Mechanisms, pathways, and dynamics of excited-state energy flow in self-assembled wheel-and-spoke light-harvesting architectures

Article abstract of DOI:10.1021/jp064000i

Static and time-resolved optical measurements are reported for two cyclic hexameric porphyrin arrays and their self-assembled complexes with guest chromophores. The hexameric hosts contain zinc porphyrins and 0 or 3 free base (Fb) porphyrins (denoted Zn₆ or Zn₃Fb₃, respectively). The guests are a tripyridyl arene (TP) and a dipyridyl- substituted free base porphyrin (DPFb), each of which coordinates to zinc porphyrins of a host via pyridyl-zinc dative bonding. Each architecture is designed to have an overall gradient of excitedstate energies that affords excitation funneling within the host and ultimately to the guest. Collectively, the studies delineate the various pathways, mechanisms, and rate constants of energy flow among the weakly coupled constituents of the host-guest complexes. The pathways include downhill unidirectional energy transfer between adjacent chromophores, bidirectional energy migration between identical chromophores, and energy transfer between nonadjacent chromophores. The energy transfer to the lowest-energy chromophore(s) within the backbone of a hexameric host (Fb porphyrins in Zn₃Fb₃ or pyridyl-coordinated zinc porphyrins in Zn₆·TP and Zn₆·DPFb) proceeds primarily via a through-bond mechanism; the transfer is rapid (～40 ps depending on the array) and essentially quantitative (≥98%). The energy transfer from a pyridyl-coordinated zinc porphyrin of the host to the Fb porphyrin guest in the Zn₆·DPFb complex is almost exclusively Foerster through-space in nature; this process is much slower (～1 ns) and has a lower yield (65%). These studies highlight the utility of cyclic architectures for efficient light harvesting and energy transfer to a designated trapping site.

Full text of DOI:10.1021/jp064000i

J. Phys. Chem. B 2006, 110, 19121-19130
19121
Mechanisms, Pathways, and Dynamics of Excited-State Energy Flow in Self-Assembled
Wheel-and-Spoke Light-Harvesting Architectures
Hee-eun Song,† Christine Kirmaier,† Jennifer K. Schwartz,† Eve Hindin,† Lianhe Yu,‡
David F. Bocian,§ Jonathan S. Lindsey,‡ and Dewey Holten*,†
Department of Chemistry, Washington UniVersity, St. Louis, Missouri 63130-4889, Department of Chemistry,
North Carolina State UniVersity, Raleigh, North Carolina 27695-8204, and Department of Chemistry,
UniVersity of California at RiVerside, RiVerside, California 92521-0403
ReceiVed: June 26, 2006; In Final Form: July 25, 2006
Static and time-resolved optical measurements are reported for two cyclic hexameric porphyrin arrays and
their self-assembled complexes with guest chromophores. The hexameric hosts contain zinc porphyrins and
0 or 3 free base (Fb) porphyrins (denoted Zn6 or Zn3Fb3, respectively). The guests are a tripyridyl arene (TP)
and a dipyridyl-substituted free base porphyrin (DPFb), each of which coordinates to zinc porphyrins of a
host via pyridyl-zinc dative bonding. Each architecture is designed to have an overall gradient of excited-
state energies that affords excitation funneling within the host and ultimately to the guest. Collectively, the
studies delineate the various pathways, mechanisms, and rate constants of energy flow among the weakly
coupled constituents of the host-guest complexes. The pathways include downhill unidirectional energy transfer
between adjacent chromophores, bidirectional energy migration between identical chromophores, and energy
transfer between nonadjacent chromophores. The energy transfer to the lowest-energy chromophore(s) within
the backbone of a hexameric host (Fb porphyrins in Zn3Fb3 or pyridyl-coordinated zinc porphyrins in Zn6‚TP
and Zn6‚DPFb) proceeds primarily via a through-bond mechanism; the transfer is rapid (∼40 ps depending
on the array) and essentially quantitative (g98%). The energy transfer from a pyridyl-coordinated zinc porphyrin
of the host to the Fb porphyrin guest in the Zn6‚DPFb complex is almost exclusively Fo¨rster through-space
in nature; this process is much slower (∼1 ns) and has a lower yield (65%). These studies highlight the utility
of cyclic architectures for efficient light harvesting and energy transfer to a designated trapping site.
I. Introduction
cyclic arrays originated with the finding that the light-harvesting
antenna complexes of photosynthetic bacteria are organized in
Understanding how to design and construct light-harvesting
architectures that absorb light and funnel the resulting energy
to a designated site remains a central objective in artificial
photosynthesis. The inspiration for the design of synthetic light-
harvesting systems stems from the natural photosynthetic
systems, where the chlorophyll pigments in the antenna
complexes can number in the hundreds. Synthetic light-
harvesting arrays generally differ from most natural systems in
(1) having fewer numbers of pigments, (2) using porphyrins as
surrogates for chlorophylls, and (3) employing covalent linkages
between the porphyrins for organization rather than noncovalent
interactions in a protein matrix. The synthesis and properties
of such multiporphyrin arrays have been extensively reviewed.1-11
Porphyrinic compounds also have been prepared that undergo
self-assembly.12-18 The synthetic light-harvesting architectures
prepared to date are less sophisticated structurally and exhibit
poorer performance than natural systems owing both to inad-
equate synthetic methodology and a lack of knowledge concern-
ing how to design efficient light-harvesting systems. Accord-
ingly, versatile molecular architectures that provide insight into
how to control the flow of energy remain of great importance.
Several years ago we reported the synthesis of cyclic
hexameric arrays of porphyrins for studies of light-harvesting
phenomena (Chart 1).19-21 The chief motivation for preparing
large cyclic assemblies. Added impetus stemmed from the
possibility of exploiting the three-dimensional architecture of
cyclic arrays for host-guest and energy-funneling studies. In
this regard, energy-transfer modeling studies have indicated that
cyclic architectures are intrinsically more efficient than linear
or dendrimeric architectures in funneling energy to a single
trapping site.22 The synthesis of the cyclic hexamers shown in
Chart 1 was facilitated by the extensive prior work of Anderson
and Sanders, who described the synthesis and host-guest
properties of diverse cyclic arrays composed of two, three, or
four porphyrins.23 In the past two years, remarkably large cyclic
arrays (containing as many as 24 porphyrins) have been
constructed.24-30 However, to date only a handful of cyclic
multiporphyrin arrays has been shown to bind a guest porphyrin,
as is desired for energy-funneling studies.31-34
Chart 1 shows structures of two cyclic hexameric porphyrins
comprised of zinc porphyrins and 0 or 3 free base (Fb)
porphyrins (denoted Zn6 or Zn3Fb3, respectively). The porphy-
rins are linked via diphenylethyne units. The cyclic arrays are
reasonably shape-persistent35 and have a cavity diameter of ∼34
Å. Chart 1 also shows a tripyridyl arene (TP) and a dipyridyl-
substituted Fb porphyrin (DPFb), each of which can coordinate
in a hexameric array via pyridyl-zinc dative bonding to give a
self-assembled host-guest complex. The structures of the self-
assembled complex of Zn6 with the TP guest (denoted Zn6‚TP)
and of Zn6 with the dipyridyl Fb porphyrin (denoted Zn6‚DPFb)
* Author to whom correspondence should be addressed. E-mail:
holten@wustl.edu.
† Washington University.
‡ North Carolina State University.
§ University of California at Riverside.
10.1021/jp064000i CCC: $33.50 © 2006 American Chemical Society
Published on Web 09/13/2006

19122 J. Phys. Chem. B, Vol. 110, No. 39, 2006
Song et al.
CHART 1
are shown in Chart 2. Both host-guest architectures assemble
with very high affinity (Kassoc > 3 × 108 M-1) in toluene at
room temperature and thus are >95% assembled upon mixing
the host and guest in 1:1 ratio at 1 µM concentrations.34
The cyclic hexameric structure of Zn3Fb3 has been confirmed
by high-angle X-ray scattering studies in toluene solution.36 Such
analyses showed that (1) the cyclic hexameric architecture is
not rigid but is distributed across a conformational ensemble
with a mean diameter that is 1.5 Å smaller than the diameter of
a symmetric, energy-minimized model structure, (2) the con-
formational envelope has limits of 3 Å positional dispersion
and full rotational freedom for all six porphyrin groups, and
(3) insertion of the TP guest to give Zn3Fb3‚TP expands the
diameter of the host conformer by 0.6 Å and decreases the
amplitude of the configurational dispersion by ∼2-fold. Mo-
lecular dynamics simulations for Zn3Fb3 show bowing and
rotations of all six porphyrin groups, particularly those porphy-
rins that contain p-ethynyl-substituted meso substituents (i.e.,
the zinc porphyrins in Zn3Fb3), while retaining the overall mac-
rocyclic architecture. The extent of conformational motion is
restricted in Zn3Fb3‚TP versus that in Zn3Fb3. The guest por-
phyrin DPFb is expected to undergo rotation about its linear
axis in the host Zn6 owing to the presence of the p-ethyne sub-
stituents in DPFb and the large cavity of the cyclic hexameric
host.
the above energy-transfer processes are operative within the
backbone of the host, in addition to subsequent transfer to the
core guest porphyrin. A key objective is to assess the magnitude
of the various processes so as to better understand how to design
more efficient light-harvesting architectures. To do so requires
an in-depth understanding of the mechanism and rate of each
of the processes.
In this paper, we report the characterization of the multiple
pathways for excited-state energy flow in cyclic hexameric
arrays (Zn6, Zn3Fb3) and the corresponding self-assembled host-
guest constructs (Zn6‚TP, Zn6‚DPFb). In the Zn6 host, all the
chromophores are isoenergetic and energy flows between the
zinc porphyrins; in the Zn3Fb3 host, energy transfer is downhill
from the zinc to the Fb porphyrins. In the Zn6‚DPFb host-
guest assembly, the lowest-energy chromophores in the back-
bone of the array are the two zinc-pyridyl-coordinated por-
phyrins, and the lowest overall energy chromophore is the Fb
porphyrin in the guest. The results reported herein show that
energy transfer within the backbone of the array is very fast,
whereas transfer from a zinc-pyridyl-coordinated porphyrin to
the Fb porphyrin guest is surprisingly slow. The companion
paper reports the characterization of architecturally more
complex arrays composed of distinct patterns of zinc and Fb
porphyrins where the lowest-energy chromophore in the back-
bone of an array is not the pyridyl-coordinated zinc porphyrin
but rather a Fb porphyrin.40 Taken together, these studies provide
valuable insight for the design of more efficient self-assembled
light-harvesting arrays.
In previous studies of acyclic diarylethyne-linked arrays, we
found that the energy-transfer process between porphyrins is
predominantly through-bond (TB) rather than Fo¨rster through-
space (TS) in nature.37 The majority of our studies concerned
unidirectional (downhill) transfer processes between a zinc
porphyrin and a Fb porphyrin. We also characterized the rate
of reversible energy transfer between identical, isoenergetic zinc
porphyrins.38 A further set of studies characterized the transfer
between nonadjacent chromophores.39
II. Experimental Methods
A. Compounds. The cyclic hexamer hosts (Zn6 and Zn3-
Fb3),19 guests (TP19 and DPFb34), reference dyads (ZnFb-m/p,
ZnFb-p/m, and ZnZn-m/p),19 and reference monomers (ZnU and
FbU)41 were prepared as described previously. The self-
assembled host-guest complexes (Zn6‚TP and Zn6‚DPFb) were
The cyclic hexameric host-guest complexes provide an
opportunity to probe energy flow in small arrays wherein all of

Self-Assembled Light-Harvesting Architectures
J. Phys. Chem. B, Vol. 110, No. 39, 2006 19123
CHART 2
B. Optical Spectroscopy. Static absorption spectra (Cary
100) and fluorescence spectra and quantum yields (Spex Tau2;
5 nm band-pass) employed nitrogen-purged solutions (1-10 µM
for absorption and 0.3-1 µM for emission). Fluorescence yields
were obtained using nondegassed ZnTPP in toluene (Φf )
0.030) as a standard.42 Fluorescence lifetime studies employed
0.3-3 µM nitrogen-purged solutions. The latter studies used
either (1) a phase modulation technique in which emission was
collected using a long-pass filter43 or (2) a single-shot method
employing 30 ps, 2 mJ, 532 nm excitation flashes and a detection
system with an ∼1 ns instrument response function in which
emission was collected with a monochromator having a 10 nm
band-pass.44 Transient absorption measurements utilized non-
degassed 50-150 µM solutions excited with ∼130 fs, ∼20 µJ,
530-550 nm excitation flashes and white-light probe pulses of
comparable duration.45 All measurements were carried out at
room temperature. Kinetic modeling utilized the program
KINSIM46 as discussed previously.38
III. Results and Discussion
A. Molecular Architectures. Chart 3 gives schematic
representations of the hexameric arrays (Zn6, Zn3Fb3) and host-
guest complexes (Zn6‚TP, Zn6‚DPFb), along with the reference
dyads (ZnZn, ZnFb), and reference arrays wherein pyridine (pyr)
is coordinated to all of the zinc porphyrins (ZnFb‚pyr, ZnZn‚
pyr2, Zn6‚pyr6, Zn3Fb3‚pyr3) or one-half of the zinc porphyrins
(ZnZn‚pyr, Zn6‚pyr3). The arrows denote the various pathways
by which energy transfer can occur. For clarity, only one process
of a given type is indicated within each array. Structures of the
reference monomers and dyads are given in Chart 4.
The key features of the self-assembled host-guest complexes
that are relevant for understanding the energy-transfer dynamics
are as follows. A dipyridyl-substituted porphyrin guest is
necessarily equidistant (17.3 Å) from the two “bookend” zinc
porphyrins to which it is coordinated (porphyrins 2 and 5 in
Chart 2B). However, the distances to the guest from the
uncoordinated porphyrins in the hexamer differ with position
(e.g., porphyrins 1 and 4 in Chart 2B; 20.9 versus 14.2 Å). All
distances were established by molecular modeling as described
previously.19 The pyridyl-coordinated zinc porphyrins (denoted
ZnC) have a first excited singlet state at a lower energy than
uncoordinated zinc porphyrins (denoted ZnU) (e.g. Charts 3A
and 3B). On the basis of the average (0,0) absorption and
fluorescence wavelengths, the excited-state energy decreases in
the order: ZnU (2.08 eV) > ZnC (2.04 eV) > DPFb (1.91 eV).
This energy ordering is a key determinant of the pathways of
energy flow in the arrays.
prepared via titrations of stock toluene solutions of the host and
guest compounds as described previously19,34 and in the Sup-
porting Information. All of the above arrays were studied in
toluene.
B. Overview of Energy-Transfer Pathways and Mecha-
nisms. An overview of operable energy-transfer processes in
the arrays simplifies the presentation of the results. Of all the
processes illustrated in Chart 3, the only one for which we have
previously measured a rate within a cyclic hexamer is the
The cyclic hexamers Zn6‚pyr6 and Zn3Fb3‚pyr3, dyad ZnZn‚
pyr2, and monomer ZnU‚pyr, in which each of the zinc
porphyrins is singly coordinated by a pyridine (pyr) ligand, were
prepared by adding a ∼104-fold molar excess of pyridine to a
toluene solution of Zn6, Zn3Fb3, ZnZn-m/p, or ZnU, respectively.
The half-ligated hexamer Zn6‚pyr3 and dyad ZnZn‚pyr were
similarly prepared using a lower concentration of pyridine. In
all cases, the concentration of pyridine was less than 0.1% v/v.
(For Zn6‚pyr3 there will be a random distribution of uncoordi-
nated and coordinated porphyrins.) The arrays Zn6‚pyr6, Zn3-
Fb3‚pyr3, ZnZn‚pyr2, ZnU‚pyr, Zn6‚pyr3, and ZnZn‚pyr were
studied in the toluene/pyridine mixture in which they were
prepared.
Zn/UFb f ZnUFb* energy transfer in Zn3Fb3 (Chart 3D).19
A
rate constant k1 ) (34 ( 4 ps)-1 was obtained, with which the
value (38 ( 4 ps)-1 obtained here agrees well. These values
for k1 in the cyclic hexamer are quite similar to those we
previously measured for both the ZnFb-m/p and ZnFb-p/m dyads
(Charts 3H and 4), for which k1 ) (40 ( 4 ps)-1 19
The latter
.
results indicate that the energy-transfer dynamics are the same
whether the zinc and Fb porphyrins are attached to the
diphenylethyne linker via m/p or p/m connectivity. We will
assume that the analogous connectivity independence is true
for other processes within the cyclic hexamers, as is implicit in

19124 J. Phys. Chem. B, Vol. 110, No. 39, 2006
Song et al.
CHART 3
the kinetic schemes shown in Chart 3. This includes Zn/ ZnC
cyclic hexamers(Chart 3A). These bidirectional processes are
also expected to occur primarily by a linker-mediated TB
mechanism.
U
f ZnUZn/ (k4; Charts 3A, 3B, and 3E) and Zn/ Fb f ZnCFb*
C
C
(k3; Chart 3C). By analogy with our prior studies of energy
transfer between diarylethyne-linked porphyrins, all of the above
processes within the cyclic hexameric hosts occur predominantly
by a linker-mediated TB electron-exchange mechanism, with
minimal contribution from the Fo¨rster TS mechanism.37
By analogy to the unidirectional and bidirectional energy
transfer steps involving adjacent porphyrins of the hexameric
wheels, processes involving nonadjacent porphyrins can also
have a significant impact on energy flow. These nonadjacent
transfers (L1 in Chart 3A) occur by a TB superexchange
mechanism mediated by the intervening porphyrins (and linkers).
Our previous studies of diarylethyne-linked multiporphyrin
arrays have shown that unidirectional nonadjacent transfers
between first nonnearest neighbors in diarylethyne-linked por-
phyrins have rate constants that are on the average only 10-
fold smaller than the analogous adjacent transfers.38,39 This point
is illustrated in Chart 5B, where the rate of energy transfer from
a zinc porphyrin to a nonadjacent Fb porphyrin through an
intervening zinc porphyrin in a diphenylethyne-linked triad
having p/p connectivity occurs with a rate constant of ∼(200
ps)-1. Our modeling further indicated that the analogous,
bidirectional process involving nonadjacent zinc porphyrins has
a rate constant of ∼(1 ns)-1 (Chart 5C). Accordingly, we expect
that nonadjacent energy transfers within the hexameric arrays
involving the first nonnearest neighbor energetically distinct
porphyrins have a rate constant L1 ≈ (300-600 ps)-1. Processes
involving the second nonnearest neighbors are expected to be
much slower and are ignored.
In addition to unidirectional energy transfers, bidirectional
energy transfer can also occur between adjacent identical zinc
porphyrins within the cyclic hexamers, namely, Zn/ ZnU
/
U
ZnUZn/ (k2; Chart 3A). The rate constant for such a process is
U
typically difficult to obtain owing to the absence of a simple
spectral handle to monitor the dynamics. In this regard, we
recently studied a large series of linear and branched architec-
tures containing multiple zinc porphyrins and an energy
trap.38,39,47 Kinetic modeling of the measured excited-state
dynamics yielded a rate constant of (30 ( 10 ps)-1 for
bidirectional energy transfer between adjacent ZnU porphyrins
joined with diphenylethyne linkers and p/p connectivity (Chart
5B). This value can be scaled to estimate k2 for the analogous
process involving m/p connectivity between adjacent ZnU
porphyrins in the cyclic hexamers (Chart 3A). The scaling factor
is obtained from the ratio of measured rates of Zn/UFb f
ZnUFb/ energy transfer in the dyads ZnFb-p/m and ZnFb-p/p
(Charts 3H, 4, and 5A), namely, (40 ps)-1 versus (24 ps)-1
.
Thus, we estimate that k2 ≈ (30 ps)-1(24/40) ) (50 ps)-1 for
Despite the underlying kinetic complexity in large multipor-
phyrin arrays, the measured and modeled time evolution of the
bidirectional Zn/ ZnU / ZnUZn/ energy transfer within the
U
U

Self-Assembled Light-Harvesting Architectures
J. Phys. Chem. B, Vol. 110, No. 39, 2006 19125
CHART 4
CHART 5
chromophore (M1-M3; Chart 3A). The final trapping step is
Zn/ DPFb f ZnCDPFb* in Zn6‚DPFb (M1; Chart 3A) because
C
(1) the “bookend” coordinated zinc porphyrins are the lowest-
energy components within the backbone and (2) the TB transfer
to these ZnC units is expected to compete effectively with TS
transfer from the ZnU units to the central trap (M2 and M3).
C. Energy Transfer in the Hexameric Hosts. To guide the
presentation of the results, Table 1 summarizes the time
constants of the kinetic components derived from the transient
absorption measurements. The table also contains the associated
fluorescence yields and lifetimes, which are dominated by the
lowest-energy site(s) in each architecture.
1. Zn6 and Zn6‚TP. Figure 1B shows absorption and fluo-
rescence spectra of Zn6. The figure also shows the corresponding
spectra of Zn6 wherein each zinc porphyrin has a pyridine axial
ligand, Zn6‚pyr6. When the zinc porphyrins are uncoordinated,
Zn6 has a split Soret absorption (due to strong exciton coupling)
with maxima at 423 and 429 nm, Q(1,0) and Q(0,0) absorption
bands at 550 and 590 nm, and Q(0,0) and Q(0,1) fluorescence
features at 599 and 646 nm, respectively. For Zn6‚pyr6, the
absorption maxima are red-shifted to 430, 437, 563, and 607
nm, as are the fluorescence bands at 614 and 663 nm.
The fluorescence yield (Φf ) 0.040) and lifetime (τ ) 2.4
ns) of the zinc porphyrins in Zn6 are similar to those of
monomeric reference porphyrins such as ZnU (Chart 4 and
Table 1). The fluorescence yield is slightly increased (0.054),
and the lifetime is slightly decreased (2.1 ns) in Zn6‚pyr6. These
differences can be traced to an increased S1 f S0 radiative rate
constant (kf) upon ligation based on the values calculated from
the formula kf ) Φf/τ or the integrated intensity of the Q-band
absorption manifold.
excited-state populations can be fit primarily with a single
exponential reflecting the effective rise of energy at the trap
site (τrise) and the lifetime of the composite zinc porphyrin
manifold. This is true even for branched architectures containing
up to 21 porphyrins in which the distant zinc porphyrins are
separated from the trap by two intervening zinc porphyrins using
p/p connectivity.38 For example, τrise increases from ∼25 to ∼50
to ∼100 ps along the series in Chart 5.
The pathways and estimated rate constants described above
are a starting point for the elucidation of the energy-transfer
dynamics among the components of the hexameric wheels. The
host-guest complexes contain one or more additional processes
underlying the ultimate trapping steps to the DPFb guest
The absorption and fluorescence spectra of the Zn6‚TP
complex (Chart 3E) are shown in Figure 1A and are in good
agreement with those reported previously.34 Comparison of the
fluorescence spectrum (Figure 1A), quantum yield, and lifetime
(Table 1) with those for Zn6 and Zn6‚pyr6 shows that fluores-

19126 J. Phys. Chem. B, Vol. 110, No. 39, 2006
Song et al.
TABLE 1: Summary of Photophysical Dataa
fluorescence
transient absorption
lifetime components (ps)
τ (ns)b
compounds
c
d
e
Φf
Zn Fbhost Fbguest
Host-Guest Complexes
Zn6‚DPFb 3.1 ( 0.7 33 ( 9 730 ( 80 0.084 0.9
11
Zn6‚TP
0.055f 2.1
Hosts
Zn6
Zn6‚pyr6
Zn3Fb3
0.040 2.4
0.054 2.1
0.11
19 ( 2
19 ( 2
13
Zn3Fb3‚pyr3
Guest
DPFb
0.13
13
Reference Compounds
0.040g 2.2g
ZnZn-m/p 2.7 ( 0.3
ZnZn‚pyr2 3.6 ( 0.8
ZnZn‚pyr
41 ( 7
ZnFb-m/p
39 ( 4h
0.11g 0.039g 13.5g
FbU
ZnU
ZnU‚pyr
0.12h
13.3h
0.034i 2.4i
0.037 2.2
a All data were obtained at room temperature. Compounds with no
pyridine (pyr) ligands on zinc porphyrins were studied in toluene,
whereas compounds with one-half or all the zinc porphyrins ligated
were studied in toluene plus pyridine. b Lifetimes (7%. c Excited-state
annihilation. d Energy transfer within the hexameric host. e Time con-
stant of energy flow from host to guest. f The same value for Zn6‚TP
(0.055) within experimental error is obtained when taking into account
the absorption and fluorescence yields of the uncoordinated and
coordinated zinc porphyrins at the excitation wavelength. g From ref
19. h From ref 50. i From ref 51.
Figure 2. Transient absorption spectra for (A) ZnZn‚pyr and (B) cyclic
hexamer Zn6‚pyr3 obtained using a 130 fs excitation flash at 540 nm.
Panel C shows the Q(0,0) ground-state absorption band (solid lines)
and spontaneous fluorescence spectra (dashed lines) for Zn6 (green)
and Zn6‚pyr6 (blue).
quantitative Zn/ ZnC f ZnUZn/ energy transfer, as has been
U
C
reported previously.34
We had anticipated performing transient absorption experi-
ments on Zn6‚TP to directly measure the rate of this process.
However, such measurements were not possible because suf-
ficiently concentrated solutions (∼50 µM) could not be obtained.
Given that the hexamers themselves are sufficiently soluble, as
is the complex containing the dipyridyl-substituted porphyrin
guest (Zn6‚DPFb), the low solubility of the TP-containing arrays
is presumably due to diminished molecular motion. Thus, we
adopted an alternative strategy using coordination with pyridine
as described in the next section.
2. Zn6‚pyr3 and ZnZn‚pyr. Solutions of the half-ligated
hexamer (Zn6‚pyr3) and dyad (ZnZn‚pyr) were excited with a
130 fs excitation flash at 540 nm, which preferentially (but not
exclusively) pumps an uncoordinated zinc porphyrin. Time-
resolved absorption difference spectra of these compounds are
shown in Figures 2A and 2B. To aid in assignment of the
features, Figure 2C shows the Q(0,0) ground-state absorption
band (solid lines) and the Q(0,0) and Q(0,1) static spontaneous
fluorescence bands (dashed lines) of Zn6 and Zn6‚pyr6.
Two main features are present in the absorption difference
spectra for both ZnZn‚pyr and Zn6‚pyr3 (Figures 2A and 2B).
The spectrum at 1 ps for each array is dominated by features
Figure 1. Absorption (solid lines) and fluorescence (dashed lines)
spectra of (A) Zn6‚TP (maroon) and (B) Zn6 (green) and Zn6‚pyr6 (blue).
cence from the complex occurs almost exclusively from the
coordinated zinc porphyrins, even when the uncoordinated
components are preferentially (∼80%) excited at 550 nm (Figure
1B). In the case of Zn6‚TP, this result is indicative of essentially

Self-Assembled Light-Harvesting Architectures
J. Phys. Chem. B, Vol. 110, No. 39, 2006 19127
with a contribution of Fb* formed directly upon excitation in a
fraction of the arrays. As energy flows from Zn/U to Fb to form
additional Fb*, the stimulated emission at ∼720 nm grows in
amplitude, and the shorter-wavelength features evolve as well.
Well before 360 ps, the spectrum has attained a constant shape
that can be assigned to Fb*.
The absorption difference spectra give a Zn/U lifetime of 19
( 2 ps for Zn3Fb3. This value is the same within experimental
error as the lifetime of 17 ( 2 ps that we obtained previously19
and is much shorter than the lifetime of 2.4 ns for Zn/U in the
absence of energy transfer (Table 1). From these data, the rate
constant and yield of Zn/ Fb f ZnUFb* transfer are k1
)
U
[1/(19 ps) - 1/(2.4 ns)]/2 ) (38 ps)-1 and Φtrans ) 1 - [2(19
ps)]/(2.4 ns) ) 0.98 (Chart 3D). The factor of 2 appears in the
calculations because each Zn/U has two adjacent Fb porphyrins
to which energy transfer may occur. The value of k1 is the same
within error as that of (40 ps)-1 for the ZnFb dyads (Chart 3H).19
Also, upon arrival on the Fb porphyrin, the excited state decays
with a (fluorescence) lifetime of ∼13 ns that is essentially the
same as the value obtained in monomeric reference porphyrins
such as FbU (Table 1). This indicates that Fb* in the cyclic
hexamer is not affected appreciably by potential competing
processes such as charge transfer.
Figure 3. Transient absorption spectra for (A) Zn3Fb3 obtained using
a 130 fs excitation flash at 540 nm. Panel B shows absorption (solid
lines) and fluorescence (dashed lines) spectra of the reference com-
pounds ZnU (green) and FbU (red).
The results obtained for Zn3Fb3‚pyr3 (Chart 3C) are similar
to those reported above for Zn3Fb3 (with the exception of
spectral shifts). In particular, the Zn/ lifetime in Zn3Fb3‚pyr3 is
C
19 ( 2 ps. Through the use of the Zn/C lifetime of 2.1 ns in the
expected for Zn/U, which is the component primarily excited by
the pump pulse. The feature at ∼600 nm contains a contribution
of bleaching of the corresponding Q(0,0) ground-state absorption
band plus Q(0,0) stimulated emission. This emission is stimu-
lated by the white-light probe pulse and has characteristics
(shape and wavelength) similar to those in the analogous
spontaneous fluorescence spectrum of Zn/U (Figure 2C, green).
The absorption difference spectrum at ∼1 ns (and much earlier)
for both half-ligated arrays exhibits features that are red-shifted
from those present at 1 ps. The red shift occurs because the
features now correspond to those expected for the formation of
Zn/C based on the static reference data (Figure 2C, blue). Thus,
these observations provide direct time-resolved evidence for the
downhill energy transfer Zn/ ZnC f ZnUZn/ , corroborating the
absence of energy transfer (Table 1), the rate constant and yield
of Zn/ Fb f ZnCFb* energy transfer in Zn3Fb3‚pyr3 are
C
calculated to be k3 ≈ (38 ps)-1 and Φtrans ) 0.98.
Collectively, the above findings on the rates and yields of
energy transfer to the lowest-energy component(s) in the
hexameric wheels are essential for understanding the dynamics
in the host-guest complexes described next and in the
companion paper.40 These values will be used in the analysis
of energy flow in the latter arrays, which exhibit the same
processes within the host plus one or more additional pathways
for energy trapping by the guest chromophore.
D. Energy Transfer in the Zn6‚DPFb Host-Guest Com-
plex. Figure 4 compares the absorption and fluorescence spectra
of the Zn6 host, DPFb guest, and the Zn6‚DPFb complex. The
fluorescence feature at 610 nm in Zn6‚DPFb is exclusively the
Q(0,0) band of the zinc porphyrins of the host while the feature
at 718 nm is exclusively the Q(0,1) band of the guest; the feature
at 652 nm derives from both components. On the basis of these
spectra, and the measured fluorescence yields of the components,
and the fact that the host absorbs 92% of the light at the 550
nm excitation wavelength, the yield of energy transfer from the
host to the guest was found to be 0.63. This value is somewhat
greater than the yield of 0.4 estimated previously from the
emission spectral data.34
U
C
findings from the fluorescence spectrum, lifetime, and yield for
Zn6‚TP.
The lifetime of photoexcited Zn/U was determined from the
time constants obtained from the time evolution of the absorption
difference spectra over the range from 550 to 770 nm. The
excited-state lifetime obtained is 41 ps for both ZnZn‚pyr and
Zn6‚pyr3.48 This Zn/U lifetime is much shorter than the value of
2.4 ns that is observed for complexes in which downhill energy
transfer cannot occur, such as the Zn6 hexamer and the ZnU
monomer (Table 1). From these values, the rate constant k4 )
1/(41 ps) - 1/(2.4 ns) ) (42 ps)-1 and yield Φtrans ) 1 - (41
ps)/(2.4 ns) ) 0.98 for Zn/ ZnC f ZnUZn/ energy transfer are
Fluorescence lifetimes give a second measure of the host-
to-guest energy-transfer yield. The fluorescence of Zn6‚DPFb
has two lifetime components, with time constants of 0.9 and
11 ns (Table 1). The longer value is only slightly shorter than
that for DPFb alone, indicating that the decay of the excited
energy trap is not appreciably affected by its presence in the
complex. We assign the shorter component to the lifetime of
the ZnC porphyrins in the host (Chart 3A), which is shorter than
the lifetime of 2.1 ns found for coordinated Zn6‚pyr6. These
U
C
obtained (Charts 3B and 3F).
3. Zn3Fb3 and Zn3Fb3‚pyr3. Transient absorption spectra for
Zn3Fb3 are shown in Figure 3A. The spectra were obtained using
excitation at 540 nm, which primarily (but not exclusively)
pumps a zinc porphyrin in each array. On the basis of the
reference static absorption and fluorescence data shown in
Figure 3B, the formation of either Zn/U or Fb* will exhibit
bleaching and/or stimulated emission at ∼590 and ∼650 nm,
albeit with different relative amplitudes. However, only Fb* will
have a feature at ∼720 nm, namely, Q(0,1) stimulated emission.
On this basis, the 1 ps spectrum is assigned primarily to Zn/U,
data afford a yield Φtrans ) 1 - (0.9/2.1) ) 0.57 for Zn/ DPFb
C
f ZnCDPFb* energy transfer in the Zn6‚DPFb complex.
Time-resolved absorption data give another measure of energy
flow within Zn6‚DPFb (Figure 5A). On the basis of the static

19128 J. Phys. Chem. B, Vol. 110, No. 39, 2006
Song et al.
TABLE 2: Summary of Rate Constantsa
rate constant
(value)-1
k1
k2
k3
38 ps
20-100 psb
38 ps
k4
M1
42 ps
1.1 ns
0.82 nsc
0.26 nsc
2.6 nsc
M2
M3
L1
g0.2 ns
a See Chart 3 for definition of rate constants. The values were derived
from the measured and modeled transient absorption kinetics unless
indicated otherwise. b We expect that (k2)-1 ≈ 50 ps but utilize the
range 20-100 ps from the kinetic modeling on the compounds studied
in ref 40. c From Fo¨rster calculations in Table 3.
amount of excitation has arrived on the guest to form DPFb*,
based on the small Q(0,1) stimulated-emission dip at ∼720 nm.
By 3.5 ns, DPFb* is essentially fully formed.
Kinetic profiles (Supporting Information) give time constants
of 3.1 ( 0.7 ps, 33 ( 9 ps, and 730 ( 80 ps. The minor, ∼3
ps phase can be assigned to Zn/UZnU/ f Zn/ ZnU excited-state
U
annihilation in a small fraction of the arrays in which two zinc
porphyrins are excited together. Such a minor component is also
observed for ZnZn-m/p (2.7 ( 0.3 ps) and ZnZn‚pyr2 (3.6 (
0.8 ps) and in previous studies of arrays containing adjacent
zinc porphyrins.38,47 The ∼35 ps component is assigned to
energy flow among the zinc porphyrins of the host. This phase
is dominated by k4 ) (42 ps)-1 for ZnU/ ZnC f ZnUZnC/ energy
transfer since each ZnU is adjacent to a ZnC (Chart 3A). On the
basis of the arguments given above, we expect that k2 ≈ (50
ps)-1; studies of the arrays studied in the companion paper40
afford k2 ) (20-100 ps)-1, which we utilized here for the kinetic
modeling on Zn6‚DPFb. This modeling also gives an upper limit
Figure 4. Absorption (solid lines) and fluorescence (dashed lines)
spectra of (A) Zn6‚DPFb (navy) and (B) Zn6 (green) and DPFb (red).
of L1 e (200 ps)-1
.
The 730 ps component is assigned to the lifetime of an excited
bookend zinc porphyrin Zn/C and is comparable to the value of
the 0.9 ns fluorescence component. Thus, the transient absorp-
tion data give a yield for energy transfer from host to guest of
Φtrans ) 1 - (0.73/2.1) ) 0.65, which is comparable to the
values of 0.63 and 0.57 derived from the fluorescence spectra
and lifetimes. Considering the errors in the various measure-
ments, we report Φtrans ) 0.65 for Zn/ DPFb f ZnCDPFb*
C
energy transfer. The rate constant for this process is best derived
by comparison of the Zn/C lifetime (from the transient absorp-
tion studies) with that of the reference compounds, which gives
M1 ) [(0.73)-1 - (2.1 ns)-1] ) (1.1 ns)-1 for Zn6‚DPFb (Chart
3A). The rate constants determined herein are summarized in
Table 2.
The distance spanned by the linker between the ZnC and DPFb
porphyrins in the Zn6‚DPFb complex is essentially the same as
that within the ZnFb-p/p dyad, in which the components are
covalently joined at their peripheries by a diarylethyne linker
with p/p connectivity (Charts 2B and 4). Energy transfer in the
latter has a rate constant of (24 ps)-1 (Chart 5) and is dominated
by a TB mechanism, as noted above. The (1.1 ns)-1 rate from
a bookend porphyrin of the host to the guest in Zn6‚DPFb is
50-fold smaller, indicating that the electronic coupling for TB
transfer via the pyridyl-zinc dative bonding (and coupling of
the metal with the porphyrin π-system) is far weaker than when
an aryl ring of a linker is attached directly to the porphyrin
macrocycle. If the process is exclusively Fo¨rster TS in nature,
then the efficiency and rate are estimated as ΦTS ) 0.73 and
kTS ) (0.82 ns)-1 (Table 3).49 This calculated rate constant is
Figure 5. Transient absorption spectra for (A) Zn6‚DPFb obtained
using a 130 fs excitation flash at 532 nm. Panel B shows absorption
(solid lines) and fluorescence (dashed lines) spectra of Zn6 (green),
Zn6‚pyr6 (blue), and DPFb (red). Note that the amplitude of the spectrum
of DPFb has been magnified 10-fold.
optical data shown in Figure 5B, the 0.5 ps transient difference
spectrum contains ground-state bleaching and stimulated emis-
sion features associated primarily with Zn/U along with a
contribution of Zn/C produced directly by the 532 nm excitation
flash in a fraction of the arrays. By 100 ps, Zn/ ZnC f ZnUZn/
U
C
energy transfer is complete and the features have shifted to the
red as expected. The 100 ps spectrum also shows that a small

Self-Assembled Light-Harvesting Architectures
J. Phys. Chem. B, Vol. 110, No. 39, 2006 19129
TABLE 3: Fo1rster Energy-Transfer Parametersa
Note Added after ASAP Publication. The authorship of the
paper has been revised from the version published September
13, 2006; the revised version was published September 20,
2006.
donor
-1
rate
R
quantum lifetime J × 1014
(kTS)
donor acceptor constant (Å) yield
(ns)
(cm6) ΦTS (ns)
ZnC DPFb
ZnU DPFb
ZnU DPFb
M1
M2
M3
17.3 0.037
14.2 0.034
20.9 0.034
2.2
2.4
2.4
4.43 0.73 0.82
5.17 0.90 0.26
5.17 0.48 2.6
Supporting Information Available: Titration data for the
formation of host-guest complexes and time-resolved absorp-
tion and kinetic data. This material is available free of charge
via the Internet at http://pubs.acs.org.
a Calculations used PhotochemCAD52 with κ2 ) 0.25. Distances were
derived from those given in ref 19. The fluorescence lifetimes and
quantum yields are from Table 1. For donor ZnC and ZnU, the
fluorescence parameters for ZnU‚pyr and ZnU, respectively, were
utilized. For acceptor DPFb, an extinction coefficient log(ꢀ) ) 5.68 at
423 nm for a toluene solution was used (obtained by solvent
replacement of a chloroform solution using the published value34 log(ꢀ)
) 5.61 at 422 nm).
References and Notes
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The wheel-and-spoke complex Zn6‚DPFb has numerous
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