Effect of methoxy substituents on the excited state properties of stilbene

Article abstract of DOI:10.1246/bcsj.81.1500

Excited state properties of stilbenes having methoxy substituents at the ortho position of the phenyl ring have been studied and the remarkable methoxy substituent effect to suppress the fluorescence quantum yield depending on the number of methoxy substi

Full text of DOI:10.1246/bcsj.81.1500

1500 Bull. Chem. Soc. Jpn. Vol. 81, No. 11, 1500–1504 (2008)
Ó 2008 The Chemical Society of Japan
Effect of Methoxy Substituents on the Excited State
Properties of Stilbene
ꢀ1
Yoshihiro Shinohara2 and Tatsuo Arai
1Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8571
2Research Facility Center for Science and Technology, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8571
Received April 11, 2008; E-mail: arai@chem.tsukuba.ac.jp
Excited state properties of stilbenes having methoxy substituents at the ortho position of the phenyl ring have been
studied and the remarkable methoxy substituent effect to suppress the fluorescence quantum yield depending on the
number of methoxy substituent was observed. The fluorescence quantum yield of the trans isomer decreased with
increasing number of methoxy substituents and 0.54 for trans-1, 0.13 for trans-2, and 0.011 for trans-3 in benzene.
Many studies have been reported concerning the photo-
chemical isomerization and fluorescence properties of stilbene
and its analogues.1–8 The excited singlet state of stilbene un-
dergoes isomerization around the C=C double bond to achieve
the perpendicular excited singlet state (1pꢀ) which deactivates
to give the perpendicular ground state (1p) finally giving the
ground state cis (1c) and trans isomers (1t). In trans-stilbene
the main deactivation process from the excited singlet state
(96%) is the above-mentioned isomerization around the
C=C double bond in the excited singlet state to give 1pꢀ
the excited state of symmetrically substituted stilbene was
strongly influenced by the solvent and consequently the photo-
physical characteristics were investigated.11
Since no systematic study has been reported on the effect of
electron-donating substituents on the photochemical and pho-
tophysical properties of stilbene, it is worth studying the effect
of methoxy substituents at different positions of the phenyl
ring. Thus, compounds with methoxy substituents at the ortho
position (Figure 1) were prepared and their excited state prop-
erties were investigated. The remarkable effect of methoxy
substituents to decrease the quantum yield of fluorescence
emission with increasing the number at the ortho position of
the phenyl ring was observed.
1
followed by isomerization to c with a quantum yield of ca.
0.5.1,2 In addition to deactivation by isomerization in the excit-
ed singlet state of trans-stilbene, nearly 4% of the excited
singlet state deactivates by fluorescence emission, but no inter-
system crossing to the triplet excited state has been observed.
Singlet cis-stilbene undergoes either isomerization around the
double bond to give perpendicular singlet excited state (1pꢀ)
with 70% efficiency and cyclization to give dihydrophenan-
threne type compounds with 30% efficiency accompanied by
very small fluorescence emission in the range of 10ꢁ2%.3
The introduction of electron-donating or electron-withdrawing
groups affects the properties of the singlet excited state. For
example, substitution of a nitro group at the para position of
a phenyl ring accelerates the intersystem crossing to the triplet
state and therefore, both singlet and triplet excited states are
responsible for the isomerization.7,8 On the other hand, elec-
tron-donating group such as amino groups at the ortho or meta
position of the phenyl ring increase the lifetime of the excited
singlet state of stilbene to give 3.7 and 7.5 ns for o- and m-
aminostilbene9,10 respectively, compared to the parent com-
pounds stilbene (60 ps) and p-aminostilbene (100 ps).
Results
The absorption spectra of methoxy-substituted stilbene in
benzene are summarized in Figure 2. The absorption maxi-
mum (ꢀmax) is observed at 330, 324, and 315 nm for trans-
2,20-dimethoxy- (1), trans-2,20,6-trimethoxy- (2), and trans-
2,20,6,60-tetramethoxystilbene (3), respectively. The maximum
wavelength of the absorption spectra shifted to shorter wave-
length with increasing number of methoxy substituents and
the absorption edge tends to shift to shorter wavelength with
increasing number of methoxy substituent. The maximum
wavelength of fluorescence emission (ꢀmax(f)) also shifted
to shorter wavelength with increasing number of methoxy
substituent as shown in Figure 3. The ꢀmax(f) value is 372,
366, and 361 nm for trans-1, trans-2, and trans-3, respectively.
The Stokes shift between the maximum wavelength of
absorption and fluorescence spectra was calculated to be ca.
3500 cmꢁ1 for trans-1, -2, and -3. The maximum wavelength
of the fluorescence excitation spectra seems to be similar
among these compounds. The singlet excitation energy (ES)
was calculated from the crossing point of the fluorescence
and absorption spectra to be 330, 332, and 336 kJ molꢁ1 for
trans-1, -2, and -3, respectively. These values are 8–17 kJ
molꢁ1 lower that of trans-4 (347 kJ molꢁ1),14 but are 33–38
kJ molꢁ1 lower than that of trans-stilbene (370 kJ molꢁ1).1d,3
The fluorescence quantum yield (ꢁf) is determined to be
Thus, the introduction of electron-donating groups to the
phenyl ring of stilbene seems to increase the fluorescence
quantum yield and fluorescence lifetime.11–13 In our previous
experiments, introduction of a methoxy substituent at the 4
meta position of the phenyl ring also increased the excited
state lifetime of stilbene to give 3 ns in hexane.12 The lifetime
increased with increasing polarity of solvent to give 10 ns in
tetrahydrofuran (THF) and 17 ns in acetonitrile. Thus, even
Published on the web November 12, 2008; doi:10.1246/bcsj.81.1500

Y. Shinohara et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008) 1501
trans-stilbene
MeO
MeO
MeO
OMe
MeO
MeO
OMe
MeO
OMe
trans-1
trans-2
trans-3
OMe
OMe
OMe
MeO
MeO
MeO
OMe
OMe MeO
cis-1
cis-2
Me
cis-3
MeO
Me
OMe
Me
Me
Me
Me
MeO
Me
Me
OMe
Me
trans-4
trans-5
Figure 1. Structure of stilbenes.
trans-6
0.54, 0.13, and 0.011 for trans-1, trans-2, and trans-3, respec-
tively. The fluorescence lifetime of trans-1 was observed to
be 0.5 ns in THF solution at room temperature. However, we
could not determine the fluorescence lifetime of trans-2 and
-3 due to the detection limit of our instrument (ca. 0.4 ns) in-
dicating that the singlet lifetime of trans-2 and -3 is consider-
ably shorter than 0.4 ns.
The absorption maximum of the cis isomer shifted to shorter
wavelength 309, 297, and 291 nm in the order of cis-1, -2, and
-3 (Figure 2). The decrease in wavelength indicates that the in-
crease in number of methoxy substituents decreases the wave-
length of absorption maximum probably due to the increase of
the steric effect to inhibit planarity and conjugation. If we
compare the absorption spectra of cis- and trans-3, the absorp-
tion maximum was shifted to shorter wavelength by ca. 25 nm
for cis-3 compared to trans-3. Different from the trans isomers,
we could not observe fluorescence spectra for all cis isomers of
1–3 in THF at room temperature.
trans-3,15 where the dihedral angle between the benzene ring
and C=C double bond is determined to be 16ꢂ.
Figure 5 shows the time dependence of the absorption spec-
tra upon irradiation starting from trans-3. The photostationary
state isomer composition ([t]/[c])pss was determined in ben-
zene to be 35.1/64.9, 13.9/86.1, and 17.3/82.7 for trans-1
(330 nm excitation), trans-2 (324 nm excitation), and trans-3
(316 nm excitation), respectively. The ([t]/[c])pss value of 3
slightly depends on the solvent polarity to give 14.2/85.8 in
THF and 13.6/86.4 in acetonitrile. The ([t]/[c])pss is correlated
to the ratio of the extinction coefficient of cis and trans isomers
and the quantum yield of isomerization from trans-to-cis (ꢁt-c)
and cis-to-trans (ꢁc-t).
([t]/[c])pss ¼ ð"c="tÞðꢁc-t=ꢁt-cÞ
The molar extinction coefficients of cis- and trans-1 at
330 nm are determined to be 5900 and 18600 Mꢁ1 cmꢁ1
ð1Þ
,
respectively. The values of cis- and trans-2 are 5200 and
21800 Mꢁ1 cmꢁ1, respectively and those of cis- and trans-3
Figure 4 shows the data for crystallographic analysis of

1502 Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008)
Photochemistry of Ortho Substituted Stilbenes
Figure 3. Fluorescene spectra of trans-1, -2, and -3 in benzene.
Figure 2. (a) Absorption spectra of trans-1, -2, and -3 in
benzene. (b) Absorption spectra of cis-1, -2, and -3 in ben-
zene.
Figure 5. Spectral change on photoirradiation of trans-3
with photoisomerization.
Figure 4. X-ray crystal structure of trans-3.
are 7600 and 26000 Mꢁ1 cmꢁ1, respectively. The quantum
yields of trans-to-cis isomerization (ꢁt-c) were determined to
be 0.20, 0.40, and 0.45, respectively for 1, 2, and 3. Thus,
one can calculate the quantum yields of cis-to-trans isomeriza-
tion (ꢁc-t) to be 0.34, 0.28, and 0.32, for 1, 2, and 3, respec-
tively. The above observed and estimated values are summa-
rized in Table 1.
cence emission is observed to be 0.4.12 The 50 fold decrease of
the fluorescence emission in trans-3 compared to trans-1
might be caused by congestion of the molecular structure of
the tetramethoxystilbene and/or electronic effects of methoxy
substituents to increase the deactivation from the excited
singlet state. We should discuss these possibilities.
It has been reported that the methyl substituent at the ortho
position accelerates the isomerization around the double bond
to give a quantum yield of isomerization as high as 0.47 and
0.48 and a quantum yield of fluorescence emission as small
as 0.003 and <0:001 for trans-2,4,6-trimethylstilbene (5) and
trans-2,20,4,40,6,60-hexamethylstilbene (6), respectively.5 In
this case, even the trans isomers of 5 and 6, the single bond
connecting the phenyl ring and the C=C double bond takes
similar arrangement to the cis-stilbene due to the steric effect
of the methyl group at the ortho position, which appears in the
similarity of the absorption spectra of trans-5 and trans-6 to
that of cis-stilbene.5
Discussion
The absorption spectra and fluorescence spectra of trans-1,
-2, and -3 appeared at longer wavelength than trans-stilbene.
These results indicate that the methoxy substituent lowers
the singlet excitation energy by electronic effect.
The fluorescence quantum yield of the trans isomer decreas-
ed with increasing number of methoxy substituents with values
of 0.54 for trans-1, 0.13 for trans-2, and 0.011 for trans-3 in
benzene (Table 1). One should mention this strong substituent
effect on the fluorescence quantum yield of stilbene deriva-
tives. In the case of methoxy substitution at the meta position
(3,30,5,50-tetramethoxystilbene, 4), the excited singlet lifetime
increased to 10 ns in THF, where the quantum yield of fluores-
The molecular congestion is reflected in the absorption
spectra as observed in 5 and 6. Although the spectral profile
of the absorption spectra is similar among trans-1, -2, and -3,

Y. Shinohara et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008) 1503
Table 1. Absorption Fluorescence, and Isomerization Properties of Stilbene and Its Derivarives 1–6 in Benzene
ꢀ
max/nm
Es
Compound
ꢁf
ꢁt-c
ꢁc-t
([t]/[c])pps
("=104 Mꢁ1 cmꢁ1
)
/kJ molꢁ1
trans-Stilbenea)
trans-1
295
330
315
345
(1.48)
340
(1.84)
330
(2.31)
310
285
330
360
0.035
0.54
0.52
0.20
0.35
0.34d)
370
330
35.1/64.9e)
13.9/86.1f)
17.3/82.7g)
(1.86)
325
(2.18)
315
(2.60)
(0.62)
350
(0.95)
345
(1.18)
trans-2
trans-3
0.13
0.40
0.45
0.28d)
0.32d)
332
336
347
0.011
trans-4b)
trans-5c)
trans-6c)
0.24
0.003
<0:001
0.31
0.47
0.48
0.39
0.40
260
a) In pentane, Refs. 1–3. b) Ref. 12. c) In methylcyclohexane:isohexane = 2:1, Ref. 5. d) Estimated from ꢁt-c and
([t]/[c])pps using eq 1. e) Irradiated at 330 nm. f) Irradiated at 325 nm. g) Irradiated at 315 nm.
the absorption maximum and the absorption edge shifts to a
shorter wavelength in the order of trans-1, -2, and -3. Even
the absorption spectra of trans-3 is observed in a considerably
longer wavelength region than that in trans-stilbene indicating
the importance of electronic effects of methoxy substituents on
the absorption and fluorescence profiles to give a bathochromic
shift. The inverse correlation of the absorption spectra with the
number of methoxy substituent may be due to the compromise
of the electronic and the steric effects by increasing the
methoxy substituent at the ortho position. Thus, the presence
of steric congestion induces the breaking of the conjugation
of the whole molecular structure to some extent in trans-3.
Actually, slight deviation from the planar conformation
of trans-3 is supported by the crystallographic analysis
(Figure 4),15 where the structure has dihedral angles between
the phenyl and the double bond planes of 16ꢂ.
1 appeared at longer wavelength than trans-stilbene. However,
the increase of the methoxy substituent at the ortho position
shifted the absorption spectra to the shorter wavelength region
as observed in trans-2 and trans-3. These results indicate that
the introduction of the methoxy group resulted in bathochro-
mic shift as observed in trans-1, but further introduction of a
methoxy substituent at the ortho position might be a compro-
mise of the bathochromic effect of the methoxy substituent
and the steric effect. Thus, finally, in trans-3, the steric effect
prevails over the bathochromic effect by increasing the twist
angle between the benzene ring and the C=C double bond
to give twist angle of 16ꢂ.
In summary, stilbene derivatives with methoxy substituents
at three and four ortho positions were successfully prepared
and for the first time it is reported that methoxy substituents
highly decrease the fluorescence quantum yield even though
the molecular structure is assumed to be almost planar and
does not seem to take a congested structure. In addition,
it is notable in the present findings that the difference in
the effect of ortho methoxy substituents from those at the meta
position where the fluorescence quantum yield and fluores-
cence lifetime increased with the introduction of methoxy
substituents. These remarkable findings of the effect of
methoxy substituents at the ortho position will contribute the
understanding of the photochemical properties of substituted
stilbenes as a most fundamental compound for photochemical
reaction.
As discussed above the decrease in the quantum yield of
fluorescence emission in trans-3 should be the consequence
of the increase of the rate constant of deactivation from the
excited state by way of trans–cis isomerization around the
C=C double bond. Actually, the quantum yield of trans-to-
cis isomerization of 3 is determined to be 0:45 ꢃ 0:05, indicat-
ing that the efficiency of twisting around the double bond in
the excited state is 0:9 ꢃ 0:1 (2 ꢄ ð0:45 ꢃ 0:05Þ). The quantum
yield of trans-to-cis isomerization is 0.20 and 0.40, respective-
ly for 1 and 2. In addition, the singlet excitation energy
increased in the order of trans-1 (330 kJ molꢁ1), trans-2
(322 kJ molꢁ1), and trans-3 (336 kJ molꢁ1). These results
are in accordance with the finding that trans-3 with the highest
excitation energy gave quite low quantum yield of fluores-
cence emission compared to trans-1 and -2 and considerably
high quantum yield of trans-to-cis isomerization. One could
imagine the change of the molecular structure in solution,
where the single bond between the aromatic ring and C=C
double bond has some twist angle and the ꢂ conjugation of
the molecule seems to be diminished in trans-3. Anyhow,
the small increase of the singlet energy in trans-3 may de-
crease the activation barrier of rotation around the double bond
Experimental
trans-3. The methoxy-substituted trans-stilbenes were pre-
pared by Wittig reaction.16 Their structures were identified by
NMR spectroscopy and elemental analysis.
Crystallographic data have been deposited with Cambridge
Crystallographic Data Centre: Deposition number CCDC
683164 for compound trans-3. Copies of the data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/consts/retrieving.
html (or from the Cambridge Crystallographic Data Centre, 12,
Union Road, Cambridge, CB2 1EZ, UK; Fax: +44 1223
336033; e-mail: deposit@ccdc.cam.ac.uk).
1 ꢀ
1 ꢀ
from t to p and increase the deactivation rate constant by
way of isomerization. The introduction of a methoxy substitu-
ent at the ortho position could cause the bathochromic shift
in absorption spectra as observed in the meta substituted
compound trans-4. Actually, the absorption spectra of trans-
To a suspension of NBS (1.4 g, 7.6 mmol) and AIBN (12.5 mg,
0.08 mmol) in carbon tetrachloride (10 mL) was added a solution
of 2,6-dimethoxytoluene (1 g, 6.6 mmol) in CCl4 (3 mL) at room
temperature. The reaction mixture was stirred for 24 h at rt and

1504 Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008)
Photochemistry of Ortho Substituted Stilbenes
heated under reflux for 24 h. Then the reaction mixture was
extracted with CH2Cl2. The organic layer was dried over anhy-
drous Na2SO4, filtered and concentrated in vacuo. Purification
by column chromatography (silica gel, CH2Cl2:hexane = 1:1)
gave 2,6-dimethoxybenzyl bromide. A solution of benzyl bromide
(1 equiv) and triphenylphosphine (1 equiv) in benzene (10 mL)
was heated at reflux. After stirring for 1 day, the mixture was
concentrated and filtered to get phosphorus compound. To a sus-
pension of 60% NaH (in oil) in THF (20 mL) was added phospho-
rus compound (493.4 mg, 1.00 mmol) at 0 ꢂC followed by stirring
for 1 h at room temperature. To the solution was added 2,6-di-
methoxybenzaldehyde (166.2 mg, 1.00 mmol) in THF (10 mL).
After stirring for 1 day at room temperature, the mixture was
poured into water and extracted with CH2Cl2. The organic layer
was dried over anhydrous Na2SO4, filtered and evaporated. The
crude product (512.1 mg) was purified by recrystallization from
hexane and CH2Cl2 to obtain a solid trans isomer (140.6 mg).
Mp: 142–143 ꢂC; 1H NMR (CDCl3, 500 MHz, TMS): ꢃ 7.73
(2H, s), 7.12 (2H, t, J ¼ 8:35 Hz), 6.57 (4H, d, J ¼ 8:35 Hz),
3.87 (12H, s). 13C NMR (CDCl3, 125 MHz): ꢃ 158.4, 127.4,
124.0, 116.6, 104.1, 55.9. Anal. Found: C, 71.68; H, 6.57%. Calcd
for C18H20O4: C, 71.98; H, 6.71%.
This work was supported by a Grant-in-Aid for Science
Research in a Priority Area ‘‘New Frontiers in Photochromism
(No. 471)’’ from the Ministry of Education, Culture, Sports,
Science and Technology (MEXT), Japan.
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trans-2: Mp: 78–79 ꢂC; 1H NMR (CDCl3, 500 MHz, TMS): ꢃ
7.87 (1H, d, J ¼ 16:8 Hz), 7.64 (1H, d, J ¼ 7:64 Hz), 7.39 (1H, d,
J ¼ 16:8 Hz), 7.19 (1H, t, J ¼ 7:75 Hz), 7.13 (1H, t, J ¼ 8:35 Hz),
6.95 (1H, t, J ¼ 7:29 Hz), 6.86 (1H, d, J ¼ 8:20 Hz), 6.56 (2H, d,
J ¼ 8:35 Hz), 3.86 (6H, s), 3.85 (3H, s). 13C NMR (CDCl3,
125 MHz): ꢃ 158.6, 156.8, 128.6, 128.0, 127.8, 127.2, 126.3,
120.7, 120.4, 115.4, 110.8, 104.0, 55.8, 55.6. Anal. Found: C,
75.56; H, 6.72%. Calcd for C17H18O3: C, 75.53; H, 6.71%.
cis-1. The methoxy-substituted cis-stilbenes were prepared by
irradiation with 365-nm light each trans-stilbene. The product was
purified by column chromatography and recrystallization from
CH2Cl2. Mp: 82–82 ꢂC; 1H NMR (CDCl3, 500 MHz, TMS): ꢃ
7.19–7.10 (4H, m), 6.86 (2H, dd, J ¼ 0:82, 8.24 Hz), 6.69 (2H,
dt, J ¼ 0:99, 7.58 Hz), 6.76 (2H, s), 3.83 (6H, s). 13C NMR
(CDCl3, 125 MHz): ꢃ 156.8, 129.8, 128.1, 126.1, 125.4, 119.8,
110.3, 55.3.
cis-2: Mp: 66–67 ꢂC; 1H NMR (CDCl3, 500 MHz, TMS): ꢃ
7.20–7.00 (2H, m), 6.95–6.90 (2H, m), 6.84 (1H, dd, J ¼ 0:81,
8.10 Hz), 6.61 (1H, t, J ¼ 7:56 Hz), 6.52 (1H, d, J ¼ 12:29 Hz),
6.46 (2H, d, J ¼ 8:37 Hz), 3.84 (3H, s), 3.55 (6H, s). 13C NMR
(CDCl3, 125 MHz): ꢃ 157.3, 156.1, 128.2, 128.0, 127.7, 127.2,
120.9, 119.5, 115.1, 109.8, 103.5, 55.4, 55.3.
cis-3: Mp: 134–135 ꢂC; 1H NMR (CDCl3, 500 MHz, TMS): ꢃ
7.07 (2H, t, J ¼ 8:30 Hz), 6.71 (2H, s), 6.42 (4H, d, J ¼ 8:30 Hz),
3.52 (12H, s). 13C NMR (CDCl3, 125 MHz): ꢃ 157.4, 127.6, 123.8,
117.7, 103.3, 55.2.
¨