Self-association and electron transfer in donor-acceptor dyads connected by meta-substituted oligomers

Article abstract of DOI:10.1021/ja9024269

The synthesis of a new series of electron donor-acceptor conjugates (5, 10, 13, and 16) in which the electron acceptor - C₆₀ - and the electron donor - π-extended tetrathiafulvalene (exTTF) - are bridged by means of m-phenyleneethynylene spacer

Full text of DOI:10.1021/ja9024269

Published on Web 08/10/2009
Self-Association and Electron Transfer in Donor-Acceptor
Dyads Connected by meta-Substituted Oligomers
Agust´ın Molina-Ontoria,† Gustavo Ferna´ndez,† Mateusz Wielopolski,‡,§
Carmen Atienza,‡ Luis Sa´nchez,† Andreas Gouloumis,† Timothy Clark,§
Nazario Mart´ın,*,†,| and Dirk M. Guldi*,‡
Departamento de Qu´ımica Orga´nica, Facultad de Qu´ımica, UniVersidad Complutense,
E-28040 Madrid, Department of Chemistry and Pharmacy and Interdisciplinary Center for
Molecular Materials (ICMM), Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, Egerlandstrasse
3, 91058 Erlangen, Germany, Computer-Chemie-Centrum and ICMM, Department of Chemistry and
Pharmacy, Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, Na¨gelsbachstrasse 25, 91052
Erlangen, Germany, and IMDEA-Nanociencia, E-28049 Madrid, Spain
Received March 26, 2009; E-mail: nazmar@quim.ucm.es; dirk.guldi@chemie.uni-erlangen.de
Abstract: The synthesis of a new series of electron donor-acceptor conjugates (5, 10, 13, and 16) in
which the electron acceptorsC60sand the electron donorsπ-extended tetrathiafulvalene (exTTF)sare
bridged by means of m-phenyleneethynylene spacers of variable length is reported. The unexpected self-
association of these hybrids was first detected to occur in the gas phase by means of MALDI-TOF
spectrometry and subsequently corroborated in solution by utilizing concentration-dependent and variable-
temperature 1H NMR experiments. Furthermore, the ability of these new conjugates to form wirelike
structures upon deposition onto a mica surface has been demonstrated by AFM spectroscopy. In light of
their photoactivity and redoxactivity, 5, 10, 13, and 16 were probed in concentration-dependent photophysical
experiments. Importantly, absorption and fluorescence revealed subtle dissimilarities for the association
constants, that is, a dependence on the length of the m-phenylene spacers. The binding strength is in 5
greatly reduced when compared with those in 10, 13, and 16. Not only that, the spacer length also plays
a decisive role in governing excited-state interactions in the corresponding electron donor-acceptor
conjugates (5, 10, 13, and 16). To this end, 5, in which the photo- and electroactive constituents are bridged
by just one aromatic ring, displayssexclusively and independent of the concentration (10-6 to 10-4
M)sefficient intramolecular electron transfer events on the basis of a “through-bond” mechanism. On the
contrary, the lack of conjugation throughout the bridges in 10 (two m-phenyleneethynylene rings), 13 (three
m-phenyleneethynylene rings), and 16 (four m-phenyleneethynylene rings) favors at low concentration (10-6
M) “through space” intramolecular electron transfer events. These are, however, quite ineffective and, in
turn, lead to excited-state deactivations that are at high concentrations (10-4 M) dominated by intracomplex
electron transfer events, namely, between exTTF of one molecule and C60 of another molecule, and that
stabilize the resulting radical ion pair state with lifetimes reaching 4.0 µs.
Introduction
the absorption of light and its conversion into power. For this
purpose, an efficient transport of electrons and holes through
The implementation of flexible optoelectronic devices1 and
more specifically organic photovoltaics2 requires the develop-
ment of molecules able to perform important functions such as
an organized network of p-typesusually soluble p-phenyle-
nevinylene (PPV) or polythiophene (PT) polymer derivativess
and n-typesthe most successful being the well-known fullerene
derivative PCBMscomponents is required.2 Prior to the mobil-
ity of the charge carriers, charge separation between the donor
and acceptor moieties at the donor-acceptor interface is needed.
This charge transfer event usually takes place between nonco-
valently linked species. A number of supramolecular model
systems have been studied to demonstrate the effectiveness in
which charge transfer can proceed.3 Thus, supramolecular
† Universidad Complutense.
‡ Department of Chemistry and Pharmacy and ICMM, Friedrich-
Alexander-Universita¨t Erlangen-Nu¨rnberg.
§ Computer-Chemie-Centrum and ICMM, Department of Chemistry and
Pharmacy, Friedrich-Alexander-Universita¨t Erlangen-Nu¨rnberg.
| IMDEA-Nanociencia.
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12218 J. AM. CHEM. SOC. 2009, 131, 12218–12229
10.1021/ja9024269 CCC: $40.75  2009 American Chemical Society

Self-Association and Electron Transfer in D-A Dyads
A R T I C L E S
donor-acceptor assemblies connected by hydrogen-bonding
arrays3a,b or coordinative metal bonds3c,d have been exploited
in the quest for long-lived charge-separated states. However,
detailed studies on charge transfer interactions in supramolecular
assemblies held together by means of π-π aromatic interactions
are yet scarce, despite their implication in naturalscharge
transport through DNA bases4a-csor artificialscharge transport
in the active layer of photovoltaic devices2,4d,essystems. An
important requisite for these studies involves the connection of
appropriate donor (D) and acceptor (A) moieties by bridges (B’s)
of different natures. Conjugates prepared by the combination
of a variety of electron donor fragments, such as ferrocenes5 or
porphyrins,6 and electron acceptor units, for instance, peryle-
nebisimides,7 have been utilized to investigate electron transfer
processes in D-B-A conjugates upon light irradiation. In our
research group, we have described a number of conjugates in
which the electron donor is 2-[9-(1,3-dithiol-2-ylidene)anthra-
cen-10(9H)-ylidene]-1,3-dithiole (exTTF)8 and the electron
acceptor unit is [60]fullerene,9 connected by different covalent
or supramolecular spacers.10 Very recently, we have reported
on the formation of supramolecular complexes from concave
exTTF-based pincerlike receptors and the convex C60 surface
in a positive homotropic cooperative manner.11 These complexes
experience an electron transfer through the π-π stacking
interactions that hold together some of these tweezers with C60
to form the corresponding supramolecular radical pair.12
In this paper we report on the synthesis and redox properties
of a new family of D-B-A conjugates in which D is an exTTF
fragment,8 A is C60, and the B groups are m-phenyleneethy-
nylene oligomers of different lengths. We have chosen this class
of spacer (i) because it is known that the promotion of an
electron from the D to the A is favored in the excited state in
only one way but not in the reverse one, that is, the charge
recombination would be unfavored,13 and (ii) because of the
well-documented ability of m-phenyleneethynylenes to switch
from random conformations in chlorinated solvents to helical
arrangements in polar solvents due to the influence of solvo-
phobic effects.14 This situation allows us to study the compe-
tence between inter- or intramolecular electron transfer pro-
cesses. Unexpectedly, these new hybrids self-associate by π-π
aromatic interactions to form aggregates as has been demon-
strated by mass spectrometry, 1H NMR experiments at different
concentrations, and atomic force microscopy (AFM) imaging.
Photophysical studies reveal the strong impact of the meta
substitution on the photophysical properties for these new
conjugates in comparison with our previous results reported for
their para-conjugated congeners15 as well as the concentration
dependence of these properties as a consequence of the self-
assembly process.
Results and Discussion
Synthesis. The synthesis of the final hybrids 5, 10, 13, and
16 was carried out by stepwise approaches based on the
preparation of asymmetrically functionalized π-conjugated
oligo(m-phenyleneethynylene) (OMPE) spacers of different
lengths endowed with a formyl group and exTTF at the terminal
positionsscompounds 4, 9, 12, and 15 in Schemes 1 and 2.
The preparation of the corresponding oligo(m-phenyleneethy-
nylene) building blocks was performed by using Pd-catalyzed
cross-coupling reactions between derivatized arylacetylenes and
aryl halides.16
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J. AM. CHEM. SOC. VOL. 131, NO. 34, 2009 12219

A R T I C L E S
Molina-Ontoria et al.
Scheme 1. Synthesis of 5a
a Reagents and conditions: (a) PdCl2(PPh3)2, CuI, THF, piperidine, TMSA, 83%; (b) K2CO3, 99%, THF/MeOH (1:1); (c) compound 3, PdCl2(PPh3)2, CuI,
THF, piperidine, 47%; (d) C60, N-octylglycine, PhCl, ∆, 40%.
Scheme 2. Synthesis of 10, 13, and 16a
a Reagents and conditions: (a) K2CO3, 2-butanone, hexyl bromide, 99%; (b) PdCl2(PPh3)2, CuI, THF, piperidine, TMSA, 80%; (c) PdCl2(PPh3)2, CuI,
THF, piperidine, TIPSA, 92%; (d) K2CO3, THF/MeOH; (e) PdCl2(PPh3)2, CuI, THF, piperidine, compound 1, 94%; (f) Bu4NF, 72% (for 11d), 69% (for
14c); (g) PdCl2(PPh3)2, CuI, THF, piperidine, compound 3, 63% (for 4), 57% (for 9), 53% (for 12), 69% (for 15); (h) C60, N-octylglycine, PhCl, ∆, 54% (for
10), 63% (for 13), 65% (for 16); (i) Pd2(dba)3, AsPh3, THF, Pri2NH, compound 1, 77%; (j) PdCl2(PPh3)2, CuI, THF, piperidine, compound 2b, 45%; (k)
Pd2(dba)3, AsPh3, THF, NEt3, 47%.
catalytic system) between the commercially available 3-iodo-
benzaldehyde and (trimethylsilyl)acetylene (TMSA) in 82%
overall yield, after trimethylsilyl group cleavage with potassium
carbonate (Scheme 1).17
we followed a literature procedure.18 In particular, commercially
available 3,5-dibromophenol (6a) led first through the formation
of phenol ether 6b and then through two sequential Sonogashira
cross-coupling reactions between 6b, TMSA, and (triisopro-
pylsilyl)acetylene (TIPSA) to compounds 7a and 7b, respec-
tively. Cleaving the TMS group with potassium carbonatesa
procedure that ensures the integrity of the TIPS groupssis the
key toward compound 7c, which was obtained in quantitative
yield. A Sonogashira cross-coupling reaction between com-
Aryldiacetylenes emerged as valuable precursors for a
bidirectional stepwise synthesis of the spacers bearing two to
four phenylethynyl groups (Scheme 2). For the stepwise
synthesis of the unsymmetrically substituted aryldiacetylene 7b
(17) Vidal-Ferran, A.; Mu¨ller, C. M.; Sanders, J. K. M. J. Chem. Soc.,
Chem. Commun. 1994, 2657–2658.
(18) Grave, C.; Lentz, D.; Schafer, A.; Samor´ı, P.; Rabe, J. P.; Franke, P.;
Schluter, A. D. J. Am. Chem. Soc. 2003, 125, 6907–6918.
9
12220 J. AM. CHEM. SOC. VOL. 131, NO. 34, 2009

Self-Association and Electron Transfer in D-A Dyads
A R T I C L E S
Table 1. Redox Potentials (V) of C60-OMPE-exTTF Conjugates
5, 10, 13, and 16 and Their Precursors
pounds 2b and 7c leads to the unsymmetrically substituted
compound 8a, which after cleavage of the TIPS group with
tetrabutylammonium fluoride (TBAF) affords 8b in 73% yield.
Similarly, the preparation of bisaryldiacetylene 11a was
carried out by a Sonogashira cross-coupling reaction, but in this
case between compounds 7a and 7c. Following cleavage of the
TMS groups, compound 11b was obtained in 98% yield. The
reaction of the latter with compound 1 via a modified Sono-
gashira copper-free protocol using Pd2(dba)3/AsPh3 as the
catalytic system,19 to avoid Glaser-type homocoupling reactions,
led to compound 11c. The final TIPS cleavage that yields 11d
was, however, performed with potassium fluoride, since TBAF
started to degrade the formyl groups.
For the synthesis of compound 14c a convergent synthetic
protocol was applied. Essentially, using the Sonogashira protocol
in the reaction between compounds 2b and 6b, compound 14a
is obtained in 45% yield. 14a and 11b were converted to 14b
via the modified Sonogashira copper-free catalytic system.
Following the treatment with potassium fluoride to cleave the
TIPS group, compound 14c was isolated in 38%.
compda
E1red
E2red
E3red
E1oxid
E2oxid
C60
5
10
13
16
4
-0.843
-0.901
-0.872
-0.869
-0.872
-1.794b
-1.984b
-2.028b
-2.036b
-2.006b
-1.294
-1.299
-1.260
-1.257
-1.250
-1.758
-1.841
-1.794
-1.799
-1.792
+0.330
+0.230
+0.226
+0.282
+0.296
+0.203
+0.216
+0.257
+1.792
+1.680
+1.626
+1.661
+1.752
+1.744
+1.683
+1.631
+1.774
9
12
15
14b
a Experimental conditions: V vs Ag/AgNO3, oDCB/CH3CN (4:1) as
solvent, GCE as working electrode, Pt as counter electrode, Bu4NClO4
(0.1 M) as supporting electrolyte, scan rate 100 mV s-1
. The
concentration of the sample in all the measurements was 0.3 mM.
b Corresponding to the aldehyde functionality.
The syntheses of the precursors bearing the exTTF moieties
(i.e., 4, 9, 12, and 15) are outlined in Schemes 1 and 2. We
applied the same catalytic protocolsPd(PPh3)4, CuI, and dry
triethylamine at 60 °C for 5-7 hsin the cross-coupling reactions
between compounds 2b, 8b, 11d, and 14c and 320 to obtain
these red solid intermediates with yields varying from 33% to
69%.
Finally, 5, 10, 13, and 16 were obtained by 1,3-dipolar
cycloaddition of the corresponding azomethine ylides, generated
in situ from N-octylglycine and aldehydes 4, 9, 12, and 15 with
C60.21 5, 10, 13, and 16 were obtained as brown solids in yields
ranging from 40% to 65%.
All compounds and intermediates were characterized by
means of analytical and spectroscopic techniques. The FTIR
spectra showed, for example, the characteristic peaks of the
carbonyl group in the precursors 4, 9, 12, and 15 at around
1700 cm-1, while the peak at around 525 cm-1 confirms the
Figure 1. Cyclic voltammograms of 13 (∼0.3 mM) in o-dichlorobenzene/
acetonitrile (4:1) utilizing 0.1 M Bu4NClO4 as the supporting electrolyte
and a glassy carbon as the working electrode at a scan rate of 100 mV s-1
.
1
presence of C60 in 5, 10, 13, and 16. The H NMR and 13C
NMR spectra reveal the expected resonance signals of aliphatic
and aromatic protons and carbons; see for details the Supporting
Information. Typical signals are, for example, those that
correspond to the pyrrolidine protons at δ ≈ 5.1, δ ≈ 5.0, and
δ ≈ 4.1.
In none of the conjugates is the highly energetic process at ∼-2
V, which corresponds to reduction of the aldehyde group,
discernible (Table 1). Regarding the anodic region of the
voltammograms, all the conjugates show two oxidation waves
at ∼+0.2 and ∼+1.7 V, respectively (Figure 1). The first
quasireversible oxidation wave corresponds to the loss of two
electrons by the exTTF moiety.22 Only for the first congener of
the series (i.e., 5) is the oxidation shifted by about 0.2 V when
compared to the rest of the series (i.e., 10, 13, and 16). Implicit
is a better electronic communication between C60 and exTTF.
The second oxidation process corresponds to that of the meta-
conjugated bridgessa conclusion that stems from the analysis
of 14b.23 In fact, only two waves are seen in the voltammogram
of 14b: One wave evolves around -2 V, a process that
corresponds to the reduction of the aldehyde functionality, see
also the reactivity of 4, 9, 12, and 15sand another wave appears
at +1.7 V, which is ascribed to the oxidation of the bridge. In
5, even the oxidation of the bridge is shifted anodically, when
Redox Properties. The electrochemical properties of conju-
gates (5, 10, 13, and 16) and their precursors (4, 9, 12, and 15)
have been investigated by cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) in a solvent mixture
containing o-dichlorobenzene (oDCB)/acetonitrile (4:1); see
Table 1 and Figure 2. Furthermore, pristine C60 and the TIPS-
protected tetramer decorated with the aldehyde functionality
(14b) have also been utilized as reference compounds.
Three quasi-reversible reduction processes refer in all C60-
based conjugates to the first, second, and third reduction of the
fullerene moiety (Figure 1). Nevertheless, the saturation of a
double bond in C60, as a consequence of the functionalization,
shifts the reduction cathodically relative to that of pristine C60.
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157–168. (b) Bryce, M. R.; Moore, A. J. J. Chem. Soc., Chem.
Commun. 1991, 22, 1638–1639. (c) Batsanov, A. S.; Bryce, M. R.;
Coffin, M. A.; Green, A.; Hester, R. E.; Howard, J. A. K.; Lednev,
I. K.; Mart´ın, N.; Moore, A. J.; Moore, J. N.; Ort´ı, E.; Sa´nchez, L.;
Saviro´n, M.; Viruela, P. M.; Viruela, R.; Ye, T. Chem.sEur. J. 1998,
4, 2580–2592.
(20) D´ıaz, M. C.; Illescas, B. M.; Seoane, C.; Mart´ın, N. J. Org. Chem.
2004, 69, 4492–4499.
(21) (a) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993,
115, 9798–9799. (b) Tagmatarchis, N.; Prato, M. Synlett 2003, 768–
779.
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A R T I C L E S
Molina-Ontoria et al.
mer and 6-mer, respectively (Figures S2-S4, Supporting
Information).26
Evidence for the ability of systems 5, 10, 13, and 16 to self-
assemble in solution has been obtained by concentration-
1
dependent H NMR experiments carried out in CDCl3. The
interaction of the geometrically and electronically complemen-
tary units of C60 and exTTF by means of π-π stacking,
solvophobic, and concave-convex interactions has been con-
firmed by the slight shiftsall resonances shift upfield, except
those corresponding to the 1,3-dithiole rings that move
downfieldsand broadening of most resonances with increasing
concentration (Figures 3 and S5, Supporting Information).3d,f,27,28
A detailed analysis of these experiments demonstrates that only
those resonances corresponding to the exTTF unit and to the
fulleropyrrolidine ring shift with increasing concentration,
leaving the signals of the oligomeric bridge practically unaltered.
These findings suggest that the self-association of the conjugates
is produced by the interaction of the geometrically and redox-
active complementary units of exTTF and C60, with the bridge
being a mere spectator in the self-assembly process.
Figure 2. MALDI-TOF mass spectra of 5 in a ditranol matrix. The inset
illustrates the formation of supramolecular oligomers (i.e., up to the 20-
mer) in the gas phase.
1
Variable-temperature H NMR studies carried out for com-
pounds 13 and 16 (Figures 4 and S7, Supporting Information)
show features similar to those observed in the concentration
dependence 1H NMR experiments and confirm the self-
association process of all these new conjugates. In addition, these
1H NMR experiments demonstrate the inversion of the spatial
arrangement of the nitrogen present in the fulleropyrrolidine
ring.29 The 1H NMR spectrum of compound 16 at room
temperature only presents one set of resonances for the geminal
protons of the pyrrolidine ring (δ ≈ 5.1 and 4.1), which implies
a fast exchange between the two extreme cis and trans
conformations for this nitrogen atom regarding the organic
addend. Decreasing the temperature below 258 K induces the
apparition of two sets of well-defined resonances at δ ≈ 5.1.
Below this temperature, the inversion barrier of the tertiary
nitrogen is frozen and two formal chiral centers, namely, the
nitrogen atom of the pyrrolidine ring and the carbon atom
supporting the organic addend, appear to generate the two
possible enantiomeric pairs in different proportions. A similar
effect is also observed for the resonance at δ ≈ 4.1, rendering
this process more energetic since only the broadening of the
signal but not the appearance of two sets of resonances is
observed in the range of temperatures studied (Figures 4 and
S6, Supporting Information).
compared to the remaining conjugates. It is most probable that
a combined effect, namely, fewer number of rings together with
a more efficient interaction with the acceptor, is responsible for
this trend. In summary, our redox datastogether with the
information extracted from the electronic absorption spectra (see
below)ssuggest the existence of weak electronic communica-
tions between the electroactive units in the new conjugates in
their ground states.
Self-Association. At first glance, the structural characterization
of 5 as performed with MALDI-TOF mass spectrometry just
confirmed the molecular mass with an m/z value of 1354.
Nevertheless, a weaker peak is seen at m/z ) 2708. Notable is
the difference between these two m/z values, which leads us to
hypothesize that a 2-mer of 5 is formed. A carefully performed
analysis of the MALDI-TOF spectrum brings as many as 20
peakssall spaced by the same molecular mass of 1354sto light
(Figure 2). Consequently, we must assume the systematic
aggregation of 5, whose aggregate size ranges from the
formation of the 2-mer all the way to the 20-mer. Such
findingssincluding our previously reported C60-exTTF
hybrids24sindicate the unexpected ability of 5 to self-associate
in the gas phase.12,25
Similar MALDI-TOF analyses have also been carried out for
10, 13, and 16. Importantly, in all the cases aggregates are
formed reflecting the spontaneous self-assembly of the mono-
meric units with molecular ions for 10 (i.e., m/z ) 1554), 13
(i.e., m/z ) 1754), and 16 (i.e., m/z ) 1954). The number of
self-assembled units of the corresponding conjugate is inversely
proportional to the molecular mass of the conjugate. In addition
to the aforementioned 5, formation of up to the 14-mer is
observed for 10, whereas 13 and 16 tend to form up to the 11-
(24) A subsequent MALDI-TOF analysis carried out for the para-
conjugated congener of dyad 5, compound 9a in ref 6c, has also
allowed the observation of this unexpected behavior.
(25) The ability of a pristine exTTF molecule to recognize [60]fullerene
or its derivative PCBM has been proven to be negligible: Otero, R.;
´
Ecija, D.; Ferna´ndez, G.; Gallego, J. M.; Sa´nchez, L.; Mart´ın, N.;
Miranda, R. Nano Lett. 2007, 7, 2602–2607.
(26) The travel time in MALDI-TOF experiments is directly proportional
to the m/z value. Therefore, the ions corresponding to the aggregates
formed from 16 need a longer time to reach the detector in comparison
with the other conjugates, and they hold a higher possibility to
disassemble. For a recent book on the utilization of the MALDI-TOF
technique to detect supramolecular species, see: Schalley, C. Analytical
Methods in Supramolecular Chemistry; Wiley: New York, 2007.
(27) (a) Pe´rez, E. M.; Capodilupo, A. L.; Ferna´ndez, G.; Sa´nchez, L.;
Viruela, P. M.; Viruela, R.; Ort´ı, E.; Bietti, M.; Mart´ın, N. Chem.
Commun. 2008, 4567–4569.
(23) We and others have experimentally observed that the oxidation
processes of π-conjugated oligomers sometimes appear as broad and
difficultly assignable waves in cyclic voltammetry. For recent ex-
amples, see: (a) Ferna´ndez, G.; Sa´nchez, L.; Veldman, D.; Wienk,
M. M.; Atienza, C.; Guldi, D. M.; Janssen, R. A. J.; Mart´ın, N. J.
Org. Chem. 2008, 73, 3189–3196. (b) Zen, A.; Bilge, A.; Galbrecht,
F.; Alle, R.; Meerholz, K.; Grenzer, J.; Neher, D.; Scherf, U.; Farrell,
T. J. Am. Chem. Soc. 2006, 128, 3914–3915. (c) Narutaki, M.;
Takimiya, K.; Otsubo, T.; Harima, Y.; Zhang, H.; Araki, Y.; Ito, O.
J. Org. Chem. 2006, 71, 1761–1768. (d) Zhao, Y.; Shirai, Y.; Slepkov,
A. D.; Cheng, L.; Alemany, L. B.; Sasaki, T.; Hegmann, F. A.; Tour,
J. M. Chem.sEur. J. 2005, 11, 3643–3658.
(28) The association constants (Ka) for compounds 5, 10, 13, and 16
determined from these CD 1H NMR investigations have been
calculated in the range of 16-34 M-1
.
(29) Lukoyanova, O.; Kitaygorodskiy, A.; Cardona, C. M.; Echegoyen, L.
Chem.sEur. J. 2007, 13, 8294–8301.
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Self-Association and Electron Transfer in D-A Dyads
A R T I C L E S
1
Figure 3. Partial H NMR spectra (CDCl3, 300 MHz, 298 K) of 10 at different concentrations.
Figure 5. AFM images (tapping mode, air, 298 K) of a drop-cast
chloroform solution of 13: (a) large AFM image, (b) expanded area. (c)
Height profile along the black line in (b).
optical measurements an asset of meta-linked phenyleneethy-
nylene systems is their lack of conjugation, which keeps the
visible range of the spectrum transparentsin contrast to the
previously studied para-linked analogues, where the conjugation
leads to a broad and intense absorption in the range of interest.
In other words, for the meta-linked phenyleneethynylene systems
the only significant contributions come from the absorptions of
C60 (i.e., 434 and 705 nm) and exTTF (i.e., 450 nm). This
opened the opportunity to test a wide range of concentrations
and follow the developments of intracomplex hybrids by
spectroscopic means.30 For example, with increasing concentra-
tions, we note that in toluene a distinct new absorption
developssred-shifted relative to that of exTTFsat 470 nm.
1
Figure 4. Partial variable-temperature H NMR spectra (298-228 K, 4
mM, CDCl3, 300 MHz) of 16.
The self-assembly of 5, 10, 13, and 16 onto solid supports
was followed by atomic force microscopy (AFM) on mica.
Figures 5 and S7 (Supporting Information) depict the AFM
images obtained upon drop-casting ∼10-7 M solutions of 5,
10, 13, and 16. In all cases, reticulated networks are visible.
These are of variable height and are constituted by interactive
necklace-like wires. Isolated, individual wires are also observed,
as demonstrated in Figure 5b,c. Height profiles that give rise to
approximately 1 nm imply the presence of only one molecular
unit.
(30) (a) Giacalone, F.; Segura, J. L.; Mart´ın, N.; Guldi, D. M. J. Am. Chem.
Soc. 2004, 126, 5340–5341. (b) de la Torre, G.; Giacalone, F.; Segura,
J. L.; Martín, N.; Guldi, D. M. Chem. Eur. J. 2005, 11, 1267–1280.
Photophysical Properties. Photophysical means provide fur-
ther insights into the organization of 5, 10, 13, and 16. For
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A R T I C L E S
Molina-Ontoria et al.
Figure 8. Excitation spectra of 10 (orange) and 13 (black) in toluene
solution monitoring the emission at 530 nm.
nm in benzonitrile. Moreover, a complementary excitation
spectrum confirms the origin of the new emission band (Figure
8). We interpret this new band as being due to charge transfer
interactions. These take the form of a shift of electron density
from exTTF to C60 to give a polar state that is solvated better
in the more polar solvents. Inspecting 10, 13, and 16, a distinct
stabilization of the interactions is reflected by a continuous red
shift from 533 nm (10) to 537 nm (13) and 554 nm (16); all
values are cited for toluene. Typical quantum yields of these
Figure 6. Absorption spectra of a toluene solution of 10 with increasing
concentrations (5.5 × 10-6, 9.3 × 10-6, 1.2 × 10-5, 1.3 × 10-5, 1.6 ×
10-5, 1.8 × 10-5, 2.0 × 10-5, 2.4 × 10-5, 2.8 × 10-5, 3.4 × 10-5, 3.9 ×
10-5, and 4.3 × 10-5 M). Arrows indicate the progression of the titration
and the development of the new features.
emissive features, which depend on the structure and the solvent
31
are on the order of 10-3
.
In particular, they tend to be lower
in polar media than in nonpolar mediasa trend that is well
understood on the basis of excited-state relaxations. For example,
10 gives rise to emission maxima at 533, 550, and 575 nm in
toluene, o-dichlorobenzene, and benzonitrile, respectively.
Notable is the correlation with the corresponding absorption
maxima at 470, 485, and 495 nm. Similar conclusions were
previously derived for C60-exTTF13 and C60-ZnP31 charge
transfer absorption and emission features. Second, in the red
region we note the C60-centered emission with a maximum
around 710 nm. The position of the maximum lacks any
susceptibility with respect to the solvent polarity. Nevertheless,
it is quenched (i.e., <3.0 × 10-4) when compared to the C60
reference (i.e., 6.0 × 10-4).10
Figure 7. Emission spectra of a toluene solution of 10 with increasing
concentrations (5.5 × 10-6, 9.3 × 10-6, 1.2 × 10-5, 1.3 × 10-5, 1.6 ×
10-5, 1.8 × 10-5, 2.0 × 10-5, 2.4 × 10-5, 2.8 × 10-5, 3.4 × 10-5, 3.9 ×
10-5, and 4.3 × 10-5 M). Arrows indicate the progression of the titration
and the development of the new features.
For a 1:1 stoichiometry, the change in fluorescence intensity
(∆I) is related to the association constant (K) by a Benesi-
Hildebrand-type equation:32
∆I/c ) K∆I∞ - K∆I
Throughout these concentration variations the C60 and exTTF
features remain visible; see Figure 6. In oDCB and benzonitrile
the absorptions shift to 485 and 495 nm, respectively. Interest-
ingly, these changes are only discernible for 10, 13, and 16. In
5, on the other hand, intramolecular C60-exTTF interactions
dominate over intermolecular C60-exTTF interactions and, in
turn, hamper in the concentration of the photophysical experi-
ments the formation of intracomplex hybrids. Steric hindrance
might also contribute to the suppression of the self-organization.
When turning to complementary emission experiments, we
note several changes. First, in the blue region the broad and
structureless exTTF emission transformssonly for 10, 13, and
16swith increasing concentration into a new band (Figure 7).
The latter is essentially a mirror image of what has been seen
in the absorption measurements. In toluene the emission band
maximizes at 533 nm, which again shifts to the red when more
polar solvents are used: 550 nm in o-dichlorobenzene and 575
where ∆I∞ is the maximum change in fluorescence intensity
when all molecules form the complex. Using this method we
(31) Guldi, D. M.; Scheloske, M.; Dietel, E.; Hirsch, A.; Troisi, A.; Zerbetto,
F.; Prato, M. Chem.sEur. J. 2003, 9, 4968–4979.
(32) The association constants (K) were determined for the explicit
association between exTTF and C60 with a 1:1 complex stoichiometry.
The complex formation is associated with a newly developing charge
transfer absorption, whose equilibrium is expressed by the Benesi-
Hildebrand method. To this end, we have used the concentration
dependence to determine the association constant according to Benesi-
Hildebrand: Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949,
71, 2703-2707. Assuming weak complexation constants (i.e., K ≈
1000 M-1) combined with appreciable absorption changes, this method
provides reasonable results: Yang, C.; Liu, L.; Mu, T.-W.; Guo, Q.-
X. Anal. Sci. 2000, 16, 537–540. In turn, it has emerged as a versatile
tool for describing the 1:1 association of organic charge transfer
complexes. Organic Charge-Transfer Complexes; Foster, R., Ed.;
Academic Press: London, 1969; p 125.
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Self-Association and Electron Transfer in D-A Dyads
A R T I C L E S
Time-resolved fluorescence measurements could not provide
further details about the competition between intramolecular and
intracomplex processes. In fact, we only see a component that
we ascribe to the intramolecular process with a rate constant of
1.5 × 109 s-1. We believe, however, that only the lack of time
resolution is responsible for this ambiguous trend.
The aforementioned trends indicated the need for further
investigations, namely, testing the C60-exTTF systems by
transient absorption measurements. In particular, excitations at
387 and 482 nm were chosen to photoexcite the ground state
of C60, exTTF, and the C60-exTTF monomer and the charge
transfer state of the intracomplex hybrids, respectively.
Photoexcitation of exTTF at 387 nm generates an exTTF-
centered excited state. Spectral characteristics of this very short-
lived excited state (1.2 ps) are transient maxima around 465,
605, and 990 nm as well as transient bleaching at <450 nm.
The short lifetimes (i.e., after 5 ps no transient absorption
remains) are rationalized by the presence of the sulfur atoms,
with a strong second-order vibronic spin-orbit coupling. Going
beyond our femtosecond experiments (i.e., 3 ns) we tested
exTTF in nanosecond experiments following 355 nm excitation.
However, outside of the 10 ns time window of the instrumental
time resolution, no notable transients were detected.
The singlet excited state of C60, on the other hand, displays
a distinctive singlet-singlet transition around 880 nm.35 The
latter undergoes a quantitative but nevertheless slow (i.e., 5 ×
108 s-1) intersystem crossing to yield the long-lived triplet
manifold, for which maxima are noted at 360 and 700 nm,
followed by a low-energy shoulder at 800 nm.
Now, when turning to the C60-exTTF systems, photoexci-
tation at 387 nm in the low concentration regime should be
discussed first. Detecting the instantaneous grow-in of the 880
nm absorption affirms the successful C60 excitationssimilar to
what has been seen for the reference experiments with C60.
Instead of seeing, however, the slow intersystem crossing
dynamics, the singlet-singlet absorption decays in the presence
of exTTF donors with accelerated dynamics. The singlet-excited-
state lifetimes, as they were determined from an average of first-
order fits of the time-absorption profiles at various wavelengths,
are on the order of 30 ps. Spectroscopically, the transient
absorption changes, taken after the completion of the decay,
bear no resemblance to the C60 triplet excited state. In particular,
the new transients reveal strong maxima in the visible rangesat
∼680 nmswhich match those of the one-electron-oxidized
exTTF radical cations, while in the near-infraredsat ∼1000
nmsfeatures evolve that are a close spectral match to those of
the one-electron-reduced C60 radical anion (Figure 10).36 In other
words, our spectroscopy confirms the formation of intramo-
lecular radical ion pair statessformed through bond in 5 and
through space in 10, 13, and 16. The latter are metastable on
the femto/picosecond time scale. This required examining the
charge-recombination dynamics on the nanosecond time scale.
Therefore, the same solutions were excited with a 6 ns laser
pulse. In this context, the spectral fingerprints of the radical
ion pair statesas seen immediately after the nanosecond laser
pulsesare useful probes (Figure 11). Important is the fact that
Figure 9. Charge transfer emission (dashed line, right x-axis) versus
C60 emission (solid line, left x-axis) quantum yields of an oDCB solution
of 10.
have calculated the binding constants (K) as 1.1 × 103 M-1
(10), 2.1 × 103 M-1 (13), and 1.4 × 103 M-1 (16). Since no
appreciable charge transfer fluorescence was observed for 5,
the evaluation of an association constant was dispensable.
In the low concentration regime (10-6 M), where the electron
donor-acceptor conjugates are present in their monomeric form,
the fluorescence quenching is likely due to intramolecular
processes. An electron transfer deactivation that takes place
between the electron-donating exTTF and the electron-accepting
C60 leads to the formation of a radical ion pair statesvide infra.14
Notable is in this context that running a solvent dependence
(i.e., toluene, o-dichlorobenzene, benzonitrile) on the fluores-
cence quenching reveals changes only for 5 (i.e., toluene, 1.2
× 10-4; o-dichlorobenzene, 7.6 × 10-5; benzonitrile, 2.2 ×
10-5). All others, that is, 10 ((1.0 ( 0.3) × 10-4), 13 ((1.2 (
0.3) × 10-4), and 16 ((1.8 ( 0.3) × 10-4), give rise to nearly
invariable quantum yields regardless of the solvent. This
indicates for 10, 13, and 16 distinct through space interactions,
while through bond interactions are likely to exist in 5. Similar
conclusions, that is, through space interactions, were noted for
flexibly linked C60-donor systems.33 Implementation of a
flexible linker is thought to enable intramolecular rearrangements
that are driven by charge transfer interactions. In other words,
C60-donor systems adapt a foldedswith close C60-donor
proximitiessrather than a stretched conformation.34 An increase
in the overall concentration (up to 10-4 M) evokes a further
decrease of the C60 fluorescence quantum yields. Since these
experiments are always set in relation to the C60 reference, we
propose that now an alternative electron transfer deactivation
dominates the photoreactivity of C60 and exTTF. Implicit is a
scenario which involves tied charge transfer interactions between
C60 and exTTF, that is, intracomplex processes in the newly
formed intracomplex hybrids. Interestingly, both emission
features track each other; the charge transfer emission band
reaches its maximum at a concentration where the C60 fluores-
cence quenching is at its peak (Figure 9).
(33) (a) Guldi, D. M. Pure Appl. Chem. 2003, 75, 1069–1076. (b) Guldi,
D. M. Chem. Soc. ReV. 2002, 31, 22–36. Herranz, M. A.; Ehli, C.;
Campidelli, S.; Gutie´rrez, M.; Hug, G. L.; Ohkubo, K.; Fukuzumi,
S.; Prato, M.; Mart´ın, N.; Guldi, D. M. J. Am. Chem. Soc., 2008, 130,
66–73.
(35) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5653. (b) Burke, K.;
Perdew, J. P.; Wang, Y. In Electronic Density Functional Theory:
Recent Progress and New Directions; Dobson, J. F., Vignale, G., Das,
M. P., Eds.; Plenum: New York, 1998.
(36) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54,
724–728. (b) Rassolov, V. A.; Pople, J. A.; Ratner, M. A.; Windus,
T. L. J. Chem. Phys. 1998, 109, 1223–1226.
(34) Schuster, D. I.; MacMahon, S.; Guldi, D. M.; Echegoyen, L.;
Braslavsky, S. E. Tetrahedron 2006, 62, 1928–1936. (a) Guldi, D. M.;
Hungerbu¨hler, H.; Asmus, K.-D. J. Phys. Chem. 1995, 99, 9380–9385.
(b) Guldi, D. M.; Sa´nchez, L.; Mart´ın, N. J. Phys. Chem. B 2001,
105, 7139–7144.
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A R T I C L E S
Molina-Ontoria et al.
Figure 10. Differential absorption spectra (visible) obtained upon femto-
second flash photolysis (387 nm) of 10 (10-6 M) in nitrogen-saturated
o-dichlorobenzene solutions with time delays between 0 and 3000 ps at
room temperature (black, 0 ps; gray, 1 ps; yellow, 2900 ps).
Figure 12. (a) Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (482 nm - 150 nJ) of 10 (2.0
× 10-4 M) in nitrogen-saturated o-dichlorobenzene solutions with time
delays between 0 and 3000 ps at room temperature (black, 0 ps; blue, 10
ps; yellow, 2900 ps). (b) Time-absorption profiles of the spectra shown
above at 510, 695, 750, and 900 nm to monitor the formation of the radical
ion pair state.
where through bond interactions lead to lifetimes of 128 ns
again, at low concentrations (Figures S8, Supporting Informa-
tion).
In stark contrast, excitation into the charge transfer bands,
that is, in the high concentration regime, evokes the immedi-
ate formation of the radical ion pair state; see Figure 12. In
particular, the exTTF radical cation and the C60 radical anion
features evolve nearly instantaneously (∼1 ps) at 680 and
1000 nm, respectively.13 This confirms the rapid formation
of the intracomplex radical ion pair state. The lifetime of
this intracomplex radical ion pair state is remarkably long,
exceeding the time scale of our femtosecond experiments. It
is on the nanosecond time scale, where the same radical ion
pair features are generated, which allows determining the
lifetimes as 4.0 ( 0.7 µs (10, 3.2 µs; 13, 4.7 µs; 16, 4.6 µs);
an example is illustrated in Figure 13. Such lifetimes infer a
significant stabilization relative to the monomer forms at low
concentrations. An intracomplex hopping mechanism is likely
to be responsible for longer lifetimes, since larger charge
separation distances are realized.
Figure 11. (a) Differential absorption spectrum (visible and near-infrared)
obtained upon nanosecond flash photolysis (355 nm) of 10 (2.0 × 10-6 M)
in nitrogen-saturated o-dichlorobenzene solutions with a time delay of 100
ns at room temperature, indicating the radical ion pair state features at 680
and 1010 nm. (b) Time-absorption profile of the spectrum shown above
at 675 nm to monitor the decay of the radical ion pair state.
Modeling. Density functional theory (DFT) and semiempirical
molecular orbital (MO) theory were used to characterize the
charge transfer features of the donor-bridge-acceptor conju-
gates. Because of the high computational demands of systems
of the size of those considered here, only monomeric 5, 10, 13,
and 16 were investigated. Geometry optimizations at the
B3PW9135/6-31G** 36 DFT37 level gave almost coplanar
the decays of both probes resemble each other and give rise to
kinetics that obey a clean unimolecular rate law. The lifetimes
are on the order of tens of nanoseconds for 10 (i.e., 26 ns), 13
(i.e., 41 ns), and 16 (i.e., 49 ns), where the reactivity is driven
by through space interactions. Different is the scenario for 5,
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Self-Association and Electron Transfer in D-A Dyads
A R T I C L E S
Figure 13. (a) Differential absorption spectrum (visible and near-infrared)
obtained upon nanosecond flash photolysis (355 nm) of 10 (2.0 × 10-4 M)
in nitrogen-saturated o-dichlorobenzene solutions with a time delay of 100
ns, indicating the radical ion pair state features at 680 and 1010 nm. (b)
Time-absorption profile of the spectrum shown above at 650 nm to monitor
the decay of the radical ion pair state.
conformations of the bridge for all isomers. Coplanarity of the
phenyl rings in oligo(phenyleneethynylene)s (OPEs) is favored
by π-overlap, but the calculations also reveal very low rotation
barriers. Nevertheless, the meta connectivity weakens the
π-conjugation in comparison to the strongly conjugated oligo(p-
phenyleneethynylene)s (OPPEs), as also suggested by the
absorption spectra, which reveal only absorption features due
to C60 and exTTF.
Figure 14. Representative frontier orbital schemes of 13 given by DFT
calculations including the orbital energies.
will address only the locally excited and the charge transfer
states of the monomers. In all systems the locally excited states
are attributed to electronic excitations of exTTF with oscillator
strengths on the order of f ≈ 0.5. The singlet-excited-state
energies calculated in vacuo (3.2 eV) are consistent with those
found throughout the absorption studies. In all C60-exTTF
systems, two distinct charge transfer states emerge that are
dominated by the HOMO f LUMO and HOMO f LUMO +
1 excitations. The orbital scheme shown in Figure 14 implies
that these states correspond to a charge transfer from exTTF
(HOMO) to C60 (LUMO/LUMO + 1). The change in the dipole
moment on excitation (µ) confirms the charge transfer nature
of these excited states. In particular, µ values that range between
43 and 157 D (Table S2, Supporting Information) are due to
charge-separated excited states. To evaluate any bridge effects,
Analysis of the frontier orbital schemes (Figure 14) cor-
roborates that the highest occupied orbitals are localized on the
electron-donating exTTF, whereas the lowest unoccupied orbit-
als are localized on C60. Important is that the bridge orbitals
are energetically well separated from the frontier orbitals,
resulting in virtually no overlap between the electron donor,
the bridge, and the electron acceptor. In line with this picture,
the HOMO - 1 to LUMO + 2 orbitals of all C60-exTTF
systems reveal almost equal orbital energies independently of
the length of the bridge (Table S1, Supporting Information).
Not only the ground-state features but also those of the excited
states are relevant for interpreting the photophysical experiments.
In this context, we have used semiempirical configuration
interaction (CI) calculations with the AM1* 38 Hamiltonian,
which includes d-orbitals on sulfur. First singles-only CI (CIS)
calculations with varying numbers of active orbitals were used
to judge the size of the CI window necessary to achieve
convergence of the results. An active window of 15 occupied
and 15 virtual orbitals gave a full description of the electronic
states of the C60-exTTF systems. In the current context, we
(37) All DFT calculations were performed using the Gaussian 03 program
package: Frisch, M. J.; et al. Gaussian 03, revision D.02; Gaussian,
Inc.: Wallingford, CT, 2004.
(38) Winget, P.; Horn, A. H. C.; Selcuki, C.; Martin, B.; Clark, T. J. Mol.
Model. 2003, 9, 408–414.
(39) Rauhut, G.; Clark, T.; Steinke, T. J. Am. Chem. Soc. 1993, 115, 9174–
9181.
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A R T I C L E S
Molina-Ontoria et al.
Scheme 3. Energy Diagram Showing the Excited-State
Deactivation Pathways in C60-exTTF at Low Concentrations (5,
10, 13, and 16) and at High Concentrations (10, 13, and 16)
Figure 15. Dependence of the calculated heats of formation, ∆Hf, on the
solvent permittivity (i.e., (ε - 1)/(2ε + 1), for the discussed states of 16 as
a representative example: locally excited state (red circles), CT state 1 (blue
triangles), CT state 2 (blue squares).
we have added the directly linked donor-acceptor system (the
0-mer; see Figure S9, Supporting Information) to our CI
calculations.
For 5 and the 0-mer, two charge transfer states were computed
at excitations between 372 and 381 nm (3.2-3.3 eV), which
are a perfect match to the experimental results. In both of these
C60-exTTF systems, these transitions are accompanied by a
change of dipole moment (i.e., µ) that is on the order of 45 D.
In 10, 13, and 16, the charge transfer transitions go along with
much larger changes in the dipole moment, reaching 97 D for
10, 125 D for 13, and 155 D for 16. These very large dipole
moments and the very low (essentially zero) oscillator strengths
confirm the charge transfer nature of the transitions and help
confirm the radical ion pair formation as found by time-resolved
absorption spectroscopy in the low-concentration regime (vide
supra). Furthermore, the dipole changes are consistent with those
previously published for the corresponding para-conjugated
systems (OPPEs).15a They correspond to charge separations (+1
and -1 electronic charges) of 20, 26, and 32 Å for 10, 13, and
16, respectively. The maximum possible distances (i.e., those
in the fully extended conformations shown in Scheme 2)
between the centers of C60 and the exTTF moiety in these three
compounds are approximately 22, 28, and 35 Å, respectively.
Obviously, the interplay between C60 and exTTF is in 5/0-mer
augmented by short bridge lengths favoring through bond charge
transfer interactions/enhanced electronic coupling. This, in turn,
lowers the activation barriers for charge separation processes
in 5. In addition, the close proximity of the donor and acceptor
moieties in 5 results in a low dipole moment in the charge-
separated state. On the other hand, the energy penalties that are
applicable in 10, 13, and 16 point to the necessities of through
space interactions/solvent stabilization, which become increas-
ingly important as the distances between the donor and acceptor
increase from 10 to 16.
charge transfer features in 5, for which already calculations in
vacuo revealed energetically lower charge transfer states in
comparison to the locally excited state (Table S2, Supporting
Information). Note that one might instinctively expect even
stronger solvent stabilization of the CT state, which has a dipole
moment of 155 D for 16. The increase in solvation energy of
approximately 30 kcal mol-1 on increasing the solvent permit-
tivity from 0.2 to 0.5 is, however, consistent with the calculated
dipole moment of the excited state. The standard Kirkwood-
Onsager expression for the free energy gives an effective
spherical cavity radius for the proposed CT state of 22 Å for
16 assuming that the dipole moment is 155 D. This value is
reasonable for 16, for which we estimate that the centers of
charge are approximately 35 Å apart. We emphasize, however,
that the SCRF model used for the calculations is far more
sophisticated than the Onsager model and that the estimate
provided above only serves to underline the reliability of both
the assignment of the CT state and the solvation calculations.
Finally, we have investigated solvent effects using the self-
consistent reaction field (SCRF) within the polarizable con-
tinuum model (PCM).39 Here, solvent stabilizations of the charge
transfer states are directly implemented. In comparison to the
calculations performed in vacuo, solvent effects assist in
lowering the energies of the charge transfer states relative to
those of the locally excited states. This implies energetic
stabilization of the charge-separated species in polar media
(Figure 15). Notably, enhanced solvent stabilizations were
indeed found for 10, 13, and 16. This proves the favorable
Conclusions
In summary, a series of new electron donor-acceptor
conjugates (5, 10, 13, and 16) have been synthesized, and
different techniques (MALDI-TOF, CD 1H NMR, and fluores-
cence and AFM imaging) demonstrate their self-organization
in the gas phase, in solution, and on solid substrates and test
them in electron transfer reactions. Important is the motif that
powers and controls the self-organization, namely, combining
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Self-Association and Electron Transfer in D-A Dyads
A R T I C L E S
convex C60sas an electron acceptorsand concave exTTFsas
an electron donorsthat are bridged by m-phenyleneethynylene
spacers. A unique asset is that the choice of m-phenyleneethy-
nylene assists in tuning the charge transfer features (i.e., charge
separation and charge recombination). In particular, implement-
ing just one m-phenyleneethynylene ring (5) leads to intramo-
lecular electron transfer that is exclusively and independent of
the concentration (10-6 to 10-4 M) driven by a “through bond”
mechanism. This is rationalized on grounds of steric hindrance,
which suppresses the self-organization. Different is the picture
when using two, three, or even four m-phenyleneethynylene
rings. At low concentration (10-6 M) “through space” intramo-
lecular electron transfer events are favored; see Scheme 3. Here,
both excited states, that is, exTTF (2.75 eV) and C60 (1.78 eV),
decay via the transient intramolecular radical ion pair state (1.2
eV). These interactions are, however, quite ineffective and, in
turn, lead to excited-state deactivations that are at high
concentrations (10-4 M) dominated by intracomplex charge
transfer events, namely, between exTTF of one molecule and
C60 of another molecule that are self-organized into wormlike
structures. In fact, distinct charge transfer features (i.e., absorp-
tion and emission) evolve. The charge transfer emissions
implysbesides large electronic coupling matrix elementssslow
radiative decays and small reorganization energies. In fact, the
inner (λV ) (λabsorption + λemission)/2) and outer (λS ) -λemission
- ∆GCR) reorganization energies were derived with the help
of the charge transfer absorption (λabsorption) and emission
(λemission) maxima. In 10, for example, the total reorganization
energies range from 0.5 eV (toluene) to 0.83 eV (benzonitrile).
Notable is the contribution of exTTF, whose substantial
structural reorganization evokes a rather large reorganization
energy. In the corresponding C60-ZnP charge transfer systems,
the lack of structural changes upon ZnP oxidation leads to
smaller values. The distinct charge transfer states in 10, 13, and
16 (2.47 eV in toluene, 2.40 eV in o-dichlorobenzene, 2.32 eV
in benzonitrile) convert upon photoexcitation into fully
C60•--exTTF•+ charge-separated states (1.2 eV in o-dichlo-
robenzene); see Scheme 3. Implicit is that, among the localized
singlet excited states, only the singlet excited state of exTTF
(2.75 eV) is able to deactivate via the charge transfer state, while
that of C60 (1.78 eV) is energetically insufficient to populate
the latter.
At higher concentration (10-4 M) through space intramo-
lecular electron transfer events are quite ineffective and, in turn,
lead to excited-state deactivations that are dominated by
intracomplex electron transfer events, namely, between exTTF
of one molecule and C60 of another molecule that are self-
organized into wormlike structures. Additionally, the self-
organization assists in stabilizing the resulting radical ion pair
state with lifetimes up to 4.7 µs. Such values reflect a remarkable
stabilization of more than 2 orders of magnitudes3.2-4.7 µs
versus 26-49 ns.
Acknowledgment. Financial support was provided by the MEC
of Spain (Projects CTQ2008-00795/BQU and Consolider-Ingenio
2010C-07-25200), CAM (MADRISOLAR Project P-PPQ-000225-
0505), UCM (Grant UCM-SCH-PR34/07-15826), Deutsche Fors-
chungsgemeinschaft (Grant SFB 583), and Office of Basic Energy
Sciences of the United States. A.M.-O., G.F., and A.G. thank the
CAM and MEC of Spain for a research grant and a Ramo´n y Cajal
contract, respectively. We are thankful to A. Soubrie´ (Centro de
Microscop´ıa y Citometr´ıa, UCM) for AFM imaging and Dr. M.
Alonso (UAM) for the MALDI-TOF spectra. We thank Dr. J. L.
Delgado for his help in the synthetic protocols.
Supporting Information Available: Experimental procedures
with complete spectroscopic and structural analysis, including
Figures S1-S10. This material is available free of charge via
the Internet at http://pubs.acs.org.
JA9024269
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