Photoswitching of an intramolecular chiral stack in a helical tetrathiazole

Article abstract of DOI:10.1039/c6cc01277a

On-off photoswitching of circularly polarized luminescence was achieved using a pyrene-bearing helical tetrathiazole, in which two pyrene fluorophores stack in a chiral fashion (folded state). The pyrene-excimer based CPL was reversibly controlled by a ge

Full text of DOI:10.1039/c6cc01277a

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DOI: 10.1039/C6CC01277A
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Photoswitching of intramolecular chiral stack in a helical
tetrathiazole†
Yuichiro Hashimoto,a Takuya Nakashima,*a Daiya Shimizub and Tsuyoshi Kawai*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
On-off photoswitching of circularly polarized luminescence was phenylthiazole units at both ends of tetrathiazole form
-
achieved with a pyrene-bearing helical tetrathiazole, in which two stacking in a helical manner. In the present study, pyrene
pyrene fluorophores stack in a chiral fasion (folded state). The fluorophores are introduced in these positions via a chiral spacer
pyrene-excimer based CPL was reversibly controlled by
geometrical change of tetrathiazole on the photoisomerization.
a
of phenylalanine (
D
-or -1o, Fig. 1). The chiral spacer is expected
L
to control the handedness of helix, twisting the pyrene-stack in a
one-handed direction. Chiral excimers of pyrenes have been
reported to exhibit relatively strong CPL with |glum| value of
1%,5,6 which is given by the equation glum = 2(IL − IR)/(IL + IR),
where IL and IR are the intensities of the left- and right-handed
circularly polarized emissions, respectively. The off-on
photoswitching of fluorophore-stacking have been demonstrated
with trans-cis isomerizations of thioindigo7 and overcrowded
alkene.8 On the other hand, the light-induced geometrical
changes of most diarylethenes and their aromatic derivatives are
rather small, which supports their photo-reactivity in crystals.9
Unlike conventional diarylethenes, the photochromism in
tetrathiazoles gives rise to a large structural change from an
open- to a ring-closed photoisomer, driving the fluorophores to
change their position and orientation with the dynamic sp2/sp3
interconversion on the reacting carbon atoms (Fig. 1).
Light is one of the external stimuli to trigger a change in the
physicochemical properties of molecules in a noninvasive
manner, promising a wide range of applications. Chiroptical
photoswitches are of particular interest in terms of advanced
informational technologies.1 Circularly polarized luminescence
(CPL) has been attracting much interest as an important
chiroptical phenomenon which offers potential applications in
sensors, display and optical storage.2 Meanwhile, the chiroptical
photoswitches have often been demonstrated by the
stereoselective isomerization of photochromic molecules,
changing circular dichroism (CD) and optical rotatory dispersion
(ORD).1 Akagi and co-workers reported the reversible changes
of CPL intensity in films of chirally aggregated conjugated
polymers bearing photoresponsive dithienylethenes.3 A photo-
generated isomer of dithienylethene with an intense visible
absorption band efficiently quenched the emission from
-
conjugated polymers in the aggregates. However, dynamic
photoswitching of CPL in solution with a unimolecular system
has still remained one of the important challenges in the
development of chiroptical photoswitches.
Here we report a chiroptical photoswitch which dynamically
modulates CPL emission using a photochromic tetrathiazole4
scaffold. In the previous work, we reported that the tetrathiazole
folds into a one-turn helical conformation with photochromic
reactivity by multiple intramolecular interactions.4b The
a. Graduate School of Materials Science, Nara Institute of Science and Technology,
Ikoma, Nara 630-0192, Japan.
Fig. 1 Photoswitching of chiral pyrene stack with tetrathiazole D-1.
E-mail: ntaku@ms.naist.jp, tkawai@ms.naist.jp
b. Advanced Chemistry Course, Osaka Prefecture University College of Technology,
Neyagawa, Osaka 572-8572, Japan.
The tetrathiazole 1o was synthesized from
a NH2-
†
Electronic Supplementary Information (ESI) available: Detailed experimental
functionalized tetrathiazole by a condensation with Boc-
methods and additional data. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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phenylalanine, a deprotection of Boc-group followed by a handed helix (Fig. 3b). Although the exciton cVoieuwpAlritinclge1O1nlionef
condensation with 1-pyrenecarboxylic acid (see the ESI† for pyrene units was not clear because of the superimposed signal
DOI: 10.1039/C6CC01277A
details). The conformational behavior of 1o in solution was originating from the main framework of tethrathiazole, the signs
characterized with variable temperature (vt) 1H NMR. The of first Cotton effect implied that
compound 1o gave an NMR profile with a broadened peak left-handed helix, respectively.4b
pattern at 30 ºC because of the limited solubility in 1,1,2,2-
D
- and
L-
1o possess a right- and
tetrachloroethane-d2 at high concentration (> 10-3 M). Each peak
(a)
0.6
1
assignment was determined based on the homonuclear H-1H
correlation spectroscopy (COSY, Fig. S3†). The two methyl
0.4
groups at the ends of tetrathiazole moiety gave a singlet peak at
2.05 ppm (Fig. S2†). The methyl signal at a very upfield chemical
shift indicated the folded helical conformation of 1o as shown in
Fig. 1, wherein the ring-current effects of the facing thiazolyl
rings operate on these methyl groups.4c As shown in Fig. 2,
amide-N-H signals (H3 and H5) showed a continuous upfield-
shift with peak sharpening upon heating to 100 ºC, indicating that
these protons participate in hydrogen bonding interactions.
Meanwhile, the protons H1 and H2 representative for pyrene-
protons exhibited a down-field shift with increasing temperature.
The pyrene rings seem, thus, to stack so closely that significant
ring-current effect operates on their protons.4b,c Furthermore, the
nuclear Overhauser effect correlation spectroscopy (NOESY)
measurement figured out the through space interactions in the
pyrene-protons region above 7.7 ppm (Fig. S3†). We also
0.2
0
(b)
(c)
40
20
0
-20
-40
D-form
L-form
D
6
4
D-form
2
0
-2
-4
-6
performed a conformational search for -1o with the MMFF94s
D
L-form
force field using the CONFLEX program.10 The calculation
study identified that the helical conformation with the stacking
of pyrene units was the most stable conformer, which is more
stable than a conformer with non-stacked pyrene moieties by 6
kcalmol-1 (Fig. S4†). These 1H NMR and computational
simulation results well supported that 1o adopts the folded
conformation with the pyrene-stack as shown in Fig 1.
300
400
500
600
700
800
Wavelength/ nm
Fig. 3 Absorption(a) and CD(b,c) spectral change of
(a) black line: 1o; green solid line: 1c; orange dashed line: PSS state. (b,c) solid
lines: 1o; dashed lines: PSS state (blue lines: -forms, red lines: -forms).
1
in chloroform (8.7×10-6 M).
L
-
L-
D
L
UV irradiation to 1o solution resulted in the emergence of
absorption band in the visible region due to the formation of 1c
1o showed relatively high coloration performance with a
quantum yield ( o-c) of 50%. The decrease in o-c relative to the
unsubstituted tetra(2-phenylthiazole)4a
o-c = 60%) may be
.


(
attributed to the light absorption of pyrene moieties which might
not contribute to the photoisomerization. While the quantum
H2
H1
H5
H4
H3
yield of cycloreversion (
high conversion ratio over 99% at the photostationary state (PSS)
suggested a suppressed value for c-o
change was recorded in the CD spectra, in which broad CD
signals at the wavelength of visible absorption band of
photoproducts emerged with changing the CD pattern below 400
nm after UV irradiation (Fig. 3c). Chiral induction of helix-
c-o) could not be determined, the very
373 K
343 K
323 K
303 K
4a
.
Meanwhile, a drastic
(ppm)
2
Vt 1H NMR spectra of L-1o in the low magnetic field region (in 1,1,2,2-
Fig.
handedness in
D
- and
L
-1o thus successfully led to a
tetrachloroethane-d2) with the part of the chemical structure of L-1o.
diastereoselective photoreaction which proceed in the
conrotatory manner. To determine the diastereo-excess (de) in 1c
the photoproducts were investigated by H NMR and chiral
HPLC. In the 1H NMR, only a set of methyl peaks were observed
due to the different electromagnetic environment of the end
methyl groups in 1c (Fig. S5†). Since the chiral groups were far
from these methyl groups, the diastereomers might gave
,
Fig. 3 summarizes the optical response of 1o with respect to
UV light ( = 365 nm) irradiation. The absorption of pyrene-
1
amide moiety was overlapped with that of tetrathiazole
framework below 400 nm, leading to the slight red-shift of
absorption peak relative to the unsubstituted tetra(2-
phenylthiazole) (Fig. 3a) with an increase in the molar absorption
identical chemical shifts. However, the
D
- and -forms of 1o
L
coefficient (
of 1o enantiomers gave mirror image profiles in the region of
transitions, suggesting the preferential formation of one-

) from 4.5 × 104 to 9.8 × 104 M-1cm-1.4a CD spectra
exhibited a single peak in a chiral HPLC with slightly different
retention times, giving an additional peak after the photoreaction
corresponding to the formation of single component 1c (Fig. S6†).

-

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Based on the results of H NMR and chiral HPLC analyses 1o memory system, which would be achieved by chiVriaewl rAartricele-eOanrlitnhe
most likely underwent almost absolute diastereoselective complexes15 with photochromic ligands.16
DOI: 10.1039/C6CC01277A
photoreaction. The
helical geometry, respectively, led to the formation of the (R, S)-
and (S, R)-form 1c 4b respectively (Fig S7†).
The tetrathiazole 1o showed blue-green emission with an
apparent quantum yield ( f) of 4%. The relatively small emission
D
- and
L
-1o with a right- and left-handed
,

quantum yield should be attributed to the light absorption by the
photochrome part which does not lead to fluorescence. The
intense and broad band at 500 nm corresponds to an
intramolecular pyrene excimer in accordance with a long lifetime
of 21 ns (Fig S8†). A weak and structured monomer emission
band was also observed at 400 nm with a shorter lifetime of 3.6
ns (Fig S8†). The intensity of both excimer and emission bands
decreased with UV irradiation (Fig. 4a). The isolated 1c gave the

f value far less than 0.1%. Both the emission from monomer and
excimer decreased in a liner fashion as a function of conversion
ratio to the colored form 1c (Fig. 4b). The quenching of monomer
emission should be attributed to an energy transfer mechanism
by the increased absorbance in the range of 380–410 nm in 1c
(Fig. 3a).12 The same mechanism partly explains the quenching
in the excimer emission at 500 nm. Although the spectral overlap
of excimer emission around 500 nm with absorption in 1c is not
as prominent as that of monomeric emission, the quenching of
excimer emission was a little more significant than that of
monomer emission (Fig. 4b). This fact might be attributed to the
non-negligible contribution of geometrical factor in 1c (Fig. 1).
Furthermore, the two amide-protons showed an upfield-shift
from 1o to 1c (Fig. S5†), indicating the dissociation of
intramolecular hydrogen bonding interactions which tether the
pyrene units close to each other.13
Fig. 4 (a) Fluorescence spectral change of L-1o upon UV irradiation in CHCl3 (4.6×10-6 M).
Solid thick black line: 1o-form, solid green line: 1c-form, dashed green line: at PSS, other
traces were measured with the irradiation interval of 5s. (b) Plots of relative emission
intensity in monomer (blue, at 390 nm) and excimer (excimer, at 500 nm) emission. I0
corresponds to the intensity at the conversion of 0. (c) CPL spectra of 1o in CHCl3 (1.7×
10-4 M). D-form: solid blue line; L- form: solid red line; dashed lines: at PSS. (d) Reversible
changes of CPL intensity at 500 nm of D-1o in chloroform with UV (365 nm) and visible (>
400 nm) irradiations.
This work was partly supported by Grant-in-Aids for
Scientific Research (B) (T. N., no. 15H03858), Challenging
Exploratory Research (T. N., no. 15K13709) and Scientific
Research on Innovative Areas “Photosynergetics” (T. K. no.
26107006) sponsored by the Ministry of Education, Culture,
Sports, Science and Technology (MEXT) Japan. The authors
wish to thank Mr. F. Asanoma and Ms. Y. Nishikawa for their
assistance with NMR and mass measurements, respectively.
The chiral induction in the helical conformation of 1o resulted
in the emergence of a strong CPL signal corresponding to the
pyrene excimer emission (Fig. 4c). The signs of CPL signal
coincide with those of the first Cotton effect observed in CD
spectra (Fig. 3b). That is, two pyrene rings are arranged
preferentially in a P- and M-configuration in the right- and left-
handed helix of
D
- and -1o, respectively. The dissymmetry
L
factor in luminesce was estimated to be |glum| ~ 1% in consistence
with previous reports.5,6 Along with the decrease in the excimer
emission intensity by the photoisomerization, the CPL intensity
decreased. Since the CPL measurement was performed at a
higher concentration (1.7 × 10-4 M) in comparison to that for
absorption spectra (Fig. 3a), the conversion ratio was not enough
to achieve the completely off-state (Fig. 4c). The on-off
photoswitching behavior was observed around 6 cycles with
alternate UV and visible irradiations as shown in Fig. 4d.
In summary, we prepared tetrathiazoles bearing two pyrene
fluorophores through chiral spacers as a chiroptical switch,
exhibiting diastereoselective photoisomerizations. The on-off
switching of intramolecular excimer CPL was demonstrated
upon the photoisomerization in a helical tetrathiazole. In addition
to the unimolecular behavior, the application of present
molecular system to chiral self-assemblies could further enhance
the switching effect14 of supramoelcular chiroptical properties.2c
Another direction may include a CPL read-out non-destructive
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