Substituent position effect on the optoelectronic properties of photochromic diarylethenes

Article abstract of DOI:10.1016/j.tetlet.2006.02.124

Photochromic diarylethenes bearing fluorine atoms at the ortho-, meta-, or para-position of both terminal phenyl rings have been synthesized. Effect of the substituent positions on their optoelectronic properties, including photochromism in solution, fluo

Full text of DOI:10.1016/j.tetlet.2006.02.124

Tetrahedron Letters 47 (2006) 3167–3171
Substituent position effect on the optoelectronic properties
of photochromic diarylethenes
Shouzhi Pu,* Tianshe Yang, Guizhen Li, Jingkun Xu and Bing Chen
Jiangxi Key Lab of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang 330013, PR China
Received 15 October 2005; revised 19 February 2006; accepted 21 February 2006
Abstract—Photochromic diarylethenes bearing fluorine atoms at the ortho-, meta-, or para-position of both terminal phenyl rings
have been synthesized. Effect of the substituent positions on their optoelectronic properties, including photochromism in solution,
fluorescence, and electrochemical properties were investigated for the first time. The results indicated that the position of the fluorine
substituent had remarkable impacts on molar absorption coefficient of the closed-ring forms of diarylethenes, quantum yields of the
cyclization and cycloreversion reactions, fluorescence intensity, as well as the oxidation potential.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Photochromic compounds, such as spirobenzopyrans,
azobenzenes, fulgides, and diarylethenes, have been
extensively investigated for their potential applications
in erasable optical memories, displays, and optical
switches.1 Among such compounds, diarylethenes with
derivatives bearing thiophene or benzothiophene ring
systems are regarded as the best candidates for applica-
tions because of their good thermal stability of both
forms, remarkable fatigue resistance, rapid response,
and high reactivity in the solid state.2–6 In particular,
diarylethenes with thiophene ring systems bearing termi-
nal phenyl groups have been of considerable interest,
because the hydrogen atoms attached to the terminal
phenyl groups can be substituted by many other elec-
tron-donating groups or electron-withdrawing groups,
which inevitably influence the optoelectronic properties
of corresponding diarylethenes.7
crease the cycloreversion quantum yield; while elec-
tron-donating substituents of the bis(2-thienyl)ethane
diarylethenes could be effective to increase the maxima
absorption of the open-ring forms and reduce the cycli-
zation quantum yield. Morimitsu et al.,9 Takami and
Irie10 revealed that bulky alkoxy substituents at 2-and
20-positions of the thiophene rings would strongly sup-
press the cycloreversion quantum yield and decrease
the thermal stability of the colored closed-ring isomers
at high temperature. Recently, Tanifuji et al.11 reported
the effect of the radical substituents on the photochro-
mic reactivity of bis(3-benzothiophene)perfluorocyclo-
pentene. In this letter, diarylethene derivatives bearing
imino nitroxide and nitronyl nitroxide have been synthe-
sized and these radical substituents reducing the quan-
tum yields of both cyclization and cycloreversion
reactions are presented.
To date, several publications concerning substituent
effect on the photochromic performance of diarylethene
have been reported. Pu et al.7 and Irie et al.8 reported
the effect of the substituents at para-positions of the ter-
minal phenyl groups on the photochromic property and
the photochemical reactivity. The results showed that
electron-donating substituents of the bis(3-thienyl)eth-
ane diarylethenes could be effective to increase the
absorption coefficient of the closed-ring forms and de-
To summarize, it can be easily concluded that these pub-
lications can be classified into two categories. One eluci-
dated that different substituents at the same position
(para-position) of the terminal phenyl rings affected
the photochromic and photochemical properties of
diarylethenes. The other mainly explained that different
substituents at the same position (2- or 3-position) of the
thiophene rings affected the photochromic performance
of diarylethenes. Yamamoto et al.12 had investigated
photochromism of diarylethenes bearing carboxyl
groups at the ortho-, meta-, and para-positions of both
terminal phenyl groups. But, they particularly empha-
sized the effect of the intermolecular hydrogen bonding
*
Corresponding author. Tel.: +86 791 3805183; fax: +86 791
3826894; e-mail: pushouzhi@tsinghua.org.cn
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.02.124

3168
S. Pu et al. / Tetrahedron Letters 47 (2006) 3167–3171
on the photochromic performance of these diarylethenes
in the single-crystalline phase. Up to now, there is no
report on the substituent position effect of the same
functional group on the optoelectronic properties of
diarylethenes.
0.8
0.6
0.4
0.2
0.0
1a
3a
2a
In this study, we have synthesized three diaryleth-
enes, 1,2-bis(2-methyl-5-(2-fluorophenyl)thiophen-3-yl)-
perfluorocyclopentene (ortho 1a), 1,2-bis(2-methyl-5-(3-
fluorophenyl)thiophen-3-yl)perfluorocyclopentene (meta
2a), and 1,2-bis(2-methyl-5-(4-fluorophenyl)thiophen-3-
yl)perfluorocyclopentene (para 3a), which have fluorine
groups at the ortho-, meta-, and para-positions of both
terminal phenyl groups. And we investigate the effect
of substituent position on the optoelectronic properties
of photochromic diarylethenes based on the absorption
spectra, fluorescence, and electrochemical properties. To
the best of our knowledge, this is the first report on the
effect of the same substituent at the different positions
(ortho-, meta-, and para-position) of the terminal phenyl
rings on the optoelectronic properties of photochromic
diarylethenes. Scheme 1 shows diarylethenes 1a–3a,
which are discussed in this letter. Diarylethenes 1a and
2a are new compounds, and compound 3a reported by
us in a previous paper.7
1b
2b
3b
300
400
500
600
700
800
Wavelength (nm)
Figure 1. Absorption spectra of diarylethenes 1a–3a in hexane solution
(C = 2.0 · 10ꢀ5 mol/L).
In hexane solution, the photochromic reactivity of
diarylethenes 1–3 was examined. Figure 1 shows their
absorption spectral changes upon photoirradiation.
Upon irradiation with 313 nm light, all of the colorless
solutions containing 1a–3a turned blue. The color
changes could be attributed to the formation of the
closed-ring forms. Upon irradiation with visible light
(k >500 nm), the colored solutions returned to their ori-
ginal colorless forms. Their absorption spectral proper-
ties are summarized in Table 1. The quantum yields of
diarylethenes 1–3 were measured in hexane using 1,2-
bis(2-methyl-5-phenyl-thiophen-3-yl)perfluorocyclopen-
tene as reference,15 and the results are also shown in
Table 1. From these data, we can see that the substituent
position effect on the molar absorption coefficient of
open-ring forms and the maxima wavelengths of both
open-ring forms and closed-ring forms are not signifi-
cant. However, the effect on the molar absorption coef-
ficient of closed-ring forms and the quantum yields are
more remarkable. Among compounds 1–3, the maxima
absorptions of the open-ring and closed-ring forms of
The synthetic route for diarylethene 1a, 2a, and 3a is
shown in Scheme 2. Suzuki coupling of three isomeric
bromobenzenes with thiophene boronic acid (4)13 gave
fluorophenylthiophene derivatives (5–7). They were lithi-
ated and then coupled with octafluorocyclopentene to
give fluorine-substituted diarylethene derivatives (1a–
3a). The structures of compounds 1a–3a14 were
1
confirmed by melting point, H NMR and 19F NMR
spectroscopy, mass spectrometry, and UV–vis spectro-
scopy.
F
F
F
F
F
F
F
F
F
F
F
F
UV
Vis
F
the meta-substituted derivative are the biggest (ko,max
=
S
S
S
S
F
F
290 nm, kc,max = 579 nm), but its quantum yields of
cyclization and cycloreversion reactions are the smallest
(Uo–c = 0.43, Uc–o = 0.012); while those of the para-
substituted derivative are the smallest and the biggest,
respectively. The values of the ortho-substituted deriva-
tive are in between those of the para- and meta-substi-
tuted derivatives. In addition, the molar absorption
F
1a: ortho-F
2a: meta-F
3a: para-F
1b: ortho-F
2b: meta-F
3b: para-F
Scheme 1. Photochromism of diarylethenes 1a–3a.
F
Br
Br
Br
5:ortho-F
6: meta-F
7: para-F
Table 1. Absorption spectral properties of diarylethenes 1–3 in hexane
at 2.0 · 10ꢀ5 mol/L
(HO)2B
S
Pd(PPh3)4
Na2CO3 aq.
S
Compound ko,max (nm)a
kc,max (nm)b
Uc
F
4
(e/L molꢀ1 cmꢀ1
)
(e/L molꢀ1 cmꢀ1
)
Uo–c Uc–o
F
F
F
286 (3.5 · 104)
577 (1.4 · 104)
579 (1.3 · 104)
570 (7.3 · 103)
0.54 0.015
0.43 0.012
0.58 0.025
F
1
2
3
BuLi,-78
C5F8
C
F
290 (3.2 · 104)
281 (3.4 · 104)
˚
F
F
1a:ortho-F
2a: meta-F
3a: para-F
a Absorption maxima of open-ring forms.
b Absorption maxima of closed-ring forms.
c Quantum yields of cyclization reaction (Uo–c) and cycloreversion
reaction (Uc–o), respectively.
S
S
F
Scheme 2. Synthetic route for the target compounds.

S. Pu et al. / Tetrahedron Letters 47 (2006) 3167–3171
3169
coefficient of closed-ring forms of the ortho-substituted
derivative is the biggest, while that of the para-substi-
tuted derivative is the smallest.
180
150
120
90
-3
1.0×10 mol/L
-4
1.0×10 mol/L
-5
5.0×10 mol/L
The fluorescence spectra of diarylethenes 1a–3a
(C = 2.0 · 10ꢀ5 mol/L) in hexane at room temperature
are illustrated in Figure 2. As shown in this figure, the
hexane solutions of diarylethenes 1a, 2a, and 3a showed
relatively strong fluorescence at 340, 348, and 348 nm
when excited at 290 nm. The fluorescence maxima of
the three compounds were observed at 340 and
348 nm, indicating that the substituent position effect
on the fluorescence peak was not significant. However,
the fluorescence intensities of diarylethenes 1a, 2a, and
3a changed dramatically along with the substituent posi-
tion. The values increased rapidly from ortho-substi-
tuted to para-substituted diarylethene derivative.
Among them, the relative fluorescence intensity of
ortho-substituted derivative is the weakest, and that of
para-substituted derivative is the strongest. The values
of the meta-substituted derivative are located in between
those of the ortho- and para-substituted derivatives.
Upon irradiation with 313 nm light, their fluorescence
intensity decreased along with the photochromism from
open-ring form to closed-ring form, suggesting that all
of the closed-ring forms of the three diarylethenes
showed no fluorescence or their fluorescence was very
weak.16 In addition, the optimal fluorescence spectra
of diarylethenes 1a, 2a, and 3a were at 340, 351, and
348 nm when excited at 289, 293, and 285 nm,
respectively.
-5
2.0×10 mol/L
60
30
0
300
350
400
450
500
550
Wavelength (nm)
Figure 3. Fluorescence spectra of diarylethene 1a in various concen-
trations in hexane at room temperature, monitored at 290 nm.
shift and the relative fluorescence intensity decreased
dramatically when the concentration increased. The
self-quench phenomenon may be resulted from the for-
mation of excimers and/or exciplexes in high concentra-
tion solution so that deactivation effect takes place
during the excited-state lifetime.17 Furthermore, the
concentration effect on the fluorescence spectra of
diarylethenes 2a and 3a were also studied, and the
results are summarized in Table 2. These results exhibi-
ted that all fluorescence of the three compounds are
remarkable concentration dependent. The fluorescence
peak showed a systematic red shift and the relative fluo-
rescence intensity decreased remarkably upon increasing
the concentration. The results also demonstrated that
molecular aggregation and fluorescence quench may
occur when the concentration increases.18
The concentration dependent fluorescence spectrum of
diarylethene 1a was measured in hexane at room
temperature, as shown in Figure 3. When the solutions
at 2.0 · 10ꢀ5, 5.0 · 10ꢀ5, and 1.0 · 10ꢀ4 mol/L were
excited at 290 nm, the maximum emission arose at
340, 347, and 354 nm, respectively. When the concentra-
tion increased to 1.0 · 10ꢀ3 mol/L, the hexane solution
showed no fluorescence. As shown in Figure 3, it can
be seen easily that the fluorescence spectra depended
heavily on the concentration. It gave a systematic red
The electrochemical properties of diarylethene are being
used for molecular switching and also can be potentially
applied to molecular-scale electronic switches. The oxi-
dative cyclization and cycloreversion of some diaryleth-
ene derivatives had been reported.19–21 In order to
investigate the substituent position effect on the electro-
chemical properties of diarylethenes, we performed elec-
trochemical examinations by linear sweep method under
the same experimental conditions using diarylethenes
1a, 2a, and 3a, respectively. Electrochemical examina-
tions were performed in a one-compartment cell by
using a Model 263 potentiostat–galvanostat (EG&G
Princeton Applied Research) under computer control
at room temperature. Platinum-electrodes (diameter
700
600
500
400
300
200
100
0
3a
2a
Table 2. The concentration effect on the fluorescence spectra of
diarylethenes 1a–3a at room temperature in hexane, monitored at
290 nm
1a
Concentration
(mol/L)
kem,max (Relative intensity)
1a
2a
3a
300
350
400
450
500
550
2.0 · 10ꢀ5
5.0 · 10ꢀ5
1.0 · 10ꢀ4
1.0 · 10ꢀ3
340 (173.18)
347 (97.49)
354 (28.08)
348 (425.50)
354 (208.70)
361 (8.39)
—
348 (677.50)
352 (231.90)
362 (16.78)
401 (7.69)
Wavelength (nm)
Figure 2. Fluorescence spectra of diarylethenes 1a–3a in hexane
solution (C = 2.0 · 10ꢀ5 mol/L) at room temperature, excited at
290 nm.
a
—
a No fluorescence spectrum peak.

3170
S. Pu et al. / Tetrahedron Letters 47 (2006) 3167–3171
the optoelectronic properties of diarylethenes was inves-
900
750
600
450
300
150
0
3b
3a
2b
1a
1b
2a
tigated for the first time. The results showed that the
optoelectronic performances of diarylethenes, including
molar absorption coefficient, photochromic quantum
yields, fluorescence intensity, and electrochemical prop-
erty, etc., were significantly dependent on the effect of
the substituent position. The oxidation potential differ-
ences of the open-ring and closed-ring forms of the
ortho-, meta-, and para-substituted diarylethene deriva-
tives were 0.03, 0.07, and 0.18 V, respectively. The rea-
son may be attributed to the different electro-
withdrawing effects when the fluorine atom was substi-
tuted on the different positions of the terminal phenyl
groups. The para-position of the terminal phenyl groups
of diarylethene on these performances is the most signif-
icant. The present results are useful for the designation
of efficient photoactive and excellent characteristic
diarylethene compounds.
1a
1b
2a
2b
3a
3b
0.3
0.6
0.9
1.2
1.5
1.8
2.1
Potential / V vs. Pt
Figure 4. The anodic polarization curves of diarylethene 1–3.
0.5 mm) served as working electrode and counter elec-
trode. Platinum wire served as a quasi-reference elec-
trode. It was calibrated using the ferrocene (Fc/Fc+)
Acknowledgments
redox couple, which has a formal potential E1/2
=
This work was supported by the Projects of National
Natural Science Foundation of China (Granted No.
20564001), the Natural Science Foundation of Jiangxi,
China (Granted No. 050017) and the Science Funds of
the Education Office of Jiangxi, China (Granted No.
[2005] 140).
+0.35 V versus platinum wire. The typical electrolyte
was acetonitrile (5 mL) containing 0.15 mol/L LiClO4
and 4.0 · 10ꢀ3 mol/L dithienylethene. All solutions were
deaerated by a dry argon stream and maintained at a
slight argon overpressure during electrochemical experi-
ments. Figure 4 shows the anodic polarization curves of
1a, 1b, 2a, 2b, 3a, and 3b. We can see from Figure 4 that
ortho- and para-fluorine substituted diarylethenes (com-
pounds 1 and 2) showed similar electrochemical behav-
ior both in closed-ring and open-ring forms. The
oxidation potential onsets of 1a, 1b, 2a, and 2b initiated
at 1.14, 1.11, 1.22, and 1.15 V, respectively. In sharp
contrast to 1 and 2, the oxidation potentials of 3a and
3b decreased significantly to 0.97 and 0.79 V, respec-
tively. Moreover, the great difference of the open-ring
form and closed-ring form of compound 3 was observed
up to 0.18 V. Furthermore, the anodic current densities
of 3a and 3b were much higher than that of 1a, 1b and
2a, 2b at given applied potentials. On the other hand,
similar phenomenon was observed when the oxidation
potentials of 1a, 2a, and 3a are all higher that those of
1b, 2b, and 3b.
References and notes
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7.12–7.19 (m, 6H), 7.44 (s, 2H), 7.54–7.58 (m, 2H); 19F
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that substituent position of fluorine atom at phenyl ring
had significant effect on the optoelectronic properties of
diarylethenes. The main reason for these may be as-
cribed to the electro-withdrawing nature of fluorine
atom at para-position of the terminal phenyl groups.
For meta-position substituent fluorine atom of diaryl-
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diarylethene 1, the electro-withdrawing effect, together
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In conclusion, three diarylethenes bearing fluorine
atoms at the ortho-, meta-, and para-positions of both
terminal phenyl groups were synthesized. The effect of
different substitution positions of fluorine atoms on

S. Pu et al. / Tetrahedron Letters 47 (2006) 3167–3171
3171
compound 3a: mp = 69–71 ꢁC; 1H NMR (400 MHz,
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7.21 (s, 2H), 7.49–7.52 (t, 4H, J = 6.8 Hz); 19F NMR
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(M) 556; compound 1b: 1H NMR (400 MHz, CDCl3) d
2.18 (s, 6H), 6.89 (s, 2H), 7.20–7.22 (d, 4H, J = 8.0 Hz),
7.37–7.40 (t, 2H, J = 6.4 Hz), 7.45–7.49 (t, 2H,
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(400 MHz, CDCl3) d 2.23 (s, 6H), 6.69 (s, 2H), 7.13–7.17
(t, 2H, J = 8.0 Hz), 7.28 (s, 2H), 7.39–7.43 (m, 4H); MS
m/z (M) 556; compound 3b: 1H NMR (400 MHz, CDCl3)
d 2.17 (s, 6H), 6.61 (s, 2H), 7.10–7.14 (t, 4H, J = 7.2 Hz),
7.53–7.59 (t, 4H, J = 6.8 Hz); MS m/z (M) 556.
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