Bisthienylethenes containing a benzothiadiazole unit as a bridge: Photochromic performance dependence on substitution position

Article abstract of DOI:10.1002/chem.200901855

A conveniently synthesized photochromic compound, BTB-1, containing an unprecedented six-mem- bered 2, 1, 3-benzothiadiazole unit as the center ethene bridge, possesses good photochromic performance, with a high cyclization quantum yield and moderate fatigue resistance in solution or an organogel system. The fluorescence of BTB-1 can be modulated by solvato- and photochromism. However, the analogue BTB-2, in which the di- methylthiophene substituents are relocated to the 5, 6-positions of benzothia- diazole, does not show any detectable photochromism. To the best of our knowledge, this is the first example of six-membered bridge bisthienylethenes (BTEs) in which the photochromism can be controlled by the substitution position. The photochromism difference is elucidated by the analysis of resonance structure, the Woodward- Hoffmann rule, and theoretical calculations on the ground-state potential- energy surface. In a well-ordered single-crystal state, BTB-1 adopts a relatively rare parallel conformation, and forms an interesting two-dimensional structure due to the presence of multiple directional intermolecular interactions, including C-H-N and C-H-S hydrogen-bonding interactions, and π stacking interactions. This work contributes to several aspects for developing novel photochromic BTE systems with fluorescence modulation and performances controlled by substitution position in different states (solution, or- ganogel, and single crystal).

Full text of DOI:10.1002/chem.200901855

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FULL PAPER
DOI: 10.1002/chem.200901855
Bisthienylethenes Containing a Benzothiadiazole Unit as a Bridge:
Photochromic Performance Dependence on Substitution Position
Weihong Zhu,* Xianle Meng, Yuheng Yang, Qiong Zhang, Yongshu Xie, and He Tian*[a]
Abstract: A conveniently synthesized
photochromic compound, BTB-1, con-
taining an unprecedented six-mem-
bered 2,1,3-benzothiadiazole unit as
the center ethene bridge, possesses
good photochromic performance, with
a high cyclization quantum yield and
moderate fatigue resistance in solution
or an organogel system. The fluores-
cence of BTB-1 can be modulated by
solvato- and photochromism. However,
the analogue BTB-2, in which the di-
methylthiophene substituents are relo-
cated to the 5,6-positions of benzothia-
diazole, does not show any detectable
photochromism. To the best of our
knowledge, this is the first example of
six-membered bridge bisthienylethenes
(BTEs) in which the photochromism
can be controlled by the substitution
position. The photochromism differ-
ence is elucidated by the analysis of
resonance structure, the Woodward–
Hoffmann rule, and theoretical calcula-
tions on the ground-state potential-
single-crystal state, BTB-1 adopts a rel-
atively rare parallel conformation, and
forms an interesting two-dimensional
structure due to the presence of multi-
ple directional intermolecular interac-
À
À
tions, including C H···N and C H···S
hydrogen-bonding interactions, and p–
p stacking interactions. This work con-
tributes to several aspects for develop-
ing novel photochromic BTE systems
with fluorescence modulation and per-
formances controlled by substitution
position in different states (solution, or-
ganogel, and single crystal).
energy surface. In
a
well-ordered
Keywords:
benzothiadiazoles
·
bisthienylethenes · density function-
al calculations
photochromism
· fluorescence ·
Introduction
ecules.[3] Among various photochromic compounds, bisthie-
nylethenes (BTEs) are known to have excellent thermal sta-
bility of two isomers (open and closed-ring isomers) and
good fatigue resistance.[4] Generally, the chemical modifica-
tions of BTEs are derived from the thiophene units and
ethene bridge. Due to the limitations of perfluorocyclopen-
tene, several groups are dedicated to the investigation of
novel bridging BTEs.[5,6] Yam et al. have reported a photoac-
tive diarylethene system containing a 1,10-phenanthroline
ligand and its metal complexes to perform photochromism.[5]
More recently, we have incorporated a naphthalimide chro-
mophore as the center ethene bridge of BTEs, exhibiting a
good photochromic performance with fluorescence modula-
tion, even successfully mimicking combined NOR and IN-
HIBIT logic gates.[6b]
Photochromic compounds are of increasing interest for their
potential application in molecular-level devices, such as pho-
toresponsive supramolecular self-assemblies, molecular
switches, logic gates, and information storage.[1] Besides ex-
tensively developing convenient syntheses of photochromic
molecules, one challenging task is to study photochromism
in different states, such as in solution, nanoparticles, or
single crystals.[2] Recently, Nakatani et al. have successfully
demonstrated the application of laser ablation as an efficient
technique for nanoparticle fabrication to photochromic mol-
[a] Prof. Dr. W. Zhu, Dr. X. Meng, Y. Yang, Q. Zhang, Prof. Dr. Y. Xie,
Prof. Dr. H. Tian
In this context, we report a convenient synthesis of two
novel bisthienylethenes (BTB-1 and BTB-2 (BTB=2,5-(bis-
methylthiophen-3yl)-2,1,3-benzothiadiazole, Scheme 1) con-
taining an unprecedented six-membered aryl ring as the
center ethene bridging unit: the 2,1,3-benzothiadiazole chro-
mophore. There are several merits in the design of these
BTE derivatives:
Key Laboratory for Advanced Materials and
Institute of Fine Chemicals
East China University of Science and Technology
Shanghai 200237 (P.R. China)
Fax : (+86) 21-6425-2758
E-mail: whzhu@ecust.edu.cn
tianhe@ecust.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901855.
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Optical properties and photo-
chromism: Both BTB-1 and
BTB-2 appeared colorless in so-
lution. As shown in Figure 1
and Figure S1 (Supporting In-
formation), they have no ab-
sorption band in the visible
range. BTB-2 has an identical
absorption peak around 370 nm
that corresponds to the ICT
transition, whereas this peak
becomes lower as a shoulder in
BTB-1. This might be ascribed
to the less efficient ICT from
the thiophene group to the
electron-withdrawing unit of
2,1,3-benzothiadiazole in BTB-
Scheme 1. Synthetic routes to BTB-1 and BTB-2.
1) The incorporated ethene bridge of the 2,1,3-benzothia-
diazole unit is conjugated to a strong electron-withdraw-
ing group, similar to traditional perfluorocyclopentene,
maleic anhydride, or maleic imide bridges, assuring good
photochromism with considerable bistability.
2) 2,1,3-Benzothiadiazoles are well-known fluorescent
building blocks in the design of functional materials,
such as organic light-emitting diodes (OLEDs), non-
linear optical (NLO) materials, and solar cells.[7] The
electron-withdrawing group of the 2,1,3-benzothiadiazole
unit and the electron-donating thiophene can form an ef-
ficient donor–p-acceptor (D-p-A) intramolecular charge-
transfer (ICT) system.
3) More importantly, the photochromism is found to be
critically dependent upon the substitution position of
2,1,3-benzothiadiazole. BTB-1, with the ethene bridge
substituted at the 4,5-positions of 2,1,3-benzothiadiazole,
possesses well-defined photochromic properties. In con-
trast, BTB-2, in which the bridge substitution is located
at the 5,6-positions, cannot undergo photocyclization
even under prolonged UV irradiation.
Results and Discussion
Synthesis: As illustrated in Scheme 1, intermediates 4,5-di-
bromo-2,1,3-benzothiadiazole and 5,6-dibromo-2,1,3-benzo-
thiadiazole were prepared according to the reported proce-
dure, in two and four steps respectively.[8] The target mole-
cules BTB-1 and BTB-2 were conveniently prepared from
the Suzuki coupling of the appropriate dibromo-substituted
2,1,3-benzothiadiazole and 2,5-dimethylthien-3-yl boronic
Figure 1. Spectral changes of UV/Vis absorption (A) and fluorescence (B,
excited at the isosbestic point of 340 nm) of BTB-1 upon irradiation at
310 nm (in cyclohexane, 2.0ꢁ10À5 mol lÀ1). The photostationary states
were obtained by irradiating solutions of BTB-1 with 310 nm light until
no further spectral changes were observed.
acid, using [PdACHTUNGTRENNUNG(PPh3)4] as a catalyst in a mixture of aqueous
Na2CO3 (2m) and 1,4-dioxane under reflux conditions. Their
chemical structures were confirmed by 1H and 13C NMR
spectroscopies and HRMS (shown in the Experimental Sec-
tion).
1 (substituted at the 4,5 positions), compared with ICT in
BTB-2 (substituted at the 5,6 positions). Additionally, both
BTB-1 and BTB-2 have a peak at 310 nm corresponding to
900
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Bisthienylethenes Containing a Benzothiadiazole Unit
FULL PAPER
Table 1. Absorbance data for BTB-1 and BTB-2 before and after UV irradiation in cyclohexane and THF.
Compound Open form Closed form
F [%][a]
(solvent) (lmax [nm]) (lmax [nm])
the p–p* transition of the 2,1,3-
benzothiadiazole moiety mixed
with that of the dimethylthio-
phene groups.
Upon UV irradiation at
310 nm (Figure 1A), the solu-
tion of BTB-1 quickly turned to
red (Figure S2 in the Support-
ing Information). Besides an in-
crease in absorption at 390 nm
G
R
G
FOÀC
FCÀO
BTB-1 (cyclohexane)
BTB-1 (THF)
BTB-2 (cyclohexane)
BTB-2 (THF)
235, 310, 368 (sh)
217, 312, 370 (sh)
233, 304, 355
230, 368, 486
237, 374, 496
–
–
21.7
24.6
–
18.5
21.3
–
239, 305, 355
–
–
[a] Quantum yields of cyclization reaction (FOÀC) and cycloreversion reaction (FCÀO) were calculated for
BTB-1 at 310 and 480 nm, respectively, according to the literature method.[10]
for BTB-1, a new broad absorption band around 500 nm
was observed that corresponds to the closed form of BTB-1
(c-BTB-1) produced by photocyclization (Scheme 2). It can
for the active conformer, the quantum yields of the photo-
cyclization for BTB-1 were determined to be about 20–30%
in solution. In addition, the fatigue resistance and the ther-
mal stability of the closed form were studied. BTB-1 could
reversibly perform the photochromism and revert back by
visible-light irradiation with moderate resistance. These col-
oration and bleaching cycles can be repeated with alternat-
ing irradiation by UV (310 nm) and visible light (>460 nm).
The irradiation time was long enough for coloration to
reach the PSS and for the color to be completely bleached.
As shown in Figure S5 in the Supporting Information, the
fatigue-resistant characteristics of BTB-1 in solution indicat-
ed that about 35% were decreased after 10 repeat cycles.
The absorbance of the closed form of BTB-1 in cyclohexane
does not show any obvious decrease in the visible range in
the dark at room temperature even after one month. Clear-
ly, BTB-1 retains the photochromic properties of parent
BTEs when replacing the five-membered cyclopentene ring
by a fluorescent benzothiadiazole unit, with a six-membered
aryl unit as the center ethene bridge. With strong electron-
withdrawing properties similar to perfluorocyclopentene,
maleic anhydride, and maleic imide, the benzothiadiazole
group might be helpful to ensure such photochromism with
considerable bistability.[6] Previously we have found that,
due to the strong electron-withdrawing effect, an incorporat-
ed naphthalimide unit can remarkably increase the lifetimes
of open merocyanine (MC) forms, almost three magnitudes
longer than that of unsubstituted spironaphthoxazine.[9]
Scheme 2. Structural change associated with photochromism of BTB-1.
1
be evidenced from H NMR spectroscopy and HPLC signals
that the p-delocalization system was far extended through
the photochromic reaction (Figures S3 and S4 in the Sup-
porting Information). Distinct differences in proton NMR
signals between the open form (BTB-1) and closed form (c-
BTB-1) in both the high- and low-field areas were observed.
As expected, protons on the benzothiadiazole moiety (Ha
and Hb) shifted remarkably to high field upon photocycliza-
tion due to great changes (extended p delocalization) in the
chemical environment. The four groups of methyl protons
on the thiophene moieties of BTB-1 were located at d=
2.03, 2.09, 2.18, and 2.22 ppm, respectively. After photocycli-
zation, the signals of these methyl protons were shifted to
d=1.80, 1.86, 2.43, and 2.45 ppm, respectively. Interestingly,
in contrast to the two thiophene protons Hc and Hh (d=6.47
and 6.73 ppm) in BTB-1, the Hc’ shifted upfield to d=
5.62 ppm, while Hh’ shifted downfield to d=7.65 ppm in c-
BTB-1, which can be attributed to the possible formation of
an intramolecular hydrogen bond between Hh’ and the N
atom on the benzothiadiazole unit upon photocyclization
(Scheme 2). Additionally, HPLC can be used to monitor the
closed form on photocyclization. For instance, the ratio be-
tween the closed form and the open form in the photosta-
tionary state (PSS) of BTB-1 was 54%, obtained from the
corresponding integrated areas of the HPLC peaks detected
at the isosbestic wavelength, 340 nm (Figure S3 in the Sup-
porting Information).
Fluorescence modulation and solvatochromism: The 2,1,3-
benzothiadiazole unit is a well-known fluorescent chromo-
phore with high efficiency and chemical stability.[7] Both
BTB-1 and BTB-2 have a typical D-p-A configuration when
two electron-donating thiophene groups are attached to the
2,1,3-benzothiadiazole unit. Upon excitation at the isosbestic
point of 340 nm, BTB-1 and BTB-2 exhibit intense lumines-
cence with efficiencies of 7.7 and 2.9%, respectively. When
irradiated at 310 nm, the solution of BTB-1 underwent pho-
tocyclization and exhibited a distinct fluorescence quenching
(Figure 1B), in which the fluorescence efficiency dropped
by 70% when reaching the PSS. Clearly, this resulted from
the closed form as a fluorescence quencher through the pos-
sible channel of Fçrester resonance energy transfer (FRET)
from the excited fluorescent chromophore unit to the ring-
closed diarylethene unit. Upon irradiation, the resulting
closed form, c-BTB-1, almost covers the whole visible
region with a broad absorption band (Figure 1A), thus re-
Notably, the color of c-BTB-1 can disappear and become
completely bleached upon irradiation with visible light (l>
460 nm). The photochromic properties of BTB-1 before and
after irradiation are summarized in Table 1. After correction
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W. Zhu, H. Tian et al.
sulting in an efficient overlap between the absorption and
emission bands. It is the FRET that successfully switches
ON and OFF the fluorescence of the incorporated 2,1,3-
benzothiadiazole unit. Accord-
simplified hexatriene framework in BTB-1 is C2 symmetry,
which is the same as traditional 1,3,5-hexatriene, whereas
that of BTB-2 is mirror symmetry (Figure 2). Considering
ingly, the bistable states of
BTB-1 with fluorescence modu-
lation show potential prospects
in molecular switches and infor-
mation storage.[11,12]
Interestingly, solvent polarity
plays a significant role in the lu-
minescent wavelength of BTB-1
and BTB-2. For BTB-2, the
redshift reaches 130 nm from
about 420 nm with a dual-expo-
nential decay containing two
components of 1.9 ns (36%)
and 16.7 ns (64%) with c2 =1.2
in nonpolar cyclohexane, to
around 550 nm containing those
Figure 2. Woodward–Hoffmann rule analysis of BTB-1 and BTB-2.
of 2.0 ns (72%) and 9.4 ns
(28%) with c2 =1.3 in polar
acetonitrile (Figures S6 and S7 in the Supporting Informa-
tion). However, for BTB-1, a smaller shift by only 50 nm is
observed from 470 nm containing two components of 2.1 ns
(52%) and 7.0 ns (48%) with c2 =1.3 in cyclohexane, to
535 nm containing those of 5.3 ns (87%) and 5.8 ns (13%)
with c2 =1.2 in acetonitrile. This might be similarly attribut-
ed to the ICT effect in BTB-1 being less efficient than in
BTB-2 because the larger difference in dipole moments be-
tween the excited and the ground states can result in the
larger solvatochromic effect.[13] The increase in the dipole
moment (Dm) of BTB-2 can be estimated to be 13 D accord-
ing to the Lippert plots, whereas for BTB-1 the data appears
to be 5 D (shown in the Supporting Information).
this, photoreaction of BTB-2 could only follow the disrota-
tory cyclization in accordance with the orbital symmetry
conservation theory. This also means that the photoreaction
could only occur on the parallel form of BTB-2, resulting in
a strained structure with two methyl groups on the same
side of the plane, which is certainly unstable given the large
steric hindrance of this conformation (Figure 2). As a result,
no obvious photochromic changes occur for BTB-2 upon
UV-light irradiation.
Second, we roughly estimated and examined the ground-
state potential-energy surfaces (PESs) in an effort to further
explain the photochromic difference between BTB-1 and
BTB-2. On the PES of BTB-1, two minima corresponding
to the open- and closed-ring isomers (noted BTB-1 and c-
BTB-1, respectively, in Figure 3) have been located. The re-
action coordinate leading from c-BTB-1 to BTB-1 is repre-
sented by the value of the distance between the reactive
carbon atoms. It shows a high energy barrier of about
52 kcalmolÀ1 for the isomerization from BTB-1 to c-BTB-1
in the ground state, indicating that the reaction can only
proceed by photoexcitation. The c-BTB-1 form is higher in
energy only by 7.8 kcalmolÀ1 with respect to BTB-1, corre-
sponding to a high energy barrier of about 44.2 kcalmolÀ1
for the isomerization from c-BTB-1 to BTB-1, which is con-
sistent with the experimental thermal stability of c-BTB-1.
However, on the PES of BTB-2, when the reaction coordi-
nate goes from 3.47 (the distance in BTB-2) to 1.4 ꢂ, no
stable isomer has been located. In addition, for the parallel
isomer of BTB-2 (noted BTB-2p in Figure 3), in spite of the
local minimum found at the shorter distance of 1.56 ꢂ, it
would transform to BTB-2 immediately because the energy
barrier for this isomerization is low (6.3 kcalmolÀ1). There-
fore, there are no thermally stable closed-ring isomers de-
tected for either BTB-2 or BTB-2p, which is coincident with
the analysis based on the Woodward–Hoffmann rule.
Photochromic difference between BTB-1 and BTB-2 in so-
lution: Unexpectedly, in contrast with the good photochro-
mic performance of BTB-1 in solution, there are no obvious
changes in the color or the absorption spectra of BTB-2
upon UV-light irradiation (Figure S2 in the Supporting In-
formation). The photochromism is found to be critically de-
pendent upon the substitution position of 2,1,3-benzothia-
diazole. To gain an insight into the interesting difference be-
tween the photochromic performances of BTB-1 and BTB-
2, we first attempt to explain this using the Woodward–
Hoffmann rule: a cyclization reaction can take place only if
the molecular orbital symmetries of reactants are the same
as that of products. Traditionally, according to the Wood-
ward–Hoffmann rule based on the p-orbital symmetries for
1,3,5-hexatriene, a conrotatory cyclization reaction to cyclo-
hexadiene is brought about by light. As can be observed
from Figure S9 in the Supporting Information, BTB-1 and
BTB-2 have the same total p-electron count and they also
have the same simplified hexatriene framework for the p
electrons on the benzothiadiazole unit to be both delocal-
ized in the plane. However, the LUMO symmetry of the
902
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