Synthesis of a π-extended TTF-perylenediimide donor-acceptor dyad

Article abstract of DOI:10.1016/j.tetlet.2008.03.091

A donor-acceptor dyad containing perylenediimide as the electron acceptor and π-extended tetrathiafulvalene as electron donor has been successfully synthesized by means of a Wittig reaction. Cyclic voltammetry and absorption spectroscopy show that both electroactive units preserve their nature, whereas preliminar photophysical investigations show a strong fluorescence quenching.

Full text of DOI:10.1016/j.tetlet.2008.03.091

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 3225–3228
Synthesis of a p-extended TTF–perylenediimide donor–acceptor dyad
a,
b
a,
*
*
´
´
Rafael Gomez , Carmen Coya , Jose L. Segura
a Departamento de Quı´mica Orga´nica, Facultad de Quı´mica, Universidad Complutense de Madrid, Avda. Complutense s/n, E-28040 Madrid, Spain
b Escuela Superior de Ciencias Experimentales y Tecnologı´a, Universidad Rey Juan Carlos, c/ Tulipa´n s/n. 28933 Mo´stoles, Spain
Received 30 January 2008; revised 17 February 2008; accepted 19 March 2008
Available online 23 March 2008
Abstract
A donor–acceptor dyad containing perylenediimide as the electron acceptor and p-extended tetrathiafulvalene as electron donor
has been successfully synthesized by means of a Wittig reaction. Cyclic voltammetry and absorption spectroscopy show that both
electroactive units preserve their nature, whereas preliminar photophysical investigations show a strong fluorescence quenching.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Perylenediimide; p-Extended tetrathiafulvalene; Wittig reaction; Electrochemistry; Fluorescence
Photoinduced electron transfer has a great importance
in both life and science. It is the fundamental process in
which photosynthesis settles and it also constitutes the
basis of molecular electronics.1 In this respect, the unique
features that organic semiconductors offer, for example
the tuning of absorption and emission properties, REDOX
behaviour or HOMO–LUMO band gaps by easy structural
modifications, have prompted the synthesis of a variety of
molecular architectures combining electron donor frag-
ments covalently linked to accepting units and exhibiting
the transfer of one electron from the donor to the acceptor
unit after photoexcitation.2
Perylenediimide (PDI) derivatives have evolved into one
of the most fascinating materials in both industry and aca-
demics due to their good chemical, thermal and photo-
stability.3 Specially remarkable for the construction of
systems showing photoinduced electron transfer are its
good electron accepting ability and high absorption of light
in the visible. Thus, PDIs have been recently linked to
conjugated polymers,4 [60]fullerene,5 oligophenylene-
vinylenes,6 pyrenes,7 oligothiophenes,8 phthalocyanines,9
porphyrins10 or tetrathiafulvalene (TTF)11 as the donor
counterpart aiming the induction of photoinduced electron
transfer. These dyads are often claimed as promising candi-
dates for the construction of organic solar cells,12 either as
active layers or to be used as light-harvesting antenna;13
or as models for the investigation of the fundamentals of
photoinduced electron transfer.
One important aspect to consider in these dyads is
the stabilization of the charge separated state (PDIÅꢀ–
DonorÅ+). The employment of TTF14-based donors in
combination with PDIs can be specially useful,11 as TTF
undergoes aromatization upon oxidation, affording
thermodynamically stable radical cationic and dicationic
species. In this respect, the substitution of TTF by its
p-extended analogue, exTTF, could be advantageous,
considering the additional improvement in the stability of
radical ion pairs produced by exTTF.15 In fact, exTTF
has been employed in the synthesis of D-A dyads showing
long lifetime charge separated states.16 This fact has been
rationalized in terms of the aromaticity and planarity that
are gained in the dicationic state of exTTF.
Thus, we decided to synthesize the dyad exTTF–PDI 5,
in which a PDI electron accepting unit has been covalently
attached to an electron donor exTTF through a relatively
rigid vinyl-benzene spacer. The presence of long alkyl
chains in the PDI unit assures the solubility of dyad 5, thus
*
Corresponding authors. Tel.: +34 91 394 5142; fax: +34 91 394 4103.
´
E-mail addresses: rafaelgomez@quim.ucm.es (R. Gomez), segura@
quim.ucm.es (J. L. Segura).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.03.091

3226
R. Go´mez et al. / Tetrahedron Letters 49 (2008) 3225–3228
enabling its complete characterization by NMR, electro-
chemical and light absorption and emission techniques.
The synthesis of dyad 5 was carried out following the
synthetic procedure depicted in Scheme 1. Although, in
general, perylenediimides are attached to a variety of sys-
tems by the direct condensation between perylenemono-
imide-monoanhydrides and amino-functionalized com-
pounds we have employed an alternative approach, namely
a Wittig reaction between the appropriately functionalized
PDI unsymmetrical diimide 3 and the corresponding tri-
phenylphosphonium salt of exTTF 4.17 The presence in 1,
readily affordable from the parent monoanhydride by
treatment with ammonium acetate,18 of an acidic imido
hydrogen enables its further and convenient functionaliza-
tion. Thus, Williamson reaction between 1 and 4-(bromo-
methyl)benzaldehyde (2)19 in the presence of cesium
carbonate as the base and a catalytic amount of sodium
iodide afforded the perylenediimide derivative 3, function-
alized with an aldehyde group. Subsequent Wittig reaction
between 3 and exTTF triphenylphosphonium salt 4 gave
dyad 5 in a moderate 65% yield.
diimide aromatic core. The vinyl protons of the exTTF
moiety appear as a singlet around 6.30 ppm and the addi-
tional signals of both the aromatic anthracene core of
exTTF and the vinyl-benzene spacer are displayed as com-
plex overlapping duplets and multiplets. One important
issue to be considered in compound 5 is the stereochemistry
of the vinylene double bond. Although Wittig reactions are
known to generally proceed without control on the stereo-
chemistry of the final alkene, in contrast to other olefin-
ation reactions such as Wittig-Horner or McMurry
couplings, it has been reported that the use of the triphen-
ylphosphonium salt 4 in similar Wittig reactions exclusively
affords the trans isomer, probably due to the bulky charac-
ter of the exTTF fragment.16 Experimental evidence of the
trans configuration arises from the observation of an ole-
finic coupling of ca. 16 Hz, which can be related to the
trans configuration. In the 13C NMR spectrum the car-
bonyl imide group of the PDI moiety is clearly visible at
163 ppm together with those signals of the aromatic car-
bons in the sp2 region, the methylene and methyne groups
linked to the nitrogen (55 ppm) and the signals of the alkyl
chains.
The presence of long and branched alkyl chains in the
perylenediimide moiety of dyad 5 provides good solubility
in common organic solvents and enables its complete char-
acterization not only by the standard spectroscopic tech-
niques,20 but also by cyclic voltammetry and absorption
and emission spectroscopy.
The electrochemical behaviour of dyad 5 has been inves-
tigated by cyclic voltammetry in dichloromethane solu-
tions, and this technique shows the ambipolar behaviour
of this material. Thus, at negative values, the two distinct
1=2
one-electron reversible waves (E ¼ ꢀ1:09 and ꢀ1.29 V)
red
1
Thus, the H NMR spectrum of dyad 5 displays at low
corresponding to the successive reduction of the PDI moi-
ety to yield PDIꢁꢀ and PDI2ꢀ species are clearly visible (see
Fig. 1).
fields the characteristic duplets arising from the perylene-
For comparison, the reference compound N,N0-(1-hexyl-
CHO
1=2
H
heptyl)perylenediimide gives peaks at E ¼ ꢀ1:11 and
red
O
N
O
ꢀ1.31 V under the same experimental conditions, which
O
N
N
O
is also similar to the values reported earlier for PDI com-
CHO
pounds.4,21 At positive potentials, a quasireversible oxida-
CsCO3
1=2
ox
+
tion process can be detected, at E ¼ ꢀ0:20 V, which
DMF, Δ
3
can be attributed to the oxidation of the exTTF unit to
Br
O
N
O
2
O
O
H15C7
C7H15
H15C7
C7H15
1
H
15C7
O
C7H15
O
tBuOK
S
S
S
S
N
Br
Toluene / Δ
(Ph)3P
4
5
N,N'-(1-hexylheptyl)PDI
O
N
O
S
S
-1.8
-0.9
E (V)
0.0
Fig. 1. Cyclic voltammogram of dyad 5 along with reference N,N0-(1-
hexylheptyl)PDI (10ꢀ2 M, Pt as working and counter electrode, TBAPF6
(0.1 M) as supporting electrolyte and a scan rate of 200 mV sꢀ1 at room
temperature. The ferrocene/ferrocenium (Fc/Fc+) couple was used as
internal reference and showed a peak at +0.35 V vs Ag/Ag+).
5
S
S
Scheme 1. Synthesis of PDI–exTTF dyad 5.

R. Go´mez et al. / Tetrahedron Letters 49 (2008) 3225–3228
3227
the corresponding aromatic and thermodynamically stable
dication exTTF2+ 22
The similar values observed for the
1.0
0.8
0.6
0.4
0.2
0.0
.
N,N'-(1-hexylheptyl)PDI
5
oxidation and the reduction processes of the two electroac-
tive moieties in dyad 5 compared to the reference com-
pounds suggest that there is no significant ground state
interaction between the electron-donating exTTF unit
and the electron-accepting PDI fragment. Many PDI deriv-
atives substituted at the imide nitrogen have already been
investigated and it has been observed that the imide substi-
tuent has a negligible influence on the electronic properties
of perylenediimides because of the presence of nodes of the
HOMO and LUMO at the imide nitrogen.3c Thus, peryl-
enediimides can be regarded as closed chromophoric sys-
tems whose electronic properties remain unaltered by the
imide substituent.23
500
550
600
650
700
Wavelength (nm)
Fig. 3. Fluorescence spectrum of dyad 5 along with reference N,N0-(1-
Thus, in these systems, the PDI and exTTF units can be
regarded as independent chromophoric systems whose elec-
tronic properties remain unaltered in spite of their covalent
attachment, as it is also observed in the UV–vis spectra.
The UV–vis absorption spectra of the donor-acceptor
dyad 5 together with that of references N,N0-(1-hexyl-hep-
tyl)perylenediimide and exTTF in diluted (ca. 5 ꢁ 10ꢀ6 M)
dichloromethane solutions are depicted in Figure 2. The
absorption spectrum of exTTF–PDI dyad 5 consists of
the approximate superposition of the absorption features
of its constitutive units, confirming the minimal interaction
between the chromophores in the ground state, in agree-
ment with the electrochemical behaviour. No new intramo-
lecular charge-transfer bands above 600 nm were detected
in dyad 5. The absorption spectrum of dyad 5 shows char-
acteristic bands arising from the presence of the PDI
groups, with maxima at 527, 489, and 455 nm, the latter
overlapping with the lowest energy band from exTTF.
Fluorescence emission spectra of PDI–exTTF dyad 5
and N,N0-(1-hexylheptyl)PDI were measured in diluted
dichloromethane solutions at fixed optical density using
an excitation wavelength of 480 nm. The emission features
hexylheptyl)PDI.
are in approximate mirror symmetry to the corresponding
absorption features. This result implies that the fluores-
cence of dyad 5 is due to the PDI units. The fluorescence
properties of the perylenediimide chromophore are greatly
affected by the presence of the exTTF moiety. Thus, a
quantitative quenching (ca. 92%) of the PDI fluorescence
emission was observed in dyad PDI–exTTF in comparison
with the fluorescence spectrum of reference N,N0-(1-hexyl-
heptyl)PDI (Fig. 3).
To explain this strong fluorescence quenching, energy
transfer can be ruled out as the PDI fragment has a smaller
optical bandgap than the exTTF moiety, that is, there is a
minimal spectral overlap between the absorption spectrum
of the exTTF moiety and the fluorescence spectrum of the
PDI fragment. Quenching by the PDI ? exTTF energy
transfer process due to the Fo¨rster mechanism will be pro-
hibited, according to the energy level of both units (2.3324
and 2.7 eV for exTTF15). Thus, we ascribe the large fluores-
cence intensity reduction in dyad 5 to photoinduced elec-
tron transfer (PET) processes between the PDI and
exTTF units. Indeed, a PET process is energetically favour-
able as the corresponding free energy DGPET was estimated
to be ꢀ2.60 eV using the Weller equation.25 However,
time-resolved absorption spectral studies (PIA) will be
needed to provide further evidence for the above
assumption.
2.0
exTTF
N.N'-(1-hexylheptyl)PDI
5
1.5
Acknowledgements
1.0
0.5
0.0
We thank the MCyT (Ref. CTQ2007-60459), Universi-
dad Complutense de Madrid (PR1/07-14894) and a
Comunidad de Madrid-Universidad Complutense joint
project (CCG07-UCM/PPQ-2126, Group no. 910759)
and URJC-CM-2007-CET-1380 for financial support. R.
´
G. is indebted to the ‘Programa Ramon y Cajal’.
300
400
500
600
700
Wavelength (nm)
References and notes
Fig. 2. UV–vis spectra of dyad exTTF–PDI 5 (solid line), N,N0-(1-
hexylheptyl)perylenediimide (dotted line), and exTTF (dashed) in dichlo-
romethane solution.
´
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8.64 (d, 4H, J = 8.0 Hz, PDI), 8.57 (d, 2H, J = 8.0 Hz, PDI), 8.53 (d,
2H, J = 8.0 Hz, PDI), 7.84 (s, 1H), 7.71–7.69 (m, 1H), 7.67 (d, 1H,
J = 8.0 Hz), 7.61 (d, 2H, J = 8.0 Hz), 7.51 (d, 1H, J = 8.0 Hz), 7.39 (d,
1H, J = 8.0 Hz), 7.38 (d, 1H, J = 8.0 Hz), 7.31 (d, 1H, Jtrans = 16.0 Hz,
Hvinyl), 7.31–7.29 (m, 1H), 7.27 (d, 1 H, Jtrans = 16.0 Hz, Hvinyl), 7.11
(s, 2H), 6.31 (s, 4H, HvinylExTTF), 5.42 (s, 2H, –CH2–N), 5.22 (m, 1H,
–CH–N), 2.29 (m, 2H, –CH2–), 1.90 (m, 2H, –CH2–), 1.58 (bs, 6H,
–CH2–), 1.36–1.24 (m, 18H, –CH2–), 0.85 (t, 6H, J = 7.0 Hz, –CH3).
13C RMN (CDCl3, 125 MHz). d (ppm) 163.77, (C@O), 137.27, 136.84,
136.19, 135.69, 135.38, 135.22, 135.16, 134.63, 134.51, 131.98, 131.95,
130.01, 129.91, 129.84, 129.79, 129.08, 128.96, 128.69, 127.05, 126.83,
126.71, 126.37, 125.67, 125.35, 124.68, 123.57, 123.48, 123.42, 123.34,
123.22, 122.52, 122.45, 117.70, 117.62, 117.51, 55.22 (C–N), 53.82
(C–N), 32.78, 32.20, 29.92, 29.62, 27.40, 23.01, 14.46. FTIR (KBr).
m = 2922, 2852, 1695, 1657, 1594, 1580, 1334 cmꢀ1. UV–vis (CH2Cl2).
k (nm) = 527, 489, 455, 434 (sh), 377, 320. m/z (FAB): 1094 (M+).
Elemental anal.: calcd for C68H58N2O2S4: C: 74.55 H, 5.34, N, 2.56.
Found: C, 74.72, H, 5.18, N, 2.52.
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+
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¨
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