Probing the donor-acceptor proximity on the physicochemical properties of porphyrin-fullerene dyads: "Tail-on" and "tail-off" binding approach

Article abstract of DOI:10.1021/ja010356q

A new approach of probing proximity effects in porphyrin-fullerene dyads by using an axial ligand coordination controlled "tail-on" and "tail-off" binding mechanism is reported. In the newly synthesized porphyrin-fullerene dyads for this purpose, the donor-acceptor proximity is controlled either by temperature variation or by an axial ligand replacement method. In o-dichlorobenzene, 0.1 M (TBA)ClO₄, the synthesized zincporphyrin-fullerene dyads exhibit seven one-electron reversible redox reactions within the accessible potential window of the solvent and the measured electrochemical redox potentials and UV-visible absorption spectra reveal little or no ground-state interactions between the C₆₀ spheroid and porphyrin π-system. The proximity effects on the photoinduced charge separation and charge recombination are probed by both steady-state and time-resolved fluorescence techniques. It is observed that in the "tail-off" form charge-separation efficiency changes to some extent in comparison with the results obtained for the "tail-on" form, suggesting the presence of some through-space interactions between the singlet excited zinc porphyrin and the C₆₀ moiety in-the "tail-off" form. The charge separation rates and efficiencies are evaluated from the fluorescence lifetime studies. The charge separation via the singlet excited states of zinc-porphyrin in the studied dyads is also confirmed by the quick rise-decay of the anion radical of the C₆₀ moiety within 20 ns. Furthermore, a long-lived ion pair with lifetime of about 1000 ns is also observed in the investigated zinc porphyrin-C₆₀ dyads.

Full text of DOI:10.1021/ja010356q

J. Am. Chem. Soc. 2001, 123, 5277-5284
5277
Probing the Donor-Acceptor Proximity on the Physicochemical
Properties of Porphyrin-Fullerene Dyads: “Tail-On” and “Tail-Off”
Binding Approach
Francis D’Souza,*,† Gollapalli R. Deviprasad,† Mohamed E. El-Khouly,‡
Mamoru Fujitsuka,‡ and Osamu Ito*,‡
Contribution from the Department of Chemistry, Wichita State UniVersity, 1845 Fairmount,
Wichita, Kansas 67260-0051, and Institute for Chemical Reaction Science, Tohoku UniVersity,
Katahira, Sendai, 980-8577 Japan
ReceiVed February 8, 2001
Abstract: A new approach of probing proximity effects in porphyrin-fullerene dyads by using an axial ligand
coordination controlled “tail-on” and “tail-off” binding mechanism is reported. In the newly synthesized
porphyrin-fullerene dyads for this purpose, the donor-acceptor proximity is controlled either by temperature
variation or by an axial ligand replacement method. In o-dichlorobenzene, 0.1 M (TBA)ClO4, the synthesized
zincporphyrin-fullerene dyads exhibit seven one-electron reversible redox reactions within the accessible
potential window of the solvent and the measured electrochemical redox potentials and UV-visible absorption
spectra reveal little or no ground-state interactions between the C60 spheroid and porphyrin π-system. The
proximity effects on the photoinduced charge separation and charge recombination are probed by both steady-
state and time-resolved fluorescence techniques. It is observed that in the “tail-off” form the charge-separation
efficiency changes to some extent in comparison with the results obtained for the “tail-on” form, suggesting
the presence of some through-space interactions between the singlet excited zinc porphyrin and the C60 moiety
in the “tail-off” form. The charge separation rates and efficiencies are evaluated from the fluorescence lifetime
studies. The charge separation via the singlet excited states of zinc porphyrin in the studied dyads is also
confirmed by the quick rise-decay of the anion radical of the C60 moiety within 20 ns. Furthermore, a long-
lived ion pair with lifetime of about 1000 ns is also observed in the investigated zinc porphyrin-C60 dyads.
Introduction
more biomimetic nature, the noncovalently linked self-assembled
donor-acceptor systems are more appealing as model com-
pounds.
During the past decade, a significant amount research has
been directed toward understanding the dynamics of photoin-
duced electron transfer in both protein and molecular systems
due to their importance in photochemical conversion and storage
of energy. As such, the natural and the artificial photosynthetic
systems rely on spatially organized units with suitable photo-
chemical and electrochemical properties.1,2 A number of co-
valently linked donor-acceptor dyads, triads, tetrads, etc., have
been designed and studied for this purpose.2 More recently, an
increasing number of noncovalently linked, by means of metal-
ligand coordinate bonds, hydrogen bonds, etc., donor-acceptor
supramolecular assemblies have also been studied.3 Due to the
Among the different donor-acceptor dyads, covalently linked
porphyrin-quinone constitutes one of the most widely studied
model compounds.2 More recently, fullerenes4 C60 and C70 have
been employed as electron acceptors5-8 due to their three-
dimensional structure,9 reduction potentials comparable to
benzoquinone,10 absorption spectrum extending most of the
visible spectrum,11 and small reorganization energy in electron-
transfer reactions.12 In the majority of the covalently linked
donor-acceptor systems,5-8 the donor and acceptor entities are
connected by a single bond. One of the problems of such model
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† Wichita State University.
‡ Tohoku University.
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10.1021/ja010356q CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/11/2001

5278 J. Am. Chem. Soc., Vol. 123, No. 22, 2001
D’Souza et al.
compounds is the free rotation and flexibility of the single
covalent bond which would give more than one conformer in
solution. This eventually is expected to modulate the electronic
interactions between the donor and acceptor entities, thus
affecting the electron transfer rates. In porphyrin-quinone
dyads, this problem has already been addressed by studying a
few rigidly linked porphyrin-quinone dyads.13
pyridine aldehyde according to a general procedure developed
by Maggini et al.14 for fulleropyrrolidine synthesis. The R-amino
acid appended porphyrin was synthesized by first preparing
5-(3′-hydroxyphenyl)-10,15,20-triphenylporphyrin, 1a, followed
by converting it to 5-(3′-bromoethoxyphenyl)-10,15,20-triphen-
ylporphyrin, 1b.15 The 5-(3′-ethoxyphenyl-amino acetic acid)-
10,15,20-triphenylporphyrin 1c was obtained by treating 1b with
glycine methyl ester in methyl ethyl ketone containing excess
potassium carbonate followed by hydrolysis of the ester in
aqueous NaOH containing a THF solution. Free-base porphy-
rin-fullerene dyads 1d and 2d were metalated with zinc
acetate16 to obtain the desired zinc porphyrin-fullerene con-
jugates 1 and 2. Generally, good yields of the reaction products
were obtained and the synthesized porphyrin-fullerene conju-
gates were found to be soluble in many organic solvents.
Electrochemical Studies. Cyclic voltammetric studies have
been performed to evaluate the redox potentials and also to
visualize any ground-state interactions between the porphyrin
and fullerene entities in the studied porphyrin-fullerene con-
jugates. Figure 1 shows the cyclic voltammograms of 1 and 2
in 0.1 M (TBA)ClO4, o-dichlorobenzene. Within the accessible
potential window of the solvent, a total of seven reversible redox
processes have been observed. The first and second redox
potentials corresponding to the oxidation of the zinc porphyrin
of 1 and 2 are virtually identical and are located at E1/2 ) 0.26
and 0.61 V vs Fc/Fc+, respectively. The reduction potentials of
the appended C60 moiety of 1 are located at E1/2 ) -1.20,
-1.55, and -2.07 V vs Fc/Fc+ and are not significantly different
from that of 2. Unlike the similarity between the oxidation
potentials of the zinc porphyrin moieties of 1 and 2, the
corresponding potentials for the porphyrin ring reduction reveal
small changes. That is, the potentials for the zinc porphyrin ring
reduction of 1 are located at E1/2 ) -1.97 and -2.36 V vs
Fc/Fc+, respectively, and are 20-40 mV negatively shifted
compared to the corresponding porphyrin ring reduction po-
tentials of 2. However, these potentials are not significantly
different from that earlier reported for tetraphenylporphyrina-
tozinc, ZnTPP, and fulleropyrrolidines.7a,17 These results col-
lectively suggest weak or no ground-state interactions between
the zinc porphyrin and C60 entities.
In the present study, we have developed a novel approach to
position the donor, zinc porphyrin, and acceptor, C60, entities
in defined spatial organization to probe the proximity effects.
For this, covalently linked porphyrin-C60 dyads capable of axial
coordination of the functionalized C60 entity via a “tail-on” and
“tail-off” binding mechanism to the central zinc ion are
developed (Scheme 1). The dyads, 2-(3′- or 4′-pyridyl)ful-
leropyrrolidine, covalently linked to one of the phenyl rings of
a tetraphenylporphyrinatozinc macrocycle through the pyrroli-
dine ring nitrogen have newly been designed and synthesized.
The donor-acceptor proximity on the efficiency of photoin-
duced electron transfer from the singlet excited zinc porphyrin
to C60 has been probed by controlling the axial coordination of
the pyridyl group attached at the 2-position of the pyrrolidine
ring of fulleropyrrolidine to the metal center of zinc porphyrin
by two mechanisms, viz., (i) temperature-dependent axial
coordination equilibrium and (ii) an intermolecular axial ligation
controlled equilibrium using 3-picoline as shown in Scheme 1.
Results and Discussion
Synthesis of Porphyrin-Fullerene Conjugates. The syn-
thetic procedure developed to probe the proximity effects in
zinc porphyrin-C60 dyads is shown in Scheme 2. This involves
first synthesizing a R-amino acid appended porphyrin followed
by condensing it with fullerene in the presence of a desired
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Addition of 3-picoline to a solution of 1 or 2, that is, to
produce the “tail-off” form according to Scheme 1b while
keeping the coordination number of the central zinc the same,
revealed no significant changes in the redox potentials. This
suggests that replacing the coordinated pyridine of the fullero-
pyrrolidine unit by externally added 3-picoline does not change
the electronic structure of the porphyrin π-system to an
appreciable extent.
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Marconi, G.; Echegoyen, L.; Bourgeois, J.-P.; Diederich, F. Chem. Eur. J.
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Temperature-Dependent “Tail-On” and “Tail-Off” Bind-
ing. The optical absorption spectrum of the zinc porphyrin-
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147, 2647.

Donor-Acceptor Proximity of Porphyrin-Fullerene Dyads
J. Am. Chem. Soc., Vol. 123, No. 22, 2001 5279
Scheme 1
fullerene conjugates revealed absorption bands characteristic of
both zinc porphyrin and fullerene entities. At room temperature,
the Soret bands of 1 and 2 are located at around 433 nm, and
this compares with a 425 nm Soret band of ZnTPP and 434 nm
band of pentacoordinated ZnTPP-pyridine complex. These
results suggest the existence of a pentacoordinated zinc por-
phyrin due to the axial coordination of the pyridyl unit of the
fulleropyrrolidine entity, a result similar to that reported by us
earlier for a pyridine-appended zinc porphyrin complex.18
Concentration dependence studies revealed no changes in the
peak maxima, indicating the absence of any intermolecular-
type interactions.
Interestingly, temperature dependence studies reveal the
occurrence of “tail-on” and “tail-off” type binding as shown in
Scheme 1a in the studied compounds. Figure 2 shows the
resulting spectral changes and the figure inset shows a Van’t
Hoff plot of -ln K vs T-1. The binding constants obtained by
the Scatchard method19 and the calculated thermodynamic
parameters are given in Table 1. An examination of Table 1
reveals that the binding constants are 5-6 times larger for the
investigated compounds than those in the earlier reported
pyridine appended zinc porphyrin18 and the majority of the
equilibrium is in the “tail-on” form. Between compounds 2 and
1, compound 2, the m-pyridyl derivative, revealed a higher
binding constant. The values of the thermodynamic parameter
suggest that the higher K value is a result of both entropy and
enthalpy contributions.
Axial Ligand Controlled “Tail-On” and “Tail-Off” Bind-
ing. To probe the effect of “tail-on” and “tail-off” forms of the
dyads on the efficiencies of photoinduced electron transfer from
singlet excited zinc porphyrin to fullerene, we have utilized an
axial ligand replacement method (Scheme 1b) to obtain the “tail-
off” form instead of the temperature variation approach to avoid
any temperature variation induced changes in the fluorescence
quantum yields. The progress of the reaction shown in Scheme
1
1b was monitored by H NMR studies (data not shown). The
(18) D’Souza, F.; Hsieh, Y.-Y.; Deviprasad, G. R. Inorg. Chem. 1996,
35, 5747.
(19) Scatchard, G. Ann. N.Y. Acad. Sci. 1949, 51, 661.

5280 J. Am. Chem. Soc., Vol. 123, No. 22, 2001
D’Souza et al.
Scheme 2
results of steady-state fluorescence emission are summarized
in Figure 3.
These results clearly indicate the effects of “tail-on” and “tail-
off” forms on the emission behavior of the zinc porphyrin in
the studied donor-acceptor dyads.
In agreement with the optical absorption behavior, the
emission bands of pentacoordinated zinc porphyrin are found
to be red-shifted as compared to the tetracoordinated zinc
porphyrin in o-dichlorobenzene. Control experiments performed
on a pentacoordinated complex formed by treating ZnTPP with
3-picoline revealed a 12% decrease of the integrated intensities
of the emission bands (curves i and ii). Interestingly, in addition
to the red shift, the emission intensities of 1 and 2 are found to
be quenched compared to the emission intensities of ZnTPP in
the presence of 3-picoline. The integrated area of the emission
bands is found to be 5% and 17%, respectively, for 2 and 1
compared to the peak area of the pentacoordinated ZnTPP-
picoline complex (curves iii and iv). Interestingly, upon addition
of 3-picoline (10 equiv) to shift the equilibrium to the “tail-
off” form, the porphyrin emission intensity increased and
reached a value of about 26-28% (curves v and vi). Addition
of a large excess of 3-picoline (up to 100 equiv) to these
solutions increased the emission intensity by another 2-4%.
The observed fluorescence enhancement upon addition of
3-picoline could be due to one or more of the following
effects: (i) proximity effects, that is, the closer distance and
orientation of the “tail-on” form as compared to the relatively
larger and flexible “tail-off” form, (ii) minor structural changes
on the porphyrin ring caused by the intramolecular coordination
of fulleropyrrolidine appended pyridine, and (iii) electronic
effects caused by the interactions between electron-rich por-
phyrin and electron-deficient C60 entities. As mentioned earlier,
cyclic voltammetric studies revealed no significant differences
in the redox potentials of the zinc porphyrin in the “tail-on”
form as compared to the “tail-off” form. Hence, effect iii may
be ruled out as a possibility for the observed fluorescence
enhancement. A recent X-ray structural study of a noncovalently
linked ZnTPP and N-methyl-2-(4′-pyridyl)fulleropyrrolidine
complex20 has revealed a center-to-center distance of ∼9.53 Å
with little or no interaction between the C60 and porphyrin π-ring

Donor-Acceptor Proximity of Porphyrin-Fullerene Dyads
J. Am. Chem. Soc., Vol. 123, No. 22, 2001 5281
Table 1. Equilibrium Constant, Ka, as Well as the Thermodynamic
Parameters for the Temperature-Controlled “Tail-On” and
“Tail-Off” Processes of Zinc Porphyrin-C60 Dyads in
o-Dichlorobenzene
∆G,
∆H,
∆S,
dyad
Ka,a M-1
kJ mol-1
kJ mol-1
J K-1 mol-1
2
1
17.29
14.43
2.99
-7.07
-6.62
-2.72
-19.17
-18.11
-45.36
-40.59
-38.52
-138.6
ZnPm∼pyb
a At 298.15 K. b From ref 18; py ) pyridine.
Figure 1. Cyclic voltammograms of 1 and 2 in 0.1 M (TBA)ClO4,
2-/3-
o-dichlorobenzene. Scan rate ) 100 mV/s. Note the overlapping C60
and ZnP0/- redox processes for 2.
Figure 3. Room-temperature steady-state fluorescence spectrum of
(i) ZnTPP, (ii) ZnTPP + picoline (10 equiv), (iii) 2, (iv) 1, (v) 2 +
picoline (10 equiv), and (vi) 1 + picoline (10 equiv) in o-dichloroben-
zene. The concentration of porphyrins was maintained at 2 µM. λex
555 nm.
)
almost similar distance of ∼9.5 Å between the porphyrin center
to the C60 center in the “tail-on” mode. In the “tail-off” form,
this distance varied between 11 and 13 Å depending up on the
different orientations of the fulleropyrrolidine group with respect
to the zinc porphyrin, suggesting that the different donor-
acceptor distances between the “tail-on” and “tail-off” forms
may be responsible for the fluorescence intensity variations. In
addition, effect ii, that is, the minor structural changes, may
also contribute to the emission changes. To verify these,
picosecond time-resolved emission studies have been performed.
Picosecond Time-Resolved Fluorescence Spectra. The
overall time-resolved fluorescence spectral results are essentially
same as those observed from steady-state measurements. The
decay time profiles for 1 and 2 along with ZnTPP in the presence
and absence of added 3-picoline are shown in Figure 4. The
evaluated fluorescence lifetimes are summarized in Table 2,
in which the charge separation rates and quantum yields,
evaluated in the usual manner,8g are also listed. A monoexpo-
nential decay for ZnTPP in o-dichlorobenzene is observed both
in the presence and absence of added 3-picoline. The calculated
lifetime for ZnTPP is found to be 10-20% smaller for the
Figure 2. UV-visible spectral changes observed for 2 as a function
of temperature in o-dichlorobenzene. The inset figure shows Van’t Hoff
plot of -ln K vs T-1
.
of the complexed fulleropyrrolidine unit. Energy minimization
calculations17a,21 and CPK model building studies revealed an
(20) D’Souza, F.; Rath, N. P.; Deviprasad, G. R.; Zandler, M. E. Chem.
Commun. 2001, 267.
(21) D’Souza, F.; Zandler, M. E.; Deviprasad, G. R.; Kutner, W. J. Phys.
Chem. A. 2000, 104, 6887.

5282 J. Am. Chem. Soc., Vol. 123, No. 22, 2001
D’Souza et al.
facilitates conformational changes favorable for through-space
charge separation. In addition, the coordinated 3-picoline in the
“tail-off” state might also cause additional quenching acting as
an electron donor, like that observed in the case of the ZnTPP-
picoline complex. Between 1 and 2, rates of charge separation,
kcs, and quantum yield, Φcs, did not show any specific trend. In
the case of 1, the kcs and Φcs increase slightly in the “tail-off”
form, while reverse trend is observed for 2. Although the
magnitude of these changes is relatively small, these results
suggest that the “tail-on” form of 1 accelerates the charge
separation, a trend similar to that seen from the steady-state
measurements. However, the different magnitudes of quenching
between the steady-state and time-resolved emission studies
suggest that in addition to the charge-separation quenching the
minor structural changes might also induce quenching (static)
of the excited porphyrin in the steady-state emission.
Nanosecond Transient Absorption Spectra. Transient
absorption spectra for 1 and 2 in the presence and absence of
3-picoline in o-dichlorobenzene are shown in Figures 5 and 6,
in which the 700 nm band was attributed to the triplet state of
the C60 moiety.23 The 850 nm band was attributed to the triplet
state of the zinc porphyrin moiety.23 The 1000 nm band was
attributed to the radical anion of the C60 moiety.24 The transient
absorption bands due to the triplet state decay smoothly with
lifetimes of 600 ns. The time profile of the radical anion of the
C60 moiety shows two components: one is quick rise-decay
and another is slower decay similar to those of the triplet states.
The absorbance ratio at 1000 nm to that at 700 nm for 1 in the
presence of 3-picoline is 0.6 () 0.08/0.13 from the spectrum at
100 ns in Figure 5b), which is large enough to assign the slow
decay part of the 1000 nm band to the anion radical of C60.
Thus, there are two types of decays in the charge-separated
states: one is fast decay and the other is slow decay.
This behavior of fast and slow decay could be due to the
following: (i) Generation of a second charge-separated state
from a triplet state formed through a quick charge recombina-
tion. The ion pair produced via the second charge separation
has the triplet state character, which shows a longer lifetime
than the ion pair with the singlet state character. (ii) The first
charge recombination occurs in three ways: the first part is to
the ground state, the second part is to the triplet excited state,
and third part remains as the long-lived ion pair, which is in
equilibrium with the triplet state. From the comparison with
the time profile at 700 nm, which is similar to the decay rate of
the slower part at 1000 nm, this interpretation seems feasible
in these dyads.
Figure 4. Fluorescence decay profiles (A and B) of (a) 2, (b) 2 +
picoline (10 equiv), (c) ZnTPP, (d) 1, and (e) 1 + picoline (10 equiv)
in o-dichlorobenzene. λex ) 555 nm.
pentacoordinated ZnTPP-picoline complex. This suggests the
occurrence of excited-state reactions most probably involving
the coordinated picoline acting as an electron donor to the singlet
excited porphyrin.22
Interestingly, the calculated lifetimes of the dyads, both in
the “tail-on” and “tail-off” forms, are found to be much smaller
than that of the pristine ZnTPP and the ZnTPP-picoline
complex, indicating the occurrence of efficient charge separation
from the singlet excited state of the zinc porphyrin both in the
“tail-on” and “tail-off” forms. The emission decay of 1 and 2
follows a biexponential decay both in the presence and absence
of 3-picoline probably due to the existence of an equilibrium
process involving the “tail-on” and “tail-off” forms as well as
one or more conformers of each of these two forms in solution.
In contrast to the results of steady-state emission where an
increase in the intensity is observed in the “tail-off” form of
the dyads, the time-resolved emission studies reveal different
trends. That is, the measured lifetimes reveal a decrease in the
overall lifetimes (both short and long components of the
biexponential decay) on addition of picoline. This suggests that
the release of the intramolecularly coordinated pyridine moiety
The quick rise-decay suggests that electron transfer from
the singlet excited zinc porphyrin to the C60 entity occurs. The
fluorescence lifetime supports such a fast charge separation. As
explained before, the slow decay component of the ion pair may
be ascribed to long-lived ion pair, because the absorption peak
at 1000 nm is observed in the spectra at 0.1 and 1.0 µs time
intervals. On addition of 3-picoline, the 1000 nm band with
quick rise-decay appears (Figure 5), indicating that the charge
separation takes place in the “tail-off” form. The relative
absorption intensity of 1 at 1000 to 700 nm in the presence of
3-picoline (0.6 at 100 ns) is slightly larger than that in the
absence of 3-picoline (0.025/0.06 ) 0.42 at 100 ns from Figure
5a). This implies that the charge separation in the “tail-off” is
as efficient as “tail-on”, which is in good agreement with the
conclusion from the time-resolved emission studies.
(23) Nojiri, T.; Watanabe, A.; Ito, O. J. Phys. Chem. A. 1998, 102, 5215.
(24) Fujitsuka, M.; Ito, O.; Yamashiro, T.; Aso, Y.; Otsubo, T. J. Phys.
Chem. A 2000, 104, 4876.
(22) Barboy, N.; Feitelson, J. J. Phys. Chem. 1984, 88, 1065.

Donor-Acceptor Proximity of Porphyrin-Fullerene Dyads
J. Am. Chem. Soc., Vol. 123, No. 22, 2001 5283
Table 2. Fluorescence Lifetimes (τf), Charge-Separation Rate Constants (kcs), Charge-Separation Quantum Yields (Φcs), Fast
f
Charge-Recombination Rate Constants (kcr ), and Slow Charge-Recombination Rate Constants (kcrS) of the Investigated Zinc Porphyrin-C60
Dyads in o-Dichlorobenzene in the Absence and Presence of 3-Picoline
compound
τf/ps (%)
2030
kcs/s-1
Φcs
kCRf/s-1 a
kCRs/s-1
ZnTPP
ZnTPP + picoline (10 equiv)
ZnTPP + picoline (200 equiv)
1
1740
1620
284 (87%)
1950 (13%)
1490 (7%)
1470 (4%)
1660 (24%)
1510 (18%)
1470 (4%)
5.1 × 109
4.7 × 109
0.93
0.92
(5 × 107)
(5 × 107)
1.3 × 106
1.6 × 106
1 + picoline (10 equiv)
1 + picoline (200 equiv)
2
2 + picoline (10 equiv)
2 + picoline (200 equiv)
197 (93%)
177 (96%)
321 (76%)
221 (82%)
177 (96%)
3.0 × 109
0.89
0.93
(5 × 107)
0.9 × 106
5.2 × 109
(5 × 107)
1.9 × 106
a
Values at first approximation.
Figure 6. Transient absorption spectra obtained by the 530 nm
nanosecond laser photolysis of (a) 2 (0.05 mM) and (b) 2 + picoline
(10 equiv) in o-dichlorobenzene; filled circle is spectrum at 100 ns
and open circle at 1000 ns. Inset: time profile at 1000 nm.
Figure 5. Transient absorption spectra obtained by the 530 nm
nanosecond laser photolysis of (a) 1 (0.05 mM) and (b) 1 + picoline
(10 equiv) in o-dichlorobenzene; filled circle is spectrum at 100 ns
and open circle at 1000 ns. Inset: time profile at 1000 nm.
through-space interactions between the singlet excited zinc
porphyrin and C60 moiety in the “tail-off” form. In the studied
dyads, the charge separation via the singlet excited states of
zinc porphyrin is also confirmed by the quick rise-decay of
the anion radical of the C60 entity within 20 ns. Furthermore, a
long-lived ion pair with a lifetime of about 1000 ns is also
observed in these systems.
For 2, similar transient absorption spectra were obtained
(Figure 6), which indicates that the charge separation via the
singlet excited state takes place in both the “tail-on” and “tail-
off” forms. The efficiency of the charge separation is also
slightly higher for the “tail-off” form.
Summary. We have successfully demonstrated a new ap-
proach of probing proximity effects in porphyrin-fullerene
based donor-acceptor dyads by using a “tail-on” and “tail-off”
binding mechanism. The cyclic voltammetric and optical
absorption spectral studies revealed no significant ground-state
interactions between the porphyrin π-ring and the C60 spheroid.
The results of steady-state and time-resolved fluorescence
emission reveal that in the “tail-off” form the charge-separation
rates and efficiency change slightly in comparison with the
results of the “tail-on” form, suggesting the presence of some
Experimental Section
Chemicals. Buckminsterfullerene, C60 (+99.95%), was from Bucky-
USA (Bellaire, TX). o-Dichlorobenzene in a sure seal bottle, methyl
glycine ester hydrochloride, pyridine carboxaldehydes, pyrrole, ben-
zaldehyde, 3-hydroxybenzaldehyde, and tetra-n-butylammonium per-
chlorate, (TBA)ClO4, were from Aldrich Chemicals (Milwaukee,
WI). All chemicals were used as received. Solvents used in the present
study are all the purest grade commercially available. The newly

5284 J. Am. Chem. Soc., Vol. 123, No. 22, 2001
D’Souza et al.
synthesized compounds were freshly purified by column chromatog-
raphy, and their purity was tested by TLC prior to spectral measure-
ments.
methanol. This solution was stirred for 30 min. The solvent was
removed under reduced pressure, and the crude product was dissolved
in CH2Cl2, washed with water, and dried with sodium sulfate. The
solution was concentrated and loaded on to a basic alumina column.
Pure 2 was eluted with chloroform:hexane (90:10 v/v). Yield 94%. 1H
NMR: 8.81 (m, 8H, â-pyrrole), 8.19 (m, 6H, o-phenyl), 7.64 (m, 9H,
m-p-phenyl), 8.03-6.72 (d, t, d, 4H, substituted phenyl), 6.78 (d br,
1H, p-pyridyl), 4.83 (s br, 1H, m-pyridyl), 4.87-4.26 (s br, s br, s br,
3H, pyrrolidine) 3.84 (s br, 4H, -CH2-), 3.68 (d br, 2H, o-pyridyl).
ESI mass in CH2Cl2, calcd 1558.3, found 1559.4.
Synthesis of 5-(3′-Hydroxyphenyl)-10,15,20-triphenylporphyrin,
1a. Compound 1a was synthesized by reacting 3-hydroxybenzaldehyde
(1.5 g, 12 mM), benzaldehyde (3.9 g, 37 mM), and pyrrole (3.3 g, 49
mM) in refluxing propionic acid. The crude product was purified on a
basic alumina column with chloroform/methanol (95:5 v/v) as eluent.
Yield 10%. 1H NMR in CDCl3, δ ppm: 8.76 (m, 8H, â-pyrrole), 8.16
(m, 6H, o-phenyl), 7.71 (m, 9H, o-p-phenyl), 7.96-7.12 (d, t, t, d,
4H, substituted phenyl), 5.87 (s, br, 1H, hydroxy), -2.76 (s, br, 2H,
imino).
Synthesis of 5-(3′-Phenoxyethyl-N-(2-(4′-pyridyl)fulleropyrroli-
dine))-10,15,20-triphenylporphyrinatozinc(II), 1. This was synthe-
sized by metalation of 1d with zinc acetate. To a solution of 1d (0.2 g,
0.3 mM) in CHCl3 was added excess zinc acetate in methanol. This
solution was stirred for 30 min. The solvent was removed under reduced
pressure, and the crude product was dissolved in CH2Cl2, washed with
water, and dried with sodium sulfate. The solution was concentrated
and loaded onto a basic alumina column. Pure 1 was eluted with
Synthesis of 5-(3′-Bromoethoxyphenyl)-10,15,20-triphenylpor-
phyrin, 1b. This was synthesized by reacting 1a (0.1 g, 0.16 mM) and
an excess of 1,2-dibromoethane in DMF containing potassium carbonate
for 24 h.14 Crude 1b was purified on basic alumina with chloroform/
1
hexane (90:10 v/v) as the eluent. Yield 87%. H NMR: 8.82 (m, 8H,
â-pyrrole), 8.18 (m, 6H, o-phenyl), 7.73 (m, 9H, m-p-phenyl), 7.83-
7.28 (d, t, d, 4H, substituted phenyl), 4.45 (t, 2H, -CH2-), 3.70 (t,
2H, -CH2-), -2.85 (s, br, 2H, imino).
1
chloroform:hexane (90:10 v/v). Yield 94%. H NMR: 8.83 (m, 8H,
â-pyrrole), 8.19 (m, 6H, o-phenyl), 7.65 (m, 9H, m-p-phenyl), 8.05-
6.63 (d, t, d, 4H, substituted phenyl), 6.48 (d br, 2H, p-pyridyl), 4.87-
4.26 (s br, s br, s br, 3H, pyrrolidine), 3.84 (s br, 4H, -CH2-), 3.42
(d br, 2H, o-pyridyl). ESI mass in CH2Cl2, calcd 1558.3, found 1559.5.
Synthesis of 5-(3′-Ethoxyphenyl-amino acetic acid)-10,15,20-
triphenylporphyrin 1c. For this, first the methyl ester of 1c was
synthesized by reacting 1b (0.5 g, 0.7 mM) and glycine methyl ester
(0.6 g, 7 mM) in methyl ethyl ketone containing excess potassium
carbonate. The crude product was purified on a silica gel column with
chloroform. 1H NMR: 8.86 (m, 8H, â-pyrrole), 8.19 (m, 6H, o-phenyl),
7.76 (m, 9H, m-p-phenyl), 7.86-7.18 (d, t, d, 4H, substituted phenyl),
4.86 (s, 2H, -CH2-), 4.52 (t, 2H, -CH2-), 3.92 (t, 2H, -CH2-),
3.14 (s, 3H, CH3), -2.89 (s, br, 2H, imino). 1c was synthesized by
refluxing the methyl ester in THF:water (95:5 v/v) with a few drops of
50% aqueous NaOH. Crude 1c was purified on a silica gel column
with chloroform:methanol (80:20 v/v). Yield 70%.
Synthesis of 5-(3′-Phenoxyethyl-N-(2-(3′-pyridyl)fulleropyrroli-
dine))-10,15,20-triphenylporphyrin, 2d. To compound 1c (0.06 g, 0.08
mM) in toluene were added 3-pyridine carboxaldehyde (0.044 g, 0.4
mM) and C60 (0.295 g, 0.4 mM), and the solution was refluxed for 12
h. The crude product was purified on a silica gel column with toluene:
ethyl acetate (90:10 v/v). Yield 28%. 1H NMR: 8.99 (d, 1H, o-pyridyl),
8.82 (m, 8H, â-pyrrole), 8.19 (m, 6H, o-phenyl), 8.55 (d, 1H, o-pyridyl),
8.08 (d, 1H, p-pyridyl), 7.72 (m, 9H, m-p-phenyl), 7.33 (q, 1H,
m-pyridyl), 8.03-7.17 (d, t, d, 4H, substituted phenyl), 6.57-5.62 (d,
s, d, 3H, pyrrolidine) 4.52 (t, 2H, -CH2-), 4.02 (t, 2H, -CH2-), -2.91
(s, br, 2H, imino).
Synthesis of 5-(3′-Phenoxyethyl-N-(2-(4′-pyridyl)fulleropyrroli-
dine))-10,15,20-triphenylporphyrin, 1d. To compound 1c (0.06 g, 0.08
mM) in toluene were added 4-pyridine carboxaldehyde (0.044 g, 0.4
mM) and C60 (0.295 g, 0.4 mM), and the solution was refluxed for 12
h. The crude product was purified on a silica gel column with toluene:
ethyl acetate (90:10 v/v). Yield 25%. 1H NMR: 8.93 (m, 8H, â-pyrrole),
8.66 (d, 2H, o-pyridyl), 8.23 (m, 6H, o-phenyl), 7.73 (d, 2H, p-pyridyl),
7.78 (m, 9H, m-p-phenyl), 8.03-7.17 (d, t, d, 4H, substituted phenyl),
5.07-4.07 (d, s, d, 3H, pyrrolidine) 4.52 (t, 2H, -CH2-), 4.02 (t, 2H,
-CH2-), -2.91 (s, br, 2H, imino-H).
Instrumentation. The UV-visible spectral measurements were
carried out with a Shimadzu model 1600 UV-visible spectrophotom-
eter. The fluorescence was monitored by using a Spex Fluorolog
spectrometer. A right angle detection method was used. The 1H NMR
studies were carried out on a Varian 400 MHz spectrometer. Tetra-
methylsilane (TMS) was used as an internal standard. Cyclic voltam-
mograms were obtained by using a conventional three-electrode system
on an EG&G model 263A potentiostat/galvanostat. A platinum or a
glassy carbon disk electrode was used as the working electrode. A
platinum wire was served as the counterelectrode. An Ag/AgCl
electrode, separated from the test solution by a fritted supporting
electrolyte/solvent bridge, was used as the reference electrode. The
potentials were referenced to an internal ferrocene/ferrocenium redox
couple. All the solutions were purged prior to spectral and electro-
chemical measurements using argon gas.
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 report.25 The picosecond
time-resolved fluorescence spectra were measured using an argon-ion
pumped Ti:sapphire laser (Tsunami) and a streak scope (Hamamatsu
Photonics). The details of the experimental setup are described
elsewhere.25
Acknowledgment. The authors thank the donors of the
Petroleum Research Fund, administered by the American
Chemical Society, for support of this work.
JA010356Q
Synthesis of 5-(3′-Phenoxyethyl-N-(2-(3′-pyridyl)fulleropyrroli-
dine))-10,15,20-triphenylporphyrinatozinc(II), 2. Compound 2 was
synthesized by metalation of 2d with zinc acetate.15 To a solution of
2d (0.2 g, 0.15 mM) in CHCl3 was added excess zinc acetate in
(25) (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.