Supramolecular triads formed by axial coordination of fullerene to covalently linked zinc porphyrin-ferrocene(s): Design, syntheses, electrochemistry, and photochemistry

Article abstract of DOI:10.1021/jp0485688

Supramolecular triads have been constructed by using covalently linked zinc porphyrin-ferrocene(s) dyads, self-assembled via axial coordination to either pyridine- or imidazole-appended fulleropyrrolidine. These triads were characterized by optical absorption, computational, and electrochemical methods. The calculated binding constants (K) revealed stable complexation and suggested the existence of intermolecular interactions between the ferrocene and fullerene entities. Accordingly, the optimized geometry obtained by ab initio B3LYP/3-21G(*) methods revealed closely spaced ferrocene and fullerene entities in the studied triads. Photoinduced charge-separation and charge-recombination processes were examined in the dyads and triads by means of time-resolved transient absorption and fluorescence lifetime measurements. In the case of zinc porphyrin-ferrocene(s) dyads, upon photoexcitation, efficient (??_CS = 0.98) to moderate (??_CS = 0.54) amounts of electron transfer from the ferrocene to the singlet excited zinc porphyrin occurred depending upon the nature of the spacer, resulting in the formation of the Fc⁺-ZnP^a?￠- radical pair. Upon formation of the supramolecular triads by axial coordination of fulleropyrrolidines, the initial electron transfer originated either from or to the singlet excited zinc porphyrin, resulting ultimately in the formation of the charge-separated states of Fc⁺-ZnP:C₆₀^a?￠- with high quantum efficiency. The calculated ratio of k_CS/k_CR. from the kinetic data was found to be a??100, indicating a moderate amount of charge stabilization in the studied supramolecular triads.

Full text of DOI:10.1021/jp0485688

J. Phys. Chem. B 2004, 108, 11333-11343
11333
Supramolecular Triads Formed by Axial Coordination of Fullerene to Covalently Linked
Zinc Porphyrin-Ferrocene(s): Design, Syntheses, Electrochemistry, and Photochemistry
Francis D’Souza,*,† Phillip M. Smith,† Suresh Gadde,† Amy L. McCarty,† Michael J. Kullman,†
Melvin E. Zandler,† Mitsunari Itou,‡ Yasuyaki 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: April 1, 2004; In Final Form: May 18, 2004
Supramolecular triads have been constructed by using covalently linked zinc porphyrin-ferrocene(s) dyads,
self-assembled via axial coordination to either pyridine- or imidazole-appended fulleropyrrolidine. These triads
were characterized by optical absorption, computational, and electrochemical methods. The calculated binding
constants (K) revealed stable complexation and suggested the existence of intermolecular interactions between
the ferrocene and fullerene entities. Accordingly, the optimized geometry obtained by ab initio B3LYP/3-
21G(*) methods revealed closely spaced ferrocene and fullerene entities in the studied triads. Photoinduced
charge-separation and charge-recombination processes were examined in the dyads and triads by means of
time-resolved transient absorption and fluorescence lifetime measurements. In the case of zinc porphyrin-
ferrocene(s) dyads, upon photoexcitation, efficient (ΦCS ) 0.98) to moderate (ΦCS ) 0.54) amounts of electron
transfer from the ferrocene to the singlet excited zinc porphyrin occurred depending upon the nature of the
spacer, resulting in the formation of the Fc+-ZnP•- radical pair. Upon formation of the supramolecular triads
by axial coordination of fulleropyrrolidines, the initial electron transfer originated either from or to the singlet
•-
excited zinc porphyrin, resulting ultimately in the formation of the charge-separated states of Fc+-ZnP:C60
with high quantum efficiency. The calculated ratio of kCS/kCR from the kinetic data was found to be ∼100,
indicating a moderate amount of charge stabilization in the studied supramolecular triads.
Introduction
effect of molecular topology and distance and orientation effects
of the donor and acceptor entities on the charge separation and
recombination processes.8
Studies on photoinduced electron transfer in molecular and
supramolecular donor-acceptor systems have undergone rapid
growth in recent years mainly to address the mechanistic details
of electron transfer in chemistry and biology, to develop artificial
photosynthetic systems for light energy harvesting,1-9 and also
to develop molecular optoelectronic devices.10 In the construc-
tion of such dyads, fullerenes11 and porphyrins12 have been
utilized as constituents owing to their rich and well-understood
redox, optical, and photochemical properties.9,13-14 In contrast
with the traditionally used two-dimensional aromatic electron
acceptors, fullerenes (C60 and C70) in donor-acceptor dyads
accelerate forward electron transfer (kCS) and slow backward
electron transfer (kCR), undoubtedly due to their small reorga-
nization energy in electron-transfer reactions.15 That is, they
form the much desired long-lived charge-separated states. The
small reorganization energy of fullerenes predicts that the kCS
lies upward along the normal region into the top region of the
Marcus curve, and the kCR to lie significantly downward into
the inverted region. The small reorganization energy is due to
the fullerene’s unique structure and symmetry, which are
ultimately responsible for its high degree of delocalization and
structural rigidity. Consequently, a number of metallotetrapy-
rrole-fullerene and luminescent transition-metal complex-
fullerene dyads have been synthesized and studied to probe the
More recently, elegantly designed porphyrin and fullerene
bearing molecular and supramolecular triads, tetrads, pentads
etc., have also been synthesized and studied.8,14,16-18 In some
of these supramolecular systems, distinctly separated donor-
acceptor radical ion pairs, in succession, are generated upon
initial electron transfer by charge migration reactions along the
well-tuned redox gradients. Photosynthetic reaction centers,
composed of self-assembled donor and acceptor entities, produce
long-lived highly energetic charge-separated states with quantum
yields close to unity using this mechanism of charge migration.19
Hence, there is a great need for developing such well-organized
molecular/supramolecular systems for harvesting solar energy
by means of fast forward electron transfer and slow backward
electron transfer and their subsequent use in building molecular
electronic devices.
Building structurally well-defined, self-assembled supramo-
lecular triads or tetrads bearing three or more photo- or redox-
active entities is tedious because of the occurrence of multiple
equilibrium reactions and the lower stability in solution of the
many utilized self-assembly approaches.8,9 Because of this, the
most successful models to achieve charge stabilization studied
to date have been the covalently linked ones.1-9,16,18 However,
it is possible to choose a covalently linked dyad or triad and
allow it to self-assemble with another donor or acceptor entity
with a well-defined binding mechanism. Alternatively, multiple
modes of binding could also be used to form structurally well-
defined, stable supramolecular systems.8,9 In the present study,
* To whom correspondence may be addressed. Tel.: +81 22 217 5608.
Fax: +81 22 217 5608. E-mail: ito@tagen.tohoku.ac.jp.
† Wichita State University.
‡ Tohoku University.
10.1021/jp0485688 CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/02/2004

11334 J. Phys. Chem. B, Vol. 108, No. 31, 2004
D’Souza et al.
Figure 1. Structure of the donors (1-4) and acceptors (5 and 6) utilized to form supramolecular triads via axial coordination of zinc porphyrin in
the present study.
we have employed the former approach of utilizing covalently
linked zinc porphyrin-ferrocene(s) dyads to form supramo-
lecular triads by the axial coordination of pyridine or imidazole
functionalized fulleropyrrolidines (Figure 1).
UV-Visible Spectral Studies. The optical absorption spectra
in the visible wavelength region of the zinc porphyrin-
ferrocene(s) dyads were similar to 1, but the peak maxima
revealed a red shift of 2-3 nm. The calculated full width at
half maximum values also revealed slight changes, suggesting
intramolecular interactions between porphyrin and the appended
ferrocene entities. No new peaks corresponding to appended
ferrocene were observed due to the low molar absorptivity, ꢀ,
of ferrocene compared to the high ꢀ values of porphyrin
absorption bands.20
As demonstrated in the present study, upon excitation of the
donor zinc porphyrin, efficient forward electron transfer to the
axially bound C60 occurs followed by a hole transfer from the
•-
ZnP•+ to the ferrocene entity to generate the Fc+-ZnP-C60
charge-separated species. As a consequence of the sequential
electron-transfer events, the charge-recombination reaction is
slowed to some extent, even though the ferrocene and fullerene
entities of the supramolecular triads exhibit some through-space
interactions.
The binding of C60 derivatives, 5 and 6, to porphyrins 1-4
resulting in the formation of the pentacoordinated zinc porphyrin
by axial coordination was monitored by optical absorption
methods. The formation of pentacoordinated complexes was
characterized by red-shifted Soret and visible bands and the
appearance of isosbestic points21 as shown for porphyrin, 4,
binding to fullerene, 5, in o-dichlorobenzene in Figure 2. Job’s
continuous variation plot also confirmed 1:1 complex formation
between the zinc porphyrin-ferrocene(s) and C60-bearing axial-
coordinating ligands. Extending the absorption wavelength to
1000 nm revealed the absence of any new charge-transfer bands.
The association constants, K, for axial coordination were
calculated from the absorption spectral data using the Scatchard
method22 (Figure 2 inset) and are listed in Table 1. For
comparison purposes, the K values for pyridine binding to the
zinc porphyrins are also listed.
Results and Discussion
In the present study, we have utilized three different zinc
porphyrin-ferrocene(s) dyads, 2-4 to self-assemble with either
pyridine or imidazole-appended fulleropyrrolidine, 5-6. The
reference compound 1, zinc tetraphenylporphyrin, is used for
control experiments. In compound 2, the zinc porphyrin and
ferrocene entities are directly linked via one of the phenyl rings
of zinc tetraphenylporphyrin, while in 3, an additional phenyl
amide spacer to increase the distance between the two entities
is introduced. Compound 4 with two directly linked ferrocene
entities is utilized to visualize the effect of a second ferrocence
entity on the spectral and photochemical properties. The syn-
thetic details are given in the Experimental Section. The struc-
1
It is clear from the data in Table 1 that (i) the K values are
2-3 times higher for 5 or 6 binding to zinc porphyrin-
ferrocene(s) dyads compared to pristine zinc porphyrin, 1, (ii)
the K values are 2-3 times higher for 5 or 6 binding to zinc
tural integrity of all of the compounds was deduced from H
NMR, electrospray ionization (ESI) mass spectrometry in
CH2Cl2 matrix, optical absorption and emission, and electro-
chemical methods.

Covalently Linked Zinc Porphyrin-Ferrocene(s)
J. Phys. Chem. B, Vol. 108, No. 31, 2004 11335
Figure 2. UV-vis spectral changes observed during the complexation of 4 (2.36 µM) and 5 (1.99 µM each addition) in o-dichlorobenzene. The
inset figure shows the Scatchard plot of the data analysis monitored at 425 nm.
TABLE 1: Formation Constants for Axial Coordination of
Fullerene Ligands to Zinc Porphyrin-Ferrocene(s) Dyads in
o-Dichlorobenzene at 298 K
K, M-1,a
compound pyridine pyridine-C60, 5 imidazole-C60,
6
ref
21d
this work
this work
this work
1
2
3
4
7.8 × 103
7.7 × 103
5.3 × 103
5.8 × 103
7.2 × 103
26.8 × 103
20.5 × 103
23.4 × 103
11.6 × 103
29.6 × 103
22.6 × 103
24.6 × 103
a Error ) +10%.
porphyrin-ferrocene(s) dyads compared to pyridine binding to
the dyads, (iii) the K values for imidazole-C60, 6, binding are
larger compared to pyridine-C60, 5, and (iv) increasing distance
between the porphyrin and ferrocene entities, or increasing the
number of ferrocene entities, decreased the binding constants
to some extent. The third observation has earlier been attributed
to the higher basicity of the imidazole ligand compared to the
pyridine ligand. The increased binding ability of zinc porphy-
rin-ferrocene(s) dyads, 2-4 to C60, 5, or 6 and not to pyridine
could be due to (i) a change in the basicity of zinc porphyrin as
a result of appended ferrocene entities or (ii) existence of
intermolecular interactions between the ferrocene and C60
entities upon axial coordination. The latter effect is more
conceivable since pyridine binding to 2-4 did not show such
an increased binding ability, and also, by increasing the
ferrocene entities in 4, a decrease in the K value was observed.
In this regard, the existence of intermolecularly interacting
ferrocene-C60 complexes is well known in the literature.23 To
visualize the existence of intermolecular ferrocene-C60 inter-
actions in the self-assembled triads, computational and electro-
chemical studies were performed and the results are discussed
in the forthcoming sections.
Figure 3. Ab initio B3LYP/3-21G(*)-optimized geometry of the
supramolecular triad formed by 2 and 6.
structure of molecular and self-assembled supramolecular dyads
and triads.25 For this, both the zinc porphyrin-ferrocene(s)
dyads and functionalized fullerenes were optimized to a
stationary point on the Born-Oppenheimer potential-energy
surface and allowed to self-assemble via axial coordination. The
structure of one of the fully optimized triads involving com-
pounds 2 and 6 is shown in Figure 3. It may be mentioned here
that the triads were very flexible and multiple minima were
possible. Under these conditions, the optimization process
required much experience and considerable computational time
to confidently achieve an optimized structure at the B3LYP/3-
21G(*) level.
The geometric parameters that are helpful to understand the
spectral and photochemical properties (kinetics of charge-
separation and charge-recombination), obtained from the B3LYP/
3-21G(*) optimized structures, are listed in Table 2. The center-
to-center distances between the ZnP and coordinated C60 entities
were found to range between 10 and 13 Å, while the edge-to-
edge distance varied between 4.7 and 9.6 Å, depending upon
the nature of the fulleropyrrolidine. The metal-metal distances
between Zn and Fe were found to be ∼10.7 Å for the directly
linked zinc porphyrin-ferrocene(s) dyads and ∼16.9 Å for the
dyad linked with an additional phenyl amide spacer. Importantly,
Ab initio B3LYP/3-21G(*) Modeling of the Triads. To gain
insights into the supramolecular geometry and possible existence
of intermolecular-type interactions, computational studies were
performed using density functional methods (DFT) at the
B3LYP/3-21G(*) level.24 Earlier, the B3LYP/3-21G(*) methods
were successfully used to predict the geometry and electronic

11336 J. Phys. Chem. B, Vol. 108, No. 31, 2004
D’Souza et al.
TABLE 2: B3LYP/3-21G(*)-Optimized Distances between
the Different Entities of the Investigated Supramolecular
Triads
of these entities, efficient charge recombination of the ferrocene
cation and C60 anion radical during photochemical electron
transfer is expected to occur.
center-to-center
distance, Å
edge-to-ede
distance, Å
Electrochemical Studies and Electron-Transfer Driving
Forces. Electrochemical studies using the cyclic voltammetric
technique were performed to visualize the intermolecular
interactions and also to evaluate the energetics of electron
transfer reactions. Figure 4 shows representative cyclic volta-
mmograms of the newly synthesized zinc porphyrin-ferrocene-
(s) dyads, while Table 3 lists their redox potential values. The
zinc porphyrin-ferrocene(s) dyads revealed redox peaks cor-
responding to the zinc porphyrin and the ferrocene entities. On
the basis of the peak-to-peak separation, ∆Epp values, and the
cathodic-to-anodic peak current ratio, all of the redox processes
of compounds 2-4 were found to be electrochemically and
chemically reversible.26 However, the peak current correspond-
ing to the oxidation of the ferrocene entity of compound 4 was
twice as much as that of the other redox waves corresponding
to the presence of two ferrocene entities. Presence of the
interacting ferrocene entity on the porphyrin π system caused
small shifts in the potential values.
supramolecular
complexa
c
e
e
ZnP-C60 ZnP-Fcd ZnP-C60 ZnP-Fce Fc-C60
1:5b
2:5
4:5
1:6b
2:6
3:6
4:6
10.4
9.9
10.3
12.3
12.1
13.0
13.0
4.7
6.4
6.8
6.9
9.6
9.6
9.6
10.7
10.8
4.3
4.3
3.7
2.8
10.4
16.9
10.5
4.3
4.2
4.3
2.6
3.8
2.8
a See Figure 1 for abbreviations. b From ref 21d. c Zinc center to the
center of fullerene sphere. d Zinc center to iron center. e Distance
between the closely located porphyrin π system carbon, ferrocene
cyclopentadienyl carbon, and/or fullerene sphere carbon (the carbon
atoms of spacer units were not considered in estimating the distances).
Next, the zinc porphyrin-ferrocene(s) dyads were titrated
with various amounts of 5 or 6 to probe the effect of axial
coordination on the redox potentials of both the ferrocene and
zinc porphyrin entities. Figure 5 shows cyclic voltammograms
obtained during the titration of 2 with various amounts of 6 in
o-dichlorobenzene, 0.1 M (TBA)ClO4. Cathodic shifts up to 60
mV corresponding to zinc porphyrin oxidations were observed;
however, no significant shift in the redox potentials correspond-
ing to the oxidation of ferrocene was observed. Similar cathodic
shifts were also observed when 2 was titrated with 5 under
similar solution conditions. The easier oxidation of zinc por-
phyrin upon axial imidazole coordination suggests that it is a
better electron donor in the triads. Additionally, the reduction
potentials corresponding to C60 reduction of coordinated 6 did
not reveal significant changes as compared to the unbound 6
(Figure 5 and Table 3). These results suggest that the inter-
molecular interactions visualized from optical and computational
studies either do not appreciably perturb the electronic structure
of ferrocene and C60 entities or such effects are within the
experimental errors of cyclic voltammetric experiments. The
electrochemical HOMO-LUMO energy gap, that is, the dif-
ference between the first oxidation potential of the donor, zinc
porphyrin, and the first reduction potential of the acceptor, C60,
was about 1.40 ( 0.3 V compared with the B3LYP/3-21G(*)
value of ∼1.20 eV. By use of the HOMO-LUMO gap, the
Figure 4. Cyclic voltammograms of (a) 3, (b) 4, and (c) 6 (0.05 mM)
in o-dichlorobenzene, 0.1 M (TBA)ClO4. Scan rate ) 100 mV/s.
intermolecular-type interactions between the C60 and ferrocene
units, such as the one shown in Figure 3 for 2:6, were observed.
That is, the edge-to-edge distances between the ferrocene and
C60 entities were found to range between 2.8 and 3.8 Å (last
column in Table 2) and are within the van der Waals interacting
distances. As pointed out earlier, such interactions between the
ferrocene and C60 entities are expected to modulate the
photochemical properties; that is, because of the close proximity
TABLE 3: Electrochemical Half-Wave Redox Potentials (vs Fc/Fc+) of the Zinc Porphyrin-Ferrocene(s) and Their
Self-Assembled with Functionalized Fulleropyrrolidine Triads in o-Dichlorobenzene, 0.1 M (TBA)ClO4
potential, V, vs Fc/Fc+
P+/2+
P0/+
Fc0/+
C60
C60
P0/-
C60
P-/2-
HOMO-LUMOa
∆Gcs°b
0/-
-/2-
2-/3-
compound
1
0.62
0.67
0.61
0.62
0.66
0.64
0.65
0.61
0.61
0.28
0.29
0.27
0.21
0.31
0.28
0.27
0.28
0.23
-1.92
-1.91
-1.93
-1.93
-1.89
-1.91
-1.91
-1.93
-1.94
-
-2.23
-2.19
-2.25
-2.27
-2.21
-2.22
-2.23
-2.25
-2.28
-
1+6c
2
-1.10
-
-1.49
-
-2.02
-
1.39
1.38
-0.52
-
0.00
-0.03
0.00
-0.01
-0.01
0.00
2+6c
3
-1.17
-1.56
-2.10
-0.53
-
3+5
3+6c
4
-1.14
-1.13
-1.55
-1.52
-2.06
-2.04
1.42
1.40
-0.52
-0.51
-
4+6c
-0.12
-1.20
-1.57
-2.10
1.43
-0.48
a HOMO-LUMO is evaluated electrochemically by P0/+-C600/- in eV. b ∆Gcs° ) E1/2(D•+/D) - E1/2(A/A•-) - ∆E0-0 + ∆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, ∆E0-0 is the energy of the 0-0 transition energy gap between the lowest excited state and the ground state, and ∆Gs refers to the solvation
1
energy, calculated by using the “Dielectric Continuum Model” according to the following equation, ∆Gs ) e2/4πꢀ0[(1/2R+ + /2R- - 1/RCC)(1/ꢀs).
From Figure 2, values of R+ ) 4.8 Å, R- ) 4.2 Å, RCC is the center-to-center distance shown in Table 2. c Obtained by addition of 4 eq. of 6 to
the zinc porphyrin-ferrocene(s) dyads.

Covalently Linked Zinc Porphyrin-Ferrocene(s)
J. Phys. Chem. B, Vol. 108, No. 31, 2004 11337
shown in Figure 7. The fluorescence emission bands were
quenched over 70% of their original intensity accompanied by
a small red shift. The fluorescence quenching data were analyzed
by Stern-Volmer plots as shown in the inset of Figure 7. The
Stern-Volmer quenching constants, Ksv, obtained from the
slopes of the linear segments of these plots were in the range
of 1.1-3.3 × 104 M-1, that is, 2-3 orders of magnitude higher
than that expected for bimolecular quenching processes involv-
ing zinc porphyrin as a fluorophore.8g These results indicate
the occurrence of intramolecular quenching processes in the
investigated self-assembled supramolecular triads. To further
understand the quenching mechanism in the zinc porphyrin-
ferrocene(s) dyads and triads and follow the kinetics of
photoinduced processes occurring in these triads, picosecond
time-resolved emission as well as nanosecond transient absorp-
tion studies was performed.
Time-Resolved Fluorescence Studies. The time-resolved
fluorescence spectral features of the zinc porphyrin-ferrocene-
(s) dyads tracked that of the steady-state measurements. Figure
8 shows the fluorescence decay time profiles of the investigated
dyads along with the reference compound, 1, in o-dichloroben-
zene. All of the investigated compounds revealed a monoex-
ponential decay. For compounds 2 and 4, rapid decay was
observed and the calculated quantum yield of fluorescence
quenching was found to be ∼0.98, a result consistent with the
steady-state emission results. For compound 3, in which the
zinc porphyrin and ferrocene entities were separated by a phenyl
amide bond, the quenching was moderate, Φ ) 0.54 (Figure
8). These results suggest the occurrence of an excited-state
quenching process in the zinc porphyrin-ferrocene(s) dyads.
The quenching of zinc porphyrin emission could occur either
from energy transfer or an electron-transfer process. In the
absence of any appreciable ferrocene absorption in the emission
wavelength region of zinc porphyrin, one could eliminate energy
transfer as a quenching via the Fo¨rster mechanism. The
Figure 5. Cyclic voltammograms of 2 (0.05 mM) in the presence of
(i) 0, (ii) 0.5, (iii) 1.0, (iv) 2, (v) 3, and (vi) 4 equiv of 6 in
o-dichlorobenzene, 0.1 M (TBA)ClO4. Scan rate ) 100 mV/s. The
dashed vertical lines indicate the cathodic potential shift observed for
the ZnP0/+ redox process upon axial coordination.
calculated free-energy change, ∆G° for electron transfer from
cs
the ferrocene entity to the excited zinc porphyrin is found to be
weakly exergonic (∆G° < 0.1 eV). These results suggest the
cs
occurrence of excited-state electron transfer in the covalently
linked zinc porphyrin-ferrocene(s) dyads.
Additional quenching of the zinc porphyrin-ferrocene dyad,
3, was observed upon axial coordination of fullerene bearing
axial ligands, 5 or 6 (Figure 8). By assumption that the
quenching is due to electron transfer from the singlet excited
porphyrin to the coordinated fullerene, the rates of charge
separation (kcssinglet) and quantum yields (Φcssinglet) were evalu-
ated in the usual manner employed for the intramolecular
electron-transfer process and the data are given in Table 4. The
data in Table 4 suggest the occurrence of efficient electron
Figure 6. Fluorescence spectra of (i) 1, (ii) 3, (iii) 2, and (iv) 4 in
o-dichlorobenzene (λex ) 551 nm). The concentration of all of the
species was held at 3.70 µM.
energy corresponding to the 0-0 transition of the zinc porphyrin,
and the solvation energy calculated from the dielectric con-
tinuum model,27 the free-energies for charge separation, ∆Gcs,
were also calculated in Table 3. The calculated ∆Gcs values
suggest the occurrence of photoinduced electron transfer from
the singlet excited zinc porphyrin to the axially bound fullerene.
Fluorescence Emission Studies. The photochemical behavior
of the zinc porphyrin-ferrocene(s) dyads was investigated,
initially, by using steady-state fluorescence measurements.
Figure 6 exhibits the fluorescence spectra of compounds 1-4
under the same solution concentrations and when excited at the
wavelength of the most intense visible band. Compounds 2 and
4 were found to be almost nonfluorescent (∼98% quenching);
however, the emission intensity of compound 3 was nearly 50%
quenched as compared with that of pristine zinc porphyrin, 1.
These results indicate the occurrence of intramolecular events
in the zinc porphyrin-ferrocene(s) dyads.
transfer from the excited zinc porphyrin to the fullerene entity
singlet
with Φcs
) 0.96. Because of the higher binding constants
of 5 and 6 to 3, there was no specific trend in the electron-
transfer rates. Further studies involving the nanosecond transient
absorption technique were performed to identify the electron-
transfer products and monitor the kinetics of charge recombina-
tion in the dyads and the self-assembled triads.
Nanosecond Transient Absorption Spectra. The nanosec-
ond transient absorption spectrum of 1 exhibited absorption
peaks at 630 and 840 nm corresponding to the triplet state.28,29
The transient absorption spectrum of compounds 2-4 revealed
a strong band around 500 nm and a weak band at 840 nm
corresponding to its triplet state28 (Figure 9). The absorption
band of the zinc porphyrin anion radical formed after electron
transfer from the ferrocene to the singlet excited zinc porphyrin
Additional quenching of the zinc porphyrin emission of 3
was observed upon axial coordination of fullerenes, 5 or 6, as

11338 J. Phys. Chem. B, Vol. 108, No. 31, 2004
D’Souza et al.
Figure 7. Fluorescence spectra of 3 (3.70 µM) in the presence of various amounts of 6 (3.30 µM each addition) in o-dichlorobenzene (λex ) 551
nm). The inset plot shows the Stern-Volmer plots for the fluorescence quenching of 3 by (a) 5 and (b) 6 in o-dichlorobenzene.
TABLE 4: Fluorescence Lifetimes (τf), Charge-Separation Rate Constants (kCS)a, and Charge-Separation Quantum Yields (ΦCS
for Zinc Porphyrin-Ferrocene(s) Dyads and Triads Formed by Axial Coordination of Fulleropyrrolidines in o-Dichlorobenzene
)
ZnP*-Fc ZnP*-C60
kCS, s-1 kCS, s-1
compoundb
τF/ps (fra %)
ΦCS
ΦCS
kCR, s-1
t
1
2
3
4
1920 (100%)
40 (100%)
890 (100%)
35 (100%)
40 (100%)
40 (100%)
80 (31%)
80 (68%)
35 (100%)
35 (100%)
2.5 × 1010
6.0 × 108
2.8 × 1010
2.5 × 1010
2.5 × 1010
6.0 × 108
6.0 × 108
2.8 × 1010
2.8 × 1010
0.98
0.54
0.98
0.98
0.98
0.54
0.54
0.98
0.98
2:5c
2:6c
3:5c
3:6c
4:5c
4:5c
2.6 × 108
1.7 × 108
1.3 × 108
1.2 × 108
890 (69%)
890 (32%)
1.2 × 1010
1.2 × 1010
0.96
0.96
a kCSsinglet ) (1/τf )sample - (1/τf )ref, Φcssinglet ) [(1/τf )sample - (1/τf )ref ]/(1/τf )sample. For biexponential fitting, τf from the initial decay component
was employed. b For abbreviations, see Figure 1. c [porphyrin] ) 0.05 mM; [fullerene] ) 0.50 mM.
Figure 8. Fluorescence decay profiles of 1 (0.05 mM), 3 (0.05 mM),
Figure 9. Transient absorption spectra of 2 (0.1 mM) in o-dichlo-
and 3 (0.05 mM) + 6 (0.50 mM) in argon-saturated o-dichlorobenzene.
robenzene at 2 µs (b), and 20 µs (O) after the 532-nm laser irradiation.
The inset is the time profile for the peak at 880 nm.
Absorbances were matched at the excitation wavelength of 532 nm.
is expected to occur at 600 nm but was apparently masked by
the strong absorption bands of the triplet state of the zinc
porphyrin. In addition, we could not observe a new band
corresponding to the formation of ferrocenium cation because
of its low molar absorptivity (ꢀ ) 1000 M-1 cm-1 at 800 nm).16e
Upon formation of the supramolecular triads by the axial
coordination of 3 with either 5 or 6, the transient absorption
spectra clearly showed the formation of radical ion pairs. A
strong absorption peak at 1010 nm characteristic of the anion
radical of fulleropyrrolidine was observed for both of the studied
triads (Figure 10). In addition, the zinc porphyrin cation radical
was also observed at 460 nm. No new peak corresponding to

Covalently Linked Zinc Porphyrin-Ferrocene(s)
J. Phys. Chem. B, Vol. 108, No. 31, 2004 11339
Figure 10. (a) Nanosecond transient absorption spectra of 3:6 (0.1:0.5 mM) in o-dichlorobenzene at 8 ns (b) and 220 ns (O) after the 565-nm laser
irradiation. Parts b and c, respectively, show the absorption time profiles monitored at 1010 and 460 nm.
the formation of ferrocenium cation, as a result of charge
migration from the ferrocene entity to the zinc porphyrin cation
radical, was observed for reasons discussed earlier.
The rate of charge recombination, kCR, monitored at 1010
nm was found to be 1.2 × 108 s-1 for the triad 3:5 (: represents
an axial coordinate bond) and 1.3 × 108 s-1 for the triad 3:6.
Similar values for kCR were obtained when the decay was
monitored at 460 nm corresponding to the zinc porphyrin cation
radical. By use of kCS and kCR thus calculated, the ratio kCS/kCR
was evaluated as a measure of the extent of charge stabilization
in the photoinduced electron-transfer process and was found to
be ∼100. This kCS/kCR ratio, reported earlier for a self-assembled
zinc porphyrin-C60 dyad without ferrocene, 1:6, was ∼2.30
These results clearly demonstrate charge stabilization in the
studied supramolecular triads as a result of sequential electron
transfer from the ferrocene entity to the zinc porphyrin cation
radical.
Energy Level Diagram and Charge Stabilization. The
energy levels in o-dichlorobenzene, which are expected to be
of significance for photoinduced electron transfer in the studied
supramolecular triads, are taken from the data in Tables 3 and
4 and are illustrated in Figure 11.
The steady-state and the time-resolved emission and transient
Figure 11. Energy-level diagram showing the different photochemical
events of the supramolecular triad composed of ferrocene, zinc
porphyrin, and fullerene entities.
absorption studies have revealed the occurrence of electron
transfer from the singlet excited zinc porphyrin of 3 to the
coordinated C60 of 5 or 6 resulting in the formation of Fc-
Nanosecond transient absorption spectra of either of the triads
3:5 or 3:6 exhibited the characteristic peak of C60 at 1010
•-
ZnP•+:C60•-. The measured rate constant (kET ) 1.2 × 1010 s-1
)
nm, although the transient spectral signature of ZnP•+ was
relatively difficult to observe because of the triplet absorption
peaks buildup of ZnP and C60 at longer time scales and the
expected rapid disappearance of ZnP•+ as a result of sequential
electron transfer. Considering a low ꢀ for Fc+, one would expect
and the quantum yield (ΦCS ) 0.96) indicate efficient formation
of Fc-ZnP•+:C60•- upon excitation of the zinc porphyrin entity
in the self-assembled triads. The fluorescence results also
indicate that the photoinduced electron transfer occurs from the
ferrocene to the singlet excited zinc porphyrin in 3 to produce
Fc+-ZnP•-. However, the electron transfer from the ferrocene
the transient absorption spectrum of Fc+-ZnP:C60 to look
•-
more like that of Fc-ZnP•+:C60•-. In this regard, the transient
to the zinc porphyrin is relatively slower (kET ) 6.0 × 108 s-1
Thus, the deactivation pathway to generate Fc+-ZnP•-:C60 in
)
spectrum recorded at 8 ns in Figure 8 is not very different from
that reported earlier for covalently linked Fc+-ZnP:C60
•-
than electron transfer from singlet-excited zinc porphyrin to C60.
species of the triad.16e
the triad is negligible. Moreover, formation of Fc+-ZnP-C60
Another intriguing question we would like to address in the
present study is the formation of the energetically favorable
•-
from Fc+-ZnP•-:C60 is energetically highly favorable (-∆G°
) 0.76 eV), that is, any Fc+-ZnP•-:C60 formed will undergo a
rapid charge migration to yield Fc+-ZnP:C60•-, although it was
not possible to obtain the rate of this reaction from the time
resolution of the utilized nanosecond transient spectroscopic
technique.
Fc+-ZnP:C60 from Fc+-ZnP•-:C60 in the case of triads
•-
formed by nonfluorescent 2 coordinated to either 5 or 6. As
pointed out earlier, photoexcitation of either 2 or 4 generates
Fc•+-ZnP•- exclusively with almost unit quantum efficiency
of charge separation because of the short spacer between the

11340 J. Phys. Chem. B, Vol. 108, No. 31, 2004
D’Souza et al.
Figure 12. (a) Nanosecond transient absorption spectra of 2:6 (0.1:0.4 mM) in o-dichlorobenzene at 8 ns (b) and 220 ns (O) after the 565-nm laser
irradiation. Parts b and c, respectively, show the absorption time profiles at 460 and 1010 nm.
two entities. Optical absorption studies have shown that both 2
and 4 bind 5 and 6 to form stable supramolecular triads. Steady-
state fluorescence studies showed additional quenching of the
already quenched (∼98%) ZnP fluorescence in 2 or 4 upon
coordination of 5 or 6 (data not shown). Figure 12 shows the
transient absorption spectrum of the triad 2:6 under solution
conditions similar to those used in Figure 10 for the supra-
molecular triad 3:6. Similar spectral features were observed for
2:5. The transient absorption spectral features of 2:5 and 2:6
are very similar to those seen in Figure 10 for 3:6, especially
the strong absorption at 1010 nm corresponding to the formation
of C60•-. The kCR values calculated from the decay of this band
were found to be very similar to those obtained for 3:5 or 3:6
(Table 4). These results clearly suggest formation of Fc+-ZnP-
Experimental Section
Chemicals. Buckminsterfullerene, C60 (+99.95%) was from
SES Research (Houston, TX). o-Dichlorobenzene in a sure-seal
bottle, sarcosine, pyrrole, and benzaldehydes were from Aldrich
Chemicals (Milwaukee, WI). Tetra-n-butylammonium perchlo-
rate, (TBA)ClO4, was from Fluka Chemicals. All chemicals were
used as received. Syntheses and purification of 1, 5, and 6 were
carried out according to the literature procedure.21d,e
4-Ferrocenylbenzonitrile. Ferrocene (3.80 g, 20.4 mmol)
was dissolved in concentrated sulfuric acid (25 mL) and stirred
for 2 h. The solution was then poured into ice cold water (100
mL). A solution of sodium nitrite (0.91 g, 13.2 mmol) in 5 mL
of ice cold water was added dropwise to a stirred solution of
4-aminobenzonitrile (1.42 g, 12.0 mmol) in 1:1 water/
concentrated HCl (25 mL) and stirred for 30 min at 0 °C. Copper
powder (1.0 g) was added to the ferrocenium solution, and the
diazonium salt solution was added dropwise. The reaction
mixture was then stirred overnight. Ascorbic acid (5.0 g) was
added, and the organic layer was extracted with methylene
chloride and dried over sodium sulfate. The compound was
purified over silica gel column using CH2Cl2/hexanes (60:40
v/v) as the eluent. Yield 1.36 g; 1H NMR (CDCl3), δ ppm, 7.53
(m, 4H), 4.69 (t, 2H), 4.42 (t, 2H), 4.04 (s, 5H); 13C NMR
(CDCl3), δ 145.8, 132.3, 126.4, 119.5, 109.0, 82.6, 70.4, 70.1,
67.1; ESI mass in CH2Cl2 calcd., 287.1; found, 287.3.
4-Ferrocenylnitrobenzene. To the 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 with further
CH2Cl2. The crude compound purified on silica gel by using
•-
C60 as the ultimate product before the occurrence of charge
recombination, irrespective of the initial path of photoinduced
electron transfer, that is, via Fc+-ZnP•-:C60 (in case of 2:5,
2:6, 4:5, and 4:6 triads) or Fc-ZnP•+:C60•- (in the case of the
3:5 and 3:6 triads).
The calculated ratio of kCS/kCR from the kinetic data was found
to be ∼100 for the supramolecular triads composed of ferrocene,
zinc porphyrin, and fullerene, indicating a moderate amount of
charge stabilization, although the low-lying charge-separated
states for all of the triads were generated with the highest
quantum yields (close to unity). The calculated lifetime for the
•-
charge-separated state of Fc+-ZnP-C60 was around 10 ns,
which is 100 times smaller than that reported earlier for
covalently linked ferrocene-zinc porphyrin-C60 triads.16e One
reason for this observation could be the overall geometry of
the supramolecular triads as shown in Figure 3 for the
supramolecular triad 2:6 and from Table 2 where presence of
the intermolecular type interactions between the ferrocene and
C60 entities was visualized. Because of the spatially close
disposition of the terminal charge bearers in the present type
of triads, relatively rapid charge recombination occurs. Impor-
tantly, the present study demonstrates utilization of the self-
assembled supramolecular approach to build triads bearing donor
and acceptor entities with a redox gradient and demonstrates
the occurrence of sequential electron transfer ultimately resulting
in the formation of long-lived charge-separated states.

Covalently Linked Zinc Porphyrin-Ferrocene(s)
J. Phys. Chem. B, Vol. 108, No. 31, 2004 11341
CH2Cl2/ hexanes (40:60). Yield (25%); ESI mass in CH2Cl2
calcd., 307.58; found, 307.12; 1H NMR in CDCl3, δ ppm, 8.14
and 7.56 (d,d, 4H, C6H4), 4.74 and 4.48 (d, d, 4H, C5H4), 4.05
(s, 5H, C5H5).
(m, 11H), 4.91 (s, 2H), 4.47 (s, 2H), 4.23 (s, 5H), -2.72 (br s,
2H); UV-vis in CHCl3, λmax nm, 419, 449, 517, 554, 592, 650;
ESI mass in CH2Cl2 calcd., 798.0; found, 798.6.
5,10,15-Triphenyl-20-(4-ferrocenylphenyl)porphyrinato-
zinc(II) (2). To a solution of 5,10,15-triphenyl-20-(4-ferrocen-
ylphenyl)porphyrin (60 mg, 0.08 mmol) dissolved in CHCl3 (100
mL) was added a solution of zinc acetate (82 mg, 0.38 mmol)
dissolved in MeOH (30 mL), and the solution was stirred for 1
h. The solution was then washed with water and dried over
sodium sulfate. The product was purified over a silica gel
column using CHCl3/hexanes (30:70 v/v) as eluent. Yield 65
4-Ferrocenylaniline.31a A mixture of granulated tin (1.0 g)
and 4-ferrocenylnitrobenzene (0.7 g, 2.28 mmol) in 1:1 HCl
and ethanol was stirred and refluxed for 2 h. The resulting
solution was then cooled to room temperature and neutralized
with 40% aqueous NaOH. The solution was extracted with
dichloromethane and purified on silica gel. Yield (80%); ESI
1
mass in CH2Cl2 calcd., 277.84; found, 277.18; H NMR in
1
DMSO, δ ppm, 7.25 and 6.67 (d,d, 4H, C6H4), 4.61 and 4.22
mg; H NMR (CDCl3), δ ppm, 9.01 (m, 8H), 8.19 (m, 8H),
(d,d, 4H, C5H4), 3.99 (s, 5H, C5 H5), 3.35 (s, 2H, NH2).
7.81 (m, 11H), 4.93 (s, 2H), 4.48 (s, 2H), 4.25 (s, 5H); UV-
vis in CHCl3, λmax nm, 425, 551, 591; ESI mass in CH2Cl2
calcd., 859.7; found, 860.3.
4-Ferrocenylbenzaldehyde. To a solution of 4-ferrocenyl-
benzonitrile (1.31 g, 4.77 mmol) dissolved in dry toluene (50
mL) and purged with argon for 15 min, DIBAL (4.6 mL, 1.0
M in CH2Cl2) was added dropwise. The solution was then stirred
for 2 h. Then methanol (8 mL) was added, and the mixture
was stirred further for another 10 min. A solution of concentrated
sulfuric acid/water (1:3; 40 mL) was added, and the organic
layer was extracted with methylene chloride and dried over
sodium sulfate. The product was purified over a silica gel
column using CH2Cl2/hexanes (80:20 v/v) as eluent. Yield 0.80
5,15-Diphenyl-10,20-bis(4-ferrocenylphenyl)porphyrina-
tozinc(II) (4). To a solution of 5,15-diphenyl-10,20-bis(4-
ferrocenylphenyl)porphyrin (55 mg, 0.06 mmol) dissolved in
CHCl3 was added a solution of zinc acetate (61 mg, 0.30 mmol)
dissolved in MeOH (30 mL), and the solution was stirred for 1
h. The solution was then washed with water and dried over
sodium sulfate. The compound was purified over a silica gel
column using CHCl3/hexanes (30:70 v/v) as eluent. Yield 58
1
1
g; H NMR (CDCl3), δ ppm, 9.97 (s, 1H), 7.79 (d, 2H), 7.59
mg; H NMR (CDCl3), δ ppm, 9.01 (m, 8H), 8.20 (m, 8H),
(d, 2H), 4.74 (t, 2H), 4.43 (t, 2H), 4.05 (s, 5H); 13C NMR
(CDCl3), δ 191.8, 147.5, 134.2, 130.1, 126.3, 83.0, 70.1, 67.3;
ESI mass in CH2Cl2 calcd., 289.8; found, 290.5.
7.81 (m, 10H), 4.93 (s, 4H), 4.48 (s, 4H), 4.25 (s, 10H); UV-
vis in CHCl3, λmax nm, 425, 552, 591; ESI mass in CH2Cl2
calcd., 1045.0; found, 1044.2.
4-Ferrocenyl-5-phenyldipyrromethane.31b To pyrrole (3.88
mL, 56.0 mmol), ferrocenylbenzaldehyde was added (400 mg,
1.4 mmol) and purged with argon for 15 min. Then TFA (10
µL) was added and stirred for 15 min. The reaction was
quenched with NaOH (50 mL, 0.1 M), 100 mL of CH2Cl2 was
added, and the organic layer was extracted over sodium sulfate.
Excess pyrrole was distilled off by vacuum distillation at room
temperature, and the compound was purified over silica gel flash
column using CH2Cl2/cyclohexane (80:20 v/v) as eluent. Yield
0.35 g; 1H NMR (CDCl3), δ ppm, 7.85 (br s, 2H), 7.40 (d, 2H),
7.10 (d, 2H), 6.66 (m, 2H), 6.15 (m, 2H), 5.92 (m, 2H), 5.40
(s, 1H), 4.59 (t, 2H), 4.28 (t, 2H), 4.02 (s, 5H); 13C NMR
(CDCl3), δ 139.9, 138.2, 132.8, 128.6, 126.6, 117.4, 108.7,
107.5, 85.4, 69.8, 69.1, 66.8, 43.9; ESI mass in CH2Cl2 calcd.,
405.8; found, 406.3.
5,10,15-Triphenyl-20-[4-(methoxycarbonyl)phenyl]por-
phyrin. To a solution of propionic acid (350 mL) containing
methyl-4-formyl benzoate (2.96 g, 18.0 mmol) was added
benzaldehyde (5.5 mL, 54.1 mmol) and pyrrole (5.0 mL, 72.2
mmol), and the mixture was refluxed for 3 h. The solvent was
distilled off, and the product was purified over basic alumina
column using CHCl3/hexanes (30:70 v/v) as eluent. Yield 600
mg; 1H NMR (CDCl3), δ ppm, 8.83 (m, 8H), 8.45 (d, 2H), 8.31
(d, 2H), 8.22 (m, 6H), 7.76 (m, 9H), 4.12 (s, 3H), -2.78 (s br,
2H); UV-vis in CHCl3, λmax nm, 418, 515, 549, 590, 648; ESI
mass in CH2Cl2 calcd., 672.2; found, 672.9.
5,10,15-Triphenyl-20-[4-(carboxy)phenyl]porphyrin. To a
solution of 5,10,15-triphenyl-20-[4-(methoxycarbonyl)phenyl]-
porphyrin (100 mg, 0.1 mmol) dissolved in 2-propanol (50 mL)
was added a solution of KOH (560 mg) dissolved in water (10
mL), and the mixture was refluxed for 6 h. The solution was
acidified with concentrated HCl and extracted with chloroform.
The organic layer was washed with saturated aqueous sodium
bicarbonate solution and dried over sodium sulfate. The product
was purified over silica gel column using CHCl3/MeOH
(90:10 v/v) as eluent. Yield 40 mg; 1H NMR (CDCl3), δ ppm,
8.84 (m, 8H), 8.51 (d, 2H), 8.35 (d, 2H), 8.23 (m, 6H), 7.77
(m, 9H), -2.78 (s br, 2H); UV-vis in CHCl3, λmax nm, 418,
514, 551, 592, 650; ESI mass in CH2Cl2 calcd., 658.3; found,
659.1.
5,15-Diphenyl-10,20-bis(4-ferrocenylphenyl)porphyrin. A
solution of 4-ferrocenyl-5-phenyldipyrromethane (100 mg, 0.25
mmol) dissolved in 200 mL of dry methylene chloride was
purged with argon for 15 min. Then benzaldehyde (25 µL, 0.25
mmol) and BF3O(Et)2 (31 µL, 0.25 mmol) were added, and the
mixture was stirred for 2 h. p-Chloranil (90 mg, 0.37 mmol)
was then added, and the reaction mixture was stirred overnight.
The solvent was evaporated under vacuum and the compound
was purified over silica gel column using CHCl3/hexanes
(30:70 v/v) as eluent. Yield 55 mg; 1H NMR (CDCl3), δ ppm,
8.88 (m, 8H), 8.16 (m, 8H), 7.78 (m, 10H), 4.89 (s, 4H), 4.45
(s, 4H), 4.21 (s, 10H), -2.73 (br s, 2H); UV-vis in CHCl3,
λmax nm, 420, 451, 518, 557, 650, 592; ESI mass in CH2Cl2
calcd., 982.0; found, 982.8.
5,10,15-Triphenyl-20-[4′-(amidophenylferrocene)phenyl]-
porphyrin. To a solution of 5,10,15-triphenyl-20-[4-(carboxy)-
phenyl]porphyrin (40 mg, 0.06 mmol) dissolved in dry toluene
(10 mL) after purging with argon for 15 min was added pyridine
(750 µL) and thionyl chloride (89 µL, 1.2 mmol), and the
mixture was refluxed under argon for 3 h. The solution was
then evaporated, and the residue was redissolved in toluene (10
mL) and pyridine (250 µL). Then 4-ferrocenylaniline (17 mg,
0.06 mmol) was added, and the solution was stirred overnight
under argon. The compound was purified over silica gel column
5,10,15-Triphenyl-20-(4-ferrocenylphenyl)porphyrin. To a
solution of 4-ferrocenyl-benzaldehyde (2.0 g, 6.9 mmol) and
benzaldehyde (2.1 mL, 20.7 mmol) dissolved in 300 mL of
propionic acid, pyrrole (1.9 mL, 27.6 mmol) was added, and
the resulting mixture was refluxed for 5 h. The propionic acid
was then distilled off, and the compound was purified over basic
alumina using CHCl3/hexanes (30:70 v/v) as eluent. Yield 0.35
g; 1H NMR (CDCl3), δ ppm, 8.91 (m, 8H), 8.18 (m, 8H), 7.81
1
using CHCl3/hexanes (40:60 v/v) as eluent. Yield 25 mg; H
NMR (CDCl3), δ ppm, 8.85 (m, 8H), 8.37 (d, 2H), 8.28 (d,

11342 J. Phys. Chem. B, Vol. 108, No. 31, 2004
D’Souza et al.
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2H), 8.22 (m, 6H), 7.76 (m, 9H), 4.69 (t, 2H), 4.34 (t, 2H),
4.09 (s, 5H), -2.77 (br s, 2H); UV-vis in CHCl3, λmax nm,
422, 516, 551, 592, 648; ESI mass in CH2Cl2 calcd., 917.8;
found, 918.3.
5,10,15-Triphenyl-20-[4′-(amidophenylferrocene)phenyl]-
porphyrinatozinc (3).16e To a solution of 5,10,15-triphenyl-
20-[4′-(amidophenylferrocene)phenyl]porphyrin (25 mg, 0.03
mmol) dissolved in CHCl3 (50 mL) was added zinc acetate (30
mg, 0.14 mmol) dissolved in MeOH (20 mL), and the solution
was stirred for 1 h. The solution was then washed with water
and dried over anhydrous sodium sulfate. The compound was
purified over silica gel column using CHCl3/hexanes (40:60 v/v)
as eluent. Yield 95%; 1H NMR (CDCl3), δ ppm, 8.96 (m, 8H),
8.24 (m, 8H), 7.76 (m, 11H), 4.70 (s, 2H), 4.36 (s, 2H), 4.10
(s, 5H); UV-vis in CHCl3, λmax nm, 423, 551, 594; ESI mass
in CH2Cl2 calcd., 978.8; found, 979.3.
Instrumentation. The UV-vis spectral measurements were
carried out with a Shimadzu Model 1600 UV-vis spectropho-
tometer. The fluorescence emission was monitored by using a
Spex Fluorolog-τ spectrometer. A right-angle detection method
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1
was used. The H NMR studies were carried out on a Varian
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300 or 400 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. 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. A ferrocene/ferrocenium redox couple was
used as an internal standard. All 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 the Gaussian 9824 software
package on high-speed computers. 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.
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) and a streak scope (Hamamatsu Photonics). The
details of the experimental setup are described elsewhere.32
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.20
Acknowledgment. The authors are thankful to the donors
of the Petroleum Research Fund administered by the American
Chemical Society and National Institutes of Health (GM 59038)
and Grants-in-Aid for Scientific Research on Primary Area (417)
from the Ministry of Education, Science, Sport and Culture of
Japan (to O.I. add Y.A.) for support of this work. P.M.S. is
thankful to the Department of Education for a GAANN
fellowship.
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