Intramolecular energy transfer within butadiyne-linked chlorophyll and porphyrin dimer-faced, self-assembled prisms

Article abstract of DOI:10.1021/ja075494f

The synthesis and photophysical properties of butadiyne-linked chlorophyll and porphyrin dimers in toluene solution and in several self-assembled prismatic structures are described. The butadiyne linkage between the 20-positions of the macrocycles results in new electronic transitions polarized along the long axes of the dimers. These transitions greatly increase the ability of these dimers to absorb the solar spectrum over a broad wavelength range. Femtosecond transient absorption spectroscopy reveals the relative rate of rotation of the macrocycles around the butadiyne bond joining them. Following addition of 3-fold symmetric, metal-coordinating ligands, both macrocyclic dimers self-assemble into prismatic structures in which the dimers comprise the faces of the prisms. These structures were confirmed by small-angle X-ray scattering experiments in solution using a synchrotron source. Photoexcitation of the prismatic assemblies reveals that efficient, through-space energy transfer occurs between the macrocyclic dimers within the prisms. The distance dependence of energy transfer between the faces of the prisms was observed by varying the size of the prismatic assemblies through the use of 3-fold symmetric ligands having arms with different lengths. These results show that self-assembly of discrete macrocyclic prisms provides a useful new strategy for controlling singlet exciton flow in antenna systems for artificial photosynthesis and solar cell applications.

Full text of DOI:10.1021/ja075494f

Published on Web 03/08/2008
Intramolecular Energy Transfer within Butadiyne-Linked
Chlorophyll and Porphyrin Dimer-Faced, Self-Assembled
Prisms
Richard F. Kelley,†,‡ Suk Joong Lee,†,‡ Thea M. Wilson,†,‡ Yasuyuki Nakamura,|
David M. Tiede,*,‡,§ Atsuhiro Osuka,*,| Joseph T. Hupp,*,†,‡ and
Michael R. Wasielewski*,†,‡
Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center,
Northwestern UniVersity, EVanston, Illinois 60208-3113, Chemistry DiVision, Argonne National
Laboratory, Argonne, Illinois 60439, and Department of Chemistry, Graduate School of Science,
Kyoto UniVersity, Kyoto 606-8502, Japan
Received July 23, 2007; E-mail: m-wasielewski@northwestern.edu
Abstract: The synthesis and photophysical properties of butadiyne-linked chlorophyll and porphyrin dimers
in toluene solution and in several self-assembled prismatic structures are described. The butadiyne linkage
between the 20-positions of the macrocycles results in new electronic transitions polarized along the long
axes of the dimers. These transitions greatly increase the ability of these dimers to absorb the solar spectrum
over a broad wavelength range. Femtosecond transient absorption spectroscopy reveals the relative rate
of rotation of the macrocycles around the butadiyne bond joining them. Following addition of 3-fold symmetric,
metal-coordinating ligands, both macrocyclic dimers self-assemble into prismatic structures in which the
dimers comprise the faces of the prisms. These structures were confirmed by small-angle X-ray scattering
experiments in solution using a synchrotron source. Photoexcitation of the prismatic assemblies reveals
that efficient, through-space energy transfer occurs between the macrocyclic dimers within the prisms.
The distance dependence of energy transfer between the faces of the prisms was observed by varying the
size of the prismatic assemblies through the use of 3-fold symmetric ligands having arms with different
lengths. These results show that self-assembly of discrete macrocyclic prisms provides a useful new strategy
for controlling singlet exciton flow in antenna systems for artificial photosynthesis and solar cell applications.
Introduction
desired low-energy transition. To address this problem, another
recent study introduced a bridging ligand into a solution of
Photofunctional materials for use in organic photovoltaics
must be capable of transferring energy efficiently due to the
relatively long distances photogenerated excitons must travel
to reach the interface at which charge carriers are generated.1,2
Several studies have examined multiporphyrin arrays constructed
with ethynyl and butadiynyl linkers for applications in photo-
voltaic devices.3,4 The linkers in these arrays provide extended
conjugation between the macrocycles resulting in new, low-
energy electronic transitions, which provide enhanced solar
spectral absorption. The same studies, however, have also shown
these linkers allow rotational conformational heterogeneity,
which can diminish the population of molecules having the
butadiyne-linked porphyrin arrays to coordinate to the porphyrin
metal centers, thus locking the chromophores in a rigid, oriented
fashion.5 The strain induced by the coordinating ligand caused
the chromophores to deviate from planarity by bending the
butadiyne linkage. This bending may result in disorder if the
systems were to be incorporated into solid-phase devices.
Coordination bond formation between tetrapyrrole metal centers
and polydentate ligands is an attractive strategy for building
supramolecular structures in which the chromophores are
planarized as the result of coordination. This coordination-driven
self-assembly strategy has become a very successful method
for constructing a variety of 3-D supermolecules.6-22 Increasing
the number of coordination bonding sites within the structure
† Department of Chemistry, Northwestern University.
‡ ANSER Center, Northwestern University.
§ Chemistry Division, Argonne National Laboratory.
| Graduate School of Science, Kyoto University.
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Condens. Matter 1999, 59, 15346-15351.
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9
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J. AM. CHEM. SOC. 2008, 130, 4277-4284
4277

A R T I C L E S
Kelley et al.
Figure 1. Structures of the molecules in this study.
enhances cooperative effects and increases control over the
assembly geometry. We recently reported on a supramolecular
architecture in which the combination of butadiyne-linked
porphyrins with triethynylpyridylbenzene resulted in the revers-
ible formation of trigonal, prismatic assemblies.23 In addition
to fixing the butadiyne-linked chromophores in the red-
absorbing, planar orientation, the prismatic architecture provides
a 3-dimensional structure that can potentially be used to bias
the orientation of the structures in the solid phase. The ability
to control aggregate assembly is beneficial in photovoltaic
applications, where efficient energy transfer is required, yet often
limited by molecular disorder. The presence of multiple chro-
mophores within close proximity allows for ultrafast energy
migration to occur between faces of the prisms. We have also
recently developed the methodology for attaching ethynyl linkers
directly to the macrocyclic cores of chlorophyll a derivatives.24
The electronic asymmetry inherent in the chlorin chromophore,
resulting from the reduced D-ring, enhances the intensity of the
Qy transition. By incorporation of chlorophyll into butadiyne-
linked dimers, the oscillator strength of the transition polarized
along the butadiyne axis is also enhanced. We now report the
structural characterization and photophysics of butadiyne-linked
chlorophyll and porphyrin dimers as well as several self-
assembled prisms comprised of them in toluene solution.
Experimental Section
The synthesis and characterization of 1 and LL (Figure 1) are
described in detail in the Supporting Information, while that of 2, PM,
ZCEPh, and SL were previously reported.23,25 Characterization was
performed with a Varian 400 MHz NMR and PE Voyager DE-Pro
MALDI-TOF mass spectrometers. High-resolution electrospray and fast
atom bombardment mass spectra were obtained with the 70-SE-4F and
Q-Tof Ultima mass spectrometers at the University of Illinois at Urbana-
Champaign.
UV-vis absorption measurements were made on a Shimadzu
spectrometer (UV1601). Steady-state fluorescence measurements were
made on a single photon counting fluorometer (PTI). All measurements
were performed at room temperature. Fluorescence studies of the dimers
were conducted on solutions with optical densities below 0.1 at the
absorption maxima in a 1-cm cuvette using right angle excitation/
emission geometry. Fluorescence studies of the prisms were conducted
on solutions with optical densities above 0.3 at the absorption maxima
in a 2-mm cuvette to ensure that the samples existed in their prismatic
states, and front-face fluorescence spectra were collected. Toluene was
purified by passing it through a series of CuO and alumina columns
immediately prior to use.
(11) Ballester, P.; Costa, A.; Deya, P. M.; Frontera, A.; Gomila, R. M.; Oliva,
A. I.; Sanders, J. K. M.; Hunter, C. A. J. Org. Chem. 2005, 70, 6616-
6622.
(12) Ballester, P.; Costa, A.; Castilla, A. M.; Deya, P. M.; Frontera, A.; Gomila,
R. M.; Hunter, C. A. Chem.sEur. J. 2005, 11, 2196-2206.
(13) Ogawa, K.; Kobuke, Y. J. Photochem. Photobiol., C 2006, 7, 1-16.
(14) Nakagawa, H.; Ogawa, K.; Satake, A.; Kobuke, Y. Chem. Commun.
(Cambridge, U.K.) 2006, 1560-1562.
X-ray scattering measurements were carried out using the undulator
beam line 12-ID at the Advanced Photon Source (APS), Argonne
National Laboratory. The X-ray scattering instrument utilized a double-
crystal Si(111) monochromator and a two-dimensional mosaic CCD
detector.26 The X-ray wavelength was set at λ ) 0.62 Å, and the sample
to detector distance was adjusted to achieve scattering measured across
the 0.007 Å-1 < q < 0.27 Å-1 region, where q ) (4π/λ) sin θ, λ is the
X-ray scattering wavelength, and 2θ is the scattering angle. Quartz
capillaries (2 mm diameter) were used as sample containers. All samples
were filtered through 200 nm PTFE filters (Whatman) prior to the
measurements. The scattering intensity was averaged over 300 mea-
surements, after which the solvent scattering intensity was analogously
measured and subtracted from the sample spectrum.
(15) Hajjaj, F.; Yoon, Z. S.; Yoon, M.-C.; Park, J.; Satake, A.; Kim, D.; Kobuke,
Y. J. Am. Chem. Soc. 2006, 128, 4612-4623.
(16) Furutsu, D.; Satake, A.; Kobuke, Y. Inorg. Chem. 2005, 44, 4460-4462.
(17) Anderson, H. L. Chem. Commun. 1999, 2323-2330.
(18) Taylor, P. N.; Anderson, H. L. J. Am. Chem. Soc. 1999, 121, 11538-
11545.
(19) Drobizhev, M.; Stepanenko, Y.; Rebane, A.; Wilson, C. J.; Screen, T. E.
O.; Anderson, H. L. J. Am. Chem. Soc. 2006, 128, 12432-12433.
(20) Screen, T. E. O.; Thorne, J. R. G.; Denning, R. G.; Bucknall, D. G.;
Anderson, H. L. J. Am. Chem. Soc. 2002, 124, 9712-9713.
(21) Dy, J. T.; Ogawa, K.; Satake, A.; Ishizumi, A.; Kobuke, Y. Chem.sEur.
J. 2007, 13, 3491-3500, S3491/3491-S3491/3498.
(22) Hoffmann, M.; Wilson, C. J.; Odell, B.; Anderson, H. L. Angew. Chem.,
Int. Ed. 2007, 46, 3122-3125.
(23) Lee, S. J.; Mulfort, K. L.; O’Donnell, J. L.; Zuo, X.; Goshe, A. J.; Wesson,
P. J.; Nguyen, S. T.; Hupp, J. T.; Tiede, D. M. Chem. Commun. (Cambridge,
U.K.) 2006, 4581-4583.
(25) Kelley, R. F.; Tauber, M. J.; Wasielewski, M. R. Angew. Chem., Int. Ed.
2006, 45, 7979-7982.
(26) Seifert, S.; Winans, R. E.; Tiede, D. M.; Thiyagarajan, P. J. Appl.
Crystallogr. 2002, 33, 782-784.
(24) Kelley, R. F.; Tauber, M. J.; Wasielewski, M. R. J. Am. Chem. Soc. 2006,
128, 4779-4791.
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Chlorophyll and Porphyrin Dimer-Faced Prisms
A R T I C L E S
Figure 3. Ground-state absorption spectra of 13-SL2 in toluene (black),
13-LL2 in toluene (red), and 1 in pyridine (blue). Inset: Spectrophotometric
titration curves of 13-SL2 in toluene (1.3 × 10-5 M) at 674 and 710 nm.
Figure 2. Ground-state absorption spectra of 1 (black), ZCEPh (red), PM
(blue), and 2 (green) in toluene.
Femtosecond transient absorption measurements were made using
a Ti:sapphire laser system as detailed in the Supporting Information.
The instrument response function (IRF) for the pump-probe experi-
ments was 150 fs. Typically 5 s of averaging was used to obtain the
transient spectrum at a given delay time. Quartz cuvettes with a 2 mm
path lengths were used, and the samples were irradiated with 0.1-1.0
µJ pulses focused to a 200-µm diameter spot. The optical density at
λex was kept between 0.4 and 0.8. Analysis of the kinetic data was
performed at multiple wavelengths using a Levenberg-Marquardt
nonlinear least-squares fit to a general sum-of-exponentials function
with an added Gaussian to account for the finite instrument response.
Fluorescence lifetime measurements were performed using a frequency-
doubled, cavity-dumped Ti:sapphire laser as the excitation source and
a Hamamatsu C4780 picosecond fluorescence lifetime measurement
system as described in the Supporting Information. The energy of the
400 nm, 25 fs pulses was attenuated to approximately 1.0 nJ/pulse for
all fluorescence lifetime experiments. The total IRF of the streak camera
system was 20 ps. The samples were prepared in 2 mm path length
quartz cuvettes, and time-resolved data were collected for each sample
at the same concentrations and experimental orientations as the steady-
state data. All fluorescence data was acquired in single photon counting
mode using the Hamamatsu HPD-TA software. The data was fit using
the Hamamatsu fitting module and deconvoluted using the laser pulse
profile.
chlorophyllide species. The spectrum of 1 is considerably
broadened, and its Q bands are red-shifted relative to ZCEPh
with its Soret band at 436 nm and Q bands at 596, 638, and
674 nm. In addition to the expected absorption bands, new
spectral features are observed at 469 and 694 nm. While the
red shifting is due to the increase in conjugation length of the
chlorophyll π-system, the spectral features at 469 and 694 are
indicative of new allowed transitions. Similar transitions have
been observed in ethynyl- and butadiynyl-linked porphyrin
dimers and are attributed to an electronic transition that is
polarized along the long axis of the molecules.3-5 In the PM
spectrum, the porphyin Soret band is observed at 443 nm and
the Q bands appear at 537, 578, and 627 nm. In the spectrum
of 2, the porphyin Soret band is split into three separate bands,
observed at 438, 460, and 493 nm, and the Q bands appear at
577, 652, and 705 nm. The red-shift and increase in amplitude
of the Q-bands is indicative of further reduction of the electronic
symmetry within both component chromophores and electronic
delocalization along the long axis of the dimer.18,29,30 It is
interesting to note that while the electronic spectrum of the
porphyrin monomer and dimer differ significantly, the chloro-
phyll dimer retains its Qy(0,0) transition. Despite the differences
between the absorption features of the component chromophores,
the lowest energy absorption features for both dimers occur at
surprisingly similar energies.
Results and Discussion
Synthesis. Homocoupling of zinc 2-octyl-1-dodecyl 3-ethyl-
20-ethynylpyrochlorophyllide a25 using Glaser-Hay methodol-
ogy23 provided 1 in 50% yield. Two sequential Sonogashira
cross-coupling reactions27 were used to synthesize LL. The first
coupling, between 4-ethynylpyridine and 1,4-dibromobenzene,
resulted in the formation of 4-(4-bromophenylethynyl)pyridine,
in 72% yield. The second coupling, between 4-(4-bromo-
phenylethynyl)pyridine and 1,3,5-triethynylbenzene,28 resulted
in LL in 59% yield. The synthesis and characterization of zinc
methyl 3-ethyl-20-phenylethynylpyrochlorophyllide a (ZCEPh),
2, porphyrin monomer (PM), and SL were reported previ-
ously.23,25
The spectrophotometric titration of SL into a toluene solution
of 1 revealed a new species of 1 with a lower intensity Soret
band, red-shifted to 440 nm, and the lowest energy Q bands
broadened and red-shifted to 676 and 712 nm, Figure 3. The
intensity of the lowest energy absorption feature reached a
maximum following the net addition of 2/3 equiv of SL to 1,
Figure 3 inset. Following the continued addition of SL, the
features of this intermediate species transition into those
observed in a pyridine solution of 1, which displays a Soret
band at 442 nm and low-energy Q bands occurring at 678 and
716 nm. The titration reveals that the observed binding
stoichiometry of the intermediate species is consistent with the
formation of the proposed prism, 13-SL2, and the enhancement
of the lowest energy absorption band spectrum of the intermedi-
Steady-State Spectroscopy. The ground-state absorption
spectra of 1, ZCEPh, 2, and PM in toluene are compared in
Figure 2. The spectrum of ZCEPh displays a Soret band at
435 nm and Q bands at 578, 628, and 665 nm, typical of zinc
(27) Wagner, R. W.; Johnson, T. E.; Li, F.; Lindsey, J. S. J. Org. Chem. 1995,
60, 5266-5273.
(29) Anderson, H. L. Inorg. Chem. 1994, 33, 972-981.
(30) LeCours, S. M.; DiMagno, S. G.; Therien, M. J. J. Am. Chem. Soc. 1996,
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63, 5568-5580.
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A R T I C L E S
Kelley et al.
Figure 5. MM+ geometry-optimized models for 13-SL2 (left) and 13-LL2
(right).
inversely proportional to the distance between their dipoles
cubed.32,33 Therefore, the longer distances between the chro-
mophores in 13-LL2 and 23-LL2 result in the smaller splittings
observed in their absorption features relative to those in 13-SL2
and 23-SL2.
Figure 4. Ground-state absorption spectra of 23-SL2 in toluene (black),
23-LL2 in toluene (red), and 2 in pyridine (blue). Inset: Spectrophotometric
titration curves of 23-SL2 in toluene (3.1 × 10-6 M) at 685 and 730 nm.
Small-Angle X-ray Scattering. To confirm the prismatic
structures of 13-SL2, 13-LL2, and 23-LL2, small-angle X-ray
ate state is consistent with the planar conformation of 1 in a
prismatic assembly.23 The titration data for 1 and SL was fit to
a three-chromophore model using multivariate factor analysis
to obtain the equilibrium constant K32 ) (2.27 ( 0.09) × 1024
M-4 for formation of 13-SL2 (see Supporting Information,
Figures S1-S3) These results suggest that the 13-SL2 prism is
fully formed and stable after the stoichiometric addition of 2/3
equiv of SL and that the prismatic assembly is maintained under
the conditions used for the photophysical measurements (see
also SAXS data given below).
scattering (SAXS) measurements were performed on 10-4
M
toluene solutions using a high-flux synchrotron source (Ad-
vanced Photon Source at Argonne National Laboratory). Similar
studies of 23-SL2 in toluene solution were previously reported.23
Solution-phase scattering patterns of 13-SL2, 13-LL2, and 23-
LL2 in toluene were obtained over the q region 0.007 Å-1 < q
< 0.27 Å-1. In the low-resolution scattering region (q < 0.2
Å-1), the scattering intensity follows the Guinier relationship,
I(q) ) I(0) exp(-q2Rg /3), which is parametrized in terms of
2
the forward scattering amplitude, I(0), and the radius of gyration,
Rg.34,35 The linearity of the Guinier plot is a measure of structural
dispersity.36 The linear Guinier plots over the range 0.003 Å-2
< q2 < 0.008 Å-2 indicate that 13-SL2, 13-LL2, and 23-LL2
each have monodisperse structures, Figures S4-S6. The least-
squares fit to the linear data reveal radii of gyration of 12.4,
16.7, and 17.9 Å for 13-SL2, 13-LL2, and 23-LL2 respectively.
MM+ geometry-optimized models for the prismatic structures
were created using Hyperchem, Figure 5.37 Coordinate-based
scattering data were then generated for each model38 and fit
using the Guinier approximation in the same manner as the
experimentally obtained data; see insets to Figures S4-S6. The
Rg values obtained from the linear fits to the simulated data for
the 13-SL2, 13-LL2, and 23-LL2 models were 12.2, 16.9, and
18.1 Å, respectively. Given that the Rg values obtained from
both the experiments and the prism models are nearly identical
and that a 3:2 ratio of dimers to ligands is observed in the
spectrophotometric titration data, we conclude that the ag-
gregates found in toluene solution are trigonal prisms.
The addition of 2/3 equiv of LL to a toluene solution of 1
resulted in an additional decrease in the intensity its Soret band
and red shifting of its Q bands to 675 and 715 nm, Figure 3.
The lowest energy Q-band also increased in intensity at the
expense of the higher energy band. The intensity differences
between the Q bands in the prisms constructed from SL and
LL are attributed to the degree of planarization within the
dimers.18 With the larger trigonal ligand, there is less steric
hindrance between the dimers in the prisms and the chro-
mophoric subunits within the dimers are most likely able to
adopt a more planar geometry relative to one another.
The spectrophotometric titration yielding 23-SL2 in toluene,
the spectrum resulting from the addition of 2/3 equiv of LL to
a toluene solution of 2, and the spectrum of 2 in pyridine are
presented in Figure 4. A detailed explanation of the spectrum
of 23-SL2 in toluene was previously reported.23 The spectrum
of 23-LL2 also displays a decrease in the intensity of the highest
energy Soret band of 2 and red shifting of the lowest energy Q
bands to 734 nm (Figure 4). The intensity differences between
the Q bands of the two porphyrin prisms are smaller than those
of the chlorophyll prisms indicating that the degree of pla-
narization within the porphyrin dimers does not change as much
with the different trigonal ligands.
Transient Absorption Spectroscopy. Excitation of 1 in
toluene with 710-nm, 120-fs pulses yields the transient absorp-
tion spectra shown in Figure 6 within the instrument response
function of the apparatus. Negative features occur at 642, 675,
and 705 nm and below 480 nm, while a broad positive feature
Exciton coupling between the dimers that comprise the faces
of the prisms results from the orientation of their transition
dipole moments, which are aligned parallel to the long molecular
axis of each dimer.31 The parallel geometry results in blue
shifting of the absorption features of the dimers within the
prisms relative to those of disaggregated 1 and 2 in pyridine.31
The magnitude of exciton splitting between the dimers is
(32) Cantor, C. R.; Schimmel, P. R. Biophysical Chemistry; W. H. Freeman:
New York, 1980; Vol. 2.
(33) Harada, N.; Nakanishi, K. Circular dichroic spectroscopy: exciton coupling
in organic stereochemistry; University Science Books: Mill Valley, CA,
1983.
(34) Glatter, O. Neutron X-ray and Light Scattering; Elsevier: Amsterdam, 1991.
(35) Guinier, A.; Fournet, G. Small Angle Scattering; Wiley: New York, 1955.
(36) Svergun, D. I.; Koch, M. H. J. Rep. Prog. Phys. 2003, 66, 1735-1782.
(37) Hyperchem(TM); Hypercube Inc.: 1115 NW 4th Street, Gainesville, FL
32601.
(31) Kasha, M.; Rawles, H. R.; El-Bayoumi, M. L. Pure Appl. Chem 1965, 11,
371-392.
(38) Zhang, R. T.; Thiyagarajan, P.; Tiede, D. M. J. Appl. Crystallogr. 2000,
33, 565-568.
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Chlorophyll and Porphyrin Dimer-Faced Prisms
A R T I C L E S
Figure 6. Transient absorption spectra of 1 in toluene following excitation
with 710-nm, 120-fs laser pulses. Inset: Transient absorption kinetics for
1 at (blue) 680 nm and at (red) 707 nm. Nonlinear least-squares fits to the
data are also shown.
Figure 8. Transient absorption spectra of 13-SL2 in toluene following
excitation with 710-nm, 120-fs laser pulses. Inset: Power-dependent
transient absorption kinetics monitored at 721 nm using 0.66 (red), 0.33
(green), and 0.11 (blue) µJ per 710 nm excitation pulse.
transitions observed in the ground-state absorption spectra of
the prismatic assemblies, Figures 3 and 4. This suggests that
the majority of the molecules excited by the wavelength-
selective laser pulse are in the conformation where the chro-
mophores are coplanar within the dimers. In this conformation,
the electronic transition selectively excited is polarized along
the butadiynyl axis. The first component in the transient kinetics
of both dimers is the result of nonlinear decay processes made
possible by the increased polarizability resulting from the
extended conjugation of the coplanar macrocycles.19,20,39 The
second component is indicative of the torsional dynamics of
the macrocycles as they rotate around the butadiyne linkage,
from twisted conformations into the coplanar conformation. This
mechanism is evidenced by the concurrent recovery of the
ground-state bleach of the planarized species and bleaching of
ground state of the nonplanarized species. The third component
corresponds to the excited-state decay of 1*1 and 1*2, as these
∼1-ns components match the fluorescence lifetimes of 1*1 and
1*2 in toluene (τ ) 1.2 ( 0.1 and 1.3 ( 0.1 ns, respectively),
which were independently measured using a fluorescence
lifetime apparatus having a 20-ps instrument response time (not
shown).
The SAXS data shows that coordination of the dimers to the
trigonal ligands ensures that the macrocycles that comprise each
dimer are locked in their coplanar conformation without
deformation of the butadiyne linkers. Assignment of the initial
negative features in the transient absorption spectra of 1 and 2
to excitation of the coplanar dimers was confirmed by excitation
of 13-SL2 and 23-SL2 in toluene with 710- and 730-nm, 120-fs
pulses, respectively. Excitation of 13-SL2 yields the transient
absorption spectrum of 1*13-SL2, with ground-state bleaching
of the Q bands at 652, 675, and 713 nm, bleaching of the Soret
band below 483 nm, and excited-state absorption between 483
and 633 nm, Figure 8. Excitation of 23-SL2 yields the transient
absorption spectrum of 1*23-SL2 with ground-state bleaching
of the Q-band at 732 nm, bleaching of the Soret band at 494
nm, and excited-state absorption between 510 and 692 nm,
Figure 9. Transient kinetics for 1*13-SL2, monitored at 721 nm,
reveal three exponential decay components with lifetimes of
Figure 7. Transient absorption spectra of 2 in toluene following excitation
with 705-nm, 120-fs laser pulses. Inset: Transient absorption kinetics for
2 at (blue) 460 nm and at (red) 491 nm. Nonlinear least-squares fits to the
data are also shown.
is observed between 480 and 620 nm. These features decay over
time to reveal bleaching that closely matches the inverted ground
state absorption spectrum of 1 in toluene, with Q bands at 635,
676, and 697 nm, the Soret band below 470 nm, and a broad
positive feature between 470 and 615 nm. Transient kinetics
monitored at 680 and 707 nm, Figure 6 inset, show three
exponential components with lifetimes (amplitudes) of 0.29 (
0.05 ps (0.50), 176 ( 23 ps (0.22), and 1.2 ( 0.1 ns (0.28),
respectively.
Excitation of 2 in toluene with 705-nm, 120-fs pulses yields
negative features at 493 and 709 nm and a broad positive feature
between 503 and 672 nm, Figure 7. These spectral features also
decay over time to reveal negative features that closely match
the inverted ground-state absorption spectrum of 2 in toluene,
with Q bands at 625, 652, and 704 nm, the Soret bands at 460
and 493 nm, and a broad absorption feature between 501 and
614 nm. Transient kinetics monitored at 460 and 491 nm, Figure
7 inset, also show three exponential components with lifetimes
of 0.11 ( 0.01 ps, 175 ( 16 ps, and 1.3 ( 0.1 ns, respectively.
The corresponding amplitudes are 0.63, 0.06, and 0.31 at 460
nm and 0.61, 0.33, and 0.06 at 491 nm.
The initial negative absorption changes in the transient spectra
of both 1 and 2 are nearly identical with the lowest energy
(39) Screen, T. E. O.; Thorne, J. R. G.; Denning, R. G.; Bucknall, D. G.;
Anderson, H. L. J. Mater. Chem. 2003, 13, 2796-2808.
9
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A R T I C L E S
Kelley et al.
Figure 9. Transient absorption spectra of 23-SL2 in toluene following
excitation with 730-nm, 120-fs laser pulses. Inset: Power-dependent
transient absorption kinetics monitored at 740 nm using 1.00 (black), 0.66
(red), 0.33 (green), and 0.11 (blue) µJ per 730 nm excitation pulse.
Figure 10. Transient absorption spectra of 13-LL2 in toluene following
excitation with 710-nm, 120-fs laser pulses. Inset: Power-dependent
transient absorption kinetics monitored at 721 nm using 1.00 (black), 0.66
(red), 0.33 (green), and 0.11 (blue) µJ per 710 nm excitation pulse.
0.31 ( 0.11 ps (0.43), 3.1 ( 0.1 ps (0.07), and 1.2 ( 0.1 ns
(0.50), Figure 8 inset, and kinetics of 1*23-SL2, monitored at
740 nm, also show three exponential decay components with
lifetimes of 0.28 ( 0.05 ps (0.35), 2.4 ( 0.3 ps (0.09), and 1.4
( 0.1 ns (0.56), Figure 9 inset. The relative amplitudes are given
for the lowest laser pump energies as indicated in the figures.
The first component in the transient decays of both prisms is
the result of the nonlinear decay process, and the ∼1 ns
component is the result of excited-state decay, which matches
the fluorescence lifetimes of τ ) 1.2 ( 0.1 and 1.4 ( 0.1 ns
for 1*13-SL2 and 1*23-SL2 in toluene, respectively, Figures S7
and S8. However, the second component, τ ) 3.1 and 2.4 ps
for 1*13-SL2 and 1*23-SL2, respectively, is not the result of
conformational dynamics as it was in the transient kinetics of
1 and 2. The amplitudes of these components display a quadratic
dependence on laser power indicative of singlet-singlet an-
nihilation, insets in Figures 8 and 9.25 Assuming annihilation is
the result of random exciton transfer between adjacent dimeric
chromophores, the time constants for exciton hopping within
each prism, τh, can be derived from the annihilation lifetime,
τa, using the formula
Figure 11. Transient absorption spectra of 23-LL2 in toluene following
excitation with 730-nm, 120-fs laser pulses. Inset: Power-dependent
transient absorption kinetics monitored at 740 nm using 1.00 (black), 0.66
(red), 0.33 (green), and 0.11 (blue) µJ per 730 nm excitation pulse.
kinetics for 13-LL2, monitored at 721 nm, show 3 exponential
decay components with lifetimes of 0.18 ( 0.05 ps (0.58), 7.3
( 1.7 ps (0.08), and 1.1 ( 0.1 ns (0.34), respectively, Figure
10 inset, and kinetics of 23-LL2, monitored at 740 nm, show 3
similar exponential decay components with lifetimes of 0.13 (
0.03 ps (0.78), 9.2 ( 1.0 ps (0.11), and 1.3 ( 0.1 ns (0.11),
respectively, Figure 11. The ∼1 ns lifetimes are once again the
result of excited-state decay, as corroborated by the fluorescence
decay data presented in Figures S9 and S10, while the
amplitudes of the 7.3 and 9.2 ps components depend quadrati-
cally on laser power indicative of singlet-singlet annihilation,
insets in Figures 10 and 11.25 Using the model described above,
N ) 3 and the measured values of τa and τh ) 14.6 ( 3.4 and
18.4 ( 2.0 ps for 1*13-LL2 and 1*23-LL2, respectively. These
laser power dependent components are longer relative to those
of 13-SL2 and 23-SL2 and are consistent with the assignment of
these components to singlet-singlet annihilation between
excited dimers in the prisms.
τh
τa )
(1)
8 sin2(π/2N)
developed by Trinkunas for cyclic arrays of N chromophores.40
Using this model, N ) 3 and the measured values of τa and τh
) 6.2 ( 0.2 and 4.8 ( 0.6 ps for 1*13-SL2 and 1*23-SL2,
respectively. Due to the parallel orientation of the transition
dipole moments along the butadiyne linkage of the dimers
comprising each face of the prismatic assemblies, anisotropy
measurements cannot be used to confirm the rates of energy
transfer. To prove that singlet-singlet annihilation is the result
of energy transfer between dimers in the prisms and not between
chromophores within the dimers, transient absorption studies
of the larger trigonal prisms are required.
Excitation of 13-LL2 and 23-LL2 in toluene with 710- and
730-nm, 120-fs pulses, respectively, reveals transient absorption
spectra that are nearly identical with those obtained for 13-SL2
and 23-SL2 in toluene, Figures 10 and 11. Transient absorption
Energy transfer between the dimers that comprise the faces
of the prisms most likely occurs entirely through the Fo¨rster
(40) Trinkunas, G. J. Lumin. 2003, 102-103, 532-535.
9
4282 J. AM. CHEM. SOC. VOL. 130, NO. 13, 2008

Chlorophyll and Porphyrin Dimer-Faced Prisms
A R T I C L E S
Table 1. Fluorescence Data and Results of Fo¨rster-Type Energy Transfer Calculations for Prisms in Toluene
a
struct
λF
τF (ns)
ΦF
τh (ps)
R (nm)b
κ2
τFRET(S1 f S1) (ps)
τFRET(S1 f S0) (ps)
13-SL2
13-LL2
23-SL2
23-LL2
733
737
749
749
1.19
1.10
1.36
1.33
0.07
0.08
0.14
0.15
6.2 ( 0.2
14.6 ( 3.4
4.8 ( 0.6
18.4 ( 2.0
1.73
2.83
1.73
2.83
1
1
1
1
10.3
157
1.1
11.1
171
0.9
19.2
11.9
a Fluorescence quantum yields use DOTC iodide59 as a reference. b Values derived from MM+ geometry-optimized models obtained using Hyperchem.37
mechanism.41-45 The competing Dexter mechanism46 requires
a direct orbital pathway between energy donors and accep-
tors,47,48 so that contributions from the Dexter mechanism in
the prismatic assemblies are most likely negligible because the
large distance between the dimer faces prohibits overlap of the
donor and acceptor orbitals. Overlap of the trigonal ligand
orbitals and the dimer orbitals is also negligible due to the small
orbital coefficient at the Zn centers of both tetrapyrrole
macrocycles.25,49
hopping rate (τh) derived from eq 1 has an increasing contribu-
tion from energy transfer from an excited chromophore to one
in its ground state. Therefore, using the S1 f S0 emission
spectrum of the donor chromophore and the S0 f S1 absorption
spectrum of the acceptor chromophore in eq 2 to calculate kFRET
becomes an increasingly better approximation as the number
of chromophores over which the exciton hops increases.53,54 For
the prismatic chlorophyll and porphyrin assemblies studied here,
there are only three chromophores/prism, so that when two S1
states are present in each prism, each of them can hop either to
one S0 chromophore or to the other S1 chromophore and
annihilate. To determine the best approximation, we will
calculate and compare both the S1 f S1 and S1 f S0 Fo¨rster
energy transfer rates.
The rate of Fo¨rster resonance energy transfer41 is expressed
as
kFRET
)
κ2ΦF
8.785 × 10-11
fD(λ)λ4ꢀA(λ) dλ (ps-1) (2)
∫
4
6
∆λ
(
)
The ground-state absorption spectra of 13-SL2, 13-LL2, 23-
SL2, and 23-LL2 in toluene are presented in Figures 3 and 4,
their fluorescence spectra are given in Figure S11, and their
fluorescence maxima (λF), quantum yields (ΦF), and lifetimes
(τF) are listed in Table 1. For all molecules, the emission features
are approximately mirror images of the corresponding Qy(0,0)
and Qy(0,1) absorption features. The most prominent features
in the transient absorption spectra of 13-SL2, 13-LL2, 23-SL2,
and 23-LL2 in toluene, Figures 8-11, are bleaches of their
ground-state absorption bands and stimulated emission. The
remaining absorption changes due to their S1 f Sn transitions
are broad and essentially featureless across most of the visible
and near-infrared region.55-57 Because it is difficult to accurately
subtract the prominent stimulated emission features from the
transient absorption spectra, we have approximated the S1 f
Sn spectra by using a constant value of the S1 f Sn absorbance
obtained from the transient absorption observed at 480-620
nm for 13-SL2 and 13-LL2 and 520-650 nm for 23-SL2, and
23-LL2, where their respective ground states have very little
absorption. The estimated S1 f Sn absorbance is assumed to
be constant throughout the region of the S1 f S0 emission from
the donor. The time constants for Fo¨rster energy transfer (τFRET
) 1/kFRET) were calculated using the PhotochemCAD software
package58 and are presented in Table 1. The estimated S1 f Sn
absorbance is an upper limit (see Figure S12 and Table S1),55-57
so that if the actual spectral overlap is smaller, the calculated
Fo¨rster energy transfer times are slower. This would result in
n R τF
where κ2 is the geometrical factor, n is the effective index of
refraction (1.496 for toluene) used to determine the Coulombic
interaction between the donor and the acceptor,50 R (nm) is the
distance between chromophores, fD(λ) is the corrected fluores-
cence intensity of the donor with the total intensity normalized
to unity, ꢀA(λ) is the extinction coefficient of the acceptor in
M-1 cm-1, and ΦF and τF are the donor fluorescence quantum
yield and lifetime, respectively. If singlet-singlet annihilation
between a pair of chromophores that are both excited to their
S1 states occurs by Fo¨rster energy transfer,51 calculating kFRET
using eq 2 requires the S1 f S0 emission spectrum of the donor
chromophore and the S1 f Sn absorption spectrum of the
acceptor chromophore.52 However, if the number of chro-
mophores in the ensemble increases beyond 2, the probability
of having energy transfer occur between an excited chromophore
and a nearby ground-state chromophore prior to singlet-singlet
annihilation also increases. As a consequence, the overall exciton
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J. AM. CHEM. SOC. VOL. 130, NO. 13, 2008 4283

A R T I C L E S
Kelley et al.
an even wider divergence between the calculated energy transfer
times and the experimentally determined values of τh given in
Table 1.
in significant spectral enhancements due in part to the reduction
in symmetry of the component chromophores and electronic
delocalization that lengthens the transition dipole. By restriction
of these dimers to structurally rigid, prismatic structures, by
the addition of 2/3 equiv of a trigonal bridging ligand, the factors
contributing to the spectral enhancements are accentuated.
Power-dependent transient absorption measurements on the
prisms with different chromophore-chromophore distances
identified one of the decay components in the transient
absorption kinetics as resulting from singlet-singlet annihilation.
From the annihilation kinetics, we are able to obtain the lifetimes
of exciton migration between the dimers which comprise the
faces of the prisms. Due to the similarity of the exciton migration
lifetimes between chlorophyll and porphyrin dimer prisms with
the same size trigonal ligand, it appears as though the energy
transfer rates are independent of the chromophores from which
the butadiyne dimers were synthesized. This independence is
indicative of the fact that energy transfer occurs from the low-
energy transition situated along the long axis of the dimers and
that these transitions for both chlorophylls and porphyrins have
similar energies. This work provides an example of structural
control over photophysical processes, knowledge of which is
crucial for the design of future photofunctional materials for
solar energy conversion.
It is interesting to note that the calculated values of τFRET for
both S1 f S1 and S1 f S0 Fo¨rster energy transfer agree to within
a factor of 2. This is most likely a consequence of the
comparable degree of overlap between the S1 f S0 emission
spectrum and both the S1 f Sn and S0 f S1 absorption spectra
for these chromophores, which has been discussed previously
for chlorophylls.53 This suggests that, for chromophore en-
sembles of chlorophylls or porphyrins, either method of
calculating τFRET gives a reasonable benchmark to compare with
experimentally determined hopping rates. However, this fortu-
itous similarity in spectral overlap is not generally true for other
chromophores.
The relative ratios of the experimental exciton hopping rates,
τh, between the chromophore faces in the larger prisms and in
the smaller prisms are about 2:1 and 4:1 for the chlorophyll
and the porphyrin faces, respectively. The analogous ratios of
τFRET are approximately 15:1 for both the chlorophyll and the
porphyrin faces, irrespective of the method used to calculate
τ
FRET. This discrepancy most likely stems from the breakdown
of the point-dipole approximation used in the formulation of
the Fo¨rster theory. The extent of the transition dipole moments
in both the chlorophyll and porphyrin dimers is comparable to
the distances between the faces of the prisms comprised of them.
Several studies involving conducting polymers60,61 and multi-
chromophoric dendrimers62,63 have shown the inadequacy of the
point-dipole approximation when the distance between the
energy donors and acceptors in a system are similar to the
overall length of the transition dipole moments of the component
chromophores. As a result, several computational methods have
recently emerged in an attempt to find better agreement between
the experimentally observed energy transfer rates and those
obtained from theoretical methods.64-69 Future studies of these
assemblies will involve applying these methods.
Acknowledgment. This work was supported by the Chemical
Sciences, Geosciences, and Biosciences Division, Office of
Basic Energy Sciences, DOE, under Grant Nos. DE-FG02-
99ER14999 (M.R.W.) and DE-FG02-87ER13808 (J.T.H.) and
under Contract DE-AC02-06CH11357 (D.M.T.). Y.N. thanks
the Japan Society for the Promotion of Science for a Young
Scientists Research Fellowship. SAXS studies at the Advanced
Photon Source were supported by the Office of Science, Office
of Basic Energy Sciences, DOE, under Contract No. DE-AC02-
06CH11357. We thank Dr. Xiaobing Zuo and Karen L. Mulfort
for assistance with the SAXS experiment.
Supporting Information Available: Experimental details
including the synthesis and characterization of the dimers and
prisms, fluorescence spectra, time-resolved fluorescence data,
additional small-angle X-ray scattering data, and Fo¨rster energy
transfer calculations. This material is available free of charge
via the Internet at http://pubs.acs.org.
Conclusions
The butadiyne linkages between chlorophyll chromophores
in 1 and between the porphyrin chromophores in 23-5,9,22 result
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