l
max [nm]
lmax [nm]
]
(ꢁ5 %)
[108 sꢀ1
]
]
[dm3 molꢀ1 cmꢀ1 B
]
2a[3a]
2b
2c
548
614
629
621
646
635
588
564
652
656
659
695
656
661
517
949
0.83
0.52
0.45
0.41
0.11
0.35
0.40
6.27
3.1
2.3
1.9
0.9
1.9
1.9
1.32
1.7
2.0
2.2
1.2
1.8
2.1
0.27
1.5
2.4
3.1
9.9
3.4
3.1
27000
39800
63000
66300
75500
67000
58400
654
2d
2e
928
1091
504
2f
2g
1878
ASamples excited at 560 nm.
BMolar absorption coefficients (e) were measured at the maximum wavelength for solution spectra (estimated error 3 %).
2a
2a
0.25
0.20
0.15
0.10
0.05
0
1200
1000
800
600
400
200
0
1.2
1.0
0.8
0.6
0.4
0.2
0
Toluene
THF
Chloroform
Acetone
Acetonitrile
Methanol
330
380
430
480
530
580
630
680
300
400
500
600
700
800
900
1000
Wavelength [nm]
Wavelength [nm]
2b
1.2
1.0
0.8
0.6
0.4
0.2
0
2b
0.25
0.20
0.15
0.10
0.05
0
1200
1000
800
600
400
200
0
Toluene
THF
Chloroform
Acetone
Acetonitrile
Methanol
330
430
530
630
730
Wavelength [nm]
300
400
500
600
700
800
900
1000
Wavelength [nm]
Fig. 4. Absorption spectra of 2a and 2b in solvents of different polarity.
Fig. 3. Normalised solution absorption (blue), and emission (red) spectra
of diketopyrrolopyrrole (DPP) derivatives 2a–2g in spectroscopic-grade
chloroform. The excitation wavelength is 560 nm. Corresponding spectra for
derivatives 2c–2g are included in the Supplementary Material.
observation of negative solvatochromism for 2a–2g is consis-
tent with the ground state being more polar than the excited
state. This result can be rationalised if there is a significant
degree of charge transfer existing between the donor and
acceptor substituents in the ground state; then excitation results
in transfer of electron density back to the donor from the
acceptor moiety in the molecule, resulting in a less-polar excited
state (Fig. 8). Mulliken and NBO atomic charges were calcu-
lated for the S0 and S1 singlet states in the same six solvent
environments, whereby it was noted that the magnitude of
charge systematically increased with more polar solvents (see
Supplementary Material). Interestingly, gas-phase atomic
charges are greater in magnitude for the excited S1 state
derivative of 2a (2a-NMe) were calculated at the B3LYP/
6–31G(d,p) level of theory. Analysis of the atomic charges
using Mulliken charges and natural bond orbital (NBO) analysis
revealed that the DPP core has an overall negative charge,
whereas the thiophens are positively charged (Fig. 7). This is
consistent with significant charge transfer between the electron-
donor thiophen and DPP core moieties.
Given that solvatochromism arises owing to differing solva-
tion of the ground and excited states of a molecule, then the

Solvatochromism in Diketopyrrolopyrrole Derivatives
1333
620
618
616
614
612
610
608
606
2a
2b
Toluene
THF
Chloroform
Acetone
Acetonitrile
Methanol
Toluene
THF
Chloroform
Acetone
Acetonitrile
550
545
540
535
0.1 0.2 0.3 0.4 0.5 0.6 0.7
0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45
ENT
ENT
2c
2d
634
632
630
628
626
624
622
630
628
626
624
622
620
Toluene
THF
Chloroform
Acetone
Toluene
THF
Chloroform
Acetone
0.10
0.15
0.20
0.25
0.30
0.35
0.10 0.15 0.20 0.25 0.30 0.35
ENT
ENT
650
648
646
644
642
640
Toluene
THF
Chloroform
Acetone
2e
2f
637
636
635
634
633
632
631
Toluene
THF
Chloroform
Acetone
0.10 0.15 0.20 0.25 0.30 0.35
0.10 0.15 0.20 0.25 0.30 0.35
ENT
ENT
635
630
625
620
2g
Toluene
THF
Chloroform
Acetone
0.10
0.15
0.20
0.25
0.30
0.35
ENT
Fig. 5. Absorbance maximum of 2a–2g versus solvent polarity (measured by the Dimroth–Reichardt
parameter). Linear fits through the data are to indicate the trend only.
(Fig. 7); however, the inclusion of solvent effects reverses the
trend and supports the explanation of the observed negative
solvatochromism. That is, the charges of the DPP core and
thiophen are smaller in magnitude in the excited S1 state
compared with the ground S0 state. This trend is particularly
evident in the NBO calculated charges, where the summed
charge on the DPP core and thio groups in the S1 state is only
approximately half that of the S0 state. The variation in Mulliken
charges between S0 and S1 states is much smaller, although the
trend remains the same.
Plots of the highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) of the ground
S0 state of 2a-NMe are presented in Fig. 9. The HOMO
is predominantly located on the DPP core for both S0 and S1

1334
R. S. Szabadai et al.
550
545
540
535
530
525
520
515
510
Ј
Ј
Ј
Ϫ
B3LYP/6-31G(d,p)
B3LYP/TZVP
PBE0/6-31G(d,p)
Ј
ϩ
Fig. 8. Possible charge-transfer transition state expected for 2a–2g.
0
20
40
60
80
Solvent dielectric constant
Fig. 6. Time-dependent density-functional theory (TD-DFT)-calculated
wavelength of S0 - S1 excitations as a function of solvent polarity.
Fig. 9. Contour plots of the HOMO (left), and LUMO (right) of the model
2a-NMe complex.
significant charge transfer that exists between the donor sub-
stituent and the diketodipyrrole core in the ground state; thus,
the HOMO resides on the DPP core. On excitation of these
molecules, the resulting electron redistribution shifts back to
the donor substituent, resulting in an excited state that is less
polar than the ground state. Further optimisation of the DPP
structure could provide useful candidates for photoexcited
electron donors in photovoltaic devices. Moderate positive
and negative solvatochromism has been noted previously for a
variety of aryl-substituted DPP derivatives.[3e]
S
S
0 Ϫ0.156 (Ϫ0.078)
1 Ϫ0.164 (Ϫ0.084)
0.077 (0.039)
0.082 (0.042)
Fig. 7. B3LYP/6–31G(d,p) gas-phase calculated natural bond orbital
(NBO) charges for the S0 and S1 states of 2a-NMe. Mulliken charges are
given in brackets. Charge indicated by colour from red (negative) to green
(positive).
Experimental
General
All reactions were performed in oven-dried glassware. Starting
materials were used without further purification. DMF was
purified by stirring with CaSO4 for 24 h, followed by vacuum
distillation over 4-A molecular sieves. THF was purified by
refluxing over sodium and benzophenone until the solution
became dark, followed by distillation. Acetone was purified by
states (69 % DPP core and 31 % thiophen contributions). The
LUMO is also mostly located on the DPP core for S0 (59 %)
and S1 (55 %) states. The distribution of orbital density
across the DPP core and thiophen substituent is consistent
with the red-shifted absorption band present in these compounds
and thus further substitution on the thiophen ring can be
expected to modify the excited state properties as noted in
this work.
˚
˚
distillation, followed by storage over 4-A molecular sieves.
CH2Cl2 was dried using a solvent-purification apparatus as
described by Pangborn et al.[6] Thin-layer chromatography
was performed on aluminium sheets precoated with silica gel,
using mixtures of EtOAc/n-hexane, Et2O/n-hexane and CH2Cl2/
n-hexane. Detection was achieved via irradiation by UV
light. Flash chromatography was performed using the method
of Still et al.[7] using silica gel grade 230–400 mesh. Automated
column chromatography was performed using a flash purifica-
tion system. NMR data were recorded on 400- and 500-MHz
instruments. Chemical shifts are expressed in parts per million
(d) using residual solvent peaks (1H NMR d 7.26 ppm for
CDCl3; 13C NMR d 77.16 ppm for CDCl3). Melting points were
obtained using an automated melting point apparatus, and are
uncorrected. Fourier-transform (FT) infrared spectra and high-
resolution electrospray ionisation mass spectra (ESI HRMS)
were recorded at The University of Melbourne.
Conclusions
DPP derivatives are finding application as the active light-
absorbing material in some organic solar cells, and the experi-
mental and theoretical results presented here provide further
insight into the nature of the electronic structure of these com-
pounds. The transition responsible for the main visible absorp-
tion band arises from intramolecular charge transfer and the
results of the present study show that the energy of this transition
is sensitive to both the nature of the substituents and the polarity
of the surrounding solvent. The negative solvatochromism
displayed by DPP derivatives 2a–2g is a reflection of the

Solvatochromism in Diketopyrrolopyrrole Derivatives
1335
Photophysics
Synthesis
UV-vis absorption spectra were recorded on a Varian Cary
5 UV-vis-NIR spectrophotometer or Varian Cary 50 Bio spec-
trophotometer. All solution absorption spectra were recorded
against solvent blanks in matched quartz cells with a 1.0-cm path
length. The error in molar absorption coefficients is estimated at
ꢁ3 % based on the uncertainty in solution concentrations and
absorbance measurements.
Compounds 2a, 2e, and 3 were prepared according to literature
procedures.[3,14]
Compound 3. Preparation of 3,6-Bis(5-bromothiophen-
2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4
(2H,5H)-dione
2,5-Bis-(2-ethyl-hexyl)-3,6-dithiophen-2-yl-2,5-dihydro-
pyrrolo[3,4c]pyrrole-1,4-dione, 2a (0.708 g, 1.35 mmol), was
dissolved in anhydrous chloroform (40 mL) and stirred under
nitrogen at room temperature for 15 min. N-Bromosuccinimide
(0.528 g, 2.97 mmol) was added to the mixture and this was
stirred at room temperature for 48 h. The solution was washed
with water, dried with magnesium sulfate, and the chloroform
was removed under vacuum giving the desired compound 3
(0.76 g, 83 %) as a dark purple solid. Mp 164.6–166.38C.[1] dH
(CDCl3, 500 MHz) 8.64 (2H, d, J 4.0), 7.22 (2H, d, J 4.5 Hz),
3.95–3.93 (4H, m), 1.37–0.81 (30H, m).[13] dC (CDCl3,
125 MHz) 161.6, 139.6, 135.5, 131.6, 131.3, 119.2, 108.2,
46.2, 39.3, 30.3, 28.5, 23.7 (CH2), 23.2, 14.2, 10.6.
Fluorescence spectra were recorded on either a Varian
Eclipse or a Horiba Jobin Yvon Fluorolog spectrofluorometer.
The optical densities of the solutions were kept below 0.20 at the
excitation wavelength to minimise reabsorption effects.
Fluorescence decay profiles were recorded by the time-
correlated single-photon counting (TCSPC) technique using
either a Jobin Yvon Fluorolog-3 spectrofluorometer (with
TCSPC accessory) or an in-house-built instrument based on a
mode-locked and cavity-dumped Ti:Sapphire laser excitation
source (Coherent MIRA/PulseSwitch) that was frequency-
doubled to 400 nm.[8] Measurements were performed in
degassed solutions. Fluorescence lifetimes were determined
by analysis of the fluorescence decay profiles using a sum of
exponentials model by iterative reconvolution procedures. In all
cases, a single exponential fit was adequate to describe the
fluorescence decay profiles as judged by inspection of the
distribution of weighted residuals and the value of the reduced
chi-square fitting parameter (example fits are provided in the
Supplementary Material). The uncertainty in fluorescence life-
times is ꢁ0.1 ns. Full details of the instrumentation and data
analysis procedures can be found elsewhere.[8]
Compound 2b. Preparation of 4,40-(5,50-(2,5-Bis
(2-ethylhexyl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo
[3,4-c]pyrrole-1,4-diyl)bis(thiophen-5,2-diyl))
dibenzaldehyde
A solution 4-formylphenyl boronic acid pinacol ester (0.21 g,
0.9 mmol) and tetrakis(triphenylphosphine) palladium(0) in
THF was degassed and purged with nitrogen. This solution
was added to a frozen degassed mixture of 3 (0.2 g, 0.30 mmol),
and 2 M potassium carbonate (0.72 mL) in THF in a Schlenk
flask. The resulting mixture was degassed by three cycles of
freeze–pump–thaw, and then refluxed at 858C for 17 h. The
reaction mixture was extracted with chloroform (15 mL), the
extract washed with water (3 ꢂ 10 mL), and dried over magne-
sium sulfate. The solvent was removed under vacuum giving the
crude product as an oily dark-blue solid. The crude product was
purified using column chromatography (ethyl acetate/hexane) to
give the pure product 2b as a dark-blue powder (0.16 g,
0.22 mmol, 73%). Mp 197.5–198.38C.[1] dH (CDCl3, 500 MHz)
10.04 (2H, s), 8.97 (2H, d, J 4.0), 7.94 (4H, d, J 8.0), 7.84 (4H, d,
J 8.0), 7.61 (2H, d, J 4.0), 4.13–4.05 (4H, m), 1.93–0.84
(30H, m).[13] dC (CDCl3, 125 MHz) 191.3, 161.7, 147.7,
138.7, 136.9 136.1, 130.7, 128.8, 126.5, 126.3, 109.1, 46.2,
39.5, 30.5, 28.7, 23.9, 23.2, 14.2, 10.7. nmax (KBr)/cmꢀ1 2957,
2928, 2857, 1700, 1664, 1599, 1550, 1454, 1401, 1309, 1234,
Fluorescence quantum yield measurements (ff) were carried
out using the comparative method against a known fluorescence
standard (cresyl violet in methanol ff ¼ 0.53).[3a] The uncer-
tainty in fluorescence quantum yield is ꢁ5 %.
Molar absorption coefficients were determined from the
gradient of a plot of absorbance (at lmax) vs concentration.
Solutions were prepared using spectroscopic-grade chloro-
form using concentrations of 5.00 ꢂ 10ꢀ6, 2.50 ꢂ 10ꢀ6, and
1.25 ꢂ 10ꢀ6 M.
Density Functional Theory Calculations
Density functional theory calculations were carried out
within the Gaussian 09 suite of programs.[9] A model
N-methyl DPP analogue was considered wherein methyl
groups were used in place of the ethylhexyl groups to limit
computation time. Geometries were optimised in the absence
of solvent with the B3LYP[10] functional in conjunction with
the 6–31G(d,p) basis set. Excited-state singlet geometries
were optimised with TD-DFT. Stationary points were char-
acterised as minima by calculating the Hessian matrix analyti-
cally at the same level of theory. All structures are minima with
no imaginary frequencies. The polarisable continuum model
(PCM)[11] self-consistent reaction field (SCRF) was used to
model solvent effects at the gas-phase optimised geometries
with a solvent field of acetonitrile, acetone, THF, chloroform,
toluene, acetonitrile, and water. Frontier MO energies were
calculated using DFT MOs with mPW91PW91,[12] PBE,[13]
B3LYP,[10] and wB97XD.[14] An SCF convergence criteria of
10ꢀ8 a.u. was employed throughout. TD-DFT calculations
were carried out at the B3LYP/6–31G(d,p) level of theory
including SCRF PCM solvent fields; 20 singlet states were
modelled. Molecular orbital analysis was carried out with the
AOMix program.[15]
1213, 1168, 1102, 833, 812, 734. lmax/nm (e/Mꢀ1 cmꢀ1
)
(CHCl3) 363, 577, 614 (39748). m/z (ESI HRMS) 733.31276;
[C44H48N2O4S2 þ H]þ requires 733.31283.
Compound 2d. Preparation of 2,5-Bis(2-ethylhexyl)-
3,6-bis(5-(4-(4-methylstyryl)phenyl)thiophen-2-yl)
pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione
A
solution (E)-4,4,5,5-tetramethyl-2-(4-(4-methylstyryl)
phenyl)-1,3,2-dioxaborolane (0.21 g, 0.65 mmol) and tetrakis
(triphenylphosphine) palladium(0) in THF was degassed and
purged with nitrogen. This solution was added to a frozen
degassed mixture of 3 (0.2 g, 0.30 mmol) and 2 M potassium
carbonate (0.72 mL) in THF in a Schlenk flask. The resulting
mixture was degassed by three cycles of freeze–pump–thaw,
and then heated at 758C for 17 h. The reaction mixture was
extracted with chloroform (15 mL), the extract washed with
water (3 ꢂ 10 mL), and dried over magnesium sulfate.

1336
R. S. Szabadai et al.
The solvent was removed under vacuum, giving the crude
product as an oily dark-blue solid. The crude product was
washed well with ethyl acetate, and purified via column chro-
matography (chloroform) to give the pure product 2d as a shiny
green powder (0.124 g, 45 %). Mp 284.3–284.78C.[1] dH (CDCl3,
500 MHz) 8.99 (2H, d, J 4.0), 7.67 (4H, d, J 8.0), 7.55 (4H, d,
J 8.0), 7.49 (2H, d, J 4.0), 7.44 (4H, d, J 8.0), 7.19 (4H, d, J 8.0),
7.16 (2H, d, J 16.0), 7.08 (2H, d, J 16.0), 4.14–4.06 (4H, m), 3.49
(2H, d, J 5.0), 2.38 (6H, s), 1.43–1.24 (16H, m), 0.95–0.87 (12H,
m).[13] dC (CDCl3, 125 MHz) 161.9, 149.6, 139.8, 138.3, 138.1,
137.1, 134.4, 132.2, 129.6, 128.9, 127.2, 126.9, 126.7, 126.4,
124.4, 108.4, 46.2, 39.4, 30.5, 28.8, 23.9, 23.3, 21.4, 14.3, 10.8.
nmax (KBr)/cmꢀ1 2956, 2926, 2855, 1666, 1555, 1511, 1494,
(0.2 g, 59 %). Mp 216.7–217.48C.[1] dH (CDCl3, 500 MHz) 9.02
(2H, d, J 4.0), 7.36 (2H, s), 6.34 (2H, d, J 4.0), 4.39–4.37 (m, 2
H), 4.30–4.27 (m, 2 H), 4.10–4.03 (4H, m), 1.41–1.27 (18H, m),
0.95–0.85 (12H, m).[13] dC (CDCl3, 100 MHz) 161.9, 142.1,
140.6, 139.6, 139.3, 136.4, 127.8, 123.7, 111.9, 108.2, 99.4,
65.4, 64.7, 46.1, 39.5, 30.6, 28.6, 23.9, 23.3, 14.2, 10.8. nmax
(KBr)/cmꢀ1 2957, 2927, 2856, 1650, 1543, 1514, 1481, 1432,
1406, 1365, 1250, 1233, 1182, 1165, 1107, 1071, 1027, 970,
909, 800, 726, 709. lmax/nm (e/Mꢀ1 cmꢀ1) (CHCl3) 358, 590,
635 (66979). m/z (ESI HRMS) 804.23892; [C42H48N2O6S4]þ
requires 804.23897.
Compound 2g. Preparation of (2Z,20Z)-3,30-(4,40-
(5,50-(2,5-Bis(2-ethylhexyl)-3,6-dioxo-2,3,5,6-
tetrahydropyrrolo[3,4-c]pyrrole-1,4-diyl)bis(thiophen-
5,2-diyl))bis(4,1-phenylene))bis(2-phenylacrylonitrile)
1446, 1433, 1401, 1233, 1100, 1074, 1027, 961, 812, 733. lmax
/
nm (e/Mꢀ1 cmꢀ1) (CHCl3) 368, 583, 621 (66332). m/z (ESI
HRMS) 908.44088; [C60H64N2O4S2]þ requires 908.44037.
A
solution (Z)-2-phenyl-3-(4-(4,4,5,5-tetramethyl-1,3,2-
Compound 2e. Preparation of DPP
dioxaborolan-2-yl)phenyl)acrylonitrile (0.22 g, 68 mmol) and
tetrakis(triphenylphosphine) palladium(0) in THF was degassed
and purged with nitrogen. This solution was added to a frozen
degassed mixture of 3 (0.21 g, 0.31 mmol) and 2 M potassium
carbonate (0.73 mL) in THF in a Schlenk flask. The resulting
mixture was degassed by three cycles of freeze–pump–thaw,
and then refluxed at 758C for 17 h. The reaction mixture was
extracted with chloroform (15 mL), the extract washed with
water (3 ꢂ 10 mL), and dried over magnesium sulfate. The
solvent was removed under vacuum, giving the crude product
as an oily dark-blue solid. The crude product was washed well
with hexane and then purified by flash chromatography (hexane/
ethyl acetate) to give the pure product 2g as a shiny green
powder (0.124 g, 45 %). Mp 246.4–247.38C. dH (CDCl3,
500 MHz) 9.00 (2H, d, J 4.0), 7.95 (4H, d, J 8.5), 7.75 (4H, d,
J 7.0), 7.69 (4H, d, J 7.0), 7.55 (2H, d, J 4.0), 7.52 (2H, s), 7.48–
7.41 (6H, m), 4.13–4.05 (4H, m), 1.55–1.25 (16H, m), 0.96–0.87
(12H, m).[13] dC (CDCl3, 100 MHz) 161.8, 148.4, 141.0, 139.9,
137.1, 135.1, 134.5, 134.1, 130.2, 129.9, 129.5, 129.3, 126.4,
126.2, 125.6, 118.1, 112.1, 108.8, 46.2, 39.5, 30.5, 28.7, 23.9,
23.3 14.2, 10.8. nmax (KBr)/cmꢀ1 2955, 2926, 2856, 2213, 1659,
1600, 1584, 1548, 1496, 1447, 1401, 1324, 1284, 1234, 1104,
1079, 1027, 812, 760, 732, 710, 688, 656, 630, 535. lmax/nm
(e/Mꢀ1 cmꢀ1) (CHCl3) 324, 392, 588 (58448). m/z (ESI HRMS)
930.39998; [C60H58N2O2S2]þ requires 930.39957.
A degassed solution of 50-hexyl-2,20-bithiophen-5-boronic
acid pinacol ester (0.16 g, 0.42 mmol) and tetrakis(triphenyl-
phosphine) palladium(0) (0.01 g) in THF was added to a frozen
degassed mixture of 3 (0.10 g, 0.14 mmol), and 2 M potassium
carbonate (0.36 mL) in THF in a Schlenk flask. The resulting
mixture was degassed by three cycles of freeze–pump–thaw,
and then refluxed at 858C for 17 h. The reaction mixture was
extracted with chloroform (15 mL), the extract washed with
water (3 ꢂ 10 mL), and dried over magnesium sulfate. The
solvent was removed under vacuum, giving a crude dark-blue
wax. The solid was washed well with hexane and acetone (until
the washings were no longer blue). After flash-chromatography
on a Biotage Isolera system (KP-Sil, 25-g cartridge, THF/
hexane, l detection 254 and 280 nm), the pure product 2e was
isolated as a dark-blue solid (0.08 g, 56 %). Mp 162.7–163.58C
(lit. 1608C[16]).[1] dH (CDCl3, 500 MHz) 8.94 (2H, d, J 4.5), 7.27
(2H, d, J 4.5), 7.19 (2H, d, J 3.5), 7.04 (2H, d, J 4.0), 7.02 (2H, d,
J 4.0), 6.71 (2H, d, J 3.5), 4.09–4.00 (4H, m), 2.81–2.79 (4H, t,
J 7.5), 1.94–1.90 (2H, m), 1.71–1.66 (4H, m), 1.44–1.24 (28H,
m), 0.94–0.87 (18H, m).[13] dC (CDCl3, 100 MHz) 161.7, 146.6,
142.7, 139.3, 139.0, 137.0, 134.22, 134.18, 128.1, 125.9, 125.2,
124.5, 124.1, 123.9, 108.5, 46.1, 39.4, 31.71, 31.67, 30.5, 30.4,
28.9, 28.7, 23.8, 23.3, 22.7, 14.3, 14.2, 10.7. nmax (KBr)/cmꢀ1
2956, 2926, 2854, 1656, 1551, 1500, 1465, 1446, 1427, 1401,
1220, 1159, 1101, 1076, 1022, 787, 733. lmax/nm (e/Mꢀ1 cmꢀ1
)
(CHCl3) 386, 615, 646 (75537). m/z (ESI HRMS) 1020.39157;
[C60H64N2O2S6]þ requires 1020.39126.
Preparation of 2-(2,3-Dihydrothieno[3,4-b][1,4]
dioxin-5-yl)-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane 4f [17]
Compound 2f. Preparation of 3,6-Bis(5-(2,3-
dihydrothieno[3,4-b][1,4]dioxin-5-yl)thiophen-2-yl)-
2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-
1,4(2H,5H)-dione
A solution of ethylenedioxythiophen (3.42 g, 24 mmol) in
anhydrous THF (40 mL) was cooled to ꢀ788C under nitrogen
and treated with a solution of 2.5M n-butyllithium (10.56 mL,
26.4 mmol). The temperature was then maintained at 08C for
20 min. The reaction mixture was cooled to ꢀ788C and a
solution of 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(4.46 g, 24 mmol) in THF (10 mL) was slowly added. The
reaction was stirred for 17 h at room temperature, then quenched
with a solution of ammonium chloride. The product was
extracted into diethyl ether, washed with water, and dried
with magnesium sulfate. The product 4f was obtained by
flash chromatography (diethyl ether/hexane) as a white solid
(5.02 g, 78 %). Mp 92.3–94.08C (lit. 928C[17]).[1] dH (CDCl3,
500 MHz) 6.62 (s, 1H), 4.31–4.29 (m, 2H), 4.19–4.17 (m, 2H),
1.34 (s, 12H).[13] dC (CDCl3, 125 MHz) 149.2, 142.5, 107.6,
84.0, 65.2, 64.4, 24.9.
A
solution 2-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane (0.25 g, 0.93 mmol)
and tetrakis(triphenylphosphine) palladium(0) (10 mg) in THF
was degassed and purged with nitrogen. This solution was added
to a frozen degassed mixture of 3 (0.29 g, 0.42 mmol), and 2 M
potassium carbonate (1 mL) in THF in a Schlenk flask. The
resulting mixture was degassed by three cycles of freeze–pump–
thaw, and then refluxed at 758C for 17 h. The reaction
mixture was extracted with chloroform (15 mL), the extract
washed with water (3 ꢂ 10 mL), and dried over magnesium
sulfate. The crude product was purified by flash chromato-
graphy giving the pure product 2f as a dark-purple powder

Solvatochromism in Diketopyrrolopyrrole Derivatives
1337
Preparation of (E)-4,4,5,5-Tetramethyl-2-(4-(4-
methylstyryl)phenyl)-1,3,2-dioxaborolane 4d
(b) A. B. Tamayo, B. Walker, T.-Q. Nguyen, J. Phys. Chem. C 2008,
112, 11545. doi:10.1021/JP8031572
(c) J. Peet, A. B. Tamayo, X. D. Dang, J. H. Seo, T.-Q. Nguyen, Appl.
Phys. Lett. 2008, 93, 163306. doi:10.1063/1.3001802
(d) S. Lunak, Jr, L. Havel, J. Vynuchai, P. Horakova, J. Kucerok,
M. Weiter, R. Hrdina, Dyes Pigments 2010, 85, 27. doi:10.1016/
J.DYEPIG.2009.09.014
(e) S. Lunak, M. Vala, J. Vynuchai, I. Ouzzane, P. Horakova,
P. Moziskova, Z. Elias, M. Weiter, Dyes Pigments 2011, 91, 269.
doi:10.1016/J.DYEPIG.2011.05.004
(f) M. Kaur, D. S. Yang, K. Choi, M. J. Cho, D. H. Choi, Dyes Pigments
2014, 100, 118. doi:10.1016/J.DYEPIG.2013.09.001
(g) J. Liu, Y. Sun, P. Moosin, M. Kuik, C. M. Proctor, J. Lin, B. B. Hsu,
V. Promarak, A. J. Heeger, T.-Q. Nguyen, Adv. Mater. 2013, 25, 5898.
doi:10.1002/ADMA.201302007
4-Formylphenylboronic acid pinacol ester (0.50 g,
2.15 mmol) and diisopropyl-4-methylbenzyl phosphonate
(0.58 g, 2.15 mmol) were dissolved in anhydrous THF (15 mL)
and then chilled to 08C under nitrogen. A solution of potassium
tert-butoxide (0.72 g, 6.45 mmol) in anhydrous THF (5 mL) was
slowly added at 08C and the reaction mixture was stirred at room
temperature for 24 h. The THF was removed under vacuum and
then water (20 mL) was added to the flask. The reaction mixture
was slowly neutralised with 1 M HCl. The white precipitate was
collected by vacuum filtration and washed thoroughly with
water. The product 4d was further dried under a high vacuum
giving a white powder (0.65 g, 94 %). Mp 124–1268C.[1] dH
(CDCl3, 500 MHz) 7.79 (2H, d, J 8.0), 7.50 (2H, d, J 8.5), 7.42
(2H, d, J 8.0), 7.17 (2H, d, J 8.0), 7.15 (1H, d, J 16.0), 7.06 (1H,
d, J 16.5), 2.36 (3H, s), 1.35 (12H, s).[13] dC (CDCl3, 125 MHz)
135.3, 129.7, 129.6, 127.8, 126.7, 125.8, 107.7, 83.9, 25.0,
21.4. m/z (ESI HRMS) 320.19421; [C21H25BO2]þ requires
320.19421.
(h) C. Kim, J. Liu, J. Lin, A. B. Tamayo, B. Walker, G. Wu, T.-Q.
Nguyen, Chem. Mater. 2012, 24, 1699. doi:10.1021/CM202852F
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1521-3900(200209)187:1,617::AID-MASY617.3.0.CO;2-5
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Preparation of (Z)-2-Phenyl-3-(4-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)phenyl)acrylonitrile 4g
A solution of benzyl cyanide (0.5 g, 4.3 mmol) and
4-formylphenyl boronic acid pinacol ester (1 g, 4.3 mmol) in
anhydrous THF (25 mL) was added dropwise to a solution of
potassium tert-butoxide (1.45 g, 12.9 mmol) and 18-crown-6
(0.04 g, 0.15 mmol) in anhydrous THF (25 mL) at 08C. The
resultant mixture was stirred at room temperature for 17 h; then
the THF was removed under vacuum. Water (25 mL) was added
and the mixture was slowly neutralised with sat. aq. NH4Cl. The
mixture was extracted into diethyl ether, washed with water, and
dried with magnesium sulfate. The crude product was chroma-
tographed with hexane/diethyl ether, giving the pure product 4g
as a clear oil (0.9 g, 60 %).[1] dH (CDCl3, 500 MHz) 7.91–7.86
(4H, m), 7.70–7.68 (2H, m), 7.55 (1H, s), 7.47–7.40 (3H, m),
1.36 (12H, s).[13] dC (CDCl3, 125 MHz) 142.3, 136.2, 135.4,
134.6, 129.5, 129.2, 128.5, 126.2, 118.0, 112.6, 84.2, 25.0. m/z
(ESI HRMS) 332.18162; [C21H22BNO2 þ H]þ requires
332.18164.
¨
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support through a CSIRO scholarship (to JR-B). The Victorian Partnership
for Advanced Computing (VPAC) and the National Computational Infra-
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