Synthesis, Photochromism, and Thermal Stability of Two New Diarylethenes Based On Spirobifluorene

Article abstract of DOI:10.1002/jhet.1015

Two novel spirobifluorene-diarylethenes compounds with furan (9a) and thiophen (8a) as heterocyclic aryl groups were successfully synthesized, and their structures were fully characterized with FTIR, NMR, mass spectra (MS), and elemental analysis. Their photochromic properties were examined. The results indicated that they showed good photochromic behaviors in hexane and acetonitrile. The fluorescence emission was quenched along with the photochromism from open ring to closed ring. Large fluorescence emission blue-shift was clearly observed in polar solvents. Furthermore, the thermal stability of 8a and 9a was greatly improved by introducing spirobifluorene group into the molecules. The 5% loss weight temperature of 9a was 59°C higher than that of 10a without spirobifluorene.

Full text of DOI:10.1002/jhet.1015

Month 2016
Synthesis, Photochromism, and Thermal Stability of Two New
Diarylethenes Based On Spirobifluorene
*
Caihua Li and Heping Zeng
Institute of Functional Molecules, School of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou 510641, China
*E-mail: hpzeng@scut.edu.cn
Received February 25, 2011
DOI 10.1002/jhet.1015
Published online 00 Month 2016 in Wiley Online Library (wileyonlinelibrary.com).
Two novel spirobifluorene-diarylethenes compounds with furan (9a) and thiophen (8a) as heterocyclic
aryl groups were successfully synthesized, and their structures were fully characterized with FTIR, NMR,
mass spectra (MS), and elemental analysis. Their photochromic properties were examined. The results indi-
cated that they showed good photochromic behaviors in hexane and acetonitrile. The fluorescence emission
was quenched along with the photochromism from open ring to closed ring. Large fluorescence emission
blue-shift was clearly observed in polar solvents. Furthermore, the thermal stability of 8a and 9a was greatly
improved by introducing spirobifluorene group into the molecules. The 5% loss weight temperature of 9a
was 59°C higher than that of 10a without spirobifluorene.
J. Heterocyclic Chem., 00, 00 (2016).
INTRODUCTION
orthogonally fused structure of the spiro-type molecules
entitles them the desired steric demand, reduce the
crystallization tendency, enhance the rigidity, and
consequently improve their thermal stability [14–17].
Therefore, spiro-type molecules are promising as
functional material to improve their thermal stability. The
molecular design strategy of introducing spirobifluorene
into 8a and 9a rendered to obtain new photochromic
materials. These molecules displayed good fluorescence
emission, stable and reversible color changes with
ultraviolet and visible light. Most importantly, they
showed high thermal stability.
Diarylethenes with furan as heterocyclic aryl groups
have rarely been studied in detail [18,25]. According to
previous reports [19], the thermal stability of the
diarylethene-type photochromic compounds can be
improved by introducing aryl groups having low aromatic
stabilization energy. Keeping in view, 9a with furan
unites was designed to improve the thermal stability of
photochromic material because furan has lower aromatic
stabilization energy than thiophen. In this study, we have
investigated not only the different photochromic
performance of 8a and 9a but also their thermal
stabilities by thermogravimetry.
Photochromic materials can reversibly switch color when
irradiated at different wavelengths [1–3]. This property is
relevant for materials applications such as optical
switching, sensing, and logic gates [4,5]. Among all the
photochromic compounds, diarylethene derivatives are the
most promising for practical applications. It is because of
their highly desirable photochromic properties such as
excellent fatigue resistance, short response time, high
quantum yields, absence of thermal isomerization, and
large changes in the absorption wavelength between the
two isomers [6–10].
Despite of expedience, diarylethene derivatives still have
not been explored for practical applications. The limitations
related to these derivatives were thermal stability, rapid
response, and so on. Among all, the most important one
was thermal stability of both isomers. Various attempts
have been carried out to stabilize the photogenerated
unstable isomers by dispersing them in high Tg polymer
matrices [11–13]; nevertheless, the stability is not enough
for practical applications. Therefore, it was strongly
desired to develop photochromic compounds in which
both isomers are intrinsically stable.
In order to improve the thermal stability of
photochromic material, we have designed and
synthesized two kinds of new diarylethenes compound 8a
and 9a based on spirobifluorene(Scheme 1). In case of
spirobifluorene, the bifluorene rings were orthogonally
arranged through a tetracoordinated carbon. The specific,
EXPERIMENTAL
Materials.
3,4-Bis(2,4,5-trimethylthiophen-3-yl)-1H-
pyrrole-2,5-dione (10a) was purchased from Tokyo
© 2016 Wiley Periodicals, Inc.

C. Li and H. Zeng
Vol 000
Scheme 1.
Chemical Industry Co. Ltd. (TCI, Shanghai). All organic
solvents were commercially available, distilled, and dried
by appropriate methods. Column chromatography was
carried out on silica gel (200–300mesh; Qingdao Haiyang
Chemical Co. Ltd, China).
Synthetic procedures.
The synthetic procedure for
compounds 8a and 9a was outlined in Scheme 1. 9, 9′-
Spirobifluorene (1) and 2-nitro-9,9′-spirobi[fluorene](2)
were obtained as described in reported literatures [20–
22]. For 8a and 9a, the central 2,5-dihydro-1H-pyrrole
was formed by a ring-closure reaction of a 1,5-diketone
via McMurry reaction [23].
Characterization and measurements. Fourier transform
infrared (FTIR) spectra were recorded on a Perkin-Elmer
1
FTIR spectrometer (Waltham, MA) using KBr pellets. H
9,9′-Spirobi[fluoren]-2-amine (3).
A suspension of 2-
NMR and 13C NMR spectra were performed on
BRUKER DRX-400 (Billerica, MA) with CDCl3 as
solvents. Chemical shifts (d) were given relative to
tetramethylsilane (TMS). The coupling constants (J) were
reported in Hz. The mass spectra were obtained on
Esquire HCT PLUS. Ultraviolet-visible (UV/Vis)
absorption and fluorescence spectra were recorded on
Hitachi U-3010 and Hitachi F-2500 fluorescence
spectrophotometers (Tokyo, Japan). The UV light source
was a high-pressure mercury lamp through a filter (365
and 650 nm, 10 mW/cm2). Thermal gravimetric analysis
(TGA) thermograms were recorded on the STA449C
thermal analysis system. Differential scanning calorimeter
(DSC) measurements were performed at a heating rate of
10°C/min up to 400°C on samples taken in an aluminum
pan with a pierced lid, in a dry nitrogen atmosphere with
an empty pan as reference.
nitro-9,9-spirobifluorene 0.72 g(1 mmol) and SnCl2 2H2O
2.26g (5 mmol)in EtOH (100 mL) was refluxed under Ar
for 5 h. The reaction mixture was concentrated, and
residue was poured into ethyl acetate (50 mL) and water
(50 mL), followed by filtering through celite pad. The
organic layer was separated, washed with water, and dried
over anhydrous MgSO4. The residue was purified by
column chromatography with CH2Cl2/petroleum ether
(from 0.5 to 1), which gave the title compound 3 (0.68 g)
as colorless solid. The yield was 68% and mp was 227–
229°C; 1H NMR (400 MHz, CDCl3) δ: 7.96 (d, 1H, J=7.6
Hz), 7.94–7.81 (m, 2H), 7.86 (d, 2H, J=7.6 Hz), 7.48–7.34
(m, 3H), 7.26(s, 1H), 7.22 (t, 1H, J= 7.6 Hz), 7.12 (t, 2H,
J=7.6 Hz), 6.76(d, 1H, J=7.6 Hz), 6.71(d, 2H, J=7.6
Hz),5.18(s, 2H); IR(KBr)ν: 3440, 3359, 1615, 1582, 1453,
756 cmÀ1. Atmospheric pressure chemical ionization
(APCI)-MS m/z (%): 332.1 (M+, 100).
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Synthesis, Photochromism, and Thermal Stability of Two New Diarylethenes
Based on Spirobifluorene
2-Bromo-1-(2,5-dimethylthiophen-3-yl)ethanone
(4)
7.6 Hz), 6.78(d, 1H, J=7.6 Hz), 6.70(d, 2H, J=7.6 Hz);
6.46(s, 2H), 4.36(s, 4H), 2.38(s, 3H), 2.23(s, 3H); 13C
NMR (100 MHz, CDCl3) δ:190.5, 159.5, 158.2, 154.4,
149.6, 143.9, 143.8, 143.2, 129.3, 128.9, 128.6, 128.0,
126.9, 126.7, 125.0, 124.3, 123.7, 122.5, 121.1, 120.3,
120.1, 112.5, 66.8, 62.3, 15.6, 14.5; APCI-MS m/z (%):
603.9 (M+, 100). IR(KBr)ν: 1682, 1557, 1449, 1377, 928,
653; Anal. Calcd for C41H33NO4: C 81.57, H 5.51, N
2.32, C 69.04; found: C 81.53, H 5.50, N 2.34.
[24]. 1-(2,5-Dimethylthiophen-3-yl)ethanone 0.15 g (1
mmol) and copper bromide 0.45 g (2 mmol) were
suspended in dry tetrahydrofuran (THF) (20 mL). The
mixture refluxed for 6 h under Ar until all of the starting
materials were consumed. The reaction was monitored
by thin layer chromatography (TLC). The solution was
filtered hot, and the filtrate was concentrated to blue
viscous oil. The oily product was dissolved in 20 mL
CH2Cl2 and washed three times with water. The crude
product was purified by column chromatography to
afford compound 4 (0.17 g) as a colorless transparent
solid. The yield was 72%, and mp was 42–43°C; 1H
NMR (400 MHz, CDCl3) δ: 7.00(s, 1H), 4.25(s, 2H),
2.69(s, 3H), 2.42(s, 3H); IR(KBr)ν: 1659, 1546,1447,
1428, 845, 619, 591 cmÀ1. APCI-MS m/z (%): 234.5
(M+, 100).
1-(9,9′-Spirobi[fluorene]-2-yl)-3,4-bis(2,5-dimethylthiophen-
3-yl)-2,5-dihydro-1H-pyrrole (8a). To a suspension of zinc
powder 1.3 g (20 mmol) in THF (30 mL) under nitrogen,
TiCl4 (1.2 mL) was added very slowly at 0°C by syringe.
The mixture was refluxed for 1 h. Then compound 6 0.1
g (0.16 mmol) in THF (60 mL) was also added very
slowly at ambient temperature. The reaction mixture
was stirred for another 24 h in darkness followed by
quenching with K2CO3 (40%, 8 mL). The solid was
filtered and washed with diethyl ether (50 mL). The
combined organic phases were concentrated to 5 mL
and water (5 mL) was added to the residue. The
product was extracted with diethyl ether (5 mL) and
dried over MgSO4, filtered, and concentrated. The
residue was purified by column chromatography over
silica gel using hexane(100%) as eluent to afford
compound 8a (47 mg) as a colorless transparent solid.
2-Bromo-1-(2,5-dimethylfuran-3-yl)ethanone (5).
synthetic approach was similar to compound 4, just
employing 1-(2,5-dimethylfuran-3-yl)ethanone instead of
The
1-(2,5-dimethylthiophen-3-yl)ethanone.
Yield
of
compound 5 was 78%, and its mp was 53–55°C; 1H
NMR (400 MHz, CDCl3) δ: 6.23(s, 1H), 4.12(s, 2H),
2.56(s, 3H), 2.27(s, 3H); IR(KBr)ν: 1682, 1570, 1377,
954, 670 cmÀ1. APCI-MS m/z (%): 216.6 (M+, 100).
2,2′-(9,9′-Spirobi[fluorene]-2-ylazanediyl)bis(1-(2,5-dimethyl
1
thiophen-3-yl)ethanone) (6).
Compound 4 0.28 g (1.2
The yield was 49%, and mp was 195–196°C; H NMR
mmol), Na2CO3 0.106 g (1 mmol), and Compound 3
0.16 g (0.5mmol) were dissolved in ethanol (20 mL;
95%). The mixture was stirred for 0.5 h at ambient
temperature and then refluxed for 4 h. The reaction
mixture was cooled and diluted with water. Solid mass
was obtained and purified through recrystallization from
ethanol to obtain yellow solide (0.2 g) [25,26]. Yield and
mp of compound 6 were 64%, 207–210°C, respectively;
1H NMR (400 MHz, CDCl3) δ: 7.99–7.91 (m, 3H), 7.88
(d, 2H, J=7.6 Hz), 7.46–7.38 (m, 3H), 7.29(s, 1H), 7.23
(t, 1H, J=7.6 Hz), 7.12 (t, 2H, J=7.6 Hz), 6.81(d, 1H,
J=7.6 Hz), 6.71(d, 2H, J=7.6 Hz);6.52(s, 2H), 4.28(s,
4H), 2.46(s, 3H), 2.29(s, 3H); 13C NMR (100 MHz,
CDCl3) δ:191.5, 155.5, 149.2, 148.4, 147.6, 146.9, 136.0,
130.3, 129.5, 128.6, 128.1, 128.0, 126.5, 126.2, 125.3,
124.4, 123.9, 122.6, 121.2, 120.4, 120.2, 110.5, 65.71,
62.3, 14.7, 13.6; IR(KBr)ν: 1679, 1569, 1400, 1357,936,
658; APCI-MS m/z (%): 634.3 (M+, 100); Anal. Calcd
for C41H33NO2S2: C 77.45, H 5.23, N 2.20, S 10.09;
found: C 77.48, H 5.22, N 2.18, S 10.07.
(400 MHz, CDCl3) δ: 7.89–7.82 (m, 5H), 7.43 (d, 2H,
J=8.0 Hz), 7.40–7.36 (m, 3H), 7.15–7.11 (m, 4H), 6.77
(d, 1H, J=8 Hz), 6.18(s, 2H), 4.38(s, 4H), 2.32(s, 6H),
2.04(s, 6H); 13C NMR (100 MHz, CDCl3) δ:150.6,
148.8, 148.3, 141.8, 140.9, 140.0, 139.3, 136.0, 134.5,
131.7, 131.5, 129.2, 127.9, 127.7, 125.2, 124.1, 121.3,
120.9, 120.2, 119.9, 119.6, 117.8, 115.7, 66.0, 60.4,
15.2, 13.8; IR(KBr)ν: 1737, 1497, 1450, 752; APCI-MS
m/z (%): 602.3 ([M+1]+, 100). Anal. Calcd for
C41H33NS2: C 81.55, H 5.51, N 2.32, S 10.62; found:
C 81.51, H 5.50, N 2.34, S 10.65.
1-(9,9′-Spirobi[fluorene]-2-yl)-3,4-bis(2,5-dimethylfuran-3-yl)-
2,5-dihydro-1H-pyrrole (9a). The synthetic procedure was
similar to compound 8a, just employing compound 7
instead of compound 6. The yield of Compound 9a was
1
42% and its mp was 183–185°C; H NMR (400 MHz,
CDCl3) δ: 7.86–7.79 (m, 5H), 7.51 (d, 2H, J=8.0 Hz),
7.45–7.39 (m, 3H), 7.21–7.18 (m, 4H), 6.78(d, 1H, J=8
Hz), 6.05(s, 2H), 4.08(s, 4H), 2.51(s, 6H), 2.32(s, 6H);
13C NMR (100 MHz, CDCl3) δ:156.7, 148.8, 146.9,
142.6, 141.5, 140.0, 138.3, 135.7, 134.9, 132.0, 131.7,
129.8, 127.1, 126.7, 125.9, 124.3, 121.9, 121.0, 120.7,
119.6, 119.0, 118.0, 115.9, 68.0, 61.5, 14.6, 11.9; IR
(KBr)ν: 1726, 1485, 1455, 740;APCI-MS m/z (%):
602.3 ([M+1]+, 100). Anal. Calcd for C41H33NO2: C
86.13, H 5.82, N 2.45, O 5.60; found: C 86.10, H 5.83,
N 2.44.
2,2′-(9,9′-Spirobi[fluorene]-2-ylazanediyl)bis(1-(2,5-dimethyl
furan-3-yl)ethanone) (7).
The synthetic procedure was
similar to compound 6; K2CO3 was used instead of
Na2CO3, which rendered compound 7. The yield was 68%
1
and mp was 216–219°C; H NMR (400 MHz, CDCl3) δ:
7.92–7.89 (m, 3H), 7.82 (d, 2H, J=7.6 Hz), 7.57–7.42 (m,
3H), 7.26(s, 1H), 7.21 (t, 1H, J=7.6 Hz), 7.19 (t, 2H, J=
Journal of Heterocyclic Chemistry
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C. Li and H. Zeng
Vol 000
RESULTS AND DISCUSSION
Synthesis. In the preparation of compounds 8a and 9a,
the synthesis of compounds 6 and 7 were considered as key
steps (Scheme 1). The reactions were carried out by taking
a 12:5 mole ratio mixture of compounds 4 (or 5) and 3 in
the presence of some alkaline salt [25,26]. To investigate
the influence of the solvent, temperature, and alkaline
salt, synthesis of compound 9a was carried out under
different conditions (Table 1). The optimum conditions
were achieved by using K2CO3 in ethanol at 80°C
(Table 1, entry 4).
Photochromic behavior of compounds 8a and 9a.
Photochromic reactivity of 8a and 9a was examined in
hexane and acetonitrile through UV-Vis spectra.
Absorption spectral changes of 9a in hexane (Fig. 1a)
and acetonitrile (Fig. 1b) were shown. Upon irradiation at
365 nm, the color of the solution turned into red, both in
hexane and acetonitrile. The absorption bands of 9a at
305–320 nm gradually weakened, and at the same time,
new absorption bands appeared at 495 and 485 nm,
which correspond to 9b in hexane and acetonitrile,
respectively. These phenomena indicated that 9a
underwent photocyclization as illustrated in Scheme 2.
The colorations of 8a in hexane and acetonitrile
irradiation at 365 nm were shown in Figure 2. Similar to
9a, new absorption bands appeared at 463 (in hexane)
and 448 (in acetonitrile) nm after irradiation
corresponding to 8b.
Figure 1. Absorption changes of 9a in hexane (a) and acetonitrile (b)
Absorption maxima of 8a and 9a before and after
irritation in hexane and acetonitrile were shown in Table 2.
It is clear that the absorption bands of 8a–9a were shifted
to hypsochromic by the solvent polarity, which is usually
called “negative solvatochromism” [27]. This effect of
8a–9a revealed that by increasing solvent polarity, the
ground-state molecules of 8a–9a was better stabilized by
solvation than the molecules in the excited state [28].
Compared with 9a containing furan moieties, 8a with
thiophen moieties were more sensitive to solvent polarity.
Because solvatochromism came from the interaction
between the dipole moments of the solvent and the
molecule, the more sensitivity to solvent polarity of 8a
means larger differences in the dipole moment at the
ground states and the excited states [29].
upon irradiation with 365 nm light.
The photocycloreversion of 8b and 9b were also
investigated in hexane using UV-Vis spectroscopy. A
hexane solution of 8a (1×10À5 mol/L ) was irradiated at
365 nm for 1 min to reach photostationary state (PSS), and
then the resulting colored solutions could be decolorized
by irradiating the solution with 650 nm light (Xenon light
500 W, with filter), which lead to disappearance of
absorption bands in the visible region (Fig. 3). It was
found that the reactive time of coloration was very short
for both 8a and 9a in solution (1×10À5 mol/L). Coloration
was completed in about 5–20 s at 365 nm light (Table 2).
However, the reactive time of bleaching for 8b and 9b was
Table 1
Synthesis of 9a under different conditions.
Entry
Solvent
Temperature (oC)
Alkaline salt
Reaction time (h)
Yield (%)
1
2
3
4
5
Ethanol
Ethanol
Ethanol
Ethanol
THF
80
80
80
80
65
Na2CO3
NaHCO3
NaOH
K2CO3
K2CO3
4
7
1
4
6
56
34
0
68
47
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Synthesis, Photochromism, and Thermal Stability of Two New Diarylethenes
Based on Spirobifluorene
Scheme 2.
The reactive time of coloration for 8a and 9a in hexane
and acetonitrile was measured with 365 nm light and
results were presented in Figure 4. It was found that the
reactive time of coloration was very short for both 8a
and 9a in solution (1×10À5 mol/L). Especially for 9a in
hexane, it was about only 10 s to complete the
coloration. In addition, it was worth noting that the
solvent polarity had significant effect on photoreactions
of 8a and 9a. As depicted in Figure 4 and Table 2,
absorbance of 9a no longer changed with the irradiation
time after 10 s in the hexane, indicating that cyclization
has completed. In contrast, it took about 20 s to
complete the coloration of 9a in acetonitrile solution.
This shows that photoreaction rate of 9a in hexane was
faster than in acetonitrile. So, solvent polarity had
significant impacts on the rate of cyclization, because
hexane is a non-polar solvent and acetonitrile is a
typical large polar solvent. Similar result was obtained
for 8a that cyclization rate in non-polar hexane was
faster than in large polar acetonitrile.
In addition, the photochromic cyclization kinetics of 8a
and 9a in hexane and in acetonitrile were also presented in
Figure 4. It can be seen that the relationships between the
absorbance and exposal time have good linearity upon
irradiation with 365 nm UV light before reaching PSS,
suggesting that the cyclization processes of 8a and 9a
belong to zeroth order reaction [30].
Fluorescence of photochromism 8a and 9a in different sol-
vents.
The fluorescence changes of 8a and 9a were
Figure 2. Absorption changes of 8a in hexane (a) and acetonitrile (b)
examined in hexane and acetonitrile during
upon irradiation with 365 nm light.
photochromism(Figs. 5 and 6). The compounds 8a and
9a exhibited photoluminescence at 361 and 367 nm in
found to be much longer than coloration. As depicted in
Figure 3, the photocycloreversion of 8b and 9b required
30 min, although they could be shifted completely back to
ring-open isomers.
acetonitrile
solution,
while
in
hexane,
their
photoluminescence peaks were at 414 and 407 nm,
respectively. These photoluminescence were attributed to
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C. Li and H. Zeng
Vol 000
Table 2
Photochemical properties of 8a–9b and TGA date of 8a–10a.
In hexane
In acetonitrile
TGA
λ
ab1/nma
λ
ab2/nmb
Timec
λ
em/nm
λ
ab1/nm
λ
ab2/nm
Time
λ
em/nm
5%
50%
8a
9a
10a
325
320
322
463
495
520
15 s
10 s
10 s
414
407
410
318
315
321
448
485
517
20 s
20 s
20 s
361
367
408
265oC
275oC
216oC
336oC
357oC
263oC
“a” refers to the absorption peak before irradiation; “b” refers to the new absorption peak after irradiation; “c” is the time of cyclization with 365 nm light.
Figure 3. Absorption changes of mixtures of 9b (9a) (a) and 8b (8a) (b)
Figure 4. (a) Absorption changes of 9a at 485 nm (hexane) and 495 nm
in hexane upon irradiation with 650 nm light.
(acetonitrile) with the irradiation time; (b) absorption changes of 8a at 463
nm (hexane) and 448 nm (acetonitrile) with the irradiation time.
the emission of dithienylethene (DTE) (8a) and
ring-closed forms. As shown in Figure 5, there were 60%
fluorescence of 9a quenched at PSS in hexne while in
acetonitrile, suppression was only 44%. Therefore, the
photochromic conversion of 9a was higher in non-polar
hexane than that in highly polar acetonitrile. Similar
finding was found for 8a while evaluating its
fluorescence changes at PSS in different solvents.
In addition, there was strong solvent effect on
fluorescence emission of 8a and 9a. As shown in
Table 2, the emission wavelength of 8a and 9a blue-
difuranylthene (9a) units [20]. The fluorescence intensity
of 8a and 9a decreased along with the photochromism
from the open ring to the closed ring upon irradiation at
365 nm. As shown in Figures 1 and 2, the ring-closed
form has the second absorption band ranging 350–600
nm. The overlapping of emission and absorption bands
suggested that efficient energy transfer to the ring-closed
form resulting in quenching of the photoluminescence
[20]. In other words, the more fluorescence were
quenched, the more 8a and 9a were transformed into
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Month 2016
Synthesis, Photochromism, and Thermal Stability of Two New Diarylethenes
Based on Spirobifluorene
Figure 5. Fluorescence changes of 9a in hexane (a) and acetonitrile (b)
upon irradiation with 365 nm light.
Figure 6. Fluorescence changes of 8a in hexane (a) and acetonitrile (b)
upon irradiation with 365 nm light.
shifted by 53 and 40 nm, respectively, by varying solvent
polarity from hexane to acetontrile. In view of previous
theoretical and experimental results [31,32], the intra-
molecular charge transfer (ICT) characteristic is the
major mechanism responsible for the large solvent-
reliant emission properties of 8a and 9a. The ICT
compounds represent a donor–acceptor (D-A) system
[33]. In 8a and 9a, thiophen and furan as electron-rich
aryl [34], are favorable electron donor, and
dihydropyrrolo group acts as electron acceptor, thus
leading to D-A structures. Interestingly, 8a and 9a with
different donor units displayed different fluorescence
solvent effect. Compared with 9a, 8a had much larger
solvent sensitivity (Table 2), which indicated more
effective ICT processes. According to ICT mechanism,
thiophen is a better electron donor than furan, therefore
8a will have a higher dipolar moment than 9a upon
excitation. Thus, it can be concluded that ICT processes
in 8a are more effective.
Figure 7. The TGA curves of 8a–10a.
given in Table 2. The thermal stability was evaluated by 5%
and 50% weight loss at minimum temperature. TGA revealed
that 8a–10a were stable up to 265, 275, and 216°C,
respectively, in nitrogen atmosphere. After that, degradation
started followed by a one-stage decomposition pattern.
Thermal stability of 8a–10a and their ring-closed com-
pounds 8b–10b
The thermogravimetric analysis of 8a–10a.
The TGA
traces of 8a–10a were shown in Figure 7, and the data were
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C. Li and H. Zeng
Vol 000
As shown in Figure 7 and Table 2, 8a with
spirobifluorene group achieved higher thermal stability
than 10a. The significant gap of 50% weight loss
temperature (from 336 to 263) between 8a and 10a may
be ascribed to the spirobifluorene group. As far as
spirobifluorene is concerned, the bifluorene rings are
orthogonally arranged through a tetracoordinated carbon.
So introduction of this component into 8a enhanced the
rigidity and consequently improved the thermal stability
of this material [35–37].
However, it was worthwhile that there were significant
differences in their thermal stability while heating the
sample to 110°C. After 100 h, 95% of 9b was recovered
unchanged in contrast to only 42% for 10b (Fig. 8). In
addition, 76% of 8b was not transformed after heated for
100 h. So, the thermal stability sequence of 9b>8b>10b
was in full accord with the sequence of 9a>8a>10a.
CONCLUSION
For 8a and 9a with thiophen and furan respectively, the
thermal stability of 9a was a little higher than 8a, which
may be explained on the basis of their different
heterocyclic aromatics. It has been reported that the
thermal stability of the diarylethene-type photochromic
compounds can be improved by introducing aryl groups
having a low aromatic stabilization energy [7].Therefore,
Furan, which has a lower aromatic stabilization energy
than thiophene (for 8b) according to the modified neglect
of diatomic overlap calculation [7], made 9a more stable
than 8a.
We have designed and synthesized two new
photochromic spirobifluorene-diarylethenes with furan
(9a) or thiophen (8a) as heterocyclic aryl groups and
investigated their photochromism in different solvents.
The results showed that both molecules showed good
photochromic behaviors in hexane and acetonitrile, and
the photochromic reaction rate and conversion in hexane
were both higher than these in acetonitrile. The
fluorescence intensity of 8a and 9a decreased along with
the photochromism from open ring to closed ring under
irradiation, and there were large fluorescence emission
blue-shift in polar solvent. Furthermore, the thermal
stability of 8a and 9a and their closed-ring forms 8b and
9b were greatly improved because of spirobifluorene or
furan groups in the molecules. The 5% loss weight
temperature of 9a was 59°C higher than that of 10a
without being substituted by spirobifluorene and only 5%
of 9b decomposed after heating at 110°C, while
decomposition of 10b was 58%.
Thermal stability of 8b–10b. Thermal stability of 8b–
10b was estimated by heating a toluene solution of a
mixture of 8b (8a)–10b (10a) in the dark [38].
A toluene solution of 8a was irradiated for 5 min by 365
nm light. The absorbance of 8b was observed at 492 nm
(A0, in Fig. 8). The solution in a sealed tube was heated
at 110°C in the dark. After 5, 10, 25, 50, 75, and 100 h,
the absorbances at 492 nm (A, in Fig. 8) were observed.
The similar procedures using 9a were carried out and the
absorbances at 460 nm were observed. Thermal stability
of the ring-closed compounds 10b was also measured in
order to compare with 8b and 9b through the
absorbances at 530 nm.
Acknowledgments. Financial support from the National Natural
Science Foundation of China (Nos. 20671036and 21371060) is
gratefully acknowledged.
It was found that upon heating at 80°C for 100 h, 8b, 9b,
and 10b were not converted to the ring-opened form.
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