Photoinduced processes of subphthalocyanine-diazobenzene-fullerene triad as an efficient excited energy transfer system

Article abstract of DOI:10.1246/cl.2008.544

Photoinduced processes of a newly synthesized subphthalocyanine- diazobenzene-fullerene triad have been studied by the time-resolved spectroscopic techniques. On photo-excitation of subphthalocyanine (SubPc) moiety, the fluorescence quenching of SubPc was

Full text of DOI:10.1246/cl.2008.544

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Chemistry Letters Vol.37, No.5 (2008)
Photoinduced Processes of Subphthalocyanine–Diazobenzene–Fullerene Triad
as an Efficient Excited Energy Transfer System
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ꢀ2;3
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Jong-Hyung Kim, Mohamed E. El-Khouly, Yasuyuki Araki, Osamu Ito, and Kwang-Yol Kay
1Department of Molecular Science and Technology, Ajou University, Suwon 443-749, Korea
2
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Sendai 980-8577
3
Department of Chemistry, Faculty of Education, Kafr El-Sheikh University, Egypt
(Received February 20, 2008; CL-080194; E-mail: kykay@ajou.ac.kr)
Photoinduced processes of a newly synthesized subphthalo-
The steady-state absorption spectra of the intense magenta
7
cyanine–diazobenzene–fullerene triad have been studied by the
time-resolved spectroscopic techniques. On photo-excitation of
subphthalocyanine (SubPc) moiety, the fluorescence quenching
of SubPc was observed, suggesting the energy-transfer process
from singlet excited energy of the light-harvesting SubPc to
C60 through diazobenzene. This finding is confirmed by the nano-
second transient absorption of the triplet excited state of C60.
solutions of SubPc–PhN=NPh–C60 and SubPc–OPh are shown
in Figure 1. The absorptions of SubPcs consist of a high-energy
B-band (between 300–310 nm) and a lower energy Q-band
(560–580 nm), which are analogous to those of porphyrins and
phthalocyanines. It is notable that the absorptions of the C60
and PhN=NPh moieties in the visible region are extremely weak
compared to the strong absorption of the SubPc in this region.
The presence of the C60 and PhN=NPh moieties was evidenced
by the higher absorptions around 300–350 and 350–420 nm,
respectively. Appreciable change was not observed between
the triad and sum of their components, suggesting absence of
the inter-component interaction in the ground state. The lack
of systematic change in absorption maxima with solvent polarity
suggests a negligible dipole moment change between the ground
state and the excited state.
Three-dimensional subphthalocyanines represent an inter-
esting class of chromophores that show a promise as building
blocks for the construction of photoactive or electro-active
assemblies.1 SubPc is particularly attractive, because their
optical and electronic features can be finely tuned by varying
their axial ligands or by functionalizing the various peripheral
positions. Moreover, the strong absorption and high emission
quantum yields of the SubPc in the visible region render them
ideal probes for energy- and electron-transfer processes. Here,
we report the energy-transfer processes of a newly synthesized
triad of SubPc connected to fullerene C60 through diazobenzene
–6
The fundamental photophysical behavior of SubPc–
PhN=NPh–C60 was investigated by using steady-state fluores-
cence observed with 510 nm excitation, which selectively
excited the SubPc moiety. The emission peak maximum of
SubPc–OPh is located around 574 nm in toluene as shown in
Figure 2, from which the singlet excited energy of the SubPc
moiety was evaluated as 2.1 eV. It is important to note that this
value is substantially higher than those for phthalocyanines
(1.7 eV) and porphyrins (2.0 eV), suggesting high potential of
(
PhN=NPh) linkage as shown in Scheme 1. In this triad mole-
cule, three-dimensional C60, which has been widely used as a
building block for the construction of artificial photosynthetic
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,8
systems, is expected to play as a triplet energy reservoir.
Role of PhN=NPh might be configuration-changeable bridge
by outside stimulus.
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the excited singlet state of the SubPc ( SubPc ) moiety as
electron-donor and energy-donor. In SubPc–PhN=NPh–C60,
the intensity of SubPc emission band was significantly quenched
by ca. 90% compared with that of SubPc–OPh. Since the excited
singlet state of PhN=NPh is higher than that of SubPc, this
SubPc–PhN=NPh–C60 (1) has been prepared as depicted
in Scheme 1. Every step of the reaction sequence proceeded
smoothly and efficiently to give a good or moderate yield of
the product (see the Supporting Information for the synthetic
details).9
N
N
OH
O2N
H2N
OH
OH
HO
2
3
O
H
N
N
O
H
O
N
OH
N
N
B
N
N
N
N
Cl
N
N
N
B
N
4
6
N
N
N
5
C8H17
N
N
O
N
N
N
B
N
N
N
Figure 1. Steady-state absorption spectra of SubPc–PhN=
NPh–C60 and SubPc–OPh in toluene (TN), o-dichlorobenzene
N
1
(
DCB), and dimethylformamide (DMF); concentrations were
ꢂ6
Scheme 1. Synthetic route of SubPc–PhN=NPh–C60 (1).
kept at 5 ꢁ 10 M.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.5 (2008)
545
Figure 2. (Left) Steady-state fluorescence spectra of SubPc–
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OPh and SubPc–PhN=NPh–C60 (5 ꢁ 10 M); ꢁex ¼ 510 nm.
Figure 4. Transient absorption spectra of SubPc–PhN=NPh–
ꢂ4
(
Right) Time-resolved fluorescence spectra of SubPc–Ph=Ph–
C60 (1 ꢁ 10 M) in DMF; ꢁex ¼ 532 nm laser light.
C60 in benzonitrile (BN); ꢁex ¼ 400 nm.
The nanosecond transient spectra of SubPc–PhN=NPh–C60
observed by excitation with 532 nm laser light exhibit only the
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characteristic peak of triplet fulleropyrrolidine ( C60 ) at
00 nm as shown in Figure 4. The spectra show no evidence of
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þ
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forming radical ion pairs like SubPc –PhN=NPh–C60 . In
nonpolar and less polar solvents, transient spectra were observed
with the similar intensity and decay of C60 to those in DMF.
These observations for both steady-state emission and time-re-
solved emission as well as nanosecond transient absorption
measurements indicate that the energy-transfer process from
the SubPc to the C60 moiety takes place in all the studied sol-
vents generating C60 from which C60 was populated by the
intersystem crossing process. Free-energy change of the charge
separation in polar solvents is almost iso-energetic to SubPc ,
suggesting slower rates than those of exothermic energy transfer.
Since the C60 moiety survives for appreciably longer time
than 10 ms (inserted time profile in Figure 4), the C60 moiety
remains keeping possibility to utilize the triplet energy to further
process to generate the singlet oxygen for the use of photody-
namic therapy in the aerobic biological systems. Further study
is in progress in these views, in addition to assessment of the
effect of the trans–cis isomerization of the PhN=NPh as length-
and configuration-tunable molecular bridge.
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1
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Figure 3. Fluorescence decay profiles of SubPc–PhN=NPh–
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C60 and SubPc–OPh (5 ꢁ 10 M) in 500–650 nm region;
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ꢁ
ex ¼ 400 nm.
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1
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observation suggests efficient quenching of the SubPc moiety
by the appended C60 moiety.
The difference in emission quenching was not observed with
changing the solvents from toluene to DMF, suggesting that
the intramolecular processes proceed with nearly activationless
kinetics processes such as energy transfer.
The time-resolved fluorescence features of SubPc–
PhN=NPh–C60 observed with the picosecond laser light pulse
at 400 nm showed the fluorescence of the SubPc moiety predom-
inantly at 100 ps after the laser pulse, whereas after 1 ns a new
References and Notes
G. de la Torre, T. Torres, F. Agull o´ -L o´ pez, Adv. Mater.
997, 9, 265.
1
1
fluorescence band appeared at 700 nm due to 1C60 even in
BN, supporting the energy transfer. The fluorescence decay of
SubPc–OPh obeyed a mono-exponential decay function with a
ꢀ
2
N. Kobayashi, T. Ishizaki, K. Ishii, H. Konami, J. Am. Chem.
Soc. 1999, 121, 9096.
3
4
N. Kobayashi, Bull. Chem. Soc. Jpn. 2002, 75, 1.
C. G. Claessens, D. Gonz a´ lez-Rodr ´ı guez, T. Torres, Chem.
Rev. 2002, 102, 835.
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lifetime (ꢀf) of 2.1 ns. As shown in Figure 3, the fluorescence
time profiles of SubPc–PhN=NPh–C60 exhibited much faster
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decay, comparing with that of SubPc–OPh reference. They
D. Gonzalez-Rodrıguez, T. Torres, D. M. Guldi, J. Rivera,
´
M. A. Herranz, L. Echegoyen, J. Am. Chem. Soc. 2004,
´
5
also could be fitted satisfactorily to a mono-exponential decay
function, giving the short lifetimes as 120, 140, and 120 ps in
toluene, BN, and DMF, respectively. These observations clearly
1
26, 6301.
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D. D. D ´ı az, H. J. Bolink, L. Cappelli, C. G. Claessens, E.
Coronado, T. Torres, Tetrahedron Lett. 2007, 48, 4657.
M. E. El-Khouly, S.-H. Shim, Y. Araki, O. Ito, K.-Y. Kay,
J. Phys. Chem., in press, doi:10.1021/jp7108668.
Y. Araki, O. Ito, in Handbook of Organic Electronics
and Photonics, ed. by H. S. Nalwa, American Scientific
Publishers, California, 2008, Vol. 2, Chap. 12, pp. 473–513.
Supporting Information is also available electronically
on the CSJ-Journal Web site, http://www.csj.jp/journals/
chem-lett/index.html.
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show quenching of the fluorescence of SubPc by the attached
C60 moiety. Based on the differences of fluorescence lifetimes
between SubPc–PhN=NPh–C60 and SubPc–OPh, the rates and
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quantum yields of the quenching process of the SubPc moiety
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were evaluated in the ranges of ð1:2{1:3Þ ꢁ 10
.97, respectively. The independency of these quenching
quantities from the solvent polarity supports that the energy
s
and 0.96–
0
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transfer predominantly takes place from the SubPc moiety to
the remote C60 moiety over the PhN=NPh linkage.
Published on the web (Advance View) April 12, 2008; doi:10.1246/cl.2008.544