Phthalimide-based π-conjugated small molecules with tailored electronic energy levels for use as acceptors in organic solar cells

Article abstract of DOI:10.1039/c5tc01877c

The design, synthesis, and characterization of seven phthalimide-based organic π-conjugated small molecules are reported. The new materials are based on a phthalimide-thiophene-CORE-thiophene-phthalimide architecture. The CORE units utilized were phthalimide (M2), diketopyrrolopyrrole (M3), isoindigo (M4), naphthalene diimide (M5), perylene diimide (M6), and difluorobenzothiadiazole (M7); they were specifically selected to progressively increase the electron affinity of the resulting compound. A small molecule with no core (M1) was synthesized for comparison. Each material was synthesized through optimized direct heteroarylation cross-coupling procedures using bench top solvents in air. Combinations of UV-visible spectroscopy (UV-vis), cyclic voltammetry (CV), differential scanning calorimetry (DSC), ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) were used to characterize each material. The use of various core acceptor building blocks with differing electron affinities resulted in the series M1-M7 having a range of energetically deep LUMO levels and a range of HOMO-LUMO gap energies. Meanwhile, the melting and crystallization temperatures of the molecules M1-M7 were also found to vary according to the change in central acceptor unit. Compounds M1-M7 were employed as acceptors in combination with either the polymeric donor P3HT or small molecule donor DTS(FBTTh₂)₂ to understand how the LUMO levels of each acceptor influences the open circuit voltage (V_oc). It was found that, in general, V_oc was only weakly related to the offset between the HOMO energy level of the donor and LUMO level of the acceptor used, with a V_oc of up to 1.2 V being achieved for M1.

Full text of DOI:10.1039/c5tc01877c

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Journal of Materials Chemistry C
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DOI: 10.1039/C5TC01877C
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Phthalimide-based π-Conjugated Small
Molecules with Tailored Electronic
Energy Levels for use as Acceptors in
Organic Solar Cells
Arthur D. Hendsbee,ad Seth. M. McAfee,ad Jon-
Paul. Sun,b Theresa M. McCormick,c Ian G. Hill,b
and Gregory C Welch,ad*
The design, synthesis, and characterization of seven phthalimide-based
organic π-conjugated small molecules are reported. The new materials are
based
on
a
phthalimide-thiophene-CORE-thiophene-phthalimide
architecture. The CORE units utilized were phthalimide (M2),
diketopyrrolopyrrole (M3), isoindigo (M4), naphthalene diimide (M5),
perylene diimide (M6), and difluorobenzothiadiazole (M7); they were
specifically selected to progressively increase the electron affinity of the
resulting compound. A small molecule with no core (M1) was synthesized
for comparison. Each material was synthesized through optimized direct
heteroarylation cross-coupling procedures using bench top solvents in air.
Combinations of UV-visible spectroscopy (UV-vis), cyclic voltammetry (CV),
differential scanning calorimetry (DSC), ultraviolet photoelectron
spectroscopy (UPS) and density functional theory (DFT) were used to
characterize each material. The use of various core acceptor building blocks
with differing electron affinities resulted in the series M1–M7 having a
range of energetically deep LUMO levels and a range of HOMO-LUMO gap
energies. Meanwhile, the melting and crystallization temperatures of the
molecules M1–M7 were also found to vary according to the change in
central acceptor unit. Compounds M1–M7 were employed as acceptors in
combination with either the polymeric donor P3HT or small molecule donor
DTS(FBTTh2)2 to understand how the LUMO levels of each acceptor
influences the open circuit voltage (Voc). It was found that, in general, Voc
was only weakly related to the offset between the HOMO energy level of
the donor and LUMO level of the acceptor used, with a Voc of up to 1.2 V
being achieved for M1.

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acceptor LUMO in a donor-acceptor solar cell, one expects a
progressive increase in open circuit voltages.
Introduction
Donor-acceptor π-conjugated materials have been
extensively used as the active components in sensing devices, thin
film transistors, photoswitches, photovoltaics and a variety of other
useful applications.1–3 With respect to solution processed
photovoltaics, the development of both donor-acceptor (D-A)
conjugated polymers and small molecules has led to the realization
of fullerene-based heterojunction (BHJ) organic solar cells (OSCs)
with power conversion efficiencies (PCE) reaching beyond 9%.4–7
More recently, donor-acceptor (D-A) organic π-conjugated
compounds have found utility as electron-accepting components
(vide infra) in solution processed BHJ OSCs, a position that is
dominated by fullerene derivatives. Key advantages of D-A
compounds compared to fullerene include the fact that electronic
energy levels, absorption profiles, solubility parameters, and self-
assembly tendencies can be precisely controlled through both
selection and functionalization of the D and A building blocks.8–10 In
addition, the majority of D-A compounds are easily synthesized
though standard organic chemistry techniques using widely
available starting materials. These advantages of D-A compounds
can potentially allow for specifically matched electron-donor
electron-acceptor pairings within the BHJ architecture where
photon harvesting by the donor (Channel I process) and acceptor
(Channel II process) are equally efficient, maximizing the photon to
electron conversion efficiency.11–13 Donor materials making use of
the D-A design strategy are well studied and thousands of materials
with different physical properties have been presented in the
literature, allowing for versatility when selecting the donor
component for use in OSCs. On the other hand, acceptor materials
based upon the D-A design strategy have traditionally been far less
studied.
Previously in our lab group we have investigated the use
a phthalimide-thiophene-thiophene-phthalimide (Phth-Th-Th-
of
Phth) architecture with respect to its utility as a charge transporter
and found it to be a good electron transporting material with
mobilities of ~0.2 cm2/Vs.36 This phthalimide-thiophene based small
molecule has also been used in our group to produce OSCs with
high Voc, ~1.0 V, when paired as an acceptor with P3HT.37 Moving
forward, we incorporated the diketopyrrolopyrrole (DPP)
chromophore as
a central building bock in this design and
demonstrated that DPP and phthalimide can be used together to
make low band gap electron transporting materials.38 In another
study we have investigated the use of the isoindigo (IS)
chromophore as a central building block and demonstrated that it
can also be used to produce low band gap materials, that when
used as the electron-accepting component in OSCs, gave devices
with a Voc of ~1.0 V.39
We have also performed a theoretical investigation on a
series of electron deficient building blocks with acceptor-donor-
acceptor-donor-acceptor (A1DA2DA1) architecture (where A1 and A2
have different electron affinities). This has demonstrated that
electronic energy levels and band gaps can be systematically tuned
by varying the terminal and/or core electron acceptor units (A1 and
A2, respectively).40
To expand on the Phth-Th-CORE-Th-Phth architecture we
have set out to incorporate naphthalene diimide (NDI), perylene
diimide (PDI), difluorobenzothiadiazole (F2BT) and phthalimide
(Phth), in addition to the previously utilized IS and DPP
chromophores, as the central acceptor units (Figure 1). Each of the
selected building blocks has a different electron affinity due to the
nature of the functional groups on their respective aromatic cores.
This change in electron-accepting character in the central acceptor
unit of our design was predicted to produce a series of molecules
with a range of LUMO energy levels. Herein, we evaluate the effect
that each electron deficient building block has on the optical,
thermal, and electronic properties of these molecular systems and
discuss the impact that the inclusion of each acceptor core has on
the Phth-Th-A-Th-Phth structure.
Within the past year there has been
a surge of
publications reporting the use of D-A conjugated polymers and
small molecules as an alternative to fullerene in BHJ OSCs with PCEs
surpassing 6% for both polymer and small molecule acceptors.14–16
In particular, the use of small molecule based D-A compounds as
electron-acceptors has been met with great success. For instance,
Sellinger et al. have used benzothiadiazaole (BT) - phthalimide
based acceptors to achieve PCEs up to 3.7 % when paired with
thiophene based donor polymers.17,18 Fluorene19–22 and fused
fluorene-thiophene14,23 building blocks have also been used in
several high performance OSC systems. Jenekhe and co-workers
have also explored imide based acceptors,24 recently reporting on
an electron deficient imide based framework to create OSCs with
PCEs reaching 5%.25 Perylene diimides are well known for their
electron deficient character and have been used to produce some
of the highest performing small molecule acceptor based solar
cells.26–28,29,30 There exist many additional examples that utilize the
D-A strategy of combining building blocks; however, for additional
details the reader is directed to a selection of recent reviews
discussing this subject.31–35
Materials Synthesis.
The synthesis of small molecules M1–M7 was completed
via direct heteroarylation (DHA) using the heterogeneous catalyst
41
SiliaCat® DPP-Pd, following our previously reported methods.27,
All precursor materials were synthesized using literature or
modified literature procedures with full details in the Supporting
Information. For each DHA reaction, catalyst loadings of 5 mol% Pd
with 20 mol% pivalic acid and potassium carbonate as the base
were used. For the phthalimide end capping units, 1-ethylpropyl
alkyl chains were added via condensation of the anhydride with 1-
ethlprophylamine. Branched alkyl chains such as this can provide
increased solubility compared to their linear counterparts while
minimizing the amount of non-conjugated moieties in the
molecule.42 For both of the CORE components NDI and PDI the
same 2-ethlypropyl chains were used which are well known to
promote the solubility of these chromophores.42,43 For the
phthalimide, IS, and DPP CORE fragments, alkylation with linear C8
In the design of new electron accepting materials one of
the key criteria sought after is relatively deep highest occupied
molecular orbital (HOMO) energy levels and low lying lowest
unoccupied molecular orbital (LUMO) energy levels to ensure
efficient Channel I and Channel II processes. The synthetic diversity
of D-A type compounds allows for precise control over electronic
energy levels thereby allowing for optimization of energy offsets.9
Specifically, by increasing the offset between the donor HOMO and

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alkyl chains, known to promote solubility of these chromophores M2 bearing a phthalimide core has absorbance maxima that is only
were added using previously reported methods.38,39
slightly red shifted compared to solution (λmax = 404 nm). The
absorbance maximum of M3 shifts towards higher energies in the
Synthesis proceeded in a similar manner for compounds solid state (λmax = 592 nm) and the absorbance profile broadens
M1–M7, and synthetic results are summarized in Table S1. Notably, with a low energy shoulder observable around 680 nm (λonset =727
small molecule M7 required the use of toluene as a solvent for the nm). Compound M4, which has a central IS unit undergoes a
reaction due to a lack of product formation when the reaction was substantial change in its absorbance profile upon thin film
attempted in polar aprotic solvents such as DMA or DMF, even over formation. The absorbance maximum for M4 shifts towards higher
a range of temperatures (See ESI). The effective use of toluene as a energies (λmax = 450 nm), and the absorbance profile is slightly
solvent for promoting direct heteroarylation reactions involving the broadened with a red shifted onset of absorption (λonset = 727 nm).
BT building block has been noted recently in the literature.44 All In contrast, compound M5, which has a NDI core shows very little
products were purified in the same manner, by first separating the change in its UV-visible absorption profile upon transitioning from
heterogeneous catalyst from the reaction mixture using a filtration solution to film with λmax = 364 nm and λonset = 611 nm. M6, which
apparatus, followed by column chromatography on silica-gel with a contains a PDI core, displayed minimal change in the absorption
pentane–CH2Cl2 gradient to elute the products. Each of the new profile upon transitioning from solution to the solid state (λmax
=
compounds M1-M7 was found to be soluble in common organic 364 nm and λonset = 611 nm). Compound M7, bearing a F2BT core
solvents used for solution processing.
also underwent minimal change in its absorption profile upon
transitioning from the solution to the solid state (λmax = 374 nm
λonset = 704 nm). It is clear from the solution and thin film
absorption experiments that each core included in the Phth-Th-
Optical Characterization of Small Molecules M1–M7
To examine the optical properties of M1–M7, the UV-Vis CORE-Th-Phth architecture has a definite effect on the optical
spectra of M1–M7 were obtained in CHCl3, spectra are shown in properties of the resultant molecule. In addition the different
Figure 2 and data is tabulated in Table 1. Due to the differing observations made for M1–M7 upon transitioning from solution to
electronic properties of the core units used in the design film indicated that the inclusion of the core acceptor unit was
architecture, small molecules M1–M7 were each a noticeably having a definite effect on the self-assembly of the molecules from
different colour. Compound M1 appears yellow in CHCl3 solution solution.
and the UV-Vis profile of M1 is broad with λmax at 424 nm and λonset
at 496 nm. Compound M2, bearing a phthalimide core exhibits a
very similar absorption profile to that of M1, and also appears
yellow in solution having λmax at 395 nm and λonset at 453 nm.
Interestingly the absorbance maxima do not change significantly
from M1 to M2, despite the relative extension of conjugation via an
additional electron withdrawing phthalimide unit. Compound M3
appears dark blue in CHCl3 solution and has λmax at 611 nm and λonset
at 666 nm. This strong low energy absorbance maxima is commonly
seen from DPP small molecules and polymers and is attributed to
intramolecular charge transfer (ICT).38,45–47 The small molecule M4
bearing an IS core absorbs light across the entire visible spectrum
and appears black in solution. The λmax is 440 nm and λonset is 678
nm for compound M4. This type of absorption profile is typical for
IS based D-A systems, which frequently produce strongly absorbing
materials with narrow band-gaps.39,48–51 Small molecule M5,
bearing a NDI core appears dark red in solution and has a broad low
energy absorbance band extending beyond 600 nm. For compound
M5 the peak absorbance occurs at 364 nm and the onset of
absorbance at 587 nm. Compound M6, which contains a PDI core
appears purple in solution and displays a similar absorbance profile
to M5 having a strong high-energy absorbance maxima (λmax = 374
nm) and a broad absorbance band at lower energies (λonset = 704
nm). The absorbance profiles of both molecules M5 and M6 display
a strong high energy absorption band and a relatively weaker low
energy absorption band, which are features often observed for
both NDI52–54 and PDI55–57 based chromophores. Small molecule M7
containing a F2BT core appears red-orange in solution and exhibits a
strong absorbance band (λmax =464 nm, λonset = 567 nm).
Thermal Characterization of Small Molecules M1–M7
In order to gain more insight into the solid phase
properties, small molecules M1–M7 were evaluated using
differential scanning calorimetry (DSC) to probe their melting and
crystallization behaviour. Samples were heated from 50 °C to 300 °C
for 3 cycles under air. Compound M1 displayed a sharp melting
transition at 277 °C and a crystallization transition at 181 °C.
Compound M2, interestingly, displayed no melting or crystallization
transitions in the temperature regime used indicating that the
incorporation of the phthalimide unit as a central acceptor in the
Phth-Th-CORE-Th-Phth architecture has a large effect on the
thermal properties of the resulting compound (M1 vs M2).
Compound M3 showed multiple transitions in the DSC curve, two
melting transitions at 144 °C and 254 °C, and two corresponding
crystallization transitions at 126 °C and 209 °C. The lower
temperature transitions are attributed to the inter chain stacking of
the alkyl groups, while the higher temperature transitions are
attributed to a lamellar melting of the aggregated π-conjugated
molecules, this type of behaviour is often seen in large π-extended
chromophores with appended alkyl chains.58,59 Compound M4
exhibited sharp melting and crystallization transitions at 259 °C and
209 °C, respectively. Small molecule M5 displays
a melting
transition in the DSC curve at 279 °C and does not display a
crystallization transition. Small molecules M6 and M7 displayed no
melting or crystallization peaks on the DSC. The lack of a melting
point in the range of temperatures studied using the DSC for
compounds M2, M6 and M7 was confirmed using a melting point
apparatus, where compound M2 was found to melt at 334-335 °C,
compound M6 began to melt at ~250 °C and continued to become
less viscous until approximately 270 °C. For compound M7 the
melting transition was observed at 305-308 °C.
The UV-Visible absorbance profiles of thin films on glass
substrates were obtained and plots of absorbance vs. wavelength
for compounds M1–M7 are shown in Figure 2, with the data
summarized in Table 1. Each of the cores used in M1–M7 can have
a different influence on the supramolecular structure of the
resultant materials when transitioning from solution into the solid
state. M1, for example has an absorbance maxima in the solid state
that is blue shifted from the solution (λmax = 468 nm). Compound
Electronic Characterization of Small Molecules M1–M7.
With the UV-visible data in hand showing a dramatic
difference in the optical band-gaps of M1–M7, caused by the

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substitution of the different core fragments to the phthalimide- Gaussian09 suit of programs.68,69 The structures for the rotational
thiophene architecture, cyclic voltammetry (CV) in CH2Cl2 solution isomers of each compound were optimized to a local minimum. The
was then used in order to gain rapid experimental insight on the lowest energy confirmation of each molecule is shown in Figure 4.
ionization energy (IE) and electron affinity (EA) for small molecules All subsequent calculations were performed on that isomer. The
M1–M7 (Figure 3). Experimental details and tabulated oxidation geometries were optimized with B3LYP/6-311+G(d)70,71 except M4
and reduction potentials can be found in the Supporting which would not converge with diffuse functionals. For M4 the
Information. The use of cyclic voltammetry data for calculating geometry was optimized with B3LYP/6-31G(d) and a single point
frontier orbital energies may not provide exact results as discussed energy calculation was performed with B3LYP/6-311+G(d) to obtain
in a recent paper;60 however, the data can still provide useful energy and orbital diagrams. The frontier orbital energies are given
insight into the relative energy levels of compounds that are in Figure 5A. The calculated HOMO and LUMO energy values for the
studied using CV under the same conditions. As shown in Figure 3, ground state molecules follow similar trends, to experimental
the EA can be systematically increased by increasing the electron values of IE and EA as determined from CV and UPS (see Figure
affinity of the central core unit. M1 and M2, with no core and a S25).
phthalimide core possessed relatively low EAs of 3.03 and 2.98 eV,
respectively. Introducing the more electron deficient DPP (M3) and All compounds exhibit distortions from planarity along their
IS (M4) cores increases the EA of the molecules to 3.44 and 3.64 eV,
respectively. Clearly the IS unit is a stronger electron-accepting unit
than DPP. Compounds M5 and M6 were found to have similar
values for EA in the range from 3.8 to 3.9 eV. The very high EAs
found for M5 and M6 are indicative of the strong electron accepting
nature of the NDI and PDI building blocks. Small molecule M7 was
found to have an EA of 3.29 eV which is to those values found for
the IS and DPP containing molecules. An especially important
feature of OSCs is the open circuit voltage of the cell (VOC), the
maximum obtainable value of which is related to the energy level
offset between the HOMO of the donor material and the LUMO of
the acceptor material. This offset should be maximized; however,
the LUMO level of the acceptor must still lie in an energetic position
such that electron transfer from the excited donor material can
take place.61 Small molecules M1–M7 all show promise in this
regard, with LUMO levels that are low enough in energy to facilitate
the transfer of electrons from donor materials material while still
having potential for high VOC when paired with common donor
materials such as P3HT.61,62 The CV plots (reduction only) are shown
in Figure 3B, full CV plots are given in the Supporting Information
for this document (Figure S22).
respective π-conjugated backbones with compounds M1, M3, M4,
and M7 having the most planar structures (Figure 4, data tabulated
in Table S3). The imide based cores of M2, M5, and M6 cause a
significant twist in the backbone due to ortho-hydrogen atom
interactions. The HOMO and LUMO orbital distributions are shown
in Figure 5. Only for compounds M1 and M2 are the orbitals fully
delocalized. Introduction of DPP (M3), II (M4), and F2BT (M7) results
in both the HOMO and LUMO only being delocalized across the
thiophene-core-thiophene portion of the molecules. For
compounds M5 and M6 the high electron affinity of the NDI and
PDI core along with the twisted backbone result in a complete
localization of the LUMOs on the core and only partial delocalized
HOMOs. These results can help explain the differences in optical
absorption profiles (Figure 2). M3 and M7 have near planar
structures and favourable HOMO-LUMO overlap and both show
λmax as the lowest energy transition. Compound M4 exhibits
favourable HOMO-LUMO overlap but the distorted structure likely
plays into the fact that both the low and high-energy bands in the
optical absorption have similar intensity. For M5 and M6, the
significant twisting of the π-conjugated backbone along with
misalignment of electronic energy levels between the thiophene
and core prevent strong HOMO-LUMO overlap resulting in weakly
absorbing low energy bands. For M2, any increase in conjugation
length afford by the phthalimide core is offset by disruption in
conjugation, thus leading to the observed similar optical absorption
spectra to M1. Indeed the predicted optical absorption profiles
match with those determined experimentally. TD-DFT calculations
were done on the optimized geometries with B3LYP/6-311+G(d).
The calculated absorption spectra are shown in Figure 5B with 0.3
eV FWHM for each transition. The TD-DFT calculations for M4 were
done using the 6-31G(d) basis set. The electronic transitions for
optimized structures of M1-M7 are tabulated in Table S5. The
lowest energy transition for all is HOMO to LUMO with the
exception of M2. For M2 the first excitation has a low oscillator
strength (f=0.099). The HOMO to LUMO for M2 is the second
singlet state excitation, and has a higher oscillator strength (f=
1.18). The oscillator strength for the twisted M5 and M6 is much
lower than the other compounds. Interestingly, the double peak
absorption observed for M3 in solution is not reproduced
theoretically, thus indicating a possible intermolecular effect.
Ultraviolet Photoelectron Spectroscopy (UPS) was used to
measure the solid-state IEs of M1–M7. The results are summarized
in Table 1, with spectra shown in the Supporting Information
(Figure S23). M1 exhibits an ionization energy (IE) of 6.0 eV, owing
to the electron withdrawing nature of the terminal phthalimide
groups. M2, with an addition central phthalimide has a slightly
higher solid-state IE of 6.1 eV. Going from M2 to M5, the NDI core
adds an electron withdrawing imide group in addition to an extra
conjugated carbon ring, two opposing effects that balance out and
result in the same IE of 6.1 eV. From M5 to M6, the addition of two
carbon rings has the effect of lowering the IE to 6.0 eV. M3 has the
lowest IE of the series. The two amides on the DPP core are less
effective at stabilizing the π-electrons, resulting in an IE of 5.4 eV.
M5, with an IS core contains the same stabilizing groups as M4, yet
bears a higher IE of 5.7 eV. The two fluorine atoms on the F2BT core
of M7 also act to withdraw electron density, resulting in an IE of 5.9
eV.
Theoretical Evaluation of Small Molecules M1-M7
It has recently been postulated that degeneracy of the
LUMO energy levels and low reorganization energies upon one
electron reduction are favourable properties for molecules
intended to be used as electron acceptors.25 Thus, to further
analyze these compounds and their potential to act as electron
transporting materials in OPVs we investigated the energy of their
frontier molecular orbitals (FMOs) in addition to the distortions
caused to the aromatic backbone upon reduction. The frontier
Molecules M1–M7 were investigated in the gas-phases using
density functional theory (DFT)63–65 and Time dependent DFT (TD-
DFT)66 to further understand the impact of the core structure on
molecular geometries, electronic structure and electronic
transitions. Structures with truncated alkyl chains (i.e. methyl
groups in place of the longer alkyl chains) were optimized to reduce
computation time.67 All calculations were conducted using the

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molecular orbital energies for all compounds from HOMO-3 to
LUMO+6 are shown in Figure 5A and tabulated in Table S4. For M1 Single Crystal X-Ray Diffraction Study of M5
the LUMOs are evenly spread out with none-being degenerate. One of the well-recognized advantages of small molecule
Interestingly, M2 has nearly energetically degenerate LUMO and semiconductors compared to polymeric structures is that they are
LUMO+1; however, the LUMO is delocalized across the molecule often more easily crystallized into large crystals suitable for single
and the LUMO+1 is localized on the core. As the electron affinity of crystal X-ray diffraction. Single crystal diffraction experiments can
the core increases from M2–M7 the energy of the LUMO decreases lend insight towards the supramolecular organization of
a
and the gap between the LUMO and LUMO+1 increases. For M3– molecular system and this in turn can lead to an understanding of
M7 the LUMO is localized on the core, indicating the energy of the the structure property relationships for this system. For both M5
LUMO is directly dictated by the electron affinity of the core. and M6, the UV-visible profiles showed strong high-energy
Conversely, the energy difference between LUMO+1 and LUMO+2 absorption bands and relatively weak low energy absorption bands.
decreases from M1 to M7 with M6 showing the closest energy in These molecules were further examined using DFT calculations
these levels. Investigation of the MO distributions shows that in where it was revealed that both M5 and M6 were expected to have
each case the core significantly contributes to the LUMO whereas large torsion angles between the core acceptor units and the
the LUMO+1 and LUMO+2 are localized on the phthalimide end- thiophene-phthalimide side arms. Large torsion angles between the
capping units. In the extreme case of M6, the LUMO+1 and donor and acceptor parts of D-A type molecules can cause
LUMO+2 are degenerate, and each resides on one phthalimide unit inefficient orbital mixing and therefore it might be expected that
(Figure 4). Thus in this series of molecules there are two competing the low energy band that is attributed to intramolecular charge
effects determining the energy of the LUMOs. 1) Energy level transfer would be relatively weak. It is also well known; however,
alignment between the core and phthalimide end-capping unit. For that when π-conjugated molecules transition from the solution to
M2, both the core and end-capping unit are phthalimide groups the solid state they have a tendency to form more planar structures
both having the same energy, thus resulting in similar energies for via π-π interactions between adjacent molecules.73–77 Therefore, we
the LUMO and LUMO+1. As the electron affinity of the core decided to investigate the single crystal structures of M5 and M6 to
increases from M2 to M7, the energy differences between the core confirm their solid-state molecular geometry.
and the end-capping phthalimide become greater, thus the LUMO
becomes more localized on the core and the LUMO+1 on the
Single crystals of M5 were grown by layering methanol
phthalimide resulting in the greater energy differences between
carefully over
a concentrated solution of M5 in CH2Cl2.
these molecular orbitals. 2) Twisting of the π-conjugated backbone.
For all compounds the end-capping phthalimide groups contribute
to the LUMO+1 and LUMO+2. From M3-M6, as the twisting of the
π-conjugated backbone increases electronic communication
between the two end-capping phthalimide groups decreases, thus
localizing the LUMO+1 and LUMO+2 on one phthalimide group
each, making them similar in energy. M7 is more planar, with good
communication between phthalimide groups in the LUMO+1. The
LUMO is localized on the core due to the high electron affinity from
the fluorine groups. Based on these results, it is clear that none of
these compounds poses a large density of states at the LUMO
which may be detrimental to their ability to act as acceptors in BHJ
type solar cells.
Crystallization of M6 was unsuccessful. Small molecule M5
crystallized in the monoclinic space group C2/c as red needle-like
crystals. Notably, the NDI core of each M5 unit is significantly
twisted at ~ 58
(Figure 6A). This was not unexpected, as the DFT calculations
predicted significant twisting of the flanking thiophene-
°
with respect to the thiophene-phthalimide arms
a
phthalimide units and the UV-Visible spectrum for this compound
showed a distinctly weak absorption in the low energy region of the
spectrum, indicating the possibility of inefficient orbital mixing
which can occur due to loss of co-planarity between two adjacent
conjugated fragments. The twisting of the side-arms with respect to
the NDI core has been observed in other small molecules containing
NDI and thiophene based building blocks, for example Patil et al.
showed that NDI cores with benzofuran side arms display significant
twisting of the core with respect to the side arms.78 For compound
M5, there is a small torsion angle of 11.5 degrees between the
phthalimide and thiophene rings that flank the NDI core. Along the
c axis direction of the unit cell (Figure 6B) these thiophene
phthalimide arms form π-stacked arrangements with a distance of
3.29 Å between stacked units. It can be seen as viewed down the c
axis that the NDI cores do not lie atop of one another in a face-to-
face fashion as the side arms do. Instead the NDI cores arranged in
a ‘zig-zag’ fashion to one another, with an angle of ~83 degrees
between the two different orientations for the NDI core (Figure 6C
and D). This packing arrangement is interesting because the NDI
fragments are not interacting in a direct face-to-face fashion that is
often seen in other single crystal studies of NDI based small
molecules. 56,79,80
To examine the effect of reduction on M1-M7 the
geometries of the reduced compounds were optimized (charge -1
doublet states). The change in bond length of a molecule upon
addition of an electron in this manner corresponds to the
delocalization of the injected electron (Figures S24).72 The smaller
the magnitude of the change in bond length the higher the
delocalization. Likewise, the location of the added electron can be
determined by examining which bonds show the greatest change.
Keeping this in mind and observing that for molecules M1 and M3
the bond length changes are roughly uniform across the entire
conjugated backbone, we expect the injected electron to be
delocalized across the entire backbone. For molecule M2 the bond
length changes are greatest on the thiophene-phthalimide arms of
the molecule indicating that the injected electron would be centred
on the side arms of the molecule. For M4-M7 the greater bond
length changes upon one electron injection occur on the conjugated
core of the molecules, indicating that the electron would be
localized on the core part of the molecules. Interestingly, the
greatest changes in bond length for all molecules M1-M7 are
approximately in line with the orbital distribution of the LUMO for
each of M1-M7. (Figure 4)
Preliminary Device Characterization of Small Molecules M1-M7
Small molecules M1–M7 possess energy levels that make
them appropriate candidates for electron transporting-light
harvesting materials for use in small molecule BJH organic solar
cells. In order to confirm their possible utility towards their

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application in BHJ-OPVs we conducted a preliminary screening of had varying reduction potentials as calculated from the CV data,
M1-M7 with two popular donor materials; DTS(FBTTh2)2 and P3HT.
which could be tuned in the range of 2.98 to 3.90 eV, simply by
exchanging the CORE component in the architecture. In addition,
Previously, we have reported on DTS(FBTTh2)2:M4 devices that the different cores introduced had a strong impact on the optical,
showed the best performance when hot-cast at 90 °C from a 20 mg thermal and electronic properties of the series. The impacts of
mL-1 solution (1:1 weight ratio) in chlorobenzene with 0.4 v/v% DIO including each core were examined experimentally with theoretical
additive at 1000 rpm.39 To probe the photovoltaic performance of results in good agreement. For instance, when DPP or isoindigo
small molecules M1–M7, devices were made with the general were included as cores a strong low energy absorption peak was
architecture ITO/PEDOT:PSS/donor:acceptor/Ca/Al and using the observed, attributed to the strong donor-acceptor nature of the
above mentioned processing conditions for active layer formation. produced compounds. When phthalimide, NDI or PDI were included
Both DTS(FBTTh2)2 and P3HT were investigated as the electron as cores; however, the effect of the donor-acceptor nature of the
donating component; however, DIO was omitted for P3HT devices. compounds was severely inhibited by steric interactions between
Prior to cathode deposition films were annealed at 70 °C for 10 the core units and side arms. This demonstrated that not only is the
minutes to drive off residual solvents. Post-cathode-deposition donor-acceptor nature of the compounds to be considered but also
annealing was also investigated for P3HT blend devices at 10 that the new geometries produced can have a strong impact on the
minute intervals. For P3HT blends with M3, M4, M5, and M6, post- properties of the new materials. Preliminary solar cell performance
cathode-deposition annealing resulted in reduced Voc and Jsc.
of M1–M7 was evaluated by screening each new acceptor with two
different donor materials and it was found that the new materials
Figure S30 shows the current density-voltage (JV) plots for were able to produce channel II photocurrent. It was also shown
the as-fabricated DTS(FBTTh2)2 devices, and P3HT devices under the that for M1–M7 there was only a weak relation between the VOC
best annealing conditions. Device performance parameters are obtained in solar cell devices and the energetic offset between
summarized in Table 2, with the best devices achieving power donor HOMO and acceptor LUMO as determined using CV. The
conversion efficiencies (PCEs) of ~ 0.5 %. The importance of ideal choice of donor acceptor pairings used in the bulk heterojunction
donor acceptor pairings is clearly illustrated in this work, a good had a strong effect on the performance of the devices, where some
example of this can be seen by comparing the devices made using molecules showed similar performance with both donors, while
M4 with each donor, for DTS(FBTH2)2-M4 devices a modest PCE of others displayed functionality with only one donor or the other.
0.43% is achieved; however, for the P3HT-M4 devices a PCE that is Further investigation of this in relation to the concept of ideal
essentially negligible was observed (Table 2). Overall the PCEs are donor acceptor pairings is underway in our research group.
significantly lower than related devices fabricated with fullerene
acceptors37,39 but based on film morphology studies (see ESI Figures
S31 and S32) we believe the PCEs obtained for M1-M7 devices are
currently limited by excessive phase separation. It is worthy to
Research Chairs, and the Canadian Foundation for Innovation. IGH
note; however, that molecule M1 with a relatively high-lying LUMO
level of -3.03 eV achieved a Voc of 1.20 and 1.23 V in devices with
the Canada Foundation for Innovation and the NSERC Photovoltaic
DTS(FBTTh2)2 and P3HT, respectively. A plot of Voc vs CV derived
acceptor LUMO level is provided in Figure 7. For both donors the
Acknowledgements
GCW acknowledges the NSERC Discovery Grants Program, Canada
acknowledges support from the NSERC Discovery Grants program,
Innovation Network. TM thanks Portland State University for
funding. ADH is grateful for an NSERC scholarship.
JPS
Voc was not found to be strictly related to the energetic offset
between donor HOMO and acceptor LUMO, although in general
molecules with a larger offset displayed an expectedly larger Voc.
This is important to note, because it clearly illustrates that other
effects such as film morphology and donor-acceptor miscibility are
more critical to the performance of the solar cell device. Figure S26
shows the external quantum efficiency (EQE) plots of devices M1–
M7 with both donors. The thin film absorbance of each donor is
provided along with the EQE to demonstrate that channel II
photocurrent is possible using several the new small molecules. For
instance, in figure S26B it can clearly be seen that small molecule
M3 contributes to the photocurrent generation of the device in the
high (< 450 nm) and low energy (>650 nm) regions of the visible
spectrum where P3HT is not an effective light harvester. M1 also
displays potential in this regard as it displays a contribution to the
photocurrent generation in the high energy (<450 nm) region of the
visible spectrum where P3HT has minimal absorbance. It is worth
noting that EQE spectra are taken at very low illumination
intensities (~ 10-4 suns per monochromator spectral bandwidth),
and tend to overstate the performance of devices where
bimolecular recombination at 1 sun is significant, as is likely the
case of DTS(FBTTh2)2:M5 devices.
acknowledges Killam Trusts, NSERC and NSERC CREATE DREAMS for
financial support.
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Figure 1: Synthetic routes towards molecules M1–M7 using direct heteroarylation conditions.
5 mol% Pd catalyst, 20 mol% pivalic acid, 2.5 eq. potassium carbonate. Solvents used in this
work include DMA, DMF and toluene. Reaction temperatures used in this work range between
85–120 °C, full synthetic details for small molecules M1–M7 and relevant precursor materials
are given in the Supporting Information, section S1.
Figure 2: UV-Visible absorption spectra of compounds M1–M7 in CHCl3 solution (solid lines),
and thin films cast from 1% wt/vol CHCl3 (dashed lines). The spectra of M1 is plotted against
each of M2–M7 for reference.
Figure 3. A) Energy level diagram showing molecules M1-M7, data was collected using cyclic
voltammetry in CH2Cl2 solution. For reference the energy levels of the donors P3HT and
DTS(FBTTh2)2 are included in an expanded energy level diagram in the ESI (Figure S30). B)
Cyclic voltammetry plots for compounds M1-M7 in CH2Cl2 solution showing reduction only, full
CV plots for M1-M7 can be found in the Supporting Information, (Figure S22).
Figure 4. A) Optimized structures and molecular orbital distributions for compounds M1-M4.
B) Optimized structures and molecular orbital distributions for compounds M1, M5-M7.
Figure 5. A) Calculated energy level diagram for M1-M7. B) Calculated absorption spectra for
M1-M7 (gas phase). Absorption spectra are predicted from molecules in the gas phase modeled
with DFT using B3LYP with the 6-31G(d) basis set
Figure 6. X-ray crystal structure of M5. A) Single unit of M5 showing torsion angles. B)
Molecular packing as viewed along the ‘b’ axis of the unit cell showing π-stacking of the
thiophene-phthalimide arms. C) Viewed along the ‘c’ axis of the unit cell. D) Viewed along the
‘a’ axis of the unit cell.
Figure 7. Acceptor LUMO vs open circuit voltage of M1-M7 devices with DTS(FBTTh2)2 and
P3HT as the donor. LUMO is estimated from CV EA. A line with slope = 1 V/eV is provided as a
guide.

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DOI: 10.1039/C5TC01877C

Journal of Materials Chemistry C
Page 16 of 18
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DOI: 10.1039/C5TC01877C

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Journal of Materials Chemistry C
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DOI: 10.1039/C5TC01877C
Table 1. Optical and electronic data for compounds M1-M7.
λmax,sol’n (nm)
λonset,sol’n (nm)
λmax,film (nm)
λonset,film (nm)
EACV (eV)
IECV (eV)
IEUPS (eV)
Tmelt (°C)
Tcryst (°C)
M1
M2
M3
M4
M5
M6
M7
426
395
611
440
364
374
464
494
453
666
678
506
704
537
413
404
592
450
364
373
468
483
483
797
738
611
709
577
3.03
2.98
3.44
3.64
3.82
3.90
3.29
5.60
5.92
5.23
5.47
5.92
5.69
5.66
6.0
6.1
5.4
5.7
6.1
6.0
5.9
277
181
-
-
144, 254
126, 209
259
209
279
-
-
-
-
-
Table 2. OPV device data for compounds M1-M7.
Jsc
(mA cm-2)
Active layer
Donor
Acceptor
Annealing temp. (°C)
Voc (V)
PCE (%)
FF
thickness (nm)
M1
M2
70
70
1.20
0.86
-0.0033
-0.028
0.00051
0.0086
0.13
0.37
120
120
M3
M4
M5
M6
M7
70
70
70
70
70
0.92
0.89
0.72
0.77
0.99
-0.64
-1.6
0.27
0.43
0.15
0.089
0.17
0.46
0.29
0.17
0.31
0.30
110
130
120
80
DTS(FBTTh2)2
-1.3
-0.37
-0.58
110
M1
M2
130
130
1.23
0.92
-1.2
-1.7
0.46
0.50
0.32
0.31
150
140
M3
M4
M5
M6
M7
70
70
0.81
0.63
0.49
0.78
0.94
-1.4
0.37
0.027
0.056
0.054
0.22
0.33
0.28
0.39
0.30
0.28
190
120
130
150
150
P3HT
-0.15
-0.29
-0.23
-0.84
70
70
130

Journal of Materials Chemistry C
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DOI: 10.1039/C5TC01877C