Design and studies on supramolecular ferrocene-porphyrin-fullerene constructs for generating long-lived charge separated states

Article abstract of DOI:10.1021/jp064504g

Supramolecular ferrocene-porphyrin-fullerene constructs, in which covalently linked ferrocene-porphyrin-crown ether compounds were self-assembled with alkylammonium cation functionalized fullerenes, have been designed to achieve stepwise electron transfer and hole shift to generate long-lived charge separated states. The adopted crown ether-alkylammonium cation binding strategy resulted in stable conjugates as revealed by computational studies performed by the DFT B3LYP/3-21G(*) method in addition to the binding constants obtained from fluorescence quenching studies. The free-energy changes for charge-separation and charge-recombination were varied by the choice of different metal ions in the porphyrin cavity. Free-energy calculations suggested that the light-induced electron-transfer processes from the singlet excited state of porphyrins to be exothermic in all of the investigated supramolecular dyads and triads. Photoinduced charge-separation and charge-recombination processes have been confirmed by the combination of the time-resolved fluorescence and nanosecond transient absorption spectral measurements. In case of the triads, the charge-recombination processes of the radical anion of the fullerene moiety take place in two steps, viz., a direct charge recombination from the porphyrin cation radical and a slower step involving distant charge recombination from the ferrocene cation moiety. The rates of charge recombination for the second route were found to be an order of magnitude slower than the former route, thus fulfilling the condition for charge migration to generate long-lived charge-separated states in supramolecular systems.

Full text of DOI:10.1021/jp064504g

25240
J. Phys. Chem. B 2006, 110, 25240-25250
Design and Studies on Supramolecular Ferrocene-Porphyrin-Fullerene Constructs for
Generating Long-Lived Charge Separated States
Francis D’Souza,*,† Raghu Chitta,† Suresh Gadde,† D.-M. Shafiqul Islam,‡
Amy L. Schumacher,† Melvin E. Zandler,† Yasuyuki Araki,‡ 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: July 16, 2006; In Final Form: August 7, 2006
Supramolecular ferrocene-porphyrin-fullerene constructs, in which covalently linked ferrocene-porphyrin-
crown ether compounds were self-assembled with alkylammonium cation functionalized fullerenes, have been
designed to achieve stepwise electron transfer and hole shift to generate long-lived charge separated states.
The adopted crown ether-alkylammonium cation binding strategy resulted in stable conjugates as revealed
by computational studies performed by the DFT B3LYP/3-21G(*) method in addition to the binding constants
obtained from fluorescence quenching studies. The free-energy changes for charge-separation and charge-
recombination were varied by the choice of different metal ions in the porphyrin cavity. Free-energy calculations
suggested that the light-induced electron-transfer processes from the singlet excited state of porphyrins to be
exothermic in all of the investigated supramolecular dyads and triads. Photoinduced charge-separation and
charge-recombination processes have been confirmed by the combination of the time-resolved fluorescence
and nanosecond transient absorption spectral measurements. In case of the triads, the charge-recombination
processes of the radical anion of the fullerene moiety take place in two steps, viz., a direct charge recombination
from the porphyrin cation radical and a slower step involving distant charge recombination from the ferrocene
cation moiety. The rates of charge recombination for the second route were found to be an order of magnitude
slower than the former route, thus fulfilling the condition for charge migration to generate long-lived charge-
separated states in supramolecular systems.
Introduction
Although substantial amount of progress in terms of charge
stabilization has been accomplished on covalently linked
polyads,1-6,9-10 achieving similar results on self-assembled
supramolecular polyads has been challenging because of the
occurrence of multiple equilibrium processes and the inferior
stability of the adopted self-assembly approaches in solution,
especially in polar solvents.10 Recently, we addressed the
stability issue by employing multiple modes of binding and
formed structurally well-defined, stable supramolecular systems.10g
An alternate supramolecular approach was also developed, in
which the covalently linked dyads were allowed to self-assemble
with either a donor or an acceptor with a well-defined binding
mechanism to form the triads.10g In the present study, we have
further explored the latter approach where covalently linked
ferrocene-porphyrin-crown ether compounds (Fc-MP-crown
ether) were self-assembled with alkylammonium cation func-
tionalized fullerenes (NH3+-C60) (Scheme 1). As demonstrated
here, upon excitation of the donor porphyrin, efficient forward
electron transfer to the fullerene entity self-assembled via crown
ether-alkylammonium cation complexation occurs followed by
Studies on photoinduced electron transfer in molecular and
supramolecular donor-acceptor polyads (triads, tetrads, etc.)
have undergone rapid growth in recent years mainly to develop
artificial photosynthetic systems capable of stabilizing charge-
separated states,1-6 which is eventually also useful for the
development of molecular optoelectronic devices.7,8 In the
construction of such polyads, porphyrins and fullerenes have
been employed as primary electron donor and acceptor,
respectively, whereas ferrocene, carotene, or tetrathiafulvalene
has often been utilized as terminal electron donors.9,10 In contrast
with the traditionally used two-dimensional aromatic electron
acceptors, fullerenes in donor-acceptor dyads and polyads
accelerate forward electron transfer (kCS) and slow backward
electron transfer (kCR) due to their small reorganization energy
in electron-transfer reactions.11 This property of fullerenes
combined with the adopted electron migration strategy of
polyads has resulted in the generation of distinctly separated
donor-acceptor radical ion pairs upon initial electron transfer
by charge migration reactions along the well-tuned redox
gradients.12-15 In a few systems, energy funneling antenna
molecules have been elegantly connected to the donor-acceptor
systems to probe sequential energy/electron-transfer processes,
thus mimicking the “antenna-reaction center” functionality of
photosynthesis.16
a hole transfer from the MP•+ to the ferrocene entity to generate
the final charge-separated states, Fc+-MP-crown:NH3+-C60
.
•-
As a consequence of the sequential electron-transfer events, the
charge-recombination reaction is slowed in these supramolecular
triads (see Supporting Information).
Results and Discussion
* To whom correspondence should be addressed. E-mail:
Francis.DSouza@wichita.edu (F.D); ito@tagen.tohoku.ac.jp (O.I.).
† Wichita State University.
The structures of the newly synthesized compounds are shown
in Scheme 1. Porphyrins, 1a-c (MP ) H2P, ZnP, and MgP,
‡ Tohoku University.
10.1021/jp064504g CCC: $33.50 © 2006 American Chemical Society
Published on Web 09/23/2006

Ferrocene-Porphyrin-Fullerene Constructs
J. Phys. Chem. B, Vol. 110, No. 50, 2006 25241
SCHEME 1: Structures of the Crown Ether Appended Porphyrins, Ferrocene-Porphyrins Dyads, and Fullerene
Derivatives
respectively) bearing donor, porphyrin, and a 18-crown-6 entity
are used as a control for compounds 2a-c possessing a
porphyrin covalently linked to a second electron donor, fer-
rocene, and a 18-crown-6 entity. We have employed free-base,
zinc, and magnesium porphyrins because of their different
emission and redox properties.17 The first oxidation potential
of these porphyrins generally follows the trend: MgP < ZnP
< H2P. As a result, it has been possible to systematically vary
the free-energy change for photoinduced electron transfer in the
studied supramolecular dyads and triads. Three derivatives of
fullerene functionalized with varying alkyl chain length am-
monium cations, 3-5, have been utilized to form the supramo-
lecular complexes via alkylammonium-crown ether complex-
ation.
No apparent absorption bands in the wavelength region covering
350-700 nm corresponding to the crown ether entities were
observed.
The optical absorption spectra in the visible wavelength region
of the ferrocene-porphyrin-crown ether systems 2a-c were
also found to be similar to the respective 1a-b analogues.
However, in each case, the peak maxima exhibited a red-shift
of 2-3 nm compared to the corresponding 1a-c derivatives.
No new peak corresponding to the appended ferrocene was
observed due to the low molar absorptivity of ferrocene
compared to the high ꢀ values of porphyrin absorption bands
in the 380-420 nm region. The binding of functionalized
fullerenes, 3, 4, or 5, to ferrocene-porphyrins 2a-c resulted
in the formation of the supramolecular triads, which was
accompanied by slightly red-shifted absorption bands of por-
phyrins (Figure 1). A weak band around 700 nm was observed
due to the absorption of 3 in this wavelength region. On
increasing the concentration of 3, the absorbance near the Soret
band increased; compared with such increases, the increment
in peak absorbance was small, supporting the supramolecular
formation. Under these conditions, it was difficult to isolate any
Optical Absorption Studies. The optical absorption behavior
of the porphyrins 1a-c was found to be similar to those of
pristine free-base porphyrin, zinc porphyrin, and magnesium
porphyrin, respectively. That is, they exhibited an intense Soret
band around 425 nm and 2-4 visible bands in the 500-650
nm range depending upon the metal ion in the porphyrin cavity.

25242 J. Phys. Chem. B, Vol. 110, No. 50, 2006
D’Souza et al.
TABLE 1: B3LYP/3-21G(*)-Optimized Distances between
the Different Entities of the 2b:3-5 Supramolecular Triads
center-to-center distance, Å
triads
structure form
Fe-Zn
Zn-C60
C60-Fc
2b:3
extended
closed
17.1
17.1
11.8
5.8
27.2
17.8
2b:4
2b:5
extended
closed
17.2
17.1
11.7
5.6
27.5
17.5
extended
closed
17.2
17.1
11.5
5.7
25.8
17.3
porphyrin and fullerene, and fullerene and ferrocene were found
to depend on the type of structure. For “closed” structures, these
distances were ∼5.7 and 17.8 Å, respectively, whereas for the
“relaxed” structures, these distances were ∼11.7 and ∼27.0 Å,
respectively. That is, there was a significant change in the donor
and acceptor distances for these two types of structures. The
gas-phase dissociation energy for the alkylammonium cation-
crown ether binding, measured as the energy difference between
the complex and the sum of the energies of the individual
porphyrin and fullerene moieties, calculated by B3LYP/3-
21G(*), was found to be ∼110 kcal mol-1. Although this small
basis set result is sure to be 20-30 kcal/mol too high, this
indicates stable complex formation through this binding mech-
anism.
Figure 1. Absorption spectral changes observed during titration of
2b (0.15 × 10-5 M) with 3 (4.2 × 10-4 M each addition) in benzonitrile.
Electrochemical Studies and Electron-Transfer Driving
Forces. Electrochemical studies using the cyclic voltammetric
technique were performed to evaluate the redox potentials of
the triads and also to evaluate the energetics of electron-transfer
reactions. The crown ether appended porphyrins, 1a-c and 2a-
c, were titrated with various amounts of functionalized fullerenes,
3-5, to probe the effect of coordination on the redox potentials
of the ferrocene, porphyrin, and fullerene entities. Figure 3
Figure 2. DFT B3LYP/3-21G (*) optimized structures of the self-
assembled triad, 2b:4 in the (a) closed and (b) extended forms.
weak band corresponding to charge-transfer type interactions
between porphyrin and fullerene entities (wide infra).
Computational Studies. To gain insights into the supramo-
lecular geometry and possible existence of intermolecular type
interactions, computational studies were performed using density
functional methods (DFT) at the B3LYP/3-21G(*) level. The
B3LYP/3-21G(*) methods have recently been successfully used
to predict the geometry and electronic structure of molecular
and self-assembled supramolecular dyads and triads.18 Here, both
the ferrocene-porphyrin-crown ether dyads and functionalized
fullerenes were optimized to a stationary point on the Born-
Oppenheimer potential energy surface and allowed to self-
assemble via crown ether-ammonium cation complexation.
Representative structures for the supramolecular triad, 2b:4 are
shown in Figure 2 and the structural parameters are given in
Table 1. For all of the studied supramolecular dyads and triads,
at least two structures were possible, namely, a closed structure
and a stretched structure. The closed structures were energeti-
cally more stable by about 2-5 kcal/mol; however, the closed
structures are not close enough to show the charge-transfer
interactions in the absorption spectra. The center-to-center
distance between the Fe atom of ferrocene and the zinc atom
of zinc porphyrin was ∼17.1 Å irrespective of the type of
structure. However, the center-to-center distances between zinc
Figure 3. Cyclic voltammograms of (a) 2a, (b) 2b, and (c) 2c in the
absence (dark lines) and in the presence (blue lines) of 5 in benzonitrile
containing 0.1 M (n-C4H9)4NClO4. The concentrations of porphyrins
and fullerene were held at 0.03 mM; scan rate ) 100 mV s-1
.
shows cyclic voltammograms obtained for compounds 2a-c
in benzonitrile containing 0.1 M t-Bu4NClO4 (dark lines); the
redox potential data are given in Tables 2 and 3. All of the
employed porphyrins revealed two one-electron oxidations
(MP0/•+ and MP•+/2+) as judged from their peak-to-peak
separation, ∆Epp, values and the cathodic-to-anodic peak current
ratios. The magnitudes of the E1/2 values were found to depend
on the metal ion present in the porphyrin cavity, which followed
the order: MgP < ZnP < H2P. In addition, for compounds 2a-
c, another reversible redox couple close to 0.0 V vs Fc/Fc+

Ferrocene-Porphyrin-Fullerene Constructs
J. Phys. Chem. B, Vol. 110, No. 50, 2006 25243
TABLE 2: Electrochemical Redox Potentials (E, V vs Fc/Fc+), Energy Levels of the Charge-Separated States (∆GRIP), and
Free-Energy Changes for Charge-Separation (∆GCS) for the Supramolecular Dyads in Benzonitrile
0/•-
b
c
E (MP0/•+
)
E (C60
)
E (MP0/•-
)
-∆GRIP(P-C)
-∆GS
CS(P*-C)
/eV
compounda
/V
/V
/V
/eV
1a
1a:4
1b
1b:4
1c
1c:4
0.55
0.54
0.29
0.28
0.15
0.14
-
-1.61
-1.63
-1.82
-1.83
-1.93
-1.90
-
-
-1.04
1.53
-
0.37
-
-1.04
-
1.31
-
1.18
0.75
0.87
-1.04
a See Scheme 1 for structures. b ∆GRIP ) Eox - Ered + ∆GS, where ∆GS ) -e2/(4πꢀ0ꢀRRCt-Ct) and ꢀ0 and ꢀR refer to vacuum permittivity and
dielectric constant of benzonitrile. c -∆GCS ) ∆E0-0 - ∆GRIP, where ∆E0-0 is the energy of the lowest excited states (2.07 eV for 1ZnP*, 2.05 eV
1
1
1
for MgP*, 1.95 eV for H2P*, and 1.72 eV for C60*).
TABLE 3: Electrochemical Redox Potentials (E, V vs Fc/Fc+), Energy Levels of the Charge-Separated States (∆GRIP),
Free-Energy Changes for Charge-Separation (∆GCS), and Hole Shift (∆GHS) for 2a-c:4 Supramolecular Triads in Benzonitrile
E(MP0/+•
)
E(Fc0/•+
)
E(C60
/V
)
E(MP0/•-
)
-∆GRIP(P-C)
-∆GRIP(F-C)
-∆GRIP(F-P)
-∆GS
-∆GHS(F-P)
0/•-
b
b
b
c
CS(P*-C)
/eV
compoundsa
/V
/V
/V
/eV
/eV
/eV
/eV
2a
2a:4
2b
2b:4
2c
2c:4
0.55
0.55
0.32
0.31
0.24
0.17
0.0
-0.01
0.0
0.0
0.0
-
-1.61
-1.62
-1.79
-1.81
-1.91
-1.95
-
-
1.61
1.63
1.79
1.81
1.91
1.96
-
-1.04
-
1.53
-
1.05
-
0.37
0.77
0.88
0.48
-1.04
-
-1.05
1.30
-
1.17
1.04
-
1.06
0.26
0.11
-0.01
a See Scheme 1 for structures. b ∆GRIP; see footnote of Table 2. c ∆GCS; see footnote of Table 2.
Figure 4. Energy level diagram showing different photochemical events of the investigated supramolecular triads.
corresponding to the covalently linked ferrocene unit was
observed. That is, the oxidation potential of the ferrocene unit
was easier than the first oxidation potential of any of these
porphyrins. During the cathodic potential scan, these porphyrins
revealed two one-electron reductions whose potentials followed
the trend: MgP > ZnP > H2P.
levels of the excited states, the driving forces (∆GCS) were
•-
evaluated. The generation of MP•+-crown:NH3+-C60 and
Fc-MP•+-crown:NH3+-C60•- were exothermic via the singlet
excited states of any of the porphyrins or the fullerene in
benzonitrile and followed the trend MgP > ZnP > H2P. A hole
shift from MP•+ to Fc was found to be feasible for all the
supramolecular triads. On the basis of these calculations, an
energy diagram for Fc-MP-crown:NH3+-C60 can be depicted
as shown in Figure 4.
Fluorescence Emission Studies. The photochemical behavior
of the ferrocene-porphyrins dyads was investigated, initially
by using steady-state fluorescence measurements. The emission
behavior of the crown ether appended porphyrins was found to
be similar to that of porphyrins with two emission bands in the
visible-near-IR regions. The fluorescence intensities of 2a-c
were found to be quenched compared with those of the crown-
ether appended porphyrins, 1a-c. These results indicate the
occurrence of intramolecular events in the ferrocene-porphyrin
dyads.
The addition of fullerenes 3-5 revealed cathodic shifts of
the peaks corresponding to the porphyrin unit and not the
ferrocene unit (blue traces in Figure 3), and such shifts were
more for easily oxidizable MgP. These observations clearly
suggest intramolecular type interactions19 between the porphyrin
and fullerene entities in support of the close structure predicted
by computational studies. During the cathodic potential scan
of the supramolecular complexes, two one-electron reductions
0/•-
•-/2-
corresponding to C60
and C60
were observed and were
independent of the nature of MP. Subsequent reductions at
higher negative potentials, depended upon M, involved either
the porphyrin (MP0/•- and MP•-/2-) or fullerene entity (C602-/3•-).
The energy levels of the charge-separated states (∆GRIP) were
evaluated using the Weller-type approach20 utilizing the redox
potentials, center-to-center distance, and dielectric constant of
the solvent as listed in Tables 2 and 3. By comparing these
energy levels of the charge-separated states with the energy
Additional quenching of the porphyrin emission was observed
upon complexation of fullerenes 3-5 to the crown ether
appended porphyrins. Representative emission changes of 2c
on increasing addition of 5 are shown in Figure 5. By using the

25244 J. Phys. Chem. B, Vol. 110, No. 50, 2006
D’Souza et al.
Figure 6. Fluorescence decays at 650 nm range of (i) porphyrin, 1a
(0.02 mM), (ii) 1a (0.02 mM) in the presence of fullerene 3 (0.15 mM),
(iii) ferrocene-porphyrin, 2a (0.02 mM), and (iv) 2a (0.03 mM) in the
presence of fullerene 3 (0.15 mM) in benzonitrile; λex ) 400 nm.
TABLE 5: Fluorescence Lifetime (τF,dyad),
Charge-Separation Rate Constant (kSCS), Charge-Separation
Quantum Yield (ΦSCS), Charge-Recombination Rate
Constant (kCR), and Lifetime of the Radical Ion Pair (τRIP
for the Investigated Porphyrin-Fullerene Conjugates in
Benzonitrile
)
Figure 5. Fluorescence spectral changes observed on increasing
addition of fullerene 5 (2.0 µM each addition) to a solution of porphyrin-
crown ether, 2c in benzonitrile. λex ) 420 nm. (Inset) Benesi-
Hildebrand plot at 620 nm constructed for measuring the binding
constant; I0 (fluorescence intensity in the absence of 5), and ∆I (changes
of fluorescence intensity on addition of 5).
a
τF,dyad
/ps (%)
kS
kCR
τRIP
CS
a
porphyrin fullerene
/s-1
ΦS
/s-1
/ns
CS
1a
1a
9510 (100)b
1350 (65)
2350 (55)
2640 (45)
1950 (100)b
250 (60)
-
-
-
-
3
4
5
-
3
6.4 × 108 0.86 1.3 × 107 80
3.2 × 108 0.75 7.1 × 106 140
2.7 × 108 0.71 1.4 × 107 70
TABLE 4: Formation Constants (K) Calculated from the
Benesi-Hildebrand Plots of the Fluorescence Data for the
Bound Porphyrin-Fullerene Conjugates in Benzonitrile at
25 °C
1b
1b
-
-
-
-
3.5 × 109 0.88 6.7 × 106 150
7.0 × 108 0.58 5.6 × 106 178
4.6 × 108 0.48 9.6 × 106 104
4
5
830 (50)
1025 (45)
6520 (100)b
420 (60)
390 (55)
490 (50)
1c
1c
-
-
-
-
K, M-1 Fullerene derivative
3
4
5
2.2 × 109 0.92 1.2 × 107 80
2.4 × 109 0.93 5.1 × 106 200
1.9 × 109 0.92 7.2 × 107 15
Porphyrin
3
4
5
1a
1b
1c
2a
2b
2c
6.4 × 104
1.9 × 105
7.1 × 104
8.0 × 104
1.7 × 105
2.8 × 104
1.7 × 103
2.5 × 104
1.8 × 104
1.5 × 104
2.5 × 104
1.0 × 104
5.9 × 103
3.2 × 103
2.5 × 103
5.3 × 103
6.7 × 103
1.9 × 103
a kSCS ) (1/τF,dyad) - (1/τF,ref), ΦSCS ) [(1/τF,dyad) - (1/τF,ref)]/(1/τF,dyad
for 1. τF,ref.
)
b
.
these two decays either to the “closed-extended” structures or
to the alkylammonium-crown ether “bound-unbound” frac-
tions. Because the fluorescence quenching is due to charge
separation from the singlet excited porphyrin to the complexed
fullerene,15a the rates (kSCS) and quantum yields (ΦSCS) of charge
separation were evaluated for both closed (or bound) and
extended (or unbound) forms in the usual manner and the data
are given in Table 5. For 1a-b, the kSCS values are on the order
of 108 s-1, whereas the kSCS values are on the order of 109 s-1
for 1c.
emission data, the binding constants (K) for the formation of
self-assembled dyads and triads were obtained by constructing
Benesi-Hildebrand plots.21 (Figure 5, inset), and the data are
listed in Table 4. The magnitude of the binding constants varied
from 103-105 M-1 and revealed stable self-assembly process
in polar benzonitrile. The binding of 3 was found to be higher
than that of either of 4 and 5.
Further picosecond time-resolved emission measurements
were performed to study the kinetics and mechanism of
quenching in these supramolecular triads.
As shown in Figure 6 for representative 2a, the fluorescence
time profiles of the ferrocene-porphyrin dyads 2a-c showed
quick decays compared with the time profiles of the references
compounds 1a-c in benzonitrile. Here also, the decay could
be fitted to a biexponential fitting curve. The evaluated
fluorescence lifetimes are summarized in Table 6. These results
suggest the occurrence of an excited-state quenching process
of MP by Fc in the ferrocene-porphyrin dyads.22 The quenching
process of the porphyrin emission could occur either from
energy-transfer or electron-transfer processes. In the absence
of any appreciable ferrocene absorption in the emission wave-
length region of porphyrins, one could eliminate energy transfer
as a quenching via the Fo¨rster mechanism. The calculated free-
Picosecond Time-Resolved Emission Studies. The time-
resolved emission spectra of the self-assembled conjugates
tracked those of steady-state spectra. Figure 6 shows the
emission decay profiles of the crown ether appended porphyrins,
1a, in the absence and presence of fullerene, 3. The appended
crown ether moieties had little influence on the lifetime of
porphyrins in 1a-c. In the presence of 3, fast fluorescence
decays were observed that could be curve-fitted to a biexpo-
nential function involving a major short lifetime and a minor
longer lifetime, as listed in Table 5. It was difficult to assign

Ferrocene-Porphyrin-Fullerene Constructs
J. Phys. Chem. B, Vol. 110, No. 50, 2006 25245
TABLE 6: Fluorescence Lifetime (τF,triad),a Charge-Separation Rate Constant (kSCS), Charge-Separation Quantum Yield (ΦSCS),
Charge-Recombination Rate Constant (kCR), and Lifetime of the Radical Ion Pair (τRIP) for the Investigated
Porphyrin-Fullerene Conjugates in Benzonitrile
c
c
τF,triad
/ps (%)
kS
kCR1
kCR2
τRIP1
/ns
τRIP2
/ns
CS
/s-1 b
b
porphyrin
fullerene
ΦS
/s-1
/s-1
CS
2a
2a
2a
2a
2b
2b
2b
2b
2c
2c
2c
2c
-
3
4
5
-
3
4
5
-
3
4
5
980 (85)
320 (78)
730 (75)
826 (73)
910 (85)
130 (76)
440 (73)
600 (70)
1240 (55)
90 (60)
(2.0 × 108)b
2.1 × 109
4.0 × 108
2.4 × 108
(2.0 × 108)b
6.5 × 109
1.2 × 109
5.6 × 108
(2.0 × 108)b
1.1 × 1010
3.3 × 109
2.1 × 109
0.18
0.67
0.30
0.20
0.18
0.86
0.52
0.34
0.18
0.99
0.80
0.72
1.6 × 107
6.7 × 106
1.7 × 107
1.0 × 107
9.4 × 106
1.5 × 107
7.0 × 106
8.0 × 106
6.5 × 107
2.6 × 105
1.0 × 106
6.3 × 105
1.6 × 105
2.8 × 105
1.6 × 105
1.4 × 105
1.5 × 105
1.5 × 105
60
150
60
3910
910
1590
100
110
70
6210
3570
6140
-
140
130
15
7140
6490
6580
240 (55)
350 (45)
a The singlet excited lifetimes (τ0) of free-base porphyrin (1a), zinc porphyrin (1b), and magnesium porphyrin (1c) were found to be 9.6, 1.9, and
5.6 ns, respectively. b kS ) (1/τF,triad) - (1/τF,2a-c), ΦS ) [(1/τF,triad) - (1/τF,2a-c)]/(1/τF,triad)) for 2. kCR2 and ΦCR2 (see Figure 4).
c
CS
CS
energy change, ∆GCS for electron transfer from the ferrocene
entity to the excited porphyrin is found to be slightly exothermic
(∆GCS ) -0.1 eV). These results indicate the occurrence of
excited-state charge-separation in the covalently linked fer-
rocene-porphyrin dyads, generating Fc+-MP•-. The kSCS values
were evaluated to be 2.0 × 108 s-1. Earlier, similar results were
obtained for covalently linked ferrocene-porphyrin dyads held
by different covalent connectivity.12c
Addition of 3 equiv of fullerene derivatives was performed
to ensure complete complexation of the porphyrins (2a-c) with
the fullerenes. Additional fluorescence quenching of MP of the
ferrocene-porphyrin(s) dyads, 2a-c, was observed upon com-
plexation of fullerene, 3. Similar observations were made for
complexation of 2a-c dyads with the fullerenes 4 and 5. The
porphyrin emission decay in the triads could be fitted satisfac-
torily to a biexponential decay curve; the lifetimes (τf) are
summarized in Table 6. On the basis of the observations that
the time-resolved fluorescence spectra did not show the ap-
pearance of the transient fluorescence peak of the C60 moiety
after the decay of the fluorescence of the porphyrin moiety, the
quenched lifetimes of porphyrins were predominantly ascribed
to charge-separation within both structural forms of the su-
pramolecular triads. Since the quenching is due to charge
separation from the singlet excited porphyrin to the complexed
fullerene,15a the kS and ΦS values were evaluated as given
Figure 7. Nanosecond transient absorption spectra of (a) the crown
ether appended porphyrin (1a, 0.04 mM) in the presence of fullero-
pyrrolidine (5, 0.07 mM) and (b) porphyrin (2a, 0.1 mM) in the presence
of fulleropyrrolidine (5, 0.15 mM) in benzonitrile after 532-nm laser
irradiation. (Inset) Absorption time profile at 1020 nm band.
CS
CS
in Table 6. Generally, the kSCS values were found to be higher
for the triads in the order of 109 s-1, with the slowest rate of
∼5 × 108 s-1 for 2a:4 and 2b:5 and the fastest rate of 1 × 1010
s-1 for 2c:3, respectively. Thus, for Fc-MP:NH3+-C60 su-
Fullerenes 3-5 showed a band at 720 nm corresponding to their
excited triplet states.
pramolecular triads, the charge-separation (CS1) generating Fc-
•
MP +-crown:NH3+-C60 is predominant compared to the
•-
Nanosecond transient spectra recorded after 532-nm laser
irradiation of ferrocene-porphyrins, 2a-c showed weak ab-
sorption peaks in the 800-840 nm region corresponding to the
excited triplet states of porphyrins.15a The absorption bands of
the porphyrin anion radicals formed after charge separation from
the ferrocene to the singlet excited porphyrins are expected to
appear at 600-640 nm; however, such absorption was not
observed, probably due to the occurrence of quick charge-
recombination of Fc+-MP•-.
charge-separation (CS2) generating Fc+-MP•-.
The data in Tables 5 and 6 indicate that the kCS values follow
the order: H2P < ZnP < MgP, that is, they depend on the
magnitude of the -∆GCS values. The ΦCS values follow the
same order: H2P < ZnP < MgP for both the dyads and triads.
Among the fullerene derivatives, 3-5, the kCS followed the trend
3 > 4 ∼ 5. The higher kCS values obtained for fullerene, 3 agreed
well with the higher K values evaluated earlier.
Further studies involving the nanosecond transient absorption
technique were performed to identify the electron-transfer
products and monitor the kinetics of charge recombination in
the dyads and the self-assembled triads.
Nanosecond Transient Absorption Studies. The nanosecond
transient absorption spectrum of 1a-c exhibited absorption
peaks at 630 and 800-840 nm corresponding to the triplet states.
The transient absorption spectra for the studied self-assembled
free-base porphyrin-fullerene dyad (1a:5) are shown in Figure
7a. In the spectra, in addition to the peaks corresponding to the
triplet excited states of porphyrin and fullerene at 700 and 800
nm, respectively, peaks around 640 nm corresponding to the
formation of porphyrin cation radical was also observed.
Furthermore, peak at 1020 nm corresponding to the formation

25246 J. Phys. Chem. B, Vol. 110, No. 50, 2006
D’Souza et al.
of fulleropyrrolidine anion radical was observed. These spectral
features clearly support that the charge separation occurred from
the singlet excited porphyrin to fullerene.15a The rates of charge
recombination kCR for MP•+-crown:NH3+-C60•- were evalu-
ated from the decay of the 1020 nm band, which obeyed the
first-order kinetics. The kCR values were found to be about 1 ×
107 s-1 with a slowest rate of 0.6 × 107 s-1 for 1b:4 and a
fastest rate of 7 × 107 s-1 for 1c:5, as listed in Table 5. The
nondecaying part after 200 ns has been attributed to the tail of
the absorption of the triplet excited states of zinc porphyrin and
fullerene because the peak at 1020 nm completely disappeared
after about 1000 ns.
of the charge-recombination process are quite characteristic of
supramolecular systems, which suggests that the kCR1 is
comparable to kHS. Indeed, the kCR1 values in the present
supramolecular systems are greater than that of covalently
bonded dyad (ZnP-C60; 700 ns in PhCN).11c Although the
molecular flexibility of the supramolecular systems appears to
accelerate the charge-recombination, substantial charge-stabi-
lization has been indeed achieved by the adopted supramolecular
strategy.
•-
In the case of covalently linked triad, the Fc+-ZnP-C60
was insensitive to O2 because of the distant doublet-doublet
spin character. This was in contrast to the ZnP•+-C60•- of the
covalently linked dyad that was quenched by O2 because of
the triplet spin character.12c In the present supramolecular dyads,
acceleration of the decay of the radical ion pair was observed
in a similar way to the covalently linked dyads. For the
supramolecular triads, further quick decay of Fc-MP•+-crown:
The transient absorption spectra for the studied self-assembled
ferrocene-free base porphyrin-fullerene triads (2a:5) are shown
in Figure 7b. Upon formation of the supramolecular triad by
the complexation of 2a with 5, the transient absorption spectra
showed a peak at 1020 nm corresponding to the formation of
fulleropyrrolidine anion radical. On the other hand, no new peak
corresponding to the formation of the porphyrin cation radical
appeared at 640 nm. Although formation of ferrocenium cation
was not observed for reasons of weak absorbance, disappearance
of the porphyrin cation radical supports the hole migration from
the zinc porphyrin cation radical to the ferrocene entity.
Interestingly, unlike the spectrum of the dyad shown in Figure
•-
NH3+-C60 was observed in the presence of O2 as expected
from the covalently linked case. However, the slow decay part
of Fc+-MP-crown:NH4+-C60 was also quenched by O2
•-
probably because of the occurrence of different phenomenon
in solution such as flexibly linked “close” forms of the
structures.
Summary. A series of supramolecular triads composed of
ferrocene, porphyrin, and fullerene were designed, synthesized,
and studied using various physicochemical techniques. The
adopted crown ether-alkylammonium cation complexation
binding strategy resulted in well-defined stable triads. The free-
energy changes for charge-separation and charge-recombination
were varied by varying the metal ion in the porphyrin cavity.
The charge-separation processes were monitored by steady-state
and time-resolved emission studies. The rate of charge-separa-
tion was found to depend on the associated free-energy changes.
The rates of charge-recombination monitored by nanosecond
transient absorption spectral studies were found to follow two
routes, namely, direct charge-recombination from the initial
charge-separated state, Fc-ZnP•+-C60•- and charge-recombina-
tion from the charge migrated state, Fc+-ZnP-C60•-. The kCR
values for the latter route were found to be 1-2 orders of
magnitude slower than the former route, thus fulfilling the
condition of charge migration to generate long-lived charge-
separated states.
•-
7a, the C60 peak at 1020 nm survived beyond 800 ns,
indicating the formation of a prolonged charge-separated state.
The transient absorption spectra for the other studied self-
assembled ferrocene-porphyrin-fullerene triads yielded similar
results.
To calculate the rate of charge recombination, the decay of
the fulleropyrrolidine anion radical peak at 1020 nm was
monitored. The charge recombination process occurred in two
steps, namely a relatively fast process and a relatively slow
process. The rates of charge recombination calculated from the
+
fast decaying component (kCR1 for Fc-MP•+-crown:NH3
-
•-
C60 in Table 6) were found to be close to the kCR values for
the dyad systems, MP•+-crown:NH3+-C60 (1a-c:3-5 in
•-
Table 5). These observations suggest that the kCR1 corresponds
to the charge-recombination process occurring before the hole
shift from MP•+ to Fc. On the other hand, slow decay at 1020
nm can be ascribed to the distant charge-recombination process
between the ferrocinium cation and fulleropyrrolidine anion
radical species in Fc+-MP-crown:NH3+-C60•-. The rate of
charge recombination, kCR2 for the slow process was found to
be 2 or 3 orders of magnitude smaller than the kCR1 in Table 6
or kCR of the dyads in Table 5. The longest lifetime of Fc+-
MP-crown:NH3+-C60•- was found for 2c:3 about 7 µs, which
is longer than that observed for 1c:3 by a factor of about 100
times. As pointed earlier, this effect could be easily rationalized
to the increased distance between the opposite charges, that is,
from about 17 Å for the dyads to 27 Å for the triads.
Experimental Section
Chemicals. Buckminsterfullerene, C60 (+99.95%) was from
SES Research, (Houston, TX). All the reagents were from
Aldrich Chemicals (Milwaukee, WI) whereas the bulk solvents
utilized in the syntheses were from Fischer Chemicals. Tet-
rabutylammonium perchloride, n-Bu4NClO4, used in electro-
chemical studies was from Fluka Chemicals. Synthesis of 4 is
given elsewhere.19b
By using the kCS and kCR thus evaluated, the ratios kCS/kCR
were evaluated as a measure of the extent of charge stabilization
in the photoinduced electron-transfer process. These values were
Synthesis of Benzo-[18-crown-6] Functionalized Ferroce-
nyl Porphyrins: 5-(4-Methoxycarbonylphenyl)dipyrrom-
ethane (1). This compound was prepared according to the
general procedure outlined for the synthesis of dipyrromethanes
by Lindsey et al.23 To pyrrole (68 mL, 0.98 mol), methyl
4-formyl benzoate (4 g, 0.024 mol) was added and purged with
argon for 15 min. Then trifluoroacetic acid (190 µL, 0.0024
mmol) was added and stirred at room temperature for 15 min.
The mixture was diluted with CH2Cl2, and the reaction was
quenched with NaOH (100 mL, 0.1 M), and the organic layer
was extracted over Na2SO4. Excess pyrrole was distilled through
vacuum at room temperature, and the compound was purified
by flash column chromatography on silica gel using hexanes:
found to be 100-150 for kCS/kCR1 and 1000-10 000 for kCS
CR2 clearly demonstrating charge stabilization in the supramo-
lecular triads.
/
k
A comparison between the results reported earlier for the
covalently linked triad bearing same Fc-ZnP-C60 entities12c
to the supramolecular triads of the present study is important.
Unlike the present supramolecular triads, a single step charge
recombination was observed for the covalently linked Fc-ZnP-
C60 triad12c where the rate for the exothermic hole-shift (HS)
process was evaluated to be >109 s-1 12c
Thus, the dual phases
.

Ferrocene-Porphyrin-Fullerene Constructs
J. Phys. Chem. B, Vol. 110, No. 50, 2006 25247
ethyl acetate (60:40 v/v) as eluent. Evaporation of the solvent
yielded pale-yellow solid. Yield: 2 g (30%).
4-Ferrocenylaniline (5).25 A mixture of granulated tin (170
mg, 1.43 mmol) was added to a solution of 4 (162 mg, 2.28
mmol) in 1:1 HCl:ethanol and refluxed for 2 h. The resulting
solution was then cooled to room temperature and neutralized
with 40% aqueous NaOH. The compound was washed with
dichloromethane and water and the organic layer was collected
and dried over anhydrous Na2SO4. Evaporation of the organic
layer yielded an orange oil which, upon refrigeration, gave the
desired compound as orange crystals. Yield: 112 mg (78%).
1H NMR (CHCl3-d): δ (in ppm): 8.05-7.90 (m, 4H, pyrrole
NH and phenyl H), 7.28 (d, 2H, phenyl H), 6.75-6.65 (m, 2H,
pyrrole H), 6.18-6.12 (m, 2H, pyrrole H), 5.92-5.87 (m, 2H,
pyrrole H), 5.54 (s, 1H, -CH-), 3.90 (s, 3H, -COOCH3).
H2-5,15-Di(p-methoxycarbonylphenyl)-10,20- -di(p-tolyl)-
porphyrin (2). This compound is prepared according to Luo
et al.24 A solution of 1 (1.6 g, 5.71 mmol) was added to
p-tolualdehyde (675 µL, 5.71 mmol) in CHCl3, and the reaction
mixture was stirred under argon for 1 h. Then BF3‚O(Et)2 (724
µL, 5.71 mmol) was added, and the reaction mixture was stirred
in the dark for 2 h. p-Chloranil (2.11 g, 8.57 mmol) was added
to the reddish-black reaction mixture, and the resultant mixture
was stirred for 18 h. Triethylamine (2.4 mL, 17.25 mmol) was
added, and the reaction mixture was stirred for 1 h and then
concentrated. Column chromatography on silica gel using
chloroform as eluent gave a mixture of five porphyrins including
the titled compound. Subsequent column chromatography of this
fraction on silica gel using toluene:CHCl3 (70:30 v/v) as eluent
yielded the titled compound as the third band. Evaporation of
the solvent yielded the desired compound as a purple solid.
Yield: 208 mg (5%).
1
ESI mass (in CH2Cl2): calcd., 277.84; found, 277.18. H
NMR (DMSO-d): δ (in ppm): 7.20 (d, 2H, phenyl H), 6.52
(d, 2H, phenyl H), 5.02 (s, br, 2H, -NH2), 4.58 (t, 2H, C5H4),
4.22 (t, 2H, C5H4), 3.98 (s, 5H, C5 H5).
H2-5-{4-[2-(4-benzo-[18-crown-6])amido]phenyl}-15-{4-[2-
(4-ferrocenylphenyl) amido]phenyl]-10,20-di(p-tolyl)porphy-
rin (6, Compound 2a in Scheme 1). 3 (78 mg, 0.11 mmol)
was taken in dry toluene (20 mL), thionyl chloride (160.5 µL,
2.2 mmol) and pyridine (160 µL) were added, and the reaction
mixture was refluxed under argon for 3 h. After cooling, the
solvent was evaporated and the resultant green compound was
redissolved in dry toluene (20 mL). Then, pyridine (516 µL)
was added followed by 5 (30.5 mg, 0.11 mmol) and 4′-amino
benzo-[18-crown-6] (36 mg, 0.11 mmol). The reaction mixture
was allowed to stir at room temperature under argon for 12 h.
The solvent was evaporated and the crude compound was
purified by column chromatography on basic alumina using
hexanes:CHCl3 (20:80 v/v). Evaporation of the second fraction
yielded the desired compound as a purple solid. Yield: 44 mg
(30%).
ESI mass (in CH2Cl2): calcd., 1299.32; found, [M+] 1299.7
(100%). 1H NMR (CDCl3-d): δ (in ppm): 8.92-8.70 (m, 8H,
â-pyrrole H), 8.35-8.10 (m, 8H, phenyl H), 8.05 (d, 4H, phenyl
H), 7.70 (d, 2H, phenyl H), 7.60-7.45 (m, 6H, phenyl H), 7.28-
7.25 (m, 1H, benzocrown phenyl H), 7.10-7.05 (m, 1H,
benzocrown phenyl H), 6.88 (d, 1H, benzocrown phenyl H),
4.65 (t, 2H, C5H4), 4.32 (t, 2H, C5H4), 4.28-4.22 (m, 2H,
crownethylene H), 4.20-4.12 (m, 2H, crownethylene H), 4.05
(s, 5H, C5H5), 3.98-3.90 (m, 4H, crownethylene H), 3.80-
3.65 (m, 12H, crownethylene H), 2.65 (s, 6H, -CH3), -2.78
(s, br, 2H, -NH).
H2-5-(p-methoxycarbonylphenyl)-10,15,20- tri(p-tolyl)por-
phyrin (7). Methyl 4-formylbenzoate (6 g, 0.037 mol) was
dissolved in propionic acid (1000 mL) and pyrrole (10.3 mL,
9.92 g, 0.15 mol) was added followed by p-tolualdehyde (13
mL, 0.111 mol). The mixture was refluxed for 3 h. The
propionic acid was removed under reduced pressure and the
compound was purified by column chromatography on basic
alumina using hexanes:CHCl3 (70:30 v/v) as eluent. Evaporation
of the solvent yielded the desired compound as a purple solid.
Yield: 1.7 g (6%).
1H NMR (CHCl3-d): δ (in ppm): 8.92-8.72 (m, 8H,
â-pyrrole H), 8.45 (d, 2H, phenyl H), 8.30 (d, 2H, phenyl H),
8.10 (d, 6H, phenyl H), 7.55 (d, 6H, phenyl H), 4.12 (s, 3H,
-COOCH3), 2.7 (s, 9H, -CH3), -2.78 (s, br, 2H, -NH).
1H NMR (CHCl3-d): δ (in ppm): 8.82 (dd, 8H, â-pyrrole
H), 8.42 (d, 4H, phenyl H), 8.25 (d, 4H, phenyl H), 8.10 (d,
4H, phenyl H), 7.58 (d, 4H, phenyl H), 4.12 (s, 6H, -COOCH3),
2.7 (s, 6H, -CH3), -2.80 (s, br, 2H, -NH).
H2-5,15-Di(p-carboxyphenyl)-10,20-di(p-tolyl)porphyrin
(3):12b A suspension of 2 (200 mg, 0.264 mmol) in THF:ethanol
(1:1) was taken in a 250 mL round-bottom flask and potassium
hydroxide (1.5 g, 0.027 mol) dissolved in water (12 mL) was
added to it. The reaction mixture was refluxed for 8 h. After
cooling, the solvent was evaporated, the residue was diluted
with water (100 mL) and then filtered. The desired compound
was obtained as the dipotassium salt. The aqueous suspension
of porphyrin dipotassium salt was acidified with concentrated
HCl (pH 2) and then filtered. The titled compound was obtained
as a purple powder and used without any further purifications.
Yield: 85 mg (44%).
1H NMR (DMSO-d): δ (in ppm): 8.8 (dd, 8H, â-pyrrole
H), 8.38 (d, 8H, phenyl H), 8.18-8.10 (d, 4H, phenyl H), 7.65-
7.55 (d, 4H, phenyl H), 2.55 (s, 6H, -CH3), -2.40-2.95 (s,
br, 2H, -NH).
4-Ferrocenylnitrobenzene (4).25 To ferrocene (3.80 g, 20.4
mmol), 25 mL of sulfuric acid was added, and the resulting
deep-blue solution was stirred at room temperature for 2 h. The
solution was then poured into ice-cold water (100 mL) and
allowed to reach room temperature. A solution of sodium nitrite
(0.91 g, 13.2 mmol) in 5 mL water at 0 °C was added dropwise
to a stirred solution of 4-nitroaniline (1.66 g, 12.0 mmol) in
1:1 water/HCl (10 mL) kept at 0 °C by an ice/water bath. The
above mixture was stirred for 30 min to ensure full diazotization.
Copper powder (1.0 g) was added to the ferrocenium solution,
and the diazonium solution was added dropwise with vigorous
solution. After 24 h of stirring at room temperature, ascorbic
acid (5 g) was added to reduce any remaining ferrocenium to
ferrocene. Dichloromethane was added, and the organic layer
was separated. The aqueous layer was extracted further with
CH2Cl2. The crude compound purified on silica gel by using
CH2Cl2/hexanes (40:60 v/v). Yield: 925 mg (25%).
H2-5-(p-carboxyphenyl)-10,15,20-tri(p-tolyl)porphyrin (8):
A mixture of 7 (840 mg, 1.18 mmol) in THF (60 mL) and
potassium hydroxide (4.62 g, 0.082 mol) in water (20 mL) was
refluxed for 20 h. After cooling, the reaction mixture was diluted
with CH2Cl2, acidified with concentrated HCl, and extracted.
The organic layer was washed with saturated NaHCO3 solution
and dried over anhydrous Na2SO4. The solvent was evaporated,
and the compound was purified by column chromatography on
silicagel using CHCl3:methanol (95:5 v/v) as eluent. Yield: 700
mg (85%).
1
ESI mass (in CH2Cl2): calcd., 307.58; found, 307.12. H
NMR (CHCl3-d): δ (in ppm): 8.14 (d, 2H, phenyl H), 7.56 (d,
2H, phenyl H), 4.74 (t, 2H, C5H4), 4.48 (t, 2H, C5H4), 4.05 (s,
5H, C5H5).

25248 J. Phys. Chem. B, Vol. 110, No. 50, 2006
D’Souza et al.
1H NMR (CHCl3-d): δ (in ppm): 8.88-8.72 (m, 8H,
â-pyrrole H), 8.60-8.48 (m, 2H, phenyl H), 8.38-8.35 (d, 2H,
phenyl H), 8.10 (d, 6H, phenyl H), 7.58 (d, 6H, phenyl H),
2.72 (s, 9H, -CH3), -2.78 (s, br, 2H, -NH).
ESI mass (in CH2Cl2): calcd., 1321.61; found, [M+] 1321.3
(100%). 1H NMR (CDCl3-d): δ (in ppm): 8.95-8.58 (m, 8H,
â-pyrrole H), 8.32-7.80 (m, 12H, phenyl H), 7.60-7.40 (m,
8H, phenyl H), 7.10-6.75 (s, br, 2H, benzocrown phenyl H),
6.38-6.22 (m, 1H, benzocrown phenyl H), 4.65-4.55 (t, 2H,
C5H4), 4.30-4.22 (t, 2H, C5H4), 4.02 (s, 5H, C5H5), 3.65-3.60
(m, 2H, crownethylene H), 2.85-2.80 (m, br, 2H, crownethylene
H), 2.78-2.62 (s, 6H, -CH3), 2.38-2.30 (m, 4H, crownethylene
H), 2.25-1.78 (m, 8H, crownethylene H), 1.68-1.52 (m, 4H,
crownethylene H).
5-{4-[2-(4-benzo-[18-crown-6])amido]phenyl}-10,15,20-tri-
(p-tolyl)porphyrinato zinc (II) (12, Compound 1b in Scheme
1). This compound was prepared from porphyrin 9 (10 mg, 0.01
mmol) according to the procedure outlined above for 10.
Chromatography on basic alumina column by using hexanes:
CHCl3 (40:60 v/v) gave the title compound. Yield: 10 mg
(90%).
H2-5-{4-[2-(4-benzo-[18-crown-6])amido]phenyl}-10,15,-
20-tri(p-tolyl)porphyrin (9, compound 1a in Scheme 1). A
solution of 8 (500 mg, 0.71 mmol), SOCl2 (521 µL, 7.15 mmol)
and pyridine (1 mL) in dry toluene (50 mL) was refluxed under
argon for 3 h. After cooling, the solvent was evaporated and
the resultant green mixture was redissolved in dry toluene (40
mL). Then, pyridine (1 mL) was added followed by 4′-amino
benzo-[18-crown-6] (470 mg, 1.43 mmol). The reaction mixture
was allowed to stir at room temperature under argon for 12 h.
The solvent was evaporated, and the crude compound was
purified by column chromatography on silica gel using hexanes:
CHCl3 (40:60 v/v). Evaporation of the solvent yielded the
desired compound as a purple solid. Yield: 500 mg (70%).
ESI mass (in CH2Cl2): calcd., 1073.58; found, [M+] 1073.4
(100%). 1H NMR (CDCl3-d): δ (in ppm): 8.95-8.62 (m, 8H,
â-pyrrole H), 8.42-8.32 (m, 1H, phenyl H), 8.05-7.82 (m, 7H,
phenyl H), 7.80-7.65 (m, 2H, phenyl H), 7.58-7.35 (m, 6H,
phenyl H), 6.95 (s, br, 1H, benzocrownphenyl H), 6.82-6.75
(s, br, 1H, benzocrown phenyl H), 6.30-6.22 (m, 1H, ben-
zocrown phenyl H), 3.20-3.05 (m, 2H, crownethylene H),
2.80-2.72 (m, 2H, crownethylene H), 2.65 (s, 9H, -CH3),
2.40-2.02 (m, 9H, crownethylene H), 1.95-1.75 (m, 6H,
crownethylene H), 0.85 (s, br, 1H, crownethylene H).
5-{4-[2-(4-benzo-[18-crown-6])amido]phenyl}-10,15,20-tri-
(p-tolyl)porphyrinato magnesium (II) (13, Compound 1c in
Scheme 1). This compound was prepared from porphyrin 9 (10
mg, 0.001 mmol) according to the procedure outlined above
for 11. Chromatography on basic alumina column with CHCl3
as eluent yielded residual 9. Elution with CHCl3:methanol (98:2
v/v) yielded a greenish-purple fraction of the desired compound.
Yield: 8 mg (80%).
ESI mass (in CH2Cl2): calcd., 1032.49; found, [M+] 1032.5
(100%). 1H NMR (CDCl3-d): δ (in ppm): 8.92-8.58 (m, 8H,
â-pyrrole H), 8.35-8.22 (m, 1H, phenyl H), 8.12-7.90 (m, 7H,
phenyl H), 7.70-7.42 (m, 8H, phenyl H), 7.05 (s, br, 1H,
benzocrownphenyl H), 6.82 (s, br, 1H, benzocrown phenyl H),
6.25 (m, 1H, benzocrown phenyl H), 2.85-2.75 (m, 2H,
crownethylene H), 2.65 (s, 9H, -CH3), 2.60-2.50 (m, 2H,
crownethylene H), 2.25-2.05 (m, 2H, crownethylene H), 1.98-
1.62 (m, 8H, crownethylene H), 1.58-1.30 (m, 4H, crowneth-
ylene H), 0.10 (s, br, 2H, crownethylene H).
Synthesis of Fullerene Derivatives: N-Boc-(2-amino)-
ethylamine (14). This compound was prepared according to
Muller et al.27a Di-tert-butyl bicarbonate (6.53 g, 0.03 mol)
dissolved in 100 mL of dry CHCl3 was added to a solution of
ethylenediamine (20 mL, 0.3 mol) in 300 mL of CHCl3 during
2.5 h with stirring and cooling in an ice bath. The reaction
mixture was stirred for additional 24 h at room temperature and
was then washed with water, extracted with CHCl3, and dried
over anhydrous Na2SO4. Evaporation of the organic layer
yielded the desired compound as a pale-yellow oil. Yield: 5.2
g (11%).
1H NMR (CHCl3-d): δ (in ppm): 5.28 (s, br, 1H, -NH),
3.25-3.08 (m, 2H, CH2), 2.8 (t, 2H, CH2), 1.65-1.40 (m, 9H,
Boc-H).
Benzyl-N-[2-(N-Boc)aminoethyl]glycinate (15). This com-
pound was prepared according to Kordatos et al.27b A solution
of benzyl 2-bromo acetate (1.53 g, 1.1 mL, 6.67 mmol) dissolved
in 30 mL of 1,4-dioxane was added to a solution of 14 (3.2 g,
0.02 mol) in 20 mL of 1,4-dioxane at 0 °C over a period of 1
ESI mass (in CH2Cl2): calcd., 1010.2; found, [M+] 1010.5
(100%). 1H NMR (CDCl3-d): δ (in ppm): 8.90-8.70 (m, 8H,
â-pyrrole H), 8.22 (m, 2H, phenyl H), 8.18-8.10 (d, 2H, phenyl
H), 8.02 (d, 6H, phenyl H), 7.54 (s, 1H, benzocrown phenyl
H), 6.98 (d, 1H, benzocrown phenyl H), 6.75 (d, 1H, ben-
zocrown phenyl H), 4.18-4.12 (m, 2H, crownethylene H),
4.06-4.02 (m, 2H, crownethylene H), 3.88-3.76 (m, 4H,
crownethylene H), 3.75-3.55 (m, 12H, crownethylene H), 2.60
(s, 9H, -CH3), -2.78 (s, br, 2H, -NH).
5-{4-[2-(4-benzo-[18-crown-6])amido]phenyl}-15-{4-[2-(4-
ferrocenylphenyl)amido]-phenyl]-10,20-di(p-tolyl)porphyri-
nato zinc (II) (10, Compound 2b in Scheme 1). The free-
base porphyrin 6 (10 mg, 0.008 mmol) was dissolved in CHCl3
(10 mL), a saturated solution of zinc acetate in methanol was
added to the solution, and the resulting mixture was refluxed
for 2 h. The course of the reaction was monitored by absorption
spectroscopy. At the end, the reaction mixture was washed with
water and dried over anhydrous Na2SO4. Chromatography on
basic alumina column by using hexanes:CHCl3 (20:80 v/v) gave
the title compound. Yield: 10 mg (90%).
ESI mass (in CH2Cl2): calcd., 1362.69; found, [M+] 1362.3
(100%). 1H NMR (CDCl3-d): δ (in ppm): 8.98-8.82 (m, 8H,
â-pyrrole H), 8.38-8.15 (m, 8H, phenyl H), 8.10 (m, 4H, phenyl
H), 7.72 (d, 2H, phenyl H), 7.60-7.48 (m, 6H, phenyl H), 7.22-
7.16 (m, 1H, benzocrownphenyl H), 7.12-7.05 (m, 1H,
benzocrownphenyl H), 6.80 (d, 1H, benzocrownphenyl H), 4.68
(t, 2H, C5H4), 4.32 (t, 2H, C5H4), 4.08 (s, 5H, C5H5), 4.0-3.86
(m, 2H, crownethylene H), 3.85-3.72 (m, 2H, crownethylene
H), 3.70-3.52 (m, 2H, crownethylene H), 3.48-3.08 (m, 14H,
crownethylene H), 2.72 (s, 6H, -CH3).
5-{4-[2-(4-benzo-[18-crown-6])amido]phenyl}-15-{4-[2-(4-
ferrocenylphenyl)amido]-phenyl]-10,20-di(p-tolyl)porphyri-
nato magnesium (II) (11, Compound 2c in Scheme 1). This
was prepared according to a general procedure developed by
Lindsey and Woodford26 for Mg porphyrin synthesis. A sample
of 6 (10 mg, 0.008 mmol) was dissolved in 10 mL of CH2Cl2.
Then triethylamine (0.022 mL, 15.6 mg, 0.15 mmol) was added
followed by MgBr2‚O(Et)2 (20 mg, 0.08 mmol). The mixture
was stirred for 20 min at room temperature. The course of the
reaction was monitored by absorption spectroscopy. The mixture
was diluted with 20 mL of CH2Cl2, washed with 5% NaHCO3,
and dried over anhydrous Na2SO4, and the filtrate was evapo-
rated. Chromatography on basic alumina column with CHCl3
as eluent yielded residual 6. Elution with CHCl3:methanol (98:2
v/v) yielded a greenish-purple fraction of the desired compound.
Yield: 8 mg (80%).

Ferrocene-Porphyrin-Fullerene Constructs
J. Phys. Chem. B, Vol. 110, No. 50, 2006 25249
h, and the reaction mixture was stirred overnight. The solvent
was evaporated under reduced pressure, and the residue was
dissolved in water and extracted with ethyl acetate. The organic
extract was dried over anhydrous Na2SO4 and the solvent was
evaporated under reduced pressure. The crude compound was
purified by column chromatography on silicagel using petroleum
ether:ethyl acetate (40:60 to 20:80 v/v). Evaporation of the
solvent yielded the desired compound as a pale-yellow oil.
Yield: 1.9 g (73%).
1H NMR (CHCl3-d): δ (in ppm): 7.45-7.40 (m, 5H, phenyl
H), 5.18 (s, 2H, CH2), 5.04 (s, br, 1H, NH), 3.45 (s, 2H, benzyl
CH2), 3.25-3.15 (m, 2H, CH2), 2.74 (t, 2H, CH2), 1.65 (s, br,
1H, NH), 1.45 (s, 9H, Boc-H).
spectrophotometer. The steady-state fluorescence emission was
monitored by using a Spex Fluorolog-tau spectrometer. A right
angle detection method was used. The H NMR studies were
1
carried out either on a Varian 400 MHz or Varian 300 MHz
spectrometers. Tetramethylsilane (TMS) was used as an internal
standard. Cyclic voltammograms were recorded on a EG&G
Model 263A potentiostat using a three electrode system in
benzonitrile containing 0.1 M n-Bu4NClO4 as the supporting
electrolyte. A platinum button or glassy carbon electrode was
used as the working electrode. A platinum wire served as the
counter electrode and a Ag/AgCl was used as the reference
electrode. Ferrocene/ferrocenium (Fc/Fc+) redox couple was
used as an internal standard. All the solutions were purged prior
to spectral measurements using argon gas.
The computational calculations were performed by the DFT
B3LYP/3-21G(*) method with the GAUSSIAN-03 software
package28 on various PCs. The ESI-Mass spectral analyses of
the self-assembled complexes were performed by using a
Fennigan LCQ-Deca mass spectrometer. The starting com-
pounds and the triads (about 0.1 mM concentration) were
prepared in CH2Cl2, freshly distilled over calcium hydride.
Time-Resolved Emission and Transient Absorption Mea-
surements. The picosecond time-resolved fluorescence spectra
were measured using an argon-ion pumped Ti:sapphire laser
(Tsunami; pulse width ) 2 ps) and a streak scope (Hamamatsu
Photonics; response time ) 10 ps). The details of the experi-
mental setup are described elsewhere.29 Nanosecond transient
absorption measurements were carried out using the SHG (532
nm) of an Nd:YAG laser (Spectra Physics, Quanta-Ray GCR-
130, fwhm 6 ns) as excitation source. For the transient
absorption spectra in the near-IR region (600-1600 nm), the
monitoring light from a pulsed Xe lamp was detected with a
Ge-avalanche photodiode (Hamamatsu Photonics, B2834).29
N-[2-(N-Boc)aminoethyl]glycine (16). To a methanolic solu-
tion of 15 (1.55 g, 5 mmol) was added 100 mg of % Pd/C, and
the mixture was stirred under a hydrogen atmosphere for 24 h.
The catalyst was removed by filtration on Celite, and the solvent
was evaporated. The residue was triturated in diethyl ether to
give the desired compound as a white solid. Yield: 500 mg
(46%).
1H NMR (D2O): δ (in ppm): 3.52 (s, 2H, CH2), 3.45 (t, 2H,
CH2), 3.18 (t, 2H, CH2), 1.45 (s, 9H, Boc-H).
N-[2-(N-Boc)aminoethyl]-2-phenyl-3,4-fulleropyrrolidine
(17). To a 200 mg (0.28 mmol) of C60 in toluene (200 mL),
122 mg (0.56 mmol) of 16 and benzaldehyde (140.4 µL) were
added and the mixture was refluxed for 2 h. After cooling to
room temperature, the solvent was evaporated under reduced
pressure. The crude compound was purified on silica gel by
flash column chromatography using toluene:ethyl acetate (98:2
v/v) as eluent. Evaporation of the solvent yielded the desired
compound as a brown solid. Yield: 123 mg (45%).
1H NMR [CDCl3:CS2 1:1 (v/v)]: δ (in ppm): 7.84-7.74 (m,
2H, phenyl H), 7.46-7.32 (m, 3H, phenyl H), 5.22-5.05 (m,
3H, NH & CH2), 4.2 (d, 1H, pyrrolidine H), 3.78-3.58 (m,
2H, pyrrolidine H), 3.45-3.30 (m, 1H, CH2), 2.78-2.65 (m,
1H, CH2), 1.52 (s, 9H, Boc-H).
To a dichloromethane solution of 17 (123 mg, 0.13 mmol),
3 mL of trifluoroacetic acid was added, and the mixture was
stirred for 3 h at room temperature. The solvent and excess acid
was removed under reduced pressure. The compound was
washed with CH2Cl2 in order to remove the unreacted starting
material. Evaporation of CH2Cl2 followed by drying yielded
the compound 18 (compound 3 in Scheme 1) as a brown solid.
Yield: 85 mg (68%).
Acknowledgment. This work is supported by the National
Science Foundation (Grant 0453464 to F.D.), the donors of the
Petroleum Research Fund administered by the American
Chemical Society, and Grants-in-Aid for Scientific Research
on Primary Area (417) from the Ministry of Education, Science,
Sport and Culture of Japan.
Supporting Information Available: Diagram of hole
transfer/electron transfer. This material is available free of charge
via the Internet at http://pubs.acs.org.
ESI mass (in CH2Cl2+CH3OH)): calcd., 883.26 (without
CF3COO-); found, [M+] 883.2 (100%).
References and Notes
N-methyl 2-(4-(2-(N-Boc)-amino) ethoxy phenyl) 3,4-
fulleropyrrolidine, 19 (Compound 5 in Scheme 1). To a 100
mg (0.138 mmol) of C60 in toluene, 109 mg (0.414 mmol) of
4-(2-Boc- amino) ethoxy benzaldehyde27c and 31 mg (0.345
mmol) of sarcosine was added and refluxed for 4 h. The solvent
was evaporated under vacuum, and purified on silica gel using
toluene and ethyl acetate [7:3]. Yield 50%. Next, to dichlo-
romethane solution of N-Boc, protected 19 (70 mg, in 5 mL),
trifluroacetic acid (3 mL), and m-cresol (50 µL) were added
and stirred for 3 h. The solvent and acid were removed under
vacuum and solid product was washed with toluene for several
times to get desired compound 19 (Yield 95%).
1H NMR (CD3OD): δ 7.71, 6.95 (d, d, 4H, phenyl), 4.98,
4.82, 4.22 (d, s, d, 3H, pyrolidine-H), 4.0, 3.51 (d, m,
-(CH2)2-), 2.78 (N-CH3), ESI mass in CH2Cl2 calcd: 913;
found: 913.3. UV-vis (MeOH, nm) 204, 254.5 nm.
Instrumentation. The UV-visible spectral measurements
were carried out with a Shimadzu Model 1600 UV-visible
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