Efficient utilization of higher-lying excited states to trigger charge-transfer events

Article abstract of DOI:10.1002/chem.201001613

Several new fullerene-heptamethine conjugates, which absorb as far as into the infrared spectrum as 800 nm, have been synthesized and fully characterized by physicochemical means. In terms of optical and electrochemical characteristics, appreciable electronic coupling between both electroactive species is deduced. The latter also reflect the excited-state features. To this end, time-resolved, transient absorption measurements revealed that photoexcitation is followed by a sequence of charge-transfer events which evolve from higher singlet excited states (i.e., S₂-fast charge transfer) and the lowest singlet excited state of the heptamethine cyanine (i.e., S ₁- slow charge transfer), as the electron donor, to either a covalently linked C₆₀ or C_70, as the electron acceptor. Finally, charge transfer from photoexcited C₆₀/ C₇₀ completes the charge-transfer sequence. The slow internal conversion within the light-harvesting heptamethine cyanine and the strong electronic coupling between the individual constituents are particularly beneficial to this process.

Full text of DOI:10.1002/chem.201001613

DOI: 10.1002/chem.201001613
Efficient Utilization of Higher-Lying Excited States to Trigger Charge-
Transfer Events
Pierre-Antoine Bouit,[a] Fabian Spꢀnig,[c] Gregory Kuzmanich,[d] Evangelos Krokos,[c]
Christian Oelsner,[c] Miguel A. Garcia-Garibay,*[d] Juan Luis Delgado,[a, b]
Nazario Martꢁn,*[a, b] and Dirk M. Guldi*[c]
Dedicated to David I. Schuster on the occasion of his 75th birthday
Abstract: Several new fullerene–hepta-
methine conjugates, which absorb as
far as into the infrared spectrum as
800 nm, have been synthesized and
fully characterized by physicochemical
means. In terms of optical and electro-
chemical characteristics, appreciable
electronic coupling between both elec-
troactive species is deduced. The latter
also reflect the excited-state features.
To this end, time-resolved, transient
absorption measurements revealed that
photoexcitation is followed by a se-
quence of charge-transfer events which
evolve from higher singlet excited
states (i.e., S2—fast charge transfer)
and the lowest singlet excited state of
the heptamethine cyanine (i.e., S1—
slow charge transfer), as the electron
donor, to either a covalently linked C60
or C70, as the electron acceptor. Finally,
charge transfer from photoexcited C60/
C70 completes the charge-transfer se-
quence. The slow internal conversion
within the light-harvesting heptame-
thine cyanine and the strong electronic
coupling between the individual con-
stituents are particularly beneficial to
this process.
Keywords:
charge transfer
·
cyanines · dyes/pigments · excited
states · fullerenes
Introduction
Natural systems that are able to efficiently transform sun-
light into chemical energy through photosynthetic processes
have inspired basic research and development into the areas
of photovoltaics[1] and photocatalysis.[2] “Light” reactions, in
which solar energy is harvested and converted into energy
carriers, play a fundamental role in these processes. The suc-
cessful mimicry of natural photosynthesis—involving a se-
quence of light-harvesting, light-funneling, charge-separa-
tion, and charge-shift processes—mandates the consider-
ation of several aspects, broad light harvesting and efficient
charge separation, combined with slow charge recombina-
tion.
For example, the delocalization of charges within the
spherical carbon framework of C60, together with the rigid,
confined structure of its aromatic p sphere, offer unique op-
portunities for stabilizing charged entities. Above all, the
small reorganization energies within fullerenes in charge-
transfer reactions have led to a notable breakthrough in syn-
thetic electron-donor–acceptor systems by providing acceler-
ated charge separation and decelerated charge recombina-
tion. The vast majority of charge-transfer reactions com-
[a] Dr. P.-A. Bouit, Dr. J. L. Delgado, Prof. Dr. N. Martꢀn
IMDEA-nanociencia, 28049 Madrid (Spain)
[b] Dr. J. L. Delgado, Prof. Dr. N. Martꢀn
Departamento de Quꢀmica Orgꢁnica, Facultad de C. C. Quꢀmicas
Universidad Complutense de Madrid
28040 Madrid (Spain)
Fax : (+34)91394-4103
E-mail: nazmar@quim.ucm.es
[c] F. Spꢂnig, E. Krokos, C. Oelsner, Prof. Dr. D. M. Guldi
Department of Chemistry and Pharmacy &
Interdisciplinary Center of Molecular Materials (ICMM)
Friedrich-Alexander-University Erlangen-Nuremberg
Egerlandstr. 3, 91058 Erlangen (Germany)
Fax : (+49)9131-85-28307
E-mail: dirk.guldi@chemie.uni-erlangen.de
[d] G. Kuzmanich, Prof. Dr. M. A. Garcia-Garibay
Department of Chemistry and Biochemistry, University of California
Los Angeles, California 90095-1569 (USA)
Fax : (+1)310-825-0767
E-mail: mgg@chem.ucla.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001613.
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FULL PAPER
mence in energetically low-lying states. Ever since the pio-
neering work by Hammartstrçm et al. on electron transfer
from the higher-lying singlet excited states (i.e., S2) of por-
phyrins to ruthenium(II) complexes,[3] follow-up studies on
this topic have been published.[4] In these systems, the large
energy gap existing between, for example, the higher-energy
singlet excited states (i.e., S2) and the lowest-energy singlet
excited state (i.e., S1), as well as the relatively long lifetime
of S2, emerged as key parameters. It has only been very re-
cently that the possibility of designing optoelectronic
switches has been suggested—relying upon electron-transfer
events that are triggered from different excited state precur-
sors in metal-containing porphyrins.[5]
Importantly—to the best of our knowledge—heptamethine
cyanines covalently linked to fullerenes have never been
probed in photoinduced charge-transfer reactions, either as
the photo- or as the redox-active component.[9]
Herein, we report on uncommon charge-transfer evolu-
tions from higher-energy singlet excited states of a heptame-
thine cyanine, as the electron donor, to a covalently linked
C60 or C70, as the electron acceptor. The slow internal con-
version within the light-harvesting heptamethine and the
strong electronic coupling between the individual constitu-
ents are beneficial to the process.
On the other hand, only a few examples have been pub-
lished of all-organic electron donors, involving aromatic-hy-
drocarbon-containing systems.[6] In this regard, the relation-
ship between structure and charge-transfer reactivity will
shape the efficiency of the conversion of solar power into
energy, which makes it as important as any other parameter.
Light-harvesting ability, on the other hand, is a parameter
that is rarely addressed, despite its paramount importance in
organic photovoltaics. Extend-
Results and Discussion
Synthesis: The syntheses of electron-donor–acceptor conju-
gates 2 and 3 were carried out in two steps starting from the
previously described heptamethine cyanine Cy.[10] To this
end, introducing p-hydroxybenzaldehyde to the chloro-hep-
tamethine cyanine Cy was achieved by substitution of the
meso-chlorine atom to form 1 (Scheme 1).[8a] The reaction of
ing the onset of light harvesting
beyond 700 nm jeopardizes the
charge-transfer efficiency by
weakening the driving forces.
Such a drawback can be partial-
ly compensated for by lowering
the redox potential of the pho-
toactive components, an altera-
tion that is, however, often ac-
companied by chemical instabil-
ity. This calls for higher-lying
excited states as a precursor for
nonadiabatic charge transfer.
In the context of light har-
vesting in the infrared section
of the spectrum, the exceptional
optical properties of heptame-
thine cyanines are a great asset.
Their stability is based upon the
delocalization of
a
positive
charge between two terminal
amino groups through a rigid,
odd-numbered
CACHTUNGTRENNUNG
(sp2) skele-
ton.[7] The complex electronic
structure of heptamethine cya-
nines results in structural iner-
tia and, in turn, in slow structur-
al relaxation/thermalization, es-
pecially of higher-energy excit-
ed states.[3b–d,4,8] All of the
aforementioned criteria moti-
vated us to probe these com-
pounds as photo- and redox-
active building blocks in con-
junction with C60, as well as C70. Scheme 1. The synthesis of fullerene–heptamethine conjugates 2 and 3.
Chem. Eur. J. 2010, 16, 9638 – 9645
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N. Martꢀn, D. M. Guldi, M. A. Garcia Garibay et al.
1 with either C60 or C70 in the presence of N-octylglycine[11]
afforded 2 or 3 as deep green solids in 40 and 30% yields,
respectively. Notably, 3 was obtained and studied as a mix-
ture of isomers due to the multiple possible positions for
adding azomethine ylides onto C70.[12] The presence of vari-
ous alkyl chains in 2 and 3 provides reasonable solubility to
these compounds in common organic media. The structures
at +0.30 V (fully reversible) and +0.93 V (quasi-reversible).
Additionally, in 2, three reversible one-electron reduction
processes—all centered on C60—were observed at ꢁ1.18,
ꢁ1.52, and ꢁ2.07 V (Figure 2). In 3, one-electron reduction
1
of 2 and 3 were confirmed by H and 13C NMR spectroscopy
and high-resolution MS experiments with molecular ion
peaks for 2 [M+H]+: 1555.6309 (calcd for C118H80N3O:
1554.6296) and 3 [M+H]+: 1675.6292 (calcd for C128H80N3O:
1674.6301)—see Figures S1–S5 in the Supporting Informa-
tion for details.
Optical properties: The absorbance of 1 has a maximum
around 782 nm that is followed by weaker transitions in the
blue region of the spectrum with maxima at 260, 387, and
440 nm (Figure 1). From this we calculated singlet excited-
Figure 1. Absorption spectra of 1 (c), 2 (b), and 3 (g) recorded
in dichloromethane (5.0ꢄ10ꢁ6 m).
Figure 2. Top: cyclic voltammogram of 2, recorded in deaerated dichloro-
methane versus the ferrocene/ferricenium couple—tributylammonium
hexafluorophosphate (TBAPF6) was employed as the supporting electro-
lyte. Bottom: the corresponding differential pulse voltammogram.
state energies of ꢀ3.0 and 1.55 eV. The spectra of 2 and 3
resemble the sum of the individual spectra of 1 and either
C60 or C70. The well-known absorption bands of C60 and C70
are discernable at 256, 320, and 420 nm[13] and 236, 359, 377,
467, and 525 nm,[10,14] respectively. Evidence for electronic
communication between the electron donor (i.e., 1) and ac-
ceptor (i.e., C60 and C70) units comes from the reduced ex-
tinction coefficients of 2 and 3 when compared to that of 1
(Figure 1).
processes of C70 were detected at ꢁ1.20, ꢁ1.51, and ꢁ2.16 V.
In comparison with pristine C60, these reduction-potential
values are cathodically shifted. Such a trend can be rational-
ized on the basis that a double bond of C60 has been saturat-
ed, which is known to raise the energy of the LUMO.[16,17] In
both cases 2 and 3, the first oxidation of the cyanine was
shifted cathodically to +0.20 V without affecting the first re-
duction of 1—see Figures S6 and S7 in the Supporting Infor-
mation. Consequently, we derive radical-ion-pair state ener-
gies of 1.38 (2) and 1.4 eV (3) in dichloromethane.
Electrochemistry: The electrochemical properties of 1, 2,
and 3 were studied in dichloromethane, at room tempera-
ture, by means of cyclic voltammetry (CV) and differential
pulse voltammetry (DPV). As described for similar cationic
heptamethines, 1 reveals an amphoteric redox character
with a completely reversible one-electron reduction process
at ꢁ1.02 V followed by a nonreversible reduction process at
ꢁ1.94 V versus the ferrocene/ferricenium couple.[15] One-
electron oxidation processes, on the other hand, are found
Photophysical properties: Next, ultrafast laser flash photoly-
sis experiments were carried out, employing either an exci-
tation wavelength of 387 nm to mainly photoactivate C60 or
C70 or an excitation wavelength of 775 nm to exclusively
excite the cyanine moiety. Excitation of C60 and C70 at
387 nm populates their singlet excited states with character-
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Charge-Transfer Events
FULL PAPER
istic absorption maxima at 610/920[11b–18] and 610/1360 nm
(Figure S8),[19] respectively. The latter undergo intersystem
crossing to the corresponding triplet manifolds with charac-
teristic lifetimes of between 0.7 and 1.4 ns and characteristic
maxima at 700 nm.[11b,16,20] The transient absorption spectra
upon 387 nm excitation of 1 (Figure S9 in the Supporting In-
formation) in dichloromethane or benzonitrile are dominat-
ed by strong ground-state bleaching at 800 nm. Directly
upon laser excitation, the higher-energy singlet excited state
features of 1 are discernable due to maxima at 600, 850, and
940 nm, minima at 510, 565, 715, and 1255 nm, and broad
bleach between 725 and 845 nm. With a lifetime of 2.6 ps,
these features transform rapidly into those of the lowest-
energy singlet excited state, with distinct maxima at 515,
560, and 1250 nm and distinct minima at 716 and 880 nm.
The recovery of the ground state follows within 1650 ps. In
contrast, if 775 nm excitation is used (Figure S10 in the Sup-
porting Information) the lowest-energy singlet excited state
(1.56 eV) is formed rapidly (<0.5 ps) bypassing any higher-
energy singlet excited states, before converting into the trip-
let excited state.
When investigating 2 and 3, the higher-energy singlet ex-
cited state features of 1 are discernable instantaneously
upon photoexcitation at 387 nm—vide supra.[21] Considering
the time window in which internal conversion and vibration-
al relaxation occurs within 1, these transient characteristics
in 2 and 3 start to transform, in benzonitrile, into the main
maxima at 600, 1015, and 1050 nm (for 2; Figure 3), as well
as at 600, 850, 1015, and 1300 nm[11b,16,22] (for 3)—the radi-
cal-ion-pair states. Support for this hypothesis was found by
using a comparison with a spectroelectrochemically and
pulse radiolytically generated radical dication of 1. In fact,
these are in perfect agreement with the transient maxima
observed at 600 and 1050 nm. The transient features, on the
other hand, at around 1015 and 850/1300 nm are in accord-
ance with previous work that used pulse radiolysis and are
assigned to the C60 and C70 radical anions, respectively.[23]
Following the charge separation (kCS1), the charge recombi-
nation (kCR) to recover the ground state sets in with a life-
time of 30 ps for 2 (Figure 4) and 45 ps for 3 (Figure S11 in
the Supporting Information).
Figure 3. Top: differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (387 nm) of 2 in benzonitrile
with two representative time delays between 0 and 10 ps at room temper-
*
*
to . Bottom: time–absorption pro-
ature—time development is from
*
*
)
files of the spectra shown in the upper part at 590 ( ) and 855 nm (
monitoring the competition between the formation of the singlet excited
state and charge separation, as well as charge recombination.
tively. In fact, a correlation between the charge-transfer rate
(i.e., charge separation and charge recombination) and the
free-energy change for the underlying reaction reveals a par-
abolic dependence—as illustrated in Figure 5—with parame-
ters of the reorganization energy (0.74 eV) and electronic
coupling (89 cmꢁ1) closely resembling those seen in recent
work involving C60.[24]
The competition between charge transfer (kCS1
; i.e.,
3.2 ps) and internal conversion/vibrational relaxation (i.e.,
2.6 ps) is notable. The as-formed singlet excited states of 1
are discernable during the initial stages of our femtosecond
experiments—shoulders at 515 and 560 nm and the 1250 nm
maximum. The latter reveals shortened lifetimes (kCS2) of 10
(2) and 14 ps (3) in benzonitrile, if compared with the intrin-
sic lifetime of 1, to generate the same radical-ion-pair state
that originated from the higher-energy singlet excited state
(Figure 4). By decreasing the solvent polarity, the lifetimes
for charge separation and charge recombination in 2 (Fig-
ure S12 in the Supporting Information) are extended to 3.5
and 14 ps, respectively. For 3, the lifetimes are 3.5 and
When turning to the red area of the spectrum, the singlet
excited state features of C60 or C70 are discernable. In the
case of C70, these evolve in a region beyond that of the hep-
tamethine cyanine, 1350 to 1600 nm. Unlike those seen for
the reference systems, the singlet excited-state features are
short-lived and transform into those of the radical-ion-pair
states—vide supra. Multiwavelength analyses provide life-
times in dichloromethane of 9 ps for 2 and 11 ps for 3.
Upon excitation of 2 (Figure 6) and 3 at 775 nm, the fea-
tures of the directly and exclusively formed singlet excited
state, that is, maxima at 515, 560, and 1250 nm, are, in con-
trast to 1, short lived. In fact, they decay (kCS2), in dichloro-
methane and benzonitrile, in 30 and 10 ps (2) and 21 and
14 ps (3) to the radical-ion-pair states. Spectroscopically, the
latter comprise, for 2, maxima at 600, 1015, and 1050 nm,
175 ps. Such lifetimes suggest charge separation (i.e., kCS1
,
kCS2) and recombination (i.e., kCR) dynamics that are in the
normal and inverted region of the Marcus parabola, respec-
Chem. Eur. J. 2010, 16, 9638 – 9645
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N. Martꢀn, D. M. Guldi, M. A. Garcia Garibay et al.
Figure 4. Top: differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (387 nm) of 2 in benzonitrile
with two representative time delays between 0 and 100 ps at room tem-
Figure 6. Top: differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (778 nm) of 2 in benzonitrile
with two representative time delays between 0 and 10 ps at room temper-
*
*
to . Bottom: time–absorption
perature—time development is from
*
*
)
profiles of the spectra shown in the upper part at 590 ( ) and 855 nm (
monitoring the charge separation and charge recombination.
*
*
ature—time development from to . Bottom: time–absorption profiles
*
*
of the spectra shown in the upper part at 590 ( ) and 855 nm ( ) moni-
toring the competition between the formation of the singlet excited state
and charge separation, as well as charge recombination.
hand is the charge separation that occurs with driving forces
around 1.5 eV. Key to this pathway is the small distance be-
tween the electron donor and the electron acceptor, which
enables strong communication between them. On the other
hand is the intrinsic deactivation to the lowest-energy singlet
excited state, which is subject to a similar driving force. Con-
sidering the underlying kinetics, we estimate a branching of
approximately 1:2 (i.e.; charge separation from heptame-
thine S2:intrinsic deactivation S2!S1). From the lowest sin-
glet excited state we observe a charge transfer that is about
10 times slower [(kCS2)=30 ps (2) and 21 ps (3), Table 1]
than the charge transfer that finds its origins in the higher-
energy singlet excited states [(kCS1)=3.2 ps (2) and 3.5 ps
(3)]. Crucial for the slower kinetics are driving forces of
about 0.15 eV. The higher-energy singlet excited-state ener-
gies of 1.79 (C60) and 1.76 eV (C70) accelerate the charge
separation (kCS3) by increasing the driving force to nearly
0.4 eV in dichloromethane in 9 ps (2). It also opens up new
pathways for the recovery of the ground state, namely
energy transfer.
*
Figure 5. The dependence of the rate constants for charge separation (
)
0
*
and charge recombination ( ) on the driving force (ꢁDG ).
whereas 3 exhibits maxima at 600, 850, 1015, and 1300 nm.
The kinetic agreement is remarkably good—charge separa-
tion and charge recombination—between the 387 and
778 nm experiments.
In summary, photoexcitation of 2 or 3, at 387 nm, with
prompt formation of higher-energy singlet excited states re-
sults in two competing processes—Figure 7. On the one
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Charge-Transfer Events
FULL PAPER
vent peaks being used as an internal standard (1H: 7.26 ppm, CHCl3; 13C:
77.36 ppm, CDCl3). High-resolution mass spectrometry measurements
were performed at the Unidad de Espectrometrꢀa de Masas of Universi-
dad Complutense de Madrid. Column chromatography was performed
on Merck 60 (40–63 mm) silica. Compound Cy was prepared according to
published procedures.[10b]
Synthesis of 1: 4-(1,3-Dioxan-2-yl)phenol (132 mg, 0.7 mmol, 1.1 equiv)
was dissolved in DMF (20 mL), under an argon atmosphere and then
NaH (19 mg, 1.2 equiv) was added. After 30 min of stirring at RT, this so-
lution was added to Cy (500 mg, 0.66 mmol, 1 equiv) in DMF (20 mL).
The solution was stirred for 6 h at RT and then poured into ice and hy-
drochloric acid (1m, 200 mL). The resulting green solid was filtered off
and dissolved in dichloromethane, and the resulting solution was washed
with water. The organic phase was then dried with MgSO4 and evaporat-
ed. The crude solid was then dissolved in the minimum amount of di-
chloromethane and precipitated in diethyl ether to afford a green solid
(380 mg, 68%). 1H NMR (300 MHz, CDCl3): d=0.9 (t, J=7 Hz, 6H),
1.1–1.5 (m, 14H), 1.63 (s, 12H), 1.7–1.8 (m, 4H), 2.0–2.1 (m, 2H), 2.7–2.8
(m, 2H), 4.12 (t, J=7 Hz, 4H), 6.14 (d, J=15 Hz, 2H), 7.05 (d, J=8 Hz,
2H), 7.1–7.4 (m, 8H), 7.78 (d, J=15 Hz, 2H), 7.97 (d, J=8 Hz, 2H),
9.92 ppm (s, 1H); 13C NMR (75 MHz, CDCl3): d=4.9, 14.0, 21.0, 22.5,
24.6, 26.7, 27.3, 27.9, 31.4, 44.9, 49.0, 100.7, 110.8, 115.4, 122.0, 122.1,
125.2, 128.7, 131.3, 132.7, 141.0, 141.2, 142.2, 162.3, 164.0, 171.9,
190.6 ppm: MS (ESI+): m/z: calcd for C49H61N2O2: 709.5 [M]+; found:
709.6.
Figure 7. An energy diagram illustrating the different excitation, charge
separation (kCS1, kCS2, kCS3—see Table 1 for details), and charge recombi-
nation (kCR) pathways of 2 and 3.
General procedure for the syntheses of 2 and 3: A solution of C60 or C70
(0.18 mmol, 3 equiv) and N-octylglycine (11 mg, 1 equiv) was refluxed in
chlorobenzene (30 mL) for 30 min. Then, a solution of 1 (50 mg, 1 equiv)
in chlorobenzene (10 mL) was added dropwise over 2 h. The solution was
refluxed for a further 2 h and the solvent was evaporated off. The crude
was purified by column chromatography on silica gel (eluent: toluene
then CH2Cl2–MeOH, 9:1). The resulting product was dissolved in the
minimum amount of dichloromethane and precipitated in pentane to
afford a green solid.
Table 1. Charge separation and charge recombination rate constants for
2 and 3 in dichloromethane and benzonitrile.
Solvent
kCS1
[1010 sꢁ1
kCS2
[1010 sꢁ1
kCS3
[1010 sꢁ1
kCR
[1010 sꢁ1
]
G
]
R
]
G
]
U
Data for compound 2: Yield: 40 mg, 40%; 1H NMR (300 MHz, CDCl3):
d=0.9 (t, J=7 Hz, 9H), 1.1–1.5 (m, 36H), 1.7–1.8 (m, 6H), 2.0–2.1 (m,
2H), 2.4–2.6 (m, 2H), 2.7–2.9 (m, 4H), 3.0–3.1 (m, 1H), 4.12 (t, J=7 Hz,
4H), 5.00 (s, 1H), 5.06 (d, J=9 Hz , 1H), 6.10 (d, J=15 Hz, 2H), 7.05 (d,
J=8 Hz, 2H), 7.1–7.4 (m, 8H), 7.85 ppm (d, J=15 Hz, 2H); 13C NMR
(125 MHz, CDCl3): d=14.4, 14.5, 15.6, 21.1, 22.4, 22.6, 24.5, 26.6, 27.2,
27.8, 27.9, 28.0, 28.3, 29.4, 29.7, 31.4, 31.9, 44.8, 48.8, 48.9, 53.4, 65.8, 66.6,
67.3, 68.9, 76.6, 81.9, 100.3, 110.7, 121.9, 122.5, 125.0, 126.4, 128.2, 128.5,
128.7, 129.0, 129.6, 131.4, 134.2, 135.6, 136.0, 136.3, 137.2, 139.0, 139.8,
140.1, 140.2, 140.8, 141.2, 141.4, 141.5, 141.6, 141.7, 141.7, 141.8, 141.9,
142.0, 142.0, 142.1, 142.1, 142.2, 142.2, 142.3, 142.5, 142.5, 142.6, 142.7,
143.0, 143.1, 144.1, 144.3, 144.4, 144.7, 144.9, 145.0, 142.1, 145.3, 142.3,
145.3, 145.4, 145.5, 145.7, 145.8, 145.9, 146.0, 146.0, 146.1, 146.1, 146.2,
146.2, 146.3, 146.4, 146.5, 147.2, 147.3, 153.3, 153.4, 154.1, 156.6, 159.4,
163.5, 171.6 ppm; HRMS (MALDI+): m/z: calcd for C118H80N3O:
1554.6296 [M+H]+; found:1555.6309.
dichloromethane
benzonitrile
dichloromethane
benzonitrile
28.6
31.3
28.6
34.5
3.33
10.0
4.76
7.14
11.1
9.1
0.71
3.33
0.57
2.22
2
3
Conclusion
This investigation documents our success in realizing the
rarely occurring scenario of a charge transfer commencing
with a higher-energy excited state, through the careful
choice of electron donor and electron acceptor. In particu-
lar, the slow internal deactivation of excited-state electron-
donating heptamethine cyanine and the acceleration of
charge-transfer reactions by the electron accepting C60/C70
are crucial requisites. Currently, we are implementing hepta-
methine cyanines in multicomponent electron-donor–accept-
or conjugates to slow down charge recombination. From a
broader perspective, we hope that this finding will provide
the impetus for breakthroughs in near-infrared responsive
photovoltaic devices.
Data for compound 3: Yield: 30%; HRMS (MALDI+): m/z: calcd for
C
128H80N3O: 1674.6301 [M+H]+; found: 1675.6292.
Electrochemistry: Electrochemical experiments were carried out by using
Princeton Applied Research Potentiostat/Galvanostat PAR 263 A.
a
Cyclic voltammetry, differential pulse voltammetry and square-wave vol-
tammetry were performed in a three electrode cell with a platinum wire
as the counter electrode and a glassy carbon disk as the working elec-
trode (Ø=2 mm) versus a silver wire as the reference electrode; ferro-
cene was applied as an internal standard with
a 0.1m solution of
(tBu)4NBF4 as the supporting electrolyte. All experiments were per-
formed under inert-gas atmosphere and a compound concentration of
0.2 mm.
Experimental Section
Spectroelectrochemistry: Spectroelectrochemical experiments were car-
ried out by using a HEKA Elektronik Potentiostat/Galvanostat PG284
and a SPECORD S600 Analytic Jena Spectrophotometer. The measure-
ments were performed in a homemade three-necked cell (ꢀ0.3 cm) with
platinum gauze as the working electrode and a platinum wire as the
counter electrode versus a silver wire, under Argon atmosphere. Tetra-n-
General synthetic procedures: NMR spectra (1H and 13C) were recorded
at room temperature on Bruker DPX 300 MHz or Bruker AVIII
500 MHz spectrometers. Data are listed in parts per million (ppm) and
are reported relative to tetramethylsilane (1H and 13C) with residual sol-
Chem. Eur. J. 2010, 16, 9638 – 9645
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9643

N. Martꢀn, D. M. Guldi, M. A. Garcia Garibay et al.
butylammonium hexafluorophosphate (Bu4NPF6; 0.2m) was used as the
support.
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Photophysics
Steady-state fluorescence spectroscopy: Spectra were collected by using a
Horiba Jobin Yvon Fluoromax 3 spectrophotometer and in a deaerated
solution, at room temperature (298 K), in a 1 cm quartz cuvette. All spec-
tra were corrected for the instrument response. The monitoring wave-
length corresponded to the maximum in the emission band.
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measurements were performed by time correlated single photon counting
(TCSPC) by using
a FluoroLog-3 spectrometer in T-configuration
(HORIBA-JOBIN YVON) including an MCP detector (R3809U-58).
Fluorescence lifetimes were measured at the emission maximum, at room
temperature, in a deaerated solution, in a 1 cm quartz cuvette.
Femtosecond transient absorption spectroscopy: Femtosecond transient
absorption studies were performed with 775 and 387 nm laser pulses
(1 kHz, 150 fs pulse width, 200 nJ) from an amplified Ti/sapphire laser
system (Model CPA 2101, Clark-MXR Inc.—output 775 nm). The transi-
ent absorption pump probe spectrometer (TAPPS) Helios from Ultrafast
systems is referred to as a two-beam setup, in which the pump pulse is
used as the excitation source for transient species and the delay of the
probe pulse is exactly controlled by an optical delay rail. As the probe
(white light continuum), a small fraction of pulses stemming from the
CPA laser system is focused by a 50 mm lens into a 3 mm thick sapphire
disc. The transient spectra were recorded by using fresh, oxygen-free sol-
utions for each laser excitation. All experiments were performed at
298 K, in a 2 mm quartz cuvette.
Pulse radiolysis: Pulse radiolysis experiments were performed by using
50 ns pulses of 15 MeV electrons from a linear electron accelerator
ꢁ
(LINAC). Dosimetry was based upon the oxidation of SCNꢁ to (SCN)2
,
which, in aqueous, N2O-saturated solution, takes place with Gꢀ6 (G de-
notes the number of species per 100 eV, or the approximate mM concen-
tration per 10 J of absorbed energy). The radical concentration generated
per pulse was varied between (1–3)ꢄ10ꢁ6 m.
Acknowledgements
The Deutsche Forschungsgemeinschaft (Grant SFB 583), the Cluster of
Excellence “Engineering of Advanced Materials”, the FCI, and the
Office of Basic Energy Sciences of the US Department of Energy are
gratefully acknowledged. M. A. G.-G and G. K. acknowledge the NSF
IGERT: Materials Creation Training Program (MCTP)—DGE-0654431
and grant NSF-CHE 0844455. This work has been supported by the
ACHTUNGTRENNUNGEuropean Science Foundation (SOHYD, MAT2006-28170-E), the MEC
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9644
www.chemeurj.org
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 9638 – 9645

Charge-Transfer Events
FULL PAPER
Santos, M. Wielopolski, C. M. Atienza, N. Martin, D. M. Guldi,
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Received: June 8, 2010
Published online: July 26, 2010
Chem. Eur. J. 2010, 16, 9638 – 9645
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9645