Remote controlled intramolecular exciplex formation and enhanced photoisomerization in stilbene-cored poly(benzyl ether) dendrimers with alkoxycarbonyl surface functional groups

Article abstract of DOI:10.1246/cl.2011.1124

Exciplex-induced isomerization played a key role in highly efficient photoisomerization of stilbene-cored poly(benzyl ether) dendrimers tethering two alkoxycarbonyl surface functional groups at the meta position of each terminating phenyl group. This is t

Full text of DOI:10.1246/cl.2011.1124

doi:10.1246/cl.2011.1124
1124
Published on the web September 23, 2011
Remote Controlled Intramolecular Exciplex Formation and Enhanced Photoisomerization
in Stilbene-cored Poly(benzyl ether) Dendrimers
with Alkoxycarbonyl Surface Functional Groups
Tsutomu Takizawa and Tatsuo Arai*
Graduate School of Pure and Applied Sciences, University of Tsukuba, Ibaraki 305-8571
(Received July 25, 2011; CL-110620; E-mail: arai@chem.tsukuba.ac.jp)
Exciplex-induced isomerization played a key role in highly
efficient photoisomerization of stilbene-cored poly(benzyl ether)
dendrimers tethering two alkoxycarbonyl surface functional
groups at the meta position of each terminating phenyl group.
This is the first report on the control of photochemical behavior
of dendrimers using intramolecular exciplex formation.
We have been studying photochemical isomerization of
various types of stilbene dendrimers which are soluble in
organic solvents as well as in water. Our first stilbene dendrimers
having benzyl ether dendrons at the meta position of the phenyl
ring of the core stilbene (G1-G3 in Scheme 1) soluble in
organic solvents underwent mutual trans-cis isomerization on
photoirradiation.1,2 On the other hand, introduction of hydro-
philic carboxylate anion groups at the periphery of the above-
mentioned dendrimers (p-wG1-p-wG3 in Scheme 1) drastically
changed the photochemical behavior to exhibit almost one-way
trans-to-cis isomerization.3,4 Thus, the substituents at the
periphery of the dendrimers may alter the photochemical as
well as photophysical properties of stilbene dendrimers, espe-
cially the introduction of carboxylate anions. In this respect, we
have decided to prepare similar stilbene dendrimers which have
two carboxylate anion groups at the meta position of each
terminating phenyl group of the dendrons (dm-wG1-dm-wG3
in Scheme 1) compared to one carboxylate anion group at the
para position. In order to prepare these dendrimers we should
prepare the ester-terminated stilbene dendrimers (dm-G1-dm-
Scheme 1. Structure and photochemical trans-cis isomerization of
TMST and stilbene dendrimers.
G3 in Scheme 1) followed by hydrolysis to give water-soluble
dendrimers. In the course of these studies, we have found that
the ester-terminated dendrimer dm-G1 exhibits quenching of the
fluorescence emission of the core stilbene, exciplex emission
in highly polar solvent DMF, and exciplex-induced trans-cis
isomerization. We wish to report here the preparation and the
photochemical processes of dm-G1 and dm-G2 induced by
peripheral electron-accepting groups at the remote position.
The dendrimers were synthesized by coupling of 3,3¤,5,5¤-
tetrahydroxystilbene with the respective dendrons. The products
and dendrons are negligible in the ground state in higher
generation dendrimers, but the surrounding dendron groups may
give some environmental effects to slightly affect the fluores-
cence profiles. In fact, the fluorescence intensities of p-Gn
and dm-Gn decreased compared to those of TMST and Gn
indicating the occurrence of charge-transfer quenching of the
excited state stilbene core by the surrounding ester-terminated
peripheries of the dendrons.
To better understand the nature of this fluorescence
quenching, fluorescence properties of dm-G1 and dm-G2 were
investigated in deaerated solvents of different polarity in
comparison with TMST (Table 1). Benzene was chosen as the
nonpolar solvent, DMF as polar solvent, and THF as inter-
mediate. As was the case in TMST, absorption maxima showed
little solvent effect while fluorescence maxima showed a red
shift in polar solvent. Interestingly, a weak broad fluorescence
band around 490 nm and a very weak shoulder around 470 nm
were observed for dm-G1 and dm-G2 respectively in DMF
1
were characterized by H NMR, 13C NMR, MS (MALDI-TOF),
and elemental analysis.12
Absorption spectra of dendrimers Gn, p-Gn, and dm-Gn in
deaerated THF solution have maxima around 310 nm resembling
that of 3,3¤,5,5¤-tetramethoxystilbene (TMST) and absorption
around 285 nm which are assigned to the dendrons. Fluorescence
spectra of p-G1 and dm-G1 in deaerated THF solution have
maxima around 375 nm and shoulders around 390 nm while
those of TMST and the rest of the dendrimers have single
maxima around 390 nm (Figure 1). These spectral profiles
indicate that electronic interactions between the core stilbene
Chem. Lett. 2011, 40, 1124-1126
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1125
Figure 1. Absorption and fluorescence spectra (Ex. 310 nm) of TMST
(dashed line), dm-G1 (dotted line), and dm-G2 (solid line) in deaerated
THF solution.
Figure 2. Absorption and fluorescence spectra (Ex. 310 nm) of TMST
(dashed line), dm-G1 (dotted line), and dm-G2 (solid line) in deaerated
DMF solution.
Table 2. Experimental values of TMST and stilbene dendrimers in
deaerated THF solution
Table 1. Experimental values of TMST, dm-G1, and dm-G2 in
deaerated solvents
¹1
Compound
Φf
Φt¼c
¸s/ns
kf/107 s
-
-
¦vss
¸
s
/ns
abs
em
Compound Solvent
Φf
¹1
TMSTa
0.27
0.43
0.27
0.14
0.43
0.39
0.21
0.38
0.32
0.27
0.30
0.32
0.26
0.50
10.0
9.7
4.6
2.6
9.3
6.8
7.4
2.7
4.4
5.9
5.4
4.6
5.7
2.8
/nm /nm /cm
G1b
TMSTa
dm-G1
dm-G2
Benzene 310
373
390
392
374
373
393
376
387
397
5600
6700
0.24
0.27
0.38
0.23
0.14
4.1
10.0
16.6
1.8
p-G1c
dm-G1
G2b
THF
309
313
311
313
316
DMF
Benzene
THF
6500
5400
5100
6200
5400
6600
7300
p-G2c
dm-G2
b
2.6
DMF
0.02 0.5/5.7
Benzene 313
0.20
0.21
5.0
7.4
aRef. 5. Ref. 2. Ref. 4.
c
THF
DMF
308
308
0.11 5.2/23.8
exciplex formation. In this case, the photoexcitation of the core
stilbene induces the charge-transfer interaction to produce the
exciplex to decrease the singlet lifetime of stilbene core. The
acceptor strength is largest in isophthalate, intermediate in
p-toluate, and smallest in benzene peripheral moiety of dm-Gn,
p-Gn, and Gn respectively. Exciplex gains better stabilization
with larger acceptor strength of the acceptor, and therefore,
excited singlet stilbene is more efficiently quenched.
aRef. 5.
solution (Figure 2). These fluorescence bands are assigned to
emission of stilbene-dendron exciplex formed through intra-
molecular charge transfer. The fluorescence decays in DMF
were biexponential. The shorter component in DMF solution
may be assigned to quenched monomer fluorescence while the
longer component is assigned to the exciplex emission. Singlet
lifetimes and quantum yields of fluorescence of all the dendri-
mers decreased compared to those of TMST in THF and DMF,
indicating the occurrence of singlet quenching. The degree of
quenching was higher in more polar solvents and lower
generation dendrimers as was calculated from the lifetimes as
follows: 74% in THF and >97% in DMF for dm-G1, and 26%
in THF and 69% in DMF for dm-G2.
The above fluorescence quenching can be followed by
lifetime measurements of fluorescence of the core stilbene.
When compared within each generation, the singlet lifetimes
decrease, except in dm-G2 and especially apparent in the first
generation dendrimers, with increasing number of alkoxycar-
bonyl surface functional groups indicating that the stilbene core
is quenched by the surrounding dendrons (Table 2). One can
suppose the effects of substituents on the phenyl ring of the
peripheral dendrons as the degree of intramolecular exciplex
formation. The through-space-charge-transfer interaction be-
tween the core stilbene as electron donor and peripheral phenyl
group with alkoxycarbonyl groups as electron acceptor results in
A rough estimation of free energy change of electron
transfer between the core and dendrons can be made from
Weller’s equation using the following numbers:
ꢀGET ¼ EoxðDÞ ꢀ EredðAÞ ꢀ E00 þ C
ð1Þ
Eox(D) = 1.11 eV (TMST), Ered(A) = ¹2.54 eV (diethyl 5-
methylisophthalate), ¹2.92 eV (methyl p-toluate) vs. Ag/AgCl
in THF solution (peak top of oxidation wave was taken for
TMST because of irreversible oxidation); E00 = 3.59 eV (first
generation), 3.55 eV (second generation); C = 0.04 eV (THF),
¹0.06 eV (DMF). ¦GET = 0.10, 0.14, 0.48, and 0.52 eV in THF
for dm-G1, dm-G2, p-G1, and p-G2, and 0.00 and 0.04 eV in
DMF for dm-G1 and dm-G2. Reasonable exciplex formation,
but not charge separation, is expected from these estimations.6
All of the dendrimers undergo trans-cis photoisomerization
as can be seen from the decrease in absorbance of the absorption
maximum and fluorescence intensity on irradiation with UV light
in deaerated THF solution (Figure 3). Isomerization quantum
yields did not decrease in spite of the shorter singlet lifetimes.
One possibility is that isomerization from the excited singlet
Chem. Lett. 2011, 40, 1124-1126
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1126
phenyl group in dm-G2 is more than 10 times less likely to
quench the core because it is more sterically crowded around the
core in dm-G2. It is obvious that the steric effect governing the
ability of the peripheral phenyl group to get into the quenching
radius of the core is active in THF as well because the rate of
singlet quenching is higher for p-G2 compared to dm-G2 in
spite of the lower acceptor strength. Stilbene surrounded with
acceptor moieties in dendrimers may create a large-charge-
transfer complex or exciplex in a whole molecular level
compared to a localized charge-transfer excited state of TMST.
As to the study on intramolecular-exciplex formation,
Vogtle et al. reported exciplex formation and emission in
cyclam-naphthalene dendrimer like structures.9 Li et al. reported
the occurrence of intramolecular electron transfer and exciplex
formation between aniline and pyrene moiety, and naphthalene
and dimethoxybenzene moiety in fan-shaped aryl ether dendri-
mers.10,11 However, these reports only concern the exciplex
formation in dendrimer-like structures. Our studies have two
different important points. First we have prepared conventional
dendrimer molecules having donor at the central core and
acceptor at the whole periphery of the dendrimers. Second, our
compounds exhibit not only the exciplex formation by fluores-
cence quenching of the core stilbene but also highly efficient
isomeization of the double bond induced by the exciplex
formation. To the best of our knowledge, this paper reports the
first experimental findings of exciplex formation followed by the
acceleration of the whole structural change of spherical dendri-
mer molecules.
Figure 3. Change of absorption spectra of dm-G2 in deaerated THF
solution on irradiation at 310 nm. Spectra of trans-isomer (solid line),
photostationary state (dashed line), and calculated cis-isomer (dotted
line) are shown. Inset shows simultaneous change of fluorescence specta.
state is accelerated. However, this seems unlikely because the
activation barrier of torsion around the double bond in the singlet
excited state was found to be 5.5 kcal mol¹1 from the temperature
dependence of singlet lifetime of dm-G1 in THF compared to
¹1
4.2 kcal mol of TMST in acetonitrile.5 Hence, it is most
probable that efficient isomerization proceeds via triplet excited
state of stilbene formed by intersystem crossing of the singlet
exciplex. Apparently, the deactivation behavior of the singlet
exciplex differs with generation. Intersystem crossing may be
more efficient and/or the nonradiative decay may be slower in
dm-G2 compared to dm-G1 leading to isomerization quantum
yields of highly enhanced 0.50 and nearly equal 0.30 respec-
tively. Many stilbene exciplexes in which stilbene is the acceptor
are known, but there are few in which stilbene acts as the donor.
Lewis reported that when intermolecular stilbene exciplex is
formed with fumaronitrile, the triplet stilbene efficiently formed
via intersystem crossing undergoes trans-cis isomerization.7
Aloisi et al. reported that quantum yield of exciplex-induced
isomerization was higher than the intrinsic value.8
Similar yet different correlation in the second generation
could be the result of complex conformations of the dendrons
which affect the distance to and the degree of crowding around
the stilbene core. Actually, the less efficient quenching of the
excited singlet state can be interpreted as the result of less
effective charge-transfer interaction due to the longer distance
between donor and acceptor.
In conclusion, it was found that in stilbene-cored dendri-
mers dm-G1 and dm-G2, the peripheral phenyl groups quench
the excited singlet state of the core stilbene by exciplex
formation via charge-transfer interactions. Moreover, exciplex-
induced isomerization via excited triplet-state stilbene formation
by intersystem crossing of singlet exciplex was found to be the
significant mechanism in highly efficient photoisomerization.
This work was supported by a Grant-in-Aid for Scientific
Research in a Priority Area “New Frontiers in Photochromism”
(No. 471) and a Grant-in-Aid for Scientific Research (B)
(No. 23350075) from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT), Japan.
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3
4
The rate constants for singlet quenching were estimated by
taking the difference of the rate constants for excited singlet state
deactivation between the dendrimers and TMST as follows:
5
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¹1
¹1
2.8 © 108 s in THF and >1.9 © 109 s in DMF for dm-G1,
¹1
¹1
4.0 © 107 s in THF and 1.3 © 108 s in DMF for dm-G2,
6
7
8
¹1
¹1
1.2 © 108 s in THF for p-G1, and 5.0 © 107 s in THF for
p-G2.12
The fact that exciplex emission was observed in both dm-
G1 and dm-G2 indicates that the peripheral phenyl groups in
these spherical dendrimers are able to approach the core stilbene
spatially to interact because direct overlap of molecular orbitals
of donor and acceptor is necessary for exciplex formation. The
fact that the rate of singlet quenching is more than 10 times
larger for dm-G1 compared to dm-G2 means that the peripheral
9
C. Saudan, V. Balzani, P. Ceroni, M. Gorka, M. Maestri, V. Vicinelli,
F. Vögtle, Tetrahedron 2003, 59, 3845.
10 Y.-Y. Li, L. Han, J. Chen, S. Zheng, Y. Zen, Y. Li, S. Li, G. Yang,
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12 Supporting Information is available electronically on the CSJ-Journal
Web site, http://www.csj.jp/journals/chem-lett/index.html.
Chem. Lett. 2011, 40, 1124-1126
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/