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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
Chem. Eur. J. 2010, 16, 899 – 906
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