Studies on covalently linked porphyrin-C60 dyads: Stabilization of charge-separated states by axial coordination

Article abstract of DOI:10.1021/jp021926r

The effect of axial ligation on the photoinduced charge separation and charge recombination of a series of covalently linked porphyrin-C₆₀ dyads is investigated. Toward this, meso-tetraphenylporphyrin and its zinc(II) derivative are functionali

Full text of DOI:10.1021/jp021926r

J. Phys. Chem. A 2002, 106, 12393-12404
12393
Studies on Covalently Linked Porphyrin-C60 Dyads: Stabilization of Charge-Separated
States by Axial Coordination
Francis D’Souza,*,† Suresh Gadde,† Melvin E. Zandler,† Klykov Arkady,†
Mohamed E. El-Khouly,‡ Mamoru Fujitsuka,‡ and Osamu Ito*,‡
Department of Chemistry, Wichita State UniVersity, 1845 Fairmount, Wichita, Kansas 67260-0051, and
Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Katahira,
Sendai, 980-8577, Japan
ReceiVed: August 21, 2002
The effect of axial ligation on the photoinduced charge separation and charge recombination of a series of
covalently linked porphyrin-C60 dyads is investigated. Toward this, meso-tetraphenylporphyrin and its zinc(II)
derivative are functionalized at the ortho or para positions of one of the aryl groups to bear a fulleropyrrolidine
entity through a flexible ethylene dioxide bridge to probe the effect of intramolecular electron transfer
phenomena. In o-dichlorobenzene, 0.1 M (TBA)ClO4, the synthesized dyads exhibit seven one electron
reversible redox reactions within in the potential window of the solvent and the measured redox potentials
and UV-visible absorption spectra reveal charge-transfer interactions between the electron donor, porphyrin,
and the electron acceptor, fullerene entities. The geometric and electronic structures of the dyads probed by
ab initio B3LYP/3-21G(*) methods also revealed the existence of charge-transfer interactions. The excited
state photochemical events are monitored by both steady state and time-resolved emission as well as transient
absorption techniques. In o-dichlorobenzene or in benzonitrile, the main quenching pathway involves charge
separation from the excited porphyrin to the moiety. The kcs and kcr are found to depend on the type of
substitution (ortho or para) and the metal ion in the porphyrin cavity. Relatively long-lived charge-separated
states are observed upon coordinating pyridine axial ligands to the central metal ion of the zinc porphyrin-
C60 dyads, and this has been attributed to the electronic and geometric effects caused by the axial coordinated
ligand.
Introduction
charge migration reactions along the well-tuned redox gradients.
Photosynthetic reaction centers composed of self-assembled
Studies on light-induced electron transfer in covalently linked
donor-acceptor systems have witnessed enormous growth in
recent years mainly to address the mechanistic details of electron
transfer in chemistry and biology, to develop artificial photo-
synthetic systems for light energy harvesting, and also to develop
molecular electronic devices.1,2 Toward constructing such dyads,
fullerenes3 and porphyrins1 have been utilized as important
constituents owing to their rich redox,4 optical,5 and photo-
chemical6 properties. In contrast to the traditionally used two-
dimensional aromatic electron acceptors, the fullerene in donor-
acceptor dyads accelerates photoinduced charge separation and
slows down charge recombination. Imahori and Sakata7 at-
tributed the slower charge recombination process to the small
reorganization energy of C60. The small reorganization energy
of C60 predicts that kcs moves upward along the normal region
into the top region of the Marcus curve, and kcr lies to be
significantly downward into the inverted region.
More recently, elegantly designed porphyrin-C60 dyads have
been synthesized to probe the effect of molecular topology and
distance and orientation effects of the donor and acceptor entities
on photoinduced charge separation and charge recombination
processes.8-11 Supramolecular triads, tetrads, pentads, etc. have
also been synthesized and studied. In these supramolecular
systems, distinctly separated donor-acceptor radical ion pairs,
in succession, are generated upon initial charge separation by
donor and acceptor entities produce long-lived, highly energetic,
charge-separated states with quantum yields close to unity by
using this mechanism of charge migration.12
In the present study, we report the effect of axial ligation on
the photoinduced charge separation and charge recombination
of covalently linked porphyrin-C60 dyads. Toward this, first, a
new synthetic methodology has been developed to covalently
link porphyrin and fullerene entities with flexible bonds, which
allows a direct special interaction between the porphyrin and
the fullerene entities. Here, meso-tetraphenylporphyrin is func-
tionalized at the ortho or para positions of one of the aryl groups
to bear a fulleropyrrolidine entity through an ethylene dioxide
bridge (Scheme 1). The ortho and para positions of the phenyl
entity are primarily chosen to probe the orientation effects. The
free-base porphyrin and zinc porphyrin are selected to visualize
the effect of the free energy changes on the kinetics of
photoinduced charge separation and charge recombination. Next,
a series of substituted pyridine ligands are utilized to form penta-
coordinated zinc porphyrin-C60 dyads (Scheme 1). Here, the
axial pyridine ligands enhance the electron donor ability of
zinc(II) tetraphenylporphyrin and are expected to slow the
charge recombination processes upon initial charge separation
of the zinc porphyrin-C60 dyad by the electronic and geometric
effects caused by the axial pyridine ligand. Time-resolved
emission and transient absorption spectral techniques have been
employed to probe these reactions.
† Wichita State University.
‡ Tohoku University.
10.1021/jp021926r CCC: $22.00 © 2002 American Chemical Society
Published on Web 11/08/2002

12394 J. Phys. Chem. A, Vol. 106, No. 51, 2002
D’Souza et al.
SCHEME 1
respective unsubstituted porphyrins. Absorption peaks corre-
sponding to the C60 are observed in the UV wavelength region.
Figure 1a shows the normalized absorption spectrum for the
Soret band of the porphyrins. The Soret peak positions are red-
shifted by 1-2 nm, and the calculated full width at half-
maximum (fwhm) values revealed slight changes suggesting
intramolecular interactions between the donor and the acceptor
entities. Extending the wavelength scan till 1100 nm revealed
less intense, new broad bands corresponding to the charge-
transfer interactions in the 600-1000 nm region (Figure 1b).
The normalized spectrum in the Q-band region also revealed
some interesting features. That is, the spectral features of ZnPo-
C60 and ZnPp-C60 exhibited features distinct from the spectrum
obtained for an equimolar mixture of (TPP)Zn and 2-propyl
fulleropyrrolidine. These features along with the red-shifted
Soret band of the dyads clearly suggest the existence of
intramolecular interactions in the dyads.
The formation of penta-coordinated zinc porphyrin-C60 dyads
by axial coordination of substituted pyridines, Py:ZnP-C60, was
monitored by optical absorption studies. The formation of penta-
coordinated Py:ZnP-C60 was characterized by red-shifted Soret
and visible bands and appearance of isosbestic points.15 Job’s
plot of a continuous variation method also confirmed 1:1
complex formation between ZnP-C60 and pyridine ligands in
solution. The association constant, K, for axial coordination was
calculated from the absorption spectral data by using the
Scatchard method,16 and values are listed in Table 1. The K
values for axial coordination of (TPP)Zn by pyridine ligands
are also given for comparison purposes. The K values in Table
1 follow the order (TPP)Zn < ZnPp-C60 < ZnPo-C60 for a
given axial ligand. The higher K values obtained for the zinc
porphyrin-C60 dyads as compared to pristine (TPP)Zn can be
ascribed to the electron deficient ZnP macrocycle of the dyads
as a result of intramolecular charge-transfer interactions
between the ZnP and the C60 entities (vide supra). From the K
values, it is also apparent that such charge-transfer interactions
are slightly higher for the ortho-substituted dyad as compared
to the para-substituted dyad. For each of the ZnP derivatives,
the K values increase in the order of substituents on pyridine;
4-acetyl- < H- < 4-dimethylamino, which is the order of
electron-donating ability of the substituents. It is important to
point out that the K values of axial coordination serve as a means
to monitor intramolecular charge-transfer interactions in co-
valently linked donor-acceptor complexes as demonstrated in
the present study.
Results and Discussion
Synthesis of Porphyrin-Fullerene Dyads. Scheme 2 depicts
the procedure adopted for synthesizing the porphyrin-C60 dyads
in the present study. This involves first synthesizing a benz-
aldehyde connected through the spacer, ethylene dioxide, to one
of the phenyl rings of the meso-tetraphenylporphyrin macro-
cycle, H2P-CHO, followed by condensing it with fullerene in
the presence of sarcosine according to a general procedure
developed by Maggini et al.13 for fulleropyrrolidine synthesis.
The porphyrins, H2P-CHO, were synthesized by first preparing
5-(2′ or 4′-hydroxyphenyl)-10,15,20-triphenylporphyrin, fol-
lowed by treating it with 4-bromoethoxy benzaldehyde in the
presence of K2CO3 in dimethyl formamide (DMF). The por-
phyrins H2P-CHO thus obtained were treated with C60 and
sarcosine in toluene to obtain the free-base porphyrin-C60
dyads, H2P-C60. The free-base porphyrin-fullerene dyads,
H2P-C60, were metalated with zinc acetate14 to obtain the zinc
porphyrin-fullerene dyads, ZnP-C60. Generally, good yields
(20-30%) of the reaction products were obtained and the
synthesized porphyrin-fullerene dyads were found to be soluble
in many organic solvents. The structural integrity of the newly
Ab Initio B3LYP/3-21G(*) Modeling of the Dyads. To gain
insights into the intramolecular interactions and the electronic
structure, computational studies have been performed by using
density functional methods (DFT) at the B3LYP/3-21G(*) level.
The DFT method over the Hartree-Fock or semiempirical
approaches was chosen since recent studies have shown that
the DFT-B3LYP method at the 3-21G(*) level predicts the
geometry and electronic structure more accurately.17 For this
study, both of the dyads were fully optimized to a stationary
point on the Born-Oppenheimer potential energy surface (Figure
2) and an ultra fine integration grid was used. Energetically,
the para-derivatized dyad is more stable by ca. 3 kcal mol-1
than the ortho-derivatized dyad.
1
synthesized compounds is deduced from H NMR, ESI mass
in CH2Cl2 matrix, optical absorption and emission, and elec-
trochemical methods. The proton resonance peaks corresponding
to the ethylene dioxide, the phenyl ring connected to the
pyrrolidine ring, and pyrrolidine ring protons of the ortho-
substituted derivatives revealed an upfield shift up to 1 ppm as
compared to their respective para-substituted derivatives, and
this has been attributed to the larger porphyrin ring currents
experienced by the ortho-substituted derivatives due to the
proximity effects.
In the optimized structure of ZnPo-C60, the center-to-center
distance between the zinc porphyrin and the C60 entities (center
of the porphyrin ring to and center of the C60 spheroid; RD-A
)
UV-Visible Spectral Studies. The optical absorption spectra
in the visible wavelength region of the free-base porphyrin-
C60 dyads and zinc porphyrin-C60 dyads are similar to their
is found to be about 6.7 Å while the edge-to-edge distance (zinc
to the closet carbon of the C60 spheroid; Ree) is found to be 3.1
Å, suggesting the occurrence of through space interactions.

Covalently Linked Porphyrin-C60 Dyads
J. Phys. Chem. A, Vol. 106, No. 51, 2002 12395
SCHEME 2
Similarly, for the ZnPp-C60 derivative, RD-A and Ree are found
to be, respectively, 6.4 and 3.9 Å, suggesting close proximity
of the two entities irrespective of the nature of the substitution
(ortho or para). For the H2Po-C60 dyad, the RD-A and Ree are
found to be slightly longer than those observed for the
corresponding ZnPo-C60 and are about 7.08 and 3.67 Å,
respectively. As shown in Figure 2, little or no distortion of the
porphyrin ring is observed for the para-substituted derivatives,
while substantial distortion of the porphyrin ring atoms near
the substituted meso-phenyl ring is observed in the case of the
ortho-substituted derivative.
The existence of intramolecular interactions between the
porphyrin ring and the C60 moiety is evidenced by the frontier
highest occupied molecular orbital (HOMO) and lowest unoc-
cupied molecular orbital (LUMO) molecular orbitals. Figure 3a
shows the HOMO and LUMO for the ZnPp-C60 dyad. The
majority of the HOMO is located on the porphyrin ring while
small amounts of the HOMO are also found on the C60 carbons
located within the interacting distances, suggesting through space
interactions. Similarly, the majority of the LUMO is located
on the C60 spheroid, with small amounts located on the porphyrin
ring atoms. The delocalization of HOMO and LUMO over the
zinc porphyrin and C60 entities suggests though space charge-
transfer interactions both in the ground state and in the excited
state of the zinc porphyrin-C60 dyads. Similar charge-transfer
interactions for the ZnPo-C60 are observed for only the HOMO
but not for the LUMO. However, for the H2Po-C60, because
of the larger separation between the porphyrin and the C60
entities, such effects are not observed; that is, the HOMO is
found entirely located on the porphyrin ring while the LUMO
is on the C60 spheroid (see Supporting Information). The gas
phase HOMO-LUMO energy gap is found to be 1.66, 1.59,
and 1.81 eV for the ZnPo-C60, ZnPp-C60, and H2Pp-C60
dyads, respectively.
Cyclic Voltammetry Studies. Determination of the redox
potentials of the newly formed molecular donor-acceptor type
systems is important to probe the existence of charge-transfer
interactions between the donor and the acceptor in the ground
state and also to evaluate the energetics of electron-transfer
reactions. With this in mind, we have performed a systematic
study to evaluate the redox behavior of the dyads using the
cyclic voltammetric technique. The cyclic voltammograms of

12396 J. Phys. Chem. A, Vol. 106, No. 51, 2002
D’Souza et al.
TABLE 1: Formation Constant for Axial Coordination of
Pyridine Ligands to Zinc-Porphyrin C60 Dyads in DCB at
298 K
a
compd
axial ligandb
K (M-1
)
ZnPo-C60
4-acetyl pyridine
pyridine
4-dimethylamino pyridine
4-acetyl pyridine
pyridine
4-dimethylamino pyridine
4-acetyl pyridine
pyridinec
1.46 × 104
1.71 × 104
13.4 × 104
1.18 × 104
1.56 × 104
11.8 × 104
1.06 × 104
0.77 × 104
5.20 × 104
ZnPp-C60
(TPP)Zn
4-dimethylamino pyridine
a Error in K values ) (10%. b Because of the limited solubility of
4-hydroxypyridine in DCB, the K values for this ligand binding to ZnP-
C60 could not be obtained. c From ref 11g.
Figure 1. (a) Normalized absorption spectra in the Soret region of
(TPP)Zn (dotted), ZnPo-C60 (solid), and ZnPp-C60 (dashed) in DCB.
(b) Normalized absorption spectra in the Q-band region of equimolar
(TPP)Zn and 2-propyl fulleropyrrolidine (solid), ZnPo-C60 (dashed),
and ZnPp-C60 (dotted) in DCB.
ZnPo-C60 and H2Po-C60 as representative examples are shown
in Figure 4, and the data for all of the derivatives are
summarized in Table 2.
Figure 2. Ab initio B3LYP/3-21G(*)-calculated geometric structures
of (a) ZnPo-C60 and (b) ZnPp-C60.
Both free-base porphyrin and zinc porphyrin dyads exhibited
a total of seven reversible redox processes within the accessible
potential window of o-dichlorobenzene (DCB) containing 0.1
M (TBA)ClO4 (Figure 4). The first and second redox potentials
corresponding to the oxidation of the zinc porphyrin in ZnPo-
more positive potential by ca. 0.2 V as compared with those of
ZnPo-C60. The reduction potentials of the appended C60 moiety
of H2Po-C60 are almost the same as those of ZnPo-C60, while
the corresponding potentials for the free-base porphyrin ring
reduction shift to less negative values. The overall electrochemi-
cal behavior of the para-substituted derivatives is found to be
similar to that of the ortho-substituted derivatives and revealed
slight changes in the redox potential values (Table 2). A
comparison of the redox potential values between the differently
substituted dyads and their starting monomeric species, por-
phyrins and fulleropyrrolidine, shows small changes, which are
C
60 are located at E1/2 ) 0.29 and 0.63 V vs Fc/Fc+, respectively.
The reduction potentials of the appended C60 moiety are located
at E1/2 ) -1.17, -1.55, and -2.07 V vs Fc/Fc+, while the
corresponding potentials for the zinc porphyrin ring reduction
are located at E1/2 ) -1.90 and -2.29 V vs Fc/Fc+, respectively.
The first and second oxidation potentials of H2Po-C60 shift to

Covalently Linked Porphyrin-C60 Dyads
J. Phys. Chem. A, Vol. 106, No. 51, 2002 12397
Figure 3. Frontier HOMO and LUMO of (a) ZnPp-C60 and (b) Py:ZnPp-C60 calculated by ab initio B3LYP/3-21G(*) methods.
in agreement with the existence of charge-transfer interactions
as shown by the spectral and computational studies. The
electrochemically measured HOMO-LUMO gap, that is, the
difference between the first oxidation potential of porphyrin and
fulleropyrolidine (Table 2), follows a trend similar to that
obtained by B3LYP/3-21G(*) methods in the gas phase (vide
supra). These results indicate a smaller HOMO-LUMO gap
for the zinc porphyrin-C60 dyads as compared to the free-base
porphyrin-C60 dyads.
It is of interest to probe the sequence of the site of electron
transfer in the investigated dyads as shown for ZnP-C60 in
Scheme 3 by ab initio B3LYP/3-21G(*) methods. In this regard,
our recent studies on ferrocene-C60-nitroaromatic entities
bearing molecular triads have demonstrated excellent tracking
of the sequence of the site of electron transfer involving the
three redox active entities by the computed HOMO and LUMO
orbitals.17b,c Figure 5 illustrates the computed HOMO and
LUMO orbitals for the ZnPp-C60 triad. Similar results were
obtained for the ZnPo-C60 and the H2P-C60 dyads. In agree-
ment with the electrochemical results (Scheme 3), a majority
of the HOMO and HOMO-1 orbitals are located on the zinc
porphyrin entity while the majority of the LUMO and LUMO+1
are located on the C60 entities. The subsequent LUMO orbitals
follow the order C60, porphyrin-C60, C60, porphyrin-C60, and
this compares with the electrochemical sequence of redox
reactions: porphyrin, C60, porphyrin, and C60.
The pyridines utilized to form the penta-coordinated species
revealed an irreversible oxidation process located at Epa ) 0.25
V vs Fc/Fc+, which is less positive than zinc porphyrin (E1/2
)
0.28 V), suggesting an increased electron donor ability of zinc
porphyrin when photoinduced charge separation takes place in
ZnP-C60. During the titrations involving ZnP-C60 and py-
ridines, the first and second oxidation peaks corresponding to
the zinc porphyrin revealed a cathodic shift of about 15-20
mV (on addition of 3-4 equiv of pyridine) as a result of axial
ligation. These results suggest that the zinc porphyrin in
Py:ZnP-C60 is slightly a better electron donor than that in
ZnP-C60. The HOMO and LUMO orbitals generated for
Py:ZnP-C60 revealed delocalization of HOMO orbitals over
the Py:ZnP entity (Figure 3b), similar to that reported earlier
for zinc porphyrin-(2-(pyridyl))fulleropyrrolidine dyad.11g Ad-
ditionally, the observed partial delocalization of HOMO on the
C60 entity and LUMO on the porphyrin entity of ZnPp-C60
(Figure 3a) is not seen in Py:ZnPp-C60 (Figure 3b). This may
be due to the increased distance between the ZnP and the C60
entities of the dyad upon axial ligation (∼0.5 Å) and the pulling
of the Zn from a 0.3 Å bias out of the porphyrin ring cavity
toward the C60 (in ZnPp-C60) to a 0.3 Å bias away from the

12398 J. Phys. Chem. A, Vol. 106, No. 51, 2002
D’Souza et al.
Fluorescence Emission Studies. The photochemical behavior
of the free-base porphyrin-C60 and zinc porphyrin-C60 dyads
is investigated first by using steady state fluorescence measure-
ments. Figure 6 shows fluorescence spectra of the zinc por-
phyrin-C60 and free-base porphyrin-C60 dyads along with ZnP
and H2P used as a reference compounds. The peak maxima
corresponding to porphyrin emission of all of the dyads revealed
a red shift of 1-2 nm as compared to their unsubstituted
derivatives. In the case of ZnPp-C60, the porphyrin emission
bands are quenched by ca. 73% while this quenching for the
ZnPo-C60 is found to be over 95% as compared with the
emission intensity of ZnP. Similar observations are made for
the free-base porphyrin dyads. The corresponding quenching
of emission of H2Pp-C60 and H2Po-C60 is found to be ca. 78
and 97%, respectively, as compared to H2P. These observations
suggest efficient quenching of the singlet excited state of the
porphyrins by the appended fullerene entity, and such quenching
for the ortho-substituted derivatives is higher than the para-
substituted ones. It may also be mentioned here that changing
the solvent from DCB to a more polar benzonitrile (BN)
increased the overall quenching efficiency by another 2-5%.
Interestingly, on extending the wavelength scan to a higher
wavelength, a weak emission at ∼715 nm was observed in the
case of zinc porphyrin derivatives corresponding to the emission
of fulleropyrrolidine entity17b (Figure 6). However, the 715 nm
emission was usually hidden by the tail of the huge emission
from zinc porphyrin itself. Hence, it was difficult to confirm
the energy transfer from the singlet excited porphyrin to the
fullerene entity in these dyads.
Figure 4. Cyclic voltammograms of (a) ZnPo-C60 and (b) H2Po-
C60 in DCB, 0.1 M (TBA)ClO4. Scan rate ) 100 mV/s.
Addition of Py (up to 10 equiv) to a solution of ZnP-C60
resulted in further red shift (∼10 nm) and a decrease of the
intensity of the emission bands (20-30%) suggesting the
formation of a penta-coordinated complex and enhanced fluo-
rescence quenching of zinc porphyrin. To further understand
the intramolecular quenching mechanism and follow the kinetics
of photoinduced processes occurring in these dyads and triads,
picosecond time-resolved emission, as well as subpicosecond
and nanosecond transient absorption studies, have been per-
formed.
SCHEME 3
C60 toward the pyridine as a result of Zn-N axial bond
formation. The delocalization of the frontier HOMO over the
axially bound pyridine ligand would dissipate the positive charge
of ZnP+• formed during initial charge separation and slow the
charge recombination process. Additionally, the slightly in-
creased distance between the ZnP and the C60 entities of Py:
ZnPp-C60 as compared to that in ZnPp-C60 might also
contribute toward slowing down the charge recombination
process.
Fluorescence Lifetimes. The time-resolved fluorescence
spectral features of the dyads track that of the steady state
measurements. The fluorescence decay-time profiles of the
investigated dyads of ZnP (monitored at 600 nm) and H2P
(monitored at 650 nm) show an enhanced decay rate as
compared with those of (TPP)Zn and (TPP)H2 in both DCB
and in BN (Figure 7). In DCB, all of the investigated compounds
TABLE 2: Electrochemical Half-Wave Redox Potentials (vs Fc/Fc+) of the Porphyrin-Fulleropyrrolidine Dyads in DCB, 0.1 M
(TBA)ClO4
potential (V vs Fc/Fc+)
0/-
-/2-
2-/3-
compd
ZnP
ZnPo-C60
ZnPp-C60
Py:ZnPo-C60c
Py:ZnPp-C60c
H2P
P+/2+
P0/+
C60
C60
P0/-
C60
P-/2-
HOMO-LUMO, Va
∆Gcso, Vb
0.62
0.63
0.61
0.61
0.60
0.79
0.81
0.80
0.28
0.29
0.27
0.26
0.26
0.53
0.50
0.51
-1.92
-1.90
-1.91
-1.91
-1.92
-1.75
-1.73
-1.74
-2.23
-2.29
-2.25
-2.29
-2.26
-2.05
-1.17
-1.18
-1.18
-1.18
-1.55
-1.55
-1.56
-1.56
-2.07
-2.06
-2.07
-2.07
1.46
1.45
1.44
1.44
-0.84
-0.86
-0.83
-0.84
H2Po-C60
H2Pp-C60
-1.18
-1.19
-1.56
-1.55
-2.10d
-2.08d
1.68
1.69
-0.44
-0.44
0/-
a HOMO-LUMO ) P0/+ - C60
.
b ∆Gcso ) E1/2(D+•/D) - E1/2(A/A•-) - ∆Eo-o + ∆Gs where E1/2(D•+/D) is the first one electron oxidation
potential of the donor porphyrin, E1/2(A/A•-) is the first one electron reduction potential of the C60 electron acceptor, ∆Eo-o is the energy of the 0-0
transition energy gap between the lowest excited state and the ground state, and ∆Gs refers to the salvation energy, calculated by using the “dielectric
continuum model” according to the following equation: ∆Gs ) e2/4πꢀo[(1/2R+ + 1/2R - 1/RD-A)(1/ꢀs) - (1/2R+ + 1/2R-)(1/ꢀR)]. From Figure 2,
values of R+ ) 4.8 Å, R- ) 4.2 Å, RD-A ) 6.7 Å for ortho and RD-A ) 6.4 Å for para derivatives were employed. c Obtained by addition of 3 equiv
of pyridine to the ZnP-C60 dyads. d Overlap of C60
and P-/2- processes.
2-/3-

Covalently Linked Porphyrin-C60 Dyads
J. Phys. Chem. A, Vol. 106, No. 51, 2002 12399
Figure 5. Ab initio B3LYP/3-21G(*)-calculated frontier (a) HOMO-1, (b) HOMO, (c) LUMO, (d) LUMO+1, (e) LUMO+2, (f) LUMO+3, (g)
LUMO+4, and (h) LUMO+5 orbitals of ZnPp-C60.
TABLE 3: Fluorescence Lifetimes (τf)a, Charge Separation Rate Constants (kcssinglet), Charge Separation Quantum Yields
(Φcssinglet), and Charge Recombination Rate Constants (kcr) for Porphyrin-C60 Dyads in DCB and BN Solvents
b
singlet b
compd
solvent
τf (ns) (fraction %)
kcssinglet (s-1
)
Φcs
kcr (s-1
)
kcs/kcr
ZnPo-C60
DCB
BN
0.33 (100)
0.26 (100)
0.29 (100)
0.23 (100)
0.70 (100)
0.64 (100)
0.61 (100)
0.54 (100)
0.87 (100)
0.44 (60)e
3.33 (100)
2.70 (100)
2.6 × 109
3.4 × 109
3.0 × 109
3.9 × 109
9.5 × 108
1.1 × 109
1.1 × 109
1.3 × 109
1.1 × 109
2.2 × 109
2.3 × 108
3.0 × 108
0.84
0.88
0.86
0.89
0.68
0.69
0.71
0.74
0.94
0.97
0.75
0.81
5.9 × 107
1.8 × 106
3.4 × 106
1.7 × 106
>6.0 × 107
3.0 × 106
5.5 × 106
2.5 × 106
4.3 × 107
4.4 × 107
1.7 × 107
2.3 × 107
44
1888
882
2294
15
366
212
660
26
Py:ZnPo-C60c
ZnPpo-C60
Py:ZnPp-C60c
H2P-C60
DCB
Pyd
DCB
BN
DCB
Pyd
DCB
BN
DCB
BN
50
14
13
H2Pp-C60
a The τf for reference compounds: (TPP)Zn ) 2.1 ns and (TPP)H2 )13.6 ns in DCB and BN. kcssinglet ) (1/τf )sample - (1/τf )ref, *Φcssinglet ) [(1/τf
- (1/τf )ref ]/(1/τf )sample. For biexponential fitting, τf from the initial decay component was employed. c Obtained by the addition of 0.1 mM
b
)
sample
pyridine. d In neat pyridine. e 0.97 ns (40%).
revealed a monoexponential decay, while in BN the decay of
H2Po-C60 could be fitted satisfactory to a biexponential decay.
Substantial quenching of the fluorescence lifetimes is observed
for the investigated dyads; the ortho derivatives clearly show a
higher efficiency of quenching than those of para derivatives.
By assuming that the quenching is due to electron transfer
from the singlet excited porphyrin to the fullerene, the rates of
charge separation (kcssinglet) and quantum yields (Φcssinglet) are
evaluated in an usual manner employed for the intra-
molecular electron-transfer process and the data are given in
Table 3. An examination of Table 3 reveals the following: (i)
The experimentally determined kcssinglet and Φcssinglet are higher
for ortho-substituted derivatives as compared to the para-
substituted derivatives. This observation is consistent with the
close proximity of the porphyrin and fullerene entities in the
ortho-substituted derivatives obtained from the fluorescence
singlet
spectral and computational methods. (ii) The kcs
is higher
for ZnP-C60 as compared with H2P-C60, which is in agreement
with the electrochemical redox potentials and the calculated free-
energy change, ∆Gcso, for photoinduced electron-transfer
reactions. This considers the reorganization energy (λ) to be
close to the ∆Gcso values of ZnP-C60 and the λ values between
the ∆Gcso values of ZnP-C60 and H2Po-C60. (iii) In agreement
singlet
singlet
with the steady state emission results, the kcs
and Φcs
are higher in more polar BN solvent as compared to the values
obtained in less polar DCB solvent.
Interestingly, upon addition of Py (10 equiv) to a solution of
either ZnPo-C60 or ZnPp-C60 in DCB to form the penta-

12400 J. Phys. Chem. A, Vol. 106, No. 51, 2002
D’Souza et al.
Figure 7. Fluorescence decay profiles of (a) (TPP)Zn, ZnPp-C60, and
ZnPo-C60 and (b) (TPP)H2, H2Pp-C60, and H2Po-C60 in BN; λex
)
410 nm. The concentrations of porphyrins were maintained at 0.05 mM.
Figure 8. Upper panel: picosecond transient absorption spectra
obtained by the 455 nm laser light excitation of H2Po-C60 (0.1 mM)
in BN. Lower panel: time profile at 455 nm.
Subpicosecond Time-Resolved Transient Absorption Spec-
tral Studies. To confirm the occurrence of a charge separation
process, the transient absorption spectra were monitored during
the first 1000 ps after pulsing the samples by a 150 fs laser
light. Figure 8 illustrates the spectral profiles obtained for
ZnPo-C60 in BN as a representative example. Immediately after
the laser pulse, an intense absorption band appeared at 455 nm
(spectrum at 0 ps; immediately after 150 fs laser pulse), which
is attributed to the S1 f Sn transition of the ZnP moiety. The
decay rate of this S1 band shown in the lower panel is about 5
× 109 s-1, which is slightly higher than that evaluated from
the fluorescence lifetime measurements (3.4 × 109 s-1) but are
within the estimated experimental error. As shown in the figure
at the 300 ps regime, the decay of the S1 band of the ZnP moiety
is accompanied by an appearance of an absorption band at 600
nm corresponding to the formation of ZnP+•. In the longer
wavelength region, a new absorption band at 1000 nm is also
observed in this time interval corresponding to the formation
of radical anion of the C60 moiety. In DCB, weaker absorption
was observed at the 600 and 1000 nm regions. On addition of
pyridines to form the penta-coordinated complex in DCB,
Figure 6. Fluorescence spectrum of (a) (i) (TPP)Zn, (ii) ZnPp-C60,
and (iii) ZnPo-C60 in DCB (λex ) 550 nm) and (b) (i) (TPP)H2, (ii)
H2Pp-C60, and (iii) H2Po-C60 in DCB (λex ) 515 nm).
coordinated species, Py:ZnPo-C60 or Py:ZnPo-C60, the fluo-
rescence decay of ZnP revealed additional quenching while
maintaining the monoexponential decay. Accordingly, the
singlet
singlet
calculated kcs
and Φcs
are found to be slightly higher
for the pyridine-coordinated dyads as compared to the corre-
sponding dyads in the absence of any axial ligand (Table 3).
These higher values of kcssinglet and Φcssinglet have been attributed
to the easier oxidation of the penta-coordinated zinc porphyrin
in the dyads (vide supra). Further increases in fluorescence decay
rates were observed in neat pyridine, suggesting that all of the
ZnPo-C60 or ZnPp-C60 are coordinated by excess pyridine,
singlet
resulting in further increases in kcs
and Φcssinglet. Further
studies involving subpicosecond and nanosecond transient
absorption technique have been performed to monitor the charge
separation and charge recombination processes in the dyads and
self-assembled triads. The forthcoming sections discuss the
results of these studies.

Covalently Linked Porphyrin-C60 Dyads
J. Phys. Chem. A, Vol. 106, No. 51, 2002 12401
Figure 9. Nanosecond transient absorption spectra obtained using 532
nm laser excitation of H2Po-C60 (0.1 mM) in Ar-saturated DCB.
Inset: time profiles for the 700 and 1000 nm bands.
Figure 10. Nanosecond transient absorption spectra obtained using
532 nm laser excitation of ZnPp-C60 (0.1 mM) in Ar-saturated neat
pyridine. Inset: time profile for the 1000 nm band.
spectral features similar to that shown in Figure 8 were observed.
For the H2P-C60 dyads both in DCB and in BN, similar transient
spectra were observed in the picosecond region thus confirming
the occurrence of charge separation from the singlet excited
porphyrin to the covalently linked fullerene moiety.
Py:ZnPp-C60, the hole (cation radical of ZnP moiety) generated
by the initial charge separation is likely to delocalize to over
the pyridine entity, thus slowing down the charge recombination
process due to an increased distance between the C60 radical
anion and the radical cation of Py:ZnP. The spectrum at 100 ns
indicates that all of the transient species observed immediately
after the nanosecond laser light pulse decayed simultaneously
within 1.0 µs. As expected, charge stabilization in neat pyridine
is found to be much more pronounced as revealed by the higher
kcs/kcr values as compared with addition of pyridine in DCB
(Table 3). Utilization of substituted pyridines as axial ligands
revealed similar observations. Smaller kcr values were obtained
when 4-hydroxypyridine or 4-acetylpyridine was used as an axial
ligand (1.1 × 106 s-1 or 1.4 × 106 s-1 for Py:ZnPp-C60 in
DCB, respectively) while for 4-dimethylamino pyridine with
an electron-donating group the kcr values (2.2 × 106 s-1 for
Py:ZnPp-C60 in DCB) were found to be almost the same as
that obtained for pyridine in DCB.
Nanosecond Time-Resolved Absorption Spectra of the
Dyads in BN. In BN, the transient absorption spectrum of
ZnPp-C60 (see Supporting Information for figure) showed a
clear absorption band of the radical anion of the C60 moiety at
1000 nm with a longer lifetime of the ion pair state (330 ns),
which is 20 times longer than that in DCB. Similarly, the
lifetime of the ion pair state of ZnPo-C60 in BN is found to be
550 ns, which is 33 times longer than that in DCB. The transient
spectrum recorded at 100 ns for the ZnP-C60 dyads revealed
the existence of a weak absorption at 620 nm, which has been
ascribed to the radical cation of the ZnP moiety. It is also
noticeable that the triplet state absorption of the C60 moiety at
700 nm decays more quickly in BN than that in DCB; the decay
rates of the triplet states are also increased and are comparable
to the decay rates of the ion pairs. This could be due to a mixing
of the radical ion pair states with the triplet states, which would
mean the existence of the radical ion pair character of the triplet
state.20 From the similarity of the kcr values in BN to those in
pyridine, the observed longer lifetime of the radical ion pairs
in BN could be ascribed to the coordination of BN to the ZnP
moiety.
Nanosecond Time-Resolved Absorption Spectra of the
Dyads in DCB. The nanosecond transient absorption spectrum
of pristine ZnP exhibited absorption peaks at 630 and 840 nm
corresponding to the triplet state.18,19 Similarly, pristine H2P
exhibited transient absorption bands at 620 and 800 nm
corresponding to its triplet state.18,19 The transient absorption
spectrum of 2-(4′-pyridyl)fulleropyrrolidine revealed a band
around 700 nm corresponding to its triplet state.18 The anion
radical of 2-(4′-pyridyl)fulleropyrrolidine is expected to reveal
a band around 1000 nm, while the cation radicals of H2P and
ZnP are expected to appear in the 620-630 nm range.
The nanosecond transient absorption spectrum of H2Po-C60
in DCB showed a fast decaying peak of the radical anion of
the C60 moiety around 1000 nm in addition to a slow decaying
band at 700 nm corresponding to the triplet C60 and 620 and
800 nm bands corresponding to the triplet state of the H2P
(Figure 9).19 A part of the transient absorption band at 626 nm
also showed quick decay, and this has been attributed to the
formation of H2P+•. The decay of the 1000 nm transient band
corresponds to the rate of the charge recombination (kcr), and
the values of kcr thus calculated are given in the last column in
Table 3.
The nanosecond transient absorption spectrum of ZnPo-C60
in DCB is quite similar to Figure 9 for H2Po-C60, showing a
fast decaying peak of the radical anion of the C60 around 1000
nm (kcr ) 5.8 × 107 s-1) in addition to the slow decaying 700
nm band of the triplet C60 and 850 nm band corresponding to
the triplet state of the zinc porphyrin. In the case of ZnPp-C60,
the transient absorption spectra also showed the 1000 nm band
(kcr g 6 × 107 s-1) in addition to the triplet states of C60 and
ZnP. A comparison between the kcr values in DCB suggests
lower rates for H2P-C60 dyads as compared to the ZnP-C60
dyads, indicating longer lifetimes of the charge-separated state
of H2P-C60 dyads. This suggests that the ∆Gcro values for both
H2P-C60 and ZnP-C60 dyads are in the inverted region and
that ∆Gcro values of ZnP-C60 dyads are near the top region of
the Marcus parabola.
Energy Level Diagrams. The results of the present inves-
tigation are summarized with the help of an energy level diagram
shown in Figure 11. By utilizing the oxidation and reduction
potentials and the calculated distance between the donor and
the acceptor, the free energies (∆G°) for light-induced electron
transfer were estimated for constructing the energy level
diagram. On the basis of these ∆G° values and the reported
energies for the singlet and triplet states, the energy levels in
Nanosecond Time-Resolved Absorption Spectra of the
Py:ZnP-C60 in Py and DCB. Figure 10 shows the nanosecond
transient absorption spectra of Py:ZnPp-C60 in neat pyridine.
For the penta-coordinated complex, the lifetime of the ion pair
states increased considerably. From the oxidation potential of

12402 J. Phys. Chem. A, Vol. 106, No. 51, 2002
D’Souza et al.
Efficient charge separation and the slower charge recombina-
tion processes have been observed upon axial coordination of
pyridine to the ZnP-C60 dyads. The higher rates of charge
separation can be attributed to the easier oxidation of the penta-
coordinated ZnP, while the slower charge recombination can
be attributed to the longer distance between radical anion and
radical cation caused by the coordinated pyridine ligand;
delocalization of the π-cation radical of ZnP to the coordinated
pyridine ligand and the longer space between them stabilizes
the radical ion pair state. More importantly, an evaluation of
the kcs/kcr ratio, a measure of the excellence in the photoinduced
electron-transfer systems, is more than 1000 for ZnPo-C60 in
neat pyridine or in BN.
Experimental Section
Chemicals. Buckminsterfullerene, C60 (+99.95%), was from
SES Research, (Houston, TX). DCB and BN in sure seal bottles,
sarcosine, pyrrole, benzaldehyde derivatives (benzaldehyde,
4-bromoethoxybenzaldehyde, and o- and p-hydroxybenzalde-
hyde), and pyridine derivatives (pyridine, 4-hydroxypyridine,
4-dimethylaminopyridine, and 4-acetylpyridine) were from
Aldrich Chemicals (Milwaukee, WI). Tetra-n-butylammonium
perchlorate, (TBA)ClO4, was from Fluka Chemicals. All
chemicals were used as received. Syntheses and purification of
(TPP)H2 and (TPP)Zn were carried out according to the
literature procedure.14
Synthesis of 5-(2′ or 4′-Hydroxyphenyl)-10,15,20-tri-
phenylporphyrin. The title compounds were synthesized by
reacting 2′- or 4′-hydroxybenzaldehyde (1.5 g, 12 mM),
benzaldehyde (3.9 g, 37 mM), and pyrrole (3.3 g, 49 mM) in
refluxing propionic acid.11f The crude product was purified on
a basic alumina column with chloroform/methanol (95:5 v/v)
1
as eluent. Yield ∼5%. H NMR in CDCl3: δ ortho-hydroxy:
8.89 (m, 8H, â-pyrrole-H), 8.27 (m, 6H, ortho-phenyl-H), 7.79
(m, 9H, meta- and para-phenyl-H), 8.08-7.21 (m, 4H, substi-
tuted phenyl-H), 5.35 (s, br, 1H, hydroxy-H), -2.81 (s, 2H,
imino-H). para-hydroxy: 8.89 (m, 8H, â-pyrrole-H), 8.27 (m,
6H, ortho-phenyl-H), 7.79 (m, 9H, meta- and para-phenyl-H),
8.08-7.21 (d,d, 4H, substituted phenyl-H), 5.35 (s (br), 1H,
hydroxy-H), -2.81 (s, 2H, imino-H).
Figure 11. Energy level diagrams showing the different photochemical
events of (a) H2P-C60 dyads and (b) ZnP-C60 dyads in DCB. The
Synthesis of H2Po-CHO and H2Pp-CHO. These were
synthesized by stirring a solution of the respective hydroxy-
derivatived porphyrins (0.1 mmol in 100 mL of DMF) and 4-(2-
bromoethoxy)benzaldehyde (0.5 mmol) in the presence of
K2CO3 for 12 h. At the end of the reaction, the DMF was
evaporated and the compound was extracted in CHCl3. Further
purification of the compounds was carried out on a silica gel
column using toluene and chloroform (95:5 v/v) as eluent. Yield
45%. 1H NMR in CDCl3: δ H2Po-CHO: 10.03 (s, 1H,
aldehyde-H), 8.83 (m, 8H, â-pyrrole-H), 8.16 (m, 6H, ortho-
phenyl-H), 7.74 (m, 9H, meta- and para-phenyl-H), 8.14-7.41
(m, 4H, substituted phenyl-H), 5.96 (d, 2H, phenyl-H), 5.53
(d, 2H, phenyl-H), 4.21, 3.51 (t,t, 2H,2H, CH2-CH2), -2.84
(s, 2H, imino-H). H2Pp-CHO: 9.90 (s, 1H, aldehyde-H), 8.84
(m, 8H, â-pyrrole-H), 8.21 (m, 6H, ortho-phenyl-H), 7.84 (m,
9H, meta- and para-phenyl-H), 7.76-7.12 (d,d, 4H, substituted
phenyl-H), 4.61 (m, 4H, phenyl-H), 4.39, 3.77 (t,t, 2H,2H, CH2-
CH2), -2.75 (s, 2H, imino-H).
energy of (Py:ZnP)•+•-C60 state in DCB (1.22 eV) and (ZnP)•+
-
-•
•-
C60 state in BN (1.12 eV) is shown as open rectangles.
Figure 11 showing the different photochemical processes of the
tetra- and penta-coordinated dyads reveal the following. The
main difference in the kcs between H2P-C60 and ZnP-C60 is
the energy differences between the singlet excited state of the
porphyrin moiety and the radical ion pair states; the reorganiza-
o
tion energy (λ) is rather near ∆Gcs values of ZnP-C60 when
o
the top region of the Marcus parabola exists between ∆Gcs
values of ZnP-C60 and H2Po-C60. The enhanced charge
separation in coordinating solvents, pyridine and BN, as
compared to that in less polar DCB for the dyads is noteworthy.
This trend is not reasonable for ZnP-C60 dyads belonging to
the Marcus inverted region. The prolonged charge recombination
processes observed for the radical ion pair of ZnP-C60 dyads
in BN and not for the H2P-C60 dyads may be attributed to the
coordination to Zn, which renders the relative position of the
radical ion pair center apart from the radical anion center, C60.
In addition, in the case of ZnP-C60 dyads, the radical ion pair
states are lower than the triplet states, while the radical ion pair
state of H2P-C60 is higher than the triplet states even in BN;
thus, the charge recombination yielding the triplet states is
efficient for H2P-C60.
Synthesis of H2Po-C60 and H2Pp-C60. These were syn-
thesized by refluxing a mixture of the respective H2P-CHO
derivatives (0.045 mmol), C60 (0.24 mmol), and sarcosine (0.24
mmol) in 100 mL of toluene for 10 h. At the end, the solvent
was evaporated and the compound was purified on a silica gel
column using toluene and hexane (95:5 v/v) as eluent. Yield

Covalently Linked Porphyrin-C60 Dyads
J. Phys. Chem. A, Vol. 106, No. 51, 2002 12403
1
56.4 mg (∼80%). H NMR in CDCl3: δ H2Po-C60: 8.76 (m,
(Tsunami) and a streak scope (Hamamatsu Photonics). The
details of the experimental setup are described elsewhere.20 The
subpicosecond transient absorption spectra were recorded by
the pump and probe method. The samples were excited with a
second harmonic generation (SHG, 388 nm) output from a
femtosecond Ti:sapphire regenerative amplifier seeded by SHG
of a Er-dropped fiber (Clark-MXRCPA-2001 plus, 1 kHz, fwhm
150 fs). The excitation light was depolarized. The monitor white
light was generated by focusing the fundamental of laser light
on flowing D2O/H2O cell. The transmitted monitor light was
detected with a dual MOS linear image sensor (Hamamatsu
Photonics, C6140) or InGaAs photodiode array (Hamamatsu
Photonics, C5890-128). Nanosecond transient absorption spectra
in the NIR region were measured by means of laser flash
photolysis; 532 nm light from a Nd:YAG laser was used as the
exciting source and a Ge-avalanche-photodiode module was
used for detecting the monitoring light from a pulsed Xe lamp
as described in our previous paper.20
8H, â-pyrrole-H), 8.16 (m, 6H, ortho-phenyl-H), 7.72 (m, 9H,
meta- and para-phenyl-H), 7.97-7.14 (d,d,t, 4H, substituted
phenyl-H), 5.94 (s, 4H, phenyl-H), 4.15, 3.48 (t, t, 2H,2H, CH2-
CH2), 4.63, 3.77 (d,d, 2H, pyrrolidine-H), 3.80 (s, 1H, pyrro-
lidine-H), 2.34 (s, 3H, pyrrolidine N-CH3), -2.77 (s.br, 2H,
imino-H). ESI mass in CH2Cl2: calcd, 1526; found, 1526.1.
UV-visible in CH2Cl2: λmax 308, 326 (sh), 418, 515, 549, 591,
648. H2Pp-C60: 8.83 (m, 8H, â-pyrrole-H), 8.19 (m, 6H, ortho-
phenyl-H), 7.75 (m, 9H, meta- and para-phenyl-H), 8.04-7.15
(d,d, 4H, substituted phenyl-H), 7.49, 7.06 (m, 4H, phenyl-H),
4.57, 4.49 (t,t, 2H,2H, CH2-CH2), 4.88, 4.14 (d,d, 2H,
pyrrolidine-H), 4.80 (s, 1H, pyrrolidine-H), 2.32 (s, 3H, pyr-
rolidine N-CH3), -2.81 (s.br, 2H, imino-H). ESI mass in CH2-
Cl2: calcd, 1526; found, 1527.1. UV-visible in CH2Cl2: λmax
306, 325 (sh), 418, 515, 551, 590, 647.
Synthesis of ZnPo-C60 and ZnPp-C60. These were syn-
thesized by metalation of the respective free-base porphyrin
derivatives (0.037 mmol) in CHCl3 by using excess of zinc
acetate in methanol. The course of the reaction was followed
spectroscopically. At the end (∼1 h), the solvent was evaporated
and the product was purified on silica gel column using toluene
as eluent. Yield 93%. 1H NMR in CDCl3: δ ZnPo-C60: 8.88
(m, 8H, â-pyrrole-H), 8.18 (m, 6H, ortho-phenyl-H), 7.71 (m,
9H, meta- and para-phenyl-H), 7.96-7.15 (m, 4H, substituted
phenyl-H), 5.91 (s, 4H, phenyl-H), 4.15, 3.51 (t,t, 2H,2H, CH2-
CH2), 4.62, 3.78 (d,d, 2H, pyrrolidine-H), 3.85 (s, 1H, pyrro-
lidine-H), 2.33 (s, 3H, pyrrolidine N-CH3). ESI mass in
CH2Cl2: calcd, 1589.4; found, 1608.5 (M + H2O). UV-visible
in CH2Cl2: λmax 309, 328 (sh), 421, 548, 584 (weak). ZnPp-
C60: 8.92 (m, 8H, â-pyrrole-H), 8.20 (m, 6H, ortho-phenyl),
7.67 (m, 9H, meta- and para-phenyl-H), 8.04-7.11 (d,d, 4H,
substituted phenyl-H), 7.29 (m, 4H, phenyl-H), 4.62, 4.53 (t,t,
2H,2H, CH2-CH2), 4.92, 4.05 (d,d, 2H, pyrrolidine-H), 4.85
(s,1H, pyrrolidine-H), 2.79 (s, 3H, pyrrolidine N-CH3). ESI
mass in CH2Cl2: calcd, 1589.4; found, 1608.7 (M + H2O).
UV-visible in CH2Cl2: λmax 308, 328 (sh), 421, 548, 585
(weak).
Acknowledgment. We are thankful to the donors of the
Petroleum Research Fund administered by the American
Chemical Society, National Institutes of Health (to F.D.), Japan
Ministry of Education, Science, Technology, Culture and Sports,
and Mitsubishi Foundation (to O.I.) for support of this work.
We are also thankful to the High Performance Computing Center
of the Wichita State University for lending SGI ORIGIN 2000
computer time.
Supporting Information Available: Ab initio B3LYP/3-
21G(*)-calculated frontier HOMO and LUMO of ZnPo-C60
and H2Po-C60. Nanosecond transient absorption spectrum of
ZnPp-C60 in BN. This material is available free of charge via
the Internet at http://pubs.acs.org.
References and Notes
(1) (a) Maruyama, K.; Osuka, A. Pure Appl. Chem. 1990, 62, 1511.
(b) Gust, D.; Moore, T. A. Science 1989, 244, 35. (c) Gust, D.; Moore, T.
A. Top. Curr. Chem. 1991, 159, 103. (d) Gust, D.; Moore, T. A.; Moore,
A. L. Acc. Chem. Res. 1993, 26, 198. (e) Wasielewski, M. R. Chem. ReV.
1992, 92, 435. (f) Paddon-Row, M. N. Acc. Chem. Res. 1994, 27, 18. (g)
Sutin, N. Acc. Chem. Res. 1983, 15, 275. (h) Bard, A. J.; Fox, M. A. Acc.
Chem. Res. 1995, 28, 141. (i) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163.
(j) Piotrowiak, P. Chem. Soc. ReV. 1999, 28, 143.
(2) (a) Ward, M. W. Chem. Soc. ReV. 1997, 26, 365. (b) Hayashi, T.;
Ogoshi, H. Chem. Soc. ReV. 1997, 26, 355. (c) Sessler, J. S.; Wang, B.;
Springs, S. L.; Brown, C. T. In ComprehensiVe Supramolecular Chemistry;
Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Vo¨gtle, F., Eds.;
Pergamon: Elmsford, NY, 1996; Chapter 9. (d) Feldheim, D. L.; Keating,
C. D. Chem. Soc. ReV. 1998, 27, 1. (e) Introduction of Molecular
Electronics; Petty, M. C., Bryce, M. R., Bloor, D., Eds.; Oxford University
Press: New York, 1995. (f) Emmelius, M.; Pawlowski, G.; Vollmann, H.
W. Angew. Chem., Int. Ed. Engl. 1989, 28, 1445.
(3) (a) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley,
R. E. Nature 1985, 318, 162. (b) Kratschmer, W.; Lamb, L. D.; Fostiro-
poulos, F.; Huffman, D. R. Nature 1990, 347, 345. (c) Fullerene and Related
Structures; Hirsch, A., Ed.; Springer: Berlin, 1999; Vol. 199.
(4) (a) Allemand, P. M.; Koch, A.; Wudl, F.; Rubin, Y.; Diederich,
F.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. J. Am. Chem. Soc. 1991,
113, 1050. (b) Xie, Q.; Perez-Cordero, E.; Echegoyen, L. J. Am. Chem.
Soc. 1992, 114, 3978.
Instrumentation. The UV-visible spectral measurements
were carried out with a Shimadzu model 1600 UV-visible
spectrophotometer. The fluorescence emission was monitored
by using a Spex Fluorolog-τ spectrometer. A right angle
detection method was used. The 1H NMR studies were carried
out on a Varian 400 MHz spectrometer. Tetramethylsilane
(TMS) was used as an internal standard. Cyclic voltammograms
were recorded on a EG&G model 263A potentiostat using a
three electrode system. A platinum button or glassy carbon
electrode was used as the working electrode. A platinum wire
served as the counter electrode, and a Ag wire (Ag/Ag+) was
used as the reference electrode. Ferrocene/ferrocenium redox
couple was used as an internal standard. All of the solutions
were purged prior to electrochemical and spectral measurements
using argon gas. The computational calculations were performed
by ab initio B3LYP/3-21G(*) methods with GAUSSIAN 9821
software package on various PCs and a SGI ORIGIN 2000
computer. The graphics of HOMO and LUMO coefficients were
generated with the help of GaussView software. The ESI-mass
spectral analyses of the newly synthesized compounds were
performed by using a Fennigan LCQ-Deca mass spectrometer.
For this, the compounds (about 1 mM concentration) were
prepared in CH2Cl2, freshly distilled over calcium hydride.
(5) Ajie, H.; Alvarez, M. M.; Anz, S. J.; Beck, R. E.; Diederich, F.;
Fostiropoulos, K.; Huffman, D. R.; Kratschmer, W.; Rubin, Y.; Schriver,
K. E.; Sensharma, E.; Whetten, R. L. J. Phys. Chem. 1990, 94, 8630.
(6) (a) Guldi, D. M. Chem. Commun. 2000, 321. (b) Guldi, D. M.
Chem. Soc. ReV. 2002, 31, 22.
(7) Imahori, H.; El-Khouly, M. E.; Fujitsuka, M.; Ito, O.; Sakata, Y.;
Fukuzumi, S. J. Phys. Chem. A 2001, 105, 325.
(8) (a) Khan, S. I.; Oliver, A. M.; Paddon-Row, M. N.; Rubin, Y. J.
Am. Chem. Soc. 1993, 115, 4919. (b) Saricciftci, N. S.; Wudl, F.; Heeger,
A. J.; Maggini, M.; Scorrano, G.; Prato, M.; Bourassa, J.; Ford, P. C. Chem.
Phys. Lett. 1995, 247, 510. (c) Liddell, P. A.; Sumia, J. P.; Macpherson, A.
N.; Noss, L.; Seely, G. R.; Clark, K. N.; Moore, A. L.; Moore, T. A.; Gust,
D. Photochem. Photobiol. 1994, 60, 537. (d) Williams, R. M.; Zwier, J.
Time-Resolved Emission and Transient Absorption Mea-
surements. The picosecond time-resolved fluorescence spectra
were measured using an argon-ion-pumped Ti:sapphire laser

12404 J. Phys. Chem. A, Vol. 106, No. 51, 2002
D’Souza et al.
M.; Verhoever, J. W. J. Am. Chem. Soc. 1995, 117, 4093. (e) Imahori, H.;
Hagiwara, K.; Aoki, M.; Akiyama, T.; Taniguchi, S.; Okada, T.; Shirakawa,
M.; Sakata, Y. J. Am. Chem. Soc. 1996, 118, 11771. (f) Kuciauskas, D.;
Lin, S.; Seely, G. R.; Moore, A. L.; Moore, T. A.; Gust, D.; Drovetskaya,
T.; Reed, C. A.; Boyd, P. D. W. J. Phys. Chem. 1996, 100, 15926. (g)
Guldi, D. M.; Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc.
1997, 119, 974. (h) Imahori, H.; Yamada, K.; Hasegawa, M.; Taniguchi,
S.; Okada, T.; Sakata, Y. Angew. Chem., Int. Ed. Engl. 1997, 36, 2626. (h)
Liddell, P. A.; Kuciauska, D.; Sumida, J. P.; Nash, B.; Nguyen, D.; Moore,
A. L.; Moore, T. A.; Gust, D. J. Am. Chem. Soc. 1997, 119, 1400.
(9) (a) Dietel, E.; Hirsh, A.; Eichhorn, E.; Rieker, A.; Hackbarth, S.;
Roder, B. Chem. Commun. 1998, 1981. (b) Gareis, T.; Kothe, O.; Daub, J.
Eur. J. Org. Chem. 1998, 1549. (c) Da Ros, T.; Prato, M.; Guldi, D.; Alessio,
E.; Ruzzi, M.; Pasimeni, L.; Carano, M.; Paolucci, F.; Ceroni, P.; Roffia,
S. In Recent AdVances in the Chemistry and Physics of Fullerenes and
Related Materials; Kadish, K. M., Ruoff, R. S., Eds.; The Electrochemical
Proceedings Series: Pennington, NJ, 1998; p 1074. (d) Martin, N.; Sanchez,
L.; Illescas, B.; Perez, I. Chem. ReV. 1998, 98, 2527. (e) Gareis, T.; Kothe,
O.; Daub, J. Eur. J. Org. Chem. 1998, 1549.
(10) (a) Imahori, H.; Tamaki, K.; Guldi, D. M.; Luo, C.; Fujitsuka, M.;
Ito, O.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 2607. (b)
Armaroli, N.; Marconi, G.; Echegoyen, L.; Bourgeois, J.-P.; Diederich, F.
Chem. Eur. J. 2000, 6, 1629. (c) Guldi, D. M.; Luo, C.; Da Ros, T.; Prato,
M.; Dietel, E.; Hirsch, A. Chem. Commun. 2000, 375. (d) Luo, C.; Guldi,
D. M.; Imahori, H.; Tamaki, K.; Sakata, Y. J. Am. Chem. Soc. 2000, 122,
6535. (e) Kuciauskas, D.; Liddell, P. A.; Lin, S.; Stone, S. G.; Moore, A.
L.; Moore, T. A.; Gust, D. J. Phys. Chem. B 2000, 104, 4307. (f) Schuster,
D. I.; Cheng, P.; Wilson, S. R.; Prokhorenko, V.; Katterie, M.; Holzwarth,
A. R.; Braslavsky, S. E.; Klihm, G.; Williams, R. M.; Luo, C. J. Am. Chem.
Soc. 2000, 121, 11599. (g) Tashiro, K.; Aida, T.; Zheng, J.-Y.; Kinbara,
K.; Saigo, K.; Sakamoto, S.; Yamaguchi, K. J. Am. Chem. Soc. 1999, 121,
9477. (h) Tkachenko, N. V.; Rantala, L.; Tauber, A. Y.; Helaja, J.; Hynninen,
P. H.; Lemmetyinen, H. J. Am. Chem. Soc. 1999, 121, 9378. (i) Imahori,
H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.;
Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 100. (j) Imahori, H.; Tkachenko,
N. V.; Vehmanen, V.; Tamaki, K.; Lemetyinen, H.; Sakata, T.; Fukuzumi,
S. J. Phys. Chem. A 2001, 105, 1750. (k) Fukuzumi, S.; Imahori, H.;
Yamada, H.; El-Khouly, M. E.; Fujitsuka, M.; Ito, O.; Guldi, D. M. J. Am.
Chem. Soc. 2001, 123, 2571. (l) Imahori, H.; Tamaki, K.; Guldi, D. M.;
Luo, C.; Fujitsika, M.; Ito, O.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc.
2001, 123, 2607. (m) Imahori, H.; Guldi, D. M.; Tamaki, K.; Yoshida, Y.;
Luo, C.; Sakata, Y.; Fukuzuzmi, S. J. Am. Chem. Soc. 2001, 123, 6617.
(11) (a) D’Souza, F.; Deviprasad, G. R.; Rahman, M. S.; Choi, J.-P.
Inorg. Chem. 1999, 38, 2157. (b) Armaroli, N.; Diederich, F.; Echegoyen,
L.; Habicher, T.; Flamigni, L.; Marconi, G.; Nierengarten, J.-F. New. J.
Chem. 1999, 77. (c) Da Ros, T.; Prato, M.; Guldi, D. M.; Alessio, E.; Ruzzi,
M.; Pasimeni, L. Chem. Commun. 1999, 635. (d) D’Souza, F.; Rath, N. P.;
Deviprasad, G. R.; Zandler, M. E. Chem. Commun. 2001, 267. (e) Da Ros,
T.; Prato, M.; Guldi, D. M.; Ruzzi, M.; Pasimeni, L. Chem. Eur. J. 2001,
7, 816. (f) D’Souza, F.; Deviprasad, G. R.; El-Khouly, M. E.; Fujitsuka,
M.; Ito, O. J. Am. Chem. Soc. 2001, 123, 5277. (g) D’Souza, F.; Deviprasad,
G. R.; Zandler, M. E.; Hoang, V. T.; Arkady, K.; VanStipdonk, M.; Perera,
A.; El-Khouly, M. E.; Fujitsuka, M.; Ito, O. J. Phys. Chem. A 2002, 106,
3243. (h) D’Souza, F.; Deviprasad, G. R.; Zandler, M. E.; El-Khouly, M.
E.; Fujitsuka, M.; Ito, O. J. Phys. Chem. B 2002, 106, 4952.
(12) Deisenhofer, J.; Michel, H. In Photochemical Energy ConVersion;
Norris, J. R., Jr., Meisel, D., Eds.; Elsevier: New York, 1989.
(13) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993, 115,
9798.
(14) Smith, K. M. Porphyrins and Metalloporphyrins; Elsevier: New
York, 1977.
(15) (a) D’Souza, F.; Hsieh, Y.-Y.; Deviprasad, G. R. Inorg. Chem. 1996,
35, 5747. (b) Nappa, M.; Valentine, J. S. J. Am. Chem. Soc. 1978, 100,
5075.
(16) Scatchard, G. Ann. N. Y. Acad. Sci. 1949, 51, 661.
(17) D’Souza, F.; Zandler, M. E.; Deviprasad, G. R.; Kutner, W. J. Phys.
Chem. A 2000, 104, 6887. (b) D’Souza, F.; Zandler, M. E.; Smith, P. M.;
Deviprasad, G. R.; Arkady, K.; Fujitsuka M.; Ito, O. J. Phys. Chem. A
2002, 106, 649. (c) Zandler, M. E.; Smith, P. M.; Fujitsuka, M.; Ito, O.;
D’Souza, F. J. Org. Chem. 2002, in press.
(18) Nojiri, T.; Watanabe, A.; Ito, O. J. Phys. Chem. A 1998, 102, 5215.
(19) (a) Ghosh, H. N.; Pal, H.; Sapre, A. V.; Mittal J. P. J. Am. Chem.
Soc. 1993, 115, 11722. (b) Fujitsuka, M.; Ito, O.; Yamashiro, T.; Aso, Y.;
Otsubo, T. J. Phys. Chem. A 2000, 104, 4876.
(20) (a) Matsumoto, K.; Fujitsuka, M.; Sato, T.; Onodera, S.; Ito, O. J.
Phys. Chem. B 2000, 104, 11632. (b) Komamine, S.; Fujitsuka, M.; Ito,
O.; Morikawa, K.; Miyata, T.; Ohno, T. J. Phys. Chem. A 2000, 104, 11497.
(c) Yamazaki, M.; Araki, Y.; Fujitsuka, M.; Ito, O. J. Phys. Chem. A 2001,
105, 8615.
(21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.7; Gaussian,
Inc.: Pittsburgh, PA, 1998.