Structure-properties relationship in diketopyrrolopyrrole based small molecules using functional terminal side chains: Via direct arylation: A joint experimental and theoretical study

Article abstract of DOI:10.1039/c6nj00556j

In this article, we report a series of diketopyrrolopyrrole based donor-acceptor small molecules, PDPP-OC8, PDPP-EC8, PDPP-EG and PDPP-CNEC8, with different phenyl containing flexible terminal side chains such as alkoxy (-OC₈H₁₇), ester ((-CO)OC₈H₁₇), ester with glycol ((-CO)(O(C₂H₄O)₃-CH₃)) and cyano acrylate with an octyl side chain ((-CN)(CO)OC₈H₁₇), respectively, via direct arylation. A comparative analysis has been done on their opto-electronic properties such as absorption, HOMO, LUMO, band gap, thermal properties, packing nature and hole mobility. The physical properties of the four small molecules were studied using absorption spectroscopy, thermal gravimetric analysis, cyclic voltammetry, differential scanning calorimetry and charge carrier mobility measurements. In comparison to molecules with alkoxy terminal side chains, those with ester and cyano acrylate terminal side chains exhibit (i) a lowered optical band gap from 1.65 eV (alkoxy) to 1.43 eV (ester) and (ii) a broad absorption covering the solar spectrum from 500 to 800 nm. From the results we are able to conclude that the terminal side chains significantly alter their optical and electronic properties. Density functional theory calculations have been carried out to substantiate the opto-electronic characteristics and further understand the effects of terminal alkyl chains.

Full text of DOI:10.1039/c6nj00556j

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Structure-properties relationship in diketopyrrolopyrrole based
small molecules using functional terminal side chains via direct
arylation: a joint experimental and theoretical study
Received 00th January 20xx,
Accepted 00th January 20xx
B. Sambathkumar,ab P. Shyam Vinod Kumar,ab K. Saurav,c S. Sundar Kumar Iyer,c
V. Subramanian,ab and N. Somanathan*ab
DOI: 10.1039/x0xx00000x
www.rsc.org/
In this article, we report a series of diketopyrrolopyrrole based donor – acceptor small molecules PDPP-OC8, PDPP-EC8,
PDPP-EG and PDPP-CNEC8, with different phenyl containing flexible terminal side chains such as alkoxy (-OC8H17), ester ((-
CO)OC8H17), ester with glycol ((-CO)(O(C2H4O)3-CH3)) and cyano acrylate with octyl side chain ((-CN)(CO)OC8H17
)
respectively via direct arylation. A comparative analysis has been done on their opto-electronic properties such as
absorption, HOMO, LUMO, band gap, thermal properties, packing nature and hole mobility. The physical properties of the
four small molecules were studied using absorption spectroscopy, thermal gravimetric analysis, cyclic voltammetry,
differential scanning calorimetry and charge carrier mobility measurements. In comparison to molecules with alkoxy
terminal side chain, those with ester and cyano acrylate terminal side chains exhibit (i) low optical band gap from 1.65 eV
(alkoxy) to 1.43 eV (ester) (ii) broad absorption covering solar spectrum from 500 to 800 nm. From the results we are able
to conclude that the terminal side chains significantly alter their optical and electronic properties. Density functional
theory calculations have been carried out to substantiate the opto-electronic characteristics and further understand the
effect of terminal alkyl chains.
(DPP) based materials have received more interest in organic solar
cells. Their well conjugated fused aromatic frame work favors
strong π-π interaction, good absorption in visible range and their
electron deficient lactam groups make DPP a strong acceptor.
1. Introduction
Combining DPP with
a variety of donor materials, offers
Over the last two decades organic photovoltaics (OPVs) based on
solution processed bulk heterojunction (BHJ) comprised of
polymer/fullerene blends have been of much attraction to research
community because of their unique advantages such as low cost,
light weight, flexibility and large area device fabrication using
various printing methodology.1 Even though impressive power
conversion efficiencies (PCE) over 10% in single junction and 11% in
tandem device have been realized in polymer based donor
materials, their batch to batch variation in molecular weight,
polydisperse nature and purity can lead to variation in their
processing properties and device performances.2 In contrast to
polymers, small molecular donor materials are highly attractive
owing to their monodisperse nature, definite molecular structure,
specific molecular weight, excellent purity and less batch to batch
variation.3 Inspiring success in their power conversion efficiencies
over 10% in single layer has been reported based on small
molecules as donor materials.4 Among various small molecular
organic semiconductors, thiophene flanked diketopyrrolopyrrole
development of wide range of push–pull or donor–acceptor (D–A)
systems. These systems exhibit low band gap due to intramolecular
charge transfer between donor and the acceptor.5 In general, such
type of D-A systems can be effectively built using traditional cross
coupling methodologies like Suzuki, Negishi, Stille, etc. The main
drawback of cross coupling is that it needs an organometallic
precursor such as organoboron or organotin components which
require multi steps for monomer synthesis, expensive purification
and release toxic byproducts during coupling. Recently a new
technique named direct arylation, alternate to traditional cross
coupling methodologies has emerged as a convenient tool for
synthesizing organic conjugated material. The advantages of direct
arylation are simplicity of reactant and absence of organometallic
precursor which makes it as cheap, time effective and
environmentally benign.6 Recently Welch and coworkers reported a
series of DPP based small molecules with different end groups and
demonstrated that α-C–H bonds of thiophene-DPP can be subjected
to direct arylation using palladium catalysis.7,8 Thus DPP flanked
thiophene segment should be the perfect candidate for the study of
direct arylation.9 However, in contrast to polymer, small molecule
based semiconductor suffers unsatisfactory film formation and poor
morphology because of their inherent crystalline nature which
results low device performance. This can be overcome by elegant
molecular design and by controlling the side chain engineering.10
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hexafluorophosphate, Triethylene glycol monomethyl ether,
N,N′-Dicyclohexylcarbodiimide, 4-(Dimethylamino)pyridine,
N,N-Dimethylacetamide, Pd(OAc)2 and Pivalic acid were
purchased from Sigma Aldrich and used without further
purification.
2.2 Instrumentation
1H and 13C NMR spectra were recorded in Bruker NB 400 MHz
using CDCl3 as the solvent and tetramethylsilane (TMS, d = 0)
as the internal standard. Molecular weights of small molecules
were confirmed using Matrix-assisted laser desorption and
ionization (MALDI) mass spectra were recorded on a Bruker
ultraflextreme MALDI-mass spectrometer by using 4-HCCA (a-
hydroxycinnamic acid) matrix. Thermal stability were analyzed
using Mettler TOLEDO TGA/SDTA 851e at a heating rate of 10°
C/min. X-ray diffraction (XRD) data of small molecules were
obtained using Bruker AXS D8 Advance X-ray diffractometer.
Cu Kα wavelength was used for XRD experiments. Differential
scanning calorimetry was performed using DSC Q100 (TA
Instruments). DSC curves were recorded at a scanning rate of
10°C min under a nitrogen flow. Electrochemical analysis were
performed by cyclic voltammetry using CH instruments, CHI
600D electrochemical work station with platinum disc as
Figure 1. Chemical structure of four small molecules.
Selecting a rigid conjugated backbone helps in enhancing π-π working electrode, Ag/AgCl as reference electrode and
interaction and also favors solid packing. Apart from using side platinum wire as counter electrode. The experiments were
chains for solubility purpose, recent study reveals that it has an carried out by coating a thin layer of small molecules in
ability to control absorption, emission, HOMO, LUMO, band gap, platinum disc electrode and measurements were taken in
morphology and molecular packing. On the other hand, there is no 0.1M
Tetrabutylammonium
hexafluorophosphate
as
appropriate protocol for selecting this flexible side chain.11 supporting electrolyte. UV-vis-NIR absorption spectra were
Therefore, designing a donor material with proper side chain with recorded using Varian Carey 50 Bio UV−visible
appropriate length and functional group are the key factors for spectrophotometer. For solid state, spin coating was done on
quartz from solution in chloroform with a concentration of
controlling their crystalline nature, solubility, miscibility and
orientation of small molecules in thin film which in turn control the 1mg ml-1.
charge separation and charge transport properties. Herein, we
2.3 Theoretical Calculations
report four different solution processed small molecules as donor
material via direct arylation, with chromophoric thiophene flanked
DPP moiety bonded to phenyl group containing various flexible
terminal side chains. The effect of different functional flexible side
chains in the optical property, band gap, packing nature, HOMO and
LUMO energy levels of the four small molecules has been
Density functional theory (DFT) calculations were performed using
the Gaussian 09 ab initio quantum chemical software package.14
The ground state geometries of all the molecules were optimized in
gas phase using the hybrid Becke’s three-parameter gradient-
investigated in detail. To gain in-depth understanding of the opto- corrected functional (B3LYP) 15 along with 6-31G(d) basis set. The
electronic profiles of the four small molecules, theoretical
calculations have been carried out using Density Functional Theory
(DFT). 12
B3LYP functional has been proved to be reliable for its optimum
prediction of properties for DPP based systems.16 Frequency
calculations were performed at the same level of theory to check
for consistency in the optimized geometries. The excited state
properties of the systems were calculated using the time
dependent-DFT theory (TD-DFT) using the hybrid PBE exchange
correlational functional (PBE0) 17 in combination with the 6-31G(d)
basis set. Single-point TD-DFT calculations for the first 20 singlet-
singlet vertical transitions were carried out using the optimized
geometries of the systems. The solvent environment was
incorporated using the polarizable continuum solvation model
2. Experimental Section
2.1 Materials
2,5-dioctyl-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-
1,4(2H,5H)-dione was prepared, according to the literature
procedure.13 4-Bromophenol, 4-Bromobenzaldehyde, Octyl (PCM) with chloroform as solvent.18
cyanoacetate, Piperidine, 4-Bromobenzoic acid, 1-Octanol,
Potassium
carbonate,
Tetrabutylammonium
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Scheme 1. Synthetic strategy for four small molecules
15 nm of electrode, rate was slowly increased to 1 A/s and after 50
nm, rate was increased to 5 A/s. Mobility was calculated by fitting
the SCLC model to the I-V characteristics from the following
equation.
2.4 Mobility Measurements
Device fabrication
ꢅ
ꢈꢉꢊꢋ
9
ꢆ ꢇ ꢆ
ꢀ ꢁ μꢂꢃꢂꢄ
8
Hole only device of the small molecules were fabricated on
patterned ITO coated glass substrate. After patterning, the
substrates were thoroughly washed with soap solution. Then
substrates were ultrasonicated in deionised water for 15 minutes.
As a final cleaning step, substrates were boiled in RCA solution for
15 minutes and then ultrasonicated in deionised water for 15
minutes. RCA solution is a mixture of H2O2 : NH4OH : H2O in the
proportion of 1:1:5. After drying the substrates with nitrogen gun,
PEDOT:PSS was spin coated. PEDOT:PSS solution was prepared by
mixing 2 parts water to 1 part PEDOT:PSS followed by filtering the
solution with 0.45 micron filter and then spin coated at 1000 RPM
for 60 seconds. Spin coated PEDOT:PSS was annealed at 120oC for
20 minutes. After letting the substrate cool at room temperature,
active material was spin coated. Active material solution was
prepared in chloroform with concentration of 10mg/ml and
solution was stirred on a magnetic stirrer for 4 hours. Solutions
were filtered with 0.45 micron filter and then spin coated at 1000
RPM for 60 seconds. The Gold (Au) electrode was deposited by
ꢌꢍ
where J is the current density, d is the film thickness of active layer,
μ is the hole mobility, εr is the relative dielectric constant of the
transport medium, ε0 is the permittivity of free space (8.85 × 10−12
Fm-1), V is the voltage applied and Vbi accounts for various barriers
in the device. The Vbi can be determined from the transition
between the ohmic region and the SCLC region.
3. Results and Discussion
3.1 Synthesis
The targeted four small molecular structures are displayed in Figure
1 and their synthetic routes are shown in Scheme 1. All the
molecules were synthesized using direct arylation under ligand free
condition using 3 equiv K2CO3, 33 mol % pivalic acid (PivOH), 5
mol % Pd(OAc)2 and anhydrous dimethylacetamide (DMA) at 110
°C for 16 h to achieve the target small molecules PDPP-EC8, PDPP-
EG, PDPP-OC8 and PDPP-CHO in yields of 57%, 61%, 58% and 47%.
Coupling or branching defect was observed in Maldi even after
purification with column chromatography.19 All the four small
molecules were carefully purified using recrystallisation thrice with
ethylacteate .The final molecular structures of
thermal evaporation under
a
vacuum of 10-5 millibar. Initial
deposition rate was 0.1 A/s and after depositing 15 nm of
electrode, rate was slowly increased under a vacuum of 10-5
millibar. Initial deposition rate was 0.1 A/s and after depositing
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Figure 2. Optimized structures of the small molecules at B3LYP/6-31G(d) level of theory and their corresponding dihedral angles.
the systems were confirmed by 1H-NMR, 13C-NMR and MALDI-TOF
(for spectra see SI). The synthesized molecules exhibit good
solubility in common organic solvent such as chloroform,
tetrahydrofuran, chlorobenzene and dichlorobenzene.
3.2 Geometry
The optimized geometries from the DFT calculations along with
dihedral angles of each molecule are displayed in Figure 2. It can be
noted that the calculated dihedral angles of the molecules vary
between 15° to 26° degrees. Also, the systems PDPP-EC8 and PDPP-
EG are showing almost similar conformational twists (≈23°) along
the backbone. The introduction of the PDPP-CNEC8 unit induces
less conformational twist in the order of ≈16° when compared to
other systems. This may be due to the limitations induced to the
steric twisting by the vinylene double bond. Among the four
molecules, PDPP-OC8 shows the larger torsions of about ≈26°.
These results elicit the influence of various side chains on the
Figure 3. Normalized UV-vis absorption spectra of PDPP-OC8, PDPP-
backbone planarity of the molecule and thereby affecting the inter-
CNEC8, PDPP-EC8 and PDPP-EG in chloroform and in film.
molecular packing and conjugation of the system.
in Eg (by ca. 0.1–0.2 eV). It is noteworthy that PDPP-CNEC8, PDPP-
3.3 Optical properties
EG and PDPP-EC8 exhibit
a broad absorption covering solar
spectrum in the range of 500 to 800 nm. Specifically ~120 nm red
shift is observed in the onset of absorption (λonset) when replacing
the alkoxy side chain by electron-withdrawing ester substituent side
chain their optical band gap in thin film is reduced from 1.65 to
1.43eV. The absorption maxima (λmax) also show red shift in both
solution and thin film when replacing alkoxy to ester. This may be
due to strong π-intermolecular interaction in solid state. Particulary
electron withdrawing side chain substituted molecules PDPP-EG
PDPP-CNEC8 and PDPP-EC8 shows a discernible vibronic shoulder
attributed to molecular rigidity and aggregating nature in solid
state7,21,22 whereas PDPP-OC8 with alkoxy side chain exhibit
narrower absorption and sharp onset. The TD-DFT results
The ultraviolet/visible (UV/vis) absorption spectra for the four small
molecules in dilute chloroform solution and spin coated thin film
are shown in Figure 3. Their corresponding optical parameters are
listed in Table 1. All the molecules exhibit two distinct absorption
bands at shorter and longer wavelength regions attributed to π-π*
and internal charge transfer. Such absorption profile reflects the
typical donor–acceptor π-conjugated system.20 Compared to
solution all the molecules exhibit red shifts in both shorter and long
wavelength regions in thin film. This is due to strong intermolecular
π–π stacking in solid state. The optical band gap in solid state
derived from the onset of absorption spectra decreases in the order
of PDPP-OC8 > PDPP-EC8 ≈ PDPP-EG > PDPP-CNEC8, with decrease
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substantiate experimental results and the absorption profiles of the their transitions. In all the molecules, the HOMOs are delocalized
molecules extend over a broad range from ≈300 to 700 nm. The mainly on the central thienyl DPP unit whereas the LUMOs extend
dominant lower energy band (≈600 to 700 nm) transitions of the from DPP to phenyl fragment except PDPP-OC8. This is because of
systems are the intra-molecular charge transfer absorption peaks the
which mainly arise from HOMO → LUMO transiꢀons. Each system
exhibit another important peak at the high energy band (≈300 to
400 nm) of π-π* nature with HOMO-2 → LUMO transiꢀons. The
simulated absorption spectra of the systems elicit that changing the
terminal end groups from PDPP-OC8 to PDPP-CNEC8 results in
bathochromic shift of the absorption maxima from ≈600 to 690 nm.
It is important to note that the molecule PDPP-CNEC8 exhibit the
maximum red-shifted absorption
electron
withdrawing
nature
of
side
chain.
Figure 6. Cyclic voltammograms for small molecules in 0.1 M
Bu4NPF6 acetonitrile solution at the rate of 0.1 Vs-1, with Ag–AgCl as
the reference electrode.
3.4 Thermal properties
In order to analyze the thermal transition of the four donor small
molecules, Differential Scanning Calorimetry (DSC) was performed
and the results are depicted in Figure 4. Their melting and
crystallization temperatures are presented in Table 1. The melting
temperatures (Tm) of PDPP-EC8, PDPP-EG, PDPP-CNEC8 and PDPP-
OC8 are 227, 197, 229 and 236 °C respectively and their
corresponding crystallizing temperatures (Tc) are of 209, 185, 215
and 225 °C. Among the four molecules, PDPP-OC8 exhibits high
melting and crystallization temperature which indicates their rigid
solid packing. Compared to PDPP-EC8, PDPP-EG exhibits low
melting and crystallization temperature due to the high flexible
nature of the glycol chain.23 PDPP-EC8 exhibit double transition 221
and 104oC probably due to liquid crystal nature and these behavior
are not shown for other three molecules because of the nature of
flexible side chain to the conjugated core which essential for LC
behavior may reduced.24 Thermal behavior of small molecules have
been further investigated using Thermogravimetric analysis (TGA)
as shown in Figure 5 and their corresponding decomposition
temperatures were listed in Table 1. All the small molecules possess
good thermal stability with decomposition temperatures greater
than 300 °C (5% weight loss) under N2 atmosphere.
Figure 4. DSC scans of PDPP-EC8, PDPP-CNEC8, PDPP-OC8 and
PDPP-EG
at about ≈690 nm. Analysing the electron density contours of the
small molecular systems is significant to understand the nature of
3.5 Electrochemical Properties
In order to understand the electronic energy levels of small
molecules, cyclic voltammetry has been carried out. The highest
occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) can be estimated from the onset of
Figure 5. TGA plots of PDPP-EC8, PDPP-CNEC8, PDPP-OC8 and
PDPP-EG at a heating rate of 10 oC min-1 in an inert atmosphere
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oxidation and reduction potential which correspond to ionization potential (IP) and electron affinity (EA) of small molecules.
Table.1 Optical, electrochemical and thermal properties of PDPP-OC8, PDPP-EC8, PDPP-EG and PDPP-CNEC8
opt
Small
λmax(nm)
λedge(nm)c Eg
EHOMO
(eV)e
ELUMO
(eV)e
Egelc (eV)f
molecule
(thin
film)
(eV)d
DSC
TGA
Td (◦C)i
434
Solutiona
Thin filmb
Tm (◦C)g
236
Tc (◦C)h
225
PDPP-OC8
PDPP-EC8
PDPP-EG
341,410,569,609 352,411,434,599,654 751
1.65
1.43
1.43
1.41
-5.31
-5.36
-5.28
-5.24
-3.62
-3.82
-3.76
-3.74
1.69
1.54
1.52
1.50
350,569,609
351,571,611
397,603,635
371,547,623,661
370,548,629,666
426,646,711
865
867
877
227
209
381
197
185
418
PDPP-CNEC8
229
215
303
aAbsorption spectra recorded in CHCl3 solution (1.0× 10-5M). bSpin coated on CHCl3 solution (1 mg/ml) on quartz plate. cOnset of absorption
e
in thin film. dOptical band gap calculated using the formula Egopt=1240/λedge,film. Calculated according to the formula EHOMO/LUMO = -e(Eox/red
+ 4.41) (eV). fEgelc= ELUMO-EHOMO.gTm = melting temperature. hTc = Crystallization temperature. iTd = decomposition temperature.
Figure 7. Electron density plots of FMOs (H atoms are not shown for clarity), optical transitions and oscillator strengths of the four small
molecules calculated using DFT B3LYP/6-31G(d) and TD-DFT PBE1PBE/6-31G(d) levels of theory.
Cyclic voltammogram of small molecules in thin film are displayed The electrochemical band gaps calculated from the above equation
in Figure 6 and their electrochemical parameters are shown in Table are in good agreement with optical band gaps. From the figure 6 all
1. From the oxidation and reduction onset values, the HOMO and the four molecules exhibit irreversible oxidation and reduction
LUMO levels can be calculated with respect to ferrocene standard wave and the similar redox process was also observed for other dpp
by the following eqn.25
based molecular system.26 Specifically, the HOMO energy level of
both PDPP-OC8 (-5.31 eV) and PDPP-EC8 (-5.36 eV) are similar,
however much variation is noticed in LUMO energy level of PDPP-
OC8 (-3.62 eV) to PDPP-EC8 (-3.82 eV). This clearly enlighten the
electron-withdrawing ester effect in PDPP-EC8. This effect
HOMO = −IP = − (Eox + 4.41) (eV)
LUMO = −EA = − (Ered + 4.41) (eV)
(1)
(2)
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significantly lowers the LUMO level for PDPP-EC8 and PDPP-EG
which reduces the band gap. The frontier molecular orbitals of the
four systems were calculated using DFT and compared with the
experimental CV results. The calculated energy levels of the small
molecules are shown in the Figure 7. Although DFT calculations
have limitations in accuracy of the predicted molecular energy
levels, the trends in variations of these energy levels can be reliable.
The theoretical results show that the HOMO and LUMO energy
levels are consistent with the experiment. It can be observed from
these energy levels that the HOMO energies range from -4.55 to -
5.13 eV while the LUMOs range between -2.35 and -3.19 eV. The
energy gaps of the various side chain functionalized small molecules
are in the order PDPP-OC8 > PDPP-EG ≈ PDPP-EC8 > PDPP-CNEC8. It
is interesting to see that the glycol and the alkoxy functionalized
systems show similar energy levels. These results indicate that the
system PDPP-CNEC8 maintains a low energy gap among the four
candidates.
3.6 X-Ray diffraction (XRD)
In order to understand the packing nature of the small molecules
thin film X-ray diffraction (XRD) was performed in thin film, spin
coated from glass substrate. The XRD patterns of four small
molecules are displayed in Figure 8. All the four small molecules
exhibit first order diffraction peak at 2Ɵ=4.76 for PDPP-OC8, 2.96
for PDPP-EC8, 3.08 for PDPP-EG and 3.44 for PDPP-CNEC8
corresponding to d–spacing values of 18.33, 29.62, 28.52 and 25.67
Å, respectively which attribute to interchain ordering. From the
figure it is clear that, PDPP-OC8 exhibit second order diffraction
peak at 2Ɵ=9.20, 13.80 which correspond to the d values of 9.63
and 6.42 Å attributed to side chain ordering and the third order
diffraction peak at 2Ɵ=18.50 correspond to 4.81 Å attributed to
intermolecular d spacing.22,27 From the above result, it was clear
that PDPP-OC8 exhibit highly ordered organization in the solid
state.
Figure 8. XRD traces of PDPP-OC8, PDPP-EC8, PDPP-EG and PDPP-
CNEC8.
3.7 Hole mobility
The hole mobility of the pristine small molecules were measured
using space charge limited current (SCLC) method with the device
structure of ITO/ PEDOT: PSS/small molecule/ Au and their SCLC
curve were shown in Figure 9. The hole motilities of PDPP-OC8,
PDPP-EC8, PDPP-EG and PDPP-CNEC8 were estimated as 1.26 x 10-4
cm2 V−1
s
−1, 7.17 x 10-6 cm2 V−1 s −1, 2.69 x 10-5 cm2 V−1 s −1 and 1.03 x
Figure 9. Current vs voltage characteristics of hole only device of
−1
10-5 cm2 V−1
s
respectively. Compared to PDPP-EC8, PDPP-EG,
small molecules.
PDPP-CNEC8, the hole mobility of PDPP-OC8 improved by two
order of magnitude probably due to the electron donating nature of
alkoxy group which was differ from the other three (electron
withdrawing substitution).The results were also well correlated with
XRD as discussed above.
4. Conclusion
In Conclusion, we successfully synthesized a series of DPP based
small molecules using direct arylation methodologies. The four
small molecules were confirmed by NMR and MALDI-TOF MS. A
comprehensive study has been performed using absorption
spectroscopy, cyclic voltammetry, XRD and hole mobility in order to
analysis how the functional side chain substitution impact on
optical, HOMO, LUMO, band gap and device performance. We find
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Liu, Y. Sun, P. Moonsin, M. Kuik, C. M. Proctor, J. Lin, B. B.
Hsu, V. Promarak, A. J. Heeger, T. Q. Nguyen, Adv. Mater.,
2013, 25, 5898; (c) S. Qu and H. Tian, Chem. Commun., 2012,
48, 3039.
that a simple replacement of alkoxy group from ester leads a drastic
change in their absorption profile (by ≈0.2 eV and 120 nm red shift
on onset) enlighten the effect of electron withdrawing nature of
terminal position. Of all the four molecules PDPP-OC8 exhibits high
carrier mobility, due to highly ordered structure and strong
intermolecular interaction which was clearly supported by XRD
data. The theoretical calculations provide more insights regarding
the nature of the small molecules. The simulated absorption
spectra and the charge density plots reveal the effect of side chains
in the optical properties of the molecules. We have also identified a
coupling defect occur in thienyl DPP and conveniently identified by
Maldi –mass. Thus a basic understanding on the selectivity of C-H
monomer in direct arylation has yet to be carefully understood and
proper optimization on reaction condition is the essential tool for
assuring the reaction yield and purity.
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Structure-properties relationship in diketopyrrolopyrrole based small
molecules using functional terminal side chains via direct arylation: a joint
experimental and theoretical study
B. Sambathkumar,ab P. Shyam Vinod Kumar ,ab K. Saurav,c S. Sundar Kumar Iyer,c V. Subramanian,ab and
N. Somanathan*ab
The role of terminal side chain on diketopyrrolopyrrole based small molecules has been analyzed and it provides
how their optoelectronic properties vary according to their side chain nature.