Effect of dual fullerenes on lifetimes of charge-separated states of subphthalocyanine - triphenylamine - fullerene molecular systems

Article abstract of DOI:10.1021/jp7108658

Photoinduced intramolecular electron-transfer events of the newly synthesized subphthalocyanine-tripheny-lamine-fullerene triad (SubPc-TPA-Co ₆₀) and subphthalocyanine-triphenylamine-bisfullerene tetrad (SubPc-TPA-(C₆₀)2) were studied. The geometric and electronic structures of the triad were probed by ab initio B3LYP/3-21G method, which predicts SubPc-TPA^.+-C₆₀^.- as a stable charge-separated state. The photoinduced events via the excited singlet state of SubPc were monitored by time-resolved emission measurements as well as transient absorption techniques. Efficient charge-separations via the excited states of SubPc were observed with the rates of a??10¹⁰ s ^_1. Compared with the SubPc - TPA dyad, a long-lived charge-separated state was observed for the SubPc - TPA - C₆₀ triad with the lifetime of the radical ion pairs (??_RIP) of 670 ns in benzonitrile. Interestingly, further charge stabilization was achieved in the charge-separated state of SubPc - TPA - (C₆₀)₂, in which the ??_RIP was found to be 1050 ns in benzonitrile. ? 2008 American Chemical Society.

Full text of DOI:10.1021/jp7108658

3910
J. Phys. Chem. B 2008, 112, 3910-3917
Effect of Dual Fullerenes on Lifetimes of Charge-Separated States of
Subphthalocyanine-Triphenylamine-Fullerene Molecular Systems
Mohamed E. El-Khouly,*,†,‡ Sun Hee Shim,§ Yasuyuki Araki,*,† Osamu Ito,† and
Kwang-Yol Kay*,§
Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Katahira, Sendai,
980-8577 Japan, Department of Chemistry, Faculty of Education, Kafr El-Sheikh UniVersity, Egypt, and
Department of Molecular Science and Technology, Ajou UniVersity, Suwon 443-749, South Korea
ReceiVed: NoVember 14, 2007; In Final Form: December 28, 2007
Photoinduced intramolecular electron-transfer events of the newly synthesized subphthalocyanine-tripheny-
lamine-fullerene triad (SubPc-TPA-C60) and subphthalocyanine-triphenylamine-bisfullerene tetrad
(SubPc-TPA-(C60)2) were studied. The geometric and electronic structures of the triad were probed by ab
•-
initio B3LYP/3-21G method, which predicts SubPc-TPA•+-C60 as a stable charge-separated state. The
photoinduced events via the excited singlet state of SubPc were monitored by time-resolved emission
measurements as well as transient absorption techniques. Efficient charge-separations via the excited states
of SubPc were observed with the rates of ∼1010 s-1. Compared with the SubPc-TPA dyad, a long-lived
charge-separated state was observed for the SubPc-TPA-C60 triad with the lifetime of the radical ion pairs
(τRIP) of 670 ns in benzonitrile. Interestingly, further charge stabilization was achieved in the charge-separated
state of SubPc-TPA-(C60)2, in which the τRIP was found to be 1050 ns in benzonitrile.
Introduction
Development of relatively simple donor-acceptor models
designed to mimic the events of the photosynthetic reaction
center has been an important goal in science and technology.
Efficient conversion of light energy into other useful energies
requires the effective formation of charge-separated (CS) states
that exhibit relatively long lifetimes and high-energy contents.1
In years past, various conjugated macrocycles such as porphy-
rins, phthalocyanines, and naphthalocyanines were widely
employed as building blocks for the photoactive and electro-
active assemblies due to their excellent light-harvesting ability
in the wide wavelength region (500-700 nm) and to their rich
redox chemistry.2
However, only a few studies have been reported for the
photophysical behavior of subphthalocyanine (SubPc), a singular
lower analogue of phthalocynanine.3 SubPc has three N-fused
diiminoisoindoline units arranged around a central boron atom
and π-electron aromatic core associated with their curved
structures, which make it different from their higher homo-
logues, phthalocyanines.4-6 Particularly attractive points of
SubPc are (1) their optoelectronic features can be finely tuned
Figure 1. Molecular structures and expected photoinduced processes
of the studied compounds.
by varying their axial ligands or by functionalizing the various
peripheral positions, (2) they are excellent antenna units that
absorb the lights in the visible region (500-700 nm) with
photosynthetic systems and as unique materials for nonlinear
excitation energy above 2.0 eV, and (3) they possess a relatively
optical applications.
low reorganization energy.7 Moreover, SubPc derivatives are
strong fluorophores with high quantum yields, which render
semblies for multicomponent photoactive systems are performed
them ideal molecules to probe electron transfer and energy
Because of the versatile chemistry of SubPcs,8 their as-
via different routes involving peripheral9 or axial approaches.10,11
transfer via the excited singlet states. Therefore, SubPc deriva-
The advantage of the axial approach is to preserve the electronic
characteristics of the macrocycles, since the substitution patterns
on the benzene rings remain unaltered. Recently, Torres et al.
reported that the exchange of the original axial halogen atom
with oxygen nucleophiles, particularly phenol, is a very
convenient method for introducing diverse functional groups
tives are appealing as new building blocks for the artificial
* To whom correspondence should be addressed. E-mail: araki@
tagen.tohoku.ac.jp; mohamedelkhouly@yahoo.com; kykay@ajou.ac.kr.
† Tohoku University.
‡ Kafr El-Sheikh University.
§ Ajou University.
10.1021/jp7108658 CCC: $40.75 © 2008 American Chemical Society
Published on Web 03/12/2008

Effect of Dual Fullerenes on Lifetimes
J. Phys. Chem. B, Vol. 112, No. 13, 2008 3911
a
SCHEME 1: Syntheses of SubPc-TPA-C60 and SubPc-TPA-(C60)2
in the axial position of the SubPc ring as well as for increasing
the solubility and stability of the SubPc derivatives.12
Our strategy is to attach the triphenylamine (TPA) unit and
the fullerene (C60) units to the SubPc moiety, expecting that
the attached units can modulate and modify the electronic
properties of SubPcs. As reported earlier, the TPA-based
compounds have been used in opto-electronic materials owing
to the high electron-donor ability and good film-forming
properties.13 Furthermore, the small reorganization energies for
the widely diffused π-electron on the spherical fullerene C60
radical anion are crucial to accelerate the initial CS process and
to decelerate exothermic charge recombination (CR) process.14,15
Thus, it is expected that the combination of SubPc with C60
and TPA may represent excellent building blocks for the
artificial photosynthetic systems and molecular photovoltaic
devices.
Taking these properties into consideration, we present in this
article the photosensitizing electron-accepting ability of SubPc
combined axially with triphenylamine (TPA) donor unit (SubPc-
TPA dyad) and the C60-connected triad and tetrad (SubPc-
TPA-(C60)n; n ) 1 and 2 in Figure 1). The electronic
characteristics of the SubPc-TPA dyad can be compared with
those of SubPc-C60 dyads reported by Torres et al.16 In the
SubPc-TPA-C60 triad, SubPc would be expected to act as a
photosensitizing electron acceptor, TPA as an electron-donor,
and C60 as an additional electron-acceptor. Furthermore, in the
SubPc-TPA-(C60)2 tetrad, the dual C60 moieties would be
expected to have synergistic effect for photoinduced CS giving
stabilized radical ion pair (RIP).
procedures depicted in Scheme 1. Every step of the reaction
sequence proceeded smoothly and efficiently to give a good or
moderate yield of the product (yields are shown in Scheme 1).
Details for syntheses of the final products are described in the
Experimental Section, which includes synthesis of SubPc-TPA;
furthermore, in the Supporting Information, syntheses of the
intermediates are included.
Molecular Orbital Calculations. For SubPc-TPA, the
optimized structure and the molecular orbital (MO) calculated
by ab intio B3LYP/3-21G method17 are shown in Figure 2. The
TPA moiety connected with SubPc via short axial linkage makes
the TPA moiety lie on just the upper position of SubPc, which
has a bent structure with upper curvature.12 Although the nearest
distance between the lowest unoccupied molecular orbital
(LUMO) and the highest occupied molecular orbital (HOMO)
is as close as 0.1 Å, the both MOs are distinctly separated,
suggesting that the formation of the CS state with the one-
electron reduced SubPc and the one-electron oxidized TPA
(SubPc•--TPA•+) is possible.
The optimized structure of SubPc-TPA-C60 is shown in
Figure 3. The molecular topology of this molecule shows that
the C60 moiety connected to one of the three phenyl rings of
TPA lies closely to the just upper position over SubPc. The
center-to-center distances (RCC) between SubPc and TPA as well
as TPA and C60 were estimated as 11.0 and 10.0 Å, respectively.
It is noticeable that the RCC value between SubPc and C60 is
also quite short (12.7 Å). The electron distribution of the HOMO
was found to be entirely located on the TPA entity, while the
electron distribution of the LUMO+3 and LUMO was found
to be entirely located over the SubPc moiety and the C60
spheroids, respectively. The nearest distance between the LUMO
of C60 and the LUMO+3 of SubPc is less than 0.5 Å, which
suggests that the through-space electron migration from SubPc•-
Results and Discussion
Synthesis and Characterization. SubPc-TPA-C60 and
SubPc-TPA-(C60)2 have been prepared according to the

3912 J. Phys. Chem. B, Vol. 112, No. 13, 2008
El-Khouly et al.
Figure 2. Frontier HOMO and LUMO of SubPc-TPA calculated by
ab intio B3LYP/3-21G method.
to C60 within SubPc•--TPA•+-C60 takes place yielding the
•-
stable final CS state like as SubPc-TPA•+-C60
.
In the case of SubPc-TPA-(C60)2, the MO calculation
predicts several optimized structures with almost the same
minimum energies. In the most optimized structure, the second
C60 approaches closely to another C60. Although the HOMO is
localized on the TPA moiety, the lower LUMOs are distributed
over the C60 units, and the upper LUMO is localized on the
SubPc moiety.
Electrochemical Measurements. The CS process from the
lowest excited singlet state of SubPc (1SubPc*) can be supported
from the viewpoint of thermodynamics. In differential pulse
voltammetry of SubPc-TPA-(C60)n in o-dichlorobenzene (o-
DCB), the first oxidation potential (Eox) of the TPA moiety was
located at 0.62 V vs Ag/AgCl, while the first reduction potentials
(Ered) of the SubPc and C60 moieties were recorded at -1.20
and -0.91 V vs Ag/AgCl. Based on these Eox and Ered values,
Figure 3. The HOMO and LUMOs of SubPc-TPA-C60 calculated
by ab intio B3LYP/3-21G method after optimization of structure.
and phthalocyanines. The absorption spectrum of SubPc-TPA
in the visible region is almost identical to that of SubPc-OPh
reference,12 revealing only weak intramolecular electronic
interactions in the ground state between the SubPc and TPA
moieties. In benzonitrile (BN), an appreciable red-shift (ca. 5
nm) was observed compared with the peak in toluene (563 nm),
which suggests the interaction of the B atom of SubPc with the
lone pair of BN. In the UV region, the new band appeared at
ca. 350 nm, which may be attributed to the substituted TPA
moiety with a triangular pyramidal structure different from
substituted TPA showing a band at 300 nm.19 The presence of
the fulleropyrrolidine unit was evidenced by the higher absorp-
tion between 250 and 350 nm and also by the weak typical
band at 432 nm, whereas the expected weak peak near 700 nm
may be hidden by the huge absorption of the SubPc unit that
dominates in this region. These absorption spectra revealed that
intramolecular electronic interactions of the C60 unit with the
SubPc and TPA moieties may be weak in the ground state.
1
the driving forces for the CS processes (-∆GCS) via SubPc*
were calculated from the Rehm-Weller equations18 as listed
in Table 1, in addition to -∆GCS values via 1C60*. The negative
driving forces of the CS processes of the studied compounds
1
(Table 1) suggest exothermic CS processes via SubPc* and
1C60* in the studied solvents. However, it has been frequently
pointed out that ∆GCS values in toluene usually contain much
estimation errors. The driving forces of CR process (-∆GCR
)
are also listed in Table 2.
Steady-State Absorption Measurements. The steady-state
absorption spectra of the intense magenta solutions of SubPc-
TPA and SubPc-TPA-(C60)n are shown in Figure 4. The
absorption spectra of the SubPc moieties consist of a high-energy
B-band (between 300-310 nm) and a lower energy Q-band
(560-580 nm)3-6 arisen from the π-π* transitions associated
with 14 π-electron systems, analogues to those of porphyrins

Effect of Dual Fullerenes on Lifetimes
J. Phys. Chem. B, Vol. 112, No. 13, 2008 3913
TABLE 1: Free Energy Changes (∆GCS), Fluorescence Lifetimes of SubPc (τf) in the 500-680 nm Region, Rate Constants
1
(kCS), and Quantum Yields (ΦCS) of Charge-Separation of SubPc-TPA, SubPc-TPA-C60 and SubPc-TPA-(C60)2 via SubPc*
c,d
-∆GCS/eVa,b
k
CS /s-1 c
ΦCS
‚-
compounds
SubPc-TPA
solvents
SubPc‚--TPA‚+-C60
SubPc-TPA‚+-C60
τf(fraction)/ps
via
1SubPc*
DMF
BN
TN
DMF
BN
TN
(0.66)e
(0.55)e
(0.13)e
0.69
0.67
0.14
120 (94%)
110 (94%)
110 (95%)
40 (94%)
30 (95%)
30 (55%)
60 (92%)
50 (92%)
8.1 × 109
8.4 × 109
8.9 × 109
2.8 × 1010
3.4 × 1010
3.4 × 1010
1.6 × 1010
2.2 × 1010
0.94
0.95
0.96
0.98
0.99
0.99
0.96
0.98
SubPc-TPA-C60
1.00
0.97
0.27
0.97
0.27
SubPc-TPA-(C60)2
BN
TN
0.67
0.14
1
1
a -∆GCS ) ∆E00 - {e(Eox - Ered) + ∆GS}, where ∆E00 is the energy of the 0-0 transition (2.1 eV for SubPc* and 1.72 eV for C60*). ∆GS
refers to the static Coulomb energy calculated by ∆GS ) -(e2/(4πꢀ0))[(1/(2R+) + 1/(2R-) - (1/RCC)/ꢀS - (1/(2R+) + 1/(2R-))/ꢀR), where R+ and
R
- are radii of the radical cation (TPA; 3.7 Å) and radical anion (SubPc (4.8 Å) and C60 (4.2 Å)). RCC; SubPc-TPA (10.0 Å) and TPA-C60 (11.0
Å). The symbols ꢀ0 and ꢀs represents vacuum permittivity and dielectric constant of solvent used for photophysical and electrochemical studies.
b -∆GCS values for SubPc•+-TPA-C60 are in the range of -0.1 - 0.0 eV in DMF and BN using Eox ) 1.04 V for SubPc in THF12b and RCC
)
•-
d
1
SubPc- C60 (12.7 Å). c Include energy transfer. ΦCS via SubPc* calculated by ΦCS ) kCS/(1/τf). e SubPc•--TPA•+
.
TABLE 2: Rate Constants of Charge-Recombination (kCR) and Lifetimes of Radical Ion-Pairs (τRIP) of SubPc-TPA,
SubPc-TPA-C60, and SubPc-TPA-(C60)2
-∆GCR/eVa,b
SubPc‚--TPA‚+-C60
SubPc-TPA‚+-C60
k
CR / s-1
τRIP /ns
‚-
compounds
SubPc-TPA
solvents
DMF
BN
TN
DMF
BN
TN
(1.44)c
(1.55)c
(1.97)c
1.41
1.43
1.96
< 108
< 108
< 108
< 10
< 10
< 10
400
SubPc-TPA-C60
1.10
1.13
1.83
1.13
1.83
2.5 × 106
1.5 × 106
1.4 × 108
9.5 × 105
2.1 × 107
670
<7
1052
47
SubPc-TPA-(C60)2
BN
TN
1.43
1.96
•-
a -∆GCR ) e (Eox - Ered) + ∆GS. b -∆GCR values for SubPc•+-TPA-C60 are in the range of 2.0-2.1 eV in DMF and BN. c SubPc•-
-
TPA•+
.
Steady-State Fluorescence Measurements. The photophysi-
cal behavior was qualitatively investigated by steady-state
fluorescence using 520-nm light excitation, which selectively
excited the SubPc moiety. As shown in Figure 5a, the
fluorescence spectrum of the SubPc-OPh reference shows a
maximum at 573 (toluene) and 577 nm (BN) with a fluorescence
(Figure 5b), suggesting that some intramolecular processes are
induced by the C60 moiety, in addition to the vicinal CS process
with TPA. Since the weak C60-fluorescence peak overlaps with
the SubPc-fluorescence peak, it is difficult to recognize the rise
of C60-fluorescence, which was usually observed for energy
transfer from 1SubPc* to C60 in nonpolar solvent. Similar
fluorescence spectra were observed for SubPc-TPA-(C60)2.
From these findings for SubPc-TPA-(C60)n that the emission
intensities of the SubPc moiety are additionally quenched by
the C60 moieties, one could speculate the energy-transfer
pathway from the 1SubPc* moiety to the C60 moieties probably
occurring through-space. However, CS pathway generating
SubPc•+-TPA-C60•- via the 1SubPc* moiety may be difficult
to occur, because of the high oxidation potential of the SubPc.12b
Fluorescence Lifetime Measurements. To complement the
emission spectral studies, the fluorescence lifetime measure-
ments (Figure 6) were performed, which tracked the above
consideration in a more quantitative way. The fluorescence
1
quantum yield of 0.45. The SubPc*-energy of SubPc-OPh
evaluated as 2.1 eV is substantially higher than those for
phthalocyanines (1.7 eV). By the axial linkage of SubPc with
TPA, the emission intensity was decreased very much without
an appreciable shift of the emission peak in toluene and BN.
The fluorescence quantum yields of SubPc-TPA were evaluated
to be less than 0.01 in toluene and BN; similar quenching was
observed in dimethylformamide (DMF). Since the possibility
of the energy transfer process from 1SubPc* to TPA is excluded
due to energetic considerations, the CS process predominantly
1
takes place via SubPc* generating SubPc•--TPA•+ in polar
and nonpolar solvents.
1
In SubPc-TPA-C60, further efficient fluorescence quenching
of the SubPc moiety was observed in all solvents employed
lifetime of SubPc*-OPh exhibited monoexponential decay
with a lifetime (τf0) of 2080 ps, which is in a good agreement
Figure 5. Steady-state fluorescence spectra of (left) SubPc-OPh and
SubPc-TPA and (right) SubPc-TPA and SubPc-TPA-C60 in toluene
(TN) and benzonirtrile (BN). The concentrations were kept at 2.2 µM;
λex ) 520 nm.
Figure 4. Steady-state absorption spectra of SubPc-OPh, SubPc-
TPA, and SubPc-TPA-C60 in toluene (TN). The concentrations were
kept at 2.2 µM.

3914 J. Phys. Chem. B, Vol. 112, No. 13, 2008
El-Khouly et al.
Figure 6. Fluorescence decay-profiles of SubPc-OPh, SubPc-TPA,
and SubPc-TPA-C60 in TN and BN in the region of 550-650 nm.
The concentrations were kept at 0.05 mM; λex ) 400 nm.
Figure 8. Nanosecond transient spectra and time profile of SubPc-
TPA-C60 in Ar-saturated DMF; λex ) 532-nm laser light.
with a depression in the 500-600 nm region in BN. Since these
absorption bands are similar to those of the SubPc-OPh, they
are assigned to the 3SubPc* unit, which is generated via
1
intersystem crossing from SubPc*. In the longer time-scale
measurements, these absorption bands were decayed within 50
µs, which were unchanged with changing the solvent polarity
(Supporting Information, Figures S1 and S2). These findings
3
also support the assignment of SubPc*. Combining with the
rapid fluorescence quenching of 1SubPc* due to the CS process,
Figure 7. Nanosecond transient spectra and time profile of SubPc-
TPA in Ar-saturated BN; λex ) 532-nm laser light.
3
the generation of SubPc*-TPA may be caused by the rapid
CR of SubPc•--TPA•+, since the energy levels of the SubPc•--
TPA•+ (2.0-1.6 eV in Table 2) are located higher than 3SubPc*
(1.4 eV)5,12,16 in both polar and nonpolar solvents (Supporting
Information, Figure S3). Since SubPc•--TPA•+ was not
observed by our nanosecond laser pulse (6 ns),22 the CR process
with the reported value.12 However, the fluorescence time
1
profiles of SubPc*-TPA could be fitted satisfactorily with
biexponential decay functions, from which the fluorescence
lifetimes (τf) of the major short-lived components were evaluated
as ca. 110 ps in polar and nonpolar solvents as listed in Table
1. The lifetime of the remaining minor slow decay part is almost
the same as that of SubPc-OPh. Based on the shortening of
the fluorescence lifetimes, the CS rates (kCS) of the SubPc-
is faster than ca. 2 × 108 s-1 23
.
This quick CR process seems
to be reasonable due to the close distance between the orbital
of the LUMO of SubPc and HOMO of TPA in Figure 2.
However, since the node at the B atom of the LUMO does not
overlap with the O atom of the HOMO in SubPc•--TPA•+,
the lifetime of the RIP (τRIP) may not be very short.
TPA dyad were estimated in the range of (8-9) × 109 s-1
.
Compared with 1SubPc*-TPA, the fluorescence lifetimes of
1SubPc*-TPA-C60 were further shortened, giving the short-
lived components as 30-40 ps in toluene, BN and DMF (Table
1), suggesting that the initial CS process takes place via 1SubPc*
The nanosecond transient absorption spectra of SubPc-
TPA-C60 observed in polar solvents show quite different
features from those of SubPc-TPA. A typical example is shown
in Figure 8 for DMF solvent; the 1000-nm band is undoubtedly
attributed to characteristic peak of the C60•- moiety and the 740-
nm band to the TPA•+ moiety. These observations suggest that
the CS state is like as SubPc-TPA•+-C60•- in the nanosecond
time region in polar solvents.24 Since the initial CT state via
1SubPc* is mainly attributed to SubPc•--TPA•+-C60, the
electron migration from SubPc•- to C60 takes place within 6 ns
before the CR between SubPc•- and TPA•+.24 As the observed
additional fluorescence quenching of 1SubPc* by C60 suggested,
the initial energy transfer from 1SubPc* to C60 may be possible
in SubPc-TPA-C60 with the kCS values of ca. 3 × 1010 s-1
,
which is larger than the kCS values for the vicinal CS process
of SubPc-TPA.20 In polar solvents, the kCS values of SubPc-
TPA-(C60)2 via 1SubPc* were estimated to be as ca. 2 × 1010
s-1. Thus, the kCS values are in the order of SubPc-TPA-C60
> SubPc-TPA-(C60)2 > SubPc-TPA in BN. Although the
acceleration role of the C60 moieties to the CS process is
prominent, the dual C60 effect is less effective; probably the
dual attachments of C60 considerably change the electronic
character of SubPc-TPA unit into an adverse tendency. Since
the kCS values are independent from solvent polarities, it is
suggested that the CS process is located near the top region of
the Marcus parabola.21
forming SubPc-TPA-1C60*, from which the CS process takes
place from vicinal TPA moiety giving SubPc-TPA•+-C60
.
•-
In BN, similar transient absorption spectra were observed
(Supporting Information, Figure S4), showing the 1000-nm band
In Table 1, the CS quantum yields (ΦCS) evaluated from the
fluorescence lifetimes of 1SubPc* are also listed. The ΦCS values
of SubPc-TPA-(C60)n are larger than the values of the vicinal
CS process for SubPc-TPA dyad, supporting that the initial
CS process of SubPc-TPA-(C60)n accompanies the energy
of C60 and 720-nm band of TPA•+. In addition, the 450-nm
•-
band of 3SubPc* was also found in the same time scale,
suggesting that SubPc-TPA•+-C60•- coexists with 3SubPc*-
TPA-C60. On the other hand, the nanosecond transient spectra
of SubPc-TPA-C60 in toluene exhibited the characteristic
1
transfer process from the SubPc* moiety to the C60 moiety.
3
Transient Absorption Measurements. By employing 532-
nm laser light, which predominately excited the SubPc moiety,
the nanosecond transient spectra of SubPc-TPA were observed
as shown in Figure 7. A main absorption band appeared at 450
nm together with broad weak bands in the 600-750 nm region
absorption bands of SubPc* moiety at 450 nm and 600-700
nm as shown in Supporting Information (Figure S5). Since the
energy levels of the CS states in toluene are higher than 3SubPc*,
the CR process may populate 3SubPc* (Supporting Information,
Figure S6).

Effect of Dual Fullerenes on Lifetimes
J. Phys. Chem. B, Vol. 112, No. 13, 2008 3915
Figure 9. Nanosecond transient spectra and time profile of SubPc-
TPA-(C60)2 in Ar-saturated benzonitrile; λex ) 532-nm laser light.
Figure 10. Energy diagram of SubPc-TPA-(C60)n: CS, charge
separation; EN, energy transfer; and CR, charge recombination.
•-
The characteristic transient absorption band of the C60
moiety was employed to determine the CR rates of SubPc-
Energy Diagram Considerations and Conclusive Remarks.
The energy level diagram constructed by utilizing the spectral,
electrochemical and photophysical data of SubPc-TPA-(C60)n
is schematically illustrates in Figure 10. The initial CS process
occurs from TPA to 1SubPc*, yielding vicinal radical ion-pairs,
SubPc•--TPA•+-(C60)n. Subsequently, it is possible to shift
an electron on SubPc•- to C60 as an exothermic process by ca.
300 mV, producing stable SubPc-TPA•+-(C60)n•-. In addition,
the through-space energy transfer process from 1SubPc* to C60
TPA•+-C60•-, since the decays of the C60 were well-fitted
•-
by a single-exponential function. The kCR value of SubPc-
•-
TPA•+-C60 was found to be 2.5 × 106 s-1 in DMF from
inset of Figure 8. Based on the kCR value, the τRIP value of
SubPc-TPA•+-C60 was evaluated as 400 ns in DMF. Such
•-
a slow CR process may be explained by the inverted region of
the Marcus parabola.21
In BN, the kCR value was evaluated to be 1.5 × 106 s-1
1
(Supporting Information, Figure S4), which is slower than that
is possible to generate the C60* moiety, from which SubPc-
•-
in DMF, giving a longer τRIP value of SubPc-TPA•+-C60
TPA•+-(C60)n can be also generated. In polar solvents, the
•-
final CS state, SubPc-TPA•+-(C60)n•-, decays directly to
populate the ground state with relatively slow rates. In nonpolar
solvent, the radical ion-pairs relaxes to 3SubPc* with the quick
CR process for SubPc-TPA-(C60)n.
as 670 ns in BN. Smaller kCR value in BN may be related to
the coexistence of 3SubPc*, which suggests that SubPc-
TPA•+-C60•- gains a triplet spin character prolonging the τRIP
value.25 These τRIP values are significantly longer compared with
those of TPA•+-C60•- dyads,24 which suggests a role of SubPc
As conclusions, high efficient CS processes of the newly
synthesized SubPc-TPA-C60 and SubPc-TPA-(C60)2 were
observed compared with SubPc-TPA by the excitation of the
huge absorption of SubPc moiety in the visible region. In polar
solvents, the CS processes via 1SubPc* generates mainly vicinal
RIP (SubPc•--TPA•+-(C60)n) and, subsequently, stable RIP
(SubPc-TPA•+-(C60)n•-). These RIPs have longer lifetimes
compared with the corresponding dyads such as SubPc•--
TPA•+ and TPA•+-(C60)n•-. Furthermore, the longer lifetime
in stabilizing SubPc-TPA•+-C60•-; that is, an electron of C60
•-
interacts with SubPc in the triad due to their close contact
(Figure 3).
In the case of SubPc-TPA-(C60)2, similar transient spectra
were observed as shown in Figure 9, in which SubPc-TPA•+-
(C60)2•- was observed in polar solvents. Interestingly, the 1000-
nm band and 750-nm band are broader than those of SubPc-
TPA•+-C60•- in BN, suggesting that the C60•- moiety interacts
with another C60 moiety or with the close SubPc moiety. The
of SubPc-TPA•+-(C60)2 than that of SubPc-TPA•+-C60
•-
•-
CR value was found to be 9.5 × 105 s-1 in BN from the inserted
was observed revealing the effect of the dual C60 moieties on
the electron-transfer processes. Such photophysical properties
afford potentials of SubPc-TPA-(C60)n for wide applications
to artificial photosynthetic systems.
k
time-profile at 1000 nm, which corresponds to the τRIP value
as 1052 ns. This τRIP value is longer than that of SubPc-TPA-
C
60 (τRIP ) 670 ns), suggesting the delocalization of the negative
charge over the two C60 units and SubPc unit in SubPc-TPA•+-
(C60)2•-. A similar prolongation of the τRIP value due to dual
C60 moieties attached TPA in TPA•+-(C60)2•- triad was recently
found in our previous paper.24b In toluene, transient spectra of
Experimental Section
Instruments. Steady-state absorption and fluorescence spectra
were measured on a JASCO V-550 spectrometer (UV-vis-
NIR) and Shimadzu spectrofluorophotometer equipped with a
photomultiplier tube having high sensitivity in the longer
wavelength region, respectively.
3
SubPc-TPA-(C60)2 showed predominantly the C60* moiety
3
at 700-750 nm, to which absorption of the SubPc* moiety
was overlapped (Supporting Information, Figure S7), suggesting
much contribution of the 3C60* moiety than the 3SubPc* moiety.
The redox values were measured using the differential pulse
voltammetry (DPV) technique by applying a BAS CV-50W
voltammetric analyzer. A platinum disk electrode was used as
the working electrode, while a platinum wire served as a counter
electrode. An Ag/AgCl electrode was used as a reference
electrode. All measurements were carried out in different
solvents containing 0.1 M (n-Bu)4NClO4 as a supporting
Compared with the initial absorbance at 1000 nm, the final
•-
yield of the RIP with C60 for SubPc-TPA-C60 was higher
than that of SubPc-TPA-(C60)2. This tendency was in good
agreement with the initial ΦCS values via 1SubPc* as evaluated
by fluorescence quenching experiments, indicating that the final
RIP yields are proportional to the observed ΦCS values. Thus,
the electron-shift process and energy-transfer/charge-transfer
process occurring after the initial CS process may be fast enough
to generate final RIP competitively with the initial CR of
SubPc•--TPA•+-(C60)n.
electrolyte. The scan rate was 0.1 V s-1
.
Frontier HOMO and LUMO of SubPc-TPA-C60 and
SubPc-TPA-(C60)2 were calculated by ab intio B3LYP/3-21G
method after optimization of the structure.17

3916 J. Phys. Chem. B, Vol. 112, No. 13, 2008
El-Khouly et al.
The picosecond time-resolved fluorescence spectra were
measured by a single-photon counting method using a second
harmonic generation (SHG, 400 nm) of a Ti:sapphire laser
(Spectra-Physica, Tsunami 3950-L2S, 1.5 ps fwhm) and a
streak-scope (Hamamatsu Photonics) equipped with a poly-
chromator as an excitation source and a detector, respectively.
Lifetimes were evaluated with software attached to the equip-
ment.
NMR, and IR spectroscopies; MALDI-TOF mass spec-
troscopies; and elemental analysis.
1H NMR spectra of SubPc-TPA-C60 and SubPc-TPA-
(C60)2 in CDCl3 are consistent with the proposed structures,
showing the expected features with the correct integration ratios.
The signals of pyrrolidine protons in SubPc-TPA-C60 and
SubPc-TPA-(C60)2 appeared as two doublets (J ) 9.5 Hz;
germinal protons), and a singlet in the δ ) 3.95-4.98 ppm
region, which is consistent with spectra obtained for similar
derivatives.30 13C NMR spectra contained the signals corre-
sponding to the sp2 and sp3 atoms of C60 and the expected signals
corresponding to the organic addends. The MALDI-TOF mass
spectra provided a direct evidence for the structures of SubPc-
TPA-C60 and SubPc-TPA-(C60)2. Compound SubPc-TPA-
C60 showed a singly charged molecular ion peak that matches
the calculated value for the molecular weight, and compound
SubPc-TPA-(C60)2 gave a peak at m/z ) 962.36 [M-2C60]+.
Further confirmation of the hybrid SubPc-fullerene structure was
obtained from UV/vis spectra of SubPc-TPA-C60 and SubPc-
TPA-(C60)2, which contain a dihydrofullerene absorption band
at around 430 nm together with the expected Soret band (around
300 nm) and Q-band (around 562 nm).
The nanosecond transient absorption measurements in the
near-IR region were measured by laser-flash photolysis; 532-
nm light from a Nd:YAG laser (Spectra-Physics and Quanta-
Ray GCR-130, 6 ns fwhm) was used as an excitation source.
The monitoring lights from a pulsed Xe-lamp were detected
via Ge-avalanche photodiode module. The samples were held
in a quartz cell (1 × 1 cm) and were deaerated by bubbling
argon gas through the solution for 20 min.
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 performed on Merck 60
silica gel (230-400 mesh). Melting points were determined on
an Electrothemal IA 9000 series melting point apparatus and
are uncorrected. NMR spectra were recorded on a Varian
Mercury-400 (400 MHz) spectrometer with the TMS peak used
as a 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.
SubPc-TPA-C60. Compound 7 (70 mg, 0.10 mmol) and
N-octylglycine (20 mg, 0.10 mmol) were added to a solution
of fullerene (70 mg, 0.13 mmol) in chlorobenzene (40 mL).29
The reaction mixture was refluxed for 16 h and then filtered
off. The filtrate was evaporated and chromatographed on silica
gel with toluene to give compound SubPc-TPA-C60 (43 mg,
1
28.7%) as a black solid. Mp > 410 °C (dec); H NMR (400
MHz, CDCl3): δ ) 8.77 (dd, J ) 8.6 Hz, J ) 2.8 Hz, 6H),
7.84 (dd, J ) 8.6 Hz, J ) 2.8 Hz, 6H), 7.15 (m, 3H), 7.10 (t,
J ) 7.6 Hz, 2H), 6.85 (m, 4H), 6.50 (d, J ) 9.5 Hz, 2H), 5.35
(d, J ) 9.5 Hz, 2H), 4.94 (d, J ) 9.5 Hz, 1H), 4.83 (s, 1H),
3.95 (d, J ) 9.5 Hz, 1H), 3.24 (t, 2H), 1.25∼1.45 (br, 12H),
0.90 (t, 3H); 13C NMR (CDCl3:CS2 ) 3:1): δ ) 151.37, 148.03,
146.78, 145.28, 144.62, 143.99, 142.51, 141.28, 139.72, 131.25,
131.06, 130.03, 129.19, 128.98, 126.13, 123.51, 122.53, 119.90,
65.88, 53.79, 32.35, 30.14, 29.80, 23.16, 19.59, 14.60. IR
(KBr): ν ) 705, 738, 896, 1122, 1265, 1421, 1598, 2306, 2987,
3054 cm-1. UV/vis (toluene): λmax (ꢀ × 10-5 /M-1 cm-1) )
300 (4.723), 431(0.488), 523 (1.241), 562 nm (4.091); MS
(MALDI-TOF) for C112H45N8BO (M ) 1529.45) m/z )
1529.33(M+); Anal. Calcd: C, 87.95%; H, 2.96%; N, 7.33%.
Found: C, 87.91%; H, 2.95%; N, 7.36%.
In Scheme 1, commercially available diphenylamine was
coupled with 4-iodoanisole under Ullmann condition26 to give
4-methoxytriphenylamine (1) in 86.0%, and subsequently,
Vilsmeier formylation27 was carried out to produce aldehyde 2
in 93.7% and 3 in 36.7%, respectively. However, direct double
formylation of 1 by the Vilsmeier reaction proved to be difficult
due to the deactivation effect of the first carbonyl group on
TPA and mainly gave monoformylated TPA 2 under normal
stoichiometry of POCl3/DMF (up to 3.5 equiv). With a large
excess of POCl3/DMF (∼10 equiv), the diformylated TPA 3
was produced with a yield of 36.7%. Subphthalocyanine (SubPc-
Cl) 6 was synthesized by condensation reaction of phthalonitrile
in the presence of boron trichloride according to the literature
procedure,8,11b and the axial chlorine atom of SubPc-Cl (6) was
then replaced with hydroxytriphenylamine aldehydes 4 and 5,
produced from corresponding methoxytriphenyl)amine alde-
hydes 2 and 3 by demethylation, to give formyl-substituted
SubPc-TPA 7 and 8 in 94.4% and 63.6%, respectively. Finally,
fulleropyrrolidine formation was achieved by 1,3-dipolar cy-
cloaddition reaction between aldehydes 7 and 8 and C60 in the
presence of excess N-octylglycine28 under the condition de-
scribed by Prato29 to give SubPc-TPA-C60 triad and SubPc-
TPA-(C60)2 tetrad in 28.7% and 10.2%, respectively. Although
SubPc-TPA-(C60)2 tetrad should be obtained as a stereoiso-
meric mixture due to the formation of two asymmetric centers
in 2-fold cycloaddition reaction, high resolution 1H NMR
spectrum (400 MHz) of tetrad showed the presence of only one
stereoisomer (see below).
SubPc-TPA-(C60)2. Compound 8 (60 mg, 0.084 mmol) and
N-octylglycine (47 mg, 0.25 mmol) were added to a solution
of fullerene (120 mg, 0.17 mmol) in chlorobenzene (50 mL).29
The reaction mixture was refluxed for 16 h and then filtered
off. The filtrate was evaporated and chromatographed on silica
gel with CS2/acetone (40:1) to give SubPc-TPA-(C60)2 (7 mg,
1
10.2%) as a black solid. Mp > 410 °C (dec); H NMR (400
MHz, CDCl3): δ ) 8.76 (dd, J ) 8.6 Hz, J ) 2.8 Hz, 6H),
7.83 (dd, J ) 8.6 Hz, J ) 2.8 Hz, 6H), 7.15 (d, J ) 9.5 Hz,
4H), 6.77 (d, J ) 9.5 Hz, 4H), 6.42 (d, J ) 9.5 Hz, 2H), 5.32
(d, J ) 9.5 Hz, 2H), 4.98 (d, J ) 9.5 Hz, 2H), 4.87 (s, 2H),
4.00 (d, J ) 9.5 Hz, 2H), 3.23 (t, 4H), 1.25∼1.45 (br, 24H),
0.90 (t, 6H). 13C NMR (CDCl3:CS2 ) 2:1): δ ) 156.03, 153.63,
153.16, 152.98, 150.93,147.38, 146.54, 146.22, 145.97, 145.70,
145.54, 145.38, 145.32, 145.04, 144.94, 144.82, 144.75, 144.50,
144.37, 143.99, 143.81, 143.58, 142.29, 142.03, 141.89, 141.68,
141.54, 141.37, 141.03, 140.78, 139.58, 139.50, 139.23, 138.50,
136.06, 135.23, 130.89, 130.58, 129.61, 128.58, 125.55, 122.19,
68.44, 66.64, 65.40, 53.15, 32.13, 30.67, 29.90, 29.53, 28.60,
27.74, 22.96, 14.42. IR (KBr): ν ) 705, 738, 896, 1122, 1265,
SubPc-TPA-C60 and SubPc-TPA-(C60)2 are very soluble
in aromatic solvents (i.e., toluene, o-dichlorobenzene, and
benzonitrile) and other common organic solvents (i.e., carbon
disulfide, acetone, CH2Cl2, CHCl3, and THF). The structure and
purity of the new compounds were confirmed by 1H NMR, 13
C

Effect of Dual Fullerenes on Lifetimes
J. Phys. Chem. B, Vol. 112, No. 13, 2008 3917
1421, 1598, 2306, 2987, 3054 cm-1; UV/vis (toluene): λmax
(ꢀ× 10-5 /M-1 cm-1) ) 301 (6.555), 435 (0.976), 528 (1.332),
565 nm (4.095); MS (MALDI-TOF) for C182H64N9BO (M )
2403.33) m/z ) 962.36 (M-2C60)+; Anal. Calcd: C, 90.96%;
H, 2.64%; N, 5.25%. Found: C, 90.91%; H, 2.62%; N, 5.26%.
SubPc-TPA. To a solution of compound 6 (45 mg, 0.10
mmol) in toluene (7 mL) was added 4-hydroxytriphenylamine
(0.11 g, 0.42 mmol, see the Supporting Information), and
refluxed for 24 h. The reaction mixture was cooled to room
temperature and evaporated. The product was chromatographed
on silica gel with dichloromethane/methanol (200:1) to give
compound SubPc-TPA (0.042 g, 63.6%) as a red solid. Mp
153∼154 °C; 1H NMR (400 MHz, CDCl3): δ ) 8.22 (dd, J )
9.3 Hz, J ) 2.7 Hz, 6H), 7.87 (dd, J ) 9.3 Hz, J ) 2.7 Hz,
6H), 7.08 (t, J ) 7.3 Hz, 4H), 6.79-6.86 (m, 6H), 6.49 (d, J )
8.6 Hz, 2H), 5.28 (d, J ) 8.6 Hz, 2H). IR (KBr): ν ) 705,
738, 896, 1052, 1133, 1265, 1421, 1596, 2306, 2987, 3054 cm-1.
UV/vis (toluene): λmax (ꢀ× 10-5 /M-1 cm-1) ) 302 (2.182),
385 (1.545), 524 (1.241), 562 nm (4.082); MS (MALDI-TOF);
m/z for C42H26BN7O Calcd. 654.71. Found 655.16. Anal.
Calcd: C, 76.96%; H, 4.00%; N, 14.96%. Found: C, 76.92%;
H, 3.99%; N, 14.98%.
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(19) SubPc-TPA dyad shows different absorption characters compared
with the reported subphthalocyanine functionalized with TPA, which shows
absorption maxima at 618 and 450 nm.12c This observation reflects the
change of the linkage style between the SubPc and TPA, shifting the B-
and Q-bands.
(20) Considerably higher fraction of long fluorescence-lifetime com-
ponent of SubPc-TPA-C60 in toluene suggests that attachment of the C60
moiety considerably changes the electronic character of SubPc-TPA unit
in this non-polar solvent.
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(22) In Figure 7, weak absorption peak seems to appear in the 600-
650 nm region after recovery of depletion in the 500-650 nm region, due
to the decay of the SubPc fluorescence, but not due to appearance of SubPc•-
at 640 nm.12b
SubPc-OPh. To a solution of phenol (0.12 g, 1.28 mmol)
in toluene (10 mL) was added compound 6 (0.12 g, 0.28 mmol)
and refluxed for 6 h.12 The reaction mixture was cooled to room
temperature and evaporated. The product was chromatographed
on silica gel with dichloromethane/methanol (200:1) to give
compound SubPc-OPh (0.11 g, 81.2%) as a red solid. Mp
1
138 °C. H NMR (400 MHz, CDCl3): δ ) 8.84 (dd, J ) 9.3
Hz, J ) 2.7 Hz, 6H), 7.90 (dd, J ) 9.3 Hz, J ) 2.7 Hz, 6H),
6.73 (m, 2H), 6.60 (m, 1H), 5.37 (d, J ) 8.6 Hz, 2H). IR
(KBr): ν ) 1036, 1052, 1238, 1506, 1585 cm-1. UV/vis
(toluene); λmax (ꢀ× 10-5 /M-1 cm-1) ) 302 (1.545), 524 (1.241),
562 nm (4.093); MS (MALDI-TOF); m/z for C42H26BN7O
Calcd. 487.51. Found 487.45. Anal. Calcd: C, 73.91%; H,
3.51%; N, 11.49%. Found: C, 73.87%; H, 3.50%; N, 11.45%.
Acknowledgment. K.-Y.K. acknowledges the financial
support from Brain Korea 21 Program in 2006.
Supporting Information Available: Synthetic procedures,
transient absorption spectra, time profiles, and energy diagrams.
This material is available free of charge via the Internet at http://
pubs.acs.org.
References and Notes
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extremely long-lived charge-separated state up to 231 µs,12b the charge-
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ns), further producing 3SubPc*. This difference may be attributed to the
shorter distance between SubPc and TPA, in addition to the position of
TPA right overhead of SubPc as shown in Figure 2, which are quite different
from SubPc-ferrocene dyad with longer linkage and side position of the
ferrocene donor.
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