Connecting two C60 stoppers to molecular wires: Ultrafast intramolecular deactivation reactions

Article abstract of DOI:10.1039/b412110d

A new series of soluble dumbbell-type architectures constituted by π-conjugated oligophenyleneethynylenes (OPEs) and two C₆₀ stoppers have been prepared by using a convergent synthetic strategy. In particular, we base our approach on the system

Full text of DOI:10.1039/b412110d

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www.rsc.org/materials | Journal of Materials Chemistry
Connecting two C60 stoppers to molecular wires: ultrafast intramolecular
deactivation reactions{
Carmen Atienza,a Braulio Insuasty,ab Carlos Seoane,a Nazario Mart´ın,*a Jeff Ramey,c
G. M. Aminur Rahmand and Dirk M. Guldi*c
Received 5th August 2004, Accepted 12th November 2004
First published as an Advance Article on the web 26th November 2004
DOI: 10.1039/b412110d
A new series of soluble dumbbell-type architectures constituted by p-conjugated
oligophenyleneethynylenes (OPEs) and two C60 stoppers have been prepared by using a
convergent synthetic strategy. In particular, we base our approach on the systematic cross-
coupling, palladium catalyzed Hagihara–Sonogashira reaction between aryl iodides and alkynyl
derivatives. The cyclic voltammetry of the correspondingly synthesized system reveals amphoteric
redox behavior and the lack of significant electronic communication between the oligo-
arylalkynyl bridge and the two C60 moieties, which behave as two independent units. Excited state
studies carried out by fluorescence and transition absorption spectroscopy show that, although a
weak competitive electron transfer could operate in the deactivation process of the excited oligo-
arylalkynyl bridges, their deactivation is mainly governed by energy transfer.
reorganization energies in electron transfer reactions, which
Introduction
renders an application of this carbon material as electron
‘‘Molecular electronics’’ is currently a hot topic in chemistry,
accepting moieties particularly appealing under aspects of
energy conversion and energy storage.6
which requires the development of efficient synthetic method-
ologies to afford a wide variety of different chemical structures
Recently, we and others have developed several different
able to perform specific functions—wires, motors, diodes
rectifiers antennas, etc.1 The rational design of such functional
approaches towards the preparation of C60/p-conjugated
oligomer architectures as integrative building blocks for the
molecules as well as the understanding of the physical
efficient conversion of solar energy.7 Interestingly, depending
processes occurring at the nanoscale molecular level are
upon the electronic nature and length of the p-conjugated
currently important challenges in science.2 In particular, the
oligomer connected—either covalently or through supramole-
study of p-conjugated oligomers has drawn interdisciplinary
cular interactions—to C60, different photophysical scenarios
attention due to their broad applicability.3 Thus,
(i.e., energy transfer versus electron transfer) evolved in the
monodisperse p-conjugated oligomers of different nature
[oligo-phenylenevinylenes (OPVs); oligo-thienylenevinylenes
corresponding C60–oligomer systems. However, in contrast to
the large number of dyads (i.e., C60–oligomers), the number of
(OTVs); oligo-phenyleneethynylenes (OPEs); etc.] have been
triads (i.e., C60–oligomer–C60) is considerably lower and only
developed as simple model systems to mimic the more complex
for a few of them (T1, T2) have their photophysical properties
features of the respective p-conjuguted polymers as well as to
test them as active components in optoelectronic devices.4
been studied and reported (Chart 1).7,8
We have recently shown that for C60–OPV–C60 triads,9a
On the other hand, with the advent of fullerenes, and in
although no significant differences were established for the
particular of C60, a new three-dimensional electron acceptor
general photoreactivity relative to the related C60–OPV
became available, which exhibits exciting characteristics. The
dyads,9b remarkable differences were found in their charge-
delocalization of charges—electrons or holes—within the
separation features. In addition, we documented decisive
˚
giant, spherical carbon framework (diameter . 7.5 A) together
evidence for molecular wire behavior through OPV systems.10
These results prompted us to study other rigid and soluble
C60–oligomer–C60 dumbbell-type triads bearing rigid oligo-
phenyleneethynylene (OPE) moieties with solubilizing alkoxy
chains.
with the rigid, confined structure of the fullerene sphere offers
unique opportunities for stabilizing charged entities. Six
equally spaced reduction waves in electrochemical experi-
ments,5 with the first reduction step resembling that of
quinones—the electron acceptor unit in PRC proteins which
is reduced to a semiquinone and finally to a hydroquinone—
are a first manifestation for conditions that guarantee the
optimal delocalization of charges. From this observation we
infer the potential of fullerenes to posses quite small
In this paper we wish to describe rational synthetic strategies
en route towards functionalized oligomers, in which the
oligomer length has been systematically increased from the
monomer to the heptamer, as well as their electrochemical
features, which were determined by cyclic voltammetry. The
photophysical study of the novel compounds, as carried out by
a series of fluorescence and transient absorption spectroscopic
measurements, complements the present study to gain a better
understanding of the excited state properties of the new triads.
{ Electronic supplementary information (ESI) available: fluorescence
spectra. See http://www.rsc.org/suppdata/jm/b4/b412110d/
*nazmar@quim.ucm.es (Nazario Mart´ın)
dirk.guldi.@chemie.uni-erlangen.de (Dirk M. Guldi)
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Scheme 1 Building blocks used in the preparation of OPEs.
palladium(II), copper(I) iodide and PPh3 in piperidine at room
temperature. Compound 3 was similarly obtained in a two-step
reaction sequence. Further deprotection of 3, by treatment
with tetrabutylammonium fluoride in THF at room tempera-
ture, afforded 4 in 98% yield. With these starting building
blocks (1–5) in hand, the preparation of the respective
oligomers was carried out by means of the Hagihara–
Sonogashira reaction, that is, using catalytic amounts of
bis(triphenylphosphine)palladium dichloride, copper(I) iodide,
and triphenylphosphine. The reactions were carried out in
piperidine or, in some cases, piperidine–toluene to improve the
solubility (see Experimental section) (Scheme 2).
Chart 1 C60–oligomer–C60 triads (T1 and T2).
Results and discussion
Synthesis
The synthesis of C60–OPE–C60 is based on the preparation of
p-conjugated oligo-phenyleneethynylenes (OPEs) that must
meet two common features: i) the presence of solubilizing long
chains, due to the well-known poor solubility of these rigid
rod-like structures, and ii) the presence of functional groups at
the terminal positions for further chemical reactions with, for
example, C60.
Although some reactions were performed with the bromo-
and iodo-arenes, the best results were obtained from the
respective iodoarenes which reacted with the Pd(0)PPh3
complexes generated in situ to give the alkynyl protected
derivatives in good yields. In these reactions the piperidine acts
as a solvent and, simultaneously, as a base to neutralize the
hydrogen iodide formed in the reaction medium.17 Thus,
compound 6 (n 5 3) bearing three repeating arylalkynyl units
was prepared from 2 and 4 (n 5 1) by using the above reaction
conditions. Deprotection of 6 (n 5 3), by fluoride treatment,
led to the free ethynyl containing compound 7 (n 5 3).
Repetitive reactions of 2 with 7 (n 5 3) under similar
experimental conditions afforded compound 8 (n 5 5) in
80% yield. Deprotection of 6 and 8 led to 7 (n 5 3) and 9 (n 5 5)
in high yields (84% and 98%, respectively) (Scheme 2).
Ethynylation of C60 has been previously reported by
Komatsu et al. as a general and versatile procedure which,
however, seems to depend on the experimental conditions.11
The ethynylation reaction has been successfully carried out by
electrochemical procedures12 and, more recently, by in situ
generation of lithium acetylide in THF.13 By following the last
mentioned synthetic strategy, Tour and coworkers have
recently reported a new series of C60–OPE–C60 triads as well
as other C60/OPE hybrids bearing three fullerene units.13
In this work we report on the synthesis of novel C60–OPE–
C
60 triads, in which the OPE moiety is covalently connected to
two fullerenes through a pyrrolidine ring by using the efficient
and widely used Prato procedure.14 Therefore, the OPE
precursors were functionalized with two terminal formyl
groups which are, in turn, required for the formation of the
respective azomethyne ylide intermediates. The synthetic
strategy for the preparation of the different monodisperse
OPEs (n 5 1, 3, 5) is based on the design of suitable building
blocks (1–5)—see Scheme 1—which were further used as
starting materials in a convergent synthetic strategy.
Formyl functionalized oligomers (OPEs) 10a–c were then
obtained by palladium(0) catalyzed Hagihara–Sonogashira
reaction of p-iodobenzaldehyde (5) with the respective OPEs
[4 (n 5 1); 7 (n 5 3); 9 (n 5 5)]. Some slight modifications of
the experimental conditions were carried out (i.e., use of
diisopropylamine as solvent) in order to improve the yields—
see Experimental section.
Finally, C60–OPE(n)–C60 (n 5 1, 3, 5) 11a–c were prepared
in moderate yields (i.e., 20–43%) as stable brown solids by 1,3-
dipolar cycloaddition reaction of the respective azomethyne
ylides, generated in situ from dialdehydes 10a–c and methyl or
octyl N-substituted glycines, to C60 by heating at 130 uC in
o-dichlorobenzene (ODCB).14 N-Octylglycine is not commer-
cially available and, therefore, it was synthesized by following
The starting building blocks are shown in Scheme 1.
Compounds 2–4 were prepared from diiodide 115 and
commercially available p-iodobenzaldehyde (5). To these
compounds the respective ethynyl groups were introduced by
using the Hagihara–Sonogashira reaction.16 Compound 2, for
example, bearing an alkynyl unit was obtained from 1 and
triisopropylsilylacetylene by cross-coupling reaction using
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Scheme 2 Reagents and conditions: a) Pd(PPh3)2Cl2, PPh3, CuI, piperidine, rt, 24 h; b) Bu4NF, THF, rt, 1 h; c) Pd(PPh3)2Cl2, PPh3, CuI,
i
piperidine, rt, 24 h or Pd(PPh3)4, Pr2NH, THF, D, 16 h; d) N-octylglycine or sarcosine, ODCB, D, 3 h.
the procedure previously reported.18 Compounds 12a–b (n 5 0)
were prepared from commercially available 4,49-diphenylethy-
nyl dialdehyde (Scheme 2).17 Although compounds 11a–c and
12a–b should be obtained as a stereoisomeric mixture due to
the formation of two stereogenic centers in the two-fold
cycloaddition reaction, high resolution 1H NMR spectra
(500 MHz) showed the presence of only one stereoisomer.
The final compounds (triads 11a–c, 12) as well as the
functionalized intermediate oligomers (OPEs) (6–10) and
starting building blocks (1–5) were fully characterized by
spectroscopic and elemental analyses. MALDI and accurate
mass analyses for triads 11a–c were carried out to confirm the
proposed structures.
Table 1 Redox potential values (¡0.01 V) for compounds (V vs.
Ag/Ag+)a
Comp. Eox
E1
E2
E3
E4
E5
E6
red
red
red
red
red
red
C60
6
8
—
20.53 20.94
—
—
—
21.42
—
—
—
21.91
1.35; 1.57
1.27; 1.54
1.46
1.43
1.37
—
—
—
—
21.72 21.93
21.73 21.92
—
—
—
—
11a
11b
11c
12b
a
20.63 21.02 21.30 21.57
—
—
21.80
—
b
20.61 21.01
20.59 21.00 21.24 21.55
20.61 20.99 21.54
21.53
—
—
Experimental conditions: ODCB–CH3CN 4 : 1 as solvent, Pt as
working electrode, Bu4NClO4 (0.1 M) as supporting electrolyte;
100 mV s21
b
.
No clear signal was observed.
show a similar trend despite the different nature of the
p-conjugated spacer.9,13,19 As expected, the reduction waves
are cathodically shifted in comparison with the parent C60
molecule. This is due to the saturation of the double bond,
which raises the LUMO energy.5,20 An additional weak
reduction wave was observed for compounds 11a–c at around
21.25 V, which corresponds to the reduction of the OPE unit.
In contrast to that recently observed for the reduction waves of
the C60 unit in triads T1, the increasing chain length of the
OPEs in 11a–c, 12a–b does not have an appreciable influence
on the reduction potential values of the fullerene moieties. This
fact could be accounted for by the two sp3 carbon atoms
existing between the C60 and the p-conjugated oligomer in
compounds 11a–c, 12, whereas only one sp3 carbon atom
exists in triads T1.13
Electrochemistry
The redox properties of the novel triads as well as the
oligomeric precursors OPE(n 5 3) (6) and OPE(n 5 5) (8),
were determined by cyclic voltammetry at room temperature in
ODCB–CH3CN (4 : 1) with a platinum wire as working
electrode. In all cases tetrabutylammonium perchlorate
was used as supporting electrolyte and the scan rate was
100 mV s21. The experimental data are collected in Table 1
together with those of C60 as reference.
All the new triads showed amphoteric redox behavior which
is rationalized by the presence of both donor (OPEs) and
acceptor (C60) moieties. It is interesting to note that despite the
presence of two C60 units connected through a p-conjugated
bridge, no electronic communication between them was
electrochemically observed in the ground state. In line with
this assumption, only three quasireversible reduction waves
were observed in the cyclic voltammogram, corresponding to
the first three reduction steps of C60. This finding is in
agreement with other related C60–oligomer–C60 triads, which
On the oxidation side, triads 11a–c showed the presence of
an oxidation wave at around 1.40 V, which relates to the
oxidation process of the OPE moiety. This oxidation wave is
slightly cathodically shifted with increasing the length of the
oligomer. However, this wave was not observed in triad 12b
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due to the weak electron donor ability of the diphenylacetylene
moiety. This oxidation wave was also observed for the parent
oligomers 6 and 8, which showed a first oxidation potential at
1.35 and 1.27 V, respectively. For these oligomers, a second
oxidation wave was observed at around 1.5 V which, however,
is not present in triads 11a–c (Table 1). In addition, oligomers
6 and 8 showed a weak reduction wave at around 21.90 V
which was, however, only observed in triad 11c. In summary,
the cyclic voltammetry data reveal the lack of significant
electronic interaction between the electroactive units OPE and
C60 in the ground state.
Photophysics
First we tested the excited state features of the reference
compounds that make up the family of novel C60–wire–C60
systems (i.e., C60 and phenylene-acetylene blocks of variable
length). For details on the C60 features we refer the reader to
the extensive literature.21 Within the context of the current
studies, only the weak fluorescing features that are maximized
at 710 nm and the strong triplet–triplet transitions at 360 and
700 nm should be mentioned explicitly.
Much less information is available on the phenylene-
acetylene building blocks. The major features include strong
visible light fluorescence with quantum yields close to unity
(i.e., 10a: 0.75; 10b: 0.78; 10c: 0.77). The fluorescence maxima,
similarly to the absorption maxima, depend on the length of
the p-conjugated oligo-phenyleneethynylenes and reach from
400 nm to 490 nm (i.e., electronic supplementary informa-
tion{–Fig. S1), in case of the monomer and heptamer,
respectively. Interestingly, the fluorescence lifetimes are also
influenced by the length of the p-conjugated oligo-phenyle-
neethynylenes with values of 2.3 ¡ 0.1 ns (10a), 0.71 ¡ 0.02 ns
(10b) and 0.6 ¡ 0.02 ns (10c). The product of this relatively
slow singlet excited state decay is the phenylene-acetylene
triplet. Spectroscopic characteristics of the triplet–triplet
spectra are maximized around 650 nm. These shift typically
to the red with increasing phenylene-acetylene length.22
When comparing the absorption spectra of the reference
systems, we note that the phenylene-acetylene building blocks
dominate most of the visible spectrum (i.e., 300–600 nm),
where the C60 absorption is marginal. Only in the UV range is
the C60 absorption cross section competitive. Relative to the
sum of the references, some degree of perturbation is found for
the C60–wire–C60 spectra. Implicit here are appreciable
electronic couplings between the different building blocks.
This, in turn, is expected to influence the deactivation rates. In
summary, visible light excitation of 11a–c will reach exclusively
the phenylene-acetylenes.
Fig. 1 a: Fluorescence spectra of 11c, 11b, and 11a—in order of
decreasing fluorescence intensity— in toluene with matching absorp-
tion at the 390 nm excitation wavelength (i.e., OD390
5 0.2). b:
nm
Fluorescence spectra of 11c, 11b, and 11a—in order of decreasing
fluorescence intensity—in benzonitrile with matching absorption at the
390 nm excitation wavelength (i.e., OD390 nm 5 0.2).
700–800 nm range of the emission spectra a new transition
emerges. In toluene, the latter is a quantitative (i.e., quantum
yield: 6 6 1024) and qualitative (i.e., fluorescence maximum:
710 nm) match of the C60 reference fluorescence. A likely
rationale for i) the absence of phenylene-acetylene fluorescence
and ii) the presence of C60 fluorescence implies an exothermic
transduction of the singlet excited state to the low lying C60
singlet excited state. Conclusive evidence for the fluorescence
trapping came from excitation spectra, that is, probing the C60
emission at 710 nm as a function of excitation wavelength. The
excitation spectra resemble the ground state absorption of the
phenylene-acetylene building blocks, with broad maxima in
the visible (i.e., 350–450 nm). Taking this evidence into
account we reach the conclusion that the origin of excited
state energy is unquestionably that of the conjugated
p-systems. Furthermore, it allows the underlying mechanism
to be attributed: an efficient intramolecular transfer of singlet
excited state energy from the photoexcited phenylene-
acetylenes to the covalently linked fullerene governs the
photophysics in 11a–c.
Insight into intramolecular deactivation processes came
from emission assays, namely, steady-state and time-resolved
fluorescence experiments. In particular, we compared the
strongly emissive features of the phenylene-acetylene units in
the 500–700 nm range upon photoexciting (i.e., in the visible at
390 nm) the references and 11a–c – see Fig. 1a and b. Dramatic
differences in the fluorescence quantum yields between the
references and 11a–c, that are as large as 1500, attest to an
almost instantaneous deactivation of the quantitatively excited
phenylene-acetylene building blocks. When inspecting the
In parallel fluorescence lifetime experiments, we made two
major observations. Firstly, in the visible region (i.e., 500–
700 nm), where typically the long-lived phenylene-acetylene
fluorescence dominates, no appreciable signal was seen that
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falls within our 100 ps time window. In other words, the
fluorescence deactivation in 11a–c is faster than 100 ps (k .
1010 s21), a consequence of the strong electronic coupling.
Secondly, in the near-infrared region (i.e., 700–800 nm) the
fluorescence that originates from the C60 termini reveals
lifetimes on the order of 1.5 ¡ 2 ns. These observations
complement the steady-state experiments nicely.
We also probed 11a–c in more polar media, such as THF
and benzonitrile. Minor differences, relative to toluene, include
slightly lower fluorescence quantum yields and slightly faster
fluorescence deactivation of the C60 contributions. In line with
previous reports on various C60–oligomer and C60–oligomer–
C60 systems (i.e., oligomer: phenylene-vinylene, etc.),23
a
competitive electron transfer deactivation is responsible for
this trend. As can be deduced from the absolute differences in
quantum yields, the electron transfer involvement is, however,
small (i.e., less than 5%). The phenylene-acetylene part, on the
other hand, remains virtually unchanged. It is important to
consider that solvent changes from toluene (e 5 2.38) to
benzonitrile (e 5 24.8) lead to marked increases in free reaction
enthalpy (2DGCSu) for intramolecular electron transfer events,
but to no or very little impact on that for energy transfer.
Better solvation of radical ion pairs assists in a polar solvent in
lowering the energy of the electron transfer product, namely,
the charge-separated radical ion pair. The excited states of C60
and phenylene-acetylene experience effects that are not worth
mentioning. Thus, the noted tendency serves as an indepen-
dent, indirect verification of excited state deactivation that is
governed in large part by energy transfer, that is, at least 95%.
A thermodynamic consideration sheds light onto our
experimental observation. As a test to validate the above
conclusion, the driving forces (2DGCSu), associated with an
intramolecular charge separation in 11a–c, were calculated in
accordance with the following approximation:24
Fig. 2 Schematic illustration of the reaction pathways in photo-
excited 11a–11c.
In the next step we employed transient absorption spectro-
scopy to test the product of the singlet–singlet transfer in 11a–
c. With the conclusion of nanosecond laser excitation at 308,
337 or 355 nm, spectroscopic and kinetic analyses suggest the
formation of just one photo-product. Spectroscopically,
transient maxima at 360 and 700 nm are unmistakable
attributes of a C60 triplet–triplet spectrum—see Fig. 3.23
Kinetically, we see transient decays that are best fitted by a
complex mixture of uni- and bimolecular dynamics with an
average lifetime of y20 microseconds. Again, this fits the
picture seen for the C60 reference under identical experimental
conditions. For example, lowering the concentration of 11a–c
and/or decreasing the laser flux leads to longer C60 triplet
lifetimes, while raising the concentration of 11a–c and/or
increasing the laser power resulted in shorter lifetimes, due to
variable contributions from intermolecular triplet–triplet
annihilations.
+
?
2DGCSu 5 2{e[E1/2(phenylene-acetylene /phenylene-
acetylene) 2 E1/2(C60/C60 2)] 2 DE*0-0
}
(1)
?
Interestingly—regardless of the solvent—in the 1000 nm
2
+
?
E1/2(phenylene-acetylene /phenylene-acetylene) and E1/2
-
?
region, where the radical anion fingerprint of C60 (i.e., C60
)
(C60/C60 2) correspond to the standard potentials for the
oxidation and reduction of phenylene-acetylene and C60,
respectively (see Table 1), while DE*0-0 represents the energy
of the photoexcited phenylene-acetylene singlet excited state
and e is the elementary charge. Considering, however, the
energy of the fullerene singlet excited state of 1.76 eV, relative
to those of the charge-separated state (in ODCB–CH3CN 4 : 1,
.2.0 eV), it is clear that the largest energy gap opens between
the two singlet excited states (i.e., phenylene-acetylene and
fullerene). Consequently, the energy transfer route is the
thermodynamically favored pathway. From this cause we can
assume that, once populated, after the completion of the
singlet–singlet energy transfer, the fullerene singlet excited
state can only decay to the triplet excited state and finally
to the singlet ground state—see the schematic sketch in Fig. 2.
Comparing the overall photoreactivity of 11a–c to
previous reports on C60–oligomer and C60–oligomer–C60
systems,7–9,11,12 intramolecular transduction of singlet excited
state energy to the energetic sink (i.e, C60) is a common
reaction pathway.
?
absorbs, no evidence for a significant contribution of electron
Fig. 3 Transient absorption spectrum (i.e., visible-near-infrared part)
recorded 100 ns upon flash photolysis of 11c (2.0 6 1025 M) at 355 nm
in deoxygenated benzonitrile, showing the fullerene triplet–triplet
features (lmax 5 360 and 700 nm).
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transfer was gathered. Also C60 triplet quantum yields of unity
fullerene forming clear, non-cracking glasses in liquid nitrogen.
In case of any other solvents, the experiments were performed
at room temperature. A 570 nm long-pass filter in the emission
path was used in order to eliminate the interference from the
solvent and stray light for recording the fullerene fluorescence.
No corrections were performed for the fluorescence, but long
integration times (20 s) and low increments (0.1 nm) were
applied. The slits were 2 and 8 nm and each spectrum was an
average of at least 5 individual scans.
are nearly the same in the C60 reference and 11a–c.
Conclusions
In summary, we have carried out the synthesis of novel C60-
based dumbbell-type triads in which highly soluble p-con-
jugated oligophenyleneethynylenes (OPEs) are covalently
connected to two C60 units. The length of the OPEs has been
systematically increased by using a cross-coupling palladium
catalysed Hagihara–Sonogashira reaction of aryl iodides and
alkynyl derivatives. Finally, the two formyl containing OPEs
at terminal positions were reacted with C60 by following
Prato’s methodology to afford the novel series of triads. In
agreement with previous studies, the cyclic voltammetry of
11a–c and 12b showed that no significant electronic interaction
takes place between the two C60 units in the ground state,
which behave as two independent systems. Photophysical
studies carried out by fluorescence and transition absorption
spectroscopy revealed that despite the good electron donor
features of the oligomer, the energy of the C60 singlet excited
state drops below the energy of the resulting charge-separated
radical ion pair features that are similar to comparable C60–
oligomer and C60–oligomer–C60 systems.7–9,11,12 Thus, we see
an outcome that is dominated by an intramolecular energy
transfer reaction, which thermodynamically (i.e., larger energy
gap) and mechanistically (i.e., efficient dipole–dipole interac-
tions) prevails.
Synthesis and characterization
IR spectra were recorded in KBr pellets. 1H and 13C NMR
spectra were recorded at 500, 300, 200 and 125, 75, 50 MHz
respectively; chemical shifts are given as d values (int.
standard: TMS). Cyclic voltammograms were recorded on a
potentiostat/galvanostat equipped with a software electro-
chemical analysis Model 250 by using platinum as working
electrode, SCE as reference electrode, Bu4N+ClO4
2
as
supporting electrolyte, ODCB–CH3CN as solvent, and at a
scan rate of 100 mV s21
.
Triad 12a
C60 (360 mg, 0.5 mmol) was dissolved in chlorobenzene
(ultrasound 30 min), 4,49-diphenylethynyl dialdehyde (58.5 mg,
0.25 mmol) and sarcosine (133.6 mg, 1.5 mmol) was added.
The mixture was heated under reflux for 10 h. After the
evaporation the solvent, the residue was purified by column
chromatography on silica gel (CS2–toluene 5 : 2). Yield 58% of
black solid. Compound insoluble.
Experimental
Picosecond laser flash photolysis experiments were carried out
with 355 nm laser pulses from a mode-locked, Q-switched
Quantel YG-501 DP Nd : YAG laser system (pulse width 18 ps,
2–3 mJ pulse21). The white continuum picosecond probe pulse
was generated by passing the fundamental output through a
D2O–H2O solution. The excitation and the probe was fed to a
spectrograph (HR-320, ISDA Instruments, Inc.) with fiber-
optic cables and was analyzed with a dual diode array detector
(Princeton Instruments, Inc.) interfaced with an IBM-AT
computer.
Triad 12b
C60 (360 mg, 0.5 mmol) was dissolved in chlorobenzene
(ultrasound 30 min), 4,49-diphenylethynyl dialdehyde (58.5 mg,
0.25 mmol) and N-octylglycine (280 mg, 1.5 mmol) was added.
The mixture was heated under reflux for 10 h. After the
evaporation of the solvent, the residue was purified by column
chromatography on silica gel (CS2–toluene 5 : 2). Yield 58% of
black solid. 1H NMR (500 MHz, CDCl3) d: 7.92 (brs, 4H),
7.67–7.65 (dd, J 5 8.22 Hz, 4H), 5.27 (d, J 5 9.18 Hz, 2H),
5.22 (s, 2H), 4.30 (dd, J 5 9.2 Hz, 2H), 3.42–3.40 (m, 4H), 2.75
(m, 4H), 2.18 (m, 2H), 2.07 (m, 2H ), 1.86 (m, 4H), 1.73 (m,
4H), 1.63 (m, 4H), 1.55 (m, 4H), 1.22 (m, 6H). 13C NMR
(125 MHz, CDCl3) d: 156.72, 154.49, 153.70, 153.49, 147.90,
147.88, 147.20, 146.97, 146.95, 146.87, 146.83, 146.79, 146.76,
146.71, 146.60, 146.56, 146.34, 146.24, 146.21, 146.07, 146.06,
145.96, 145.93, 145.89, 145.86, 145.84, 145.77, 145.32, 145.28,
145.01, 144.99, 143.79, 143.74, 143.66, 143.34, 143.25, 143.22,
143.21, 142.90, 142.87, 142.77, 142.72, 142.67, 142.66, 142.63,
142.50, 142.49, 142.32, 142.20, 140.90, 140.89, 140.65, 140.22,
138.17, 137.49, 137.17, 136.51, 136.34, 136.31, 132.65, 129.95,
124.29, 91.32, 83.04, 77.14, 69.38, 67.56, 54.12, 33.12, 30.89,
30.59, 29.57, 28.70, 24.13, 15.45. FTIR (KBr) n 2918, 1848,
526 cm21. lmax (CH2Cl2): 431, 321.02, 257.01 nm.
Nanosecond laser flash photolysis experiments were per-
formed with laser pulses from a Molectron UV-400 nitrogen
laser system (337.1 nm, 8 ns pulse width, 1 mJ pulse21). The
photomultiplier output was digitized with a Tektronix 7912 AD
programmable digitizer. A typical experiment consisted of
5–10 replicate pulses per measurement. The averaged signal
was processed with an LSI-11 microprocessor interfaced with a
VAX-370 computer.
Time-resolved emission fluorescence lifetimes were mea-
sured with a Laser Strope Fluorescence Lifetime Spectrometer
(Photon Technology International) with 337 nm laser pulses
from a nitrogen laser fiber-coupled to a lens-based T-formal
sample compartment equipped with a stroboscopic detector.
Details of the Laser Strobe systems are described on the
manufacturer’s web site, http://www.pti-nj.com.
Compound 10a
Steady-state emission and excitation spectra were recorded
with a SLM 8100 Spectrofluorometer. Fluorescence spectra
were measured in methylcyclohexane solutions containing
Compound 4 (600 mg, 1.84 mmol) and compound 5 (852.8 mg,
3.68 mmol) were dissolved in dry piperidine (20 mL), and the
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system was flushed with argon. Bis(triphenylphospine)-
palladium dichloride (64.48 mg, 0.092 mmol), copper(I) iodine
(19.69 mg, 0.1 mmol) and triphenylphosphine (29.96 mg,
0.1 mmol) were added. The mixture was stirred for 24 h at
room temperature. Chloroform was added to the residue and
washed with saturated NH4Cl, neutralized with saturated HCl
10% and washed with NaCl. The organic phase was dried with
MgSO4 and the solvent was removed. The crude product was
purified by column chromatography on silica gel, hexane–
6.96 (brs, 2H), 6.93 (brs, 2H), 4.13–3.91 (m, 12H), 1.86–1.54
(m, 12H), 1.34–0.88 (m, 60H), 0.0053 (s, 36H). 13C NMR
(50 MHz, CDCl3) d: 154.35, 153.53, 153.34, 153.28, 118.02,
117.30, 116.64, 114.45, 114.34, 114.23, 114.05, 69.89, 69.70,
69.30, 31.66, 31.60, 29.43, 29.31, 25.83, 25.64, 22.60, 18.69,
14.02, 13.98, 11.39, 0.98. FTIR (KBr) n 636, 657, 677, 769 ,
812, 1029, 1211, 1274, 1340, 1386, 1419, 1458, 1473, 2148,
2864, 2941 cm21. EM m/z 1238, 1132, 1131, 1050, 685. lmax
(CH2Cl2): 417; 315 nm. Anal. Calcd for C80H126O6Si2: C 77.49,
H 10.24, O 7.74. Found: C 78.19, H 9.89%.
1
AcOEt (9 : 1).Yield 61% of yellow solid. Mp 125–126 uC. H
NMR (200 MHz, CDCl3) d: 10.02 (s, 2H), 7.89–7.85 (dd,
J 5 8.55 Hz, 2H), 7.69–7.65 (dd, J 5 8.05 Hz, 2H), 7.04 (s,
2H), 4.08–4.02 (t, J 5 6.35 Hz, 4H), 1.93–1.80 (m, 4H), 1.57–
1.25 (m, 12H), 0.93–0.86 (m, 6H). 13C NMR (50 MHz, CDCl3)
d: 191.32, 153.89, 135.45, 132.03, 129.67, 129.56, 116.90,
113.96, 94.24, 90.01, 69.65, 31.56, 29.26, 25.73, 22.61, 13.99.
EM m/z 543 (100), 463 (9), 450 (15), 379 (19), 366 (100). FTIR
(KBr) n 1218, 1282, 1697, 2320, 2570 cm21. lmax 395.99;
327.92 nm. Anal. Calcd for C36H38O4: C 80.87, H 7.16, O
11.97. Found: C 80.95, H 7.16%.
Tris(2,5-dihexyloxy-1,4-(ethynylbenzyl)benzene 7
Compound 6 (1.238 mg, 1 mmol) was dissolved in THF
(30 mL) and tetrabutylammonium fluoride (574.2 mg,
2.2 mmol) was added to the solution. The solution was stirred
at room temperature for 10 h. The mixture was extracted with
chloroform and washed with water. The organic phase was
dried with MgSO4. The solvent was removed and the product
was crystallized with hexane. Yield 84% of yellow solid. 1H
NMR (200 MHz, CDCl3) d: 7–6.97 (brs, 6H), 4.04–3.96 (m,
12H), 3.34 (s, 2H), 1.86–1.80 (m, 12H), 1.55–1.25 (m, 36H),
0.90–0.87 (m, 18H). 13C NMR (50 MHz, CDCl3) d: 154.17,
153.55, 153.34, 118.02, 117.33, 117.12, 115.01, 114.33, 112.63,
91.54, 80.04, 69.71, 69.65, 31.58, 31.51, 29.29, 29.24, 29.14,
25.63, 25.59, 22.60, 22.56, 13.98. FTIR (KBr) n 601, 640, 860,
Triad 11a
To a solution of C60 (360 mg, 0.5 mmol) in chlorobenzene
(100 mL), compound 10a (58.5 mg, 0.25 mmol) and
N-octylglycine (280 mg, 1.5 mmol) was added. The mixture
was heated under reflux for 10 h. After evaporation of the
solvent, the residue was purified by column chromatography
1062, 1220, 1346, 1384, 1464, 1496, 2854, 2925, 2950 cm21
.
1
on silica gel (CS2–toluene 5 : 2). Yield 43% of black solid. H
Compound 10b
NMR (500 MHz, CDCl3) d: 7.71 (brs, 4H), 7.50–7.49 (dd,
J 5 8.37 Hz, 4H), 6.89 (s, 2H), 5.04–5.00 (m, 4H), 4.07–4.02
(m, 2H), 3.92–3.81 (m, 4H), 3.15–3.13 (m, 4H), 2.50–1.91 (m,
4H), 1.91–1.81 (m, 4H), 1.80–1.70 (m, 8H), 1.58–1.53 (m, 4H),
1.47–1.17 (m, 20H), 0.85–0.71 (m, 12H). 13C NMR (125 MHz,
CDCl3) d: 156.41, 154.12, 153.57, 153.28, 147.29, 146.65,
146.41, 146.29, 146.20, 146.14, 146.01, 145.92, 145.73, 145.51,
145.31, 145.24, 145.15, 144.70, 144.61, 144.37, 143.14, 142.97,
142.66, 142.55, 142.29, 142.23, 142.12, 142.01, 141.86, 141.65,
141.51, 140.16, 140.12, 139.85, 139.52, 137.73, 136.90, 136.54,
135.81, 135.64, 131.81, 129.41, 123.43, 116.84, 113.86, 94.70,
86.67, 82.31, 69.57, 68.88, 66.80, 53.23, 31.91, 31.54, 29.54,
29.32, 29.22, 28.28, 27.54, 25.68, 22.71, 22.61, 14.16, 14.06.
FTIR (KBr) n 526, 1375, 279, 2848 cm21. lmax (CH2Cl2): 431,
371.97, 266.92 nm. EM (MALDI-TOF) m/z 2226.60.
Compound 7 (920 mg, 1 mmol) and 5 (464 mg, 2 mmol)
were dissolved in dry piperidine (20 mL) and toluene (6 mL),
the system was flushed with argon. Bis(triphenylphosphine)-
palladium dichloride (35.09 mg, 0.05 mmol), copper(I) iodine
(18.66 mg, 0.098 mmol) and triphenylphosphine (25.70 mg,
0.098 mmol) were added. The mixture was stirred for 48 h at
room temperature. Chloroform was added to the residue and
washed with saturated NH4Cl, HCl (10%) and NaCl. The
organic phase was dried with MgSO4 and the solvent was
removed. The crude product was purified by column
chromatography on silica gel, hexane–AcOEt (9 : 1). Yield
84% of yellow-orange solid. Mp 119–120 uC. 1H NMR
(200 MHz, CDCl3) d: 10–02 (s, 2H), 7.89–7.84 (dd, J 5 8.3 Hz,
4H), 7.68–7.64 (dd, J 5 8.05 Hz, 4H), 3.96–3.91 (m, 12H),
1.89–1.82 (m, 12H), 1.56–1.51 (m, 12H), 1.36–1.25 (m, 24H),
0.91–0.88 (m, 18H). 13C NMR (50 MHz, CDCl3) d: 191.34,
155.00, 153.94, 153.58, 153.50, 153.40, 135.35, 131.98, 129.82,
129.55, 117.85, 117.16, 117.03, 115.29, 114.35, 113.04, 91.98,
91.42, 90.27, 69.80, 69.72, 69.56, 31.58, 29.30, 25.75, 25.65,
22.61, 22.58, 14.00. FTIR (KBr) n 731, 777, 1024 (m), 1217 (s),
1274 (m), 1386 (m), 1493 (m), 1596 (m), 1654 (m), 1701 (s),
2854 (s), 2925 (s) cm21. EM m/z 1158, 1135, 925, 737, 612, 296.
lmax (CH2Cl2): 434.99, 329.02, 274.97 nm. Anal. Calcd for
C76H94O8: C 80.38, H 8.34. Found: C 80.35, H 8.83%.
Tris(2,5-dihexyloxy-1,4-triisopropylsilylethynylbenzyl)benzene.
6
Compounds 4 (326 mg, 1 mmol) and 2 (1.168 mg, 2 mmol)
were dissolved in dry piperidine (20 mL) and the system was
flushed with argon. Bis(triphenylphosphine) dichloride
(35.09 mg, 0.005 mmol), copper(I) iodide (19.04 mg, 0.1 mmol)
and triphenylphosphine (26.23 mg, 0.1 mmol) were added. The
mixture was stirred for 48 h at room temperature. Chloroform
was added and washed with NH4Cl, neutralized with saturated
HCl 10% and washed with NaCl. The organic phase was dried
with MgSO4 and the solvent was removed. The crude product
was purified by column chromatography on silica gel, using
toluene as the eluent. Yield 78% of yellow solid. Mp 100–
Triad 11b
C60 (360 mg, 0.5 mmol) was dissolved in chlorobenzene
(100 mL), compound 10b (58.5 mg, 0.25 mmol) and
N-octylglycine (280 mg, 1.5 mmol) was added. The mixture
1
101 uC. H NMR (200 MHz, CDCl3) d: 6.99–6.97 (brs, 2H),
130 | J. Mater. Chem., 2005, 15, 124–132
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was heated under reflux for 10 h. After the evaporation the
solvent, the residue was purified by column chromatography
the product was crystallization with hexane. Yield 98% of
1
yellow solid. Mp 95–96 uC. H NMR (300 MHz, CDCl3) d:
1
on silica gel (CS2–toluene 5 : 2). Yield 35% of black solid. H
7.02–7.00 (brs, 10H), 4.05–4.00 (m, 20H), 3.36 (s, 2H), 1.85–
1.81 (m, 20H), 1.58–1.52 (m, 20H), 1.40–1.26 (m, 40H), 0.92–
0.89 (m, 30H). 13C NMR (75 MHz, CDCl3) d: 155.36, 154.53,
153.89, 153.69, 118.30, 118.17, 117.60, 117.41, 115.80, 115.34,
114.78, 114.67, 114.52, 112.92, 92.00, 91.96, 91.62, 82.71,
80.45, 79.75, 76.99, 70.29, 70.11, 70.05, 32.02, 31.94, 30.11,
29.69, 29.64, 29.54, 26.08, 26.05, 26.02, 23.05, 14.45. FTIR
(KBr) n 1265, 2304, 2385, 3419 cm21. EM m/z 1549, 1443,
1361, 1201, 1088.
NMR (500 MHz, CDCl3) d: 7.73 (brs, 4H), 7.52–7.50 (d,
J 5 8.37 Hz, 4H), 6.91–6.89 (s, 6H), 5.05–5.01 (m, 4H), 4.07–
4.02 (m, 2H), 3.94–3.89 (m, 12H), 3.16–3.15 (m, 4H), 2.51 (m,
4H), 1.91–1.81 (m,4H), 1.79–1.72 (m,16H), 1.59–1.17 (m,
48H), 0.85–0.71 (m,24H). 13C NMR (125 MHz, CDCl3) d:
156.31, 154.95, 154.75, 154.03, 153.60, 153.48, 153.30, 147.30,
146.63, 146.39, 146.30, 146.25, 146.20, 146.14, 146.00, 145.93,
145.74, 145.51, 145.31, 145.25, 145.16, 144.70, 144.62, 144.37,
143.14, 142.97, 142.67, 142.56, 142.28, 142.22, 142.13, 142.01,
141.87, 141.65, 141.51, 140.12, 139.86, 139.53, 137.75, 136.95,
136.58, 135.79, 135.63, 131.82, 129.44, 123.50, 117.79, 117.63,
117.22, 117.03, 115.35, 114.40, 114.26, 113.92, 113.73, 112.54,
94.62, 92.11, 91.60, 91.48, 91.39, 86.77, 82.32, 79.92, 79.59,
79.33, 69.66, 69.56, 68.86, 66.76, 53.26, 31.58, 30.92, 29.60,
29.32, 29.25, 29.10, 28.22, 27.53, 25.69, 25.63, 22.71, 22.61,
14.15, 14.01. FTIR (KBr) n 549, 1186, 2395 cm21. lmax
(CH2Cl2): 420.06, 321.96, 254.96 nm. EM (MALDI-TOF) m/z
2825.01.
Compound 10c
Compound 9 (63 mg, 0.041 mmol) and 5 (19.03 mg,
0.082 mmol) were dissolved in dry THF (20 mL), and the
system was flushed with argon. Tetrakis(triphenylphosphine)-
palladium (4.76 mg, 0.004 mmol), copper(I) iodine (0.5 mg,
0.004 mmol) were added. The mixture was heated for 46 h at
70–80 uC. Chloroform was added to the residue and washed
with saturated NH4Cl, HCl (10%) and NaCl. The organic
phase was dried with MgSO4 and the solvent was removed.
The crude product was purified by column chromatography
on silica gel (hexane–AcOEt 9 : 1). Yield 43% of yellow-orange
Compound 8
Compound 7 (820 mg, 1 mmol) and 2 (1.096 mg, 2 mmol) were
dissolved in dry piperidine (20 mL) and toluene (20 mL). The
system was flushed with argon. Bis(triphenylphosphine)-
palladium dichloride (35.09 mg, 0.05 mmol), copper(I) iodide
(19.04 mg, 0.1 mmol) and triphenylphosphine (26.23 mg,
0.1 mmol) were added. The mixture was stirred for 48 h at
room temperature. The residue was washed with saturated
NH4Cl, HCl (10%), NaCl and extracted with chloroform. The
organic phase was dried with MgSO4. The crude was purified
by column chromatography on silica gel, using toluene as
1
solid. Mp 119–120 uC. H NMR (200 MHz, CDCl3) d: 10–03
(s, 2H), 7.89–7.87 (d, J 5 8.09 Hz, 4H), 7.70–7.67 (d,
J 5 8.09 Hz, 4H), 7.03 (s, 10H), 4.08–4.02 (t, J 5 6.45 Hz,
20H), 2.05–1.82 (m, 20H), 1.81–1.56 (m, 20H), 1.54–1.23 (m,
40H), 0.9–0.84 (m, 30H). 13C NMR (50 MHz, CDCl3) d:
191.45, 153.91, 153.54, 153.50, 153.45, 135.32, 132.01, 129.85,
129.59, 117.23, 117.08, 116.94, 115.26, 114.47, 114.29, 114.09,
112.92, 93.92, 92.05, 91.68, 91.68, 91.56, 91.34, 90.33. FTIR
(KBr) n 731, 777, 1024, 1217, 1274, 1386, 1493, 1596, 1654,
1701, 2854, 2925. lmax (CH2Cl2): 432.99, 324.01, 279 nm. EM
m/z 1735.
1
eluent. Yield 80% of yellow solid, mp 118–119 uC. H NMR
(300 MHz, CDCl3) d: 7.02–6.95 (brs, 10H), 4.06–3.93 (m,
20H), 1.88–1.75 (m, 20H), 1.53–1.36 (m, 20H), 1.35–1.14 (m,
40H), 0.92–0.88 (m, 30H). 13C NMR (75 MHz, CDCl3) d:
155.45, 155.38, 155.17, 155.10, 154.74, 153.95, 153.90, 153.72,
153.65, 152.91, 151.96, 131.31, 129.23, 123.63, 119.73, 118.27,
118.19, 118.02, 117.60, 117.44, 117.10, 116.90, 116.62, 115.93,
115.82, 114.88, 114.76, 114.69, 114.66, 114.45, 114.41, 114.38,
114.09, 114.03, 113.83, 112.92, 103.61, 103.43, 103.00, 96.91,
96.52, 96.18, 94.78, 92.66, 92.12, 92.03, 92.00, 91.76, 88.03,
80.02, 79.76, 77.64, 70.58, 70.21, 70.12, 69.92, 69.79, 69.62,
69.08, 32.11, 32.04, 31.95, 30.12, 29.92, 29.85, 29.78, 29.71,
29.62, 29.57, 29.53, 29.44, 26.28, 26.09, 26.04, 23.05, 23.02,
19.12, 14.49, 14.45, 11.79. FTIR (KBr) n 929, 1215, 1508, 1539,
1558, 1575, 2148, 2399 cm21. EM m/z 1861, 1838, 1755, 1672,
1492, 1317, 1201. lmax (CH2Cl2): 433; 272 nm. Anal. Calcd for
Triad 11c
To a solution of C60 (76.32 mg, 0.106 mmol) was dissolved in
chlorobenzene (50 mL), compound 10c (93 mg, 0.053 mmol)
and sarcosine (14.15 mg, 0.159 mmol) was added. The mixture
was heated under reflux for 3 h. After the evaporation the
solvent, the residue was purified by column chromatography
1
on silica gel (CS2–toluene 5 : 2). Yield 13% of black solid. H
NMR (500 MHz, CDCl3) d: 7.74 (brs, 2H), 7.73–7.72 (d,
J 5 3.33 Hz, 2H), 7.63–7.56 (d, J 5 8.15 Hz, 4H), 7.03–7.01 (s,
10H), 4.06–4.02 (m, 20H), 2.85 (s, 3H), 1.70–1.60 (m, 20H),
1.58–1.45 (m, 20H), 1.4–1.2 (m, 40H), 0.92–0.75 (m, 30H). 13
C
NMR (125 MHz, CDCl3) d: 168.15, 156.56, 154.34, 154.06,
153.93, 153.90, 153.61, 153.44, 147.73, 147.06, 146.84, 146.74,
146.69, 146.64, 146.57, 146.52, 146.44, 146.37, 146.35, 146.17,
145.95, 145.84, 145.75, 145.69, 145.65, 145.59, 145.11, 145.03,
144.79, 143.56, 143.50, 143.41, 143.11, 143.01, 142.99, 142.96,
142.68, 142.65, 142.53, 142.45, 142.31, 142.10, 141.96, 140.57,
140.30, 140.00, 137.71, 137.26, 136.89, 136.30, 136.12, 132.87,
131.26, 129.20, 117.70, 117.50, 114.88, 114.75, 114.70, 114.19,
91.99, 91.89, 83.75, 70.46, 70.11, 70.01, 69.48, 68.56, 40.41,
39.14, 32.33, 32.02, 32.01, 31.99, 31.31, 30.77, 30.10, 30.06,
29.76, 29.71, 29.33, 26.13, 26.08, 26.06, 24.16, 23.38, 23.09,
C
120H182O10Si2: C 78.29, H 9.96. Found: C 77.52, H 9.76%.
Compound 9
Compound 8 (155 mg, 0.085 mmol) was dissolved in THF
(15 mL) and tetrabutylammonium fluoride (0.17 mL,
0.179 mmol) was added to the solution. The solution was
stirred at room temperature for 12 h. The mixture was
extracted with chloroform and washed with water. The organic
phase was dried with MgSO4. The solvent was removed and
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1999, 10, 2445–2457; (c) From Synthesis to Optoelectronic
23.03, 14.51, 14.47, 14.44, 14.43, 11.36. lmax (CH2Cl2): 430.9,
311.98, 255.94 nm. EM m/z 1616, 1524, 1373, 969.
Properties, ed. D. M. Guldi and N. Mart´ın, Kluwer Academic
Publishers, Dordrecht, The Netherlands, 2002; (d) D. M. Guldi,
Chem. Commun., 2000, 321–327; (e) D. M. Guldi and N. Mart´ın,
J. Mater. Chem., 2002, 12, 1978–1992; (f) D. M. Guldi and
M. Prato, Chem. Commun., 2004, 2517–2525.
Acknowledgements
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2005, in press; (b) J. F. Nierengarten, Solar Energy Mater. Solar
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The authors acknowledge the Secretar´ıa de Estado de
Educacio´n y Universidades and MECD of Spain. This work
has been supported by the MCYT of Spain (Projects
BQU2002-00855, BQU2000-0790). Part of this work was
carried out with support from the EU (RTN network
‘‘WONDERFULL’’) SFB 583 and the Office of Basic
Energy Sciences of the U.S. Department of Energy
(Contribution No. NDRL-4580 from the Notre Dame
Radiation Laboratory).
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(b) S. Fukuzumi, H. Imahori, H. Yamada, M. E. El-Khouly,
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Carmen Atienza,a Braulio Insuasty,ab Carlos Seoane,a Nazario Mart´ın,*a
Jeff Ramey,c G. M. Aminur Rahmand and Dirk M. Guldi*c
aDepartamento de Qu´ımica Orga´nica, Facultad de Ciencias Qu´ımicas,
Universidad Complutense, E 28040 Madrid, Spain.
E-mail: nazmar@quim.ucm.es; Fax: 91 394 4103; Tel: 91 394 4227
bDepartamento de Qu´ımica, Facultad de Ciencias, Universidad del Valle,
AA-25360, Cali, Colombia
cUniversita¨t Erlangen, Institute for Physical Chemistry, 91058 Erlangen,
Germany. E-mail: dirk.guldi.@chemie.uni-erlangen.de
dRadiation Laboratory, University of Notre Dame, Notre Dame,
IN 46556, USA
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