Multi-triphenylamine-substituted porphyrin-fullerene conjugates as charge stabilizing Antenna - Reaction Center mimics

Article abstract of DOI:10.1021/jp073121v

A new concept of charge stabilization via derealization of the π-cation radical species over the donor macrocycle substituents in a relatively simple donor-acceptor bearing multimodular conjugates is reported. The newly synthesized multimodular systems were composed of three covalently linked triphenylamine entities at the meso position of the porphyrin ring and one fulleropyrrolidine at the fourth meso position. The triphenylamine entities were expected to act as energy transferring antenna units and to enhance the electron donating ability of both free-base and zinc(II) porphyrin derivatives of these pentads. Appreciable electronic interactions between the meso-substituted triphenylamine entities and the porphyrin π-system were observed, and as a consequence, these moieties acted together as an electron-donor while the fullerene moiety acted as an electron-acceptor in the multimodular conjugates. In agreement with the spectral and electrochemical results, the computational studies performed by the DFT B3LYP/3-21G(*) method revealed delocalization of the frontier highest occupied molecular orbital (HOMO) over the triphenylamine entities in addition to the porphyrin macrocycle. Free-energy calculations suggested that the light-induced processes from the singlet excited state of porphyrins are exothermic in the investigated multimodular conjugates. The occurrence of photoinduced charge-separation and charge-recombination processes was confirmed by the combination of time-resolved fluorescence and nanosecond transient absorption spectral measurements. Charge-separated states, on the order of a few microseconds, were observed as a result of the derealization of the π-cation radical species over the porphyrin macrocycle and the meso-substituted triphenylamine entities. The present study successfully demonstrates a novel approach of charge-stabilization in donor-acceptor multimodular conjugates.

Full text of DOI:10.1021/jp073121v

8552
J. Phys. Chem. A 2007, 111, 8552-8560
Multi-Triphenylamine-Substituted Porphyrin-Fullerene Conjugates as Charge Stabilizing
“Antenna-Reaction Center” Mimics
Francis D’Souza,*,† Suresh Gadde,† D.-M. Shafiqul Islam,‡ Channa A. Wijesinghe,†
Amy L. Schumacher,† Melvin E. Zandler,† 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 23, 2007
A new concept of charge stabilization via delocalization of the π-cation radical species over the donor
macrocycle substituents in a relatively simple donor-acceptor bearing multimodular conjugates is re-
ported. The newly synthesized multimodular systems were composed of three covalently linked triphenylamine
entities at the meso position of the porphyrin ring and one fulleropyrrolidine at the fourth meso position.
The triphenylamine entities were expected to act as energy transferring antenna units and to enhance the
electron donating ability of both free-base and zinc(II) porphyrin derivatives of these pentads. Appreciable
electronic interactions between the meso-substituted triphenylamine entities and the porphyrin π-system were
observed, and as a consequence, these moieties acted together as an electron-donor while the fullerene moiety
acted as an electron-acceptor in the multimodular conjugates. In agreement with the spectral and electrochemical
results, the computational studies performed by the DFT B3LYP/3-21G(*) method revealed delocalization
of the frontier highest occupied molecular orbital (HOMO) over the triphenylamine entities in addition to
the porphyrin macrocycle. Free-energy calculations suggested that the light-induced processes from the singlet
excited state of porphyrins are exothermic in the investigated multimodular conjugates. The occurrence of
photoinduced charge-separation and charge-recombination processes was confirmed by the combination
of time-resolved fluorescence and nanosecond transient absorption spectral measurements. Charge-separated
states, on the order of a few microseconds, were observed as a result of the delocalization of the π-cation
radical species over the porphyrin macrocycle and the meso-substituted triphenylamine entities. The present
study successfully demonstrates a novel approach of charge-stabilization in donor-acceptor multimodular
conjugates.
Introduction
accumulators are qualities that enhance their performance.14 As
a consequence, fullerenes in donor-acceptor dyads and polyads
accelerate forward electron-transfer (kCS) and slow down
backward electron-transfer (kCR), thus generating the much
desired long-lived charge-separated states.14 Hence, a combina-
tion of both chromophores (i.e., porphyrins and fullerenes) seems
ideal for enhancing the light-harvesting efficiency throughout
the solar spectrum and for converting the harvested light into
the high-energy state of the charge-separation by PET.15-18
Over the last three decades, a great deal of research effort
has been invested in the study of artificial constructs that mimic
the photoinduced electron-transfer (PET) lying at the heart of
photosynthetic solar energy conversion and in developing
molecular electronics.1-12 Both covalently linked and supramo-
lecularly assembled donor-acceptor dyads and polyads (triads,
tetrads, etc.) have been elegantly designed and studied. In the
construction of such dyads and polyads, porphyrins4,10 and
fullerenes12 have been employed as primary electron-donor and
-acceptor entities, respectively, although ferrocene, carotene, or
tetrathiafulvalene entities have often been utilized as terminal
electron-donors.11 Porphyrins as electron-donors, as well as
sensitizers, are suitable for efficient electron-transfer. The rich
and extensive absorption features of porphyrins guarantee an
increased absorption cross-section and an efficient use of the
solar spectrum.4 Fullerenes are excellent electron-acceptors for
incorporation into artificial reaction centers.13 Their relatively
negative reduction potentials, low solvent and internal reorga-
nization energies, low susceptibility to solvent stabilization of
the radical anion, and potential ability to act as electron
More recently, researchers have begun incorporating not only
photoinduced charge-separation but also antenna functionality
into artificial photosynthetic constructs.19 The structures of
photosynthetic reaction centers of bacteria,3 a “nanobiological
machine” of Mother Nature, and other organisms have provided
important hints on the design of photosynthesis mimics. Natural
antennae, including chlorophylls and carotenoids, collect large
amounts of solar energy and redirect it as electronic excitation
energy to the reaction center, where subsequent conversion into
chemical energy takes place through redox active molecules such
as chlorophylls and quinones across a photosynthetic membrane.
In the developed artificial systems, energy funneling antenna
molecules have been elegantly connected to the donor-acceptor
systems to probe sequential energy and electron-transfer pro-
cesses, thus mimicking the complex “antenna-reaction center”
functionality of photosynthesis.19
* To whom correspondence should be addressed. (F.D.) E-mail:
Francis.DSouza@wichita.edu; (O.I.) E-mail: ito@tagen.tohoku.ac.jp.
† Wichita State University.
‡ Tohoku University.
10.1021/jp073121v CCC: $37.00 © 2007 American Chemical Society
Published on Web 07/04/2007

Charge Stabilizing “Antenna-Reaction Center” Mimics
J. Phys. Chem. A, Vol. 111, No. 35, 2007 8553
SCHEME 1: Molecular Structure Depicting the
Photochemical Events of the Multimodular
Donor-Acceptor Systems Developed in the Present Study
Figure 1. Optical absorption spectrum of 1 and 2 in o-DCB.
Results and Discussion
The structures of the newly synthesized donor-acceptor
bearing multimodular systems, 5-[2-(4′′-benzoic acid-4′-phenyl
ester)-N-methyl-3,4-fulleropyrrolidine]-10,15,20-tri(N,N-diphe-
nylaminophenyl) porphyrin (1) and 5-[2-(4′′-benzoic acid-4′-
phenyl ester)-N-methyl-3,4-fulleropyrrolidine]-10,15,20-tri(N,N-
diphenylaminophenyl) porphyrinatozinc(II) (2), and their reference
compounds that bear a phenyl moiety instead of the fullerene
moiety, 5-(benzoic acid-4′-phenyl ester)-10,15,20-tri(N,N-diphe-
nylaminophenyl) porphyrin (3) and 5-(benzoic acid-4′-phenyl
ester)-10,15,20-tri(N,N-diphenylaminophenyl) porphyrinatozinc-
(II) (4) are shown in Scheme 1. We have employed both free-
base and zinc porphyrins because of their different emission
and redox properties.10a,20 The first oxidation potential of these
porphyrins generally follows the trend zinc porphyrin < free-
base porphyrin. As a result, it has been possible to probe the
variation in the free-energy change of photoinduced electron-
transfer in the studied compounds.
Absorption and Fluorescence Studies. Figure 1 shows the
optical absorption spectra of conjugates 1 and 2 in o-dichlo-
robenzene (o-DCB). Both compounds revealed a band around
307 nm corresponding to the triphenylamine entities. The
porphyrin Soret and visible bands of compounds 1-4 were
broad and red-shifted by nearly 20 nm as compared to pristine
free-base and zinc tetraphenylporphyrins (H2TPP and ZnTPP),
suggesting interactions between the porphyrin π-system and
peripheral substituents. Similar results were also obtained in a
polar solvent, benzonitrile (PhCN). Importantly, the absorption
band of the triphenylamine entity in compounds 1-4 had no
appreciable overlap with the porphyrin absorption in the 300-
380 nm spectral region. Control experiments performed using
pristine triphenylamine revealed a lack of absorption in the 400-
700 nm spectral region. Hence, irradiation of compounds 1-4
at 307 nm is expected to selectively excite triphenylamine units,
although exciting these compounds at any one of the porphyrin
absorption peaks is expected to selectively excite the porphyrin
macrocycle. The appended fullerene entity in 1 and 2 appeared
as a shoulder at 328 nm.
When excited at the most intense visible band maxima (430-
440 nm), the fluorescence spectra of 1 and control compound
3 (which has a phenyl entity instead of a fullerene entity)
revealed emission bands at 670 and 732 nm, respectively.
Additionally, the emission bands of 2 and control compound 4
were located at 620 and 665 nm, respectively. However, the
emissions of 1 and 2 are quenched by over 90% as compared
to their control compounds. As will be demonstrated later, this
is due to PET from the singlet excited porphyrin to the fullerene
entity. It should be mentioned here that the porphyrin emission
maxima are red-shifted by 20-25 nm as compared to their
tetraphenylporphyrin analogues, which is a trend similar to that
observed for absorption maxima of these compounds.
In the present study, we report novel multimodular systems
as charge stabilizing antenna-reaction center mimics. The
structures of the studied molecules are comprised of three
entities of triphenylamines, which absorb energy in the UV
region and act as a transferring antenna in addition to the
electron-donating entities, and a porphyrin moiety, which acts
as an energy-acceptor from the triphenylamines and promotes
electron-transfer to the fullerene entity by using the excitation
energy, with the fullerene being the electron-acceptor (Scheme
1). The employed triphenylamine is a strong fluorophore,
absorbing at about 300 nm and emitting in the 480 nm region.20
Because of the close proximity of the triphenylamine entities
to the porphyrin π-ring in the multimodular conjugates, modula-
tion of the electronic properties of porphyrin, that is, delocal-
ization of the porphyrin π-system to the triphenylamine entities,
is expected. As demonstrated here, such delocalization will result
in slowing down the undesired charge-recombination process
during electron-transfer, thus generating relatively long-lived
charge-separated species. Hence, the present multimodular
conjugates are not only novel systems to probe sequential
antenna-reaction center events but are also involved in generat-
ing relatively long-lived charge-separated states as a result of
delocalization of the donor, porphyrin π-system to the antenna
units. The sequential energy-transfer followed by electron-
transfer results in the generation of long-lived charge-separated
states, which replicates different aspects of natural photosyn-
thesis.

8554 J. Phys. Chem. A, Vol. 111, No. 35, 2007
D’Souza et al.
Figure 2. Fluorescence spectrum of 1, 2, and control compounds 3
and 4 in o-DCB. The concentration of porphyrins was held at 20 µM,
and the triphenylamine moieties were excited at λmax ) 307 nm. The
asterisk indicates instrument grating line.
Figure 3. Cyclic voltammograms of
(n-Bu4N)ClO4. Scan rate ) 100 mV/s.
1 in o-DCB, 0.1 M
TABLE 1: Electrochemical Redox Potentials (V vs Fc/Fc+),
Energy Levels of the Charge-Separated States (∆GRIP), and
Free-Energy Changes for Charge-Separation (∆GCS) for the
Multimodular Systems
As shown in Figure 2, when compounds 3 and 4 were excited
at 307 nm, corresponding to the absorption maxima of the
peripheral triphenylamine substituents, a new emission band
around 480 nm was observed. Control experiments performed
using triphenylamine confirmed that this band is due to the
emission of peripheral substituents. As compared with the
pristine triphenylamine fluorescence, the triphenylamine emis-
sion in the case of the porphyrin derivatives was considerably
quenched. For controls 3 and 4, scanning the emission wave-
length into the near-IR region also revealed additional bands
corresponding to porphyrin emissions, which were not observed
for the pristine porphyrins (H2TPP and ZnTPP) with the 307
nm excitation. Excitation spectra of 3 and 4, recorded by fixing
the emission monochromator to the porphyrin emission maxima
(670 and 732 nm for the zinc and free-base porphyrins,
respectively), revealed spectra with the triphenylamine absorp-
tion shown in Figure 1 (also see Figure S1 Supporting
Information). These results indicate that, in case of compounds
3 and 4, the observed porphyrin emission in the near-IR region
(Figure 2) is a result of singlet energy-transfer from the
peripheral triphenylamine entities to porphyrin and is not a
consequence of the direct excitation of porphyrin macrocycles.
Interestingly, for compounds 1 and 2, the emission correspond-
ing to the peripheral substituents at 480 nm and the porphyrin
emission in the near-IR region were almost completely quenched,
which suggest the occurrence of additional photochemical
events. An excited energy-transfer from the peripheral substit-
uents to the porphyrin followed by electron-transfer from the
excited porphyrin to the covalently linked fullerene entity is
envisioned. In addition, a direct energy- or electron-transfer from
the excited triphenylamine to the distinctly located fullerene
entity must be considered to be the cause of the complete
quenching of the excited triphenylamine. In the more polar
benzonitrile solvent, more efficient quenching of the triphenyl
and porphyrin emissions was observed.
Eox
(TPA-P0/•+
(V)
Ered
(C60
(V)
S
b
0/•-
a
)
)
-∆GRIP -∆GCS
compound solvent
(eV)
(eV)
1
o-DCB
PhCN
0.37
0.39
-1.16
-1.01
1.41
1.35
0.54
0.60
2
o-DCB
PhCN
0.15c
-1.15
-1.02
1.18
1.15
0.89
0.92
0.18c
a ∆GRIP ) Eox - Ered + ∆GS, where ∆GS ) -e2/(4πꢀ0ꢀRRCt-Ct) and
ꢀ0 and ꢀR refer to the vacuum permittivity and the dielectric constant
of the employed solvents, respectively. b -∆GCS ) ∆E0-0 - ∆GRIP
,
where ∆E0-0 is the energy of the lowest excited states (2.07 eV for
1
1ZnP*, and 1.95 eV for H2P*). c Quasi-reversible.
reduction of all of the studied porphyrins (1-4) were found to
be reversible, as judged from their peak-to-peak separation
(∆Epp) values and from their cathodic-to-anodic peak current
ratios.22 The oxidation potentials were cathodically shifted over
100 mV as compared to their tetraphenylporphyrin analogues.
Scanning the potential further into the anodic direction revealed
additional peaks; however, at these potentials, films started to
form on the electrode surface.
The HOMO-LUMO gap for 1, calculated from the first
oxidation potential and the first fullerene spheroid centered
reduction potential, was 1.53 and 1.40 V in o-DCB and PhCN,
respectively. For 2, these values were 1.30 and 1.20 V in o-DCB
and PhCN, respectively. It is also important to note that the
calculated HOMO-LUMO gap for 1 and 2 is smaller by about
100 mV as compared to the earlier reported tetraphenylporphy-
rin-fullerene derivatives in the literature,17,18 which is due to
the effects caused by the triphenylamine entities.
Further, to aid in the interpretation of the transient spectral
data discussed in the latter part of the present paper, it was
essential to spectrally characterize the porphyrin π-cation radical
species because of the different nature of the porphyrin
macrocycle employed in the present study. Figure 4 shows the
spectra of chemically oxidized porphyrins 3 and 4. Similar
spectral results were obtained during spectroelectrochemical
measurements. Both free-base and zinc porphyrins exhibited
distinct spectral bands in the near-IR region. The free-base
porphyrin π-cation radical of compound 3 revealed bands at
500 and 750 nm, and the zinc porphyrin π-cation radical of
compound 4 revealed bands at 770 and 1294 nm in o-DCB. It
is important to note that the position of the 1294 nm band was
far away from the spectral region where the triplet absorption
of porphyrin and fullerene is expected to appear (500-720 nm
region). Such spectrally isolated cation radical bands should help
us not only in identifying the electron-transfer products but also
Electrochemistry, Spectral Characterization of Porphyrin
π-Cation Radicals, and Free-Energy Calculations. Further-
more, electrochemical studies using cyclic voltammetric tech-
niques were performed to evaluate the redox potentials of the
conjugates and also to evaluate the energetics of electron-transfer
processes. The electrochemistry of free-base and zinc porphyrins
has been extensively studied in the literature.21 These compounds
are known to undergo two one-electron reductions, which lead
to the formation of π-anion radical and dianion species, and
two one-electron oxidations, which lead to the formation of
π-cation radical and dication species, respectively. Figure 3
shows a cyclic voltammogram of representative conjugate 1 in
o-DCB containing 0.1 M (n-Bu4N)ClO4, and the redox potential
data are listed in Table 1. The first oxidation and the first

Charge Stabilizing “Antenna-Reaction Center” Mimics
J. Phys. Chem. A, Vol. 111, No. 35, 2007 8555
states, the driving forces (∆GCS) were also evaluated. The
generations of ((Ph2N)3-ZnP)•+-C60•- in 2 and ((Ph2N)3-H2P)•+-
C60•- in 1 were exothermic via the singlet excited states of the
porphyrin or the fullerene in PhCN as well as in o-DCB. On
the basis of these calculations, an energy diagram for 2 in PhCN
is shown in Figure 6. A similar energy level diagram could also
be envisioned for 1 that possesses free-base porphyrin as the
primary electron-donor unit. Furthermore, the energy of the
charge-separated states is lower than the porphyrin triplet state
(3H2P* and 3ZnP*), suggesting the possibility of charge-
separation via the porphyrin triplet states in PhCN. However,
such calculations suggested that the energy level of the charge-
separated state of 1 is higher than the porphyrin triplet states in
o-DCB, hence the electron-transfer from the triplet excited-state
of the donor is endothermic for 1 in o-DCB, whereas charge-
recombination to the triplet state is possible.
Time-Resolved Emission Studies. The time-resolved emis-
sion results tracked those of the earlier discussed steady-state
emission results. Figure 7a shows the emission decay profiles
of 1 and 3 in o-DCB and PhCN by the excitation of the
porphyrin moiety. The fluorescence decay of the free-base
porphyrin moiety in 3 showed mono-exponential decay (traces
i and ii), with a lifetime that was close to that of the pristine
free-base tetraphenylporphyrin. In the studied solvents, the
fluorescence of the singlet excited porphyrin in 1 showed fast
decays (traces iii and iv) that could be curve-fitted to a
biexponential function involving a major short lifetime and a
minor longer lifetime as listed in Table 2. As shown in Figure
7b for 4 (traces i and ii), the fluorescence time profile of the
zinc porphyrin moiety also showed mono-exponential decay,
with a lifetime that was close to that of the pristine zinc
tetraphenylporphyrin. However, the fluorescence time profiles
of 2 showed quick decays (traces iii and iv) as compared to the
time profiles of reference compound 4 in both o-DCB and
PhCN. Here also, the decay could be fitted to a biexponential
fitting curve. The evaluated fluorescence lifetimes are sum-
marized in Table 2.
These results suggest the occurrence of an excited singlet
state quenching process of the porphyrins by the fullerene moiety
in 1 and 2. The quenching process of the porphyrin emission
could occur from either energy-transfer or electron-transfer
processes. On the basis of the observations for 2 that the time-
resolved fluorescence spectra did not show the appearance of
the transient fluorescence peak of the fullerene moiety after the
decay of the fluorescence of the porphyrin moiety, the short
lifetimes of porphyrins were predominantly ascribed to charge-
separation, which was supported by the finding that the
fluorescence decay rates were faster in more polar PhCN.
Therefore, the rates (kSCS) and the quantum yields (ΦCSS) of
charge-separation were evaluated for 1 and 2 in the usual
manner, and the data are shown in Table 2. The magnitudes of
kSCS and ΦSCS reveal efficient charge-separation from the singlet
excited-state of porphyrin moiety to fullerene in 1 and 2.
Figure 4. Visible-NIR spectra of neutral (dark line) and chemically
oxidized, using equimolar nitronium hexafluoroantimonate, (red line)
of (a) 3 and (b) 4 in o-DCB.
in following the kinetics of electron-transfer without the
interference of triplet absorption bands, a problem often
encountered in traditionally used tetraphenylporphyrin based
donor-acceptor systems.17,18
DFT B3LYP/3-21G(*) Studies. To gain insights into the
geometry and the electronic structure, computational studies
were performed using density functional methods (DFT) at the
B3LYP/3-21G(*) level on 2. The B3LYP/3-21G(*) methods23
have recently been successfully used to predict the geometry
and the electronic structure of molecular and supramolecular
donor-acceptor assemblies.24 Compound 2 was optimized to a
stationary point on the Born-Oppenheimer potential energy
surface whose structure is shown in Figure 5a. The center-to-
center distance between the porphyrin (Zn center) and the
fullerene entities was ∼18.5 Å with no significant interactions
between them. The distance between the Zn and the N atom of
the triphenylamine entities was 9.2 Å, and this value was
compared to the distances 18.3, 22.8, and 27.4 Å between the
center of the C60 to the N atoms of the triphenylamine entities.
These results clearly show triphenylamine entities disposed away
from the C60 entity, but they are much closer to the porphyrin.
In the optimized structure, the majority of the HOMO was
located on the porphyrin macrocycle with considerable contribu-
tion also on the peripheral triphenylamine entities, whereas the
LUMO was fully localized on the fullerene entity (Figure 5b).
These results suggest the triphenylamine-substituted porphyrin
is an electron-donor and fullerene is an electron-acceptor in
electron-transfer reactions.24 The computed gas phase HOMO-
LUMO gap was 1.47 eV, which compared well with the
electrochemically measured HOMO-LUMO gap in nonaqueous
solvents (1.53 and 1.40 V in o-DCB and PhCN, respectively).
The appearance of part of the HOMO over the peripheral
triphenylamine substituents is expected to stabilize the radical
cation by delocalization and is further slowed down in the
charge-recombination process during the light-induced electron-
transfer process.
For 2, the kSCS values were (4-5) × 109 s-1, and the ΦSCS
values were 0.85-0.90 in o-DCB and PhCN, which are larger
than those for 1 and suggest a relatively faster and more efficient
charge-separation process in ZnP than in H2P in polar solvents.
It should be noted that because of the delocalization of the
HOMO into the peripheral triphenylamine entities in Figure 5b,
the (Ph2N)3-MP moiety is expected to act as a single unit during
electron-transfer from the singlet excited-state of porphyrin.
Thus, these results suggest the occurrence of excited-state
The energy levels of the charge-separated states (∆GRIP) were
evaluated using the Weller-type approach,25 utilizing the redox
potentials and the dielectric constant of the employed solvents
as listed in Table 1. By comparing the energy levels of the
charge-separated states with the energy levels of the excited
•-
charge-separation in 1 and 2, generating ((Ph2N)3- H2P)•+-C60
and ((Ph2N)3-ZnP)•+-C60 via the excited-singlet states of
•-

8556 J. Phys. Chem. A, Vol. 111, No. 35, 2007
D’Souza et al.
Figure 5. B3LYP/3-21G(*) optimized (a) structure and (b) the HOMO and LUMO of 2.
Figure 6. Energy level diagram showing different photochemical
events of 2 in PhCN.
porphyrins and triphenylamine moieties. The evaluated kCSS
and ΦSCS values shown in Table 2 generally follow the order 1
< 2, that is, they depend upon the magnitude of the -∆GCS
values, which suggests that this process belongs to the normal
region of the Marcus parabola.
Figure 7. Fluorescence decays (a) at 660-680 nm range of (i) 3 (0.02
mM in o-DCB), (ii) 3 (0.02 mM in PhCN), (iii) 1 (0.02 mM in o-DCB),
and (iv) 1 (0.02 mM in PhCN); and (b) at 600-620 nm range of (i) 4
(0.02 mM in o-DCB), (ii) 4 (0.02 mM in PhCN), (iii) 2 (0.02 mM in
o-DCB), and (iv) 2(0.02 mM in PhCN); λex ) 400 nm.
Further studies involving the nanosecond transient absorption
technique were performed to identify the electron-transfer
products and monitor the kinetics of charge-recombination.
Nanosecond Transient Absorption Studies. Nanosecond
transient absorption spectra of 2 observed with the 532 nm laser

Charge Stabilizing “Antenna-Reaction Center” Mimics
J. Phys. Chem. A, Vol. 111, No. 35, 2007 8557
TABLE 2: Fluorescence Lifetime (τF) of Porphyrins,
Charge-Separation Rate-Constant (kSCS),a and Charge-
Separation Quantum Yield (ΦSCS) via Singlet Excited States
of Porphyrins for the Multimodular Systems
τ
F(P) (ns) /
compound
solvent
(fraction)
kSCS(s-1
)
ΦSCS
1
o-DCB
PhCN
1.11 (93%)
0.79 (75%)
0.7 × 109
0.82
0.87
1.1 × 109
2
o-DCB
PhCN
0.21 (90%)
0.17 (95%)
4.3 × 109
5.2 × 109
0.89
0.90
a kSCS ) ((1/τRef ) - (1/τConjugate)), τRef ) 6.27 ns (in o-DCB) and
5.69 ns (in PhCN) for 3, and τRef ) 1.85 ns (in o-DCB) and 1.72 ns (in
PhCN) for 4.
Figure 9. Nanosecond transient absorption spectra of 1 (0.08 mM) in
Ar-saturated PhCN obtained by 532-nm laser light irradiation; inset:
Absorption time profile at 1020 nm.
charge-separation rate evaluated from the fluorescence decay
(5.2 × 109 s-1), as the half-rise should be 200 ps. Thus, the
slow rise was attributed to the second charge-separation via the
excited triplet species, after the fast charge-separation via the
singlet excited-state of ZnP and followed by fast charge-
recombination.14c The rate of charge-separation (kCTS) via 3ZnP*
was evaluated from the rise of the 1000 and 1300 nm bands
for ((Ph2N)3-ZnP)•+-C60 and was about 1.3 × 107 s-1 in
•-
PhCN. The radical ion pair (Ph2N)3-ZnP)•+-C60 generated
•-
3
via ZnP* may have triplet spin character, which was sup-
ported by the easy quenching with added O2 (see Supporting
Information).26 The rate of charge-recombination (kCTR) for
((Ph2N)3-ZnP)•+-C60•- with triplet spin character evaluated from
the decay of the 1000 and 1300 nm bands was about 1.0 × 106
s-1, which corresponds to the lifetime of 1000 ns in PhCN and
may be far slower than the fast charge-recombination with
singlet spin character.
Figure 8. Nanosecond transient absorption spectra of 2 in Ar-saturated
PhCN obtained by 532-nm laser light irradiation. Inset: Absorption
time profile at 1300 nm.
light excitation of the ZnP moiety in deaerated PhCN are shown
in Figure 8. Absorption bands appeared at 720, 840 (shoulder),
1000, and 1200-1300 nm at 100 ns after the 6 ns laser pulse
excitation. Similar transient absorption spectra were also
observed with 355 nm laser light excitation of 2, where the
(Ph2N)3 moiety was predominantly excited, in addition to a small
fraction of the ZnP and fullerence moieties. Importantly, a peak
at 1000 nm, corresponding to the formation of the fulleropyr-
rolidine anion radical (C60•-), and an intense and broad
absorption band at 1200-1300 nm, corresponding to the cation
radical [(Ph2N)3-ZnP]•+, were observed. As mentioned earlier,
the absorption peak position of [(Ph2N)3-ZnP]•+ was different
from that typically observed for the ZnP cation radicals, where
only a broad band in the 500-700 nm region was reported.15-18
The intense absorption band at 720 nm was also due to
[(Ph2N)3-ZnP]•+. These results clearly demonstrate the genera-
tion of a charge-separated state [((Ph2N)3-ZnP)•+-C60•-] as the
main product by the excitation of either the zinc porphyrin entity
or the triphenylamine entities of the multimodular system (2)
in PhCN.
For compound 1 in PhCN, the nanosecond transient absorp-
tion spectra recorded following 532 nm laser irradiation
exhibited absorption peaks at 700, 800-840, and 1020 nm at
100 ns in PhCN (Figure 9). The transient absorption band at
1020 nm was attributed to C60•-, and the cation radical of the
free-base porphyrin moiety appeared around 750 nm, as was
expected from Figure 4a. These spectral features support the
occurrence of the excited-state charge-separation from the
free-base porphyrin to the fullerene moiety, generating
((Ph2N)3-H2P)•+-C60•-. The time profile at 1020 nm showed the
quick decay within the laser pulse (ca. 10 ns) that was followed
by the slow rise, generating prolonged ((Ph2N)3-H2P)•+-C60
.
•-
The quick decay may correspond to the fast charge-recombina-
tion of the radical ion pair with singlet spin character. From
the slow rise, the rate for slow charge-separation via the triplet
states was kTCS ) 1.0 × 106 s-1 in PhCN. The rate of charge-
T
recombination (kCR) for ((Ph2N)3-H2P)•+-C60 was evaluated
•-
from the decay of the 1020 nm band and is about 5 × 105 s-1
,
The inset of Figure 8 shows the decay time profile of
((Ph2N)3-ZnP)•+ at 1300 nm in PhCN; almost the same decay
profile was observed for the C60•- at 1000 nm. These diagnostic
peaks are useful probes for examining the reaction course of
((Ph2N)3-ZnP)•+-C60•-. Both spectral time profiles exhibited the
same rise and the decay, and their profiles followed a mono-
exponential decay. The initial rise was slow, reaching a
maximum at 200 ns, which does not correspond to the
which corresponds to the 2000 ns lifetime in PhCN and is slower
than kTCR for ((Ph2N)3-ZnP)•+-C60•-. This suggests that they are
in the inverted region of the Marcus parabola.2,11
In less polar o-DCB with 532 nm laser excitation of 1 and 2,
quite different transient absorption spectra were observed, in
which most of the bands were attributed to the triplet states
(see Supporting Information), suggesting that the initial charge-

8558 J. Phys. Chem. A, Vol. 111, No. 35, 2007
D’Souza et al.
TABLE 3: Charge-Separation Rate-Constant (kTCS) via
Chemicals. Tetrabutylammonium perchloride, (n-Bu4N)ClO4
used in electrochemical studies was from Fluka Chemicals.
Triplet Excited State, Charge-Recombination Rate Constant
(kTCR), and Lifetime of the Radical Ion-Pair (τRIP) with Triplet
Spin Character for the Multimodular Conjugates in PhCN
Synthesis of Conjugates 1 and 2 and Control Compounds
3 and 4. 5-(4′-Hydroxy phenyl)-10,15,20-tri(N,N-diphenylami-
nophenyl) porphyrin, 1a: In a round-bottom flask, 4 equiv of
pyrrole (1.25 g, 18.5 mmol), 3 equiv of diphenyl aminoben-
zaldehyde (3.75 g 13.7 mmol), and 1 equiv of para hydroxy-
benzaldehyde (0.565 g, 4.6 mmol) were added to 250 mL of
propionic acid, and the mixture was refluxed for 4 h. The solvent
was removed under vacuum, and the crude product was adsorbed
onto basic alumina and was purified by column chromatography
on basic alumina with chloroform/methanol (91:9 v/v) as eluent.
Yield ∼4.6%. 1H NMR in CDCl3, δ (ppm) 9.0 (m, 8H,
â-pyrrole-H), 8.1 (m, 6H, ortho-phenyl-H), 7.42 (m, 6H, meta-
phenyl-H), 8.9, 8.1 (d, d, 4H, substituted phenyl-H), 7.0-7.16
(m, m, 30 H, N-phenyl), 5.35 (s (br), 1H, hydroxy-H), -2.79
(s, 2H, imino-H).
kTCS
kTCR
τRIP
(ns)
compound
(s-1
)
(s-1
)
kSCS/kTCR
1
2
1.1 × 106
5.0 × 105
2000
1000
2200
5200
1.3 × 107
1.0 × 106
recombination is faster than the l0-ns laser pulse; in addition,
the second charge-separation via the triplet states is an inefficient
process.
As was mentioned earlier, with the 355-nm laser pulse
primarily exciting the triphenylamine moiety of 1 and 2, energy-
transfer from the excited-state of the triphenylamine moieties
to the porphyrin moieties generates their excited states, from
which electron-transfer takes place. In addition, direct photo-
ejection from the triphenylamine moieties to the fullerene may
occur, although this process is minor in PhCN as compared to
the charge-separation process via the excited states of the
porphyrin moieties.
5-{4′′-Formyl benzoic acid-4′-phenyl ester}-10,15,20-tri(N,N-
diphenylaminophenyl) porphyrin, 1b: In a round-bottom flask,
100 mg of 5-(4′-hydroxyphenyl)-10,15,20-tri(N,N-diphenyl-
aminophenyl) porphyrin (0.088 mmol), 1 equiv of 4-carboxy-
benzaldehyde (14 mg, 0.088 mmol), and
5 equiv of
By using the kSCS and kCTR thus evaluated in PhCN, the ratios
kSCS/kTCR were evaluated as a measure of the extent of charge-
stabilization in the photoinduced electron-transfer process. These
values were 2200 for 1 and 5200 for 2 (Table 3). The
magnitudes of kTCR, evaluated for the present multimodular
systems, are smaller than the reported ones for a number of
tetraphenylporphyrin-fullerene based dyads in the literature,14c
which is a prominent effect induced by the introduction of three
triphenylamine moieties on the porphyrin core. This clearly
demonstrates charge-stabilization in the present multimodular
systems due to the delocalization of the radical cations as
supported by the optical spectral data (Figures 1 and 3) and by
the HOMO in Figure 5.
4-di(methylamino)pyridine (DMAP, 53 mg, 0.44 mmol) were
dissolved in 50 mL of dry CH2Cl2. The reaction mixture was
cooled to 0 °C, and 5 equiv of N,N′-dicyclohexylcarbodiimide
(DCC, 90 mg, 0.44 mmol) was slowly added. The reaction
mixture was stirred for 6 h at room temperature, and the solvent
was removed. The crude compound was washed with water
several times and was extracted with CHCl3. Further purification
of the compound was carried out on a silica gel column using
1
toluene and chloroform (95:5 v/v) as eluent. Yield 65%. H
NMR in CDCl3, δ (ppm) 10.1 (1H, -CHO), 9.05 (m, 8H,
â-pyrrole-H), 8.1 (m, 6H, ortho-phenyl-H), 7.47 (m, 6H, meta-
phenyl-H), 8.9, 8.3 (d, d, 4H, substituted phenyl-H), 8.55, 7.65
(d, d, 4H, phenyl-CHO), 7.1-7.2 (m, 30 H, N-phenyl), -2.79
(s, 2H, imino-H).
Summary
5-[2-(4′′-Benzoic acid-4′-phenyl ester)-N-methyl-3,4-fullero-
pyrrolidine]-10,15,20-tri(N,N-diphenylaminophenyl) porphyrin,
1: To 100 mL of dry toluene, compound 1b (72 mg, 0.057
mmol), 3 equiv of C60 (123 mg, 0.171 mmol), and 3 equiv of
sarcosine (15 mg, 0.171 mmol) were added, and the mixture
was refluxed for 12 h. Solvent was removed under vacuum,
and the crude compound was adsorbed onto silica gel and was
purified on a silica gel column using toluene and ethyl acetate
(92:8 v/v) as eluent. Yield 37%. ESI mass in CH2Cl2 matrix:
calculated, 2009.7; found, 2010.3. 1H NMR in CDCl3, δ (ppm)
8.97 (m, 8H, â-pyrrole-H), 8.1 (m, 6H, ortho-phenyl-H), 7.41
(m, 6H, meta-phenyl-H), 8.9, 8.25 (d, d, 4H, substituted phenyl-
H), 8.42, 7.61 (d, d, 4H, phenyl-pyrrolidine), 7.1-7.2 (m, 30H,
N-phenyl), 5.1 (s, 1H, pyrrolidine-H), 5.05, 4.31 (d, d, 2H,
pyrrolidine-H) 2.84 (s, 3H, pyrrolidine N-CH3), -2.8 (s, 2H,
imino-H). UV-vis in o-DCB, λ (nm), 305, 333, 435.5, 525.6,
566.3, 597, 657.1.
Novel multimodular systems, composed of three entities of
triphenylamine, porphyrin, and fullerene, were designed, syn-
thesized, and studied to investigate photoinduced processes by
time-resolved spectroscopic methods. The spectral and compu-
tational studies revealed appreciable electronic interactions
between the porphyrin π-system and the meso-substituted
triphenylamine entities. The free-energy change for charge-
separation and charge-recombination were evaluated and com-
pared between (Ph2N)3-ZnP-C60 and (Ph2N)3-H2P-C60. The
charge-separation processes were monitored by steady-state and
time-resolved emission studies, which revealed the occurrence
of sequential energy-transfer and electron-transfer processes. The
rate of charge-separation was found to depend on the free-energy
changes. The rate of charge-recombination, as monitored by
nanosecond transient absorption spectral studies, revealed
charge-stabilization in these multimodular conjugates. Long-
lived charge-separated states on the order of microseconds were
obtained in polar PhCN, and this was attributed to the delocal-
ization of the porphyrin π-cation radical to the triphenylamine
entities. The present study offers a new route to generate long-
lived charge-separated states.
5-[2-(4′′-Benzoic acid -4′-phenyl ester)-N-methyl-3,4-fulle-
ropyrrolidine]-10,15,20-tri(N,N-diphenylaminophenyl) porphy-
rinatozinc(II), 2: Pentad 1 (25 mg, 0.0124 mmol) was dissolved
in 30 mL of CHCl3, and an excess of zinc acetate (50 equiv) in
methanol was added. The course of the reaction was monitored
spectroscopically. At the end of the reaction (∼1 h), the solvent
was evaporated, and the product was purified on silica gel
column using toluene as eluent. Yield 95%. ESI mass in CH2-
Experimental Section
Chemicals. Buckminsterfullerene (C60, +99.95%) was ob-
tained from SES Research (Houston, TX). All of the reagents
were from Aldrich Chemicals (Milwaukee, WI), whereas the
bulk solvents utilized in the syntheses were from Fischer
1
Cl2 matrix: calculated, 2073.1; found, 2074.6. H NMR (CS2:
CDCl3 (1:1 v/v)) δ (ppm) 9.04 (m, 8H, â-pyrrole-H), 8.08 (m,
6H, ortho-phenyl-H), 7.43 (m, 6H, meta-phenyl-H), 9.0, 8.25

Charge Stabilizing “Antenna-Reaction Center” Mimics
J. Phys. Chem. A, Vol. 111, No. 35, 2007 8559
(d, d, 4H, substituted phenyl-H), 8.46, 7.61 (d, d, 4H, phenyl-
pyrrolidine), 7.1-7.2 (m, 30H, N-phenyl), 5.04 (s, 1H, pyrro-
lidine-H), 5.0, 4.31 (d, d, 2H, pyrrolidine-H), 2.84 (s, 3H,
pyrrolidine N-CH3). UV-vis in o-DCB, λ (nm), 306, 330,
437.5, 554.64, 597.6.
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).26
Acknowledgment. This work is supported by the National
Science Foundation (Grant No. 0453464 to F. D.), by the donors
of the Petroleum Research Fund as administered by the
American Chemical Society, and by Grants-in-Aid for Scientific
Research on Primary Area (417) from the Ministry of Education,
Science, Sport, and Culture of Japan.
5-[Benzoic acid-4′-phenyl ester))-10,15,20-tri(N,N-diphenyl-
aminophenyl) porphyrin, 3: In a round bottom flask, 100 mg
of compound 1a (0.088 mmol), 1.5 equiv of benzoyl chloride
(15.3 µL, 0.132 mmol), and 1.5 equiv of pyridine (10 µL, 0.132
mmol) were combined in 25 mL of dry CH2Cl2, and the mixture
was refluxed for 5 h. The solvents were removed under vacuum.
The crude product was washed with water and was extracted
in CHCl3. Further purification of the compound was carried out
on a silica gel column using hexanes and chloroform (9:1 v/v)
Supporting Information Available: Excitation spectrum
and transient absorption spectra. This material is available free
of charge via the Internet at http://pubs.acs.org.
1
as eluent. Yield 82%. H NMR in CDCl3, δ (ppm) 9.02 (m,
References and Notes
8H, â-pyrrole-H), 8.09 (m, 6H, ortho-phenyl-H), 7.47 (m, meta-
phenyl-H), 8.92, 8.39 (d, d, 4H, substituted phenyl-H), 8.29,
7.74 (d, m, 5H, phenyl), 7.1-7.2 (m, 30H, N-phenyl), -2.79 (s,
2H, imino-H).
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5-[Benzoic acid-4′-phenyl ester))-10,15,20-tri(N,N-diphenyl-
aminophenyl) porphyrinatozinc(II), 4: Compound 3 (50 mg,
0.0404 mmol) was added to 20 mL of CHCl3, and an excess
of zinc acetate in methanol was added. The mixture was
stirred, and the course of the reaction was monitored spectro-
scopically. At the end of the reaction (∼3 h), the solvent was
evaporated, and the product was purified on a silica gel column
using toluene as eluent. Yield 93%. ESI mass in a CH2Cl2
matrix: calculated, 1299.84; found, 1298.41. 1H NMR CDCl3,
δ (ppm) 9.12 (m, 8H, â-pyrrole-H), 8.07 (m, 6H, ortho-phenyl-
H), 7.47 (m, 6H, meta-phenyl-H), 9.02, 8.29 (d, d, 4H,
substituted phenyl-H), 8.41, 7.74 (d, m, 5H, phenyl-pyrrolidine),
7.14-7.2 (m, 30H, N-phenyl), UV-vis in o-DCB, λ (nm), 308,
438, 556, 598.
Instrumentation. The UV-vis spectral measurements were
carried out with a Shimadzu model 1600 UV-vis spectropho-
tometer. The steady-state fluorescence emission was monitored
by a Varian Eclipse spectrometer. A right angle detection
1
method was used. The H NMR studies were carried out on
Varian spectrometers at either 400 or 300 MHz. Tetramethyl-
silane (TMS) was used as an internal standard. Cyclic voltam-
mograms were recorded on an EG&G model 263A potentiostat
using a three electrode system in benzonitrile containing 0.1 M
(n-Bu4N)ClO4 as the supporting electrolyte. A platinum button
or a glassy carbon electrode was used as the working electrode.
A platinum wire served as the counter electrode, and Ag/AgCl
was used as the reference electrode. Ferrocene/ferrocenium (Fc/
Fc+) redox couple was used as an internal standard. All of the
solutions were purged with argon gas prior to spectral measure-
ments.
The computational calculations were performed by the DFT
B3LYP/3-21G(*) method with the GAUSSIAN-03 software
package.23 The graphics of frontier orbitals were generated using
the GaussView software.
Time-Resolved Emission and Transient Absorption Mesure-
ments. The picosecond time-resolved fluorescence spectra were
measured using an argon-ion pumped Ti:sapphire laser (Tsu-
nami; pulse width ) 2 ps) and a streak scope (Hamamatsu
Photonics; response time ) 10 ps). The details of the experi-
mental setup are described elsewhere.26 Nanosecond transient
absorption measurements were carried out using the third
harmonic generation (THG, 355 nm) and second harmonic
generation (SHG, 532 nm) of an Nd:YAG laser (Spectra
Physics, Quanta-Ray GCR-130, full-width at half-maximum
(fwhm) 6 ns) as an excitation source. For the transient absorption

8560 J. Phys. Chem. A, Vol. 111, No. 35, 2007
D’Souza et al.
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