A photochromic phenoxyquinone based cyanide ion sensor

Article abstract of DOI:10.1016/j.tetlet.2011.02.105

We have developed a new chemosensor system for cyanide ion that is based on a photochromic material. We observed that addition of cyanide anion to a UV irradiated solution of a phenoxynaphthacenequinone derivative brought about a significant change in the absorption spectra that enabled detection of cyanide ion in a selective and sensitive manner. A carbanion intermediate was shown to be responsible for the long wavelength absorption band (630-940 nm) that is generated by cyanide addition.

Full text of DOI:10.1016/j.tetlet.2011.02.105

Tetrahedron Letters 52 (2011) 2454–2457
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Tetrahedron Letters
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A photochromic phenoxyquinone based cyanide ion sensor
⇑
In Sung Park, Eun-Joo Heo, Jong-Man Kim
Department of Chemical Engineering, Hanyang University, Seoul 133-791, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
We have developed a new chemosensor system for cyanide ion that is based on a photochromic material.
We observed that addition of cyanide anion to a UV irradiated solution of a phenoxynaphthacenequinone
derivative brought about a significant change in the absorption spectra that enabled detection of cyanide
ion in a selective and sensitive manner. A carbanion intermediate was shown to be responsible for the
long wavelength absorption band (630–940 nm) that is generated by cyanide addition.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 15 December 2010
Revised 18 February 2011
Accepted 25 February 2011
Available online 4 March 2011
Keywords:
Cyanide sensor
Photochromic
Phenoxyquinone
Photochromic materials undergo structural changes that inter-
convert two distinct isomeric forms upon irradiation at specific
wavelengths.1–3 Owing to their light driven molecular switching
properties, photochromic compounds have been incorporated into
many intriguing systems, including molecular switches, optical
memory devices, holographic gratings, and drug delivery
vesicles.4–19 Because of these wide ranging applications, various
photochromic molecules, such as azobenzenes, spiropyrans, phen-
oxyquinones, and bisthienylethenes have been developed.
addition to carbon (a) would form an adduct like 3 (Scheme 1b).
Similarly, the carbon center (b) should also be reactive toward
nucleophiles.
A survey of the literature suggests that only
amine–quinone adducts, arising from reaction at carbon (a), have
been prepared thus far from the photochemically generated
‘ana’-quinone form.24
We felt that it would be intriguing to investigate the reactivity
of the photochemically produced ‘ana’-quinone form toward other
Among the many photochromic compounds described to date,
phenoxyquinone derivatives are unique in several ways.20–24 First,
these molecules participate in a photoinduced reversible phenyl
group migration process. For example, the ‘trans’-quinone form of
1-phenoxy-5,12-naphthacenequinone (1) undergoes photochemi-
cal rearrangement to form the ‘ana’-quinone form upon irradiation
with UV light (Scheme 1a). The reverse conversion of the ‘ana’-
quinone to ‘trans’-quinone form occurs readily upon irradiation
with visible light. Second, unlike azobenzene or spiropyran deriva-
tives, phenoxyquinones display negligible thermal interconversion
of these forms at room temperature. Third, phenoxyquinone deriv-
atives usually show an excellent resistance to fatigue. For example,
it has been reported that the photochemical rearrangement pro-
cess can be repeated 500 times without decomposition of the
materials.24 Lastly, the ‘ana’-quinone form of phenoxyquinones is
known to undergo nucleophilic substitution reactions with amines
to form amine–quinone adducts (Scheme 1b).24
a
a
O
O
O
O
O
UV
Vis
O
b
1
(trans)
2
(ana)
Et
O
c
b
R
NH
O
O
O
Close inspection of the structure of the photochemically
generated ‘ana’-quinone form indicates that it contains two
electrophilic sites at carbons a and b (Scheme 1a). Nucleophilic
O
3
4
Scheme 1. (a) Photoinduced isomerization of 1-phenoxy-5,12-naphthacenequi-
none, (b) structure of an amine–quinone adduct, (c) structure of a phenoxyquinone
derivative investigated in this study.
⇑
Corresponding author. Tel.: +82 2 2220 0522; fax: +82 2 2298 4101.
E-mail address: jmk@hanyang.ac.kr (J.-M. Kim).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.02.105

I. S. Park et al. / Tetrahedron Letters 52 (2011) 2454–2457
2455
nucleophiles, such as cyanide, acetate, and iodide anions since
covalent adduct formation would be expected to promote changes
in the electronic absorption or emission properties of the quinone
chromophore. Furthermore, if the spectral changes are sensitive
and specific to the type of nucleophile used, phenoxyquinones
would serve as selective chemosensors for anions. Although
numerous probe molecules have been described, those that
serve as photochromism-based chemosensors are exceptionally
rare. Until recently, we25 and others26 were the only groups
that have investigated photochromic spiropyran-based cyanide
ion chemosensors.27
In Figure 1 are shown the UV–visible absorption spectral
changes that occur upon UV-irradiation (365 nm) of an acetonitrile
solution (0.1 mM) containing 6-(40-ethylphenoxy)-5,12-naphtha-
cenequinone 4. The spectral changes show that characteristic
peaks in the 400–550 nm region, corresponding to the ‘ana’-
quinone form of the phenoxynaphthacenequinone, increase
(Fig. 1a) with increased irradiation time. In order to investigate
the possible use of the photochemically generated ‘ana’-quinone
form as a chemosensor, various anions, including CNꢀ, Fꢀ, Clꢀ, Br
ꢀ, Iꢀ, NO3ꢀ, H2PO4ꢀ, and AcOꢀ, with n-Bu4N+ as a common counter-
cation were added to the UV irradiated solution of 4. As displayed
in Figure 1b, addition of cyanide anion to the UV irradiated phen-
oxyquinone 4 results in the generation of a broad absorption peak
in the 630–940 nm (visible and infrared) region. This spectral
change can be detected even when the cyanide anion concentra-
a
1.0
0.8
0.6
0.4
0.2
0.0
tion is as low as 18.7
lM (Fig. 1c). The detection limit was found
to be 5.26 M when a lower concentration (0.05 mM) of 4 was
l
300
350
400
450
500
550
600
used (Fig. S5, Supplementary data). Fluoride anion addition also re-
sults in formation of the broad peak while addition of other anions,
including Clꢀ, Brꢀ, Iꢀ, NO3ꢀ, H2PO4ꢀ, does not. Importantly, the
magnitude of the increase in absorption in the visible-infrared re-
gion promoted by fluoride is much lower than that generated by
cyanide addition. In addition, the color of the irradiated acetonitrile
solution of 4 was found to change from yellow to pale brown when
cyanide anion is added (Fig. 1b inset). It should be noted that a
solution containing unirradiated phenoxyquinone 4 does not un-
dergo any spectral changes in the presence of the anions tested
including cyanide (Fig. S1, Supplementary data). This observation
demonstrates that the spectral change observed originates from a
Wavelength (nm)
1.0
b
CN-
0.8
0.6
0.4
0.2
0.0
Cl-, Br, I-, NO3 ,
-
-
CN-
-
-
H2PO4 , AcO
F-
1.0
0.6
0.8
0.6
0.4
0.2
0.0
0.4
0.2
0.0
300
400
500
600
700
800
900
Wavelength (nm)
0.25
0.20
0.15
0.10
0.05
0.00
c
TFA
800
Wavelength (nm)
400
500
600
700
900
0.10
0.08
0.06
0.04
0.02
0.00
400
600
800
Wavelength (nm)
Figure 2. Absorption spectral changes of an acetonitrile solution containing
365 nm irradiated (20 min, 1 mW/cm2) 4 (0.1 mM) and cyanide anion (10 molar
equiv to 4) upon standing in the dark. Spectra were obtained 30 s after addition of
cyanide anion. The inset contains absorption spectra before (red) and immediately
after (blue) addition of TFA (1 equiv to cyanide anion used).
0
20
40
60
80
100
CN-(mM)
0.0
0.2
0.4
0.6
08.
1.0
CN-(mM)
CN OH
CN
O
O
Figure 1. (a) UV–visible absorption spectra of an acetonitrile solution containing
0.1 mM of the phenoxyquinone 4 upon irradiation with 365 nm UV light (1 mW/
cm2). (b) Absorption spectra of 365 nm UV-irradiated (20 min, 1 mW/cm2) aceto-
nitrile solutions (0.1 mM) of the phenoxyquinone 4 after addition of CNꢀ, Fꢀ, Clꢀ,
Brꢀ, Iꢀ, NO3ꢀ, H2PO4ꢀ, and AcOꢀ (10 molar equiv to 4). The spectra were obtained
30 s after addition of the anions. The inset shows photographs of a cyanide anion-
treated vial containing photoirradiated 4. (c) Plot of absorbance at 885 nm as a
function of cyanide ion concentration. The inset contains a plot of the absorption
ratio vs cyanide anion concentration in the micromolar range.
base
acid
OH CN
5
CN
6
Figure 3. Structure of CN-adduct 5 and its soluton colors in acid/base condition.

2456
I. S. Park et al. / Tetrahedron Letters 52 (2011) 2454–2457
Et
Et
O
Et
O
Et
O
O
O
O
O
O
O
O
CN-
hν
365nm
CN
H
OH CN
9
O
4
7
8
CN
O
CN
O
CN-
O
CN
OH CN
10
6
Scheme 2. Proposed mechanism for the formation of the intermediate and CN-adduct 5.
reaction of the ‘ana’-quinone form of phenoxyquinone 4. The cya-
nide-induced spectral change in the visible and infrared region was
observed even in acetonitrile–HEPES buffer (5 mM, pH 7.4) (99:1,
is apparent that irradiation of the ‘trans’-quinone form of phenoxy-
quinone 4 promotes an excited state aryl group migration reaction
to generate the ‘ana’-quinone form 7 (Scheme 2). Nucleophilic
addition of cyanide anion to one of the highly electrophilic carbons
in 7 then produces the phenolate intermediate 8. Tautomerization
by way of a 1,4-proton shift yields the carbanion 9 that is believed
to be the intermediate which absorbs light in the visible-infrared
region. Addition of a second cyanide anion to 9 involves displace-
ment of a p-ethylphenolate anion to form 10, which likely exists
as tautomer 6 under basic conditions.
An analogy for this pathway is found in the work of Happ et al.
who reported that N-methylacridium ion 11 forms the cynide
adduct 12 through a nucleophilic addition reaction (Scheme 3).28
Interestingly, these workers observed that a solution of 12 in the
presence of excess cyanide anion displays broad absorption bands
in the 530–600 nm region. Based on the results of additional exper-
iments, they concluded the long wavelength absorption band was
a consequence of the formation of the carbanion intermediate 13.
These observations support the assignment of carbanion 9 (Scheme
2) as the transient intermediate formed by cyanide addition to 7.
In addition, rapid quenching of the broad absorption peak upon
addition of acid serves as further support for this proposal (Fig. 2,
inset). Importantly, Happ et al. reported that carbanion 13 is highly
reactive toward molecular oxygen, a phenomenon we observed
with the long wavelength absorbing intermediate 9, which
disappears more rapidly in an oxygen purged solution than in
solutions that are either unpurged or purged with nitrogen
(Fig. S4, Supplementary data). Tae and coworkers also reported
an acridium ion based colorimetric cyanide anion sensor.29
In conclusion, this study has led to the development of a new
chemosensor system for cyanide ion that is based on a photochromic
material. Specifically, addition of cyanide anion to a UV irradiated
solution of phenoxynaphthacene quinone 4 brings about a significant
change in the absorption spectra that enables detection of cyanide
v/v) solution with a detection limit of 27.5
Supplementary data).
lM (Figs. S6 and S7,
Observations made in several experiments provided informa-
tion about the nature of the visible-infrared absorbing substance
formed by adding cyanide. First, the substance generated in this
process was found to be unstable, as evidenced by the gradual
decrease in the intensity of broad peak in the visible-infrared
region (Fig. 2). The broad peak disappears completely in 2 h.
Second, addition of trifluoroacetic acid to the solution containing
the formed substance results in immediate quenching of the
absorption peaks in the visible-infrared region (Fig 2, inset).
The results described above clearly indicate that a relatively
short-lived, acid sensitive intermediate is formed in the reaction
of the photochemically generated ‘ana’-quinone form of 4 with
cyanide anion. We believed that structural identification of the sta-
ble product formed eventually in this process would provide valu-
able information about the nature of the unstable intermediate
produced by cyanide addition. In order to produce a sufficient
quantity of the final reaction product, a preparative scale photo-
chemical reaction of phenoxyquinone 4 was conducted in an ace-
tonitrile solution containing cyanide anion (10 equiv to 4). Thin
layer chromatographic (TLC) analysis of the crude mixture showed
that the process generates a single product, which can be isolated by
using silica gel column chromatography (see Supplementary data).
Interestingly, data gained from 1H NMR, IR (Fig. S2, Supplementary
data) and mass spectroscopic analyses showed that the product
formed in the reaction of the photochemically generated ‘ana’-
quinone form of 4 with cyanide anion is 6,12-dihydroxytetra-
cene-5,11-dicarbonitrile 5 (Fig. 3).
An acetonitrile solution of the CN-adduct 5 is yellow and it
undergoes a yellow-to-purple color transition upon addition of
base (Fig. 3), a change that is likely a result of base-promoted
deprotonation of the phenolic hydroxyl groups. Addition of acid
to the basic solution of 5 results in regeneration of the yellow
color, confirming that color change is reversible. Monitoring of
these color transitions by using UV–visible spectroscopy reveals
that the CN-adduct 5 and the deprotonated form 6 have maximum
absorption wavelengths at 434 nm and 527 nm, respectively
(Fig. S3, Supplementary data).
CN
H
CN
CN-
CN-
N
N
N
Me
Me
Me
11
12
13
The mechanisms for formation of the long wavelength absorb-
ing intermediate and the CN-adduct 5 are worthy of comment. It
Scheme 3. Mechanism of the formation of Happ’s carbanion 13.

I. S. Park et al. / Tetrahedron Letters 52 (2011) 2454–2457
2457
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Acknowledgments
The authors gratefully thank National Research Foundation of
Korea for financial support through Basic Science Research Program
(20100018438), Center for Next Generation Dye-sensitized Solar
cells (20100001844), and International Research & Development
Program (K20901000006-09E0100-00610). I.S.P. is a recipient of
Hi Seoul Science Fellowship from Seoul Scholarship Foundation.
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Supplementary data
Supplementary data (experimental procedures, 1H NMR spec-
tra, IR spectrum, and additional experimental data) associated with
this article can be found, in the online version, at doi:10.1016/
j.tetlet.2011.02.105.
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