Synthesis of a doubly strapped light-harvesting porphyrin bearing energy donor molecules hanging on to the straps: An attempt toward macroscopic control over molecular conformation that affects the efficiency of fluorescence resonance energy transfer

Article abstract of DOI:10.1246/bcsj.20100232

We report here the synthesis of a light-harvesting molecule 1, in which 5,5'-diphenyl-2,2'-bithiophene units (energy donors) and a doubly strapped porphyrin (energy acceptor) are three-dimensionally connected through four alkyl chains to form a "universal

Full text of DOI:10.1246/bcsj.20100232

40
Bull. Chem. Soc. Jpn. Vol. 84, No. 1, 40–48 (2011)
© 2011 The Chemical Society of Japan
Selected Papers
Synthesis of a Doubly Strapped Light-Harvesting Porphyrin
Bearing Energy Donor Molecules Hanging on to the Straps:
An Attempt toward Macroscopic Control over Molecular Conformation
that Affects the Efficiency of Fluorescence Resonance Energy Transfer
Soichiro Ogi,1,2 Kazunori Sugiyasu,*1 and Masayuki Takeuchi*1,2
1Macromolecules Group, Organic Nanomaterials Center, National Institute for Materials Science (NIMS),
1-2-1 Sengen, Tsukuba 305-0047
2Department of Materials Science and Engineering, Graduate School of Pure and Applied Sciences,
University of Tsukuba, 1-1-1 Tennoudai, Tsukuba 305-8571
Received August 19, 2010; E-mail: SUGIYASU.Kazunori@nims.go.jp
We report here the synthesis of a light-harvesting molecule 1, in which 5,5¤-diphenyl-2,2¤-bithiophene units (energy
donors) and a doubly strapped porphyrin (energy acceptor) are three-dimensionally connected through four alkyl chains to
form a “universal joint”-like architecture. From the results of the optical properties of 1 in solution, fluorescence
resonance energy transfer (FRET) takes place from the donor to the acceptor in 1 with the FRET efficiency of 99.7%. We
prepared 1/polydimethylsiloxane (PDMS) elastomeric films in which 1 is connected to a polysiloxane network through
covalent bonds. When we stretched the 1/PDMS film (elongation: up to 60%), the FRET efficiency decreased by 13.1%.
The theoretical analysis suggests that the FRET efficiency of 1 (E) is virtually uninfluenced by any changes in the distance
between the donor and acceptor (r). Therefore, the observed change in FRET efficiency should have been derived from the
orientation factor (¬2), which describes the relative orientation of the emission transition dipole of the donor and the
absorption transition dipole of the acceptor. The anisotropic absorption spectral measurements support the notion that the
transition dipoles of the fluorophores became orthogonally aligned upon stretching, as expected from the molecular
design.
Synchronizing molecular motion and human-scale phenom-
reversible switching of molecular functions.9,10 This concept
should be conducive to the enhancement of inherent molecular
properties and even to the creation of unprecedented smart
materials that can link molecular-level functions to human-
scale dynamics. Moore and co-workers recently provided a
comprehensive review of mechanically induced chemical
changes in polymeric materials: a subject that is related
conceptually to the topic of this present paper.6 Inevitably,
rational molecular design is of great importance to transmit the
dynamic motion over different hierarchies of size.
Toward this goal, we have designed a light-harvesting
molecule 1 featuring 5,5¤-diphenyl-2,2¤-bithiophene units as
energy donors and a porphyrin unit as an acceptor, i.e., an
optimal combination for generating fluorescence resonance
energy transfer (FRET) (Scheme 1a).27-29 The three-dimen-
sional architecture of 1 resembles that of a universal joint (or
Cardan joint), which enjoys mechanical motion. By manipu-
lating the molecular motion through macroscopic mechanical
force, we expect that the light-harvesting ability of 1 can be
controlled (see below).
ena, the dimensions of which differ by approximately five to
seven orders of magnitude (e.g., from nanometers to milli-
meters), is attracting much attention. Machinery based on
molecular motion®for instance, rotational and/or translational
motion in interlocked molecules or structural isomerization
induced by photo- or electrical stimuli®has been extended to
visible shape changes in macroscopic materials by application
of self-assembled supramolecular systems.1-5 These systems
have potential applications as actuators and mechanical devices
that take advantage of changes in molecular momentum. In
clear contrast to these studies, manipulation of molecular-scale
phenomena through macroscopic mechanical stimulation has
become an attractive challenge in the past few years.6-26 The
application of a macroscopic mechanical force®one that is
independent of thermal, photonic, or electronic input®can lead
to novel molecular-level phenomena that cannot be realized
using conventional methods. For example, ultrasound-induced
mechanical forces can accelerate and alter the course of
electrocyclic ring-opening reactions, yielding products that are
not obtainable from purely thermal or light-induced reactions.7
In addition, conformational changes induced by variations in
the surface pressure at air-water interfaces have enabled
FRET is transfer of the excited-state energy from the initially
excited donor to an acceptor. Since FRET efficiency (E)
depends on the distances between the donor and acceptor
Published on the web December 25, 2010; doi:10.1246/bcsj.20100232

S. Ogi et al.
Bull. Chem. Soc. Jpn. Vol. 84, No. 1 (2011)
41
O
ing the elastomeric film, we have observed the changes in the
luminescence properties of 1 due to the changes in the
efficiency of FRET, which is very much a molecular level
phenomenon.
(H2C)4
(a)
RO
(CH2)4
O
N
H
O
O
S
S
N
N
S
S
H
O
O
OR
O
N
(CH2)4
(H2C)4
O
Results and Discussion
R =
1
Synthesis of the Light-Harvesting Molecule.
The
synthetic scheme of the light-harvesting molecule 1 and its
two structural subunits (i.e., A1 and D1) is shown in Scheme 2.
The meso-phenyl doubly strapped porphyrin A1 was synthe-
sized from the bis(formylphenyl) and bis(dipyrrolylmethyl-
phenyl) straps 3 and 4, respectively under Lindsey conditions.
This synthetic approach enables selective formation of
¡,¢,¡,¢-linked doubly strapped porphyrin.29 The bromoben-
zene at the center of each strap is for the Suzuki-Miyaura
coupling reaction with 5, which yields the light-harvesting
molecule 1 featuring 5,5¤-diphenyl-2,2¤-bithiophene units as
energy donors and a porphyrin unit as an acceptor. The subunit
of donor molecule (D1), which lacks porphyrin acceptor, was
synthesized from 5 and 1-bromo-3,5-dibutoxybenzene (6)
through Suzuki-Miyaura coupling reaction. Compound 1 was
reacted with pentamethyldisiloxane in Pt-catalyzed hydrosily-
lation conditions to yield compound 1cap, which is for a
reference experiment (see below and Figure S9 in the Support-
ing Information). Williamson ether synthesis of phloroglucinol
with 6-bromo-1-hexene in the presence of K2CO3 led to
compound 2, which is a crosslinker for preparation of PDMS
elastomeric films (Scheme 3).
Spectroscopic Measurements of 1 in Solution. Figure 1
displays the optical properties of compound 1 and its two
structural subunits, D1 and A1, in solution (see the Supporting
Information for the syntheses and characterization of 1, D1, and
A1). Because the absorption spectrum of 1 is identical to the
sum of those of the reference compounds D1 and A1, electronic
coupling between the two chromophores in the ground state is
negligibly small (Figure 1a). Figure 1b presents the emission
spectra of 1, D1, and A1 excited at 370 nm, which is close to
the absorption maximum (-max) of the donor molecule; the
emission of D1 (at ca. 450 nm) is almost completely quenched
in 1, with the porphyrin emission (at ca. 650 nm) of A1
exhibiting a 4.4-fold increase in intensity in 1. This confirms
that light harvesting occurred from the 5,5¤-diphenyl-2,2¤-
bithiophene moieties to the porphyrin unit in 1 as a result
of suitable spectral overlap between the emission of the donor
D1 and the absorption of the acceptor A1. We calculated the
efficiency of energy transfer (E) to be 99.7% from a
comparison of the spectral integrals of the donor emissions
of D1 and 1, collected at the same excitation wavelength
(370 nm). Perfect overlap between the excitation spectrum and
the corresponding absorption spectrum of 1 supports the notion
that the fluorescence quenching is entirely a result of the energy
transfer in the donor-acceptor system (Figure 1c). In addition,
the absorption, fluorescence, and excitation spectra of 1 are
virtually independent of solvent polarity (PhMe, CHCl3, THF,
and DMF), thus indicating that the contribution of electron
transfer from the donors to the porphyrin acceptor is negligible
(see Figure S5 in the Supporting Information). This optical
property is derived from the average conformation of 1 as it
moves randomly in solution.
Elastomeric film
Stretch
(b)
Perpendicular orientation
Random
Scheme 1. (a) Chemical structure of 1 and schematic
representation of the “universal joint.” (b) Cartoon
representation of the concept developed in this study.
molecules, FRET-based spectral changes can be a reliable
“ruler” for measuring the distances between two sites on
macromolecules and proteins.30-35 In addition, Clark et al. and
Sijbesma et al. have recently reported a polymeric material in
which changes in stress of the polymer matrix is translated into
changes in FRET efficiency because the macroscopic deforma-
tion affects the donor-acceptor distance.16,17 Unlike these
studies, we have focused our attention on the fact that FRET is
the result of long-range dipole-dipole interactions between the
donor and acceptor; the FRET efficiency (E) depends on the
orientation factor (¬2), which describes the relative orientation
of the emission transition dipole of the donor and the
absorption transition dipole of the acceptor.30,36-39 Thus, the
orientation factor is sensitive to molecular conformational
changes. In solution, the donor transition dipoles of 1 are
directed randomly toward the transition dipole of the porphyrin
because they are connected through flexible alkyl chains. In
this case, the orientation factor (¬2) is approximated to be equal
to 2/3, which is an average of the values for the donors and
acceptor randomized through rotational diffusion prior to
energy transfer. The orientation factor (¬2) can vary from 0
to 4 depending on the angle between the transition dipoles (ªD):
perpendicular (90°) and parallel (0°) orientations correspond to
values of ¬2 of 0 and 4, respectively. We harnessed the donor
molecules in 1 to the acceptor platform at the skew positions of
the ¡,¢,¡,¢-linked doubly strapped porphyrin. Given the
molecular structure of 1, the donor dipoles should be oriented
perpendicular to the acceptor dipole when a tensional force
is applied to its termini; thus, the efficiency of FRET would
be changed as a result of the controlled molecular conformation
of 1.
Herein, we report the synthesis and the photophysical studies
of compound 1 in solution. In addition, we have prepared
elastomeric materials in which the molecular conformation of
compound 1 can be controlled dynamically in response to a
macroscopic stretching operation (Scheme 1b). Upon stretch-

42
Bull. Chem. Soc. Jpn. Vol. 84, No. 1 (2011)
Mechanical Control of FRET
OR
Br
O
O
S
S
O
O
Br
CHO
OHC
3
O
O
(CH2)4
O
(H2C)4
O
O
(CH2)4
Pyrrole
TFA
34%
Na2CO3
[Pd(PPh3)4]
1) BF3
2) chloranil
O
(H2C)4
O
N
HN
NH
O
O
N
CHCl3, EtOH
13%
Toluene
EtOH, H2O
50%
N
O
HN
NH
O
N
O
O
Br
(H2C)4
(CH2)4
O
(H2C)4
O
(CH2)4
O
O
O
Br
O
O
A1
N
H
N
H
S
HN
NH
O
O
4
S
B
S
S
OR
5
OR
1: R =
5
Br
O
Na2CO3
[Pd(PPh3)4]
CH3
Si
CH3
O
O
H
O
Si CH3
CH3
S
S
Toluene
EtOH, H2O
50%
OR
O
CH3
Karstedt's catalyst
Toluene
86%
D1
6
H3C
O
CH3
Si
H3C
CH3
Si
1cap: R =
OH
OR
Br
CH3
K2CO3
DMF
42%
HO
OH
RO
OR
2, 5, D1: R =
2
Scheme 2. Syntheses and chemical structures of the light-harvesting molecule 1, its two structural subunits (A1 and D1, which are
acceptor and donor units in 1, respectively), 1cap, and crosslinker 2 for preparation of PDMS elastomeric films.
The FRET efficiency (E) can be described using eq 1, where
r (¡) is the distance between the donor and acceptor molecules
and R0 (¡) is the Förster distance at which the energy-transter
efficiency becomes 50%:
The orientation factor (¬2) is given by eq 3:
¬2 ¼ ðsin ªD sin ªA cos º ꢂ 2 cos ªD cos ªAÞ
2
ð3Þ
where ªD and ªA are the angles of the emission transition dipole
of the donor and the absorption transition dipole of the acceptor
(with respect to the vector joining the donor and the acceptor),
respectively, and º is the angle between the planes. When
the acceptor transition dipole is placed parallel to one axis,
the orientation factor is simply represented as the angle (ªD in
Figure 2c) between the transition dipoles of donor and acceptor
moieties in 1; in this case, eq 3 transforms into eq 4:
6
6
E ¼ R0 =½R0 þ r6ꢀ
ð1Þ
ð2Þ
6
R0 ¼ 8:79 ꢁ 10ꢂ5½¬2nꢂ4QDJð-Þꢀ
Using eq 2, we calculated the Förster distance (R0) to be 36.9 ¡
(Figure 2a): here, we used an orientation factor (¬2) of 2/3, a
refractive index (n) for the solvent of 1.43, a fluorescence
quantum yield for the donor (QD) of 0.19, and a spectral
overlap integral for the donor emission and the acceptor
absorption [J(-)] of 9.50 © 1014 M¹1 cm¹1 nm.4 The relation-
ship between the values of E and r for 1 obtained from eq 1
reveals that the value of E is greater than 99.5% even at a value
of rmax of 14.8 ¡, the farthest possible distance between the
donor and acceptor units connected through these alkyl chains
(Figure 2b). Therefore, we would not expect the FRET
efficiency (E) of 1 to be affected by any changes in the
distance between the donor and acceptor units (see below).
¬2 ¼ 4 cos2 ªD
ð4Þ
Thus, the FRET efficiency (E) is a function of ªD, which can be
graphed as shown in Figure 2d; perpendicular (ªD = 90°)
orientation results in E to be zero.
Spectroscopic Measurements of 1/PDMS Elastomeric
Films before and after the Macroscopic Stretching Oper-
ation.
macroscopic mechanical force, we have prepared elastomeric
In order to control the orientation factor (¬2) by

S. Ogi et al.
Bull. Chem. Soc. Jpn. Vol. 84, No. 1 (2011)
43
H
H
H
P1
1
Pt catalyst
Toluene
H
H
H
P1
1
P1-1-P1
Pt catalyst
Toluene
1/PDMS
Elastomeric film
H
H
H
H
P2
2
Pt catalyst
Toluene
2
2-P2
1 cm
O
(H2C)4
O
(CH2)4
O
O
N
O
H
S
S
N
N
S
S
H
O
(H2C)4
O
O
O
1
N
(CH2)4
O
1
O
2
O
O
2
Scheme 3. Schematic representation of the preparation of 1/PDMS film and a photograph of the final product.
films in which compound 1 is incorporated through covalent
bonds (Scheme 3).40-43 We chose polydimethylsiloxane
(PDMS) to be the polymeric matrix because its transparency
allowed us to evaluate the optical properties of the incorporated
1. Compound 1 is readily introduced into PDMS networks
through hydrosilylation of its two terminal olefin moieties. P1
is a hydride-terminated polydimethylsiloxane (Mn, 6000; d,
0.97; viscosity, 100 cSt; n, 1.403) that we reacted in excess
(50 mg, 20 ¯mol of SiH units) with 1 (8 nmol) in the presence
of Karstedt’s catalyst (1.5 ¯L) in toluene (50 ¯L) under an Ar
atmosphere at 30 °C for 1 h, thereby yielding P1-1-P1, in
which 1 was present in the main chains of PDMS. GPC
chromatogram of the product obtained after the hydrosilylation
of 1 with P1 clearly reveals that 1 had reacted with P1
quantitatively (see Figure S6 in the Supporting Information).
P2 is a copolymer of dimethylsiloxane and methylhydrosilox-
ane (Mn, 1900-2000; methylhydrosiloxane, 15-18 mol %; d,
0.97; viscosity, 25-35 cSt; n, 1.400); we synthesized the macro
crosslinker 2-P2 from 2 (30 mg, 80 ¯mol) and P2 (21 mg,
40 ¯mol of SiH units) in the presence of Karstedt’s catalyst
(5 ¯L) in toluene (500 ¯L) at room temperature. The resultant
solution of 2-P2 and 2 was added to a mixture of P1-1-P1
(8 nmol) and P1 (300 mg) in toluene (700 ¯L) and the mixture
was stirred at room temperature. After 15 s, the resultant
solution was transferred onto a Teflon substrate (20 mm ©
40 mm) and an elastomeric film was obtained by a crosslinking
hydrosilylation reaction under gradual evaporation of toluene at
30 °C overnight. FT-IR spectra recorded for the samples before
and after the crosslinking reaction revealed that the elastomeric
film did not feature SiH bonds (no stretching vibrations at
2127 cm¹1),42 indicating that all of the hydride-terminated
polydimethylsiloxane, including P1-1-P1, had crosslinked
with 2-P2 in the elastomeric film (see Figure S7 in the
Supporting Information). The obtained films, which were ca.
20 mm long, 10 mm wide, and 1 mm thick, were about twice
stretchable (elongation: up to ca. 100%, see Figure S8 in the
Supporting Information).
Figure 3a reveals that the absorption spectrum of the
1/PDMS elastomeric film was identical to that of 1 in solution
(Figure 1a), indicating that 1 had been incorporated into the
elastomeric film homogeneously. For reference, when 1cap
was doped (but not connected covalently) within a PDMS film,
the Soret band of the porphyrin unit underwent a hypsochromic
shift (see Figure S9 in the Supporting Information). Our results
indicate that the covalent crosslinking of 1 in the PDMS
network prevented the fluorophores from undergoing intra- and
intermolecular aggregation, which is a very important require-
ment for the following photophysical studies of 1/PDMS films.
The fluorescence spectrum of the 1/PDMS film excited at
370 nm prior to stretching revealed that efficient FRET also
occurred in the film, similar to the situation in solution (cf.
purple lines in Figures 1b and 3b), because the donor and
acceptor units were harnessed within the Förster distance and
the directions of their transition dipoles remained random (i.e.,

44
(a)
Bull. Chem. Soc. Jpn. Vol. 84, No. 1 (2011)
Mechanical Control of FRET
0.5
0.4
0.3
When we stretched the 1/PDMS film up to 60% of
elongation, the fluorescence of the donor was enhanced relative
to that of the acceptor (Figure 3b). In contrast, elastomeric
films prepared from only D2 or A1 resulted in weaker
fluorescence upon stretching, because the films became thinner
(see Figure S11 in the Supporting Information, for this reason,
normalized fluorescence spectra are shown in Figure 3b).
Hence, the recovery of the donor fluorescence in the stretched
1/PDMS film implied that the FRET efficiency decreased as
a result of a macroscopic mechanical force. The inset to
Figure 3b presents the ratio of the fluorescence intensities of
the donor and acceptor (ID/IA) plotted with respect to the
stretching ratio. Upon stretching, the ID/IA ratio increased.
Because the ID/IA ratio is a function of the FRET efficiency, we
calculated the change in the FRET efficiency (¦E) induced
upon elongation of 30 and 60% to be 5.6 and 13.1%,
respectively (see Figure S12 in the Supporting Information).
As mentioned above, we would expect the FRET efficiency of
1 to be virtually uninfluenced by any changes in the distance
between the donor and acceptor. In addition, 1 in the films is so
diluted that we can rule out the contribution of intermolecular
FRET (see Figure S10 in the Supporting Information). There-
fore, the observed change in FRET efficiency (¦E) should have
been derived from the orientation factor (¬2); according to
eqs 1 and 2, as the orientation factor (¬2) decreases, the FRET
efficiency (E) decreases.44,45 Importantly, the control of the
FRET efficiency was reversible in response to the mechanical
operation, although the reverse process was slow (several
hours), presumably because the conformational relaxation was
hampered in such a viscoelastic material.
To provide evidence for the controlled angle of the transition
dipoles, we conducted anisotropic absorption spectral measure-
ments with a polarizer placed between the film and the light
source. The absorption spectrum of the as-prepared film
revealed no anisotropy, indicating a random orientation of the
transition dipoles. In contrast, the film elongated by 30%
clearly revealed that the transition dipoles of the donor and
acceptor units were oriented parallel and perpendicular,
respectively, to the stretching direction (Figure 3c).46 This
supports the notion that the transition dipoles of the fluoro-
phores became orthogonally aligned upon stretching, as
expected from the molecular design; consequently, the effi-
ciency of the FRET was controllable.
0.2
1
A1
0.1
D1
0
300
400
500
600
700
wavelength / nm
1 104
8000
6000
4000
2000
0
(b)
500
D1
1
250
A1
0
400
500
600
700
wavelength / nm
400
500
600
700
wavelength / nm
0.15
0.10
0.05
0
(c)
400
500
600
wavelength / nm
Figure 1. (a) Absorption and (b) fluorescence spectra of 1
(purple line), D1 (green line), and A1 (red line) at room
temperature in a mixed solvent system of toluene and
siloxane oligomer (3:7, v/v): [1] = [A1] = 1 © 10¹6 M;
[D1] = 2 © 10¹6 M; -ex = 370 nm (wavelength at which
the donor molecules are excited selectively). (c) Absorp-
tion (purple solid line) and fluorescence excitation ( )
spectra of 1 in a mixed solvent system of toluene and
Conclusion
In summary, we have synthesized a light-harvesting mole-
cule 1, the structure of which is reminiscent of a universal joint.
Due to its unique three-dimensional architecture, the light-
harvesting ability can be controlled in response to molecular
conformational changes. To synchronize the molecular level
motion with human-scale dynamics, we have incorporated 1
within PDMS film and examined changes in FRET efficiency
of 1 upon stretching the film. Although the observed change
was small (¦E is 13.1% at 60% of elongation), we have
demonstrated that elastomeric materials can link the human-
scale dynamics to molecular-level function. “Mechano-based
FRET” systems have potential application to detect internal
damage of polymeric materials: the previously reported
concepts are based on the changes in the distance between
siloxane oligomer (3:7, v/v): [1] = 0.3 © 10¹6 M; -mon
=
653 nm; RT.
the orientation factor remained equal to 2/3). Note that the
contribution of intermolecular FRET was negligible, as
indicated by the observation that elastomeric films prepared
using the donor D2 as a crosslinker preserved the fluorescence
intensity of D2 even in the presence of the acceptor molecule
A1 ([A1]/[D1] = 0-1.0; see Figure S10 in the Supporting
Information).

S. Ogi et al.
(a)
Bull. Chem. Soc. Jpn. Vol. 84, No. 1 (2011)
45
(b)
100
80
60
40
20
0
O
O
S
S
O
O
O
100
99
R0
99.6%
0
20
40
60
80
100
98
97
D-A distance (r) / Å
96
14.8 Å
0
5
10
15
20
25
30
PDMS
H3C
D-A distance (r) / Å
(c)
Si
O
CH3
(d)
100
S
80
60
40
20
0
S
O
O
(CH2)4
O
(H2C)4
θD
N
O
HN
N
NH
O
70
75
80
85
90
O
θ
D / °
Figure 2. (a) The relationship between the values of the energy-transter efficiency (E) and the distance between the donor and
acceptor molecules (r) for 1 obtained from eq 1. The Förster distance (R0) is calculated to be 36.8 ¡ at which energy-transter
efficiency becomes 50%. (b) The enlarged graph of the red square area in Figure 2a. The graph revealed that the value of E is
greater than 99.5% even at a value of rmax of 14.8 ¡, the farthest possible distance between the donor and acceptor units connected
through these alkyl chains. (c) The partial molecular structure of 1 that is connected to a polysiloxane network; ªD is defined as the
angle between the transition dipoles of the donor and acceptor moieties in 1. (d) The graphic image of the relationship between the
values of E and ªD for 1 obtained from eqs 1, 2, and 4, in which the value of r is 14.8 ¡ (rmax).
the donor and acceptor (i.e., “r” in eq 1).16,17 To the best of our
knowledge, this system is the first example of controlling
FRET efficiency through a mechanism based upon the
orientation factor (¬2) being affected by a macroscopic
mechanical force. We expect that optimization of the polymer
structures and their mechanical properties will improve the
mechanical responsiveness of the incorporated molecules.
Studies along this line and manipulation by hand of more
functional molecular machines are now in progress.
spectrophotometer and a Hitachi F-7000 spectrophotometer,
respectively. Relative fluorescence quantum yield of D1 was
determined using quaterthiophene (Φ = 0.18, benzene) as
standard.47
Materials.
A hydride-terminated polydimethylsiloxane
P1 and a copolymer of dimethylsiloxane and methylhydro-
siloxane P2 were purchased from GELEST, Inc. Platinum(0)-
(1,1,3,3-tetramethyl-1,3-divinyldisiloxane) complex in xylene
(Karstedt’s catalyst) was purchased from Aldrich. All other
reagents and solvents were purchased from Aldrich, Kanto
Chemical Co., or Wako Chemicals and used as received.
Experimental
Measurements.
1H NMR spectra were recorded on a
Synthesis of A1.
A mixture of 1-bromo-3,5-bis[4-(2-
Bruker Biospin DRX-600 (600 MHz) spectrometer. Chemical
shifts are reported in ppm downfield from tetramethylsilane
(0 ppm for 1H) or residual CHCl3 (77 ppm for 13C) as an
internal standard. Mass spectral data were obtained using a
SHIMADZU AXIMA-CFR Plus MALDI TOF mass spectrom-
eter. Melting points were determined with a Yanako NP-500P
micro melting point apparatus. UV-vis absorption spectra and
fluorescence spectra were obtained on a Hitachi U-2900
formylphenoxy)butoxy]benzene (3) (604 mg, 1.12 mmol) and
compound 4 (863 mg, 1.12 mmol) in CHCl3 (1000 mL), EtOH
(8 mL) was bubbled with argon for 2 h. Boron trifluoride
diethyl etherate (196 ¯L, 1.56 mmol) was added to the solution
under argon atmosphere and the mixture was stirred at room
temperature. After 3 h, chloranil (760 mg, 3.35 mmol) was
added and the resulting solution was stirred for 3 h. After
purification through flash column chromatography (SiO2;

46
Bull. Chem. Soc. Jpn. Vol. 84, No. 1 (2011)
Mechanical Control of FRET
(a)
1.5
1.05-1.10 (8H, m, -O-CH2-(CH2)2-CH2-O-), 2.55 (8H, t,
J = 6.3 Hz, -O-CH2-), 3.92 (8H, t, J = 5.4 Hz, -O-CH2-),
4.06 (2H, t, J = 2.1 Hz, C6H3-Br), 6.14 (4H, d, J = 1.8 Hz,
C6H3-Br), 7.28-7.32 (8H, m, -O-C6H4-), 7.71 (4H, t,
J = 8.1 Hz, -O-C6H4-), 7.86 (4H, dd, J = 7.8, 1.8 Hz, -O-
C6H4-), 8.74 (8H, s, ¢-pyrrole). l3C NMR (150 MHz, CDCl3,
298 K): ¤ 24.99, 25.18, 67.15, 68.10, 99.09, 110.28, 112.34,
115.69, 119.67, 122.01, 129.61, 131.76, 135.46, 158.87,
159.74. MALDI-TOF-MS (dithranol): Found m/z = 1272.48,
Calculated for [M]+ (C72H64Br2N4O8) = 1272.31.
1.0
1/PDMS elastomeric film
0.5
0
Synthesis of D1. A mixture of 2,3-dimethyl-2,3-butylidene
5¤-[4-(5-hexenyloxy)phenyl]-2,2¤-bithiophene-5-boronate (5)
(133 mg, 285 ¯mol), 1-bromo-3,5-dibutoxybenzene (6) (72.3
mg, 240 ¯mol), sodium carbonate (170 mg, 1.37 mmol) in
toluene (9.0 mL), EtOH (2.2 mL), water (2.2 mL) was bubbled
with argon for 2.5 h. Tetrakis(triphenylphosphine)palladium(0)
(19.8 mg, 17.1 ¯mol) was added to the solution and the mixture
was refluxed for 6.5 h. After cooling to room temperature, the
reaction mixture was washed with water and the organic layer
was dried over anhydrous Na2SO4. The filtrate was evaporated
and the crude product was purified by column chromatography
(SiO2; n-hexane/CHCl3) to produce a yellow powder (66.8 mg,
400
500
600
700
wavelength / nm
3.0
2.0
1.0
0
(b)
60%
0
30 60
Elongation / %
30%
0% (unstretched)
l
50%). Mp 102.4-103.0 °C. H NMR (CDCl3, 298 K): ¤ 0.99
(6H, t, J = 7.5 Hz, -O-CH2-(CH2)2-CH3), 1.48-1.55 (4H, m,
-O-CH2-(CH2)2-CH3), 1.56-1.62 (2H, m, -O-CH2-(CH2)3-
CH=CH2), 1.76-1.87 (6H, m, -O-CH2-(CH2)3-CH=CH2 and
-O-CH2-(CH2)2-CH3), 2.13-2.16 (2H, m, -O-CH2-(CH2)3-
CH=CH2), 3.97-4.01 (6H, m, -O-CH2-(CH2)4-CH=CH2 and
-O-CH2-(CH2)2-CH3), 4.98-5.07 (2H, m, -O-CH2-(CH2)3-
CH=CH2), 5.81-5.87 (1H, m, -O-CH2-(CH2)3-CH=CH2),
6.40 (1H, t, J = 2.1 Hz, p-H in C6H3), 6.74 (2H, d, J = 1.8 Hz,
o-H in C6H3), 6.91 (2H, d, J = 8.4 Hz, m-H in C6H4), 7.11-
7.14 (3H, m, C4H2S), 7.21 (1H, d, J = 3.6 Hz, C4H2S), 7.52
(2H, d, J = 8.4 Hz, o-H in C6H4). l3C NMR (150 MHz, CDCl3,
298 K): ¤ 13.85, 19.25, 25.29, 28.66, 31.31, 33.41, 67.82,
67.87, 100.56, 104.37, 114.77, 114.88, 122.63, 123.98, 123.99,
124.42, 126.69, 126.86, 135.59, 135.70, 136.83, 138.48,
142.85, 143.25, 158.83, 160.62. MALDI-TOF-MS (dithranol):
Found m/z = 560.20, Calculated for [M]+ (C34H40O3S2) =
560.24.
400
500
600
700
wavelength / nm
(c)
After stretching
90°
0°
Acceptor
0.01
0
Synthesis of 1.
A mixture of compound A1 (171 mg,
134 ¯mol), 2,3-dimethyl-2,3-butylidene 5¤-[4-(5-hexenyloxy)-
phenyl]-2,2¤-bithiophene-5-boronate (5) (150 mg, 322 mmol),
sodium carbonate (191 mg, 1.54 mmol) in toluene (10.2 mL),
EtOH (2.4 mL), water (2.4 mL) was bubbled with argon for 3 h.
Tetrakis(triphenylphosphine)palladium(0) (22.3 mg, 19.3 ¯mol)
was added to the solution and the mixture was refluxed for 6 h.
After cooling to room temperature, the reaction mixture was
diluted with CH2Cl2 and washed with water and the organic
layer was dried over anhydrous Na2SO4. The filtrate was
evaporated and the crude product was purified by column
chromatography (SiO2; n-hexane/CHCl3, 1:1) to produce a
: unstretched
: 30% elongated
-0.01
Donor
350
375
400
425
450
wavelength / nm
Figure 3. (a) Absorption spectrum of the 1/PDMS film.
(b) Fluorescence spectral changes of the 1/PDMS
elastomeric film upon stretching: Intensities are normalized
at 653 nm, -ex = 370 nm. Inset to (b): ID/IA ratio plotted
with respect to the stretching ratio. (c) Differential
anisotropic absorption spectra of the 1/PDMS film before
(black line) and after (red line) stretching.
l
purple powder (121 mg, 50%). Mp 161.1-162.1 °C. H NMR
(CDCl3, 298 K): ¤ ¹2.55 (2H, s, inner-NH), 0.83-0.88 (8H, m,
-O-CH2-(CH2)2-CH2-O-), 1.09-1.13 (8H, m, -O-CH2-
(CH2)2-CH2-O-), 1.55-1.60 (4H, m, -O-CH2-(CH2)3-
CH=CH2), 1.79-1.83 (4H, m, -O-CH2-(CH2)3-CH=CH2),
2.13 (4H, q, J = 7.2 Hz, -O-CH2-(CH2)3-CH=CH2), 2.59
(8H, t, J = 6.3 Hz, -O-CH2-(CH2)2-CH2-O-C6H3), 3.94 (8H,
CHCl3) and column chromatography (SiO2; n-hexane/CHCl3,
from 1:3 to 1:4), a purple powder was obtained (180 mg, 13%).
Mp 156.0-158.0 °C. H NMR (CDCl3, 298 K): ¤ ¹2.61 (2H, s,
l
inner-NH), 0.80-0.88 (8H, m, -O-CH2-(CH2)2-CH2-O-),

S. Ogi et al.
Bull. Chem. Soc. Jpn. Vol. 84, No. 1 (2011)
47
l
t, J = 5.7 Hz, C6H4-O-CH2-(CH2)2-CH2-O-), 3.98 (4H, t, J =
6.6 Hz, -O-CH2-(CH2)4-CH=CH2), 4.15 (2H, t, J = 2.1 Hz,
C6H3), 4.97-5.06 (4H, m, -O-CH2-(CH2)3-CH=CH2), 5.80-
5.86 (2H, m, -O-CH2-(CH2)3-CH=CH2), 6.26 (4H, d,
J = 2.4 Hz, C6H3), 6.89 (4H, d, J = 9.0 Hz, C6H4), 6.98 (2H,
d, J = 3.6 Hz, C4H2S), 7.02 (2H, d, J = 3.6 Hz, C4H2S), 7.05
(2H, d, J = 3.6 Hz, C4H2S), 7.07 (2H, d, J = 3.6 Hz, C4H2S),
7.26 (4H, t, J =7.5 Hz, C6H4), 7.30 (4H, d, J = 7.8 Hz, C6H4),
7.48 (4H, d, J = 8.4 Hz, C6H4), 7.70 (4H, t, J = 8.0 Hz, C6H4),
7.93 (4H, dd, J = 7.2, 1.8 Hz, C6H4), 8.76 (8H, s, ¢-pyrrole).
l3C NMR (150 MHz, CDCl3, 298 K): ¤ 25.07, 25.27, 28.65,
33.40, 67.02, 67.85, 68.15, 99.10, 104.52, 112.33, 114.76,
114.86, 115.71, 119.64, 122.58, 123.66, 123.88, 124.30,
126.68, 126.83, 129.59, 131.78, 134.93, 135.43, 135.60,
136.53, 138.47, 142.69, 143.12, 158.80, 158.94, 159.63.
MALDI-TOF-MS (dithranol): Found m/z = 1792.73, Calcu-
lated for [M + H]+ (C112H103N4O10S4) = 1792.66.
matter (583 g, 42%). H NMR (CDCl3, 298 K): ¤ 1.53-1.58
(6H, m, -O-CH2-(CH2)3-CH=CH2), 1.75-1.80 (6H, m, -O-
CH2-(CH2)3-CH=CH2), 2.10-2.14 (6H, m, -O-CH2-(CH2)3-
CH=CH2), 3.91 (6H, t, J = 6.6 Hz, -O-CH2-(CH2)4-
CH=CH2), 4.96-5.05 (6H, m, -O-CH2-(CH2)3-CH=CH2),
5.79-5.86 (3H, m, -O-CH2-(CH2)3-CH=CH2), 6.06 (3H, s,
C6H3). l3C NMR (150 MHz, CDCl3, 298 K): ¤ 25.30, 28.65,
33.40, 67.72, 93.73, 114.70, 138.51, 160.88. MALDI-TOF-MS
(dithranol): Found m/z = 373.31, Calculated for [M + H]+
(C24H37O3) = 373.27.
This study was supported partially by KAKENHI
(No. 20750097) for K.S. and the “Nanotechnology Network
Project” of the Ministry of Education, Culture, Sports, Science
and Technology of Japan (MEXT).
Supporting Information
Synthesis of 1cap. A mixture of compound 1 (20.5 mg,
11.4 ¯mol) and Platinum(0)-(1,1,3,3-tetramethyl-1,3-divinyldi-
siloxane) complex (1.5 ¯L, 2%Pt in xylene, Aldrich) in toluene
(1.0 mL) was added to pentamethyldisiloxane (433 mL,
2.29 mmol) and the mixture was stirred under argon atmo-
sphere at room temperature overnight. The reaction mixture
was purified by column chromatography (SiO2, n-hexane/
CHCl3, 1:4) to produce a purple powder (20.1 mg, 86%). Mp
Synthesis and characterization of light-harvesting molecule
1 and its reference compounds; preparation procedure, charac-
terization, and optical properties of the elastomeric films; the
relationship between the FRET efficiency (E) and the ratio of
the fluorescence intensities of the donor and acceptor (ID/IA).
This material is available free of charge on the Web at:
http://www.csj.jp/journals/bcsj/.
l
131.6-132.6 °C. H NMR (CDCl3, 298 K): ¤ ¹2.54 (2H, s,
References
inner-NH), 0.04 (12H, s, -Si(CH3)2-O-), 0.06 (18H, s, -O-
Si(CH3)3), 0.51-0.53 (4H, m, -O-CH2-(CH2)4-CH2-Si-),
0.83-0.88 (8H, m, -C6H4-O-CH2-(CH2)2-CH2-O-), 1.09-
1.13 (8H, m, -C6H4-O-CH2-(CH2)2-CH2-O-), 1.33-1.41 (8H,
m, -O-CH2-(CH2)4-CH2-Si-), 1.44-1.49 (4H, m, -O-CH2-
(CH2)4-CH2-Si-), 1.76-1.81 (4H, m, -O-CH2-(CH2)4-CH2-
Si-), 2.58 (8H, t, J = 6.6 Hz, -C6H4-O-CH2-(CH2)2-CH2-
O-), 3.94 (8H, t, J = 5.4 Hz, -C6H4-O-CH2-(CH2)2-CH2-O-),
3.97 (4H, t, J = 6.6 Hz, -O-CH2-(CH2)4-CH=CH2), 4.14-
4.15 (2H, m, C6H3), 6.26 (4H, d, J = 2.4 Hz, C6H3), 6.91 (4H,
d, J = 9.0 Hz, C6H4), 6.98 (2H, d, J = 3.6 Hz, C4H2S), 7.01
(2H, d, J = 3.6 Hz, C4H2S), 7.05 (2H, d, J = 3.6 Hz, C4H2S),
7.07 (2H, d, J = 3.6 Hz, C4H2S), 7.26 (4H, t, J =7.5 Hz, C6H4),
7.30 (4H, d, J = 8.4 Hz, C6H4), 7.48 (4H, d, J = 9.0 Hz, C6H4),
7.70 (4H, t, J = 8.0 Hz, C6H4), 7.93 (4H, dd, J = 7.2, 1.8 Hz,
C6H4), 8.75 (8H, s, ¢-pyrrole). l3C NMR (150 MHz, CDCl3,
298 K): ¤ 0.34, 1.98, 18.30, 23.21, 25.09, 25.29, 25.75, 29.17,
33.10, 67.04, 68.15, 99.11, 104.54, 112.35, 114.89, 115.72,
119.66, 122.57, 123.67, 123.88, 124.31, 126.64, 126.84,
129.60, 131.81, 134.96, 135.45, 135.60, 136.56, 142.70,
143.18, 158.89, 158.96, 159.65. MALDI-TOF-MS (dithranol):
Found m/z = 2088.80, Calculated for [M + H]+ (C122H135N4-
O12S4Si4) = 2088.81.
1
a) Y. Yu, M. Nakano, T. Ikeda, Nature 2003, 425, 145. b) T.
Ikeda, J. Mamiya, Y. Yu, Angew. Chem., Int. Ed. 2007, 46, 506.
c) M. Yamada, M. Kondo, J. Mamiya, Y. Yu, M. Kinoshita, C. J.
Barrett, T. Ikeda, Angew. Chem., Int. Ed. 2008, 47, 4986.
2
a) K. Ichimura, S.-K. Oh, M. Nakagawa, Science 2000,
288, 1624. b) J. Berná, D. A. Leigh, M. Lubomska, S. M.
Mendoza, E. M. Pérez, P. Rudolf, G. Teobaldi, F. Zerbetto, Nat.
Mater. 2005, 4, 704.
3
a) R. Eelkema, M. M. Pollard, J. Vicario, N. Katsonis, B. S.
Ramon, C. W. M. Bastiaansen, D. J. Broer, B. L. Feringa, Nature
2006, 440, 163. b) D. Pijper, M. G. M. Jongejan, A. Meetsma,
B. L. Feringa, J. Am. Chem. Soc. 2008, 130, 4541. c) A. Bosco,
M. G. M. Jongejan, R. Eelkema, N. Katsonis, E. Lacaze, A.
Ferrarini, B. L. Feringa, J. Am. Chem. Soc. 2008, 130, 14615.
4
S. Kobatake, S. Takami, H. Muto, T. Ishikawa, M. Irie,
Nature 2007, 446, 778.
5
a) H. Yu, T. M. Swager, IEEE J. Oceanic Eng. 2004, 29,
692. b) C. Song, T. M. Swager, Org. Lett. 2008, 10, 3575.
6
M. M. Caruso, D. A. Davis, Q. Shen, S. A. Odom, N. R.
Sottos, S. R. White, J. S. Moore, Chem. Rev. 2009, 109, 5755.
C. R. Hickenboth, J. S. Moore, S. R. White, N. R. Sottos, J.
Baudry, S. R. Wilson, Nature 2007, 446, 423.
A. Piermattei, S. Karthikeyan, R. P. Sijbesma, Nat. Chem.
2009, 1, 133.
7
8
Synthesis of 2.
A mixture of phloroglucinol (472 mg,
9
J. Kim, T. M. Swager, Nature 2001, 411, 1030.
3.74 mmol) and potassium carbonate (6.20 g, 44.9 mmol) in
N,N-dimethylformamide (15 mL) was stirred at 70 °C for 2 h,
then, 6-bromo-1-hexene (2.00 mL, 15.0 mmol) was added. The
reaction mixture was further stirred at 70 °C for 6 h. After
cooling to room temperature, the reaction mixture was diluted
in ether, washed with water, and the organic layer was dried
over anhydrous Na2SO4. The filtrate was evaporated and
obtained liquid material was purified by column chromatog-
raphy (SiO2; n-hexane/CHCl3, 7:3) to produce colorless oily
10 K. Ariga, T. Nakanishi, Y. Terasaka, H. Tsuji, D. Sakai, J.
Kikuchi, Langmuir 2005, 21, 976.
11 T. Ikawa, T. Shiga, A. Okada, J. Appl. Polym. Sci. 1997, 66,
1569.
12 H. Spanggaard, M. Jørgensen, K. Almdal, Macromolecules
2003, 36, 1701.
13 M. Kinami, B. R. Crenshaw, C. Weder, Chem. Mater. 2006,
18, 946.
14 F. Donati, A. Pucci, C. Cappelli, B. Mennucci, G. Ruggeri,

48
Bull. Chem. Soc. Jpn. Vol. 84, No. 1 (2011)
Mechanical Control of FRET
J. Phys. Chem. B 2008, 112, 3668.
Chem., Int. Ed. 2001, 40, 3621.
15 D. A. Davis, A. Hamilton, J. Yang, L. D. Cremar, D.
van Gough, S. L. Potisek, M. T. Ong, P. V. Braun, T. J. Martínez,
S. R. White, J. S. Moore, N. R. Sottos, Nature 2009, 459, 68.
16 N. Bruns, K. Pustelny, L. M. Bergeron, T. A. Whitehead,
D. S. Clark, Angew. Chem., Int. Ed. 2009, 48, 5666.
17 S. Karthikeyan, R. P. Sijbesma, Macromolecules 2009, 42,
5175.
37 F. D. Lewis, L. Zhang, X. Zuo, J. Am. Chem. Soc. 2005,
127, 10002.
38 A. Iqbal, S. Arslan, B. Okumus, T. J. Wilson, G. Giraud,
D. G. Norman, T. Ha, D. M. J. Lilley, Proc. Natl. Acad. Sci. U.S.A.
2008, 105, 11176.
39 K. Börjesson, S. Preus, A. H. El-Sagheer, T. Brown, B.
Albinsson, L. M. Wilhelmsson, J. Am. Chem. Soc. 2009, 131,
4288.
40 A. E. Mera, R. E. Morris, Macromol. Rapid Commun.
2001, 22, 513.
41 S. A. Ponomarenko, E. A. Tatarinova, A. M. Muzafarov,
S. Kirchmeyer, L. Brassat, A. Mourran, M. Moeller, S. Setayesh,
D. de Leeuw, Chem. Mater. 2006, 18, 4101.
42 V. V. Antić, M. P. Antić, M. N. Govedarica, P. R. Dvornić,
J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 2246.
43 S. Ivanovici, G. Kickelbick, J. Sol-Gel Sci. Technol. 2008,
46, 273.
44 The fluorescence spectrum of the donor units in the
stretched 1/PDMS film (Figure 3b) was blue-shifted compared
to that of D1 in solution (Figure 1b) or that of D2 in the
elastomeric films (see Figure S11 in the Supporting Information).
It is unclear at present why the spectral shift was induced upon
stretching. It should be noted that the blue-shifted donor fluores-
cence changes the spectral overlap integral [J(-)] that is the factor
affecting the FRET efficiency (E) as represented in eq 2. In fact,
the blue-shifted donor fluorescence results in a larger spectral
overlap integral [J(-)]. Importantly, eqs 1 and 2 suggest that the
FRET efficiency (E) will increase upon stretching in case a spectral
overlap integral [J(-)] increases. This predicted phenomenon is
opposite to our results in which we observed the decrease of E
upon stretching. Thus, we could at least rule out the effect of the
blue-shifted donor fluorescence on our proposed mechanism.
45 The orientation factors (and average angles, ªD) of 1 when
elongated by 30 and 60% were calculated to be 0.045 (83.9°) and
0.019 (86.1°), respectively.
18 J. M. Ribó, J. Crusats, F. Sagués, J. Claret, R. Rubires,
Science 2001, 292, 2063.
19 A. Tsuda, M. A. Alam, T. Harada, T. Yamaguchi, N. Ishii,
T. Aida, Angew. Chem., Int. Ed. 2007, 46, 8198.
20 Y. Sagara, T. Kato, Nat. Chem. 2009, 1, 605.
21 S. Mizukami, H. Houjou, K. Sugaya, E. Koyama, H.
Tokuhisa, T. Sasaki, M. Kanesato, Chem. Mater. 2005, 17, 50.
22 Y. Sagara, T. Mutai, I. Yoshikawa, K. Araki, J. Am. Chem.
Soc. 2007, 129, 1520.
23 Y. Sagara, T. Kato, Angew. Chem., Int. Ed. 2008, 47, 5175.
24 H. Ito, T. Saito, N. Oshima, N. Kitamura, S. Ishizaka, Y.
Hinatsu, M. Wakeshima, M. Kato, K. Tsuge, M. Sawamura, J. Am.
Chem. Soc. 2008, 130, 10044.
25 J. Kunzelman, M. Kinami, B. R. Crenshaw, J. D.
Protasiewicz, C. Weder, Adv. Mater. 2008, 20, 119.
26 S. Yagai, T. Seki, T. Karatsu, A. Kitamura, F. Würthner,
Angew. Chem., Int. Ed. 2008, 47, 3367.
27 F. Würthner, M. S. Vollmer, F. Effenberger, P. Emele, D. U.
Meyer, H. Port, H. C. Wolf, J. Am. Chem. Soc. 1995, 117, 8090.
28 M. S. Vollmer, F. Würthner, F. Effenberger, P. Emele, D. U.
Meyer, T. Stümpfig, H. Port, H. C. Wolf, Chem.®Eur. J. 1998, 4,
260.
29 K. Sugiyasu, M. Takeuchi, Chem.®Eur. J. 2009, 15, 6350.
30 J. R. Lakowicz, Principles of Fluorescence Spectroscopy,
2nd ed., by Springer, New York, 1999, Chap. 13, pp. 367-394.
31 L. Stryer, Annu. Rev. Biochem. 1978, 47, 819.
32 R. H. Fairclough, C. R. Cantor, Methods Enzymol. 1978,
48, 347.
33 J. R. Lakowicz, I. Gryczynski, W. Wiczk, G. Laczko, F. C.
Prendergast, M. L. Johnson, Biophys. Chem. 1990, 36, 99.
34 H. Sahoo, D. Roccatano, A. Hennig, W. M. Nau, J. Am.
Chem. Soc. 2007, 129, 9762.
35 A. K. Woźniak, G. F. Schröder, H. Grubmüller, C. A. M.
Seidel, F. Oesterhelt, Proc. Natl. Acad. Sci. U.S.A. 2008, 105,
18337.
46 60% elongating induces deformation of the film and
decrease of the absorbance (thickness of the film), which makes
the absorption measurement difficult. Therefore, the data of only
30% elongated film is shown. Inportantly, this deformation is not a
issue in fluorescence measurements, because we evaluate the
FRET efficiency using the ratio of ID/IA.
47 R. S. Becker, J. S. de Melo, A. L. Maçanita, F. Elisei,
J. Phys. Chem. 1996, 100, 18683.
36 L. Giribabu, A. A. Kumar, V. Neeraja, B. G. Maiya, Angew.