Structure-property relationship in charge transporting behaviour of room temperature liquid crystalline perylenebisimides

Article abstract of DOI:10.1039/c4tc01806k

A homologous series of pentadecyl phenol functionalized perylenebisimide (PBI) terminated with trialkoxy gallate esters was synthesized, where the terminal alkyl chain length was varied from n = 4 to 12 (PBI-En). The thermotropic liquid crystalline (LC) c

Full text of DOI:10.1039/c4tc01806k

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Revised Manuscript submitted to Journal of Materials Chemistry C
Manuscript ID TCꢀARTꢀ08ꢀ2014ꢀ001806)
(
StructureꢀProperty Relationship in Charge Transporting Behaviour of Room
Temperature Liquid Crystalline Perylenebisimides
.
a,b
a
a,b,c
a,b,c
K. P. Prajitha, S, Chithiravel, K. Krishnamoorthy*
and S. K. Asha*
a) Polymer Science and Engineering Division, CSIRꢀNational Chemical Laboratory, Dr
Homi Bhabha Road, Pune 411008, India.
b) Academy of Scientific and Innovative Research, New Delhi, India
c) CSIRꢀNetwork Institutes of Solar Energy, New Delhi, India
Corresponding Author: S. K. Asha, Eꢀmail: sk.asha@ncl.res.in Fax: 0091ꢀ20ꢀ25902615.
K. Krishnamoorthy, Eꢀmail: k.krishnamoorthy@ncl.res.in
1

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Abstract:
A homologous series of pentadecyl phenol functionalized perylenebisimide (PBI) terminated
with trialkoxy gallate esters were synthesized, where the terminal alkyl chain length was
varied from n = 4 to 12 (PBIꢀEn). The thermotropic liquid crystalline (LC) characteristics of
the molecules were analyzed using differential scanning calorimetry (DSC), polarized light
microscopy (PLM) combined with variable temperature wide angle Xꢀray diffraction
(WXRD) techniques. A clear oddꢀeven oscillation was observed in the melting as well as
isotropization enthalpies as a function of alkyl spacer length in the terminal gallate unit with
the even spacers exhibiting higher values. The higher members of the series with n > 8
exhibited thermotropic liquid crystalline textures in the PLM which remained stable until
room temperature. The nature of the LC phase was identified to be columnar rectangular and
columnar hexagonal based on detailed analysis of the WXRD pattern recorded in the LC
phases. The WXRD pattern of the room temperature LC frozen samples indicated a nearly
constant intra columnar stack distance of ~3.7 Å for all the members. The space charge
limited current (SCLC) values of the LC frozen sample films were analyzed for dependence
of bulk mobility estimate on the nature of the LC phase. The columnar hexagonal phase
ꢀ3
2
ꢀ1 ꢀ1
ꢀ4
exhibited a mobility value one order (10 cm V s ) higher than that of crystalline (10
2
ꢀ1 ꢀ1
ꢀ5
2
ꢀ1 ꢀ1
cm V s ) and two orders higher than that of columnar rectangular phase (10 cm V s )
indicating a strong dependence of packing on bulk mobility.
2

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Introduction
Charge transport in organic semiconducting materials is a very active research area
because of their potential applications in optoelectronic materials such as organic field effect
transistors (OFET), organic light emitting diodes (OLED), organic photovoltaic cells (OPV)
1
ꢀ4
etc. In this perspective columnar liquid crystalline (LC) materials are well recognized
because of their ability to self organize in to an ordered architecture allowing for one
5
ꢀ8
dimensional charge transport resulting in a high charge carrier mobility. In the columnar
liquid crystals, the disc like mesogens are stacked one on top of another surrounded by the
flexible alkyl chain resulting in mesophase formation. The disc like molecules can form
stable columns (typical intra columnar distance of 3.5ꢀ3.9 Å) due to the strong πꢀπ stacking
between the poly aromatic core with significant overlap of pꢀorbital, which results in
9
effective charge transport. Upon thermal annealing, these molecules can form single large
domain with less number of grain boundaries as a result of their inherent self healing
1
0ꢀ12
property due to their partial liquidꢀlike behavior.
Therefore, columnar liquid crystalline
materials have several advantages over organic single crystalline materials including easy
processing, ability to form thin films, long range self assembly, less number of grain
1
3, 14
boundaries etc which are the key factors for efficient charge transporting.
But the main
challenge in this area is the attainment of mesogenicity in a temperature range that is
1
5
adaptable on a device substrate. The clearing temperatures should be low and ideally the
liquid crystalline phase should be retained at room temperature.
However, the design of a suitable LC material for a desired application requires a
basic understanding of the structureꢀproperty relationship. Depending on the lateral chain
length of the substituent, the inter columnar distance is usually in the range of 20ꢀ40 Å. The
interactions between the adjacent molecules within the same column is much stronger than
the interactions between neighboring columns which results in better one dimensional charge
transport. Depending on the extent of coreꢀcore packing, the columnar phases can be further
8
, 9
classified in to ordered and disordered columnar mesophase.
For devise application,
columnar ordered phases are preferred because of the effective πꢀπ stacking. Further,
depending on the core and spacer interactions the columnar phase can self assemble in to
columnar hexagonal, columnar rectangular, columnar oblique, columnar lamellar or
columnar plastic mesophases. Among these the columnar hexagonal (Col ) and columnar
h
3

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rectangular (Col ) phases are the two commonly observed mesophases in the discotic
r
1
6, 17
systems.
Columnar plastic phase (Col ) is the highest ordered phase having three
p
1
8
dimensional positional order as in crystalline state along with rotational mobility. Because
of these characteristics columnar plastic phase has been considered as an ideal charge
1
9
transporting system and better mobility values have been observed in these systems.
Further understanding of packing and charge transport in these systems are required as only
1
9, 20
few reports are available in literature.
The liquid crystalline and charge transporting properties of p type semiconducting
systems like triphenylenes, pthalocyanin, hexabenzocoronene etc are well studied in
2
1ꢀ23
literature.
crystalline materials are concerned.
semiconducting material with inherent mesogenicity possessing high thermal and photo
However, there are only limited choices as far as n type semiconducting liquid
2
4, 25
Perylenebisimides (PBI) are an n type organic
2
6
stability along with high absorption coefficient. Aromatic πꢀπ stacking, van der waals
interaction, hydrogen bonding, chirality etc are some of the major tools that have been
2
7
reported to self assemble PBI based systems to desired architectures. With suitable
molecular design, the PBI derivatives can be made to exhibit a wide array of liquid
crystalline phases. Struijk et al. reported a linear aliphatic chain substituted liquid crystalline
2
ꢀ1 ꢀ1
PBI derivative with a mobility of 0.1 cm V s determined by pulseꢀradiolysis timeꢀresolved
28
microwave conductivity (PRꢀTMC) method. Marder et al. reported liquid crystalline
2
ꢀ1 ꢀ1
dodecyloxy ester derivative of PBI with a mobility of 1.3 cm V s by SCLC method which
2
9
is one of the highest charge carrier mobility reported so far in the LC phase. Recently,
Thelekkat et al. compared the charge carrier mobility in crystalline vs liquid crystalline
ꢀ3
2
ꢀ1 ꢀ
perylenebisimide and observed that the LC PBI exhibited SCLC mobility of 7x10 cm V s
1
30
which was 2 orders higher compared to their crystalline counterpart. Similar report has
come from the group of Zhang et al., where a liquid crystalline PBI showed high mobility
3
1
value compared to the crystalline state.
Based on the above evidence from literature, it is clear that the charge transport
properties of the PBIs could be fineꢀtuned by controlling the molecular structure factors that
decide the crystalline or liquid crystalline ordering. In this scenario we have undertaken a
systematic study on the spacer effect on mesogenic and charge transport properties in a series
of 3, 4, 5ꢀtri alkoxy ester derivative of symmetrically substituted petadecyl phenol based
4

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perylenebisimides. It has been well established that the methylene units in the even spacer
can pack efficiently compared to the odd spacers, which is directly reflected in their
3
2
mesophase behavior as well as phase transitions. Previously Percec et al. systematically
studied the effect of spacer on the perylenebisimide core and on the terminal spacer in
3
3, 34
dendritic PBI molecules.
Very recently, Grozema et al. reported systematic study on a
series of PBI derivatives terminated with linear alkylester chains where the length of the
alkyl chain as well as the position of the ester group on the aliphatic unit was varied and the
2
ꢀ1 ꢀ1
charge transport characteristics was probed. Mobilities as high as 0.22 cm V s was
observed using the PRꢀTMC technique and results showed that the mobility values could be
3
5
tuned by varying the relative position of the ester group in the side chain.
Previously we had reported the liquid crystalline ordering of 3,4,5ꢀtri dodecyloxy
substituted pentadecyl phenol based PBI ester which was shown to exhibit room temperature
3
6
columnar mesophase. Taking advantage of the unique self assembling characteristics
afforded by the pentadecyl phenol imide substitution on PBI, we have designed a series of
PBI derivatives (PBI-En; n: 4ꢀ12) where the terminal spacer in the alkoxy chain was varied
from 4 to 12 for understanding the effect of spacer length on the intrinsic properties. The
molecules were structurally characterized and their mesophase characteristics were
determined using differential scanning calorimetry (DSC), polarizing light microscope
(PLM) and variable temperature wide angle Xꢀray diffraction (VTꢀWXRD) studies. The
charge transport properties were explored using space charge limited current (SCLC)
measurements. The important aspects that were explored in this work are highlighted below.
1
)
A detailed structureꢀproperty analysis relating the role of the varying terminal
trialkoxy spacer length on the liquid crystalline ordering in the PBIꢀEn series.
2
)
Correlation of the bulk mobility estimate, determined using the SCLC method among
the crystalline and different LC phases.
EXPERIMENTAL SECTION
Materials: Peryleneꢀ3,4,9,10ꢀtetracarboxylic dianhydride (PTCDA), 3ꢀpentadecyl phenol,
zinc acetate, imidazole, methylꢀ3,4,5ꢀtrihydroxybenzoate, triethylamine, 1ꢀbromobutane, 1ꢀ
bromopentane, 1ꢀbromohexane, 1ꢀbromoheptane, 1ꢀbromooctane,1ꢀbromononane, 1ꢀbromo
decane, 1ꢀbromoundecane and 1ꢀbromododecane were purchased from Sigma–Aldrich and
5

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used without further purification. Sodium nitrite and potassium carbonate were purchased
from Merck Chemicals Ltd and used as such. Thionyl chloride, triꢀethyl amine, dimethyl
formamide (DMF), dichloromethane (DCM), tetrahydrofuran (THF) and ethanol were
purchased from Merck Chemicals Ltd and were purified using standard procedures.
Instrumentation:
1
13
HꢀNMR and CꢀNMR spectra of PBI-En molecules were recorded on a BrukerꢀAVANCE
0
2
00 MHz spectrometer. Chemical shifts are reported in ppm at 25 C using CDCl as solvent
3
containing small amount of tetramethylsilane (TMS) as internal standard. The purity of
samples was confirmed using elemental analysis, which was done using a Thermo Finnigan
Flash EA 1112 series CHNS analyzer. Gel Permeation Chromatography (GPC) was carried
out on Polymer Laboratories PLꢀGPCꢀ220 using CHCl as eluent. The flow rate of CHCl
3
3
was maintained as 1 µL/min throughout the experiments and the sample solutions at
concentrations 2ꢀ3 mg/ml were filtered through syringe filter and injected for recording the
0
chromatograms at 30 C. The mass spectral analysis was carried out in reflecting mode with
an accelerating voltage of 25 kV using a VoyagerꢀDeꢀSTRMALDIꢀTOF (Applied
Biosystems, Framingham, MA, USA) instrument equipped with 337 nm pulsed nitrogen laser
used for desorption and ionization. The sample was made in CHCl and premixed with
3
dithranol matrix before spotting on 96ꢀwell stainless steel MALDI plate by dried droplet
method. Infrared spectra were recorded using Bruker FTꢀIR (ATR mode) spectrophotometer
ꢀ1
in the range of 4000ꢀ600 cm . The thermal stability of the PBIꢀEn were analyzed using
PerkinElmer STAꢀ6000 thermogravimetric analyser (TGA) under nitrogen atmosphere from
0
0
4
0ꢀ800 C at 10 C/min. Differential Scanning Calorimetry (DSC) was performed using a TA
Q10 model. About 2–3 mg of the samples were taken in aluminium pan, sealed and scanned
0
at 10 C/min under nitrogen atmosphere. The instrument was calibrated with indium
standards before measurements. The phase behaviour of the molecules was analyzed using
LIECA DM2500P polarized optical microscope equipped with Linkam TMS 94 heating and
cooling stage connected to a Linkam TMS 600 temperature programmer. The Transition
from isotropic to liquid crystalline phase was monitored by the evolution of characteristic
textures. Xꢀray diffraction of all the annealed samples were recorded by a DY 1042ꢀ
Empyrean XRD with Programmable Divergence Slit (PDS) and PIXcel 3D detector using Cu
6

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Kα (1.54 Å) emission. The spectra were recorded in the range of (2θ) 2–50° and analyzed
using X’pert software. Variable temperature in situ XRD experiments were carried out in an
AntonꢀPaar XRK900 reactor.
Device Fabrication: SCLC electronꢀonly devices were fabricated using the following
structure: glass/Al/active layer/Al. The glass substrates were cleaned using the following
3
7
sequence in an ultrasonic bath: water, acetone, and 2ꢀpropanol. The bottom aluminum
electrode was deposited by thermal evaporation technique with a thickness of 100 nm under
vacuum in a glove box. 20 mg/ml of PBI-En samples were dissolved in dry chloroform and
7
0 micro liters was drop cast on top of the aluminum electrode. The drop cast films were
annealed above their clearing temperature for 20 minutes and cooled to room temperature.
Aluminum counter electrodes were evaporated through a shadow mask on top of the active
layer to a thickness of ∼100 nm in a thermal evaporation chamber. The active device area
2
was found to be in the range of 0.08 ꢀ 0.12 cm . The mobility measurements were carried out
by measuring the current–voltage characteristics with a Keithley 2400 source meter.
Results and Discussion
Synthesis and characterization
A series of trialkoxy ester terminated derivatives of perylenebisimide (PBI-En; n = 4ꢀ12)
was designed and developed by esterification reaction between pentadecyl phenol substituted
perylenebisimide and 3,4,5ꢀtrialkoxy benzoyl chloride. The alkoxy spacer length was varied
from 4 to 12 and the synthetic route is shown in schemeꢀ1. The synthesis of the pentadecyl
phenol substituted perylenebisimide as well as the 3,4,5ꢀtridodecyloxy substituted PBI
3
6
molecule, (PBI-E12) was described in detail in an earlier report. The peripheral 3,4,5ꢀ
trialkoxyphenyl substitution is known to be helpful in inducing mesogenicity in
perylenebisimides, however the long C15 alkyl chain in the ortho position to the imide
linkage proved crucial in widening the liquid crystalline window as well as to retain the
liquid crystalline order until room temperature. The resulting PBI-En molecules were
structurally characterized using proton NMR spectroscopy, mass analysis using MALDIꢀ
TOF and purity confirmed by single peak in Gel Permeation Chromatography (GPC) as well
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as elemental analysis. The proton NMR spectra as well as the other structural
characterization data are given in the supporting information (figure S1 to S3).
Mesophase Characteristics of PBI-En
The thermal stability of the PBI-En series of molecules was determined by TGA under N2
atmosphere. Figure S4 shows the TGA curves of the series recorded at a heating rate of 10
0
0
0
C/min from 40ꢀ800 C. All molecules were observed to be thermally stable up to 370 C.
Tableꢀ1 shows the 10 wt % decomposition temperature of the molecules. The thermotropic
liquid crystalline characteristics of the PBI-En series were studied using differential scanning
calorimetry (DSC) analysis coupled with polarized light microscope (PLM) as well as
temperature dependant Xꢀray diffraction (VTXRD). Most of the molecules exhibited multiple
transitions both in the heating and cooling cycles. Figure 1 shows the DSC second heating
and cooling scans of the entire series and the transition temperatures and corresponding
enthalpies in the cooling cycle are summarized and given in tableꢀ1. PBI-E4 exhibited two
0
0
transitions both in the heating (22 C, 1.99 kJ/mol; 264.4 C, 71.86 kJ/mol) as well as cooling
0
0
cycle (21.05 C, 2.06 kJ/mol; 250.5 C, 71.43 kJ/mol). Multiple transitions (more than two)
were observed in the case of almost all other molecules in the series. Figure 2a shows the plot
of the transition enthalpies for the clearing (while heating) and the crystallization (while
cooling) transitions as a function of the number of carbon atoms in the terminal alkoxy
spacer length. An interesting oddꢀeven oscillation could be observed in the change in
enthalpy (ꢀH), with the even members exhibiting higher values as a function of the number
of carbon atom in the terminal alkoxy spacer segment. The clearing and the crystallization
temperature were also plotted as a function of the number of carbon atom in the terminal
alkoxy spacer segment (figure 2b). A weak oddꢀeven oscillation with a steady decrease in the
transition temperature was observed as the spacer length increased from 4 to 9; thereafter a
strong oddꢀeven oscillation was observed for members 9 to 12. It is a well studied and
understood concept that the even spacer can pack more efficiently compared to the odd one.
The highly packed structure requires more energy for melting (endothermic) and similarly
releases more energy (exothermic) while crystallizing as compared to the weakly packed
molecules. This is reflected in the clearing transitions as well as entropies and enthalpy
values. The enthalpy of clearing transitions was almost equal to the first transition from
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isotropic to crystalline / liquid crystalline phase, which indicated a thermodynamically stable
3
4
phase rather than a kinetically driven one. Although oddꢀeven oscillation in transition
3
8
39, 40
temperature has been reported for a variety of mesogens like azobenzene, biphenyl,
4
1
and oligo phenylene vinylenes, there has so far been no reports on similar observation in
the perylenebisimide chromophores.
The phase identification of the various transitions observed in the DSC thermogram
was carried out using Polarized Light Microscopic (PLM) analysis enabled with a
programmable hot stage. The experimental procedure involved placing a trace amount of the
0
0
sample on a cover glass plate, heating to melt at 10 C/min, holding at isotropic state (10 C
0
higher than the clearing temperature) for 10 minutes followed by slow cooling of 5 C/ min.
The images captured at various temperature intervals are shown in figure S6. Figure 3 shows
the texture retained at room temperature for all the samples. Although two transitions were
0
observed both in heating and cooling cycle for PBI-E4, the transition at 250.5 C while
0
cooling corresponded to crystallization and that at 264 C while heating was the clearing
transition. A fast crystallization with growth of sharp needle like textures which remained
until room temperature, confirmed the crystalline nature for PBI-E4. Similarly, PBI-E5,
PBI-E6 and PBI-E7 also exhibited fast crystallization upon cooling from the isotropic melt
which confirmed the absence of liquid crystallinity in these samples. PBI-E8 formed lancetꢀ
0
like textures upon cooling from isotropic state to 212 C. These were characteristic columnar
textures and it remained unchanged until room temperature. PBI-E9 exhibited clearing
0
transition at 202 C; upon cooling from the isotropic melt it exhibited a characteristic snowꢀ
0
0
flake like texture identified as columnar mesophase at 190 C. Upon slow cooling to 180 C
the domain size seemed to increase. In the DSC cooling cycle also a clearly distinguishable
transition was observed around this temperature, indicating a phase transition. The columnar
0
mesophase texture remained stable until room temperature (25 C). The transitions observed
during cooling could be attributed to a columnar disordered to a columnar ordered one. In all
cases, the transition observed below room temperature in the DSC (which could not be traced
4
2
under PLM), could be the reorganization of the alkyl segments. PBI-E10 exhibited
0
beautiful leafꢀ like textures at 213 C upon cooling from the isotropic melt which remained
0
stable until room temperature. Similarly PBI-E11 also had a clearing point at 200 C during
0
heating which on cooling formed the leafꢀlike textures at 192 C. The mesophase
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3
6
characterization of PBI-E12 was reported previously. In short, it had a clearing point at 215
0
0
C and upon cooling snow flake like textures appeared at 200 C corresponding to columnar
hexagonal disordered phase. Further cooling resulted in a large dendritic growth
corresponding to formation of columnar ordered phase. Thus, based on the observation under
the PLM it could be concluded that the PBIꢀEn molecules with shorter terminal spacer
exhibited a tendency to crystallize while those with longer spacer length exhibited typical
columnar mesophases upon cooling from the isotropic melt.
Wide angle X-ray diffraction (WXRD) analysis of the liquid crystalline phase
The phase identification of the PBI-En series was undertaken with the help of wide
angle Xꢀray diffraction (WXRD) studies from 2θ = 2ꢀ35°. The details of the analysis of the
3
6
WXRD pattern of PBI-E12 were provided in our earlier report. In short, a sharp reflection
at 2θ = 2.09° having a d spacing of 42.19 Å was indexed as the d100 reflection. The other
peaks which followed the characteristic ratios of 1:1/√4:1/√7:1/√12 were indexed as the
(100), (200), (210), (220) reflections of the hexagonal lattice. Variable temperature wide
angle Xꢀray diffraction (VTꢀWXRD) studies were undertaken for the samples PBIꢀE8, PBI-
E9, PBI-E10 and PBI-E11 and are given in Figure 4aꢀd. Figure 4a shows that no reflections
0
0
were observed for PBI-E8 in the isotropic state at 235 C. Around 212 C, sharp reflections
appeared at 2θ values of 2.45°, 5.28° and 5.63° followed by small intense reflections in the
0
2
θ range from 5°ꢀ22°. The DSC thermogram had also indicated a transition at 212.6 C,
0
which was supported by the observation of lancet like patterns under PLM around 212 C.
Further cooling of the sample resulted in more sharp reflections and shift in peak positions,
especially for reflections at 5.28°, and 5.63° and at 2θ = 21.24°. The sharp reflection at
3
6
2
θ = 2.45° having a d spacing of 35.12 Å was indexed as the d100 reflection. In PBI-E12 a
o
reflection at 2θ =5.35 was indexed as the (200) plane. However in PBI-E8 the reflection
o
around 2θ =5 was split into two equal intense reflections at 5.28°, and 5.63° which is
2
0
characteristic of a columnar rectangular phase. A similar split was observed for the
o
reflection around 2θ =5.5 for PBI-E9, PBI-E10 and PBI-E11 also, which indicated that all
of them belonged to the same columnar rectangular phase (the supporting figure S7 compares
o
the expanded 2θ region from 5 to 10 for PBI-E12 and PBI-E11 highlighting the split of the
o
peak around 2θ =5.5 ). Tableꢀ2 shows the temperature and the corresponding d spacing of
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peaks which followed the characteristic pattern of a columnar rectangular organization (a
ratio of ≅ 1: 4/2: 5/2 for the d spacing) which was in good agreement with the reported
2
0
ratio. The reflection at 2θ = 21.24° (4.18 Å) and 2θ = 21.46° (4.14 Å) corresponded to the
πꢀπ stacking of the perylene core. Upon cooling a gradual shift was observed in the π
0
stacking peaks to higher theta region. At room temperature (25 C) the πꢀπ stack distance
0
reduced to 3.7 Å (from 4.1 at 230 C), indicating strong overlapping of the aromatic core. A
split due to diffraction from two separate planes was observed in the π stacking reflection
around 3.7 Å, which was a characteristic signature of the columnar plastic phase. The
1
8
splitting indicated better packing in these systems. Plastic phase is characterized by higher
crystallinity along with fluidity. Similar VTWXRD studies conducted for the other higher
member samples also revealed some common trends. No shift was observed in the d100
reflection as a function of temperature, however regular shifts to higher 2θ values were
observed for the double reflections around 5.28° and 5.63° as well as for the reflection
around 2θ = 21.24° corresponding to the πꢀπ stacking of the perylene aromatic core (inset
expanded plot 2θ = 6.5° ꢀ 30° in figure 4aꢀd). In contrast to the appearance of the plastic
phase at higher temperature observed in the case of PBI-E8, the higher spacer samples like
PBI-E9, PBI-E10, and PBI-E11 exhibited a split of the πꢀπ stacking reflection around 2θ =
2
1.46° (4.14 Å) only upon cooling to room temperature. Another striking difference in
behavior exhibited by the odd membered samples like PBI-E9 and PBI-E11 in comparison
with their even membered analogues was the formation of an initial disordered phase at high
temperature characterized by absence of the columnar πꢀπ stacking (see the supporting figure
S8) this was clearly observed in the DSC thermogram also during the cooling cycle for PBI-
E9. The cooling cycle in the DSC thermogram of PBI-E9 (figure 1b) had two exothermic
0
0
transitions at 191.38 C (9.37 kJ/mol) and 181.4 C (3.76 kJ/mol) – the first one
corresponding to the isotropicꢀ disordered columnar rectangular phase and the second
transition at 181 corresponding to the transition of the disordered phase to a more ordered
columnar one. The reflection for the πꢀπ stacking interaction became prominent in the
0
WXRD pattern only below 180 C. PBI-E11 also exhibited a similar trend, although the
disordered to ordered transition was observed only as a shoulder in the DSC thermogram.
o
Even after carrying out the DSC scan at a cooling rate of 1 C/minute, the shoulder could not
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be resolved into a separate peak (supporting figure S5). However, the variable temperature
WXRD pattern of PBI-E11 clearly showed the absence of the peak corresponding to πꢀπ
o
o
stacking at higher temperature (~ 190 C); and it appeared while cooling to 180 C. Thus, it
could be confirmed that PBI-E11 also had the disordered to ordered transition at a high
temperature following a similar trend as that of PBI-E9. Figure 5 compares the room
temperature WXRD data of all the samples in the 2θ range 5° ꢀ 30° collected after heating to
isotropic followed by cooling. It could be seen from the figure that irrespective of the
terminal spacer length, the πꢀπ stacking distance was same for all the samples at 3.7 Å. On
the other hand, a regular increase was observed in the d spacing for the d100 reflection as the
number of carbon atoms in the terminal spacer increased (figure S9). The lower homologues
PBI-E4, PBI-E5, PBI-E6 and PBI-E7 did not exhibit any liquid crystalline textures as
explained earlier; however they also showed a tendency to form columnar stacks as
observable from figure S10. Thus the entire series from spacer 4 to spacer 12 could be
classified into 3 groups – a lower member group PBI-E4 to PBI-E7 which did not exhibit
mesophase, a middle spacer series from PBI-E8 to PBI-E11 ordered columnar plastic phases
and the highest member PBI-E12 which formed hexagonal ordered columnar phase.
SCLC mobility measurements
SCLC measurement is preferred over the FET measurement due to the stacking of the
molecule parallel to the substrate. Therefore, the charge transport is expected to be
perpendicular to the substrate. The electron transport characteristics of all members of the
PBI-En series were investigated by spaceꢀchargeꢀlimited current (SCLC) method. This
method allows for determination of the macroscopic bulk mobility of a material in thin film.
The semiconducting PBIs were sandwiched between an electron injecting Aluminum
electrode as top and bottom contact with a configuration of glass/Al/PBI-En/Al. Aluminum
electrodes were chosen for the electron only devices due to the work function (4.3 eV) match
4
3
with the LUMO of perylenebisimdes. The samples were heated to isotropic and then cooled
to room temperature (25 °C), following which the top Al electrode was deposited. The charge
carrier mobilities were evaluated by the current – voltage (IꢀV) measurement under inert gas
atmosphere. The JꢀV curves present two regimes i) at low voltages; ohmic region where the
current measured is potentially limited by the charge traps and a linear relation between J and
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V exists; ii) at high voltages; SCLC region where the charge transport is through bulk with
2
less number of traps and J α V . The slope of the region was determined from the logꢀlog JꢀV
plot. The charge carrier mobility was calculated from the slope 2 region, which was
determined from the logarithmic plot. The charge carrier mobility in this SCLC region can be
4
4
calculated from the JꢀV curve by using MottꢀGurney equation:
9
= ꢁεꢂ
8
ꢄꢅ
ꢆꢇ
ꢀ
ꢁεꢃꢁμꢁ
Where J is the current density, ε is the dielectric constant of the organic semiconductor
r
4
5
(
assumed to be 3 in our calculations), ε is the permittivity of free space, ꢁ is the charge
0
carrier mobility, L is the thickness of the active layer, and V is the applied voltage across the
device. The SCLC mobility was measured for 5 devices each, the active layer thickness
ranged from 5ꢀ14 μꢈ (determined using optical profilometer), and covered an area of ~ 0.08
2
–
0.12 cm . J−V characteristics of annealed films of PBI-En together with the respective fits
according to the Mott−Gurney equation are shown in Figure 6. The short spaced (n < 8)
ꢀ4
2
ꢀ1 ꢀ1
crystalline molecules exhibited a mobility in the order of 10 cm V s , where as the middle
ꢀ5
2
ꢀ1
spaced (n = 8 to 11) columnar rectangular phase showed mobility in the range of 10 cm V
ꢀ1
s . The electron mobility value for the columnar hexagonal phase of PBI-E12 (n= 12) was
ꢀ3
2
ꢀ1 ꢀ1
obtained as 2.02 x10 cm V s , which was two orders of magnitude higher compared to
the other liquid crystalline analogues. This was the highest value observed among the series.
The WXRD and PLM studies had indicated that PBI-E12 assembled in the plastic columnar
hexagonal phase upon cooling from the isotropic phase. The higher order with flexibility
which is a characteristic of the plastic LC phase supported better electron mobility in PBIꢀ
1
9
E12 compared to the samples assembled in the columnar rectangular phase.
Although it was anticipated that the packing difference between the odd and even
spaced PBI derivatives would be reflected in their mobility values also, we observed that the
bulk mobility estimate was largely influenced by the similar morphologies and crystallinity.
The impact of oddꢀeven effect on charge carrier mobility would be pronounced if all the
molecules formed similar morphologies and crystallinity. In LC systems, the morphologies
and crystallinity were different as a function of alkyl chain length, hence comparison was not
straightforward. This was corroborated by the fact that the charge carrier mobility was found
to vary by few orders when the crystallinity and morphology varied among the different
13

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members. Thus, the highest SCLC mobility was exhibited by PBI-E12, in the columnar
ꢀ3
2
ꢀ1 ꢀ1
hexagonal phase (10 cm V s ), followed by the crystalline molecules PBI-E4 to PBI-E7
ꢀ4
2
ꢀ1 ꢀ1
(
10 cm V s ), and the lowest mobility was observed for the PBI-En (n = 8 to 11) in the
ꢀ5
2
ꢀ1 ꢀ1
columnar rectangular phase (10 cm V s ) indicating a strong dependence of packing on
bulk mobility.
Conclusion
A systematic study of the effect of molecular packing on the nꢀtype charge transport
characteristics in a novel series of highly soluble pentadecyl phenol substituted
perylenebisimide ester derivatives was presented. The homologous PBI-En series constituted
a 3, 4, 5ꢀtri alkoxy ester derivative of a symmetrical petadecyl phenol substituted
perylenebisimide, where the terminal alkyl spacer length was varied from n = 4 to 12.
Detailed characterization using DSC, PLM and variable temperature WXRD established that
the members of the homologous series with n > 8 exhibited columnar liquid crystalline
phases at higher temperature and froze into a columnar plastic phase at room temperature as
evidenced by the split appearance of the πꢀπ stacking reflection around 2θ = 22°. The lower
members of the series with n < 8 were crystalline in nature. The PBI derivative with the
longest spacer length n = 12 (PBI-E12) formed columnar hexagonal phase characterized by
beautiful dendritic textures under the PLM. The enthalpy of the clearing and crystallization
transitions exhibited a strong oddꢀeven oscillation as a function of the number of carbon
atoms in the terminal alkoxy unit. The even members exhibited higher enthalpy values due to
the more efficient packing of the even membered alkyl spacers in the solid state compared to
the odd ones. The bulk mobility of the molecules of the series annealed from the isotropic
melt was estimated by the SCLC method using the device configuration of glass/Al/PBI-
ꢀ4
En/Al. The crystalline members with n < 8 gave a bulk mobility estimate in the order of 10
2
ꢀ1 ꢀ1
cm V s while the intermediate spacers with n = 8 to 11 showed mobility in the range of
ꢀ5
2
ꢀ1 ꢀ1
ꢀ3
2
ꢀ1 ꢀ1
1
0 cm V s only. A high mobility estimate of 2.02 x 10 cm V s was observed for
PBI-E12, which indicated the direct relation between charge transport and appropriate
molecular ordering with better core packing. The liquidꢀlike flexibility combined with the
better core packing of the columnar hexagonal plastic phase facilitated slightly better charge
transport compared to the three dimensional crystalline packing. The present study thus
14

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highlighted (a) the important role played by the flexible alkyl chains in fine tuning the
mesophases in a homologous series of PBI derivatives functionalized with pentadecyl phenol
and trialkoxy gallate terminal units. The rigidꢀflexible balance required for the observation of
mesogenicity was attained beyond a terminal alkoxy spacer length of 8 methylene units. The
C15 alkyl chain on the pentadecyl phenol unit incorporated into the PBI molecular design
also had a crucial role in shaping the liquid crystalline characteristics as well as packing in
this series of PBI-En molecules. The second highlight of the study was the (b) correlation of
the bulk mobility estimate among the different LC and crystalline phases. The SCLC
mobility analysis clearly brought out the importance of molecular structural factors that are
to be taken into consideration while designing materials for better charge transport.
Acknowledgment:
We thank the DST Nano Mission project SR/NM/NSꢀ1028/2012 and CSIR network project
NWP054 for financial support. The authors sincerely thank Professor V. A. Raghunathan and
Mrs. Vasudha. K. N., from Raman Research Institute (RRI) Bangalore for variable
temperature XRD measurements. Prajitha thanks CSIRꢀNew Delhi, India, for Senior
Research Fellowship.
Supporting Information:
1
Synthetic details for PBI–En molecules, HꢀNMR spectra, MALDIꢀTOF, Gel permeation
chromatogram (GPC), Thermo gravimetric analysis (TGA) of all PBI–En molecules; PLM
images of liquid crystalline PBI–En molecules; Room temperature XRD of crystalline PBI–
En molecules; d spacing and variation of d spacing with spacer length for all the PBI–En
molecules.
15

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Table 1. Transition temperature and corresponding enthalpies of PBI-En molecules during
the second heating and cooling cycles.
a
b
T
T
cl
a
b
b
c
b
c
0
0
0
ꢀ
Hcl
T ( C)
ꢀHc
ꢀHc
T
(
C)
( C)
c
D
Name
0
(
Lc/Cꢀ (KJ/mol) IꢀLc/C (KJ/mol) (LcꢀC) (KJ/mol)
( C)
I)
/(CꢀC)
PBIꢀE4
PBIꢀE5
264
72.83
37.46
251
231
53.17
35.10
10
3.81
6.28
379
375
244
6
PBIꢀE6
PBIꢀE7
PBIꢀE8
239
227
221
48.23
39.8
230
215
212
48.68
38.64
46.16
47
ꢀ6
1.23
13.65
17.76
372
376
376
44.82
ꢀ12.7
PBIꢀE9
PBIꢀE10
PBIꢀE11
PBIꢀE12
202
212
200
215
33.22
43.58
29.17
16.72
191
205
192
202
31.08
42
10
ꢀ2.64
ꢀ8
34.91
23
370
373
372
370
28.17
15.31
16.65
52.55
13
a
b
clearing transition and corresponding enthalpy values during heating cycle, phase transitions and
c
corresponding enthalpy values during cooling cycles, 10 % weight loss under N atmosphere during
2
TGA.
19

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Table 2. dꢀspacing values and phase behavior of the Liquid crystalline PBI-En molecules at
various temperatures.
Sample
Temperature and dꢀspacing (Å)
0
0
0
0
0
0
2
30 ( C)
211 ( C)
196 ( C)
175 ( C)
100 ( C)
25 ( C)
3
1
5.92
6.17
35.94
16.45
15.49
4.12
35.97
16.21
15.27
4.10
35.99
15. 98
15.64
4.05
36.00
15.52
14.77
3.93
36.12
15.43
14.57
3.81
PBI – E8
15.72
4
4
.17
.13
4.07
4.05
4.01
3.90
3.72
Colrp
Colrp
Colrp
Colrp
Colrp
Colrp
0
0
0
0
0
1
1
90 ( C)
3
1
175 ( C)
100 ( C)
75 ( C)
25 ( C)
8.49
6.8
15.88
Colrd
38.52
16.45
15.73
4.15
38.54
16.20
15.57
4.09
38.56
15.64
14.98
4.05
38.58
14.89
14.53
3.82
3.75
Colrp
PBI – E9
PBIꢀ E10
Colro
4.05
3.9
Col
130 ( C)
Col
25 ( C)
rp
rp
0
0
0
0
90 ( C)
160 ( C)
3
1
9.98
7.54
40.01
17.44
16.75
4.13
40.03
17.12
16.48
4.05
40.07
15.27
15.14
3.81
16.80
.3
4
Colro
4.11
4.03
3.72
Colrp
Colrp
Colrp
0
0
0
0
0
1
91 ( C)
183 ( C)
127 ( C)
149 ( C)
25 ( C)
4
1
0.97
6.71
41.03
16.54
15.96
4.12
41.06
16.04
15.27
4.03
41.1
16
15.47
4.01
3.9
41.13
15.41
15.12
3.8
PBIꢀ E11
16.20
Colrd
Colro
4.01
3.7
Colrp
Colrp
Colrp
Note: Col ꢀ columnar rectangular plastic phase, Col ꢀ columnar rectangular disordered
rp
rd
phase and Col ꢀ columnar rectangular ordered phase.
ro
20

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Table 3. Maximum and average SCLC mobilities of the annealed PBI-En series of
molecules.
Sample
Maximum
mobility
ꢁe max (cm /Vs) ꢁ Avg (cm /Vs)
Average
mobility
Mean
deviation
2
2
e
ꢀ
4
ꢀ4
ꢀ4
ꢀ4
PBI-E4
PBI-E5
PBI-E6
PBI-E7
PBI-E8
PBI-E9
PBI-E10
PBI-E11
PBI-E12
3.35 × 10
2 × 10
± 0.85 × 10
± 2.65 × 10
ꢀ4
ꢀ4
8.9× 10
8.5× 10
1.2× 10
ꢀ
4
ꢀ4
ꢀ4
1.7 × 10
± 0.40× 10
ꢀ
4
ꢀ4
ꢀ5
ꢀ4
3.01× 10
1.81× 10
1.85× 10
± 0.67× 10
ꢀ5
ꢀ5
3.65× 10
± 1.82× 10
± 2.05× 10
± 1.81× 10
± 1.24× 10
± 0.67× 10
ꢀ5
ꢀ5
ꢀ5
ꢀ5
ꢀ5
5.65 x 10
3.35 x 10
4.45 x 10
2.05 x10
1.81 x10
ꢀ5
ꢀ5
ꢀ5
ꢀ3
ꢀ5
3.6 x10
ꢀ
3
ꢀ3
3.67 x 10
2.02 x 10
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Scheme 1: Synthesis and chemical structures of PBI esters (PBI –En).
OH
O
O
O
O
O
O
C H
31
1
5
NH2
PTCDA
Zn (OAc) , Imidazole
2
O
1
60 C, 24 h.
O
N
O
O
N
O
HO
OH
C H
C H
15 31
1
5
31
PER-PDP-Diol
O
H2n+1C O
n
Et N, THF
3
H2n+1C O
n
0 OC, 4 h.
Cl
H2n+1C O
n
O
N
O
O
N
O
OC H
2n+1
H2n+1C O
n
n
O
O
O
O
OC H
2n+1
H2n+1C O
n
n
C H
C H
15 31
1
5
31
OC H
2n+1
H2n+1C O
n
n
PBI - En, n = 4 - 11
22

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Figure captions
Figure 1. DSC thermograms of the PBI-En molecules in the second (a) heating and (b)
0
cooling cycle at 10 C /min.
Figure 2. Odd–Even oscillations in the (a) clearing and the crystallization enthalpies and (b)
clearing and the crystallization temperatures of PBI-En molecules as a function of number of
carbon atoms in the terminal tri alkoxy spacer unit.
Figure 3. Polarized Light microscopic images of PBI-En molecules (under crossed polarizer)
at room temperature. a) PBI-E4 b) PBI –E5 c) PBI –E6 d) PBI –E7 e) PBI –E8 f) PBI-E9
g) PBI –E10 h) PBI – E11 and i) PBI – E12.
Figure 4. Variable temperature wide angle Xꢀray diffraction patterns of liquid crystalline
PBIꢀEn molecules (a) PBI-E8; b) PBI-E9; c) PBI-E10 and d) PBI-E11.
Figure 5. Wide angle Xꢀray diffraction pattern of annealed samples of PBI-En molecules
o
from 2 θ = 5ꢀ30 .
Figure 6. Double logarithmic plot of the current density (J) versus applied voltage (V)
0
measured for the PBI-En molecules at room temperature (25 C) in devices with area of
2
0
.10cm and thickness 0f 10 ꢁm. The drop cast sample films were annealed above their
clearing temperature for 20 minutes and cooled to room temperature before the measurement.
23

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Figure 1: DSC thermograms of the PBI-En molecules in the second (a) heating and (b)
0
cooling cycle at 10 C /min.
b) Cooling cycle
a) Heating cycle
PBI-E12
PBI-E12
PBI-E11
PBI-E11
PBI-E10
PBI-E10
PBI-E9
PBI-E8
PBI-E9
PBI-E8
PBI-E7
PBI-E6
PBI-E5
PBI-E7
PBI-E6
PBI-E4
PBI-E5
PBI-E4
-50
0
50 100 150 200 250 300 350 -50
0
50 100 150 200 250 300 350
O
O
Temperature ( C)
Temperature ( C)
24

Page 25 of 30
Journal of Materials Chemistry C
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DOI: 10.1039/C4TC01806K
Figure 2: Odd–Even oscillations in the (a) clearing and the crystallization enthalpies and (b)
clearing and the crystallization temperatures of PBI-En molecules as a function of number of
carbon atoms in the terminal tri alkoxy spacer unit.
80
70
60
50
40
30
20
10
ꢀ
ꢀ
HC
a)
HI-C/LC
b)
TC
260
240
220
200
180
TI-C/LC
4
6
8
10
12
No of carbon atoms in spacer (n)
25

Journal of Materials Chemistry C
Page 26 of 30
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DOI: 10.1039/C4TC01806K
Figure 3: Polarized Light microscopic images of PBIꢀEn molecules (under crossed polarizer)
at room temperature. a) PBI-E4 b) PBI –E5 c) PBI –E6 d) PBI –E7 e) PBI –E8 f) PBI-E9
g) PBI –E10 h) PBI – E11 and i) PBI – E12.
26

Page 27 of 30
Journal of Materials Chemistry C
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DOI: 10.1039/C4TC01806K
Figure 4. Variable temperature wide angle Xꢀray diffraction patterns of liquid crystalline
PBIꢀEn molecules (a) PBI-E8; b) PBI-E9; c) PBI-E10 and d) PBI-E11.
3
2
2
1
1
0
.0
.5
.0
.5
.0
.5
3.0
2.5
2.0
a)
PBI-E8
b) PBI-E9
8
1
0
1
2
1
4
1
6
1
8
2
0
2
2
2
4
2
6
2
8
3 0
0
2
5 C
8
1 0 1 2 1 4 1 6 1 8 2 0 2 2 2 4 2 6 2 8 3 0
0
25 C 1.5
0
0
1
00 C
75 C
0
0
175 C
100 C 1.0
0
0
1
96 C
175 C
0.5
0
0
2
30 C
190 C
0
0
215 C
0.0
2
35 C
0
3
.0
.0
c)
PBI-E10
d)
PBI-E11
3.0
2
2
1
1
0
0
.5
.0
.5
.0
.5
.0
2.5
2.0
8
1 0 1 2 1 4 1 6 1 8 2 0 2 2 2 4 2 6 2 8 3 0
0
2
5 C
1.5
8
10 12 14 16 18 20 22 24 26 28 30
0
0
2
5 C
127 C
0
0
149 C 1.0
1
30 C
0
0
1
60 C
1
83 C
0
0
0.5
0.0
1
90 C
191 C
0
0
2
15 C
215 C
5
10 15 20 25 30 5
10 15 20 25 30
2θ (degrees)
27

Journal of Materials Chemistry C
Page 28 of 30
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DOI: 10.1039/C4TC01806K
Figure 5. Wide angle Xꢀray diffraction pattern of annealed samples of PBI-En molecules
o
from 2 θ = 5ꢀ30 .
2.4
2.0
PBI-E12
1
.6
PBI-E11
PBI-E10
PBI-E9
1.2
PBI-E8
PBI-E7
0.8
PBI-E6
PBI-E5
0.4
PBI-E4
0.0
5
10
15
20
25
30
2θ(deg)
28

Page 29 of 30
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C4TC01806K
Figureꢀ6: Double logarithmic plot of the current density (J) versus applied voltage (V)
0
measured for the PBI-En molecules at room temperature (25 C) in devices with area around
2
0
.08 – 0.12 cm and thickness 0f 10 ꢁm. The drop cast sample films were annealed above
their clearing temperature for 20 minutes and cooled to room temperature before the
measurement.
1E-3
1E-4
1E-5
1E-6
PBI-E12
SCLC region
Ohmic region
1
E-7
E-4
PBI-E8
PBI-E9
PBI-E10
PBI-E11
1
1E-5
1E-6
1E-7
1
E-8
E-3
1
PBI-E4
PBI-E5
PBI-E6
PBI-E7
1E-4
1E-5
1E-6
1E-7
1
10
100
log V
(
Voltage
)
29