Dyads and triads containing perylenetetracarboxylic diimide and porphyrin: Efficient photoinduced electron transfer elicited via both excited singlet states

Article abstract of DOI:10.1021/jp045163e

Synthesis, characterizations, and photophysical properties of new photoactive dyads and triads containing perylenetetracarboxylic diimide (PIm) and porphyrin (free-base porphyrin (H₂P) and zinc porphyrin (ZnP)), in which both entities were connected with a short ether bond, were examined with the aim of using these systems for molecular photonics. The porphyrin(P)-PIm systems absorbed strongly across the visible region, which greatly matched the solar spectrum. The geometric and electronic structures of the dyads and triads were probed using density function theory method at the B3LYP/3-21G level. It was revealed that the majority of the highest-occupied molecular orbital was located on the porphyrin entity, while the lowest-unoccupied molecular orbitals were entirely on the PIm entity. The excited-state electron-transfer processes were monitored by both steady-state and time-resolved emission as well as transient-absorption techniques in polar solvent benzonitrile. Upon excitation of the P (H₂P and ZnP) moieties, efficient fluorescence quenching of the P moiety was observed, suggesting that the main quenching paths involved charge separation from the excited singlet porphyrin (¹P*) to the PIm moiety. Upon excitation of the PIm moiety, fluorescence quenching of the ¹PIm* moiety was also observed. The nanosecond transience of spectra in near-IR region revealed the charge separation process from the P moieties to the PIm moiety via their excited singlet states. The lifetimes of the charge-separated states were evaluated to be 7-14 ns, depending on the solvent polarity. Photosensitized electron mediation systems were also revealed in the presence of methyl viologen and sacrificial electron donor. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp045163e

3658
J. Phys. Chem. B 2005, 109, 3658-3667
Dyads and Triads Containing Perylenetetracarboxylic Diimide and Porphyrin: Efficient
Photoinduced Electron Transfer Elicited via Both Excited Singlet States
Shengqiang Xiao,† Mohamed E. El-Khouly,‡ Yuliang Li,*,† Zhenhai Gan,‡ Huibiao Liu,†
Li Jiang,† Yasuyuki Araki,‡ Osamu Ito,*,‡ and Daoben Zhu†
CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences,
Bejing, 100080 China, and Institute of Multidisciplinary Research for AdVanced Materials,
Tohoku UniVersity, Katahira, Aoba-ku, Sendai, 980-8577 Japan
ReceiVed: October 22, 2004; In Final Form: December 12, 2004
Synthesis, characterizations, and photophysical properties of new photoactive dyads and triads containing
perylenetetracarboxylic diimide (PIm) and porphyrin (free-base porphyrin (H2P) and zinc porphyrin (ZnP)),
in which both entities were connected with a short ether bond, were examined with the aim of using these
systems for molecular photonics. The porphyrin(P)-PIm systems absorbed strongly across the visible region,
which greatly matched the solar spectrum. The geometric and electronic structures of the dyads and triads
were probed using density function theory method at the B3LYP/3-21G level. It was revealed that the majority
of the highest-occupied molecular orbital was located on the porphyrin entity, while the lowest-unoccupied
molecular orbitals were entirely on the PIm entity. The excited-state electron-transfer processes were monitored
by both steady-state and time-resolved emission as well as transient-absorption techniques in polar solvent
benzonitrile. Upon excitation of the P (H2P and ZnP) moieties, efficient fluorescence quenching of the P
moiety was observed, suggesting that the main quenching paths involved charge separation from the excited
singlet porphyrin (1P*) to the PIm moiety. Upon excitation of the PIm moiety, fluorescence quenching of the
1PIm* moiety was also observed. The nanosecond transience of spectra in near-IR region revealed the charge
separation process from the P moieties to the PIm moiety via their excited singlet states. The lifetimes of the
charge-separated states were evaluated to be 7-14 ns, depending on the solvent polarity. Photosensitized
electron mediation systems were also revealed in the presence of methyl viologen and sacrificial electron
donor.
Introduction
counterpart donors, porphyrin analogues (P ) ZnP and H2P)
displayed many attractive photophysical and chemical fea-
tures.29,30 Porphyrins contain an extensively conjugated π system
with high electron donor ability, and such a delocalized π system
is suitable for efficient electron-transfer processes.
Recently, developing renewable energy sources has stimulated
tremendous interest in construction of light energy conversion
systems.1-8 Development of molecular photoelectronic devices
requires the creation of molecular arrays that undergo electron-
transfer processes after photoexcitation with light of specific
wavelengths. For this purpose, several organic compounds have
been applied to construct solar cells. Among them, perylene-
tetracarboxylic diimide (PIm) dyes seem to be ideal for light-
based applications due to their excellent photophysical prop-
erties. The PIm dyes have strong absorption with high molar
absorption coefficients (ꢀ ≈ 58 000 M-1 cm-1 at 490 nm in
CHCl3), a long fluorescence lifetime, near unity fluorescence
quantum yields, and very high photostability.9,10 Although many
perylenetetracarboxylic diimide derivatives have poor solubility
in common organic solvents, their solubility can be considerably
enhanced by the introduction of bulky substituents.
To the best of our knowledge, there are fewer examples of
connected systems prepared from porphyrin and PIm, revealing
their unique photophysical and photochemical properties.
Wasielewski’s group prepared a molecular optical switch
comprised of a PIm dye attached to two free base porphyrins
via short N-phenyl linkers.20 Excitation of the PIm and porphyrin
moieties resulted in electron transfer from porphyrin to the PIm,
showing that the PIm unit participated as an electron acceptor
with the porphyrin as a donor in their excited singlet states.
Another series of porphyrin (H2P, ZnP, and MgP)-PIm systems
were reported by Lindsey, Holten, Bocian, et al. using a longer
linker in an attempt to suppress electron-transfer quenching
while maintaining energy transfer.21,22,24,25
To date, many redox active chromophores were involved in
these donor-acceptor systems based on the PIm derivatives.11-28
Incorporating PIm within molecular donor-acceptor systems,
in which PIm acts as an electron acceptor linked with appropriate
photosensitizers and/or electron donors, is a viable route to
design photovoltaics and molecular electronics.11-28 Among
In the present paper, we report the synthesis, characterizations,
and photophysical properties of new photoactive dyads and
triads of porphyrin-PIm systems that would be applied to
improve the light absorption efficiency and achieve the efficient
charge-separation states for appropriately long times.
Results and Discussion
* Authors to whom correspondences may be addressed. E-mail:
ylli@iccas.ac.cn (Y.L.); ito@tagen.tohoku.ac.jp (O.I.).
† Chinese Academy of Sciences.
Synthesis. 5-(4-Hyroxyphenyl)-10,15,20-tris-(3′,4′,5′-tri-
methoxyphenyl)- porphyrin (H2P) as shown in Scheme 1 was
‡ Tohoku University.
10.1021/jp045163e CCC: $30.25 © 2005 American Chemical Society
Published on Web 01/28/2005

Perylenetetracarboxylic Diimide and Porphyrin
J. Phys. Chem. B, Vol. 109, No. 8, 2005 3659
SCHEME 1: Structure of H2P and ZnP
SCHEME 2: Synthesis of H2P-PIm and ZnP-PIm
Figure 1. Optimized structure and the HOMO and LUMO of
H2P-PIm obtained by DFT at the ab initio B3LYP/3-21G level.
SCHEME 4: Structure of SMP-PIm and DMP-PIm
SCHEME 3: Structures of H2P-PIm-H2P and ZnP-
PIm-ZnP
stationary point of the Born-Openheimer potential-energy sur-
face. For H2P-PIm, the H2P plane and PIm plane were almost
in the same plane. The distance between the center of H2P and
the center of the PIm (RCC) was estimated to be 8.3 Å. The
highest-occupied molecular orbital (HOMO) and lowest-unoc-
cupied molecular orbital (LUMO) for H2P-PIm obtained by
using the B3LYP/3-21G method were shown in Figure 1. The
electron distribution of the HOMO was found to be entirely lo-
cated on the H2P entity, while the electron distribution of the
LUMO was found to be entirely located on the PIm, suggesting
the hole (radical cation) and electron (radical anion) distribution
in the charge-separated state, respectively. In the case of ZnP-
PIm, the RCC value was found to be 9.3 Å, and the HOMO and
LUMO were calculated (Supporting Information, Figure S1).
A similar picture can be summarized for H2P-PIm-H2P and
ZnP-PIm-ZnP triads, where the porphyrin plane and PIm plane
were almost perpendicular. The estimated RCC between the
centers of H2P and PIm was found to be 8.3 Å, while the
distance between the centers of ZnP and PIm was found to be
9.3 Å. The HOMO and LUMO were also calculated as shown
in Figure 2, in which the HOMO located on the two porphyrin
rings, while the LUMO located on the PIm moiety.
conveniently synthesized in 10% yield from the acid-catalyzed
condensation of 3,4,5-trimethoxy benzaldehyde and 4-hydroxy-
benzaldehyde with pyrrole at an appropriate portion.31 Anhy-
drous potassium carbonate mediated the condensation of H2P
with N,N′-dioctyl-1-bromoperylene-3,4:9,10-tetracarboxylic-bis-
(imide) to afford the dyad H2P-PIm (Scheme 2). The triad
H2P-PIm-H2P (Scheme 3) was prepared by the same method
with N,N′-dioctyl-1,7-dibromoperylene-3,4:9,10-tetracarboxylic-
bis(imide) as starting PIm. Preparations of all Zn(II)-porphyrined
samples were carried out by treatments of porphyrin-containing
samples with Zn(OAc)2. SMP-PIm and DMP-PIm as shown
in Scheme 4 were also prepared as reference compounds in the
1
same way. All the new compounds were characterized by H
NMR, 13C NMR, matrix-assisted laser desorption ionization
time-of-flight (MALDI-TOF) mass spectra, and FT-IR spectra
(see Experimental Section).
Optimized Molecular Structures. To gain insights into the
intramolecular interactions and the electronic structure, com-
putational studies have been performed by using density func-
tional theory (DFT) methods at the B3LYP/3-21G level. For
this aim, both the dyads and triads were fully optimized to a
Redox Potentials and Free-Energy Changes. The deter-
mination of the redox potential in the donor-acceptor systems

3660 J. Phys. Chem. B, Vol. 109, No. 8, 2005
Xiao et al.
(2)
-∆GCS ) ∆E0-0 - (-∆GCR
)
where ∆E0-0 refers to the energy of the 0-0 transition energy
gap between the lowest excited state and the ground state of P
and PIm depending on the excitation wavelength and ∆Gs refers
to the solvation energy that can be calculated according to eq
332
-∆GS ) (e2/(4πꢀ0))[(1/(2R+) + 1/(2R-) - 1/RD-R)/ꢀS -
(1/(2R+) + 1/(2R-))/ꢀR] (3)
where R+ refers to the radii of the radical cation of P and
R- refers to the radii of the radical anion of PIm (Figure 1), e,
ꢀ0, ꢀS, and ꢀR refer to elementary charge, vacuum permittivity,
and static permittivities of the solvents used for rate measure-
ments and redox potential measurements, respectively. In
benzonitrile, the charge-separated state of ZnP•+-PIm•- located
at 1.53 eV above the ground state, 0.56 eV lower than the first
excited singlet state of ZnP and 0.85 eV lower than the first
excited state of PIm (Table 2). In general, the charge-separation
process via the excited singlet state of the porphyrin (1P*) and
perylene (1PIm*) was exothermic and favorable in benzonitrile.
UV-Vis Spectral Studies. UV-vis spectroscopy is a simple
method for determining the presence of ground-state interactions
between the moieties in the connected systems. Steady-state
absorption spectra of H2P-PIm and ZnP-PIm in CHCl3 were
shown in Figure 3, together with the absorption bands of the
components. An obvious feature can be observed that the PIm
moiety absorbed strongly in the region between the porphyrin
Soret and Q bands of the ZnP moiety. The absorption spectra
of H2P-PIm and H2P-PIm-H2P showed superposition fea-
tures of their components, which indicated no evidence for
strong electronic interactions among the component chro-
mophores in the ground state. In Figure 4, absorption bands of
H2P-PIm-H2P and ZnP-PIm-ZnP were shown together with
their components. These spectra also indicated the absence of
interaction between the components. These results were con-
sistent with the conclusion drawn from the electrochemistry data.
On addition of tetrakis(dimethylamino)ethylene (TDAE),
which is well-known as a strong reducing organic reagent, the
color of the solution containing SMP-PIm changed, and the
new absorption bands appeared in the 700-1000-nm region as
shown in Figure 5. The absorption bands at 720, 800, 850, and
950 nm were attributed to PIm•-.33 On further addition of octyl
viologen dication (OV2+), the absorption bands of PIm•-
disappeared, and new absorption bands with a peak at 600 nm
appeared, indicating the generation of the radical cation of octyl
viologen (OV•+) by the electron transfer from PIm•-.
Fluorescence Studies. The photoexcited states of PIm, H2P,
H2P-PIm, and H2P-PIm-H2P were investigated by steady-
state fluorescence measurements (Figure 6). The measurements
of Figure 6a were carried out at an excitation wavelength of
490 nm, where the PIm was exclusively excited. The fluores-
cence intensity of the PIm moiety at 540 nm decreased in H2P-
PIm and H2P-PIm-H2P in toluene and benzonitrile compared
with that of PIm,34 suggesting that energy transfer and charge
separation took place via 1PIm* and formed 1H2P*-PIm
(1H2P*-PIm-H2P) and H2P•+-PIm•- (H2P•+-PIm•--H2P or
H2P-PIm•--H2P•+), which can be supported by negative
∆GCS values. The quenching of fluorescence intensity of
H2P-PIm-H2P was more efficient compared with that of
H2P-PIm. For ZnP-PIm and ZnP-PIm-ZnP, the similar
tendency was observed. These findings suggested that two H2P
(or ZnP) moieties were cooperating to enhance energy migration.
Figure 2. Optimized structure and the HOMO and LUMO of H2P-
PIm-H2P obtained by density function method at ab initio B3LYP/
3-21G level.
TABLE 1: Half-Wave Potentials (E1/2 vs SCE) for
Porphyrin-PIm Compounds, Detected by CV (scan rate
-
50mV/s) in o-Dichlorobenzene Solutions (0.1 M n-Bu4N+PF6
as Supporting Electrolyte) at Room Temperature
oxidation potentials/V
H2P/ZnP
reductio n potential s/V
PIm H2P/ZnP
compounds
H2P
H2P-PIm
H2P-PIm-H2P
ZnP
ZnP-PIm
ZnP-PIm-ZnP
SMP-PIm
DMP-PIm
Eox(1)
Eox(2)
Ered(1) Ered(2) Ered(1) Ered(2)
+1.06
+1.06
+1.07
+0.85
+0.86
+0.86
-1.31 -1.62
-0.67 -0.86 -1.29 -1.58
-0.67 -0.87 -1.30 -1.60
-1.62 -1.90
+1.08
+1.09
+1.09
-0.67 -0.86 -1.63 -1.90
-0.67 -0.87 -1.63 -1.90
-0.66 -0.86
-0.68 -0.86
is important to evaluate the energy of electron-transfer reactions.
We have performed a systematic study to evaluate the redox
behavior of the dyads and triads using the cyclic voltammetry
(CV) technique by sweeping an applied voltage to the compound
in solution with a suitable electrolyte. The oxidation potentials
(EOX) for donor moieties and reduction potentials (ERED) for
acceptor moieties were evaluated as summarized in Table 1. A
comparison between the EOX potentials of the H2P-PIm and
ZnP-PIm dyads with those of the H2P and ZnP recorded under
similar conditions was revealed to be almost the same, sug-
gesting the absence of ground-state interactions between the H2P
(and ZnP) and the PIm moieties. Similar electrochemical
behaviors were observed for the triads (H2P-PIm-H2P and
ZnP-PIm-ZnP). These results collectively suggested the
absence of weak interactions between the different entities of
the triads.
The free-energy changes for charge separation (-∆GCS) and
charge recombination (-∆GCR) can be calculated based on the
electrochemical data by Rehm-Weller (eqs 1-2)32
-∆GCR ) Eox - Ered + ∆GS
(1)

Perylenetetracarboxylic Diimide and Porphyrin
J. Phys. Chem. B, Vol. 109, No. 8, 2005 3661
TABLE 2: Free-Energy Changes for Charge Separation
(∆GCS) and Charge Recombination (∆GCR) of P•+ - PIm•-
in Toluene and Benzonitrilea
at 650 nm of H2P-PIm and H2P-PIm-H2P were evaluated
from the single exponential decays as listed in Tables 3 and 4.
On the other hand, the fluorescence lifetimes of the ZnP moiety
at 600 nm of ZnP-PIm and ZnP-PIm-ZnP showed dual
exponential decays. The lifetimes were also listed in Tables 3
and 4.
Attaching the PIm moiety to H2P or ZnP introduced a new
quenching pathway to reduce the lifetimes of the 1PIm*, 1ZnP*,
or1H2P* in toluene and benzonitrile, which was in agreement
with the steady-state results. This quenching was due to the
photoinduced charge separation between excited singlet states
of porphyrins and PIm to yield charge-separated states
(P•+-PIm•- and P•+-PIm•--P).
-∆GCS/eV
solvent
via 1P*
via 1PIm*
-∆GCR/V
H2P-PIm
BN
toluene
BN
toluene
BN
toluene
BN
0.17
-0.08
0.16
-0.09
0.56
0.10
0.56
0.10
0.65
0.40
0.64
0.39
0.85
0.39
0.85
0.39
1.73
1.98
1.74
1.99
1.53
1.99
1.53
1.99
H2P-PIm-H2P
ZnP-PIm
ZnP-PIm-ZnP
toluene
a E0-0 for 1PIm* is 2.38 eV, for 1H2P* 1.90 eV, and for 1ZnP*
2.08 eV.
The rate constant for fluorescence quenching can be calcu-
lated from eq 4
When the H2P moiety of H2P-PIm and H2P-PIm-H2P was
excited at 420 nm, the fluorescence intensity of them at 653
nm decreased much more compared with that of reference
compound H2P in benzonitrile (Figure 6b),35,36 suggesting that
kq ) (1/τf)sample - (1/τf)reference
(4)
1
1
where (τf)sample is the lifetime of the PIm*, ZnP*, or1H2P in
the dyads and triads, while (τf)reference refers to the lifetime of
reference samples. The kq values were calculated out in the range
of 1.6 × 109 - 1.7 × 1010 in toluene and benzonitrile. In Table
1
charge separation took place via H2P* and formed H2P•+-
PIm•- (or H2P•+-PIm•--H2P). Efficient fluorescence quench-
ing of H2P-PIm-H2P more than H2P-PIm was also observed.
For ZnP-PIm and ZnP-PIm-ZnP, the similar tendency was
observed as well.
Time-resolved emission and nanosecond transient absorption
studies were performed for both the dyad and triad systems to
follow the kinetics of photoinduced processes.
1
1
3, the kq values via H2P* were larger than those via PIm*
both in H2P-PIm and H2P-PIm-H2P. The kq values via 1H2P*
were also larger than those via 1ZnP. For ZnP-PIm-ZnP, the
1
kq values via PIm* were quite large. The quantum yields of
the charge-separated states were calculated according to eq 5
Time-Resolved Fluorescence Studies. The time-re-
solved fluorescence measurements of PIm, H2P-PIm, and
H2P-PIm-H2P shown in Figure 7 were consistent with those
of the steady-state fluorescence measurements. Figure 7 showed
the fluorescence decay profiles of the H2P in benzonitrile (λex
) 400 nm) where the PIm moiety was exclusively excited. The
fluorescence time profiles of SMP-PIm, H2P-PIm, and
H2P-PIm-H2P at 540 nm exhibited single-exponential de-
cays with the lifetimes (τf) of 5.0, 0.17, and 0.61 ns, respec-
tively. Similarly, the fluorescence lifetimes of the H2P moiety
Φq ) kq(τf)sample
(5)
The Φq values were larger than 0.90 for all processes. In the
1
case of fluorescence lifetimes of the PIM* moiety, the kq and
Φq values included both the energy transfer (kEN and ΦEN) and
charge separation (kCS and ΦCS), respectively. While in the case
1
1
of fluorescence lifetimes of the H2P* and ZnP* moiety, the
kq and Φq values corresponded to kCS and ΦCS, respectively.
Even if energy transfer took place in toluene, the ΦCS values
Figure 3. The absorption spectra of H2P-PIm, ZnP-Pim, and reference compounds H2P, ZnP, and SMP-PIm in chloroform at the fixed concentration
of 1 µM at room temperature. The dotted lines are fluorescence spectra of H2P, ZnP, and SMP-PIm in chloroform at room temperature obtained
upon exciting the porphyrin unit at Soret band and exciting PIm at 490 nm. All spectra are normalized to the same amplitude in their corresponding
panels.

3662 J. Phys. Chem. B, Vol. 109, No. 8, 2005
Xiao et al.
Figure 4. The absorption spectra of H2P-PIm-H2P, ZnP-PIm-ZnP, and reference compounds H2P, ZnP, and DMP-PIm in chloroform at the
fixed concentration of 1 µM at room temperature. The dotted lines are fluorescence spectra of H2P, ZnP, and DMP-PIm in chloroform at room
temperature are obtained upon exciting the porphyrin unit at Soret band and exciting PIm at 490 nm. All spectra are normalized to the same
amplitude in their corresponding panels.
The lifetimes of the charge-separated states were evaluated
from the decay of the PIm•- moiety at 820-1020 nm. The
decays of P•+-PIm•- can be attributed to the recombination to
the ground state. The lifetime of the charge-separated states were
calculated in the range of 7-14 ns in benzonitrile as listed in
Tables 3 and 4.
In benzonitrile, similar transient absorption spectra were
observed as shown in Figure 9, in which the 700-nm bands were
unclearly observed because of stronger emission or scattering
of the light in this region.
Steady-State Photolysis in the Presence of External
Electron Donors and Electron Acceptors. The formation of
Figure 5. (Solid lines) Absorption spectra of SMP-PIm (7 × 10-3
the charge-separated state of the investigated systems was further
mM) in benzonitrile in the presence of TDAE (0.0-0.2 mM). (Dotted
confirmed in deoxygenated solutions under irradiation upon
lines) Further addition of octyl viologen (OV2+, 0.4 mM) in the presence
adding methyl viologen (MV2+) or octyl viologen (OV2+) as
of TDAE (0.2 mM) and SMP-PIm (7 × 10-3 mM) in deaerated
an external sacrificial electron acceptor and 1-benzyl-1,4-
dihydronicotinamide (BNAH) as an external electron donor.37
A comparison between the one-electron reduction potential of
the PIm unit (-0.67 vs SCF) in H2P-PIm and MV2+ (-0.45
V vs SCF) indicated that an electron transfer from PIm•- to
MV2+ was exothermic by 0.22 eV. Also, on addition of an
external electron donor, benzyldihydro nicotinamide (BNAH)
(Eox ) 0.57 V vs SCF), the hole shift from H2P•+ in H2P-PIm
to BNAH was expected to be exothermic by 0.49 eV. Under
these conditions, once the charge-separation state was obtained
upon photoirradiation of P-PIm and P-PIm-P, the oxidation
of BNAH and reduction of MV2+ were expected to take place
simultaneously (Scheme 5). The process was experimentally
confirmed by photolysis of the H2P-PIm/ MV2+-BNAH
system with monochromatized light of 423 nm in Ar-satu-
rated DMF/H2O (95:5, v/v) solution. The steady-state photoly-
hfsis of the dyad produced a progressive increase in the
characteristic absorption band of MV•+ (λmax ) 605 and 398
nm) and a progressive decrease in the absorption band of
BNAH at 347 nm according to the time of the irradiations as
shown in Figure 10. By contrast, no reaction occurred in the
benzonitrile.
in benzonitrile became predominant, taking into consideration
the more negative ∆GCS values in benzonitrile than those in
toluene.
Transient Absorption Measurements. The nanosecond
transient absorption spectra of P-PIm and P-PIm-P were
observed in Ar-saturated toluene by applying 532-nm laser light
excitation. In the case of H2P-PIm, both H2P and PIm were
photoexcited. Whereas the 532-nm laser light exclusively excited
the PIm moiety in the case of ZnP-PIm (Figure 8). The transient
absorption spectra revealed sharp absorption bands at 720, 800,
850-860, and 920-940 nm with fast rise and decay. These
absorption peaks can be ascribed to PIm•-. In some transient
spectra, absorption peaks at 720 and 800 nm were hidden within
the emission of H2P-PIm and ZnP-PIm in this region. The
rapid rise indicated that the charge-separation process took place
from ZnP to 1PIm*. In the case of H2P-PIm, charge separation
1
took place in both directions: from H2P to PIm* and from
1H2P* to PIm due to sufficiently negative ∆GCS values in both
processes (Table 2).

Perylenetetracarboxylic Diimide and Porphyrin
J. Phys. Chem. B, Vol. 109, No. 8, 2005 3663
Figure 6. Steady-state fluorescence spectra (a) (i) PIm, (ii) H2P-Pim, and (iii) H2P-PIm-H2P obtained by 490-nm excitation laser light. (b) (i)
H2P, (ii) H2P-Pim, and (iii) H2P-PIm-H2P obtained by 420-nm excitation laser light. The concentrations of compounds were maintained at 5.0
× 10-6 M in benzonitrile.
zinc analogue systems, the same steady-state photolysis results
were also obtained (see Supporting Information).
The electron-mediating process was confirmed by applying
532-nm laser excitation of ZnP-PIm (0.05 mM) in the presence
of OV2+ (4 mM) in Ar-saturated benzonitrile, as shown in the
transient absorption spectra and time profiles in Figure 11. The
presence of OV•+ was observed at 1.0 µs by building up of
absorption around 620 nm and paralleling a disappearance of
PIm•- at 840 and 920 nm.
These observations indicated that, in the presence of MV2+
,
laser irradiation of PIm induced electron transfer from ZnP to
1PIm*. The produced PIm•- successively donated its excess
electron to MV2+ yielding MV•+ as indicated by the very fast
disappearance of PIm•- that we could not record due to the
instrument limitation. Here it is worth mentioning that the slow
rise of MV2+ might also arise from the intermolecular interaction
between MV2+ and the excited triplet states of porphyrins. In
Figure 7. Fluorescence decay profiles of (a) SMP-PIm, (b)
H2P-Pim, and (c) H2P-PIm-H2P in benzonitrile. λex ) 400 nm. The
concentrations were maintained at 0.05 mM.
dark or in the absence of H2P-PIm under photoirradiation. From
the H2P-PIm-H2P/MV2+/BNAH system and corresponding
1
1
1
TABLE 3: Photophysical Properties of Porphyrin-Perylene Systems via H2P*, ZnP*, and PIm*a in Tolune where λex ) 420
nm
kq /s-1
b
Φq
k
CR/s-1 (τCS/ns)
b
excited state
τf/ns
H2P-PIm
1H2P*
1PIm*
1H2P*
1PIm*
1ZnP*
1PIm*
1ZnP*
1PIm*
0.31(31%) 2.20(69%)
0.24(52%) 3.10(48%)
0.32(33%) 2.10(67%)
0.06(100%)
0.20(52%) 2.45(48%)
0.22(71%) 3.14(29%)
0.12(61%) 1.42(39%)
0.14(100%)
2.7 × 109 (9.5 × 107)c
3.6 × 109 (9.5 × 107)c
2.6 × 109 (9.5 × 107)c
1.6 × 1010
0.83 (0.15)c
0.87 (0.15)c
0.83 (0.15)c
0.97
8.4 × 107
(12)
H2P-PIm-H2P
ZnP-PIm
1.5 × 108
(11)
4.5 × 109 (3.1 × 108)c
4.1 × 109 (4.7 × 108)c
7.9 × 109 (1.2 × 109)c
6.7 × 109
0.91 (0.39)c
0.88 (0.49)c
0.94 (0.70)c
0.93
9.6 × 107
(14)
ZnP-PIm-ZnP
1.4 × 108
(11)
a τf values for H2P, ZnP, and SMP-PIm are 13.6, 2.1, and 5.0, respectively. b Calculated from the fast component. c Calculated from the average
values of both fast and slow components.
1
1
1
TABLE 4: Photophysical Properties of Porphyrin-Perylene Systems via H2P*, ZnP*, and PIm*a in Benzonitrile
kq /s-1
Φq
0.99
0.97
99
0.99
k
CR/s-1 (τCS/ns)
b
b
excited state
τf/ns
H2P-PIm
1H2P*
1PIm*
1H2P*
1PIm*
1ZnP*
1PIm*
1ZnP*
1PIm*
0.06 (100%)
0.17 (100%)
0.06 (100%)
0.61 (100%)
0.21 (66%) 2.51 (33%)
0.18 (44%) 3.14 (56%)
0.12 (100%)
1.7 × 1010
1.5 × 108
5.7 × 109
(7)
H2P-PIm-H2P
ZnP-PIm
1.7 × 1010
2.0 × 108
(5)
1.6 × 109
4.2 × 109 (2.8 109)c
5.4 × 109 (2.3 109)c
6.7 × 109
0.90 (0.60 )c
0.96 (0.42 )c
0.94
1.5 × 108
(7)
ZnP-PIm-ZnP
1.4 × 108
0.08 (100%)
1.2 × 1010
0.98
(7)
a τf values for H2P, ZnP, and SMP-PIm are 13.6, 2.1, and 5.0, respectively. b Calculated from the fast component. c Calculated from the average
values of both fast and slow components.

3664 J. Phys. Chem. B, Vol. 109, No. 8, 2005
Xiao et al.
Figure 8. Transient absorption spectra obtained by 532 nm laser photolysis of (a) H2P-PIm, (b) H2P-PIm-H2P, (c) ZnP-Pim, and (d)
ZnP-PIm-ZnP (0.1 mM) in Ar-saturated toluene.
Figure 9. Transient absorption spectra obtained by 532 nm laser photolysis of (a) H2P-PIm, (b) H2P-PIm-H2P, (c) ZnP-Pim, and (d)
ZnP-PIm-ZnP (0.1 mM) in Ar-saturated benzonitrile.
SCHEME 5: Summarization of the Whole
Photosensitized Electron-Mediation/Hole-Shift Processes
within the Dyad of H2P-Pim
Conclusions
In conclusion, we demonstrated here the synthesis and the
photoinduced electron-transfer process of new photoactive dyads
and triads containing porphyrin (H2P and ZnP), as an electron
donor, and perylenetetracarboxylic diimide (PIm), as an electron
acceptor. Thermodynamically, the electron transfer process from
porphyrin to perylene in their excited singlet states was
exothermic and favorable in benzonitrile. The steady-state, time-
resolved emission and transient absorption results revealed the
occurrence of electron transfer mainly from the porphyrin to
perylene entities in the polar solvent by observing the anion
radical of the perylene moiety. The formation of the CS state
was evidenced by the formation of MV•+ in the presence of
BNAH as illustrated by the energy diagram shown in Figure
12. Further studies on photophysics of the dyads and triads and
this case, PIm acted as an electron mediator in addition to a
photosensitizing electron acceptor. The whole photosensitized
electron-mediating/ hole-shift processes were summarized in
Scheme 5.

Perylenetetracarboxylic Diimide and Porphyrin
J. Phys. Chem. B, Vol. 109, No. 8, 2005 3665
Figure 10. (a) Absorption spectral changes observed in the steady-state photolysis with monochromatized light (λ ) 423 nm) of H2P-PIm (1.0
× 10-6 M) in the presence of BNAH (4.0 × 10-4 M) and MV2+ (8.0 × 10-4 M) in an Ar-saturated DMF/H2O (95:5, v/v) solution. (b) Differences
in spectral changes between after and before steady-state photolysis (the inset shows the absorption band at 605 nm with a multiple of 10).
was prepared from commercially available octyl viologen
bromide. All solvents were purified using standard pro-
cedures. Evaporation and concentration in vacuo were done at
water aspirator pressure, and compounds were dried at 10-2
Torr. Column chromatography (CC): SiO2 (160-200 meshes).
TLC glass plates coated with SiO2 F254 were visualized by UV
light.
Instrumental Techniques. All spectroscopic data were
collected at room temperature. UV-vis spectra were measured
on a Hitachi U-3010 spectrometer. FT-IR spectra were recorded
as KBr pellets on a Perkin-Elmer System 2000 spectrometer.
1H NMR (400-MHz) and 13C NMR spectra were recorded on a
Bruker ARX400 spectrometer. The solvent signal was used as
an internal reference for both 1H and 13C NMR spectra. MALDI-
Figure 11. Transient absorption spectra obtained by 532-nm laser
photolysis of ZnP-PIm in the (a) absence and (b) presence of OV2+
(0.3 mM) in Ar-saturated benzonitrile.
TOF mass spectrometric measurements were performed on
Bruker Biflex â MALDI-TOF.
Cyclic voltammetry (CV) was performed on a CHI voltam-
metric analyzer (BAS 100w, Bioanalytical Systems) at room
temperature with a three-electrode configuration in o-dichlo-
robenzene solution containing a supporting electrolyte. A glassy
carbon (L3 mm) disk served as the working electrode; a
platinum foil and a SCE were the counter and the reference
electrodes, respectively. Both the counter and the reference
electrodes were directly immersed in the electrolyte solution.
The surface of the working electrode was polished with
commercial alumina (No. 1C, Alpha Micropolish, Aldrich;
particle size 1.0 µm) prior to use. Tetra-n-butylammonium
hexafluorophosphate (>99%, Fluka) was recrystallized twice
from ethanol and dried in a vacuum at 100 °C overnight prior
to use and was employed as the supporting electrolyte (0.1 M).
Solutions were stirred and deaerated by bubbling nitrogen for
about 10 min prior to each voltammetric measurement. The scan
rate was 50 mV s-1 unless otherwise specified.
Figure 12. Energy-level diagrams of ZnP-PIm systems by applying
The time-resolved fluorescence spectra were measured by a
single-photon-counting method using a second harmonic gen-
eration (SHG, 410 nm) of Ti:sapphire laser (Spectra-Physics,
Tsunami 3950-L2S, 1.5 ps full width at half maximum (fwhm))
and a streak-scope (Hamamatsu Photonics, C43334-01) equipped
with a polychromator (Action Research, SpectraPro 150) as an
excitation source and a detector, respectively. Lifetimes were
evaluated with software attached to the equipments.
excitation laser light in the presence of MV2+ and BNAH.
their application as photoactive materials in photovoltaic devices
is currently underway.
Experimental Section
General. Reagents were purchased reagent grade from
Acros or Aldrich Corporation and were utilized as received
unless indicated otherwise. N,N′-Dioctyl-1-bromoperylene-
3,4:9,10-tetracarboxylic bisimide,38 N,N′-dioctyl-1,7-dibromo-
perylene-3,4:9,10-tetracarboxylic bisimide,38 H2P,31 ZnP,31 and
DMP-PIm31 were prepared as described in the literature. TDAE
was commercially available. Octyl viologen (OV2+) pechlorate
The nanosecond transient absorption measurements were
carried out using SHG (532 nm) of a Nd:YAG laser (Spectra-
Physics and Quanta-Ray GCR-130, 6 ns fwhm) as an excitation
source. For transient absorption spectra in the near-IR region
(600-1200 nm), monitoring light from a pulsed Xe lamp was
detected with a Ge-APD (Hamamatsu Photonics, B2834). All

3666 J. Phys. Chem. B, Vol. 109, No. 8, 2005
Xiao et al.
samples in a quartz cell (1 × 1 cm) were deaerated by bubbling
with saturated sodium bicarbonate aqueous solution and water
successively and dried over anhydrous sodium sulfate, and then
the solvent was removed under reduced pressure. Column
chromatography on silica gel with chloroform as an eluent
afforded the corresponding Zn(II)-porphyrin containing targets.
ZnP-Pim. (Rf ) 0.68, CHCl3/CH3COOC2H5, 10:1.) 1H
NMR (CDCl3): 9.52 (d, 1H, J ) 8.2 Hz), 9.10 (d, 2H, J ) 4.5
Hz), 9.08 (m, 6H), 8.60 (s, 1H), 8.44 (s, 2H), 8.30-8.36 (m,
4H), 7.91 (br, 1H), 7.57 (d, 2H, J ) 7.8 Hz), 7.45 (s, 6H), 4.18
(s, 9H), 4.16 (br, 2H), 3.92 (s, 18H), 3.66 (m, 2H), 1.76 (m,
2H), 1.27-1.45 (m, 22H), 0.84-0.89 (m, 6H). MALDI-TOF
MS: 1574.6. FT-IR (KBr, ν(cm-1)): 2926, 2853, 1696, 1658,
15937, 1497, 1458, 1406, 1346, 1237, 1204, 1165, 1126, 1001,
940, 808, 722.
Ar through the solution for 15 min.
Steady-State Photolysis. A square quartz cell (10 mm inside
diameter) containing a deaerated DMF-water (95:5, v/v)
solution of H2P-PIm (1.0 × 10-6 M), MV2+ (8.0 × 10-4 M),
and BNAH (4.0 × 10-4 M) was irradiated with monochroma-
tized light of λ ) 423 nm from a 150-W metal halogen lamp
through a filter. The light intensity was determined as 2.3
mW/cm2. The photochemical reactions were monitored by
measuring the absorption spectra at different irradiation times.
H2P-PIm. N,N′-Dioctyl-1-bromoperylene-3,4:9,10-tetracar-
boxylic bisimide (70 mg, 0.1 mmol) in 40 mL of dry toluene
containing K2CO3 (2 equiv) and 18-crown-6 (2 equiv) was
stirred under N2 for 20 min, and subsequently H2P (100 mg,
1.1 equiv) was added. The mixture was heated to 100 °C and
continuously stirred for 3 h until the reaction finished from the
detection of TLC. When the solvent was evaporated under
reduced pressure, the resulting mixture was loaded to a column
chromatography to afford 90 mg of pure target product
H2P-PIm (Rf ) 0.78, CHCl3/CH3COOC2H5, 10:1, yield 60%)
eluted by chloroform. 1H NMR (CDCl3): 9.75 (d, 1H, J ) 8.3
Hz), 9.02 (d, 2H, J ) 4.7 Hz), 8.98 (s, 4H), 8.97 (d, 2H, J )
4.7 Hz), 8.79 (d, 1H, J ) 8.3 Hz), 8.71 (s, 1H), 8.70 (d, 1H, J
) 8.1 Hz), 8.67 (s, 2H), 8.61 (d, 1H, J ) 8.1 Hz), 8.32 (d, 2H,
J ) 8.5 Hz), 7.59 (d, 2H, J ) 8.5 Hz), 7.49 (s, 6H), 4.23 (m,
4H), 4.19 (s, 9H), 3.98 (s, 18H), 1.81 (m, 4H), 1.26-1.48 (m,
20H), 0.88 (t, 3H, J ) 7.0 Hz), 0.86 (t, 3H, J ) 7.0 Hz). 13C
NMR (CDCl3): 163.4, 163.2, 162.7, 155.5, 154.8, 151.5, 139.2,
138.0, 137.5, 136.4, 134.2, 134.1, 133.5, 131.9, 130.7, 129.8,
128.9, 128.6, 128.5, 126.7, 125.8, 125.1, 124.4, 124.0, 123.5,
123.1, 122.7, 122.4, 120.2, 118.6, 117.6, 113.0, 113.0, 61.3,
56.4, 40.9, 31.8, 29.7, 29.4, 29.3, 28.2, 27.2, 22.6, 14.1. MALDI-
TOF: 1512.9. FT-IR (KBr, ν(cm-1)): 3455, 2926, 2853, 1696,
1658, 1593, 1500, 1465, 1407, 1352, 1236, 1206, 1169, 1127,
1008, 975, 925, 854, 805, 730.
ZnP-PIm-ZnP. (Rf ) 0.48, CHCl3/CH3COOC2H5, 10:1.)
1H NMR (CDCl3): 9.64 (br, 2H), 9.05-9.11 (m, 16H), 8.65
(s, 2H), 8.30 (4H), 7.80 (2H), 7.46-7.59 (m, 16H), 4.15 (s,
18H), 3.92 (s, 36H), 3.65 (m, 4H), 1.1-1.4 (m, 24H), 0.87 (t,
6H, J ) 6.7 Hz). MALDI-TOF MS: 2533.9. FT-IR (KBr,
ν(cm-1)): 2929, 2853, 1697, 1659, 1591, 1497, 1460,1406,
13476, 1237, 1165, 1126, 1073, 1001, 941, 799, 722.
Reference compounds SMP-PIm and DMP-PIm31 were
prepared according to the general method described for the
synthesis of compound H2P-PIm except that p-methyl phenol
was used as the starting materials instead of H2P.
1
SMP-Pim. H NMR (CDCl3): 9.43 (d, 1H, J ) 8.3 Hz),
8.44-8.58 (m, 5H), 8.16 (s, 1H), 7.27 (d, 2H, J ) 9.6 Hz),
7.06 (d, 2H, J ) 9.6 Hz), 4.12 (m, 4H), 2.42 (s, 3H), 1.72 (m,
4H), 1.28-1.50 (m, 20H), 0.87 (t, 6H, J ) 7.2 Hz). MALDI-
TOF MS: 720.6. FT-IR (KBr, ν (cm-1)): 2926, 2854, 1695
(s), 1657 (s), 1594 (s), 1505, 1407, 1343 (s), 1260 (s), 1201,
808, 747.
Acknowledgment. This work was supported by the Major
State Basic Research Development Program and the National
Natural Science Foundation of China (20151002, 50372070).
This research was partially supported by a Grant-in-Aid for the
COE Project, Giant Molecules and Complex Systems, 2002.
This work was also supported by a Grant-in-Aid for Scientific
Research on Priory Area (417) from the Ministry of Education,
Culture, Sports, Science, and Technology of the Japanese
Government.
H2P-PIm-H2P. N,N′-Dioctyl-1,7-dibromoperylene-3,4:9,10-
tetracarboxylic diimide (39 mg, 0.05 mmol) in 40 mL of dry
toluene containing K2CO3 (2 equiv) and 18-crown-6 (2 equiv)
was stirred under N2 for 20 min and subsequently H2P (100
mg, 2.2 equiv) was added. The mixture was heated to 100 °C
and continuously stirred until the reaction finished from the
detection of TLC. When the solvent was evaporated under
reduced pressure, the resulting mixture was loaded to a column
chromatography to afford 78 mg of pure target product
H2P-PIm-H2P (Rf ) 0.70, CHCl3/CH3COOC2H5, 10:1, yield
65%) eluted by chloroform. 1H NMR (CDCl3): 9.93 (d, 2H, J
) 8.3 Hz), 9.01 (d, 4H, J ) 4.0 Hz), 8.98 (m, 8H), 8.89 (d,
2H, J ) 8.3 Hz), 8.80 (s, 2H), 8.32 (d, 4H, J ) 8.3 Hz), 7.61
(d, 4H, J ) 8.3 Hz), 7.48 (s, 12H), 4.30 (m, 4H), 4.19 (s, 18H),
3.98 (s, 36H), 1.86 (m, 4H), 1.52 (m, 4H), 1.26-1.43 (m, 20H),
0.88 (t, 6H, J ) 6.6 Hz). 13C NMR (CDCl3): 163.4, 163.1,
155.3, 154.9, 151.5, 139.0, 138.0, 137.5, 136.4, 133.5, 131.5,
130.7, 129.6, 129.3, 128.0, 125.6, 124.9, 124.7, 124.4, 122.7,
120.2, 118.7, 117.6, 117.4, 113.0, 112.9, 61.3, 56.4, 40.9, 31.8,
29.7, 29.4, 29.3, 28.2, 27.3, 14.1. MALDI-TOF: 2411.7 (M +
H+), 2434.6 (M + Na+), 2451.6 (M + K+). FT-IR (KBr,
ν(cm-1)): 3472, 2929, 2154, 1698, 1659, 1593, 1580, 1499,
1464, 1407, 1356, 1235, 1128, 1105, 1007, 973, 924, 855, 801,
732.
Supporting Information Available: Optimized structure
and HOMO and LUMO of ZnP-PIm and absorption spectral
changes observed in the steady-state photolysis of ZnP-PIm,
H2P-PIm-H2P, and ZnP-PIm-ZnP in the presence of BNAH
and MV2+ in an Ar-saturated solution. This material is available
free of charge via the Internet at http://pubs.acs.org.
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