Aromaticity-controlled thermal stability of photochromic systems based on a six-membered ring as ethene bridges: Photochemical and kinetic studies

Article abstract of DOI:10.1002/chem.201200354

Three photochromic compounds-2-butyl-5,6-bis[5-(4-methoxyphenyl)-2- methylthiophen-3-yl]-1 H-benzo[de]isoquinoline-1,3(2 H)-dione (BTE-NA), 4,5-bis[5-(4-methoxyphenyl)-2-methylthiophen-3-yl]benzo[c][1,2,5]thiadiazole (BTA), and BTTA, which contain naphtha

Full text of DOI:10.1002/chem.201200354

FULL PAPER
DOI: 10.1002/chem.201200354
Aromaticity-Controlled Thermal Stability of Photochromic Systems Based on
a Six-Membered Ring as Ethene Bridges: Photochemical and Kinetic Studies
Yuheng Yang,[a] Yongshu Xie,[a] Qiong Zhang,[a] Keitaro Nakatani,[b] He Tian,[a] and
Weihong Zhu*[a]
Abstract: Three photochromic com-
pounds—2-butyl-5,6-bis[5-(4-methoxy-
phenyl)-2-methylthiophen-3-yl]-1H-
benzo[de]isoquinoline-1,3ACTHUNRTGNE(GNU 2H)-dione
(BTE-NA), 4,5-bis[5-(4-methoxyphen-
tion by both photochromism and solva-
tochromism. The three ethene bridges
with different degrees of aromaticity
can provide a systematic comparison of
the thermal stability evolution for their
corresponding closed forms (c-BTE-
NA, c-BTA, and c-BTTA). c-BTE-NA
shows first-order decay in various sol-
vents from cyclohexane to acetonitrile.
c-BTA only shows first-order decay in
polar solvents such as chloroform,
whereas it is stable in nonpolar sol-
vents like toluene. In contrast, the less
aromatic property of BTTA gives rise
to its unprecedented thermal stability
in various solvents even at elevated
temperatures in toluene (328 K). More-
over, the small energy barrier between
the parallel and antiparallel conformers
allows the full conversion from BTTA
to c-BTTA. In well-ordered crystal
states, all three compounds adopt a
parallel conformation. Interestingly,
BTTA forms a twin crystal of asym-
metric nature with interactions be-
tween the electron-rich oxygen atom of
the methoxy group and the carbon
atom of the electron-deficient benzo-
bisthiadiazole moiety. This work con-
tributes to the understanding of aroma-
ticity-controlled thermal stability of
photochromic systems based on a six-
membered ring as an ethene bridge,
and a broadening of the novel building
blocks for photochromic bisthienyl-
yl)-2-methylthiophen-3-yl]benzo[c]-
ACHTUNGTRENNUNG[1,2,5]thiadiazole (BTA), and BTTA,
which contain naphthalimide, benzo-
thiadiazole, and benzobisthiadiazole as
six-membered ethene bridges with dif-
ferent aromaticities—were systemati-
cally studied in solution, sol–gel, and
single-crystal states. They exhibit typi-
cal photochromic performance with
considerably high cyclization quantum
yields. BTE-NA, BTA, and BTTA
form a typical donor–p–acceptor (D–
p–A) system with significant intramo-
lecular charge transfer (ICT) between
HOMO and LUMO upon excitation,
thus realizing the fluorescence modula-
Keywords: aromaticity · benzobis-
thiadiazoles
· charge transfer ·
AHCTUNGTREGeNNUN thene systems.
diarylethenes · photochromism
Introduction
assemblies, molecular switches, logic gates, and information
storage.[1] In particular, bisthienylethenes (BTEs) have at-
tracted much more attention on account of their thermal ir-
reversibility and outstanding fatigue resistance among vari-
ous photochromic systems, in which the 1,3,5-hexatriene sec-
tion could adopt an appropriate conformer to undergo con-
rotatory 6p-electron photocyclization upon excitation with
UV light.[2] To date, most studies have focused on molecular
design, especially the symmetric and asymmetric synthesis
of organic frameworks in diarylethenes with different
Research on photochromic systems has received much at-
tention over the past two decades on account of their distin-
guishable absorption spectra in two different states and
their potential applications in ophthalmic lenses, smart mo-
lecular materials in which they act as photoresponsive self-
[a] Y. Yang, Prof. Dr. Y. Xie, Dr. Q. Zhang, Prof. Dr. H. Tian,
Prof. Dr. W. Zhu
heteroACHTNUTRGNEaNUG ryl units such as thiophene or benzothiophene, furan
Shanghai Key Laboratory of Functional
Materials Chemistry
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
and benzofuran, thiazole, indole, pyrazole, and pyrrole
rings.[3] However, exploitation of the central ethene bridge
has been confined to a cyclopentene unit or its strong elec-
tron-withdrawing analogues, such as perfluorocyclopentene,
maleic anhydride, or maleic imide.[4] Alternatively, several
groups have chosen six-membered rings as a BTE central
bridge to perform photochromism. For instance, Yam and
co-workers took advantage of the complexation ability of
E-mail: whzhu@ecust.edu.cn
[b] Prof. Dr. K. Nakatani
PPSM, ENS Cachan, CNRS, UniverSud
61 av Prꢀsident Wilson, 94230 Cachan (France)
Fax : (+33)147402454
the 1,10-phen
ACHTUNGTRENUNaNG nthroline ligand to form the backbone of a
novel diarylethene photochromic complex, which was utiliz-
AHCTUNGTRENNUNG
ed to tune the excitation wavelength and photochromic be-
havior.[5] Yokoyama and co-workers exploited the fluores-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200354.
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cence property of coumarin to construct a functional ethene
bridge, thus achieving dual-mode fluorescence switching.[6]
On the other hand, on account of the strong aromaticities
of six-membered ethene bridges, the greatly enhanced
ground-state energy difference (Hꢂckel rule) between the
open and closed forms ultimately results in the thermal in-
stability of the closed form in BTEs.[7] With this in mind, we
thiadiazole unit ultimately leads to the unexpected thermal-
ly irreversible photochromic system (BTTA). Moreover, the
small energy barrier between the parallel and antiparallel
conformers allows the full conversion from BTTA to c-
BTTA. For the first time, the three ethene bridges with dif-
ferent degrees of aromaticity provided a systematical com-
parison of the thermal-stability evolution of their corre-
sponding closed forms. This work contributes to the under-
standing of aromaticity-controlled thermal stability of pho-
tochromic systems based on six-membered rings as ethene
bridges, and broadens the novel building blocks for photo-
chromic BTE systems.
have recently synthesized
a novel photochromic BTE
system based on the specific benzobisthiadiazole unit that
shows excellent thermal stability for the closed form in vari-
ous solvents from nonpolar to polar, and even at elevated
temperatures.[8] Since the ethene bridge with a six-mem-
bered ring has its own advantages such as a higher quantum
yield on ring closure and longer absorption wavelength, we
envision that the six-membered ring with low or non-aroma-
ticity as the center ethene bridge might greatly extend the
diversity of the thermally irreversible photochromic systems.
However, up to now, insight into the ethene bridges has
seldom been provided. Accordingly, we systematically
report herein the synthesis and photochromic properties of
three ethene bridges with different degrees of aromaticity
(2-butyl-5,6-bis[5-(4-methoxyphenyl)-2-methylthiophen-3-
Results and Discussion
Molecular design and synthesis: Naphthalimide, benzothia-
diazole, and benzobisthiadiazole are excellent candidates for
acceptor moieties on account of their attractive photophysi-
cal properties and their priority in electron-withdrawing ca-
pabilities. As shown in Scheme 1, we designed novel BTE
derivatives with these building blocks based on the following
points: 1) given the similarly strong electron-withdrawing
properties of perfluorocyclopentene, maleic anhydride, or
maleic imide, the remarkable electron-withdrawing units of
naphthalimide, benzothiadiazole, and benzobisthiadiazole
are supposed to assure good photochromism with considera-
ble bistability, high cyclization quantum yield, and fatigue
resistance; 2) naphthalimide, benzothiadiazole, and benzo-
bisthiadiazole are essentially well-known fluorescent build-
yl]-1H-benzo[de]isoquinoline-1,3
4,5-bis[5-(4-methoxyphenyl)-2-methylthiophen-3-
yl]benzo[c][1,2,5]thiadiazole (BTA), and BTTA; Scheme 1),
A
(BTE-NA),
ACHTUNGTRENNUNG
which is expected to achieve clear thermal stability evolu-
tion, and realize the fluorescence modulation by solvato-
and photochromism in the six-membered ethene-bridged
photochromic systems. As expected, an extremely low aro-
matic property of the center ethene bridge with a benzobis-
Scheme 1. Synthetic routes to BTE-NA, BTA, and BTTA.
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Photochromic Systems
FULL PAPER
ing blocks in the design of functional materials such as sen-
sors, OLEDs, nonlinear optics (NLO), and solar cells.[9]
Thus, the electron-withdrawing groups and the electron-do-
nating anisole can form an efficient donor–p–acceptor (D–
p–A) system with significant charge transfer between
HOMO and LUMO upon excitation to realize the fluores-
cence modulation by means of both photochromism and sol-
vatochromism;[10] and 3) the three ethene bridges with dif-
ferent degrees of aromaticity can provide a systematical
comparison of the thermal stability evolution for their corre-
sponding closed forms.
Scheme 1 summarizes the synthetic routes to BTE-NA,
BTA, and BTTA in this study. 5-(4-Methoxyphenyl)-2-meth-
ylthiophen-3-ylboronic acid was prepared by the reported
procedure[11] and was used as a building block to synthesize
the desired target molecules. The key intermediates of 4-
bromo-N-butyl-3-iodo-1,8-naphthalimide,[11]
2,1,3-benzothiadiazole,[12] and 4,5-dibromobenzo[1,2-c:3,4-
c’]bis
[1,2,5]thiadiazole[8] were conveniently synthesized from
4-bromo-1,8-naphthalic anhydride, 4-bromobenzene-1,2-di-
ACHTUNGTRENNUNG
amine, and benzothiadiazole,[13] respectively. The Suzuki
4,5-dibromo-
ACHTUNGTRENNUNG
coupling between 4-bromo-N-butyl-3-iodo-1,8-naphthali-
mide, 4,5-dibromo-2,1,3-benzothiadiazole, 4,5-dibromoben-
zo[1,2-c:3,4-c’]bis
yl)-2-methylthiophen-3-ylboronic acid in the presence of a
[Pd(PPh3)4] catalyst in a mixture of aqueous Na2CO3 (2m)
ACHTUNGTRENNUNG[1,2,5]thiadiazole, and 5-(4-methoxyphen-
ACHTUNGTRENNUNG
and 1,4-dioxane under reflux conditions gave the target mol-
ecules of BTE-NA, BTA, and BTTA, which were fully char-
1
acterized by H and 13C NMR spectroscopy as well as high-
resolution MS (HRMS; shown in the Experimental Section
and the Supporting Information).
Optical properties and photochromism: All three com-
pounds were found to easily dissolve in toluene to give a
colorless solution without any absorption bands in the visi-
ble range. In general, the electronic absorption spectra for
the open forms of BTE-NA, BTA, and BTTA in toluene at
298 K show an intense absorption band in the range of 290–
350 nm and a moderately intense band at 378–430 nm
(Figure 1). Upon UV irradiation at 365 nm, the correspond-
ing closed forms (c-BTE-NA, c-BTA, and c-BTTA) were
produced by the typical photocyclization as the colorless sol-
ution changed to yellow green, bright green, and then dark
green, respectively, thereby resulting in broad absorption
bands centered at 719 (c-BTE-NA), 657 (c-BTA), and 497
and 655 nm (c-BTTA). The significant difference in absorp-
tion bands of the closed forms compared to their open
forms is mainly due to the increase in p conjugation, which
dramatically changes the electronic structure as a whole in
such a way that new electronic transitions are observed in
the visible region. The longer wavelength of c-BTE-NA in
the visible region than that of c-BTTA strongly indicates
that the energy gap between the HOMO and LUMO orbi-
tals of c-BTE-NA is smaller than that of c-BTTA, which re-
sults from the strong conjugation between 2-(4-methoxy-
phenyl)-5-methylthiophene and N-butyl-1,8-naphthalimide
units. The typical photochromic properties of BTE-NA,
Figure 1. Absorption changes upon irradiation at 365 nm: A) BTE-NA in
toluene (2.31ꢃ10ꢀ5 molLꢀ1); B) BTA in toluene (1.85ꢃ10ꢀ5 molLꢀ1);
C) BTTA in toluene (1.14ꢃ10ꢀ5 molL ꢀ1). The corresponding inset pho-
tographic images are BTE-NA (2.31ꢃ10ꢀ4 molLꢀ1), BTA (1.85ꢃ
10ꢀ4 molLꢀ1), and BTTA (1.14ꢃ10ꢀ4 mol Lꢀ1) in toluene upon irradiation
at 365 nm.
BTA, BTTA and their corresponding closed forms (c-BTE-
NA, c-BTA, and c-BTTA) are summarized in Table S1 in
the Supporting Information. Notably, the ring-closure quan-
tum yield of these three ethene bridges decreased with the
loss of aromaticity. The ring-closure quantum yields for
BTE-NA, BTA, and BTTA are 43.13, 38.21, and 10.96% in
cyclohexane, respectively. This might be ascribed to the
competition between different pathways to the ground state.
The fluorescence quantum yields for BTE-NA, BTA, and
BTTA were found to be 0.76, 0.12, and 0.48% in cyclohex-
ane, respectively. The extremely low fluorescence quantum
yield is indicative of the inefficient radiative pathway to the
ground state. Accordingly, the excited molecules could be
predominately deactivated through photocyclization or
other non
ACHTUNGTRENrNUNG adiative approaches. Here the decrease in photo-
cyclization quantum yield for BTTA is probably the result
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W. Zhu et al.
of the increased spin–orbit coupling and possible triplet
decay due to the increased inclusion of sulfur atoms in the
ring systems.
conformers of BTTA in [D6]benzene is 64:36. When irradi-
ated at 365 nm, the open form was converted to the closed
form (c-BTTA) with absorption bands in the visible region.
The existence of c-BTE-NA, c-BTA, and c-BTTA could also
1
NMR spectroscopic changes before and after irradiation: In
fact, distinct 1H NMR spectroscopic changes could be ob-
served in photochromic systems due to the large chemical
environment changes between the open and closed forms.
be evidenced by H NMR spectroscopy. For BTE-NA (Fig-
ure S1A in the Supporting Information), the signals of Ha,
Hb, and Hc on the butyl group after photocyclization (in
[D6]benzene) do not show any obvious changes with respect
to the open form due to the distance far away from the p-
delocalization of naphthalimide, whereas the methylene hy-
drogen signals (Hd) attached to the imide nitrogen shift by
0.07 ppm from d=4.37 to 4.30 ppm. The two groups of
methyl protons (Ho, Hp) on the thiophene ring in BTE-NA
shift from d=1.82 and 2.13 ppm to d=2.37 and 2.50 ppm,
respectively. There is a similar case for the two methyl pro-
tons (Hi, Hj) on the thiophene group of BTA (in
[D6]benzene), which shift downfield from d=1.98 and
2.03 ppm to d=2.46 and 2.50 ppm, respectively (Figure S1B
in Supporting Information).
1
The H NMR spectra of BTE-NA and BTA show only one
set of signals for the methyl protons on thiophene rings lo-
cated at d=1.82 and 2.13 ppm for BTE-NA and d=1.98 and
2.03 ppm for BTA in [D6]benzene (Figure S1 in the Support-
ing Information). The different chemical shifts of the two
methyl protons are only on account of the asymmetric or-
ganic framework of the central ethene bridge. However, the
1H NMR spectrum of BTTA in [D6]benzene shows two well-
resolved sets of signals for the methyl protons that corre-
spond to the parallel (d=2.04 ppm) and antiparallel (d=
1.98 ppm) conformers (Figure 2). In fact, the two conforma-
tions—parallel (photochromic inactive) and antiparallel
(photochromic active) conformers—undergo very fast
single-bond rotation in BTE-NA and BTA, thereby resulting
in only one set of time-averaged signals in the 1H NMR
spectra. Only in the cases of BTEs with large energy barri-
ers between two conformers can the rotation of the aryl
groups be slowed down sufficiently to give two sets of sig-
Generally speaking, the proton resonances on the thio-
phene heterocycles and six-membered ethene bridges shift
upfield upon ring-closure of diarylethene,[14] which is due to
the aromaticity loss of heterocycles in the ring-closure reac-
tion. In the ring-open form, the resonances of the protons
attached to thiophene units and six-membered rings appear
1
in the H NMR spectroscopic aromatic region. In contrast,
1
nals.[7] As found by the area integration in H NMR spectra
the resonances for these protons in the ring-closed form
appear in the region characteristic of alkenes. As expected,
after the ring closure of BTE-NA upon irradiation at
365 nm, the aromaticity of the naphthalimide and thiophene
groups is disturbed, and the protons (He, Hf, Hg, and Hh) on
the naphthalimide moiety shift remarkably to the high field
upon photocyclization (Figure S1A in the Supporting Infor-
mation). A typical triplet peak that corresponds to Hf moves
from d=7.14 to 6.98 ppm. The singlet peak of Hh shifts sig-
nificantly from d=8.91 to 8.24 ppm. Additionally, the two
protons (Hi and Hv) located on thiophene are d=6.94 and
6.84 ppm for BTE-NA, whereas they shift to d=6.86 and
6.61 ppm after photocyclization. Similarly in the case of
BTA, the chemical shifts of Ha and Hb on the benzothiadia-
zole (BTA) and Hc move to high field (Figure S1B in the
Supporting Information). Interestingly, in contrast to the
two protons (Hc and Hp) located on thiophene (d=6.95 and
7.16 ppm) for BTA, H’c shifts upfield to d=6.39 ppm,
whereas H’p shifts downfield to d=8.43 ppm for c-BTA.
Also in the system of BTTA, the protons of Hb and H’b on
the thiophene rings of BTTA shift downfield from d=7.05
and 7.25 ppm in the open form to d=8.48 ppm after photo-
cyclization (Figure 2). The dramatic downfield shifts of H’p
for BTA could be attributed to the possible formation of an
intramolecular hydrogen bond between H’p and the N atom
on the benzothiadiazole moiety (Figure S1B in the Support-
ing Information). Similarly, the intramolecular hydrogen
bonds between Hb, H’b, and the N atom on the benzobisthia-
diazole units ultimately lead to the downfield shift of Hb
and H’b upon photocyclization (Figure 2).
(Figure 2), the ratio between the parallel and antiparallel
Figure 2. 1H NMR spectroscopic changes of BTTA in [D6]benzene. Note:
the inset photographic images were taken of BTTA in toluene (1.14ꢃ
10ꢀ4 molLꢀ1) upon irradiation at 365 nm.
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Photochromic Systems
FULL PAPER
Interestingly, in case of BTTA (in [D6]benzene), the two
sets of well-resolved proton signals on the thiophene and
methyl groups of methoxy on the phenyl rings are converted
into one set of proton signals (they move from d=1.98 and
2.04 to 2.50 ppm; d=3.22 and 3.26 to 3.19 ppm, respective-
ly). It is the same case for the protons on phenyl rings after
photocyclization. The simultaneous disappearance of the
two sets of signals indicates that a slow interconversion be-
tween parallel and antiparallel conformations takes place in
the system, and that the chemical environment of the two
sets of methyl groups becomes identical after photocycliza-
tion. It might be due to the small energy barrier between
the parallel and antiparallel conformers of BTTA and c-
BTTA in the absorption spectra that allows the full conver-
sion from BTTA to c-BTTA as demonstrated in the
1H NMR spectra (Figure 2).
Fluorescence modulation and solvatochromism: As shown
in Table S1 in the Supporting Information, the absorption
bands of the open and closed forms in various solvents
remain in the range of 378–430 nm and 650–719 nm, thus in-
dicating that the solvent dependence of the absorption band
is rather small according to the Franck–Condon principle.
All three compounds show noticeable fluorescence, which
could be sufficiently modulated by solvato- and photo-
chromism. Indeed, upon excitation at 719 (BTE-NA), 657
(BTA), and 655 nm (BTTA), no significant fluorescence sig-
nals could be recorded from the closed form. The fluores-
cence quenching in the photostationary state (PSS) in tol-
uene is 77% for BTE-NA, 92% for BTA, and 94% for
BTTA (excited at the isobestic points of 329, 324, and
324 nm, respectively), which are fully compatible with the
conversion yield (Table S1 in the Supporting Information).
Thus, the fluorescence quenching of BTE-NA, BTA, and
BTTA is simply due to the disappearance of the open form
upon photocyclization and the nonfluorescent property of
the corresponding closed form (Figure 3A and Figures S3–
S5 in the Supporting Information). Moreover, the solvent
polarity plays a decisive role in the emission wavelength on
account of the strong electron-withdrawing nature of naph-
thalimide, benzothiadiazole, and benzobisthiadiazole, and
the moderate electron-donating ability of anisole rings.
When the three compounds were excited, the dipole
moment was largely enhanced for all of them. In nonpolar
solvents like cyclohexane, the dipolar moments could not be
stabilized, and hence, the emission wavelength was shorter.
In polar solvents like acetonitrile, however, the stabilized di-
polar moment yielded a small energy gap between the
HOMO and LUMO, and thus a longer emission wavelength
was observed (Figure 3B). The peak of the emission bands
appear at 464 and 601 nm in cyclohexane and THF for
BTE-NA, 494 and 670 nm in cyclohexane and acetonitrile
for BTA, and 488 and 636 nm in cyclohexane and acetoni-
trile for BTTA, respectively (Tables S2–S4 in the Supporting
Information). Clearly, the solvatochromism of all three com-
pounds arises from the possible photoinduced charge trans-
fer from the donor anisole group to the electronegative cen-
Figure 3. A) Fluorescence spectral changes of BTA in toluene with the
excitation at isobestic point 324 nm. B) Normalized fluorescence spectra
of BTA in various solvents (from left to right: cyclohexane, toluene,
THF, chloroform, acetone, and acetonitrile).
tral ethene bridge. The positive linearity of the Lippert–
Mataga plots also illustrates that there is a higher dipole
moment in the excited state than in the ground state (Fig-
ure S6 in the Supporting Information).
Effects of the ethene bridge on the thermal stability of the
closed forms in various solvents: The energy barrier, which
correlates to the ground-state energy difference between the
open and the closed forms, contributes to the stability of the
photogenerated closed-ring isomers. In fact, the change in
aromatic character of the six-membered ethene from its
open to closed form during the course of photocyclization
can control the ground-state energy difference. With the
strong aromaticity of the central ethene bridge, the energy
barrier for the cycloreversion thus becomes smaller. Hence,
the thermal back-reaction is ready to take place.[7] As ex-
pected, c-BTE-NA is unstable even at 293 K due to the
large difference in the ground-state energy before and after
irradiation. As illustrated in Table 1, the absorbance at
around 710 nm for BTE-NA could be completely bleached
Table 1. Spectrokinetic data of thermal bleaching for BTE-NA in various
solvents at 293 K.[a]
Solvents
lmax [nm]
t1/2 [s]
k [10ꢀ3 sꢀ1
]
A0
cyclohexane
toluene
THF
708
717
716
719
647.5
581.4
215.3
182.2
1.01
1.27
3.10
3.71
0.863
0.919
0.660
0.150
acetonitrile
[a] The values k and A1 are the fitted parameters from Equation (1);
lmax, t1/2, and A0 were obtained from the experimental data.
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W. Zhu et al.
crease in aromaticity, c-BTA
shows no clear thermal back-
reaction in nonpolar solvents
like toluene, even at 328 K.
However, c-BTA is not stable
in polar solvents such as
chloroform, which was found
to follow similar first-order
decay with a constant of 1.52ꢃ
10ꢀ5 sꢀ1 (Figure S7 in the Sup-
porting Information), four
magnitudes slower than that of
BTE-NA (1.80ꢃ10ꢀ1 sꢀ1). As
shown in Figure 5 and Table 2,
BTA exhibits first-order decay
in chloroform with a half-life
(t1/2 =4.58ꢃ104 s) much slower
than that of BTE-NA (3.89 s).
The different thermal back-re-
action rates in the system of
BTE-NA and BTA are ap
ACHTUNGTRENNUNGpar-
AHCTUNGTRENNUNG
ence in activation energy of
the thermal cycloreversion in
various solvents.
In contrast, no obvious ther-
mal back-reaction could be ob-
served for BTTA in the dark
Figure 4. The monoexponential (first-order) decay model and experimental thermal fading kinetics of BTE-
NA monitored at 708, 717, 716, 719 nm in A) cyclohexane (8.67ꢃ10ꢀ5 molLꢀ1), B) toluene (8.26ꢃ
10ꢀ5 molLꢀ1), C) THF (6.23ꢃ10ꢀ5 molLꢀ1), and D) acetonitrile (1.43ꢃ10ꢀ5 molLꢀ1) at 293 K, respectively
(black line: data measured; gray dot: the first-order decay model fitted; solid line: the absorbance residuals at
testing time). Inset: the first-order decay model parameters. Note: Due to the fast bleaching rate of BTE-NA
in acetonitrile and low signal-to-noise ratio, the experiment of picture D was carried out on a CARY instru-
ment in scanning kinetic mode. Thus the residuals were omitted due to the small amount of data points.
and 273 K for
a prolonged
time. As matter of fact,
a
BTTA preserves very promis-
ing thermal stability with
within 35 min, thereby indicating the great loss of aromatici-
ty after the photocyclization reaction. The thermal back-re-
action rates for c-BTE-NA are 1.01ꢃ10ꢀ3, 1.27ꢃ10ꢀ3, 3.10ꢃ
10ꢀ3, and 3.71ꢃ10ꢀ3 sꢀ1 in cyclohexane, toluene, THF, and
acetonitrile, respectively, as monitored by using a charge-
coupled device (CCD) camera mounted on a spectrometer
and on a CARY instrument in the kinetic mode (Figure 4).
More impressively, the decay becomes much faster in
chloroform (1.80ꢃ10ꢀ1 sꢀ1; Table 2). The experimental data
were fitted with Equation (1),[15] in which A0, A1, and A are
the initial absorbance, infinite absorbance, and absorbance
at t(s), respectively.
almost flat decays, and does not show any thermal back-re-
action in various solvents like cyclohexane, toluene, chloro-
form, and even in acetonitrile in the dark and 293 K (Fig-
AHCTUNGTREGuNNNU res S8–S10 in the Supporting Information). Even the pure
closed form was successfully separated on aluminum oxide
eluted with CCl4 and dichloromethane (volume ratio 4:1;
Figure S25 in the Supporting Information). At elevated tem-
peratures (328 K) in toluene, BTTA does not show any obvi-
ous decrease in its absorbance at 655 nm after 800 min (Fig-
ure S9 in the Supporting Information).
Furthermore, the absorption bands of BTTA in the visible
region could be gradually bleached as a result of the photo-
chromic back-reaction upon irradiation at 575 nm, thus
showing a typical photoresponsive back-reaction. However,
for BTE-NA and BTA, the photochromic reaction could be
reverted either by thermal back-reaction in the dark in
polar solvents or upon irradiation at 575 nm. Thus, consider-
ing the high polarity and electron-withdrawing tendency of
the benzobisthiadiazole unit, the incorporation of an ex-
tremely highly polar, low aromatic unit with benzobisthia-
diazole as the center ethene bridging unit can ultimately
lead to a thermally irreversible photochromic system that
can even be comparable to the photochromic performances
of parent BTEs, such as the widely known five-membered
hexafluorocyclopentene-based counterparts.
kt ¼ ꢀln½ðAꢀA1Þ=ðA0ꢀA1Þꢁ
ð1Þ
Generally, the decrease in aromaticity can greatly increase
the bistability of the BTE system. On account of the de-
Table 2. Spectrokinetic data of thermal bleaching for BTE-NA, BTA,
and BTTA in chloroform at 293 K
lmax [nm]
t1/2 [s]
k [sꢀ1
]
A0
BTE-NA
BTA
BTTA
719
657
655
3.89
1.80ꢃ10ꢀ1
0.28
1.63
0.87
4.58ꢃ104
1.52ꢃ10ꢀ5
[a]
[a]
–
–
[a] Data could not be obtained.
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Photochromic Systems
FULL PAPER
BTE-NA, BTA, and BTTA in the single-crystal state and or-
ganogel systems: Single crystals of the open forms of BTE-
NA, BTA, and BTTA were grown by slow evaporation of
the corresponding solutions in mixed solvents of THF/
MeOH, THF/H2O, and CHCl3/MeOH, respectively. X-ray
crystal-structure analyses revealed that in all these crystals
only parallel conformations are observed, thereby indicating
that all three of these systems cannot undergo photocycliza-
tion in the single-crystal state. For the crystal of BTE-NA
(Figure S11 in the Supporting Information), two molecules
are arranged in a head-to-tail fashion to afford a dimeric
ꢀ
moiety by the intermolecular C H···O hydrogen bonds with
ꢀ
H···O distances of 2.43 ꢄ, and C H···O angles of 1748. Inter-
estingly, weak O···C interactions are observed between
naphthalimide and methoxy units, with O···C distances of
3.120(2) ꢄ, which is smaller than the sum of the van der
Waals radii for O and C atoms. These interactions might be
understood as electrostatic interactions between the elec-
tron-rich O atom and the electron-deficient naphthalimide
carbon atom. For the crystal of BTA (Figure 6), multiple in-
Figure 5. Thermal back-reaction of BTE-NA, BTA, and BTTA in chloro-
form at 293 K. Inset: the enlarged graph is the thermal back-reaction of
BTE-NA in chloroform at 293 K.
ꢀ
Theoretical calculations: To gain further insight into the aro-
maticity-controlled thermal stability, the calculations were
performed with the Gaussian 09 program.[16] The geometries
were optimized with the Perdew–Burke–Ernzerhof (PBE0)
termolecular C H···O hydrogen bonds are found that in-
ꢀ
volve the methoxy O atoms and phenyl C H moieties, with
ꢀ
H···O distances of 2.48 ꢄ and C H···O angles of 1608. Thus,
interesting supramolecular linear structures are formed by
these interactions.
functional[17] and the 6-31G
ACTHNUTRGNE(NUG d,p) basis set. The energies ob-
tained from the small basis-set optimizations are not accu-
rate enough. On the basis of the optimized structures, more
accurate energies can be achieved by performing single-
For the crystal of BTTA (Figure 7), S···p interactions are
observed with the shortest S···C distances of 3.412(5) ꢄ.
Similar to that observed in the crystal of BTE-NA, weak
O···C interactions are also observed between benzobisthia-
diazole and methoxy units, with O···C distances of
3.12(5) ꢄ. Thus, a linear structure is formed by these inter-
actions (Figure 7c). From these observations, it is reasonable
to assert that the multiple intermolecular interactions stabi-
lize and fix the conformations of the molecules. Thus, only
the parallel conformations are observed. In contrast, the
previously reported BTTE without methoxy groups adopts
an antiparallel conformation on account of the lack of wide-
spread occurrence of intermolecular interactions.[8] Clearly,
the methoxy group plays an important role in these crystals
due to the presence of the electron-rich O atoms. Moreover,
BTTA forms a twin crystal. The asymmetric nature of the
twin crystal and cooperative intermolecular interactions fi-
nally lead BTTA to form a three-dimensional tubelike struc-
ture. Apparently, the variation in the six-membered ethene
bridges from naphthalimide and benzothiadiazole to benzo-
bisthiadiazole plays a key role in modulating intermolecular
interactions, thus affecting the packing of these molecules.
Furthermore, the distance of the double bonds for central
ethene bridges within naphthalimide (C3=C4), benzothiadia-
zole (C6=C11), and benzobisthiadiazole (C4=C5) are
1.398(2), 1.383(4), and 1.371(5) ꢄ, respectively (Figures 6
and 7, and Figure S11 in the Supporting Information). Gen-
erally, the typical bond length is approximately 1.54 ꢄ for a
point calculations with the larger 6-311+GACTHUNRTGNEUNG(d,p) basis set.
The solvent effect is taken into account within the polariza-
ble continuum model (PCM)[18] in the single-point calcula-
tions.
For BTTA, BTA, and BTE-NA, the open-ring isomers are
more stable than the closed-ring isomers both under
vacuum and in solvents. Destabilization on account of the
destruction of the aromatic thiophene rings and central
ethene bridges during the course of cyclization increases the
ground-state energy of the closed-ring isomers.[7] In BTE de-
rivatives, the cycloreversion reaction in the ground states
generally has to overcome the energy barriers that correlate
to the ground-state energy differences. The calculated
values of ground-state energy difference between the open
and closed isomers for BTTA, BTA, and BTE-NA are
ꢀ4.93, ꢀ10.16, and ꢀ20.44 kcalmolꢀ1 under vacuum, respec-
tively (Tables S5 and S6 in the Supporting Information).
When the ground-state energy difference is large, the
energy barrier becomes small and the cycloreversion reac-
tion readily takes place. On the other hand, the reaction bar-
rier becomes large when the ground-state energy difference
is small. In the case of c-BTTA, the cycloreversion reaction
could not occur easily in the ground state relative to c-BTE-
NA and c-BTA. Accordingly, the destabilization energy,
which correlates to the decrease in aromaticity during the
course of photocyclization, is lower in BTTA than in BTA,
and much lower than in BTE-NA. The small energy differ-
ence between the open and closed forms results in the excel-
lent thermal stability of c-BTTA in a variety of solvents.
ꢀ
C C single bond and 1.33 ꢄ for a double bond. Among
these three compounds, the relatively short bond length in-
ꢀ
dicates that the bonding between C4 C5 in the BTTA
system is nearer to a typical double bond. It is consistent
Chem. Eur. J. 2012, 00, 0 – 0
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W. Zhu et al.
Figure 7. A) ORTEP representation of the crystal structure of BTTA
with displacement ellipsoids shown at the 25% probability level. Selected
ꢀ
ꢀ
ꢀ
bond lengths [ꢄ]: C5 C6 1.452(5), C3 C4 1.454(5), C4 C5 1.371(5).
B) Top view of a linear structure of BTTA formed by cooperative inter-
molecular S···C and O···C stacking interactions and a photographic image
of a single crystal of BTTA obtained from slow evaporation of BTTA in
ꢀ
a mixed solvent of CHCl3 MeOH. C) Side view of the linear structure of
BTTA. Hydrogen atoms are omitted for clarity. Selected atom distances
[ꢄ]: S···C 3.412(5), O···C 3.121(5).
Figure 6. A) ORTEP representation of the crystal structure of BTA with
displacement ellipsoids shown at the 25% probability level. Selected
ꢀ
ꢀ
ꢀ
bond lengths [ꢄ]: C6 C7 1.442(4), C10 C11 1.435(4), C6 C11 1.383(4).
ꢀ
B) View of a linear structure formed by multiple C H···O hydrogen
BTTA, which could be bleached by irradiation with visible
light at 575 nm.
bonds in the crystal of BTA.
with the aromaticity tendency: benzobisthiadiazole<benzo-
thiadiazole<naphthalimide.
Conclusion
However, considering the good photochromic perform-
ance of BTE-NA, BTA, and BTTA in organic solvents, it is
expected that the parallel arrangements in the single-crystal
state can shift to the antiparallel conformation in solution
due to the loss of the above-mentioned cooperative interac-
tions in the well-ordered crystal state. Therefore, it is rea-
sonable that the photochromic behavior can be distinctively
different between the crystalline and solution phases. As il-
lustrated by a further case (Figure S12 in the Supporting In-
formation), BTE-NA, BTA, and BTTA also exhibit excel-
lent photochromic properties and defined thermoreversible
properties in an organogel system.[19] A stable gel with BTE-
NA (1.66ꢃ10ꢀ3 wt%), BTA (1.28ꢃ10ꢀ3 wt%), and BTTA
(1.59ꢃ10ꢀ3 wt%) dopants were formed around 20 w/w%
Poloxamer 407 (Boluoshamu 407 with a molecular distribu-
tion of 9840–14600) in water, and showed an extraordinary
sol–gel phase-transition temperature around 288C (Fig-
ure S12 in the Supporting Information). Irradiation of the
thermoreversible organogel with UV light at 365 nm leads
to the appearance of new absorbance bands around 719 nm
for BTE-NA, 655 nm for BTA, and 497 and 655 nm for
Three compounds—BTE-NA, BTA, and BTTA, which con-
tain electron-withdrawing groups of naphthalimide, benzo-
thiadiazole, and benzobisthiadiazole as six-membered ring
bridges—were specifically designed and synthesized. The
three ethene bridges with different degrees of aromaticity
could provide a systematical comparison in the thermal sta-
bility evolution of their corresponding closed forms. They
show typical photochromic performance with considerably
high cyclization quantum yields and high fatigue resistance
in solution. The decrease in the aromaticity in ethene
bridges was able to increase the thermal stability of the
closed form at room temperature. c-BTE-NA shows first-
order decay in various solvents, whereas c-BTA is only
stable in nonpolar solvents. The lower aromaticity of BTTA
gives rise to its unprecedented thermal stability in various
solvents, even at elevated temperature in toluene. In a well-
ordered crystal state, all three compounds adopt a parallel
conformer, and BTTA forms a three-dimensional tubelike
structure on account of the strong p···p stacking interactions
and weak electrostatic interactions between the electron-
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Photochromic Systems
FULL PAPER
126.93, 127.17, 127.21, 127.56, 130.92, 131.72, 133.07, 133.46, 134.57,
134.94, 135.73, 136.46, 137.38, 139.73, 139.94, 140.72, 158.98, 159.23,
164.08, 164.34 ppm; HRMS (TOF MS ESI+): m/z: calcd for
C40H36NO4S2: 658.2086 [M+H]+; found: 658.2089.
rich oxygen atom on the methoxy group and the carbon
atom on the electron-deficient benzobisthiadiazole moiety.
Furthermore, the fluorescence of BTE-NA, BTA, and
BTTA could be modulated by photochromism and solvato-
chromism. This work contributes to the understanding of ar-
omaticity-controlled thermal stability of the ethene bridge
as well as novel building blocks for the photochromic BTE
systems.
Synthesis of BTA: BTA was prepared by a similar procedure to BTE-NA
by Suzuki coupling between 4,5-dibromo-2,1,3-benzothiadiazole and 5-(4-
methoxyphenyl)-2-methylthiophen-3-ylboronic acid (250 mg, yield:
1
ꢀ
57%). H NMR (400 MHz, CDCl3): d=2.07 (s, 3H; CH3), 2.23 (s, 3H;
ꢀ
ꢀ
ꢀ
CH3), 3.81 (s, 3H; OCH3), 3.82 (s, 3H; OCH3), 6.85–6.88 (m, 5H;
phenyl-H), 7.04 (s, 1H; thiophene-H), 7.36 (d, J=8.8 Hz, 2H; phenyl-H),
7.42 (d, J=8.8 Hz, 2H; phenyl-H), 7.71 (d, J=9.2 Hz, 1H; phenyl-H),
8.02 ppm (d, J=8.8 Hz, 1H; phenyl-H); 1H NMR (400 MHz,
ꢀ
ꢀ
ꢀ
[D6]benzene): d=1.98 (s, 3H; CH3), 2.03 (s, 3H; CH3), 3.22 (s, 3H;
Experimental Section
ꢀ
OCH3), 3.23 (s, 3H; OCH3), 6.68 (d, J=6.0 Hz, 2H; phenyl-H), 6.71 (d,
J=6.0 Hz, 2H; phenyl-H), 6.95 (s, 1H; thiophene-H), 7.34 (d, J=8.8 Hz,
1H; phenyl-H), 7.44 (d, J=8.8 Hz, 2H; phenyl-H), 7.47 (d, J=8.8 Hz,
2H; phenyl-H), 7.79 ppm (d, J=8.8 Hz, 1H; phenyl-H); 13C NMR
(100 MHz, CDCl3): d=14.23, 14.53, 55.37, 114.18, 114.27, 119.95, 124.30,
124.98, 126.83, 126.94, 127.10, 127.41, 128.27, 133.25, 133.76, 135.17,
136.59, 136.66, 137.46, 140.00, 140.16, 154.26, 155.16, 158.93, 159.06 ppm;
HRMS (TOF MS ESI+): m/z: calcd for C30H25N2O2S3: 541.1078 [M+H]+
; found: 541.1078.
General: 1H and 13C NMR spectra were recorded using Bruker AM-400
spectrometers with tetramethylsilane (TMS) as an internal reference.
CDCl3 and [D6]benzene were used as solvents. HRMS spectra were re-
corded using a Waters LCT Premier XE spectrometer with methanol or
acetonitrile as solvents. Absorption and fluorescence spectra were re-
corded using Varian Cary 500 and HORIBA Fluoromax 4 instruments,
respectively. The photochromic reaction was induced in situ by continu-
ous wavelength irradiation using an Hg/Xe lamp (Hamamatsu, LC8
Lightningcure, 200 W) equipped with narrow-band interference filters of
appropriate wavelengths (Semrock Hg01 for lirr =365 nm, Semrock
BrightLine FF01-575/25-25 for lirr =575 nm). The irradiation power was
measured using a photodiode from Ophir (PD300-UV). The photochro-
mic quantum yields were determined by probing the sample with a
xenon lamp during the photochromic reaction. Absorption changes were
Synthesis of BTTA: BTTA was prepared by a similar procedure to BTE-
NA by Suzuki coupling between 4,5-dibromobenzo[1,2-c:3,4-c’]bis-
AHCTUNGTRENNUNG
ꢀ
ꢀ
CH3, antiparallel conformer), 2.20 (s, 3H; CH3, parallel conformer),
ꢀ
ꢀ
3.81 (s, 3H; OCH3, parallel conformer), 3.83 (s, 3H; OCH3, antiparal-
lel conformer), 6.84 (d, J=8.4 Hz, 2H; phenyl-H, parallel conformer),
6.88 (d, J=8.8 Hz, 2H; phenyl-H, antiparallel conformer), 6.92 (s, 1H;
thiophene-H, parallel conformer), 7.13 (s, 1H; thiophene-H, antiparallel
conformer), 7.38 (d, J=8.8 Hz, 2H; phenyl-H, parallel conformer),
7.44 ppm (d, J=8.4 Hz, 2H; phenyl-H, antiparallel conformer); H NMR
(400 MHz, [D6]benzene): d=1.98 (s, 3H; CH3, antiparallel conformer),
2.04 (s, 3H; CH3, parallel conformer), 3.22 (s, 3H; OCH3, parallel con-
former), 3.26 (s, 3H; OCH3, antiparallel conformer), 6.69 (d, J=8.4 Hz,
2H; phenyl-H, parallel conformer), 6.74 (d, J=8.4 Hz, 2H; phenyl-H, an-
tiparallel conformer), 7.05 (s, 1H; thiophene-H, parallel conformer), 7.25
(s, 1H; thiophene-H, antiparallel conformer), 7.48 (d, J=8.4 Hz, 2H;
phenyl-H, parallel conformer), 7.51 ppm (d, J=8.8 Hz, 2H; phenyl-H,
antiparallel conformer); 13C NMR (100 MHz, CDCl3): d=14.85, 55.37,
114.24, 124.43, 124.57, 126.93, 127.03, 127.09, 130.75, 130.91, 132.42,
132.57, 137.07, 137.85, 140.33, 140.54, 147.48, 156.85, 157.15, 159.09 ppm;
HRMS (TOF MS ESI+): m/z: calcd for C30H22N4O2S4: 598.0626 [M]+;
found: 598.0626.
monitored by
a CCD camera mounted on a spectrometer (Ocean
Optics). The fluorescence quantum yields were measured by the compari-
son of BTE-NA, BTA, and BTTA with standard 0.05m quinine sulphate
in sulfuric acid. 4-Bromo-N-butyl-3-iodo-1,8-naphthalimide,[11] 4,5-dibro-
1
mo-2,1,3-benzothiadiazole,[12]
4,5-dibromobenzo[1,2-c:3,4-c’]bis-
ACHTUNGTRENNUNG
[1,2,5]thiadiazole,[8] and 5-(4-methoxyphenyl)-2-methylthiophen-3-ylbor-
ꢀ
onic acid were prepared according to the established method.[12] All
other reagents were of analytical purity and used without further treat-
ment.
ꢀ
ꢀ
ꢀ
Synthesis of BTE-NA: 4-Bromo-N-butyl-3-iodo-1,8-naphthalimide
(0.68 g, 1.5 mmol) was dissolved in dioxane (100 mL). Then [PdACTHUNTRGNE(UNG PPh3)4]
(0.35 g) was added, and the resulting mixture was stirred for 15 min at
room temperature. Then aqueous Na2CO3 (100 mL, 2.0 molLꢀ1) was
added. The reactive mixture was heated to reflux at a temperature of
608C, and the solution of 5-(4-methoxyphenyl)-2-methylthiophen-3-ylbor-
onic acid (0.74 g, 3.0 mmol) was added dropwise with a syringe. Subse-
quently the mixture was heated to reflux for 24 h and cooled to room
temperature. The reactive mixture was poured in H2O and extracted with
diethyl ether. The organic layer was separated and dried with Na2SO4.
After concentration, the compound was purified by column chromatogra-
phy on silica gel (CCl4/ethyl acetate=20:1 v/v) to yield a yellow solid
Pure closed-form c-BTTA: BTTA was irradiated with UV light and sepa-
rated on aluminum oxide eluted with CCl4/dichloromethane (4:1 v/v).
H NMR (400 MHz, [D6]benzene): d=2.50 (s, 6H; CH3), 3.19 (s, 6H;
OCH3), 6.66 (d, J=8.8 Hz, 4H; phenyl-H), 7.64 (d, J=8.8 Hz, 4H;
1
ꢀ
ꢀ
1
(340 mg, yield: 35%). H NMR (400 MHz, CDCl3): d=0.98 (t, J=7.6 Hz,
phenyl-H), 8.48 ppm (s, 2H; thiophene-H).
ꢀ
ꢀ
ꢀ
3H; CH2CH3), 1.48–1.53 (m, 2H; CH2CH3), 1.72–1.80 (m, 2H;
Photochromism of BTE-NA, BTA, and BTTA doped in thermosensitive
organogel: The sol–gel system was based on commercially available Po-
loxamer 407 (Boluoshamu 407) with a molecular distribution of 9840–
14600. The thermosensitive organogel with a dopant of BTE-NA, BTA,
and BTTA was prepared as follows. A solution (200 mL) of BTE-NA
(1.66ꢃ10ꢀ3 wt%), BTA (1.28ꢃ10ꢀ3 wt%), or BTTA (1.59ꢃ10ꢀ3 wt%) in
THF was added to an aqueous solution of Poloxamer 407 (3.0 g; 20% in
weight ratio). The viscous solution was cooled to 58C and sonicated until
the solution became transparent without obvious bubbles. The phase
transition for thermosensitive organogel system was then about 288C.
When keeping the temperature above 288C, Poloxamer 407 aqueous sol-
ution (20% in weight ratio) became an organogel.
ꢀ
ꢀ
ꢀ
ꢀ
NCH2CH2 ), 1.99 (s, 3H; CH3), 2.34 (s, 3H; CH3), 3.80 (s, 3H;
ꢀ
ꢀ
ꢀ
OCH3), 3.84 (s, 3H; OCH3), 4.21 (t, J=7.6 Hz, 2H; NCH2 ), 6.72 (s,
1H; thiophene-H), 6.81 (d, J=8.8 Hz, 1H; phenyl-H), 6.89 (d, J=8.8 Hz,
1H; phenyl-H), 6.97 (s, 1H; thiophene-H), 7.30 (d, J=8.8 Hz, 2H;
phenyl-H), 7.44 (d, J=8.8 Hz, 2H; phenyl-H), 7.70 (t, J=7.6 Hz, 1H;
naphthalene-H), 8.17 (d, J=8.4 Hz, 1H; naphthalene-H), 8.62 (d, J=
7.2 Hz, 1H; naphthalene-H), 8.67 ppm (s, 1H; naphthalene-H); 1H NMR
ꢀ
(400 MHz, [D6]benzene): d=0.90 (t, J=7.6 Hz, 3H; CH2CH3), 1.39–
ꢀ
ꢀ
ꢀ
1.45 (m, 2H; CH2CH3), 1.82 (s, 3H; CH3), 1.87–1.93 (m, 2H;
ꢀ
ꢀ
ꢀ
ꢀ
NCH2CH2 ), 2.13 (s, 3H; CH3), 3.22 (s, 3H; OCH3), 3.26 (s, 3H;
ꢀ
ꢀ
OCH3), 4.36 (t, J=7.2 Hz, 2H; NCH2 ), 6.69 (d, J1 =8.4 Hz, 2H;
phenyl-H), 6.73 (d, J1 =8.8 Hz, 2H; phenyl-H), 6.84 (s, 1H; thiophene-
H), 6.94 (s, 1H; thiophene-H), 7.40 (d, J=8.4 Hz, 2H; phenyl-H), 7.45
(d, J=8.8 Hz, 2H; phenyl-H), 7.99 (d, J=8.4 Hz, 1H; naphthalene-H),
8.63 (d, J=6.8 Hz, 1H; naphthalene-H), 8.91 ppm (s, 1H; naphthalene-
H); 13C NMR (100 MHz, CDCl3): d=13.91, 14.06, 14.17, 20.45, 30.29,
40.33, 55.36, 55.41, 114.21, 114.35, 121.83, 122.83, 124.29, 125.09, 126.84,
X-ray crystallography for BTE-NA: C40H35NO4S2; Mr =657.81 gmolꢀ1
;
yellow block; 0.35ꢃ0.15ꢃ0.05 mm3; triclinic; space group P1; a=
¯
11.4898(12), b=11.9884(13), c=14.1549(15) ꢄ;
1.342 gmolꢀ1; m
(MoKa)=0.208; T=133 K; 12582 measured reflections,
7033 unique reflections (Rint =0.0296), 5972 with Iꢂ2s(I) used in refine-
FACHTUNRGTENN(GU 000)=692.0; 1calcd =
AHCTUNGTRENNUNG
Chem. Eur. J. 2012, 00, 0 – 0
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W. Zhu et al.
ment, final wR2 =0.1434, R1 (I>2s(I))=0.0416; GoF=1.088; largest
gawa, T. Kawai, Eur. J. Org. Chem. 2007, 3212–3218; d) K. Yagi, M.
Irie, Bull. Chem. Soc. Jpn. 2003, 76, 1625–1628; e) S. Z. Pu, G. Liu,
L. Shen, J. K. Xu, Org. lett. 2007, 9, 2139–2142; f) Y. Wu, S. J. Chen,
Y. H. Yang, Q. Zhang, Y. S. Xie, H. Tian, W. H. Zhu, Chem.
Commun. 2012, 48, 528–530.
final difference peak/hole= +0.43/ꢀ0.35 eꢄꢀ3
.
X-ray crystallography for BTA: C30H24N2O2S3; Mr =540.69 gmolꢀ1
;
yellow block; 0.40ꢃ0.20ꢃ0.18 mm3; monoclinic; space group P21/n; a=
7.5004(9), b=28.647(3), c=12.1653(14) ꢄ; (000)=1128.0; 1calcd
1.375 gmolꢀ1; m
(MoKa)=0.316; T=133 K; 19898 reflections measured
F
N
=
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ACHTUNGTRENNUNG
using a Bruker SMART Apex diffractometer, of which 5698 (Rint
0.0307), 19898 measured reflections, 5698 unique reflections (Rint
=
=
0.0307), 4963 with Iꢂ2s(I) used in refinement, final wR2 =0.1435, R1 (I>
2s(I))=0.0533; GoF=1.158; largest final difference peak/hole= +0.574/
ꢀ0.448 eꢄꢀ3
.
X-ray crystallography for BTTA: C30H22N4O2S4; Mr =598.76 gmolꢀ1
;
Kelly block; 0.23ꢃ0.11ꢃ0.09 mm3; triclinic; space group P1; a=
¯
12.301(4), b=12.559(4), c=19.099(6) ꢄ;
FACHTNUTRGNEG(UN 000)=1240.0; 1calcd =
1.426 gmolꢀ1; m
ACHTUNGTRENNUNG
9518 unique reflections (Rint =0.0306), 5763 with Iꢂ2s(I) used in refine-
ment, final wR2 =0.1362, R1 (I>2s(I))=0.0518; GoF=1.011; largest final
difference peak/hole= +0.216/ꢀ0.220 eꢄꢀ3
.
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CCDC-862019 (BTE-NA), 862020 (BTA), and 862021 (BTTA) contain
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Acknowledgements
This work was supported by the NSFC of China, the National 973 Pro-
gram (2011CB808400), the Oriental Scholarship, SRFDP (200802510011
and 20100074110015), the Program for New Century Excellent Talents in
University (NCET), the Innovation Program of Shanghai Municipal Edu-
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Received: February 2, 2012
Revised: May 9, 2012
Published online: && &&, 0000
&
10
&
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Photochromic Systems
FULL PAPER
Photochromism
Y. Yang, Y. Xie, Q. Zhang,
K. Nakatani, H. Tian,
W. Zhu* ........................ &&&& — &&&&
Aromaticity-Controlled Thermal
Stability of Photochromic Systems
Based on a Six-Membered Ring as
Ethene Bridges: Photochemical and
Kinetic Studies
Theory of evolution: The evolution of
aromaticity with different ethene
bridges (see scheme) was found to
greatly affect the thermal stability of
photochromic systems. Closed-form
BTE-NA (c-BTE-NA) shows first-
order decay in various solvents,
whereas c-BTA is only stable in non-
polar solvents. The less aromatic prop-
erty of c-BTTA gave rise to its unpre-
cedented thermal stability in various
solvents.
Chem. Eur. J. 2012, 00, 0 – 0
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
11
&
ÞÞ
These are not the final page numbers!