Activating Multistep Charge-Transfer
A R T I C L E S
coupling in multicomponent systems in solution,6 one can expect
that the effects of this parameter are even more critical in solid-
state photovoltaic devices.2 In fact, in the absence of a solvent
medium, the π-conjugated photoactive molecules are forced to
pack via intimate van der Waals interactions and conformational
or orientational motions are severely limited. Most of the
synthetic donor-acceptor models described so far fail to
reproduce this situation where, due a higher degree of orbital
overlap between the confined molecules, the fate of the harvested
energy may be controlled by mechanisms different to those
operating in molecularly dissolved systems.
We and others have previously demonstrated that covalent
systems that combine subphthalocyanine (SubPc)7 and C60
fullerene do not exhibit a large driving force for photoinduced
charge separation. Despite the relatively high excitation energy
of SubPcs (ca. 2.1 eV), these macrocycles typically exhibit high
or moderate oxidation potentials, which results in a high-energy
SubPc·+-C60·- charge-separated state. Instead, energy transfer
processes arise as the dominant mechanism operating after
photoexcitation.8 Only when the SubPc core is substituted with
strong electron-donating groups at the periphery (i.e., amines),
thereby lowering its oxidation potential, are electron transfer
processes observed in these SubPc-C60 hybrids.8b,f So far this
has been proven in weakly coupled conjugates, where the SubPc
axial position served to connect C60 through partially conjugated
spacers.
However, we recently described C60-SubPc capped conju-
gates where electron transfer can compete with energy transfer
mechanisms due to an intimate π-π contact between the two
photoactive units.9 Here, we profit from the synthetic versatility
of SubPcs and coupled an additional ferrocene (Fc) electron-
donating unit at the axial position of the macrocycle. In these
C60-SubPc-Fc the SubPc light-harvesting unit is situated
between the acceptor and the donor moieties and can thus
mediate their electronic communication. We show that multistep
charge-transfer mechanisms, ultimately leading to the spatially
separated C60·--SubPc-Fc·+ radical ion pair, are only triggered
when the C60 and the SubPc chromophores are rigidly main-
tained at a short distance and forced to strongly interact through
their complementary curved π-surfaces.10 This π-π distance,
and hence the degree of orbital overlap, could be finely adjusted
by just varying the linkage between the three semirigid spacers
that anchor the SubPc to the C60: a shorter C-C bond (-C series)
or a slightly longer C-O-C bond (-O series). A full account
of the synthesis, characterization, and our studies of the ground-
and excited-state electronic interactions occurring in these
C60-SubPc-Fc systems, as well as in their respective
C60-SubPc and SubPc-Fc reference compounds, is presented
in this article.
Results and Discussion
Synthesis. Two different synthetic routes to the SubPc-Fc
conjugates (S-Fc), C60-SubPc (C60-S) conjugates, and
C60-SubPc-Fc (C60-S-Fc) conjugates were designed, depend-
ing on the connection of the spacer to the SubPc macrocycle:
a direct C-C bond (-C series) or an oxygen atom (-O series).
The compounds of the -C series (Scheme 1) were prepared
starting from a chloro-SubPc peripherally substituted with three
iodine atoms (3),11 which was synthesized by cyclotrimerization
reaction of 4-iodophthalonitrile (4) in the presence of BCl3.12
The axial chlorine atom in 3 was then replaced either by 4-tert-
octylphenol or 4-ferrocenylphenol, leading respectively, after
chromatographic separation of the 1:3 mixture of C3/C1 regioi-
somers, to SubPcs 2-C and 2-Fc-C. These compounds were then
subjected to a Suzuki reaction with (2-hydroxyphenyl)pinacol
boronate,13 affording trihydroxy derivatives 1-C and 1-Fc-C in
good yields. Finally, the acylation reaction between 1-C or
1-Fc-C and ethyl malonyl chloride led to SubPc S-C and
SubPc-Fc dyad S-Fc-C, respectively.
On the other hand, the route leading to the products in the
-O series (Scheme 2) starts with the condensation reaction of
the TBDMS-protected phthalonitrile 6. The silylether groups
in this reagent are sufficiently stable to resist the harsh conditions
of the cyclotrimerization reaction with BCl3. It is known that
chloro-SubPcs that are peripherally functionalized with electron-
donating groups can easily undergo axial exchange reactions.7b
Therefore, in order to avoid the formation of the corresponding
axially hydroxy-substituted SubPc during chromatographic
workup, we decided to carry out the axial replacement of the
chlorine atom with the phenol derivatives just after the
condensation reaction, without isolation of the corresponding
chloro-SubPcs. This led to compounds 5-O and 5-Fc-O, which
are axially equipped with 4-tert-octylphenoxy and 4-ferroce-
nylphenoxy units, respectively. Deprotection of the TBDMS
groups in 5-O and 5-Fc-O in the presence of TBAF13a led to
the corresponding trihydroxy SubPcs 1-O and 1-Fc-O. As we
did with SubPcs 1-C and 1-Fc-C, compounds 1-O and 1-Fc-O
were finally subjected to an acylation reaction with ethyl malonyl
chloride, yielding SubPc S-O and SubPc-Fc dyad S-Fc-O,
respectively. The isolation of the C3 regioisomer was carried
(5) (a) Guldi, D. M.; Hirsch, A.; Scheloske, M.; Dietel, E.; Troisi, A.;
Zerbetto, F.; Prato, M. Chem.sEur. J. 2003, 9, 4968–4979. (b)
Albinsson, B.; Eng, M. P.; Pettersson, K.; Winters, M. U. Phys. Chem.
Chem. Phys. 2007, 5847–5864. (c) Araki, Y.; Ito, O. J. Photochem.
Photobiol. C 2008, 9, 93–110.
(6) (a) El-Khouly, M. E.; Ito, O.; Smith, P. M.; D’Souza, F. J. Photochem.
Photobiol., C 2004, 5, 79–104. (b) Fukuzumi, S. Phys. Chem. Chem.
Phys. 2008, 10, 2283–2297. (c) D’Souza, F.; Ito, O. Organic
Electronics and Photonics; Nalwa, H. R., Ed.; American Scientific
Publishers: Stevenson Ranch, CA, 2008; Vol. 1, Chap. 13. (d) Ohkubo,
K.; Fukuzumi, S. Bull. Chem. Soc. Jpn. 2009, 82, 303–315.
(7) (a) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guillard,
R., Eds.; Academic Press: San Diego, 2003; Vol. 15. (b) Claessens,
C. G.; Gonza´lez-Rodr´ıguez, D.; Torres, T. Chem. ReV. 2002, 102, 835–
853. (c) Torres, T. Angew. Chem., Int. Ed. 2006, 45, 2834–2837. (d)
Claessens, C. G.; Medina, A. J. Porphyrins Phthalocyanines 2009,
13, 446–454.
(10) (a) Giaimo, J. M.; Gusev, A. V.; Wasielewski, M. R. J. Am. Chem.
Soc. 2002, 124, 8530–8531. (b) Veldman, D.; Chopin, S. M. A.;
Meskers, S. C. J.; Groeneveld, M. M.; Williams, R. M.; Janssen,
R. A. J. J. Phys. Chem. A 2008, 112, 5846–5857. (c) Giaimo, J. M.;
Lockard, J. V.; Sinks, L. E.; Scott, A. M.; Wilson, T. M.; Wasielewski,
M. R. J. Phys. Chem. A 2008, 112, 2322–2330.
(8) (a) Gonza´lez-Rodr´ıguez, D.; Torres, T.; Guldi, D. M.; Rivera, J.;
Echegoyen, L. Org. Lett. 2002, 4, 335–338. (b) Gonza´lez-Rodr´ıguez,
D.; Torres, T.; Guldi, D. M.; Rivera, J.; Herranz, M. A.; Echegoyen,
L. J. Am. Chem. Soc. 2004, 126, 6301–6313. (c) Iglesias, R. S.;
Claessens, C. G.; Torres, T.; Rahman, G. M. A.; Guldi, D. M. Chem.
Commun. 2005, 2113–2115. (d) Iglesias, R. S.; Claessens, C. G.;
Rahman, G. M. A.; Herranz, M. A.; Guldi, D. M.; Torres, T.
Tetrahedron 2007, 63, 12396–12404. (e) Kim, J.-H.; El-Khouly, M. E.;
Araki, Y.; Ito, O.; Kay, K.-Y. Chem. Lett. 2008, 37, 544–545. (f)
Gonza´lez-Rodr´ıguez, D.; Torres, T.; Herranz, M. A.; Echegoyen, L.;
Carbonell, E.; Guldi, D. M. Chem.sEur. J. 2008, 14, 7670–7679.
(9) Gonza´lez-Rodr´ıguez, D.; Carbonell, E.; Guldi, D. M.; Torres, T.
Angew. Chem., Int. Ed. 2009, 48, 8032–8036.
(11) Claessens, C. G.; Torres, T. Tetrahedron Lett. 2000, 41, 6361–6365.
(12) Claessens, C. G.; Gonza´lez-Rodr´ıguez, D.; del Rey, B.; Torres, T.;
Mark, G.; Schuchmann, H.-P.; von Sonntag, C.; MacDonald, J. G.;
Nohr, R. S. Eur. J. Org. Chem. 2003, 2547–2551.
(13) (a) Gonza´lez-Rodr´ıguez, D.; Torres, T. Eur. J. Org. Chem. 2009, 1871–
1879. (b) Gonza´lez-Rodr´ıguez, D.; Mart´ınez-D´ıaz, M. V.; Abel, J.;
Echegoyen, L.; Perl, A.; Huskens, J.; Torres, T. Org. Lett. 2010, 12,
2970–2973.
9
J. AM. CHEM. SOC. VOL. 132, NO. 46, 2010 16489