Photoinduced electron transfer in a fullerene-oligophenylenevinylene dyad

Article abstract of DOI:10.1039/b9nj00259f

A dialkylamino-subtituted oligophenylenevinylene (OPV) derivative bearing a fullerene subunit (F-D) has been prepared. The electrochemical properties of F-D have been investigated by cyclic voltammetry. Whereas the first reduction is centered on the C₆₀ unit, the oxidation is centered on the dialkylamino subunit of the OPV rod. In F-D, both the OPV and the fullerene-centered fluorescence bands are quenched and this suggests the presence of photoinduced electron transfer from the amino-substituted OPV to the carbon sphere. By means of bimolecular quenching experiments, transient absorption spectral fingerprints of the radical cationic species have been detected in the VIS (670 nm) and NIR (1300-1500 nm) regions, along with the much weaker fullerene anion band at λ_max = 1030 nm. Intramolecular photoinduced electron transfer occurs in the investigated dyad and a relatively long-lived charge-separated state has been detected, with a lifetime of 135 and 85 ns in toluene and benzonitrile, respectively. The longer lifetime in the less polar solvent suggests a Marcus inverted region behavior for the charge recombination process.

Full text of DOI:10.1039/b9nj00259f

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Photoinduced electron transfer in a
fullerene–oligophenylenevinylene dyad
a
a
Aline Gegout, Juan Luis Delgado, Jean-Franc¸
ois Nierengarten,*a
´
´
b
c
c
Beatrice Delavaux-Nicot,* Andrea Listorti, Claudio Chiorboli,
Abdelhalim Belbakrac and Nicola Armaroli*c
Received (in Montpellier, France) 12th June 2009, Accepted 17th July 2009
First published as an Advance Article on the web 26th August 2009
DOI: 10.1039/b9nj00259f
A dialkylamino-subtituted oligophenylenevinylene (OPV) derivative bearing a fullerene subunit
(F–D) has been prepared. The electrochemical properties of F–D have been investigated by cyclic
voltammetry. Whereas the first reduction is centered on the C60 unit, the oxidation is centered on
the dialkylamino subunit of the OPV rod. In F–D, both the OPV and the fullerene-centered
fluorescence bands are quenched and this suggests the presence of photoinduced electron transfer
from the amino-substituted OPV to the carbon sphere. By means of bimolecular quenching
experiments, transient absorption spectral fingerprints of the radical cationic species have been
detected in the VIS (670 nm) and NIR (1300–1500 nm) regions, along with the much weaker
fullerene anion band at lmax = 1030 nm. Intramolecular photoinduced electron transfer occurs in
the investigated dyad and a relatively long-lived charge-separated state has been detected, with a
lifetime of 135 and 85 ns in toluene and benzonitrile, respectively. The longer lifetime in the less
polar solvent suggests a Marcus inverted region behavior for the charge recombination process.
charge-separated state is lower than that of the fullerene
Introduction
singlet. Indeed, such a favorable thermodynamic driving force
is mainly dependent on the donating ability of the conjugated
oligomer associated to the fullerene acceptor. For moderate
donors, the charge-separation process shows a strong solvent
polarity dependence. For example, Hummelen, Janssen and
co-workers have reported the photophysical properties of an
oligophenylenevinylene (OPV)–C60 conjugate in solvents of
different polarity.5 In apolar solvents such as toluene, the
charge-separated state is higher in energy than the first
fullerene singlet and triplet excited states. Therefore, the light
energy absorbed by the OPV fragment, promptly conveyed to
the fullerene lowest singlet excited state via energy transfer,
cannot yield charge separation anymore. In contrast, the
energy of the charge-separated state drops below that of the
first fullerene singlet excited state in more polar solvents
allowing occurrence of electron transfer after the initial energy
transfer event. Similar findings have been reported by Martin,
Guldi and co-workers6 for a series of oligonaphthylenevinylene–
fullerene dyads and by us for fullerene derivatives covalently
linked to OPV fragments7 or dendritic wedges.8 In this paper,
we now report a new C60–oligophenylenevinylene (OPV)
derivative in which the OPV unit has been functionalized with
a dialkylamine group to improve its donating ability.9 As a
result, photoinduced electron transfer is observed upon the
initial energy transfer whatever the solvent.
Owing to their low reduction potentials and low reorganization
energy in electron-transfer reactions, fullerene derivatives
are outstanding electron acceptors in photoactive molecular
devices.1 Actually, an impressive amount of work has been
devoted to the development of molecular systems in which
C60 is covalently linked to electron donors.1 As part of this
research, hybrid compounds combining the carbon sphere
with p-conjugated oligomers are of particular interest.2
Effectively, their peculiar electronic properties led to the
development of promising molecular materials for photovoltaic
applications.3 In this particular case, the occurrence of
photoinduced electron transfer is of fundamental importance
as charges have to be produced upon light absorption to
generate the photocurrent. However, for C60–(p-conjugated
oligomer) hybrid systems, the anticipated electron transfer
process is in competition with an ultrafast energy transfer.4
Hence, fullerene derivatives are also excellent energy acceptors
and the low-lying fullerene singlet and triplet excited states are
quite often the final products of the photoinduced processes.4
Sometimes, however, the initial energy transfer event can be
followed by an electron transfer when the energy level of the
a
´
´
Laboratoire de Chimie des Materiaux Moleculaires,
Ecole Europeenne de Chimie, Polymeres et Materiaux,
´
`
´
´
Universite de Strasbourg et CNRS (UMR 7509), 25 rue Becquerel,
67087 Strasbourg Cedex 2, France
E-mail: nierengarten@chimie.u-strasbg.fr
b Laboratoire de Chimie de Coordination du CNRS,
205 route de Narbonne, 31077 Toulouse Cedex 4, France.
E-mail: Beatrice.Delavaux-Nicot@lcc-toulouse.fr
Results and discussion
Synthesis
c
`
Istituto per la Sintesi Organica e la Fotoreattivita, Consiglio
In the design of the C60–OPV conjugate, we have decided to
introduce a spacer unit between the two photoactive moieties
Nazionale delle Ricerche, via Gobetti 101, 40129 Bologna and Via
Borsari 46 44100 Ferrara, Italy. E-mail: armaroli@isof.cnr.it
ꢀc
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in order to prevent fast charge recombination upon electron
transfer and thus to facilitate the observation of the charge-
separated state. The synthesis is based on the esterification
reaction of a fullerene carboxylic acid building block with an
OPV derivative bearing one hydroxy group. In the first
synthetic approach, we have prepared OPV oligomer D
bearing a terminal bromobenzene subunit (Scheme 1). The
latter should give us access to the targeted OPV building block
with an alcohol group by lithiation followed by quenching
with formaldehyde.
hindrance resulting from the presence of the octyloxy group
in the ortho position of the reactive phosphonate groups may
explain the lack of E : Z selectivity.12 Isomerization to the
all-E derivative 6 was easily achieved by treatment with a
catalytic amount of iodine in refluxing toluene and compound
6 was thus obtained in 52% yield. The E stereochemistry of the
double bond in 6 was confirmed by the coupling constant of
ca. 16.5 Hz observed for the AX system corresponding to the
signal of the vinylic protons in the 1H NMR spectrum.
Treatment with trifluoroacetic acid (TFA) in CH2Cl2–H2O
afforded aldehyde 7 in 80% yield. Subsequent reaction with
phosphonate 813 under Wadsworth–Emmons conditions
Reaction of 4-bromophenol (1) with 2-bromoethanol under
Williamson conditions (K2CO3, DMF) gave alcohol 2 in 83%
yield. Subsequent treatment with CBr4–PPh3 in THF afforded
bromide 3 in 70% yield. Alkylation of 4-hydroxybenzaldehyde
with 3 gave aldehyde 4 in 88% yield. Reaction of 4 with
phosphonate 510 in the presence of t-BuOK in THF afforded
the targeted stilbene derivative, but as an E–Z isomer mixture.
The reaction of benzylic phosphonates with aromatic aldehydes
under Wittig–Horner conditions is generally stereoselective,11
leading to the E isomer only. In the present case, steric
1
afforded the all-E OPV derivative D as attested by H NMR
spectroscopy showing coupling constants of ca. 16 to 16.5 Hz
for the two AX systems of the vinylic protons. The structure of
D was further confirmed by its MALDI-TOF mass spectrum,
which shows the expected molecular ion peak at m/z 824.95.
Lithiation of compound D followed by quenching with
formaldehyde was attempted under different experimental
conditions to introduce an alcohol group on the OPV building
block. However, in our hands, the targeted alcohol derivative
was never obtained from D. Indeed, bromide D was found
to be poorly reactive under all the attempted lithiation
conditions. This prompted us to develop a new synthetic route
in which the alcohol unit is introduced at an earlier stage of the
synthesis (Scheme 2).
Reduction of aldehyde 4 with lithium aluminium hydride
followed by reaction of the resulting 9 with triisopropylsilyl
chloride (TIPSCl) afforded protected alcohol 10. Treatment
with t-BuLi and subsequent quenching with N,N-dimethyl-
formamide (DMF) afforded aldehyde 11 in 80% yield. The
preparation of the OPV backbone from compound 11 was
then achieved by following the synthetic route developed for
compound D. Reaction of 11 with phosphonate 5 under
Wadsworth–Emmons conditions followed by treatment with
a catalytic amount of iodine in refluxing toluene gave 12
which, after treatment with CF3CO2H in CH2Cl2–H2O,
yielded aldehyde 13. Subsequent reaction of 13 with phospho-
nate 8 in the presence of t-BuOK afforded OPV derivative 14
(Scheme 3). Alcohol 15 was finally obtained in 61% yield by
treatment with tetra-n-butylammonium fluoride (TBAF) in
THF at 0 1C.
Reaction of alcohol 15 with carboxylic acid 16 under
esterification conditions using N,N0-dicyclohexylcarbodiimide
(DCC), 4-dimethylaminopyridine (DMAP) and 1-hydroxy-
benzotriazole (HOBt) afforded compound F–D in 65% yield.
Both 1H- and 13C-NMR spectra were in full agreement
with the structure of compound F–D. In addition, the
molecular constitution of F–D was further confirmed by its
MALDI-TOF mass spectrum which depicted the expected
molecular ion peak at m/z 2209.54.
Electrochemistry
The electrochemical properties of compounds D, F and F–D
were investigated by cyclic voltammetry (CV). All the
experiments were performed at room temperature in CH2Cl2
solutions containing tetra-n-butylammonium tetrafluoroborate
(0.1 M) as supporting electrolyte, with a Pt wire as the working
Scheme 1 Reagents and conditions: (a) 2-bromoethanol, K2CO3,
DMF, 80 1C (83%); (b) CBr4, PPh3, THF, 0 1C (70%), (c) 4-hydroxy-
benzaldehyde, K2CO3, DMF, 80 1C (88%); (d) t-BuOK, THF, 0 1C to
room temp., then I2, toluene, D (52%); (e) CF3CO2H, H2O, CH2Cl2,
room temp. (80%); (f) t-BuOK, THF, 0 1C to room temp. (82%).
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dialkoxyphenyl moiety.15 The cyclic voltammograms recorded
for hybrid compound F–D shows the characteristic electro-
chemical features of both constitutive units, i.e. methano-
fullerene and OPV. The comparison of the E1/2 potentials of
F–D with the corresponding model compounds clearly shows
that the three first reduction waves correspond to fullerene-
centered processes, while the two oxidation processes are
centered on the conjugated OPV backbone. Comparison of
the redox potentials of F–D with those of the corresponding
model compounds F and D reveals no particular electronic
interactions between the two moieties.
Ground state absorption and luminescence properties
The electronic absorption spectra of D, F and F–D in toluene
are reported in Fig. 2. For dyad F–D, absorption features at
l o 350 nm and l 4 450 nm are attributable to the
methanofullerene moiety, whereas the wide and intense band
around 425 nm is related to transitions centered on the OPV
moiety. The absorption spectra of dyad F–D exhibit minor
differences when compared with the sum of the spectra of the
corresponding reference compounds, indicating negligible
ground state electronic interactions in good agreement with
the electrochemical data (vide supra).
Oligomer D is a quite strong fluorophore with an emission
quantum yield of 0.46 and an excited state lifetime of 1.2 ns in
toluene solution. This fluorescence signal, attributable to the
deactivation of the lowest singlet OPV electronic level,7 is
dramatically quenched in dyad F–D (f = 0ꢂ031 and t o 0.5 ns)
suggesting the occurrence of OPV–C60 excited state inter-
actions. As extensively discussed in the past for similar
OPV–C60 structures,7 the quenching mechanism is ultrafast
1OPV* - 1C60 singlet energy transfer. Importantly, sensitization
of the carbon sphere via the OPV moiety (lexc = 450 nm) does
not result in any fullerene fluorescence even in apolar toluene.
Model compound F is characterized by a fluorescence band
Scheme 2 Reagents and conditions: (a) LiAlH4, THF, 0 1C (84%);
(b) TIPSCl, imidazole, DMF, 0 1C to room temp. (90%); (c) t-BuLi,
THF, ꢁ78 1C, then DMF, then HCl (80%); (d) 5, t-BuOK, THF,
0 1C to room temp., then I2, toluene, D (80%); (e) CF3CO2H, H2O,
CH2Cl2, room temp. (70%).
electrode and a saturated calomel electrode (SCE) as a
reference. Potential data for all of the compounds are collected
in Table 1. Typical examples of cyclic voltammograms are
shown in Fig. 1.
centered at 705 nm with t = 1.5 ꢃ 0.1 ns and f = 3 ꢄ 10ꢁ4 16
,
corresponding to the deactivation of the lowest-lying singlet
level (1F*). In both toluene and benzonitrile, 1F* is
substantially quenched in F–D when compared to reference
F (Fig. 2). Moreover no singlet oxygen sensitized emission is
detected in the NIR at 1270, also ruling out the population of
the fullerene triplet level.17 These results can be rationalized
assuming that the electronic excited states centered on the two
moieties are quenched due to the presence of a low-lying
charge-separated state indicated as Fꢁ–D+, the positive
charge being located on the amine terminal residue of the
OPV moiety and the negative one on the C60 unit (Fig. 3).
Compound F exhibits the classical behavior observed for
methanofullerene derivatives.14 Whereas the first reduction
observed at ꢁ0.51 V vs. SCE is always reversible, the second
is irreversible at low scan rates but becomes reversible upon
increasing the scan rate. The third wave gradually disappears
when the second one becomes reversible, so that it probably
implies reduction of the product formed after the second
reduction.14 For model compound D, two quasi-reversible
oxidation processes are observed at +0.61 and +0.85 V vs.
SCE. The first one corresponds to the oxidation of the
terminal tertiary amine group while the second one is
most likely ascribed to an oxidation centered on the central
Transient absorption spectroscopy
In order to get evidence for the occurrence of photoinduced
electron transfer, transient absorption investigations were
carried out in benzonitrile (BN). First, bimolecular quenching
experiments were accomplished, with the aim of revealing the
spectral features of the involved cationic and anionic species.
Transient absorption spectra in the nano- and microsecond
timescale obtained with a solution of F (0.1 mM) in the
presence of D (0.1 mM) in oxygen-free BN are depicted in
Fig. 4. The broad absorption band peaked at 700 nm and
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Scheme 3 Reagents and conditions: (a) 8, t-BuOK, THF, 0 1C to room temp. (60%); (b) TBAF, THF, 0 1C (61%); (c) DCC, DMAP, HOBt,
CH2Cl2, 0 1C to room temp. (65%).
appearing immediately after the ns-laser pulse is attributed to
the fullerene triplet (3F*);16 for comparison, see the typical
long-lived triplet absorption features of F alone in benzonitrile
(Fig. 4b). The early triplet spectral trace of F (700 ns) then
n
Fig. 1 Cyclic voltammograms of F, D and F–D on a Pt electrode at v = 0.1 V sꢁ1 in CH2Cl2 + 0.1 M Bu4NBF4.
ꢀc
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Table 1 Electrochemical data of F, D and F–D determined by CV on
a Pt working electrode in CH2Cl2 + 0.1 M nBu4NBF4 at room
temperature
Oxidation
Reduction
a
E1
a
E2
a
b
c
E1
E2
E3
F
D
F–D
—
+0.62
+0.61
—
+0.87
+0.85
ꢁ0.51
—
ꢁ0.50
ꢁ0.89
—
ꢁ0.87
ꢁ1.12
—
ꢁ1.12
a
Values for (Epa + Epc)/2 in V vs. SCE at a scan rate of 100 mV sꢁ1
Peak potential value at a scan rate of 100 mV sꢁ1, irreversible
.
b
c
process. Waves corresponding to an electrogenerated species
obtained after the second reduction step.
Fig. 3 Energy level diagram summarizing the photoinduced intra-
molecular processes in F–D (EnT: energy transfer, ElT: electron
transfer, CR: charge recombination). The energy of the lowest singlet
excited states (1D* and 1F*) were estimated from the highest energy
feature of fluorescence spectra in Fig. 2. The energy of the lowest C60
triplet states (3F*) was taken from earlier work.7 The values of the first
oxidation and reduction potentials (Table 1) allow the determination
of the free energy change (ꢁDG) of the corresponding redox process
leading to the charge-separated (CS) state Fꢁ–D+
.
Fig. 2 Absorption spectra of F–D (grey) and its component chromo-
phores F (dotted black) and D (black) at 298 K. Dashed lines:
fluorescence spectra of D (black) and F–D (grey, multiplied by a factor
of 100); lexc = 420 nm, toluene. Inset: fullerene-centered fluorescence
spectra: F (dotted black) and F–D (grey), lexc = 550 nm, toluene.
disappears and a new spectral profile grows up with two broad
bands in the VIS and NIR regions at lmax = 640 and 1300 nm.
Such bands are assigned to the radical cation of D18 and their
rise occurs within about 15 ms after excitation (Fig. 4a, inset).
By adding oxygen to the solution, where an intermolecular
3
energy transfer from C60* to oxygen takes place,19,20 the
electron transfer event is suppressed and the novel spectral
traces are no longer detected. The latter observation confirmed
the occurrence of electron transfer from the fullerene triplet
level. Despite several efforts, a detailed Stern–Volmer kinetic
analysis of the bimolecular quenching process, with determi-
nation of the electron transfer yield (Fet) and rate (ket),18,20,21
is not possible due to the substantial overlapping of the
fullerene triplet spectral features with the intense radical cation
bands, which prevents a reliable evaluation of the pseudo-first
Fig. 4 (a) Transient absorption spectra of an oxygen-free benzonitrile
solution containing F (1 ꢄ 10ꢁ4 M) and D (1 ꢄ 10ꢁ4 M); depicted
spectral traces are at 1, 2, 11, 70, 170 and 350 ms (from bigger to
smaller circles) after the laser pulse. Inset: absorbance decays at
600 (grey) and 1020 nm (light grey). lexc = 532 nm; energy:
0.5 mJ per pulse. (b) Transient absorption spectra of F in benzonitrile;
depicted spectral traces are taken between 1 and 80 ms (from bigger to
smaller circles). Inset: absorbance decay at 700 nm. lexc = 532 nm;
energy: 0.5 mJ per pulse.
3
order decay rate constant of C60*.
Transient absorption spectra were then recorded for dyad
F–D in deaerated benzonitrile and toluene in the ns–ms
timescale. The spectral traces obtained for F–D are depicted
in Fig. 5 and exhibit the ion pair features, i.e. D radical cation
(670, 1300 nm) and F radical anion (1030 nm), the latter as a
neat spike emerging over the intense cation background. They
are compelling evidence of intramolecular electron transfer.
The first order decay kinetic traces (Fig. 5, inset) are associated
to the charge recombination process, yielding a lifetime of the
charge-separated state of 135 and 85 ns in toluene and
benzonitrile, respectively. The longer lifetime in the less polar
solvent suggests a back-electron transfer process occurring in
the Marcus inverted region,22 as often found in photoinduced
electron transfer involving fullerene moieties23–26 that are
characterized by an intrinsically low reorganization energy.27
Notably, compared to bimolecular quenching experiments
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analyses were performed by the analytical service at the
Laboratoire de Chimie de Coordination (Toulouse, France).
Synthesis
Compound 2. A mixture of K2CO3 (21.00 g, 0.15 mol),
4-bromophenol (10.00 g, 0.06 mol), and 2-bromoethanol
(5.10 mL, 0.07 mol) in DMF (100 mL) was heated at 80 1C
for 72 h. The mixture was then filtered over Celite, evaporated
to dryness and taken up with CH2Cl2. The organic layer was
washed with a saturated aqueous NaCl solution, dried
(MgSO4), filtered and evaporated. Column chromatography
(SiO2, hexane–CH2Cl2 1 : 1) yielded 2 (10.00 g, 83%) as a
1
white solid. H NMR (300 MHz, CDCl3): 7.37 (m, 2H), 6.79
(m, 2H), 3.98 (m, 4H), 2.18 (t, J = 6 Hz, 1H).13C NMR
Fig. 5 Upper panel (a) transient absorption spectra of F–D in
toluene; profiles from 0.08 ms (black) to 1.4 ms (black dotted line).
Bottom panel (b) transient absorption spectra of F–D in benzonitrile;
profiles from 0.05 ms (black) to 1.6 ms (black dotted line). Insets:
absorbance decays at 600 nm for both panels; lexc = 532 nm; energy:
0.5 mJ per shot.
(75 MHz, CDCl3): 157.8, 132.4, 116.4, 113.3, 69.48, 61.3.
Compound 3. CBr4 (10.70 g, 0.03 mmol) and PPh3 (8.39 g,
0.03 mmol) were added to a solution of 2 (5.00 g, 0.02 mmol)
in anhydrous THF (200 mL) at 0 1C under argon atmosphere.
The mixture was then stirred for 4 h. The solvent was removed
under reduced pressure and the residue purified by column
chromatography (SiO2, hexane–CH2Cl2 4 : 1) to yield 3
(vide supra) no triplet features are observed at the earliest time
following nanosecond laser excitation, suggesting an ultrafast
forward intramolecular electron transfer step from the
fullerene singlet level.8
1
(3.92 g, 70%) as a white solid. H NMR (300 MHz, CDCl3):
7.40 (m, 2H), 6.79 (m, 2H), 4.28 (t, J = 6 Hz, 2H), 3.65 (t, J =
6 Hz, 2H). 13C NMR (75 MHz, CDCl3): 157.2, 132.4, 116.6,
113.7, 68.1, 28.9. Anal. calcd for C8H8OBr2: C 34.32,
H 2.88%. Found: C 34.30, H 2.65%.
Conclusions
Compound 4. As described for 2, with 4-hydroxybenzalde-
hyde (1.72 g, 14.08 mmol), 3 (4.73 g, 16.89 mmol) and K2CO3
(4.86 g, 35.20 mmol) in DMF (25 mL). Column chromato-
graphy (SiO2, hexane–CH2Cl2 1 : 1) yielded 4 (4.00 g, 88%) as
a white solid. 1H RMN (300 MHz, CDCl3): 9.90 (s, 1H), 7.86
(d, J = 9 Hz, 2H), 7.40 (d, J = 9 Hz, 2H), 7.06 (d, J = 9 Hz,
2H), 6.84 (d, J = 9 Hz, 2H), 4.37 (m, 4H). 13C NMR (75 MHz,
CDCl3): 190.8, 163.5, 157.6, 132.4, 132.0, 130.3, 116.5, 114.9,
113.5, 66.7, 66.5. Anal. calcd for C15H13O3Br: C 56.10,
H 4.08%. Found: C 55.81, H 3.93%.
We have prepared and investigated a C60–OPV hybrid system
in which a dialkylamino group was attached to the OPV
moiety, to serve as potential electron donor for the fullerene
acceptor in photoinduced electron transfer. This process was
deduced by the lack of the typical OPV and fullerene-centered
fluorescence in the multicomponent array and unambiguously
evidenced by means of transient absorption spectroscopy.
A relatively long-lived charge-separated state was detected
and the dependence of its charge recombination kinetics on
the solvent polarity (135 and 85 ns in toluene and benzonitrile,
respectively) suggests an inverted region regime.
Compound 6. t-BuOK (0.22 g, 1.95 mmol) was added to a
solution of 4 (0.50 g, 1.5 mmol) and 5 (1.03 g, 1.71 mmol) in
anhydrous THF (30 mL) under argon atmosphere at 0 1C. The
mixture was stirred for 1 h at 0 1C, then overnight at room
temperature. The mixture was then filtered over Celite,
evaporated to dryness and taken up with CH2Cl2. The organic
layer was washed with water, dried (MgSO4), filtered and
evaporated. Column chromatography (SiO2, hexane–CH2Cl2
3 : 2) gave 6 as an E–Z isomer mixture. The E–Z mixture
obtained after the first chromatographic purification was
directly isomerized as follows: a solution of the E–Z mixture
and I2 in toluene (10 ml) was refluxed for 12 h and then cooled
to room temperature. The resulting toluene solution was
washed with an aqueous 0.3 M Na2S2O3 solution and water,
dried (MgSO4), filtered and evaporated. Column chromato-
graphy (SiO2, hexane–CH2Cl2 3 : 2) yielded 6 (0.60 g, 52%) as
an orange solid that was used in the next step without further
purifications. 1H NMR (300 MHz, CDCl3): 7.46 (d, J = 9 Hz,
2H), 7.39 (d, J = 9 Hz, 2H), 7.34 (d, J = 16.5 Hz, 1H), 7.19
(s, 1H), 7.08 (s, 1H), 7.05 (d, J = 16.5 Hz, 1H), 6.93 (d, J =
9 Hz, 2H), 6.84 (d, J = 9 Hz, 2H), 5.75 (s, 1H), 4.31 (m, 4H),
Experimental
Materials and methods
All reagents were used as purchased from commercial sources
without further purification. Compounds 5,10 8,13 1628 and F28
were prepared according to a previously reported procedure.
Solvents were dried using standard techniques prior to use. All
reactions were performed in standard glassware. Evaporation
was done using water aspirator and then drying in vacuo
at 10ꢁ2 Torr. Column chromatography: silica gel 60
(230–400 mesh, 0.040–0.063 mm) was purchased from E.
Merck and treated with Et3N (1% in CH2Cl2) for the purification
of compounds bearing NEt2 groups. TLC: precoated glass
sheets with silica gel 60 F254 (Merck), visualization by UV
light. Gel permeation chromatography was performed on
Biorad, Biobeads SX-1 under the use of CH2Cl2 as eluent.
NMR spectra were recorded on a Bruker AM 300 (300 MHz)
with solvent signal as reference. MALDI-TOF-MS were
obtained on a Bruker BIFLEXt mass spectrometer. Elemental
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4.02 (t, J = 6 Hz, 4H), 3.72 (AB, J = 11 Hz, 4H), 1.82
mixture was stirred overnight at room temperature. Water
(100 mL) was then added to the reaction mixture and it was
extracted with ether (3 ꢄ 20 mL). The organic layer was
washed with water, dried (MgSO4), filtered and evaporated.
The residue was recrystallized from hexane to yield 10 (2.00 g,
(m, 4H), 1.32 (m, 23H), 0.89 (m, 6H), 0.81 (s, 3H).
Compound 7. TFA (3 mL) was added dropwise to a solution
of 6 (0.56 g, 0.73 mmol) in CH2Cl2–H2O 1 : 1 (12 mL).
The mixture was then stirred for 2 h, washed with H2O
(until pH was near neutrality), dried (MgSO4), filtered and
evaporated. Column chromatography (SiO2, hexane–CH2Cl2
1
90%). H NMR (300 MHz, CDCl3): 7.38 (d, J = 9 Hz, 2H),
7.28 (d, J = 9 Hz, 2H), 6.92 (d, J = 9 Hz, 2H), 6.85 (d, J =
9 Hz, 2H), 4.73 (s, 2H), 4.31 (s, 4H), 1.11 (m, 21H). 13C NMR
(75 MHz, CDCl3): 157.5, 157.2, 134.5, 132.3, 127.2, 116.5,
114.7, 113.3, 66.6, 66.5, 64.7, 18.1, 12.1. Anal. calcd for
C24H35O3BrSi: C 60.11, H 7.36%. Found: C 60.03, H 7.60%.
1
3 : 2) yielded 7 (0.40 g, 80%) as an orange solid. H NMR
(300 MHz, CDCl3): 10.45 (s, 1H), 7.50 (d, J = 9 Hz, 2H), 7.40
(d, J = 9 Hz, 2H), 7.36 (d, J = 16.5 Hz, 1H), 7.32 (s, 1H), 7.20
(d, J = 16.5 Hz, 1H), 7.15 (s, 1H), 6.95 (d, J = 9 Hz, 2H), 6.84
(d, J = 9 Hz, 2H), 4.32 (m, 4H), 4.10 (t, J = 6 Hz, 2H), 4.02
(t, J = 6 Hz, 2H), 1.85 (m, 4H), 1.30 (m, 20H), 0.88 (m, 6H).
13C NMR (75 MHz, CDCl3): 189.1, 158.7, 157.8, 156.3, 150.6,
134.7, 132.4, 131.8, 130.6, 128.3, 124.0, 121.1, 116.5, 114.9,
110.3, 110.1, 69.3, 69.1, 66.8, 66.5, 31.8, 29.4, 29.35, 29.3, 26.2,
26.15, 22.7, 14.1.
Compound 11. A 1.7 M solution of t-BuLi in THF (2.40 mL,
4.06 mmol) was added dropwise to a solution of 10 (0.97 g,
2.03 mmol) in anhydrous THF (10 mL) at ꢁ78 1C under argon
atmosphere. After 3 h at ꢁ78 1C, dry DMF (0.24 ml,
3.05 mmol) was added and the resulting mixture was allowed
to warm to room temperature. The mixture was stirred for 2 h,
then an aqueous 1 M HCl solution was added. The solvent was
removed under reduced pressure and the residue taken up with
CH2Cl2. The organic layer was washed with water, dried
(MgSO4), filtered and evaporated. Column chromatography
(SiO2, hexane–CH2Cl2 2 : 1) yielded 11 (0.69 g, 80%) as a
Compound D. t-BuOK (93 mg, 0.83 mmol) was added to a
solution of 7 (0.45 g, 0.66 mmol) and 8 (0.30 g, 0.99 mmol) in
anhydrous THF (13 mL) at 0 1C under argon atmosphere. The
reaction mixture was stirred for 1 h at 0 1C, then for 2 h at
room temperature. The mixture was then filtered over Celite,
evaporated and taken up with CH2Cl2. The organic layer was
washed with water, dried (MgSO4), filtered and evaporated.
Column chromatography (SiO2, CH2Cl2) yielded D (0.45 g,
1
white solid. H NMR (300 MHz, CDCl3): 9.90 (s, 1H), 7.85
(d, J = 9 Hz, 2H), 7.29 (d, J = 9 Hz, 2H), 7.06 (d, J = 9 Hz, 2H),
6.92 (d, J = 9 Hz, 2H), 4.78 (s, 2H), 4.38 (m, 4H), 1.11
(m, 21H). 13C NMR (75 MHz, CDCl3): 190.8, 163.7, 157.4,
134.6, 131.9, 130.2, 127.1, 114.9, 114.4, 66.9, 66.3, 64.6, 18.1,
12.0. Anal. calcd for C25H36O4Si: C 70.05, H 8.47%. Found:
C 69.89, H 8.33%.
1
82%) as an orange solid. H NMR (300 MHz, CDCl3): 7.47
(d, J = 9 Hz, 2H), 7.42 (d, J = 9 Hz, 2H), 7.40 (d, J =
9 Hz, 2H), 7.33 (s, 1H), 7.27 (d, J = 16.5 Hz, 1H), 7.14
(d, J = 16 Hz, 1H), 7.10 (s, 1H), 7.06 (d, J = 16 Hz, 1H), 7.05
(d, J = 16.5 Hz, 1H), 6.94 (d, J = 9 Hz, 2H), 6.85 (d, J = 9
Hz, 2H), 6.68 (d, J = 9 Hz, 2H), 4.31 (m, 4H), 4.04 (2t, J = 6
Hz, 4H), 3.39 (q, J = 7 Hz, 4H), 1.87 (m, 4H), 1.31 (m, 20H),
1.19 (t, J = 7 Hz, 6H), 0.89 (m, 6H). 13C NMR (75 MHz,
CDCl3): 158.0, 157.8, 151.1, 150.7, 147.3, 132.3, 131.6, 128.9,
127.9, 127.7, 127.5, 126.0, 125.5, 121.9, 118.5, 116.6, 114.9,
113.3, 111.7, 110.8, 110.1, 69.8, 69.6, 66.8, 66.5, 44.4, 31.9,
31.85, 29.65, 29.6, 29.5, 29.45, 29.4, 26.3, 22.75, 22.7, 14.1,
12.7. Anal. calcd for C50H66O4BrN: C 72.80, H 8.06, N 1.70%.
Found: C 73.04, H 8.15, N 1.48%. MS-MALDI: calcd for
C50H66O4BrN 824.96. Found: 824.95 [M]+.
Compound 12. As described for 6, with 11 (0.44 g, 1.01
mmol), 5 (0.73 g, 1.21 mmol) and t-BuOK (0.15 g, 1.22 mmol)
in anhydrous THF (20 mL). Column chromatography
(SiO2, hexane–CH2Cl2 1 : 1) yielded 12 (0.69 g, 80%) as an
orange glassy product. 1H NMR (300 MHz, CDCl3): 7.46
(d, J = 9 Hz, 2H), 7.39 (d, J = 9 Hz, 2H), 7.34 (d, J =
16.5 Hz, 1H), 7.19 (s, 1H), 7.08 (s, 1H), 7.05 (d, J = 16.5 Hz, 1H),
6.93 (d, J = 8 Hz, 2H), 6.92 (d, J = 8 Hz, 2H), 5.75 (s,1H),
4.77 (s, 2H), 4.31 (s, 4H), 4.02 (2t, J = 6 Hz, 4H), 3.72
(AB, J = 11 Hz, 4H), 1.82 (m, 4H), 1.32 (m, 23H), 1.1
(m, 21H), 0.89 (t, J = 7 Hz, 6H), 0.88 (s, 3H). 13C NMR
(75 MHz, CDCl3): 158.2, 157.6, 151.0, 150.3, 141.1, 134.4,
130.3, 128.5, 128.1, 127.7, 127.2, 126.9, 121.8, 114.9, 114.5,
114.2, 111.5, 110.5, 91.1, 69.6, 69.4, 69.2, 66.6, 64.7, 31.85,
31.8, 30.3, 29.7, 29.5, 29.45, 29.4, 29.35, 29.3, 26.3, 26.1, 23.2,
22.75, 22.7, 21.9, 18.1, 14.1, 12.1.
Compound 9. A 1 M LiAlH4 solution in THF (10 mL,
10 mmol) was added dropwise to a solution of 4 (2.50 g,
7.78 mmol) in anhydrous THF (150 mL) at 0 1C. The mixture
was stirred for 2 h. Precipitate of the aluminium salts was
obtained upon addition of methanol and H2O. The mixture
was then filtered over Celite and evaporated to dryness. The
residue was recrystallized from hexane to yield 9 (2.10 g, 84%).
1H NMR (300 MHz, CDCl3): 7.39 (d, J = 9 Hz, 2H), 7.30
(d, J = 9 Hz, 2H), 6.94 (d, J = 9 Hz, 2H), 6.84 (d, J = 9 Hz, 2H),
4.63 (s, 2H), 4.30 (s, 4H). 13C NMR (75 MHz, CDCl3): 158.2,
157.8, 133.6, 133.3, 128.7, 116.5, 114.8, 113.4, 66.6, 66.5, 65.0.
Anal. calcd for C15H15O3Br: C 55.75, H 4.68%. Found:
C 55.84, H 4.71%.
Compound 13. As described for 7, with 12 (0.40 g,
0.46 mmol) and TFA (3 mL) in CH2Cl2–H2O 1 : 1 (10 mL).
Column chromatography (SiO2, hexane–CH2Cl2 1 : 1) yielded
13 (0.25 g, 70%) as a red glassy product. 1H NMR (300 MHz,
CDCl3): 10.47 (s, 1H), 7.46 (d, J = 9 Hz, 2H), 7.33
(d, J = 16.5 Hz, 1H), 7.31 (d, J = 9 Hz, 2H), 7.18 (s, 1H),
7.05 (d, J = 16.5 Hz, 1H), 7.01 (s, 1H), 6.99 (d, J = 8 Hz, 2H),
6.95 (d, J = 8 Hz, 2H), 4.80 (s, 2H), 4.37 (s, 4H), 4.00
(2t, J = 6 Hz, 4H), 1.85 (m, 4H), 1.39–1,25 (m, 20H), 1.10
(m, 21H), 0.89 (t, J = 7 Hz, 6H). 13C NMR (75 MHz, CDCl3):
189.1, 158.9, 157.5, 156.3, 150.6, 134.7, 134.4, 131.8, 130.4,
128.2, 127.2, 123.9, 121.0, 115.0, 114.4, 110.3, 110.1, 69.2, 69.1,
Compound 10. TIPSCl (1.20 mL, 4.64 mmol) and imidazole
(0.76 g, 11.14 mmol) were added to a solution of 9 (1.50 g,
4.64 mmol) in DMF (12 mL) at 0 1C under argon. The reaction
ꢀc
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66.7, 66.6, 64.7, 31.8, 29.7, 29.35, 29.3, 29.25, 26.2, 26.1, 22.7,
18.1, 17.7, 14.1, 12.1.
0.88 (t, J = 7 Hz, 12H). 13C NMR (75 MHz, CDCl3): 166.4,
163.0, 160.5, 158.9, 145.25, 145.2, 145.15, 145.1, 145.0, 144.9,
144.7, 144.6, 144.5, 144.45, 143.9, 143.8, 143.0, 142.9, 142.2,
142.2, 141.9, 141.8, 140.9, 140.85, 139.8, 138.4, 136.6, 130.4,
127.95, 127.85, 127.8, 127.8, 127.75, 127.7, 127.65, 127.6,
127.55, 127.4, 114.9, 110.1, 107.4, 101.7, 71.2, 69.8, 69.6,
69.1, 68.2, 67.3, 66.6, 66.5, 66.45, 62.7, 31.95, 31.9, 31.85,
29.7, 26.3, 26.3, 26.1, 22.7, 14.1. Anal. calcd for
Compound 14. As described for D, with 13 (0.30 g,
0.38 mmol), 8 (0.24 g, 0.72 mmol) and t-BuOK (56 mg,
0.45 mmol) in anhydrous THF (20 mL). Column chromato-
graphy (SiO2, hexane–CH2Cl2 1 : 1) yielded 14 (213 mg, 60%)
as an orange glassy product. 1H NMR (300 MHz, CDCl3):
7.49 (d, J = 8 Hz, 2H), 7.40 (d, J = 8 Hz, 2H), 7.37 (d, J = 17
Hz, 1H), 7.32 (d, J = 8 Hz, 2H), 7.28 (d, J = 17 Hz, 1H), 7.12
(s, 1H), 7.10 (s, 1H), 7.06 (d, J = 17 Hz, 1H), 6.98 (d, J = 17
Hz, 1H), 6.97 (d, J = 8 Hz, 2H), 6.96 (d, J = 8 Hz, 2H), 6.70
(d, J = 8 Hz, 2H), 4.65 (s, 2H), 4.36 (s, 4H), 4.05 (2t, J =
7 Hz, 4H), 3.41 (q, J = 7 Hz, 4H), 1.87 (m, 4H), 1.39–1.25
(m, 20H), 1.20 (t, J = 7 Hz, 6H), 1.10 (m, 21H), 0.88 (t, J =
7 Hz, 6H).13C NMR (75 MHz, CDCl3): 158.1, 157.6, 134.4,
127.8, 127.6, 127.1, 121.8, 118.5, 118.5, 115.9, 114.4, 111.7,
110.1, 69.8, 69.6, 66.65, 66.6, 64.7, 44.4, 31.9, 31.85, 31.8, 29.7,
29.6, 29.55, 29.5, 29.45, 29.35, 29.3, 29.25, 26.3, 22.7, 18.1,
14.1, 12.7, 12.1.
C
155H141NO12: C 84.25, H 6.43, N 0.63%. Found: C 84.22,
H 6.60, N 0.75%. MS-MALDI: calcd for C155H141NO12
2209.82. Found: 2209.54 [M]+.
Electrochemistry
The cyclic voltammetric measurements were carried out with a
potentiostat Autolab PGSTAT100. Experiments were
performed at room temperature in a homemade airtight
three-electrode cell connected to a vacuum/argon line. The
reference electrode consisted of a saturated calomel electrode
(SCE) separated from the solution by a bridge compartment.
The counter electrode was a platinum wire of ca. 1 cm2
apparent surface. The working electrode was a Pt microdisk
(0.5 mm diameter). The supporting electrolyte [nBu4N][BF4]
(Fluka, 99% electrochemical grade) was used as received and
simply degassed under argon. Dichloromethane was freshly
distilled over CaH2 prior to use. The solutions used during the
electrochemical studies were typically 10ꢁ3 M in compound
and 0.1 M in supporting electrolyte. Before each measurement,
the solutions were degassed by bubbling Ar and the working
electrode was polished with a polishing machine (Presi P230).
Under these experimental conditions, Fc+/Fc is observed at
+0.54 ꢃ 0.01 V vs. SCE.
Compound 15. A 1 M solution of TBAF in THF (0.40 mL,
0.40 mmol) was added dropwise to a solution of 14 (168 mg,
0.18 mmol) in anhydrous THF (10 mL) at 0 1C under argon
atmosphere. The reaction mixture was stirred for 2 h and
water was added. The solvent was removed under reduced
pressure and the residue taken up with CH2Cl2. The organic
layer was washed with water, dried (MgSO4), filtered and
evaporated. Column chromatography (SiO2, hexane–CH2Cl2
7 : 3) yielded 15 (85 mg, 61%) as an orange glassy product.
1H NMR (300 MHz, CDCl3): 7.49 (d, J = 8 Hz, 2H), 7.40
(d, J = 8 Hz, 2H), 7.37 (d, J = 17 Hz, 1H), 7.32 (d, J = 8 Hz,
2H), 7.28 (d, J = 17 Hz, 1H), 7.12 (s, 1H), 7.10 (s, 1H), 7.06
(d, J = 17 Hz, 1H), 6.98 (d, J = 17 Hz, 1H), 6.97 (d, J = 8 Hz,
2H), 6.96 (d, J = 8 Hz, 2H), 6.70 (d, J = 8 Hz, 2H), 4.65
(s, 2H), 4.36 (s, 4H), 4.05 (m, 4H), 3.41 (q, J = 7 Hz, 4H), 1.88
(m, 4H), 1.39–1.25 (m, 20H), 1.20 (t, J = 7 Hz, 6H), 0.88
(t, J = 7 Hz, 6H). 13C NMR (75 MHz, CDCl3): 161.0, 158.3,
158.0, 151.1, 150.7, 140.2, 133.6, 131.5, 128.8, 128.7, 128.6,
127.8, 127.7, 127.65, 127.6, 126.0, 121.8, 114.9, 114.8, 111.9,
110.7, 110.1, 70.1, 69.8, 69.6, 67.1, 66.6, 66.5, 65.0, 55.9, 45.6,
44.6, 44.5, 42.3, 38.2, 31.9, 31.8, 29.6, 29.55, 29.45, 29.3, 29.3,
26.3, 22.75, 22.7, 14.1, 12.6.
Photophysics
The photophysical investigations were carried out in benzo-
nitrile and toluene (Carlo Erba, spectrofluorimetric grade).
Absorption spectra were recorded with a Perkin-Elmer l40
spectrophotometer. Emission spectra were obtained with an
Edinburgh FLS920 spectrometer (continuous 450 W Xe
lamp), equipped with a Peltier-cooled Hamamatsu R928
photomultiplier tube (185–850 nm) or
a Hamamatsu
R5509-72 supercooled photomultiplier tube (193 K,
800–1700 nm range). Emission quantum yields were deter-
mined according to the approach described by Demas and
Crosby29 using quinine sulfate in 1 N H2SO4 (F = 0.546)30 as
standard. Emission lifetimes were determined with the time
correlated single photon counting technique using an
Edinburgh FLS920 spectrometer equipped with a laser diode
head as excitation source (1 MHz repetition rate, lexc = 407 or
635 nm, 200 ps time resolution upon deconvolution) and an
Hamamatsu R928 PMT as detector. Transient absorption
spectra in the nanosecond–microsecond time domain were
obtained by using the nanosecond flash photolysis apparatus
described previously.31 Experimental uncertainties are
estimated to be 8% for lifetime determinations, 20% for
emission quantum yields, 10% for relative emission intensities
in the NIR, 1 nm and 5 nm for absorption and emission peaks,
respectively.
Compound F–D. DCC (20 mg, 0.10 mmol) was added to a
solution of 15 (50 mg, 0.07 mmol), 16 (143 mg, 0.10 mmol) and
DMAP (4.80 mg, 0.04 mmol) in CH2Cl2 (50 mL) at 0 1C. The
solution was stirred for 1 h at 0 1C and after that period a
catalytic amount of HOBt was added. The mixture was then
stirred for 2 h at 0 1C, then for 12 h at room temperature. The
mixture was then filtered and evaporated. Column chromato-
graphy (SiO2, hexane–CH2Cl2 7 : 3) yielded F–D (100 mg,
65%) as a brown glassy product. 1H NMR (300 MHz,
CDCl3): 7.46 (d, J = 8 Hz, 2H), 7.41 (d, J = 8 Hz, 2H),
7.23–7.34 (m, 4H), 7.10 (s, 1H), 7.08–7.01 (m, 3H), 6.93
(d, J = 8 Hz, 2H), 6.92 (d, J = 8 Hz, 2H), 6.66 (d, J =
8 Hz, 2H), 6.59 (d, J = 2 Hz, 2H), 6.39 (t, J = 2 Hz, 1H), 5.42
(s, 2H), 5.20 (s, 2H), 4.97 (s, 2H), 4.32 (s, 4H), 4.03 (2t, J =
7 Hz, 4H), 3.86 (t, J = 7 Hz, 4H), 3.38 (q, J = 7 Hz, 4H), 1.86
(m, 4H), 1.73 (m, 4H), 1.39–1.25 (m, 72H), 1.18 (t, J = 7 Hz, 6H),
ꢀc
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11 Electronic Materials the Oligomer Approach, ed. K. Mullen and
¨
G. Wegner, Wiley-VCH, Weinheim, 1998.
Acknowledgements
12 F. Langa, M. J. Gomez-Escalonilla, J. M. Rueff, T. M. Figueira
Duarte, J.-F. Nierengarten, V. Palermo, P. Samori, Y. Rio,
G. Accorsi and N. Armaroli, Chem.–Eur. J., 2005, 11, 4405.
13 S. Zheng, S. Barlow, T. C. Parker and S. R. Marder, Tetrahedron
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This research was supported by the CNRS, the CNR
(Commessa PM.P04.010, MACOL), the EC (contract
HPRN-CT-2002-00171-FAMOUS and PITN-GA-2008-
215399-FINELUMEN) and the Agence Nationale de la
14 F. Cardullo, P. Seiler, L. Isaacs, J.-F. Nierengarten, R. F. Haldimann,
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J.-P. Gisselbrecht and M. Gross, Helv. Chim. Acta, 1997, 80, 343;
Recherche (Solaire Photovoltaıque-Nanorgysol). A. G. thanks
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the Agence de l’Environnement et de la Maıtrise de
l’Energie—Region Alsace—for her fellowship.
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B. Knight, N. Martın, T. Ohno, E. Ortı, C. Rovira, J. Veciana,
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2182 | New J. Chem., 2009, 33, 2174–2182