Substituent effect on the photochemical and photophysical properties of 4,4′-distyrylbiphenyl derivatives

Article abstract of DOI:10.1246/bcsj.20170259

Photochemical properties of 4,4′-distyrylbiphenyl (DSBP) derivatives have been studied. While the introduction of methoxy groups at meta-positions does not influence the photoisomerization of DSBP, the introduction of methyl substituents at the biphenyl moiety (MDSBP) changed the photoisomerization from one-way cis-to-trans photoisomerization for DSBP to cistrans mutual photoisomerization for MDSBP.

Full text of DOI:10.1246/bcsj.20170259

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with hydrophilic carboxylate groups at the termini exhibit dramatic
change in isomerization quantum yield and photostationary state of
the stilbenoid core9,10 to give highly selective trans-to-cis photo-
isomerization. Moreover, we have revealed that the dendritic effect
could induce the photoisomerization of DSBP moiety in aqueous
solution (Figure 2, w-pG1 and w-pG2).8 Carboxylate anion ter-
minated DSBP dendrimers (w-pG1 and w-pG2) undergo trans-to-
cis photoisomerization in aqueous solution which is not observed
in ester terminated lipophilic dendrimer in THF.8 Dendritic effect
on DSBP is probably due to the electronic effect of benzyloxy
substitution at the meta-position and/or steric effect caused by a
congested dendritic environment around the core in water. How-
ever, the mechanism of this effect is still under discussion.
Here, a DSBP derivative having methoxy groups at the meta-
positions of benzene rings of styryl moieties (compound 2) instead
of benzyl ether-type dendron has been synthesized to investigate
the photochemical and photophysical properties of the meta-
substituted core of previously reported dendrimers (Figure 2,
w-pG1 and w-pG2). In addition, we also investigated these prop-
erties of 2-methyl DSBP (compound 3) to evaluate the effect of
the π-electronic conjugation in the biphenyl moiety. These effects
are discussed on the basis of the comparison of photochemical
properties with 1.
Absorption and fluorescence spectra were measured on a
Shimadzu UV-1600 spectrophotometer and on a Hitachi F-7000
fluorimeter, respectively. The concentration for steady state fluo-
rescence measurements was adjusted so that the absorbance at
the excitation wavelength was lower than 0.1 for each sample. The
fluorescence quantum yields were determined by using anthracene
(ΦFl = 0.27 in ethanol) as a standard. A correction in the difference
in refractive index among the solvents was made for each sample.
Benzene, THF, chloroform and acetonitrile used as solvents
were spectroscopic grade and used as received. Sample solu-
tions were deoxygenated by bubbling with highly purified argon
(>99.999%).
Short Article
Substituent Effect on the
Photochemical and Photophysical
Properties of 4,4¤-Distyrylbiphenyl
Derivatives
Hiroya Sakurai and Tatsuo Arai*
Graduate School of Pure and Applied
Sciences, University of Tsukuba,
T. Arai
Tsukuba, Ibaraki 305-8571
E-mail: arai@chem.tsukuba.ac.jp
Received: August 16, 2017; Accepted: September 12, 2017;
Web Released: September 21, 2017
Abstract
Photochemical properties of 4,4¤-distyrylbiphenyl (DSBP)
derivatives have been studied. While the introduction of
methoxy groups at meta-positions does not influence the
photoisomerization of DSBP, the introduction of methyl
substituents at the biphenyl moiety (MDSBP) changed the
photoisomerization from one-way cis-to-trans photoisomeri-
zation for DSBP to cis-trans mutual photoisomerization for
MDSBP.
Stilbenoid compounds are widely investigated for their inter-
esting photophysical and photochemical behavior.1-3 The excited
singlet state of stilbene twists around the C=C double bond to
achieve the perpendicular excited singlet state which deactivates to
give the ground state of cis- and trans-isomers. Photoisomerization
and fluorescence properties of stilbenoid compound are dependent
on the molecular structure. For example, introduction of electron-
donating groups at the meta-position of the phenyl rings (Figure 1,
tetramethoxystilbene; TMST) prolongs the fluorescence lifetime
of stilbene and leads to the considerably high fluorescence quan-
tum yield4,5 for TMST. Phenyl or styryl substitution at the para-
positions of trans-stilbene extends electronic conjugation and leads
to the suppression of photoreactivity.6,7 We have already reported
that the trans,trans isomer of 4,4¤-distyrylbiphenyl (compound 1)
cannot twist around the C=C double bonds in the excited singlet
state due to a high energy barrier8 for rotation around the C=C
double bond to give the perpendicular excited singlet state
followed by deactivation to the ground state finally giving the
isomer.
The quantum yield of trans-cis isomerization of 3 was deter-
mined by comparison of the rate of initial absorbance change in the
optically matched solution at 308 nm with that of the trans-TMST
under the same irradiation conditions.
Absorption and Fluorescence Properties. Figure 3 shows the
absorption and fluorescence spectra of 1-3 in THF under Ar; their
spectroscopic parameters are summarized in Table 1. Compound
1 and 2 show typical absorption bands around 350 nm attributed
to π-π* transitions with high ¾ values of 7.3 © 104 and 7.7 © 104
We have been interested in the effect of dendron groups on
photoisomerization of stilbenoid compounds. For example, hydro-
philic dendrimers having hydrophobic benzyl ether-type dendron
O
O
O
O
stilbene
TMST
Figure 2. Structures
derivatives.
of
4,4¤-distyrylbiphenyl
(DSBP)
Figure 1. Structures of stilbene and TMST.
Bull. Chem. Soc. Jpn. 2017, 90, 1337–1339 | doi:10.1246/bcsj.20170259
© 2017 The Chemical Society of Japan | 1337

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M
¹1 cm¹1, respectively. Compound 3 having a methyl group at the
the trans,trans isomers of 1 and 2 are photostable. A decrease in
absorbance was observed in 3, indicating that photoisomerization
around the C=C double bond took place to give corresponding
cis isomer (Scheme 1). On irradiation of cis,cis isomer of 3 gave
cis,trans isomer as an initial product and the cis,trans isomer gave
trans,trans isomer as observed by HPLC (Figure S1). The occur-
rence of photoisomerization should be the consequence of the
distortion at the biphenyl moiety to increase the excited state
energy of trans isomer and decrease the barrier for isomerization
in the excited state. In other words, the introduction of torsion is
effective to induce trans-to-cis photoisomerization. In our previous
work, water-soluble denrimers, w-pG1 and w-pG2, also isomer-
ized from trans isomer to cis isomer in water upon photoirra-
2-position of biphenyl shows blue shift of 16 nm compared to 1
with a relatively low ¾ value of 4.90 © 104 M¹1 cm¹1. The marked
change in the absorption spectra observed in 3 compared to 1
should be caused by the non-planar conformation due to the
presence of the methyl substituent on the biphenyl moiety in the
ground state. Fluorescence spectra in THF were almost the same
between 1 and 2, but blue shift was observed for 3. Non-planar
conformations of 1 and 2 in the ground state give structureless
absorption spectra, but rapid relaxation to nearly planar conforma-
tion in the excited states lead to the fluorescence spectra with some
vibrational structure. The structureless fluorescence spectrum of 3
is caused by the conformation in the emission state being non-
planar at the biphenyl moiety due to the presence of the methyl
substituent. The 0-0 transition energies estimated from the inter-
section of normalized absorption and fluorescence spectra are 73.9,
73.5 and 76.9 kcal/mol for 1, 2 and 3, respectively. Photochemical
and photophysical properties of 2 did not depend on solvent
polarity (Table 1), indicating that the dendritic effect on DSBP
core8 is not caused by solvent effect.
1
2
Photoisomerization. Figure 4 shows the changes in absorp-
tion spectra of 1-3 on irradiation with 350-nm light under argon.
Almost no change in absorption spectra for 1 and 2 indicates that
w-pG28)
3
8
(a)
1
2
3
6
4
2
Figure 4. Absorption spectra observed before photoirradiation
(0 s) and after irradiation for 1200 s (1 and 2) or 6000 s (3).
0
250
300
350
400
450
500
(a) Cis-to-trans one-way photoisomerization of 1
Wavelength / nm
(b)
1
2
3
trans,trans-1
cis,trans-1
cis,cis-1
(b) Mutual photoisomerization of 3
350
400
450
500
550
600
Wavelength / nm
trans,trans-3
cis,trans-3
cis,cis-3
Figure 3. (a) Absorption and (b) fluorescence spectra of
1-3 in THF.
Scheme 1. Photoisomerization of compound (a) 1 and (b) 3.
Table 1. Absorption and fluorescence parameters and isomerization quantum yields
¾/
E0-0
kcal mol
/
¹1
Compound
Solvent
-abs/nm
-fl/nm
¦Ess/cm
ΦF
Φiso
¹1
¹1
104 M¹1 cm
1
THF
350
7.30
400, 422
3570
73.9
0.95
®
2
3
Acetonitrile
THF
Chloroform
Benzene
350
354
353
354
6.98
7.75
7.63
7.12
401, 422
402, 425
404, 427
403, 426
3630
3370
3580
3440
74.1
73.5
73.4
73.4
0.90
0.86
0.86
0.75
®
®
®
®
THF
334
4.90
390, 409
4300
76.9
0.87
0.0047
1338 | Bull. Chem. Soc. Jpn. 2017, 90, 1337–1339 | doi:10.1246/bcsj.20170259
© 2017 The Chemical Society of Japan

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(a) 1
(b) 3
Figure 5. Calculated relative energies in ground and singlet excited states of (a) 1 and (b) 3.
diation.8 The excited state properties studied here for 1-3 indicate
that the dendritic environment in water could induce torsion at the
core by hydrophobic interaction and the isomerization.
Quantum Yields. The fluorescence quantum yields in THF
are determined to be 0.95, 0.86 and 0.87 for 1-3, respectively
(Table 1). Despite the non-planar conformation of 3, the fluores-
cence quantum yield and rate constant are close to that of 1 and the
isomerization quantum yield is a relatively low value.
1 (73.9 kcal/mol) and 2 (73.5 kcal/mol). As discussed previously,
the substituent affects the excitation energy of planar excited state
more than that of the perpendicular excited state.7 In other words,
the planar excited state of 3 is higher than those of 1 and 2, but the
energy of perpendicular excited state is expected to be similar
among 1-3 and therefore, the energy barrier for rotation in the
excited singlet of 3 should become lower than those of 1 and 2
resulting in the opening of the isomerization from trans,trans
isomer to the perpendicular conformation in the excited state
finally giving cis,trans isomer and cis,cis isomer of 3.
We have investigated the electronic and the steric effect on
DSBP derivatives to reveal the potential energy surfaces. The
photophysical properties of 2 are similar to that of 1, indicating
that the electronic effect by introducing methoxy groups at meta-
positions is small. Methylation at the 2-position of biphenyl induce
trans-to-cis photoisomerization by the steric effect around biphen-
yl moiety. These results can be used to build photoresponsive
rigid, linear systems. On the basis of the obtained results, the
dendritic effect in water-soluble DSBP cored dendrimer would
come from steric effects due to a congested dendritic environment.
Potential Energy Surfaces of Photoisomerization.
DFT
calculations at RB3LYP 6-31G(d) level were performed on the
three configurational isomers of compounds 1 and 3. The calculated
relative energies in the ground state are shown in the potential
energy surface of 1 and 3 (Figure 5). The excited singlet energies
(E0-0) were estimated from the absorption spectra. Stilbene under-
goes mutual photoisomerization between cis and trans isomers
through perpendicular conformation in the excited singlet state by
direct irradiation. However, 1 and 2 underwent isomerization from
trans,trans isomers to neither cis,trans nor cis,cis isomer, which
may proceed through the potential energy surfaces in Figure 5a. In
this case, trans,trans isomers could not twist around the double
bond to achieve the perpendicular excited state (1t,p*) and the
1
deactivation should proceed solely at the singlet state t,t*. When
Supporting Information
we introduced the methyl group at the 2-position of biphenyl
moiety, trans,trans isomer underwent isomerization around the
C=C double bonds to give cis,trans isomer and cis,cis isomer.
Thus, one may propose the potential energy surfaces of isomer-
ization as shown in Figure 5b.
Synthetic procedures of 1-3 and HPLC chromatogram of 3
before and after photoirradiation. This material is available on
http://dx.doi.org/10.1246/bcsj.20170259.
These changes of isomerization behavior of 1-3 compare to
stilbene and the effect of the methy group on the biphenyl moiety
could be explained as follows. The conjugation of the whole
molecular structure induces the stabilization of both 1t,t* and 1c,t*,
but may not influence the perpendicular singlet (1t,p*) energy
and therefore, the energy difference between trans,trans and
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Bull. Chem. Soc. Jpn. 2017, 90, 1337–1339 | doi:10.1246/bcsj.20170259
© 2017 The Chemical Society of Japan | 1339