A novel gated photochromic reactivity controlled by complexation/ dissociation with BF3

Article abstract of DOI:10.1039/c1cc15824d

A photochemically active dithiazolethene BN was designed and synthesized, exhibiting a specific gated photochromism. That is, the photochromic reactivity of BN is prevented to a great extent by BF₃, showing a "Lock" gate.

Full text of DOI:10.1039/c1cc15824d

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COMMUNICATION
A novel gated photochromic reactivity controlled by complexation/
dissociation with BF3w
Yue Wu, Shangjun Chen, Yuheng Yang, Qiong Zhang, Yongshu Xie, He Tian and
Weihong Zhu*
Received 20th September 2011, Accepted 20th October 2011
DOI: 10.1039/c1cc15824d
A photochemically active dithiazolethene BN was designed and
synthesized, exhibiting a specific gated photochromism. That is,
the photochromic reactivity of BN is prevented to a great extent
by BF3, showing a ‘‘Lock’’ gate.
from its open form BN to the closed form c-BN. A well-defined
isosbestic point appeared at 374 nm, suggesting a two component
mixture. The photostationary state (PSS) was reached after
irradiation at 365 nm (0.5 mW) for nearly 40 s. BN can be
photochemically regenerated from c-BN by irradiation with
visible light (546 nm), meanwhile, the characteristic red color
of the closed form returns to its originally colorless open form.
Impressively, upon the alternating irradiation with UV (365 nm)
and visible light (546 nm), compound BN can be toggled
repeatedly between the open-form and closed-form, remaining
intact without any obvious degradation (Fig. 2b).
Diarylethenes are the most promising candidates for photo-
electronic applications due to their fatigue resistant and
thermally irreversible properties.1 Their potential applications
could be in molecular level devices, such as photoresponsive
self-assemblies, molecular switches, logic gates and information
storage.2 Although much progress has been made in diarylethene
systems, a property that is strongly desirable, but is still inadequate
is gated photochromic reactivity, which has rarely been
reported in recent years. Gated reactivity is the property that
the ON/OFF photoactivity of molecules is dependent on the
external stimuli, such as reaction,3 heat4 and coordination.5
The threshold reactivity is, in particular, moving forward data
processing on the molecular level, with potential applications
in sensing and labeling, as well as data manipulation. More-
over, the photocyclization quantum yield is also very critical to
the photochromic system. Recently, a number of dithiazol-
ethenes have been studied,6 exhibiting a very high photo-
cyclization quantum yield.7 With this in mind, we designed
and synthesized a photochromic molecule BN (Fig. 1a) with
gated reactivity and high quantum yield.
As is well-known, diarylethenes possess two conformations
in solution, with the two rings in a parallel conformation or
anti-parallel conformation. Since the photocyclization reaction
proceeds only from the anti-parallel conformation, the quantum
yield is about 50% at most. The Fo-c of dithiazolethene BN
and the reference dithienylethene BT (Fig. 1b) in THF was
41.2 and 21.4%, respectively (Table 1).9 Apparently, dithiazol-
ethene BN displays a higher cyclization quantum yield than BT,
The dithiazolethene BN was synthesized from 2,3-dibromo-
benzo[b]thiophene-1,1-dioxide8 and the corresponding borate
of 4-bromo-5-methyl-2-phenylthiazole through a Suzuki reaction
1
with 45% yield. The structure was confirmed with H NMR
and 13C NMR, HRMS spectra and X-ray crystallography
(ESIw).
Photochromic properties of BN was studied in CH3CN
solution. Upon light irradiation at 365 nm, the colorless
open-ring form BN promptly turned red (lmax = 526 nm,
Figs 1a and 2a), which can be ascribed to the transformation
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. E-mail: whzhu@ecust.edu.cn; Fax: (+86) 21-6425-2758
w Electronic supplementary information (ESI) available: Synthesis
and characterization, photochromic properties of BN and BT. CCDC
843533. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1cc15824d
Fig. 1 (a) The proposed mechanism and photographic images of the
‘‘Lock and Key’’ gated process of BN. (b) The structural changes of
the reference compound BT under the alternative irradiation of UV
and visible light after the addition of BF3ꢀEt2O.
c
528 Chem. Commun., 2012, 48, 528–530
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Fig. 3 (a) The single-crystal structure of BN with displacement
ellipsoids shown at the 50% probability level. Non-hydrogen bonded
hydrogen atoms are omitted for clarity. Selected bond lengths (A):
C9ꢀ ꢀ ꢀC19, 3.700(3); N1ꢀ ꢀ ꢀH5, 2.680; N2ꢀ ꢀ ꢀO2, 3.223; N2ꢀ ꢀ ꢀO1, 3.828;
C6–C7, 1.483(3); C17-S1, 1.787(3); C7–C17, 1.341(3). (b) A view of the
one-dimensional structure of BN formed by hydrogen bonding (light
blue line), with non-hydrogen bonded hydrogens omitted for clarity.
Selected bond lengths: Nꢀ ꢀ ꢀH, 2.53 A. (c) Color changes of BN in the
crystal state.
Fig. 2 (a) UV/Vis absorption changes of BN (2.0 ꢁ 10ꢂ5 M) in CH3CN
upon irradiation with 365 nm light every 5 s until 80 s. (b) Fatigue
resistance of BN upon prolonged irradiation with alternation of 365 and
546 nm for forward and backward reactions in toluene, respectively.
(c) Absorption changes of BN in CH3CN before irradiation with 365 nm
light (black line) and upon addition of BF3ꢀEt2O (6.0 ꢁ 10ꢂ2 M) with
365 nm light irradiation (red line), finally irradiation with 365 nm light
(blue line) after adding Et3N (6.0 ꢁ 10ꢂ2 M). (d) Absorption changes of
reference compound BT in CH3CN solution before (black line) and after
irradiation with 365 nm light (red line), or irradiation with 365 nm light
after adding BF3ꢀEt2O (blue line). Note: the absorption band resulting
from the photocyclized closed form is much lower than that of BN due to
the low photocyclization conversion yield (8.1%) in the system of BT
(Table 1), which might be a result of the forming of methylthiophene
CHꢀꢀꢀO hydrogen bonds in BT.3e
‘‘Lock’’ gate. Moreover, the gate behavior is reversible. Upon
adding Et3N, the photochromic property could be recovered
(Fig. 2c). Apparently, Et3N is considered as a ‘‘Key’’ to
‘‘Lock’’ (BF3). Furthermore, H+ and metal ions, such as
Zn2+, Mn2+, Mg2+, Ca2+ and Ba2+ (Fig. S1w) had no effect
on the photochromism of BN.
Table 1 Quantum yields of BN and BT in THF
Meanwhile, dithienylethene BT was studied as a reference
(Fig. 1b). BT could be changed between colorless open and
colored closed forms by alternated irradiation with UV and
visible light (Fig. S2w). As shown in Fig. 2d, the photochro-
mism of BT is characteristically photoactive. In contrast, when
adding BF3ꢀEt2O and, then, upon irradiation with 365 nm, the
absorption of BT was almost the same as that without BF3
(Fig. 2d). With respect to BN, the gated reactivity of BT was
not possible in that there is no center (like a N atom) to
coordinate with BF3. Therefore, in the system of dithiazolethene
BN, we can rule out the interaction of the ethene bridge
(benzo[b]thiophene-1,1-dioxide) with BF3.
Compounds
Fo-c (%)
Fc-o (%)
Conversion at PSS (%)
BN
BT
41.2
21.4
2.4
11.6
77.3
8.1
and the PSS coversion of BN in THF was 77.3%, 9.6-fold higher
than that of reference BT (8.1%).
The single crystal of dithiazolethene BN was obtained by the
diffusion method from a mixture of ethyl acetate and octane at
room temperature.z The structure has a typical anti-parallel
conformation (Fig. 3a), with a distance of 3.700(3) A between
the photoactive carbons (C9ꢀꢀꢀC19), which meets the prerequisites
for a molecule to undergo the photochromic reaction10 from
colorless to a violet color in the crystalline phase (Fig. 3c).
In addition, a weak CHꢀ ꢀ ꢀN hydrogen bond occurs with a
N1ꢀ ꢀ ꢀH5 distance of 2.68 A, which is shorter than the sum of
the van der Waals radii of H (1.2 A) and N (1.55 A). This is
consistent with Kawai’s report.7b Consequently, the weak
CHꢀ ꢀ ꢀN hydrogen bonding in BN (Fig. 3a) can stabilize the
anti-parallel conformation, thus resulting in an increase in the
observed cyclization quantum and conversion yield. Further-
more, intermolecular CHꢀ ꢀ ꢀN hydrogen bonds (Fig. 3b) are
also observed, with a Hꢀ ꢀ ꢀN distance of 2.53 A, which is even
shorter than the intramolecular length.
As shown in Fig. 1a, we propose the specific ‘‘Lock’’ gate in
BN via the coordination model of BN_BF3 with BF3ꢀEt2O.
That is, the thiazole rings rotate to coordinate with BF3, resulting
in a rigid seven-membered ring11 and a longer distance between
the two active carbon atoms. Meanwhile, the geometry becomes
closer to the parallel conformer, by which the initial photochromic
reaction is blocked. Consequently, the photochromism in the
system of BN is not allowed, and the ‘‘Lock’’ gate is achieved by
BF3. When adding Et3N to the system, the coordination of BN
with BF3 becomes interrupted, thus recovering the original
characteristic photochromicity. Accordingly, Et3N can play a
role as a ‘‘Key’’ to the ‘‘Lock’’ (BF3). The proposed mechanism
1
was further evidenced by the H NMR titration and theoretical
Interestingly, no obvious UV absorption spectra changes
could be observed with irradiation at 365 nm after the addition
of BF3ꢀEt2O (Fig. 2c). In other words, the photochromism of
BN was prevented to a great extent by BF3, showing a specific
calculation.
To evaluate the ‘‘Lock’’ mechanism with BF3, we performed
a BF3-dependent 1H NMR titration on BN in CDCl3, wherein
the distinct change for the characteristic methyl groups in the
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 528–530 529

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BT due to the existing weak CHꢀꢀꢀN hydrogen bonds, as
demonstrated by X-ray crystallography.
We are grateful for support from NSFC/China, the Oriental
Scholarship, SRFDP 200802510011, and the Fundamental
Research Funds for the Central Universities (WK1013002).
Notes and references
z Crystallographic data for BN: C28H20N2O2S3, M = 512.64, mono-
clinic, space group P21/n, a = 14.2273(13) A, b = 13.0726(12) A, c =
14.5059(13) A, a = 901, b = 109.866(2)1, g = 901, V = 2537.4(4) A3,
Z = 4, rcalcd = 1.342 gcmꢂ3, T = 296(2)K, 13187 reflections
measured, 5638 unique (Rint = 0.0202) which were used in all
calculations. The final wR(F2) was 0.1724 (all data). R = 0.0450,
Rw= 0.1420, GOF = 1.088. CCDC 843533 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
Fig. 4 Changes in the 1H NMR spectra of BN in the absence and the
presence of BF3.
thiazole units were observed (Fig. 4). Before adding BF3ꢀEt2O,
the two signals corresponding to the photoactive methyl
hydrogens appear at 2.10 and 2.04 ppm, respectively. Notably,
in contrast to the reference BT (1:1, Fig. S6w), the integration ratio
of the lower- and upper-field signals is 1 : 0.06 (Fig. 4), indicating
that there exists a predominant anti-parallel conformation in
BN,7b,12 which can be further certified by hydrogen bonds of
single crystals and contributes to the relative high conversion
and cyclization quantum yield (Table 1). Upon adding BF3,
the signals of methyl hydrogens in the system of BN were
shifted downfield to 2.60 and 2.50 ppm with an integration
ratio of 1 : 1, respectively, possibly arising from the deshielding
effect of the coordination of BF3. This is consistent with the
proposed mechanism of a ‘‘Key and Lock’’ process of the
photochromic reaction (Fig. 1a). Obviously, the resulting
conformation becomes rigid when the nitrogen in the thiazole
rings are coordinated with BF3.
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In order to further unravel the underlying mechanism of the
gated photochromism of BN in response to BF3, the optimized
geometries of all of the isomers and complexes are depicted in
Table S1.w Some critical structural parameters and relative
energies of the complexes are tabulated in Table S2.w Due to
the unsymmetrical nature of the bridging unit of BN, there are
four other isomers located, including the parallel isomers
BN-2-a and BN-2-b and the anti-parallel isomers BN-3-a and
BN-3-b. It clearly shows that all the isomers, except for BN-2-b,
can form complexes with BF2. Energy calculations have excluded
the presence of BN-1_BF2 in the formed complexes. Energies of
the complexes demonstrate that BN-3-a_BF2 and BN-3-b_BF2
are the most stable complexes formed, which are 6.02 and 7.77
kcal molꢂ1 more favorable than BN-1_BF2 and BN-2-a_BF2,
respectively. Furthermore, the significantly large distances
between the reactive carbons in both BN-3-a_BF2 and BN-
3-b_BF2 are around 5.31 A. Therefore, it can be concluded from
the theoretical calculations that the gated photochromism of BN
in response to BF3 can be reasonably ascribed to the formation
of the photochromically deactivated rigid boron-coordination
conformation, such as BN-3-a_BF2 and BN-3-b_BF2.
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In summary, we designed and synthesized a triangle terarylene
with gated photochromic reactivity. The dithiazolethene BN
exhibits an obvious gated reactivity, photochromic activity of
which can be manipulated by a reversible process between
BF3ꢀEt2O and Et3N. Meanwhile, the photocyclization quantum
yield of BN is higher than the corresponding dithienylethenes
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c
530 Chem. Commun., 2012, 48, 528–530
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