Enhanced photochemical sensitivity in photochromic diarylethenes based on a benzothiophene/thiophene nonsymmetrical structure

Article abstract of DOI:10.1002/ejoc.201402774

Photosensitivity of molecules is one of the central subjects of organic photochemistry. In this article, we present how and why a benzothiophene moiety impacts the photochromic quantum yield for the photochemical pericyclic ring closing of the hexatriene moiety. Two compounds were prepared: a symmetrical one incorporating two fluorenylthiophene units, and its corresponding nonsymmetrical partner, where one of the thiophene units was replaced by a benzothiophene group. Interestingly, the latter presents relatively high cyclization quantum yield (0.74), a value close to the largest one so far reported. Based on crystal structures, VT-NMR data and DFT calculations, we demonstrate that introducing a benzothiophene in the molecular scaffold induces skeleton stiffening by means of CH-π interactions and steric repulsions, and leads to increased population of the reactive conformer in the ambient conditions.

Full text of DOI:10.1002/ejoc.201402774

Job/Unit: O42774
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Date: 26-09-14 13:01:04
Pages: 10
FULL PAPER
DOI: 10.1002/ejoc.201402774
Enhanced Photochemical Sensitivity in Photochromic Diarylethenes Based on a
Benzothiophene/Thiophene Nonsymmetrical Structure
[a]
[a]
[a]
[a]
Olivier Galangau, Yuka Kimura, Rui Kanazawa, Takuya Nakashima, and
Tsuyoshi Kawai*[
a]
Keywords: Photochromism / Noncovalent interactions / Density functional calculations / Diarylethenes
Photosensitivity of molecules is one of the central subjects of
organic photochemistry. In this article, we present how and
why a benzothiophene moiety impacts the photochromic
quantum yield for the photochemical pericyclic ring closing
of the hexatriene moiety. Two compounds were prepared: a
symmetrical one incorporating two fluorenylthiophene units,
and its corresponding nonsymmetrical partner, where one of
the thiophene units was replaced by a benzothiophene
group. Interestingly, the latter presents relatively high cycli-
zation quantum yield (0.74), a value close to the largest one
so far reported. Based on crystal structures, VT-NMR data
and DFT calculations, we demonstrate that introducing a
benzothiophene in the molecular scaffold induces skeleton
stiffening by means of CH–π interactions and steric repul-
sions, and leads to increased population of the reactive con-
former in the ambient conditions.
charge transfer (TICT) nature in the excited state is believed
to suppress the ring cyclization reaction, which rises espe-
cially in standard DAEs with central ethene units bearing
Introduction
Diarylethenes (DAEs) and their aromatic analogues have
been the subject of intense research over the past two dec-
ades. These classes of compounds undergo a reversible
photochromic reaction between two stable forms: the ring-
strong electron-withdrawing groups in polar environ-
[8]
ments. Some DAEs show Φ values close to unity in in
o/c
[9]
single-crystalline samples. Amongst the several DAE-re-
opened form with the hexatriene backbone and the ring-
lated substances displaying high Φ values in the solution
o/c
closed form with the cyclohexadiene backbone.[
1,2]
Their
[10]
phase, some, in which the DAE backbone was modified
high fatigue resistance and low thermal reactivity make
these derivatives highly promising for optoelectronics appli-
for stabilizing the reactive conformation with noncovalent
attractive interactions between noncarbon atoms, have also
[
3–7]
cations.
The photochemical parameter that governs the
[11–16]
displayed significant Φo/c values.
photocyclization sensitivity is well defined as the quantum
yield Φo/c. The quantum yield has been shown to predo-
minantly depend on the ground state equilibrium between
the nonreactive parallel and the reactive antiparallel con-
formers, because their ultrafast photo-excited state lifetimes
of less than several picoseconds restrict rotational isomer-
ization in the excited state.[ The photoisomerization serves
as “snapshot” of the geometry of the open form in equilib-
rium with the reactive and the nonreactive conformations.
Meanwhile, the contribution of a twisted intramolecular
Since these approaches require fiddly and specific designs
with limited choice of molecular structures, it is still worth
studying the relationship between photoreactivity and mo-
lecular structure of the usual DAE backbone.
In the class of standard perfluoroDAEs (DAEs contain-
ing a perfluorocyclopentene central moiety), the highest
Φo/c value so far reported (0.79) has been established for a
3]
structurally simple, nonsymmetrical compound featuring a
[17]
benzothiophene moiety and a thiophene unit.
A refer-
ence compound with symmetrical structure has denoted
suppressed photoreactivity. Yet, no clarification has been
provided to unveil the causes of such a Φo/c high value and
the effect of its nonsymmetric backbone structure. This lack
of systematic knowledge motivated us to synthesize two
analogous compounds, TT and BT, and to study their
structural and photophysical properties in condensed and
solution phases (see Scheme 1).
[
a] Graduate School of Materials Science, Nara Institute of Science
and Technology, NAIST,
8
916-5 Takayama-cho, Ikoma, Nara 630-0192, Nara, Japan
E-mail: tkawai@ms.naist.jp
http://mswebs.naist.jp/LABs/kawai/english/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201402774.
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O. Galangau, Y. Kimura, R. Kanazawa, T. Nakashima, T. Kawai
FULL PAPER
Scheme 1. Ring cyclization and ring cycloreversion photoreactions of TT (top) and BT (bottom) compounds.
Results and Discussion
Compounds TT and BT were prepared in moderate
yields according to a reported procedure of CH direct aryl-
Syntheses
ation developed by Shinokubo et al., using the strong reac-
tivity of the thiophene’s α-proton (Scheme 4). The H NMR
spectrum (see Supporting Information) displaying a singlet
signal located in the aromatic region (δ = 7.18 ppm for BT
1
Our approach to synthesizing compounds TT and BT
hinges on a final and flexible C–H organometallic arylation
coupling between the central photochromic core compound
and δ = 7.33 ppm for TT in CDCl ) and X-ray data (see X-
3
7
(or compound 6) and the commercially available 2-iodo-
ray discussion) are consistent with the presence of a
hydrogen atom placed on the thiophene’s β position.
[
18]
fluorene.
To this end, we first focused on preparing the
corresponding intermediate derivatives 1–3 (Scheme 2).
Commercially available benzothiophene was successfully
methylated at the α-position at low temperatures and then
converted into the corresponding β-iodo derivative 2 using
conventional strong iodination conditions.[ Finally, Li/I
Photophysical Studies
19]
The spectrophotometric properties of both compounds
exchange at low temperatures and the subsequent nucleo- were investigated in toluene. Their open form isomers,
philic substitution onto the perfluorocyclopentene eventu- namely TTo and BTo, feature absorption maxima located
[20]
ally afforded precursor 3 in excellent yields. In the mean- in the UV part of the optical absorption spectra (Table 1).
time, a “halogen dance” reaction[ on 2,5-dibromothio- Under UV light irradiation at 313 nm, both absorption
phene afforded 4 in excellent yield. Li/Br exchange followed spectra undergo deep variations, testifying of the emergence
by a substitution onto TMSCl or TiPSCl resulted in ob- of new bands safely assigned to the corresponding closed-
taining derivatives 5a or 5b in 72% yields. It must be noted form isomers TTc and BTc (Figure 1 and Scheme 1). As the
that the both 3 and 5 were obtained with an overall yield photoreaction proceeds, the spectra display isosbestic
of 77% and 60%, respectively, on a scale of up to several points located at 355 nm and 354 nm, suggesting two-com-
grams. Final coupling between compounds 3 and 5 through ponent photoisomerizations. Visible light irradiation
Li/Br exchanges followed by nucleophilic substitutions onto (Ͼ 500 nm) drives the cycloreversion reaction with the same
compound 3 led to the targeted intermediate 6 in good isosbestic points and allows a full recovery of the starting
yield. Compound 7 was obtained from a standard nucleo- open form spectra. These phototransformations can be
philic substitution onto the commercially available perfluo- done repetitively without signs of any degradation, al-
21]
rocyclopentene (Scheme 3).
though some occurred under prolonged 254 nm irradia-
Scheme 2. Synthesis of compounds 1–3.
2
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Enhanced Photochemical Sensitivity in Nonsymmetrical Diarylethenes
Scheme 3. Syntheses of compounds 4–7.
Scheme 4. Syntheses of compounds TT and BT.
1
tions. The photocyclizations were monitored by H NMR the extinction coefficient. We also measured the ring cycli-
in [D ]toluene to assess their conversion (see Supporting zation quantum yield for the both compounds. While TTo
8
Information) and were evaluated to be 0.94 for both deriva- shows a classical value of 0.62 as observed for symmetrical
[
22]
tives.
DAEs,
BTo displays a higher value of 0.74, close to the
highest ring cyclization quantum yield so far reported. On
Table 1. Photophysical properties of TT and BT derivatives in tolu-
these bases, we investigated the conformational equilibria
ene. Solutions were not degassed.
1
of TTo and BTo by Variable Temperature HNMR (VT-
[a]
[b]
λ
max
ε (λmax–
)
Φ
o/c
Φ
c/o
χ
NMR) in order to determine whether or not these values
1
–1
[nm]
[Lmol cm ]
originate from a conformational issue.
TTo
TTc
BTo
BTc
332
610
327
562
79500Ϯ100[
0.62Ϯ0.01
–
0.94
0.94
[23]
Following the seminal work of Coudret et al. and as-
c]
25100Ϯ100
–
0.0013
–
0.022
suming a fast equilibrium between an antiparallel (photo-
reactive) and a parallel (non-reactive) conformers, we evalu-
ated the population ratio of antiparallel conformer at
35200Ϯ100[
c]
0.74Ϯ0.01
19300Ϯ100
–
[
a] Reported as absolute values at 313 nm. [b] Reported as absolute
293 K. We monitored as a function of the probe’s tempera-
values at 510 nm. [c] Closed form isomers were separated by re-
versed-phase HPLC.
ture the methyl signals chemical shifts (Figure 2, Table 2
and Supporting Information).
Both open-form compounds absorb in a similar wave-
For both compounds, no signal splitting was observed,
length range, with a maximum of absorption slight red- meaning that the fast equilibrium still took place even at the
shifted by 5 nm for TTo. Interestingly, replacing one fluor- low temperatures. Due to the symmetrical nature of TTo, a
enylthiophene with a benzothiophene reduces drastically sole singlet signal was subjected to a large shift of 0.35 ppm
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O. Galangau, Y. Kimura, R. Kanazawa, T. Nakashima, T. Kawai
FULL PAPER
Figure 2. 1H NMR monitoring of the methyl group’s chemical
shifts as functions of probe temperature for TTo (top), BTo-benzo-
thiophene signal (middle) and BTo–thiophene signal (bottom).
Figure 1. UV/Vis monitoring of the ring cylization processes of
TTo (top) and BTo (bottom). Black curve: open forms, red curve:
photostationary state.
Table 2. VT-NMR spectroscopic data for compounds TTo and
BTo.
Δδ
ppm]
ΔG0
[kcalmol ]
Population of anti-
parallel isomers
–1
[
TTo
BTo
CH3,thiophene
0.35
0.40
65Ϯ5%
upon raising the temperature. Estimation of the antiparallel
population ratio gave a 65% value, which is consistent with
0.48
0.30
0.70
0.50
71Ϯ8%
70Ϯ6%
the ring cyclization quantum yield observed. For BTo, the CH3,benzothiophene
signals of two methyl groups were monitored as functions
of the probe temperature. The methyl attached to the thio-
tive conformer in the media and not by photodynamic pro-
cesses such as contribution of TICT states. Inserting one
benzothiophene moiety instead of a thiophene ring,
though, results in more energetically demanding conforma-
tional change, probably originating from an efficient stabili-
zation of the reactive conformer. On those bases, we investi-
gated the solid-state structures of TTo and BTo.
phene ring was subjected to a downfield shift of 0.48 ppm
upon heating, while the signal of the methyl group on the
benzothiophene moiety shifted by 0.30 ppm from 193 K to
313 K. These outcomes suggest that the magnetic environ-
ment of the methyl group attached to the thiophene ring
changed in more pronounced way than for the other methyl
group during the interconversion. Moreover, the anti-
parallel population at 293 K was determined to be approxi-
mately 70%, a value consistent with the measured quantum
yield. Standard Gibbs free energy differences between the
conformations can also be roughly estimated and were cal-
Crystal Structures
Crystals of compounds TTo[25] and BTo[26] were obtained
by slow evaporation of solvents from toluene solutions. TTo
–
1
–1
culated to be 0.40 kcalmol for TTo and 0.60 kcalmol
¯
for BTo (average value). Although the equilibria are likely crystallized into a triclinic system [P1 (#2) space group]
to be disfavoured in both situation, the benzothiophene while BTo crystallized into an orthorhombic system [Pbca
core hampers even more the conformational change. The (#61) space group] (Figure 3). Both derivatives crystallized
cycloreversion quantum yields of the compounds follow the into the photoreactive antiparallel conformation. The two
well-established tendency that the DAEs with longer π-con- reactive carbon centers are 3.56 Å apart for TTo and 3.89 Å
[
24]
jugation system exhibit lower cycloreversion reactivity.
apart for BTo, which are shorter distances than the typical
These results suggest that the ring cyclization quantum limiting photoreactivity distance in the crystal state of
[
27]
yields are strictly limited by the relative population of reac- 4.2 Å.
4
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Enhanced Photochemical Sensitivity in Nonsymmetrical Diarylethenes
Figure 3. Crystal structures of TTo (top) and BTo (bottom).
Both crystals have presented photochromic properties In the case of TTo, for example, the distances in the both
(not shown). The TTo crystal structure featured torsion an- sides (see Supporting Information) were evaluated to be
gle values around 43° (see Supporting Information) be- 2.83 Å and 2.75 Å. These values are consistent with typical
[
28]
tween the side aryl units and the central bridge unit, re- CH–π interaction distances.
This also suggests that a
flecting the molecule’s symmetrical nature. On the other third interaction contributes to maintaining the molecular
hand, BTo’s structure showed a marked difference in the scaffold in the antiparallel conformation. However, BTo
torsion angles. Indeed, while the torsion angle between the shows a completely different feature. Because the benzo-
thiophene and the bridge is 45°, similar to TTo, the benzo- thiophene moiety has a large torsion angle with respect to
thiophene and the bridge form a large torsion angle of 66°. the central bridge, the distance between the methyl group
Both crystals feature intramolecular noncovalent interac- on the thiophene unit and the π-system on the benzothio-
tions, which are believed to stabilize their reactive confor- phene core is 2.76 Å, which allows a CH–π interaction for
mations. One should note that the distances between the only one of the methyl groups. Consequently, the symmetry
fluorine atoms of the bridge and the β-hydrogen of the thio- breaking on the hexatriene backbone results in a strong and
phenes are shorter than the sum of their Van der Waals mono-directional CH–π interaction, resulting in a highly
radii (rH = 1.20 Å and rF = 1.47 Å). For example, TTo sterically constrained structure (space filling views, see the
showed slightly different distance values of 2.41 Å and Supporting Information) in which the benzothiophene core
2.55 Å, signifying strong hydrogen bonds in both sides of is sandwiched between the perfluoro-bridge on one side and
the molecule, which helps to maintain the conformation the methyl thiophene on the other. As opposed to TTo,
into the antiparallel orientation. Although, BTo showed a BTo’s interactions seem to strongly rigidify the molecular
significantly larger distance of 2.63 Å, it suggests that this scaffold, and should impact on the photochromic properties
interaction must be weak, but still exists. In addition, 1D in solution.
1
9
F NMR experiments were carried out to confirm the exis-
All together, these data suggest that the replacement of
tence of these CH–F interactions in solution (see Support- one side group of TT by a benzothiophene unit induces a
ing Information). For both molecules, signal broadening large conformational change in the crystal structure. It also
was observed at 20 °C when decoupling protons from fluor- brings the emergence of intramolecular interactions of vari-
ine atoms, clearly indicating H–F interactions, especially in ous natures (F–H bonds, CH–π interactions, steric repul-
BTo. Moreover, the two X-ray structures display very clear sions) that stabilize the antiparallel conformation, and re-
CH–π interactions occurring between hydrogen atoms of sults in a loss of molecular flexibility. Although, solute–
the methyl groups and the opposite heteroaromatic rings. solvent interactions are important, these crystal properties
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O. Galangau, Y. Kimura, R. Kanazawa, T. Nakashima, T. Kawai
FULL PAPER
only partly explain the observed conformational differences tween the HOMO–LUMO gaps (4.26 eV for TTo, 4.31 eV
between the two derivatives in the solution phase.
for BTo). However, introducing a benzothiophene drasti-
cally diminished the oscillating strength by more than a fac-
tor 2, which is consistent with the observed variation of
extinction coefficient values (S ǞS , f = 2.1 for TTo, f =
DFT and TD-DFT Calculations
0
3
0
.9 for BTo). On the contrary, the molecular orbitals of the
We also performed calculations on TTo/TTc and on BTo/
BTc to gain a better insight into their electronic properties.
Optimizations were carried out at the B1B95 6-311g (d,p)
level of theory, which is known to provide the best compro-
mise between the geometrical minimization and theoretical
corresponding closed forms TTc and BTc were distributed
similarly over the two molecules. As commonly noticed for
the closed forms of DAEs, the lowest energy transition was
assigned to the HOMO–LUMO transition. Compared to
TTc, BTc showed a large blueshift of 66 nm, induced by the
loss of a fluorene moiety. Indeed, introducing a benzothio-
phene decreased the HOMO level, while the energy of the
LUMO was barely affected (see Supporting Information).
These calculations provided a good insight into the elec-
tronic properties of TT and BT compounds. They also pre-
dicted in both open forms the existence of stabilizing intra-
molecular interactions (CH–π, H–F bonding).
[
29]
UV spectra.
Simulations of IR spectra were also per-
formed to make sure that optimization outcomes corre-
sponded to true minima. Finally, TD-DFT calculations
were performed at the same level of theory. From the struc-
tural point of view, calculated structures of TTo and BTo
match the experimental ones (Table 3) with reasonable devi-
ation.
Table 3. Comparison of the structural parameters obtained by
DFT (calcd.) and by X-ray diffraction (exp).
Conclusions
T
calc
T
exp
dH–F calcd.
dH–F exp.
dCH–π calcd.
dCH–π exp.
[
°]
[°]
[Å]
[Å]
[Å]
[Å]
We have designed two new push–pull DAEs, namely BT
and TT derivatives, and studied their structural and photo-
physical properties in solution, as well as in the solid state.
Both compounds exhibited photochromic reactions in solu-
tion and condensed phases. The highest ring cyclization
quantum yield in solution was obtained for compound BT,
in which strong CH–π interactions and H–F bonding inter-
actions were involved, as shown by X-ray, VT-NMR and
TTo
BTo
44
4
42
5
47
43
45
66
2.49
2.58
2.46
–
2.41
2.55
2.63
–
2.72
2.81
2.73
–
2.75
2.83
2.76
–
3
7
As expected, DFT optimization provided a reliable
model of these two derivatives. In particular, this method
predicted the appearance of CH–π interactions[30] as well as
19
F NMR data. Combined with the loss of molecular flexi-
a strong steric hindrance between the perfluoro group and
the benzothiophene moiety. The two corresponding closed
forms, namely TTc and BTc, displayed similar flat struc-
tures, typical of these compounds.
bility, the intramolecular interactions must drive the molec-
ular conformation toward the most reactive one, and there-
fore must enhance the photocyclization efficiency. Solvent
polarity might also impact the equilibria by destabilizing
the parallel conformer, consistent with the slight enhance-
ment of Φo/c we observed in the case of BTo. Nonetheless,
several groups reported that for compounds analogous to
BT, various efficiencies as functions of the nature of the
aromatic groups connected to the thiophene ring, hence
TD-DFT and molecular orbitals (see Supporting Infor-
mation) also provided us interesting information. In both
compounds in their open forms, the LUMO orbital contrib-
uted to the photocyclization reaction, presenting the ade-
quate bonding interaction between the two carbon centers
in the excited state. While the HOMO in TTo extends to
the two fluorenylthiophene moieties, that of BTo is more
localized onto the fluorenylthiophene unit. In both systems,
the HOMO to LUMO transition is accompanied by an elec-
tron density transfer from the side units to the center of the
molecule, illustrating their weak charge transfer nature. The
HOMO–LUMO transition of BTo exhibited a significant
[
32]
highlighting its key role.
We are now focusing our re-
search efforts on studying the influence of this aromatic ring
nature upon the overall ring cyclization quantum yield.
Experimental Section
charge-transfer (CT) nature, which originated from its non- General: Chemicals were purchased from Tokyo Chemical Indus-
symmetrical molecular structure. The CT nature in the ex- tries, Wako Chemicals and Sigma Aldrich, and were used without
cited states of π-conjugated substances generally tends to any further treatment. Dry solvents were purchased from Nacalai
Tesque and Sigma Aldrich, and were kept under nitrogen atmo-
stabilize the twisted excited state (TICT) and thus the CT
sphere. Spectroscopic grade solvents were purchased from Wako
nature in the DAE backbone has been thought to suppress
1
13
the photocyclization reactivity.[
tendency in the photoreactivity suggests that the photoreac-
tivity in TTo and BTo is dominated by the ground state
8b–8e]
chemicals. H and C NMR spectrum were recorded with JEOL
JNM-AL-300 (300 MHz) and JEOL JNM-ECP400 (400 MHz)
spectrometers . Chemical shifts were referenced to the solvent resi-
due peaks. Mass spectra were measured with a JEOL AccuTOF
JMS-T100LC (ESI) and JML-700 (EI) instruments. ATR-IR spec-
tra were recorded with a JASCO FT/IR-4200 spectrometer with
The present opposite
geometry but not by the excited state electronic structures
in nonpolar solvents.[
31]
The HOMO–LUMO gap was
rather unaffected when the thiophene was replaced by a ATR PRO410-S. Melting points were measured with a capillary
benzothiophene unit, as seen in the small difference be- MEL-TEMP apparatus and are not corrected. Purifications by
6
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Enhanced Photochemical Sensitivity in Nonsymmetrical Diarylethenes
automatic column chromatography were performed on a YAM-
AZEN W-Prep 2XY system. Compounds that were studied by
spectroscopic measurement of any kind were first purified by
HPLC using 8391 NEXT HPLC from Japan Analysis Industry. 1534, 1622, 1654, 2323, 2343, 2855, 2946 cm . HRMS-EI: calcu-
Separations of open- and closed-form isomers were carried out on
a HITACHI LaChrom ELITE Reversed Phase HPLC system.
127.4, 138.2, 142.1, 142.4 ppm, m.p. 55–60 °C. IR: ν˜ = 500, 536,
571, 814, 840, 887, 938, 965, 984, 1021, 1040, 1051, 1097, 1111,
1133, 1150, 1187, 1200, 1251, 1296, 1290, 1334, 1356, 1432, 1442,
–
1
+
lated for C19
12 6 2
H F S 418.0285; found 418.0289 [M ].
3
-{2-[2-Ethyl-5-(9H-fluoren-2-yl)thiophen-3-yl]-3,3,4,4,5,5-hexa-
Photophysical Studies: Absorption spectra were recorded with a
UV–Vis spectrophotometer (JASCO V-670 or V-660). Absolute
ring cyclization quantum yield values were obtained with a Shim-
adzu QYM-01 setup. Prior to the photophysical experiments, sam-
ples were subjected to GPC purification and were evaporated under
high vacuum in the dark during almost a week. Light intensities
were evaluated by using CUSTOM UV 340 and ADVANTES
Q8221 as power meters, for UV and visible irradiations, respec-
tively.
fluorocyclopent-1-enyl}-2-methylbenzo[b]thiophene (BTo): A two-
necked round-bottomed flask equipped with a magnetic stirring
bar, a refluxed condenser and capped with a rubber septum was
charged with PdCl
2
(38 mg, 30 mol-%) and 2,2Ј-bipyridyl (34 mg,
30 mol-%). The flask was evacuated and backfilled with argon sev-
eral times. Dry m-xylene (1.2 mL) was added, and the mixture was
stirred at 60 °C (bath temperature) over a period of 40 min. Then,
compound 6 (300 mg, 0.7 mmol, 1 equiv.), 2-iodofluorene (629 mg,
2 3
2.2 mmol, 3.0 equiv.) and Ag CO (297 mg, 1.1 mmol, 1.5 equiv.)
were added under argon. Dry m-xylene (1.2 mL) was added, and
the mixture was heated at 130 °C overnight. The mixture was co-
oled to room temp., and CH Cl was added to dilute the mixture.
2 2
The solution was filtered through a pad of silica to remove the
remaining solids. The filtrate was concentrated under reduced pres-
sure and purified by an automatic chromatography column, using
Synthetic Procedures: Compounds 1, 2, 3, 4 and 5a were synthe-
sized according reported procedures.[
18,19]
(4-Bromo-5-methylthiophen-2-yl)triisopropylsilane (5b): A Schlenk
tube equipped with a magnetic stirring bar and capped with a rub-
ber septum was charged with 3,5-dibromo-2-methylthiophene
(
(
4.0 g, 16.0 mmol, 1 equiv.) followed by the addition of dry THF
80 mL). The solution was cooled to –78 °C. At that temperature,
silica gel and a mixture of hexane and CHCl
3
as eluent, affording
a white solid (150 mg, 34% yield). H NMR (300 MHz, CDCl ): δ
1.91 (s, 3 H), 2.29 (s, 3 H), 3.82 (s, 2 H), 7.18–7.71 (m, 12 H)
1
3
a hexane solution of nBuLi (1.6 m, 9.8 mL, 17.2 mmol, 1.1 equiv.)
was added dropwise with stirring. After 1 h at low temperature,
neat chlorotriisopropylsilane was added dropwise (2.8 mL,
=
13
ppm. C NMR (75 MHz, CDCl
3
): δ = 14.7, 14.8, 36.8, 119.9,
1
1
1
1
5
20.2, 120.3, 122.0, 122.1, 124.3, 124.5, 124.9, 125.0, 125.1, 126.8,
26.9, 131.6, 138.2, 141.0, 141.4, 141.5, 142.3, 142.5, 143.3,
2
3.5 mmol, 1.5 equiv.), and the mixture was slowly warmed to
room temperature. The reaction was quenched by carefully adding
water, followed by an extraction with Et O. The organics were com-
43.9 ppm, m.p. 140–145 °C. IR: ν˜ = 607, 688, 849, 1071, 1207,
–1
2
294, 2847, 2900 cm . HRMS-EI: calculated for C32
20 6 2
H F S
bined, dried with anhydrous magnesium sulfate and filtered, and
the solvents were removed under reduced pressure. The crude mate-
rial was subjected to silica gel column chromatography using hex-
+
82.0911; found 582.0909 [M ].
3
,3Ј-(Perfluorocyclopent-1-ene-1,2-diyl)bis(2-methylthiophene) (8):
A flame-dried Schlenk tube equipped with a magnetic stirring bar
was charged with (4-bromo-5-methylthiophen-2-yl)triisopropylsi-
lane (4 g, 12 mmol, 2.2 equiv.) under a flux of nitrogen before dry
THF (30 mL) was added. The solution was cooled to –78 °C before
a hexane solution of nBuLi (1.6 m, 7.5 mL, 12 mmol, 2.2 equiv.)
was added dropwise with stirring. Stirring was maintained over 1 h
at low temperature, then perfluorocyclopentene was quickly added
(733 μL, 5.5 mmol, 1.0 equiv.), and the mixture was allowed to
slowly warm to room temperature. Finally, a TBAF solution in
THF (1 m, 13.7 mL, 14 mmol, 2.5 equiv.) was added dropwise. The
reaction was quenched by addition of water. The biphasic system
ane as the eluent. Purification afforded white crystals (4.87 g, 94%
1
yield) with satisfactory purity. H NMR (300 MHz, CDCl
3
): δ =
1
.08–1.10 (m, 18 H), 1.29 (m, 3 H), 2.43 (s, 3 H), 7.02 (s, 1 H) ppm.
C NMR (75 MHz, CDCl ): δ = 11.6, 14.7, 18.5, 110.5, 131.7,
3
1
3
1
1
1
3
37.7, 139.3 ppm, m.p. 50 °C. IR: ν˜ = 749, 921, 1139, 1157, 1168,
194, 1208, 1229, 1306, 1334, 1365, 1381, 1419, 1507, 1524, 1542,
–
1
633, 1732, 2889 cm . HRMS-EI: calculated for C14
H
25BrSSi
+
32.0630; found 332.0622 [M ].
3
-[3,3,4,4,5,5-Hexafluoro-2-(2-methylthiophen-3-yl)cyclopent-1-
enyl]-2-methylbenzo[b]thiophene (6): A two-necked round-bottomed
flask equipped with a magnetic stirring bar and capped with a rub-
ber septum was charged with compound 5 (2.0 g, 8.0 mmol,
2
was extracted by adding Et O. The organics were washed several
times with water, dried with anhydrous magnesium sulfate, filtered,
and the solvents were evaporated under reduced pressure. The
crude product was purified by a silica column chromatography
using hexane as eluent, to afford transparent crystals (1.31 g, 65%
1
equiv.). The flask was degassed and backfilled with argon several
times. Dry THF (80 mL) was added, and the solution was cooled
to –78 °C using a dry ice/acetone bath. A hexane solution of nBuLi
(
1.6 m, 5.5 mL, 8.8 mmol, 1.1 equiv.) was added dropwise with stir-
ring. After 45 min at –78 °C, a THF (dry) solution of compound
(3.0 g, 8.8 mmol, 1.1 equiv.) was added. After stirring at –78 °C
for 1 h, a THF solution of TBAF (1 m, 9.6 mL, 9.6 mmol,
.2 equiv.) was added dropwise, and the mixture was warmed to
1
yield) with satisfactory sample purity. H NMR (300 MHz,
3
CDCl ): δ = 1.87 (s, 6 H), 7.07 (d, J = 6.0 Hz, 4 H), 7.16 (d, J =
3
13
6
1
6
1
2
3
3
.0 Hz, 4 H) ppm. C NMR (75 MHz, CDCl ): δ = 14.1, 123.6,
24.9, 127.1, 141.7 ppm, m.p. 64 °C. IR: ν˜ = 603, 628, 637, 649,
64, 694, 713, 741, 804, 824, 843, 886, 893, 925, 981, 1042, 1055,
104, 1155, 1190, 1203, 1268, 1336, 1362, 1380, 1436, 1542, 1635,
854, 2924, 2957 cm . HRMS-EI: calculated for C15
1
room temp. overnight. Water was carefully added to quench the
reaction, and diethyl ether was added to extract the product. The
–1
10 6 2
H F S
organic layer was washed with brine, dried with anhydrous MgSO
4
,
+
68.0128; found 368.0149 [M ].
and the solids were filtered off. The filtrate was concentrated under
reduced pressure. The crude product was purified by a chromatog-
3,3Ј-(Perfluorocyclopent-1-ene-1,2-diyl)bis[5-(9H-fluoren-2-yl)-2-
methylthiophene] (TTo): A flame-dried Schlenk tube equipped with
raphy on a silica gel column, using hexane as the eluent, to afford
1
a white solid (2.34 g, 63% yield). H NMR (300 MHz, CDCl
3
): δ a magnetic stirring bar was charged with PdCl
2
(10 mg, 10 mol-
=
6
1
1.95 (s, 3 H), 2.25 (s, 3 H), 6.96 (d, J = 6 Hz, 1 H), 7.04 (d, J = %), 2,2Ј-bipyridyl (8 mg, 30 mol-%). The flask was evacuated and
Hz, 1 H), 7.30–7.36 (m, 2 H), 7.53–7.56 (m, 1 H), 7.71–7.74 (m, backfilled with argon several times. Dry m-xylene (0.9 mL) was
H) ppm. 13C NMR (75 MHz, CDCl
22.0, 122.1, 122.2, 123.3, 124.2, 124.4, 124.9, 125.1, 126.8, 127.1,
): δ = 14.5, 14.7, 120.3,
added, and the mixture was stirred at 60–70 °C (bath temperature)
3
1
over a period of 60 min. Then, 3,3Ј-(perfluorocyclopent-1-ene-1,2-
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Job/Unit: O42774
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Date: 26-09-14 13:01:04
Pages: 10
O. Galangau, Y. Kimura, R. Kanazawa, T. Nakashima, T. Kawai
FULL PAPER
diyl)bis(2-methylthiophene) (200 mg, 0.5 mmol, 1 equiv.), 2-iodo-
fluorene (476 mg, 1.6 mmol, 3.0 equiv.) and Ag CO (450 mg,
.6 mmol, 3 equiv.) were added under argon. Dry m-xylene
0.9 mL) was added, and the mixture was heated at 120–130 °C for
4 h. The mixture was cooled to room temp., and CH Cl was
added to dilute the mixture. The solution was filtered through a
pad of silica to remove the remaining solids. The filtrate was con-
centrated under reduced pressure and purified by an automatic
chromatography column, using silica gel and a mixture of hexane
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(
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6
2
2
[
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as eluent, affording a pale yellow solid (100 mg, 26%
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H), 7.30–7.81 (m, 16 H) ppm. 13C NMR (75 MHz, CDCl
): δ =
4.6, 36.7, 120.0, 120.3, 122.0, 122.1, 124.5, 125.1, 125.9, 126.9,
27.0, 131.7, 141.0, 141.6, 142.7, 143.4, 144.0 ppm, m.p. 240 °C.
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3
1
1
[
IR: ν˜ = 531, 563, 677, 732, 766, 823, 980, 995, 1058, 1108, 1192,
–
1
[13] F. Stellacci, C. Bertarelli, F. Toscano, M. Z. Gallazi, G. Zotti,
1
263, 1340, 1429, 1449, 2850, 2965, 3050 cm . HRMS-EI: calcu-
+
G. Zerbi, Adv. Mater. 1999, 11, 292–295.
lated for C41
26 6 2
H F S 696.1380; found 696.1364 [M ].
[
[
[
[
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Supporting Information (see footnote on the first page of this arti-
2
1
13
cle): Copies of H NMR and C NMR spectra, VT-NMR spectro-
scopic data, NMR monitoring of the photoreaction, DFT calcula-
tions details, X-ray structural data.
2
3
Acknowledgments
O. G. thanks the Japan Society for the Promotion of Science and
the Centre National pour la Recherche Scientifique (CNRS) for
financial support. This work was also partly supported by the Min-
istry of Education, Culture, Sports, Science and Technology
[
[
[
(MEXT), Japan (Grant-in-Aid for Scientific Research on Innov-
19] S. Kawai, T. Nakashima, Y. Kutsunugi, H. Nakagawa, H. Nak-
ano, T. Kawai, J. Mater. Chem. 2009, 19, 3606–3611.
ative Area, “Application of Cooperative Excitation into Innovative
Molecular Systems with High-Order Photofunctions” by the Nan-
otechnology Platform Program running in Japan Advanced Insti-
tute of Science and Technology, JAIST). The authors are grateful
to Mr. M. Asanoma and Mr. M. Katao, technical staff members
at NAIST, for their help with VT-NMR experiments and X-ray
characterizations, respectively. Finally, the authors show their grati-
tude to Prof. Delbaere, Prof. Maurel, Dr. Sliwa, Dr. Aloïse and Dr.
Perrier for fruitful discussions.
20] H. Nishi, T. Namari, S. Kobatake, J. Mater. Chem. 2011, 21,
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1] G. H. Brown (Ed.), Photochromism, Techniques in Chemistry,
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3] M. Irie, Chem. Rev. 2000, 100, 1684–1716.
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[
5] V. Balzani, A. Credi, M. Venturi, in: Molecular Devices and
Machines: A Journey into the Nano World, Wiley-VCH,
Weinheim, Germany, 2003.
25] Crystallographic data of TTo: C H F S , C H F S , a =
41 26
6
2
32 20 6 2
9
1
7
.9787(2) Å,
597.53(6) Å ,
b
=
α
11.4122(2) Å,
70.9671(7)°,
c
=
15.1487(3) Å,
89.4662(7)°,
V
γ
=
=
3
=
β
=
[
6] J. A. Delaire, K. Nakatani, Chem. Rev. 2000, 100, 1817–1816.
7] Y. Hasegawa, T. Nakagawa, T. Kawai, Coord. Chem. Rev. 2010,
3
8.8813(7)°, ρcalcd. = 1.448 g/cm , triclinic. Of the 27724 reflec-
[
tions measured, up to 2θ = 55.0° were collected, 4731 were
unique (Rint = 0.0237). Hydrogen atoms were calculated, in:
riding positions. CCDC-987958 contains the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
254, 2643–2651.
[
8] a) S. Nakamura, T. Kobayashi, A. Takata, K. Uchida, Y. As-
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[26] Crystallographic data of BTo:
a
=
14.1418(9) Å,
b
=
=
3
7.7792(5) Å, c = 47.021(3) Å, V = 5172.8(6) Å , ρcalcd.
1.496 g/cm , orthorhombic. Of the 67657 reflections measured
up to 2θ = 55.0° were collected, 7323 were unique (Rint =
3
23529–23538.
[
9] a) M. Irie, Proc. Jpn. Acad., Ser. B 2010, 86, 472–483; b) T.
0.1590). CCDC-961406 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Yamada, S. Muto, S. Kobatake, M. Irie, J. Org. Chem. 2001,
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6, 6164–6168; c) M. Irie, T. Lifka, S. Kobatake, N. Kato, J.
Am. Chem. Soc. 2000, 122, 4871–4876; d) K. Shibata, K. Muto,
8
www.eurjoc.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O42774
/KAP1
Date: 26-09-14 13:01:04
Pages: 10
Enhanced Photochemical Sensitivity in Nonsymmetrical Diarylethenes
[
[
27] a) S. Kobatake, K. Uchida, E. Tsuchida, M. Irie, Chem. Com-
mun. 2002, 2804–2805; b) M. Morimoto, M. Irie, Chem. Com-
mun. 2005, 3895–3905.
[31] Ring cyclization quantum yield of BTo in acetonitrile, a more
polar solvent, was about 0.8Ϯ0.1. A VT-NMR experiment in
the same solvent afforded 84%Ϯ7% population of the photo-
0
28] a) T. Nakashima, K. Yamamoto, Y. Kimura, T. Kawai, Chem.
Eur. J. 2013, 19, 16972–16980; b) H. Suezama, T. Yoshida, Y.
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reactive conformer, associated with a ΔG (293 K) value of 1.0
–
1
kcalmol , which is in good consistency with the above-men-
tioned ring cyclization (see the Supporting Information). One
could expect that weak interactions such as H–F bonding
would be weaker in a more polar solvent such as acetonitrile,
and result in a decrease of the ring cyclization quantum yield.
And yet, a slight increase of Φo/c was observed in acetonitrile
vs. toluene. This difference might stem from a larger destabili-
zation of the parallel conformer compared to the antiparallel
one when changing the solvent polarity, while the energy bar-
3
148–3155.
29] A. Fihey, A. Perrier, F. Maurel, J. Photochem. Photobiol. A
012, 247, 30–35.
[
[
2
30] In BTo, DFT calculations also predict a C–H bond length con-
traction (0.0008 Å) for the methyl involved in the noncovalent
interaction with the benzothiophene unit, when compared to
the carbon–hydrogen bond lengths for the two hydrogens of
TTo (used here as a reference). This contraction is often corre-
lated to CH–π interactions, and confirms the benzothiophene’s
capacity to generate the present robust CH–π bonding interac-
tion (see A. Baggioli, S. V. Meille, G. Raos, R. Po, M. Brink-
mann, A. Famulari, Int. J. Quantum Chem. 2013, 113, 2154–
BTo (toluene)
BTo (acetonitrile)
rier would remain unaffected (ΔG
Ͻ ΔG
).
[
32] G. Liu, S. Pu, C. Fan, S. Cui, Dyes Pigm. 2012, 95, 553–562.
Received: June 19, 2014
Published Online: ᭿
Published Online: ᭿
2162).
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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Job/Unit: O42774
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Date: 26-09-14 13:01:04
Pages: 10
O. Galangau, Y. Kimura, R. Kanazawa, T. Nakashima, T. Kawai
FULL PAPER
Diarylethenes
We provide evidence (VTNMR, X-ray
data, DFT simulations) that a standard
nonsymmetric diarylethene containing a
benzothiophene unit achieves high ring
cyclization quantum yield by means of
favorable intramolecular interactions (CH–
π, H–F bonding or steric hindrance).
O. Galangau, Y. Kimura, R. Kanazawa,
T. Nakashima, T. Kawai* ................ 1–10
Enhanced Photochemical Sensitivity in
Photochromic Diarylethenes Based on a
Benzothiophene/Thiophene Nonsymmetri-
cal Structure
Keywords: Photochromism / Noncovalent
interactions / Density functional calcu-
lations / Diarylethenes
10
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