Dual-mode fluorescence switching of photochromic bisthiazolylcoumarin

Article abstract of DOI:10.1039/c1cc16516j

Three new photochromic coumarins were synthesized. Fluorescence of the open form of 7-hydroxy-3,4-bisthiazolyl-coumarin increased to 1400% by changing the pH only slightly from 6.05 to 7.58. This was subsequently quenched to 1.5% of the maximum intensity at the UV photostationary state in water-methanol media.

Full text of DOI:10.1039/c1cc16516j

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Dual-mode fluorescence switching of photochromic bisthiazolylcoumarinw
Kazushi Suzuki, Takashi Ubukata and Yasushi Yokoyama*
Received 20th October 2011, Accepted 21st November 2011
DOI: 10.1039/c1cc16516j
Three new photochromic coumarins were synthesized. Fluorescence
of the open form of 7-hydroxy-3,4-bisthiazolyl-coumarin increased
to 1400% by changing the pH only slightly from 6.05 to 7.58. This
was subsequently quenched to 1.5% of the maximum intensity at
the UV photostationary state in water–methanol media.
compounds exhibiting good reversibility in fluorescence intensity
have been reported, most of them have relatively low fluores-
cence quantum yields and fluorescence switching efficiency.7 It is
also well documented that coumarin-type dyes possess excellent
fluorescence quantum yields and that their fluorescence intensity
is increased when an electron-donating group is substituted on
C-7 of the coumarin framework.11 Although a photochromic
hexatriene structure possessing a coumarin as one of the terminal
double bonds has been reported,12 its fluorescence switching
efficiency is not very impressive.
Photochromic compounds, which change their properties
reversibly in response to light irradiation, hold the potential
to work as molecular switches which can regulate various useful
functions.1 Diarylethenes2 are one of the most promising
thermally irreversible photochromic families due to their
remarkable fatigue resistance, enabling them to be used for
molecular switches. However, structurally less versatile hexa-
fluorocyclopentene is usually employed as the central ethene
moiety so that modifications on the ethene moiety to append novel
switchable functions in place of the hexafluoropropano-bridge
have recently attracted much attention.3–5 When a p-conjugating
functional group is placed as the central ethene moiety and its
property is altered by changing the p-conjugation, then a
photo-triggered molecular switch can be constructed.5 Among
the chemical and physical properties that a molecule can
switch, fluorescence is especially interesting and useful6 since
fluorescent molecules can be used as probes for intracellular
processes7 as well as for optical recording materials with
fluorescence readout.8 Moreover, in relation to the former
use, biological fluorescence imaging techniques have been
developed in order to visualize biological materials. For
example, ultra high resolution imaging that we could hardly
have previously imagined has recently been achieved.9 To
clarify the flow of materials in a living cell before and after
an event such as stimuli applied to the cell, it is essential that
the fluorescent ability of the probe is activated and that this
should coincide with the stimulation at the same point where it
is applied. It is also more useful if the fluorescence intensity is
sensitive to the environment so that it works as a fluorescent
probe when placed in a particular environment, e.g., a light-
switchable probe which works at a specific pH.10
We report here on the synthesis, photochromism, and
fluorescence switching of molecules having two thiazole rings
and a coumarin skeleton as the central ethene moiety. Upon
UV irradiation, we expected the fluorescence to be efficiently
quenched by photocyclization due to the destruction of the
coumarin structure. We have designed 3,4-bisthiazolylcoumarin
1O, its 7-methoxy derivative 2O, and 7-hydroxy derivative 3O
(Scheme 1). A stronger fluorescence of 2O than that of 1O
could be expected due to the larger electron-donating ability of
the 7-methoxy group. As for 3O, in addition to the photochromic
interconversion, its fluorescence can be expected to become
stronger by the generation of the phenolate anion upon base
treatment10,11c so that it could be regarded as a dual-mode
fluorescence switching molecule.
The synthetic key-step of 1O, 2O and 3O to introduce
thiazole groups to the coumarin skeleton was performed by
Suzuki–Miyaura coupling reactions of the corresponding
coumarin, the 3- and 4-positions of which were substituted by a
bromine and a trifluoromethanesulfonyloxy group, respectively.13
The intermediate coumarins were synthesized according to
previously reported procedures.13a Demethylation of 2O by
boron tribromide gave 3O. Characterizations of the novel
compounds were carried out by 1H NMR, FT-IR, UV-vis,
and mass spectroscopy.
The photochromic reactions and fluorescent properties of 1,
2 and 3 were investigated in various solvents with different
polarities.14 The conversion ratio to 2C at the photostationary
state (pss) upon 313 nm light irradiation in CH2Cl2 was
It is clear that such a switchable fluorescence imaging
system requires both high fluorescence quantum yield and
high fluorescence switching efficiency. Although photochromic
Department of Advanced Materials Chemistry, Graduate School of
Engineering, Yokohama National University, Hodogaya, Yokohama,
240-8501, Japan. E-mail: yyokoyam@ynu.ac.jp
w Electronic supplementary information (ESI) available: Experimental
details and Fig. S1–S26. See DOI: 10.1039/c1cc16516j
Scheme 1 Photochromism of the bisthiazolylcoumarin derivatives.
Chem. Commun., 2012, 48, 765–767 765
c
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determined by HPLC to be 98%. When the pss solution was
irradiated with 578 nm light, the original absorption spectrum
of the initial open form was restored, and HPLC analysis proved
that all the closed form molecules were converted to the original
open form again. As the conversion ratios of 1 and 2 were
remarkably high (92–93% for 1, 96–98% for 2) in various
solvents,14 it has been proved that they are suitable for the switch.
As we expected, the fluorescence intensity of 1O and 2O
decreased drastically by photochromic ring closure upon
irradiation with 313 nm light due to the deformation of the
coumarin structure. The fluorescence intensity was recovered
upon successive irradiation with 578 nm light due to the
restoration of the coumarin structure.14 The fluorescence
switching efficiency upon irradiation with 313 nm is excellent
owing to the high conversion ratio. The fluorescence quantum
yields of 1O and 2O excited with 330 nm light in hexane were
0.0025 and 0.0043, respectively. The larger quantum yield of
2O over 1O is attributed to the electron-donating ability of the
methoxy group on C-7 of the coumarin structure.11b
With 2, repetitive fluorescence switching was carried out by
alternate irradiation with 313 and 578 nm light. Repetition of
fluorescence switching could be successfully conducted without
decrease in fluorescence intensity for five cycles.14 However,
the fluorescence quantum yields of both 1O and 2O were
unsatisfactory. Therefore, we next examined the properties of
3O which can be changed to phenolate by base treatment. As
the electron-donating ability of phenolates is much stronger
than that of the methoxy group, we expected the quantum
yield of the fluorescence to increase.
Upon alternate irradiation with 313 and 578 nm light in
dichloromethane and acetonitrile, 3 showed good photo-
chromism. High conversion ratios to 3C at the pss upon 313 nm
light irradiation were also achieved at 98% and 95% for
dichloromethane and acetonitrile, respectively. When 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) was added to a dichloro-
methane solution of 3O in order to generate the phenolate, a
new absorption band centred at 404 nm appeared and reached
saturation when 12.0 mole equivalents of DBU was added
Fig. 1 (a) Change in absorption spectra of 3O upon addition of DBU
in CH2Cl2. Equivalence of DBU: 0 to 12. (b) Change in absorption
spectra of 3Oanion during irradiation with 405 nm light in the presence
of 12.0 equiv. DBU in CH2Cl2. Irradiation time: 0–40 min. (c) Change
in fluorescence spectra of 3Oanion upon 405 and 578 nm light irradiation
in the presence of excess DBU in CH2Cl2. excitation: 330 nm.
(Fig. 1a). The appearance of this new band is ascribable to the
15
the phenol of 3O was determined to be 6.84, which is consistent
with that of 7-hydroxycoumarin.11a This pKa value is suitable
in controlling the changes between 3O and 3Oanion by the slight
alteration of the pH in the biological systems. In fact, 14-fold
increase was observed by changing the pH from 6.05 to 7.58.
The fluorescence quantum yield of 3Oanion with the excitation
generation of a phenolate anion 3Oanion.
chromic reaction between 3Oanion and its pss (3Canion/3Oanion
A reversible photo-
4
10016) in dichloromethane was observed by means of alternate
irradiation with 405 nm (Fig. 1b) and 578 nm light.
We then examined the light-triggered switching behavior of
fluorescence for 3anion in dichloromethane. The fluorescence
switching of 3Oanion upon alternate irradiation with 405 and
578 nm light was demonstrated as shown in Fig. 1c, and the
fluorescence quantum yield of 3Oanion in dichloromethane was
determined to be 0.019. Although this value is more than four
times as large as that of 2O in hexane, it is still unsatisfactory
for biological use. As these experiments were carried out in
organic solvents, we then moved to solvents containing water
as the major component in order to assess the applicability of 3
to biological systems.
To our great delight, the fluorescence of 3O was strongly
enhanced in water/methanol (2/1 v/v) with base treatment, as
compared to that in organic solvents. Fig. 2 illustrates the
change in the fluorescence spectra upon acid–base titrations
using aqueous sodium hydroxide solution. The pKa value of
Fig. 2 Change in fluorescence spectra of 3O with pH of the medium.
Solvent: CH3OH/NaNO3 aqueous buffer solution = 1/2 (v/v), concen-
tration: 5.1 Â 10À6 mol dmÀ3, excitation wavelength: 365 nm, pH of
the solution: 2.64, 6.05, 6.38, 7.01, 7.58, 9.65.
c
766 Chem. Commun., 2012, 48, 765–767
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‘‘New Frontier in Photochromism (No. 471)’’ from the Ministry
of Education, Culture, Sports, Science and Technology
(MEXT), Japan.
Notes and references
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coumarins 1–3. Among them, under the basic conditions,
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7-hydroxy-3,4-bis(5-methyl-2-phenyl-4-thiazolyl)coumarin
3
changed its fluorescence intensity (quantum yield of 0.25 at
pH 9.48 in buffered aqueous solution containing methanol)
with photochromism as well as changes in the pH of the
media. The intensity ratio of the photostationary state/open
form at pH 9.48 was 1.5% while the intensity ratio of the open
form at pH 7.58/pH 6.05 was 14-fold. These results show that
the fluorescence intensity of 3O can be modulated by the
photochromic reactions as well as by the pH of the media.
The authors gratefully acknowledge the financial support
of a Grant-in-Aid for Scientific Research in Priority Areas
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 765–767 767