Multi-electron-acceptor dyad and triad systems based on perylene bisimides and fullerenes

Article abstract of DOI:10.1002/chem.201003092

Fullerene (C₆₀) and 3,4,9,10-perylene tetracarboxylic diimide (PTDCI) were used as building blocks for an electron acceptor dyad (C ₆₀-PTCDI) and triad (C₆₀-PTCDI-C₆₀). As the first reduction potentials for C

Full text of DOI:10.1002/chem.201003092

FULL PAPER
DOI: 10.1002/chem.201003092
Multi-Electron-Acceptor Dyad and Triad Systems Based on Perylene
BisACTHNUTRGNEiNUG mides and Fullerenes
Thomas W. Chamberlain, E. Stephen Davies, Andrei N. Khlobystov,* and
Neil R. Champness*[a]
Abstract: Fullerene (C60) and 3,4,9,10-
perylene tetracarboxylic diimide
mation of
a
series of well-defined
four electrons can be reversibly inject-
ed into the dyad C60–PTCDI and up to
six into the triad C60–PTCDI–C60
system. The optical absorption proper-
ties in the UV/Vis range are also cru-
cially defined by the distribution of
electrons between the acceptor parts,
as the injection/removal of electrons
causes drastic colour changes in the
dyad and the triad systems.
anionic species in which electrons asso-
ciated with the fullerene or the PTDCI
components of the molecule can be
clearly distinguished. In total, up to
(PTDCI) were used as building blocks
for an electron acceptor dyad (C60–
PTCDI) and triad (C60–PTCDI–C60).
As the first reduction potentials for C60
and PTCDI are very close, simulta-
Keywords: cyclic voltammetry
·
·
ACHTUNGTRENNUNGneous introduction of two or three
EPR spectroscopy
perylene bisimides
spectroscopy
· fullerenes
electrons is possible into the dyad and
triad, respectively. Further stepwise
electrochemical reduction leads to for-
·
UV/Vis
Introduction
unpaired electrons can be controlled through the structure
of the dyad. Such tunable multi-electron molecular systems
in which information can be stored and processed in a form
of electron spins have been considered as candidate
materials for quantum information processing.[7]
In this study we have investigated acceptor–acceptor mo-
lecular systems based on fullerene (C60) and 3,4,9,10-pery-
lene tetracarboxylic diimide (PTCDI) derivatives. Both ful-
lerenes and PTCDI-based species are an attractive target in
forming multi-component acceptor–donor systems due to
their extensive redox chemistry and photochemical proper-
ties.[4,8–14] Fullerene absorbs light in the UV/Vis range[2] and
PTCDI strongly absorbs light in the Vis/near IR range,[15,16]
Molecules comprising electron-donor and electron-acceptor
components and possessing light-absorbing properties are
currently attracting significant attention in the context of
photovoltaic devices and photocatalysis applications.[1–3] Dis-
tribution of photo- or electrochemically generated electrons
in donor–acceptor dyads is usually predictable due to the
significant differences between redox potentials for the
donor and acceptor components. However, when two-elec-
tron-acceptor parts with similar redox potentials are com-
bined within the same dyad, the location of incoming elec-
trons is more difficult to predict as the two acceptors are
competing for the electrons.[4] As a result, each part of the
acceptor–acceptor dyad may accommodate an unpaired
electron, giving rise to species with a high total electron
spin, important for molecular electronic and magnetic appli-
cations.[5,6] Unpaired electrons residing on such a dyad will
be spatially separated by a bridge linking its constituent
parts. Since the separating distance is dictated by the length
of that bridge, the strength of spin–spin interactions between
and
both
possess
excellent
electron-acceptor
characteristics[17–19] and have been identified as candidates
for the development of photovoltaic devices.[20–22] For such
applications it is imperative that the electronic/redox and
optical properties of the molecular species are fully under-
stood.
Fullerene cages appended at the “bay region” (1-, 6-, 7-
and/or 12-positions) of the perylene core play the role of
redox active sites capable of accommodating up to six elec-
trons. Considering that any chemical functionalisation of
fullerene cages introduces sp3-carbon atoms within the
bridge linking PTCDI and C60, no orbital overlap between
the two moieties is expected. As a result, we demonstrate
that the empty orbitals of pendant fullerene groups and the
perylene core (designated as “LUMOs” to emphasise the
fact that they are localised either on the C60 or PTCDI part
of the dyad) are capable of accommodating electrons inde-
pendently of each other. This enables reversible electro-
chemical switching of the fullerene–perylene conjugate mol-
ecules to polyanionic states, possessing up to three spatially
[a] Dr. T. W. Chamberlain, Dr. E. S. Davies, Dr. A. N. Khlobystov,
Prof. N. R. Champness
School of Chemistry, University of Nottingham
University Park, Nottingham, NG7 2RD (UK)
Fax : (+44)115-95-13555
E-mail: Andrei.Khlobystov@notttingham.ac.uk
Neil.Champness@nottingham.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003092. It includes details of
experimental and synthetic methods employed; additional figures and
data for square wave voltammetry, EPR spectra and UV/Vis spec-
troelectrochemistry.
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separated unpaired electrons and some interesting optical
properties.
related system has been reported for a fullerene–PTCDI–
porphyrin triad system,[14] there have not been extensive
There are a small number of previous studies of PTCDI–
fullerene species.[11–14] The majority of these studies differ
significantly from our study in that the fullerene moiety is
attached to the PTCDI group by means of the imide nitro-
gen.[11–13] Molecular orbital calculations on PTCDI
derivatives reveal a node in the HOMO at the nitrogen
atoms of the imide functions, precluding electronic commu-
nication pathways between imide appendages and the fuller-
ene.[23] Of additional impor-
combined
electrochemical/spectroelectrochemical/EPR
studies of the multiple redox states of such systems.
Results and Discussion
Fullerene-perylene compounds 2 and 4 were prepared as
outlined in Schemes 1 and 2, respectively. Following the ini-
tance is the group used to func-
tionalise the fullerene moiety.
Previous studies have exploited
either a cyclopropyl[11,12] or pyr-
rolidine
coupling,[13,14]
the
former of which has been dem-
onstrated to be unstable in
some instances upon electro-
chemical reduction.[24] By ex-
ploiting pyrrolidine coupling at
the fullerene and “bay region”,
PTCDI functionalisation should
lead to the most chemical and
electrochemically
robust
PTCDI derivatives and hence
to the formation of stable ac-
ceptor–acceptor arrays. Al-
Scheme 1. Synthetic procedures for 1, 2 and 5. a) 4-boronic acid benzaldehyde, CsF, Ag2O, PdACTHNURTGNE(UNG PPh3)4, THF,
though an extensive and ele- 668C, 78%; b) C60, Et-NHCH2COOH, toluene, 1108C, 42%; c) p-tolualdehyde, Et-NHCH2COOH, toluene,
1108C, 56%.
gant photophysical study of a
Scheme 2. Synthetic procedures for 3 and 4. a) 4-boronic acid benzaldehyde, CsF, Ag2O, PdCATHUGNTRENUN(NG PPh3)4, THF, 668C, 64%; b) C60, Et-NHCH2COOH, toluene,
1108C, 18%. A mixture of both 1,7- and 1,6-Br2–PTCDI were used in reaction a) and the resultant isomers of 3 were reacted further in reaction b).
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Acceptor—Acceptor Systems
FULL PAPER
Table 1. Redox potentials (CV) of C60–PTCDI derivatives 2 and 4, PTCDI derivatives 1 and 3 and C60 deriva-
tive 5.[a]
tial introduction of bromine
substituents to the perylene-
red
1st
red
2nd
red
3rd
red
4th
ACHTUNGTRENNUNG
tetracarboxylic acid core,[25] the
E
E
E
E
DEFc+/Fc
aldehyde functionality of the in-
termediate compounds 1 and 3
were introduced by the Suzuki
coupling of the mono-/bis-
1
2
3
4
5
À0.96 (0.08)
À0.96
À1.15 (0.08)
À1.14
0.08
À1.03 (0.11)[b]
À1.03
À1.16 (0.07)[c]
À1.17
À1.43 (0.06)[d]
À1.43
À1.98 (0.07)[d]
À1.98
0.07
0.07
0.19
0.07
À0.93 (0.09)
À0.92
À1.12 (0.09)
À1.11
ACHTUNGTRENNUNG(bromo)-substituted perylenes
À1.13 (0.14)[b]
À1.14
À1.29 (0.08)[c]
À1.25
À1.50 (0.10)[d]
À1.48
À2.06 (0.10)[d]
À2.04
with 4-boronic acid benzalde-
hyde.[26] The [3 +2] cycloaddi-
tion of the azomethine ylide,
formed by the addition of N-
ethylglycine to the aldehyde in-
termediate, to one or two
equivalents of C60 resulted in
the formation of the five-mem-
bered pyrrolidine ring contain-
ing products 2 and 4 in good
yields.[27] As discussed above a
À1.06 (0.07)
À1.05
À1.44 (0.07)
À1.44
À1.98 (0.07)
À1.98
[a] All potentials reported as E1/2 (=(Eap +Epc)/2) in V vs. Fc+/Fc and quoted to the nearest 0.01 V; the values
in brackets are (EpaÀEpc) for couples; glassy carbon electrode as the working electrode; 1, 2, 3 and 5 in CH2Cl2
[BF4]; scan rate=100 mVsÀ1
containing 0.4m [NBu4]ACTHNUGTRENNUG[BF4]; 4 in 1,2-dichlorobenzene containing 0.4m [NBu4]HCATUNGTRENNUGN .
Values in italics taken from square wave voltammetry. [b] Reduction potentials associated with combined ful-
lerene + perylene reductions. [c] Reduction potentials associated with perylene reduction. [d] Reduction
potentials associated with fullerene reduction.
AHCTUNGTRENNUNG
pyrrolidine linker was selected to functionalise the C60
moiety due to the enhanced stability of this linker to reduc-
tion in comparison to cyclopropyl linkers.[24]
In the case of compound 4 preparation of the dibromo-
substituted PTCDI, N,N’-bisACHTNUTRGNE(NUG n-butyl)dibromo-3,4,9,10-pery-
lenetetracarboxylic bisimide, yields a mixture of both 1,7
and 1,6 isomers in an estimated 80:20 ratio, which it has not
proven possible to separate.[28] The 1,7 isomer is always the
major product from the bromination of perylene-3,4,9,10-
tetracarboxylic acid and as a result the same isomer is also
the major product from substitution reactions using the “as-
synthesised”
N,N’-bisACTHNGUTERNNU(G n-alkyl)dibromo-3,4,9,10-perylene-
ACHTUNGTRENNUNG
of the 1,6 isomer is present in the final product, 4 (approxi-
1
mately 10% by H NMR spectroscopy) used for all charac-
terisation methods. Compound 5 {1,2-(1-ethyl-2-(4-methyl-
benzene)-pyrrolidine)[60]fullerene}, was prepared by the
[3+2] cycloaddition of C60 with p-tolualdehyde and N-ethyl-
glycine and was used as a reference molecule for the fuller-
ene moiety in the fullerene–perylene compounds.
Figure 1. Cyclic voltammograms recorded for 1, 2, 3 and 5 in CH2Cl2 con-
taining [Bu4N][BF4] (0.4m) and for 4 in 1,2-dichlorobenzene containing
[Bu4N][BF4] (0.4m). Results of square wave voltammetry studies are pre-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
sented in Figure S1 in the Supporting Information.
Electrochemical studies: Since both fullerene and perylene
moieties in compounds 2 and 4 are efficient electron accep-
A
Conversely, the first reductions for reference compounds 1
(À0.96 V) and 3 (À0.92 V) demonstrate that the phenylalde-
hyde group(s) withdraw electron density from the PTCDI
core leading to more facile first reductions than for simpler
phenyl-substituted analogues.[28]
Hence, from these results it was anticipated that the ap-
pendage of one C60–PTCDI (2) or two C60–PTCDI–C60 (4)
fullerene cages to the perylene core would result in the
overlap of first reductions on the PTCDI and C60 moieties
and as a consequence increase the electron uptake to four
for 2 (two electrons each from PTCDI and C60 components)
or six for 4 (two electrons from PTCDI and four electrons
from the two C60 components) in the 0 to À1.5 V range of
potentials (Figure 1, Table 1 and Figure S1 in the Supporting
Information file). In agreement with this, the CV of 2 and 4
bution of electrons in reduced forms of these molecules
[2]nÀ and [4]nÀ is a challenging task that can be only
achieved by the careful comparison of the redox and spec-
troscopic properties of these compounds with those of their
corresponding reference compounds; 1 and 3 representing
perylene moieties in 2 and 4, respectively and 5 representing
the fullerene moiety in both 2 and 4. Results from cyclic vol-
tammetry (CV) measurements of 1–5 are given in Table 1
and Figure 1. The results confirm that the first reductions of
2 and 4 involve the PTCDI moiety. It is worth noting that
the shift in potential of the first reduction associated with
the mono- and di-C60-substituted compounds, 2 (À1.03 V)
and 4 (À1.14 V), respectively, indicates that attachment of
C60 groups increases electronic density in the PTDCI core.
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N. R. Champness, A. N. Khlobystov et al.
gave an overlap of the first reduction potentials of the fuller-
ene and perylene moieties allowing 2 and 4 to accept more
than one electron in the initial reduction step. Quantitative
analysis by coulometry shows simultaneous reduction of C60
and PTDCI moieties in 2 and 4 (Figure 1) that converts
them to [2]2À and [4]3À species at potentials corresponding to
the first reduction process (À1.03 V and À1.14 V, respective-
ly). Taking into account the redox potentials of the refer-
ence compounds (Table 1), distribution of electrons in [2]2À
CÀ CÀ
should be expected as C60 –PTCDI and in [4]3À as C60
–
CÀ
CÀ
CÀ
PTCDI –C60 . This hypothesis was confirmed by the results
of UV/Vis spectroelectrochemical studies discussed below.
UV/Vis spectroelectrochemical studies: The UV/Vis spec-
troelectrochemical data for compounds 1–5 in neutral and
reduced states are given in Table 2. Both the PTCDI and
Table 2. UV/Vis spectroelectrochemical data for compounds 1–5 in neu-
tral and reduced states.
Figure 2. UV/Vis spectra recorded in CH2Cl2 containing [Bu4N]ACHTUNGTRENNUNG[BF4]
(0.4m) using spectroelectrochemical methods for 2 at 273 K; the spec-
trum of 2 is represented by the solid line, [2]2À (dashed line), [2]3À
(dotted line) and [2]4À (dashed-dotted line).
lmax [nm] (eꢁ10À4 [molÀ1 m3 cmÀ1])
[1]
262 (3.5), 323 (1.4), 379 (0.4), 498 (2.9), 534 (4.2)
[1]À 235 (5.3), 326 (1.0), 355 (0.7), 378 (0.7), 404 (0.7), 490 (0.3), 542
(0.3), 721 (6.1), 809 (3.1)
[1]2À 231 (4.6), 277 (4.8), 398 (0.7), 435 (0.9), 588 (5.7)
[2]
244 (8.9), 328 (2.9), 430 (0.7), 506 (1.8), 539 (2.3)
[2]2À 242 (10.0), 726 (4.4), 812 (2.1), 871 (0.8)
[2]3À 262 (8.7), 579 (4.3), 602 (3.7), 643 (1.9)
[2]4À 267 (8.5), 579 (4.4), 601 (3.9), 642 (2.0), 709 (0.5), 867 (0.6)
[3]
261 (3.6), 322 (2.3), 391 (0.4), 543 (2.6)
[3]À 235 (4.5), 265 (4.1), 356 (0.9), 378 (0.9), 418 (0.9), 492 (0.4), 526
(0.4), 733 (0.4), 822 (2.0), 876 (1.0)
[3]2À 230 (4.1), 279 (4.9), 406 (1.0), 602 (4.5)
[4]
432 (0.5), 562 (0.8)
[4]3À 750 (1.6), 835 (0.8)
[4]4À 595 (1.6), 621 (1.8), 653 (1.1)
[4]6À 595 (1.7), 621 (1.9), 652 (1.2), 867 (0.4)
[5]
256 (6.3), 306 (2.0), 325 (1.9), 430 (0.2)
[5]À 263 (5.6), 766 (0.2)
[5]2À 266 (5.1), 721 (0.4), 863 (0.6)
[5]3À 748 (0.4), 863 (0.5)
C60 parts of 2 (Figure 2) and 4 (Figure 3) absorb light in the
UV/Vis range, which enables one to follow the addition of
electrons into “LUMOs” of these molecules by optical spec-
troscopy. When correlated with the spectra of the reference
compounds (see the Supporting Information), the optical
absorption spectra of the fullerene–perylene conjugates
appear as simple superpositions of fullerene and perylene
spectra thus indicating little or no orbital overlap in the
charge-neutral ground state of these molecules. As a result
of the perylene chromophore having a significantly higher
extinction coefficient than that of the fullerene, the spectra
of 2 and 4 are dominated by the strong absorption bands of
the PTCDI moiety masking the fullerene absorption bands;
however, a characteristic band at 430 nm in reference com-
pound 5 is evident in both 2 (430 nm) and 4 (432 nm). The
[2]2À dianion formed at the first reduction step shows new
absorbance bands including two very strong peaks at 726
and 812 nm (Figure 2), which clearly indicate that some of
Figure 3. UV/Vis spectra recorded in 1,2-dichlorobenzene containing
[Bu4N]ACHTUNTRGNEUNG[BF4] (0.2m) using spectroelectrochemical methods for 4 at 273 K;
the spectrum of 4 is represented by the solid line, [4]3À (dashed line),
[4]4À (dotted line) and [4]6À (dashed-dotted line).
the electrons are located on the “LUMO” of the perylene
part of 2. Comparison with the spectra obtained for the ref-
erence perylene compound [1]À confirmed that only one
electron resides on the PTCDI moiety in [2]2À (Figure S2,
Supporting Information), and therefore the second electron
should reside on the C60 part, so that [2]2À exists as a bi-
CÀ
CÀ
AHCTUNGRTEUNNvGN ersibly cycled between the C60–PTCDI and C60 –PTCDI .
AHCTUNGTRENrGNUN adical dianion C60 –PTCDI . The state of 2 can be re-
CÀ CÀ
A similar strategy applied to compound 4 showed that it
undergoes a reduction to the trianion [4]3À (Figure 3); how-
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Acceptor—Acceptor Systems
FULL PAPER
ever, in contrast to compound 2, reductions of 4 have not
demonstrated complete chemically reversibility in the
spectroACHTUNGTRENNUNGelectrochemical cell. It was noted that no evident
various reduced states have been generated and studied by
electron paramagnetic resonance (EPR) spectroscopy. One-
electron reduction of reference compounds 1, 3 and 5 indi-
cates that the position and shape of the EPR spectrum ob-
tained from a single electron occupying either a fullerene-
or a PTCDI-based “LUMO” was distinctly different (Fig-
bands associated with decomposition were present and the
lack of complete return of original spectra might result from
the extended timescales that were required in order to ach-
ieve the various reduction levels under the conditions of this
experiment. The profiles of species generated upon reduc-
tion of 4 were consistent with analogous reductions of 2. Ac-
cording to optical spectroelectrochemical measurements
[4]3À contains just one electron on its PTCDI moiety, and
therefore, taking into account the large difference between
the 1st and 2nd reduction potentials of the fullerene cage
(Table 1), the trianion species should exist as a triradical
AHCTUNGTREGuNNNU res S5–S8 in the Supporting Information). For PTCDI con-
taining 1 and 3, reduction to the monoanions [1]À and [3]À,
gives EPR active species with extensive hyperfine interac-
tions and giso values of 2.0036 and 2.0034, respectively (Fig-
ure S5 and S7 in the Supporting Information). Subsequent
reduction to the dianions, [1]2À and [3]2À, results in a loss of
EPR activity as the two electrons pair in the PTCDI orbital;
hence only residual signals are recorded. For fullerene based
5, reduction to the monoanion generates a paramagnetic
species with giso =2.0003, the signal of which is depleted
upon reduction to [5]2À (Figure S8 in the Supporting Infor-
mation).
CÀ CÀ
CÀ
C60 –PTCDI –C60 .
Radical anions [2]2À and [4]3À have one electron per each
of their “LUMOs”, and therefore addition of any further
electrons will result in the pairing of incoming electrons
with those already present in the molecules. Formation of
[2]3À is accompanied by the disappearance of the strong ab-
sorption bands at 726 and 812 nm associated with the single-
electron-doped perylene core and the emergence of a new
set of bands at 579, 602 and 643 nm (Figure 2) consistent
with two electrons on the perylene “LUMO” (c.f. [1]2À, Fig-
ure S2, Supporting Information). Therefore the electron dis-
Given that both 2 and 4 have two possible “LUMO” or-
AHCTUNGTREGbNNNU itals, and from the results of (spectro)electrochemical
studies these are populated at the same or very similar po-
tentials such that the first reduction should yield [2]2À and
[4]3À, respectively, we have attempted to sequentially reduce
2 to [2]2À and [4] to[4]3À by the addition of single equivalents
of electrons in order to observe the distribution of these
electrons in the system. However, the actual distribution of
these electrons is determined by thermodynamic equilibria
and as such we note the form of the CV (Figure 1) in each
case. For 2 the first reduction results from an overlap of one
electron reductions on both PTCDI and fullerene moieties.
Thermodynamics would dictate that whilst we can add a
single equivalent of electrons into 2, these electrons could
CÀ
tribution in the [2]3À radical trianion is described as C60
–
PTCDI2À. The optical spectrum of [2]4À is very similar to
that of trianion [2]3À, indicating no changes in the state of
the perylene core; however, an additional broad feature is
developed at low energy (867 nm) that is consistent with a
band for C60 in [5]2À (at 863 nm), so that the electron dis-
2À
tribution in [2]4À can be described as C602À–PTCDI2À. The
anions [2]3À and [2]4À can be interconverted reversibly;
hence compound 2 can be reversibly switched between C60–
CÀ
CÀ
be distributed as C60 –PTCDI and C60–PTCDI , or equili-
CÀ
CÀ
brated as C60 –PTCDI and C60–PTCDI. Addition of a
second equivalent of electrons should give C60 –PTCDI
PTCDI, C60 –PTCDIÀ, C60 –PTCDI2À and C602À–PTCDI2À
levels.
CÀ
CÀ
CÀ CÀ
only, as a dianion with two unpaired electrons and therefore
this would have both singlet and triplet electronic states. We
have estimated that the distance between spin distributions
based on one on C60 and the other on PTCDI is 0.86–
1.21 nm and at this relatively large separation the exchange
interaction that separates the energies of the singlet and
triplet states is expected to be less than 1 cmÀ1. Therefore at
room temperature both states should have similar popula-
The absorption spectrum of [4]4À (Figure 3) clearly indi-
cates that the “LUMO” of perylene moiety is populated
with two paired electrons (PTCDI2À), which means that
each of the fullerene cages will have an unpaired electron,
so that the electron distribution in [4]4À can be expressed as
CÀ
CÀ
C60 –PTCDI2À–C60 . Similar to the transition between [2]3À
and [2]4À, the optical spectrum of [4]6À is very similar to that
of tetraanion [4]4À indicating no changes in the state of the
perylene core; however, an additional broad feature is de-
veloped at low energy (867 nm) that is consistent with a
CÀ
CÀ
tions and the C60 –PTCDI dianion should be considered
as a biradical.[30] A similar consideration can be made for 4
although the first reduction process is an overlap of two ful-
2À
band for C60 in [5]2À (at 863 nm), so that the electron dis-
ACHTUNGTRENNUNG
tribution in [4]6À can be described as C602À–PTCDI2À–C60
.
not locked to a single species in solution until C60 –
2À
CÀ
CÀ
PTCDI – C60 is generated by the addition of three elec-
trons (Scheme 3). At this point the EPR spectrum will result
from the magnitude of interactions between three electrons,
one in each separate “LUMO” of the triad. Reductions
beyond [2]2À and [4]3À will result in the spin-pairing of elec-
trons in these “LUMOs”.
EPR spectroscopic studies: The results from spectroelectro-
chemical studies have given an insight into the nature of the
molecular orbital into which the electron is added for the
C60–PTCDI conjugates at stages in the reduction sequence,
this is now extended to the investigation of how these elec-
trons interact when in these orbitals.
Addition of sub-two equivalents of electrons to a red solu-
tion of 2 (experimentally this was achieved in two ways,
either by the addition of one equivalent of electrons to [2]
Coulometry has allowed us the control of the number of
electrons added per molecule (1–5) in solution so that the
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N. R. Champness, A. N. Khlobystov et al.
The three-electron reduction
of compound 4 to [4]3À can be
similarly examined step by step
using the bulk solution electrol-
ysis technique coupled with
EPR spectroscopy. The initial
purple solution of 4 was elec-
trolysed to give [4]À as a green
solution. The EPR signal for
[4]À has a giso value of about
2.003 and a very broad shape
and as for [2]À (giso =2.0032)
might indicate that the first
electron is located in an orbital
containing PTCDI character.
Addition of a second and third
equivalent of electrons leads to
the formation of [4]2À and [4]3À
as brown and green solutions,
respectively. The addition of a
second equivalent of electrons
to [4]À resolves a signal with
g
iso =2.0014 for [4] ACTHNGUTERNNUG
2À(Figure 6)
that is very similar in shape and
position to the broad signal
noted for [2]2À at giso =2.0018
and on this basis we assign this
signal to that of a biradical,
Scheme 3. Proposed electron configurations in the HOMOs of a) dyad [2]nÀ (n=0-4) and b) triad [4]nÀ (n=0–
6) acceptor–acceptor systems. The individual PTCDI and fullerene units are represented by a rectangle and
circle respectively. The unpaired electrons which interact strongly to generate a single signal in solution EPR
are grouped with dashed lines.
as in Figure S9 in the Supporting Information or in reverse
by first generating [2]2À then allowing the slow re-oxidation
of the sample as in Figure 4) resulted in the generation of a
green solution, which is consistent with the formation of
mono-anion based on PTCDI in each case. The main EPR
signal of [2]À is relatively low in intensity (Figure 4a) and
has a broad profile with giso =2.0032, suggesting that this
represents an electron residing on a “LUMO” containing
PTCDI character (the presence of hyperfine splitting is
noted but not sufficiently resolved for analysis). From the
results of optical measurements for [2]2À it would be expect-
ed that one electron occupies an orbital based on fullerene
and the other occupies a PTCDI based orbital. Addition of
electrons up to two equivalents leads to the formation of
[2]2À (Figure 4 f) as a green/blue solution the EPR spectrum
of which gives a signal at giso =2.0018. This value corre-
sponds to an average g-value between [2]À (with an electron
based on PTCDI; giso =2.0032) and [5]À (with an electron
Figure 4. Experimental EPR spectra of [2]nÀ (0<nꢀ2), Spectra a)–e)
were obtained by the slow re-oxidation of [2]2À, spectrum f) is [2]2À. All
CÀ
based on C60 ; giso =2.0003). This is evidence for assignment
of [2]2À as a biradical, with strong exchange coupling be-
spectra were recorded in CH2Cl2 containing [Bu4N]
ent temperature. Analogous data recorded in 1,2-dichlorobenzene con-
taining [Bu4N]
[BF4] (0.2m) for [2]À, [2]2À and[2]3À are presented in Fig-
ACHTUNGTREN[NNUG BF4] (0.2m) at ambi-
tween the two unpaired electrons, in this case with one elec-
CÀ
CÀ
AHCTUNGTRENNUNG
tron on PTCDI and the other on C60 in the C60 –PTCDI
ure S9 in the Supporting Information.
dianion. The two smaller signals noted to higher and lower
field of giso =2.0018 in Figure 4 f are attributed to the pres-
CÀ
CÀ
ence of C60 –PTCDI and C60–PTCDI , respectively, result-
ing from slight re-oxidation of the sample during manipula-
tion.
analogous to [2]2À, with strong exchange interaction between
CÀ
unpaired electrons on PTCDI and C60 in the C60
–
CÀ
PTCDI –C60 dianion. On acquisition of the third electron
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Acceptor—Acceptor Systems
FULL PAPER
the trianion [4]3À is formed and results from optical spectro-
scopic measurements suggest a C60 –PTCDI –C60·À distribu-
tion of electrons. The EPR spectrum of [4]3À retains the
broad signal with giso =2.0014 assigned as the biradical of
[4]2À and develops a signal at giso =2.0004; this is consistent
with an unpaired electron on a fullerene cage. The lack of
apparent perturbation of the biradical signal of [4]2À in the
spectrum of [4]3À and the presence of a new C60 anion signal
is not inconsistent with the presence of distinct (non-inter-
acting) biradical and doublet states for [4]3À.
CÀ
CÀ
From the results of UV/Vis spectroelectrochemistry the
CÀ
electron distribution in [2]3À is described as C60 –PTCDI2À
and this is supported by the results of EPR spectroscopy
(Figure 5); the signal at giso =2.0003 is solely due to the un-
Figure 6. Experimental EPR spectra of [4]À (upper trace), [4]2À, (upper
middle trace), [4]3À, (lower middle trace) and [4]4À(lower trace). All spec-
tra were collected in 1,2-dichlorobenzene containing [Bu4N]
at ambient temperature.
ACHTUNGTRENN[UGN BF4] (0.2m)
The absorption spectrum of [4]4À (Figure 3) clearly indi-
cates that the “LUMO” of perylene moiety is populated
with two paired electrons (PTCDI2À), which means that
each of the fullerene cages will have an unpaired electron so
that the electron distribution in [4]4À can be expressed as
CÀ
CÀ
C60 –PTCDI2À–C60 . EPR spectroscopy confirms that the
two unpaired electrons are solely located on each of the ful-
AHCTUNGTERNNGlUN erene “LUMO”s (giso =2.0004) and that their spins are es-
sentially non-interacting in this biradical. It is worth noting
that the intensity of the signal derived from the two un-
paired electrons for the fullerene anions in [4]4À is consider-
ably greater than that of the corresponding signal in [4]3À,
which was also anticipated to arise from the same two un-
paired electrons on fullerene. This indicates a change in the
observed electron coupling and is consistent with the loss of
the biradical associated with the strong exchange interaction
between one electron on a fullerene and one on PTCDI as a
second electron is introduced into the PTCDI orbital.
Hence [4]4À would represent a biradical in which the two un-
paired electrons, one on each of the fullerenes, show a weak
exchange interaction. The EPR spectrum of [4]6À showed
that the hexaanion is essentially a diamagnetic species with
all electrons paired (a residual signal comprising 11% of the
double integrated intensity of [4]4À is noted but not studied
further; Figure S11 in the Supporting Information) hence
each of the electrons added enters a fullerene based orbital
and is spin-paired with the electron already present thus
Figure 5. Experimental EPR spectra of [2]2À (upper trace) and [2]3À
(lower trace). All spectra were collected in CH2Cl2 containing [Bu4N]-
ACHTUNGTRENNUNG[BF4] (0.2m) at ambient temperature. The high field signal in the upper
trace is attributed to a small quantity of [2]3À present in the sample as a
result of slight over reduction.
paired electron present on the fullerene based “LUMO”,
since the shape and position of the EPR signal is almost
identical to that of reference compound [5]À (giso =2.0003)
(Figure S8 in the Supporting Information). The third elec-
tron enters the PTCDI orbital and spin-pairs with the elec-
tron already present (consistent with results of introducing a
second electron in PTCDI models 1 and 3) to give a system
with a single unpaired electron on fullerene. The fourth
electron added to 2 goes into the fullerene “LUMO” so that
the two electrons present on the fullerene cage pair up lead-
ing to an effective quenching of EPR signal of tetraanion
[2]4À relative to [2]3À (a residual signal comprising 12% of
the double integrated intensity of [2]3À is noted but has not
been investigated further; Figure S10 in the Supporting In-
formation) to give C602À–PTCDI2À and consistent with the
results from the UV/Vis spectroscopy of [2]4À.
2À
giving an electron distribution of C602À–PTCDI2À–C60 and
confirming the results of optical spectroscopic studies.
Chem. Eur. J. 2011, 17, 3759 – 3767
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www.chemeurj.org
3765

N. R. Champness, A. N. Khlobystov et al.
Conclusion
[1] K. Domen, J. N. Kondo, H. Hara, T. Takata, Bull. Chem. Soc. Jpn.
2000, 73, 1307.
The comparison of electrochemical (CV and square wave
voltammetry) and UV/Vis spectroelectrochemical data for 2
and 4 with that of their reference compounds (1, 3 and 5)
has allowed unambiguous assignment to the order of orbital
[2] G. Yu, J. Gao, J. Hummelen, C. F. Wudl, A. J. Heeger, Science 1995,
270, 1789.
[3] a) H. Kurreck, M. Huber, Angew. Chem. 1995, 107, 929; Angew.
Chem. Int. Ed. Engl. 1995, 34, 849; b) H. A. Staab, A. Feurer, R.
Hauck, Angew. Chem. 1994, 106, 2542; Angew. Chem. Int. Ed. Engl.
1995, 33, 2428; c) M. R. Roest, J. W. Verhoeven, W. Schiddeboon,
M. Warman, J. M. Lawson, M. N. Paddon-Row, J. Am. Chem. Soc.
1996, 118, 1762; d) D. Gust, T. A. Moore, A. L. Moore, Pure Appl.
Chem. 1998, 70, 2189; e) D. Gust, T. A. Moore, A. L. Moore, Acc.
Chem. Res. 1993, 26, 198; f) M. R. Wasielewski, Chem. Rev. 1992, 92,
435; g) M. N. Paddon-Row, Acc. Chem. Res. 1994, 27, 18; h) A. J.
Bard, M. A. Fox, Acc. Chem. Res. 1995, 28, 141; i) T. J. Meyer, Acc.
Chem. Res. 1989, 22, 163; j) P. Piotrowiak, Chem. Soc. Rev. 1999, 28,
143; k) J. L. Delgado, P-A. Bouit, S. Filippone, M. ꢂ. Herranz, N.
Martꢃn, Chem. Commun. 2010, 46, 4853.
[4] B. M. Illescas, N. Martꢃn, C. R. Chim. 2006, 9, 1038.
[5] L. Bogani, W. Wernsdorfer, Nature Mater. 2008, 7, 179.
[6] W. Y. Kim, K. S. Kim, Acc. Chem. Res. 2010, 43, 111.
[7] S. C. Benjamin, A. Ardavan, G. A. D. Briggs, D. A. Britz, D. Gun-
lycke, J. Jefferson, M. A. G. Jones, D. F. Leigh, B. W. Lovett, A. N.
Khlobystov, S. A. Lyon, J. J. L. Morton, K. Porfyrakis, M. R. Sam-
brook, A. M. Tyryshkin, J. Phys.: Condens. Matter 2006, 18, S867.
[8] M. Eiermann, R. C. Haddon, B. Knight, Q. C. Li, M. Maggini, N.
Martꢃn, T. Ohno, M. Prato, T. Suzuki, F. Wudl, Angew. Chem. 1995,
107, 1733; Angew. Chem. Int. Ed. Engl. 1995, 34, 1591.
[9] T. Ohno, N. Martꢃn, B. Knight, F. Wudl, T. Suzuki, H. Yu, J. Org.
Chem. 1996, 61, 1306.
[10] B. Knight, N. Martꢃn, T. Ohno, E. Ortꢃ, C. Rovira, J. Veciana, J.
Vidal-Gancedo, P. Viruela, R. Viruela, F. Wudl, J. Am. Chem. Soc.
1997, 119, 9871.
[11] J. Baffreau, S. Leroy-Lhez, N. Vꢄn Anh, R. M. Williams, P. Hud-
homme, Chem. Eur. J. 2008, 14, 4974.
[12] R. Gꢅmez, J. L. Segura, N. Martin, Org. Lett. 2005, 7, 717.
[13] J. Hua, F. Meng, F. Ding, F. Li, H. Tian, J. Mater. Chem. 2004, 14,
1849.
[14] S. Xiao, Y. Li, Y. Li, J. Zhuang, N. Wang, H. Liu, B. Ning, Y. Liu, F.
Lu, L. Fan, C. Yang, Y. Li, D. Zhu, J. Phys. Chem. B 2004, 108,
16677.
CÀ
CÀ
CÀ
population for [2]2À, [2]3À and [2]4À as C60 –PTCDI , C60
–
PTCDI2À and C602À–PTCDI2À, respectively and for [4]3À,
CÀ
CÀ
CÀ
CÀ
CÀ
[4]4À and [4]6À as C60 –PTCDI –C60 , C60 –PTCDI2À–C60
and C602À–PTCDI2À–C602À, respectively.
Results from fluid solution EPR spectroscopic studies are
equally unambiguous; [2]2À appears as a biradical with
strong exchange interaction between unpaired electrons on
PTCDI and C60. Trianion [4]3À has a similar form to that of
[2]2À with an additional contribution from an unpaired elec-
tron on a C60 moiety, whereas [2]3À appears as a monoradical
based on fullerene due to spin-pairing of the electrons on
PTCDI. The [2]4À and [4]6À ions are diamagnetic with all
their electrons paired in fullerene and PTCDI orbitals.
Anion [4]4À exists as a biradical with the two unpaired elec-
trons of fullerene non-interacting and spin-pairing of the
electrons in the PTCDI core. These results are completely
consistent with the results from optical spectroscopy and
demonstrate a range of electron interactions for these com-
pounds.
The thorough understanding of the electronic and optical
properties of these acceptor–acceptor dyads and triads re-
veals the formation of a series of reduced states that are
readily accessible allowing tuning of the optical properties
and magnetic states of these molecules. The results are of
significance not only for the photochemical and chemical
properties of the species reported but are crucial for further
development of applications in photovoltaics and quantum
information processing devices. Our studies continue to
pursue these goals.
[15] L. Schmidt-Mende, A. Fechtenkçtter, K. Mꢆllen, E. Moons, R. H.
Friend, J. D. MacKenzie, Science 2001, 293, 1119.
[16] H. Quante, P. Schlichting, U. Rohr, Y. Geerts, K, Mꢆllen, Macro-
mol. Chem. Phys. 1996, 197, 4029.
[17] N. Martꢃn, L. Sꢇnchez, B, Illescas, I. Pꢈrez, Chem. Rev. 1998, 98,
2527.
Experimental Section
[18] H. Imahori, Y. Sakata, Adv. Mater. 1997, 9, 537.
[19] S. Fukuzumi, K. Ohkubo, H. Imahori, D. M. Guldi, Chem. Eur. J.
2003, 9, 1585.
[20] B. Maennig, J. Drechsel, D. Gebeyehu, P. Simon, F. Kozlowski, A.
Werner, F. Li, S. Grundmann, S. Sonntag, M. Koch, K. Leo, M.
Pfeiffer, H. Hoppe, D. Meissner, N. S. Sariciftci, I. Riedel, V. Dyako-
nov, J. Parisi, Appl. Phys. A 2004, 79, 1.
[21] B. E. Hardin, E. T. Hoke, P. B. Armstrong, J. H. Yum, P. Comte, T.
Torres, J. M. J. Frechet, M. K. Nazeeruddin, M. Graetzel, M. D.
McGehee, Nat. Photonics 2009, 3, 406; S. Ferrere, A. Zaban, B. A.
Gregg, J. Phys. Chem. B 1997, 101, 4490.
[22] Y. Shibano, T. Umeyama, Y. Matano, N. Y. Tkachenko, H. Lemme-
tyinen, H. Imahori, Org. Lett. 2006, 8, 4425.
Details of and information relating to all synthetic procedures are given
in supporting information. Elemental analyses (CHN) were performed
by the University of Nottingham, School of Chemistry Microanalytical
Service on a Perkin–Elmer 240B instrument. Infra-red spectra were mea-
sured as solutions using a Nicolet Avatar 380 FT-IR spectrometer over
1
the range 400–4000 cmÀ1. H and 13C NMR spectra were obtained using a
Bruker DPX 300 spectrometer. Mass spectrometry was carried out using
a FAB-LSIMS spectrometer and a MALDI-TOF spectrometer at the
EPSRC National Mass Spectrometry Service Centre in Swansea. Further
details of experimental techniques, electrochemical/spectroelectrochemi-
cal and EPR spectroscopic measurements, are given in Supporting Infor-
mation.
[23] H. Langhals, S. Demmig, H. Huber, Spectrochim. Acta, Part A 1988,
44, 1189.
Acknowledgements
[24] J. Fan, Y. Wang, A. J. Blake, C. Wilson, E. S. Davies, A. N. Khlobys-
tov, M. Schroder, Angew. Chem. 2007, 119, 8159; Angew. Chem. Int.
Ed. 2007, 46, 8013.
[25] H. Langhals, S. Kirner, Eur. J. Org. Chem. 2000, 365.
[26] N. Miyaura, A. Suzuki, Chem. Rev. 1995, 95, 2457.
[27] M. Maggini, G. Scorrano, M. Prato, J. Am. Chem. Soc. 1993, 115,
9798.
We would like to gratefully acknowledge the support of the Engineering
and Physical Sciences Research Council (EP/D048761/1), the Royal Soci-
ety and the European Science Foundation for funding. N.R.C. gratefully
acknowledges the receipt of a Royal Society Leverhulme Trust Senior
Research Fellowship.
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Chem. Eur. J. 2011, 17, 3759 – 3767

Acceptor—Acceptor Systems
FULL PAPER
[28] F. Wꢆrthner, V. Stepanenko, Z. Chen, C. R. Saha-Mçller, N. Kocher,
D. Stalke, J. Org. Chem. 2004, 69, 7933.
[30] F. Arena, F. Bullo, F. Conti, C. Corvaja, M. Maggini, M. Prato, G.
Scorrano, J. Am. Chem. Soc. 1997, 119, 789.
[29] C.-C. Chao, M.-k. Leung, Y. O. Su, K.-Y. Chiu, T.-H. Lin, S.-J. Shieh,
S.-C. Lin, J. Org. Chem. 2005, 70, 4323.
Received: October 26, 2010
Published online: March 1, 2011
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www.chemeurj.org
3767