Columnar Self-Assembly of Electron-Deficient Dendronized Bay-Annulated Perylene Bisimides

Article abstract of DOI:10.1002/chem.201705290

Three new heteroatom bay-annulated perylene bisimides (PBIs) have been synthesized by microwave-assisted synthesis in excellent yield. N-annulated and S-annulated perylene bisimides exhibited columnar hexagonal phase, whereas Se-annulated perylene bisimide exhibited low temperature columnar oblique phase in addition to the high temperature columnar hexagonal phase. The cup shaped bay-annulated PBIs pack into columns with enhanced intermolecular interactions. In comparison to PBI, these molecules exhibited lower melting and clearing temperature, with good solubility. A small red shift in the absorption was seen in the case of N-annulated PBI, whereas S- and Se-annulated PBIs exhibited blue-shifted absorption spectra. Bay-annulation increased the HOMO and LUMO levels of the N-annulated perylene bisimide, whereas a slight increase in the LUMO level and a decrease in the HOMO levels were observed in the case of S- and Se-annulated perylene bisimides, in comparison to the simple perylene bisimide. The band gaps of PBI and PBI-N were almost same, whereas an increase in the band gaps were observed in the case of S- and Se-annulated PBIs. The tendency to freeze in the ordered glassy columnar phase for PBI-N and PBI-S will help to overcome the charge traps due to crystallization, which are detrimental to one-dimensional charge carrier mobility. These solution processable electron deficient columnar semiconductors possessing good thermal stability may form an easily accessible promising class of n-type materials.

Full text of DOI:10.1002/chem.201705290

A Journal of
Accepted Article
Title: Columnar Self-assembly of Electron deficient Dendronized bay-
annulated Perylene Bisimides
Authors: Ravindra Kumar Gupta, D. S. Shankar Rao, S. Krishna
Prasad, and Achalkumar Ammathnadu Sudhakar
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To be cited as: Chem. Eur. J. 10.1002/chem.201705290
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10.1002/chem.201705290
Chemistry - A European Journal
Columnar Self-assembly of Electron deficient Dendronized bay-annulated
Perylene Bisimides
Ravindra Kumar Gupta,a D. S. Shankar Rao,b S. Krishna Prasad b and
Ammathnadu S. Achalkumar*,a
aDepartment of Chemistry,
Indian Institute of Technology Guwahati, Guwahati, 781039, Assam, India.
b Centre for Nano and Soft Matter Sciences, Jalahalli, P. B. No. 1329, Bangalore,
560013, India.
Abstract: Three new heteroatom bay-annulated perylene bisimides (PBIs) have been
synthesized by microwave-assisted synthesis in excellent yield. N-annulated and S-
annulated perylene bisimides exhibited columnar hexagonal phase, while Se-
annulated perylene bisimide exhibited low temperature columnar oblique phase in
addition to the high temperature columnar hexagonal phase. The cup shaped bay-
annulated PBIs pack into columns with enhanced intermolecular interactions. In
comparison to PBI, these molecules exhibited lower meting and clearing temperature,
with good solubility. A small red shift in the absorption was seen in the case of N-
annulated PBI, while S- and Se-annulated PBIs exhibited blue-shifted absorption
spectra. Bay-annulation increased the HOMO and LUMO levels of the N-annulated
perylene bisimide, while a slight increase in the LUMO level and a decrease in the
HOMO levels were observed in the case of S- and Se-annulated perylene bisimides, in
comparison to the simple perylene bisimide. The band gaps of PBI and PBI-N were
almost same, while an increase in the band gaps were observed in the case of S- and
Se-annulated PBIs. The tendency to freeze in the ordered glassy columnar phase for
PBI-N and PBI-S will help to overcome the charge traps due to crystallization, which
are detrimental to one-dimensional charge carrier mobility. These solution
processable electron deficient columnar semiconductors with good thermal stability
may form an easily accessible promising class of n-type materials.
I. Introduction
Perylene bisimides are receiving increasing attention from the scientific
community working in the field of organic electronics and self-assembly. They have
been tested for variety of applications like organic solar cells,1 field effect transistors,2
light emitting diodes3 and light harvesting antennae.4 Although there are several p-
type organic semiconductors reported, the number of n-type semiconductors are less.
The progress in organic electronics always faced with a bottleneck, i.e. the dearth of
suitable n-type organic semiconductors with high mobility and stability. Perylene
bisimides due to their electron withdrawing imide substituents serve as electron
deficient n-type semiconductors. PBIs, when prepared by reacting perylene
tetracarboxylic acid dianhydride (PTCDA) with alkyl/aryl amines with peripheral
substitution have shown the tendency to stabilize various functional supramolecular
self-assemblies like J-aggregates, light harvesting assemblies, vesicles, liquid crystals
and chiral discriminators.5 Further modification in their physical and self-assembling
properties, were realized by bay-substitution. But it has to be noted that bay
1
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10.1002/chem.201705290
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substitution affects the planarity of the molecule and hence the self-assembly behavior
of the PBI derivatives. Recently we have realized bay-annulated perylene tetraesters
(PTEs), which were planar and exhibited columnar self-assembly over a wide thermal
range.6 This is interesting due to the fact that, by hetero atom bay-annulation, one can
modify the physical properties of these molecules without sacrificing the planarity
and self-assembly. Dendronized PBIs prepared by connecting the various benzyl
amines with peripheral substitution with flexible alkyl chains have shown to self-
assemble into columnar (Col) phases.7,9 Here the central PBI unit is a very rigid
aromatic unit exhibiting strong π-π interactions, while the peripheral trialkoxy benzyl
group provides a flexible unit. Organization of these n-type semiconductors to form
one-dimensional (1D) columnar structures, have advantages in comparison to the
polymeric or single crystalline organic semiconductors.9 This is because such self-
assembling molecular motifs have better solubility, ease of purification,
reproducibility and ease of handling in comparison to polymers and single crystals.8
Col phases formed by the stacking of disc-shaped molecules provide the anisotropic
charge carrier mobility along the columns, thus can be compared to molecular wires.
PBIs could be a possible alternative to fullerenes, which are popular as electron
accepting materials in organic solar cells. However, to become an efficient alternative
to fullerene based acceptors, these molecules have to be synthetically modified and
their properties have to be studied. There have been reports on the introduction of
various hetero-atoms like oxygen, nitrogen, sulphur and selenium in the bay region of
perylene derivatives by annulation. This has led to transformed aromatic systems with
fascinating properties. Incorporation of sulfur and selenium in organic semiconductors
is widely explored due to the intermolecular S···S and Se···Se interactions, which
also help to stabilize the molecular self-assembly and hence improve 1D
conductivity.6b-d,10 Motivated by our success in stabilizing luminescent Col self-
assembly in hetero atom bay-annulated PTEs over a wide thermal range,6 we
envisaged the synthesis of their bisimide counterparts.
II. Results and discussion
II.1 Synthesis and molecular structural characterization
The synthetic route is depicted in Scheme 1. Experimental details and
characterization data are provided in Electronic Supporting Information (ESI).
Perylene-3,4,9,10-tetracarboxylic dianhydride was subjected to hydrolysis and the
tetracarboxylate salt obtained was acidified to obtain the corresponding tetraacid. This
on refluxing in the presence of Aliquat 336 and n-bromobutane yielded the
corresponding tetraester (1) in good yield. The tetraester obtained was nitrated under
mild condition to obtain the bay-substituted mono nitro derivative 2 in good yield.
The nitro compound 2 on triethyl phosphite mediated Cadogan reaction yielded the N-
annulated perylene tetraester (3).6 The heating of nitro compound in presence of
sulphur or selenium powder in anhydrous NMP yielded the corresponding S and Se
annulated PTEs (Compound 6 and 8).6 The N-annulated perylene tetraester (3) was N-
ethylated in presence of sodium hydride as a base to give compound 4. The bay-
2
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10.1002/chem.201705290
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annulated tetraesters 4, 6 and 8 on heating in the presence of p-toluene sulphonic acid
(PTSA) monohydrate, yielded the respective anhydrides (5, 7 and 9). These bay-
annulated perylene dianhydrides were treated with tris-dodecyloxy benzyl amine
under microwave conditions, to yield the corresponding bisimides PBI-N, PBI-S and
PBI-Se in excellent yields. The application of microwave assisted synthesis in
presence of zinc acetate and imidazole improved the yields ranging from 80-90%, in
comparison to the conventional method.7a The synthesis of the key intermediate tris-
dodecyloxy benzyl amine (14) is provided in scheme 2 (SI). Ethyl gallate was O-
alkylated with n-dodecyl bromide under Williamson’s ether conditions to obtain
compound 10. This was reduced to corresponding benzyl alcohol (11) on treating with
lithium aluminum hydride (LAH) under anhydrous conditions. On treatment with
thionyl chloride, corresponding benzyl chloride (12) was prepared. Reaction of
Scheme 1. Synthesis of bay-annulated perylene bisimides
O
O
O
O
O
O
OC12H25
C12H25
O
C12H25
O
O
O
O
C4H9OOC
COOC4H9
O
N
O
OC12H25
OC12H25
C4H9OOC
COOC4H9
(i)
(v)
(vi)
Se
Se
OC12H25
Se
O
N
O
C4H9OOC
C4H9OOC
COOC4H9
8
O
O
O
C4H9OOC
COOC4H9
O
N
O
N
9
1
OC12H25
OC12H25
(ii)
(viii)
COOC4H9
C4H9OOC
COOC4H9
OC12H25
O
N
O
PBI-Se
C12H25
O
O
(iii)
NO2
NH
OC12H25
OC12H25
C12H25
OC12H25
PBI-N
OC12H25
C4H9OOC
COOC4H9
(vii)
C4H9OOC
COOC4H9
(iv)
O
N
O
3
2
(vi)
O
O
O
O
O
O
S
C4H9OOC
COOC4H9
C4H9OOC
COOC4H9
(v)
(vi)
(v)
S
N
S
N
O
N
O
C12H25
O
O
C12H25
C4H9OOC
COOC4H9
O
O
O
C4H9OOC
COOC4H9
O
O
O
OC12H25
4
6
7
5
PBI-S
o
Reagents and conditions. i) KOH, H2O, 70 C, 0.5 h, 1M HCl, Aliquat 336, KI,1-
o
bromobutane, reflux, 12 h (76%); (ii) NaNO2, HNO3, 0 C, 1 h (90%); (iii)Triethyl
o
phosphite, 160 C, reflux, 4 h, N2 (60 %); (iv) NaH, 1-iodoethane, Dry THF, reflux,
17 h (75%); (v) p-toluene sulphonic acid mono hydrate, toluene, 100 oC, 30 h (90%);
o
(vi) 14, zinc acetate, imidazole, 165 C, microwave, 30 min. (80-90%); (vii) Sulfur
o
o
powder, anhyd.NMP, N2, 70 C, 0.5 h, 180 C, 17 h (55%); (viii) Selenium powder,
anhyd. NMP, N2, 70 oC, 0.5 h, 180 oC, 17 h (55%).
3
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Hc
Ha
Hb
Hc
Ha
Hb
Hc
Ha
Hb
Ha
Hb
ꢁꢂꢃ
ꢁꢉꢃ
ꢁꢈꢃ
ꢀꢁ
ꢃ
ꢀꢁꢀꢃ
ꢀꢁꢇꢃ
ꢀꢁꢆꢃ
ꢀꢁꢃꢃ
ꢀꢁꢅꢃ
ꢀꢁꢄꢃ
δ (ppm)
Figure 1. Overlay of the expanded region of the 1H NMR spectra (CDCl3, 600 MHz)
of PBI and bay-annulated PBIs.
compound 12 with phthalimide yielded it’s N-benzylated derivative 13, which on
hydrolysis yielded the corresponding benzyl amine 14, as reported earlier.7c We have
prepared simple perylene bisimide from the amine 14 by reacting it with perylene
bisanhydride in order to compare with the thermal and photophysical behavior of the
corresponding hetero atom bay-annulated PBIs. All the intermediates and final
1
compounds were characterized with H NMR, 13C NMR, IR, MALDI-TOF/ESI-
1
HRMS techniques. The overlay of the H NMR spectra shows the proton adjacent to
the hetero atom is more deshielded, in comparison to other protons on perylene ring
(Fig.1). Among the bay-annulated PBIs, the proton Hc in PBI-S is more deshielded.
This is in contradiction to bay-annulated perylene tetraesters, where the proton Hc of
selenium annulated perylene tetraester resonated down field.6c
II. 2. Thermal behavior
The thermal stability of these compounds was investigated with the help of
thermo gravimetric analysis (TGA). S-annulated compound PBI-S exhibited higher
thermal stability in contrast to parent PBI, N- and Se-annulated PBIs (Fig.S35).
Thermotropic mesomorphic behavior of the PBIs was investigated as follows. The
samples sandwiched between a glass slide and a coverslip were tested for the
shearability and birefringence on heating in a programmable hot stage associated with
a polarizing optical microscope (POM). The phase transitions and associated
enthalpies were obtained by differential scanning calorimetry (DSC) thermograms,
while the mesophases are assigned with the help of X-ray diffraction (XRD) studies.
In comparison to perylene bisimide without annulation the bay-annulated PBIs
exhibited lower clearing temperatures (Fig.2). The phase transition temperatures and
associated enthalpy changes are presented in Table 1.
o
Compound PBI-N on heating showed a transition around 102 C, with an
increase in the birefringence along with the fluidity, which corresponds to a crystal to
mesophase transition, before reaching the clearing temperature of 156 oC (Fig.3a). On
4
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o
cooling the isotropic liquid at rate of 5 C/min), showed the formation of highly
birefringent mosaic pattern starting at 152 oC. At around 69 oC, a transition with an
Columnar Oblique phase
Columnar hexagonal phase
PBI
PBI-N
PBI-S
PBI-Se
230
62
104 146 188
20
Temperature (oC)
PBI
PBI-N
PBI-S
PBI-Se
Figure 2. Molecular structures of PBI and bay-annulated PBIs and the bargraph
representing their thermal behavior (based on first cooling scan)
(a)
(b)
Colh1
I
I
Colob
cooling
heating
Cr
Colh
Colh2
I
I
Colh
cooling
heating
cooling
heating
Colh2
Colh
Colh
Colob
Colh1
(c)
I
I
Figure 3. DSC thermograms obtained for PBI-N (a); PBI-S (b) and PBI-Se (c) in the
first cooling and second heating scans.
Table 1. Phase transition temperatures a (oC) and corresponding enthalpies (kJ/mol) of
DLCs
Phase sequence
Entry
PBI-N
PBI-S
Second Heating
Colh2 77.8 (0.7) Colh1 156.1 (1.4) I
Colh 200.6 (2.3)
First Cooling
c
I 152 (1.7) Colh1 69.4 (1.5) Colh2
c
I 196.4 (2.9) Colh
b, d
PBI-Se Cr 78.6 (2.3) Colob 125.1 (9.4) Colh 202.5 (2.2) I I 198.6 (2.7) Colh 70 Colob
aPeak temperatures in the DSC thermograms obtained during the second heating and first cooling
cycles at 5 oC/min; bThe transition observed only under microscope. the mesophase freezing in the
c
glassy state; d Crystallization was not detected on cooling; Cr = Crystal phase; Colh = Columnar
hexagonal phase; Colob = Columnar oblique phase; I = Isotropic phase.
enthalpy change of 1.5 kJ was observed, but no change was observed in the optical
texture, which remained unchanged till room temperature, without any signs of
o
crystallization (Fig.3a). In the second heating a transition at 78 C (ΔH = 0.7 kJ/mol)
with a slight increase in the brightness and fluidity was noted. This phase existed till
o
156 C, after which the phase transformed into an isotropic liquid. These changes
were corresponding to high and low temperature phases, which were difficult to
5
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distinguish, except a decrease in the birefringence in the low temperature phase
(Fig.4a and b). Thus we decided to investigate these mesophases by XRD studies at
different temperature intervals. XRD studies of the sample carried out at 145 oC after
(a)
(b)
T = 145 oC
T = 40 oC
(c)
(d)
Figure 4. POM images obtained for PBI-N (a) and (b); the XRD profiles depicting
the intensity against the 2θ obtained for the Colh1 phase at 145 oC (c) and Colh2 phase
28 oC (d).
cooling from its isotropic melt showed a intense sharp peak at low angle with a Bragg
spacing d = 27.97 Å in the low angle region. Further several reflections with d-
spacing of 16.12, 13.94 and 10.52 Å were observed in the middle-angle region (5o <
2θ < 10o). These reflections could be indexed into Miller indices of 100, 110, 200 and
210 with the ratio of 1:1/√3:1/√4:1/√7 and therefore these values fit very well with the
lattice of the Colh phase (Fig.4c). In the wide-angle region two diffused peaks were
observed at a d-spacing of 4.64 and 3.54 Å. The first of these peaks can be attributed
to the packing of floppy alkyl chains and the second one is arising from the packing
of aromatic cores within the column. It is interesting to note that within the Col phase
these molecules are packing with an intimate overlap of aromatic cores, which is
beneficial for 1D charge migration. The calculated lattice parameter ‘a’ was found to
be 32.3 Å, which is approximately 35% lesser than the molecular diameter obtained
from the molecular model in its all trans conformation (Fig.5, Table 2). This decrease
may be due to the folding of the peripheral flexible chains, which decrease the
effective diameter of the molecule in the Col phase or due to the interdigitation of the
flexible chains of PBI-N of one column with that of the neighbouring column. From
6
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Table 2. Results of (hkl) indexation of XRD profiles of PBI-N and PBI-S at a given
temperature (T) of mesophasesa
Compounds
(D/Å)
Phase
(T/oC)
dobs (Å)
dcal (Å)
Miller
indices
(hkl)
100
110
200
Lattice parameters (Å),
Lattice area S (Å2),
Molecular volume V (Å3)
a = 32.30
c = 3.54
S = 903.6
145
27.97
27.97
16.15
13.99
10.57
(Colh1) 16.12
13.94
10.52
210
V = 3203
4.64 (ha)
3.54 (hc)
Z = 1.1
001
100
27.89
27.89
16.10
13.95
10.54
100
110
200
210
a = 32.21
c = 3.51
S = 898.2
V = 3157.1
Z = 1.1
(Colh1) 16.08
13.91
RG-381
PBI-N
10.50
4.57 (ha)
3.51 (hc)
(49.45)
001
100
40
32.11
32.11
(diffuse)
a = 37.08
c = 3.53
S = 1190.8
V = 4205.6
Z = 1.5
(Colh2) 9.08
4.47 (ha)
3.53 (hc)
194
28.15
28.15
16.25
14.08
10.64
100
110
200
210
a = 32.51
c = 3.51
S = 915.2
V = 3208.4
Z = 1.1
(Colh) 16.21
14.03
10.60
4.71 (ha)
3.51 (hc)
27.69
001
100
110
200
210
300
120
27.69
15.99
13.84
10.47
9.23
a = 31.97
c = 3.46
S = 885.3
V = 3064.1
Z = 1.1
(Colh) 15.94
13.81
10.42
9.18
4.62 (ha)
RG-378
PBI-S
3.46 (hc)
27.47
(Colh) 15.81
13.69
001
100
110
200
210
80
27.47
15.86
13.74
10.38
a = 31.72
c = 3.44
S = 871.5
V = 3000.6
Z = 1.1
(48.37)
10.33
4.54 (ha)
3.44 (hc)
27.93
001
100
110
200
40
27.93
16.13
13.97
a = 32.25
c = 3.43
S = 900.9
V = 3092.5
Z = 1.1
(Colh) 16.13
13.86
9.58
4.48 (ha)
3.43 (hc)
(diffuse) 300
001
aThe diameter (D) of the disk (estimated from Chem 3D Pro 8.0 molecular model
software from Cambridge Soft). dobs: spacing observed; dcal: spacing calculated
(deduced from the lattice parameters; a for Colh phase; c is height of the unit cell).
The spacings marked ha and hc correspond to diffuse reflections in the wide-angle
region arising from correlations between the alkyl chains and core regions,
respectively. Z indicates the number of molecules per columnar slice of thickness
hc estimated from the lattice area S and the volume V.
the lattice parameter ‘a’, we could calculate the number of molecules (Z) present in
the unit hexagonal cell with the height (hc) of 3.54 Å and found to be one. This means
that each stratum of the column is formed by single molecule, which is packed with
7
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an intermolecular distance of 3.54 Å. The XRD pattern obtained at lower temperature
(at 100 oC) in high temperature phase regime was almost same with a lowering of ‘a’
and the core-core distance (hc) (see the table and Fig.S36a).
49.5 Å
PBI-N
hc = 3.54 Å
ha = 4.64 Å
d110
a = 32.3 Å
Columnar hexagonal phase
d100
Figure 5. Schematic showing the energy minimized structure of PBI-N and its self-
o
assembly into columnar hexagonal (Colh1) phase (XRD data at 145 C is considered
for the illustration).
We were curious to know the nature of the low temperature phase, thus the
XRD studies were carried out at 40 oC. The XRD pattern obtained at 40 oC, exhibited
a single sharp peak at low angle, along with two diffused peak at wide angle, which
confirms the phase to be a Colh. A diffused peak with a d-spacing of 9.1 Å was
observed and the ratio of this with the first d-spacing was found to be 1:1/√3, which
usually corresponds to the Miller indices 220 in a hexagonal lattice (Fig.4c). Though
it is not explicit to assign the Colh phase from the observation of single sharp peak at
low angle, the absence of other peaks has been ascribed to a minimum in the form
factor.11The optical texture remained unaffected even at ambient temperature, and the
sample was not shearable, suggesting the freezing of Colh phase in the glassy state.
For the sake of identification, we have denoted the high temperature Colh phase as
Colh1 and the low temperature Colh phase as Colh2. Such transitions between Colh
phases are rarely observed.12The self-assembly of PBI-N in Colh1 phase is illustrated
in Fig.5.
8
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(a)
(b)
T = 194 oC
T = 28 oC
(c)
(d)
Figure 6. POM images obtained for PBI-S (a) and (b); the XRD profiles depicting the
intensity against the 2θ obtained for the Colh phases at 194 oC (c) and at 40 oC (d).
PBI-S and PBI-Se exhibited enantiotropic Colh phase, but with higher melting and
clearing temperatures than PBI-N, which may be due to the enhanced S…S and Se…Se
interactions (intra or intercolumnar).6a,c,10 In addition, we should note that these
compounds do not have an ethyl substitution near to the core as in the case of PBI-N,
which might reduce the core-core interactions. The thermal range of Col phase of
both these molecules was found to be almost same. Compound PBI-S, on heating,
o
melted at ≈123 C, with a large enthalphy change (ΔH = 34.2 kJ/mol) corresponding
to Cr-mesophase transition as noted in DSC scans in concurrence with the POM
o
observations. This mesophase cleared at ≈ 201 C. On cooling the isotropic liquid,
pseudofocal conic fan-like texture appeared which persisted till room temperature
(Fig.6a,b). DSC cooling scan did not show any signs of crystallization even up to 0 oC
(Fig.3b), however the material was not shearable, which confirmed the freezing of the
mesophase in glassy state. XRD studies at different temperature intervals, like 194 oC,
o
o
o
120 C, 80 C and 40 C were performed. All the XRD profiles exhibited a similar
o
pattern corresponding to Colh phase. For example, the XRD pattern at 194 C
(Fig.6c), exhibited a strong reflection with a d-spacing of 28.2 Å at low angle region
along with low intensity reflections at the middle angle region with d-spacings of
16.21 Å, 14.03 Å, 10.6 Å. The reciprocal spacing ratio was found to be 1:√3:√4:√7.
These d-spacings can be indexed into Miller indices of 100, 110, 200 and 210 of a
hexagonal lattice (Table 2). At wide angle, diffused spacings corresponding to 4.71 Å
and 3.51 Å were observed, which are corresponding to the alkyl chain packing and
core-core stacking respectively. This confirms the Colh phase, and further the XRD
9
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10.1002/chem.201705290
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profiles obtained at low temperature (Fig.S36b,c and Fig.6d) also corresponds to the
same confirming the monomesomorphic behavior of PBI-S.
Table 3. Results of (hkl) indexation of XRD profiles of PBI-Se at a given
temperature (T) of mesophasesa
Compounds
Phase
(T/oC)
dobs (Å)
dcal (Å)
Miller
indices
(hkl)
lattice parameters (Å),
lattice area S (Å2),
(D/Å)
molecular volume V (Å3)
195
(Colh)
28.24
16.25
14.07
10.62
4.69 (ha)
3.51 (hc)
27.86
16.03
13.88
28.24
16.30
14.12
10.67
100
110
200
210
a = 32.60
c = 3.51
S = 920.6
V = 3233.9
Z = 1.1
001
100
110
200
210
140
(Colh)
27.47
15.86
13.74
10.38
a = 31.72
c = 3.48
S = 871.5
V = 3035.6
Z = 1
RG-379
PBI-Se
10.48
4.59 (ha)
3.48 (hc)
27.43
15.81
13.69
(48.468)
001
100
110
200
210
80
(Colh)
27.86
16.09
13.93
10.53
a = 32.17
c = 3.45
S = 896.5
V = 3099.7
Z = 1.1
10.34
4.43 (ha)
3.45 (hc)
45.98
001
01
11
10
02
03
22
04
33
16
45
RT/24
45.98
27.24
26.33
22.99
15.33
13.62
11.50
9.08
a = 28.04
b = 48.96
c = 3.44
27.24
(Colob) 26.33
22.96
15.21
13.58
11.31
9.08
γ = 69.9o
S = 1289.2
V = 4434.8
Z = 1.5
8.26
6.47
8.15
6.52
4.31
4.26
diffuse
4.26
-54
-55
-63
-65
3.99
3.84
3.44
4.01
3.88
3.52
aThe diameter (D) of the disk (estimated from Chem 3D Pro 8.0 molecular model
software from Cambridge Soft). dobs: spacing observed; dcal: spacing calculated
(deduced from the lattice parameters; a for Colh phase; a and b for Colob phase; c
is height of the unit cell). The spacings marked ha and hc correspond to diffuse
reflections in the wide-angle region arising from correlations between the alkyl
chains and core regions, respectively. Z indicates the number of molecules per
columnar slice of thickness hc estimated from the lattice area S and the volume V.
o
Selenium annulated PBI (PBI-Se) on heating melted at ≈130 C, with a large
enthalphy change (ΔH = 32.8 kJ/mol) corresponding to Cr-mesophase transition. This
was in accordance with the POM observation that showed increased fluidity and
birefringence.
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(b)
(a)
T = 197 oC
T = 28 oC
(c)
(d)
Figure 7. POM images obtained for PBI-Se (a) and (b); the XRD profiles depicting
the intensity against the 2θ obtained for the Colh phase at 195 oC (c) and for the Colob
phase at 24 oC (d).
48.5 Å
γ = 69.9o
b
= 49 Å
a
= 32.2 Å
a
= 28 Å
Columnar hexagonal phase
Columnar oblique phase
Figure 8. Schematic showing the energy minimized structure of PBI-Se and its self-
o
assembly into Colh phase (XRD data at 80 C is considered for the illustration) and
Colob phase (XRD data at 24 oC is considered for the illustration).
The clearing point was almost same with that of the PBI-S, but the melting point was
higher than that of PBI-S. As per the DSC scan in cooling cycle a transition
11
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10.1002/chem.201705290
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corresponding to isotropic to mesophase was observed (Fig.3c). It was very hard to
detect any change in the pseudo focal conic texture except the blurring of the dark
o
homeotropic background (Fig.7a,b), which was seen around 70 C. Thus we have
carried out XRD at different temperature intervals. The XRD patterns obtained at 195
o
o
oC, 140 C and 80 C were almost same corresponding to Colh phase (Fig.7c and
Fig.S36d,e, Table 3), while the one obtained at room temperature (Fig.7d) showed
several peaks at low and middle angle region. These d-spacings could be indexed to a
Col phase with a parallelogram lattice of an oblique symmetry (Colob), with the lattice
parameters a = 28.04 and b = 48.96 Å with a tilt angle γ = 69.9o. The core-core
stacking distance ‘c’ was found to be 3.44 Å. Thus the number of discs present in a
unit cell (Z) turns to be 1.5. Thus, PBI-Se exhibited a bimesomorphic behavior, with
high temperature Colh phase and low temperature Colob phase. This phase appears to
be 3D-ordered, although the XRD peaks can be assigned to Colob phase. The presence
of a non-negligible diffuse maximum at wide angles does suggest enough disorder in
the medium, which could be of either plastic or glassy in nature. Conclusive evidence
was not found, as no peak required all non-zero hkl values. The observation of Colob
phase corresponds to the enhanced intermolecular interactions in PBI-Se in
comparison to the other two PBIs, which is also reflected in its higher melting point in
comparison to PBI-S. However, the second heating showed a cold crystallization at ≈
o
79 C (2.3 kJ/mol), which was not noticed during the first cooling cycle in DSC
(Fig.3c). It also should be noted that the XRD at room temperature was taken in the
first cooling cycle. Thus somewhere below this temperature crystallization occurs,
which was not seen in the case of PBI-N and PBI-S.
During these X-ray studies we had some interesting findings, which is due to the
minute structural differences arising from the bay annulation. The XRD patterns
obtained for PBI-N at low temperature i.e. Colh2 phase shows that this phase is less
ordered in comparison to the high temperature Colh1 phase. This is in contradiction
to what is expected on lowering the temperature. Usually the phase becomes more
ordered, due to the reduced mobility of the molecules along with a lowered core-
core distance. But the observed difference, where the peaks at low angle become
broad and diffuse was found to be reproducible and genuine. Similar effect was
noticed in the case of PBI-S, but it was not that drastic. The main structural aspect
that should be borne in mind is that in the case of PBI-N where the effect is drastic
has an ethyl group substitution at the bay-nitrogen position. This can, in principle
be considered like an extended arm of the hydrocarbon having substantial degree of
freedom to move about the plane of the perylene core. At high temperatures, i.e. in
the Colh1 phase, when the terminal chains have a substantial folded character
(molten chains), the effect due to the ethylene part is less dominant. Thus the
interfaces between the lattice planes are reasonably flat, a feature that is evident
from the stronger signal for the peak associated with the core-core correlation.
Concomitantly the core-core distance is also lower. At much lower temperature, in
the Colh2 phase, a large increase in the diameter (a) of the columns is seen
(increasing by ~ 5 Å from the value in the Colh1 phase) signifies that the terminal
12
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chains are more spread out. Intuitively, this should have led to a flatter lattice plane
and consequently better-packed molecular cores. However, the experiments show
the opposite as seen by reduced number of sharp peaks at low angles, and a far
weaker core-core peak. This must be due to the increasing influence of the wagging
by the ethyl group in the core region. The reduced overall entropy of the system
must be making the ethyl group to occupying regions above or below the plane of
the core, and less biased towards being in that plane; this should increase the core-
core distance as is clear from the X-ray studies. This diminishes the interaction
between the neighbouring discs, a feature evident from the weakened core-core
peak, making the lattice plane interfaces rough and in turn reducing the number of
low-angle peaks. The presence of the sulphur atom in the PBI-S case also has
some of these features, but the effect is much smaller, as sulphur is less bulky
compared to the ethyl group. It is possible that the rarely observed transition
between two hexagonal columnar phases (Colh1-Colh2) is caused by the presence of
the ethyl group in the core-region. In the selenium case (PBI-Se) probably the
disturbance leads towards the tilting of the columnar lattice (Colob phase), although
only after a three-dimensional ordering (as seen by a number of sharp peaks) sets
in.
Density functional theory studies
We have used density functional theory (DFT) calculations to visualize the energy-
minimized structures of the PBIs and the energy levels of the frontier molecular
orbitals (HOMO-LUMO) of all the compounds (See the SI). The energy minimized
structures for PBI, PBI-N, PBI-S and PBI-Se showed in Fig.9a. Images of frontier
molecular orbitals (FMOs) and the differences in their energies provide knowledge
about the length of conjugation and band gap level, which helps in comparing the
perylene bisimide with its bay-annulated derivatives. In the energy minimized
structures, we can notice the planar perylene and two out of plane benzyl rings. Side
view presented in Fig.9b distinguishes the bay-annulated PBIs from the parent PBI. In
the case of parent PBI, the benzyl wings are positioned at two different planes, while
in the case of bay-annulated PBIs, two benzyl wings are on the same side of the
central perylene ring, leading to a cup shape. Such an orientation helps the molecules
to pack into more ordered columns as shown in Fig.5 and Fig. 8.7a
The contours of the HOMO and LUMO of PBIs are shown in Fig. 9c and
d.The HOMO-LUMO energy levels of PBIs are distributed on the entire aromatic ring.
PBIs are considered as closed chromophoric (electronically decoupled) systems due
to the presence of nodes of the HOMO and LUMO orbitals on imide nitrogen atoms.
Thus the substitution at imide nitrogen does not alter the optical properties
significantly.13 Bay-annulation increased the HOMO and LUMO levels in comparison
to the simple perylene bisimide. In the case of annulated derivatives, HOMO and
LUMO levels were found to be spreading over heteroatom. The theoretical band gaps
for PBI, PBI-N, PBI-S and PBI-Se were found to be 2.27 eV, 2.3 eV, 2.14 eV and
2.32 eV respectively.
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LUMO (EL)
HOMO (EH)
-3.50 eV
PBI
-5.77 eV
-3.27 eV
PBI-N
-5.57 eV
-3.50 eV
PBI-S
-5.64 eV
-3.45 eV
PBI-Se
-5.77 eV
(a)
(b)
(c)
(d)
Figure 9. Optimized geometry of compounds PBI, PBI-N, PBI-S and PBI-Se (a);
side view of the same (b); HOMO (c) and LUMO (d) frontier molecular orbitals of
compounds PBI, PBI-N, PBI-S and PBI-Se at the B3LYP/6-31G(dp) level. EH and
EL denote energies of the highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO), respectively.
Photophysical and electrochemical studies
Photophysical properties of these compounds were explored by obtaining their
absorption and emission spectra in micromolar chloroform solutions (Table 4 and
Fig.10). The absorption spectra of these PBIs were well-structured and showed the
characteristic four vibronic bands, with the longest absorption wavelength was
centered in the range of 503-532 nm. Compound PBI-N was slightly red-shifted in
comparison to PBI, while PBI-S and PBI-Se exhibited slightly blue shifted
absorption spectra. All the PBIs exhibited high molar extinction coefficients at their
absorption maxima (ε: 14,700 to 20,300 Lmol-1cm-1). The optical band gap calculated
from the absorption onset showed that the optical band gap of PBI and PBI-N were
almost matching while that of the PBI-S and PBI-Se shown an increase. Emission
spectra of these compounds showed two main bands, with the high intensity band was
in the range of 517-545 nm. The Stoke’s shift was found to be less in the range of
448-596 cm-1.
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Table 4. Photophysical properties of PBIs in solution
Solution state
Absorption
(nm)
ε (Lmol-1cm-1) Emissionb
Stokes shift
(cm-1)
ΔEcg, opt
(eV)
(nm)
PBI
528, 491, 460 430
18259
544, 580
557
2.28
PBI-N
PBI-S
PBI-Se
532, 496, 468, 438
503, 470, 442, 417
510, 476, 449, 421
14797
16600
20310
545, 584
517, 551
526, 560
448
538
596
2.27
2.39
2.34
a micromolar solutions in CHCl3; b the excitation wavelength λex = 460, 468, 442 and 449 nm
respectively for compound PBI, PBI-N, PBI-S and PBI-Se; c calculated from the red edge of
the absorption band;
Cyclic voltammetry (CV) studies are carried out to understand the electronic energy
levels that decide the energy and electron transfer process and reversibility of a redox
process. CV studies were carried out in anhydrous dichloromethane solutions (Table
5). PBIs are known to be electron deficient n-type semiconductors and known to have
high electron affinity. Among these compounds, only PBI and PBI-N exhibited two
reversible reduction peaks (Fig.11a). It was difficult to obtain the oxidation potentials
of these electron deficient compounds even with a CV measurement window up to +1
V vs Ag/AgNO3. Thus we have calculated the HOMO levels, by subtracting the
LUMO energy values from the optical band gap that was obtained from the
absorption onset of the compounds in solutions. From the CV studies it is found that
the bay-annulated PBIs showed increased LUMO levels in comparison to PBI
(Fig.11b). A small increase of 0.18 eV in the HOMO level was noted in the case of
PBI-N, while PBI-S and PBI-Se displayed a lowering of HOMO levels by 0.08 and
0.04 eV respectively (Fig.11b). The values obtained from CV studies followed the
same general trend that was obtained from DFT calculations.
(a)
(b)
Figure 10. (a) Absorption and (b) emission spectra in micromolar THF solution for
PBI, PBI-N, PBI-S and PBI-Se.
15
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(a)
(b)
PBI-Se
PBI-S
PBI-N
PBI
-3
-3.66 eV
-3.65 eV
-3.51 eV
-3.68 eV
LUMO
-4
-5
-6
2.28
eV
2.34
eV
2.39
eV
2.27
eV
HOMO
-5.78 eV
-5.96 eV
-6.0 eV
-6.04 eV
Figure 11. (a) Cyclic voltammograms of PBI and bay-annulated PBIs; b) Energy
band level diagram showing HOMO- and LUMO- energy levels of the same.
Table 5. Electrochemical properties of PBIs in solutiona,b
Entry
PBI
PBI-N
PBI-S
PBI-Se
Ec
Ed,e
-3.68
-3.51
-3.65
-3.66
Ed,f
-5.96
-5.78
-6.04
-6.00
ΔEd,g
g, opt
2.28
2.27
2.39
2.34
1red
LUMO
HOMO
-0.57
-0.74
-0.60
-0.59
ain Dichloromethane solutions; bExperimental conditions:
Ag/AgNO3 as reference electrode, Glassy carbon working
electrode, Platinum wire counter electrode, TBAP (0.1 M) as
a supporting electrolyte, room temperature; c in volts (V); d in
eV; eEstimated from the formula by using ELUMO = – (4.8 –
E1/2, Fc/Fc+ + Ered, onset) eV; Estimated from the formula
f
EHOMO = ELUMO – Eg, opt) eV; gcalculated from the red edge
of the absorption band (544, 547, 519, 530 nm respectively).
E1/2, Fc/Fc+ = 0.55
Conclusions
In conclusion, we have synthesized three new electron deficient hetero atom
bay-annulated perylene bisimides stabilizing ordered columnar phase over a wide
temperature range. The N- and S-annulated perylene bisimides showed columnar
hexagonal phase, while the Se-annulated perylene bisimide exhibited columnar
oblique phase in addition to the columnar hexagonal phase. The Col phase on cooling
freezes into glassy state, this is quite impressive to obtain the highly ordered structure
in the devices, by thermal annealing. These molecules exhibit high molar extinction
coefficients along with the absorption spectra covering the most of the visible
spectrum, which will be important for solar cells with a scope to improve further by
the structure modulation. The optical band gaps are low with minor differences
observed in the HOMO and LUMO levels. Further structural modifications are
needed to approach the fullerene’s HOMO and LUMO levels, however this serves as
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a starting point. DFT calculations revealed the cup-like conformation of these
molecules in the energy-minimized structure, which may support their columnar self-
assembly along with the π-π interactions and nanophase segregation. Considering the
scarcity of n-type semiconductors in literature, these compounds could give rise to a
variety of alternatives to synthesize highly ordered n-type columnar self-assemblies.
The microwave-assisted synthesis further improved the synthetic access to such
materials in good yields.
Acknowledgements. ASA sincerely thanks Science and Engineering Board (SERB),
DST, Govt. of India and BRNS-DAE for funding this work through project No.
SB/S1/PC-37/2012 and No.2012/34/31/BRNS/1039 respectively. We thank the
Ministry of Human Resource Development for Centre of Excellence in FAST (F. No.
5-7/2014-TS-VII). We thank CIF, IIT Guwahati for analytical facilities.
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