Exceptionally strong electronic communication through hydrogen bonds in porphyrin-C60 pairs

Article abstract of DOI:10.1002/anie.200601264

(Figure Presented) Hooking up: Self-assembly of a porphyrin amidine and fulleropyrrolidine carboxylic acid based on a two-point amidinium-carboxylate motif leads to supramolecular dyads of high stability (K_a ≈ 10⁷ M^-1 in toluene/acetonitrile (9:1)). The synergy of the hydrogen bonds and electrostatic interactions has been shown to be particularly beneficial in terms of electronic coupling between both electroactive components of the dyads.

Full text of DOI:10.1002/anie.200601264

Angewandte
Chemie
Electron Transfer
DOI: 10.1002/anie.200601264
Exceptionally Strong Electronic Communication
through Hydrogen Bonds in Porphyrin–C60
Pairs**
Luis Sµnchez, Maria Sierra, Nazario Martín,*
Andrew J. Myles, Trevor J. Dale, Julius Rebek, Jr.,*
Wolfgang Seitz, and Dirk M. Guldi*
In memory of Roger Taylor
Proton pumping in the photosynthetic reaction center and
conduction of electrons in cytochrome C are significant
examples of biological processes governed by supramolecular
interactions.[1] The key function of these examples is electron
transfer, which is powered through a network of hydrogen
bonds. As a result, hydrogen-bonded donor–acceptor assem-
blies have emerged as a benchmark for biomimetic model
systems: they provide the means for understanding and
controlling the role of hydrogen-bonding networks either as a
simple molecular interface or, more interestingly, as an
actively functioning motif (namely, by assisting more efficient
electron-transfer events). These effects largely result from the
high efficiency that this supramolecular motif offers in
controlling through-bond-mediated electron-transfer process-
es as well as long-range electronic coupling between donors
and acceptors.[2] Pioneering work carried out by Sessler and
[*] Dr. L. Sµnchez, M. Sierra, Prof. Dr. N. Martín
Departamento de Química Orgµnica
Facultad de Química
Universidad Complutense de Madrid, 28040 Madrid (Spain)
Fax: (+34)91-394-4103
E-mail: nazmar@quim.ucm.es
Dr. A. J. Myles, T. J. Dale, Prof. Dr. J. Rebek, Jr.
The Skaggs Institute for Chemical Biology and Department of
Chemistry
The Scripps Research Institute
10550 North Torrey Pines Road (MB26), La Jolla, CA 92037 (USA)
Fax: (+34)91-394-4103
E-mail: jrebek@scripps.edu
W. Seitz, Prof. Dr. D. M. Guldi
Institut für Physikalische und Theoretische Chemie
Universität Erlangen-Nürnberg
Egerlandstrasse 3, 91058 Erlangen (Germany)
Fax: (+49)9131-852-830
E-mail: dirk.guldi@chemie.uni-erlangen.de
[**] This work was supported by the MEC of Spain and Comunidad de
Madrid (CTQ2005-02609/BTQ and P-PPQ-000225-0505), the Deut-
sche Forschungsgemeinschaft(SFB 583), FCI, the Office of Basic
Energy Sciences of the US Department of Energy (contribution No.
NDRL-4670 from the Notre Dame Radiation Laboratory), and the
Skaggs Institute for Chemical Biology. M.S. is indebted to MEC of
Spain for a PhD fellowship, A.J.M. is a NSERC postdoctoral fellow,
and T.J.D. is a Skaggs predoctoral fellow.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Therien have demonstrated that the electronic communica-
tion through hydrogen-bonding interfaces is more efficient
than those found in comparable s- or p-bonding networks.[3]
Electron-transfer events also impart on the function of
artificial devices, such as organic photovoltaics.[4] The incen-
tives for creating (supra)molecular heterojunctions by, for
example, integrating photo-/electroactive donors and accept-
ors that bear complementary binding sites into functional
architectures are 1) facilitation of charge transport and
2) increase in energy conversion efficiency.[5]
The relevance of fullerenes as spherical electron acceptors
in the construction of novel artificial photosynthetic models
and in high-performance organic solar cells is incontesta-
ble.[4–6] The combination of the characteristic selectivity and
directionality of hydrogen bonds with an efficient electron-
Scheme 1. Amidinium–carboxylate interfaced porphyrin–C60 assemblies
(1·2).
transfer process through
a noncovalent framework is
expected to set new milestones, especially in terms of
achieving longer-lived radical-ion-pair states. In fact, a
plethora of covalent C60–donor conjugates have stimulated
evolutionary advances and revolutionary breakthroughs in
the context of converting light into high-energy chemical
products or electrical currents.[7] Very little is, however,
known about noncovalent C60-based hybrid ensembles.[8] To
our knowledge, no C60-based examples of electron-transfer
processes that occur exclusively through hydrogen-bonded
networks have been reported. We present herein a set of
noncovalently associated C60–porphyrin ensembles (1·2)
interfaced by a two-point amidinium–carboxylate pair that
facilitates an efficient charge-separation process to afford
microsecond-lived PC+–C60CÀ radical pairs. The latter is partic-
ularly stable, even in highly polar solvents, as a result of the
synergy of hydrogen bonds and electrostatic interactions.[9]
Beneficial effects also materialize from other salt pairs, that is,
guanidinium–carboxylate, especially in terms of structural
aspects.[10] Notably, the noncovalent binding motif utilized
herein, amidinium–carboxylate, diminishes the possible
bonding modes and, therefore, favors the linearity of the
final interfaced pair. In addition, the rational design, that is,
placing the amidinium and carboxylate functionalities at the
porphyrins and C60, respectively, reinforces the strength of the
hydrogen-bonding network and ensures an optimal pathway
for the motion of charges and the electronic coupling between
both electroactive units.[9a]
Figure 1. 1H NMR shifts upon titration of 2 into a solution of 1a in
[D8]THF.
The synthesis of the new 1·2 ensembles started from a
previously reported porphyrin amidine donor 1a,b[11] and
fulleropyrrolidine carboxylic acid 2.[12] Directly mixing both
components readily yields target ensembles, namely, non-
covalently bonded hybrids 1·2 (see Scheme 1 and the
Supporting Information).
Importantly, no further changes are observed upon addition
of more than one equivalent of 2, thus confirming the
exclusive presence of a strong 1:1 complex.[14]
Further details on the formation of 1a·2 and 1b·2 were
obtained from experiments in which dilute solutions of 1a and
1b in toluene/acetonitrile (9:1, v/v) or THF were titrated with
variable amounts of 2 and probed by absorption and
fluorescence spectroscopy (see Figure 2 and the Supporting
Information in which a typical example for an absorption
measurement is given). Relative to the component spectra
(1b and 2), a number of differences are apparent. For 1b,
Soret- and Q-band transitions at 428 and 558 nm, respectively,
shift incrementally to the red (431 and 560 nm). Moreover,
the presence of isosbestic points indicate the transformation
Complex formation between 1a and 2 was monitored by
1H NMR spectroscopy (3 mm, [D8]THF). Upon titration of
one equivalent of 2 into a solution of 1a, a series of resonance
shifts are observed (Figure 1). The most dramatic shift
involves the aryl protons ortho to the amidine functionality
(Dd = 0.35 ppm).[13] Although the amidine protons are not
1
resolved in the H NMR spectrum of 1a, formation of 1a·2
resolves two broad downfield resonances at d = 9.4 and
13.1 ppm. These new peaks each integrate for two protons
and are attributed to the two different amidine protons.
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reduction and oxidation steps (Figure 3). For 2, three
reduction steps are discernable. The corresponding zinc
complex 1b·2 reveals cathodic shifts of the reduction and
oxidation steps relative to the free-base complex 1a·2 (see the
Supporting Information). In both 1·2 complexes, the redox
patterns of the two constituents, namely porphyrin amidines 1
and C60 carboxylic acid 2, are essentially preserved (Figure 3).
Figure 2. Fluorescence spectra (lexc =433 nm) of 1b (1.08”10À6 m)
and variable concentration of 2 (0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6,
2.4, 3.9, 9.1, and 23.7”10À6 m) in toluene/acetonitrile (9:1, v/v) at
room temperature. Insert displays the relationship of I/I0 versus [2]
that was used to determine the association constant. I=current
intensity in relative units.
of 1b (the starting point of the titration) into 1b·2 (the end
point of the titration). The changes in absorption were further
employed to confirm the stoichiometry of 1a·2 and 1b·2
through Job plots. In this light, Gaussian relationships with
maxima at 0.5 unequivocally confirm the 1:1 complex
Figure 3. Cyclic voltammograms of 1b·2 and its precursors 1b and 2,
measured in THF (5”10À4 m) at100 mVs À1 (V vs. Ag/Ag+).
1
stoichiometries that were evoked from the H NMR experi-
ments (see the Supporting Information).
The observed slight shifts in the redox potentials of the 1·2
assemblies could be accounted for by a pronounced electronic
coupling between both electroactive components (see below).
Neglecting significant electronic interactions, we estimate
values of the radical-ion-pair-state energies as the sum of the
first reduction potential of 2 and the first oxidation potential
of either 1a or 1b to be 1.9 and 1.62 eV for 1a·2 and 1b·2,
respectively. In other words, radical-ion-pair formations that
evolve from the singlet excited states are highly exothermic.
Finally, transient absorption studies showed the fate of the
porphyrin excited states and the identification of the photo-
products. Pumping light into the ground state of 1a or 1b with
short 387-nm (namely, 150 fs) or 532-nm (namely, 6 ns) laser
pulses led to the population of their singlet excited states. The
latter undergo intersystem crossing ( ꢀ 108 sÀ1) to the long-
lived and molecular-oxygen-sensitive triplet states. Triplet
spectra of 1a or 1b reveal, besides bleaching in the Soret- and
Q-band region, characteristic triplet maxima at 780 (1a) or
840 nm (1b).[6d]
Initially, the porphyrin chromophores in 1a·2 and 1b·2
produce notable singlet fingerprints for 1a and 1b upon
photoexcitation. This behavior attests to the successful
excitation of 1a and 1b. However, instead of the slow
intersystem crossings, the singlet excited states decay with
rates of about 1010 sÀ1, from which we deduce an electronic
coupling of 36 cmÀ1 between both electroactive elements
(that is, 1b·2). At the end of the intrinsically fast decay,
differential absorption changes in the visible region are
governed by broad absorptions at 600–800 nm, thus indicating
1a- or 1b-centered oxidation products. In the near-infrared
region, on the other hand, the signature of a 2-centered
In excitation experiments with 1a and 1b, strong fluores-
cence emissions are seen in the red region, with quantum
yields of 0.2 and 0.04, respectively. When variable concen-
trations of 2 are present, the fluorescence emission intensities
decreased exponentially (see Figure 2 and the Supporting
Information). Nonlinear least-square analyses, that is, fluo-
rescence intensity versus concentration of 2, allowed the
evaluation of the binding constants of 1·2, which are excep-
tionally high. In solvents that do not interfere significantly
with either the electrostatic or the hydrogen-bonding inter-
actions (toluene or toluene/acetonitrile (9:1, v/v)), binding
constants as high as 2.1 ” 107 mÀ1 were deduced. The binding
constants in THF, in which interactions are mostly based on
electrostatic attractions, showed values between 1.3 ” 105 mÀ1
(1a·2) and 3.3 ” 105 mÀ1 (1b·2).
Potential C60 interactions with 1a or 1b were tested in a
series of reference assays. For example, in the absence of the
amidine and carboxylic acid functionalities, the absorption
spectra remain virtually as the superimpositions of the
component spectra throughout the titrations. The lack of
mutually interacting systems was also confirmed in fluores-
cence experiments. Overall, only a quenching of less than 5%
is noted, relative to > 95% quenching in 1a·2 or 1b·2.
The redox features of interfaced 1·2 pairs were deter-
mined by cyclic voltammetry in THF at room temperature
(see the Supporting Information). These values provide
useful information about the energies of the radical ion
pairs formed upon photoexcitation (see below). The cyclic
voltammograms of both porphyrins, free base (1a) and zinc
(1b) at concentrations of 5 ” 10À4 m, show sets of reversible
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reduction is seen at 1000 nm.[15] Figure 4 exemplifies the
spectral changes seen in the cases of 1b·2 and 1a·2,
respectively (see the Supporting Information also). The
decay kinetics of both signatures—on the nanosecond
10252 – 10253; b) J. L. Sessler, M. Sathiosatham, C. T. Brown,
T. A. Rhodes, G. Wiederrecht, J. Am. Chem. Soc. 2001, 123,
3655 – 3660; c) A. J. Myles, N. R. Branda, J. Am. Chem. Soc.
2001, 123, 177 – 178; d) R. K. Castellano, S. L. Craig, C. Nuckolls,
J. Rebek, Jr., J. Am. Chem. Soc. 2000, 122, 7876 – 7882; e) T. H.
Ghaddar, E. W. Castner, S. S. Isied, J. Am. Chem. Soc. 2000, 122,
1233 – 1234.
[3] a) J. L. Sessler, B. Wang, A. Harriman, J. Am. Chem. Soc. 1993,
115, 10418 – 10419; b) P. J. F. De Rege, S. A. Williams, M. J.
Therien, Science 1995, 269, 1409 – 1413.
[4] a) W. Ma, C. Yang, X. Gong, K. Lee, A. J. Heeger, Adv. Funct.
Mater. 2005, 15, 1617 – 1622; b) G. Li, V. Shrotriya, J. Huang, Y.
Yao, T. Moriarty, K. Emery, Y. Yang, Nat. Mater. 2005, 4, 864 –
868; c) I. Riedel, E. von Hauff, J. Parisi, N. Martꢀn, F. Giacalone,
V. Dyakonov, Adv. Funct. Mater. 2005, 15, 1979 – 1987; d) M. W.
Wienk, J. M. Kroon, W. J. H. Verhees, J. Knol, J. C. Hummelen,
P. A. van Hal, R. A. J. Janssen, Angew. Chem. 2003, 115, 3493 –
3497; Angew. Chem. Int. Ed. 2003, 42, 3371 – 3375; e) C. J.
Brabec, N. S. Sariciftci, J. C. Hummelen, Adv. Funct. Mater. 2001,
11, 15 – 26.
[5] a) C.-H. Huang, N. D. McClenaghan, A. Kuhn, J. W. Hofstraat,
D. M. Bassani, Org. Lett. 2005, 7, 3409 – 3412; b) J. L. Brꢁdas, D.
Beljonne, V. Coropceanu, J. Cornil, Chem. Rev. 2004, 104, 4971 –
5003; c) M. A. Fox, Acc. Chem. Res. 1999, 32, 201 – 207.
[6] a) H. Imahori, Y. Sakata, Adv. Mater. 1997, 9, 537 – 546; b) N.
Martꢀn, L. Sꢂnchez, B. Illescas, I. Pꢁrez, Chem. Rev. 1998, 98,
2527 – 2548; c) D. M. Guldi, N. Martꢀn, J. Mater. Chem. 2002, 12,
1978 – 1992; d) D. M. Guldi, Chem. Soc. Rev. 2002, 31, 22 – 36;
e) J.-F. Nierengarten, Top. Curr. Chem. 2003, 228, 87 – 110;
f) J. L. Segura, N. Martꢀn, D. M. Guldi, Chem. Soc. Rev. 2005, 34,
31 – 47; g) D. M. Guldi, G. M. A. Rahman, C. Ehli, V. Sgobba,
Chem. Soc. Rev. 2006, 35, 471 – 487.
[7] For recent examples, see: a) C. M. Atienza, G. Fernꢂndez, L.
Sꢂnchez, N. Martꢀn, I. Sꢂ Dantas, M. W. Wienk, R. A. J. Janssen,
G. M. A. Rahman, D. M. Guldi, Chem. Commun. 2006, 514 –
516; b) G. Kodis, Y. Terazono, P. A. Liddell, J. Andrꢁasson, V.
Garg, M. Hambourger, T. A. Moore, A. L. Moore, D. Gust, J.
Am. Chem. Soc. 2006, 128, 1818 – 1827; c) L. Sꢂnchez, M. Sierra,
N. Martꢀn, D. M. Guldi, M. W. Wienk, R. A. J. Janssen, Org. Lett.
2005, 7, 1691 – 1694; d) T. Oike, T. Kurata, K. Takimiya, T.
Otsubo, Y. Aso, H. Zhang, Y. Araki, O. Ito, J. Am. Chem. Soc.
2005, 127, 15372 – 15373; e) D. M. Guldi, F. Giacalone, G.
de la Torre, J. L. Segura, N. Martꢀn, Chem. Eur. J. 2005, 11,
7199 – 7210; f) L. Sꢂnchez, M. A. Herranz, N. Martꢀn J. Mater.
Chem. 2005, 15, 1409 – 1421; g) R. S. Iglesias, C. G. Claessens, T.
Torres, G. M. A. Rahman, D. M. Guldi, Chem. Commun. 2005,
2113 – 2115; h) K. Ohkubo, H. Kotani, J. Shao, Z. Ou, K. M.
Kadish, G. Li, R. K. Pandey, M. Fujitsuka, O. Ito, H. Imahori, S.
Fukuzumi, Angew. Chem. 2004, 116, 871 – 874; Angew. Chem.
Int. Ed. 2004, 43, 853 – 856; i) F. Giacalone, J. L. Segura, N.
Martꢀn, D. M. Guldi, J. Am. Chem. Soc. 2004, 126, 5340 – 5341;
j) M. Gutierrez-Nava, G. Accorsi, P. Masson, N. Armaroli, J. F.
Nierengarten, Chem. Eur. J. 2004, 10, 5076 – 5086.
Figure 4. Transient absorption spectrum (NIR part) of 1b·2 in toluene
at room temperature recorded 50 ps following a 150 fs laser pulse
(lexc =387 nm).
scale—reflect the return of the radical-ion-pair states to the
electronic ground states. The lifetime of the newly formed
radical-ion-pair state, as derived by analyzing several wave-
lengths under unimolecular conditions, are 9.3 Æ 0.1 and 7.9 Æ
0.5 ms for 1a·2 and 1b·2, respectively, in THF (see the
Supporting Information).
In summary, the two-point amidinium–carboxylate bind-
ing motif guarantees an extraordinary stabilization for a set of
noncovalently interfaced ensembles (1·2). Association con-
stants reach up to 107 mÀ1. Exceptionally strong electronic
couplings stem from such binding, which in turn facilitate a
faster, more efficient, and longer-lived formation of radical-
ion-pair states (that is, ꢀ 10 ms in THF) relative to similar
covalent C60 conjugates (namely, ꢀ 1 ms in THF).[15] Most
importantly, such remarkable radical-ion-pair lifetimes out-
perform previously reported ensembles based on 1) a non-
amidinium–carboxylate binding motif[8] or 2) non-fullerene
electron acceptors[2,9] by several orders of magnitude. These
results point unmistakably to the fundamental advantages of
strong and highly directional hydrogen-bonding networks in
assisting electron-transfer processes and pave the way to the
construction of efficient photovoltaic devices inspired by
biomimetic principles.
[8] a) F. Diederich, M. Gꢃmez-Lꢃpez, Chem. Soc. Rev. 1999, 28,
263 – 278; b) L. Sꢂnchez, N. Martꢀn, D. M. Guldi, Angew. Chem.
2005, 117, 5508 – 5516; Angew. Chem. Int. Ed. 2005, 44, 5374 –
5382; c) Special Issue on Supramolecular Chemistry of Fullerenes
(Eds.: N. Martꢀn, J. F. Nierengarten), Tetrahedron 2006, 62,
1905 – 2132; d) U. Hahn, M. Elhabiri, A. Trabolsi, H. Hersch-
bach, E. Leize, A. Van Dorsselaer, A.-M. Albrecht-Gary, J.-F.
Nierengarten, Angew. Chem. 2005,117, 5472 – 5475; Angew.
Chem. Int. Ed. 2005, 44, 5338 – 5341; e) N. D. McClenaghan, Z.
Grote, K. Darriet, M. Zimine, R. M. Williams, L. De Cola, D. M.
Bassani, Org. Lett. 2005, 7, 807 – 810; f) J. L. Sessler, J. Jaya-
wickramarajah, A. Gouloumis, T. Torres, D. M. Guldi, S.
Maldonado, K. J. Stevenson, Chem. Commun. 2005, 1892 – 1894.
Received: March 31, 2006
Published online: June 22, 2006
Keywords: electron transfer · electronic coupling · fullerenes ·
.
hydrogen bond
[1] a) J. A. Stubbe, D. G. Nocera, C. S. Yee, M. C. Y. Chang, Chem.
Rev. 2003, 103, 2167 – 2201; b) A. Namslauer, A. Aagaard, A.
Katsonouri, P. Brzezinski, Biochemistry 2003, 42, 1488 – 1498.
[2] a) A. P. H. J. Schenning, J. V. Herrikhuyzen, P. Jonkheijm, Z.
Chen, F. Würthner, E. W. Meijer, J. Am. Chem. Soc. 2002, 124,
4640 www.angewandte.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angewandte
Chemie
[9] a) J. P. Kirby, J. A. Roberts, D. G. Nocera, J. Am. Chem. Soc.
1997, 119, 9230 – 9236; b) N. H. Damrauer, J. M. Hodgkiss, J.
Rosenthal, D. G. Nocera, J. Phys. Chem. B 2004, 108, 6315 –
6321.
[10] a) M. Haj-Zaroubi, N. W. Mitzel, F. P. Schmidtchen, Angew.
Chem. 2002, 114, 111 – 114; Angew. Chem. Int. Ed. 2002, 41, 104 –
107; b) V. Krµl, F. P. Schmidtchen, K. Lang, M. Berger, Org. Lett.
2002, 4, 51 – 54; c) M. Berger, F. P. Schmidtchen, Angew. Chem.
1998, 110, 2840 – 2842; Angew. Chem. Int. Ed. 1998, 37, 2694 –
2694.
[11] J. Otsuki, K. Iwasaki, Y. Nakano, M. Itou, Y. Araki, O. Ito,
Chem. Eur. J. 2004, 10, 3461 – 3466.
[12] M. Segura, L. Sµnchez, J. de Mendoza, N. Martín, D. M. Guldi, J.
Am. Chem. Soc. 2003, 125, 15093 – 15100.
[13] 2D ROESY spectrometry was performed to confirm peak
assignments in the 1D 1H NMR spectrumof 1a.
[14] The high stability of the 1a·2 complex prevents reliable
determination of the association constants using NMR techni-
ques; see Ref. [9].
[15] H. Imahori, H. Yamada, D. M. Guldi, Y. Endo, A. Shimomura, S.
Kundu, K. Yamada, T. Okada, Y. Sakata, S. Fukuzumi, Angew.
Chem. 2002, 114, 2450 – 2453; Angew. Chem. Int. Ed. 2002, 41,
2344 – 2347.
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