Interrupted energy transfer: Highly selective detection of cyclic ketones in the vapor phase

Article abstract of DOI:10.1021/ja202277h

We detail our efforts toward the selective detection of cyclic ketones, e.g. cyclohexanone, a component of plasticized explosives. Thin films comprised of a conjugated polymer are used to amplify the emission of an emissive receptor via energy transfer. W

Full text of DOI:10.1021/ja202277h

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pubs.acs.org/JACS
Interrupted Energy Transfer: Highly Selective Detection of Cyclic
Ketones in the Vapor Phase
Jason R. Cox, Peter M€uller, and Timothy M. Swager*
Department of Chemistry and Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge,
Massachusetts 02139, United States
S Supporting Information
b
ABSTRACT: We detail our efforts toward the selective
detection of cyclic ketones, e.g. cyclohexanone, a compo-
nent of plasticized explosives. Thin films comprised of a
conjugated polymer are used to amplify the emission of an
emissive receptor via energy transfer. We propose that the
energy transfer is dominated by an electron-exchange
mechanism to an upper excited state of the fluorophore
followed by relaxation and emission to account for the
efficient energy transfer in the absence of appreciable
spectral overlap. Exposure to cyclic ketones results in a
ratiometric fluorescence response. The thin films show
orthogonal responses when exposed to cyclic ketones versus
acyclic ketones. We demonstrate that the exquisite selec-
tivity is the result of a subtle balance between receptor
design and the partition coefficient of molecules into the
polymer matrix.
espite significant advances in fluorescence-based chemosen-
D
sing technology,1 the detection of energetic materials
Figure 1. (a) Synthesis of squaraine reporter dye 1. (i) 3,5-Bis-
(trifluoromethyl)phenyl isocyanate, CH2Cl2, rt, 12 h. (ii) Squaric acid,
toluene/n-butanol (3:1), reflux, 6 h. (b) Molecular structure of PPE 2 (R
= C14H29). (c) Absorbance (solid line) and photoluminescence (circles)
spectra of PPE 2 (blue) and squaraine 1 (red). PPE 2 photolumines-
cence spectra are from a thin film; all other measurements taken in
CHCl3 (2 λex = 380 nm, 1 λex = 660 nm). The red shift of the emission
maximum of 1 in thin films is likely due to stabilization of the excited
state of 1 in the polymer matrix.
remains an important area of research. Recent events worldwide
have highlighted the need for a portable and highly sensitive
explosives sensing device capable of high-throughput screening
in a variety of environmental conditions.2 However, explosive
compositions containing 1,3,5-trinitro-1,3,5-triazacyclohexane
(RDX) remain particularly challenging targets. Indeed, a low
equilibrium vapor pressure of 6 ppt at 25 °C and an unfavorable
reduction potential add to the complexity of this problem.2,3 To
address this need, we focused our efforts on the detection of
cyclohexanone, a ketone used in the recrystallization of RDX
with an equilibrium vapor pressure of 4470 ppm.4 However,
optical detection of cyclohexanone vapor in an operational
environment is not without its own challenges; specifically, the
ubiquity of ketones in nature requires a highly selective sensing
system. This Communication describes a new approach to
this goal.
used in this work. The conjugated polymer emission displays
negligible spectral overlap with the squaraine absorbance even in
thin films. However, closer inspection of the dye absorbance
reveals a small transition centered at ∼415 nm. This transition
has been observed in other squaraine systems and has been
attributed to an S0fSn excitation as the S0fS2 is forbidden by
symmetry.7 Given that the efficiency of FRET processes are
dependent on the oscillator strength of the coupled transitions,
and exchange processes are not,8 we propose that the observed
energy transfer process consists of an electron exchange from the
excited polymer to the Sn state of the squaraine followed by rapid
relaxation to the S1 state and subsequent emission. It must be
noted, however, that in thin films both mechanisms are
Our transduction mechanism, predicated on interrupted
electronic energy transfer (EET), is schematically outlined in
Figure 1d. A stable and highly emissive conjugated polymer (2) is
used as a light-harvesting unit to amplify the emission from a
fluorophore (squaraine 1) embedded in a thin film via EET. Two
primary mechanisms exist for singletÀsinglet energy transfer, a
long-range dipoleÀdipole interaction commonly referred to as
F€orster resonance energy transfer (FRET)5 and an electron-
exchange pathway formulated by Dexter.6 Figure 1c shows the
absorbance and photoluminescence data for both components
Received: March 12, 2011
Published: August 02, 2011
r
2011 American Chemical Society
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Figure 3. Change in thin-film fluorescence intensity of polymer (blue)
and squaraine dye (red) after 30 s exposure to the saturated vapor of the
indicated ketone (λex = 400 nm)(mean of three different films).
Figure 2. Thin-film fluorescence spectra before (blue) and after (red)
exposure to the saturated vapor of acetone (top) and cyclohexanone
(bottom) for 30 s (λex = 400 nm).
potentially operative given such small separations between the
donor and acceptor.9 Given the negligible spectral overlap
between the two components in this work and the fact that there
is a finite overlap in the absorbance of both components, the
energy transfer cascade is more consistent with an exchange
mechanism.8À10 Of critical importance to this work is the fact
that the efficiency of energy transfer, by an exchange mechanism,
should be extremely sensitive to intermolecular distance changes
of only a few angstroms.8À11 Herein, we report that ratiometric
and selective detection of ketones can be realized using rationally
designed acceptor molecules that modulate energy transfer
efficiency based on binding-induced displacement of the accep-
tor from the conjugated polymer backbone.
The key feature of this approach is the ability to detect analytes
lacking low-lying electronic states that do not alter the electronic
properties of either the donor or acceptor and also do not
undergo photoinduced electron transfer (PET) with either
component. Instead, binding interactions between the analyte
and dye cause small molecular movements (0.5À2 Å) that
diminish orbital interactions (or modulate the orientation)
between the dye and luminescent polymer backbone, yielding
a quench in the reporter dye emission and a concomitant increase
in the conjugated polymer emission.
The synthesis of squaraine 1 is outlined in Figure 1a. A urea
based receptor was chosen due to the extensive use of this motif
in organocatalysis12 involving carbonyl-containing substrates,
and was linked to a near-infrared (NIR) squaraine fluorophore
in a fashion that enables the formation of a strong intramolecular
hydrogen bond. This interaction rigidifies the dye scaffold there-
by increasing the quantum yield of the fluorophore and enhances
the efficiency of energy transfer by limiting nonradiative decay
pathways.13 In addition, the rigidity between the receptor
periphery and the fluorophore core ensures that a binding event
will displace the core from the polymer backbone. The resulting
acceptor dye 1 undergoes facile energy transfer when colocalized
in a thin film of poly(p-phenylenethynylene) (PPE) 2 at a dye
loading of 0.5 wt % as shown in Figure 2 (rationale for this dye
loading is discussed in the Supporting Information).
Figure 4. Mean frequency shift of a QCM crystal coated with a thin film
of the sensor formulation after 30 s exposure to the saturated vapor of
the indicated ketone (mean of three sequential measurements).
Exposure of these films to the saturated vapor of hydrogen-
bond acceptors such as cyclohexanone results in a pseudoratio-
metric ‘polymer-on’/‘dye-off’ response (Figure 2). Interestingly,
the films do not elicit the same response when exposed to the
saturated vapors of acyclic ketones such as acetone. This result
was surprising to us as we anticipacted that acetone would elicit a
much larger response given its compact size and volatility.
Exposure to other acyclic ketones such as 3-pentanone and
diisopropyl ketone also behaved differently than cyclohexanone.
In fact, the emission intensity of both components tends to
increase to a small degree (5À30%) when exposed to these
vapors (Figure 3). This response can be explained by a swelling
mechanism whereby the ketones swell the conjugated polymer
film and reduce interpolymer πÀπ interactions that act as
nonemissive low-energy exciton traps. These excitons are then
free to migrate to sites bearing squaraine molecules and undergo
energy transfer followed by emission.
Exposure to other cyclic ketones such as cyclopentanone and
cycloheptanone gave responses similar to cyclohexanone. We
initially suspected that the observed selectivity may be due to
differential uptake of the analytes into the hydrophobic polymer
matrix. To test this hypothesis, a quartz crystal microbalance
(QCM) was used to measure the change in frequency of the
oscillating quartz crystal (coated with the polymer matrix) upon
exposure to various analytes (Figure 4). The resulting frequency
changes are linearly related to the uptake of mass into the films by
Sauerbrey’s equation.14 We expected the ratiometric response of
the films to trend in the same manner as the partition coefficients
of the different analytes.
As shown in Figures 3 and 4, this hypothesis is in line with the
observed response of the films to acyclic ketone vapor. Clearly,
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energy transfer mechanisms. The resulting films display exqui-
site selectivity for cyclic ketones, including cyclohexanone
which is a component of plasticized explosives. The selectivity
appears to be due to a subtle balance of receptor specificity and
the ability of analytes to partition into the polymer matrix.
These films exhibit a limit of detection for cyclohexanone of
4.76 ppm.15 Further investigations of this approach and its
utility in explosives detection are currently underway in our
laboratory.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details includ-
b
ing synthetic procedures, characterization of all compounds,
and spectral data for the quenching experiments. This ma-
terial is available free of charge via the Internet at http://pubs.
acs.org.
’ AUTHOR INFORMATION
Corresponding Author
tswager@mit.edu
Figure 5. X-ray structure of 1 (1:1 cocrystal with DMF) (gray = carbon,
green = fluorine, blue = nitrogen, red = oxygen, yellow = hydrogen, light
blue dashed lines = hydrogen bonds).
’ ACKNOWLEDGMENT
This work was funded by the Army Research Office through
the Institute for Soldier Nanotechnologies. We thank Dr.
Augustus W. Fountain III, Juan C. Cajigas, and Christopher B.
Steinbach for assistance determining the limit of detection. We
also thank Dr. Trisha L. Andrew and Dr. Aimee Rose for
insightful discussions. J.R.C. thanks the Corning Foundation
for a graduate fellowship. The diffractometer was purchased with
the help of funding from the National Science Foundation under
Grant No. CHE-0946721.
the swelling response of the films in Figure 3 follows the same
trend as the frequency shifts shown in Figure 4. However, this
hypothesis fails to explain the disparate response between
cyclopentanone and diisopropyl ketone. Additionally, one would
expect the largest response from cycloheptanone (with the
greatest relative uptake into the films); however, the films display
a greater response to cyclopentanone and cyclohexanone. These
data lend credence to two important conclusions: (1) the
receptor is involved in the proposed transduction mechanism
—the response for cyclic ketones is not the result of film swelling
—and (2) the observed selectivity is related to the steric bulk
surrounding the ketone functionality and the partition coefficient
of the analyte into the film.
To further probe these points, we investigated the response of
the films to 2-methylcyclopentanone. We expected that if the
selectivity was related to steric bulk in the vicinity of the carbonyl
functional group, then the sensor should behave differently in the
presence of 2-methylcyclopentanone versus cyclopentanone. In
accord with our hypothesis, the response to the two analytes is
orthogonal (Figure 3), despite having very similar uptake profiles
into the polymer films (Figure 4). This result supports both the
notion that the receptor is involved in the selectivity of the
response and that the binding pocket of squaraine 1 requires
sterically unhindered ketone functionalities to induce a ratio-
metric response.
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