Pentadecyl phenol- and cardanol-functionalized fluorescent, room-temperature liquid-crystalline perylene bisimides: Effect of pendant chain unsaturation on self-assembly

Article abstract of DOI:10.1002/chem.201101011

A new perylene bisimide (PBI) building block based on pentadecyl phenol (PDP) or cardanol was developed, which upon esterification with 3,4,5-tridodecyloxy gallate resulted in highly emissive, room-temperature liquid-crystalline (LC) molecules. The self a

Full text of DOI:10.1002/chem.201101011

DOI: 10.1002/chem.201101011
Pentadecyl Phenol- and Cardanol-Functionalized Fluorescent, Room-
Temperature Liquid-Crystalline Perylene Bisimides: Effect of Pendant Chain
Unsaturation on Self-Assembly
Ghanashyam A. Bhavsar and S. K. Asha*[a]
Abstract:
A
new perylene bisimide
methylcyclohexane (MCH). Surface
morphology in dropcast films were
characterized using microscopic tech-
niques such as SEM, TEM, and atomic
force microscopy (AFM). The liquid-
crystalline properties were studied
using differential scanning calorimetry
(DSC), polarized light microscopy
(PLM), and variable-temperature X-
ray (small-angle X-ray scattering
(SAXS) and wide-angle X-ray diffrac-
tion (WXRD)) studies. Variable-tem-
perature X-ray studies in the LC phase
indicated strong p–p stacking interac-
tion present in the PDP-based PBI de-
rivative, whereas the stacking was
absent in the LC phase of the carda-
nol-based PBI. The latter formed self-
organized structures of extremely short
length due to the presence of cis
double bonds in the C15 alkyl side
chain, whereas the saturated alkyl side
chain in PDP could pack efficiently,
thereby resulting in nanofibers that
were several micrometers in length.
(PBI) building block based on penta-
decyl phenol (PDP) or cardanol was
developed, which upon esterification
with 3,4,5-tridodecyloxy gallate result-
ed in highly emissive, room-tempera-
ture liquid-crystalline (LC) molecules.
The self assembly in solution was stud-
ied in detail by NMR spectroscopy,
UV/Vis absorption, and fluorescence
spectroscopy. In solution both PDP-
and cardanol-based PBI exhibited simi-
lar behavior. They were molecularly
dissolved in chloroform (CHCl3) but
formed rotationally displaced H-type
aggregates that emitted at 640 nm in
Keywords: aggregation
·
liquid
crystals · mesophases · pi interac-
tions · self-assembly
Introduction
3,4,5-trialkoxyphenyl substitution into the periphery has
been shown to induce mesogenicity—usually columnar hex-
agonal mesophases—in perylene bisimides. However, retain-
ing the liquid-crystalline order at room temperature is an
important property as far as long-term sustainability is con-
cerned. Since the first report on liquid-crystalline perylene
bisimides by Cormier et al. in 1997 based on linear and
branched poly(oxyethylene) derivatives,[14,15] several reports
have appeared that made use of several building motifs such
as long linear alkyl chains,[16] swallowtail,[17] and dodecyloxy
benzoyl substituents[18–23] to induce liquid crystallinity in per-
ylene bisimides. Bay substitution was another approach that
extended the range of absorption of perylene bisimides and
at the same time improved their solubility and also de-
creased the clearing temperature.[20] However, it resulted in
a twisting of the aromatic core, which hindered self-assem-
bly. Recently, reports are available in which proper and
well-designed bay substitution resulted in new two dimen-
sional organization in the solid state with improved charge-
carrier mobilities.[24–27] Cardanol and its saturated analogue
pentadecyl phenol (PDP) are bio-based amphiphilic mole-
cules that can function as molecular building blocks that
have all the features necessary for self-assembly.[28–32] They
have a phenolic group, which enables further functionaliza-
tion, and a long C15 alkyl chain, which is not easily avail-
able by synthetic routes.[33–38] This C15 alkyl side chain can
self-organize by interdigitation, whereas the aromatic unit
can do so by p–p stacking interactions. Cardanol has an un-
Liquid-crystalline materials that can act as charge-transport
channels are highly attractive as potential materials for ap-
plication in organic electronic devices such as field-effect
transistors (FET),[1] electroluminescent displays,[2] photovol-
taic cells.[3] Recently various examples of discotic molecules
such as triphenylenes,[4,5] hexabenzocoronenes,[6] and phtha-
locyanines[7–9] have been reported to form p stacks that ex-
hibited increased charge-carrier mobilities due to the higher
molecular order imparted by the p-stacking arrangement.
All of the above-mentioned molecules possessed electron-
rich aromatic cores that gave rise to columnar p stacks with
p-type semiconducting properties. In this context, p-stacked
liquid-crystalline n-type perylene bisimides (PBIs) are im-
portant because there are not many choices for n-type or-
ganic molecules.[10–13] The p stacking in perylene molecules
is expected to result in an increase in the exciton diffusion
length and stabilize the charge-separated states, thereby
slowing down the charge recombination. The introduction of
[a] G. A. Bhavsar, Dr. S. K. Asha
Polymer & Advanced Material Laboratory
Polymer Science & Engineering Division
NCL (CSIR), Pune-411008, Maharashtra (India)
Fax : (+91)20-25902615
E-mail: sk.asha@ncl.res.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101011.
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FULL PAPER
saturation in this C15 side chain that differentiates it from
PDP in terms of self-assembly.[30] There are several reports
that describe the self-assembled nanostructures formed in
systems based on cardanol or PDP; however, there are no
reports on the combination of the properties of both carda-
nol or PDP with that of the well-studied perylene bisimides.
This prompted us to design a novel building block based on
these two important ingredients along with the 3,4,5-tri(do-
decyloxy)phenyl substitution to impart mesogenicity with
the expectation that the interesting mix and match of prop-
erties that this combination generated would definitely
make this molecular design a sought-after one for future re-
search.
In this article, we introduce a novel building block that
combines perylene with PDP or cardanol and possesses a
terminal phenolic functionality. These nonluminescent build-
ing blocks were converted to highly fluorescent derivatives
by means of coupling with a 3,4,5-tri(dodecyloxy)phenyl
group through ester linkages. The resulting molecules exhib-
ited room-temperature liquid crystallinity (LC) and also re-
tained their emission in the LC phase. Alkoxy phenyl substi-
tution is usually known to result in an almost complete
quenching of fluorescence in solution, probably due to pho-
toinduced electron transfer from the electron-rich alkoxy-
phenyl groups to the perylene core.[20] In fact, very few ex-
amples are reported for perylene bisimides in which their
dye aggregates have been shown to have intense emis-
sion.[24,39,40] Thus, without additional noncovalent interaction
aids like hydrogen bonding or metal ion interaction, and
making use of only p–p stacking, we have been able to ach-
ieve self-organized dye aggre-
Results and Discussion
The novel building blocks that combine perylene tetracar-
boxylic dianhydride (PTCDA), pentadecyl phenol (PDP), or
cardanol were synthesized as described in Scheme 1. In
short, PTCDA was first coupled with 2.2 equivalents of 4-
amino-3-pentadecyl phenol or its unsaturated analogue
based on cardanol to obtain the novel phenol-terminated
building blocks 1 and 2. Cardanol, which was obtained from
cashew-nut-shell liquid (CNSL), is a phenol that has a mix-
ture of unsaturations (mono-, di-, and tri-) in the pendent
C15 alkyl chain. Due to the difficulty in separating the mix-
ture of mono-, di-, and tri-unsaturated cardanol, all three
derivatives were present in the amine derivative (as con-
firmed by LC-MS data given in Figure S1 of the Supporting
Information). The MALDI-TOF mass spectra of 2 (Fig-
ure S2 in the Supporting Information) also showed multiple
peaks that correspond to various possible combinations of
unsaturation in the side chain. The two diols differed in ap-
pearance with 1 being red in the solid state, whereas 2 was
maroon. The diols were coupled with 3,4,5-tri(dodecyloxy)-
benzoyl chloride to obtain the final molecules 3 and 4. The
molecules were subjected to purification procedure as de-
scribed in detail in the Experimental Section. The purity of
the final products was confirmed by elemental analysis and
unimodal distribution curves in the size-exclusion chroma-
tography (SEC; Figure S4 in the Supporting Information).
The cardanol-based perylene bisimide ester 4 differed in
mass by 4 units compared to the PDP-based perylene bis-
AHCTUNGTREGiNNNU mide ester 3, in their SEC chromatograph. The single peak
gates that were liquid crystal-
line and exhibited intense
emission in solution as well as
in the solid state. Room-tem-
perature liquid crystallinity is
an important property that
helps to retain the self-organi-
zation over long periods of
time after thermal processing.
Most of the reported PBIs are
either monotropic or display
an LC range at higher temper-
ature.[16] In addition, they also
have multiple transitions (crys-
talline or LC) within a narrow
range of temperature. As the
charge-carrier mobility is dif-
ferent in different LC phases,
a material with single discotic
LC phase over a wide temper-
ature range is desirable for
practical device application.
Scheme 1. Synthesis of perylene bisimides 3 and 4.
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12647

G. A. Bhavsar and S. K. Asha
in the SEC coupled with the mass spectra (MALDI-TOF)
reaffirmed the high purity of the molecules under study. The
PBI derivatives were structurally characterized by proton
NMR spectra (Figures S5 and S6 in the Supporting Informa-
tion).
showed that the aggregates remained stable even upon heat-
ing up to 708C. Similar studies (variable temperature and
concentration) were carried out for 4 (Figures S7 and S8 in
the Supporting Information). The temperature- and concen-
tration-dependent proton NMR spectroscopic studies clearly
showed the existence of p–p stacked aggregates in nonpolar
solvent like methylcyclohexane.
Temperature- and concentration-dependent 1H NMR spec-
troscopy studies: The proton NMR spectra of the perylene
bisimides 3 and 4 recorded in CDCl3 exhibited a simple pat-
tern of sharp signals with a doublet of doublet for the eight
perylene protons (labeled a and b in Figure 1), which indi-
Photophysical studies: Building blocks 1 and 2 were non-
fluorescent, whereas after ester functionalization the fluores-
cence was regained. The solubility of 1 and 2 in common or-
ganic solvents like CHCl3 was very poor; however, they
were soluble in THF and other polar solvents. The absorp-
tion in THF indicated typical features of perylene bisimide
with peaks at 521, 486, and 455 nm (Figure S9 in the Sup-
porting Information), but the quantum yield in THF was
very low (F=0.07).
The ester derivatives 3 and 4, on the other hand, were
completely soluble in common organic solvents like chloro-
form, dichloromethane, methylcyclohexane, and so on. In
chloroform, they exhibited the characteristic features of per-
ylene bisimide absorption with four peaks in the range of
400 to 530 nm, which corresponded to the 0–0, 0–1, 0–2, and
0–3 electronic transitions, respectively, whereas aggregation
was observed in MCH. In MCH the absorption was broad
and peak maximum was blueshifted. Compound 4 also
showed a similar behavior. Table 1 shows a comparison of
the values of lmax in these two solvents for both molecules 3
and 4. Variable-concentration (1ꢁ10ꢀ6 to 5ꢁ10ꢀ5 m) and var-
iable-temperature studies were carried out for both 3 and 4
in MCH.
1
Figure 1. Temperature-dependent H NMR (400 MHz) spectra of 3 (1.2ꢁ
10ꢀ2 m) in [D14]MCH at 0 to 858C and in CDCl3 at 258C. (The signals la-
beled a and b are the four inner and four outer perylene ring protons,
and the asterix indicates solvent).
Table 1. Photophysical properties in CHCl3 and MCH (in brackets) at
approximately 0.1 OD.
cated that they were present in the molecularly dissolved
form in chloroform. The H NMR spectrum in nonpolar sol-
1
[a]
[b]
Sample
lmax (abs)
[nm]
lmax (emission)
[nm]
fFL
tavg
[ns]
c2[b]
vent methylcyclohexane (MCH, deuterated) was different
from that in CDCl3. At 208C, the spectrum was broad and
peaks overlapped with each other due to the presence of
different aggregated species. Figure 1 compares the expand-
ed aromatic proton region (d=10–6 ppm) of PDP derivative
3 recorded in CDCl3 at 258C and in [D14]MCH at different
temperatures from 0 to 858C. At temperatures below 608C,
the spectra remained complex and broad, thus indicating
that the aggregates were stable. Upon increasing the temper-
ature beyond 608C, the complex pattern simplified and
sharpened. The dotted line traces the four inner-ring and
four outer-ring protons of the perylene bisimide aromatic
core as it shifted from downfield to upfield due to shielding
by the p systems of close neighbors in the aggregates.[21] Di-
lution is also known to result in breakup of aggregates in so-
lution. Figure S8A in the Supporting Information shows a
3
527
(514)
537
(521)
0.91
(0.50)
3.1[c]
1.12[c]
(1.01)[d]
(1.22)[e]
1.04[c]
(3.2)[d]
(23.4)[e]
3.2[c]
4
527
(514)
537
(521)
0.83
(0.51)
(3.4)[d]
(20.2)[e]
(0.99)[d]
(1.09)[e]
[a] Excitation wavelength is 488 nm in CHCl3 and MCH. Quantum yields
were calculated using 0.1 OD rhodamine 6G solution at 488 nm in etha-
nol. [b] LED 458 nm; 0.1 OD at 458 nm. [c] Solvent: CHCl3 and observed
at 537 nm. [d] Solvent: MCH and observed at 520 nm. [e] Solvent: MCH
and observed at 640 nm.
Figure 2a shows the absorption spectra for 3 as a function
of concentration, which shows that the spectra became
broader and less structured with increase in concentration,
thus reflecting the electronic coupling between the chromo-
phores. Furthermore, a new broad bathochromically shifted
band appeared at 538 nm. Figure 2b shows the transition
from the aggregated state to the molecularly dissolved state
as a function of temperature for a solution of 1ꢁ10ꢀ6 m con-
centration. At the highest temperature of 708C, the absorp-
1
comparison of the H NMR spectra of 3 recorded at 258C at
two different concentrations, ꢀ1.2ꢁ10ꢀ2 and 1.2ꢁ10ꢀ3 m.
However, the sample remained fully aggregated at the low
concentration as confirmed by the broad peak positions.
UV/Vis absorption studies (discussed in detail later on) of
samples of much lower concentration (5ꢁ10ꢀ5 m) also
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Perylene Bisimides
FULL PAPER
degree of aggregation as a function of temperature could be
fit very well to an isodesmic sigmoidal curve (Figure S12 in
the Supporting Information). Table 2 shows a comparison of
a0.5(T) (the temperature at which a=0.5) obtained for these
Table 2. Degree of aggregation and stack length of 3 in MCH at various
concentrations. The data in brackets are those for 4.
[b]
Concentration [mm]
Tm[a] (at a0.5
)
a[b]
DPN
1
5
10
50
32 (28)
44 (42)
53 (47)
57 (53)
0.81 (0.78)
0.97 (0.96)
0.99 (0.98)
0.99 (0.99)
2.31 (2.15)
5.77 (4.78)
8.86 (6.89)
38.48 (10.62)
[a] Tm =melting temperature of aggregates that have an aggregation
value of 0.5. [b] Values determined at 208C.
different concentrations. It could be seen that this tempera-
ture increased from 328C at low concentration (1ꢁ10ꢀ6 m)
to 578C at high concentration (5ꢁ10ꢀ5 m), thus indicating
better stability of the aggregates. Similar plots were also ob-
served for 4 (Figure S13). The a0.5(T) values were slightly
lower for 4 (Table 2) than for the saturated counterpart,
thus suggesting lower thermal stability for the aggregates.
From the degree of aggregation, the average stack length
(DPN) of the aggregates was calculated by using Equa-
tion (2):
Figure 2. a) Variable-concentration and b) variable-temperature absorp-
tion spectra of 3 in MCH.
p
DPNðTÞ ¼ 1= 1ꢀaðTÞ
ð2Þ
tion spectra resembled that of molecularly dissolved species
as observed in chloroform. These spectral features were in-
dicative of formation of face-to-face stacked rotationally dis-
placed H-type aggregates. Isosbectic points were observed
at 469, 490, and 529 nm during this transition from the mo-
lecularly dissolved to the aggregated species. Similar varia-
ble-temperature measurements were carried out for differ-
As expected, the size of the aggregates decreased with an
increase in temperature. Table 2 compares the average stack
length (DPN) at room temperature as a function of increase
in concentration. The stack length increased linearly with
concentration and at the highest concentration studied that
is, 5ꢁ10ꢀ5 m (50 mm), at which the sample remained aggre-
gated even at the highest temperature of 708C, the stack
length was as high as 38 for the PDP derivative (3). This is
typical of isodesmic assembly by which the stack length of
the aggregated species increased slowly and only at high
concentrations associated with high association constants,
larger-sized architectures were formed. For 4, the transition
temperatures as well as the stack length were lower, with
ent concentrations that ranged from 1ꢁ10ꢀ6 to 5ꢁ10ꢀ5
m
(Figure S10 in the Supporting Information; Figure S11
shows the plot for 4). At the highest concentration of 5ꢁ
10ꢀ5 m, the aggregates remained stable until the highest tem-
perature of 708C, thus indicating the thermal stability of the
aggregates. The aggregates in solution are dynamic and
polydisperse in nature and can usually be described by the
isodesmic equilibria that assumes a single equilibrium con-
stant for every monomer addition to the growing aggre-
gate.[41] The isodesmic model could be fitted to both varia-
ble-concentration as well as variable-temperature data.
However, compared to variable-concentration measure-
ments, variable-temperature studies spanned a wider set of
data points, thereby resulting in a more accurate determina-
tion of parameters.[42] The propensity for aggregation can be
quantified by calculating the degree of aggregation (a) using
Equation (1):
the stack length at the highest concentration of 5ꢁ10ꢀ5
m
being less than half the value relative to its saturated coun-
terpart.
The emission characteristics of the perylene bisimide mol-
ecules were studied in CHCl3 by excitation at 458 nm. Both
the molecules gave the typical emission with peak maxima
at 537, 576, and 626 nm. The quantum yields were calculated
using rhodamine 6G as the standard; the values are given in
Table 1. The quantum yield values were 0.91 and 0.83 for 3
and 4, respectively, in CHCl3, thereby indicating that upon
formation of the ester linkage, the hitherto nonluminescent
building blocks 1 and 2 converted to highly luminescent PBI
derivatives. In MCH, a new redshifted peak was observed at
643 nm, and the onset was also redshifted. These are all sig-
natures of face-to-face p-stacking H aggregation, which con-
firms the observation from the UV/Vis absorption spectra.
aðTÞ ¼ eðTÞꢀeðMÞ=eðAÞꢀeðMÞ
ð1Þ
in which e(T) is the extinction coefficient at temperature T,
and e(M) and e(A) are those for the molecularly dissolved
species and fully aggregated species, respectively. The
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G. A. Bhavsar and S. K. Asha
for the melt-cooled sample,
which had an emission only at
640 nm without any monomer
emission. The inset in Figure 4
shows the normalized UV/Vis
absorption spectra of the LC
films for the two ester deriva-
tives. All these results indicat-
ed the highly emissive nature
of both the derivatives 3 and 4
in dropcast film as well as solid
melt-cooled state, which are
highly desirable for practical
application in devices.
Fluorescence lifetimes were
studied for nitrogen-bubbled
0.1 OD solutions (OD=optical
density) in chloroform as well
as in MCH for the two ester
derivatives, 3 and 4. The decay
was monitored both at the mo-
nomer band of 520 nm as well
as the aggregate band at
640 nm. Table 1 gives the fit
parameters, for which the data
in brackets represents the
decay obtained in MCH (Fig-
ures S15 and S16 in the Sup-
porting Information). When
the data was fitted to single-ex-
ponential function, high c2
values were obtained for the
fit. A greatly improved fit with
Figure 3. a) Concentration-dependent fluorescence spectra of 3 in MCH (excited at 458 nm) without normali-
zation and b) after normalization at 524 nm. c) Temperature-dependent fluorescence spectra of 3 in MCH at
5ꢁ10ꢀ5 m concentration without normalization and d) after normalization at 524 nm. e) Temperature-depen-
dent fluorescence spectra of 3 in MCH at 1ꢁ10ꢀ6 m without normalization and f) after normalization at
524 nm.
Figure 3a shows the evolution of the new peak at 643 nm
as a function of increasing concentration in MCH. Fig-
ure 3c–f shows the increase in intensity of the emission at
643 nm as a function of temperature for solutions of increas-
ing concentrations. For the highest concentration of 5ꢁ
10ꢀ5 m, even at 708C, which was the highest temperature of
measurement, emission from the aggregates was pronounced
with a noticeable difference in the peak intensity ratios that
is suggestive of H aggregates. The intensity of the aggregate
emission at 640 nm at the lowest temperature of 108C was
20 times higher for 5ꢁ10ꢀ5 m solution relative to that for the
lowest concentration of 1ꢁ10ꢀ6 m, thus indicating that the
aggregates were highly emitting species. Similar spectral fea-
tures were observed for 4 (Figure S14 in the Supporting In-
formation).
c2 values around 1 was obtained using a double-exponential
function. The resulting short and long lifetimes were approx-
imately 1.7 and 3.4 ns, respectively, for the monomer decay
in both chloroform and MCH for both samples 3 and 4. The
average lifetime decay (taking into account their percentage
contribution) was 3.1 ns in chloroform and around 3.2 and
Thin-film samples were prepared by dropcasting the solu-
tions from MCH. The emission from film did not have the
monomer peak at 520 nm, only aggregate emission at
640 nm. Thin films of melt-cooled samples were also pre-
pared on glass substrates by heating the samples to isotropic
melt and then slowly cooling to room temperature. Polar-
ized light microscopy (PLM) studies confirmed that these
melt-cooled samples retained the LC textures upon cooling
to room temperature. Figure 4 shows the intense emission
Figure 4. Fluorescence spectra of 3 and 4 in the melt-cooled state. Inset:
Corresponding normalized UV/Vis absorption spectra.
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Perylene Bisimides
FULL PAPER
3.4 ns, respectively, in MCH for samples 3 and 4, which was
similar to literature reports.[21] As discussed by Winnik et al.
for a perylene-labeled polymer, a double-exponential fit
does not imply that there were two distinct species with dif-
ferent lifetimes.[43] A single emitting species could exhibit
nonexponential decay, which is represented by sums of two
or more exponentials due to solvent relaxation processes.[44]
The aggregate band at 640 nm was observed only in MCH
and it exhibited a much slower decay with a value of
tagg(av) =23 and 20 ns, respectively, for samples 3 and 4.[21]
Thus, the aggregate fluorescence lifetime was six times
higher than the monomer fluorescence lifetime, and also
among the two systems, the PDP-based perylene derivative
had longer aggregate fluorescence lifetime, thereby indicat-
ing a more stable species.
Figure 5. Energy-minimized structure (using density functional theory) of
compounds 1 and 2 viewed orthogonally to the polyaromatic plane.
Room-temperature wide-angle X-ray diffraction (WXRD)
studies: The only structural difference between the two
building blocks 1 and 2 was the presence of double bonds in
the C15 side chain in 2. Both 1 and 2 were high melting
compounds (>4008C), and due to their high melting tem-
peratures, it could not be confirmed by DSC or PLM studies
whether they formed LC phases. The difference in the solid-
state packing in these two building blocks was studied using
WXRD. Room-temperature WXRD studies clearly indicat-
ed the difference in the packing of the two molecules as a
consequence of the unsaturation in the side chain (Fig-
ure S17 in the Supporting Information).
The PDP derivative 1 was more crystalline with sharp
peaks at 2q=4.70 (18.77 ꢂ), 9.50 (9.29 ꢂ), 11.06 (7.99 ꢂ),
and 12.438 (7.12 ꢂ). It also had a broad peak around 2q
ꢁ208 (4.25 ꢂ), which corresponded to the loosely packed
alkyl chains, and a peak around 2q=24.78 (3.59 ꢂ) that indi-
cated the presence of p–p stacking. Compound 2 also had
similar peaks, but the intensity of the peaks was lower and
the peaks were diffused. The main difference between 1 and
2 was observed in the alkyl chain region. Sharp shoulders
could be observed in the 2qꢁ208 region in the case of 1 in
contrast to the broad featureless hump present in this region
for 2. Additionally, the peak at 2q=24.78 (3.59 ꢂ) that cor-
responded to p–p stacking was absent in 2. The cis double
bonds in the latter made the alkyl chain packing difficult
and reduced close contact between the aromatic core. Mo-
lecular modeling using density functional theory clearly
demonstrated this difference in alkyl chain packing. Figure 5
shows the energy-minimized structures of the two molecules
obtained using DFT.[45–50] In both molecules, the perylene
core was perfectly flat with the phenyl ring perpendicular to
the perylene plane.
wide-angle X-ray diffraction studies. The DSC analysis gives
information about the phase transitions and the enthalpies
associated with these transitions; PLM helps identify the
liquid-crystalline mesophases based on the texture observed;
and X-ray diffraction gives detailed information about the
molecular organization within the various phases at different
temperatures.
Figure 6 shows the second heating and first cooling cycles
of 3 and 4 at a scanning rate of 108Cminꢀ1. In DSC, com-
pound 3 showed three reversible transitions in the heating
cycle at 20 (60.52 kJmolꢀ1), 167 (5.84 kJmolꢀ1), and 2158C
(16.72 kJmolꢀ1), whereas the corresponding transitions
during cooling were observed at 202 (15.31 kJmolꢀ1), 163
(12.65 kJmolꢀ1), and 138C (52.55 kJmolꢀ1) (Table 3). The
Thermal and liquid-crystal characteristics: The thermal sta-
bility of the perylene bisimides were studied using thermog-
ravimetric analysis (TGA) under a nitrogen atmosphere.
They were found to be thermally stable until 3808C. The
thermotropic liquid-crystalline tendency of the series was
studied using DSC analysis coupled with PLM and tempera-
ture-dependent small-angle X-ray scattering (SAXS) and
Figure 6. DSC thermograms of 3 and 4 showing second heating and first
cooling cycle at 108Cminꢀ1 with respective transition temperatures.
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12651

G. A. Bhavsar and S. K. Asha
first transition in the cooling corresponded to disordered
columnar hexagonal (Colhd) mesophase as confirmed by the
snow-crystal-like dendritic texture observed under PLM.
Figure 7a and b show the large dendritic textures character-
istic of a Colh mesophase obtained after annealing the
sample at 2008C.[21]
and cooling scans during DSC and X-ray analysis as well as
when observed under PLM.
Variable-temperature small-angle X-ray scattering studies
were undertaken both during heating and cooling cycles.
Figure 8A shows the SAXS data as a plot of intensity versus
2q (1.0–3.08) at 25 (curve a) and 2308C (curve b) during
heating, and at 200, 160, 100, and 258C (curves c, d, e, and f)
during cooling for 3. Only a single peak that corresponded
to d100 reflection was observed in the 2q region between 1.0
and 3.08 that was studied.[51] The d100 value increased from
38.96 ꢂ at 2008C (2q=2.268) to 43.69 ꢂ at 258C (2q=
Table 3. Phase-transition temperatures and enthalpies of mesophases.
Sample
Phase transitions[a] [T in 8C (DH in kJmolꢀ1)]
3
Cr 20 (60.52)!Colhp 167 (5.84)!Colhd 215 (16.72)!I
I 202 (15.31)!Colhd 163 (12.65)!Colhp 13 (52.55)!Cr
Cr 29 (42.78)!Colhd 1958C (6.16)!I
1.988). The calculated ahex lattice parameter (ahex
=
4
2p
(4= d2100)) increased from 44.99 ꢂ at 2008C to 50.45 ꢂ at
I 169 (3.49)!Colhd 17 (40.66)!Cr
3
258C. The fully extended length of the molecule from one
end to the other end was calculated to be around 57 ꢂ using
ChemDraw. The ahex value at 258C was closer to this molec-
ular length than that at higher temperature. Figure 8B
shows the stack plot of the wide-angle X-ray diffraction
studies taken during the cooling scan from the isotropic
temperature at 2308C and cooling to 258C for 3.
The first pattern in the isotropic phase (2308C) showed
only one broad diffuse peak around 2q=15–208 (4–5 ꢂ)
that corresponded to loosely packed liquidlike alkyl
chains.[17] The second pattern for the sample annealed at
2008C showed only a single sharp peak at 2q=4.628
(19.13 ꢂ). No long-range intracolumnar order was observed
at this temperature. The two peaks at 2q=2.26 (SAXS) and
4.628 (WXRD) in the LC phase at 2008C could be indexed
to the 100, 200 Bragg reflec-
[a] Phase-transition temperature [8C] observed during the first cooling
and second heating cycle in DSC thermogram.
X-ray diffraction studies were also in accordance with a
columnar hexagonal ordering of mesogens.[17–19] Although
DSC showed a transition at 1638C and shearability was also
lost beyond this temperature, the dendritic textures did not
show any visible changes under PLM until room tempera-
ture (258C). But the X-ray data clearly showed changes that
occur around 1608C as will be discussed in detail below.
DSC showed further transition around 138C; however, the
polarized light microscopic studies as well as the X-ray scat-
tering studies could not be performed below room tempera-
ture. The LC nature was reversible with repeated heating
tion with the typical ratio of
p
1:1/ 4 for a hexagonal system,
and the structural information
is comparable to reported liter-
ature.[18] The DSC thermogram
showed
a transition around
1608C, and the WXRD pattern
at 1608C showed multiple
sharp diffractions both in the
small-angle and wide-angle
region. The appearance of the
sharp peaks in the diffuse halo
region of 15–208 indicated the
crystallization of the alkyl
chains. Also noticeable was the
appearance of the sharp peak
at 2q=21.58 (4.1 ꢂ), which
corresponded to perylene p–p-
stacking interaction. For the
powder pattern at 1608C, the
characteristic ratios of 1:1/
p
p
p
4:1/ 7:1/ 12 of the (100),
(200), (210), (220) reflections
of the hexagonal lattice found
reasonable agreement with the
observed peaks (Table S1 in
the
Supporting
Informa-
Figure 7. a),b) Polarization optical microscopic images of the snow-crystal-like dendritic texture obtained for 3
at 2008C and c),d) for 4 showing spherulitic textures with maltase crosses at 1858C.
tion).[18] The absence of any
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Perylene Bisimides
FULL PAPER
Figure 9. Schematic diagram showing the molecular ordering of 3 in the
liquid crystalline phase.
Variable-temperature SAXS and WXRD studies were car-
ried out for 4 at various temperature intervals during heat-
ing and cooling (Figure S18 in the Supporting Information).
The SAXS showed a single peak around 2q=2.09–2.268 sim-
ilar to the observation for the PDP derivative. At room tem-
perature (258C), a single peak was observed at 2q=2.098
that corresponded to a d100 value of 42.19 ꢂ. The ahex lattice
2p
(4= d2100)) of 48.72 ꢂ.
3
parameter was calculated to be (ahex
=
In the mesophase at 1008C, the 2q increased to 2.188, which
corresponded to a d spacing of 40.36 ꢂ. This peak totally
vanished in the isotropic phase at 2308C, and during cooling,
the single peak reappeared at 1658C at 2q of 2.268 that cor-
responded to a d spacing of 38.84 ꢂ. This also corresponded
with the first transition observed in DSC upon cooling at
1698C. Thereafter upon cooling further to 258C, although
no visible transition was observed in the DSC, the SAXS re-
vealed an increase in d spacing to 40.36 ꢂ at 1008C and fi-
nally to 42.19 ꢂ at 258C. The d spacing in the mesophase
during heating and cooling were identical. It is interesting to
note that the d100 value at the start of the LC phase—2008C
for 3 and 1658C for 4—were exactly identical and corre-
sponded to a value of 38.84 ꢂ. This corresponded to the dis-
ordered hexagonal phase in both systems. Upon cooling, the
d100 value increased for both the molecules; however, unlike
the PDP system in which the pentadecyl chain could stabi-
lize in the fully extended form, the cis double bond in the
cardanol chain restricted a fully extended packing. Hence
the unit-cell dimension for 4 was smaller (ahex =48.71 ꢂ)
than that of 3 (ahex =50.45 ꢂ).
In sharp contrast to the PDP derivative 3, the cardanol
derivative 4 did not show many sharp peaks in the WXRD.
Also noticeable was the absence of the peak around 2q=
22–268 that corresponded to the p–p-stacking interaction.
The lower propensity of the pendant cardanol chain to pack
effectively resulted in this lower p–p-stacking interaction in
4. The sample that crystallized as solution at 258C showed
sharp peaks at 2q=4.9 and 5.88 and two broad peaks
around 2q=10–15 and 15–258. The broad peak between 15–
Figure 8. Variable-temperature a) SAXS and b) WXRD traces of 3 at
various temperature intervals.
other peaks in the wide-angle region in the LC phase at
2008C indicated the existence of a disordered columnar
packing at higher temperatures. Upon cooling, the phase
shifted from the 2D disordered hexagonal system at 2008C
to a highly ordered 3D monoclinic system at 258C. Corre-
spondingly, the p–p stacking also became stronger (de-
creased from 4.1 ꢂ (2q=21.58) at 1608C to 3.77 ꢂ (2q=
23.58) at 258C). SAXS had shown an expansion of the ahex
parameter upon cooling from 160 to 258C, which indicated
an extended conformation of the alkyl chains at 258C. All
these facts along with the observation that the shearability
was lost below 1608C suggested a plastic columnar hexago-
nal phase (Colhp) at lower temperatures. Figure 9 shows
schematically the molecular ordering in the LC phase at var-
ious temperatures.
Compound 4 showed only two reversible transitions in the
heating cycle (DSC) at 29 (42.78 kJmolꢀ1) and 1958C
(6.16 kJmolꢀ1), whereas during cooling transitions were ob-
served at 169 (3.49 kJmolꢀ1) and 178C (40.66 kJmolꢀ1
;
Figure 6). The clearing temperature at 1958C was approxi-
mately 208C lower than the saturated analogue, which was
attributed to loosely packed nature of 4. Upon cooling from
the isotropic phase, spherulitic textures with maltese crosses,
typical for columnar hexagonal organization (Figure 7c and
d), were observed around 1858C.
Chem. Eur. J. 2011, 17, 12646 – 12658
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12653

G. A. Bhavsar and S. K. Asha
258, which is usually attributed to alkyl chain packing,
showed sharp spikes. The second pattern was for the LC
phase at 1008C during heating and it showed a sharp peak
around 2q=4.778. In the isotropic state at 2308C, this peak
disappeared and only the two broad peaks at around 2q=
10–15 and 15–258 remained. The WXRD pattern at 1908C
also showed that the sample was still in the isotropic phase.
But at 1808C, a small peak appeared around 2q=4.498. Al-
though DSC and SAXS did not show any noticeable change,
observation under the PLM had showed that spherulitic pat-
tern started to appear around this temperature. Compared
with the WXRD pattern at 1008C during heating, the inten-
sity of the peak at 2q=4.778 remained very low, even when
cooling to 258C. Thus, although both perylene derivatives 3
and 4 exhibited columnar LC
Solid-state morphology: Evidence for aggregate formation
in both the perylene bisimides 3 and 4 were obtained from
microscopy techniques such as SEM, TEM, and contact-
mode AFM. SEM was recorded on silicon wafers for drop-
cast samples from MCH at a concentration of 1ꢁ10ꢀ4 m.
Figure 10a–c show the images obtained for 3, which re-
vealed featherlike patterns. These were observed uniformly
over the entire sample and were several micrometers long
with diameters in the nanometer range. The TEM images of
3 gave more insight into the featherlike morphology.
Figure 11a–c shows TEM images obtained for 1ꢁ10ꢀ5
m
sample dropcast from MCH onto 200 mesh-sized carbon-
coated copper grids. Long cylindrical bundles of nanorods
that were several micrometers in length with average bundle
phases, there were also sharp
contrasts in their packing that
were highlighted by the ab-
sence of p–p-stacking interac-
tion in the latter. The cis
double bond in the C15 side
chain of 4 made a difference in
the LC packing as well as the
LC pattern observed since it
reduced the overall ordering
of the alkyl chain.
Another comparison be-
tween the two molecules was
the total enthalpy changes ob-
served during the heating and
cooling cycles in DSC. The en-
thalpy change in the heating
cycle for 3 was 83.08 kJmolꢀ1,
whereas
for
4
it
was
48.94 kJmolꢀ1, thus indicating
more ordered phases for the
former. This could be also sup-
ported by the enthalpy of iso-
tropization, which was higher
for 3 (16.72 kJmolꢀ1), since the
energy required to break the in-
itially formed ordered struc-
tures was more for 3, whereas
in the case of 4, less-ordered
structures could be easily
broken with a small enthalpy
change of 6.16 kJmolꢀ1. Impor-
tantly, both the samples re-
mained frozen in the LC phase
up to room temperature, and
textures were stable for several
weeks, which is a very useful
property as far as device fabri-
cation in the LC phase is con-
cerned.[52–53]
Figure 10. SEM images of a)–c) 3 and d)–f) 4 dropcast from MCH.
Figure 11. TEM images of a)–c) 3 and d–f) 4 dropcast from MCH.
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Perylene Bisimides
FULL PAPER
diameter in the range of 300–400 nm could be observed.
The observation from TEM was in accordance with that
from SEM, with the bundles not being in one straight length
but sickle-shaped with bifurcation at regular intervals. Simi-
lar studies were carried also out for 4. Figure 10d–f shows
the SEM images recorded for a 1ꢁ10ꢀ4 m sample dropcast
from MCH. It could be observed that the features were sim-
ilar except for the shorter length. The TEM images for 4
(Figure 11d–f) were in sharp contrast to that for 3. The
sickle-shaped nanorod bundles were extremely short, and
many of them were observed to twist around into coils or
springs with an average bundle diameter of around 250 nm.
This observation was very much in agreement with the ob-
servation from solution studies in MCH whereby shorter
average aggregate length (DPN) was obtained for the carda-
nol derivative than the PDP derivative. Figure 12a–c shows
the AFM images of 3 taken in contact mode for samples
dropcast from MCH onto silicon wafers at a concentration
of 1ꢁ10ꢀ4 m. The AFM images also showed features identi-
cal to that observed using TEM, that is, sickle-shaped bun-
dles of nanorods with widths in the range of 300–400 nm,
lengths in micrometers, and vertical distances of 20–30 nm
obtained from the height profile. The AFM images of the
cardanol derivative in Figure 12d–f also clearly show the
short V-shaped aggregates in conformity with the SEM
images.
dicated a clear difference in the packing interaction of the
two molecules. The sample of 3—both in the RT as well as
in the LC-RT—had a sharp peak in the wide-angle region
that corresponded to a p–p-stacking interaction of 3.77 ꢂ,
which was clearly absent in the cardanol-based PBI deriva-
tive 4. The difference in packing in the two systems, espe-
cially in the solid state, was driven by the packing con-
straints of the C15 alkyl side chain of cardanol, which had
the cis double bond. In both the PDP as well as the cardanol
system, the p–p stacking of the perylene core was rotation-
ally displaced by the steric hindrance of the C15 chain. Al-
though this difference was not very strong in the solution, in
the bulk samples the initially formed aggregates of 3 packed
into columns with pendant C15 alkyl chains. Such packing
was hindered by the twist caused by the cis double bond
present in the cardanol-based PBI 4. By using ChemDraw,
the total length of the molecule was calculated to be approx-
imately 5.7 nm. Taking into consideration the p–p-stacking
distance of 3.77 ꢂ along with sufficient alkyl chain interdigi-
tation, it could be assumed that around 100 molecules were
arranged laterally in each bundles and approximately 3000
molecules were stacked to form continuous rods in the PDP
derivative. This sort of extensive packing was not possible in
the cardanol derivative due to the presence of the kink in
the C15 alkyl chain, thereby resulting in short twisted and
coiled structures.
Based on this direct evidence, a model for the self-organi-
zation of these novel PBI derivatives could be established.
These molecules showed fibrous aggregate formation driven
by strong p–p-stacking interactions. Often fibrous network
formation observed in small molecules is driven by addition-
al forces in the form of hydrogen bonding.[29] It is interesting
to ponder the driving forces that lead to high levels of ag-
gregate formation in the PBI derivatives reported here. The
difference in the aggregate size in the two samples indicated
different packing in the two systems. The X-ray data also in-
Conclusion
Two novel perylene bisimide esters based on cardanol and
pentadecyl phenol were synthesized and fully characterized.
These perylene bisimide esters not only exhibited hexagonal
columnar liquid-crystalline mesophases that were retained
at room temperature but were also found to be highly emis-
sive both in solution and melt-cooled solid film. A compari-
son of structure and self-as-
sembly in solution as well as in
bulk for the pentadecyl phenol
versus cardanol perylene bisi-
mide derivative provided val-
uable insight into the packing
in these two systems. The pres-
ence of the long C15 alkyl
chain in the ortho position to
the imide linkage did not nega-
tively affect the propensity of
these novel PBIs to form H-
type aggregates. Interestingly, a
kink in the alkyl chain intro-
duced by way of the cis double
bond also did not result in a
change in the type of aggregate
formation, but there was
a
drastic reduction in the aggre-
gate length. The X-ray meas-
urements highlighted the simi-
Figure 12. AFM images of a)–c) 3 and d)–f) 4 dropcast from MCH.
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12655

G. A. Bhavsar and S. K. Asha
tion of the residuals. The thermal stability of the PBIs was analyzed using
a Perkin–Elmer STA 6000 thermogravimetric analyzer under a nitrogen
atmosphere from 40 to 8008C at 108Cminꢀ1. Differential scanning calo-
rimetry was performed using a TA Q10 differential scanning calorimeter.
Typically, 2–3 mg of samples was placed in aluminum pan, sealed proper-
ly, and scanned at 108Cminꢀ1. The instrument was calibrated with indium
standards before measurements. The phase behaviors of the molecules
were analyzed using a Leica DM2500P polarized optical microscope
equipped with a Linkam TMS 94 heating and cooling stage connected to
a Linkam TMS 600 temperature programmer. The transition from an iso-
tropic to liquid-crystalline phase can be monitored by the evolution of
characteristic textures. To remove the thermal history of the samples, the
first heating cycles of DSC were neglected.
larities as well as differences in packing brought about by
the presence of the cis double bond in the pendant alkyl
substituent. Although both molecules exhibited columnar
hexagonal mesophases, the LC window as well as the melt-
ing temperature were different due to the difference in their
columnar ordering. The novel PBIs reported here remained
in the LC phase at room temperature (258C) with low clear-
ing temperature, which is highly desirable for practical
device applications.
WXRD: WXRDs were recorded using a Philips analytical diffractometer
with CuKa emission, and the spectra were recorded in the (2q) range of
3–508 and analyzed using X’pert software. Powder X-ray diffraction of all
the samples was carried out using a PANalytical X’pert Pro dual goniom-
eter diffractometer. An X’celerator solid-state detector was employed in
wide-angle experiments. The radiation used was CuKa (1.54 ꢂ) with a Ni
filter, and the data collection was carried out using a flat holder in
Bragg–Brentano geometry. Care was taken to avoid sample displacement
effects. Variable-temperature in situ XRD experiments were carried out
using an Anton–Paar XRK900 reactor.
Experimental Section
General procedures: Perylene-3,4,9,10-tetracarboxylic dianhydride
(PTCDA), 3-pentadecyl phenol, zinc acetate, imidazole, methyl-3,4,5-tri-
hydroxybenzoate, triethylamine, and 1-bromododecane were purchased
from Sigma–Aldrich and used without further purification. Other re-
agents were purchased locally and purified using standard procedures.
Cardanol was purified by double-vacuum distillation of cashew-nut-shell
liquid (CNSL). All solvents were of analytical reagent grade and were
used as received. DMF and THF were dried using the standard proce-
dure.[54] 1H and 13C NMR spectra were recorded using a Bruker AVENS
200 MHz spectrometer. Chemical shifts (d) are reported in ppm at 258C
using CDCl3 or CDCl3/TFA as solvent with a small amount of tetrame-
thylsilane (TMS) as internal standard. Variable-temperature measure-
ments were recorded using a Bruker AVENS 400 MHz spectrometer
with [D14]MCH as solvent, and the spectra were calibrated against TMS.
The purity of the compounds was determined by elemental analysis as
well as MALDI-TOF in combination with SEC. SEC was performed
using polystyrene standards for the calibration in CHCl3 as eluent. The
flow rate of CHCl3 was maintained as 1 mLminꢀ1 throughout the experi-
ments, and the sample solutions at concentrations 3–4 mgmLꢀ1 were fil-
tered and injected for recording the chromatograms at 308C. Elemental
analysis was done using a Thermo Finnigan Flash EA 1112 series CHNS
analyzer. Infrared spectra were recorded using a Bruker FTIR (ATR
mode) spectrophotometer in the 4000–600 cmꢀ1 range. UV/Vis spectra
were recorded using a Perkin–Elmer Lambda ꢀ35 UV/Vis spectrometer
with a Peltier setup in the 0–1008C range for variable temperature meas-
urements. Steady-state fluorescence emission, variable-temperature, and
time-resolved fluorescence lifetime measurements were performed using
a Fluorolog HORIBA Jobin Yvon fluorescence spectrophotometer. The
emission as well as excitation slit width was maintained at 1 nm through-
out the experiments, and the data was obtained in S1c/R1 mode (to ac-
count for the variations in lamp intensity). Measurements were made at
908 positions for solutions and 22.58 in front-face films. The fluorescence
quantum yields of the perylene derivatives were determined in CHCl3
using rhodamine 6G in ethanol (F=0.95) as the standard by exciting at
488 nm. The optical density at l488 was maintained at (0.1ꢂ0.05) to avoid
reabsorption artifacts. Temperature-dependent fluorescence was recorded
by using a Peltier sample compartment with a Peltier Sample Cooler F-
3004 attached to a thermoelectric temperature controller (model no. LFI-
3751) with an autotune PIDw supplied by Wavelength Electronics. The
temperature was set manually for each reading and it was equilibrated
for 10 min at each temperature before recording the spectrum. The toler-
ance range for each set temperature was maintained at 0.58C. For life-
time measurements, decay curves were obtained by the time-correlated
SAXS: SAXS measurements were made using a Bruker Nanostar (CuKa
radiation). The Nanostar was equipped with an 18 kW rotating anode
generator and a 2D multiwire detector, with a sample-detector distance
of roughly 1 m. The entire X-ray flight path including the sample cham-
ber was evacuated to minimize air scattering. Samples were placed in ca-
pillaries and were mounted on the Bruker heating stage. The two-dimen-
sional SAXS data was azimuthally averaged, and 1D data is presented
after background subtraction.
SEM: SEM images were recorded using an FEI Quanta 200 3D scanning
electron microscope with tungsten filament as electron source. Samples
were prepared by dropcasting the sample (10 mL, 10ꢀ4 m concentration)
from MCH onto a silicon wafer and allowed to dry for 4 to 5 days at am-
bient temperature followed by gold coating (3 nm) before subjection to
SEM analysis.
TEM: An FEI Tecnai F30 electron microscope operating at 100 kV was
used for HRTEM sample observation. It was equipped with a Gatan digi-
tal camera for recording micrographs. Samples were prepared from 1ꢁ
10ꢀ5 m MCH solution dropcast onto a holey carbon-coated copper grid of
200 mesh size (Ted Pella) and allowed to dry at room temperature for 3
to 4 days. The size of nanorods was analyzed from TEM images using Im-
agePro software.
AFM: AFM images were taken by a Multimode scanning probe micro-
scope equipped with a Nanoscope IV controller from Veeco Instruments,
Inc. in the contact mode using an SiN probe, with a maximum scan size
of 10 mmꢁ10 mm and with a vertical range of 2.5 mm. For the AFM stud-
ies, samples were prepared by dropcasting 1ꢁ10ꢀ4 m solution in MCH
onto silicon wafers and allowed to dry before being subjected to AFM
analysis.
MALDI-TOF: Solution (1 mm) was premixed with dithranol matrix in
CHCl3 and mixed well before spotting on 96-well stainless steel MALDI
plate by dried droplet method. It was then used for MALDI analysis.
The mass spectral analysis was carried out using a Voyager-De-STR
MALDI-TOF (Applied Biosystems, Framingham, MA, USA) instrument
equipped with 337 nm pulsed nitrogen laser used for desorption and ioni-
zation. The mode of operation was in a reflector mode with an accelerat-
ing voltage of 25 kV. All spectra were acquired by accumulating 50 single
laser shots over each sample spot with all the analysis was done in 5 to 6
replications. They were processed for advanced baseline correction and
noise removal using Data Explorer software (Applied Biosystems).
single-photon counting (TCSPC) technique using
a HORIBA Jobin
Yvon Nano-LED source with wavelength 458 nm. Emission was collected
at a monomeric emission of 520 nm and the aggregate emission at
640 nm. The samples were prepared by making the optical density (0.1ꢂ
0.05) at excitation wavelength (lex ꢁ459 nm) in CHCl3 as well as in
MCH. All samples were purged with gentle flow of nitrogen for 5 min.
All experiments were performed under identical conditions. Fluorescence
lifetime values were determined by deconvoluting the data with exponen-
tial decay using DAS6 decay analysis software. The quality of fit was
judged by fitting parameters such as c2 ꢁ1, as well as the visual inspec-
Molecular modeling: All the calculations were carried out using density
functional theory using the Turbomole suite of programs.[45] Geometry
optimizations were performed using the B–P 86 functional.[46, 47] The elec-
tronic configuration of the atoms was described by a triple-x basis set
augmented by
a polarization function (TURBOMOLE basis set
TZVP).[48] The resolution of identity (RI)[49] along with the multipole ac-
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Perylene Bisimides
FULL PAPER
celerated resolution of identity (marij)[50] approximations were employed
for an accurate and efficient treatment of the electronic Coulomb term in
the density functional calculations.
Compound 4: This was synthesized following a similar procedure as for 3
but using 2. The crude product was then purified by column chromatog-
raphy in hexane. Yield: 0.325 g (28%); m.p. 1958C; 1H NMR (CDCl3):
ꢀ
d=8.7–8.8 (dd, 8H; perylene), 7.45 (s, 4H; Ar H), 7.25–7.40 (m, 6H;
Synthesis of 4-amino-3-pentadecyl phenol: 4-Amino-3-pentadecyl phenol
was synthesized form pentadecyl phenol (PDP) by following the litera-
ture procedure.[55]
ArH-cardanol), 5.20 (br, 4H; CH2-CH=CH-CH2), 4.06 (t, 12H; Ar-O-
CH2), 2.48 (t, 4H; cardanol-Ar-CH2), 1.0–1.95 (m, 164H; aliphatic CH2),
0.87 ppm (t, 24H; terminal CH3); 13C NMR: d=164.72, 163.33, 153.00,
151.51, 143.05, 141.99, 134.94, 131.90, 131.19, 129.89, 126.67, 123.92,
123.41, 122.86, 120.49, 108.60, 73.61, 69.30, 31.96, 31.17, 30.40, 29.60,
29.40, 29.22, 28.94, 27.15, 26.14, 22.72, 14.14 ppm; FTIR: n˜ =3079 (n(=
CꢀHstretch)), 2960 (n(-CH)stretch), 2925, 2853, 1731 (n(C=Oester)), 1703
(n(C=Oimide)), 1664, 1594, 1503, 1464, 1430, 1359, 1338, 1260, 1197, 1103,
1021, 863, 804, 750 cmꢀ1; MALDI-TOF (dithranol matrix): m/z calcd for
C152H226N2O14: 2303.70; found: 2326.89 [M+Na]; elemental analysis calcd
(%): C 79.19, H 9.88, N 1.22; found: C 78.85, H 9.64, N 1.24.
Cardanol was obtained from double vacuum distillation of CNSL and
was converted to the amine derivative by following the same procedure
as that for PDP derivative.
Synthesis of 1: PTCDA (5 g, 0.013 mol, 1 equiv) was heated with 4-
amino-3-pentadecylphenol (8.9 g, 0.028 mol, 2.2 equiv) and Zn (OAc)2
(0.5 g) in imidazole (40 g) at 1608C for 4 h. The reaction mixture was
then cooled to room temperature and precipitated by adding 2n HCl and
washed with deionized water (1 L). The crude product was purified by
dissolving in a minimum amount of THF and precipitating into methanol.
The methanol washing was continued until the washings were clear and
finally washed with hexane. The purified dark red solid was dried in a
vacuum oven at 808C for 12 h. Yield: 11.0 g (74%); m.p.>4008C;
1H NMR (CDCl3 +TFA): d=8.91 (s, 8H; perylene), 6.89–7.16 (m, 6H;
ꢀ
ꢀ
ꢀ
ArH-PDP), 2.39 (t, 4H; Ar CH2), 1.53 (t, 4H; Ar CH2 CH2), 1.0–1.4
(br, 48H; aliphatic CH2), 0.84 ppm (t, 6H; terminal CH3); 13C NMR: d=
166.27, 163.21, 162.65, 160.62, 155.86,143.08, 136.66, 134.16, 130.32,
127.00, 126.14, 124.92, 123.12, 122.59, 115.03, 111.82, 106.19, 32.17, 31.28,
29.92, 29.60, 22.85, 13.81 ppm; FTIR: n˜ =3359 (n(-OHstretch)), 2922, 2853,
1698 (n(C=Oimide)), 1656, 1592, 1498, 1403, 1360, 1348, 1300, 1250, 1231,
Acknowledgements
We thank the network project NWP0023 and DST project GAP 277826
for financial support. The authors thank Mr. Ketan Bhotkar, Mr. Naren-
dra Raj Karthi, and Ms. Pooja Mudellu, NCL-Pune for SEM, TEM, and
AFM measurements, respectively. The authors would also like to ac-
knowledge Dr. Nandini Devi and Dr. Kumar Vanka, NCL for useful dis-
cussions on X-ray analysis and DFT calculations, respectively.
1198, 1176, 967, 862, 813, 796, 750 cmꢀ1
matrix): m/z calcd for C66H78N2O6: 995.34; found: 996.65 [M+1], 1018.58
[M+Na].
; MALDI-TOF (dithranol
Synthesis of 2: Compound 2 was synthesized by following the above pro-
cedure by using 4-amino-3-pentadec(8-ene-yl)phenol as the amine. The
final maroon solid was dried in a vacuum oven at 808C for 12 h. Yield:
8.00 g (63%); m.p.>4008C; 1H NMR (CDCl3 +TFA): d=8.91 (s, 8H;
perylene), 6.89–7.17 (6H; ArH-cardanol), 5.23 (s, 4H; CH2-CH=CH-
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CH2), 2.39 (t, 4H; Ar CH2), 1.9 (m, 8H; CH2-CH=CH-CH2), 1.53 (t,
4H; Ar-CH2-CH2), 1.0–1.4 (br, 32H; aliphatic CH2), 0.83 ppm (t, 6H; ter-
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ꢀ
25.09, 22.81, 13.77 ppm; FTIR: n˜ =3333 (n(-OHstretch)), 3006 (n(=C
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1
ꢀ
H NMR (CDCl3): d=8.7–8.9 (dd, 8H; perylene), 7.44 (4H, s; Ar H),
ꢀ
7.25–7.40 (m, 6H; ArH PDP), 4.06 (t, 12H; Ar-O-CH2), 2.48 (t, 4H;
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142.93, 142.13, 135.08, 132.04, 131.16, 129.96, 126.78, 123.86, 123.39,
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1702(n(C=Oimide)), 1663, 1593, 1503, 1465, 1429, 1359, 1337, 1260, 1199,
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Received: April 2, 2011
Published online: September 28, 2011
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