Cyclodextrin-covered organic microrods and microsheets derived from supramolecular self assembly of 2,4-dihydroxyazobenzene and 4-hydroxyazobenzene inclusion complexes

Article abstract of DOI:10.1246/bcsj.20130255

The self assembly behavior of 2,4-dihydroxyazobenzene (DHAB) and 4-hydroxyazobenzene (HAB) inclusion complexes were studied by SEM and TEM. Formation of inclusion complexes were characterized by FT-IR, DSC, PXRD, ¹HNMR, UVvisible, fluorescence,

Full text of DOI:10.1246/bcsj.20130255

© 2013 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 87, No. 2, 283-293 (2014)
283
Cyclodextrin-Covered Organic Microrods and Microsheets Derived
from Supramolecular Self Assembly of 2,4-Dihydroxyazobenzene
and 4-Hydroxyazobenzene Inclusion Complexes
Govindaraj Venkatesh, Jayachandran Saravanan, and Narayanasamy Rajendiran*
Department of Chemistry, Annamalai University, Annamalainagar-608002, Tamilnadu, India
Received September 13, 2013; E-mail: drrajendiran@rediffmail.com
The self assembly behavior of 2,4-dihydroxyazobenzene (DHAB) and 4-hydroxyazobenzene (HAB) inclusion
complexes were studied by SEM and TEM. Formation of inclusion complexes were characterized by FT-IR, DSC, PXRD,
1H NMR, UV-visible, fluorescence, time-resolved fluorescence, and molecular modeling techniques. Micrometer-sized
rods were observed in DHAB:α-cyclodextrin (α-CD) and DHAB:β-cyclodextrin (β-CD) inclusion complexes while
microsheets were obtained in HAB/α-CD and HAB/β-CD complexes. In DHAB/CD and HAB/CD individual “barrel”
type (head to head arrangement) of 2:2 inclusion complexes form microrods and microsheets through intermolecular
hydrogen bonding between the neighboring complexes. Molecular modeling calculations suggest that (i) DHAB and
HAB form stable complexes with α-CD and β-CD, (ii) intermolecular hydrogen bonds form between the guests and CDs.
Investigations of energetic, thermodynamic, and electronic properties by PM3 method confirmed the stability of the
inclusion complex.
Developments in nano- and microscale supramolecular self
assembly materials have become great initiation for new
technology with potential applications to photoelectric con-
version devices,1 separation and filtration media,2 nonlinear
optics,3 single molecular devices,4 and organic memory
devices.5 Different morphologies of supramolecular materials
with nano- and microstructure have been reported from a
wide range of guest and different cyclodextrins (CDs).6-8 The
homologous series of cyclic oligomers (CDs) consists of 6 to 8
α-1,4-linked D-(+)-glucopyranose residues. In particular 6, 7,
and 8 member homologous molecules named α-, β-, and γ-CD
have been a focus for important supramolecular host mate-
rials. Among them β-CD is a familiar interesting host material
for numerous applications in the area of catalysis, molecular
recognition, increasing solubility, bioavailability, drug deliv-
ery, and environmental protection.9 Due to their wide applica-
tions in a vast area, a number of studies have been undertaken
to understand the nature of CD inclusion complexes.10
one-step synthesis of carbon nanorod-like structure from direct
pyrolysis of α-CD.14 From the above reports it has been shown
that the construction of supramolecular architecture can be
achieved by interfacial supramolecular interactions. All these
recent studies establish that CDs formed supramolecular archi-
tecture through host-guest aggregates.11-14 With these reports
on hand, the intension of our present work is to characterize
the formation of nanorod aggregate and to find the role of
the dimension of CDs in the self assembly. We have already
reported formation of inclusion complexes of 2,4-dihydroxy-
azobenzene (DHAB) and 4-hydroxyazobenzene (HAB) with
β-CD in aqueous solution,15 but in this paper we analyze
the micro self assembly of α-CD and β-CD induced by DHAB
1
and HAB using SEM, TEM, FT-IR, DSC, PXRD, H NMR,
absorption, fluorescence, fluorescence decay spectroscopy, and
molecular modeling.
Results and Discussion
It is well known that CDs self aggregate through inter-
molecular hydrogen bonding to form nano- or microstructures.
Absorption, fluorescence, and infrared spectra (FT-IR), differ-
ential scanning colorimetriy (DSC), powder X-ray diffraction
(PXRD), scanning electron microscopy (SEM), and trans-
mission electron microscopy (TEM) are general tools exploited
to characterize these nano- or microstructures. Many reports
have demonstrated these supramolecular architectures through
involvement of appropriate guest molecules.11-13 Chen et al.
reported carbonaceous nanofiber membrane functionalized
by β-CD for molecular filtration.11 Mandal et al. reported that
γ-CD host-guest complex nanotubes aggregate by fluorescence
correlation spectroscopy.12 Sun et al. reported controlled trans-
formation from nanorods to vesicles induce by cyclomalto-
heptose.13 St. Dennis et al. reported their results in the simple,
Nanomaterial Observations. SEM images clearly show
the different morphologies of the inclusion complex nano-
materials: (i) α-CD has prismatic, (ii) β-CD has sheeted and
platted form, and (iii) DHAB and HAB dyes have uniform
cubic crystal shape and agglomerated irregular shape, respec-
tively. As seen from the SEM images, DHAB/α-CD (Figure 1)
inclusion complex present in agglomerated uniform crystal
shape, whereas DHAB/β-CD (Figure 1) presents in plate form
and this morphology differs from that of the pure DHAB
compound. Similarly, HAB/α-CD and HAB/β-CD inclusion
complexes were also present in different shapes than that of
pure HAB. Modification of these morphologies can be taken as
a proof for the formation of inclusion complex nanomaterials.
TEM images of HAB/α-CD, HAB/β-CD, DHAB/α-CD,
and DHAB/β-CD are shown in Figure 2. The thickness for
Published on the web November 27, 2013; doi:10.1246/bcsj.20130255

284
Bull. Chem. Soc. Jpn. Vol. 87, No. 2 (2014)
Cyclodextrin-Covered Azobenzene Microrods and Microsheets
(a)
(b)
(d)
(c)
(e)
(g)
(h)
(f)
Figure 1. SEM photographs of (a) α-CD, (b) β-CD, (c) DHAB, (d) DHAB/α-CD, (e) DHAB/β-CD, (f) DHAB, (g) HAB/α-CD,
and (h) HAB/β-CD inclusion complex.
DHAB/β-CD (Figures 2c and 2d) microrod was approximately
in the range of ca. 1.46 ¯m and height is about ca. 9.8 ¯m, we
observed that the microrod has uniform surface and the corner
was smoothly ended, whereas, DHAB/α-CD (Figures 2a
and 2b) microstructure has different morphology i.e., many
microrod-agglomerated structures. The typical average dimen-
sion of DHAB/α-CD microrods is ca. 360 nm width and
ca. 3 ¯m length. We already reported that both DHAB and
HAB bound with the CD cavity to form a 1:1 (host:guest)
inclusion complexes then these 1:1 inclusion complexes are
further developed into a higher order 2:2 (host:guest) “barrel”
type of inclusion complexes.15 Both DHAB/α-CD and DHAB/
β-CD microstructures suggest that the individual “barrel” type
(head-to-head arrangement) of 2:2 inclusion complexes form
various dimensions of microrods based on supramolecular
pseudorotaxane mechanism. The considerable torsion angle
flexibility of the azo functional group (-N=N-) lead to both
azo dye molecules (DHAB and HAB) being present in the
inside of the CD cavity forming 2:2 inclusion complexes.
Liu et al. reported that a linear structure was attained from 4-
aminoazobenzene with β-CD complex by intra- and interdimer
hydrogen bonding. Intradimer π-π interactions and wave-type
structure were attained respectively.16
α-CD and DHAB/β-CD) different template of microrod mak-
ing possible formation of a single line head to head arrange-
ment 2:2 inclusion complex (Figure 3d). The small individual
nanorods are secondary self-assembled through intermolecular
hydrogen bonds by CD hydroxy groups, these types of higher
stoichiometry (2:2) inclusion complexes have the tendency to
form linear CD pseduopolyrotaxanes.17-19 Further, the key to
the exploitation of the microscale rod structure is believed to be
related to the various forces and hydrogen bonding. According
to previous reports8,10 the self aggregation proceeds mainly
from the contribution of hydrogen bonding or electrostatic
interactions, however these reports indicates that the aggrega-
tion between CD inclusion complexes is not only hydrogen
bonding and nonbonding van der Waals forces are also respon-
sible for the formation of microrod structures. Therefore the
above two forces beyond doubt exist between the DHAB/CD
inclusion complexes and the resultant microrods are formed.
HAB/α-CD (Figures 2e and 2f) complexes form a self-
assembled monolayer whereas HAB/β-CD (Figures 2g and 2h)
forms a thick microsheet. HAB/β-CD complexes show the
strong agreement between the center and margin, which is a
distinctive typical microsheet structure (Figures 2g and 2h).
Since CDs having two types of hydroxy groups on the end of
the cavities, one is a primary hydroxy group (tail) and another
one is a secondary hydroxy group (head). Hence, there is a
possibility of three types of arrangement modes which CDs
A proposed pseudorotaxane mechanism shown in Figure 3.
There are no massive end groups in either complex (2:2 barrel
type) of barrel arrangements are self assembled into a (DHAB/

G. Venkatesh et al.
Bull. Chem. Soc. Jpn. Vol. 87, No. 2 (2014)
285
(a)
(b)
(c)
0.5 μm
0.5 μm
1 μm
3 μm
2 μm
1 μm
1 μm
(a)
(c)
(e)
(b)
(d)
(d)
(e)
Figure 3. Proposed mechanism for the formation of micro-
rod from inclusion complex. (a) DHAB, (b) CD, (c) 2:2
barrel type inclusion complexes through head-to-head
arrangement, (d) 2:2 inclusion complexes secondary self
assembled to form smaller nanorod, (e) individual nano-
rods are again self assembled with another nanorods
through hydrogen bonding to form cluster of microrod.
(f)
CDs. The aromatic C-H stretching of DHAB appeared at 2920
cm moved to a higher frequency in the DHAB/α-CD (2926
cm¹1) and DHAB/β-CD (2925 cm¹1) complexes. Azo group
(-N=N-) stretching appeared at 1455 cm moved to lower
frequency at 1420 cm in the inclusion complex nanomate-
rials. The FT-IR spectrum of HAB shows the absorption
frequency at 3397 cm for the -OH stretching and 678 cm
¹1
0.2 μm
(g)
(h)
¹1
¹1
Figure 2. TEM images of (a and b) DHAB/α-CD, (c and d)
DHAB/β-CD, (e and f) HAB/α-CD, and (g and h) HAB/
β-CD inclusion complex.
¹1
¹1
frequency due to -OH deformation vibration (Figure S1d). The
¹1
can have interacted with each others, namely head-to-head, tail-
to-tail, and head-to-tail.18,19 From the above argument mono-
layer architecture formed by HAB/α-CD, which related to the
weaker supramolecular interaction, whereas microsheets were
formed by HAB/β-CD due to the stronger van der Waals force
and hydrogen bond between neighboring complex.
frequency at 1452 cm can be attributed to N=N stretching
vibration. The -OH stretching appeared as a single broad band
¹1
¹1
at 3377 cm in HAB/α-CD (Figure S1e) and 3382 cm in
HAB/β-CD (Figure S1f) complex nanomaterials. This result
suggests that the formation of a monomeric dispersion of the
HAB molecule is a consequence of the interaction with CDs,
which can result from inclusion of HAB in the hydrophobic
cavities. Further, DHAB and HAB hydroxy group stretching
band shapes changed in the corresponding inclusion complex
nanomaterials spectra indicating that -OH of these molecules
interacts strongly with CDs.
DSC Thermograms. DSC profiles of α-CD, β-CD, DHAB,
HAB, and the corresponding inclusion complexes are presented
in Figure 4. The thermal curve of DHAB shows a sharp endo-
thermic peak at 170.6 °C, which corresponds to the melting
point of the DHAB. DSC curves of β-CD show a broad endo-
thermic peak at 128.6 °C and α-CD shows three endothermic
peaks at 79.2, 109.1, and 137.5 °C, these endothermic peaks
Infrared Spectral Studies. FT-IR is useful to identify
host-guest interactions, Figures S1 and S2 shows the FT-IR
spectra of the inclusion complex nanomaterials, physical mix-
tures, and pure azo dyes (DHAB and HAB). The above figures
indicate that the physical mixture of the guests and hosts are
different from the inclusion complex nanomaterials. For pure
¹1
DHAB, the absorption peak at 3380 cm can be attributed to
the -OH stretching vibration and 628 cm¹1 is reflected -OH out
plane deformation. A substantial increase in intensity as well as
broadening of the -OH band was noted in the DHAB/α-CD
(Figure S1b) and DHAB/β-CD (Figure S1c) complexes sug-
gest that the -OH group of the DHAB molecule interacts with

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Bull. Chem. Soc. Jpn. Vol. 87, No. 2 (2014)
Cyclodextrin-Covered Azobenzene Microrods and Microsheets
(h)
(g)
(a)
99.8 °C
109.7 °C
93.8 °C
(b)
(c)
(f)
(e)
(d)
152.7 °C
118.5 °C
(d)
(e)
98.6 °C
(c)
170.6 °C
(b)
(a)
128.6 °C
109.1 °C
(f)
79.2 °C
137.5 °C
2θ /degree
40
60
80
100
120 140 160 180
200 220
Temperature/°C
Figure 5. PXRD patterns of (a) DHAB, (b) DHAB/α-CD,
(c) DHAB/β-CD, (d) HAB, (e) HAB/α-CD, and (f) HAB/
β-CD inclusion complex.
Figure 4. DSC thermograms of (a) α-CD, (b) β-CD,
(c) DHAB, (d) DHAB/α-CD, (e) DHAB/β-CD, (f) HAB,
(g) HAB/α-CD, and (h) HAB/β-CD inclusion complexes.
are attributed to crystal water loss from CDs. A broader endo-
thermic effect was recorded for α-CD, β-CD, and respective
inclusion complex nanomaterials as a consequence of water loss
from the CDs. When compared with isolated DHAB and CDs,
DHAB/CD (Figures 4d and 4e) inclusion complexes did not
show the entire distinguishable peak which indicates the forma-
tion of inclusion complex nanomaterials. The DSC thermogram
of DHAB/CD complexes did not show peaks corresponding
to the isolated DHAB and CD, instead new peaks appeared at
98.6 and 118.5 °C for DHAB/α-CD and DHAB/β-CD complex
nanometers respectively. In the case of HAB/α-CD (Figure 4g)
and HAB/β-CD (Figure 4h) the absence of the characteristic
HAB endothermic peak at 152.7 °C, in the thermograms, indi-
cates trapping of HAB in the CDs cavities. The disappearances
of endothermic peaks of DHAB and HAB in the DSC curves
of all the respective inclusion complexes give strong evidence
for the formation of inclusion complex nanomaterials.20
Powder X-ray Diffractograms. Powder X-ray diffrac-
tometry (PXRD) is a useful method for detection of CD com-
plexation in powder or microcrystalline states. The diffraction
patterns of the complex nanomaterials are distinct from that of
the pure components (free guest and CD) is taken as evidence
for the inclusion complex formation.21 As shown in Figure 5,
PXRD patterns of DHAB show intense, sharp peaks that prove
the crystalline nature of the compounds (Figure 5a). DHAB
had strong crystallinity peaks at 2ª of 13.8, 17.1, 24.8, and
26.1° and several minor peaks at 2ª of 14.1, 19.8, 20.9,
28.6, and 32.1°. On the other hand, DHAB/α-CD (Figure 5b)
and DHAB/β-CD (Figure 5c) inclusion complex patterns are
clearly distinct diffraction patterns when compared to free
DHAB (Figure 5a) suggesting the formation of inclusion com-
plex nanomaterials. Moreover many sharp peaks appeared in
the range of 10 to 25°, confirming the formation of DHAB/CD
inclusion complex nanomaterials. In the case of HAB, major
intense diffraction peaks appear at 2ª of 11.8, 23.6, 25.2, and
27.8° is shown in Figure 5d. Many of these peak intensities are
dramatically increased and new peaks appeared in the HAB/α-
CD (Figure 5e) and HAB/β-CD (Figure 5f) inclusion complex
diffraction patterns. The above results are taken as evidence for
the formation of HAB/CD inclusion complex nanomaterials.
Overall, the variation in the peak intensities of the DHAB/CD
and HAB/CD complex diffraction patterns from the corre-
sponding free molecule diffraction patterns suggest that new
complex nanomaterials were formed.
Proton Magnetic Resonance Spectral Studies. Generally,
the chemical shift values of the guest protons tend to show
appreciable changes if the guest molecules are included in the
1
CDs cavities.22 Hence, H NMR spectra of DHAB and HAB
complexes are acquired at 25 °C in DMSO-d6 and compared
with those of pure compounds, which are shown in Figures S3
to S5. As can be seen from Table S1, the ¤ value of Ha and Hb
are the hydroxy protons of DHAB, shifts up field in α-CD

G. Venkatesh et al.
Bull. Chem. Soc. Jpn. Vol. 87, No. 2 (2014)
287
1.00
800
600
800
0.46
Abs
DHAB/α-CD
DHAB/α-CD
If
400
0.38
450 nm
0
7
1
380nm
10
0
5
10
15
7
1
[α-CD] × 10-3/M
0.30
0.50
0
5
15
400
[α-CD] × 10-3/M
200
0
0
200
300
Wavelength/nm
400
500
300
350
400
450
500
550
600
Wavelength/nm
0.75
0.50
600
450
046
Abs
0.38
600
If
HAB/α-CD
HAB/α-CD
300
7
1
340nm
10
450 nm
10
0
0.30
0
5
15
7
[α-CD] × 10-3/M
0
5
15
[α-CD] × 10-3/M
300
0.25
0
1
150
0
200
275
350
425
Wavelength/nm
300
350
400
450
500
550
600
Figure 6. Absorbance spectra of DHAB and HAB in differ-
ent α-CD concentrations (M): (1) 0, (2) 0.001, (3) 0.002,
(4) 0.004, (5) 0.006, (6) 0.008, and (7) 0.01. Insert figure
absorbance vs. α-CD concentrations.
Wavelength/nm
Figure 7. Fluorescence spectra of DHAB and HAB in
different α-CD concentrations (M): (1) 0, (2) 0.001, (3)
0.002, (4) 0.004, (5) 0.006, (6) 0.008, and (7) 0.01. Insert
figure fluorescence intensity vs. α-CD concentrations.
complex and shifts down field in β-CD complex. The aromatic
protons of the DHAB molecules (Hc, Hd, He, and Hf ) shift up
field as compared with the corresponding free DHAB. These
results indicate that these above aromatic protons interact
with CD cavity protons. Further, considering the higher chem-
ical shift of the Hf protons (DHAB/α-CD = ¹0.024 ppm and
DHAB/β-CD = ¹0.026 ppm) suggests that the Hf protons of
the DHAB molecule have more interaction with the CDs. As
can be seen from the Table S1 the chemical shift data for the
inclusion complex nanomaterials was different from the free
compound. In particular, the resonance of the Ha (-OH) protons
of HAB molecule showed remarkable up field shift in the
HAB/α-CD = ¹0.078 ppm and HAB/β-CD = ¹0.089 ppm
inclusion complex nanomaterials. This higher chemical shift
value of the Ha proton of the HAB indicates that this proton
may be hydrogen bonded with the CD hydroxy protons. In
HAB molecule, the aromatic ring protons Hb and Hd are up
field shifted in the complex, which suggests that the aromatic
ring is shielded largely in the complex and it must penetrate
deeply into the CDs cavities.
same indicating that both azo molecules were present in the CD
cavities in solution without decomposition due to the formation
of inclusion complex. The absorbance of DHAB and HAB
increases in both CDs solutions indicating that the encapsula-
tion of these molecules into the CD cavities. Figures 7 and S7
represent the fluorescence spectra of DHAB and HAB in
different concentrations of α-CD and β-CD respectively. In
both CD solutions, the fluorescence spectra of the above guest
molecules are more sensitive than the absorption spectra. As
the CD concentrations increased, the emission maxima of both
hydroxy azo compounds were largely red shifted from 430 to
460 nm for DHAB and 390 to 450 nm for HAB. Such a change
in the emission maxima of both molecules caused by the
introduction of both CD indicates the formation of an inclusion
complex nanomaterials. The above unusual red shift emission
maxima due to the formation of 2:2 inclusion complexes were
already explained in our earlier reports.15,23-26
To determine the stoichiometry of the inclusion complexes,
a Job’s method27 for the absorption/fluorescence was applied
keeping the sum of initial concentration of the guests and CDs
constant and the molar ratio of CD changing from 0 to 1. The
absorption/fluorescence intensity of the guest in the absence
of (I0) and presence of CD (I) were determined respectively.
A plot of (A ¹ A0)/A0 and (I ¹ I0)/I0 versus the molar fraction
of CDs were provided in Figure 8. It shows that, (A ¹ A0)/A0
and (I ¹ I0)/I0 values versus molar fraction gives the straight
Absorption and Fluorescence Spectral Studies. Figures 6
and S6 depicts the absorption spectral data of DHAB, HAB,
and AB in aqueous solution containing different concentrations
of α-CD and β-CD respectively. The absorption spectrum of
DHAB (-abs ca. 382, 249, 220 nm to 379, 256, 226 nm) and
HAB molecules (-abs ca. 342, 228 nm to 340, 240 nm) shows a
slight blue shift with increasing concentration of both CDs. The
absorbance of CD solutions recorded after 12 h remains the

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Cyclodextrin-Covered Azobenzene Microrods and Microsheets
line, indicating a 1:1 stoichiometry of the azodyes with CDs
in the inclusion complex.
profiles for DHAB and HAB molecules are shown in Figure S8
and the fitted lifetime values are displayed in Table 1. The decay
profile for the DHAB and HAB molecules in water shows a
two exponential decay, but in the CD solutions it fits with three
exponentials. From Table 1 it can be observed that in CD
solution DHAB and HAB molecules show higher lifetime than
compared with water medium. This suggests that fluorescence
emission of these azo molecules was stabilized by both CDs
cavities. For example, in water, DHAB is found to be 1.22 ns
is less than that of DHAB/α-CD (1.85 ns) and DHAB/β-CD
(3.01 ns) inclusion complexes. The fluorescence decay profile
of DHAB and HAB are shown triexponential fit, indicates that
three different species were present in solutions.
Fluorescence Lifetime. In order to further characterize
the properties of inclusion complex nanomaterials of DHAB
and HAB, we measured fluorescence lifetimes of the above
fluorophores in water as well as in 0.01 M α-CD and β-CD. The
fluorescence decay profile for the DHAB and HAB molecules
were measured in the excitation wavelength of 360 and 340
nm respectively. Multiexponential decays were significant for
organic compounds in solution, and it is often difficult to mech-
anically assign the various components of decay. In the aqueous
medium, DHAB and HAB have multiple hydrogen bonding
sites in the ground and excited states. The above behavior gives
rise to different charge distribution structures with varying
fluorescence decay lifetimes. As an alternative of giving impor-
tance to individual decay components, we define the second-
order average lifetime h¸i for these azo molecules in solution as
described by Guchhait and his co-workers.28 The typical decay
Molecular Modeling.
In addition to providing exper-
imental results we also applied the molecular modeling studies,
which offer a molecular level explanation with ability to
estimate the participation of specific functional groups and
their interaction in the complex nanomaterials stabilization.
As stated in the earlier studies15,23 azo form only exists in the
aqueous solution, in order to confirm this we also performed
molecular optimization of both azo and hydrazo tautomers
of the DHAB and HAB molecules with CDs. The minimum
energies of the optimized DHAB/CD and HAB/CD complexes
given in Table 2 and Table 3, and the minimized energy struc-
ture of these guests and the complex nanomaterials are shown
in Figures 9 to 11.
0.140
(a)
DHAB/α-CD
DHBA/β-CD
HAB/α-CD
HBA/β-CD
0.105
(A-A0)/A0
0.070
Two different structures could be written corresponding
to the molecular composition of DHAB and HAB molecules
namely, azo and hydrazo (Figure 9). This was due to the migra-
tion of the hydrogen atom of a hydroxy group (which is
attached to C4 carbon) to the azo nitrogen atom. A further
tautomeric structure was also possible in the case of DHAB as a
result of the transfer of the -OH proton to the nitrogen atom of
azo group. Azo as well as hydrazo forms have been geometri-
cally optimized using PM3 level and the results are compiled
in the Table 2 and Table 3 respectively. Analysis of the opti-
mized molecular geometries reveals that the azo and hydrazo
tautomers were essentially nonplanar in the ground states since
significant torsion angles were detected between the plane
of the azo linkage and the C1 and C7 carbons (Figure 9). We
optimized both azo and hydrazo inclusion complex nano-
materials and the energy change accompanying the formation
of DHAB/CD and HAB/CD complex nanomaterials calcu-
lated using the following equation
0.035
0
0
0.20
0.40
0.60
0.80
1.00
1.20
[CD]/[CD]+[Guest]
24.0
18.0
(b)
(I-I0)/I0
12.0
DHAB/α-CD
DHBA/β-CD
HAB/α-CD
HBA/β-CD
6.0
0
0
0.20
0.40
0.60
0.80
1.00
1.20
[CD]/[CD]+[Guest]
ꢀE ¼ Ecomplex ꢀ ðECD þ Edye
where Ecomplex, Edye, and ECD are the energies of the complex
nanomaterials, the free guest, α-CD, and β-CD (host) respec-
Þ
ð1Þ
Figure 8. Job’s plot using (a) absorption and (b) fluores-
cence intensity at pH ca. 7. The total concentration was 2 ©
10¹5 M. Molar fraction is given by [CD]/([CD] + [guest]).
Table 1. Fluorescence Decay Parameters of DHAB and HAB in Water and 0.01 M CD Solution
Lifetime/ns
Pre exponential factor
Dyes
Medium
h¸i
¸
1
¸
2
¸
3
a1
a2
a3
DHAB
Water
α-CD
β-CD
Water
α-CD
β-CD
0.96
0.31
0.11
0.47
0.36
0.31
3.35
1.09
1.79
4.97
1.10
2.11
0.56
0.86
0.55
0.26
0.16
0.88
0.02
0.51
0.30
0.02
0.14
0.39
1.22
1.85
3.01
2.44
2.04
2.94
4.65
4.65
0.04
0.11
HAB
4.69
5.08
0.02
0.13

G. Venkatesh et al.
Bull. Chem. Soc. Jpn. Vol. 87, No. 2 (2014)
289
Table 2. Energetic Features, Thermodynamic Parameters, and HOMO-LUMO Energy Calculations for Azo Form of DHAB, HAB, AB,
and Its Inclusion Complexes by PM3 Method
Properties
DHAB HAB
α-CD
β-CD
DHAB/α-CD DHAB/β-CD
HAB/α-CD
HAB/β-CD
E
E
HOMO/eV
LUMO/eV
¹8.79 ¹8.97
¹0.86 ¹0.79
¹10.37
1.26
¹11.63
11.34
¹10.35
1.23
¹11.58
12.29
¹8.90
¹0.98
¹7.93
11.61
¹8.80
¹0.65
¹8.15
11.87
¹8.96
¹0.83
¹8.13
10.69
¹1210.47
¹7.87
¹527.51
¹1.54
¹401.10
¹5.78
0.417
¹8.90
¹0.67
¹8.23
10.86
¹1421.62
¹9.01
¹643.75
¹4.62
¹500.23
¹8.20
0.481
EHOMO ¹ ELUMO/eV ¹7.93 ¹8.18
Dipole moment/D
1.13
9.68
1.17
45.02
Ea)
¹1247.62 ¹1457.63 ¹1253.99
¹16.05
¹1467.84
¹19.89
¹686.02
¹10.56
¹541.97
¹20.76
0.483
¹0.036
867.14
0.00
¦Ea)
Ga)
114.06 150.39 ¹676.37
¹789.52
¹667.55
0.409
¹568.13
¹5.83
¹440.97
¹17.20
0.426
¹0.037
761.73
0.00
¦Ga)
Ha)
147.07 175.52 ¹570.84
¦Ha)
Sb)
0.110
0.117
0.353
¦Sb)
¹0.053
759.29
0.00
¹0.045
863.33
0.00
ZPEa),c)
129.08 121.97 635.09
0.00 0.00 0.00
740.56
0.00
Mulliken charge
a) kcal mol¹1. b) kcal mol¹1 K¹1. c) ZPE: zero point vibration energy.
Table 3. Energetic Features, Thermodynamic Parameters, and HOMO-LUMO Energy Calculations for Hydrazo Form of DHAB, HAB,
and Its Inclusion Complexes by PM3 Method
Properties
DHAB
HAB
DHAB/α-CD
DHAB/β-CD
HAB/α-CD
HAB/β-CD
E
E
HOMO/eV
LUMO/eV
¹8.46
¹1.29
¹7.16
1.12
¹8.53
¹1.44
¹7.30
1.17
¹8.63
¹1.45
¹7.18
9.82
¹1241.03
¹5.66
¹552.10
12.81
¹428.38
¹3.88
0.414
¹0.056
762.16
0.00
¹8.56
¹1.27
¹7.29
10.64
¹1453.86
¹8.48
¹627.37
50.70
¹528.49
¹7.28
0.482
¹0.044
867.01
0.00
¹9.28
¹0.77
¹8.51
11.12
¹1164.06
¹6.93
¹494.43
¹1.10
¹359.16
¹5.12
0.410
¹8.68
¹1.36
¹7.32
11.07
¹1404.01
¹9.49
¹633.19
¹2.4
¹483.28
36.35
0.469
¹0.054
863.19
0.00
EHOMO ¹ ELUMO/eV
Dipole moment/D
Ea)
12.25
45.02
¦Ea)
Ga)
111.45
146.34
0.117
150.39
175.52
0.117
¦Ga)
Ha)
¦Ha)
Sb)
¦Sb)
¹0.056
755.20
0.00
ZPEa),c)
125.19
0.00
121.97
0.00
Mulliken charge
a) kcal mol¹1. b) kcal mol¹1 K¹1. c) ZPE: zero point vibration energy.
H1
H10
H9
C11
H9
H1
H2
H10
O1
C1
H2
C2
C12
C11
C3
C5
H3
O2
C12
C7
C3
H8
H3
C2
H8
N1
C7
C8
N2
N1
C1
C10
C10
C4
O1
C4
C5
N2
C9
H7
C6
H5
C9
H7
C8
H6
C6
H5
H4
H4
H6
(a)
(b)
N2
H9
C7
H10
H2
H9
O1
C1
H2
H8
C11
H1
C2
H10
N1
H1
N1
C12
C12
C3
C3
C11
C2
H3
N2
H8
C7
C8
H7
O1
C4
O2
C4
C10
C10
C1
C8
C9
H7
C5
C6
H5
C9
H6
C5
C6
H4
H6
H5
H4
H3
(c)
(d)
Figure 9. PM3 optimized structures of (a) DHAB-azo, (b) HAB-azo, (c) DHAB-hydrazo, and and (d) HAB-hydrazo tautomers.

290
Bull. Chem. Soc. Jpn. Vol. 87, No. 2 (2014)
Cyclodextrin-Covered Azobenzene Microrods and Microsheets
2.35 Å
2.56 Å
4.41 Å
2.67 Å
(a)
(b)
2.55 Å
2.25 Å
(c)
(d)
Figure 10. PM3 optimized azo tautomers structures of (a) DHAB/α-CD, (b) DHAB/β-CD, (c) HAB/α-CD, and (d) HAB/β-CD
inclusion complex.
tively. This provides quantitative measures of the interaction
forces driving the complexation process. In overall azo tautomer
complex nanomaterials were most stable because it has higher
negative ¦E value. For example in HAB azo/α-CD complex
The HOMO/LUMO orbital picture of both azo and hydrazo
tautomers of the DHAB, HAB, and AB molecules was shown
in Figure S9. Generally the difference between this HOMO
and LUMO is responsible for the stability of the complexes.30
Hence HAB/β-CD complex was more stable because it has
higher energy difference between the HOMO and LUMO
(¹8.23 eV) than the other complexes. The net Mulliken charge
distribution analysis reveals that all studied complexes were not
having charge-transfer character.
Thermodynamics of Inclusion Complexes. To investi-
gate the thermodynamics of the most stable inclusion complex
nanomaterials, we applied statistical thermodynamic calcula-
tions at 1 atm and 298.15 K by PM3 method and the results
are listed in Tables 1 and 2. In the light of our data obtained
from PM3, the azo forms were found to be more thermo-
dynamically stable than the hydrazo tautomers. The negative
enthalpy changes suggest that these inclusion processes are
enthalpy driven in nature. As compared to hydrazo tautomer,
the enthalpy change (¦H) of azo tautomer of HAB/α-CD is
more negative (¹5.78 kcal mol¹1), suggesting that the azo
inclusion complex nanomaterials are more stable and that
these inclusion process were enthalpy driven. In the case of
DHAB/β-CD complexes, the high negative ¦H values (¹20.76
kcal mol¹1) indicated that this complex nanomaterial is more
stable and occurred spontaneously at room temperature.31
Gibbs free energy changes are negative for complexation azo
tautomer in both HAB and DHAB molecules, which imply that
these occur spontaneously at room temperature. But in the case
¹1
nanomaterials has an energy of ¹7.87 kcal mol which was
more negative than that of corresponding hydrazo tautomer
(¹6.93 kcal mol¹1). Similar results were observed in other
DHAB/CD and HAB/CD inclusion complex nanomaterials.
Azo tautomer inclusion complex nanomaterials were energeti-
cally favorable, otherwise in the process of energy minimization
these HAB and DHAB molecules pass the apolar cavity of CD,
neither formatting hydrogen bonds between the CD and guest
molecules (Figures 10 and 11). This suggested that the com-
plexes were stabilized predominantly by dispersion of van der
Waals interactions but as one would expect additional hydrogen
bond lowers the energies of DHAB/CD and HAB/CD inclu-
sion complex nanomaterials and thus increased the stability.
Changes in the dipole moment as a result of complexation
for each tautomeric form are quite clear. The dipole moment
changes accompanying complexation in the case of azo-
hydrazo forms are not very high compared to corresponding
isolated host molecules, but all the values are positive in both
tautomers. This implies that after inclusion complexation, the
polarity of the host (CD) molecule decreased. These substantial
changes in the dipole moment were due to the specific inter-
actions of the guest molecules with the CD.29 In view of these
facts, the formed inclusion complex nanomaterials are more
stable than the corresponding isolated host and guest molecules.

G. Venkatesh et al.
Bull. Chem. Soc. Jpn. Vol. 87, No. 2 (2014)
291
2.86 Å
2.70 Å
4.22 Å
3.85 Å
(a)
(b)
3.54 Å
3.37 Å
2.49 Å
2.19 Å
(c)
(d)
Figure 11. PM3 optimized hydrazo tautomers structures of (a) DHAB/α-CD, (b) DHAB/β-CD, (c) HAB/α-CD, and (d) HAB/
β-CD inclusion complex.
of DHAB the hyrazo tautomer has positive ¦G of value 12.81
and 50.70 kcal mol¹1 for α-CD and β-CD respectively, this sug-
gests that they hardly occur spontaneously at room temperature.
DHAB/β-CD inclusion complexes show high negative ¦G
value ¹10.56 kcal mol which implies that the DHAB/β-CD
complex nanomaterials formed more spontaneously than other
complex nanomaterials.
The geometric parameters like bond length, bond angles, and
dihedral angles of the guest molecule changed drastically before
and after complexation (Tables S2 and S3). The dihedral angle
between the azo groups was more affected by the complexation,
while only small changes were observed in the bond length and
bond angle, which indicated that the DHAB and HAB adopted a
specific conformation to form a stable complex nanomaterials.
The internal diameter of β-CD is approximately 6.5 ¡ and α-CD
is approximately 5.0 ¡ and the height of the both CD is 7.8 ¡.
Considering the shape and dimensions of CD, both molecules
cannot be completely embedded into the CD nanocavity. The
vertical distance and length of the DHAB, HAB, and AB were
greater than the upper/lower rim of the CD. Hence, the entire
molecule cannot be fully encapsulated in the CD cavity.
nanomaterials were studied by SEM and TEM. The inclusion
complex nanomaterials of α-CD and β-CD with DHAB and
HAB were investigated by UV-vis, steady-state and time-
resolved fluorescence, FT-IR, DSC, PXRD, and 1H NMR
molecular modeling techniques. DHAB/CD and HAB/CD
microstructure formed by individual “barrel” type (head-to-head
arrangement) of 2:2 inclusion complexes through intermolec-
ular hydrogen bonding between the neighboring complexes.
The thickness for DHAB/β-CD microrod was approximately
in the range of ca. 1.46 ¯m and height is about ca. 9.8 ¯m,
whereas, typical average dimension of DHAB/α-CD micro rod
is ca. 360 nm width and ca. 3 ¯m length. HAB/α-CD complexes
form a self-assembled monolayer and HAB/β-CD forms a thick
microsheet. From the computational study, we find that negative
Gibbs energy and enthalpy changes for the inclusion complexes
indicate that the formation of these complexes is spontaneous
and exothermic and hydrogen-bonding interactions play a major
role in the inclusion process.
¹1
Experimental
Materials. DHAB, HAB, α-CD, and β-CD were purchased
from Sigma-Aldrich chemical company and used without
further purification. The purity of the compound was checked
for similar fluorescence spectra when excited with different
wavelengths.
Conclusion
The self assembly behavior of 2,4-dihydroxyazobenzene
(DHAB) and 4-hydroxyazobenzene (HAB) inclusion complex

292
Bull. Chem. Soc. Jpn. Vol. 87, No. 2 (2014)
CD (1 mmol) was
Cyclodextrin-Covered Azobenzene Microrods and Microsheets
Preparation of Nanomaterials.
guest molecule was initially placed onto the Z axis. The position
of the guest was determined by the Z coordinate of one selected
atom of the guest. The inclusion process was simulated by
putting the guest on one end of the CD and then letting it pass
through the CD cavities. Since the semiempirical PM3 method
has been proven to be a powerful tool in the conformational
study of CD inclusion complexes and has high computational
efficiency,33 we selected semiempirical PM3 method to study
the inclusion process of CDs with the DHAB and HAB.
dissolved in 40 mL of distilled water and DHAB or HAB
(1 mmol) in 10 mL of methanol and was slowly added to
the CD solution. This mixture was stirred at 50 °C overnight.
Then the final solution was refrigerated overnight at 5 °C. The
precipitated DHAB/CD and HAB/CD complexes were recov-
ered by filtration and washed with a small amount of ethanol
and water to remove uncomplexed DHAB, HAB, and CD,
respectively. This precipitate was dried in vacuum at room
temperature for two days and stored in an airtight bottle. This
powder sample was used for further analysis.
This work was supported by the Council of Scientific
Industrial Research [No. 01(2549)/12/EMR-II] and University
Grants Commission [F. No. 41-351/2012 (SR)] New Delhi,
India. One of the authors G. Venkatesh is thankful to the UGC
for the award of BSR fellowship. The authors thank to Dr. P.
Ramamurthy, Director, National Centre for Ultrafast Processes,
Madras University for allowing the fluorescence lifetime mea-
surements for this work.
Preparation of CD Solution. The concentration of stock
solution of DHAB and HAB was 2 © 10¹3 M. The stock
solution (0.2 mL) was transferred into 10 mL volumetric flasks.
¹3
To this, varying concentration of CD solution (1.0 © 10
to 1.0 © 10¹2 M) was added. The mixed solution was diluted
to 10 mL with triply distilled water and shaken thoroughly.
The final concentration of DHAB and HAB in all the flasks
was 4 © 10¹5 M. The experiments were carried out at room
temperature.
Supporting Information
Instruments. Scanning electron microscopy (SEM) photo-
graphs were collected on a JEOL JSM 5610LV instrument. The
morphology of DHAB and HAB encapsulated CDs inclusion
complexes was investigated by transmission electron micros-
copy (TEM) using a TECNAI G2 microscope with accelerating
voltage 200 kV, for the TEM analysis using carbon-coated
copper TEM grid (200 mesh). FT-IR spectra of DHAB, HAB,
both CDs and the inclusion complexes were measured between
4000 and 400 cm¹1 on a Nicolet Avatar 360 FT-IR spectrometer
by using KBr to make pellets. One-dimensional 1H NMR
spectra for DHAB, HAB, and CDs were recorded on a Bruker
AVANCE 400 MHz spectrometer (Germany) using DMSO-d6
(99.98%) as a solvent. The differential scanning calorimetry
(DSC) was recorded using a Mettler Toledo DSC1 fitted with
STRe software (Mettler Toledo, Switzerland), temperature
scanning range was from 25 to 220 °C with a heating rate of
10 °C min¹1. Powder X-ray diffraction (PXRD) spectra were
recorded with a BRUKER D8 advance diffractometer (Bruker
AXS GmbH, Karlsruhe, Germany) and the patterns were
measured in the 2ª angle range between 5 and 80° with a
scan rate 5° min¹1. Absorption spectral measurements were
carried out with a Shimadzu (model UV 1650 PC) UV-visible
spectrophotometer and steady-state fluorescence measurements
were made by using a Shimadzu spectrofluorimeter (model RF-
5301). pH of the solution was measured on a Elico pH meter
(model LI-120). The fluorescence lifetime measurements were
performed using a picosecond laser and single photon counting
setup from Jobin-Vyon IBH (Madras University, Chennai).
In the Supporting file we are provide three tables and nine
figures. Table contains 1H NMR chemical shift values and geo-
metrical parameters of azo and hydrazo forms for the DHAB
and HAB molecules with α-CD and β-CD. Figures contain
1
FT-IR spectra, H NMR spectra, absorption and fluorescence
spectra, and fluorescence decay curves for DHAB and HAB
with α-CD and β-CD inclusion complexes. Further, HOMO
and LUMO pictures of DHAB and HAB are also provided in
the figures. This material is available free of charge on the web
at http://www.csj.jp/journals/bcsj/.
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