Solid state supramolecular structure of diketopyrrolopyrrole chromophores: Correlating stacking geometry with visible light absorption

Article abstract of DOI:10.1039/c6ce02157c

Mono- and di-alkylated 1,4-diketo-3,6-dithiophenylpyrrolo[3-4-c]pyrrole derivatives (TDPPs) have been synthesised and their solid state packing and absorption properties have been correlated. In this library of compounds the bulkier substituents distort the geometry of the chromophores and shift the lowest energy absorption band as a consequence of reduced π-π stacking and inter-chromophore overlap. Longitudinal displacement of the conjugated core is affected by donor-acceptor intermolecular interactions and twisting of the thiophene ring out of the plane of the DPP core, whereas lateral displacement was correlated to distortion of the N_Lactam-C(R) bond out of the plane of the DPP core. The di-substituted TDPP with hexyl units exhibit high molecular planarity, strong close packing of the conjugated core and significant red shift of the maximum of absorption in the solid, whereas the mono-substituted compounds with hexyl and ethyl acetate units are the least distorted of the series because of strong intermolecular hydrogen bonding that increases the molecular overlap and planarity of the chromophores. Therefore the family of mono-substituted TDPPs and more specifically the ones with ethyl acetate substituents show good potential for modulating the molecular geometry and optimizing the charge transport in materials for organic electronic applications.

Full text of DOI:10.1039/c6ce02157c

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Page 1 of 13
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DOI: 10.1039/C6CE02157C
Solid state supramolecular structure of diketopyrrolopyrrole
chromophores: Correlating stacking geometry with visible light absorption
Flavia Pop,* a,b William Lewis a and David B. Amabilino* a,b
a School of Chemistry, The University of Nottingham, Nottingham NG7 2RD, UK
b The GSK Carbon Neutral Laboratories for Sustainable Chemistry, The University of Nottingham
Jubilee Campus, Triumph Road, Nottingham NG7 2TU, UK
Mono- and di-alkylated 1,4-diketo-3,6-dithiophenylpyrrolo[3-4-c]pyrrole derivatives (TDPPs) have been synthesised and their solid state
packing and absorption properties have been correlated. In this library of compounds the bulkier substituents distort the geometry of the
chromophores and shift the lowest energy absorption band as a consequence of reduced π-π stacking and inter-chromophore overlap.
Longitudinal displacement of the conjugated core is affected by donor-acceptor intermolecular interactions and twisting of the thiophene
ring out of the plane of the DPP core, whereas lateral displacement was correlated to distortion of the NLactam-C(R) bond out of the plane of
the DPP core. The di-substituted TDPP with hexyl units exhibit high molecular planarity, strong close packing of the conjugated core and
significant red shift of the maximum of absorption in the solid, whereas the mono-substituted compounds with hexyl and ethyl acetate units
are least distorted of the series because of strong intermolecular hydrogen bonding that increases the molecular overlap and planarity of
the chromophores. Therefore the family of mono-substituted TDPPs and more specifically the ones with ethyl acetate substituents show
good potential for modulating the molecular geometry and optimizing the charge transport in materials for organic electronic applications.
Introduction
The development of new materials for organic solar cells (OSCs) lead to continually improving device efficiencies,
in great measure through control of the supramolecular organization of the material in thin films of the active layer
incorporating the donors and acceptors.1 The molecular approach makes use of various functionalities – either at the end of
the conjugated chain and/or sideways to the core - in attempts to control the supramolecular organization through π-π
stacking, hydrogen bonding2 and other supramolecular interactions.3 Both covalent and multicomponent supramolecular
structures (primary and secondary)4 as well as hierarchical superstructures determine the properties of the materials.5 In
organic semiconductors in general the electron mobility is sensitive to intermolecular electronic interactions that depend on
the shape of the frontier orbitals and on the relative orientation and position of the aromatic cores of the adjacent molecules.
Thus the fundamental properties of materials for OSCs are controlled by modulating the molecular structure, thin film
ordering and optimizing the device architecture. In highly crystalline small molecule thin films absorption and charge carrier
transport are determined by packing and orbital overlap but limited by the crystalline domain sizes and their
interconnection.6 Derivatives of small molecules such as 1,4-diketo-pyrrolo[3-4-c]pyrroles (DPPs, Scheme 1) have become an
attractive alternative to polymers for photovoltaic applications due to their facile synthesis and purification, solution
processability,7 as well as efficient charge transport and high light absorption.8 DPPs are thus a class of intensively studied
organic building blocks being used as high-performance pigments9 and light harvesting chromophore in OSCs.10 DPP
chromophores consist of two condensed lactam units that are habitually decorated at the "end" of the conjugated core by
aromatic rings and at the lateral amide-nitrogen atoms by alkyl chains.11 The two electron withdrawing amide units on the
core make DPPs good electron acceptor moieties that can be associated to different electron donating or withdrawing
aromatic units resulting in an increase of π-conjugation and control over the frontier orbitals energies. Of the wide variety
of chromophores derived from DPP, the series of 1,4-diketo-3,6-dithienylpyrrolo[3-4-c]pyrroles (TDPP) derivatives is one of
the most investigated for a number of reasons, including synthetic accessibility, and high planarity (small dihedral angle
between the DPP core and the thiophene rings) which results in high π-delocalization and favourable π-π intermolecular
interactions. The importance of the donor group as well as the presence of one or two alkyl side units on the solid state
packing, photophysical and charge transport properties has been recently highlighted for a series of di-hexyl-phenyl, -furyl,
-thiophenyl and –selenophenyl,12 and di- and mono-hexyl-phenyl and -thiophenyl DPPs,13 respectively. Based on single
crystal X-ray diffraction, a higher torsion angle of the aromatic unit and the DPP core of about 33˚ was found for phenyl DPP
because of steric repulsion between the proton of the phenyl unit and the alkyl side chain attached to the DPP core, whereas
five-membered rings remain almost coplanar with the DPP unit. On the other hand, mono-hexyl TDPP has been found to
have superior charge carrier mobility to its di-alkylated analogue apparently due to improved close packing driven by
intermolecular hydrogen bonding between the free amide groups. Similar studies on TDPPs in the solid state revealed the
importance of the alkyl units (branched and linear hydrocarbons, and hydrophilic ethyleneoxy units) in preserving high
planarity and therefore close packing of the molecules.14 Hydrophilic ethyleneoxy side chains reduce the twisting out of plane
of the thiophene unit, but the observed intermolecular π-π distance (4.021 Å compared to 3.614 Å and 3.785Å for hexyl and
ethylhexyl, respectively) suggests reduced π-π interactions as a result of random orientation of the chain possibly to
maximize favourable intermolecular interactions between the ethyleneoxy and thiophene units. In phenyl DPPs possessing
benzyl substituents favourable benzyl based intermolecular interactions have been shown to improve the overlap of the π-

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π co-facial dimers and the charge transfer properties.15 Similarly, higher co-planarity and intramolecular interaction in TDPPs
View Article Online
compare to phenyl DPPs favour close packing of chromophores allowing singlet exciton fission to occurDsOuIg: g10e.s1t0in39g/Cth6eCE02157C
potential of TDPPs of enhancing the efficiency of solar cells devices.16 These observations highlight the importance of
molecular design in controlling the alignment and the π-π overlap of DPPs as ways of optimizing the charge transport in
organic conducting materials. We present herein the synthesis, spectroscopic properties and thorough solid state structural
analysis of two new series of mono- and di- alkylated TDPP derivatives, with a focus on the differences in the solid state
packing caused by the nature of the side-appended alkyl units.
Results and discussions
The TDPPs discussed here were synthesized according to similar procedures described previously17 by using either one or
two equivalents of the alkyl halide derivatives as shown in Scheme 1. When two equivalents of alkylating agent were added
TDPPs 1 were obtained exclusively whereas using fewer equivalents both derivatives 1 and 2 were obtained. They were
separated by flash column chromatography (see experimental section in the ESI for detailed conditions). Synthesis of
unsymmetrical TDPP 1CD was performed in two successive steps, reacting TDPP in a first step with di-tert-butyl-dicarbonate
to give 2C, followed by subsequent reaction with 1,6-dibromo hexane (Scheme S1, SI).
Scheme 1. Synthesis of TDPPs 1A-E and 2A-E.
Crystal structure analysis
Single crystals with needle and plate shapes were grown by evaporation from dichloromethane/ethyl acetate for most
compounds, and dichloromethane (1C) and a mixture of dichloromethane and tetrahydrofuran (2A). Single crystals of 1B14
and 2B13 have been described elsewhere and the crystallographic data for the comparative discussion were taken from the
available cif files. The homogeneity of the crystalline forms was checked by comparing the powder X-ray diffraction of the
solid samples with theoretical patterns simulated from their single-crystal structures (Figure S1, SI). The experimental peaks
in the region of 5−30° match well with simulated ones for the tested compounds. 2E shows very weak diffraction peaks
indicating a lower degree of crystallinity in the powder compared to other derivatives. In general, the molecular packing in
the powders is very similar to that in single crystals, and there is no clear evidence of polymorphism. The molecular and
single crystals structures of all TDPP derivatives discussed are shown in Figure 1.

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Figure 1. Structure of the discussed TDPP. The structures of 1B and 2B have been taken from previously reported files.13 14
,
TDPPs 1A-D and 2A-D crystallize in centrosymmetric space groups of the monoclinic and orthorhombic systems whereas
chiral compounds 1E and 2E in the non-centrosymmetric space groups P21 and R3 of the monoclinic and trigonal system,
respectively (Table S1 and S2, SI). At the molecular level in the crystals, the thiophene rings are arranged in an anti orientation
in most of the di-substituted compounds 1 and in 2C, and a syn orientation in the mono-alkylated 2A and 2D, in both cases
the sulphur atoms of the thiophene rings face the alkyl chain. In 2C the sulphur atoms of the thiophene units are pointing
towards the DPP carbonyl moiety whereas in 1C, 1CD and 2E the thiophenyl units shows structural disorder of the sulphur
atom (Figure 1). The planarity of the molecules has been found to be related to the nature of the appended alkyl substituents
and will be addressed later in this work as the degree of twisting of the thiophenyl units and the -N-C(R) bonds out of the
plane of the DPP core (Tables 1 and 2).
At the supramolecular level, in the crystal packing, the TDPPs bearing two alkyl units (compound family 1) show slipped
stacking in a classical edge-to-face herringbone fashion similar to the packing of previously described dihexyl-phenyl DPP and
thiophene DPP 1B (Figure S2, SI).12 The mono-alkylated derivatives form strong hydrogen bonds between two molecules
through the free amide functionalities and the dimers thus formed further adopt slipped packing as a result of π-π stacking.
Most of the mono-alkylated TDPPs (compound family 2) prefer to pack in a slipped co-facial herringbone arrangement similar
to the packing observed in previously reported 2B13 except for 2D (which has a slipped one dimensional π stacking of the
dimers) and 2E, that shows the classical herringbone type seen in di-alkylated DPPs (Figure 2, Figure S3, SI).

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Figure 2. Molecular packing of 2D showing anisotropy of the π stacking along a axis (top) and 2A showing co-facial
herringbone arrangement with a highlight of the dimer formed by hydrogen bonding (bottom).
The longitudinal (long axis) slipped stacking is probably a consequence of the thiophenyl units overlapping with the lactam
rings of neighbouring molecules, resulting in donor-acceptor reinforced π-π interactions as suggested by the short
intermolecular distances observed between the centroids of the electron donor thiophene ring (D) and the electron deficient
lactam unit (A) (Figure S4, SI). Significant short D-A intermolecular π-π distances of as little as 3.40 Å and up to 3.82 Å were
observed for 1 (A, B, D, E and CD) and 2B-D supported as well by the short plane to plane distances (Figures 3 and 4). In
contrast, molecules 1C, 2A and 2E show high D-A centroid – centroid distances of 4.13, 4.81 and 4.04; 4.73 Å, respectively,
as a consequence of both longitudinal and lateral (short axis) slipping (Tables 1 and 2). Furthermore as a result of high lateral
slipping 2E shows inter-planar distance between head-to-head molecules of about 3.00 Å and between head-to-tail
molecules of 3.45 Å. The latter has a short intermolecular D-A distance (4.04 Å) suggesting stronger π-π stacking in a head-
to-tail fashion rather than head-to-head observed in all the other derivatives (Figure 4 and Figure S5, SI).
Table 1. Measured values (Å) of the thiophene•••lactam centroids, inter-planar distances and molecular displacement (along
the long and short axes) together with dihedral angles (˚) between the planes of DPP and the adjacent thiophene groups
(DPP-Th) and the planes of lactam unit and CLactam-N-C(R) of the alkyl substituent, for TDPP derivatives 1.
TDPP
Th•••lactam
centroids (Å)
3.40; 3.57
3.61
Inter-planar distance (Å)
Displacement (Å)
DPP-Th (˚)
Lactam-[CLactam-N-C(R)] (˚)
Long
4.59
4.09
Short
1A
1B14
1C
1D
1E
3.35
3.50
3.40
3.37
a: 3.58
b: 3.35
2.85 and 4.66
9.67
14.18
5.48 and 7.64
4.96
24.96
4.13
4.77
3.56; 3.60
a: 3.69; 3.74
b: 3.69; 3.72
3.82
4.18
8.83
2.61
a: 4.32
b: 4.49
4.99
a: 18.24 and 14.57
b: 20.70 and 16.84
17.90 (C)
a: 3.43 and 4.35
b: 3.98 and 4.39
31.76 (C)
1CD
3.54
2.33

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2.66 (D)
0.71 (D)
DOI: 10.1039/C6CE02157C
Table 2. Measured values (Å) of the thiophene•••lactam centroids, inter-planar distances and molecular displacement (along
the long and short axes) together with dihedral angles (˚) between the planes of DPP and the adjacent thiophene groups
(DPP-Th) and the planes of lactam unit and CLactam-N-C(R) of the alkyl substituent, for TDPP derivatives 2.
TDPP
Th•••lactam Inter-planar distance (Å)
Displacement (Å) DPP-Th (˚)
Lactam-[CLactam-N-C(R)] (˚)
centroids (Å)
Long
Short
3.34
2A
2B13
2C
4.81
3.67; 3.71
3.61; 3.64
3.35
3.32
3.42
0.79; 3.89 (R)
3.46; 11.62 (R)
4.20; 13.7 (R)
9.77
1.30
3.58
5.09
a: 4.51
b: 5.97
a: 9.11
b: 2.66
a: 14.13
b: 17.49
2D
3.57; 3.60
3.34
3.36
3.66
0.45
a: 3.99; 6.49 (R)
b: 1.97; 2.59 (R)
6.11; 19.82 (R)
2E, H-to-H 4.73
2E, H-to-T 4.04
3.00
3.45
0.65
2.37
4.60
2.34
In both series of DPPs inter-planar distances, displacement between parallel molecules (“slipping”) and planarity have been
correlated with the nature of the substituent (linear or branched) attached to the nitrogen atom of the lactam ring. Any
distortion of planarity of the molecule is expected to have an influence on the charge delocalization along the molecular
backbone and is believed to influence their packing in the solid, limiting the contact of the nearby molecules. For analogous
phenyl-DPPs the large dihedral angle between the phenyl and lactam rings of 34.13˚ was associated with steric interactions
between the proton of the phenyl ring and the alkyl substituents.12 In thiophenyl-DPP these steric repulsions are significantly
reduced, so that the out of plane twisting of the thiophene ring has a relatively small influence on packing. Nevertheless the
impact of the bulkiness of the chain is significant and worthy of further discussion. Large dihedral angles between the
thiophene and the DPP core originate from steric repulsions between the thiophene rings and the nearby substituents on
the nitrogen atoms of the lactam rings irrespective of the orientation of the thiophenyl units (Tables 1 and 2). Beside the
thiophene ring twisting out of the molecular plane, distortion of the NLactam-C(R) bond was also found to be highly dependent
on the substituent’s nature and its bulkiness. Thus the highest inter-planar distances, molecular slipping and planarity
distortion have been observed for branched alkyl units C and E (Figures 3, 4 and Figure S7, SI). The structures of 1C and 1E
show twisting of the thiophene rings out of the plane of the DPP unit by 14.2˚ and 14.6-20.7˚, respectively, suggesting this is
mainly caused by the bulkiness of the substituent and not the disposition of the thiophenyl units with respect to the alkyl
chains (anti disposition of the sulphur atoms facing the lactam carbonyl group in 1C and the alkyl chain in 1E). This effect can
be easily observed in 1CD bearing both linear and bulky chains. The dihedral angle with the DPP core of the thiophenyl unit
interacting with the tert-butoxycarbonyl (BOC) substituent (C) is 17.9˚ whereas the one in the vicinity of the hexyl-bromide
(D) remains almost coplanar with the DPP unit (2.66˚). In contrast, mono-alkylated TDPPs (family 2) have different dihedral
angles of the planes of DPP moiety and the two thiophenyl groups ranging from 0.79˚ to 6.11˚ for the thiophene ring adjacent
to the free lactam unit and from 2.59˚ to 19.8˚ for the thiophene ring interacting with the alkyl chain, demonstrating the
influence of the substituent on the overall molecular conformation (Table 2, Figures S8 and S9, SI).

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Lateral displacement
Interplanar distance
Longitudinal displacement
4.59
4.09
1A
1B
3.35
3.50
4.77
4.99
1C
3.40
3.54
1CD
2.33
4.18
1D
1E
3.37
4.32
4.49
3.58
3.35
Figure 3. Side and top views of the crystal packing of 1A-E and 1CD, plane to plane distances (blue) together with molecular
plane displacement values (longitudinal in red and lateral in green) given in Å.

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Interplanar distance
Lateral displacement
Longitudinal displacement
3.34
3.58
2A
3.35
2B
3.32
3.42
2C
2D
5.09
3.69
0.45
3.34
3.36
0.65
4.60
2E
2E
3.00
3.45
H-to-H
H-to-T
2.37
2.34
Figure 4. Side and top views of the crystal packing of 2A-E, plane to plane distances (blue) and molecular displacement
plane values (longitudinal in red and lateral in green) given in Å.
Secondly, compared to other members of their series, 1C, 1CD and 2E also exhibit higher dihedral angles between the plane
of the lactam ring and the corresponding CLactam-N-C(R) plane (Tables 1 and 2, Figure S6, SI). The twisting of the BOC unit out
of the DPP plane is surely associated with electrostatic repulsion between the carboxylate directly attached to the nitrogen
atom and the carbonyl of the lactam ring, whereas in 1A and 2A, with the carboxylate one methylene carbon removed from
the lactam ring, the NLactam-C(R) bond is less distorted; the values of the corresponding dihedral angles are similar to the ones

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of the linear branched derivatives. Therefore, in some cases, flexibility of the molecule overcomes the impact of the alkyl
unit bulkiness ensuring its planarity. Additionally the crystallization energy and intermolecular interactioDnOs I:co10u.1ld03e9i/tCh6eCrE02157C
overcome or impose strains in the packing of the substituents (Figure S7, SI). Similar observations were made for polymorphs
of other thiophenyl-DPPs bearing two BOC units where the deviation of planarity and conformation of the BOC along the
NLactam-C(R) bond were found to be directly related.18 In that case one phase contains almost planar molecules with a very
small twist of the NLactam-C(R) bond (5.28 ˚) whereas the second phase has a higher distortion from planarity and dihedral
angle of the CLactam-N-C(R) plane and the lactam unit (23.4˚). The distortion out of the molecular plane of the CLactam-N-C(R)
bond could also result from the large lateral displacement of the molecules and thus increased steric impediments between
the substituents and the aromatic rings of the nearby molecules. Thus, molecules 1C, 1CD and 2E that have the largest
dihedral angles of the CLactam-N-C(R) plane and the lactam unit (25.0˚, 31.8˚ and 17.5˚) have the most significant lateral
displacement of adjacent molecules in the stack (Tables 1 and 2). Despite the high distortion of planarity 1C, 1E and 2E (head-
to-head) are showing short inter-planar distances very likely because of the large molecular displacement. (Figure 3). Shorter
inter-planar distances in 1A and 2A could be associated to increased planarity resulting from significant intramolecular
carbonyl-thiophene CHTh•••OLactam interactions reinforced by alkyl-CH•••OLactam intrastack intermolecular interactions
arising from superior flexibility of the substituent compared to BOC unit (Tables S4-S6, Figure S7, SI). In the two families of
compounds discussed in here the longitudinal displacement has been associated to either strong D-A interactions or
distortion of the thiophenyl ring out of the plane of the DPP whereas in derivatives with lateral displacement different
degrees of NLactam-C(R) bond distortion have been observed. In order to further correlate the π-π packing with molecular
planarity and intermolecular electronic coupling solution and solid absorption investigations have been performed on both
families of compounds and are discussed below.
Spectroscopic properties
The UV-Visible absorption spectra of all compounds have been examined in solution and using diffuse reflectance
spectroscopy in the solid state. All TDPP derivatives exhibit dual band absorption profiles common for compounds with a
donor-acceptor primary structure.19 The absorption profiles show intense low energy bands at approximately 500 nm and
weaker high energy bands at around 340 nm attributed to strong intramolecular interactions from the thiophenyl donor to
the DPP acceptor and π-π transitions localized on either the donor or the acceptor, respectively (Figure 5, Tables 3 and 4).20
Along the series of alkyl units a shift of the absorption maximum correlates with the electron withdrawing effect of the
substituent. Therefore in both mono- and di- alkyl DPPs a blue shift occurs in the order B (or E), A, C when moving from
simple hydrocarbons to carboxylate. The electron withdrawing effect of the substituent can be further seen in non-
symmetrical 1CD that is blue shifted compared to analogous symmetrical 1B and red shifted compared to 1C (Figure 5, top
left).

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Figure 5. Absorption spectra of compounds 1A-C, E and 2A-C, E in solution (CH2Cl2 10-5 M, top), as powder (by diffuse
reflectance mode and transformed to Kubelka-Munk units,21 middle) and as film (bottom).
In all derivatives the absorption peaks of the solids (powder and film) are broader and red shifted by about 50 nm compared
to the solution state bands, which is indicative of increased intermolecular interactions in the solid state. In the absorption
spectra of the films the low energy regions show shoulder peaks that result from vibronic coupling of adjacent molecules.22
The emerging shoulder of the lower energy band has been associated to the formation of H-type aggregates and the red
shift of the lower energy band to the formation of J-type aggregation.23 The maximum of absorption of 1E is only slightly red
shifted whereas 1B possessing linear chains shows higher red shift which is consistent with J-type aggregation and increased
conjugation24 because of the highly planar conformation in the solid state (Figure 6 and Table 3). A similar effect was
observed in the solid state spectra of 1A, a higher red shift suggesting a more dominant J type stacking compared to 1C,
probably as consequence of reduced steric impediments of the A substituent and thus increased molecular planarity. These
observations are in line with previous results on simple di-alkylated TDPP derivatives that show complex J and H type
aggregation depending on the nature of the hydrocarbon chain, linear versus branched.25
Figure 6. Comparison of absorption spectra in solution and solid of 1A and 1C (top left), 1B and 1E (top right) and 1CD
(bottom).
Table 3. Absorption data in dichloromethane solution, conc. 10-5 M, as powder and film for TDPP derivatives 1.
TDPP
λmax (nm)
solution
538
548
485
548
528
ε M−1cm−1
λmax (nm)
powder
578
603
544
λmax (nm)
film
567
601
540
1A
1B
1C
1E
1CD
34500
31055
30270
31055
27435
575
568
544
Table 4. Absorption data in dichloromethane solution, conc. 10-5 M, as powder and film for TDPP derivatives 2.
ε M−1cm−1
TDPP
λmax (nm)
λmax (nm)
λmax (nm)

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solution
531
538
514
538
powder
film
569
559
552
571
2A
2B
2C
2E
37067
33705
33791
24505
572
563
560
573
The absorption maximum in the solid has been correlated with the molecular geometry and relative displacement in the
stacks for the two series of derivatives (Figure 7). In the di-alkylated series the highest red shift of absorption maximum of
the lowest energy band has been observed for molecule 1B whose geometry proved to be least distorted in the crystal (small
dihedral angle between the DPP and the adjacent thiophenyl unit). Therefore the molecular overlap and red shift is increasing
in the order 1C, 1E, 1A and 1B in the di-alkylated series and 2E, 2C, 2B and 2A in the mono-alkylated series. In the mono-
alkylated series differences of the maximum of absorption with the nature of the alkyl unit are less significant (Figure 5, right
and Table 4). Thus a slight or no red shift of the maximum of absorption of the lowest energy transition was observed for 2A
and 2B compared to 2E and 2C possibly because of small molecular displacement and therefore higher contribution to the
absorption band of the H-type aggregation. This feature might be because intermolecular hydrogen bonding allows the
alignment of the transition dipoles,26 and therefore strong close packing and π-π stacking. In both series the molecular
planarity is increasing in the order E, C, B and A (Figure 7, bottom). The molecular overlap in 2A is highest of the series as a
consequence of planarity and intermolecular hydrogen bonding. Therefore mono-alkyl TDPP based on A alkyl units are
expected to have superior charge carrier mobility reported for example on 2B, because of improved close packing.13
Figure 7. Maximum of absorption in the solid and solution plotted against the molecular displacement for 1 (top left) and 2
(top right), and in the solid against the dihedral angles between the planes of DPP and the adjacent thiophene rings
(bottom).

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Conclusions
The intermolecular packing of TDPP derivatives in single crystals has been found to be highly dependent on the nature of the
alkyl units at the lateral amide-nitrogen atoms. The spectroscopic properties of the two series of TDPPs were directly
correlated to the geometry of the aromatic core that was found to be strongly influenced by the side substituent. As
previously shown linear chains allow close packing of the molecules due to reduce steric impediments whereas branched
chains will increase their solubility. Nevertheless branched solubilising chains have to be carefully chosen as they not only
influence the close packing of the molecules through their bulkiness but have as well a huge impact on the planarity of the
molecule and thus affecting both the close packing and the conjugation. In both series of compounds, bulkier substituents,
methyl-butyl (E) and BOC (C), distort the planarity of the conjugated system showing high dihedral angles between the DPP
core and the adjacent thiophene or the CLactam-N-C(R) plane. Moreover 1C, 1CD and 2E that have the largest dihedral angles
of the CLactam-N-C(R) plane and the corresponding lactam rings show the most significant lateral displacement of neighbouring
molecules. These packing effects influence directly the visible light absorption properties of the materials. The maximum of
absorption of solid 1E is only slightly red shifted with respect to solution whereas 1B possessing linear chains shows higher
red shift which is consistent with J type aggregation and increased conjugation because of the higher planar conformation.
All mono-alkylated TDPPs show similar absorption maxima in the solid (differences of about 10 nm) because of strong close
packing as a consequence of both increased planarity because of reduced steric impediments and intermolecular hydrogen
bonding of the free lactam units. Molecular displacement correlated with the red shift of the lowest energy band suggests
linear alkyl chains (type B) as ideal substituents for close packing of the molecules in the solid, while favourable π-core
geometry is also achieved with the ethyl acetate substituent (type A). Beside the effect on the molecular planarity in the
solid, ethyl acetate substituents could be useful in switching the solubility of the TDPPs towards polar solvents potentially
giving a control over the packing in the bulk materials, a possibility we are exploring presently. These results are particularly
pertinent given the very recent report on the geometry dependence of singlet fission in TDPP derivatives,27 an effect which
is promising for the application of these materials in optoelectronic devices. We believe that the identification of these
multifarious aggregation modes will also aid in the interpretation of the supramolecular arrangements in devices in which
precise structural characterisation is not possible.
Acknowledgements
We thank the EPSRC, GSK, and the School of Chemistry at the University of Nottingham for funding, and STFC/Diamond
for access to Diamond Light Source Beamline I19.28
Notes and references
‡ Electronic supplementary information (ESI) available: detailed experimental section and synthetic procedures,
additional solid and solution data and crystallographic data tables. CCDC 1508451-1508459 contain the supplementary
crystallographic data for this paper.
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Molecular planarity, close packing of the conjugated core and shift of the maximum of absorption in the solid of mono- and
di-alkylated TDPPs are highly affected by the nature of the appended substituent and intermolecular hydrogen bonding.