Rapid intramolecular hole hopping in meso-meso and meta-phenylene linked linear and cyclic multiporphyrin arrays

Article abstract of DOI:10.1021/ja908605s

A series of zinc porphyrin arrays comprised of a meso - meso linked porphyrin dimer, a metaphenylene linked dimer, gable-like tetramers consisting of the meso - meso linked dimers bridged via a meta-phenylene linker, and a dodecameric ring composed of this alternating dimeric pattern were singly oxidized, and intramolecular hole hopping between the porphyrin moieties was probed using electron paramagnetic resonance (EPR) spectroscopy. Electron nuclear double resonance (ENDOR) spectroscopy was also used to probe hole hopping within the dimers. Rapid hole hopping occurs between both porphyrins within both dimers and among three porphyrins of the tetramers with rates > 10⁷ s^-1 at 290 K. Additionally, the hole hops among 8-12 porphyrins in the dodecameric ring with a rate that is > 10⁷ s^-1 at 290 K, but hopping is slow at 180 K. These results show that hole hopping is rapid even though the meta-phenyl bridges and direct meso - meso linkages do not provide optimal electronic coupling between the porphyrins within these multiporphyrin arrays. This greatly expands the scope of possible structures that can be employed to tailor the design of long distance charge transport systems.

Full text of DOI:10.1021/ja908605s

Published on Web 01/05/2010
Rapid Intramolecular Hole Hopping in meso-meso and
meta-Phenylene Linked Linear and Cyclic Multiporphyrin
Arrays
Thea M. Wilson,† Takaaki Hori,‡ Min-Chul Yoon,§ Naoki Aratani,‡ Atsuhiro Osuka,*,‡
Dongho Kim,*,§ and Michael R. Wasielewski*,†
Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center,
Northwestern UniVersity, EVanston, Illinois 60208-3113, Department of Chemistry, Graduate
School of Science, Kyoto UniVersity, Kyoto 606-8502, Japan, and Spectroscopy Laboratory for
Functional π-Electronic Systems and Department of Chemistry, Yonsei UniVersity,
Seoul 120-749, Korea
Received October 9, 2009; E-mail: m-wasielewski@northwestern.edu; osuka@kuchem.kyoto-u.ac.jp;
dongho@yonsei.ac.kr
Abstract: A series of zinc porphyrin arrays comprised of a meso-meso linked porphyrin dimer, a meta-
phenylene linked dimer, gable-like tetramers consisting of the meso-meso linked dimers bridged via a
meta-phenylene linker, and a dodecameric ring composed of this alternating dimeric pattern were singly
oxidized, and intramolecular hole hopping between the porphyrin moieties was probed using electron
paramagnetic resonance (EPR) spectroscopy. Electron nuclear double resonance (ENDOR) spectroscopy
was also used to probe hole hopping within the dimers. Rapid hole hopping occurs between both porphyrins
within both dimers and among three porphyrins of the tetramers with rates >107 s-1 at 290 K. Additionally,
the hole hops among 8-12 porphyrins in the dodecameric ring with a rate that is >107 s-1 at 290 K, but
hopping is slow at 180 K. These results show that hole hopping is rapid even though the meta-phenyl
bridges and direct meso-meso linkages do not provide optimal electronic coupling between the porphyrins
within these multiporphyrin arrays. This greatly expands the scope of possible structures that can be
employed to tailor the design of long distance charge transport systems.
Introduction
linked array systems typically connect the porphyrins using
phenylene, phenylene-ethynylene, and butadiyne linkers. Peng
The photosynthetic proteins in green plants and photosynthetic
bacteria have long provided the inspiration for developing
synthetic architectures to explore the energy and electron transfer
functions of these natural solar energy conversion systems.1,2
Porphyrin building blocks have been used in the quest to build
large circular chromophore arrays similar to those found in the
light-harvesting antenna complexes (LH1 and LH2) of photo-
synthetic purple bacteria3-6 largely because of their synthetic
accessibility relative to the natural chlorophylls, photochemical
stability, and similar optical and redox properties. Numerous
covalently linked7-17 and self-assembled18-21 multiporphyrin
cyclic arrays have been prepared previously. The covalently
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† Northwestern University.
‡ Kyoto University.
§ Yonsei University.
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10.1021/ja908605s  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 1383–1388 1383

A R T I C L E S
Wilson et al.
et al.22 prepared the first cyclic array that uses a direct
meso-meso linkage between two porphyrins. Six meso-meso
linked porphyrin dimers were coupled using a meta-phenylene
bridge in an alternating fashion to form a rigid dodecameric
ring with an ∼35 Å diameter. Efficient energy migration within
this cyclic array is attributed to its rigid structure and strong
dipole-dipole interactions between neighboring subunits.23-25
However, ground-state intramolecular hole hopping and/or
charge delocalization have yet to be explored in such systems,
a property which could render these materials useful in a variety
of applications as electronic materials.
and rings, generally consisting of 4-6 porphyrins, also exhibit
hole hopping phenomena as probed by EPR.12,35,39
In this study, we have used EPR and ENDOR spectroscopies
to explore hole hopping in a series of mono-oxidized multi-
porphyrin systems, including the meso-meso linked porphyrin
dimer, Z2B, the meta-phenylene linked dimer, MPD, gable-
like tetramers comprised of the meso-meso linked dimers
bridged by a meta-phenylene linkage (4ZA and 4ZB), and a
dodecameric ring (C12Z) comprised of this alternating pattern
(Figure 1). In these solution phase studies we found rapid hole
hopping between the two porphyrins within both dimers and
among three porphyrins within the tetramers. Additionally, hole
hopping within the dodecameric ring was observed among 8-12
porphyrins on the EPR time scale (>107 s-1). These results give
further insight into how meta-phenyl bridges and direct
meso-meso linkages influence hole mobilities in multiporphyrin
arrays. This is important for designing extended molecular
charge transport arrays.
Electron paramagnetic resonance (EPR) and electron nuclear
double resonance (ENDOR) spectroscopies are well-established
probes of charge hopping in both chemical and biological
systems.12,26-30 Notably, these techniques provide convincing
evidence that the oxidized primary electron donor in the bacterial
photosynthetic reaction center consists of a bacteriochlorophyll
(BChl) “special pair” over which the positive charge is de-
localized. The single-line, inhomogeneously broadened EPR
spectrum of the BChl2+• special pair narrows by a factor of 1/ꢀ2
over the signal obtained from monomeric BChl+•, while ENDOR
spectroscopy shows a 2-fold reduction of the isotropic hyperfine
Experimental Section
Synthesis. The synthesis of MPD40 and C12Z22,41,42 has been
described previously, while that of the model compounds Z2B,
4ZA, and 4ZB is detailed in the Supporting Information and is
minimally modified from literature procedures41,42 by adding phenyl
groups at the terminal meso positions of each acyclic array to
preclude oligomerization upon chemical oxidation with AgClO4/
I2. Briefly, N-bromosuccinimide was used to dibrominate the
meso-meso linked diporphyrin or H-terminated tetramers. A Suzuki
coupling between the dibromo compounds and phenylboronic acid
gave the phenyl-terminated oligomers Z2B, 4ZA, and 4ZB in 84,
89, and 85% yields, respectively.
Optical Spectroscopy. Steady-state absorption measurements
were performed on a Shimadzu 1601 UV-vis spectrometer. A 1.0
mm quartz cuvette was used, and all measurements were performed
at room temperature. Nonstabilized HPLC grade dichloromethane
(DCM) from Fischer and inhibitor-free HPLC grade tetrahydrofuran
(THF) from Aldrich were dried using a Glass Contour solvent
purification system.
EPR and ENDOR Spectroscopy. EPR and ENDOR spectra
were acquired with a Bruker Elexsys E580 spectrometer, fitted with
the DICE ENDOR accessory, EN801 resonator, and an ENI A-500
RF power amplifier. Applied RF powers ranged from 250 to 300
W across the 4 MHz scanned range, and the microwave power
ranged from 0.6 to 100 mW. The sample temperature was controlled
by a liquid nitrogen flow system. Solutions of ZnTPP+•, MPD+•,
Z2B+•, 4ZA+•, 4ZB+•, and C12Z+• ((0.25 -1.0) × 10-4 M) in
CH2Cl2/THF (9:1, v:v) were generated by addition of a solution of
I2 and AgClO4 dissolved in acetonitrile. The oxidation process was
monitored using UV-vis absorption spectroscopy to maximize the
production of the singly oxidized species. Oxidized samples were
loaded into 2.35 mm I.D. quartz tubes and sealed under vacuum
following several freeze-pump-thaw cycles. A spline fit baseline
correction was applied to the ENDOR spectra following integration.
+•
coupling constants (hfcc’s) of BChl2 relative to those of
BChl+•.31-33 Prior studies of radical cation hopping and/or
delocalization in mono-oxidized multiporphyrin systems have
examined dimers connected by phenylene, diphenylethyne, and
ethyne linkages.12,34-38 In one case, hole delocalization over
at least seven porphyrins has been observed in long linear arrays
on the EPR time scale (107 s-1),38 which is set by the hyperfine
coupling between the hole (unpaired electron) and the nitrogen
and hydrogen nuclei in the porphyrins. Other small two- or
three-dimensional porphyrin architectures such as boxes, stars,
(22) Peng, X.; Aratani, N.; Takagi, A.; Matsumoto, T.; Kawai, T.; Hwang,
I. W.; Ahn, T. K.; Kim, D.; Osuka, A. J. Am. Chem. Soc. 2004, 126,
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Results and Discussion
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Steady-State Spectroscopy. The normalized UV-vis absorp-
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Intramolecular Hole Hopping in Multiporphyrin Arrays
A R T I C L E S
Figure 1. Molecular structures.
meso-meso linked dimer Z2B,44,45 only the parallel Bx transi-
tions couple effectively, while the other dipole interactions
should be nearly zero for an averaged perpendicular conforma-
tion between the two zinc porphyrins. This results in a split of
the Z2B Soret band into a red-shifted Bx component at 461 nm
and unperturbed By and Bz components at 426 nm. The Q-band
of Z2B is also slightly red-shifted to 569 and 612 nm. Due to
the gable geometry of MPD,40 both H- and J-type exciton
interactions are present, which results in a split B-band that is
both red- and blue-shifted to 420 and 434 nm, respectively. In
the tetramers 4ZA and 4ZB the gable geometry is formed by
combining the meta-phenylene linkage in MPD and the directly
meso-meso linked zinc porphyrin geometry in Z2B in an
alternating fashion, such that in 4ZA there is one meso-meso
linkage and in 4ZB there are two. Thus, the intensity of the
red-shifted Bx band is larger in 4ZB than 4ZA. The absorption
spectra of the tetramers and C12Z are also further red-shifted
compared to that of Z2B, which is indicative of strong excitonic
coupling between the diporphyrin Z2B subunits.23
Upon partial chemical oxidation of the zinc porphyrin species
(Figures 3 and S1-S6), absorption changes are observed across
the spectrum that are consistent with those observed previously
for meso-substituted Zn porphyrin radical cations.46 In the case
of the exciton coupled dimers, tetramers, and cyclic array, a
decrease in the red-shifted B band is observed, as seen in the
Figure 2. UV-vis absorption spectra of ZnTPP (a), Z2B (b), MPD (c),
4ZA (d), 4ZB (e), and C12Z (f) in CH2Cl2/THF (9:1).
presented in Figure 2. It is well established that there is a
relatively large energy separation in metalloporphyrin mono-
mers, such as ZnTPP, between the S2 (B or Soret band) and S1
(Q-band) states, absorbing at 424 nm (B band) and 556 and
595 nm (Q bands). The UV-vis spectra of the dimers exhibit
perturbations in the B-bands due to strong excitonic interactions
between the two zinc porphyrins.43 The Soret band of the zinc
porphyrin monomer has two degenerate perpendicular compo-
nents Bx and By. However, as previously reported for the
(43) Kasha, M.; Rawles, H. R.; El-Bayoumi, M. L. Pure Appl. Chem. 1965,
11, 371–392.
(44) Aratani, N.; Osuka, A.; Kim, Y. H.; Jeong, D. H.; Kim, D. Angew.
Chem., Int. Ed. 2000, 39, 1458–1462.
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Chem. Soc. 1970, 92, 3451–3459.
9
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A R T I C L E S
Wilson et al.
Table 1. EPR Line Widths (∆H), Line Width Ratios of ZnTPP+•
Monomer to Porphyrin N-mers (∆HM/ ∆HN), and Number of
Porphyrins over Which the Radical Cation Is Shared (NS) Based
on Eq 1
molecule
∆H (G)
∆HM/ ∆HN
NS
ZnTPP+•
MPD+•
Z2B+•
5.8
4.2
4.3
3.5
3.4
2.1
-
-
1.4
1.3
1.7
1.7
2.8
1.9
1.8
2.7
2.9
7.6
4ZA+•
4ZB+•
C12Z+•
The number of sites over which the radical cation is shared
by either hole hopping or delocalization can be estimated from
the narrowing of Gaussian EPR lines using the equation of
Norris et al.,31
Figure 3. UV-vis absorption spectra of neutral Z2B (black) and partially
oxidized Z2B•+ (red) in CH2Cl2/THF (9:1).
1
∆HN )
∆HM
(1)
(
)
N
√
S
which relates the line width of the monomer, ∆HM, to the line
width when an unpaired spin is shared over NS molecular sites,
∆HN. The ratios of ∆HM to ∆HN are reported in Table 1 along
with the number of hopping sites NS, according to eq 1, over
which the charge is rapidly hopping on the EPR time scale (>107
s-1). More specifically, the EPR time scale is governed by the
hyperfine interactions between the unpaired spin and the nitrogen
and hydrogen nuclei of the porphyrins, such that a typical
porphyrin nitrogen hfcc of 1.54 G corresponds to a frequency
of 4.3 × 106 s-1, the rate at which charge hopping between
porphyrins must exceed in order to observe EPR line narrowing.
The unpaired spin is fully shared by the two porphyrins in the
MPD+• and Z2B+• dimers. However, in both tetramers, 4ZA+•
and 4ZB+•, the unpaired spin is only rapidly hopping among
three porphyrins. This could be due to the fact that the oxidation
potentials (HOMO energies) of the four porphyrins in the
tetramers are not sufficiently similar to allow thermally activated
hopping among all four porphyrins at room temperature. In this
case, the hole would spend more time on the two inner
porphyrins which have a slightly lower oxidation potential than
the two outer porphyrins, such that the overall charge hopping
process, as revealed by EPR line width narrowing, becomes
slow.48,49 According to eq 1, which assumes a Gaussian line
shape, the unpaired spin is rapidly hopping among nearly eight
porphyrins in C12Z+•.
These results were analyzed further by simulating the EPR
spectra using WinSIM.50 The four nitrogen and eight ortho-
phenyl hydrogen isotropic hfcc’s of ZnTPP+• were obtained
by fitting the EPR spectrum using a Gaussian line shape, which
gave nitrogen (aN) and proton (aH) coupling constants consistent
with the literature value, aN ) 1.54 G and aH ) 0.28 G.46,51
Spectra of the multiporphyrin arrays were then simulated by
adding the appropriate number of extra nuclei and dividing the
isotropic hfcc by the additional number of porphyrins, while
keeping the line width parameter constant. In the case of Z2B+•
for example, eight nitrogens were used (four for each porphyrin)
with aN ) 0.77 G and 12 ortho-phenyl hydrogens were used
Figure 4. CW-EPR spectra of ZnTPP+• (a), MPD+• (b), Z2B+• (c), 4ZA+•
(d), 4ZB+• (e), and C12Z+• (f) in CH2Cl2/THF (9:1) at 290 K. Microwave
power was 0.6 mW (a), 20 mW (b, c, f), or 60 mW (d, e) with a modulation
amplitude of 0.1 G (a) or 1 G (b-f) at 25 kHz. Simulations are shown in
red, and the simulation parameters are presented in Table S1.
UV-vis spectra of Z2B+• (Figure 3), as the exciton coupling
is lost upon the formation of the mono-oxidized species. The
degree of oxidation was maintained well under 50, 25, and 8%
for the dimers, tetramers, and dodecamer, respectively, to ensure
that each species contains a single radical cation.
EPR and ENDOR Spectroscopy. The EPR spectrum of
ZnTPP+• shown in Figure 4a is typical of a porphyrin π-cation
2
radical in the A2u ground state, exhibiting the characteristic
nine hyperfine lines due to the interaction of the unpaired spin
with the four pyrrole nitrogens and broadening of these lines
from the interaction of the unpaired spin with the phenyl protons.
The EPR spectra of MPD+•, Z2B+•, 4ZA+•, 4ZB+•, and C12Z+•
are inhomogeneously broadened into single unresolved lines
owing to the large number of electron-nuclear hyperfine
interactions within each molecule. At the same time, rapid
hopping or delocalization will reduce the magnitude of each
hyperfine interaction.31 As seen in Figure 4, the peak-to-peak
EPR line width decreases from 5.8 to 2.1 G as the number of
porphyrins increases from just a single ZnTPP to 12 porphyrins
in C12Z. The measured peak-to-peak line widths, ∆H, are
summarized in Table 1. While both dimers exhibit Gaussian
EPR line shapes, the line shapes of the larger systems 4ZA+•,
4ZB+•, and C12Z+• show ∼20-30% deviations from a pure
Gaussian (see below). All radical cations have measured
g-values of 2.0022 ( 0.0002, which is typical for organic
aromatic molecules.47
(48) Wilson, T. M.; Tauber, M. J.; Wasielewski, M. R. J. Am. Chem. Soc.
2009, 131, 8952–8957.
(49) Muller, U.; Baumgarten, M. J. Am. Chem. Soc. 1995, 117, 5840–
5850.
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(51) Fajer, J.; Davis, M. S. In The Porphyrins; Dolphin, D., Ed.; Academic
Press: New York, 1979; Vol. IV, pp 197-256.
(47) Gerson, F.; Huber, W. Electron spin resonance spectroscopy of organic
radicals; Wiley-VCH: Weinheim, 2003.
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Intramolecular Hole Hopping in Multiporphyrin Arrays
A R T I C L E S
Figure 5. Experimental CW-EPR spectrum of C12Z+• and simulated
spectra based on the extent of charge hopping between porphyrin units
within the ring on the EPR time scale. Simulation parameters are reported
in Table S1.
Figure 6. 1H-ENDOR spectra of ZnTPP+• (a), MPD+• (b), and Z2B+• (c)
at 290 K in CH2Cl2/THF. Microwave powers were 20-100 mW, and RF
power was 240-400 W with a modulation frequency of 100 kHz.
porphyrins is a thermally activated process, likely dependent
upon torsional motion between the porphyrin planes. Localiza-
tion of the radical cation in singly oxidized multiporphyrin
systems when the sample temperature is lowered to near the
freezing point or below has also been observed with diphenyl-
ethyne bridging units.12,35,36 However, porphyrins linked at their
meso positions to an ethyne group exhibited temperature
independent charge motion, which in fact may indicate that the
charge is fully delocalized within a HOMO that extends over
the entire multiporphyrin π system of this array.38 Counterions
and solvent polarity often play a decisive role as to whether a
from the six meso-substituted aryl groups with aH ) 0.14 G.
The simulated spectra are shown in red overlaying the experi-
mental data in Figure 4, and the simulation parameters are
reported in Table S1. Again, the dimer spectra were well
simulated by a spectrum having parameters consistent with hole
hopping between both porphyrins within the dimer, while the
tetramer spectra were best simulated with hole hopping among
only three porphyrins (Figures S7 and S8). However, assuming
a Gaussian line shape for the simulation of C12Z+• does not
yield a particularly good fit (Figure 5). Nonetheless, curve fitting
using a Gaussian line shape provides the best agreement with
the EPR spectrum of C12Z+• if the hole is hopping rapidly
-
charge will migrate throughout a molecule. The ClO4 coun-
terion used in these studies is considered to be a weakly
complexing ion with respect to its interactions with porphy-
rins;51,56 however, it is still likely that the hole localization
observed at low temperatures is a result of the inability of the
counterion to move between the porphyrins sufficiently fast to
accommodate charge hopping on a >107 s-1 time scale.
among all 12 units of the cyclic array at a rate >107 s-1
.
Simulations performed using either a pure Lorentzian (Figures
S9 and S10, Table S2) or a mixed Gaussian/Lorentzian (Voigt)
line shape (not shown) also indicated hole hopping among
10-12 porphyrins. In view of these simulations and the
measured ∆H values, it is concluded that the hole hops between
8 and 12 porphyrins in C12Z+• at a rate >107 s-1. The
observation of mixed Gaussian/Lorentzian (Voigt) line shapes
has also been reported in other large multiporphyrin38 arrays
and multi-BChl proteins.52-54 One possibility for the emergence
of Lorentzian line shapes along with narrowing is spin exchange;
however we consider this unlikely with the dilute oxidation
conditions used in these experiments. Instead, we hypothesize
that the hopping rate lies in an intermediate motion regime, in
which EPR line shapes have been shown to deviate from their
Gaussian shape observed at either the fast or slow hopping
limits.55
Upon lowering the temperature to 180 K, near the freezing
point of the solvent, the CW-EPR spectra of all radical cation
species broadened to approximately the width of the ZnTPP+•
monomer (Figure S11). The samples neither showed signs of
broadening until the temperature decreased below 250 K nor
exhibited narrowing upon heating to 310 K, except for 4ZB+•
and C12Z+•, which showed slight narrowing from 3.4 to 3.2 G
and from 2.1 to 2.0 G, respectively. This indicates that hole
hopping between meso-meso linked and meta-phenylene linked
ENDOR spectroscopy is often used to obtain individual
isotropic hfcc’s that cannot be easily resolved from inhomoge-
neously broadened EPR spectra. However, 1H-ENDOR spectra
for the systems larger than the Z2B and MPD dimers could
not be obtained. We suspect this may be due to changes in
(nuclear) relaxation times of the larger systems, in addition to
anticipated very small hyperfine splittings. Isotropic hfcc’s for
ZnTPP+•, Z2B+•, and MPD+• were obtained from ENDOR
spectroscopy in liquid CH2Cl2/THF (9:1, v:v) using the ENDOR
resonance condition νE(NDOR ) |νn ( aH/2|, where νE(NDOR are the
ENDOR transition frequencies and νn is the proton Larmor
frequency.57 The 0.79 MHz hyperfine splitting of ZnTPP+•
shown in Figure 6 is assigned to the phenyl protons.51 Both
Z2B+• and MPD+• show essentially identical spectra with a
hyperfine splitting of 0.42 MHz (Figure 6), nearly half that of
the monomer ZnTPP+• spectrum. This indicates that rapid hole
hopping is occurring in both dimers at a rate >107 s-1. While
these results are in agreement with previous studies on MPD,34
they contradict earlier results of Segawa and co-workers on
mono-oxidized meso-meso linked porphyrins.58,59 These in-
vestigators used NaAuCl4 as the oxidant in CHCl3, a combina-
(52) Kolbasov, D.; Srivatsan, N.; Ponomarenko, N.; Jager, M.; Norris, J. R.
J. Phys. Chem. B 2003, 107, 2386–2393.
(56) Fajer, J.; Borg, D. C.; Forman, A.; Felton, R. H.; Vegh, L.; Dolphin,
D. Ann. N.Y. Acad. Sci. 1973, 206, 349–364.
(53) Srivatsan, N.; Kolbasov, D.; Ponomarenko, N.; Weber, S.; Ostafin,
A. E.; Norris, J. R. J. Phys. Chem. B 2003, 107, 7867–7876.
(54) Srivatsan, N.; Weber, S.; Kolbasov, D.; Norris, J. R. J. Phys. Chem.
B 2003, 107, 2127–2138.
(57) Kurreck, H.; Kirste, B.; Lubitz, W. Electron Nuclear Double Reso-
nance Spectroscopy of Radicals in Solution; VCH: New York, 1988.
(58) Nakazaki, J.; Senshu, Y.; Segawa, H. Polyhedron 2005, 24, 2538–
2543.
(55) Tang, J.; Dikshit, S. N.; Norris, J. R. J. Chem. Phys. 1995, 103, 2873–
2881.
(59) Segawa, H.; Machida, D.; Senshu, Y.; Nakazaki, J.; Hirakawa, K.;
Wu, F. P. Chem. Commun. 2002, 3032–3033.
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A R T I C L E S
Wilson et al.
tion that results in stronger ion pairing. Using these same
experimental conditions, we also observed charge localization
onto one of the porphyrins of Z2B+• (Figure S12). These
findings exemplify how important the surrounding environment
is toward facilitating charge hopping.
among 8-12 porphyrins, nearly the same extent observed
among BChls in the natural light-harvesting systems.
Conclusions
Hole transfer among two different sets of singly oxidized
porphyrin dimers, either directly connected with a meso-meso
linkage or bridged by a meta-phenylene linkage, and larger
arrays comprised of these subunits, including two gable-like
tetramers and a cyclic dodecamer, were studied using EPR and
ENDOR spectroscopies. Thermally activated hole hopping at
rates faster than 107 s-1 was found to occur in both of the dimers
at 290 K. In the case of the tetramers, it was found that rapid
hole hopping occurs only between three porphyrin molecules,
likely due to small energetic differences in the HOMOs between
the outer and inner porphyrins. In the cyclic array, hole hopping
among 8-12 porphyrins was observed at 290 K (but not at 180
K), which is a step closer toward reaching the extent of charge
hopping observed in cyclic BChl arrays within natural photo-
synthetic light-harvesting systems. Our results expand upon the
initial studies of charge transfer in meta-phenylene linked
porphyrins34 and demonstrate for the first time that charge
hopping occurs readily in nearly orthogonal meso-meso linked
porphyrins. Hole hopping is rapid in these systems, even though
the meta-phenyl bridges and direct meso-meso linkages do not
provide optimal electronic coupling between the porphyrins
within these multiporphyrin arrays. This greatly expands the
scope of possible structures that can be employed to tailor the
design of long distance charge transport systems.
The porphyrin meso positions have large spin densities, so
that despite the nearly orthogonal orientation of the porphyrin
planes, which limits electronic coupling, charge hopping still
occurs. Other examples of charge hopping in orthogonally linked
molecular arrays include those of oligo(9,10-anthrylene)s, in
which both hole and electron hopping are observed,49 and our
recent findings of electron hopping in N-N linked perylene-
3,4:9,10-bis(dicarboximide) (PDI) oligomers.48 The PDI oligo-
mers differ from the meso-meso linked porphyrins in that the
PDI LUMO has nodes at its imide nitrogen atoms, while the
porphyrin HOMO has large orbital coefficients at the meso
positions. This suggests that the degree of electronic coupling
between adjacent π systems necessary to achieve rapid charge
hopping between them is relatively modest.
The dodecameric porphyrin array C12Z is to our knowledge
the largest artificial cyclic light-harvesting array in which EPR
experiments have been performed to probe charge migration
within the ring. In singly oxidized B880 light-harvesting proteins
isolated from photosynthetic bacteria, EPR line width narrowing
was interpreted as hole hopping or delocalization over ∼12
BChls.60,61 Further studies on B880+• explored in detail the
mechanisms and temperature dependence of the EPR line
narrowing, finding that at extremely low temperatures charge
transport becomes slower so that the hole is localized on one
or two BChls.53,54 In the larger cyclic array of BChls within
the LH1 protein, which contains about 32 BChls, charge hopping
Acknowledgment. This research was supported by the Chemical
Sciences, Geosciences, and Biosciences Division, Office of Basic
Energy Sciences, DOE under Grant No. DE-FG02-99ER14999. The
work at Kyoto University was partly supported by Grant-in-Aid
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan (No. 1920006(A), and 20108001 “pi-Space”).
T.H. acknowledges the Research Fellowships of the JSPS for Young
Scientists. The work at Yonsei University was supported by the
World Class University (no. 2008-8-1955) Programs of the Ministry
of Education, Science and Technology of Korea, and Fundamental
R&D Program for Core Technology of Materials funded by the
Ministry of Knowledge Economy (D.K.).
was found to occur over ∼11 BChls with a rate constant of
52
108-109 s-1
.
Again, the EPR line shapes were of mixed
Gaussian/Lorentzian character, and charge localization was
observed upon lowering the temperature. One of the primary
differences between the natural light-harvesting arrays and cyclic
porphyrin dodecamer C12Z is structural. C12Z is entirely
covalently linked with large dihedral angles between the π
systems of several of its porphyrins, while the natural systems
are self-assembled and have continuous networks of cofacially
overlapping BChls. Despite this large structural difference, the
rigidity of C12Z results in a system having porphyrins with
very similar oxidation potentials, which allows for hole hopping
Supporting Information Available: Details regarding the
synthesis and characterization of the molecules presented in this
study, and additional EPR/ENDOR data. This material is
available free of charge via the Internet at http://pubs.acs.org.
(60) Gingras, G.; Picorel, R. Proc. Natl. Acad. Sci. U.S.A. 1990, 87, 3405–
3409.
(61) Picorel, R.; Lefebvre, S.; Gingras, G. Eur. J. Biochem. 1984, 142,
305–311.
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