Synthesis, characterisation and photoswitchability of a new [2]rotaxane of α-cyclodextrin with a diazobenzene containing π-conjugated molecular dumbbell

Article abstract of DOI:10.1080/10610278.2012.660529

In this work, a new [2]rotaxane consisted of a diazobenzene containing α-conjugated linear compartment, including the 4,4-bipyridyl moiety and α-cyclodextrin (-CD) as the macrocyclic compartment, was synthesised with yields of nearly 57% and fully characterised. α-CD easily assembled with the linear compartment and suitable bulky ends (stoppers) in water to give a new [2]rotaxane. The characterisation of this supramolecular compound was accomplished using several spectroscopic techniques such as ¹H NMR, ¹³C NMR and 2D NMR spectroscopy, powder X-ray diffraction, fourier transform infrared spectroscopy, induced circular dichroism and UV-vis spectrophotometry, as well as scanning electron microscopy and Energy Dispersive X-ray. Furthermore, the reversible E-Z photoisomerisation of both [2]rotaxane and its molecular dumbbell was investigated by irradiation with UV light.

Full text of DOI:10.1080/10610278.2012.660529

Supramolecular Chemistry
Vol. 24, No. 5, May 2012, 333–343
Synthesis, characterisation and photoswitchability of a new [2]rotaxane of a-cyclodextrin with a
diazobenzene containing p-conjugated molecular dumbbell
Ioanna Deligkiozia, Raffaello Papadakisa,b and Athanase Tsolomitisa*
aLaboratory of Organic Chemistry, School of Chemical Engineering, National Technical University of Athens (NTUA), 15780 Athens,
b
Greece; CNRS, Universite Paul Cezanne, UMR6263, ISM2/BiosCiences, Marseille 13397, France
´
´
(Received 4 November 2011; final version received 19 January 2012)
In this work, a new [2]rotaxane consisted of a diazobenzene containing p-conjugated linear compartment, including the 4,40-
bipyridyl moiety and a-cyclodextrin (a-CD) as the macrocyclic compartment, was synthesised with yields of nearly 57% and
fully characterised. a-CD easily assembled with the linear compartment and suitable bulky ends (stoppers) in water to give a
new [2]rotaxane. The characterisation of this supramolecular compound was accomplished using several spectroscopic
1
techniques such as H NMR, 13C NMR and 2D NMR spectroscopy, powder X-ray diffraction, fourier transform infrared
spectroscopy, induced circular dichroism and UV–vis spectrophotometry, as well as scanning electron microscopy and
Energy Dispersive X-ray. Furthermore, the reversible E–Z photoisomerisation of both [2]rotaxane and its molecular
dumbbell was investigated by irradiation with UV light.
Keywords: a-cyclodextrin; molecular switches; [2]rotaxane; diazobenzene; viologens
Introduction
self-assembling, with a new synthesised linear molecule.
What is interesting with this [2]rotaxane is that its linear
molecular part presents high p-conjugation. Several works
have been published in the past concerning the use of
conjugated (either polymeric or oligomeric) compounds for
the preparation of molecular wires due to their electrical
conductivity attributed to their conjugation (15–17).
Herein, we report the synthesis of a new [2]rotaxane with
such a linear compartment.
Rotaxanes are a relatively new class of nanomaterials (1–3),
which have been intensively investigated in the last two
decades because of their ability to act as molecular switches
by giving controllable, reversible transformations (4, 5).
These are materials consisting of one or more linear mole-
cular fragments, and one or more macrocyclic molecules
threaded through the first ones, and stoppered with suitable
side bulky moieties (the so-called stoppers) (6–8). Many
attempts have been made in the past and have yielded in
challenging rotaxanes and pseudorotaxanes, with potential
applications in nanotechnology and molecular electronics (9,
10), serving as molecular switches (11–14), molecular wires
(15–17) and molecular memory devices (18). [2]Rotaxanes,
the structurally simplest rotaxanes, consist of one linear and
one macrocyclic compartment as well as two bulky moieties
at both sides of the linear part, preventing the macrocyclic
molecule from slipping away, hence stabilising the inclusion
complex. A very interesting class of rotaxanes includes the
inclusion complexes of suitable linear molecules with
cyclodextrins (CDs). CDs are structurally 1 ! 4-a-linked
anhydroglucopyranose cyclic oligomers and have the
geometry of truncated cones, the cavities of which are
hydrophobic and they can easily create inclusion complexes
by self-assembling with a wide class of organic molecules,
including many aromatic and other compounds (19, 20).
In this work, a-cyclodextrin (a-CD) has been used
for the preparation of a new [2]rotaxane, interacting by
Additionally, it is known that diazobenzene-based
compounds can easily undergo a reversible E–Z
isomerisation of the type shown in Scheme 1. Since, the
Z diastereomer is more unstable than the E one, the energy
needed for E ! Z isomerisation is higher than that needed
for the Z ! E isomerisation. Consequently, the first one
occurs photochemically whereas the second one thermally.
This is the reason why diazobenzene-based ‘linear’
molecules have been used in the past for the preparation of
switchable rotaxanes (11). This controllable isomerisation
can lead to the movement of the threaded macrocycle (in
our case a-CD) to another position due to stereochemical
reasons, in other words the CD moiety can shuttle between
the azo group in response to an external stimulation,
e.g. UV light. Thus, a diazobenzene [2]rotaxane can act as
a molecular shuttle. In this work, they are used for the
same reason, and shuttling of a-CD in the case of the
synthesised rotaxane is investigated.
*Corresponding author. Email: tsolom@chemeng.ntua.gr
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2012 Taylor & Francis
http://dx.doi.org/10.1080/10610278.2012.660529
http://www.tandfonline.com

334
I. Deligkiozi et al.
azodianiline (ADA), in the solid phase the precursor (new
product) linear compound 2 as shown in Scheme 2. This is
a reaction which in our hands did not yield compound 2 in
solution. However, it is remarkable that in the solid phase
this reaction is more favourable probably because of the
removal of the 2,4-dinitroaniline (leaving group) through
sublimation.
R
R
N
N
UV Irradiation
Thermally
N
N
R
Compound 2 was then reacted with an excess of fluoro-
2,4-dinitrobenzene and a-CD in water to yield through a
self-assembling ‘threading’ reaction of the desired
[2]rotaxane.
E
Z
R
The threading technique is a widely used synthetic
strategy for rotaxane synthesis (24) during which a
pseudorotaxane is initially formed. The pseudorotaxane
corresponds to the inclusion complex of a-CD with the
linear compound 2.
Scheme 1. Switchability of a diazobenzene derivative.
Results and discussion
Synthesis
This complex is not stabilised with any side bulky
substituent; that is why it is relatively unstable. In the
presence of an excess of fluoro-2,4-dinitrobenzene, the as-
formed pseudorotaxane is transformed to the desired
[2]rotaxane. The 2,4-dinitrophenyl moieties are bulky
enough concerning the dimensions of a-CD, which is the
smallest CD (19), to prevent decomposition of the rotaxane.
As depicted in both Schemes 3 and 4, the compounds
3a and 4a (salts with counter anions Cl2 and F2) were
transformed to the corresponding hexafluorophosphates 3b
and 4b (salts with counter anions PF26 ) via an ion exchange
reaction of 3a and 4a with the salt NH4PF6.
The reason why this ion exchange reaction was chosen
is that as reported in the past (25), the hexafluorophosphate
salts of viologens (substituted 4,40-bipyridines) are soluble
in various polar organic solvents (Acetonitrile, Dimethyl
sulfoxide (DMSO), Dimethylformamide, acetone, etc.)
whereas the halogen (F2, Cl2, Br2) salts of viologens are
soluble in water but insoluble or slightly soluble in organic
solvents. Consequently, this helped to easily dissolve the
compounds 3b and 4b to suitable solvents for the needs of
characterisation.
In this work, we report the synthesis of a new [2]rotaxane
with a p-extended conjugated linear compartment. Several
works have been published in the past concerning
[2]rotaxanes without conjugation in their linear compart-
ments. Although the high conjugation induces several
interesting properties such as conductivity or semicon-
ductivity to the final material, little has been done in the
field of rotaxanes which contain highly conjugated
compartments, due to possible difficulties in their
synthetic procedures.
For the preparation of the title [2]rotaxane, first of all
the ‘linear’ compound 2 (Scheme 2) had to be synthesised.
For that purpose 4-(2,4-dinitrophenyl)-40-bipyridinium
chloride 1 (a Zincke salt) was used as the starting material
synthesised according to procedures published in the
past (22, 23) to give after a Zincke reaction with 4,40-
Cl
O2N
N
N
NO2
The dumbbell compounds 4a,b (Scheme 4) were also
synthesised to help the interpretation of the structure of the
[2]rotaxanes 3a,b, by comparing their spectral features.
For the synthesis of the compounds 4a,b, the same
conditions as those used for the synthesis of the
[2]rotaxanes were chosen.
N
H2N
NH2
N
NH2
NO2
NO2
Solid Phase, Zincke reaction,
overnight, 250°C
Characterisation
For the characterisation of the synthesised compounds,
several spectroscopic techniques were employed. As
mentioned before, the comparison of the spectroscopic
data of the dumbbell-like compound 4b with those of the
final [2]rotaxane 3b made the characterisation feasible. In
this work, compound 4b (the hexafluorophosphate [2]rotax-
ane salt) has been fully characterised through various
spectroscopic techniques, as it will be analysed.
Cl
Cl
N
N
N
(2)
N
N
N
Scheme 2. Synthesis of linear compound 2.

Supramolecular Chemistry
335
Cl
N
N
N
Cl
N
N
N
(i) a-CyD, H2O
"Threading'
NO2
O2N
F
a-CyD
NO2
Cl
N
Cl
N
F
F
O2N
N
N
N
N
NO2
O2N
(ii) NH4PF6, H2O
[2]Rotaxane
NO2
O2N
N
N
N
N
N
N
NO2
O2N
4[PF6]
Scheme 3. Synthetic route for the preparation of the [2]rotaxane 3a,b.
1H NMR, 13C NMR and rotating frame Overhauser
effect spectroscopy
easily deduced that both these two components are
assembled in equimolar proportion in the [2]rotaxane
molecule 3b. Additionally, the corresponding neighbourly
protons of the quaternary nitrogen atoms lose their
symmetry due to the presence of a-CD. This is a strong
evidence of the formation of an inclusion complex (27–30)
(FigureS2ofSupplementaryInformation, availableonline).
Furthermore, a broadening of almost all aromatic and
a-CD peaks was observed which is probably attributed to
the interaction of the two compartments of the inclusion
complex 3b. Concerning the 13C NMR spectra, it has been
observed that for the case of 3b both the carbon atoms of
a-CD and of the dumbbell compound are present (see
‘Experimental’ section).
The formation of the inclusion complex between the
dumbbell-like molecule and a-CD is clearly confirmed
by 1H NMR, 13C NMR and 2D NMR spectroscopy
experiments in D6DMSO. The NMR study gave clearly
strong indications that an interaction between the dumb-
bell compound and a-CD takes place (Figure 1; Figures S1
and S2 of Supplementary Information, available online).
This result supports the formation of the [2]rotaxane
structure. It is also observed that the interior H3 and H5
protons located in the internal a-CD cavity, shifted upfield
as expected (Figure S1 of Supplementary Information,
available online) which indicates that these signals are
strongly sensitive to the electronic environment inside the
cavity with the aromatic phenyl protons b and a of the
dumbbell molecule (Scheme 5) (26).
To obtain more detailed information, the stable
conformation of the [2]rotaxane was additionally fully
1
characterised by 2D H rotating frame Overhauser effect
spectroscopy (ROESY), which is depicted in Figure 1. 2D
NMR spectroscopy has recently become an important
method for studying the structures of CD-containing
rotaxanes, since one can conclude that two protons are
Taking into account the proportion of the protons of
a-CDand of the protons of thedumbbell molecule, obtained
from the relative integrals of the NMR spectra of 3b, it is

336
I. Deligkiozi et al.
Cl
N
N
N
Cl
N
N
N
NO2
F
O2N
NO2
Cl
Cl
F
F
O2N
N
N
N
N
N
N
NO2
O2N
NH4PF6, H2O
NO2
O2N
N
N
N
N
N
N
NO2
O2N
4[PF6]
Scheme 4. Synthesis of the dumbbell-like molecules 4a,b.
closely enough located in space if a cross-peak is detected
between the relevant proton signals in the rotating frame
Overhauser effect (ROE) spectrum.
additional evidence that the aromatic guest is included in
the CD cavity between the interior H3 and H5 protons.
In the ROE spectrum of Figure 1, the cross-peaks of a-
CD and the aromatic phenyl protons a and b (Scheme 5)
are clearly shown, and that corresponds to the fact that the
a-CD ring is located around the azobenzene unit, an
Determination of the structure in solution
2D ROESY experiments provided us with the information
on the averaged relative inter- and intramolecular proton
distances, for the closeness between the dumbbell
compound and a-CD. In the 2D spectrum of Figure 2,
intense cross-peaks are observed between the aromatic
protons of the dumbbell compound and those of the inert
protons of the CD cavity (H3, H5) as it was noted
previously. The relative volumes of the most important
cross-peaks have been summarised in Table S1 (Sup-
plementary Information, available online). No correlation
is detected between the dinitrophenyl group and any of the
a-CD protons and this fact indicates that the stoppers of
the molecule are placed outside the cavity.
X-ray diffraction
The powder X-ray diffraction (XRD) is a method generally
used to characterise CD-based complexes, because the
patterns of these compounds are different from those of the
native CDs. The XRD pattern of the dumbbell compound
differs from that of the [2]rotaxane. Additionally, they both
Figure 1. The 2D 1H ROE NMR spectrum of 3b ([2]rotaxane),
600 MHz in D6DMSO at 298 K; mixing time 300 ms.

Supramolecular Chemistry
337
NO2
i
O2N
N
N
N
h
g
e
d
c
f
b
a
Scheme 5. Designation of hydrogen atoms for compounds 3 and 4.
differ from the diffraction pattern of the free a-CD. The
diffractogram of a-CD has shown intense and sharp peaks,
indicating its crystalline nature. The a-CD shows character-
istic reflectionsat2u ¼ 12.2, 14.3, 21.7, 13.5, 5.3 and 9.78. In
contrast, the dumbbell compound shows a few sharp peaks
with characteristic reflections at about 2u ¼ 21.5, 20.1, 26.9,
23.5, 39.6 and 10.88 having less intensities (Table S2
Supplementary Information, available online). This fact
indicates the semi-crystalline nature of the dumbbell
compound. However, these characteristic reflections dis-
appeared in the case of the [2]rotaxane (3b), indicating that
the arrangement of a-CD in the complex ([2]rotaxane) is
modified after the rotaxanation. Additionally, the inclusion
complexes of a-CD with the dumbbell compound showed
undefined, broad, diffused peaks with low intensities. The
diffraction pattern of the [2]rotaxane (3b), exhibits a wide-
spreading peak from 2u ¼ 10–308. All the samples were
analysed in the 2u angle range of 5–608 (Figure 3).
Scanning electron microscopy (SEM) observations,
depicted in Figure 4, gave some insights concerning the
different morphology of the synthesised compounds. SEM
experiments were carried out on all compounds, as shown
in Figure 4(a)–(c). When the magnification of SEM was
£ 400, the morphologies of a-CD and the compounds 3b
and 4b were different (for better resolution, see also Figure
S6 of Supplementary Information, available online).
As shown in Figure 4(a), the morphology of a-CD
corresponds to aggregates of crystalline grains of irregular
form with the size varying from 20 to 100 mm. On the other
hand, the dumbbell compound is a semi-crystalline material
and finally the [2]rotaxane (Figure 4(b),(c)) is amorphous.
Additional evidence of the successful rotaxanation has
been obtained through the Energy Dispersive X-ray (EDX)
measurement (elemental analysis) of the compounds 3b
and 4b (Figure S6 of Supplementary Information, available
online). The elemental analysis is in agreement with the
expected one for the rotaxanated dumbbell compound 3b.
Mass spectroscopy
The mass spectra of the [2]rotaxane and the dumbbell
compound were also confirmed by their MALDI-TOF-MS
and ESI-HR-MS. The structure of the [2]rotaxane under
investigation was confirmed additionally by ESI mass
spectrometry. In the mass spectrum of the [2]rotaxane salt,
the molecular ion corresponds to the tetra-anion formed by
the loss of four hexafluorophosphate anions. Figure S3 of
Supplementary Information, available online, shows the
mass spectrum of the [2]rotaxane, in which the
corresponding peaks are listed. The mass spectra showed
peaks at m/z values for M–4PF6 þ 1H, corresponding to
the loss of four hexafluorophosphate counter ions. In the
case of the dumbbell compound, peaks at m/z values were
observed, corresponding to the loss of four hexafluoropho-
sphate counter ions M–4PF6 þ 1H. Additionally, in the
case of the [2]rotaxane, major fragment ions like the
dumbbell compound and the a-CD were observed.
UV–vis spectroscopic and circular dichroism study
In Figure 5, the UV–vis and induced circular dichroism
(ICD) spectra of the dumbbell compound and the
[2]rotaxane are shown, all recorded in DMSO. In the UV–
vis spectra of both compounds, a broad band in the region
300–400 nm, which is attributed to the p–p excitation of
*
the group (–N ¼ N–) (31) is observed.
A bathochromic shift of this band is observed when
rotaxanation takes place. The maximum wavelength for the
dumbbellcompoundis337 nmwhereasthemeasuredonefor
the case of the compound 3b ([2]rotaxane) is 351 nm. This
has to dowith the intercomponent interaction between the a-
CD cavity and the (–N ¼ N–) group of the dumbbell-like
Figure 2. Partial view of ROE corresponding cross-peaks.

338
I. Deligkiozi et al.
Figure 3. XRD patterns of a-CD and of the compounds 3b and 4b. Diffraction intensity in arbitrary units.
Figure 4. SEM images of a-CD (left), and compounds 3b (middle) and 4b (right).

Supramolecular Chemistry
339
Figure 5. ICD (top) and UV–vis (bottom) spectra of 3b and 4b.
compound, which causes a decrease in the energy difference
between the ground and excited state of the azo compound.
Hence, the energy needed for the p–p excitation is lower
the case of the compounds studied in this work, evidence
based on the differences of the fourier transform infrared
spectroscopy (FTIR) spectra of the 3b and 4b is obtained.
As shown in Figure S7 of Supplementary Information,
available online, the signals attributed to aromatic
absorptions due to the stretching of the (C ¼ C) bond
and of the azo (–N ¼ N–) bond are shifted to higher wave
numbers in case of 3b, due to the interaction of the azo
group and the neighbouring aromatic nuclei with a-CD.
As observed, the stretching of the (C ¼ C) and the (–
N ¼ N–) bonds become energetically harder, thus
the observed increases in the corresponding absorption
wave numbers, in case of the compound 3b are induced.
Additionally, a sharp absorption band, noticed at
1484 cm21 due to the pyridine ring stretching vibrations,
of the 4,40-bipyridine moiety of the dumbbell compound,
was also reduced about 40 cm21 (1446 cm21).
*
(bathochromism) (32). This ‘environment-responsive’ char-
acter of the p–p excitation band of the (–N ¼ N–) group
*
has also been observed in solutions of 4b in different
hydroxylic solvents such as ethylene-glycol and methanol
(solvatochromic effect).
The ICD spectra of the compounds 3b and 4b were
different. A characteristic positive band at about 330 nm
with molar ellipticity of about 3 mdeg was observed in the
caseofthe[2]rotaxane(3b). A shoulderatabout 430 nmwas
also observed as well as a negative band in the region 450–
520 nm (u < 2 0.3 mdeg). These are characteristic fea-
tures also observed in the case of other relevant [2]rotaxanes
with azo-containing viologen-based molecular dumbbells
assembled with a-CD (lacking conjugation) (36). The
situation was much different in the case of compound 4b.
The ICDspectrumof themolecular dumbbell lacksall of the
mentioned features and presents a positive peak at about
300 nm with u < 1 mdeg. This is an additional information
which reflects the diversity of molecules 3b and 4b.
Photoswitchability and light-driven molecular shuttle
function
Photoswitchable azo-containing molecular shuttles have
been studied for several years. Herein, we report the
photoswitchability of the [2]rotaxane (3b) and the
corresponding molecular dumbbell 4b. As shown in Figure
5, two absorption bands appear in the UV–vis spectra of 3
and 4b. First of all, a low energy n–p visible band at
*
about 450 nm of low absorbance, which typically appears as
ashoulder of asecondband of much higher absorbance. The
Infrared spectroscopy
Finally, infrared spectroscopy was also employed for the
characterisation of the compounds 3b and 4b. This
spectroscopic technique has been used in the past for the
characterisation of [2]rotaxanes since it is a tool to prove
the presence of both host and guest components (33). In
latter corresponds to a p–p excitation of the (–N ¼ N–)
*

340
I. Deligkiozi et al.
group, and lies in the ultraviolet region at about 330–
350 nm (34, 35). When compounds 3b and 4b are irradiated
with UV light (as described in the ‘Experimental’ section),
they undergo E–Z isomerisations. In both cases, the
E-isomer is more stable than the Z. Thus, visible light or
heating of the produced by irradiation Z-isomer, can lead
back to the initial more stable E-state (34, 36). In the case of
[2]rotaxane 3b, this photoisomerisation results in the
movement of the threaded a-CD molecule to another
position (‘station’) because of hindrance, as also proved by
2D NMR experiments. This E ! Z isomerisation which is
reversible (37, 38) induces a motion of the described
molecular shuttle upon irradiation. A comparison of the
E–Z isomerisation, through UV–vis spectrophotometry
for the cases of 3b and 4b, is shown in Figure 6 and Figures
S10 and S11 of Supplementary Information, available
isomerisationbecomesmoredifficultinthepresenceofthea-
CD ring (41), as described for other similar cases in the past.
Concluding remarks
In summary, in this work, we have described the design,
synthesis and characterisation of a new photo driven a-
CD-based [2]rotaxane with a p-extended conjugated
molecular dumbbell. The intercomponent interactions
responsible for the stability of this [2]rotaxane (3b) were
revealed through several spectroscopic techniques, which
helped to characterise the final material. The dumbbell-like
molecule 4b was used as a reference for the analysis of the
[2]rotaxane. Interesting phenomena such as some asym-
metries in the NMR signals of 3b, as well as
differentiations of the XRD patterns of 3b in comparison
with those of the dumbbell compound and of a-CD,
associated with the formation of the inclusion complex 3b,
were observed and analysed. The UV–vis properties of the
[2]rotaxane have shown that the rotaxanation by a-CD
reduces the absorbance intensity compared to that of the
free dumbbell compound. SEM and EDX analysis also
proved to be really useful for the confirmation of the
success of the rotaxanation reaction. Additionally through
XRD, FTIR and NMR analysis, it was proved that a-CD is
threaded in the region near the (–N ¼ N–) bond as
depicted in Scheme 3. Finally, the photoswitchability of the
reported new [2]rotaxane, which resulted in its reversible
molecular shuttling, was successfully investigated.
online. In both cases a decrease in the p–p band and an
*
increase of the n–p visible band was observed, indicating
*
the formation of the Z-isomer (36, 39). A slight hypso-
chromic shift of the p–p band of 3b upon irradiation and
*
E–Z isomerisation corresponds to the slipping of a-CD
away of the (–N ¼ N–) group. (As mentioned, rotaxana-
tion and stoppage of a-CD close to the azo-group cause the
bathochromic shift of the p–p band of compound 3b
*
(Figure S11 Supplementary Information, available online).
This was not observed in the same extent for 4b.
The characteristic p–p and n–p absorption bands
*
*
and conversions at the photostationary states of the
[2]rotaxane and the dumbbell in DMSO, after 1–6h of
irradiation are summarised in Tables 1 and 2 (plots of E–Z
conversion vs. irradiation time, in Figures S8 and S9 of
Supplementary Information, available online). E–Z conver-
sions were calculated by the fraction jAt–Apj/At, where
A ¼ p–p absorbance at trans state and A ¼ p–p
Experimental section
Physical measurements
NMR spectra were obtained using a Varian Gemini 300 or
1
a Varian 600 MHz spectrometer (300 MHz H, 75 MHz
*
*
t
p
absorbance at photostationary state (40). We have observed
that the spectral changes for the dumbbell compound were
quite greater (39.33%) than those of the [2]rotaxane
(20.54%). This difference in the conversion could be
attributed to the fact that it is easier for the dumbbell
compound to undergo E/Z photoisomerisation. This is not
surprising in contrast with the [2]rotaxane because the
13C or 600 MHz 1H). 1H and 13C NMR spectra were
recorded either in D6DMSO at 258C (for the compounds
3b and 4b) or in D2O (1H) and D2O/D6DMSO (13C)
(compound 2). The ROESY experiment was carried out
with a spin lock mixing time of 300 ms. In all cases, the
residual solvent peaks were used as internal reference
(a)
(b)
1.2
2.1
1.8
1.5
1.0
0.8
π-π *
1.2 π-π *
n-π*
n-π*
0.6
0.4
0.2
0.0
0.9
0.6
0.3
0.0
300 350 400 450 500 550 600
300 350 400 450 500 550 600 650
λ (nm)
λ (nm)
Figure 6. UV–vis spectra of (a) dumbbell compound and (b) [2]rotaxane without irradiation and after 1–6 h of irradiation.

Supramolecular Chemistry
Table 1. p–p Band absorbances for 3b before and after irradiation recorded in DMSO.
341
*
t
Ap
At
jAt–Apj/At
% E–Z
ln(jAt–Apj/At)
0
60
2.171
2.171
2.171
2.171
2.171
2.171
2.171
2.141
1.942
1.919
1.884
1.849
1.814
1.801
–
–
–
0.10
0.11
0.13
0.15
0.18
0.20
10.25
11.57
13.64
15.79
18.03
20.54
22.27
22.15
21.99
21.84
21.71
21.58
120
180
240
300
360
Table 2. p–p Band absorbances for 4b before and after irradiation recorded in DMSO.
*
t
Ap
At
jAt–Apj/At
% E–Z
ln(jAt–Apj/At)
0
60
1.254
1.254
1.254
1.254
1.254
1.254
1.254
1.254
1.069
1.055
1.014
0.976
0.940
0.900
–
–
–
0.17305893
0.18862559
0.23668639
0.28483607
0.33404255
0.39333333
17.31
18.86
23.67
28.48
33.40
39.33
21.75412309
21.66799122
21.44101926
21.25584147
21.09648689
20.93309785
120
180
240
300
360
(2.5 ppm for D6DMSO concerning Proton Magnetic
Resonance (PMR), and 39.52 ppm for D6DMSO concern-
ing 13C NMR; 4.79 ppm for D2O concerning NMR and
39.39 ppm for D6DMSO in D2O concerning 13C NMR)
(21). Abbreviations used for multiplicity in the text:
s ¼ single; d ¼ doublet; pd ¼ pseudo doublet and m ¼
multiplet. IR spectra were recorded on a Perkin-Elmer
Spectrum 1 FTIR spectrophotometer in the solid state
(without any preparation of the samples) using the
attenuated total reflectance technique (ATR) in the region
600–4000 cm21. UV–vis spectra were recorded using a
Varian CARY 1E UV–vis spectrophotometer at 25 ^ 18C
in DMSO solutions. The concentrations of the solutions
used were about 50 ppm and they were prepared right
before each measurement. Electrospray ionisation ESI-
HR-MS spectra were obtained on a Waters, Inc. Q-TOF
Premier Mass Spectrometer (High Resolution Mass
Spectroscopy facility, Iowa University, Iowa, USA).
Melting points were determined in open capillary tubes
using a Gallenkamp HFB-595 melting point apparatus and
are uncorrected. Powder XRD measurements were carried
out using a Siemens D5000 diffractometer using Cu Ka1,
1 cm quartz cell. The distance between the lamp and the
sample cell was about 5 cm (37).
Materials and procedures
Fluoro-2,4-dinitrobenzene and 4,40-ADA were purchased
from ACROS Organics, a-CD was obtained from Fluka in
99.0% purity and was dried in vacuo over P2O5 prior to use,
and finally 4,40-bipyridinewas obtained from Fluka as well,
and it was used without further purification. The solvents
used were degassed for several minutes with argon before
use. Water was purified with a Barnstead EASYpure RF
compact ultrapure water system and then distilled twice. In
all cases of complex formation reactions, the reaction
mixture was kept under argon atmosphere. All the reactions
were monitored through thin layer chromatography (TLC).
The silica gel TLC plates were ALUGRAM SIL G/UV254
purchased from Macherey-Nagel. TLC was also used in
order to check the absence of unreacted a-CD, or of the
unreacted linear molecules in the final materials. The final
products were dried under vacuum over anhydrous CaCl2
or P2O5 in case of CD complexes, for several days until they
were brought to a constant weight.
˚
radiation l ¼ 1.5405 A, and a graphite monochromator.
Finally, SEM images and EDX measurements were
obtained using a FEI Quanta 200 SE-Microscope. For the
photoswitchability investigation, all spectra were recorded
using a Varian CARY 1E UV–vis spectrophotometer at
258C, before and after illumination by external light
irradiation with a 365 nm UV light lamp. The UV light
sources were obtained from a low pressure Mercury lamp,
and the light beam with a wavelength of about 365 nm,
achieved through suitable filters, in a sealed Ar-saturated
Synthesis of linear compound 2
Compound 1 (1.01 g, 2.83 mmol) and 4,40-ADA (0.3 g,
1.41 mmol) were mixed in a round flask in molar proportion
2:1, respectively, and heated overnight without solventin an
oil bath regulated at 150–1708C. The solid mass was then
dissolved in MeOH and compound 2 was precipitated by
adding Et2O and then filtered, washed with Et2O and dried

342
I. Deligkiozi et al.
in vacuo. Red brown powder (0.54 g, 68%), mp: . 2508C;
1H NMR (300 MHz, D2O, 268C): d ¼ 9.43 (d, J ¼ 6.3 Hz,
4H; C5H5N), 8.97 (d, J ¼ 6.3 Hz, 4H; C5H5N), 8.74
(d, J ¼ 7.2 Hz, 4H; C5H5N), 8.35 (d, J ¼ 8.4 Hz, 4H; Ph),
8.29 (d, J ¼ 4.8 Hz, 4H; C5H5N), 8.18 (d, J ¼ 8.1 Hz, 4H;
Ph); 13C NMR (75 MHz, D2O/D6DMSO, 268C):
d ¼ 153.48, 152.78, 150.80, 145.48, 144.22, 140.86,
126.56, 125.37, 124.27, 122.29. MS-ESI (m/z): calculated
for C32H25N6Na (M 2 2Cl þ Na þ H), 563.57 found:
563.49.
143.08, 132.09, 130.39, 127.02, 126.75, 126.61, 125.43,
124.42, 122.23, 121,58; HR-MS-ESI (m/z): calculated for
C44H30N10O8 (M 2 4PF6 þ 1H), 826.2242 found:
826.2245. UV–vis (DMSO, lnm (logemax)): 337 (4.569),
(p–p ); IR (ATR): n ¼ 1595 (n–N¼N–), 1522 (nNO2),
*
1484, 1373 (nNO2), 1218 cm21
.
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*
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compound 3b. Red solid (0.2 g, 80%), mp: . 2508C; H
NMR (300 MHz, D6DMSO, 268C): d ¼ 9.83 (d,
J ¼ 5.4 Hz, 3H, C5H5N), 9.74 (d, J ¼ 5.4 Hz, 3H,
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