Pairing fullerenes and porphyrins: Supramolecular wires that exhibit charge transfer activity

Article abstract of DOI:10.1021/ja101937w

A concept is elaborated of pairing electron donors and electron acceptors that share a common trait, wire-like features, as a powerful means to realize a new and versatile class of electron donor-acceptor nanohybrids. Important variables are fine-tuning (i) the complexation strength, (ii) the electron/energy transfer behavior, and (iii) the solubilities of the resulting architectures. In particular, a series of supramolecular porphyrin/fullerene hybrids assembled by the hydrogen bonding of Hamilton receptor/cyanuric acid motif has been realized. Putting the aforementioned variables into action, the association constants (K_ass), as they were determined from ¹H NMR and steady-state fluorescence assays, were successfully tweaked with values in the range of 10⁴-10⁵ M ^-1. In fact, our detailed studies corroborate that the latter reveal a dependence on the nature of the spacer, that is, p-phenylene-ethynylene, p-phenylene-vinylene, p-ethynylene, and fluorene, as well as on the length of the spacer. Complementary performed transient absorption studies confirm that electron transfer is indeed the modus operandi in our novel class of electron donor-acceptor nanohybrids, while energy transfer plays, if any, only a minor role. The accordingly formed electron transfer products, that is, one-electron oxidized porphyrins and one-electron reduced fullerenes, are long-lived with lifetimes that reach well into the time domain of tens of nanoseconds. Finally, we have used the distance dependence on electron transfer, charge separation and charge recombination, to determine for the first time a ss value (0.11^-1) for hydrogen-bonding-mediated electron transfer.

Full text of DOI:10.1021/ja101937w

Published on Web 07/15/2010
Pairing Fullerenes and Porphyrins: Supramolecular Wires
That Exhibit Charge Transfer Activity
Florian Wessendorf,† Bruno Grimm,‡ Dirk M. Guldi,‡,* and Andreas Hirsch*,†
Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials
(ICMM), Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, Henkestraꢀe 42, 91054 Erlangen,
Germany and Egerlandstraꢀe 3, 91058 Erlangen, Germany
Received March 13, 2010; E-mail: andreas.hirsch@chemie.uni-erlangen.de;
dirk.guldi@chemie.uni-erlangen.de
Abstract: A concept is elaborated of pairing electron donors and electron acceptors that share a common
trait, wire-like features, as a powerful means to realize a new and versatile class of electron donor-acceptor
nanohybrids. Important variables are fine-tuning (i) the complexation strength, (ii) the electron/energy transfer
behavior, and (iii) the solubilities of the resulting architectures. In particular, a series of supramolecular
porphyrin/fullerene hybrids assembled by the hydrogen bonding of Hamilton receptor/cyanuric acid motif
has been realized. Putting the aforementioned variables into action, the association constants (Kass), as
they were determined from 1H NMR and steady-state fluorescence assays, were successfully tweaked
with values in the range of 104-105 M-1. In fact, our detailed studies corroborate that the latter reveal a
dependence on the nature of the spacer, that is, p-phenylene-ethynylene, p-phenylene-vinylene, p-
ethynylene, and fluorene, as well as on the length of the spacer. Complementary performed transient
absorption studies confirm that electron transfer is indeed the modus operandi in our novel class of electron
donor-acceptor nanohybrids, while energy transfer plays, if any, only a minor role. The accordingly formed
electron transfer products, that is, one-electron oxidized porphyrins and one-electron reduced fullerenes,
are long-lived with lifetimes that reach well into the time domain of tens of nanoseconds. Finally, we have
used the distance dependence on electron transfer, charge separation and charge recombination, to
determine for the first time a ꢀ value (0.11 Å-1) for hydrogen-bonding-mediated electron transfer.
Introduction
harvest and convert sunlight, cascades of short-range/multistep
electron transfer events have been the subject of extensive
One of the most interesting challenges in the field of
nanoscale electronics lies in the mediation of charges/electrons
between mutually interacting components.1 To this end, a
combination of the rapidly evolving fields of nanostructured
materials and supramolecular chemistry provides an attractive
strategy for constructing large and complex, yet highly ordered,
molecular and supramolecular entities, with specific functions.2-13
Inspired by the natural photosynthetic apparatus, in which
chlorophylls (i.e., bacteriochlorophyll dimer, cytochcrome C)
studies.2-4 Because of the rich photo- and redox chemistry of
porphyrins and metalloporphyrins, they have been probed as
integrative components in photosynthetic reaction center
models.8-10 Here, porphyrins/metalloporphyrins serve several
functions ranging from harvesting light through most of the
visible part of the solar spectrum to electron transfer and electron
transport. Multifunctional fullerenes, on the other hand, are
(8) (a) Imahori, H.; Tamaki, K.; Yamada, H.; Yamada, K.; Sakata, Y.;
Nishimura, Y.; Yamazaki, I.; Fujitsuka, M. O.; Ito, O. Carbon 2000,
38, 1599. (b) Fukuzumi, S.; Imahori, H.; Yamada, H.; El-Khouly,
M. E.; Fujitsuka, M.; Ito, O.; Guldi, D. M. J. Am. Chem. Soc. 2001,
123, 2571–2575.
† Friedrich-Alexander-Universita¨t Erlangen-Nu¨rnberg, Henkestraꢀe 42.
‡ Friedrich-Alexander-Universita¨t Erlangen-Nu¨rnberg, Egerlandstraꢀe 3.
(1) Carroll, R. L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 4378–
4400.
(9) Imahori, H.; Tamaki, K.; Guldi, D. M.; Luo, C.; Fujitsuka, M.; Ito,
O.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 2607–
2617.
(2) Kirmaier, C.; Holton, D. In The Photosynthetic Reaction Center;
Deisenhofer, J., Norris, J. R., Eds.; Academic Press: San Diego, CA,
1993; Vol. II, pp 49-70.
(10) Imahori, H.; Guldi, D. M.; Tamaki, K.; Yoshida, Y.; Luo, C.; Sakata,
Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 6617–6628.
(11) (a) Imahori, H.; Hagiwara, K.; Akiyama, T.; Aoki, M.; Taniguchi, S.;
Okada, T.; Shirakawa, M.; Sakata, Y. Chem. Phys. Lett. 1996, 263,
545–550. (b) Tkachenko, N. V.; Guenther, C.; Imahori, H.; Tamaki,
K.; Sakata, Y.; Fukuzumi, S.; Lemmetyinen, H. Chem. Phys. Lett.
2000, 326, 344–350.
(3) (a) Remy, A.; Gerwert, K. Nat. Struct. Biol. 2003, 10, 637–644. (b)
Okumura, M. Y.; Paddock, M. L.; Graige, M. S.; Feher, G. Biochim.
Biophys. Acta 2000, 1458, 138–163.
(4) Boxer, S. G. Annu. ReV. Biophys. Bioeng. 1990, 19, 267.
(5) The Reaction Center of Photosynthetic Bacteria; Michel-Beyerle,
M. E., Ed.; Springer: Berlin, 1996.
(6) (a) Fukuzumi, S.; Imahori, H. In Electron Transfer in Chemistry;
Balzani, V., Ed.; Wiley-VCH: Weinheim, 2001; Vol. 2, pp 927-975.
(b) Fukuzumi, S.; Guldi, D. M. In Electron Transfer in Chemistry;
Balzani, V., Ed.; Wiley-VCH: Weinheim, 2001; Vol. 2, pp 270-337.
(c) Fukuzumi, S. Org. Biomol. Chem. 2003, 1, 609–620.
(7) Guldi, D. M.; Kamat, P. V. In Fullerenes, Chemistry, Physics, and
Technology; Kadish, K. M., Ruoff, R. S., Eds.; Wiley-Interscience:
New York, 2000; pp 225-281.
(12) Vehmanen, V.; Tkachenko, N. V.; Imahori, H.; Fukuzumi, S.;
Lemmetyinen, H. Spectrochim. Acta, Part A 2001, 57, 2229–2244.
(13) (a) Imahori, H. Org. Biomol. Chem. 2004, 2, 1425–1433. (b) Imahori,
H.; Yamada, H.; Guldi, D. M.; Endo, Y.; Shimomura, A.; Kundu, S.;
Yamada, K.; Okada, T.; Sakata, Y.; Fukuzumi, S. Angew. Chem., Int.
Ed. 2002, 41, 2344–2347. (c) Imahori, H.; Tamaki, K.; Araki, Y.;
Sekiguchi, Y.; Ito, O.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc.
2002, 124, 5165–5174.
9
10786 J. AM. CHEM. SOC. 2010, 132, 10786–10795
10.1021/ja101937w  2010 American Chemical Society

Hydrogen Bonding in Electron Transfer
A R T I C L E S
known for their unique electron-accepting features. In fact, these
three-dimensional electron acceptors hold great promise on
account of their small reorganization energies in electron transfer
reactions and have exerted noteworthy impact on the improve-
ment of light-induced charge-separation.14 In short, porphyrins/
metalloporphyrins and fullerenes are molecular architectures
ideally suited for devising integrated, multicomponent model
systems to transmit and process solar energy.15-17 Photoexci-
tation of the porphyrin/metalloporphyrin by visible light is
readily followed by an electron transfer to guarantee the
formation of a radical ion pair state, that is, the one-electron
oxidized radical cation and the one-electron reduced radical
anion of the porphyrin and the fullerene, respectively.8-13
The most far-reaching observation is that charge-recombina-
tion in metalloporphyrin/fullerene couples is located deep in
the “inverted region” of the Marcus parabola, regardless of
linkage, distance, and orientation. By contrast, lowering the
driving force via replacing the metalloporphyrins with the better
electron donors ferrocene or tetrathiafulvalene, while keeping
all other parameters (i.e., distance, acceptor, solvent, tempera-
ture, etc.) constant, shifts the dynamics into the normal region.
This variation is of great advantage in determining parameters
such as electronic coupling (V), reorganization energy (λ), and
attenuation factor (ꢀ) with high accuracy.18 These parameters
have key character for material design considerations with the
objective to prolong the lifetime of the energetic radical ion
pair states, while, simultaneously, optimizing the efficiency of
charge separation.
Figure 1. Schematic representation of the complementary hydrogen-
bonding motif of a cyanuric acid derivative and a Hamilton receptor.
comprising noncovalent electrostatic interactions, where op-
positely charged fullerenes and porphyrins/cytochrome C interact
tightly with each other. Sufficiently strong electronic couplings
powered an intrahybrid charge separation in these nanohybrids.15
This approach turned out to be very promising for constructing
photovoltaic devices with efficient solar energy conversion
performances.19
Nevertheless, the most compelling supramolecular motif is
hydrogen bonding. Although hydrogen bonding bears great
potential for the realization of highly directional self-assemblies,
only a few examples of electron donor-acceptor nanohybrids,
using porphyrins/metalloporphyrins and fullerenes, have been
reported so far.17,19 We have recently introduced the Hamilton-
receptor/cyanuric acid binding motif (Figure 1)20 to self-
assemble porphyrins/metalloporphyrins together with fulleroden-
drimers17a,b as well as hydrogen-bonded complexes containing
N,N-dimethylanilines, flavines,21a and supramolecular wires.21b
Importantly, the six-point hydrogen-bonding motif leads to
comparatively strong binding between hosts and guests with
association constants Kass that range in apolar solvents (i.e.,
Up to now, most electron donor-acceptor systems are based
on the use of covalent linkages. Hereby, the linkage mediates
the distance, spatial orientation, and flexibility between the donor
and acceptor components. Much less is, however, known about
noncovalent linkages en-route toward electron donor-acceptor
nanohybrids and the function of the intervening spacers.17 In a
recent example, we introduced electron donor-acceptor systems
dichloromethane and toluene) from 103 to 1012 M-1
.
(14) Guldi, D. M. Chem. Soc. ReV. 2002, 31, 22–36.
(19) (a) Sanchez, L.; Martin, N.; Guldi, D. M. Angew. Chem. 2005, 117,
5508–5516; Angew. Chem., Int. Ed. 2005, 44, 5374-5382. (b)
Sanchez, L.; Sierra, M.; Martin, N.; Myles, A. J.; Dale, T. J.; Rebek,
J., Jr.; Seitz, W.; Guldi, D. M. Angew. Chem. 2006, 118, 4753–4757;
Angew. Chem., Int. Ed. 2006, 45, 4637-4641. (c) Sandanayaka,
A. S. D.; Araki, Y.; Ito, O.; Chitta, R.; Gadde, S.; D’Souza, F. Chem.
Commun. 2006, 4327–4329. (d) D’Souza, F.; Chitta, R.; Gadde, S.;
Zandler, M. E.; Sandanayaka, A. S. D; Araki, Y.; Ito, O. Chem.
Commun. 2005, 1279–1281. (e) D’Souza, F.; Chitta, R.; Gadde, S.;
McCarty, A. L.; Karr, P. A.; Zandler, M. E.; Sandanayaka, A. S. D.;
Araki, Y.; Ito, O. J. Phys. Chem. B 2006, 110, 5905–5913. (f) D’Souza,
F.; El-Khouly, M. E.; Gadde, S.; Zandler, M. E.; McCarty, A. L.;
Araki, Y.; Ito, O. Tetrahedron 2006, 62, 1967–1978. (g) D’Souza,
F.; Chitta, R.; Gadde, S.; Rogers, L. M.; Karr, P. A.; Zandler, M. E.;
Sandanayaka, A. S. D.; Araki, Y.; Ito, O. Chem.-Eur. J. 2007, 13,
916–922.
(15) (a) D’Souza, F.; Deviprasad, G. R.; El-Khouly, M. E.; Fujitsuka, M.;
Ito, O. J. Am. Chem. Soc. 2001, 123, 5277–5284. (b) Regev, A.; Galili,
T.; Levanon, H.; Schuster, D. I. J. Phys. Chem. A 2006, 110, 8593–
8598. (c) Guldi, D. M.; Zilbermann, I.; Anderson, G.; Li, A.; Balbinot,
D.; Jux, N.; Hatzimarinaki, M.; Hirsch, A.; Prato, M. Chem. Commun.
2004, 6, 726–727. (d) Balbinot, D.; Atalick, S.; Guldi, D. M.;
Hatzimarinaki, M.; Hirsch, A.; Jux, N. J. Phys. Chem. B 2003, 107,
13273–13279. (e) Hartnagel, U.; Balbinot, D.; Jux, N.; Hirsch, A. Org.
Biomol. Chem. 2006, 4, 1785–1795.
(16) (a) D’Souza, F.; El-Khouly, M. E.; Gadde, S.; Zandler, M. E.; McCarty,
A. L.; Araki, Y.; Ito, O. Tetrahedron 2005, 62, 1967–1978. (b) Guldi,
D. M. Chem. Commun. 2000, 5, 321–327. (c) Sessler, J. S.; Wang,
B.; Springs, S. L.; Brown, C. T. In CromprehensiVe Supramolecular
Chemistry; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Vo¨gtle,
F., Eds.; Pergamon: New York, 1996; Chapter 9. (d) Piotrowiak, P.
Chem. Soc. ReV. 1999, 28, 143–150. (e) Wasielewski, M. R. Acc.
Chem. Res. 2009, 42, 1910–1921.
(20) (a) Chang, S.-K.; Hamilton, A. D. J. Am. Chem. Soc. 1988, 110, 1318–
1319. (b) Hager, K.; Franz, A.; Hirsch, A. Chem.-Eur. J. 2006, 12,
2663–2679. (c) Hager, K.; Hartnagel, U.; Hirsch, A. Eur. J. Org. Chem.
2007, 12, 1942–1956. (d) Maurer, K.; Hager, K.; Hirsch, A. Eur. J.
Org. Chem. 2006, 15, 3338–3347. (e) Franz, A.; Bauer, W.; Hirsch,
A. Angew. Chem. 2005, 117, 1588–1561; Angew. Chem., Int. Ed. 2005,
44, 1564-1567.
(17) (a) Sessler, J. L.; Wang, B.; Harriman, A. J. Am. Chem. Soc. 1993,
115, 10418–10419. (b) de Rege, P. J. F.; Williams, S. A.; Therien,
M. J. Science 1995, 269, 1409–1413. (c) Rosenthal, J.; Hodgkiss, J. M.;
Young, E. R.; Nocera, D. G. J. Am. Chem. Soc. 2006, 128, 10474–
10483. (d) D’Souza, F.; Gadde, S.; Islam, D.-M. S.; Pang, S.-C.;
Schumacher, A. L.; Zandler, M. E.; Horie, R.; Araki, Y.; Ito, O. Chem.
Commun. 2007, 480–482. (e) Gadde, S.; Islam, D.-M. S.; Wijesinghe,
C. A.; Subbaiyan, N. K.; Zandler, M. E.; Araki, Y.; Ito, O.; D’Souza,
F. J. Phys. Chem. C 2007, 111, 12500–12503. (f) Wessendorf, F.;
Gnichwitz, J.-F.; Sarova, G. H.; Hager, K.; Hartnagel, U.; Guldi, D. M.;
Hirsch, A. J. Am. Chem. Soc. 2007, 129, 16057–16071. (g) Gnichwitz,
J.-F.; Wielopolski, M.; Hartnagel, K.; Hartnagel, U.; Guldi, D. M.;
Hirsch, A. J. Am. Chem. Soc. 2008, 130, 8491–8501. (h) D’Souza,
F.; Venukadasula, G. M.; Yamanaka, K.-I.; Subbaiyan, N. K.; Zandler,
M. E.; Ito, O. Org. Biomol. Chem. 2009, 7, 1076–1080.
(21) (a) Murakami, M.; Ohkubo, K.; Hasobe, T.; Sgobba, V.; Guldi, D. M.;
Wessendorf, F.; Hirsch, A.; Fukuzumi, S. J. Mater. Chem. 2010, 20,
1457–1466. (b) Wessendorf, F.; Hirsch, A. Tetrahedron 2008, 64,
11480–11489.
(22) (a) Goldsmith, R. H.; Sinks, L. E.; Kelley, R. F.; Betzen, L. J.; Liu,
W.; Weiss, E. A.; Ratner, M. A.; Wasielewski, M. R. Proc. Natl. Acad.
Sci. U.S.A. 2005, 102, 3540–3545. (b) Weiss, E. A.; Tauber, M. J.;
Kelley, R. F.; Ahrens, M. J.; Ratner, M. A.; Wasielewski, M. R. J. Am.
Chem. Soc. 2005, 127, 11842–11850. (c) Tauber, M. J.; Kelley, R. F.;
Giaimo, J. M.; Rybtchinski, B.; Wasielewski, M. R. J. Am. Chem.
Soc. 2006, 128, 1782–1783. (d) Goldsmith, R. H.; Wasielewski, M. R.;
Ratner, M. A. J. Phys. Chem. A 2006, 110, 20258–20262.
(18) (a) Guldi, D. M.; Fukuzumi, S. J. Porphyrins Phthalocyanines 2002,
6, 289–295. (b) Guldi, D. M. Pure Appl. Chem. 2003, 75, 1069–1075.
9
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Wessendorf et al.
Figure 2. Synthesized fullerene 1 and porphyrin 2 derivatives.
Notable in the case of the aforementioned metalloporphyrin/
fullerodendrimer assemblies with flexible alkyl chains, which
were introduced as spacers, may have well been the inception
to inter- and/or intramolecular folding. Conjugated spacers (i.e.,
p-phenylene-ethynylene, p-phenylene-vinylene, p-ethynylene,
and fluorene), on one hand, provide wire-like behavior in terms
of electron transfer/electron transport and, on the other hand,
restrict the flexibility of the Hamilton receptor and cyanuric acid
functionalities. The wire-like behavior is key to guaranteeing
an efficient electronic coupling between the electroactive units
if the following factors are provided: (i) matching the donor/
acceptor and bridge energy levels, (ii) good electronic coupling
between the donor and acceptor by means of the bridge
orbitals,20 and (iii) a small attenuation factor (ꢀ).23 Intramo-
lecular electron transfer/electron transport along π-conjugated
oligomers, such as o-phenylenevinylene (oPV), has been tested
in several donor-acceptor conjugates involving porphyrins,24
anilins,25 or ferrocenes26 as electron donors and C60 as electron
acceptor. In fact, recent studies have shown that the electron
transfer along oPVs is much more efficient than that along
o-fluorenes (oFl) or o-phenylene-ethynylenes (oPE). Such a
trend is caused by a complete delocalization of π-electrons in
oPVs, whereas the electron delocalization in oPE resides mostly
on the phenyl rings. The electron delocalization in oFls can be
found between both extremes.27
Here, we report on the study of electron transfer along
π-conjugated spacers in a novel series of supramolecular
assembled porphyrin/fullerene hybrids. In light of the afore-
mentioned, we have integrated p-phenylene-ethynylene, p-
phenylene-vinylene, p-ethynylene, or fluorene units as spacers
between fullerenes and the Hamilton receptors as well as a
p-phenylene-ethynylene bridged porphyrinato cyanuric acid
(Figure 2) and have analyzed the attenuation factor (ꢀ).
(23) (a) Muellen, K.; Wegner, G. Electronic Materials: The Oligomer
Approach; Wiley-VCH: Weinheim, Germany, 1998. (b) Atienza-
Castellanos, C.; Wielopolski, M.; Guldi, D. M.; van der Pol, C.; Bryce,
M. R.; Filippone, S.; Martin, N. Chem. Commun. 2007, 5146–5166.
(24) (a) Redmore, N. P.; Rubstov, I. V.; Therien, M. J. J. Am. Chem. Soc.
2003, 125, 8769–8778. (b) Screen, T. O.; Thorne, J. R. G.; Denning,
R. G.; Bucknall, D. G.; Anderson, H. L. J. Mater. Chem. 2003, 13,
2796–2808.
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Scheme 1. Syntheses of Hamilton Receptor Bearing 4 and 8a
a (i) Pd(PPh3)2Cl2, CuI, NEt3, THF, room temperature; (ii) Pd(PPh3)2Cl2, CuI, HNEt2, toluene, 75 °C; (iii) TBAF, THF, 0 °C; (iv) Pd(PPh3)2Cl2, CuI,
NEt3, THF, room temperature.
Results
receptor bearing fullerenes that are not only connected by
p-phenylene-ethynylene chains 1a,b, but also by p-ethynylene
1c, p-phenylene-vinylene 1d,e and fluorene 1f spacers, were
designed. The p-ethynyl precursor 9 was accessible by reacting
3,3-diethoxyprop-1-yne with iodo Hamilton receptor 3 under
Pd0/CuI catalysis in tetrahydrofurane and diethylamine. Depro-
tection of the acetal by adding TFA (trifluoro acetic acid) and
subsequent neutralization with sodium bicarbonate afforded the
ethynyl Hamilton receptor 10, Scheme 2.
Synthesis. From the synthetic point of view, iodo
compound21b 3 (Scheme 1) is the key compound en-route toward
1. Subsequent C-C cross coupling reactions were performed
under Sonogashira conditions involving terminal alkynes and
aryl halides.28
The need for further functionalization of the Hamilton
receptor via, for example, the Prato reaction of C60 required
29
the implementation of formyl groups in para position relative
to the cross-coupled aryl groups. To combine both synthetic
necessities, 4-ethynylbenzaldehyde was used as a promising
reagent toward iodo compound 3. The coupling of both
compounds was nearly quantitative at room temperature21a
(Scheme 1).
Extension of the p-phenylene-ethynylene chain was performed
by using 4-bromobenzaldehyde and (2-(4-ethynylphenyl)ethy-
nyl)triisopropylsilane 5, which were coupled using Pd0/CuI
catalysis at 75 °C (Scheme 1). Deprotecting the resulting
p-phenylene-ethynylene using TBAF (tetrabutyl ammonium
fluoride) as fluoride source led to the terminal ethynyl derivative
7. The latter was subsequently coupled to the Hamilton receptor
building block, as described above for 4-ethynyl benzaldehyde,
to afford 8 (Scheme 1). To investigate the influence of the
bridge’s nature on the photophysical properties, Hamilton
To introduce a p-phenylene-vinylene spacer, we synthesized
4-ethenyl benzaldehyde 11 by adding n-BuLi and DMF to
4-bromostyrene at -78 °C. Heck conditions were then applied
for coupling 4-ethenylbenzaldehyde and iodo Hamilton receptor
3 to give the p-phenylene-vinylene Hamilton receptor 12 using
Pd2dba3 (tris(dibenzylideneacetone)dipalladium(0)) as Pd0 source
and triphenyl arsine as additional ligand, Scheme 2.30 The
fluorene containing precursor 14 was obtained by Suzuki
coupling of boronic acid ester 13 and Hamilton receptor 3 under
the action of Pd(PPh3)4 and potassium carbonate (Scheme 2).31
To increase the solubility of the p-phenylene-ethynylene and
p-phenylene-vinylene containing fullerene derivatives in com-
mon organic solvents, we synthesized N-dodecyl glycine 15 by
adding iodo acetic acid to dodecylamine at 0 °C in a mixture
of ethanol and water (Scheme 3).32
The wire-like Hamilton receptor fullerenes 1 were then
synthesized following the standard Prato protocol, that is,
allowing C60 to react with aldehydes 4, 8, 10, 12, 14, and
sarcosine or N-dodecyl glycine 15 in THF/toluene under reflux
(Scheme 4).29
(25) (a) Thomas, K. G.; Biju, V.; Guldi, D. M.; Kamat, P. V.; George,
M. V. J. Phys. Chem. B 1999, 103, 8864–8869. (b) Guldi, D. M.;
Swartz, A.; Luo, C.; Gomez, R.; Segura, J. L.; Martin, N. J. Am. Chem.
Soc. 2002, 124, 10875–10886.
(26) Dong, T.-Y.; Chang, S.-W.; Kin, S.-F.; Lin, M.-C.; Wen, Y.-S.; Lee,
L. Organometallics 2006, 25, 2018–2024.
(27) Wielopolski, M.; Atienza, C.; Clark, T.; Guldi, D. M.; Martin, N.
Chem.-Eur. J. 2008, 14, 6379–6390.
(28) (a) Nagy, A.; Novak, Z.; Kotschy, A. J. Organomet. Chem. 2005, 69,
4453–4461. (b) Tykwinski, R. R. Angew. Chem., Int. Ed. 2003, 42,
1566–1568.
(30) Stewart, J. J. P. J. Comput. Chem. 1989, 209, 221–264.
(31) Heck, R. F.; Nolley, J. P., Jr. J. Org. Chem. 1972, 37, 2320–2322.
(32) (a) Miyaura, N.; Yamada, K.; Suzuki, A. Tetrahedron Lett. 1979, 36,
3437–3440. (b) Suzuki, A. J. Organomet. Chem. 1999, 576, 147–
168.
(29) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993, 115,
9798–9799.
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Wessendorf et al.
Scheme 2. Syntheses of Hamilton Receptor Bearing 10, 12, and 14a
a (i) Pd(PPh3)2Cl2, CuI, HNEt2, THF, room temperature; (ii) (1) TFA, CH2Cl2, room temperature, (2) H2O/NaHCO3, room temperature; (iii) (1) n-BuLi,
THF, -78 °C, (2) DMF, -78 °C, (3) H2O/NH4Cl, room temperature; (iv) Pd2dba3, AsPh3, NEt3, THF, 70 °C; (v) Pd(PPh3)4, K2CO3, DMF, room temperature.
Scheme 3. Synthesis of N-Dodecyl Glycine 15a
calculations. In addition, it assists in shedding light onto their
rigidity as an important means to guarantee efficient electron
transfer along the π-conjugated spacers and to eliminate through-
space electron transfer fostered by inter- or intramolecular
folding. All calculations, PM3 geometry optimization in the gas
a (i) Iodo acetic acid, EtOH/H2O, 0 °C.
phase,30 revealed rigid geometries, regardless of the conjugated
spacer type. In terms of spacer lengths, varying distances were
obtained for p-phenylene-vinylene, p-ethynylene, and fluorene-
based Hamilton receptor fullerenes. Moreover, introduction of
an additional p-phenylene-ethynylene unit goes in hand with
an increase in distance from 12.2 to 19.1 Å; see Figure 3 and
Table 1.
The cyanuric acid bearing porphyrin 2, on the other hand,
was synthesized in accordance with our recently published
procedure.21b
Molecular Modeling. To determine the length of the π-con-
jugated spacers (i.e., p-phenylene-ethynylene, p-phenylene-
vinylene, p-ethynylene, and fluorene) as part of the Hamilton
receptor carrying fullerenes, we carried out molecular mechanics
NMR-Titration Experiments. Initial insight into the formation
1
of the 1·2 complexes was lent from H NMR spectroscopy.
Scheme 4. Syntheses of the Hamilton Receptor Bearing Fullerene Derivatives 1a
a (i) C60, toluene, reflux.
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10790 J. AM. CHEM. SOC. VOL. 132, NO. 31, 2010

Hydrogen Bonding in Electron Transfer
A R T I C L E S
Figure 3. PM3-optimized structure of 1b.
Table 1. Spacer Distance, Association Constants Kass, Absorption Decrease, Fluorescence Quenching Efficiency, Charge Separation, and
Charge Recombination Dynamics, Determined via Femtosecond and Nanosecond Transient Absorption Measurements
c
log Kass
rate constantsd
b
,
spacer distance,a
Å
log Kass
CDCl3
oDCB
CH2Cl2
absorption decrease, %
quenching efficiency, %
kCS, s-1
kCR, s-1
1a·2
1b·2
1c·2
1d·2
1f·2
12.2
19.1
7.9
12.0
13.8
5.318 ( 1.11
3.528 ( 0.3
2.182 ( 0.5
3.719 ( 0.4
2.698 ( 0.7
4.667
4.332 ( 0.4
4.201 ( 0.4
13
17
4
30
25
22
3.1 × 109
1.4 × 107
not detectable
3.3 × 107
2.5 × 107
1.9 × 107
not observed
not detectable
1.1 × 1010
5.3 × 109
24
16
9
5.175 ( 1.05
5.163 ( 1.09
4.089 ( 0.4
4.104 ( 0.4
4.1 × 109
a See Molecular Modeling section. b Obtained from 1H NMR titration experiments. c Obtained from fluorescence titration experiments. d Rate
constants for charge separation (kCS) and charge recombination (kCR) were calculated from the transient absorption data.
solutions. In each case, the spectrum remained unchanged during
a time period of 60 min, indicating a fast equilibrium.
Whereas the NH1- and NH2-protons undergo a shift to lower
fields, the NH3-protons of the cyanuric acid moieties at 13 ppm
are subject to an opposite effect. The corresponding signals
undergo a high field shift during addition of 2. Furthermore,
they broaden, and, finally, they disappear. The reason for the
disappearance in the presence of an excess of 2 is a fast
equilibrium (coalescence regime) between bound and free
cyanurate. On the basis of these titration experiments, the
association constants Kass (Table 1) were determined with the
help of Chem-Equili.33
A plot of the chemical shift (δ [ppm]) of the NH protons as
a function of mole fraction X of titrated cyanuric acid bearing
porphyrin 2 reveals the characteristic sigmoidal shape with a
plateau at X ) 1; see Figure 5. We take the latter as preliminary
evidence for the 1·2 formation with a 1:1 stoichiometry.
Photophysical Characterization. Further information on the
Figure 4. Binding motif between 1a,e,f and 2 with indication of the NH
protons NH1, NH2, and NH3 (top) and 300 MHz H NMR spectra of 1a
1
supramolecular complexation was obtained by optical absorption
spectroscopy and steady-state fluorescence spectroscopy. To this
end, variable amounts of the different Hamilton receptors (i.e.,
1a, 1b, 1c, 1d, 1e, or 1f) were added to porphyrinato cyanuric
acid (2) containing ortho-dichlorobenenzene or dichloromethane
solutions. Importantly, upon increasing gradually the concentra-
tion of 1a, 1b, 1c, 1d, 1e, or 1f, the Soret band of 2 is slightly
decreased without, however, giving rise to any spectral shifts.17c
Notable is, however, that the overlapping absorptions of 1a,
1b, 1c, 1d, 1e, or 1f and 2 prevent the formation of clear
isosbestic points in the 415 and 429 nm range.
at a concentration of 3.3 mM in CDCl3 in the presence of various equivalents
of 2 (bottom).
Here, the determination of the association constant Kass was
achieved through a complementary series of titration experi-
ments in CDCl3. In particular, the characteristic downfield shifts
of the NH1- and NH2-protons of the Hamilton-receptor moieties
in1emergedassuitablemarkersforthecomplexformation.17a,20,21
In Figure 3, the corresponding shifts of the protons of 1 are
depicted as they change as a function of added fullerene 2.
Additionally, the shifts of the NH3-protons within the cyanuric
acid moiety of 2 were investigated (Figure 4). In a typical
experiment, 500 µL of a 2 mM solution of 1a, 1e, 1f was titrated
with 100 µL of a 2 mM solution of porphyrin 2. The 1H NMR
spectra were recorded approximately 30 min after mixing the
(33) (a) Solov’ev, V. P.; Vnuk, E. A.; Strakhova, N. N.; Reavsky, O. A.
VINITI; Moscow, 1991. (b) Solov’ev, V. P.; Baulin, V. E.; Strahova,
N. N.; Kazachenko, V. P.; Belsky, V. K.; Varnek, A. A.; Volkova,
T. A.; Wipff, G. J. Chem. Soc., Perkin Trans. 1 1998, 1489–1498.
9
J. AM. CHEM. SOC. VOL. 132, NO. 31, 2010 10791

A R T I C L E S
Wessendorf et al.
Figure 5. Plot of the chemical shift (δ [ppm]) of the NH1 and NH2 as a
function of mole fraction X of titrated cyanuric acid bearing porphyrin 2.
Figure 7. Steady-state fluorescence spectra of 2 (2.3 × 10-6 M) recorded
at different concentrations of 1d (0-2.10 × 10-5 M) in CH2Cl2 at room
temperature (λexc ) 416 nm).
Figure 6. Changes in the Zn porphyrin 2 (2.83 × 10-6 M) electronic
absorption spectrum in CH2Cl2 upon addition of increasing concentrations
of C60 derivative 1d (0-1.0 × 10-5 M) at room temperature.
Figure 8. Plot of I/I0 versus concentration of 1d used to determine the
association constant (according to eq 1).
Here, Io refers to the initial fluorescence intensity, co is the total
porphyrin concentration, cD is the total concentration of the
added fullerene, and Kass is the association constant (Figure 8).34
The association constants deduced from the fluorescence
experiments are listed in Table 1. Considering the error margins,
which are 10% in the case of 1a, 1d, 1e, and 1f and 25% in the
case of 1b, a reasonably good agreement is achieved with the
It is interesting to note that the strength of the spectral changes
seems to relate to the nature of the spacer (Figure 6). For 1a·2,
1d·2, 1e·2, and 1f·2, the spectral changes, as they evolve during
the titration, are rather marginal, whereas noticeable alterations
emerge for 1c·2. Opposing to the aforementioned trend, titrating
2 with 1b resulted in what is best described as the simple
superimposition of the individual spectra; see the Supporting
Information. In general, our current observations, detectable but
rather weak spectroscopic changes, are in stark contrast to
previous studies.17a A likely rationale implies that introducing
rigid spacers constrains the ground-state interactions between
1 and 2.
1
association constants based on the H NMR experiments.
Next, we turned to time-resolved fluorescence. To this end,
porphyrinato cyanuric acid 2 was excited at 403 nm at different
ratios with 1a, 1b, 1c, 1d, 1e, or 1f, and the resulting
fluorescence time profiles were analyzed by mono-, bi-, or even
poly exponential decay fitting functions. In the absence of 1a,
1b, 1c, 1d, 1e, or 1f, the fluorescence decay of 2 was strictly
fitted by a monoexponential fitting function that yielded lifetimes
of about 1.5 ns in ortho-dichlorobenzene as well as in
chloroform. Addition of 1a, 1b, 1c, 1d, 1e, or 1f led to the
following consequence. Now, a satisfying fit of the fluorescence
time profiles is only achieved when a biexponential fitting
function is employed. Besides the 1.5 ns lifetime, a shorter
lifetime in the range of hundreds of picoseconds emerges. While
the earlier is clearly associated with the intrinsic decay of 2,
the latter reflects the accelerated excited-state deactivation in
successfully formed 1·2. Interestingly, no appreciable changes
resulted in the presence of 1b, just the 1.5 ns lived component.
To gain quantitative details on the binding strength, comple-
mentary fluorescence titration experiments were carried out. In
line with earlier work, the prominent fluorescence of 2 (ΦF )
0.04) was monitored while variable amounts of 1a-d were
added. Figure 7 illustrates an exemplary case, in which 2 was
titrated with 1d in dichloromethane. The gradual quenching at
the long wavelength maximum of the emission was used to
estimate the association constant according to eq 1:
If
2
1
1
1
) 1 -
+ c0 + cD
-
+ c0 + cD - 4c0cD
[
(
)
]
I0
2cD Ks
K
ꢀ
s
(1)
(34) Valeur, B. Molecular Fluorescence; Wiley-VCH: Weinheim, 2002.
9
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Hydrogen Bonding in Electron Transfer
A R T I C L E S
In complementary experiments, we varied the concentration of
1a, 1b, 1c, 1d, 1e, or 1f incrementally. Here, analyses of the
pre-exponential factors of both lifetimes demonstrated that an
increase of 1a, 1b, 1c, 1d, 1e, or 1f causes an increase of the
pre-exponential factor of the short-lived contributions and,
simultaneously, a decrease of the pre-exponential factor of the
long-lived component. Importantly, the absolute values of the
short- and the long-lived components remained unchanged
throughout these assays.
Results from the steady-state and time-resolved experiments
prompt one to a fairly fast and efficient deactivation of
photoexcited 1·2, presumably via electron transfer to the
electron-accepting fullerene, while energy transfer cannot be
ruled out. Notable is in this context that excited-state contribu-
tions (i.e., singlet and triplet excited states) as they originate
from an inefficient fluorescence quenching in 1·2 and/or
incomplete association of 1·2 render the quantification of a
transduction of excited-state energy intrinsically difficult. Thus,
femtosecond transient absorption spectroscopic measurements
were performed to unveil the deactivation process and, in
addition, to analyze its kinetics. The differential absorption
changes, taken right after 150 fs laser pulse excitation of 2 at
420 nm in ortho-dichlorobenzene, show in the absence of any
1a, 1b, 1c, 1d, 1e, or 1f the prompt transformation (i.e., 5.6 ×
108 s-1) of the high lying singlet excited states of the porphyrin
into the lowest vibrational state of the porphyrin singlet excited
state.35,36 In particular, marked transitions develop in the region
between 600 and 1100 nm. Besides these characteristics, bleach
features as they relate to the ground-state absorptions are found
around 420 nm as well as at 550 and 600 nm. This singlet
excited state with an energy of about 2.00 eV deactivates slowly
via intersystem crossing to the energetically lower lying triplet
excited state, which is located at around 1.53 eV.37 The rate
constant of intersystem crossing was determined from a mul-
tiwavelength analysis to be 4.6 × 108 s-1. The newly developing
band at 840 nm reflects the diagnostic signature of the triplet
excited state of 2 with a lifetime of 45 µs. In the presence of
molecular oxygen, the triplet excited state experiences a
concentration-dependent deactivation process to form singlet
oxygen quantitatively.36
Figure 9. Differential absorption spectra (visible and near-infrared) obtained
upon femtosecond flash photolysis (550 nm) of 1a·2 in argon-saturated
ortho-dichlorobenzene with time delays of 0 and 292 ps at room temperature.
visible light excitation at 420 or 550 nm is expected to lead
predominantly to the formation of the porphyrin singlet excited
state; see Figure S2 for the transient absorption spectrum of 2.
This is confirmed by transient features such as transient
bleaching at 420, 550, and 600 nm and transient absorption from
570 to 750 nm. However, this species undergoes a fast decay
in sharp contrast to the slow decay of 2 to generate the triplet
excited state. Concomitant with the singlet excited-state decay
of 2 in the 400-800 nm range, new features appeared in the
800-1200 nm range. In particular, a maximum at 1000 nm is
a reliable attribute of the one-electron reduced form of fullerenes,
indicating that deactivation of the singlet excited state of 2
occurs by electron transfer.39 Evidence for the one-electron
oxidized porphyrins was found in the 600-800 nm range, where
its diagnostic absorption developed in parallel; see Figure 9.40
A multiwavelength analysis led to a major decay component
with a lifetime of 326 ps, which we attributed to charge
separation affording the radical ion pair state. The rates of charge
separation were derived for all systems and are listed in Table
1. Importantly, the radical ion pair species is stable on the 3 ns
time scale of our femtosecond experiments.
Upon excitation of 1a, 1b, 1c, 1d, 1e, or 1f at 387 nm, the
lowest vibrational state of the initially generated singlet excited
state undergoes quantitative intersystem crossing with a rate
constant of 5 × 108 s-1 to the energetically lower lying long-
lived triplet excited state. Characteristic absorptions of the singlet
excited state are seen at 510 and 920, while maxima at 360 and
720 nm and a low-energy shoulder around 800 nm are associated
with the triplet features.9,37,38 In the absence of molecular
oxygen, the triplet excited states of 1a, 1b, 1c, 1d, 1e, or 1f
have a lifetime of up to 20 µs.
Interestingly, introduction of a second p-phenylene-ethynylene
repeat unit (i.e., 1a·2 versus 1b·2) eliminates the electron
transfer process with no evidence for any radical ion pair state
formation, Figure 10. This finding is well in line with our steady-
state and time-resolved fluorescence measurements. In fact, it
supports the notion that in photoexcited 1b·2, intrinsic deactiva-
tion pathways, as for example, fluorescence and intersystem
crossing, dominate rather than electron transfer that has been
seen to be the case in 1a·2.
In the final part, nanosecond transient absorption measure-
ments were employed to follow the charge recombination
processes in 1·2 upon 6 ns lasting 532 nm excitation in argon-
saturated ortho-dichlorobenzene solutions. The product of charge
separation, that is, the one-electron oxidized porphyrin and the
one-electron reduced fullerene, was partly masked by the strong
excited-state features of free, uncomplexed 2, 1a, 1c, 1d, 1e,
and 1f. These emerge in the 580-900 nm range despite the
The presence of 1a, 1c, 1d, 1e, or 1f impacts the reactivity
of the cyanuric bearing porphyrin 2. Because of the strong and
dominant absorption of 2 in 1a·2, 1c·2, 1d·2, 1e·2, and 1f·2,
(35) (a) Hoffman, M. Z.; Bolletta, F.; Moggi, L.; Hug, G. L. J. Phys. Chem.
Ref. Data 1989, 18, 219–543. (b) Rodriguez, J.; Kirmaier, C.; Holten,
D. J. Am. Chem. Soc. 1989, 111, 6500–6509. (c) Murov, S. L.;
Carmichael, I.; Hug, G. L. Handbook of Photochemistry; Marcel
Dekker Inc.: New York, 1993;
(36) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin
Complexes; Academic Press: London, 1992.
(39) (a) Guldi, D. M.; Hungerbuehler, H.; Janata, E.; Asmus, K. D. J. Chem.
Soc., Chem. Commun. 1993, 6, 84–86. (b) Kato, T.; Kodama, T.; Shida,
T.; Nakagawa, T.; Matsui, Y.; Suzuki, S.; Shiromaru, H.; Yamauchi,
K.; Achiba, Y. Chem. Phys. Lett. 1991, 180, 446–450.
(37) Luo, C.; Guldi, D. M.; Imahori, H.; Tamaki, K.; Sakata, Y. J. Am.
Chem. Soc. 2000, 122, 6535–6551.
(38) Nojiri, T.; Watanabe, A.; Ito, O. J. Phys. Chem. A 1998, 102, 5215–
5219.
(40) Guldi, D. M.; Hirsch, A.; Scheloske, M.; Dietel, E.; Troisi, A.; Zerbetto,
F.; Prato, M. Chem.-Eur. J. 2003, 9, 4968–4979.
9
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A R T I C L E S
Wessendorf et al.
Figure 10. Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (420 nm) of 1b·2 in argon-
saturated ortho-dichlorobenzene with time delays of 0 and 512 ps at room
temperature.
Figure 12. Energy diagram showing the deactivation pathways in 1·2.
were observed in both cases, and the corresponding slopes were
used to derive a ꢀ factor of 0.11 Å-1. Notable is, however, that
the determination of ꢀ infers spacers with different electron
transfer activities. Still, this approximation sheds first light on
supramolecular-based molecular wire systems with a ꢀ factor
that lies between the extremes of covalent p-phenylene-
ethynylene27 and fluorene23b systems.
Discussion
This work sheds comprehensively and newly light onto a
novel set of Hamilton receptor fullerenes (1a-f) and a cyanuric
acid linked porphyrin (2). In particular, we have investigated
the synthesis, self-assembly, and photophysical properties of
novel supramolecular wire-like donor-acceptor nanohybrids
using π-conjugated spacers of defined geometry, p-phenylene-
ethynylene, p-phenylene-vinylene, p-ethynylene, and fluorene.
Importantly, the 6-fold hydrogen-bonding motif prevents rota-
tional freedom/structural flexibility and, in turn, strengthens the
rigid confinement of the electron donor-acceptor nanohybrids.
The rigidity was verified and illustrated by PM3 Molecular
Modeling calculations. Dodecyl or hexyl chains were used to
ensure sufficient solubility in a variety of solvents.
Figure 11. Differential absorption spectra (visible and near-infrared)
obtained upon nanosecond flash photolysis (532 nm) of 1c·2 in Ar-saturated
ortho-dichlorobenzene with a time delay of 80 ns.
rather high association constants of 103-104 M-1, derived from
1
the fluorescence assays, and 105 M-1, derived from H NMR
experiments. Only around 1000 nm is the one-electron reduced
fullerene seen with ease for 1a·2, 1c·2, 1d·2, and 1f·2. This
led us to perform the same experiments but in the presence of
molecular oxygen. Through the triplet deactivation of 1 and 2
to form singlet oxygen, the diagnostic features of the one-
electron oxidized porphyrin came to light; see Figure 11.
Multiwavelength analyses of the transient absorption spectra
in the absence of the triplet quenching oxygen gave rise to a
biexponential decay behavior. On one hand, it is the lifetime
of the triplet excited states of 1 (ca. 20 µs) and/or 2 (ca. 45 µs)
and, on the other hand, a short lifetime that is on the order of
several tens of nanoseconds. By purging the solutions with
molecular oxygen, the triplet excited-state lifetimes of 1 and 2
were shortened from microseconds to hundreds of nanoseconds,
without, however, affecting the short-lived component. Taking
the aforementioned into consideration, we assign the second
component to the charge recombination process. Please note
that the accordingly determined rate constants for 1a·2, 1c·2,
1d·2, and 1f·2 are listed in Table 1.
The self-assembly of 2 and 1a-f was probed by means of
1H NMR and fluorescence assays. Because of the exceptional
strength of the 6-fold hydrogen bonding, association constants
1
evolve in H NMR experiments that are on the order of 105
M-1, at millimolar concentrations. When, however, the associa-
tion constants were considered that were derived from the
characteristic changes in the fluorescence intensity, values
emerged that are on the order of 104 M-1 for p-phenylene-
ethynylene, p-phenylene-vinylene, and p-ethynylene spacers,
respectively, at micromolar concentrations. The only notable
exception is 1b with an association constant of 102 M-1 that
lacks electron transfer activity and, thus, just reflects a lower
limit.
The spacer dependence, that is, length and nature, was
corroborated by means of time-resolved techniques. In terms
of simple distance dependence, transient absorption measure-
ments confirm that the charge separation rate constants are 3.1
× 109 and 1.1 × 1010 s-1 for 1a·2 and 1c·2, respectively.
Interestingly, for 1b·2 no electron transfer activity was found
on the time scale of up 3000 ps, which suggests that incorpora-
tion of an extra p-phenylene-ethynylene shuts down the electron
transfer in 1b. On the other hand, the rate constants in 1a·2,
Finally, we calculated the ꢀ factor by plotting the rate
constants of charge separation and charge recombination versus
donor-acceptor distances; see Figure S3. Linear dependences
9
10794 J. AM. CHEM. SOC. VOL. 132, NO. 31, 2010

Hydrogen Bonding in Electron Transfer
A R T I C L E S
1d·2, and 1f·2 increase with decreasing attenuation factor of
the spacer. Clearly, a correlation to a spacer-mediated depen-
dence is likely. In fact, previous work has shown that the
attenuation factors of π-conjugated oligomers increase in the
order p-phenylene-vinylene > fluorene > p-phenylene-ethy-
nylene.27 Notable is the agreement with the results from our
fluorescence assays. In particular, the fastest electron transfer,
5.3 × 109 s-1, was observed for 1d·2, followed by 4.1 × 109
(1f·2) and 3.1 × 109 s-1 (1a·2). Again, the fluorene-based 1f·2
is a more efficient π-conjuaged spacer than the p-phenylene-
ethynylene-based 1a·2. Energy transfer certainly emerges as a
feasible alternative to the electron transfer scenario. However,
contributions from an inefficient fluorescence quenching and
an incomplete association toward the generation of singlet and
triplet excited states mask the spectroscopic interpretation.
employed spacer help to rationalize the conclusion of the
fluorescence assays. Overall, a notably small ꢀ factor of 0.11
Å-1 was obtained upon considering all electron donor-acceptor
nanohybrids.
Conclusion
In short, we have demonstrated unambiguously that the
electronic communication in rigid electron donor-acceptor
nanohybrids that are brought together by the Hamilton receptor/
cyanuric acid motif is controlled by the nature of the conjugated
spacer moieties. Selective photoexcitation of the porphryins (2.0
eV) triggers in most hybrids an electron transfer to yield the
one-electron reduced fullerenes and the one-electron oxidized
porphyrins (1.4 eV); see Figure 12. Any appreciable contribu-
tions from a competitive energy transfer to populate the fullerene
singlet excited state (1.78 eV) are masked by intrinsic forma-
tions. Spacers with good electron transfer properties (i.e., small
ꢀ values) facilitate electron transfer along the supramolecular
bridge and, in addition, cause an increase in the apparent
association constants.
An overall resembling trend is seen for the charge recombina-
tion processes. The following trends correlate either with the
nature of the spacer or with the length of the spacer. To this
end, in the p-phenylene-ethynylene-based systems, the values
of 1.4 × 107 (1a·2) and 3.3 × 107 s-1 (1c·2) reflect the change
in spacer length, 12.2 versus 7.9 Å. It is worth re-emphasizing
the lack of electron transfer activity in 1b·2. On the other hand,
varying the spacer motif from p-phenylene-ethynylene, fluorene,
to p-phenylene-vinylene (i.e., 1a·2, 1f·2, and 1d·2) results in
a steady increase in charge recombination rate constant from
1.4 × 107 to 1.9 × 107 and to 2.5 × 10-7 s-1. Here, the rate of
charge recombination is, again, mainly governed by the nature
of the spacer and not by its length. At this point, it is fair to
conclude that the charge separation and charge recombination
kinetics as they reflect either the length or the ꢀ factor of the
Acknowledgment. We thank the Fonds der Chemischen In-
dustrie (FCI), Deutsche Forschungsgemeinschaft (SFB 583, Re-
doxaktive Metallkomplexe - Reaktivita¨tssteuerung durch molekulare
Architekturen, and Exzellenzcluster, EAM - Engineering of Ad-
vanced Materials), for financial support.
Supporting Information Available: Experimental details and
figures. This material is available free of charge via the Internet
at http://pubs.acs.org.
JA101937W
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