Synthesis and photoinduced intramolecular processes of light-harvesting silicon phthalocyanine-naphthalenediimide-fullerene connected systems

Article abstract of DOI:10.1002/chem.200900165

Photoinduced intramolecular processes of a newly synthesized pentad composed of silicon phthalocya- nine (SiPc) that is connected with two units of naphthalenediimide (NDI) and fullerene C₆₀ to form SiPc-(NDI) ₂- (C₆₀)

Full text of DOI:10.1002/chem.200900165

FULL PAPER
DOI: 10.1002/chem.200900165
Synthesis and Photoinduced Intramolecular Processes of Light-Harvesting
Silicon Phthalocyanine–Naphthalenediimide–Fullerene Connected Systems
Mohamed E. El-Khouly,[a] Jung Hoon Kim,[b] Kwang-Yol Kay,*[b] Chan Soo Choi,[c]
Osamu Ito,[d] and Shunichi Fukuzumi*[a]
Abstract: Photoinduced intramolecular
processes of newly synthesized
SiPc-(NDI)2-(C60)2 and SiPc-(NDI)2,
but not NDI-C60 for which the energy
transfer is dominant. The occurrence of
electron-transfer processes of SiPc-
(NDI)2-(C60)2 and SiPc-(NDI)2 were
studied by time-resolved emission and
transient absorption techniques and
confirmed by redox measurements and
molecular orbital calculations with ab
initio B3LYP/3–21G(*) methods. Fast
and efficient charge-separation process-
es via the singlet excited states of NDI
and SiPc were monitored, followed by
charge recombination to populate the
a
pentad composed of silicon phthalocya-
nine (SiPc) that is connected with two
units of naphthalenediimide (NDI) and
fullerene C60 to form SiPc-(NDI)2-
(C60)2 have been studied and the results
are compared with the reference com-
pounds, namely, the SiPc-(NDI)2 triad
and NDI-C60 dyad. Upon photoexcita-
tion, the main quenching pathway in
polar solvents involved electron trans-
fer via the singlet excited states of
C
60 and SiPc triplet states. The lifetimes
of charge-separated states were esti-
mated as 1000 and 250 ps for SiPc-
Keywords: electron transfer · ful-
lerenes · laser chemistry · naphtha-
(NDI)2-(C60)2
respectively.
and
SiPc-(NDI)2,
lenediimides
·
photochemistry
·
AHCTUNGTRENNUNG
phthalocyanines
Introduction
an active area of research. Phthalocyanines with their excel-
lent light-harvesting properties in a wide range of the solar
light spectrum (the maximum around 700 nm, at which the
maximum of the solar photon flux occurs) are promising
materials for these applications. In addition, phthalocyanines
exhibit rich redox chemistry, characteristic electrical, ther-
mal, optical, light stabilities, semiconducting and catalytic
properties.[1–8] Among the reported phthalocyanines, the ax-
ially substituted silicon phthalocyanines (SiPc) have gained
attention recently, because they are not able to aggregate in
solution due to their special structural features; in addition
they have longer fluorescence lifetimes than the more
common zinc phthalocyanines.[9] A number of accessory pig-
ments have been examined in conjunction with silicon
phthalocyanines, including carotenoid,[10] azobenzene,[11] tet-
rathiafulvalene,[12] ferrocene,[13] porphyrin,[14] cadmium sele-
nide (CdSe) nanoparticles,[15] fullerene,[16] and perylene.[17]
Compared with porphyrins, phthalocyanines emerge as at-
tractive molecular building blocks for the artificial photo-
synthetic systems, in which they function as antennae with a
wider absorbing range of the solar spectrum at which por-
phyrins fail to exhibit appreciable absorption. Although sev-
eral systems composed of porphyrins with multi-electron-ac-
cepting entities have been developed to study multistep
electron-transfer processes mimicking the photosynthetic
The synthesis of near-infrared (NIR) light-absorbing and
-emitting organic chromophores for photochemical energy
conversion and storage, as well as optical data processing is
[a] Dr. M. E. El-Khouly, Prof. Dr. S. Fukuzumi
Department of Material and Life Science
Graduate School of Engineering
Osaka University
SORST, Japan Science and Technology Agency (JST)
Suita, Osaka 565–0871 (Japan)
Fax : (+82)-31-213-3652
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
[b] Dr. J. H. Kim, Prof. Dr. K.-Y. Kay
Department of Molecular Science and Technology
Ajou University, Suwon 443–749 (South Korea)
Fax : (+82)-31-219-1615
E-mail: kykay@ajou.ac.kr
[c] Dr. C. S. Choi
Department of Applied Chemistry
Daejeon University, Daejeon 300–716 (South Korea)
[d] Prof. Dr. O. Ito
Institute of Multidisciplinary Research for Advanced Materials
Tohoku University, Katahira
Sendai, 980–8577 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900165.
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A
5) The electron shift between the electron-accepting NDI
and C60 entities of 1 is anticipated, which may contribute
in stabilizing the charged-separated states.
of phthalocyanines with multi-electron-accepting entities
have yet to be explored.
In this context, we report herein the integration of silicon
phthalocyanine with two units of different electron accept-
ors, 1,4,5,8-naphthalenenediimide (NDI) and fullerene (C60),
through the axial position to form the SiPc-(NDI)2-(C60)2
pentad (1), which emerges as a promising material for solar-
In the current work, we are concerned in exploring the in-
tramolecular events of the newly synthesized SiPc-(NDI)2-
(C60)2 pentad, in which SiPc would be expected to act as a
photosensitizing electron donor, and NDI and C60 act as
planar and spherical electron acceptors, respectively. The re-
sults are compared with SiPc-(NDI)2 (2) and NDI-C60 (3) as
reference compounds. From the spectroscopic characteriza-
tion by time-resolved emission and transient absorption
techniques, we could obtain a clear picture about the photo-
induced intramolecular processes via the singlet-excited
states of the NDI and SiPc entities.
Results and Discussion
Synthesis and characterization: The most straightforward
preparation of SiPc-(NDI)2-(C60)2 (1) and its reference com-
pounds, SiPc-(NDI)2 (2) and NDI-C60 (3), can be envisaged
to proceed through a reaction sequence of the following
steps as depicted in Scheme 1. Every step of the reaction se-
quence proceeded smoothly and efficiently to give a good or
moderate yield of the product (see Experimental Section for
the synthetic details).
Commercially available 1,4,5,8-naphthalenetetracarboxylic
dianhydride (NDA) was treated with p-anisidine to give
naphthalenemonoimide 4 in 45.0%, which was then convert-
ed by using p-aminobenzyl alcohol to the corresponding
naphthalenediimide (NDI) 5 in 52.0%. The alcohol group in
5 was oxidized with PCC to give aldehyde 6 in 74.0%, and
then fulleropyrrolidine formation was achieved by 1,3-dipo-
lar cycloaddition reactions between aldehyde 6 and C60 in
the presence of excess N-octylglycine,[24] under conditions
described by Prato,[25] to give 7 in 40.5%. Subsequently, de-
methylation from the methoxy group of 7 was carried out
using BBr3 to afford hydroxyl group containing 8 in 44.5%.
Finally, the reaction between 8 and silicon phthalocyanine
dichloride was performed in the presence of sodium hydride
to produce the pentad 1 in 12.2%. Reference compounds 2
and 3 were synthesized in a similar fashion. The compounds
1–3 are very soluble in aromatic solvents (i.e., toluene, o-di-
chlorobenzene, benzonitrile) and other common organic sol-
vents. The structure and purity of the new compounds were
energy-harvesting purposes. To the best of our knowledge,
this is the first report of the synthesis and photoinduced in-
tramolecular processes of a donor–acceptor system com-
posed of silicon phthalocyanine as electron donor, linked co-
valently with multi-electron-accepting entities. A number of
considerations led to design of 1 including:
1) The pentad 1 serves as a light-harvesting architecture
with good spectral coverage in the visible and NIR re-
gions.
1
confirmed by H and 13C NMR spectroscopy, MALDI-TOF
2) Planar NDI was chosen as an electron acceptor, because
of its higher energy excited state than SiPc and C60, pro-
viding a large driving force for the charge separation
(CS) process, as well as its lower reduction potential.[22,23]
3) From a photophysical standpoint, the small reorganiza-
tion energies for the widely diffused p-electron on the
C60 radical anion offer unique opportunities for stabiliz-
ing charged entities.[19–21]
mass spectrometry, and elemental analysis (see Experimen-
tal Section).
The H NMR spectra of 1, 2, and 3 in CDCl3 are consis-
tent with the proposed structures, showing the expected fea-
tures with the correct integration ratios. 13C NMR spectra
showed characteristic aliphatic and aromatic carbon peaks
as well as C60 resonance lines in the expected region. These
patterns of signals are unambiguously diagnostic for the pro-
posed structures of 1, 2, and 3. The MALDI-TOF MS pro-
vided a direct evidence for the structures of 1, 2, and 3 by a
1
4) The geometry of 1 is suitable for fast and efficient intra-
molecular charge-separation processes.
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Light-Harvesting Systems
FULL PAPER
the individual chromophores in
their ground-state configura-
tion.
The photophysical behavior
was qualitatively investigated
by steady-state fluorescence
using the 620 nm light excita-
tion, which selectively excited
the SiPc entity. As shown in
Figure 2, the fluorescence spec-
trum of SiPc reference shows a
maximum at 690 nm with
shoulders at 660 and 620 nm.
By the axial linkage of SiPc
with NDI, the 1SiPc* emission
intensity of 2 was significantly
decreased in polar tetrahydro-
furan and benzonitrile, but not
in toluene. Such fluorescence
quenching in polar solvents sug-
gests a charge-separation (CS)
Scheme 1. Synthesis of 1, 2, and 3. a) p-Anisidine, DMF, reflux, 4 d. b) 4-Aminobenzylalcohol, ZnACHTUNRGTNEUNG(OAc)2, tolu-
ene, reflux, 3 d for 5, 2 d for 11. c) PCC, dichloromethane, 1 h. d) C60, N-octylglycine, chlorobenzene, reflux,
3 d. e) BBr3, dichloromethane, 08C, 1 h then RT, 24 h. f) Silicon phthalocyanine dichloride, NaH, toluene,
1
process via SiPc* with respect
to NDI, generating the charge-
+
CÀ
.
reflux, 3 d. g) Octylamine, DMF, reflux, 3 d. h) 4-Aminophenol, ZnACTHNUGTRENUNG(OAc)2, pyridine, reflux, 2 d.
C
separated
SiPc -(NDI)2
singly charged molecular ion peak at m/z=3155.28, 1478.74
and 1328.44, respectively.
UV/Vis spectral and fluorescence emission studies: The
steady-state absorption spectra of 1 and 2 in benzonitrile are
shown in Figure 1 with that of SiPc reference. The absorp-
tion spectrum of the SiPc reference consists of high-energy
B-bands (peak positions=334 and 355 nm) and lower
energy Q-bands (peak positions=610 and 690 nm) arising
from p–p* transitions. In 2, the SiPc moiety exhibits similar
absorption features to that of the SiPc reference. The NDI
entity has absorption bands in the 300–400 nm region with a
maxima at 381, 359, and 339 nm. Compound 1 has addition-
al absorption peaks corresponding to the fulleropyrrolidine
entity that are observed below 350 nm, in addition to weak
absorptions at 432 and 704 nm. The absorption spectra of 1
and 2 showed no significant electronic interaction between
Figure 2. Steady-state fluorescence spectra of SiPc, 2 and 1 in toluene
and benzonitrile. Concentrations were kept at 5.5ꢁ10À6
620 nm.
m and lex=
1
Since the energy of SiPc* (1.77 eV) is much lower than the
energy of 1NDI* (3.10 eV),[26] the energy transfer from
1SiPc* to NDI is energetically unfavorable.
1
In case of 1, further decreases of the SiPc* emission in-
tensity were observed in polar solvents, which may be inter-
preted by the additional CS from 1SiPc* to C60 through
+
C
CÀ
NDI, generating SiPc -(NDI)2-(C60)2 , in addition to the
1
energy transfer from SiPc* to C60. In toluene, in which the
CS process is energetically unfavorable, it is most likely that
1
the energy-transfer process from SiPc* to C60 over the NDI
moiety is the quenching pathway.
Fluorescence lifetime studies: To complement the emission
spectral studies, fluorescence lifetime measurements
(Figure 3) were performed, which supported the above con-
Figure 1. Steady-state absorption spectra of SiPc, 1 and 2 in benzonitrile.
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S. Fukuzumi, K.-Y. Kay et al.
Figure 4. Femtosecond transient absorption spectra of 3 in deaerated ben-
zonitrile after laser excitation at 380 nm.
380 nm. We rule out the excitation of C60 due to the low ex-
tinction coefficient at this wavelength. Photons at this wave-
length exclusively pump the NDI component. The spectrum
at 1 ps with peak at 480 nm and shoulder at 600 nm is attrib-
uted to the NDI singlet excited state (1NDI*). The spectrum
Figure 3. Fluorescence decay profiles of 1SiPc* of 2 (top) and 1 (bottom)
in deaerated solvents.
1
sideration in a more quantitative way. The fluorescence
decay profiles were fitted satisfactorily with mono-exponen-
at 3000 ps shows the concomitant formation of C60* with
the decay of 1NDI*, indicating that the major fraction of
1NDI* decayed by an energy-transfer process to populate
1C60*. The rate of the energy-transfer process was estimated
1
tial function. The fluorescence of SiPc* reference decayed
with a lifetime (tf0) of 5800 ps.[16b] Figure 3 shows the fluo-
rescence decay profiles of 1SiPc*-(NDI)2 in different sol-
vents. In toluene, no shortening of the lifetime (tf) was ob-
served. In polar solvents, the fluorescence lifetimes were
evaluated as 1050 and 930 ps in tetrahydrofuran and benzo-
nitrile, respectively. It is most likely that the quenching pro-
1
1
from the decay of NDI*, as well as the formation of C60*,
as 7.5ꢁ108 sÀ1. The absence of radical-ion pairs indicates
that the CS process is not involved in the quenching process
of 1NDI*. On the other hand, the nanosecond transient
spectra at 0.25 and 2.50 ms (Figure 5) showed only the char-
acteristic C60 triplet state (3C60*) at 700 nm.
1
cess is due to the charge-separation from the SiPc* to the
attached NDI entities. The rates (kCS) (and quantum yields
FCS) of the CS processes were evaluated as 5.2ꢁ108 sÀ1
(0.75) and 9.1ꢁ108 sÀ1 (0.84) in tetrahydrofuran and benzo-
nitrile, respectively.[27]
Compared with 1SiPc*-(NDI)2, the fluorescence decay
profiles of 1SiPc*-(NDI)2-(C60)2 exhibit short-lived compo-
nents with lifetimes of 570 and 440 ps in tetrahydrofuran
and benzonitrile, respectively. Such quenching processes in
the polar solvents can be explained mainly by the electron
1
transfer from SiPc* to the attached electron-accepting NDI
and C60. The rates (kCS) (and quantum-yields FCS) of the CS
processes were evaluated as 1.58ꢁ109 sÀ1 (0.90) and 2.10ꢁ
109 sÀ1 (0.93) in tetrahydrofuran and benzonitrile, respective-
1
ly. In toluene, the shortening of the SiPc* lifetime (830 ps)
1
can be attributed only to the energy transfer from SiPc* to
Figure 5. Nanosecond transient absorption spectra of 3 in deaerated ben-
zonitrile.
C60 with a rate constant of 1.0ꢁ109 sÀ1 and a quantum yield
of 0.86.
To further understand the reaction mechanism and follow
the kinetics of photoinduced intramolecular processes,
femto- and nanosecond laser photolysis studies were per-
formed on 1–3 as in the forthcoming sections.
Figure 6 shows the energy-level diagram summarizing the
observed photoinduced intermolecular events of 3 in benzo-
nitrile. Upon photoirradiation of 3 by 380 nm, the 1NDI* de-
1
cayed by singlet–singlet energy transfer to populate C60*,
Time-resolved absorption spectra of 1–3: Figure 4 shows
representative transient absorption difference spectra for 3
in benzonitrile following excitation with 130 fs flash at
which relaxes to the C60 triplet state (3C60*). Finally, the trip-
let state decayed to the ground state with a rate constant
1.5ꢁ105 sÀ1.
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Light-Harvesting Systems
FULL PAPER
nential function, the rate constant of the CR process (kCR
)
)
was found to be 4.0ꢁ109 sÀ1, from which the lifetime (tRIP
+
C
CÀ
of the radical-ion pair, SiPc -(NDI)2 , was estimated as
250 ps. The nanosecond transient absorption spectra of 2 in
deaerated benzonitrile (Figure 8) show no characteristic ab-
sorption of the radical-ion pair, but instead the triplet–trip-
let (T–T) absorption of 3SiPc*-(NDI)2 was observed at
500 nm that decays to the ground state with a rate constant
of 1.4ꢁ104 sÀ1.
Figure 6. Energy-level diagram shows the different intramolecular events
of 3 in benzonitrile.
When turning to 1 and 2, the time-resolved spectra
showed quite different behavior compared with 3. Upon
photoexcitation of a deaerated solution containing 2 in ben-
zonitrile at 380 nm, which selectively excites the NDI entity,
+
C
the transient absorption band of SiPc radical cation (SiPc
)
was clearly observed at 865 and 550 nm (Figure 7).[16b,17] In
addition, the observed absorption bands at 484, 555, 585,
635, 713 and 744 nm were recorded and assigned to NDI
Figure 8. Nanosecond transient absorption spectra of 2 in deaerated ben-
CÀ
CÀ
radical anion (NDI ) by comparison with those of NDI
zonitrile.
produced by the one-electron reduction of NDI with tetra-
methylsemiquninone radical anion and tetrakis(dimethyl-
amino)ethylene (Figures S4–S6 in the Supporting Informa-
The occurrence of photoinduced CS and CR processes of
1 via 1NDI* was confirmed by femtosecond laser flash meas-
urements with an excitation wavelength of 380 nm, as shown
in Figure 9. The excitation of SiPc and C60 of 1 was ruled
out due to the low extinction coefficient at this wavelength.
Upon photoirradiation of a deaerated solution of 1 in ben-
zonitrile, the characteristic absorption bands of the SiPc
and NDI were observed, generating SiPc -(NDI)2 -(C60)2.
In contrast to the nanosecond transient spectra of 2, we
note a weak absorption at around 1000 nm for 1 that may
tion).[28] These observations clearly indicate the formation of
+
C
CÀ
SiPc -(NDI)2 within 9 ps. The characteristic absorption
+
C
band of SiPc was employed to determine the rates of CS
and charge recombination (CR) processes. From the inset of
+
C
Figure 7, the time-profile of SiPc at 865 nm shows quick
+
C
rise followed by slow decay. From the quick rise, the rate
constant of CS process (kCS) was found to be 2.4ꢁ1011 sÀ1,
which is larger than that of the CS process via 1SiPc*-
(NDI)2 as evaluated from fluorescence lifetime (9.1ꢁ108 sÀ1
in bezonitrile). By fitting the slow decay to a single-expo-
+
CÀ
C
CÀ
Figure 7. Femtosecond transient absorption spectra of 2 in deaerated ben-
Figure 9. Femtosecond transient absorption spectra of 1 in deaerated ben-
zonitrile after laser excitation at 380 nm.
zonitrile after laser excitation at 380 nm.
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S. Fukuzumi, K.-Y. Kay et al.
CÀ
arise from the electron shifts from the NDI to the adjoin-
found to be same as that of 2, while the Ered were located at
À992 and À1420 mV versus Ag/AgNO3 (Figure S3 in the
Supporting Information). From the redox measurements, we
note a strong overlap of the first Ered of the C60 and NDI en-
tities of 1, supporting the electron-shift between the NDI
and C60 entities of 1 as shown in Figure 9. In addition, a neg-
ative shift of 20 mV was recorded for 1 as compared to 2.
The driving forces for charge-recombination processes
(ÀDGCR) of the CS states of 1 and 2 in benzonitrile were
found to be 1.65 and 1.68 eV, respectively.[29] The higher cal-
+
C
CÀ
ing C60 to generate SiPc -(NDI)2-(C60)2 . The rate constant
of CS process was found to be 3.0ꢁ1011 sÀ1, which is larger
1
than that of 2. The kCS values via NDI* are much larger
than that via SiPc* as evaluated from the fluorescence life-
1
time (2.1ꢁ109 sÀ1 in bezonitrile). By following the decay of
the CS state in benzonitrile—throughout the visible and
NIR regions—the rate of CR process was found to be 1.0ꢁ
109 sÀ1, from which a lifetime of 1000 ps was derived. The
finding that the tRIP of 1 was found to be longer than that of
2, suggesting that the electron shift between the electron-ac-
cepting NDI and C60 entities prolongs the life time of the
radical-ion pair.
1
culated values of ÀDGCR of 1 and 2 via NDI* suggest that
the CR process belongs to the inverted region of the Marcus
parabola.[30–32] The higher ÀDGCR values predict the occur-
rence of relatively slow charge recombination for phthalo-
cyanine-based compounds 1 and 2, and the experimentally
determined tCS value (1000 ps) readily agrees with this.
The decay of the CS state of 1 in benzonitrile affords the
triplet excited state of C60 (3C60*) as well as the ground state
as confirmed by nanosecond laser measurements
(Figure 10). The nanosecond spectra show no absorption
1
From the energy of NDI*, the driving forces of the CS pro-
cess (ÀDGCS) were found to be 1.45 and 1.42 eV for 1 and 2,
respectively. High exothermicity for the CS process via
1NDI* support the observed larger kCS values. On the other
1
hand, the ÀDGCS values of 1 via 1SiPc* and C60* were
found to be 0.12 and 0.10 eV, respectively. Similarly, the
1
ÀDGCS of 2 via SiPc* was found to be 0.09 eV. Such a low
1
1
exothermicity for the CS processes via SiPc* and C60* sup-
1
port the observed smaller kCS values via SiPc*.
Moreover, the charge separation processes of the cova-
lently linked SiPc-NDI-C60 were supported by studying the
electronic structures on the basis of molecular orbital calcu-
lations by using ab initio B3LYP/3–21G (*) methods
(Figure 11). The electron-density distribution of the highest
occupied molecular orbital (HOMO) was found to be locat-
ed over the SiPc entity, while the electron distributions of
the lowest unoccupied molecular orbitals LUMO+1 and
LUMO are entirely located over the C60 and NDI entities,
Figure 10. Nanosecond transient absorption spectra of 1 in deaerated
benzonitrile.
+
+
C
CÀ
C
respectively, suggesting SiPc -(NDI)2-(C60)2 and SiPc
-
CÀ
(NDI)2 -(C60)2 are charge-separated states. Although the
bands of the radical-ion pair, but instead the decay of SiPc-
LUMO and LUMO+1 in Figure 11 indicate slightly lower
+
(NDI)2--3
ACHTUNGTRENNUNG(C60)2* was observed at 720 nm accompanied with
energy for SiPc -(NDI)2 -(C60)2 than SiPc +-(NDI)2-
C
CÀ
C
3
the formation of SiPc*-(NDI)2-(C60)2 at 500 nm. The decay-
ACTHNUTRGNEUNG
rate of SiPc-(NDI)2--3(C60)2* (9.0ꢁ104 sÀ1) seems coincide
with the formation of 3SiPc*-(NDI)2-(C60)2, which in turn de-
cayed to the ground state with a rate constant of 1.4ꢁ
104 sÀ1. Similar nanosecond transient spectra were obtained
by employing 355 and 532 nm laser excitation.
1
1
The charge-separation processes via the NDI* and SiPc*
were supported from the viewpoint of thermodynamics of
electron-transfer processes. Electrochemical measurements
were carried out by using cyclic voltammetry (CV) and dif-
ferential pulse voltammogram (DPV) techniques at a glassy
carbon electrode in carefully dried benzonitrile containing
0.1m
tetra-n-butylammonium
hexafluorophosphate
(TBAPF6; 0.1m) at room temperature (see the Supporting
Information). The first oxidation potential (Eox) of the SiPc
was located at 636 mV versus Ag/AgNO3, while the reduc-
tion potentials (Ered) of 2 were located at À1012 and
À1432 mV versus Ag/AgNO3 (Figure S2 in the Supporting
Information). When turning to 1, the Eox of the SiPc was
Figure 11. Frontier HOMO and LUMOs of SiPc-NDI-C60 calculated by
ab initio B3LYP/3–21G(*) methods.
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Light-Harvesting Systems
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CÀ
(C60)2 , the energy difference is quite small as revealed by
The CS state of 2 decayed to populate the SiPc triplet
state; however, the direct production of the C60* state by
3
the CV measurements.
As depicted in Figure 12 for the photoinduced intramo-
lecular processes of 1 and 2 in benzonitrile, the 380 nm laser
excitation pumps up the NDI moiety to its singlet excited
charge recombination of 1 is likely to be the pathway. From
3C60*, the T–T energy-transfer process of 1 was confirmed by
the nanosecond transient measurements to populate low-
ENACTHNUTRGNEUNG
lying 3SiPc* (k (T-T)=6.3ꢁ104 sÀ1). Finally, the triplet
state decayed to the ground state with a rate constant 1.4ꢁ
104 sÀ1.
Conclusion
Details of the photoinduced intramolecular processes of the
newly synthesized pentand composed of SiPc and two differ-
ent acceptors (1: SiPc-(NDI)2-(C60)2) in comparison with the
triad without C60 (2: SiPc-(NDI)2) have been revealed by
using time-resolved fluorescence and absorption techniques
by the selective excitation of NDI and SiPc. Our results
strongly support the view that upon photoirradiation, the
1
1
CS processes via NDI* and SiPc* take place in polar sol-
+
C
CÀ
vents forming the charge-separated state SiPc -(NDI)2
-
(C60)2. Importantly, a stabilization of the radical-ion pair
state has been observed when two naphthalenediimide/two
fullerene electron acceptors were employed rather than just
two naphthalenediimide electron acceptors. The longer tRIP
of 1 (1000 ps) compared to that of 2 (250 ps) in benzonitrile
reflects the role of the spherical C60 on increasing tRIP
through the electron shift from the NDI to C60 entities gen-
+
C
CÀ
erating SiPc -(NDI)2-(C60)2 . A high kCS/kCR ratio up to
1
about 3000 was observed for 1 via NDI*, a measure of the
excellence in the photoinduced electron-transfer systems,
suggest the usefulness of 1 and 2 as light-harvesting systems.
Experimental Section
Instruments: Steady-state absorption and fluorescence spectra were mea-
sured on a JASCO V-550 spectrometer (UV/Vis) and Shimadzu spectro-
fluorophotometer equipped with a photomultiplier tube with high sensi-
tivity in the longer wavelength region.
Figure 12. Energy-level diagram shows the different photoinduced intra-
Electrochemical measurements were performed on an ALS630B electro-
chemical analyzer in deaerated benzonitrile containing 0.1m TBAPF6 as
supporting electrolyte at 298 K. A conventional three-electrode cell was
used with platinum working electrode (surface area of 0.3 mm2) and a
platinum wire as the counter electrode. The Pt working electrode (BAS)
was routinely polished with BAS polishing alumina suspension and
rinsed with acetone before use. The measured potentials were recorded
with respect to the Ag/AgNO3 (0.01m) reference electrode. All electro-
chemical measurements were carried out under an atmospheric pressure
of argon.
molecular events of 2 (top) and 1 (bottom) in benzonitrile.
state, from which the CS process takes place quite efficiently
as confirmed by femtosecond laser measurements. From the
time-resolved emission measurements, the CS processes
from 1SiPc* to the ground-state NDI were recorded. The
1
findings that the rates of CS processes via NDI* (ꢀ1011 sÀ1)
Density-functional theory (DFT) calculations were performed on
a
1
are much faster than those via SiPc* (ꢀ109 sÀ1) can be ex-
COMPAQ DS20E computer. Geometry optimizations were carried out
using the B3LYP functional and 3-21G(*) basis set,[33] with the unrestrict-
ed Hartree–Fock (UHF) formalism and as implemented in the Gaussi-
an 03 program.[34] Graphical outputs of the computational results were
generated with the Gauss View software program (ver. 3.09) developed
by Semichem, Inc.
plained in terms of increase of the ÀDGCS values, suggesting
1
that the CS process via SiPc* moiety is located near the
foot of the normal region of Marcus parabola, while the CS
1
via NDI* may be located near the top of Marcus parabola,
because relatively large reorganization energies similar to
The picosecond time-resolved fluorescence spectra were measured by a
single-photon counting method by using a second-harmonic generation
1
the DGCS via NDI* have been reported for molecules in-
cluding the NDI units.
(SHG, 400 nm) of a Ti:sapphire laser (Spectra-Physica, Tsunami
Chem. Eur. J. 2009, 15, 5301 – 5310
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5307

S. Fukuzumi, K.-Y. Kay et al.
3950 L2S, 1.5 ps fwhm) and
a
streak-scope (Hamamatsu Photonics)
phthalocyanine dichloride (30 mg, 0.046 mmol) was added to the mixture,
which was then heated under reflux for 3 d. After cooling, the crude ma-
terial was purified by column chromatography over silica gel with di-
chloromethane/acetone (98:2) to yield compound 2 (27 mg, 37%) as a
green solid. M.p. > 4008C (decomp); 1H NMR (400 MHz, CDCl3): d=
9.65 (m, 8H), 8.58 (d, J=8.2 Hz, 4H), 8.43 (d, J=8.2 Hz, 4H), 8.33 (m,
8H), 5.50 (d, J=8.2 Hz, 4H), 4.12 (d, J=8.2 Hz, 4H), 2.62 (d, J=8.1 Hz,
4H), 1.21 (m, 24H), 0.81 ppm (t, J=8.1 Hz, 6H); IR (KBr): n˜ =3008,
2911, 2855, 1738, 1522, 1298, 1178 cmÀ1; UV/Vis (toluene): lmax =360,
382, 588, 615, 648, 684 nm; MS (MALDI-TOF): m/z calcd for
C88H66N12O10Si: 1479.63; found 1478.74; elemental analysis calcd (%): C
71.43, H 4.50, N 11.36; found: C 71.42, H 4.54, N 11.33.
equipped with a polychromator as an excitation source and a detector,
respectively. Lifetimes were evaluated with software attached to the
equipment.
For the nanosecond measurements, the solutions containing 1, 2, and 3
were excited by a Panther OPO pumped by Nd:YAG laser (Continuum,
SLII-10, 4–6 ns fwhm) with the powers of 1.5 and 3.0 mJ per pulse. The
transient absorption measurements were performed by using a continu-
ous xenon lamp (150 W) and an InGaAs-PIN photodiode (Hamamatsu
2949) as a probe light and a detector, respectively. The output from the
photodiodes and a photomultiplier tube was recorded with a digitizing
oscilloscope (Tektronix, TDS3032, 300 MHz). Femtosecond transient ab-
sorption spectroscopy experiments were conducted using an ultrafast
source: Integra-C (Quantronix), an optical parametric amplifier: TOPAS
(Light Conversion Ltd.) and a commercially available optical detection
system: Helios provided by Ultrafast Systems LLC. The source for the
pump and probe pulses were derived from the fundamental output of In-
tegra-C (780 nm, 2 mJ per pulse and fwhm=130 fs) at a repetition rate
of 1 kHz. 75% of the fundamental output of the laser was introduced
into TOPAS, which has optical frequency mixers resulting in tunable
range from 285 to 1660 nm, while the rest of the output was used for
white-light generation. Typically, 2500 excitation pulses were averaged
for 5 s to obtain the transient spectrum at a set delay time. Kinetic traces
at appropriate wavelengths were assembled from the time-resolved spec-
tral data. All measurements were conducted at 298 K. The transient spec-
tra were recorded using fresh solutions in each laser excitation.
Preparation of NDI-C60 (3): Compound 12 (0.1 g, 0.21 mmol), fullerene
(0.16 g, 0.22 mmol), and N-octylglycine (0.10 g, 0.50 mmol) were added to
chlorobenzene (50 mL) and heated under reflux for 3 d. After cooling,
the solvent was evaporated and the product was purified by column chro-
matography on silica gel with dichloromethane/hexane (3:1) to give com-
pound
3 (76 mg, 27%) as a brown solid. M.p. >4008C (decomp);
1H NMR (400 MHz, CDCl3): d=8.81 (d, J=7.1 Hz, 4H), 8.05 (br, 2H),
7.39 (d, J=8.0 Hz, 2H), 5.19 (s, 1H), 5.08 (d, J=8.2 Hz, 1H), 4.18 (d, J=
8.2 Hz, 1H), 4.16 (t, J=8.1 Hz, 2H), 3.38 (m, 2H), 1.21 (m, 24H),
0.79 ppm (m, 6H); 13C NMR (400 MHz, CDCl3): d=156.11, 155.80,
154.47, 154.11, 147.80, 146.51, 146.37, 146.11, 145.86, 145.41, 145.08,
145.05, 144.98, 144.95, 144.51, 144.46, 143.54, 142.86, 142.62, 142.28,
141.54, 141.10, 140.71, 140.57, 139.63, 139.23, 138.77, 136.24, 134.76,
133.88, 132.52, 132.65, 130.06, 129.11, 128.83, 128.58, 127.51, 126.35,
125.77, 125.41, 123.85, 122.61, 122.33, 120.82, 119.61, 84.21, 76.39, 67.32,
47.12, 44.23, 43.81, 41.62 ppm; IR (KBr): n˜ =2971, 2930, 2854, 1664, 1489,
1390, 1281, 1184 cmÀ1; UV/Vis (toluene): lmax =330, 362, 382, 434 nm;
MS (MALDI-TOF): m/z calcd for C98H45N3O4: 1328.42; found 1328.44;
elemental analysis calcd (%): C 88.60, H 3.41, N 3.16; found: C 88.59, H
3.43, N 3.15.
Materials: Reagents and solvents were purchased as reagent grade and
used without further purification. All reactions were performed using dry
glassware under nitrogen atmosphere. Analytical TLC was carried out on
Merck 60 F254 silica gel plate and column chromatography was per-
formed on Merck 60 silica gel (230–400 mesh). Melting points were deter-
mined on an Electrothermal IA 9000 series melting point apparatus and
are uncorrected. NMR spectra were recorded on a Varian Mercury-400
(400 MHz) spectrometer with TMS peak as reference. IR spectra were
recorded on a Nicolet 550 FT infrared spectrometer and measured as
KBr pellets. MALDI-TOF MS spectra were recorded with an Applied
Biosystems Voyager-DE-STR. Elemental analyses were performed with a
Perkin-Elmer 2400 analyzer.
Acknowledgements
K.-Y. Kay acknowledges the financial support from Brain Korea 21 Pro-
gram in 2006. This work was also supported by a Grant-in-Aid (No.
19750034) and a Global COE program, “the Global Education and Re-
search Center for Bio-Environmental Chemistry” from the Ministry of
Education, Culture, Sports, Science and Technology (Japan).
In Scheme 1, the synthetic procedures and their spectral and analytical
data of the final compounds 1–3 are described as below (preparation of
the precursors are described in Supporting Information).
Preparation of SiPc-(NDI)2-(C60)2 (1):
A mixture of NaH (0.1 g,
4.1 mmol) and 8 (0.1 g, 0.08 mmol) in toluene (20 mL) was stirred for 1 h.
To the mixture was added silicon phthalocyanine dichloride (20 mg,
0.033 mmol) and heated under reflux for 3 d. The mixture was cooled to
room temperature, and the solvent was removed under reduced pressure.
The product was purified by column chromatography over silica gel with
dichloromethane/acetone (40:1) to give compound 1 (12 mg, 12.2%) as a
green solid. M.p. > 4008C (decomp); 1H NMR (400 MHz, CDCl3): d=
9.65 (m, 8H), 8.68 (d, J=7.6 Hz, 4H), 8.55 (d, J=7.5 Hz, 4H), 8.30 (m,
8H), 8.01 (br, 4H), 7.41 (d, J=8.1 Hz, 4H), 5.50 (d, J=8.2 Hz, 4H), 5.18
(s, 2H), 5.10 (d, J=8.2 Hz, 2H), 4.18 (d, J=8.2 Hz, 2H), 3.42 (m, 4H),
2.65 (d, J=8.1 Hz, 4H), 1.21 (m, 24H), 0.81 ppm (t, J=8.1 Hz, 6H);
13C NMR (400 MHz, CDCl3); d=154.21, 154.10, 151.63, 148.17, 147.10,
146.66, 146.32, 146.14, 146.09, 146.07, 146.01, 145.96, 145.90, 145.74,
145.57, 145.47, 145.35, 145.27, 145.24, 145.14, 145.10, 145.08, 145.05,
144.98, 144.95, 144.51, 144.46, 144.20, 142.98, 142.80, 142.48, 142.41,
142.37, 142.08, 142.06, 141.97, 141.92, 141.88, 141.84, 141.82, 141.60,
141.51, 141.47, 141.41, 140.53, 140.23, 139.99, 139.78, 139.45, 138.82,
135.94, 135.73, 135.66, 131.44, 131.33, 131.10, 130.76, 130.60, 129.82,
129.15, 128.39, 128.18, 126.69, 126.35, 125.98, 125.22, 124.85, 124.36,
122.74, 122.43, 83.01, 77.44, 69.92, 68.53, 43.12, 40.17 ppm; IR (KBr): n˜ =
3056, 2928, 2846, 1721, 1336, 1081 cmÀ1; UV/Vis (toluene): lmax =336,
357, 434, 565, 614, 651, 683 nm; MS (MALDI-TOF): m/z calcd for
C224H80N14O10Si: 3155.21; found 3155.28; elemental analysis calcd (%): C
85.27, H 2.56, N 6.21; found: C 85.25, H 2.58, N 6.20.
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Received: January 20, 2009
Published online: April 3, 2009
5310
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Chem. Eur. J. 2009, 15, 5301 – 5310