Single-color pseudorotaxane-based temperature sensing

Article abstract of DOI:10.1039/c0nj00541j

Colored pseudorotaxane solutions can be used to assess temperature changes over large temperature windows. The color intensity of our novel pseudorotaxane systems decreases gradually from -50 to +50 °C with no shift in absorption maximum, making these and similar pseudorotaxanes attractive candidates for single-wavelength colorimetric temperature sensors.

Full text of DOI:10.1039/c0nj00541j

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Single-color pseudorotaxane-based temperature sensingw
Isurika R. Fernando, Semere G. Bairu, Guda Ramakrishna* and Gellert Mezei*
Received (in Victoria, Australia) 10th July 2010, Accepted 30th July 2010
DOI: 10.1039/c0nj00541j
Colored pseudorotaxane solutions can be used to assess
temperature changes over large temperature windows. The color
intensity of our novel pseudorotaxane systems decreases gradually
from ꢀ50 to +50 1C with no shift in absorption maximum,
making these and similar pseudorotaxanes attractive candidates
for single-wavelength colorimetric temperature sensors.
Pseudorotaxanes are supramolecules comprised of an axle-like
molecule threaded through a wheel-like molecule, where the
two components are held together by intermolecular forces
such as ion–dipole interactions, hydrogen bonding, p–p
stacking, electrostatic interactions or coordination bonds.
Attachment of bulky groups (stoppers) to the two ends of
the axle component results in a rotaxane, in which the wheel
component is mechanically locked onto the axle component.1
One of the most interesting features of such molecular archi-
tectures is the possibility of switching the position of the wheel
Scheme
1 Temperature-dependent pseudorotaxane formation
along the axle in a controlled and reversible fashion, triggered
by external stimuli such as chemical (pH change, metal ion,
etc.), electrochemical or light input. The resulting ‘‘molecular
machines’’ can potentially be used in a variety of molecular-
level applications, including mechanical (shuttles, nanovalves,
elevators, muscles, etc.) and electronic devices (information
storage, components for molecular electronics such as switches
and single-molecule transistors, etc.).2 Herein we show that a
colored pseudorotaxane solution has a hitherto unexplored
potential: temperature sensing at a single wavelength over a
temperature window of 100 1C. Although the influence of
temperature on the color intensity of some pseudorotaxane
systems has occasionally been noticed,3,4 to our knowledge
this is the first reported systematic study of the temperature/
optical absorption relationship in these systems.
equilibrium.
color to the pseudorotaxane system and serves as a visual
indicator for the self-assembly process. C4BP is prepared from
the corresponding bromide salt4 by anion exchange in H2O
using a saturated aqueous solution of NH4PF6. Single-crystals
were obtained from 1 : 1 CH3CN/CHCl3 solutions by Et2O or
iPr2O vapour diffusion.w BN32C8 is a novel crown ether
reported here for the first time, while BPP34C105 and
BN38C106 are prepared according to previously published
procedures. BN32C8 is prepared in two steps similarly to
BN38C10,6 from 1,5-dihydroxynaphthalene and tri(ethylene
glycol) ditosylate in 13% overall yield. Single-crystals were
i
grown from CHCl3 by Et2O or Pr2O vapour diffusion.w
Pseudorotaxane solutions are prepared by mixing the
axle- and wheel-components in various solvents/solvent
mixtures, depending on the solubility of the individual
components. C4BP is soluble in acetone and acetonitrile, while
the wheel molecules are soluble in chloroform and dichloro-
methane. BPP34C10 is also soluble in acetone and methanol.
Both the axle- and wheel-components are colorless; upon
mixing at room temperature, a dark orange (with BPP34C10)
or red (with BN38C10 or BN32C8) color appears. A charge
transfer band centred at 448, 490 and 501 nm, respectively, is
observed in the UV-Vis spectra of the resulting solutions
in (CH3)2CO/CH2Cl2 (2 : 1), indicating pseudorotaxane
formation. 1H NMR spectroscopy in (CD3)2CO/CDCl3
(2 : 1) solutions also confirms complex formation, as indicated
by significant shifts of proton signals relative to the individual
components.w The most affected peaks are those of the
moieties involved in charge transfer: the bipyridinium protons
of the axle (up to 0.78 ppm shift) and aromatic protons of the
The axle-component of the pseudorotaxanes used in this work
consists of N,N0-bis[3-(carboethoxy)propyl]-4,40-bipyridinium
hexafluorophosphate (C4BP), while the wheel-component is
an aromatic crown ether: bis-para-phenylene-34-crown-10
(BPP34C10), bis-1,5-naphtho-38-crown-10 (BN38C10) or bis-
1,5-naphtho-32-crown-8 (BN32C8) (Scheme 1). The driving
force for pseudorotaxane assembly using the above mentioned
components is the combined effect of p–p stacking
and ion–dipole interactions between the electron-deficient
bipyridinium dication (axle) and the electron-rich crown-ether
(wheel). The resulting charge-transfer imparts an orange/red
Department of Chemistry, Western Michigan University, Kalamazoo,
MI 49008, USA. E-mail: gellert.mezei@wmich.edu,
rama.guda@wmich.edu; Tel: +1 269 387 2859
w Electronic supplementary information (ESI) available: Synthesis,
crystallographic, NMR and UV-Vis details. CCDC reference numbers
780506–780508. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0nj00541j
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Fig. 2 (a) Optical absorption spectra of 1.0 mM C4BP/BPP34C10
pseudorotaxane (1 : 1) in (CH3)2CO/CH2Cl2 (2 : 1) as a function of
temperature (ꢁ0.1 K); (b) plot of extinction coefficients as a function
of temperature for the different pseudorotaxanes investigated.
ꢀ
Fig. 1 Crystal structure of C4BP/BPP34C10 (two PF6 counterions
naphthalene units of the crown ether wheel and the electron
acceptor bipyridinium dication axle. Similarly colored charge
transfer complexes formed by the bipyridinium moiety with
various electron donor systems are well documented in the
literature.4,9 The continuous and reversible change in absorption
with temperature observed in the present systems is indicative
of an equilibrium between the colored pseudorotaxane and the
colorless axle- and wheel-components in solution over a
100 1C window. Interestingly, the color of C4BP/BPP34C10
pseudorotaxane crystals is lost abruptly at 107–109 1C before
melting at 191–193 1C. The pseudorotaxane solutions studied
here are stable over time and the absorption values are
reproducible. These results suggest that the present pseudo-
rotaxane systems could be used to measure temperature.
Fig. 2b shows the plot of extinction coefficients of the three
systems studied as a function of temperature. The obtained
calibration curves can be employed as scales for colorimetric
thermometers. Most known colorimetric thermometers
contain thermochromic pigments based on liquid crystals.
The color change in liquid crystals,10 as in most inorganic11
and organic12 thermochromic compounds, occurs at specific
temperatures over a very narrow range. Only a few examples
of individual chemical systems are known today where the
color change spans a large temperature range.13 In these
systems, the temperature dependent color change often
involves a shift in the absorption maximum. For application
purposes, such as in microfluidic devices14 and other complex
systems where conventional thermometers are inappropriate,
it would be advantageous to have a colorimetric thermometer
that can be read at a single wavelength.
are omitted for clarity), color code: O—red, N—blue, C—black,
H—pink.
wheel (up to 0.50 ppm shift). Similar results have been
observed in related systems.4,7 Addition of excess axle or
wheel component to a 1 : 1 pseudorotaxane solution leads
only to slight changes in chemical shifts but not to the
appearance of new peaks, indicating a fast exchange equili-
brium between individual components and the corresponding
pseudorotaxane at room temperature.
Dark orange crystals of C4BP/BPP34C10 pseudorotaxane,
suitable for X-ray diffraction, were obtained in 90% yield from
1 : 1 (CH3)2CO/CH3OH or (CH3)2CO/CHCl3 solutions by
i
layering with Pr2O at ꢀ20 1C.z The crystal lattice contains
one [2]pseudorotaxane (wheelꢀand axle lie about a common
inversion center) and two PF6 counterions per unit (Fig. 1).
The absence of interstitial solvent molecules accounts for the
stability of the crystals in air at room temperature. Within the
pseudorotaxane units, the two phenylene rings of the crown-
ether are symmetrically located above and below the centre of
the bipyridinium core, with a 3.55(1) A separation between the
centres of the donor and acceptor units (similar to previously
characterized related systems).8 Pseudorotaxane units within
the crystal lattice interact weakly with each other through p–p
stacks between adjacent crown-ether phenylene groups
(center-to-center distance: 5.24(1) A).
During previous work on pseudomorphism and crystal
growth of similar pseudorotaxane systems at different
temperatures, we observed a variation of color intensity with
temperature: the absorption more than doubled on cooling a
solution from room temperature to ꢀ40 1C.4 To better under-
stand the temperature dependence of absorption and to
describe the thermodynamics of pseudorotaxane formation
in solution, optical absorption measurements were carried out
on three systems using C4BP as the axle and three different
wheels, BPP34C10, DN38C10 and DN32C8. Fig. 2a shows the
optical absorption spectra of C4BP/BPP34C10 at different
temperatures from ꢀ50 1C to +50 1C (similar results were
obtained with BN38C10 and BN32C8).w Since the individual
axle and wheel components do not absorb in the visible region,
the absorptions of the three different pseudorotaxane
solutions centred at 448, 490 and 501 nm, respectively, are
attributed to charge transfer complexation as a result of
the interaction between the electron donor benzene or
While in the solid state the 1 : 1 axle/wheel ratio is clearly
indicated by the crystal structure of C4BP/BPP34C10, the
stoichiometry of pseudorotaxane formation in solution was
investigated by optical absorption measurements using Job’s
plot. The absorption maximum at 0.5 mole fractions for all
three systems investigated confirms a 1 : 1 complexation.w
Benesi–Hildebrand analysis15 of concentration-dependent
absorption measurements provides the corresponding
association constants and extinction coefficients (Table 1).
The larger association constants in the case of BN38C10 and
BN32C8 suggest stronger interactions of the C4BP axle with
naphthalene- than benzene-based wheels. Thermodynamic
parameters (free energy change, enthalpy and entropy) were
determined from the results of temperature-dependent UV-Vis
measurements (Table 1) and found to be consistent with
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Table 1 Thermodynamic parameters for the formation of the three pseudorotaxanes studied in 1.0 mM (CH3)2CO/CH2Cl2 (2 : 1) solutions
(* 294 K, ** 298 K)
Wheel component
Ka* (102 Mꢀ1
)
e (103 Mꢀ1 cmꢀ1
)
DH/kcal molꢀ1
DS/cal molꢀ1 Kꢀ1
DG**/kcal molꢀ1
BPP34C10
BN38C10
BN32C8
1.0 (ꢁ0.2)
2.4 (ꢁ0.3)
1.7 (ꢁ0.3)
0.63 (ꢁ0.08)
1.0 (ꢁ0.1)
1.3 (ꢁ0.1)
ꢀ11 (ꢁ0.2)
ꢀ8.1 (ꢁ0.2)
ꢀ7.1 (ꢁ0.2)
ꢀ26 (ꢁ1)
ꢀ16 (ꢁ1)
ꢀ13 (ꢁ1)
ꢀ3.2 (ꢁ0.1)
ꢀ3.4 (ꢁ0.1)
ꢀ3.2 (ꢁ0.1)
literature reports on similar systems.8,16 w Pseudorotaxane
formation from its components in the present systems is
barrierless and is driven by entropy, with higher temperatures
G. M. and G. R. thank Western Michigan University for
start-up funds. We also thank Prof. Ekkehard Sinn for access
to the X-ray diffractometer and Mary Sajini Devadas for help
with the electrochemistry measurements.
favoring dissociation. It is important to note that
a
temperature-dependent color intensity change would not
be observable in the case of related rotaxanes, where
dissociation of the wheel from the axle is prevented by the
bulky stoppers.
Notes and references
z Crystal data: C4BP/BPP34C10: C50H70F12N2O14P2, M = 1213.02,
ꢀ
T = 100(2) K, triclinic, space group P1, a = 10.5908(2), b =
Preliminary electrochemical studies show a correlation
between the reduction/oxidation potential of the axle and
wheel, respectively, and the absorption spectrum of the
corresponding pseudorotaxane. Square-wave voltammograms
(Fig. 3) show two reversible reduction peaks (ꢀ0.76 V and
ꢀ1.20 V) for C4BP and two partially reversible oxidation
peaks (0.84 V and 0.96 V) for BPP34C10. As expected, in the
case of the corresponding pseudorotaxane these peaks shift to
slightly more negative/more positive potentials, respectively,
due to the charge transfer. The higher oxidation potential
of BPP34C10 (0.84 V) than BN38C10 (0.70 V) and BN32C8
(0.63 V) is in accord with the higher energy charge transfer
absorption band of C4BP/BPP34C10 (448 nm) than C4BP/
BN38C10 (490 nm) and C4BP/BN32C8 (501 nm).
11.5200(3), c = 13.0613(5) A, a = 102.140(2)1, b = 101.609(2)1,
g = 109.620(1)1, V = 1402.07(7) A3, Dc = 1.437 g cmꢀ3, m =
0.182 mmꢀ1, Z = 1, reflections collected = 78 662, unique = 8541
(Rint = 0.0354), final R indices [I > 2sI]: R1 = 0.0435, wR2 = 0.1537,
GoF = 1.271.
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2100 New J. Chem., 2010, 34, 2097–2100 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010