Short Articles
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Dendritic Effect on the
Photoisomerization of
4,4¤-Distyrylbiphenyl
in Aqueous Solution
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R = OMe ; pG1
OH ; w-pG1
pG0
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Hiroya Sakurai and Tatsuo Arai*
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Graduate School of Pure and Applied
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Sciences and Tsukuba Research Center
for Interdisciplinary Materials Science (TIMS),
University of Tsukuba, Tsukuba, Ibaraki 305-8571
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R = OMe ; pG2
OH ; w-pG2
Figure 1. Structures of new dendrimers pGn and w-pGn.
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T. Arai
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E-mail: arai@chem.tsukuba.ac.jp
Received: April 1, 2016; Accepted: May 2, 2016;
Web Released: May 19, 2016
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4
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pG0
pG1
pG2
w-pG1
w-pG2
Abstract
Newly synthesized 4,4¤-distyrylbiphenyl-cored water-
soluble dendrimers underwent trans-to-cis photoisomeriza-
tion, while the core 4,4¤-distyrylbiphenyl underwent cis-to-
trans one-way photoisomerization.
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Wavelength / nm
Introduction
Figure 2. Absorption spectra of pGn of THF and w-pGn in
Dendrimers have a well-defined branched structure1 and the
surrounding dendron groups can give a special microenviron-
ment around the core which can be used for a specific reaction
field to control photoreaction.2-6 The fundamental properties
inside a dendritic structure have been studied using an aromatic
olefin molecule. A water-soluble diphenylbutadiene dendrimer7
which has 1,4-diphenylbuta-1,3-diene unit as a core and car-
boxy groups at the terminal is a typical photoresponsive den-
dritic molecule. In aqueous solution, the dendrimer dissolves
like a unimolecular micelle and the dendron groups shrink due
to the hydrophobic interactions of the dendrimer interior, which
induces a twisted or distorted structure of the core around C=C
double bond or a different π-π dihedral angle. As a con-
sequence, the photoisomerization quantum yield of the di-
2 mM KOH (aq) at 298 K under Ar.
aqueous solution of w-pGn exhibit the absorption band
(S0 ¼ S1) of the 4,4¤-distyrylbiphenyl unit located in the 300-
400 nm region (Figure 2). Absorption edges of w-pGn were
slightly red shifted and molar extinction coefficients of w-pGn
were slightly smaller compared to pGn. The results indicate
the formation of aggregates and an interaction between two
cores in w-pGn. Aggregates were confirmed by dynamic light
scattering (DLS) measurement of w-pGn in aqueous solution
(Figure S1).
The photoisomerization behavior of pG0, w-pG1, and
w-pG2 was investigated by monitoring the changes of their
absorption spectra upon irradiation with 327 nm light for pG0
and 350 nm light for w-pGn. The results are displayed in
Figure 3. Irradiation of the THF solution of pG0 in their cc
forms with 327 nm light led to the cis-to-trans isomerization
(Figure 4a) as observed by the changes of the absorption band
(Figure 3a). This cis-to-trans photoisomerization of cc-pG0 is
quantitative as demonstrated in changes of absorption spectral
shape and tt-pG0 is not photoreactive. The lipophilic dendri-
mers tt-pG1 and tt-pG2 displayed similar behavior as tt-pG0
(Figure S2). In contrast, the absorption spectra of tt form of the
water-soluble dendrimers w-pG1 and w-pG2 changed with
UV irradiation at 350 nm (Figures 3b and 3c). This result indi-
cates that w-pGn underwent trans-to-cis photoisomerization
(Figure 4b). This isomerization process could be also followed
by 1H NMR as represented in Figure 5. The isomerization from
tt form to ct form was confirmed by the appearance of the
peak around 6.6 ppm (J = 12.8 Hz) of cis olefin proton. The
phenylbutadiene unit of the dendrimer is much higher (Φiso
0.86)7 than the core compound in THF (Φiso = 0.26).
=
These precedented results forced us to study novel water
soluble dendrimers (Figure 1, w-pGn) and corresponding lipo-
philic compounds (Figure 1, pGn) having 4,4¤-distyrylbiphenyl
unit as a core. The core molecule 4,4¤-distyrylbiphenyl is a
well-known structure as a highly fluorescent material for fluo-
rescent brightening agent and organic light-emitting diode.8,9
Two aromatic olefin units are connected via biphenyl to expand
the π-conjugation. The photochemical behavior of w-pGn was
investigated and the specific trans-to-cis isomerization by
dendritic effect was observed.
Results and Discussion
Absorption Spectra and Photoisomerization Behaviors.
Absorption spectra of THF solution of pGn and 2 mM KOH
© 2016 The Chemical Society of Japan | 911