Switchable V-type [2]pseudorotaxanes

Article abstract of DOI:10.1021/ol9011794

A V-type molecule comprising a 2-[2-[4-(dimethylamino)phenyl]ethenyl] pyridinium cyanine branch and a p-aminophenoxy ethyl side arm was synthesized and can form quite different [2]pseudorotaxanes with cucurbit[7]uril (CB[7]) as a model thread in aqueous s

Full text of DOI:10.1021/ol9011794

ORGANIC
LETTERS
2009
Vol. 11, No. 15
3234-3237
Switchable V-Type [2]Pseudorotaxanes
Hongyuan Zhang, Qiaochun Wang,* Minhua Liu, Xiang Ma, and He Tian*
Key Laboratory for AdVanced Materials and Institute of Fine Chemicals,
East China UniVersity of Science & Technology, Shanghai 200237, P.R. China
tianhe@ecust.edu.cn; qcwang@ecust.edu.cn
Received May 26, 2009
ABSTRACT
A V-type molecule comprising a 2-[2-[4-(dimethylamino)phenyl]ethenyl]pyridinium cyanine branch and a p-aminophenoxy ethyl side arm was
synthesized and can form quite different [2]pseudorotaxanes with cucurbit[7]uril (CB[7]) as a model thread in aqueous solution. The CB[7]
ring can be switched reversibly from the cyanine branch to the aminophenoxy ethyl side arm by protonation of the aniline group, and the
color of the solution was changed from orange red to yellow.
A [2]pseudorotaxane is a type of supramolecular assembly
where a rodlike molecule threads a cyclic molecule. In recent
years, switchable [2]pseudorotaxanes become one of the hot
topics for their applications on the construction of molecular
devices such as switches,1 logic gates,2 sensors,3 valves,4
and machines.5 The integrating/disintegrating behavior and
the migration of the ring component between two binding
stations on the thread are the two fundamental switching
motions of a [2]pseudorotaxane. Compared with the disin-
tegrating switching, the migrating switching acts more like
a machine. However, in a [2]pseudorotaxane which com-
prises no bulky terminal groups on both ends of the axle,
once the two stations are arranged separately at about 180°,
the encircling of two rings on the rod to form a [3]pseu-
dorotaxane occurs, and the switching system becomes
complicated.6 To avoid the over-encircling, partially overlap-
ping the two stations is the strategy conventionally adopted.7
Herein we describe a novel method which generates steric
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Yang, Y.-W.; Patel, K.; Stoddart, J. F.; Zink, J. I. Angew. Chem., Int. Ed.
2008, 120, 2222–2226.
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Commun. 2001, 1860–1861. (b) Jeon, W. S.; Ziganshina, A. Y.; Lee, J. W.;
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4097–4100. (c) Silvi, S.; Arduini, A.; Pochini, A.; Secchi, A.; Tomasulo,
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10.1021/ol9011794 CCC: $40.75
Published on Web 07/02/2009
 2009 American Chemical Society

hindrance to prevent the over threading by settling two CB[7]
binding sites on the two arms of a V-type molecule (DP) at
an angle of about 60°. The CB[7] ring can be switched
between the two stations by pH stimuli, and two different
[2]pseudorotaxanes are originated.
The DP molecule is a 2-[2-[4-(dimethylamino)phenyl]ethe-
nyl]pyridinium cyanine dye with a p-aminophenoxy ethyl
side arm, and the dimethylamino phenyl moiety (DMA) and
p-aminophenoxy group (OA) on the two branches are
potential binding sites for the CB[7] macrocycle, as shown
in Figure 1. CB[7] has a 9.1 Å depth and a 7.3 Å equatorial
effects, and those near the carbonyl rim are scarcely
affected.10 The 1H NMR spectra of DP and DP⊂CB (DP in
the presence of 1.3 equiv of CB[7]) in D2O at pD 9.6 were
thus recorded to obtain the coconformation details of
DP⊂CB, as shown in Figure 2a and 2b. Compared with pure
Figure 2
.
1H NMR spectra (D2O, 500 MHz) of DP (a), DP⊂CB
(b), ADP (c), and ADP⊂CB (d). Spectra a and b were recorded at
pD 9.6 and c and d at pD 4.7.
Figure 1. Formation and the switching behaviors of the two
[2]pseudorotaxnes DP⊂CB and ADP⊂CB.
DP, the aromatic protons H7 and H8 on DMA conduct
distinct shielding effects (∆δH7 ) -0.05 and ∆δH8 ) -0.88
ppm) while the two vinyl protons H5 and H6 undergo
deshielding effects (∆δH5 ) +0.66 and ∆δH6 ) +0.20 ppm),
width,8 which is relatively larger than the distance of the
two branches.9 So it is impossible for two CB[7] rings to
encircle simultaneously both arms of DP because of steric
hindrance. The fact that same proton resonances of DP were
found in the presence of 1.3 and 2.5 equiv of CB[7] in D2O
at pD 9.6 (Figure S3, Supporting Information) also supports
the impossibility of the formation of a [3]pseudorotaxane.
The NMR integration of the appropriate proton resonances
of DP in the presence of 0.8 equiv of CB[7] indicates that
there are 79% of DP combine with CB[7] (Figure S3) and
the formation of a [2]pseudorotaxane (DP⊂CB) from sto-
ichiometric complexation was thus validated.
and the chemical shift of H9 is nearly unaltered (∆δH9
)
+0.03 ppm). These spectroscopic results indicate that, among
the [2]pseudorotaxane, the CB[7] macrocycle resides over
the benzene ring of the cyanine arm, the vinyl group stands
out from one opening of CB[7] and the two methyl groups
locate on the vicinity of the other. Those protons, H1, H2,
H3, H4 on pyridinium unit, and H12, H13 on OA, are all found
to move downfield (∆δH1 ) +0.08, ∆δH2 ) +0.21, ∆δH3
)
+0.17, ∆δH4 ) +0.18, ∆δH12 ) +0.57 and ∆δH13 ) +0.17
ppm), which is coincident with their location outside the
CB[7] cavity. It is interesting that the CB[7] in DP⊂CB is
away from the pyridinium and encircles the tail of the
cyanine dye, which is quite different from those common
CBs-based inclusion complexes where the CBs would stay
around the N+ and to the best of our knowledge is first
observed.
For the coconformational identification of cucurbituril-
based inclusion complexes, it is a common rule that the
protons inside the hydrophobic cucurbituril cavity undergo
shielding effect while the outside ones conduct deshielding
(6) (a) Tsortos, A.; Yannakopoulou, K.; Eliadou, K.; Mavridis, I. M.;
Nounesis, G. J. Phys. Chem. B 2001, 105, 2664–2671. (b) Yuan, L.; Wang,
R.; Macartney, D. H. J. Org. Chem. 2007, 72, 4539–4542.
(7) For examples using a common N+ for the stations, see: (a)
Sobransingh, D.; Kaifer, A. E. Org. Lett. 2006, 8, 3247–3250. (b) Sindelar,
V.; Silvi, S.; Parker, S. E.; Sobransingh, D.; Kaifer, A. E. AdV. Funct. Mater.
2007, 17, 694–701.
The pKa of OA is 5.2511 and that of DMA is determined
as 2.81,12 so the V-type molecule could exsist in DP form
(10) (a) Mock, W. L.; Shih, N.-Y. J. Org. Chem. 1986, 51, 4440–4446.
(b) Blanch, R. J.; Sleeman, A. J.; White, T. J.; Arnold, A. P.; Day., A. I.
Nano Lett. 2002, 2, 147–149. (c) Sindelar, V.; Cejas, M. A.; Raymo, F. M.;
Kaifer, A. E. New J. Chem. 2005, 29, 280–282. (d) Feng, K.; Wu, L.-Z.;
Zhang, L.-P.; Tung, C.-H. Dalton Trans. 2007, 3991–3994. (e) Ko, Y. H.;
Kim, H.; Kim, Y.; Kim, K. Angew. Chem., Int. Ed. 2008, 47, 4106–4109.
(8) Lagona, J.; Mukhopadhyay, P.; Chakrabarti, S.; Isaacs, L. Angew.
Chem., Int. Ed. 2005, 44, 4844–4870.
(9) The distance between the two benzene rings is about 7.3 Å from
the energy-minimized structure of DP; see Figure S2, Supporting Informa-
tion.
Org. Lett., Vol. 11, No. 15, 2009
3235

(pH > 5.25) and protonized ADP form (OA is protonized
while the cyanine arm remains intact at 2.81 < pH < 5.25).
Noticing that CBs favor highly stable inclusion complexes
lamino unit through the styrene group to the pyridinium
ring is shut down, and as a result, the cloud density of
the dimethylaminostyrene group increases while the
pyridinium ring decreases, which are coincident with the
above detected upfield shift of the protons on dimethy-
laminostyrene group and the downfield shift of the protons
on the pyridinium ring.
More importantly, the switching of the CB[7] ring from
the cyanine branch of DP⊂CB to the aminiumphenoxy
side arm of ADP⊂CB is easily detected by a change of
color. Figure 4 displays the absorption spectra of DP,
with cationic organic guest,13 especially those of the -NH3
+
group, which can develop favorable hydrogen-bonding,
ion-dipole interaction, and hydrophobic effects with CBs,
it is expected that the CB[7] macrocycle could move to the
ammonium side branch of ADP from the original cyanine
branch to form a new [2]pseudorotaxane. In this case, the
pKa value of OA is the critical point for the pH switching.
However, it should be noted that the pKa value of an amino
group would become bigger upon complexation,14 which
means that the pKa’ value of OA would not be less than
1
5.25 when forming an inclusion with CB[7]. The H NMR
spectra of ADP in the presence of 1.3 and 2.5 equiv of CB[7]
in D2O at pD 4.7 were then recorded and the observed same
proton signals of ADP again supports the non-[3]pseudoro-
taxane formation (Figure S4, Supporting Information).
1
The H NMR measurements of ADP and ADP in the
presence of 1.3 equiv of CB[7] (ADP⊂CB) in D2O at pD
4.7 were then carried out for the [2]pseudorotaxane formation
investigation, as shown in parts c and d, Figure 2. Compared
with ADP, the aromatic protons H12, H13 of the p-amini-
umphenoxy group on ADP⊂CB are observed obviously shift
to higher field (∆δH12 ) -0.15 and ∆δH13 ) -0.40 ppm),
and the ethyl protons H10 and H11 exhibit downfield shift
(∆δH10 ) +0.16 and ∆δH11 ) +0.26 ppm). These facts
reveal that ADP⊂CB is also a [2]pseudorotaxane where the
CB[7] ring encircles the p-aminiumphenoxy unit as assumed
with the ethyl group staying outside.
Figure 4
.
UV-vis absorption spectra of DP, ADP, DP⊂CB,
and ADP⊂CB (in H2O, 1.0 × 10-5 M). Curves a and b are
relevant to the absorption of DP⊂CB and ADP⊂CB after 10
swithing circles.
The protons of the cyanine branch, which locate outside
of the CB[7] ring, are thus expected to shift downfield as
a result of the deshielding effect of the ring. However,
except the downfield-shifted protons on the pyridinium,
those protons on the dimethylaminostyrene unitsH5, H6,
H7, H8 and H9sshift to higher field. The energy-minimized
structure of ADP⊂CB gives useful information, as il-
lustrated in Figure 3. The steric exclusion of CB[7]
ADP, as well as the [2]pseudorotaxane DP⊂CB and
ADP⊂CB. The colors of the aqueous solution of DP (at
pH 9.6) and ADP (at pH 4.7) are orange yellow and
exactly the same with the maximum absorption peaks both
at 445 nm. Compared with the identical color of the two
unbound molecules, the two corresponding inclusions with
CB[7] ring show obvious color changes. On one hand,
DP⊂CB conducts a color of orange red in the aqueous
solution with the maximum absorption peak at 459 nm,
which is a 14 nm bathochromic shift with respect to that
of DP. This finding might be attributed to the shielding
effects on the dimethyl amino group (donor) and the
deshielding effects on the pyridinium ring (acceptor) when
DMA was encapsulated inside the hydrophobic cavity of
CB[7], which result in the enhancement of intramolecular
charge-transfer effects among the cyanine branch. On the
(11) Hall, W. E.; Higuchi, T.; Pitman, I. H.; Uekama, K. J. Am. Chem.
Soc. 1972, 94, 8153–8156.
(12) The pKa value was determined by monitoring the absorption change
of DP (2.5 × 10-5 M) at λ ) 327 nm upon pH variation from 6.0 to 1.0;
see the Supporting Information.
(13) (a) Eelkema, R.; Maeda, K.; Odell, B.; Anderson, H. L. J. Am.
Chem. Soc. 2007, 129, 12384–12385. (b) Liu, Y.; Shi, J.; Chen, Y.; Ke,
C.-F. Angew. Chem., Int. Ed. 2008, 47, 7293–7296. (c) Gadde, S.; Batchelor,
E. K.; Weiss, J. P.; Ling, Y.; Kaifer, A. E. J. Am. Chem. Soc. 2008, 130,
17114–17119. (d) Rauwald, U.; Scherman, O. A. Angew. Chem., Int. Ed.
2008, 47, 3950–3953.
Figure 3
.
Energy-minimized structure of ADP⊂CB.
macrocycle forces the styrene group to turn at an angle
and detach the plane of pyridinium ring. Consequently,
the electron-donating effect origins from the dimethy-
(14) Praetorius, A.; Bailey, D. M.; Schwarzlose, T.; Nau, W. M. Org.
Lett. 2008, 18, 4089–4092, and references therein.
3236
Org. Lett., Vol. 11, No. 15, 2009

other hand, ADP⊂CB exhibits a 17 nm hypsochromic
shift relative to ADP with the peak maximum at 428 nm
and the color of the solution is yellow. This phenomenon
should result from the reduced conjugate degree of the
cyanine branch in view of the distortion mentioned above.
It should be noted that the switching between DP⊂CB
and ADP⊂CB is reversible. A switching circle where
DP⊂CB is converted to ADP⊂CB and then turned back to
DP⊂CB was achieved by adjusting the pH value of DP⊂CB
aqueous solution (1.0 × 10-5 M 250 mL, pH 9.6) to 4.7 and
afterward again to 9.6 using diluted HCl and NaOH aqueous
solutions (both 1.0 M). Such a switching circle can be
repeated many times. As shown in Figure 4, the absorption
curves of DP⊂CB and ADP⊂CB after 10 switching circles
are exactly the same with their original ones. The reversibility
of the switching process is thus validated.
Figure 5. Change of the absorption maximum of ADP aqueous
solution (2.5 × 10-5M) in the presence of 1.3 equiv of CB[7] upon
pH variation from 4.0 to 11.0.
The fact that the absorption maximum of the two [2]pseu-
dorotaxanes are different allowed us to estimate the pKa′
value of OA upon complexation. The moritoring of the
absorption maximum change upon the pH variation of
ADP⊂CB aquoeus solution from 4.0 to 11.0 were carried
out as illustrated in Figure 5. The pKa′ value was found to
be 7.31, which is a 2.06 shift corresponding to pKa. It can
also be seen that the selected pH values for states DP⊂CB
(pH 9.6) and ADP⊂CB (pH 4.7) are appropriate from this
titration curve.
nating the aniline group and such inclusion exchange is
accompanied with a color readout.
Acknowledgment. We thank the NSFC/China (50673025,
20603009), National Basic Research 973 Program
(2006CB806200), the Key Project of Chinese Ministry of
Education (107044), the Foundation for the Author of
National Excellent Doctoral Dissertation of PR China
(200758), Shanghai Science and Technology Development
Funds (07QA14012), and the Scientific Committee of
Shanghai for funds.
In conclusion, two CB[7] recognition sites were ar-
ranged on both arms of a new V-type cyanine molecule
consisting of an aminophenoxy ethyl side branch. The
steric hindrance between the two stations provides a novel
way to prohibit the formation of a [3]pseudorotaxane but
to favor [2]pseudorotaxane inclusions. It should be noted
that the CB[7] ring can be switched from the cyanine
branch to the aminophenoxy ethyl side branch by proto-
Supporting Information Available: Synthetic details,
NMR spectra, figures, and curves mentioned in the text. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL9011794
Org. Lett., Vol. 11, No. 15, 2009
3237