Photoswitching and sensor applications of a spiropyran-polythiophene conjugate

Article abstract of DOI:10.1039/b926211c

A new FRET-based, photoswitchable fluorescent dyad derived from a spiropyran-polythiophene conjugate was developed and applied to the detection of cyanide anion.

Full text of DOI:10.1039/b926211c

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Photoswitching and sensor applications of a spiropyran–polythiophene
conjugatew
In Sung Park,a Young-Sik Jung,*b Kee-Jung Leea and Jong-Man Kim*a
Received (in Cambridge, UK) 11th December 2009, Accepted 3rd February 2010
First published as an Advance Article on the web 4th March 2010
DOI: 10.1039/b926211c
A new FRET-based, photoswitchable fluorescent dyad derived
from a spiropyran–polythiophene conjugate was developed and
applied to the detection of cyanide anion.
clearly shows that the MC form of the photochromic
compound is able to absorb the emission from P3HT
efficiently as a result of large overlap of the emission and
absorbance spectra. As a consequence, the spiropyran–P3HT
system is ideally suited for incorporation into a FRET based
photoreversible switching system.
Upon irradiation at specific wavelengths, photochromic materials
undergo photochemically reversible structural changes that
result in the interconversion of two distinct isomeric forms.1–3
Owing to this intriguing light-driven molecular switching pheno-
menon, photochromic compounds have been extensively investi-
gated as key materials in optical memory, switching and sensor
fields.4–13 Recently, fluorescence-coupled photochromism has
been a central focus among many potential applications of
photochromic compounds.14–24 In these systems, fluorescent
chromophores are either covalently incorporated into or physi-
cally mixed with photochromic dyes. Fluorophore-coupled
spiropyran derivatives also have gained substantial attention
owing to the unique optical properties associated with the
spiropyran units.25–29 The spirocyclic (SP) and ring opened
merocyanine (MC) forms of these fluorophore-coupled
photochromic compounds participate as ‘‘Switch On’’ and
‘‘Switch Off’’ functions (Scheme 1). Specifically, fluorescence
emission from the fluorophore is observed when the photochromic
compound is in the SP form. In contrast, emission is severely
quenched when the photochromic compound is in the MC form
as a result of fluorescence resonance energy transfer (FRET)
from the excited state of the fluorophore to the MC form of the
photochromic compound. Irradiation with visible light results in
transformation of the MC to the SP form of the compound and
recovery of the fluorescence. By using this strategy, a variety of
spiropyran-derived, fluorophore-coupled (covalently bonded or
physically mixed) photochromic compounds have been designed
and probed in molecular switching applications.25–29
Encouraged by the photophysical properties related to the
donor emission and acceptor absorption described above, we
designed the photoswitchable spiropyran–polythiophene
conjugate P(TSP/3HT) shown in Scheme 2. The thienyl group
containing spiropyran monomer TSP was readily prepared in
76% yield by use of a EDC coupling reaction between
commercially available 10-(2-hydroxyethyl)-30,30-dimethyl-6-
nitrospiro[1(2H)-benzopyran-2,20-indoline] (SP-OH) and
4-(3-thieyl)butanoic acid (TBA) (see ESIw for experimental
details). Copolymerization of the monomer TSP with
3-hexylthiophene (3-HT) was carried out with a respective
1 : 5 molar feed ratio in the presence of FeCl3 in chloroform.
The copolymer, P(TSP/3HT), obtained as a dark brown
powder in 55% yield after precipitation from and thorough
washing with MeOH, was determined to have an average
molecular weight (Mw) of 120 000, compared to a polystyrene
standard, by using gel permeation chromatography (GPC)
(polydispersity (PD) = 5.28). Interestingly, 1H NMR analysis
showed that the composition of P(TSP/3HT) was almost the
same as the molar ratio of the monomer feed employed in its
preparation. Finally, the copolymer is soluble in common
organic solvents, such as chloroform, THF, methylene chloride
and toluene but insoluble in acetonitrile and methanol.
In order to explore the feasibility of fluorescence switching,
a THF (0.1 mM) solution of P(TSP/3HT) was irradiated with
365 nm light (1 mW cmꢀ2). Inspection of the UV-visible
absorption and emission spectra monitored before and after
UV irradiation (Fig. 2) shows that UV-irradiation for 2 min
results in the formation of a new absorption band at 585 nm,
corresponding to the MC form of the spiropyran (Fig. 2A). In
addition, UV irradiation results in a substantial decrease in
fluorescence intensity, dropping to 12% of the initial value
For effective functioning of a fluorescence-coupled spiro-
pyran photoswitch, it must incorporate a FRET system in
which emission from the fluorophore is absorbed efficiently by
the MC form of the photochromic molecule. In this regard,
polythiophenes (PTs) are very attractive substances. Fig. 1
shows the emission spectrum of poly-3-hexylthiophene
(P3HT) and the absorption spectrum of the MC form of
10,30-dihydro-10,30,30-trimethyl-6-nitrospiro[2H-1-benzopyran-
2,20-(2H)-indole] (SPP) in THF. Inspection of these spectra
a Department of Chemical Engineering, Hanyang University,
Seoul 133-791, Korea. E-mail: jmk@hanyang.ac.kr;
Fax: +82-2-2298-4101; Tel: +82-2-2220-0522
b Bioorganic Science Division, Korea Research Institute of Chemical
Technology, Daejeon 305-606. E-mail: ysjung@krict.re.kr
w Electronic supplementary information (ESI) available: Experimental
details and NMR spectra of the monomer and the polymer. See DOI:
10.1039/b926211c
Scheme 1 Fluorescence switching of a fluorophore-coupled spiropyran
system.
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Fig. 1 Emission spectrum of P3HT (—) and absorption spectrum of
the MC form of spiropyran (-ꢂ-) in THF (0.1 mM). P3HT is excited at
448 nm.
Fig. 2 Absorbtion (A) and emission (B) spectra of P(TSP/3HT) ((A):
0.1 mM, (B): 0.8 mM) in THF before (—) and after (-ꢂ-) UV
irradiation (365 nm, 1 mW cmꢀ2) for 2 min. The emission spectra
were recorded with excitation at 435 nm. Inset displays concentration
dependent changes in fluorescence intensity.
Scheme 2 Preparation of the spiropyran–polythiophene conjugate,
P(TSP/3HT).
(Fig. 2B). The UV-induced fluorescence quenching is caused by
effective energy transfer from polythiophene moieties to the MC
form of the spiropyran. In addition, the degree of fluorescence
quenching increases as the concentration of the polymer
increases (Fig. 2B, inset). This phenomenon is likely a con-
sequence of increased interchain energy transfer when the
concentration of the polymer is high. It should be noted that
UV irradiation of a THF solution of the polymer results in the
appearance of a new emission band around 614 nm. This result
indicates that part of the energy transferred from excited state
polythiophene moieties is used to promote excitation of the MC
form of the spiropyran molecule, since the MC form displays a
weak fluorescence band near 614 nm when excited at 435 nm.
The UV irradiation induced decrease in the fluorescence
intensity of P(TSP/3HT) is substantially recovered (ca. 94% of
the initial value) by irradiation with visible light. This is a result
of the fact that the ring-closed SP form of the spiropyran moiety
produced upon visible light irradiation does not quench the
fluorescence of the polythiophene moieties. As shown by the
plot displayed in Fig. 3, the spiropyran-polythiophene dyad is
capable of undergoing a number of photoswitching cycles
between high (c, e, g, i, k) and low (b, d, f, h, j) fluorescence
intensity values by repetitive sequential irradiation of the
solution containing P(TSP/3HT) with UV and visible light.
The final phase of the current investigation focused on
demonstrating the utility of the spiropyran–polythiophene
conjugate as a FRET-based chemosensor. Recently, Shiraishi
Fig. 3 Relative fluorescence intensity of a THF solution (0.8 mM)
containing P(TSP/3HT) monitored at 556 nm (excitation at 435 nm)
before (a) and after UV (b, d, f, h, j) and visible light (c, e, g, i, k)
irradiation.
and co-workers described an interesting observation made in
studies of the UV irradiation of a solution of a spiropyran
containing various anions.30 These workers showed that
cyanide ion could be selectively detected (over other aꢀnions
such as Fꢀ, Clꢀ, Brꢀ, Iꢀ, AcOꢀ, H2PO4ꢀ, HSO4ꢀ, NO3 and
ClO4ꢀ) by monitoring the formation of an adduct, derived by
nucleophilic addition of cyanide anion to the ring-opened MC
form of the spiropyran (Scheme 3). Formation of the adduct
results in a significant blue shift of the absorption maximum
from 519 to 421 nm.
We envisioned that the spiropyran–polythiophene
conjugate would also interact with cyanide anion after being
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photoswitching system based on a spiropyran–polythiophene
conjugate. Thus, UV irradiation of a solution containing the
photochromic polymer results in a drastic decrease in fluorescence
intensity owing to effective FRET. The fluorescence intensity is
recovered by visible light irradiation of the solution. The
spiropyran–polythiophene conjugate system can also be used to
detect cyanide anion in a highly selective and sensitive manner.
The authors gratefully thank National Research Foundation
of Korea for financial support through Basic Science Research
Program (20090083161), Center for Next Generation Dye-
sensitized Solar cells (2009–0063368), and International
Research & Development Program (K20901000006-09E0100-
00610).
Scheme 3 Reaction between MC form of the spiropyran and cyanide
anion.
transformed to the MC form by UV-irradiation. We also
thought that the significant hypsochromic shift associated with
adduct formation would render the CN adduct incapable of
quenching efficiently the fluorescence emitted from polythio-
phene backbone. Thus, unlike the colorimetric CN sensor
reported by Shiraishi, the spiropyran–polythiophene
conjugate system would allow fluorescence-based detection
of cyanide anion.31
Notes and references
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In order to test this proposal, THF solutions (0.1 mM) of
P(TSP/3HT) containing various anions were irradiated with
365 nm UV light for 2 min and their fluorescence intensities
were monitored (Fig. 4). Interestingly, the degree of the
fluorescence quenching promoted by irradiation of the
solution containing cyanide anion is much smaller than that
caused by other anions or that monitored in the absence of
anions (Ctrl). In fact, no significant differences in the fluores-
cence quenching behavior of the polymer were seen when
other ions, such as fluoride, chloride, bromide, or nitrate, were
present. In addition, the color of the cyanide containing
polymer solution becomes yellow after 2 min irradiation while
the blue color of the solution remains unchanged or only
slightly changed when other anions are present. Greater than
90% of the initial fluorescence intensity is maintained when
the concentration of cyanide anion is 40.5 mM, indicating
that most of the spiropyran moieties form adducts upon
UV-irradiation in the presence of cyanide (Fig. 4, inset).
The detection limit for cyanide anion was determined to be
ca. 10 mM (Fig. 4, inset and Fig. S1, ESIw).
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Fig. 4 Emission spectra of P(TSP/3HT) (0.1 mM in THF) monitored
after 365 nm UV irradiation for 2 min in the presence of 0.1 mM of
anions (counter cation: n-Bu4N+). The inset is a plot of fluorescence
intensity as a function of CNꢀ concentration (mM); 0 (a), 0.001 (b),
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Chem. Commun., 2010, 46, 2859–2861 | 2861