Photon-quantitative reaction of a dithiazolylarylene in solution

Article abstract of DOI:10.1002/anie.201006844

Ready for action: An excellent quantum yield was observed for a photocoloration reaction of a photochromic molecule based on a triangular terarylene structure. The molecule is brought into a conformation favorable for photocyclization by multiple intramolecular interactions, including CHi-N hydrogen bonding and Si-N and CHi-π interactions (see picture).

Full text of DOI:10.1002/anie.201006844

DOI: 10.1002/anie.201006844
Photochromic Switches
Photon-Quantitative Reaction of a Dithiazolylarylene in Solution**
Sayo Fukumoto, Takuya Nakashima, and Tsuyoshi Kawai*
In the cascade of mammalian vision, the cis-to-trans photo-
isomerization reaction of the retinal chromophore bound to
the opsin apoprotein acts as the first trigger for the conversion
of light signals into electrical pulses, which are finally
transmitted to the brain. One of the specific chemical
characteristics of retinal chromophore is its high quantum
yield of cis-to-trans photoisomerization, Fcis–trans = 65%,
which affords the high sensitivity of the rod rhodopsin, an
opsin–retinal complex required for night vision.[1] Interest-
ingly, the Fcis–trans value of retinal decreased to less than 30%
in solution without the protein. This observation suggests that
the environmental conditions of the binding pocket in
rhodopsin may be responsible for the high photochemical
reactivity of retinal. The hydrophobic pocket in rhodopsin is
thought to favor appropriate noncovalent bonding to regulate
the geometry of the guest molecule in preparation for the
photoisomerization reaction.[2] With the aim of developing
highly efficient and sensitive photochromic molecules on the
basis of the opsin–retinal system, we designed an organic
photochromic molecule with a terarylene structure and
demonstrated its photon-quantitative reaction.
cyclization reaction in the crystalline state, when their
conformation is fixed in the antiparallel conformation and
the distance between reactive carbon atoms is sufficiently
short: typically less than 0.4 nm.[6] These observations indicate
that the ground-state conformation plays a crucial role in the
photoinduced pericyclization reaction. Therefore, various
attempts have been made to regulate the ground-state
geometry of diarylethenes so that they have C2 symmetry.[7]
We herein propose a concept for the design of highly reactive
photochromic hexatriene molecules that is inspired by the
design of foldamers;[8] thus, multiple intramolecular interac-
tions were used to stabilize the photoreactive conformation of
a terarylene.
The 2,3-dithiazolylbenzothiophene 1a was designed as a
representative photochromic molecule whose conformation is
expected to be stabilized in the photoreactive conformation
through intramolecular interactions, as depicted by dotted
lines in Scheme 1. The rotation of one side-chain thiazolyl
Photochromic terarylenes are composed of three aromatic
rings connected to form a hexatriene backbone,[3] which is
converted into a cyclohexadiene structure upon UV irradi-
ation, as similarly observed for the photochromism of diary-
lethenes.[4] Recent studies on terarylenes have highlighted
their high photocyclization quantum yields, which exceed
60%[3b,d,5] and are apparently higher than those of conven-
tional diarylethenes.[4] Since the photoelectrocyclic reactions
typically occur in less than 200 fs, the photocyclization
quantum yield is generally determined by the electronic
structure in the excited state, which is directly interpreted as
the molecular conformation in the ground state. Diaryle-
thenes are known to possess two conformations in solution,
with the two rings in mirror symmetry (parallel conformation)
or C2 symmetry (antiparallel conformation). As the photo-
cyclization reaction proceeds only from the C2-symmetrical
conformation, the quantum yield is about 50% at most.[4a] In
contrast, some diarylethenes undergo a quantitative photo-
Scheme 1. Photochromism of dithiazolylbenzothiophene 1a and the
reference compound 2a (dithienylbenzothiophene).
ring would be suppressed by an S–N heteroatom-contact
interaction.[9] Such interactions are considered to be respon-
sible for the planarity of conjugated polymers composed of,
for example, co-thiophene-benzo[c]1,2,5-thiadiazole structur-
es.[9a] S–N close contacts shorter than the sum of the
van der Waals radii were also observed for crystal structures
of p-conjugated oligomers consisting of alternate N- and S-
heteroaromatic rings with a coplanar conformation.[9b,c] Weak
[*] S. Fukumoto, Dr. T. Nakashima, Prof. T. Kawai
Graduate School of Materials Science
Nara Institute of Science and Technology, NAIST
8916-5 Takayama, Ikoma, Nara 630-0192 (Japan)
Fax: (+81)743-72-6179
E-mail: tkawai@ms.naist.jp
[**] We thank Y. Nishikawa for the measurement of mass spectra. This
research was partly supported by the Ministry of Education, Culture,
Sports, Science and Technology (MEXT) of Japan with Grants-in-Aid
for Scientific Research on the Priority Area “New Frontiers in
Photochromism” (471).
[3b,10]
À
CH N hydrogen bonding
would tether the other thia-
zolyl ring. In combination with these interactions, which favor
the coplanarity of the three heteroaromatic rings, steric
hindrance between the methyl groups at the reactive carbon
centers causes the side-chain thiazolyl rings to tilt slightly and
direct the adoption of the photoreactive conformation in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006844.
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1565

Communications
which the hexatriene moiety has C2 symmetry. The 2,3-
dithienylbenzothiophene 2a was also designed as a reference
compound.
The 2,3-diarylbenzothiophenes 1a and 2a were synthe-
sized by conventional cross-coupling reactions of the corre-
sponding arylene units. Their chemical structures were
confirmed by high-resolution mass spectroscopy, 1H and
13C NMR spectroscopy, and elementary analysis. Both 1a and
2a could be recrystallized from solutions in hexane, and their
crystal structures were determined by X-ray crystallographic
analysis.[11] The single crystal of 1a turned blue upon
irradiation with UV light, whereas 2a showed no photo-
chromism in the crystal. The crystal structure of 1a showed
the photoreactive conformation with C2 symmetry of the
hexatriene moiety (Figure 1a). In particular, the distance
Figure 2. Temperature-dependent 1H NMR spectra of 1a in
[D8]toluene. The arrow indicates the signal of hydrogen atom H1
(Figure 1).
high field for methyl-substituted thiazole derivatives. The
upfield shift of methyl hydrogen atoms would originate from
the ring-current effect of the neighboring thiazole ring and
indicates CH–p interactions between the methyl groups and
side-chain thiazole units. Upon heating, the H1 signal showed
a continuous upfield shift from d = 8.12 ppm at 293 K to d =
7.99 ppm at 353 K, as indicated by arrows in Figure 2. This
observation suggests that the CH–N interaction became weak
as the temperature was raised. The signals of other arylene
hydrogen atoms between d = 7.9 and 8.0 ppm showed a slight
upfield shift by 0.05 ppm at most; thus, these signals were not
shifted to such a significant degree as the signal of H1.
Moreover, the signal for methyl hydrogen atoms underwent a
progressive downfield shift and splitting upon heating
(Figure 2). This behavior indicates the weakening of CH–p
interactions and a consequent decrease in the structural
symmetry around the hexatriene moiety. In contrast, the
signal of H1 exhibited no marked temperature dependence in
CD3OD. These experimental 1H NMR spectroscopic data
strongly suggest that CH–N hydrogen bonding and CH–p
interactions stabilize the photoreactive conformation of 1a at
room temperature in nonpolar solvents; such an effect is not
expected in protic solvents.
Figure 1. ORTEP drawings (showing 50% probability displacement
ellipsoids) of the open-ring isomer in a) crystal 1a and b) crystal 2a.
between H1 and N2 was estimated to be 0.269 nm, which is
shorter than the sum of the van der Waals radii of the H
(0.12 nm) and N atoms (0.155 nm). The distance between the
N1 and S1 atoms, 0.295 nm, was also shorter than the sum of
the van der Waals radii of the S (0.185 nm) and N atoms
(0.155 nm). These atomic contacts clearly indicate noncova-
lent interactions of the sort mentioned above, which are
considered to stabilize the reactive quasi-C2-symmetrical
conformation of 1a in the crystalline phase. Furthermore,
CH–p interactions were also suggested by the short distances
between the hydrogen atoms on reactive methyl carbon
atoms and the molecular planes of the thiazolyl rings (0.31
and 0.32 nm). On the other hand, the geometry of 2a in the
crystal is apparently unsuitable for a photocyclization reac-
tion (Figure 1b).
To evaluate intramolecular interactions in solution, we
performed temperature-dependent 1H NMR spectroscopy on
compound 1a in the nonpolar solvent [D8]toluene. The signal
of H1 (in Figure 1), which is expected to participate in CH–N
hydrogen bonding, appeared at d = 8.12 ppm at 293 K, as
indicated by the arrow in Figure 2. This chemical shift is well
downfield of those for other arylene hydrogen atoms in 1a,
whereas it was observed below d = 8.0 ppm and overlapped
with those of other arylene hydrogen atoms in CD3OD (see
the Supporting Information). A chemical shift at such a low
field was also reported for intramolecular weak CH–N
hydrogen bonding in 4-arylthiazolyl derivatives.[12] Mean-
while, the hydrogen atoms at the reactive methyl carbon
atoms gave a single peak at d = 1.79 ppm, and thus at a very
The results of a density functional theory (DFT)[13] study
at B3LYP/6-31G* to optimize the structure of 1a were in good
agreement with the results of X-ray crystallography and
1H NMR spectroscopy. A photoreactive conformation of 1a
was obtained as the optimized structure, which was almost
identical to the crystal structure shown in Figure 1a. The
atomic contacts between H1 and N2 and between S1 and N1
(estimated distance: H1–N2, 0.26 nm; S1–N1, 0.32 nm) are
consistent with CH–N and S–N intramolecular interactions.
The distances between hydrogen atoms on reacting methyl
carbon atoms and the molecular planes of side-chain thiazole
units were evaluated to be about 0.31 and 0.35 nm, which are
also short enough for CH–p interactions.
Finally, we investigated the photochromic performance of
1a in solution. Irradiation of a solution of 1a in hexane with
light of wavelength 313 nm resulted in the immediate changes
in the UV/Vis absorption spectrum that are typical for
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that the absorbed photons quantitatively induce the photo-
isomerization of 1a to 1b. We also measured the progression
curve of the concentration of ring-closed isomer 1b (Cc) on
the basis of the absorbance at 600 nm as a function of UV
irradiation time to verify these quantum-yield values
(Figure 3, inset). It was numerically reproduced by using an
equation assuming the ideal photochemical reaction scheme
with cyclization and cycloreversion quantum yields of 0.98
and 0.008, respectively. In contrast, 1a showed a marked
decrease in cyclization quantum yield, Fo–c = 0.54, in meth-
anol. The decrease in Fo–c in a protic solvent also supports the
hypothesis that the weak CH–N hydrogen bonding is
predominantly responsible for the stabilization of the photo-
reactive conformation, in agreement with the results of the
1H NMR spectroscopic study described above. In contrast,
the photocyclization quantum yields of 2a were substantially
lower and showed less-prominent solvent dependence than
that observed for 1a.
Figure 3. Absorption spectral changes of a) 1 (4.7ꢀ10À5 m) and b) 2
(1.9ꢀ10À5 m) in hexane. Spectra for the photostationary state under
irradiation with light of wavelength 313 nm and for the closed form
(1b or 2b) are depicted by bold black and gray lines, respectively.
Inset: Progression of the generation of ring-closed isomer 1b by UV
irradiation. The plot was fitted by a kinetic equation in which Fo–c and
Fc–o were assumed to be 0.98 and 0.008, respectively.
In summary, we designed and synthesized a triangle
terarylene with high photochromic-coloration performance.
The photocyclization quantum yield of compound 1a in
hexane was almost 100%, whereas that in methanol was
considerably lower. Multiple intramolecular noncovalent
interactions that tether the geometry of 1a in the photo-
reactive conformation, including weak CH–N hydrogen
bonds as well as S–N and CH–p interactions, were demon-
diarylethenes and terarylenes. The absorption band of 1a at
307 nm observed in the original state decreased, and a new
absorption band appeared at around 600 nm (Figure 3), which
corresponds to the formation of 1b. The chemical structure of
1
1b was confirmed by H NMR spectroscopy. The change in
the absorption spectrum upon photoirradiation was accom-
panied by an isosbestic point at 330 nm, which clearly
indicated the two-component photochromic reaction between
the open form 1a and the closed form 1b. Irradiation of the
colored solution with visible light led to the recovery of an
absorption spectrum identical to that of the initial solution of
1a. Similar photochromic behavior was also observed for 2.
The conversion ratios between the colorless and colored
photostationary states upon irradiation with UV light (l =
313 nm) were estimated to be more than 96% for 1 and 90%
for 2. The photochromic properties of 1 and 2, including the
reaction quantum yields, are summarized in Table 1. The
1
strated by X-ray crystallography, H NMR spectroscopy, and
DFT calculations. The molecular design based on supra-
molecular interactions provided an artificial photochromic
system with greater sensitivity than that of a natural biological
system, such as an opsin–retinal complex, as a result of precise
control of the ground-state conformation.
Received: November 1, 2010
Published online: January 7, 2011
Keywords: heterocycles · hydrogen bonds · photochromism ·
.
supramolecular chemistry
Table 1: Absorption maxima and coefficients of the open- and closed-
ring isomers of 1 and 2, together with the quantum yields in solution.
Isomer
lmax [nm][a]
Fo–c
Fc–o
(e [104 mÀ1 cmÀ1][a])
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Angew. Chem. Int. Ed. 2011, 50, 1565 –1568
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