Probing charge separation in structurally different C60/exTTF ensembles

Article abstract of DOI:10.1021/jo034432e

The scope of the present work is to highlight the effects stemming from different C₆₀/exTTF linkages (exTTF = 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene)-either via an anthracene unit or a dithiole ring. Particular emphasis is place

Full text of DOI:10.1021/jo034432e

P r obin g Ch a r ge Sep a r a tion in Str u ctu r a lly Differ en t C60/exTTF
En sem bles
Marta C. D´ıaz, Ma Angeles Herranz, Beatriz M. Illescas, and Nazario Mart´ın*
Departamento de Qu´ımica Orga´nica, Facultad de Ciencias Qu´ımicas, Universidad Complutense,
E 28040 Madrid, Spain
Nicolas Godbert and Martin R. Bryce*
Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, UK
Chuping Luo, Angela Swartz, Greg Anderson, and Dirk M. Guldi*
Radiation Laboratory, University of Notre Dame, Indiana 46556
nazmar@quim.ucm.es
Received April 4, 2003
The scope of the present work is to highlight the effects stemming from different C60/exTTF linkages
(exTTF ) 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene)seither via an anthracene unit
or a dithiole ring. Particular emphasis is placed on photoinduced electron-transfer features.
Therefore, we devised a new series of C60-exTTF ensembles, synthesized via 1,3-dipolar cycloaddition
and Diels-Alder cycloaddition reactions, in which exTTF units are separated from C60 by two single
bonds (3a -c, 4), one vinylene unit (5a ), or two vinylene units (5b). The cyclic voltammetry reveals
an amphoteric redox behavior with remarkably strong electron-donor ability of the trimethyl-
substituted exTTF moiety in 4 and 5a ,b. Steady-state and time-resolved photolytic techniques show
that the fullerene singlet excited state in (3a -c, 4, and 5a ,b) is subject to a rapid electron-transfer
quenching. The resulting charge-separated states, that is C60•--exTTF•+, were identified by transient
absorption spectroscopy. We determined radical pair lifetimes of the order of 200 ns in benzonitrile.
This suggests (i) that the positive charge of the exTTF•+ is delocalized over the entire donor rather
than localized on one of the 1,3-dithiole rings and (ii) that linking exTTF via the anthracene or
1,3-dithiole ring has no appreciable influence. Increasing the donor-acceptor separation via
implementing one or two vinylene units as spacers led to improved radical pair lifetimes (5a : τ )
725 ns; 5b: τ ) 1465 ns).
In tr od u ction
Tetrathiafulvalene (TTF) and especially p-quinonoid
π-extended tetrathiafulvalenes (exTTFs) are strong elec-
tron donor molecules, which are oxidized at relatively low
anodic potential values to form the respective π-radical
cation (TTF.+) and dication (TTF2+ and exTTF2+) species.
In the past decade, fullerenes emerged as a molecular
tecton ideally suited for devising multicomponent model
systems to transmit and process solar energy. Thus, the
design of novel fullerene derivatives currently constitutes
an active research area toward artificial light harvesting
antenna and reaction center mimics. In these model
systems, the specific composition of electron donors linked
to C60 has been utilized to tune the electronic couplings
between donor and acceptor sites.1 Fullerenes are three-
dimensional electron acceptor modules able to accept up
to six electrons in solution.2 On account of C60’s small
reorganization energy in electron-transfer reactions, it
has exerted a noteworthy impact on the improvement of
light-induced charge-separation.3
All the oxidized species (i.e., TTF.+, TTF2+, and exTTF2+
)
are thermodynamically stable products, a consequence
that stems primarily from the aromatic character of the
generated dithiolium cations.4
Thus, integration of TTFsa donor whose oxidized state
is aromaticsand C60san acceptor that accelerates charge
separation and decelerates charge recombinationsinto
novel molecular donor-acceptor ensembles holds great
expectations to ultimately improve light-induced charge-
separation processes. Up to now, a wide variety of C60-
TTF5 and C60-exTTF6 dyads as well as C60/TTF and C60/
exTTF triads7 were designed and probed as artificial
reaction centers. Most remarkably, the radical pair
lifetimes found, when using TTF and exTTF donor
moieties, turned out to be some of the longest values
* To whom correspondence should be addressed. Fax: +34-
913944103. Phone: +34-913944227.
(1) (a) Guldi, D. M., Mart´ın, N., Eds. Fullerenes: From Synthesis to
Optoelectronic Properties; Kluwer Academic Publishers: Dordrecht,
2002. (b) Prato, M., Mart´ın, N., Eds. Special issue on Functionalized
Fullerene Materials. J . Mater. Chem. 2002, 12, 1931-2159.
(2) Echegoyen, L.; Echegoyen, L. E. Acc. Chem. Res. 1998, 31, 593.
(3) Guldi, D. M. Chem. Commun. 2000, 321.
(4) (a) Mart´ın, N.; Sa´nchez, L.; Seoane, C.; Ort´ı, E.; Viruela, P. M.;
Viruela, R. J . Org. Chem. 1998, 63, 1268. (b) Mart´ın, N.; Sa´nchez, L.;
Seoane, C.; Ferna´ndez, C. Synth. Met. 1996, 78, 137.
10.1021/jo034432e CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/10/2003
J . Org. Chem. 2003, 68, 7711-7721
7711

D´ıaz et al.
CHART 1
(C60-exTTF-exTTF: τ ∼ 100 µs) ever reported for in-
tramolecular charge-recombination in donor-acceptor
systems that are comparable in composition, donor-
acceptor separation, etc.8 Very recently, even a trannu-
to the increase in aromaticity and planarity that exTTFs
undergo upon oxidation of the buckled neutral state,
exTTF derivatives reveal a much stronger propensity
for stabilizing charge-separated radical ion pairs
(C60-exTTF: τ ∼ 200 ns), relative to the parent TTF
derivatives (C60-TTF: τ ∼ 2 ns), which transform into
aromatic compounds upon oxidation, with little change
in conformation.
lene, that is, a highly fluorinated fullerene (C60F18
)
derivative, has been used in the design of a novel
multicomponent donor-acceptor arraysC60F15-exTTFs
which exhibits a 870 ns charge-separated state.9 Owing
The simplest C60-exTTF dyad (2) is depicted in Chart
1, in which the closest 1,3-dithiole ring of the exTTF
(5) (a) Mart´ın, N.; Sa´nchez, L.; Seoane, C.; Andreu, R.; Gar´ın, J .;
Orduna, J . Tetrahedron Lett. 1996, 37, 5979. (b) Mart´ın, N.; Sa´nchez,
L.; Herranz, M. A.; Guldi, D. M. J . Phys. Chem. A 2000, 104, 4648. (c)
Guldi, D. M.; Gonza´lez, S.; Mart´ın, N.; Anto´n, A.; Gar´ın, J .; Orduna,
J . J . Org. Chem. 2000, 65, 1978. (d) Mart´ın, N.; Sa´nchez, L.; Seoane,
C.; Andreu, R.; Gar´ın, J .; Orduna, J .; Ort´ı, E.; Viruela, P. M.; Viruela,
R. J . Phys. Chem. Solids 1997, 58, 1713. (e) Mart´ın, N.; Sa´nchez, L.;
Illescas, B.; Gonza´lez, S.; Herranz, M. A.; Guldi, D. M. Carbon 2000,
38, 1577. (f) Kreher, D.; Cariou, M.; Liu, S.-G.; Levillain, E.; Veciana,
J .; Rovira, C.; Gorgues, A.; Hudhomme, P. J . Mater. Chem. 2002, 12,
2137 and references therein.
(6) (a) Mart´ın, N.; Pe´rez, I.; Sa´nchez, L.; Seoane, C. J . Org. Chem.
1997, 62, 5690. (b) Mart´ın, N.; Sa´nchez, L.; Guldi, D. M. Chem.
Commun. 2000, 113. (c) Herranz, M. A.; Mart´ın, N.; Sa´nchez, L.;
Seoane, C.; Guldi, D. M. J . Organomet. Chem. 2000, 599, 2. (d)
Gonza´lez, S.; Mart´ın, N.; Swartz, A.; Guldi, D. M. Org. Lett. 2003, 5,
557.
(7) (a) Sa´nchez, L.; Pe´rez, I.; Mart´ın, N.; Guldi, D. M. Chem. Eur.
J . 2003, 9, 2457. (b) Herranz, M. A.; Illescas, B.; Mart´ın, N.; Luo, Ch.;
Guldi, D. M. J . Org. Chem. 2000, 65, 5728. (c) Segura, J . L.; Priego, E.
M.; Mart´ın, N. Tetrahedron Lett. 2000, 41, 7737. (d) Segura, J . L.;
Priego, E. M.; Mart´ın, N.; Luo, Ch.; Guldi, D. M. Org. Lett. 2000, 2,
4121. (e) Gonza´lez, S.; Herranz, M. A.; Illescas, B.; Segura, J . L.;
Mart´ın, N. Synth. Met. 2001, 121, 1131. (f) Gonza´lez, S.; Mart´ın, N.;
Guldi, D. M. J . Org. Chem. 2003, 68, 779. (g) Kodis, G.; Liddell, P. A.;
de la Garza, L.; Moore, A. L.; Moore, Th. A.; Gust, D. J . Mater. Chem.
2002, 12, 2100.
(8) (a) Imahori, H.; Sakata, Y. Adv. Mater. 1997, 9, 537. (b) Mart´ın,
N.; Sa´nchez, L.; Illescas, B.; Pe´rez, I. Chem. Rev. 1998, 98, 2527. (c)
Imahori, H.; Sakata, Y. Eur. J . Org. Chem. 1999, 2445, 5. (d) Guldi,
D. M.; Maggini, M.; Mart´ın, N.; Prato, M. Carbon 2000, 38, 1615.
7712 J . Org. Chem., Vol. 68, No. 20, 2003

Probing Charge Separation in C60/ exTTF Ensembles
SCHEME 1 a
a
t
Reagents and conditions: (a) py, AcOH, 70 °C; (b) BuPh2SiCl/DMF; (c) LDA/THF, -78 °C; (d) Bu4NF/THF; (e) PBr3/CCl4/0 °C; (f)
NaI, 18C6; (g) C60/toluene, ∆.
moiety is separated by six bonds from C60. A contrasting
pattern is created in the structure of dyad 1: now only
two bonds are placed between the nearest dithiole ring
and C60. This topological issue points to a general and
sometimes controversially handled question: is the gain
of donor aromaticity accountable for the long radical pair
lifetimes in dyads of type 2 or, alternatively, is this effect
due to larger donor (dithiole ring)-acceptor (C60) separa-
tions?
The scope of the current work is to shed light onto this
question through molecular tailoring of C60-extendedTTF
dyads, such as the newly developed series of (3a -c, 4),
in which the 1,3-dithiole ring is connected through two
single bonds, similarly to the scenario in dyad 1 and
probe them in photophysical experiments. Our strategy
involved two basically different connectivities, that is,
Diels-Alder cycloaddition and 1,3-dipolar cycloaddition
of azomethine ylides to C60. This allowed a comparison
of the effect that the connection through one (4) or two
(3a -c) connectivities has on the electron-transfer proper-
ties. In addition, we have carried out the synthesis of C60-
exTTF dyads (5a ,b) in which the distance between the
dithiole ring and the C60 unit has been systematically
increased by adding vinylene units of different length.
Resu lts a n d Discu ssion
Syn th esis. The preparation of dyads 3a -c10 was
carried out in a multistep synthetic procedure from
commercially available anthrone (6). First, reaction of 6
with dithiolium iodide (7)11 produced compound 8 in 54%
yield. Forming the bis-hydroxymethyl-substituted
exTTFs 12a -c required protection of the hydroxy groups
in 8, which was done with tert-butyldiphenylsilyl chloride
in 81% yield. Subsequent Wittig-Horner reaction of 9
with the anion of phosphonate esters 10a -c12sgenerated
in basic mediumsled to exTTFs 11a -c. Finally, exTTFs
11a -c were reacted with tetrabutylammonium fluoride
to give bis-hydroxymethyl exTTFs 12a -c in 58-93%
yields (Scheme 1).
The precursors to dyads 3a -csdibromomethyl exTTFs
(13a -c)swere prepared from 12a -c by treating them
with phosphorus tribromide in THF/CCl4 at 0 °C. Since
13a -c undergo iodide-induced 1,4-dehydrohalogenation
in the presence of 18-crown-6 to afford “in situ” generated
o-quinodimethane heteroanalogues,13 we trapped them
with dienophilic C60. Dyads 3a -c were obtained as
(10) A preliminary communication reporting dyad 3b has been
previously published. Herranz, M. A.; Mart´ın, N. Org. Lett. 1999, 2005.
(11) Fox, M. A.; Pan, H.-L. J . Org. Chem. 1994, 59, 6519.
(12) (a) Moore, A. J .; Bryce, M. R. J . Chem. Soc., Perkin Trans. 1
1991, 157. (b) Bryce, M. R.; Moore, A. J . Synthesis 1991, 26. (c) Parg,
R. P.; Kilburn, J . D.; Ryan, T. G. Synthesis 1994, 195 and references
therein.
(9) Burley, G. A.; Avent, A. G.; Boltalina, O. V.; Goldt, I. V.; Guldi,
D. M.; Marcaccio, M.; Paolucci, F.; Paolucci, D.; Taylor, R. Chem.
Commun. 2003, 148.
J . Org. Chem, Vol. 68, No. 20, 2003 7713

D´ıaz et al.
TABLE 1. Red ox P oten cia l Va lu es ((0.01 V) of Novel
Common to the electrochemical features of all the
dyads is the presence of three quasireversible reduction
waves, which correspond to the first three reduction steps
of the fullerene moiety. As expected, these reduction
values are cathodically shifted in comparison with the
parent C60, a consequence that stems from saturating a
double bond of the C60 core, which raises the LUMO
energy. Interestingly, dyads 4 and 5a ,b bearing the
trimethyl substituted exTTF donor showed an additional
cathodic shift (40-60 mV) of the first reduction potential
in comparison to 3a -c. A possible rationale lies in the
electron donor ability of the trimethyl-substituted exTTF
unit in 4 and 5a ,b, which is much stronger than those of
the exTTF units in dyads 3a -c. This finding points to
electronic interactions between the electroactive units in
the ground state, rendering the reduction process in
dyads 4 and 5a ,b slightly more difficult.
For the oxidation process, all dyads showed a quasi-
reversible oxidation wave involving a two-electron process
to form the dication species.6,17 The trimethyl substituted
exTTFs in dyads 4 and 5a ,b showed oxidation potentials
at values that are positively shifted by 180-280 mV
relative to the exTTF units in dyads 3a -c.18 The re-
reduction process of the exTTF dications back to the
neutral molecules is observed in all cases, and occurs with
a separation of around 300-600 mV relative to the anodic
oxidation peak. The large potential gap between oxidation
and rereduction is indicative of the difficulties that a
planar and aromatic dication species experiences by
forming a highly distorted neutral molecule.
Dya d s a n d Rela ted Com p ou n d s (V vs SCE)
compd
E1
E1
E1
E2
E3
red
ox (p a)
ox (p c)
red
red
a
C60
-0.60
-0.67
-0.66
-0.69
-0.73
-0.71
-0.70
-1.07
-1.12
-1.17
-1.09
-1.12
-1.10
-1.09
-1.64
-1.72
-1.77
-1.67
-1.67
-1.62
-1.63
3a a
0.58
0.68
0.66
0.40
0.44
0.44
0.45
0.53
0.57
0.57
0.51
0.45
0.40
-0.04
-0.05
0.12
0.12
0.13
3ba
3ca
4b
5a b
5bb
0.10
exTTF (18)c
12a c
12bc
12cc
15c
0.02
0.16
0.14
0.24
-0.22
-0.13
-1.76
-1.48
-1.36
16c
17c
a
Experimental conditions: Tol/CH3CN 4:1 as solvent, GCE as
working electrode, Bu4NClO4 (0.1 M) as supporting electrolyte;
b
100 mV s-1
.
In ODCB/CH3CN 4:1. c In CH2Cl2.
yellow-brown solids in 16-34% yields (51-77% based on
recovered C60).
The electronic spectra of dyads 3a -c are dominated,
in addition to the typical fullerene bands in the UV
region, by absorption bands in the 420-450 nm region,
which correspond to the visible transitions of the exTTF
moieties. These bands mask, however, the 430 nm
absorption band of dihydrofullerenes. In the 13C NMR,
the quaternary sp3-hybridized carbons, where the organic
addends are attached to the C60 sphere, are at around
66 ppm. Furthermore, the number of 13C NMR lines
observed for C60 derivatives 3a -c is consistent with a
C2v symmetry. The structures were also confirmed by
MALDI-TOF mass spectrometry with molecular ions at
m/z 1126 (3a ), 1218 (3b), and 1216 (3c).
Dyads 4 and 5a ,b, on the other hand, were obtained
by 1,3-dipolar cycloaddition involving the respective
formyl-substituted exTTFs (15-17) and C60 in the pres-
ence of N-substituted glycine.14 The formyl-containing
exTTFs (16, 17) were prepared by Wittig-Horner reac-
tion from formyl-exTTF (15)sa product of a previously
reported multistep synthetic procedure15sand phospho-
ranylideneacetaldehyde to form 16 (81% yield). After a
follow-up reaction of 16 with a second molecule of
phosphorane, we isolated exTTF 17 in 55% yield. Further
reaction of aldehydes 15-17 with N-(3,6,9-trioxadecyl)-
glycine16 in refluxing chlorobenzene led to the respective
azomethine ylides. In the final step, the in situ generated
azomethine ylides undergo 1,3-dipolar cycloaddition to
C60 giving dyads 4 and 5a ,b in moderate yields (24-63%)
as highly soluble yellow-brown solids (Scheme 2).
Electr och em istr y. The redox properties of the novel
dyads were studied by cyclic voltammetry at room
temperature in a 4:1 toluene/acetonitrile solvent mixture.
The data are shown in Table 1 along with those of some
of the intermediates (12a -c and 15-17), C60, and the
parent π-extended 9,10-bis(1,3-dithiol-2-ylidene)-9,10-
dihydroanthracene (18).17
The redox potentials of intermediate donors 12a -c and
15-17 have also been included in Table 1. Compounds
12a -c showed slightly better electron-donor ability than
that observed in dyads 3a -c, which can be accounted
for by the electronic effect of the two hydroxymethyl
groups as substituents on the 1,3-dithiole ring.
An interesting trend was seen in compounds 15-17
since the proximity of the conjugated formyl group exerts
a strong impact on the oxidation of the 1,3-dithiole rings.
In particular, compound 15, in which the carbonyl group
is directly linked to the dithiole ring shows a more
positive oxidation potential value than 16 (∆Eox ) 60 mV)
bearing a vinylene spacer, and 17 (∆Eox ) 110 mV)
bearing two vinylene units as spacer between the formyl
and exTTF moieties (Table 1). Compounds 15-17 present
an additional reduction wave at negative values corre-
sponding to the reduction of the carbonyl group which is
also influenced by the distance to the 1,3-dithiole ring.
P h otop h ysics. The fullerene fluorescence, although
principally weak in nature (i.e., ∼6.0 × 10-4 for most
investigated monoadducts), has been proven to be a
sensitive and useful marker for probing excited-state
interactions, as they may prevail in C60-based donor-
acceptor systems.19 For instance, connecting an electron
donor, such as exTTF, to C60 leads to ensembles in which
the fullerene still acts as the primary photosensitizer in
(17) (a) Yamashita, Y.; Kobayashi, Y.; Miyashi, T. Angew. Chem.,
Int. Ed. Engl. 1989, 28, 1052. (b) Bryce, M. R.; Moore, A. J .; Hasan,
M.; Ashwell, G. J .; Fraser, A. T.; Clegg, W.; Hurshouse, M. B.;
Karaulov, A. I. Angew. Chem., Int. Ed. Engl. 1990, 29, 1450.
(18) (a) Farges, J .-P., Ed. Organic Conductors. Fundamentals and
Applications; Marcel Dekker: New York, 1994. (b) For a recent review
on TTFs, see: Segura, J . L.; Mart´ın, N. Angew. Chem., Int. Ed. 2001,
40, 1372.
(13) For a review of o-quinodimethanes, see: Segura, J . L.; Mart´ın,
N. Chem. Rev. 1999, 99, 3199.
(14) Prato, M.; Maggini, M. Acc. Chem. Res. 1998, 31, 519.
(15) Bryce, M. R.; Finn, T.; Moore, A. J .; Batsanov, A. S.; Howard,
J . A. K. Eur. J . Org. Chem. 2000, 51.
(16) Da Ros, T.; Prato, M.; Novello, F.; Maggini, M.; Banfi, E. J .
Org. Chem. 1996, 61, 9070.
(19) Guldi, D. M.; Prato, M. Acc. Chem. Res. 2000, 33, 695.
7714 J . Org. Chem., Vol. 68, No. 20, 2003

Probing Charge Separation in C60/ exTTF Ensembles
a
SCHEME 2
a
Reagents and conditions: (a) Ph3PdCH-CHO, toluene, ∆; (b) C60, CH3(OCH2CH2)3NHCH2CO2H, ClPh, ∆.
the 320-360 nm range. Taking the fullerene’s suscepti-
bility toward electrons, especially that of the singlet
excited state, and the strong donor properties of exTTF
into concert, charge separation is expected to dominate
the deactivation of the photoexcited fullerene. One im-
portant aspect of the current investigation is to explore
the consequence of different functionalization methodolo-
gies of the fullerene core, that is, the specific linkage of
the exTTF, on the electron-transfer dynamics.
In an effort to characterize the photophysics of C60-
exTTF dyads (in 3a -c, 4, and 5a ,b) a series of time-
resolved and steady-state experiments were conducted
in different solvents, ranging from toluene (ꢀ ) 2.38) to
benzonitrile (ꢀ ) 24.8). In particular, the steady-state
fluorescence spectra of 3a -c, 4, and 5a ,b in toluene and
benzonitrile were probed as a first, crude measure to
indicate excited state interactions. The reference, namely
N-methylpyrrolidino[3′,4′:1,2][60]fullerene itself shows
only a weak solvent dependence. Specifically, the *0-0
transition shifts progressively from 713 nm (toluene) to
719 nm (benzonitrile), while the emission intensity
remains unaffected.
The picture is, however, fundamentally different for
the currently investigated processes in 3a -c, 4, and
5a ,bssee Figure 1. Their fluorescence is subject to a
strong quenching relative to the corresponding fullerene
reference, which is in line with our previous studies on
a series of exTTF moieties linked either to a methano-
fullerene (i.e., [6-6]-closed and [6-5]-open)6b or a fullero-
pyrrolidine.7a Importantly, the pattern of the fullerene
fluorescence with strong *0-0 maxima round 715 nm is
fully preserved, despite the overall quenching. A closer
inspection of the fluorescence quantum yields (see Tables
2 and 3) reveals several trends in 3a -c and 4, 5a ,b
J . Org. Chem, Vol. 68, No. 20, 2003 7715

D´ıaz et al.
TABLE 2. P h otop h ysica l a n d Th er m od yn a m ic P r op er ties of Dya d s 3a -c
solvent
3a
3b
3c
-∆GCS
°
tolueneb
bzcnb
0.51 eV
0.58 eV
1.25 eV
1.18 eV
0.68 × 10-4
0.25 × 10-4
8.8
0.42 eV
0.49 eV
1.34 eV
1.27 eV
1.24 × 10-4
0.41 × 10-4
4.8
0.41 eV
0.48 eV
1.35
-∆GCR
°
toluenea
bzcnb
toluene
bzcn
toluene
bzcn
toluene
bzcn
1.28
fluorescence quantum yield (Φ)
fluorescence quenching I0(ref)/I(dyad)
lifetime (τ) C60 singlet excited state
lifetime (τ) radical pair
1.02 × 10-4
0.32 × 10-4
5.9
24
14.6
18.8
0.21 ns
0.07 ns
13 ns
0.31 ns
0.12 ns
20 ns
0.24 ns
0.11 ns
12 ns
toluene
bzcn
174 ns
172 ns
167 ns
a
b
Sum of oxidation and reduction potentials: see Table 1. Determined by eqs 1-3.
substitution on the exTTF donor (3a , 4) yielding fluores-
cence quantum yields as low as 0.1 × 10-4. On the
contrary the highest values, namely, 1.24 × 10-4 and 1.46
× 10-4 were found in toluene for the -S-CH3 substitu-
tion (3b) and the donor-acceptor dyad with two vinylene
units (5b), respectively. A rationalization for these quali-
tative values infers that the fullerene singlet excited
state, as the predominantly populated state, is readily
deactivated by an intramolecular electron-transfer event.
In the current dyads, it is expected that this evolves from
a reaction between the singlet excited state (1.76 eV) of
the electron accepting fullerene moiety and the exTTF
donor and yields C60•--exTTF•+
.
Probably the most important observation is that the
chemical coupling between the donor and acceptor leads
to insignificant effects on the quenching at least in the
fulleropyrrolidine,7a Diels-Alder adducts,21 and 3a -c.
This can be rationalized by reference to the fullerene
reduction potentials, which remain virtually unchanged
in all these C60 derivatives. A slightly different outline
is deduced once we consider the better electron donor
features in 4 and 5a ,b, for which the oxidation potential
of the donor is around 0.4 eV, appreciably less anodic
than those found in 3a -c (∼0.6 eV). The donor strength
is known to be one of the central parameters in modulat-
ing the energy of charge-separated states and, thereby,
impacting the free energy changes for an intramolecular
electron transfer. To substantiate this energetic consid-
eration we estimated the driving forces (-∆GCR° (eV)) (eq
1) for the intramolecular charge-recombination (CR)
processes via22
F IGURE 1. Emission spectra of dyads 3a -c and a N-meth-
ylpyrrolidino[3′,4′:1,2][60]fullerene as reference in (a) toluene
and (b) in benzonitrile with matching absorption at the 337
nm excitation wavelength (i.e., OD340 nm ) 0.2).
-∆GCR° ) E1/2(D•+/D) - E1/2(A/A•-) + ∆GS (1)
where E°1/2(D•+/D) is the first one-electron oxidation
potential of the exTTF donor moiety, while E°1/2(A/A•-)
refers to the first one-electron reduction potential of the
C60 electron acceptor. In the next step, we elucidate the
role of the solvent (∆GS)23 on the relative energy of the
regarding the magnitude of fluorescence quenching. First,
in 3a -c and 4 the quenching correlates well with the
oxidation strength of the exTTF donor. Second, in 3a -c
and 4, 5a ,b the quenching increases progressively with
the solvent polarity. Third, increasing the donor-acceptor
separation from 4 to 5a and 5b led to a reactivation of
the fullerene fluorescence. These findings are in agree-
ment with those previously reported for related struc-
tures bearing a ferrocene moiety connected to the C60 core
through different vinylene units.20 Extreme scenarios in
each series are benzonitrile as a solvent and H- or CH3-
(21) Herranz, M. A.; Mart´ın, N.; Ramey, J .; Guldi, D. M. Chem.
Commun. 2002, 2968.
(22) (a) Weller, A. Z. Phys. Chem. (Wiesbaden) 1982, 93, 1982. (b)
Imahori, H.; Hagiwara, K.; Aoki, M.; Akiyama, T.; Taniguchi, S.;
Okada, T.; Shirakawa, M.; Sakata, Y. J . Am. Chem. Soc. 1996, 118,
11771. (c) van Dijk, S. I.; Groen, C. P.; Hartl, F.; Brouwer, A.;
Verhoeven J . W. J . Am. Chem. Soc. 1996, 118, 8425.
(23) Determined from the following relations: R+ ) radius donor;
RD-A ) donor-acceptor separation; R- ) radius acceptor (4.4 Å); ꢀS
solvent dielectric constant (toluene ) 2.39; benzonitrile ) 24.8); ꢀR
)
)
(20) Guldi, D. M.; Maggini, M.; Scorrano, G.; Prato, M. J . Am. Chem.
Soc. 1997, 119, 974.
solvent dielectric constant for electrochemical measurements (9.412).
7716 J . Org. Chem., Vol. 68, No. 20, 2003

Probing Charge Separation in C60/ exTTF Ensembles
TABLE 3. P h otop h ysica l a n d Th er m od yn a m ic P r op er ties of Dya d s 4, 5a ,b
solvent
4
5a
5b
-∆GCS
°
tolueneb
bzcnb
0.63 eV
0.7 eV
1.13 eV
1.06 eV
0.16 × 10-4
0.1 × 10-4
37.5
0.61 eV
0.7 eV
0.62 eV
0.73 eV
1.14 eV
1.03 eV
1.46 × 10-4
0.65 × 10-4
4.1
-∆GCR
°
toluenea
1.15 eV
1.06 eV
0.62 × 10-4
0.1 × 10-4
9.7
60
0.35 ns
0.14 ns
bzcnb
toluene
bzcn
toluene
bzcn
toluene
bzcn
fluorescence quantum yield (Φ)
fluorescence quenching I0/I
lifetime (τ) C60 singlet excited state
lifetime (τ) radical pair
60
9.2
0.19 ns
0.08 ns
21 ns
0.51 ns
0.23 ns
189 ns
toluene
bzcn
180 ns
725 ns
1465 ns
a
b
Sum of oxidation and reduction potentials; see Table 1. Determined by eqs 1-3.
charge-separated state by referring to the “dielectric
continuum model” (eq 2):
e2
1
1
1
1
∆GS )
+
-
-
(
[
)
4πꢀ0 2R+ 2R- RD-A ꢀS
1
1
1
+
(2)
(
)
]
2R+ 2R- ꢀR
Furthermore, the driving forces (-∆GCS° (eV)) for the
intramolecular charge-separation (CS) processes were
determined by eq 3:
-∆GCS° ) ∆E0-0 - (-∆GCR
)
(3)
F IGURE 2. Picosecond transient absorption spectrum (vis-
ible-near-infrared part) recorded 50 ps (dashed line) and 5000
ps (solid line) upon flash photolysis of dyad 4a (5.0 × 10-5 M)
at 355 nm in deoxygenated benzonitrile, indicating the fullerene
singlet-singlet (λmax at 880 nm).
Hereby, ∆E0-0 is the energy of the 0-0 transition
energy gap between the lowest excited state and the
ground state (1.76 eV). The accordingly calculated -∆GCS
°
and -∆GCR° values are given in Tables 2 and 3. Clearly,
the better electron donor in 4 and 5a ,b increases the
energy gap between the excited state of the fullerene and
the charge-separated state by about 0.2 eV relative to
3a -c.
Comparable trends were noted in the picosecond
transient absorption studies. These experiments were
conducted to complement the emission experiments and,
more fundamentally, to examine the deactivation dy-
namics of the photoexcited fullerene core. In addition, this
technique enabled us not only to characterize the initially
generated state, which is expected to be the fullerene
singlet excited state, but also the resulting photoproducts.
Thus, to probe the singlet and triplet excited-state
behavior of the C60-exTTF dyads, time-resolved dif-
ferential absorption spectra were recorded immediately
following pico- and nanosecond laser excitation. The
pump wavelength was alternatively 355 or 337 nm.
Specifically, 355 nm illumination of dyads in toluene
leads to the instantaneous formation of a new transient
absorption, which maximizes around 900 nm. In line with
the above assumption, we assign the instantaneously
formed transient to the fullerene singlet excited state;
see Figure 2.19 This singlet-singlet absorption decays
rapidly with time constants that depend, similar to the
fluorescence quenching, on the donor strength (i.e.,
compare 3a and 3b), the solvent polarity, and the donor-
acceptor separation (i.e., compare 4 and 5b). For example,
a qualitatively similar picture was noted in polar sol-
vents, such as benzonitrile, with the exception that the
singlet excited state deactivation is significantly acceler-
F IGURE 3. Transient absorption spectrum (visible-near-
infrared part) recorded 50 ns upon flash photolysis of dyad 4a
(2.0 × 10-5 M) at 337 nm in deoxygenated benzonitrile,
indicating the charge-separated radical pair features (λmax at
680 and 1000 nm).
ated relative to nonpolar toluene. Typically, they range
between 500 and 50 ps and, thus, corroborate qualita-
tively the trend seen in the emission experiments.
At this point, it is worth noting that all the fullerene
references, such as methanofullerene, pyrrolidino-
fullerene, and triazolinofullerene, disclose after the comple-
tion of the initial excitation process a slow, spin-forbidden
intersystem crossing affording the energetically lower
lying triplet excited state.19 The absorption decays in the
J . Org. Chem, Vol. 68, No. 20, 2003 7717

D´ıaz et al.
900 nm region, while a simultaneous grow-in of a new
absorption dominates the absorption changes in the 700
nm region, corresponding to the fullerene triplet-triplet
absorption. Importantly, none of these new dyads show
any appreciable triplet absorption (in the range between
690 and 740 nm).19 In contrast to the fast rates, seen for
the C60-based donor-acceptor systems, the intersystem
crossing processes in these references take place on a
time scale of 1.8 ns. These quantitatively formed triplet
states are characterized by sets of transient absorption
bands with maxima around 360 and 700 nm. A clean
monoexponential recovery of the singlet ground state
follows at low fullerene concentration and low laser
power, affording a triplet lifetime of nearly 100 µs.
Employing, however, higher fullerene concentration and/
or higher laser power, the kinetics become more compli-
cated: they are impacted primarily by efficient (i) triplet-
triplet and (ii) triplet-ground-state annihilation processes.
Although substantial, these annihilation processes had,
within the applied concentration range (e5.0 × 10-5 M),
no significant effect on the fullerene singlet lifetime.
Evidence for the electron-transfer came from time-
resolved photolysis studies, namely, as a consequence of
the fast singlet decay, the grow-in of a new transient
developed in the monitored wavelength range (i.e., 450-
960 nm) with maxima at 660 nm (Figure 3).24
The product of electron transfer, namely, the charge-
separated radical pair, was unequivocally confirmed by
the spectral fingerprints of the one-electron oxidized
electron donor-π-radical cation of the exTTF moiety,
exTTF•+, and the one electron reduced electron acceptor-
fullerene π-radical anion, C60•-. The strongest absorption
of the exTTF•+ moiety shifted in accordance with the
substitution pattern of the donor and covered a range
between 650 and 680 nm. Similarly, the absorption of
C60•- lies at around 1000 nm. The exact location of these
absorption bands was independently confirmed in a set
of complementary pulse radiolysis experiments focusing
on the radical-induced oxidation and reduction of the
exTTF-donor and fullerene-acceptor, respectively.
units exerts a profound impact with lifetimes as long as
∼1500 ns.
Con clu sion s
We have carried out the synthesis of two new series of
C60-exTTF dyads in which the exTTF donor is connected
through one (4, 5a ,b) or two (3a -c) single bonds by using
the 1,3-dipolar cycloaddition reaction of in situ generated
azomethine ylides and the Diels-Alder cycloaddition
reaction, respectively.
The electrochemical data show the stronger electron
donor ability of the trimethyl substituted exTTF in
compounds 4, 5a ,b in comparison with 3a -c and suggest
an electronic interaction between the electroactive units
in the ground state. The amphoteric redox behavior of
these dyads is confirmed by the reduction waves associ-
ated with the fullerene unit which are cathodically
shifted in comparison with the parent [60]fullerene.
Fluorescence and time-resolved and steady-state ex-
periments in different solvents were carried out to
determine the photophysical behavior of the new com-
pounds (3a -c, 4, 5a ,b). In all cases, the fullerene singlet
excited state deactivated rather rapidly via an intra-
molecular electron transfer process, affording the charge-
separated radical pair. The latter which was unequivo-
cally confirmed by the spectral fingerprints of C60•- (1000
nm) and exTTF•+ (650-680 nm).
One of the most important conclusions is that the slow
recombination kinetics are only marginally effected by
the type of functionalization of the C60 moiety and,
therefore, the lifetimes determined for the charge sepa-
rated state are quite similar (∼200 ns in benzonitrile) in
structurally analogous fulleropyrrolidines and Diels-
Alder cycloadducts. These findings confirm that the
positive charge of the formed exTTF radical cation is
delocalized over the whole donor moiety rather than on
one of the two dithiole rings. As a result, the radical pair
lifetimes are not significantly influenced by the an-
thracene or dithiole linkage to the C60 unit.
The strong absorptions of the one-electron reduced
fullerenes (C60•-) and of the one-electron oxidized exTTF
(exTTF•+) allow an accurate determination of the lifetime
of the radical pair; see Tables 2 and 3. An important
conclusion in this context is the fact that the charge
recombination kinetics are only marginally effected by
the type of functionalization, which, in turn, determines
the distance separating the donor from the acceptor.
Compare, for example, the stabilization of the charge-
separated in the pyrrolidinofullerene (∼200 ns in
benzonitrile)7a and the Diels-Alder adducts (∼200 ns in
benzonitrile).21 These data also confirm the hypothesis
that the positive charge, evolving from the intramolecular
electron transfer, is delocalized over the entire donor,
rather than stabilized on one of the dithiole rings. As a
consequence, linking the donor via the anthracene or
alternatively via the dithiole moiety has no profound
effects on the lifetime of the radical pair. The radical pair
lifetime changes, however, substantially when increasing
the donor-acceptor distances. In particular, separating
the donor in the 4, 5a ,b series by one or two vinylene
Exp er im en ta l Section
Gen er a l P r oced u r es. 4,5-Bis(hydroxymethyl)-2-(meth-
ylthio)-1,3-dithiolium iodide 711 and phosphonate esters (10a -
c)12 were obtained by previously reported procedures. Aldehyde
15 was obtained by a route different from that reported
previously.15 The corresponding hydroxymethyl derivative25
was treated with selenium dioxide in refluxing THF-dioxane
for 15 h, followed by column chromatography on silica with
EtOAc/hexane (1:1) as eluent to afford 9 in 51% yield.
9-[4,5-Bis(h yd r oxym eth yl)-1,3-d ith iol-2-ylid en e]-9,10-
d ih yd r oa n th r a cen -10-on e (8). To a stirred solution of an-
throne 6 (190 mg, 0.98 mmol) in pyridine/acetic acid (44 mL,
3:1) was added 4,5-bis(hydroxymethyl)-2-(methylthio)-1,3-
dithiolium iodide (7) (340 mg, 1.01 mmol). The resulting deep
red solution was heated at 60-70 °C for 16 h. The solvent was
then removed under vacuum and the residue extracted with
ethyl acetate (2 × 100 mL). The combined organic extracts
were washed with water (2 × 100 mL) and aqueous sodium
bicarbonate 1 M (2 × 50 mL). After drying over MgSO4, the
solvent was removed under reduced pressure to afford a red
solid, which was further purified by column chromatography
on silica gel using hexane: ethyl acetate (1:1) to remove the
(24) Guldi, D. M.; Sa´nchez, L.; Mart´ın, N. J . Phys. Chem. B 2001,
105, 7139.
(25) Godbert, N.; Bryce, M. R.; Dahaoui, S.; Batsanov, A. S.; Howard,
J . A. K.; Hazendonk, P. Eur. J . Org. Chem. 2001, 749.
7718 J . Org. Chem., Vol. 68, No. 20, 2003

Probing Charge Separation in C60/ exTTF Ensembles
unreacted anthrone, followed by ethyl acetate to get compound
8. Additionally, recrystallization from ethyl acetate afforded
8 as thin red needles: 54% yield; mp 261-263 °C (lit. 245-
247 °C dec);26 1H NMR (DMSO-d6, 200 MHz) δ 8.13 (d, 2H, J
) 7.8 Hz), 7.94 (d, 2H, J ) 7.8 Hz), 7.78 (t, 2H, J ) 7.8 Hz),
7.50 (t, 2H, J ) 7.8 Hz), 5.59 (t, 2H, J ) 5.9 Hz), 4.35 (d, 4H,
J ) 5.9 Hz); 13C NMR (DMSO-d6, 50 MHz) δ 182.0, 156.4,
142.5, 138.3, 132.2, 130.5, 129.4, 126.4, 126.3, 115.5, 56.7; FTIR
(KBr) 3217, 2852, 1618, 1587, 1488, 1460, 1437, 1398, 1302,
1244, 1180, 1057, 1034, 1018, 1001, 905, 688 cm-1; UV-vis
(CH2Cl2) λmax 472, 366, 274, 254 nm; MS (EI) m/z 354 (M+).
Anal. Calcd for C19H14O3S2: C, 64.40; H, 3.99; S, 18.10.
Found: C, 64.30; H, 4.33; S, 18.90.
9-[4,5-Bis(ter t-bu tyldiph en ylsilyloxym eth yl)-1,3-dith iol-
2-ylid en e]-9,10-d ih yd r oa n th r a cen e (9). To a solution of 8
(300 mg, 0.85 mmol) in dry DMF (10 mL) was added tert-
butylchlorodiphenylsilane (0.65 mL, 2.61 mmol) followed by
imidazole (1.23 g, 18.1 mmol) and the reaction stirred under
argon for 16 h at 20 °C. The crude reaction product was
dissolved in ethyl acetate, washed with water, and dried over
MgSO4. After removal of the solvent in vacuo, column chro-
matography on silica gel with hexane/dichloromethane (2:1)
as eluent afforded compound 9: 81% yield; mp 118-120 °C
dec (lit.26 mp 128-129 °C); 1H NMR (CDCl3, 300 MHz) δ 8.32
(d, 2H, J ) 7.8 Hz), 7.94 (d, 2H, J ) 8.0 Hz), 7.66 (t, 2H, J )
7.8 Hz), 7.60-7.57 (m, 8H), 7.47-7.26 (m, 14H), 4.20 (s, 4H),
1.02 (s, 18H); 13C NMR (CDCl3, 75 MHz) δ 183.6, 151.8, 147.0,
143.8, 139.2, 135.5, 132.4, 131.5, 130.5, 129.9, 129.2, 127.8,
127.0, 126.3, 126.1, 116.7, 59.0, 26.7, 19.2; FTIR (KBr) 2928,
2855, 1961, 1896, 1828, 1643, 1589, 1567, 1486, 1361, 1262,
1113, 940, 821, 771, 609 cm-1; UV-vis (CH2Cl2) λmax 490, 370,
270, 254 nm; MS (FAB+) m/z 832 (M+). Anal. Calcd for
4.02 (d, 2H, AB, J AB ) 13.2 Hz), 2.31 (s, 6H), 0.92 (s, 18H);
13C NMR (CDCl3, 50 MHz) δ 135.5, 135.3, 134.6, 134.2, 132.6,
129.8, 128.4, 127.7, 126.2, 125.8, 125.7, 125.4, 125.3, 59.2, 26.7,
19.2; FTIR (KBr) 2930, 2856, 1527, 1446, 1427, 1113, 1080,
1059, 823, 781, 756, 737, 700, 677, 644, 606 cm-1; UV-vis
(CH2Cl2) λmax 440, 370, 272, 246 nm; MS (FAB+) m/z 1009 (M+).
Anal. Calcd for C56H56O2S6Si2: C, 66.60; H, 5.60; S, 19.00.
Found: C, 65.60; H, 5.72; S, 18.80.
9-[4,5-Bis(ter t-bu tyldiph en ylsilyloxym eth yl)-1,3-dith iol-
2-ylid en e]-10-(4,5-et h ylen ed it h io-1,3-d it h iol-2-ylid en e]-
9,10-d ih yd r oa n th r a cen e (11c): 65% yield; mp 102-104 °C;
1H NMR (CDCl3, 200 MHz) δ 7.72-7.69 (m, 2H), 7.52-7.50
(m, 10H), 7.40-7.27 (m, 16H), 4.17 (d, 2H, AB, J AB ) 12.8 Hz),
4.10 (d, 2H, AB, J AB ) 12.8 Hz), 3.29 (s, 4H, 4H), 1.01 (s, 18H);
13C NMR (CDCl3, 50 MHz) δ 135.2, 134.7, 134.6, 132.7, 132.6,
129.8, 128.4, 128.1, 127.7, 126.3, 125.6, 125.5, 125.3, 125.1,
120.7, 59.1, 29.6, 26.7, 19.2; FTIR (KBr) 2928, 2855, 2359,
1588, 1516, 1474, 1457, 1445, 1427, 1361, 1282, 1187, 1112,
1080, 1006, 941, 823, 780, 755, 739, 701, 675, 644, 622, 606
cm-1; UV-vis (CH2Cl2) λmax 446, 378, 276, 246 nm; MS (FAB+)
m/z 1007 (M+). Anal. Calcd for C56H54O2S6Si2: C, 66.80; H,
5.40; S, 19.00. Found: C, 66.50; H, 5.80; S, 19.40.
π-Exten d ed Tetr a th ia fu lva len es 12a -c. Gen er a l P r o-
ced u r e. To a solution of the corresponding bis(tert-butyldi-
phenylsilane) derivative previously synthesized (11a -c) (0.08
mmol) in dry THF (10 mL) at 20 °C was added tetrabutyl-
ammonium fluoride (72 mg, 0.23 mmol) over a period of 10
min. Stirring was continued under argon atmosphere for 16
h. THF was removed in vacuo, the reaction crude was dissolved
in dichloromethane, washed with water, dried over MgSO4,
and solvent was removed under vacuo. Chromatography of the
residue on silica gel using hexane/ethyl acetate (1:2) as eluent
afforded compounds 12a -c as yellow solids.
C
51H50O3S2Si2: C, 73.70; H, 6.10; S, 7.70. Found: C, 73.60; H,
6.30; S, 7.70.
9-[4,5-Bis(h ydr oxym eth yl)-1,3-dith iol-2-yliden e]-10-(1,3-
Wittig-Hor n er Rea ction s for th e P r ep a r a tion of Do-
n or System s Der ived fr om Bis(ter t-bu tyld ip h en ylsila n e).
Gen er a l P r oced u r e. To a solution of the appropriate phos-
phonate ester (10a -c) (1 mmol) in dry THF (20 mL) at -78
°C and under argon atmosphere was added lithium diisopropyl
amide (LDA) (1.1 mmol). After 1 h at -78 °C, a solution of
compound 9 in dry THF (150 mg, 0.18 mmol) was added with
a syringe. The mixture was stirred for 1 h at -78 °C and then
allowed to warm to 20 °C and kept at this temperature
overnight. The THF was evaporated under reduced pressure,
water (50 mL) added, and the residue extracted with CH2Cl2
(3 × 50 mL). The combined extracts were dried (MgSO4) and
filtered, and the solvent was removed under reduced pressure.
Purification of products was achieved by column chromatog-
raphy on silica gel using hexane/dichloromethane (2:1) as
eluent.
9-[4,5-Bis(ter t-bu tyldiph en ylsilyloxym eth yl)-1,3-dith iol-
2-y lid e n e ]-10-(1,3-d it h io l-2-y lid e n e -9,10-d ih y d r o a n -
th r a cen e (11a ): 68% yield; mp 112-114 °C; 1H NMR (CDCl3,
300 MHz,) δ 7.66-7.64 (m, 4H), 7.55-7.51 (m, 8H), 7.34-7.21
(m, 16H), 6.25 (s, 2H), 4.10 (d, 2H,AB, J AB ) 13.0 Hz), 4.07 (d,
2H, AB, J AB ) 13.0 Hz), 0.96 (s, 18H); 13C NMR (CDCl3, 50
MHz) δ 135.6, 135.4, 135.2, 135.0, 132.7, 129.8, 128.4, 127.7,
125.9, 125.6, 125.3, 124.9, 122.7, 117.1, 59.2, 26.7, 19.2; FTIR
(KBr) 2928, 2854, 2710, 1545, 1518, 1458, 1445, 1427, 1362,
1261, 1188, 1113, 1078, 1055, 999, 939, 823, 800, 781, 756,
739, 700, 675, 644, 606 cm-1; UV-vis (CH2Cl2) λmax 436, 368,
270, 244 nm; MS (FAB+) m/z 917 (M+). Anal. Calcd for
dith iol-2-yliden e)-9,10-dih ydr oan th r acen e (12a): 93% yield;
mp 252-254 °C; H NMR (DMSO-d6, 300 MHz) δ 7.65-7.62
1
(m, 2H), 7.58-7.55 (m, 2H), 7.35-7.32 (m, 4H), 6.72 (s, 2H),
5.42 (t, 2H, ABX, J AX ) J BX ) 5.7 Hz), 4.26 (dd, 2H, ABX, J AB
) 13.6 Hz, J BX ) J AX ) 5.7 Hz), 4.18 (dd, 2H, ABX, J AB ) 13.6
Hz, J AX ) J BX ) 5.7 Hz); 13C NMR (DMSO-d6, 75 MHz) δ 136.9,
135.1, 134.8, 133.9, 133.5, 129.8, 126.5, 125.6, 125.1, 121.5,
121.1, 118.4, 56.5; FTIR (KBr) 3432, 3064, 2923, 2854, 1617,
1546, 1517, 1455, 1444, 1281, 1171, 1048, 999, 883, 802, 781,
755, 675, 644, 624 cm-1; UV-vis (CH2Cl2) λmax 432, 366, 270,
244 nm; MS (EI) m/z 440 (M+), 422 (M+ - 18).
9-[4,5-Bis(h ydr oxym eth yl)-1,3-dith iol-2-yliden e]-10-(4,5-
d im e t h y lt h io -1,3-d it h io l-2-y lid e n e )-9,10-d ih y d r o a n -
th r a cen e (12b): 79% yield; mp 199-201 °C; 1H NMR (DMSO-
d6, 300 MHz) δ 7.61-7.58 (m, 2H), 7.51-7.48 (m, 2H), 7.40-
7.36 (m, 4H), 5.43 (t, 2H, ABX, J AX ) J BX ) 5.8 Hz), 4.27 (dd,
2H, ABX, J AB ) 13.7 Hz, J BX ) J AX ) 5.8 Hz), 4.19 (dd, 2H,
ABX, J AB ) 13.7 Hz, J AX ) J BX ) 5.8 Hz), 2.40 (s, 6H); 13C
NMR (DMSO-d6, 50 MHz) δ 134.4, 134.1, 133.6, 129.6, 129.4,
126.7, 126.2, 125.3, 125.0, 124.8, 123.6, 120.8, 56.0, 18.4; FTIR
(KBr) 3421, 2922, 2853, 1528, 1499, 1445, 1385, 1283, 1171,
1049, 999, 947, 858, 779, 756, 675, 644, 625 cm-1; UV-vis
(CH2Cl2) λmax 430, 364, 266, 242 nm; MS (EI) m/z 532 (M+),
514 (M+ - 18). Anal. Calcd for C24H20O2S6: C, 54.10; H, 3.80;
S, 36.10. Found: C, 54.50; H, 4.20; S, 36.10.
9-[4,5-Bis(h ydr oxym eth yl)-1,3-dith iol-2-yliden e]-10-(4,5-
e t h yle n e d it h io-1,3-d it h iol-2-ylid e n e )-9,10-d ih yd r oa n -
th r a cen e (12c): 58% yield; mp 192-194 °C; 1H NMR (DMSO-
d6, 300 MHz) δ 7.60 (m, 2H), 7.43-7.36 (m, 6H), 5.44 (t, 2H,
ABX, J AX ) J BX ) 5.7 Hz), 4.27 (dd, 2H, ABX, J AB ) 13.6 Hz,
C
54H52O2S4Si2: C, 70.70; H, 5.70; S, 14.00. Found: C, 70.50;
H, 5.70; S, 13.90.
9-[4,5-Bis(ter t-bu tyldiph en ylsilyloxym eth yl)-1,3-dith iol-
2-ylid en e]-10-(4,5-d im eth ylth io-1,3-d ith iol-2-ylid en e)-9,-
10-d ih yd r oa n th r a cen e (11b): 73% yield; mp 160-162 °C;
1H NMR (CDCl3, 200 MHz) δ 7.66-7.61 (m, 2H), 7.49-7.48
(m, 12H), 7.31-7.18 (m, 14H), 4.09 (d, 2H, AB, J AB ) 13.2 Hz),
J BX ) J AX ) 5.7 Hz), 4.18 (dd, 2H, ABX, J AB ) 13.7 Hz, J BX
)
J AX ) 5.7 Hz), 3.30 (s, 4H); 13C NMR (DMSO-d6, 50 MHz) δ
134.3, 133.6, 129.4, 128.8, 128.1, 127.6, 126.8, 126.2, 125.3,
124.5, 120.6, 110.2, 56.0, 29.1; FTIR (KBr) 3382, 2919, 2850,
1596, 1514, 1456, 1445, 1373, 1281, 1240, 1170, 1048, 1019,
858, 780, 755, 720, 675, 643, 622, 455 cm-1; UV-vis (CH2Cl2)
(26) Christensen, C. A.; Bryce, M. R.; Batsanov, A. S.; Becher, J .
Org. Biomol. Chem. 2003, 1, 511.
λmax 440, 374, 270, 244 nm; MS (EI) m/z 530 (M+), 514 (M+
-
J . Org. Chem, Vol. 68, No. 20, 2003 7719

D´ıaz et al.
18). Anal. Calcd for C24H18O2S6: C, 54.30; H, 3.40; S, 36.20.
Found: C, 54.90; H, 4.10; S, 35.90.
643, 604, 588, 574, 552, 526 cm-1; UV-vis (CH2Cl2) λmax 436,
422, 370, 328, 258 nm; MS (MALDI-TOF) m/z 1126 (M+), 406
(diene).
π-Exten d ed Tetr a th ia fu lva len es 13a -c. Gen er a l P r o-
ced u r e. To a solution of the appropriate bis(hydroxymethyl)
derivative 12a -c (0.05 mmol) in dry THF (1 mL), under argon,
and at 0 °C was added carbon tetrachloride (1 mL). While the
mixture was stirred, phosphorus tribromide (0.01 mL, 0.07
mmol) was added slowly, and the reaction mixture was kept
under these conditions for 1 h. The reaction crude was poured
into ethyl acetate (50 mL) and washed with brine. The organic
layer was dried over MgSO4, and evaporation of the solvent
(below 40 °C) gave an oily orange residue, which was chro-
matographed using silica gel and a mixture of hexane/ethyl
acetate (5:1) as the eluent.
9-[4,5-Bis(br om om eth yl)-1,3-d ith iol-2-ylid en e]-10-(1,3-
dith iol-2-yliden e)-9,10-dih ydr oan th r acen e (13a): 54% yield;
mp >300 °C; 1H NMR (CDCl3, 300 MHz) δ 7.74-7.71 (m, 2H),
7.60-7.58 (m, 2H), 7.33-7.30 (m, 4H), 6.30 (s, 2H), 4,18 (s,
4H); 13C NMR (CDCl3, 75 MHz) δ 135.4, 134.5, 133.5, 129.1,
128.4, 126.4, 126.0, 125.1, 123.9, 121.6, 117.2, 22.1; FTIR (KBr)
2924, 2853, 2633, 1593, 1560, 1545, 1508, 1445, 1205, 756, 675,
644 cm-1; UV-vis (CH2Cl2) λmax 420, 354, 268, 234 nm; MS
(EI) m/z 566 (M+), 486 (M+ - 80), 406 (M+ - 160). Anal. Calcd
for C22H14S4Br2: C, 46.60; H, 2.50; S, 22.60. Found: C, 47.60;
H, 3.10; S, 21.60.
9-[4,5-Bis(br om om eth yl)-1,3-d ith iol-2-ylid en e]-10-(4,5-
d im e t h y lt h io -1,3-d it h io l-2-y lid e n e )-9,10-d ih y d r o a n -
th r a cen e (13b): 71% yield; mp >300 °C; 1H NMR (DMSO-d6,
300 MHz) δ 7.58-7.54 (m, 4H), 7.44-7.40 (m, 4H), 4.70 (s,
4H), 2.38 (s, 6H); 13C NMR (CDCl3, 75 MHz) δ 134.6, 134.5,
129.2, 129.0, 126.5, 126.3, 125.8, 125.5, 125.2, 123.5, 123.4,
22.1,19.2; FTIR (KBr) 2852, 1593, 1526, 1497, 1458, 1445,
1431, 1201, 1024, 779, 754, 675, 642 cm-1; UV-vis (CH2Cl2)
λmax 432, 364, 276, 246 nm; MS (EI) m/z 578 (M+ - 80), 498
(M+ - 160). Anal. Calcd for C24H18S6Br2: C, 43.80; H, 2.75; S,
29.20. Found: C, 44.90; H, 2.95; S, 28.10.
Cycloa d d u ct of 9-[4,5-bis(br om om eth yl)-1,3-d ith iol-2-
ylid en e]-10-(4,5-d im eth ylth io-1,3-d ith iol-2-ylid en e)-9,10-
d ih yd r oa n th r a cen e a n d C60 (3b): 25% yield (57% based on
recovered C60); 1H NMR (CDCl3, 300 MHz) δ 7.84-7.79 (m,
2H); 7.57-7.54 (m, 2H), 7.38-7.29 (m, 4H), 4.37 (d, 2H, AB,
J AB ) 14.8 Hz), 4.33 (d, 2H, AB, J AB ) 14.8 Hz), 2.43 (s, 6H);
13C NMR (CDCl3/CS2 1/1, 75 MHz) δ 155.8, 155.5, 155.1, 148.2,
147.1, 146.8, 146.2, 146.1, 146.0, 145.4, 145.2, 143.6, 143.2,
142.8, 142.6, 142.2, 140.7, 136.4, 136.3, 135.6, 135.2, 131.3,
129.5, 128.7, 128.0, 127.1, 126.9, 126.8, 126.3, 126.0, 125.7,
124.4, 66.4, 41.1, 19.7; FTIR (KBr) 2915, 1522, 1496, 1443,
1426, 1281, 1182, 968, 766, 753, 696, 674, 642, 623, 575, 553,
526 cm-1; UV-vis (CH2Cl2) λmax 442, 424, 316, 260 nm; MS
(MALDI-TOF) m/z 1218 (M+), 498 (diene).
Cycloa d d u ct of 9-[4,5-bis(br om om eth yl)-1,3-d ith iol-2-
ylid en e]-10-(4,5-eth ylen ed ith io-1,3-d ith iol-2-ylid en e)-9,-
10-d ih yd r oa n th r a cen e a n d C60 (3c): 16% yield (51% based
on recovered C60); 1H NMR (CDCl3, 300 MHz) δ 7.84-7.81 (m,
2H), 7.61-7.56 (m, 2H), 7.40-7.34 (m, 4H), 4.32 (d, 2H,AB,
J AB ) 13.9 Hz), 4.30 (d, 2H, AB, J AB ) 13.9 Hz), 3.35-3.33
(m, 4H); 13C NMR (CDCl3/CS2 1/1, 75 MHz) δ 155.3, 154.9,
147.5, 146.4, 146.1, 145.6, 145.5, 145.4, 145.3, 1449, 144.8,
144.6, 144.5, 143.0, 142.9, 142.5, 142.1, 141.9, 141.5, 140.1,
135.8, 135.7, 1351, 135.0, 134.6, 128.1, 126.4, 126.3, 126.1,
125.5, 125.1, 123.8, 122.6, 65.7, 40.4, 29.7; FTIR (KBr) 2918,
2844, 1507, 1444, 1281, 1261, 1212, 1187, 1095, 1021, 802, 766,
753, 697, 673, 642, 622, 574, 554, 526 cm-1; UV-vis (CH2Cl2)
λ
max 448, 388, 316, 260 nm; MS (MALDI-TOF) m/z 1216 (M+),
496 (diene).
Com p ou n d s 16 a n d 17. Gen er a l P r oced u r e. To a stirred
solution of (triphenylphosphoranylidene)acetaldehyde (1 mmol)
in dry toluene (20 mL) and under argon atmosphere was added
the corresponding aldehyde (15, 16) (0.5 mmol). The mixture
was refluxed for 12 h and then allowed to reach 20 °C. Hexane
(30 mL) was added, and the triphenylphosphine oxide pre-
cipitated was separated by filtration. The solvent was evapo-
rated under vacuum, and the pure product was chromato-
graphed on a silica gel column using hexane/CH2Cl2 in the
proportion indicated below as eluent.
9-[4,5-Bis(br om om eth yl)-1,3-d ith iol-2-ylid en e]-10-(4,5-
e t h yle n e d it h io-1,3-d it h iol-2-ylid e n e )-9,10-d ih yd r oa n -
th r a cen e (13c): 87% yield; mp >300 °C; 1H NMR (CDCl3, 200
MHz) δ 7.62-7.58 (m, 2H), 7.55-7.51 (m, 2H), 7.37-7.31 (m,
4H), 4.19 (s, 4H), 3.30 (s, 4H); 13C NMR (CDCl3, 75 MHz) δ
134.6, 134.5, 132.4, 130.0, 129.5, 129.1, 128.3, 126.4, 125.7,
125.2, 123.5, 110.8, 29.6, 22.0; FTIR (KBr) 2921, 2851, 1653,
1594, 1515, 1456, 1445, 1282, 1262, 1201, 1148, 1102, 1056,
9-[4-(2-For m ylvin yl)-5-m eth yl-1,3-d ith iol-2-ylid en e]-10-
(4,5-d im e t h yl)-1,3-d it h iol-2-ylid e n e )-9,10-d ih yd r oa n -
th r a cen e (16): hexane/CH2Cl2 (1:1); 81% yield; mp 252 °C;
1H NMR (CDCl3, 200 MHz) δ 9.57 (d, 1H, J ) 7.3 Hz), 7.70-
7.57 (m, 4H), 7.35-7.30 (m, 4H), 6.75 (d, 1H, J ) 15.6 Hz),
5.99 (dd, 1H, J 1 ) 15.6 Hz, J 2 ) 7.3 Hz) 2.22 (s, 3H), 1.93 (s,
6H); 13C NMR (CDCl3, 75 MHz) δ 191.6, 135.2, 134.9, 134.4,
130.4, 129.8, 129.5, 125.9, 125.7, 125.3, 125.2, 120.8,120.0;
939, 919, 886, 857, 801, 779, 756, 703, 675, 643, 620, 597 cm-1
;
UV-vis (CH2Cl2) λmax 438, 370, 280, 242 nm; MS (EI) m/z 496
(M+ - 160). Anal. Calcd for C24H16S6Br2: C, 43.90; H, 2.46; S,
29.30. Found: C, 44.80; H, 2.68; S, 28.70.
Diels-Ald er Cycloa d d u cts 3a -c. Gen er a l P r oced u r e.
To a refluxing solution of [60]fullerene (50 mg, 0.07 mmol),
sodium iodide (47 mg, 0.31 mmol), and 18-crown-6 (55 mg, 0.21
mmol) in toluene (50 mL) was added dropwise a solution of
the respective bis(bromomethyl) derivative (13a -c) (0.08
mmol) in toluene (5 mL). The resulting green reaction mixture
was refluxed for a variable period of time (from 30 min to 2
h). The solvent was removed under vacuo, and the residue was
chromatographed on silica gel using cyclohexane/toluene as
eluent. Further purification was accomplished by washing the
obtained solid three times with hexane, methanol, and diethyl
ether, respectively.
FTIR (KBr) 2924, 2852, 1676, 1595, 1560, 1132, 756, cm-1
;
UV-vis (CH2Cl2) λmax 441, 309, 257 nm; MS m/z 476 (M+).
9-[4-(4-F or m yl-1,3-bu ta d ien yl)-5-m eth yl-1,3-d ith iol-2-
ylid en e]-10-(4,5-d im eth yl)-1,3-d ith iol-2-ylid en e)-9,10-d i-
h yd r oa n th r a cen e (17): Hexane/CH2Cl2 (1:4); 55% yield; mp
197 °C; 1H NMR (CDCl3, 300 MHz) δ 9.52 (d, 1H, J ) 7.1 Hz),
7.68-7.54 (m, 4H), 7.28-7.22 (m, 4H), 7.08-7.03 (m, 1H),
6.72-6.39 (m, 2H), 6.15 (dd, 1H, J 1 ) 15.0 Hz, J 2 ) 7.1 Hz)
2.06 (s, 3H), 1.87 (s, 6H); 13C NMR (CDCl3, 75 MHz) δ 191.8,
135.4, 135.1, 134.6, 133.6, 131.5, 130.6, 130.0, 129.7, 128.7,
126.2, 126.1, 125.9, 125.5, 125.4, 125.3, 124.9, 122.7, 120.9,
13.6, 13.2; FTIR (KBr) 2922, 2852, 2331, 1676, 1654, 1558,
1521, 1458, 1155 cm-1; UV-vis (CH2Cl2) λmax 441, 309, 257
nm; MS m/z 502 (M+).
Cycloa d d u ct of 9-[4,5-bis(br om om eth yl)-1,3-d ith iol-2-
y lid e n e ]-10-(1,3-d it h io l-2-y lid e n e )-9,10-d ih y d r o a n -
th r a cen e a n d C60 (3a ): 34% yield (77% based on recovered
1
C
60); H NMR (CDCl3, 300 MHz) δ 7.78-7.75 (m, 2H), 7.72-
7.69 (m, 2H), 7.33-7.30 (m, 4H), 6.34 (s, 2H), 4.35 (d, 2H, AB,
J AB ) 13.3 Hz), 4.31 (d, 2H, AB, J AB ) 13.3 Hz); 13C NMR
(CDCl3/CS2 1/1, 75 MHz) δ 155.2, 154.8, 147.5, 146.4, 146.1,
145.6, 145.4, 145.3, 144.8, 144.7, 144.5, 142.9, 142.4, 142.1,
141.9, 141.5, 140.0, 136.5, 135.7, 135.6, 135.3, 134.9, 134.3,
126.3, 126.1, 125.9, 125.0, 124.9, 122.9, 121.9, 117.2, 65.7, 40.3;
FTIR (KBr) 2920, 1573, 1542, 1514, 1456, 1443, 1281, 1259,
1214, 1187, 1161, 1095, 944, 857, 801, 777, 767, 753, 697, 674,
F u ller op yr r olid in es (4, 5a ,b). Gen er a l P r oced u r e. To
a solution of C60 (0.5 mmol) in chlorobenzene (100 mL) were
added N-(3,6,9-trioxadecyl)glycine (1.5 mmol) and the corre-
sponding formyl-exTTF derivative 15-17 (0.5 mmol). The
mixture was refluxed for 16 h under argon atmosphere. The
solvent was partially removed under reduced pressure and was
then loaded on a silica gel column, using toluene/ethyl acetate
(9:1) as eluent. Further purification was accomplished by
7720 J . Org. Chem., Vol. 68, No. 20, 2003

Probing Charge Separation in C60/ exTTF Ensembles
repetitive precipitation and centrifugation with methanol and
diethyl ether as solvents.
4: 63% yield (78% based on recovered C60); H NMR (CS2,
5b: 14% yield (23% based on recovered C60); 1H NMR (CS2,
300 MHz) δ 7.79-7.75 (m, 4H), 7.44-7.34 (m, 4H), 7.00-6.95
(m, 1H),6.69-6.39 (m, 3H),5.34 (d, 1H, J ) 9.3 Hz), 4.85 (d,
1H, J ) 8.9 Hz), 4.42 (d, 1H, J ) 8.9 Hz), 4.23-3.19 (m, 15H),
2.39 (s, 3H), 2.26 (s, 3H), 2.22 (s, 3H); 13C NMR (CDCl3/CS2
1/1, 125 MHz) δ 156.53, 154.74, 153.92, 152.98, 147.58, 147.37,
147.14, 147.08, 146.68, 146.51, 146.41, 146.27, 146.19, 145.92,
145.87, 145.70, 145.63, 145.53, 145.09, 145.04, 144.74, 143.53,
143.39, 143.04, 142.95, 142.69, 142.61, 142.53, 142.41, 142.06,
140.58, 140.46, 137.83, 137.76, 136,89, 136.47, 136.32, 136.13,
135.94, 135.68, 135.27, 134.76, 134.64, 134.44, 132.81, 132.72,
132.29, 131.84, 131.68, 130.61, 130.44, 128.79, 128.75, 126.46,
126.24, 126.19, 126.13, 125.76, 125.68, 125.58, 123.12, 122.88,
122.88, 122.80, 122.75, 121.34, 120.95, 120.83, 81.22, 81.13,
76.11, 72.65, 71.46, 71.32, 71.23, 69.92, 67.91, 59.06, 53.13,
13.87, 13.65; FTIR (KBr) 2961, 2920, 2851, 1636, 1444, 1261,
1097, 1023, 803, 527 cm-1; UV-vis (CHCl3) λmax 432, 256, 245
nm; MS m/z 1382 (M+).
1
300 MHz) δ 7.60 (m, 4H), 7.21 (m, 4H), 5.31 (s, 1H), 5.17 (d,
1H, J ) 10.0 Hz), 4.21 (d, 1H, J ) 10.0 Hz), 3.90-3.15 (m,
15H), 2.24 (s, 3H), 2.00 (s, 6H); 13C NMR (CDCl3/CS2 1/1, 125
MHz) δ 155.67, 155.43, 152.97, 152.73, 147.16, 146.21, 146.05,
145.85, 145.72, 145.48, 145.42, 145.34, 145.30, 145.18, 145.17,
145.10, 144.70, 144.23, 142.91, 142.84, 142.63, 142.59, 142.52,
142.24, 142.06, 141.74, 141.61, 141.46, 140.25, 140.11, 139.82,
136.78, 135.14, 134.72, 133.93, 133.20, 127.80, 127.46, 125.94,
125.74, 125.55, 125.48, 125.29, 125.20, 125.04, 124.97, 121.93,
121.69, 121.61, 121.54, 121.07, 120.99, 120.89, 120.83, 75.90,
75.77, 72.18, 71.01, 70.80, 70.69, 67.59, 65.18, 58.61, 52.83,
14.87, 14.67, 13.27; FTIR (KBr) 2912, 2856, 2807, 1619, 1510,
1442, 1384, 1114, 1037, 752, 643, 527 cm-1; UV-vis (CHCl3)
λmax 441, 309, 257 nm; MS m/z 1329 (M+).
5a : 24% yield (37% based on recovered C60); 1H NMR (CDCl3/
CS2 1/1, 300 MHz) δ 7.57 (m, 4H), 7.24 (m, 4H), 6.86 (d, 1H, J
) 15 Hz), 6.49-6.63 (m, 1H), 6.05-6.95 (m, 1H), 5.09 (d, 1H,
J ) 9.8 Hz), 4.59 (d, 1H, J ) 9.8 Hz), 4.19-2.90 (m, 16H),
2.14 (s, 3H), 1.98 (s, 3H), 1.91 (s, 3H); 13C NMR (CDCl3/CS2
1/1, 125 MHz) δ 156.36, 154.68, 153.76, 152.64, 152.46, 147.59,
147.34, 147.07, 146.70, 146.52, 146.42, 146.24, 145.88, 145.54,
145.09, 144.74, 143.52, 143.40, 143.03, 142.96, 142.69, 142.43,
142.05, 140.71, 140.56, 140.46, 140.28, 138.05, 137.83, 137.00,
136.89, 136.36, 136.29, 136.13, 135.92, 135.72, 135.66, 135.52,
135.26, 135.14, 134.74, 134.63, 132.82, 132.72, 131.31, 130.60,
130.49, 130.44, 129.48, 126.63, 126.47, 126.24, 126.13, 126.06,
125.78, 125.66, 125.58, 125.31, 125.22, 125.15, 123.44, 123.13,
122.87, 121.45, 121.35, 121.29, 121.09, 120.94, 120.71, 81.46,
81.13, 75.97, 72.66, 71.46, 71.33, 71.24, 69.85, 67.91, 59.07,
53.13, 13.92, 13.66; FTIR (KBr) 2912, 2864, 1630, 1519, 1443,
1109, 754, 527 cm-1; UV-vis (CHCl3) λmax 433, 257, 247 nm;
MS m/z 1356 (M+).
Ack n ow led gm en t. This work has been supported
by the MCYT of Spain (Project BQU2002-00855). Part
of this work was supported by the Office of Basic Energy
Sciences of the U.S. Department of Energy (Contribu-
tion No. NDRL-4478 from the Notre Dame Radiation
Laboratory). The work in Durham was funded by the
University of Durham (studentship to N.G.) and Durham
County Council under the Science and Technology for
Business and Enterprise Program SP/082.
Su p p or tin g In for m a tion Ava ila ble: Emission Spectra
of dyads 4, 5a , and 5b in toluene, o-dichlorobenzene, and
benzonitrile. This material is available free of charge via the
Internet at http://pubs.acs.org.
J O034432E
J . Org. Chem, Vol. 68, No. 20, 2003 7721