Electrochemical properties of 3,5-diphenylaniline units encapsulated within a crown ether. Effects of the macrocycle's aromatic functionality and ring size

Article abstract of DOI:10.1016/j.tetlet.2010.11.043

This Letter describes a series of [2]rotaxanes featuring a 3,5-diphenylaniline terminus in their dumbbell-shaped component and crown ethers as the macrocyclic component, prepared through imine formation and hydrogen bond - guided self-assembly. Electroche

Full text of DOI:10.1016/j.tetlet.2010.11.043

Tetrahedron Letters 52 (2011) 240–243
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Electrochemical properties of 3,5-diphenylaniline units encapsulated within
a crown ether. Effects of the macrocycle’s aromatic functionality and ring size
Yuji Tokunaga a, , Takuya Iwamoto a, Satoshi Nakashima a, Eiichi Shoji b, Ryuji Nakata c
⇑
a Department of Materials Science and Engineering, Faculty of Engineering, University of Fukui, Fukui 910-8507, Japan
b Department of Human and Artificial Intelligent Systems, Faculty of Engineering, University of Fukui, Fukui 910-8507, Japan
c Natural Science Education Laboratory, Faculty of Education and Regional Studies, University of Fukui, Fukui 910-8507, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
This Letter describes a series of [2]rotaxanes featuring a 3,5-diphenylaniline terminus in their dumbbell-
shaped component and crown ethers as the macrocyclic component, prepared through imine formation
and hydrogen bond—guided self-assembly. Electrochemical studies of these [2]rotaxanes revealed that
the oxidation potential of the aniline moiety when positioned within the cavity of a crown ether was
shifted negatively relative to that of the corresponding dumbbell-shaped compound, and that a crown
ether possessing a small cavity and a large number of aromatic rings had a more negative effect on
the oxidation potential of the aniline moiety than did a large-cavity crown ether featuring no aromatic
rings. UV experiments showed that absorption band of the rotaxanes bearing small crowns shifted to
longer wavelengths as compared to those of the rotaxanes having large crowns.
Received 27 August 2010
Revised 22 October 2010
Accepted 4 November 2010
Keywords:
Rotaxane
Aniline
Crown ether
Redox
Ó 2010 Elsevier Ltd. All rights reserved.
Molecular wire
Polyanilines have attracted much interest because of their
applications as electrochromic devices, sensors, electromechanical
actuators, and rechargeable batteries and for their use as hole-
transporting materials in organic light emitting diodes.1 One of
the attractive features of polyanilines is that they are simple to
synthesize in large quantities through oxidation of inexpensive
aniline.
polyaniline unit. We used differential pulse voltammetry and
UV to systematically investigate the effects of macrocyclic rings
on the electrochemical and photochemical properties of
p-conju-
gated system as a primary step of oligo- and polyanilines.
The crown ethers that we chose for this investigation were
[24]crown-8 (24C8, 1a), dibenzo[24]crown-8 (DB24C8, 1b),
dinaphtho[24]crown-8 (DN24C8, 1c),11 p-phenylenebenzo[26]
crown-8 (DB26C8, 1d),12 trinaphtho[27]crown-9 (TN27C9, 1e),
and dinaphtho[30]crown-10 (DN30C10, 1f),13 which contain 24-,
26-, 27-, and 30-membered rings and various aromatic units
(Fig. 1, Scheme 1). Although we have previously synthesized the
[2]rotaxanes 4a–c and the dumbbell-shaped molecule 4g,14 the
methodology was not optimal: (i) acylation of the [2]rotaxane salts
3 afforded 4 in poor yields when hexafluorophosphate was used as
the counter anion, meaning that counter ion exchange was neces-
sary prior to acylation; (ii) the selectivity of the acylation reactions
of the dialkylammonium moieties in the [2]rotaxanes 3 was poor.
Therefore, we have modified our previous method to synthesize
the [2]rotaxanes 4 more efficiently.
We synthesized the [2]rotaxanes 3a–f from the aldehyde 2,
the crown ethers 1a–f, and 3,5-diphenylaniline15 through ther-
modynamic covalent chemistry (reversible imine formation)16,17
and hydrogen bond—guided self-assembly (secondary dialkylam-
monium ion/crown ether complexation).18 Reduction of the
imine units and subsequent spontaneous salt formation in the
air gave the [2]rotaxanes 3a–f as bicarbonate salts. Selective
N-acylation of the dialkylammonium moieties afforded the
corresponding [2]rotaxanes 4a–e and the dumbbell-shaped
The properties of [2]rotaxanes containing p-conjugated systems
in their dumbbell-shaped components are highly tunable through
changes in the structure of their macrocyclic component; the de-
gree of stabilization,2 the electrochemical and/or photochemical
properties,3,4 and the three-dimensional structures of
p-conju-
gated moieties5 can all be regulated in the presence of encircling
macrocyclic species, including cyclophanes,6 cyclodextrins,2b,c,7
cucurbiturils,2a,5b,8 and crown ethers.9,10
Although the syntheses and characteristics of many [2]rotax-
anes containing
p-conjugated systems have been discussed, the
effects of the size and functionality of the macrocyclic compo-
nents on the properties of the dumbbell-shaped component are
rarely examined sufficiently or systematically, possibly because
the preparation of [2]rotaxanes featuring functionalized macrocy-
cles is somewhat complex. In this study, we prepared a series of
[2]rotaxanes comprising crown ether macrocyclic components (of
various sizes and aromatic functionalities) and a diphenylaniline-
terminated dumbbell-shaped component to function as a small
⇑
Corresponding author. Tel./fax: +81 776 27 8758.
E-mail address: tokunaga@matse.u-fukui.ac.jp (Y. Tokunaga).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.11.043

Y. Tokunaga et al. / Tetrahedron Letters 52 (2011) 240–243
241
reaction for the [2]rotaxane 4a,19 presumably because the hydro-
gen bonds in 3a were the strongest among all of the [2]rotaxanes
3a–f and, therefore, a weak base (Et3N) could not completely
deprotonated 3a. In contrast, the strong base (NaOBut) readily
deprotonated the dialkylammonium center in the [2]rotaxane
3a, causing the 24C8 unit to interact only weakly with the dial-
kylamino group, which, therefore, attacked the electrophile
(BOC2O) selectively, causing the 24C8 unit to hydrogen bond
weakly to the aniline unit, which has a relatively high Brönsted
acidity and hydrogen bond donating ability. Changing the base
from Et3N to NaOBut also improved the yield of the acylation
of 4e, in this case because of the low solubility of 3e in dichloro-
ethane. Notably, we could not isolate the acylated [2]rotaxane 4f
featuring the 30-membered-ring crown ether after tert-butoxy-
carbonylation, presumably because the diphenylaniline terminus
was not sufficiently bulky to prevent dethreading at high
temperature.20
O
O
O
O
O
O
O
O
1a: 24C8
1b: DB24C8
1c: DN24C8
O
O
O
O
O
O
O
O
O
O
O CH2
3
1d: DB26C8
1e: TN27C9
1H NMR spectra of the [2]rotaxanes 4a–e and the dumbbell-
shaped molecule 4g revealed evidence for the selective acylation
of the dialkylamine moieties of the [2]rotaxanes 3a–f and, there-
fore, the movement of the crown ether units from the dialkylamine
stations to the aniline stations. For example, in the 1H NMR spec-
trum of the [2]rotaxane 4a (Fig. 2c), the signals of the benzylic pro-
tons Hc and Hd (of 3a, Fig. 2b) have shifted to higher field, and
similar to those of the dumbbell-shaped molecule 4g ( Fig. 2a),
attributable to the loss of deshielding effects even though the
neighboring nitrogen atom has been acylated. Shielding effects of
the aromatic moieties are also responsible for the upfield shifts
of the signals of the aromatic protons Ha and Hb in 4a. In addition,
the signals of the protons He and Hg moved to lower field, a likely
result of shielding by the aromatic rings of the macrocyclic compo-
nent. Moreover, the aniline moiety appears to exist as a free base
because the integration of its NH unit corresponded to a single pro-
ton. And the correlation of signals between crown protons and Hb
in 3a and between crown protons and Hg in 4a was observed in
NOESY spectra of 3a and 4a, respectively.21
Table 1 reveals the differential pulse voltammetric behavior of
the [2]rotaxanes 4a–e and the dumbbell-shaped molecule 4g.
These experiments were performed in MeCN using a glassy carbon
electrode as the working electrode and LiClO4 (0.5 M) as the sup-
porting electrolyte. We observed oxidation processes for all of
the [2]rotaxanes 4a–e and the dumbbell-shaped molecule 4g.
The oxidation potentials of the aniline moieties in the [2]rotaxanes
4a–e were significantly shifted to more negative values (by up to
0.257 V for 4c) relative to that for 4g.
O
O
O
O
O
O
O
O
O
O
1f: DN30C10
Figure 1.
1) 3,5-diphenylaniline,
+
Ph
crown ethers 1
N
H2
CHO
-
2) LiAlH4
3) air (CO2)
Ph
PF6
Hd
2
Ph
Hc
Hb
Hh
Ph
Boc2O
+
Ph
Ha
N
N
Hf
H2
tBuONa in DMF
or Et3N in (ClCH2)2
Hg
-
Ph
He
HCO3
3d: ring = DB26C8 51%
3a: ring = 24C8 77%
3b: ring = DB24C8 79% 3e: ring = TN27C9 22%
3c: ring = DN24C8 49%
3f: ring = DN30C10 31%
(3f as a PF6 salt)
Ph
Hh
Hb Hc
Hd
Ph
Ha
Hf
N
Two possible interactions between the two components might
be responsible for the negative shifts of oxidation potentials: p–p
Boc
N
Ph
Hg
He
Ph
stacking and hydrogen bonding interactions in the rotaxanes 4.
For the [2]rotaxanes 4a–c, which all possess 24-membered macro-
cyclic components, we observed an effect of the presence of aro-
matic moieties in the crown ether units on the oxidation
potentials: the potential shifted to more-negative values upon
increasing the number of aromatic rings in the crown ether unit
(from zero in 1a, to two in 1b, to four in 1c), suggesting that p–p
stacking interactions primarily increased electron density of the
aniline moieties in these cases.
4a: ring = 24C8 75% (24%)*
4b: ring = DB24C8 72% (57%)*
4c: ring = DN24C8 84% (65%)*
4d: ring = DB26C8 80% (74%)*
4e: ring = TN27C9 70% (15%)*
4f: ring = DN30C10 (0%)*
4g: no ring
*( ): wnen Et3N was used as a base
Notably, however, we did not observe a similar effect for the
aromatic rings in the [2]rotaxane 4d, the crown ether component
of which features one hydroquinone unit and one catechol unit;
the oxidation potential for 4d was close to those of 4a, which fea-
tures a 24C8 unit (i.e., no aromatic units). The 1H NMR spectra
(DMSO-d6, 353 K) of the [2]rotaxanes 4a–e provide a clue regard-
ing the position of the hydroquinone ring of 4d; the signals for
the protons He in 4a–c and 4e appear in the range from 4.67 to
4.96 ppm, whereas that in 4d appears at 3.76 ppm. This high field
shift for 4d suggests a shielding effect of the hydroquinone ring
1) Boc2O, Et3N 87%
4g
2
2) 3,5-diphenylaniline
3) NaBH4 (85% for 3 steps)
Scheme 1.
molecule 4g. Exchanging Et3N for sodium tert-butoxide had a
particularly large effect on improving the yield of the acylation

242
Y. Tokunaga et al. / Tetrahedron Letters 52 (2011) 240–243
Figure 2. 1H NMR spectra (500 MHz, DMSO-d6, 353 K) of (a) the dumbbell-shaped molecule 4g, (b) the NH2þ-centered [2]rotaxane 3a, and (c) the acylated [2]rotaxane 4a.
Table 1
Oxidation potentials and absorption maxima of the [2]rotaxanes 4a–e and the dumbbell-shaped molecule 4g
Rotaxane
Ring
4a
24C8
4b
DB24C8
4c
DN24C8
4d
DB26C8
4e
TN27C9
4g
None
a
Oxidation potential
kmax
+0.306
348
5
1
+0.218
346
5
1
+0.200
346
5
1
+0.311
342
5
1
+0.331
342
5
1
+0.457
333
5
1
b
c
c
c
a
D
V values versus ferrocene. All experiments were performed twice and the average is shown in table. Substrates in acetonitrile containing supporting electrolyte LiClO4
(0.5 M); recorded in acetonitrile containing LiClO4 (0.5 M). Pulse amplitude 50 mV, pulse with 50 ms, scan rate 5 mV/s, pulse interval 0.3 s.
b
UV–vis spectra of rotaxanes and the dumbbell-shaped molecule (34
Ref. 14.
lM) in 1% triethylamine-acetonitrile.
c
over the ArCHN unit; therefore, it does not effectively overlap the
aniline moiety.
rotaxanes 4a–c are similar, although the oxidation potentials were
influenced by the aromatic rings. The respective shielding effects of
macrocycles on HOMO and LUMO orbitals display similar tenden-
cies among these rotaxanes 4a–c. Probably, the aromatic effects
are canceled by the hydrogen bonding and/or dipole interactions.
In contrast, the kmax of rotaxanes 4d and 4e, which feature large
crown ethers, are shorter than those of 4a–c, although some effects
could be achieved by the macrocycles. Therefore, we suspect that
the long-wave shifting of aniline moiety in UV spectra resulted
from the effectiveness of interactions, which depend on the ring
size of macrocyclic components.
In summary, we have used reversible imine formation, hydro-
gen bonding between crown ethers and dialkylammonium ions,
and selective acylation of dialkylammonium moieties to construct
the five [2]rotaxanes 4a–e featuring aniline moieties in their
dumbbell-shaped units and crown ethers as their macrocyclic
components. Differential pulse voltammetry and UV experiments
of these [2]rotaxanes and the corresponding dumbbell-shaped
molecule 4g revealed negative shifts in the oxidation potentials
and long-wave shifting of the aniline moieties when situated with-
in crown ether cavities, influenced by both the nature of the aro-
matic units and the ring size of the crown ether components.
The oxidation potential of 4e was more positive than that of 4a.
We suspect that hydrogen bonding was important to increase the
electron density of aniline moieties. In the 1H NMR spectra of all
the [2]rotaxanes 4a–e, the presence of concentration-independent
signals for the NH units (in the range 4.98–5.72 ppm) supports the
presence of hydrogen bonds between the aniline NH units and
the crown ether moieties in their ground states. We suspect that
the strength of the hydrogen bonds would follow the order
4a > 4b ꢀ 4c > 4d and 4e, because 24-membered rings provide
stronger hydrogen bonds12a and because oxygen atoms of dialkyl
ethers have higher basicity relative to those of alkyl aryl ethers.
Therefore, the negative shifts in the oxidation potentials resulted
from the combination of effects from
bonding interactions.
p–p stacking and hydrogen
The absorption maxima (kmax) of rotaxanes 4a–e and their
dumbbell-shaped molecule 4g are shown in Table 1. The wave-
length shifts of rotaxanes revealed the shielding effects of macro-
cycles; noncovalent interactions between the aniline moiety and
crown ethers seem to cause a decrease in the energy gaps between
HOMO and LUMO orbitals. Interestingly, the kmax values of

Y. Tokunaga et al. / Tetrahedron Letters 52 (2011) 240–243
243
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Acknowledgments
We thank Dr. T. Hoshi (Tohoku University) for performing the
spectroscopic measurements. This study was supported by the
University of Fukui.
Supplementary data
Supplementary data (NOESY spectra of [2]rotaxanes 3a and 4a,
differential pulse voltammograms of the [2]rotaxanes 4a–e and the
dumbbell-shaped molecule 4g, and UV spectra of [2]rotaxanes
4a–e and the dumbbell-shaped molecule 4g) associated with this
article can be found, in the online version, at doi:10.1016/
j.tetlet.2010.11.043.
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