A 'chemically-gated' photoresponsive compound as a visible detector for organophosphorus nerve agents

Article abstract of DOI:10.1039/c1cc13685b

We describe a versatile and convenient visible detection method for organophosphorus compounds based on a colorless 'pro-photoresponsive' organic molecule that undergoes photochemical ring-closing to produce a colored isomer only after it reacts with vapo

Full text of DOI:10.1039/c1cc13685b

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COMMUNICATION
A ‘chemically-gated’ photoresponsive compound as a visible detector for
organophosphorus nerve agentsw
Farahnaz Nourmohammadian,z Tuoqi Wu and Neil R. Branda*
Received 20th June 2011, Accepted 15th August 2011
DOI: 10.1039/c1cc13685b
We describe a versatile and convenient visible detection method
for organophosphorus compounds based on colorless
ambient conditions (light or heat) to trigger an optical signal
that is visually obvious to anyone present regardless of their
technical background or expertise. Although fluorescence is a
more sensitive optical read-out technique, the convenience of a
readily observable change in color of a bulk material cannot be
downplayed. Such a material could be processed into films
or applied to almost any surface as a coating without the need
for electronic and/or optical controls. In this communication,
we demonstrate how a molecular photoswitch provides
an observable readout when it undergoes ‘chemically-gated’
photochromism.
a
‘pro-photoresponsive’ organic molecule that undergoes photo-
chemical ring-closing to produce a colored isomer only after it
reacts with vapors of the phosphorylating agent.
Today there is still an urgent need for cheap and easy ways to
detect lethal airborne agents such as those used in chemical
warfare and as pesticides. Among the most noteworthy
targets are the odorless, colorless and volatile neurotoxic
organophosphates such as sarin, cyclosarin, soman and tabun
(Chart 1).1 The huge lethality of these highly potent and
rapidly acting compounds is due to their irreversible inhibition
of the cholinesterase enzymes, which give rise to subsequent
accumulation of the neurotransmitter acetylcholine in the brain
and its periphery resulting in hypersecretion, bronchoconstriction,
miosis, muscular twitching, mental confusion, convulsive seizures,
flaccid paralysis, respiratory distress and eventual death.
In molecular systems that exhibit ‘chemically-gated’ photo-
chromism, a change in color in response to a photochemical
stimulus occurs only after the molecule has undergone a
chemical reaction with another compound.11 Because the
reactivity of organophosphate nerve agents involves conver-
sion of an active hydroxyl group of an enzyme to a phosphate
ester,12 a detection system that possesses competitive function-
ality seems the most appropriate. The colorless dithienylethene
shown in Scheme 1 (1o) is an excellent candidate for this
role. Using similar compounds,13 Irie has shown that intra-
molecular proton transfer of the phenolic hydrogen efficiently
quenches the excited states of the molecules and prevents
ring-closing to form its colored isomer (1c). In the absence
of the intramolecular hydrogen bond between the –OH and
the oxygen atom of the imide carbonyl group, these systems
undergo photochemically induced ring-closing as is typical
for this photoresponsive class of compounds.14 The authors
originally demonstrate this fact by acetylating the phenol of
their ‘pro-photochromic’ compound with acetic anhydride. In
our studies, we show that this compound works equally well in
solution and in an immobilized state when exposed to vapors
of an organophosphate to generate colorless compound 2o,
which undergoes ring-closing (2o - 2c) when exposed to UV
light and produces an easily observable change in color.
Visible light resets the system by triggering the reverse reaction
and regenerating the ring-open isomer.
Most of the reported examples of detectors for organo-
phosphates use complicated biochemical assays (fluorometric2
or amperometric3), changes in the optical properties4 (color,5
fluorescence,6 diffraction) of small molecules, polymers7 or
gels,8 conduction and resistivity in electronic devices,9 or
physical changes in piezoelectric devices10 to monitor the levels
of airborne organophosphate toxins. In many cases, the read-
out method relies on specific equipment or protocols limiting
their widespread use in public spaces. We suggest that a more
convenient detection method would take advantage of the
Chart 1 Some of the most common organophosphorus nerve agents.
4D LABS, Department of Chemistry, Simon Fraser University,
8888 University Drive, Burnaby, BC, Canada V5A 1S6.
E-mail: nbranda@sfu.ca; Fax: +1 778 782 3765;
The appeal of using this specific photochromic backbone
lies at many levels. The reverse ring-opening reactions tend to
be only photochemical driven so the colored readout signal
will always be visible to those in the vicinity. The backbone
can be decorated with functional groups to fine-tune the type
of light needed to trigger the ring-closing reaction as well as
the color produced when the cyclization reaction occurs.14
Tel: +1 778 782 8051
w Electronic supplementary information (ESI) available: Detailed
descriptions of experimental methods, synthetic procedures, charac-
terization of new compounds and additional absorption spectra. See
DOI: 10.1039/c1cc13685b.
z On sabbatical leave from Department of Organic Colorants, Insti-
tute for Color Science and Technology, Tehran, Iran.
c
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Fig. 1 Changes in the UV-vis absorption spectrum of (a) a cyclo-
hexane (top) and a CH3CN (bottom) solution of 1o (1.5–2.5 ꢀ 10ꢁ4 M),
and (b) a cyclohexane solution of 2o (1.5 ꢀ 10ꢁ4 M) as it is exposed to
254 nm light. (c) Changes in the absorption at 575 nm of cyclohexane
and acetonitrile solutions of 1o, and a cyclohexane solution of 2o as
they are exposed to 254 nm light. All concentrations are 1.5 ꢀ 10ꢁ4 M.
(d) Visual changes in the color of the cyclohexane solution of 2o as it is
exposed to 254 nm light, and a piece of paper treated with a solution of
1o and air-dried in a sealed vial containing a few drops of diethyl
chlorophosphate while being exposed to 365 nm light (left). The vial on
the right shows an identical piece of treated paper in a vial without added
diethyl chlorophosphate but with exposure to UV light.
Scheme 1 Synthesis and photochemical behavior of compounds 1
and 2.
This allows the tailoring of the detection system so it can be
activated using the particular environmental light source
present, whether it’s the sun, LEDs or even the fluorescent
lighting typically found in interior spaces. Our detection
system also relies only on a one-step reaction, which gives it
some advantages over the elegant examples that use two-step
processes.6b,d Another appealing feature is that the toxic
organophosphate is sequestered by the system until it is reset
using visible light to trigger the back reaction (2c - 2o) and
hydrolysis (2o - 1o).
time to drive the photochemical ring-closing of typical dithienyl-
ethene derivatives. This is not the case for the phosphorylated
version 2o. Cyclohexane (or hexanes) solutions of this com-
pound undergo immediate changes in their UV-vis absorption
spectra when exposed to UV light as shown in Fig. 1b. As is
typical when dithienylethenes undergo photochemical ring-
closing, the high energy absorption bands at 290 nm decrease
in intensity along with the appearance of a new set of broad
bands in the visible region of the spectrum centered at 578 nm.
This spectroscopic change explains the distinguishable color
change of the solution from pale yellow to blue. Exposing the
colored solutions to visible light (at wavelengths greater than
510 nm, for example) drives the reverse, ring-opening reactions
and regenerates both the original color and the absorption
spectra (Fig. S4, ESIw). These observations support the claim
that intramolecular proton transfer of the phenolic hydrogen is
the cause for the suppression of photochromism in compounds
such as 1o.13 The claim is supported by the analogous spectro-
scopic and color changes that phenol 1o undergoes when it is
irradiated in solvents that compete for hydrogen bonding as
illustrated in Fig. 1a, bottom, which shows the changes of a
CH3CN solution of 1o when it is exposed to UV light.
The ring-open isomer of the phenolic dithienylethene (1o)
was synthesized by following the procedures published for the
preparation of a similar compound.w13 The synthesis is shown
in Scheme 1 and involves the double Suzuki coupling of two
known starting materials, 3,4-dibromo-furan-2,5-dione15 and
(2-methyl-5-phenylthiophen-3-yl)boronic acid,16 to produce
an anhydride (3) bearing the required hexatriene. This
anhydride is converted to the maleimide 1o in good yield by
reacting it with o-hydroxyaniline. The phosphorylated version
of the phenolic compound (2o) was prepared by treating 1o
with diethyl chlorophosphate. This phosphorylating agent was
used in all studies described in this communication as it
represents a less toxic mimic of sarin.
As expected, compound 1o is not significantly photorespon-
sive as long as it is irradiated in non-polar solvents. For
example, exposing cyclohexane solutions of 1o (1.5 ꢀ 10ꢁ4 M)
to UV light (254 nm)y produces only minor changes in their
UV-vis absorption spectra even after almost 3 minutes of
irradiation (Fig. 1a top), which is usually more than enough
Fig. 1c shows the comparative rates of growth of the
absorbances corresponding to the ring-closed isomers of
phenol 1o and phosphate 2o when solutions of them (cyclo-
hexane and CH3CN) are exposed to UV light. The increased
rate of photochemical ring-closing for the phosphate (2o - 2c)
in cyclohexane over the parent phenol (1o - 1c) in the more
c
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competitive solvent CH3CN is likely due to the fact that
(1) this solvent does not completely eliminate the intra-
molecular hydrogen bonding between the phenol hydrogen
and the carbonyl oxygen, and/or (2) this solvent allows a
greater contribution of intramolecular charge transfer from a
twisted conformation of the chromophore, which has been
shown to suppress the photocyclization reaction.17
z Although this longer wavelength light does not induce as rapid of a
photochromic response as does the higher energy light (254 nm) used
for the cuvette studies, 365 nm light was used to minimize the amount
of UV light filtered by the glass from the vial.
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The demonstration of compound 1’s ability to act as a visual
detector of phosphorylating agents is shown in the last panel
of Fig. 1. Two samples were prepared treating each with one
drop of a solution containing 1o (2 mg) dissolved in cyclo-
hexane (0.2 mL). After air-drying the two pieces of paper, they
were placed in separate vials. The vial shown on the left side of
the image in Fig. 1 also contained a piece of filter paper soaked
with 5 drops of diethyl chlorophosphate (placed at the bottom
of the vial). Each vial was sealed and exposed to 365 nm light.z
Only the paper in the vial containing the phosphorylating
agent turned blue-green (after merely a few seconds) showing
that the vapors of diethyl chlorophosphate reacted with 1o to
produce 2o, which immediately ring-closed to the colored 2c.
The molecular system described in this manuscript is very
sensitive to ambient light and even the fluorescent lighting
conditions in the laboratory were enough to convert the vials
from their light to dark states.
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In this communication we have highlighted how a relatively
simple detection system for organophosphorus compounds
can be constructed using the visible change in color when a
photoresponsive dithienylethene undergoes ‘gated’ photo-
chromism. The color changes are easily apparent to the naked
eye and can be tuned by judicious modifications to the molecular
backbone. Future generation of these types of detection systems
will focus on color tuning by mixing more than one photo-
chromic system, and on enhancing the rate of the coloration
reaction by introducing oximes as has been demonstrated by
Rebek’s recent report18 to ensure degradation of any toxic and
reactive phosphate ester, although this will limit the reversibility.
The ability for analogous systems to respond to different
environmental light conditions will also be demonstrated.
This research was supported by the Natural Sciences
and Engineering Research Council (NSERC) of Canada, the
Canada Research Chairs Program, and Simon Fraser University
through the Community Trust Endowment Fund.
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Notes and references
y All solution-state studies of the photochemical ring-closing reactions
of compounds 1o and 2o were carried out using the light source from a
lamp used for visualizing TLC plates at 254 nm or 365 nm (Spectroline
E-series, 470 W cmꢁ2).
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c
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This journal is The Royal Society of Chemistry 2011