Photoinitiated charge transport in supramolecular assemblies of a 1,7,N,N′-tetrakis(zinc porphyrin)-perylene-3,4:9,10-bis(dicarboximide)

Article abstract of DOI:10.1021/ja0664741

We report the synthesis and photophysical characterization of a multichromophore array, (Z3PN)₄PDI, consisting of four zinc 5-phenyl-10,15,20-tri(n-pentyl)porphyrins (Z3PN) attached to the 1,7,N,N′-positions of perylene-3,4:9,10-bis(dicarboximi

Full text of DOI:10.1021/ja0664741

Published on Web 02/24/2007
Photoinitiated Charge Transport in Supramolecular
Assemblies of a 1,7,N,N′-Tetrakis(zinc
porphyrin)-perylene-3,4:9,10-bis(dicarboximide)
Richard F. Kelley, Won Suk Shin, Boris Rybtchinski, Louise E. Sinks, and
Michael R. Wasielewski*
Contribution from the Department of Chemistry and International Institute for Nanotechnology,
Northwestern UniVersity, EVanston, Illinois 60208-3113
Received September 7, 2006; E-mail: m-wasielewski@northwestern.edu
Abstract: We report the synthesis and photophysical characterization of a multichromophore array,
(Z3PN)4PDI, consisting of four zinc 5-phenyl-10,15,20-tri(n-pentyl)porphyrins (Z3PN) attached to the
1,7,N,N′-positions of perylene-3,4:9,10-bis(dicarboximide) (PDI). The dynamics of energy and charge
transport within this system were compared to those of two model compounds, N,N′-(Z3PN)2PDI and 1,7-
(Z3PN)2PDI. The symmetry of the lowest unoccupied and highest occupied molecular orbitals of PDI results
in significantly different electronic couplings between Z3PN and PDI when they are connected at the 1,7-
positions vs the N,N′-positions of PDI. This results in two distinct pathways for electron transfer in
(Z3PN)4PDI. Using a combination of metal-ligand binding with the bidentate ligand 1,4-diazabicyclo[2.2.2.]-
octane (DABCO) and π-π stacking, (Z3PN4)PDI forms a supramolecular assembly, [[(Z3PN)4PDI]2-
DABCO4]2, in toluene solution. The structure of this hierarchical assembly is characterized with the use of
solution-phase X-ray scattering techniques and demonstrates both efficient light harvesting and facile charge
separation and transport using multiple pathways.
Introduction
The properties of the covalent units frequently define a self-
assembly pattern, which in turn determines the function of the
Well-ordered assemblies of photofunctional chromophores are
responsible for the efficient energy and electron transfer found
in photosynthetic light-harvesting and reaction center proteins.1-5
Natural photosynthetic systems make repetitive use of chloro-
phyll molecules having different environments determined by
the surrounding protein to promote directional energy or electron
transfer. In a similar fashion, organization of chromophores into
ordered structures is critical to the development of advanced
photonic materials designed to carry out complex energy- and
electron-transport functions.6-8 Self-assembly of relatively
simple molecular modules into large supramolecular structures
using noncovalent interactions is one of the most promising
strategies for the preparation of photofunctional materials.6,8-31
(13) Rybtchinski, B.; Sinks, L. E.; Wasielewski, M. R. J. Phys. Chem. A 2004,
108, 7497-7505.
(14) Rybtchinski, B.; Sinks, L. E.; Wasielewski, M. R. J. Am. Chem. Soc. 2004,
126, 12268-12269.
(15) van der Boom, T.; Hayes, R. T.; Zhao, Y.; Bushard, P. J.; Weiss, E. A.;
Wasielewski, M. R. J. Am. Chem. Soc. 2002, 124, 9582-9590.
(16) Ajayaghosh, A.; George, S. J.; Praveen, V. K. Angew. Chem., Int. Ed. 2003,
42, 332-335.
(17) Brunsveld, L.; Vekemans, J. A. J. M.; Hirschberg, J. H. K. K.; Sijbesma,
R. P.; Meijer, E. W. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4977-4982.
(18) Cheng, X. H.; Diele, S.; Tschierske, C. Angew. Chem., Int. Ed. 2000, 39,
592-595.
(19) El-Ghayoury, A.; Schenning, A. P. H. J.; van Hal, P. A.; van Duren, J. K.
J.; Janssen, R. A. J.; Meijer, E. W. Angew. Chem., Int. Ed. 2001, 40, 3660-
3663.
(20) Engelkamp, H.; Middelbeek, S.; Nolte, R. J. M. Science 1999, 284, 785-
788.
(21) Fenniri, H.; Deng, B.-L.; Ribbe, A. E. J. Am. Chem. Soc. 2002, 124, 11064-
11072.
(1) McDermott, G.; Prince, S. M.; Freer, A. A.; Hawthornthwaite-Lawless, A.
M.; Papiz, M. Z.; Cogdell, R. J.; Isaacs, N. W. Nature 1995, 374, 517-
521.
(22) Gesquiere, A.; Jonkheijm, P.; Hoeben, F. J. M.; Schenning, A. P. H. J.; De
Feyter, S.; De Schryver, F. C.; Meijer, E. W. Nano Lett. 2004, 4, 1175-
1179.
(2) McLuskey, K.; Prince, S. M.; Cogdell, R. J.; Isaacs, N. W. Biochemistry
2001, 40, 8783-8878.
(23) van Herrikhuyzen, J.; Syamakumari, A.; Schenning, A. P. H. J.; Meijer, E.
W. J. Am. Chem. Soc. 2004, 126.
(3) Pullerits, T.; Sundstro¨m, V. Acc. Chem. Res. 1999, 29, 381-389.
(4) Savage, H.; Cyrklaff, M.; Montoya, G.; Kuhlbrandt, W.; Sinning, I.
Structure 1996, 4, 243-252.
(24) Hirschberg, J. H. K. K.; Brunsveld, L.; Ramzi, A.; Vekemans, J. A. J. M.;
Sijbesma, R. P.; Meijer, E. W. Nature 2000, 407, 167-170.
(25) Jonkheijm, P.; Miura, A.; Zdanowska, M.; Hoeben, F. J. M.; Feyter, S. D.;
Schenning, A. P. H. J.; Schryver, F. C. D.; Meijer, E. W. Angew. Chem.,
Int. Ed. 2004, 43, 74-78.
(5) Walz, T.; Jamieson, S. J.; Bowers, C. M.; Bullough, P. A.; Hunter, C. N.
J. Mol. Biol. 1998, 282, 833-845.
(6) Ikkala, O.; ten Brinke, G. Science 2002, 295, 2407-2409.
(7) Kim, D.; Osuka, A. Acc. Chem. Res. 2004, 37, 735-745.
(8) Lehn, J.-M. Science 2002, 295, 2400-2403.
(26) Kleppinger, R.; Lillya, C. P.; Yang, C. J. Am. Chem. Soc. 1997, 119, 4097-
4102.
(9) Reinhoudt, D. N.; Crego-Calama, M. Science 2002, 295, 2403-2407.
(10) Ahrens, M. J.; Sinks, L. E.; Rybtchinski, B.; Liu, W.; Jones, B. A.; Giaimo,
J. M.; Gusev, A. V.; Goshe, A. J.; Tiede, D. M.; Wasielewski, M. R. J.
Am. Chem. Soc. 2004, 126, 8284-8294.
(27) Thalacker, C.; Wu¨rthner, F. AdV. Funct. Mater. 2002, 12, 209-218.
(28) Wu¨rthner, F.; Thalacker, C.; Sautter, A. AdV. Mater. 1999, 11, 754-758.
(29) Haycock, R. A.; Hunter, C. A.; James, D. A.; Michelsen, U.; Sutton, L. R.
Org. Lett. 2000, 2, 2435-2438.
(11) Fuller, M. J.; Sinks, L. E.; Rybtchinski, B.; Giaimo, J. M.; Li, X.;
Wasielewski, M. R. J. Phys. Chem. A 2005, 109, 970-975.
(12) Li, X.; Sinks, L. E.; Rybtchinski, B.; Wasielewski, M. R. J. Am. Chem.
Soc. 2004, 126, 10810-10811.
(30) Haycock, R. A.; Yartsev, A.; Michelsen, U.; Sundstrom, V.; Hunter, C. A.
Angew. Chem. 2000, 39, 3616-3619.
(31) Mak, C. C.; Bampos, N.; Sanders, J. K. M. Angew. Chem. 1998, 37, 3020-
3023.
9
10.1021/ja0664741 CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 3173-3181
3173

A R T I C L E S
Kelley et al.
self-assembled array. In this hierarchical approach, the symmetry
of the molecular building blocks not only controls the geometry
of the assemblies formed from these subunits but also the
directionality of photoinduced electron transfer.
nearly degenerate and have electron density distributions that
differ greatly.56-58 The a1u orbital has significant electron density
at the â-carbons of the porphyrin with little density at the meso
positions, while the opposite is true for the a2u orbital. The
energetic ordering of these two orbitals in the metalloporphyrin
depends on the nature and the position of the substituents
attached to the porphyrin.58 Substituting electron-donating
groups, e.g., phenyl rings or alkyl chains, at the meso positions
(5,10,15,20) of the porphyrin raises the energy of the a2u orbital
above that of the a1u orbital, making the a2u orbital the HOMO,
while substitution of the â-positions with similar electron-
releasing groups results in the a1u orbital being the HOMO.58,59
Generally, increased electron density at particular peripheral
carbon atoms of the porphyrin results in stronger electronic
coupling to molecules attached to those positions, and conse-
quently in faster electron-transfer rates.49 For example, Hayes
et al.53 showed that a zinc porphyrin having a pyromellitimide
electron acceptor attached to a meso-phenyl group has a charge
recombination rate that is 3 times faster than that of a porphyrin
in which the acceptor is attached to a â-phenyl group. The
intense, complementary electronic spectra of Z3PN and PDI
result in absorption of a large portion of the solar spectrum
between 400 and 650 nm. It has already been shown that
photoinduced electron transfer occurs rapidly and efficiently
from 1*Z3PN to PDI derivatives.60,61 Each of these examples is
consistent with dominant through-bond, superexchange-mediated
electron transfer.62-66
Perylene-3,4:9,10-bis(dicarboximide) (PDI) chromophores are
excellent building blocks for covalent and noncovalent photo-
functional arrays.10-15,23,27,28,32-41 PDI has two distinct sites to
which electron donors and/or acceptors can be easily attached:
the nitrogen atoms of the imides and the 1,6,7,12-carbon atoms
at its long edges. Electronic structure calculations show that
both the HOMO and LUMO of PDI have nodal planes that
bisect the molecule along the N-N axis, so that the π-electron
density in these orbitals is much greater at the 1,6,7,12-carbon
atoms than at the imide nitrogen atoms.42 Rates of electron
transfer kDA, are given by kDA V2DA(FCWD), where VDA is
the electronic coupling matrix element and FCWD is the
Franck-Condon weighted density of states.43-45 Through-bond
Dexter-type energy transfer,46 when viewed formally as two
electron transfers, can also be described using a similar
expression involving the product of the electronic coupling
matrix elements for each electron transfer.47,48 Since VDA
depends strongly on orbital overlap between the frontier
molecular orbital of the donor with that of the acceptor,49 rates
of electron transfer and Dexter-type energy transfer depend
strongly on the electron density distributions within these
orbitals.50,51 Thus, attaching identical electron donors to sites
on PDI having orbital coefficients that differ greatly should in
principle result in large differences in electron-transfer rates,
making it possible to tailor the rates of individual electron-
transfer pathways in a complex donor-acceptor array.
Coordination bond formation using metal centers and poly-
dentate ligands is an attractive strategy for building self-
assembled structures in which the geometries of the covalent
building blocks dictate the structure of the assemblies. Increasing
the number of coordination bonding sites within the structure
enhances allosteric effects and increases control over the
assembly geometry. Multiporphyrin arrays, in which coordina-
tion bonds are formed between bidentate ligands and the
porphyrin metal centers, represent an important controllable
assembly motif.67-77 There are many examples of multipor-
It is well-known that the rates of both Dexter-type energy
transfer and electron transfer to or from metalloporphyrins
depend on which position of the macrocycle the donor or
acceptor is attached.50,52-55 The two highest occupied molecular
orbitals (HOMO and HOMO-1) within metalloporphyrins are
(32) Wu¨rthner, F. Chem. Commun. 2004, 1564-1579.
(33) Xiao, S.; El-Khouly, M. E.; Li, Y.; Gan, Z.; Liu, H.; Jiang, L.; Araki, Y.;
Ito, O.; Zhu, D. J. Phys. Chem. B 2005, 109, 3658-3667.
(34) Liu, Y.; Wang, N.; Li, Y.; Liu, H.; Li, Y.; Xiao, J.; Xu, X.; Huang, C.;
Cui, S.; Zhu, D. Macromolecules 2005, 38, 4880-4887.
(35) He, X.; Liu, H.; Li, Y.; Liu, Y.; Lu, F.; Li, Y.; Zhu, D. Macromol. Chem.
Phys. 2005, 206, 2199-2205.
(54) Strachan, J.-P.; Gentemann, S.; Seth, J.; Kalsbeck, W. A.; Lindsey, J. S.;
Holten, D.; Bocian, D. F. J. Am. Chem. Soc. 1997, 119, 11191-11201.
(55) Osuka, A.; Marumo, S.; Maruyama, K.; Mataga, N.; Tanaka, Y.; Taniguchi,
S.; Okada, T.; Yamazaki, I.; Yoshinobu, N. Bull. Chem. Soc. Jpn. 1995,
68, 262-276.
(36) You, C.-C.; Wu¨rthner, F. J. Am. Chem. Soc. 2003, 125, 9716-9725.
(37) You, C.-C.; Wu¨rthner, F. Org. Lett. 2004, 6, 2401-2404.
(38) Prodi, A.; Chiorboli, C.; Scandola, F.; Iengo, E.; Alessio, E.; Dobrawa,
R.; Wu¨rthner, F. J. Am. Chem. Soc. 2005, 127, 1454-1462.
(39) You, C.-C.; Dobrawa, R.; Saha-Moeller, C. R.; Wu¨rthner, F. Top. Curr.
Chem. 2005, 258, 39-82.
(56) Gouterman, M. Porphyrins 1978, 3, 1-165.
(57) Weiss, C.; Kobayashi, H.; Gouterman, M. J. Mol. Spectrosc. 1965, 16,
415-450.
(58) Fajer, J.; Borg, D. C.; Forman, A.; Dolphin, D.; Felton, R. H. J. Am. Chem.
Soc. 1970, 92, 3451-3459.
(40) Beckers, E. H. A.; Meskers, S. C. J.; Schenning, A. P. H. J.; Chen, Z.;
Wu¨rthner, F.; Marsal, P.; Beljonne, D.; Cornil, J.; Janssen, R. A. J. J. Am.
Chem. Soc. 2006, 128, 649-657.
(59) Atamian, M.; Wagner, R. W.; Lindsey, J. S.; Bocian, D. F. Inorg. Chem.
1988, 27, 1510-1512.
(60) Gosztola, D.; Niemczyk, M. P.; Wasielewski, M. R. J. Am. Chem. Soc.
1998, 120, 5118-5119.
(41) An, Z.; Yu, J.; Jones, S. C.; Barlow, S.; Yoo, S.; Domercq, B.; Prins, P.;
Siebbeles, L. D. A.; Kippelen, B.; Marder, S. R. AdV. Mater. (Weinheim)
2005, 17, 2580-2583.
(61) Hayes, R. T.; Wasielewski, M. R.; Gosztola, D. J. Am. Chem. Soc. 2000,
122, 5563-5567.
(42) Burquel, A.; Lemaur, V.; Beljonne, D.; Lazzaroni, R.; Cornil, J. J. Phys.
Chem. A 2006, 110, 3447-3453.
(62) Anderson, P. W. Phys. ReV. 1950, 79, 350-356.
(63) Anderson, P. W. Phys. ReV. 1959, 115, 2-13.
(64) McConnell, H. M. J. Chem. Phys. 1961, 35, 508-515.
(65) Marcus, R. A. Chem. Phys. Lett. 1987, 133, 471.
(66) Ogrodnik, A.; Michel-Beyerle, M. E. Z. Naturforsch. 1989, 44a, 763-
764.
(43) Hopfield, J. J. Proc. Natl. Acad. Sci. U.S.A. 1974, 71, 3640-3644.
(44) Jortner, J. J. Chem. Phys. 1976, 64, 4860-4867.
(45) Marcus, R. A. J. Chem. Phys. 1984, 81, 4494-4500.
(46) Dexter, D. L. J. Chem. Phys. 1953, 21, 836-850.
(47) Closs, G. L.; Johnson, M. D.; Miller, J. R.; Piotrowiak, P. J. Am. Chem.
Soc. 1989, 111, 3751-3753.
(67) Tabushi, I.; Kugimiya, S.; Kinnaird, M. G.; Sasaki, T. J. Am. Chem. Soc.
1985, 107, 4192-4199.
(48) Closs, G. L.; Piotrowiak, P.; MacInnis, J. M.; Fleming, G. R. J. Am. Chem.
Soc. 1988, 110, 2652-2653.
(68) Hunter, C. A.; Meah, M. N.; Sanders, J. K. M. J. Chem. Soc., Chem.
Commun. 1988, 694-696.
(49) Plato, M.; Mo¨bius, K.; Michel-Beyerle, M. E.; Bixon, M.; Jortner, J. J.
Am. Chem. Soc. 1988, 110, 7279-7285.
(69) Hunter, C. A.; Leighton, P.; Sanders, J. K. M. J. Chem. Soc., Perkin Trans.
1 1989, 547-552.
(50) Holten, D.; Bocian, D. F.; Lindsey, J. S. Acc. Chem. Res. 2002, 35, 57-
(70) Camara-Campos, A.; Hunter, C. A.; Tomas, S. Proc. Natl. Acad. Sci. U.S.A.
2006, 103, 3034-3038.
69.
(51) Tsai, H.; Simpson, M. C. Chem. Phys. Lett. 2002, 353, 111-118.
(52) Redmore, N. P.; Rubtsov, I. V.; Therien, M. J. J. Am. Chem. Soc. 2003,
125, 8769-8778.
(71) Ballester, P.; Oliva, A. I.; Costa, A.; Deya, P. M.; Frontera, A.; Gomila,
R. M.; Hunter, C. A. J. Am. Chem. Soc. 2006, 128, 5560-5569.
(72) 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.
(53) Hayes, R. T.; Wasielewski, M. R. J. Phys. Chem A 2004, 108, 2375-
2381.
9
3174 J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007

Photoinitiated Charge Transport
A R T I C L E S
Figure 1. Structures of molecules used in this study.
phyrin systems in which coordination bonds between the
bidentate ligand 1,4-diazabicyclo[2.2.2]octane (DABCO) and
porphyrin metal centers are used to prepare porphyrin
dimers.31,67,68,71,73,78,79
(Z3PN4)PDI forms a supramolecular assembly, [[(Z3PN)4PDI]2-
DABCO4]2, in toluene solution. The structure of this assembly
is characterized using solution-phase X-ray scattering techniques
and demonstrates both efficient light harvesting and facile charge
separation and transport using multiple pathways.
There are numerous reports on the self-assembly of both zinc
porphyrin and PDI derivatives alone, yet there are relatively
few examples in which both of these important donor-acceptor
building blocks are incorporated into specific covalent structures
to elicit different supramolecular photofunctional assem-
blies.15,33,37,38 We reported earlier on supramolecular assemblies
in which four PDI derivatives were appended to the para position
of each phenyl within zinc meso-tetraphenylporphyrin.15 These
molecules formed large, ordered supramolecular assemblies in
which the assembly was driven mainly by π-π interactions of
the PDI electron acceptors. Here we report the synthesis and
photophysical characterization of a multichromophore array
constructed from four zinc 5-phenyl-10,15,20-tri(n-pentyl)-
porphyrins (Z3PN) attached to the 1,7,N,N′-positions of PDI to
give (Z3PN4)PDI, Figure 1. We also report the dynamics of
energy- and charge transport of two model compounds, N,N′-
(Z3PN)2PDI and 1,7-(Z3PN)2PDI, which help to interpret the
data obtained from the full array. As mentioned above, the
symmetry of the lowest unoccupied molecular orbital of PDI
results in significantly different electronic couplings between
Z3PN and PDI when they are connected at the 1,7-positions vs
the N,N’-positions of PDI. This results in two distinct pathways
for electron transfer in (Z3PN)4PDI. Using a combination of
metal-ligand binding with (DABCO) and π-π stacking,
Experimental Section
The syntheses and characterization of (Z3PN)4PDI and model
compounds, 1,7-(Z3PN)2PDI and N,N′-(Z3PN)2PDI, Figure 1, are
described in detail in the Supporting Information. UV-vis absorption
measurements were made with a Shimadzu spectrometer (UV1601).
All solvents were reagent grade or better and were purified using
standard procedures. Toluene used for spectroscopic and X-ray scat-
tering measurements was purified by passing it through a series of CuO
and alumina columns (GlassContour) immediately prior to use.
Electrochemical measurements were performed using a CH Instruments
model 660A electrochemical workstation. Reversible half-wave po-
tentials were determined in butyronitrile containing 0.1 M n-Bu4N+-
BF-4 using a Pt working electrode, a Pt wire counter electrode, a Ag/
AgxO reference electrode, and ferrocene/ferrocenium (Fc/Fc+, 0.42 vs
SCE) as an internal reference.
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.80 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 Å-1 < q < 0.1 Å-1 region, where q ) (4π/λ) sinθ, λ is the X-ray
scattering wavelength and 2θ is the scattering angle. Glass capillaries
(0.2 mm diameter) were used as sample containers. All samples were
filtered through 200 nm PTFE filters (Whatman) prior to the measure-
ment. The scattering intensity was averaged over three measurements,
after which the solvent scattering was systematically measured and
subtracted from the sample spectrum.
(73) Ballester, P.; Costa, A.; Castilla, A. M.; Deya, P. M.; Frontera, A.; Gomila,
R. M.; Hunter, C. A. Chem.-Eur. J. 2005, 11, 2196-2206.
(74) Ogawa, K.; Kobuke, Y. J. Photochem. Photobiol., C 2006, 7, 1-16.
(75) Nakagawa, H.; Ogawa, K.; Satake, A.; Kobuke, Y. Chem. Commun.
(Cambridge) 2006, 1560-1562.
Femtosecond transient absorption measurements were obtained using
an instrument outfitted with a CCD array detector for the collection of
spectral data at multiple delay times following photoexcitation of the
(76) Hajjaj, F.; Yoon, Z. S.; Yoon, M.-C.; Park, J.; Satake, A.; Kim, D.; Kobuke,
Y. J. Am. Chem. Soc. 2006, 128, 4612-4623.
(77) Furutsu, D.; Satake, A.; Kobuke, Y. Inorg. Chem. 2005, 44, 4460-4462.
(78) Hunter, C. A.; Meah, M. N.; Sanders, J. K. M. J. Am. Chem. Soc. 1990,
112, 5773-5780.
(79) Bampos, N.; Marvaud, V.; Sanders, J. K. M. Chem.-Eur. J. 1998, 4, 335-
343.
(80) Seifert, S.; Winans, R. E.; Tiede, D. M.; Thiyagarajan, P. J. Appl.
Crystallogr. 2002, 33, 782-784.
9
J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007 3175

A R T I C L E S
Kelley et al.
Figure 2. Ground-state electronic absorption spectra of the indicated
compounds in toluene with 1% pyridine (v:v). (Inset) Expansion of spectra
between 450 and 650 nm.
Figure 3. Comparison of the ground-state electronic absorption spectra of
(Z3PN)4PDI with 1% pyridine and with 2 equiv of DABCO in dry toluene.
(Inset) Expansion of spectra between 500 and 650 nm.
sample.81 The total instrument response 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. Cuvettes with a 2-mm path
length were used, and the samples were irradiated with 1.0 µJ/pulse
focused to a 200-µm spot. The optical density was kept between 0.2
and 0.4 at λex. 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
instrument response function.
due to different Z3PN/PDI ratios, the spectra of the two model
systems are almost identical to that of (Z3PN)4PDI. The spectra
are indicative of pyridine ligation to each zinc atom with
porphyrin Soret bands appearing at 433 nm, porphyrin Q bands
at 538, 573, and 615 nm,56 and additional absorbance between
470 and 590 nm due to the PDI core. As in (Z3PN-py)4PDI,
the spectra of the model compounds between 460 and 665 nm
are dominated by porphyrin absorption bands, due to the 2:1
ratio of Z3PN to PDI. The spectra of (Z3PN-py)4PDI, N,N′-
(Z3PN-py)2PDI and 1,7-(Z3PN-py)2PDI can be approximately
reconstructed using a weighted sum of the component Z3PN
and PDI absorptions, which indicates weak ground-state elec-
tronic coupling between the chromophores.85 Steady-state
fluorescence from (Z3PN-py)4PDI, N,N′-(Z3PN-py)2PDI, and
Results and Discussion
Donor-Acceptor Building Blocks. The featured compound,
(Z3PN)4PDI, was synthesized from 1,7-dibromoperylene-3,4:
9,10-tetracarboxydianhydride15 via sequential imide condensa-
tion and Stille cross-coupling reactions with 20-(p-aminophenyl)-
5,10,15-tripentylporphyrin81 and zinc 20-(p-tri(n-butyl)tinphenyl)-
5,10,15-tripentylporphyrin, respectively. Metalation of the free-
base N,N′-porphyrins using zinc acetate in CHCl3/MeOH
resulted in the final product, (Z3PN)4PDI. Model compounds,
N,N′-(Z3PN)2PDI and 1,7-(Z3PN)2PDI, were synthesized to
independently study electron-transfer pathways through the
imide and 1,7-positions. The syntheses for both models were
analogous to that of (Z3PN)4PDI, where 4-tributyltintoluene82
and p-toluidine were substituted for zinc 20-(p-tri(n-butyl)-
tinphenyl)-5,10,15-tripentylporphyrin and 20-(p-aminophenyl)-
5,10,15-tripentylporphyrin to yield N,N′-(Z3PN)2PDI and
1,7-(Z3PN)2PDI, respectively. Additional model compounds,
zinc 5,10,15,20-tetra(n-pentyl)porphyrin (Z4PN)83 and N,N′-
bis(cyclohexyl)-1,7-diphenylperylene-3,4:9,10-bis(dicarboxim-
ide) (1,7-(Ph)2PDI),84 serve as references for the individual
component chromophores.
1,7-(Z3PN-py)2PDI is almost completely quenched (φF
<
0.001), indicating that efficient photoinduced electron transfer
occurs in these systems (see below).
Supramolecular Assemblies. The ground-state absorption
spectrum of (Z3PN)4PDI in dry toluene with two equivalents
of DABCO changes significantly, relative to that of (Z3PN-
py)4PDI in toluene, Figure 3. The S0 f S2 transition of Z3PN
in monomeric (Z3PN-py)4PDI occurs at 433 nm, which is typical
of amine-ligated Zn porphyrins,31,86 while replacement of
pyridine by DABCO blue shifts this transition to 428 nm. It is
well-known that cofacial porphyrin dimers show exciton
coupling between the two chromophores as evidenced by blue-
shifting of the porphyrin Soret bands.71,73 The porphyrin Q bands
at 538, 570, and 613 nm are perturbed to a lesser extent relative
to those of (Z3PN-py)4PDI. The contribution of PDI to the
ground-state spectrum is seen as a shoulder at 525 nm in (Z3PN-
py)4PDI, Figure 3. For (Z3PN)4PDI with 2 equiv of DABCO
the 525 nm feature intensifies. Cofacial dimers and higher
aggregates of PDI, in which the transition dipole moments are
parallel, show an enhanced (0,1) vibronic band relative to the
(0,0) band.15,32,87-89 Overall, these spectral changes are consis-
The ground-state absorption spectrum of (Z3PN)4PDI in
toluene with 1% pyridine (py) at room temperature is compared
with those of model compounds N,N′-(Z3PN)2PDI and 1,7-
(Z3PN)2PDI in Figure 2. Apart from differences in absorbance
(81) Kelley, R. F.; Tauber, M. J.; Wasielewski, M. R. J. Am. Chem. Soc. 2006,
128, 4779-4791.
(82) Justicia, J.; Oltra, J. E.; Cuerva, J. M. J. Org. Chem. 2004, 69, 5803-
(85) Greenfield, S. R.; Svec, W. A.; Gosztola, D.; Wasielewski, M. R. J. Am.
Chem. Soc. 1996, 118, 6767-6777.
(86) Nappa, M.; Valentine, J. S. J. Am. Chem. Soc. 1978, 100, 5075-5080.
(87) Langhals, H.; Ismael, R. Eur. J. Org. Chem. 1998, 1915-1917.
(88) Li, A. D. Q.; Wang, W.; Wang, L.-Q. Chem.-Eur. J. 2003, 9, 4594-
4601.
5806.
(83) Lindsey, J. S.; Schreiman, I. C.; Hsu, H. C.; Kearney, P. C.; Marguerettaz,
A. M. J. Org. Chem. 1987, 52, 827-836.
(84) Chao, C.; Leung, M.; Su, Y.; Chiu, K.; Lin, T.; Shieh, S.; Lin, S. J. Org.
Chem. 2005, 70, 4323-4331.
9
3176 J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007

Photoinitiated Charge Transport
A R T I C L E S
tent with self-association of at least two (Z3PN)4PDI molecules
to form [(Z3PN)4PDI]2-DABCO4 but do not provide informa-
tion about the possible formation of higher aggregates or
structural information about these aggregates.
In order to determine the stability of [(Z3PN)4PDI]2-
DABCO4 insolution,aspectrophotometrictitrationof(Z3PN)4PDI
(4.6 × 10-7 M in chloroform) was conducted, Figure S1. Prior
to the addition of DABCO the porphyrin Soret band occurs at
424 nm. Following initial addition of DABCO to the chloroform
solution, the absorption band at 424 gives way to a new band
centered at 428 nm, which is assigned to the formation of face-
to-face porphyrin dimers in which the two porphyrins are bound
by a single DABCO.71 The 428 nm absorption feature increases
very little after the addition of 2 equiv of DABCO. However,
after the addition of ∼104 equiv of DABCO, the band at 428
nm decreases, and a new band at 433 nm appears. This band is
due to dissociation of the dimer into two fully ligated, open
complexes, (Z3PN)4PDI-DABCO4. The presence of two isos-
bestic points in the UV-vis titration data indicates that only
three chromophores are present in significant quantity, Figure
S2.71 The equilibrium constants applicable to the three species
model are defined as follows:
Figure 4. Small-angle X-ray scattering data for ((Z3PN)4PDI)2-DABCO4
in toluene (2 × 10-4 M) solution. (Inset) SAXS data for (Z3PN)4PDI in
wet toluene (7 × 10-4 M) solution. Guinier fits to the data are also shown.
between two Z3PN moieties. [(Z3PN)4PDI]2-DABCO4 also
shows resonances at 9.6 ppm, due to the porphyrin â-protons.
The cofacial proximity of the two porphyrin π-systems in this
assembly causes these signals to experience upfield ring-current-
induced chemical shifts relative to the corresponding signals in
the monomeric (Z3PN-py)4PDI complexes. Methodology other
than conventional spectroscopy must be used to elucidate these
structures in toluene.
K41 ) [[(Z3PN)4PDI] - DABCO4]/
[DABCO]4[(Z3PN)4PDI] (1)
K42 ) [[(Z3PN)4PDI]2 - DABCO4]/
[DABCO]4[(Z3PN)4PDI]2 (2)
The structure of [(Z3PN)4PDI]2-DABCO4 in toluene was
determined by performing small-angle X-ray scattering (SAXS)
measurements on (2-6) × 10-4 M samples, see Supporting
Information for details. Comparable data in CHCl3 cannot be
obtained due to the strong absorption of X-rays by the Cl atoms
of CHCl3. The scattering intensity is a function of the scattering
vector q, which is related to the scattering angle 2θ by the
relationship q ) [(4π/λ) sinθ], where λ is the X-ray wavelength.
In the low-resolution scattering region (q < 0.2 Å-1), the
scattering follows the Guinier relationship, I(q) ) I(0) exp(-
K42T41 ) (K41)2/K42
(3)
The titration data was fit to a three-chromophore model using
multivariate factor analysis (SPECFIT V3.090), Figure S3, to
yield the stability constants K41 ) (9.7 ( 3.0) × 1031 M-5 and
K42 ) (1.2 ( 0.2) × 1018 M-4. From these stability values, the
equilibrium constant for the exchange between the two species
was determined to be K42T41 ) 1.5 × 104 M-3. These results
suggest that the [(Z3PN)4PDI]2-DABCO4 dimer is fully formed
and stable after the stoichiometric addition of 2 equiv of
DABCO to (Z3PN)4PDI and that a large excess of ligand is
required to disrupt the dimer. This knowledge is important for
understanding the structure-function relationship of the dimer
in solution.
2
q2Rg /3), which is parametrized in terms of the forward scattering
amplitude, I(0), and the radius of gyration, Rg.78,91,92 Guinier
plots of (Z3PN)4PDI in wet toluene (0.1%, to ensure complete
disaggregation) and [(Z3PN)4PDI]2-DABCO4 in dry toluene
are presented in Figure 4. The linearity of the Guinier plot is a
measure of the compound dispersity.92,93 (Z3PN)4PDI shows
Additional evidence for the formation of a minimal [(Z3PN)4
PDI]2-DABCO4 structural unit upon addition of two equivalents
of DABCO to a solution of (Z3PN)4PDI is provided by its H
NMR spectrum in CDCl3 (not shown). Unfortunately, the H
an essentially linear Guinier plot over the range 0.004 Å-2
<
1
q2 < 0.010 Å-2. The least-squares fit to the linear data reveals
Rg ) 11.6 ( 0.03 Å. Similar monodispersity and Rg were
observed for a THF solution of (Z3PN)4PDI, Figure S4. UV-
vis spectra obtained at the conditions of SAXS measurements
(10-4 M THF and wet toluene solutions) show spectra typical
of disaggregated molecules, indicating that (Z3PN)4PDI is
monomeric in both wet toluene and THF.
1
NMR signals of [(Z3PN)4PDI]2-DABCO4 in toluene-d8 are
severely broadened, and its UV-vis spectrum is inconclusive
regarding both the size and detailed structure of the aggregate.
1
The H NMR resonances of (Z3PN)4PDI in CDCl3 are broad
at room temperature, but the aggregated species exhibits a signal
centered around -4.75 ppm, characteristic of DABCO sand-
wiched between two metalloporphyrins.78 The absence of signals
around 3 and -3 ppm (characteristic of free DABCO and
DABCO coordinated to a single zinc porphyrin, respectively)
indicates that all of the DABCO ligands in solution reside
(Z3PN)4PDI in dry toluene with 2 equiv of DABCO shows
an essentially linear Guinier plot, Figure 4, in the analogous
range of 0.004 Å-2 < q2 < 0.01 Å-2 and upward deviation
from linearity in the region q2 < 0.004 Å-2. Linearity over the
higher q2 range indicates that the assemblies formed with the
(89) Rybtchinski, B.; Sinks, L. E.; Wasielewski, M. R. J. Phys. Chem. A 2004,
(91) Glatter, O. Neutron, X-ray and Light Scattering; Elsevier: Amsterdam,
1991.
(92) Guinier, A.; Fournet, G. Small Angle Scattering; Wiley: New York, 1955.
(93) Svergun, D. I.; Koch, M. H. Rep. Prog. Phys. 2003, 66, 1735-1782.
108, 7497-7505.
(90) SPECFIT, version 3.0.38 ed.; Spectra Software Associates: Marlborough,
MA, 2006.
9
J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007 3177

A R T I C L E S
Kelley et al.
Table 1. One-Electron Redox Potentials vs SCE (EOX, ERED),
Excitation Energies (ES1), and Free Energiesa of Charge
Separation (∆GCS) and Charge Recombination (∆GCR
)
cmpd
EOX
ERED
ES1
∆GCS
∆GCR
N,N′-(Z3PN)2PDI
1,7-(Z3PN)2PDI
0.62
0.62
-0.49
-0.49
2.04
2.04
-0.26
-0.43
-1.78
-1.61
a Calculated using an expression derived by Weller as described in the
text.
Figure 6. Transient absorption spectra of (Z3PN)4PDI in toluene with 1%
pyridine following excitation with 612 nm, 120 fs laser pulses. (Inset)
Transient absorption kinetics for (Z3PN)4PDI at 565 nm (blue) and 764
nm (red) in toluene with 1% pyridine following excitation with 612 nm,
120 fs laser pulses. Nonlinear least-squares fits to the data are also shown.
Figure 5. MM+ calculated structure of [[(Z3PN)4PDI]2-DABCO4]2.
Views are along the stacking direction (top) and an axis 90° to that direction
(side). Simulated Rg ) 17.6 Å.
distances using the Weller formalism,96 Table 1 (see Supporting
Information for details). The calculated values of ∆GCS predict
that photoinduced charge separation should occur readily in both
model systems as well as in (Z3PN)4PDI in toluene.
addition of DABCO are largely monodisperse. The upward
deviation from linear Guinier behavior is characteristic of weakly
associating molecular systems94 and can be attributed to the π-π
interactions characteristic of porphyrin systems.95 The least-
squares fit to the linear data reveals Rg ) 18.0 ( 0.2 Å. The
Guinier plots also yield the extrapolated intensity at zero
scattering angle, I(0), which is proportional to the overall
electron density and can be used to estimate the molecular
weight of the monodisperse aggregate by comparison to an I(0)
value of a monodisperse reference compound of similar structure
with known molecular weight.10,93 By comparing the I(0)
obtained from monomeric (Z3PN)4PDI in wet toluene to that
obtained from the higher aggregates of [(Z3PN)4PDI]2-
DABCO4, we estimate that the assemblies are most likely
constructed of two [(Z3PN)4PDI]2-DABCO4 complexes. Com-
parison of the experimentally obtained values for Rg with those
generated from a variety of models suggests that the structure
of the assembly is a slipped-stacked dimer of [(Z3PN)4PDI]2-
DABCO4, see Figure 5 (see also Figure S5).
Femtosecond transient absorption spectra of (Z3PN-py)4PDI
in toluene with 1% pyridine (v:v) were obtained using 120 fs,
612 nm pulses, which selectively excite the S0 f S1 transition
of Z3PN, Figure 6. The initial spectrum shows 1*Z3PN
absorption,60,61 which evolves in time to that of (Z3PN)4
+• 60,61
and PDI-• 97 with absorption features centered around 450 and
744 nm respectively, and ground-state bleaches occurring at 531
and 568 nm for PDI and 614 nm for Z3PN. Transient absorption
kinetic traces at 565 and 746 nm, Figure 6 (inset), reveal that
the radical ion pair appears with τCS ) 5.0 ( 1.0 ps and
subsequently decays biexponentially with τCR ) 81 ( 6 ps
(60%) and 480 ( 60 ps (40%). Studies on the relevant model
systems were performed to clarify the biexponential nature of
the charge recombination kinetics within (Z3PN)4+ •-PDI-•.
Femtosecond transient absorption measurements on N,N′-
(Z3PN-py)2PDI and 1,7-(Z3PN-py)2PDI in toluene with 1%
pyridine (v:v) using 120 fs, 612 nm pulses show that the spectral
features in the transient spectra of both model compounds, see
Figures S6 and S7, are nearly identical to those of (Z3PN-
py)4PDI. The initial spectrum shows features due to 1*Z3PN,
which evolve in time to those of (Z3PN-py)4+•-PDI-•. Transient
absorption kinetic traces at 565 and 745 nm for N,N′-(Z3PN-
py)2PDI reveal that the charge separated state appears with τCS
Photoinduced Energy and Electron Transfer. The free
energies for photoinduced charge separation (∆GCS) and charge
recombination (∆GCR) for N,N′-(Z3PN)2PDI and 1,7-(Z3PN)2PDI
were calculated using their experimentally determined redox
potentials and the lowest singlet excited-state energy (ES1) of
Z3PN, along with computed ionic radii and donor-acceptor
(94) Ducruix, A.; Guilloteau, J. P.; Ries-Kautt, M.; Tardieu, A. J. Cryst. Growth
1986, 168, 28-39.
(95) Tiede, D. M.; Zhang, R.; Chen, L. X.; Yu, L.; Lindsey, J. S. J. Am. Chem.
Soc. 2004, 126, 14054-14062.
(96) Weller, A. Z. Phys. Chem. (Munich) 1982, 133, 93-98.
(97) Gosztola, D.; Niemczyk, M. P.; Svec, W. A.; Lukas, A. S.; Wasielewski,
M. R. J. Phys. Chem. A 2000, 104, 6545-6551.
9
3178 J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007

Photoinitiated Charge Transport
A R T I C L E S
Table 2. Time Constants and Electronic Coupling Matrix Elements
for Charge Separation and Charge Recombination in Toluene
) 9.0 ( 0.5 ps and decays with τCR ) 1070 ( 30 ps, Figure
S6 (inset). The corresponding transient absorption kinetics at
570 and 747 nm for 1,7-(Z3PN-py)2PDI reveal that the charge
separated state appears with τCS ) 10.0 ( 0.5 ps and decays
with τCR ) 77 ( 3 ps, Figure S7 (inset).
τCS
(ps)
VDA
(cm-
τCR
(ps)
VDA
1
1
cmpd
)
(cm-
)
N,N′-(Z3PN-py)2PDIa
1,7-(Z3PN-py)2PDIa
(Z3PN-py)4PDIa
9.0 ( 0.5
10.0 ( 0.5
5.0 ( 1.0
21
18
1070 ( 30
77 ( 3
250
380
The transient absorption spectra of (Z3PN-py)4PDI, N,N′-
(Z3PN-py)2PDI and 1,7-(Z3PN-py)2PDI do not show long-lived
absorption changes indicative of the formation of significant
yields of either 3*Z3PN or 3*PDI triplet states upon charge
recombination. Although the energies of 3*Z3PN and 3*PDI are
about 1.7 eV98 and 1.2 eV,99 respectively, significant yields of
triplet-state formation by either spin-orbit intersystem crossing
or by radical pair intersystem crossing do not occur. In the
former case, the rates of electron transfer are all far faster than
spin-orbit intersystem crossing in either 1*Z3PN and 3*PDI,
while in the latter case, the lifetimes of the radical ion pairs are
too short and the spin-spin exchange interactions between the
radicals are most likely too large for efficient radical pair
intersystem crossing.100
The transient absorption kinetics for N,N′-(Z3PN-py)2PDI and
1,7-(Z3PN-py)2PDI show that the rate constants for CS are
comparable, while those for CR differ by a factor of ∼12.
Electron-transfer rates depend on both the free energy of reaction
and the electronic coupling matrix element, VDA, for the
process.15 Considering the free energies of reaction, ∆GCS for
1,7-(Z3PN-py)2PDI (-0.43 eV) is slightly more negative than
that for N,N′-(Z3PN-py)2PDI (-0.26 eV). This is primarily due
to greater Coulombic stabilization in 1,7-(Z3PN-py)2PDI be-
cause the ion pair distance is shorter. The total reorganization
energy for electron transfer in these systems is due largely to
internal nuclear motions with Z3PN and PDI, where λI = 0.32
eV,81 since the solvent contribution, λS, = 0.101 In the Marcus
model, both CS reactions are close to the peak of the rate vs
free energy dependence, so that kCS for 1,7-(Z3PN-py)2PDI
should be comparable to that observed for N,N′-(Z3PN-py)2PDI,
provided that the values of VDA are also comparable. The
observed values of kCS for N,N′-(Z3PN-py)2PDI and 1,7-(Z3PN-
py)2PDI are within experimental error of one another supporting
the notion that the values of VDA for the CS reactions are indeed
comparable. It is well-known that increased orbital overlap
between redox partners leads to a larger value of VDA for electron
transfer, which, in turn, produces faster electron-transfer
rates.54,102,103 Electron transfer from 1*Z3PN occurs essentially
from the LUMO of Z3PN, which has a large fraction of its π
charge density at the â-pyrrole positions of the porphyrin
macrocycle. In addition, the LUMO of PDI has considerable
electron density at both the carbonyl oxygen atoms and at the
1,6,7,12-carbon atoms,42 so that the interaction of electron
density at the â-pyrrole positions of 1*Z3PN with the LUMO
of PDI should be significant irrespective of whether Z3PN is
substituted at the N,N′- or 1,7-positions of PDI. The interaction
of these LUMOs should occur by means of superexchange with
orbitals centered on the intervening phenyl joining Z3PN to PDI.
81 ( 6
480 ( 60
83 ( 4
480 ( 60
[[(Z3PN)4PDI]2-DABCO4]2
3.2 ( 0.2
a Measured in toluene with 1% pyridine to prevent aggregation
We have calculated the value of VDA using the semi-classical
expression of Jortner:44
2π
1
Sn
∞
2
kET
)
|VDA
|
exp[-S]
∑
p
4πλ k T
n!
x
n)0
S
B
-(∆G + λS + npω)2
exp
(4)
[
]
4λSkBT
where λS and λI are respectively the solvent and internal
reorganization energies for the electron-transfer reactions, ∆G
is either ∆GCS or ∆GCR for the respective charge separation
and recombination reactions, ω is the characteristic vibration
coupled to the electron-transfer process (usually a C-C stretch-
ing frequency of 1500 cm-1), T is the temperature, S ) λI/ω.
Details of the parameters and the calculation are given in the
Supporting Information. The values of VDA for the charge
separation reactions for N,N′-(Z3PN-py)2PDI and 1,7-(Z3PN-
py)2PDI, Table 2, are comparable and approximately 20 cm-1
in accord with our expectations based on the similar values of
kCS for the two molecules.
For the CR reactions, the difference in ∆GCR between N,N′-
(Z3PN-py)2PDI (-1.78 eV) and 1,7-(Z3PN-py)2PDI (-1.61 eV)
is also small, but because the CR is in the Marcus inverted
region of the rate vs free energy profile, theory predicts that
CR for N,N′-(Z3PN-py)2PDI should be somewhat slower than
that for 1,7-(Z3PN-py)2PDI. In addition to energetic consider-
ations, a slower CR rate for N,N′-(Z3PN-py)2PDI is predicted
by examining the electronic overlap between the donors and
acceptors in the model compounds. As mentioned above, the
electron density within the PDI-• differs considerably between
the imide nitrogens and the carbon atoms at the 1,7-positions
due to the nodal plane, which bisects the two imide nitrogens.42
In addition, it is well-known that the HOMO of Z3PN+• has
substantial charge (and spin) density at the 5,10,15,20-carbon
atoms,58,59 one of which serves as the attachment point to PDI-•.
Therefore, the electronic coupling matrix element for CR
through the 1,7-positions should be considerably larger than that
for CR through the imide nitrogen atoms. The calculated value
of VDA for 1,7-(Z3PN-py)2PDI is 380 cm-1 and is about 50%
larger than that for N,N′-(Z3PN-py)2PDI, which supports the
arguments given above.
The shorter of the two charge recombination lifetimes of
(Z3PN-py)4PDI (81 ( 6 ps) is identical within experimental
error to that of 1,7-(Z3PN-py)2PDI (τCR ) 77 ( 3 ps). Since
photoexcitation of either the N,N-Z3PN or the 1,7-Z3PN
chromophores within (Z3PN-py)4PDI is equally probable given
their nearly identical electronic absorption spectra, the 81 ps
component of CR within (Z3PN)4PDI is most likely due to
(98) Wasielewski, M. R.; Johnson, D. G.; Svec, W. A.; Kersey, K. M.; Minsek,
D. W. J. Am. Chem. Soc. 1988, 110, 7219-7221.
(99) Ford, W. E.; Kamat, P. V. J. Phys. Chem. 1987, 91, 6373-6380.
(100) Till, U.; Hore, P. J. Mol. Phys. 1997, 90, 289-296.
(101) Marcus, R. A. J. Chem. Phys. 1956, 24, 966-978.
(102) del Rosario Benites, M.; Johnson, T. E.; Weghorn, S.; Yu, L.; Rao, P.
D.; Diers, J. R.; Yang, S. I.; Kirmaier, C.; Bocian, D. F.; Holten, D.;
Lindsey, J. S. J. Mater. Chem. 2002, 12, 65-80.
(103) Loewe, R. S.; Lammi, R. K.; Diers, J. R.; Kirmaier, C.; Bocian, D. F.;
Holten, D.; Lindsey, J. S. J. Mater. Chem. 2002, 12, 1530-1552.
9
J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007 3179

A R T I C L E S
Kelley et al.
Figure 7. Energy level diagram and time constants for the photoproducts of (Z3PN)4PDI in toluene with 1% pyridine (v:v).
charge recombination that occurs through the 1,7-positions. The
second observed component of charge recombination in (Z3PN-
py)4PDI, however, is considerably shorter (480 ( 60 ps) than
the charge recombination lifetime observed for N,N′-(Z3PN-
py)2PDI (1070 ( 30 ps). This result can be explained by
assuming that statistical formation of N,N′-(Z3PN-py)2+•-1,7-
(Z3PN-py)2PDI-• and N,N′-(Z3PN-py)2-1,7-(Z3PN-py)2
-
+•
PDI-• occurs, and that CR within these ion pairs is competitive
with an equilibrium in which the positive charge is transferred
between the N,N-Z3PN and 1,7-Z3PN chromophores. Using the
kinetic scheme shown in Figure 7, the biexponential decay of
CR within (Z3PN-py)4PDI can be modeled using equal rate
constants of 1.3 × 109 s-1 for hole transfer between the N,N′-
Z3PN and 1,7-Z3PN chromophores leading to an equilibrium
constant of unity, and therefore ∆G ) 0 for this process. Not
too surprisingly, this implies that the values of ∆GCR calculated
in toluene using the Weller formalism are uncertain by about
0.1 eV.
Femtosecond photoexcitation of (Z3PN-py)4PDI (toluene with
1% pyridine) with 414 nm or 548 nm laser pulses yields transient
absorption spectra and time constants identical to those obtained
with 612 nm excitation (data not shown). This indicates that
both efficient internal conversion from S2 f S1 as well as
efficient energy transfer from PDI to Z3PN (S1 f S1) occurs
within the 150 fs instrument response. Thus, the excitation
energy is efficiently harvested from 400 to 650 nm and funneled
into Z3PN, which leads to quantitative charge separation to form
the radical ion pair (Z3PN-py)2+•PDI-•.
Transient absorption spectra for [[(Z3PN)4PDI]2-DABCO4]2
in toluene were obtained using 120 fs, 612 nm pulses and are
shown in Figure 8. The transient spectrum comprises the ground-
state bleaches of PDI and Z3PN (occurring at 527 and 569 nm
for PDI and 614 nm for Z3PN) as well as the absorbance
features of Z3PN+• and PDI-• centered at 450 and 741 nm,
respectively. The most significant difference between this
spectrum and that of disaggregated (Z3PN-py)4PDI shown in
Figure 6 is that the PDI-• absorbance in the spectrum in Figure
8 is significantly broadened and its maximum is slightly blue-
shifted by 2 nm. Such broadening and shifting of the PDI-•
Figure 8. Transient absorption spectra of [[(Z3PN)4PDI]2-DABCO4]2 in
toluene following excitation with 612 nm, 120 fs laser pulses. (Inset)
Transient absorption kinetics for [[(Z3PN)4PDI]2-DABCO4]2 (blue) at 565
nm and (red) at 747 nm in toluene following excitation with 612 nm, 120
fs laser pulses. Nonlinear least-squares fits to the data are also shown.
spectrum is consistent with a cofacial geometry of π-π stacked
PDI chromophores.15,89 Representative transient kinetics at 565
and 745 nm, Figure 8 (inset), reveal that the radical ion pair
appears with τCS ) 3.2 ( 0.2 ps and decays biexponentially
with τCR ) 83 ( 4 and 480 ( 60 ps, see Table 2.
These data show that charge separation within [[(Z3PN)4
PDI]2-DABCO4]2 occurs 1.5 times faster than that in (Z3PN-
py)4PDI, while charge recombination in both systems occurs
with the same biexponential time constants. Based on the SAXS
data, dimerization of [(Z3PN)4PDI]2-DABCO4 in toluene to
form [[(Z3PN)4PDI]2-DABCO4]2 results from pairwise π-π
interactions between the N,N′-(Z3PN) of one [(Z3PN)4PDI]2-
DABCO4 and the 1,7-(Z3PN) of another [(Z3PN)4PDI]2-
DABCO4, Figure 5. The small increase in the charge separation
rate constant upon formation of [[(Z3PN)4PDI]2-DABCO4]2
may result from small changes in free energy for charge
separation due to the close association of the PDI molecules
within each [(Z3PN)4PDI]2-DABCO4 within the dimer.
9
3180 J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007

Photoinitiated Charge Transport
A R T I C L E S
Conclusions
Since the time constants for biexponential charge recombina-
tion in [[(Z3PN)4PDI]2-DABCO4]2 are the same within ex-
perimental error as those observed for the (Z3PN-py)4PDI, it is
likely that the charge recombination dynamics within [[(Z3PN)4
PDI]2-DABCO4]2 are dominated by the same equilibrium hole
transfer between N,N′-(Z3PN) and 1,7-(Z3PN) that occurs within
(Z3PN-py)4PDI. In [[(Z3PN)4PDI]2-DABCO4]2 there is an
additional hole migration pathwaysthe through-space pathway
from N,N′-(Z3PN) to 1,7-(Z3PN) between the two dimers, which
is characterized by shorter distance between the porphyrins. The
geometry optimized models reveal center-to-center distances
between π-stacking porphyrins of only 3.9 Å. The same models
reveal distances between the analogous porphyrins in the
covalent system to be 13.8 Å. Unfortunately, the transient
absorption spectral features due to Z3PN+• within [[(Z3PN)4-
PDI]2-DABCO4]2 do not show conclusive evidence for hole
hopping between the Z3PN molecules that are bound either by
DABCO or by noncovalent interactions, even though the spectra
are consistent with electron hopping between adjacent PDI
molecules. If the hole-hopping rates between the stacked Z3PN
molecules in [[(Z3PN)4PDI]2-DABCO4]2 are very fast, the
charge recombination kinetics we observe should still be limited
by the intrinsic rates observed in the (Z3PN-py)4PDI monomer.
Electron-nuclear double resonance (ENDOR) experiments on
a single Z3PN+• produced within the [[(Z3PN)4PDI]2-
DABCO4]2 assembly are capable of determining whether hole
Directional charge transport in tailored, self-assembling
molecular materials is essential for the development of organic
photovoltaics. The studies reported here reveal that it is possible
to rapidly transfer charge between adjacent sites in an ordered
array of identical chromophores, as a result of structural and
electronic asymmetry in the array at both the molecular and
supramolecular level. This asymmetry creates different elec-
tronic couplings and Coulombic interactions between the
photoinduced charges, depending on their location on the array,
making one pathway for charge recombination favorable over
the other. This allows control over charge recombination
directionality and may prove to be an important design principle
for the development of new supramolecular materials for organic
photovoltaics.
Acknowledgment. This research was supported by the Office
of Naval Research under Grant No. N00014-05-1-0021. We
thank Drs. Xiaobing Zuo and David M. Tiede (Chemistry
Division, Argonne National Laboratory) for assistance with
SAXS experiments. Use of the Advanced Photon Source was
supported by the U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences, under Contract No. W-31-
109-ENG-38.
Supporting Information Available: Experimental details,
including the synthesis and characterization of the molecules,
SAXS, modeling, and transient absorption. This material is
available free of charge via the Internet at http://pubs.acs.org.
hopping in this system occurs with rate constants >107 s-1 104
,
and will be reported in a future publication.
(104) Tauber, M. J.; Kelley, R. F.; Giaimo, J. M.; Rybtchinski, B.; Wasielewski,
M. R. J. Am. Chem. Soc. 2006, 128, 1782-1783.
JA0664741
9
J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007 3181