Synthesis and Liquid Crystalline Properties of Unsymmetrically Substituted Naphthalenediimides with a Polar Headgroup: Effect of Amide Hydrogen Bonding and Alkyl Chain Length

Article abstract of DOI:10.1002/open.201700151

A series of new unsymmetrically substituted naphthalenediimide (NDI) moieties NDI-1 to NDI-6 were synthesized. The structures of these compounds were confirmed by means of FT-IR, ¹H NMR, ¹³C NMR, ESI-mass and HRMS spectroscopic measu

Full text of DOI:10.1002/open.201700151

DOI: 10.1002/open.201700151
Synthesis and Liquid Crystalline Properties of
Unsymmetrically Substituted Naphthalenediimides with a
Polar Headgroup: Effect of Amide Hydrogen Bonding and
Alkyl Chain Length
Namdev V. Ghule,[a] Rajesh S. Bhosale,[a] Sidhanath V. Bhosale,*[a] Turlapati Srikanth,[b]
Nandiraju V. S. Rao,*[b] and Sheshanath V. Bhosale*[c]
A series of new unsymmetrically substituted naphthalenedii-
mide (NDI) moieties NDI-1 to NDI-6 were synthesized. The
structures of these compounds were confirmed by means of
FT-IR, 1H NMR, 13C NMR, ESI-mass and HRMS spectroscopic
measurements. UV/Vis and fluorescence spectroscopy were
employed to investigate the photophysical properties of the
prepared compounds in solution and in the solid state. Using
the onset of UV/Vis absorption, the optical band gaps were
calculated. Cyclic voltammetry measurements were performed
to study the electrochemical behavior and to calculate the
LUMO energy levels. The thermal properties of NDI derivatives
were studied by differential scanning calorimetry. The meso-
morphic birefringent behavior of the NDI derivatives was inves-
tigated with polarizing optical microscopy. Among all of the
studied NDI derivatives, only NDI-1, NDI-2, and NDI-3 showed
liquid crystalline texture, owing to the presence of an amide
linkage for H-bonding along with aromatic moieties for p–p-
stacking.
1. Introduction
The design, synthesis, and optical properties of organic p-con-
jugated systems are studied in different fields, which have at-
tracted considerable attention from various research groups in
recent years. The self-assembled and self-organized p-conju-
gated systems possess excellent potential to be employed in
organic semiconductor devices and charge-transporting devi-
ces. Furthermore, their ability to exhibit liquid crystalline (LC)
behavior is very interesting.[1–3] During the synthesis of LC ma-
terials, researchers were troubled by the solubility issue with
higher molecular analogues. Hence, researchers turned their
attention towards lower-molecular-weight analogue derivatives
to overcome the solubility issue and succeeded to realize the
LC behavior. Liquid crystallinity is the fourth state of matter,
that is, an intermediate state between solid crystals and con-
ventional liquid, and is distinguished by different molecular ori-
entations, alignments, and ordered states.
LC materials are electrical insulators and are employed in flat
panel displays and for developing semiconductor materials.[4]
Mostly, p-conjugated systems have been used during the con-
struction of LC materials, because it assists in inducing p–p
stacking and hydrogen-bonding interactions to promote self-
assembly and self-organization. Whereas the self-organization
asserts aggregation and exhibits appropriate orientations for
LC materials, the polar headgroups also play a vital role during
the alignment of LC materials, not only for thermotropic, but
also for lyotropic LC phases.[5]
[a] N. V. Ghule, Dr. R. S. Bhosale, Dr. S. V. Bhosale
Polymers and Functional Materials Division
CSIR-Indian Institute of Chemical Technology
Hyderabad 500007, Telangana (India)
E-mail: bhosale@iict.res.in
[b] Dr. T. Srikanth, Prof. Dr. N. V. S. Rao
Chemistry Department, Assam University
Silchar 788011, Assam (India)
Naphthalenediimide (NDI) and its derivatives are excellent n-
type semiconductor materials with high electron affinities,
owing to their self-organization manifested by the p–p stack-
ing.[6,7] NDI is an ideal building block for the construction of
self-assembled nanostructures such as nanowires, nanoflowers,
nanosheets, vesicles, nanospheres, golf-ball-like morphology,
nanotubes, nanowires, rods, and ribbons because of their high
p–p stacking interactions.[8–18] Nevertheless, NDIs are lower ho-
mologues among other rylenediimides and have been studied
extensively in the field of optoelectronics, fabrication of field-
effect transistors, organic light-emitting diodes, and flexible LC
display, owing to charge carrier mobility and p-electron deloc-
alization, which leads to conductivity.[19–24] Yitzchaik and co-
workers[25] first studied the molecular layer epitaxy of NDI de-
E-mail: nandirajuv@gmail.com
[c] Dr. S. V. Bhosale
School of Science, RMIT University
GPO Box 2476V, Melbourne, VIC-3001 (Australia)
E-mail: bsheshanath@gmail.com
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under https://doi.org/10.1002/
open.201700151.
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rivatives and LC properties. Schab-Balcerzak et al. reported the
synthesis and thermotropic LC properties of azomethine–NDI
for optoelectronic applications.[26] Reckzek and co-workers es-
the appropriate amines. These amines were further used for
coupling with the acid chloride of 4-cyano benzoic acid in the
presence of triethylamine/DCM to give the desired products
NDI-1 to NDI-3. Furthermore, all compounds were fully charac-
terized by using spectroscopic techniques. Furthermore, NDI-4
to NDI-6 were prepared via imide formation reaction by using
NDA, alkyl amine, and 4-amino benzonitrile, as outlined in
Scheme 1. Similarly, the chemical structures of NDI-4 to NDI-6
were also confirmed by spectral and elemental analysis.
tablished the charge transfer in columnar LC materials using
[27]
NDI (acceptor) and alkoxy anthracene (donor).
Recently,
Xiao et al. demonstrated LC properties of NDI-based dyads,
triads with a substituted triphenylene core as the donor.[28]
The changes in alkyl chain length are well documented to
affect the aggregation ability, thermal, and electronic proper-
ties of the material.[29] Furthermore, Lee et al. demonstrated
that, by tuning the alkyl chain length of the NDI polymer, the
electron mobility and solar cell efficiency in organic photovol-
taics are enhanced.[30]
The obtained analytical data of NDI-1 are in good agree-
1
ment with their chemical structure. The H NMR spectrum of
compound NDI-1 is discussed; a peak for four protons of the
NDI core appeared as a singlet at 8.85–8.81 ppm. The nitrile-
bearing phenyl ring exhibits one singlet peak for four protons
at 7.80 ppm. The amide proton peak was observed at 7.64–
7.62 ppm as a broad singlet. In the 13C NMR spectrum, the
characteristic peak of ÀCN carbon appeared at d 115.61 ppm.
The obtained HRMS value, 551.2289, confirms the structure of
compound NDI-1. The FT-IR peaks at 1658 and 1702 cmÀ1 of
NDI-1 confirmed the presence of amide and imide carbonyl
functional groups, respectively, along with the ÀCN stretching
peak at 2231 cmÀ1. In compounds NDI-2 to NDI-6, the elemen-
tal analysis deviation for carbon, hydrogen, and nitrogen was
found to be more than 0.3%, which could be attributed to the
presence of solvent trapped in the LC compounds. Usually, it is
very difficult to remove the trapped solvent from such p-con-
jugated small molecules.
The above comprehensive list of applications of NDI, includ-
ing the donor–acceptor motif, motivated us to design and syn-
thesize unsymmetrical NDI derivatives with alkyl chains at one
end and a cyanophenyl moiety at the other terminal end.
Herein, we report the synthesis of NDI derivatives NDI-1 to
NDI-6, and demonstrated the distinct influence of an amide
linkage and long alkyl chains to manifest the LC properties.
The p–p stacking and amide hydrogen bonding would assist
for self-organization and a polar headgroup such as a nitrile
would be beneficial, which takes part in enhancing the dipole
moment and assigns appropriate orientations in the liquid
crystal.[31]
The synthesized compounds were further characterized by
using UV/Vis and fluorescence spectroscopy techniques.
2. Results and Discussion
2.1. Synthesis and Characterization
2.2. UV/Vis and Fluorescence Study
The synthetic details for compounds NDI-1 to NDI-3 are
shown in Scheme 1. A one-step imide formation reaction be-
tween 1,4,5,8-naphthalene tetracarboxylic dianhydride (NDA),
aliphatic amines, and monoboc ethylenediamine in dry dime-
thylformamide (DMF) yields compounds 4–6. Compounds 4–6
were readily subjected to boc deprotection in the presence of
1:1 trifluoroacetic acid (TFA)/dichloromethane (DCM) to yield
UV/Vis absorption and emission spectroscopy were employed
to study the photophysical properties of NDI-1 to NDI-6. The
UV/Vis absorption spectra were measured both in tetrahydro-
furan (THF) solution and in a thin film coated onto a glass sub-
strate (Figure 1). Absorption spectra of NDI-1 to NDI-6 showed
two intense peaks at 358 and 378 nm along with one shoulder
Figure 1. Normalized UV/Vis spectra of NDI-1 to NDI-6 in 1ꢁ10À5 m THF so-
Scheme 1. Synthetic scheme of NDI-1 to NDI-6 compounds.
lution (solid lines) and thin films (dashed line).
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peak at 340 nm. The absorption peaks above 340 nm are char-
acteristic peaks for NDIs and attributes the p–p* transition. Sig-
nificant redshifts in the absorption peaks of thin films of these
synthesized NDIs are observed with respect to all NDIs in solu-
tion, reflecting the conformations of p–p stacking and aggre-
gation. The photophysical data of NDI-1 to NDI-6 are detailed
in Table 1.
2.3. Electrochemical Properties
The electrochemical properties of NDI-1 to NDI-6 were mea-
sured by using cyclic voltammetry (CV). The obtained voltam-
mograms of the NDI derivatives are presented in Figure 3. The
Table 1. Photophysical data of NDI-1 to NDI-6 in solution in THF (1ꢁ
10À5 m) and in thin films on a glass substrate.[a]
Compound lmax (abs)
e
lmax (em) UV/Vis (film)
e
[nm]
[lmÀ1 cmÀ1
]
[nm]
[nm]
[lmÀ1 cmÀ1
]
NDI-1
NDI-2
NDI-3
NDI-4
NDI-5
NDI-6
379
379
378
379
379
379
27700
13400
13500
34700
29400
19700
403
401
405
401
405
403
388
388
387
387
388
388
65553
34260
69478
89953
64948
97169
[a] e=Absorbance.
Figure 3. Overlapped cyclic voltammograms of NDI-1 to NDI-6 in THF
(1ꢁ10À5 m) with a scan rate of 50 mVsÀ1, 0.1m Bu4 VHClO4 supporting elec-
trolyte, a Pt disk working electrode, a Pt wire counter electrode, and a satu-
rated calomel reference electrode.
Upon excitation at 355 nm, compounds NDI-1 to NDI-6
shows emission peaks at 403, 401, 405, 401, 405, and 403 nm,
respectively (Figure 2 and Table 1). The thin film fluorescence
NDI-1 to NDI-6 compounds exhibited a single electrochemical
reduction process characteristic of NDI between potentials of
À0.7 and À0.6 V. Determination of onset reduction potential
values allowed the calculation of the lowest unoccupied mo-
lecular orbital (LUMO). The optical band gap values (Eg ꢀ3.17
to 3.18 eV) for the NDI derivatives were calculated from the
onset of the UV/Vis absorption band, as enlisted in Table 2. Uti-
lizing the LUMO values and optical band gap, we determined
the HOMO values of NDI-1 to NDI-6.
Table 2. Electronic parameters of NDI-1 to NDI-6.
Compound
l(onset)
Eg =1240/l(onset)
[nm]
E(onset, red)
[V]
LUMO
[eV][a]
HOMO
[eV]]b]
[nm]
NDI-1
NDI-2
NDI-3
NDI-4
NDI-5
NDI-6
389
389
390
390
389
389
3.18
3.17
3.18
3.17
3.18
3.18
À0.75
À0.67
À0.78
À0.96
À0.67
À0.64
À4.05
À4.13
À4.02
À3.84
À4.13
À4.16
À7.23
À7.30
À7.20
À7.01
À7.31
À7.34
Figure 2. Normalized UV/Vis spectra (solid line) of NDI-1 to NDI-6 and fluo-
rescence emission (lex =355 nm) spectra (dashed line) in THF (1ꢁ10À5 m).
emission spectra were recorded on a quartz substrate with ex-
citation at 355 nm; it can be clearly seen that the 403, 401,
and 405 nm peaks for NDI-1 to NDI-3 are present, with addi-
tional broad emission peaks at 530, 528, and 550 nm, respec-
tively (Figure S1). However, for NDI-4 to NDI-6, the additional
emission peaks appeared at 455, 460, and 462 nm, respectively
(Figure S1). The appearance of additional emission peaks could
be attributed to the p–p stacking of the NDI core in the solid
state. Nevertheless, the appearance of pronounced redshifts in
emission peaks of NDI-1 to NDI-3 compared to NDI-4 to NDI-6
are attributed to p–p stacking of the NDI core along with the
presence of amide hydrogen-bonding interactions in thin film.
[a] LUMO [eV]=À[Ered + 4.8]. [b] HOMO [eV]=LUMO-Eg
2.4. Phase Behavior and LC Properties
2.4.1. Differential Scanning Calorimetry (DSC) Measurements
The thermal properties of the compounds show multiple tran-
sitions, as estimated by DSC during heating and cooling cycles.
Mostly, on the second heating cycle, the phase transitions dis-
appeared, which could be assigned to the melting of crys-
tals,[32] whereas the second cooling cycle assigns the molten
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Table 3. Phase-transition temperatures of compounds NDI-1 to NDI-6, recorded for the second heating cycle (first row) and second cooling cycle (second
row) at 58CminÀ1 from DSC and confirmed with POM. The enthalpies (DH in kJmolÀ1) and entropies (DS in JmÀÀ1) are presented in parentheses.
Crystalline
Liquid crystalline
Isotropic
Compound
Heating [8C]
Cooling [8C]
Heating [8C]
Cooling [8C]
NDI-1
NDI-2
206.0 (3.06, 6.39)
140.0 (2.97, 7.20)
197.7 (2.27, 4.82)
135.4 (2.79, 6.84)
264.8 (22.8, 42.4)
260.7 (15.5, 29.2)
258.6 (21.9, 41.1)
254.2 (3.0, 5.73)
LC1 261.5 (0.66, 1.23)
251.5 (9.40, 17.9)
260.8 (38.7, 72.7)
NDI-3
NDI-4
NDI-5
NDI-6
126.6 (2.17, 5.46)
140.7 (5.47, 10.6)
123.8 (3.59, 9.06)
139.1 (6.89, 13.4)
252.6 (6.61, 15.2)
287.1 (36.0, 64.2)
285.1/273.1
262.5 (30.3, 56.7)
256.6 (29.8, 56.3)
phase of the liquid. The second heating and first cooling DSC
thermograms at 108CminÀ1 were recorded and the results are
presented in Table 3. The DSC thermograms include the series
of synthesized unsymmetrical NDI derivatives NDI-1 to NDI-6.
NDI derivatives NDI-1 to NDI-3 bearing an amide linkage show
the transition phase at different temperatures, which is shown
in Figure 4. In NDI-4 to NDI-6, such behavior is not observed
(Figure S2).
2.4.2. Polarized Optical Microscopy (POM)
Compounds NDI-1 to NDI-3 were studied for their mesomor-
phic properties through the observation of optical textures
with POM. Upon slow cooling, compound NDI-1 exhibited a
birefringent texture at 2588C (Table 3), which stabilized later
with a consistent pattern and developed throughout the entire
region of thin film. The texture resembles the textures ob-
served for a discotic nematic phase, as shown in Figures 5a
and 5b). The sample crystallized at 1988C (Table 3). Similarly,
compound NDI-2 exhibited a birefringent optical texture in
the cooling cycle at 2548C (Table 3), as shown in Figure 5c.
NDI-3 also exhibited the LC texture at temperatures below
2518C (Table 3), as shown in Figure 5d.
As the alkyl chain length increases from C8 to C12 and C16,
the transition temperature decreases from 258 to 254 and
2518C, and the NDI derivatives exhibit a discotic nematic
phase, birefringent, and crystalline phase, respectively. These
Figure 5. POM images of NDI derivatives: a) growth of mesomorphic bire-
fringent texture at 2578C of NDI-1; b) fully developed birefringent texture
256.78C; c) mesomorphic texture of NDI-2 at 2608C. d) Indicates mesomor-
phic texture of NDI-3 at 2608C.
Figure 4. DSC thermograms of model compounds: a) NDI-1, b) NDI-2, and
c) NDI-3 recorded at a heating/cooling rate of 58CminÀ1
.
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results suggest the change in alkyl chain length could change
the LC phase with the respective transition temperature.
Hence, it is concluded that the nature of the LC texture de-
pends on the flexibility of the spacer, the linkage group, and
the polar end group, which creates the transverse dipole
moment that supports molecular interactions.[31] The LC prop-
erties were disrupted in the presence of an increasing percent-
age of amide linkages.[33] However, without an amide linkage,
the NDI derivatives did not exhibit multiple phase transitions
during the heating and cooling cycles. It implies that NDI-4 to
NDI-6 (Figure S3) could not exhibit LC behavior, and we
assume that the spacer as well as the type of linkage for mo-
lecular interactions plays an important role in inducing meso-
morphism.
Optical Measurements
Electronic absorption and emission spectra were recorded on an
Agilent Technologies Cary 5000 eclipse UV/Vis-NIR spectrophotom-
eter and Agilent technologies Cary eclipse fluorescence spectro-
photometer, respectively. For DSC, the phase transition tempera-
tures and associated enthalpies were measured on a PerkinElmer
Pyris-1 system with a heating/cooling rate of 58CminÀ1
.
Synthetic Procedures
Synthesis of Compound 4–6
In the synthesis of compounds 4, 5, and 6, imidation reactions
were performed by modifying a reported procedure.[16] 1,4,5,8-Nap-
thalenetetracarboxylic dianhydride (NDA) (1 g, 0.0037 mol) and
mono boc-ethylene diamine (1.2 equiv) were mixed in 15 mL of
dry DMF under a N2 atmosphere, and then the reaction mixture
was heated under reflux to yield a clear brown solution. Then, the
aliphatic amine (1 equiv) was added into the reaction mixture. Fur-
thermore, the reaction mixture was heated under reflux for 16 h in
a N2 atmosphere. The completion of the reaction was monitored
by TLC analysis. After completion of reaction, the reaction mixture
was cooled to room temperature. The solvent was removed under
reduced pressure by using a rotary evaporator, which gave the
crude product as a brown solid. This was re-precipitated by using
methanol and removal of the solvent was subsequently achieved
by filtration. The obtained product, a faint brown solid, was puri-
fied by column chromatography using silica gel (60–120 mesh),
eluting with a mixture of DCM and hexane (8:2, v/v), followed by
evaporation of solvent on a rotary evaporator under vacuum to
yield the pure product as a brown solid (0.92 g, 48%).
3. Conclusions
We have studied the effect of the amide linkage and different
alkyl chain lengths on the liquid crystalline (LC) properties of
NDI-based derivatives. In NDI-1 to NDI-3, as the alkyl chain
length increases from C8 to C12 and C16, it was observed that
the transition temperature decreases from 258 to 254 and
2518C, and different LC properties are observed, going from a
discotic nematic phase to a birefringent and crystalline phase,
respectively. Whereas, NDI-4 to NDI-6, without a spacer or
amide linkage, do not display any LC properties. These results
imply the importance of the alkyl chain length as well as the
amide linkage for hydrogen bonding in LC behavior. We be-
lieve that such an LC materials may have future potential ap-
plications in display devices such as TVs, mobile phones, com-
puter monitors, and so forth.
Compounds 5 and 6 were synthesized by following the procedure
adopted for synthesis of compound 4.
Spectroscopic Data of Compound 4
1
FT-IR (KBr, cmÀ1): 3352, 2924, 1704, 1686, 1656; H NMR (500 MHz,
Experimental Section
CDCl3, TMS) d ppm:8.77–8.74 (m, 4H, J=7.6 Hz), 4.87 (br, 1H),
4.39–4.37 (t, 2H, J=5.64 Hz), 4.20–4.17 (t, 2H, J=7.26 Hz), 3.56–
3.55 (t, 2H), 1.77–1.71 (m, 2H),1.30–1.25 (m, 19H) 0.89–0.86 (t, 3H,
J=6.7 Hz); 13C NMR (125 MHz, CDCl3, TMS) dppm: 163.09, 162.66,
156.09, 130.96, 130.80, 126.58, 126.37, 79.22, 40.96, 40.60, 39.10,
31.74, 29.64.29.22, 29.13, 28.11, 28.01, 27.04, 22.57, 14.03; ESI-MS
m/z (%): 544 [M+Na]+; HRMS: Calculated. for C29H36N3O6 [M+H]+:
522.2598; found: 522.2598, Calculated. for C29H35N3O6Na [M+Na]+:
544.2417; found: 544.2418.
1,4,5,8-Naphthalenetetracarboxylic dianhydride, aliphatic amines,
and ethylenediamine were purchased from Sigma Aldrich (Benga-
luru, Karnataka, India); 4-amino benzonitrile and 4-cyano benzoic
acid were purchased from Spectrochem (Hyderabad, Telangana,
India) and were used without further purification. Furthermore, all
other solvents and chemicals were distilled and dried before use
such as DMF, DCM, and triethylamine. Spectroscopic-grade solvents
were used for all optical measurements.
Spectroscopic Data of Compound 5
1
FT-IR (KBr, cmÀ1): 2920, 1697, 1660; H NMR (400 MHz, CDCl3, TMS)
Spectroscopic Measurements
d ppm:8.76 (m, 4H), 4.86 (br, 1H), 4.40–4.37 (t, 2H, J=5.38 Hz),
4.21- 4.17 (t, 2H, J=7.45 Hz), 3.56 (t, 2H), 1.78–1.70 (m, 2H), 1.25–
1.22 (m, 27H), 0.89–0.85 (t, 3H, J=6.62 Hz); 13C NMR (100 MHz,
CDCl3, TMS) d ppm: 161.90, 161.47, 132.25, 131.90, 127.42, 125.58,
78.73, 41.91, 40.90, 38.76, 31.93, 29.63, 29.24, 27.92, 27.45, 22.68,
13.99. ESI-MS m/z (%): 610 [M+Na]+.
1H NMR spectra were recorded on AVANCE 300 and 500 MHz in-
struments. 13C NMR spectra were recorded on 75, 100, and
125 MHz NMR spectrometers, using tetramethylsilane (TMS) as the
internal standard and CDCl3-d1 or [D6]DMSO as the solvent. Mass
spectrometric data were obtained by using the positive electron
spray ionization (ESI-MS) technique on an Agilent Technologies
1100 Series (Agilent Chemistation Software) mass spectrometer.
High-resolution mass spectra (HRMS) were obtained by using an
ESI-QTOF mass spectrometr. FT-IR spectra were recorded on a Per-
kinElmer spectrum-100 spectrometer. Elemental analysis was car-
ried out by using a Vario Micro Cube.
Spectroscopic Data of Compound 6
FT-IR (KBr, cmÀ1) (6): 3425, 2916, 1703, 1659; 1H NMR (500 MHz,
CDCl3, TMS) d ppm: 8.77–8.74 (m, 4H, J=7.6 Hz), 4.86 (br, 1H),
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4.39–4.37 (t, 2H, J=5.64 Hz), 4.20–4.17 (t, 2H, J=7.26 Hz), 3.56–
3.55 (t, 2H), 1.75–1.72 (m, 2H) 1.25 (m, 35H), 0.89–0.86 (t, 3H, J=
7.1 Hz); 13C NMR (100 MHz, CDCl3, TMS) d ppm: 163.19, 162.76,
156.12, 131.03, 130.86, 126.74, 126.66, 79.28, 41.01, 40.61, 39.15,
31.90, 29.67, 29.33, 28.11, 27.07, 22.61, 14.11;ESI-MS m/z (%):535
[M+H]+.
Spectroscopic Data of Compound NDI-3
MP: 2708C; FT-IR (KBr, cmÀ1): 3413, 3280, 2917, 2233, 1702, and
1
1657; H NMR (500 MHz, CDCl3, TMS) d ppm: 8.84–8.80 (m, 4H, J=
7.62 Hz), 7.80 (s, 4H), 7.60 (brs, 1H) 4.62–4.60 (t, 2H, J=5.18 Hz),
4.23–4.19 (t, 2H, J=7.62 Hz), 4.00–3.38 (t, 2H), 1.73 (m, 2H), 1.25
(m, 26H), 0.88–0.86 (t, 3H); 13C NMR (100 MHz, CDCl3, TMS) d ppm
: 164.25, 163.27, 136.77, 132.96, 131.59, 131.88, 127.75, 126.95,
125.84, 118.62, 115.79, 112.96, 110.13, 41.57, 40.48, 39.98, 31.91,
29.68, 29.24, 27.94, 26.99, 22.68, 14.06; ESI-MS m/z [%, -ve mode]:
662 [M-]+; Elem. anal.: Calculated C: 72.48%; H: 7.00%; N: 8.45%;
found C: 70.32%; H: 6.97%; N: 8.36%.
Synthesis of NDI-1
After boc deprotection of compound 4, 5, or 6 using TFA/DCM
(1:1) at room temperature for 4–5 h, it was immediately used for
further reactions to synthesize compounds NDI-1 to NDI-3.
Synthesis of NDI-4
A catalytic amount of DMF was added to 4-cyano benzoic acid
(0.05 g, 0.00034 mol) in a 50 mL round-bottom flask. Distilled thio-
nyl chloride (3.5 mL, 0.047 mol) was added dropwise at 08C to the
aforementioned solution. The reaction mixture was heated under
reflux for 6 h. The excess of thionyl chloride was evaporated under
vacuum by using a rotary evaporator under a N2 atmosphere. De-
protected amine was mixed in dry DCM, and then the mixture was
added dropwise to the acid chloride solution. Then, triethylamine
(0.5 mL) was added at once to a stirred mixture of amine and acid
chloride. The reaction mixture was stirred vigorously for 30 min
under a N2 atmosphere. Then, the reaction mixture was heated
under reflux for 8 h. The completion of the reaction was monitored
by TLC, following which the reaction mixture was cooled to room
temperature. Evaporation of solvent under reduced pressure yields
a white residue, which was washed with saturated NaHCO3 and ex-
tracted with CH2Cl2 (2ꢁ25 mL). The obtained crude product was
purified by column chromatography using DCM/MeOH (98:2, v/v)
as an eluent, yielding pure product NDI-1 (0.083 g, 64%) as a solid.
NDA (0.5 g, 0.0018 mol) and 4-amino benzonitrile (1.2 equiv) were
added to a 50 mL round-bottom flask containing dry DMF (15 mL),
which was refluxed until a clear solution was obtained; then, ali-
phatic amine (1.0 equiv) was added to the reaction mixture and re-
fluxed for a further 12 h under a N2 atmosphere. Completion of the
reaction was monitored by TLC, following which the reaction mix-
ture was cooled at room temperature to yield a solid, which was
filtered and the residue was washed with MeOH to get the crude
product. This was further purified by column chromatography
using DCM/hexane (95:5, v/v) as the eluent to obtain a pure white
solid NDI-4 (0.39 g) in 46% yield.
For the synthesis of NDI-5 and NDI-6, the same experimental pro-
cedure described for the preparation of NDI-4 was employed.
Spectroscopic Data of Compound NDI-4
MP: 2758C; FT-IR (KBr, cmÀ1): 2921, 2232, 1706, 1661; 1H-NMR
(500 MHz, CDCl3, TMS) d ppm: 8.81 (s, 4H), 7.89–7.87 (d, 2H, J=
8.54 Hz) 7.49–7.48 (d, 2H, J=8.54 Hz), 4.22–4.19 (t, 2H), 1.79–1.73
(m, 2H), 1.25 (m, 10H), 0.89–0.86 (t, 3H); 13C NMR (75 MHz, CDCl3,
TMS) d ppm: 162.56, 133.29, 131.57, 130.99, 129.86, 127.24, 126.05,
117.96, 113.22, 41.07, 31.73, 29.64, 29.11, 28.01, 27.02, 22.57, 14.08;
ESI-MS m/z (%) : 480 [M+H]+;Elem. anal.: Calculated: C: 72.64%; H:
5.25%; N: 8.76%; Found: C: 72.35%; H: 5.69%; N: 8.10%.
Compounds NDI-2 and NDI-3 were synthesized following the pro-
cedure for the synthesis of NDI-1.
Spectroscopic Data of Compound NDI-1
MP: 2628C; FT-IR (KBr, cmÀ1) : 3279, 2923, 2231, 1702, and 1658;
1H NMR (500 MHz, CDCl3, TMS) d ppm: 8.85–8.81(m, 4H, J=
7.62 Hz), 7.80 (s, 4H,), 7.64–7.62 (brs, 1H) 4.62–4.60 (t, 2H, J=
5.18 Hz), 4.23–4.20 (t, 2H, J=7.62 Hz), 4.02–3.92(t, 2H), 1.77–1.71,
(m, 2H), 1.26 (m, 10H), 0.89–0.86 (t, 3H); 13C NMR (75 MHz, CDCl3,
TMS) d ppm: 164.07, 162.88, 136.85, 132.78, 131.64, 131.23, 127.69,
127.09, 126.77, 125.83, 115.61, 112.65, 41.29, 40.57, 39.84, 31.75,
29.13, 27.98, 27.01, 22.59, 14.02; ESI-MS m/z (%): 551 [M+H]+;
HRMS m/z (%): calc. for C32H31N4O5: 551.2300; found: 551.2289.
Spectroscopic Data of Compound NDI-5
MP: 2908C; FT-IR (KBr, cmÀ1): 2920, 2228, 1703, 1660; 1H-NMR
(300 MHz, CDCl3, TMS) d ppm: 8.82 (s, 4H), 7.90–7.87 (d, 2H, J=
8.39 Hz) 7.50–7.47 (d, 2H, J=8.39 Hz), 4.23–4.18 (t, 2H), 1.80–1.71
(m, 2H), 1.25 (m, 19H), 0.90–0.85 (t, 3H); 13C NMR (75 MHz, CDCl3,
TMS) dppm: 162.25, 131.61, 133.30, 131.02, 129.89, 127.32, 126.11,
117.96, 113.28, 41.12, 31.90, 29.67, 29.30, 28.06, 27.07, 22.66, 14.09;
ESI-MS m/z (%): 535 [M+]+; Elem. anal.: Calculated: C: 76.38%; H:
6.41%; N: 5.24%; Found C: 72.56%; H: 6.23%; N: 8.19%.
Spectroscopic Data of Compound NDI-2
MP: 2728C; FT-IR (KBr, cmÀ1): 3281, 2920, 2233, 1702, and 1657;
1H NMR (400 MHz, CDCl3, TMS) d ppm: 8.85–8.81(m, 4H, J=
7.62 Hz), 7.80 (s, 4H,), 7.64–7.62 (brs, 1H) 4.62–4.60 (t, 2H, J=
5.18 Hz), 4.23–4.20 (t, 2H, J=7.62 Hz), 4.01–3.99 (t, 2H), 1.77–1.70
(m, 2H), 1.26 (m, 19H), 0.89–0.86 (t, 3H); 13C NMR (100, MHz, CDCl3,
TMS) d ppm: 164.27, 163.30, 132.99, 131.61, 127.75, 126.96, 125.86,
117.42, 115.61, 112.65, 41.59, 40.51, 39.99, 31.90, 29.61, 29.32,
27.93, 26.98, 22.67, 14.06; ESI-MS m/z -ve mode, (%): 605 [M-1]+;
Elem. anal.: calculated: C: 71.27%; H: 6.31%; N: 9.23%; Found: C:
68.47%; H: 6.06%; N: 7.53%.
Spectroscopic Data of Compound NDI-6
MP: 2658C; FT-IR (KBr, cmÀ1): 2919, 2225, 1703, 1661; 1H-NMR
(300 MHz, CDCl3, TMS): d ppm: 8.81 (s, 4H), 7.89–7.87 (d, 2H, J=
8.39 Hz) 7.49–7.47 (d, 2H, J=8.39 Hz), 4.22–4.19 (t, 2H), 1.79–1.73
(m, 2H), 1.25 (m, 26H), 0.89–0.86 (t, 3H); 13C NMR (75 MHz, CDCl3,
TMS) dppm: 162.53, 133.30, 131.61, 131.02, 129.86, 117.96, 113.25,
41.10, 31.88, 29.63, 29.32, 27.02, 22.6690, 14.08; ESI-MS m/z (%):
590 [M+]+; Elem. anal.: Calculated: C: 75.10%; H: 6.98%; N: 5,24%;
Found C: 73.04%; H: 7.04%; N: 6.44%.
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UV/Vis Absorption Spectral Measurements
Conflict of Interest
UV/Vis spectra were recorded by using a Shimadzu UV-1800 spec-
trophotometer at room temperature. UV/Vis experiments were run
in a 3 mL quartz cuvette.
The authors declare no conflict of interest.
Keywords: alkyl chains · amide hydrogen bonding · liquid
crystals · naphthalenediimide · polar headgroup
Film Preparation for UV/Vis Measurements
For recording solid-state UV/Vis spectra of thin films, the solution
was drop-cast onto a glass substrate and allowed to dry for 5 mins
under high vacuum at 408C prior to the measurements. The film
thickness was varied by changing the concentration of the
solution.
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For recording solid-state fluorescence emission spectra of thin
films, solutions of the NDI derivatives 1ꢁ10À5 m were prepared in
THF. The solution was drop-cast onto a quartz substrate and al-
lowed to dry for 5 mins under high vacuum at 408C prior to the
measurements.
CV Experiments
Cyclic voltammograms of NDI-1 to NDI-6 amphiphiles in THF (1ꢁ
10À5 m) were recorded under nitrogen atmosphere with a scan rate
of 50 mVsÀ1, 0.1m Bu4 NHClO4 supporting electrolyte, a Pt disk
working electrode, a Pt wire counter electrode, and a saturated cal-
omel reference electrode.
DSC
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The LC properties were observed and characterized by using POM
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Acknowledgements
S.V.B. (IICT) is grateful for financial support from SERB (Project
Code: SB/S1/IC-09/2014), New Delhi, India. N.V.G. acknowledges
financial support from CSIR, New Delhi for SRF fellowship. S.V.B.
(RMIT) acknowledges financial support from the Australian Re-
search Council under a Future Fellowship Scheme (FT110100152).
R.S.B acknowledges financial support from CSIR, New Delhi under
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Received: September 12, 2017
Version of record online && &&, 2017
ChemistryOpen 2017, 00, 0 – 0
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ꢀ 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

FULL PAPERS
N. V. Ghule, R. S. Bhosale, S. V. Bhosale,*
T. Srikanth, N. V. S. Rao,* S. V. Bhosale*
Bringing it to a head: Unsymmetrically
substituted naphthalenediimide (NDI)
moieties (NDI-1 to NDI-6) are synthe-
sized via imide formation followed by
amide coupling reactions. The meso-
morphic birefringent behavior of the
NDI derivatives is investigated with po-
larizing optical microscopy (POM).
Among all of the NDI derivatives, only
NDI-1, NDI-2, and NDI-3 showed liquid
crystalline texture.
&& – &&
Synthesis and Liquid Crystalline
Properties of Unsymmetrically
Substituted Naphthalenediimides with
a Polar Headgroup: Effect of Amide
Hydrogen Bonding and Alkyl Chain
Length
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ÝÝ These are not the final page numbers!