A study on the synthesis, characterization, structural optimization, and conformational behaviors of bromo-substituted pyromelliticdiimide-based [2+2] macrocycle as structural units of covalently linked molecular tubes

Article abstract of DOI:10.1016/j.molstruc.2020.128164

Synthesis and structural, photo physical, and conformational behaviors at variable temperature and structural optimization of the pyromelliticdiimide-based bromo-substituted [2 + 2] macrocycles are described. Cyclization of the diamine (3) with dianhydride (4) in THF, followed by dehydration of the resultant amic acids resulted in the isolation of the bromo-substituted [2 + 2] macrocycle 1 (4.5%). The dynamic temperature-dependent ¹H NMR spectra and MO calculations revealed the presence of two possible conformers for the [2 + 2] macrocycle 1. The UV/Vis spectrum of 1 reveals the presence of a weak intramolecular CT interaction of electron-withdrawing pyromelliticdiimide moiety with the electron-donating hexyloxy-substituted xylyl moiety. The cyclic voltammetric measurement shows two two-electron reversible reduction processes.

Full text of DOI:10.1016/j.molstruc.2020.128164

Journal Pre-proof
A study on the synthesis, characterization, structural optimization, and conformational
behaviors of bromo-substituted pyromelliticdiimide-based [2+2] macrocycle as
structural units of covalently linked molecular tubes
Md. Ershad Halim, Arkasish Bandyopadhyay, Md. Elius Hossain, Teruo Shinmyozu
PII:
S0022-2860(20)30489-0
DOI:
https://doi.org/10.1016/j.molstruc.2020.128164
MOLSTR 128164
Reference:
To appear in: Journal of Molecular Structure
Received Date: 17 August 2019
Revised Date: 28 March 2020
Accepted Date: 30 March 2020
Please cite this article as: M.E. Halim, A. Bandyopadhyay, M.E. Hossain, T. Shinmyozu, A study on the
synthesis, characterization, structural optimization, and conformational behaviors of bromo-substituted
pyromelliticdiimide-based [2+2] macrocycle as structural units of covalently linked molecular tubes,
Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/j.molstruc.2020.128164.
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A study on the synthesis, characterization, structural optimization, and
conformational behaviors of bromo-substituted pyromelliticdiimide-
based [2+2] macrocycle as structural units of covalently linked molecular
tubes
Md. Ershad Halim1 *, Arkasish Bandyopadhyay2, Md. Elius Hossain3, Teruo Shinmyozu2
1Department of Chemistry, University of Dhaka, Dhaka 1000, Bangladesh
2Institute for Materials Chemistry and Engineering (IMCE), Kyushu University, 6-10-1 Hakozaki, Higashi-ku,
Fukuoka 812-8581, Japan
3Department of Chemistry, Bangladesh University of Engineering and Technology, Dhaka 1000, Bangladesh
Keywords: Artificial Molecular Tube, Macrocycle, PyromelliticDiimide, Conformational Behavior, Structure
Optimization
ABSTRACT: Synthesis and structural, photo physical, and conformational behaviors at variable temperature and
structural optimization of the pyromelliticdiimide-based bromo-substituted [2+2] macrocycles are described. Cy-
clization of the diamine (3) with dianhydride (4) in THF, followed by dehydration of the resultant amic acids re-
sulted in the isolation of the bromo-substituted [2+2] macrocycle 1 (4.5%). The dynamic temperature-dependent
1H NMR spectra and MO calculations revealed the presence of two possible conformers for the [2+2] macrocycle
1. The UV/Vis spectrum of 1 reveals the presence of a weak intramolecular CT interaction of electron-
withdrawing pyromelliticdiimide moiety with the electron-donating hexyloxy-substituted xylyl moiety. The cyclic
voltammetric
measurement
shows
two
two-electron
reversible
reduction
processes.
Corresponding Author
gen bonds, [6] π-π interactions, [7] or van der Waals
*E-mail: ershadhalim@yahoo.com
contacts [8]. We have been investigating a directed
and controlled approach for the synthesis of nano-
tubes held together via covalent bonds [9].
Introduction
Supramolecular structures with spherical and tubular
shapes have practical implications in material science
and molecular electronics [1]. The discovery of the
highest covalent bond containing carbon nanotubes
has created a flurry of activity in the design and syn-
thesis of other forms of nanotubes [2]. The tubular
structure increases the stability of the molecule by
chemical bonding as well as offers synthetic control
of the conformations. Encapsulation [3] of atoms or
molecules as well as their transportation [4] is also
desired in the empty cylindrical cavity of tubular mol-
ecules. The tubular molecules known till date are de-
signed by self-assembly, [5] held together by hydro-
Macrocycles with their interesting structures and
properties are a promising candidate to build up the
supramolecular structure. Till now there exist very
little reports in the literature on the covalently linked
nanotubes [10] based on macrocycles, presumably
due to difficulties in synthesizing suitably tethers
macrocyclic precursors and poor solubility of bulkier
macrocycles. So, we are interested to incorporate
macrocyclic motif into nanotubes framework. For the
construction of macrocycles as structural units, we
have selected pyromelliticdiimide moiety [11] since
this moiety has four connecting sites; two nitrogen
atoms for the construction of the macrocycles, and the
carbon atoms at the para-positions of the benzene
ring for the connection of the macrocycles. In addi-
tion to that, pyromelliticdiimide moiety can take the

conformation with a deep cavity, in which
π
-planes of
toluene was obtained by refluxing and distillation
over CaH2. Dry THF and Et2O were prepared accord-
ing to the literature [14]. The melting point of the
product was determined by BUCHI M-565 Melting
Point Apparatus. H and C NMR spectra were rec-
orded using a JEOL ECS-400, ECA-500 or a Bruker
AVANCE 600 spectrometer. Chemical shifts are re-
the benzene rings face to the center of the cavity. Py-
romelliticdiimides have electron affinity, thus the se-
lective inclusion of an electron-rich guest via a charge
transfer (CT) interaction is also desirable [2, 12]. Fur-
thermore, photo physical and redox properties of py-
romelliticdiimide are also interesting and the carbonyl
oxygen atoms of the moiety can serve as hydrogen
acceptor for the formation of supramolecules [13].
1
13
ported (δ-scale) relative to internal tetramethylsilane
(TMS). Mass spectra were recorded on a JEOL LMS-
HX-110 spectrometer. FAB MS spectra were meas-
ured with 3-nitrobenzyl alcohol (NBA) as the matrix.
The absorption spectra were recorded using JASCO
U-4000 spectrometer. Redox potentials were carried
out on a BAS-100B/W electrochemical analyzer. CV
measurements were performed using a cell equipped
with glassy carbon as the working electrode, platinum
wire as the counter electrode, and Ag/AgNO3 as the
reference electrode. All electrochemical measure-
ments were performed under an Ar atmosphere at
As a part of our continuing effort toward the synthesis
of the pyromelliticdiimide-based molecular tube, we
aimed to synthesize functionalized macrocycle 1.
Synthesis of nanotube can be done by connecting
suitable spacer molecules (Sonogashira coupling)
with the functionalized pyromelliticdiimide-based
macrocycles, followed by connecting the resultant
capsule with another macrocycle (Scheme 1). We now
report herein the synthesis of bromo-substituted mac-
rocycles as structural units of molecular tubes and
their properties, based on the electronic spectra and
redox potentials, structural optimization and confor-
mational behavior at variable temperature. The solu-
bility in common organic solvents for attachment of
hexyloxy groups to the xylyl moiety will also allow
the investigation of electroscopic and electrochemical
properties. The introduction of methylene linker be-
tween diimide and xylyl moiety should create electron
affinity for host-guest chemistry with electronically
richer molecules.
room temperature in o-dichlorobenzene solution (1 x
-4
10
M)
containing
0.10
M
tetra-n-
butylammoniumhexafluorophosphate as a supportin
electrolyte at a scan rate of 100 mV/s. The ferro-
cine/ferrocenium (Fc/Fc+) couple was used as an in-
ternal standard.
Scheme 2. Synthesis of Br4[2+2] MC 1.
Scheme 1. Synthetic strategy toward molecular cap-
sule and tube.
Preparation and characterization
Synthesis
of
1,4-bis(aminomethyl)-2,5-
Experimental
bis(hexyloxy)benzene 3: 1,4-Bis(hexyloxy)benzene
3(a): To a mixture of hydroquinone (66.0 g, 0.6 mol)
in degassed acetone (1200 mL), were added K2CO3
(248.8 g, 1.8 mol) and n-bromohexane (210.0 mL, 1.5
mol). The suspension was then stirred at 80 oC for 3d.
The reaction mixture was then allowed to attain the
Materials and methods
Chemicals were used without further purification un-
less otherwise, stated. Column chromatography was
carried out using Kanto Chemical silica gel 60N. Dry

room temperature and solid was filtered off and
washed several times with EtOAc. Solvent was re-
moved under reduced pressure in the rotary evapora-
tor and the crude product was poured into ice cold
water (500 mL), extracted with ethyl acetate. The or-
ganic extract was washed with saturated brine solu-
tion (200 mL). The organic layer was dried over
Na2SO4, and solvent was removed to get the crude
product as a brown solid. It was separated by column
chromatography on silica gel with hexane as eluent to
get the desired product, 1,4-Bis(hexyloxy)benzene
3(a) (155.0 g, 0.55 mol, 93%)asa colorless solid: mp
forC18H28O2Br2: C, 49. 56; H, 6.47. Found: C, 49.47;
H, 6.44.
1, 4-Dicyano-2, 5-bis(hexyloxy)benzene 3(c): A
mixture of 1,4-Dibromo-2,5-bis(hexyloxy)benzene
(109.0 g, 0.25 mol) and CuCN (89.5 g, 1.0 mol) in
o
DMF (800 mL) was heated at 150 C for 24 h. The
reaction mixture was then allowed to attain the room
temperature. Into that mixture was added slowly a
solution of FeCl3.6H2O (135.0 g, 0.5 mol) in 800 ml
1.6 N HCl and stirred at room temperature for 12 h in
a hood with very good ventilation. Solid was filtered
off and washed several times with water. Solid was
then taken in CH2Cl2 (500 mL) and was poured into
ice cold water (500 mL), extracted with CH2Cl2 (3 x
200 mL). The organic extract was washed with satu-
rated brine solution (200 mL). The organic layer was
dried over Na2SO4, and solvent was removed to get
the crude product as a brown solid. It was separated
by column chromatography on silica gel with CH2Cl2
as eluent to get the desired product, 1,4-Dicyano-2,5-
bis(hexyloxy)benzene 3(c) (66.7 g, 0.20 mol, 81%) as
41-42 °C; 1H NMR (CDCl3, 500 MHz)
δ 6.82 (s, 4H),
3.90 (t, J = 6.6 Hz, 4H), 1.75 (quint, J = 7.2 Hz, 4H),
1.45 (quint, J = 7.2 Hz, 4H), 1.33 (quint, J = 3.6 Hz,
13
8H), 0.90(t, J = 6.6 Hz, 6H); C NMR (CDCl3, 125
MHz)
δ 153.2, 115.4, 68.6, 31.6, 29.4, 25.7, 22.6,
14.1; HRMS (FAB-MS, NBA-positive) calcd. for
C18H30O2 (M+): 278.2246, found: 278.2254.
Anal.Calcd for C18H30O2: C, 77.65; H, 10.86. Found:
C, 76.90; H, 10.78.
1
1,4-Dibromo-2,5-bis(hexyloxy)benzene 3(b): To a
mixture of 1,4-Bis(hexyloxy)benzene (139.0 g, 0.5
mol) and I2 (1.27 g, 5.0 mmol) in CH2Cl2 (800 mL),
a colorless solid: mp 150-151 °C; H NMR (CDCl3,
500 MHz)
δ 7.13 (s, 2H), 4.03 (t, J = 6.6 Hz, 4H),
1.84 (quint, J = 7.2 Hz, 4H), 1.48 (quint, J = 7.2 Hz,
4H), 1.33 (quint, J = 3.5 Hz, 8H), 0.91 (t, J = 6.6 Hz,
6H); 13C NMR (CDCl3, 125 MHz)
δ 154.1, 116.8,
114.9, 106.9, 70.0, 31.3, 28.7, 25.4, 22.4, 14.0;
HRMS (FAB-MS, NBA-positive) calcd. for
C20H28O2N2 (M+): 328.2151, found: 328.2152.
Anal.Calcd forC20H28O2N2: C, 73. 14; H, 8.59; N,
8.53. Found: C, 72. 96; H, 8.53; N, 8.56.
o
was added Br2 (71.3 mL, 1.25 mol) at 0 C. The mix-
ture was then stirred at room temperature for 12 h.
Excess bromine in the reaction mixture was then de-
stroyed by slow addition of saturated aqueous solu-
tion of NaOH, until the red color disappears. The
mixture was then extracted with CH2Cl2 (3 x 200 mL).
The organic extract was washed with saturated brine
solution (200 mL). The organic layer was dried over
Na2SO4, and solvent was removed to get the crude
product as a white solid. It was separated by column
chromatography on silica gel with hexane as eluent to
1,4- Bis(aminomethyl)-2,5-bis(hexyloxy)benzene 3:
To
a
solution
of
1,4-Dicyano-2,5-
bis(hexyloxy)benzene(3.38 g, 0.01 mol) in dry tolu-
ene (100 mL) was added diisobutylaluminium hydride
(1.0 M toluene solution, 50 mL, 0.05 mol) at room
get
the
desired
product,
1,4-Dibromo-2,5-
bis(hexyloxy)benzene 3(b) (213.0 g, 0.49 mol,
1
o
98%)asa colorless solid: mp 61-62 °C; H NMR
temperature. The mixture was then heated at 100 C
(CDCl3, 500 MHz)
δ
7.08 (s, 2H), 3.94 (t, J = 6.6 Hz,
for 4 h. The reaction mixture was then allowed to at-
tain the room temperature and poured to an ice-cold
aqueous solution of KF (9.5 g, 0.25 mol). Solid was
filtered off and washed several times with Et2O. The
filtrate was extracted with Et2O (3 x 50 mL). The or-
ganic extract was washed with saturated brine solu-
tion (200 mL). The organic layer was dried over
Na2SO4, and solvent was removed to get the crude
4H), 1.79 (quint, J = 7.2 Hz, 4H), 1.48 (quint, J = 7.2
Hz, 4H), 1.33 (quint, J = 3.5 Hz, 8H), 0.90 (t, J = 6.6
Hz, 6H); 13C NMR (CDCl3, 125 MHz)
δ 149.9, 118.2,
111.0, 70.2, 31.4, 29.0, 25.6, 22.5, 14.0; HRMS
(FAB-MS, NBA-positive) calcd. for C18H28O2Br2
(M+): 434.0456, found: 434.0454. Anal.Calcd

product as a yellow liquid (3.22 g, 9.6 mmol, 96%). It
KMnO4 was destroyed by caution addition of ethanol
(60ml). The hot solution was filtered to separate
MnO2. The filtrate was acidified with aqueous HCl
(5N). After solvent was removed under reduced pres-
sure, colorless compound obtained. Acetone was add-
ed to this solution and stirred for 40 min. It was fil-
tered to remove NaCl and filtrate was concentrated
under reduced pressure to obtain 20g (47%) product.
The colorless powder from acetone .13C-NMR (Ace-
was then used for next cyclization reaction without
1
further purification. H NMR (CDCl3, 500 MHz)
δ
6.77 (s, 2H), 3.95 (t, J = 6.6 Hz, 4H), 3.78 (s, 4H),
1.78 (quint, J = 7.2 Hz, 4H), 1.61 (bs, 4H), 1.46 (quint,
J = 7.2 Hz, 4H), 1.34 (quint, J = 3.5 Hz, 8H), 0.90 (t,
J = 7.2 Hz, 6H); 13C NMR (CDCl3, 125 MHz)
δ 150.5,
130.9, 112.4, 68.5, 42.7, 31.5, 29.4, 25.8, 22.5, 13.9;
HRMS (FAB-MS, NBA-positive) calcd. for
C20H36O2N2 (M+): 336.2777, found: 336.2777.
tone-d6, 75 MHz)
δ = 116.41m (CBr), 138.39 (C-
COOH), 165.87 (C=O).
3, 6-Dibromodurene 4 (a): A solution of durene 12
(26.8 g, 200.0 mmol) and I2 (1.0 g, 4.0 mmol) in
CH2Cl2 (350 mL) were added Br2 (26 mL, 500.0
mmol) in CH2Cl2 (50 mL) at room temperature. After
adding the Br2, the reaction mixture was refluxed for
overnight. After cooling to room temperature, the
mixture was quenched saturated aqueous NaHCO3 to
color disappearing, and the resultant mixture was ex-
tracted with CH2Cl2 (20 mL × 3) and the combined
extracts were dried over sodium sulfate and filtered.
The solutions evaporated to leave a residue, which
was recrystallized from CH2Cl2-methanol (5:1) to
give 3, 6-Dibromodurenecolorless needle crystals;
yield 51.4 g (88%); mp 208-209 oC; 1H NMR (CDCl3,
3, 6-Dibromopyromellitic dianhydride 4: Though
preparation of this compound is already known, we
here describe our own procedure. A suspension of 3,
6-dibromopyromellitic acid (4.12 g, 0.01 mol) in
Ac2O (19 mL) was heated at 100 oC for 2 h. The reac-
tion mixture was allowed to attain the room tempera-
ture slowly and further cooled by dipping the flask in
ice. The precipitate was collected by filtration and
was washed with dry Et2O (3 x 10 ml). The solid was
o
dried under reduced pressure at 90 C to obtain crude
product as a white solid (2.63 g, 7.0 mmol, 70%). It
was then used for next cyclization reaction without
13
further purification: mp > 300 °C; C NMR (CDCl3,
600 MHz)
MHz)
δ
2.48 (s, 12H); 13C NMR (CDCl3, 150
125 MHz)
δ 158.5, 138.3, 116.8; HRMS (FAB-MS,
NBA-positive) calcd. for C10H6Br2 (M+): 373.8062,
found: 373.8047. Anal.Calcd for C10H6Br2: C, 31. 95.
Found: C, 31.91.
δ
22.27, 128.13, 135.03; FAB-MS (NBA, posi-
tive): m/zcalcd for C10H12Br2N2 292.00 (M+), found
292.01 (M+), peak ration of 79Br and 81Br isotopes (1 :
1).
Tetrabromo [2+2] pyromelliticdiimide-based mac-
rocycle 1: To 100 mL dry THF were simultaneously
added a solution of the diamine 3 (3.38 g, 0.01 mol)
and the dianhydride 4 (3.76 g, 0.01 mol) both in dry
THF (200 mL) over a period of 4 h at 60 oC. The mix-
Synthesis
of
3,6-dibromo-1,2,4,5-
Benzenetetracarboxylic acid 4 (b): To a 3L four
necked round bottom flask fitted with refluxed con-
denser and mechanical stirrer, were added 1,4-
dibromotetramethylbenzene (30g, 102.73 mmol),
pridine (1250 ml) and water (180 ml). The reaction
mixture was heated at 100°C with constant stirring.
To this reaction mixture, KMnO4 (82.5g, 522.15
mmol) was added in small aliquots in 45 min. After
complete addition the reaction mixture was refluxed
for next 5h. The hot solution was filtered to remove
MnO2 from reaction mixture and the solvent concen-
tration of at vacuo. To the residual solid were added
water (1500 ml) and NaOH (70 g). The reaction mix-
ture was stirred and heated at 100°C. KMnO4 (82.5 g,
522.15 mmol) was added in small aliquots in 1h. The
reaction mixture was refluxed for next 5h. Excess of
o
ture was then heated at 70 C for 24 h. The reaction
mixture was then allowed to attain room temperature
and solvent was completely removed under reduced
pressure to get a yellow color mixture of amic acids.
The solid was taken in a mixture of Ac2O (50.0 ml)
o
and NaOAc (8.2 g, 0.1 mol) and heated at 110 C for
2 h. The reaction mixture was then allowed to attain
the room temperature and poured to water. The re-
sultant solid was filtered off and washed with water
and then air-dried. The collected precipitate was ex-
tracted continuously with CH2Cl2 (400 mL). The sol-
vent was removed to get the crude product as red sol-
id. It was separated by column chromatography on

silica gel with hexane-EtOAc (20:1, v/v) as the eluent
and followed by separation with recyclic HPLC (GPC
column, CHCl3) to obtain the [2+2] macrocycle 1
(0.20 g, 0.148 mmol, 4.5%) as a red solid: mp 214-
Figure 1. Possible syn - and anti-conformers of Br4
[2+2] MC 1 in CD2Cl2 solution
1
215 °C. H NMR (CDCl3, 600 MHz) δ 6.76 (s, 2H),
4.78 (s, 2H), 3.78 (t, J = 6.3 Hz, 4H), 1.72 (quint, J =
7.2 Hz, 4H), 1.42 (quint, J = 7.2 Hz, 4H), 1.28 (quint,
J = 3.5 Hz, 8H), 0.84 (t, J = 7.2 Hz, 6H); 13C NMR
(CDCl3, 150 MHz) δ 161.4, 149.6, 135.2, 123.8,
114.9, 113.4, 68.5, 36.3, 31.0, 30.4, 28.6, 24.7, 13.1;
HRMS (FAB-MS, NBA-positive) calcd. for
C60H64O12N4 Br4 (M+): 1348.1254, found: 1348.1329.
Figure 2. Optimized structures of 1 (syn-1, left
and anti-1, right) by B3LYP/6-31G*(d) level of
MO calculations
Computational methods
All calculations were done with the Gaussian
software package. After computing the optimized
structure, the molecular orbital calculations were
conducted using B3LYP/6-31G (d) level of theory
for analyzing the HOMO–LUMO energies. The
HF/6-31G* minimization of [2+2] cp was carried
out as well. The distances between two benzene
rings and two pyromelliticdiimide motifs are
9.859A° and 7.026 A°, respectively. The cavity
size in [2+2] macrocycle, which is suitable for
inclusion of pyrrol and anisole, toluene, xylene
and so on as per6-31G minimization by using
Gaussian program.
Figure 3. HOMO (left) and LUMO (right) orbitals of
the macrocycle 1 by B3LYP/6-31G*(d) level of theo-
ry
9.818 A°
Results and Discussion
7.034 A°
1
The temperature-dependent H NMR spectra of 1 in
Front view
Side view
Figure 4.HF/6-31G*optimized structure of [2+2] MC
1
CD2Cl2 show no shifts but a broadening of some sig-
nals until -20 °C; however, below -30 °C, a slight
shift and coalescence of the signals were observed.
These observations are presumably due to the pres-
ence of two conformers of 1 (Figure 1). The MO cal-
culations (B3LYP/6-31G (d)) of 1 suggested that anti-
1 and syn-1 have similar stability (Figure 2). The
HF/6-31G optimized structure [2+2] macrocycle
along with cavity size were calculated to understand
the types of guest molecules it can accommodate
(Figure 4).
VT 1H NMR spectrum behavior of Br4 [2+2] MC 1 in
CD2Cl2 and the existence of two conformers along
with the comparable thermodynamic stability of the
two isomers is expected. Two conformational iso-
mers, syn and anti, are possible (Figure 1) in relation
to the overlapping mode of the facing
bis(hexyloxy)benzene moieties. The benzylic proton
signal at 4.86 ppm (Hg+Hg’) appears as a broad singlet
at room temperature in CD2Cl2, indicating free rota-
tion of the bis(hexyloxy)benzene moieties between
synand anti-conformers (Figure 5). The sharp singlet
of the aromatic proton signals (Hh and Hh’) of the
bis(hexyloxy)benzene moiety of 1 also supports rapid
rotation of the bis(hexyloxy)benzene moieties at room

temperature (Figure 6). The singlet due to the ben-
zylic proton at 4.86 ppm (Hg+Hg’) at room tempera-
ture significantly broadens as the temperature lowers,
and then the signal begins to split and becomes two
π-π* transitions at the pyromelliticdiimide moiety.
For long axis polarized π-π* transition, the corre-
sponding absorption coefficient 0.68) is slightly high-
er than that of the short axis polarized π-π* transition
(0.63). The UV/Vis spectrum over 380 nm (Figure 7)
shows the tail of the longer absorption band in 1. This
tail is assigned for out-of-plane polarized n-π* transi-
tion (390-410 nm), enter into the visible region (> 400
nm). This may be due to a weak intramolecular CT
interaction of electron-withdrawing pyromellit-
icdiimide moiety with the electron-donating hex-
yloxy-substituted xylyl moiety. The intramolecular
CT interaction is also supported by MO calculations
(B3LYP/6-31G*). The calculation showed the HO-
MO and LUMO orbitals of 1 to be localized in the
xylyl andpyromelliticdiimide moieties, respectively,
through the methylene linker (Figure 3).
o
double doublets below -70 C, indicating that the ro-
tation of the bis(hexyloxy)benzene moiety is frozen.
As a result, two conformers exist and the comparable
thermodynamic stability of the two isomers is ex-
pected.
Table 1
UV/Vis absorption bands of the macrocycles 1 and
their parent compounds in CHCl3
λmax (logε)
1
Figure 5. The temperature-dependent H NMR spec-
tra (recorded at rt to -90oC) of Br4 [2+2] MC 1 in
Br4[2+2] 1
303.5 (0.88), 352.5 (0.63), 367.5 (0.68)
307 (0.79)
CD2Cl2
[2+2]
Figure 7. UV/Vis absorption bands of MC 1 and their
parent compounds in CHCl3
The cyclic voltammetric traces of the macrocycle 1
were recorded in o-dichlorobenzene solution with
tetra-n-butylammonium hexafluorophosphate as sup-
porting electrolyte (Figure 8). The macrocycle 1 ex-
hibited first two-electron reduction process, followed
by the second two-electron reduction. Each reduction
process was split into two waves (Table 2). Pyro-
melliticdiimide moiety generally shows two reversi-
ble one electron reductions. The first and second
waves are due to radical anion and dianion species,
respectively. The splitting of the first two-electron
reduction is due to electronic repulsion between radi-
1
Figure 6. The temperature-dependent H NMR spec-
tra of the benzylic proton signal of Br4 [2+2] MC 1 in
CD2Cl2
UV/Vis spectrum of the macrocycle 1 was measured
in CHCl3. The spectrum shows three absorption max-
ima (λmax) over 250 nm [303.5 (logε) 0.88), 352.5
(logε) 0.63), 367.5 nm (logε) 0.68)] (Figure 7 and
Table 1). The absorption bands at 352.5 and 367.5 nm
were presumably due to short and long axis polarized

cal anion and neutral species and then second two-
electron reduction splitting is due to electronic repul-
sion between radical anion and dianion spe-
cies.Surprisingly, long distance between two pyro-
melliticdiimide π-faces (ca. 7.1 Å) has not make bar-
rier for making weak electronic interaction between
the radical anion and neutral diimide moiety. The first
radical anion for one pyromelliticdiimide moiety in-
creases the electron density on the other diimide moi-
ety. So, another one-electron reduction of the radical
anion species become harder compare to the first one-
electron reduction.
ties, respectively. The UV/Vis spectrum revealed the
presence of a weak intramolecular CT interaction be-
tween electron-withdrawing pyromelliticdiimidemoie-
ty with the electron-donating hexyloxy-substituted
xylyl moiety. The cyclic voltammetric measurement
shows two two-electron reduction processes. Moreo-
1
ver, VT H NMR spectrum behavior of Br4 [2+2] MC
2 in CD2Cl2 gives us idea about not only the existence
of two conformers but also the comparable thermody-
namic stability of the two isomers at lower tempera-
ture. These new findings could be applied for the con-
formational study, free and restricted rotation in a
molecule when the NMR- temperature is varied. Our
study towards covalently bonded organic nanotubes
based on pyromelliticdiimidecyclophane is in pro-
gress and the results will be reported soon.
Table 2
Reduction potentials for Br4[2+2]MC in o-di
chlorobenzene (0.1 mM in o-dichlorobenzene; 0.1 M
nBu4NPF6 supporting electrolyte; scan rate 100 mVs-
1
)
Acknowledgment
We are sincerely thankful for the financial support
from the G-COE Program, Science for Future Molec-
ular Systems, Kyushu University, Japan. The compu-
tations were carried out using the computer facilities
at the Research Institute for Information Technology,
Kyushu University. Moreover, Md. Ershad Halim
1Ered
2Ered
Macrocycle
Br4[2+2]MC
-1.38, -1.51 -2.16, -2.31
is extremely grateful to his PhD supervisor, Pro-
fessor TeruoShinmyozu, for his kind support and
cordial co-operation in every respect until com-
pletion of his PhD research.
References
Figure 8: Cyclic voltammetric traces and reversible
sequential reduction processes of the Br4[2+2]MC in
o- dichlorobenzene (0.1 mM in o-dichlorobenzene;
0.1 M nBu4NPF6 supporting electrolyte; scan rate
100 mVs-1)
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Highlights
•
•
•
•
•
Bromo-substituted pyromelliticdiimide-based [2+2] macrocycle has been synthesized.
The macrocycle structure has been elucidated by NMR, HRMS and elemental analyses.
The conformational behavior has been studied by variable temperature NMR.
The macrocycle structure has been optimized by computational method.
HOMO and LUMO orbitals were calculated by B3LYP/6-31G*(d) level of theory.

The authors have no conflict of interest regarding this research.