Optically sensed, molecular shuttles driven by acid-base chemistry

Article abstract of DOI:10.1039/b710708k

A pair of bistable [2]rotaxane, molecular shuttles were prepared that combine 1,2-bis(pyridinium)ethane and benzylanilinium recognition sites; acid-base controlled shuttling of DB24C8 was accompanied by a change in colour and/or fluorescence intensity. The Royal Society of Chemistry.

Full text of DOI:10.1039/b710708k

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www.rsc.org/chemcomm | ChemComm
Optically sensed, molecular shuttles driven by acid–base chemistry{
Sarah J. Vella, Jorge Tiburcio and Stephen J. Loeb*
Received (in Austin, TX, USA) 12th July 2007, Accepted 28th August 2007
First published as an Advance Article on the web 10th September 2007
DOI: 10.1039/b710708k
A pair of bistable [2]rotaxane, molecular shuttles were prepared
that combine 1,2-bis(pyridinium)ethane and benzylanilinium
recognition sites; acid–base controlled shuttling of DB24C8 was
accompanied by a change in colour and/or fluorescence
intensity.
A number of molecular switches based on the concept of a
molecular shuttle have been reported.1 The design of such a
bistable rotaxane requires two different recognition sites on the
axle and a method for controlling the positioning of the wheel
component. Ideally, this equilibrium situation can be tuned such
that each of the two possible positions of the wheel can be
stabilised almost exclusively resulting in the apparent observation
of two distinct states; ON and OFF. Conversion between the two
states can then be induced by altering the position of the
equilibrium via some type of external perturbation.2 In addition,
a desirable feature would be the ability to observe this ON/OFF
molecular switching by some easily detectable optical signal such
as a change in colour or fluorescence intensity.3
Scheme 1 Preparation of [2]rotaxane molecular shuttle 32+. Reagents
We recently reported the formation of [2]pseudorotaxanes
and conditions: (i) MeCN, reflux, 48 h; (ii) 5 equiv. DB24C8 in a two-layer
utilising benzylanilinium axles and dibenzo-24-crown-8 ether
(DB24C8) wheels.4 This motif combines the strong N–H
…
MeNO2/NaOTf (aq) mixture at room temperature for 7 days; in the
absence of crown this produces the axle 42+. All cations are isolated as the
O
CF3SO3 (OTf2) salts.
2
hydrogen bonding and acid/base control of the dialkylammonium
template with the enhanced p-stacking properties found for the
recognition of 1,2-bis(pyridinium)ethane dications.5 We have also
reported control over [2]pseudorotaxane formation for a specific
type of 1,2-bis(pyridinium)ethane axle and DB24C8 due to the
switching ON/OFF of an intramolecular charge transfer (ICT).6
We report herein, the synthesis of two acid–base driven [2]rotaxane
molecular shuttles that incorporate both the benzylanilinium and
1,2-bis(pyridinium)ethane recognition sites.
the crown ether, and the proton resonances b, g and h shift upfield
due to p-stacking interactions. In contrast, protons belonging to
the benzylaniline portion of the axle, i, k, l and m, do not shift
indicating DB24C8 only interacts with the 1,2-bis(pyridinium)-
ethane recognition site. This is to be expected since unprotonated
benzylaniline is incapable of complexing with DB24C8.4
Single crystals of [3][OTf]2 were grown by slow evaporation of
an MeCN solution. Fig. 1 shows the structure of 32+ and verifies
that DB24C8 resides at the 1,2-bis(pyridinium)ethane binding
The [2]rotaxane molecular shuttles were synthesized as shown in
Scheme 1. A Suzuki coupling reaction was used to synthesize
4-pyridylaniline7 which was subsequently reacted in neat 1,2-
dibromoethane to form the bromoethylpyridinium species 1+.
Further reaction with 3,5-lutidine, which also conveniently acts as
a stopper,8 gave 22+ which was combined in a two-phase solvent
system with five equivalents of DB24C8 and half an equivalent of
3,5-bis(trifluoromethyl)benzyl bromide to give the [2]rotaxane
site.{ The crown ether adopts a less common C-shaped
conformation9 instead of the typical S-shaped one seen for most
molecular shuttle 32+
.
Comparing the 1H NMR spectrum, in MeCN-d3, of [2]rotaxane
32+ with that for the stoppered axle 42+ indicates that the ethylene
protons d and e, as well as the ortho pyridinium protons c and f
shift downfield since they are involved in hydrogen-bonding with
Department of Chemistry and Biochemistry, University of Windsor,
Windsor, Ontario, Canada. E-mail: loeb@uwindsor.ca;
Fax: 1 519 973 7098; Tel: 1 519 253 3000
{ Electronic supplementary information (ESI) available: Detailed synthesis
of compounds 1–5. See DOI: 10.1039/b710708k
Fig. 1 A ball-and-stick representation of the X-ray crystal structure of
molecularshuttle32+ showingthenumberingscheme.Blue=nitrogen,red=
oxygen, green = fluorine, black = carbon. Carbon atoms are numbered
continuously along the pyridinium axle and around the macrocycle wheel.
4752 | Chem. Commun., 2007, 4752–4754
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other crystal structures with this binding motif.10 This is attributed
to the presence of a significant intramolecular charge transfer from
the aniline nitrogen to the pyridinium nitrogen which reduces the
positive charge at the pyridinium nitrogen and increases the
electron density of the pyridinium ring thus disfavouring
p-stacking. Consequently, DB24C8 interacts in a double p-stacking
manner with the lutidinium ring; centroid to centroid distances are
indicating this site does not interact with DB24C8. In contrast,
protons associated with the benzylanilinium site, i, j, k and l shift
downfield as a result of hydrogen-bonding and h and m shift
upfield due to p-stacking.
1
Thus, the H NMR spectra in CD2Cl2 of unprotonated and
protonated molecular shuttles 32+ and H-33+ provide direct
evidence that when the axle is unprotonated DB24C8 prefers the
1,2-bis(pyridinium)ethane site and when the axle is protonated the
crown prefers the benzylanilinium site. This implies that in
CD2Cl2, this new molecular shuttle behaves very much like a
bistable ON/OFF switch. Addition of base (NEt3) returns the
shuttle to its original state and the shuttling process can be cycled
by repeated addition of acid (CF3SO3H) and base without
significant degradation of the compound as observed by 1H
NMR spectroscopy.
˚
3.82 and 3.75 A. Both the N–CH2CH2–N and C–NH–CH2–C
chains adopt the stepped anti conformation favourable for
interaction with DB24C8.
Since we have previously shown that a benzylanilinium group
(i.e. a protonated aniline) will bind DB24C8 quite effectively,4 it
was of interest to add H+ to 32+ to create H-33+ and set up a
competition for DB24C8 between the two types of recognition
sites. Fig. 2 shows the 1H NMR spectra, in CD2Cl2, of the
molecular shuttle states 32+ and H-33+. For both 32+ and H-33+
,
As designed, this new molecular shuttle contains a 4-pyridinium
aniline ICT chromophore.6 Thus, the switching between 32+ and
H-33+ is accompanied by a vivid colour change from bright orange
to colourless. Fig. 3 shows the absorption spectra for the two states
of the shuttle. The ICT between the aniline nitrogen and the
pyridinium nitrogen in 32+ produces an absorption peak at 410 nm
(CH2Cl2), but upon protonation of the aniline nitrogen, which
triggers the crown to move to the benzylanilinium site, the ICT is
eliminated and the intense colour disappears. Therefore, the
only one set of peaks is observed. For 32+, the spectrum is very
similar to that observed in MeCN-d3 and indicates that DB24C8
resides at the 1,2-bis(pyridinium)ethane site. While for H-33+, it
appears that upon protonation DB24C8 prefers to reside at the
1
benzylanilinium recognition site. A comparison of the H NMR
spectra of rotaxane H-33+ and axle H-43+ shows that protons a–f,
those located at the 1,2-bis(pyridinium)ethane site, do not shift
1
shuttling of DB24C8 can be verified by H NMR spectroscopy
and indirectly signalled by a visible colour change detected with the
naked eye.
Another attractive feature of this molecular shuttle design is the
ability to make modular structural changes. To demonstrate the
versatility of this system, we conceptually exchanged the 3,5-
bis(trifluoromethyl)benzyl group for an anthracenyl group to give
52+. The synthesis of 52+ was the same as outlined for 32+ except
9-bromomethylanthracene was used in place of 3,5-bis(trifluoro-
methyl)benzyl bromide (see ESI{). The inclusion of an anthracene
group introduces the ability to monitor the molecular shuttling
behaviour by fluorescence spectroscopy.11
As with 32+ and H-33+, the 1H NMR spectra for 52+ and H-53+
demonstrate that, in CD2Cl2, this molecular shuttle act as a
bistable switch showing only one set of resonances for each
spectrum. The UV-visible spectra of 52+ and H-53+ are also very
similar to 32+ and H-33+; the unprotonated species having a large
absorption band at 420 nm which disappears upon protonation.
Fig. 2 A comparison of the 1H NMR spectra of 32+ (top) and H-33+
Fig. 3 UV-vis absorption spectra for 32+ and H-33+ in CH2Cl2 solution
at a concentration of 2.0 6 1023 M.
(bottom) in CD2Cl2 at 298 K.
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1 For a general review, see: (a) V. Balzani, A. Credi and M. Venturi,
Molecular Devices and Machines, Wiley-VCH, Weinheim, 2003; (b)
A. Mateo-Alonso, G. Fioravanti, M. Marcaccio, F. Paolucci,
G. M. A. Rahman, C. Ehli, D. M. Guldi and M. Prato, Chem.
Commun., 2007, 1945; (c) D. S. Marlin, D. G. Cabrera, D. A. Leigh and
A. M. Z. Slawin, Angew. Chem., Int. Ed., 2006, 45, 1385; (d)
H. Murakami, A. Kawabuchi, R. Matsumoto, T. Ido and
N. Nakashima, J. Am. Chem. Soc., 2005, 127, 15891; (e) G. Bottari,
F. Dehez, D. A. Leigh, P. J. Nash, E. M. Perez, J. K. Y. Wong and
F. Zerbetto, Angew. Chem., Int. Ed., 2004, 42, 5886; (f) G. Bottari,
D. A. Leigh and E. M. Perez, J. Am. Chem. Soc., 2003, 125, 13360; (g)
for the original report of a molecular shuttle, see: P. L. Anelli, N. Spencer
and J. F. Stoddart, J. Am. Chem. Soc., 1991, 113, 5131.
2 (a) A. Mateo-Alonso, C. Ehli, G. M. A. Rahman, D. M. Guldi,
G. Fioravanti, M. Marcaccio, F. Paolucci and M. Prato, Angew. Chem.,
Int. Ed., 2007, 46, 3521; (b) A. B. Braunschweig, W. R. Dichtel,
O. S. Miljanic, M. A. Olson, J. M. Spruell, S. I. Khan, J. R. Heath and
J. F. Stoddart, Chem.–Asian J., 2007, 2, 634; (c) V. Balzani,
M. Clemente-Leon, A. Credi, M. Semeraro, M. Venturi, H.-R. Tseng,
S. Wenger, S. Saha and J. F. Stoddart, Aust. J. Chem., 2006, 59, 193; (d)
K. Noergaard, B. W. Laursen, S. Nygaard, K. Kjaer, H.-R. Tseng,
A. H. Flood, J. F. Stoddart and T. Bjoernholm, Angew. Chem., Int. Ed.,
2005, 44, 7035; (e) A. Altieri, G. Bottari, F. Dehez, D. A. Leigh,
J. K. Y. Wong and F. Zerbetto, Angew. Chem., Int. Ed., 2003, 42, 2296;
(f) for an example containing a 1,2-bis(pyridinium) recognition site, see:
A. M. Elizarov, S. H. Chiu and J. F. Stoddart, J. Org. Chem., 2002, 67,
9175.
Fig. 4 Fluorescence emission spectra for 52+ and H-53+ in CH2Cl2
solution at a concentration of 2.0 6 1023 M. Excitation wavelength =
250 nm, * denotes an artefact of the instrumentation which occurs at
double the excitation wavelength; 500 nm.
With the elimination of the ICT absorption band, three absorption
maxima of the anthracene fragment become apparent at 389, 368
and 348 nm. Fig. 4 shows the emission spectra of 52+ and H-53+ in
CH2Cl2. The spectrum of 52+ is complicated but definitely shows
stunted emission, probably due primarily to quenching of
fluorescence by the lone pair of electrons on the aniline nitrogen.11
Protonation partially restores normal anthracene fluorescence as
3 (a) E. M. Perez, D. T. F. Dryden, D. A. Leigh, G. Teobaldi and
F. Zerbetto, J. Am. Chem. Soc., 2004, 126, 12210; (b) D. H. Qu,
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Q. C. Wang, D. H. Qu, J. Ren, K. C. Chen and H. Tian, Angew. Chem.,
´
Int. Ed., 2004, 43, 2661; (d) D. A. Leigh, M. A. F. Morales, E. M. Pe´rez,
J. K. Y. Wong, C. G. Saiz, A. M. Z. Slawin, A. J. Carmichael,
D. M. Haddleton, A. M. Brouwer, W. J. Burma, G. W. H. Wurpel,
S. Leo´n and F. Zerbetto, Angew. Chem., Int. Ed., 2005, 44, 3062.
4 S. J. Loeb, J. Tiburcio and S. J. Vella, Org. Lett., 2005, 7, 4923.
5 (a) S. J. Loeb and J. A. Wisner, Angew. Chem., Int. Ed., 1998, 37, 2838;
(b) S. J. Loeb and J. A. Wisner, Chem. Commun., 1998, 2757; (c)
S. J. Loeb and J. A. Wisner, Chem. Commun., 2000, 845; (d)
A. L. Hubbard, G. J. E. Davidson, R. H. Patel, J. A. Wisner and
S. J. Loeb, Chem. Commun., 2004, 138; (e) S. J. Loeb and J. A. Wisner,
Chem. Commun., 2000, 1939; (f) S. J. Loeb and D. A. Tramontozzi,
Org. Biomol. Chem., 2005, 3, 1393.
6 S. J. Vella, J. Tiburcio and S. J. Loeb, Org. Lett., 2006, 8, 3421.
7 This is a known compound which is also commercially available.
8 (a) G. J. E. Davidson, S. J. Loeb, N. A. Parekh and J. A. Wisner,
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S. J. Vella, Chem. Commun., 2006, 1598.
seen in the spectrum of H-53+
.
Molecular shuttles combining 1,2-bis(pyridinium)ethane and
benzylanilinium recognition sites and utilising DB24C8 as the
mobile fragment have been described. The system incorporates a
4-pyridinium aniline chromophore unit, and is modular enough to
allow easy incorporation of bulky stoppers that may also possess
1
useful spectroscopic handles. While H NMR spectroscopy was
ultimately used to elucidate the position of the crown ether on the
axle, the protonation/deprotonation steps used to drive switching
could be sensed optically by a measurable change in colour or
fluorescence intensity and thus provide indirect evidence of
molecular shuttling.
9 For examples of the C-conformation for DB24C8 with this recognition
motif see ref. 4, and D. J. Hoffart and S. J. Loeb, Angew. Chem., Int.
Ed., 2005, 44, 901.
10 S. J. Loeb, J. Tiburcio, S. J. Vella and J. A. Wisner, Org. Biomol. Chem.,
2006, 4, 667.
Notes and references
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G. Nishimura and T. Hirai, Org. Lett., 2005, 7, 2611; (e) V. Amendola,
L. Fabbrizzi, C. Mangano, P. Pallavicini, A. Perotti and A. Taglietti,
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{ X-Ray crystal data were collected on a Bru¨ker APEX CCD
diffractometer following standard procedures.12,13 The crystals were of
poor quality which is reflected in the quality of the data as represented by
Rint. However, the solution was straightforward and the metric parameters
chemically reasonable. One of the triflate anions was poorly defined and
restrained to have the same geometry as the well defined one. Summary of
crystal data for [3][OTf]2?2H2O: C55H65F12N3O16S2, M = 1316.22, T =
173(2) K, monoclinic, space group P21/c, a = 23.985(10), b = 15.436(6),
3
c = 17.782(7) A, b = 102.815(8)u, V = 6420(4) A , Dc = 1.362 g cm23, m =
0.183 mm21, Z = 4, reflections collected = 33416 (Rint = 0.1162), final R
indices [I . 2s(I)]: R1 = 0.1189, wR2 = 0.3152, R indices (all data): R1 =
0.2993. wR2 = 0.4122, GoF = 0.976 with data/variables/restraints = 11273/
784/31. CCDC 656476. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/b710708k
˚
˚
12 G. M. Sheldrick, SHELXTL 5.03 Program Library, Bru¨ker Analytical
Instrument Division, Madison, WI, USA.
13 DIAMOND 3.1 - Visual Crystal Structure Information System
CRYSTAL IMPACT, Postfach 1251, D-53002 Bonn, 2006.
4754 | Chem. Commun., 2007, 4752–4754
This journal is ß The Royal Society of Chemistry 2007