Polarity effects on the photophysics of dendrimers with an oligophenylenevinylene core and peripheral fullerene units

Article abstract of DOI:10.1002/chem.200400157

Highly soluble dendritic branches with fullerene subunits at the periphery and a carboxylic acid function at the focal point have been prepared by a convergent approach. They have been attached to an oligophenylenevinylene (OPV) core bearing two alcohol f

Full text of DOI:10.1002/chem.200400157

FULL PAPER
Polarity Effects on the Photophysics of Dendrimers with an
Oligophenylenevinylene Core and Peripheral Fullerene Units
Manuel Gutierrez-Nava,[a] Gianluca Accorsi,[b] Patrick Masson,[a] Nicola Armaroli,*[b] and
Jean-François Nierengarten*[a, c]
Dedicated to Dr. Jean-Pierre Sauvage on the occasion of his 60th birthday
Abstract: Highly soluble dendritic
branches with fullerene subunits at the
periphery and a carboxylic acid func-
tion at the focal point have been pre-
pared by a convergent approach. They
have been attached to an oligophenyle-
nevinylene (OPV) core bearing two al-
cohol functions to yield dendrimers
with two, four or eight peripheral C60
groups. Their photophysical properties
have been systematically investigated
in solvents of increasing polarity; that
is, toluene, dichloromethane, and ben-
zonitrile. Ultrafast OPV!C60 singlet
energy transfer takes place for the
whole series of dendrimers, whatever
the solvent. Electron transfer from the
fullerene singlet is thermodynamically
allowed in CH2Cl2 and benzonitrile,
but not in apolar toluene. For a given
solvent, the extent of electron transfer,
signaled by the quenching of the fuller-
ene fluorescence, is not the same along
the series, despite the fact that identical
electron transfer partners are present.
By increasing the dendrimer size, elec-
tron transfer is progressively more dif-
ficult due to isolation of the central
OPV core by the dendritic branches,
which hampers solvent induced stabili-
zation of charge separated couples.
Compact structures of the hydrophobic
dendrimers are favored in solvents of
higher polarity. These structural effects
are also able to rationalize the unex-
pected trends in singlet oxygen sensiti-
zation yields.
Keywords: dendrimers
transfer energy transfer
fullerenes · singlet oxygen
· electron
·
·
Introduction
tion of donor–fullerene arrays. On one hand, such deriva-
tives have been used as the active layer in organic photovol-
taic cells.[2–4] This molecular approach appears to be particu-
larly interesting since the behavior of a unique molecule in
a solar cell and the study of its electronic properties means
one can easily obtain structure/activity relationships leading
to a better understanding of the photovoltaic conversion.[4]
On the other hand, covalently linked fullerene–(p-conjugat-
ed oligomer) systems show excited state interactions making
them excellent candidates for fundamental photophysical
studies. Generally, the light energy absorbed by the p-conju-
gated system is promptly convoyed to the fullerene sphere
by an ultrafast singlet–singlet energy transfer process.[2–6] For
example, this particular behavior has been used for the
design of dendritic systems with light-harvesting proper-
ties.[7] In these compounds, a fullerene core unit acting as
terminal energy receptor has been functionalized with den-
dritic branches bearing an array of peripheral chromophores
collecting the light energy. It can be added that the initial
energy transfer event observed in fullerene–(p-conjugated
oligomer) ensembles populates the lowest fullerene singlet
excited state which is capable of promoting electron transfer
from the oligomer to the C60 unit.[2–7] Actually, this is only
the case when the charge separated state is lower in energy
The unique electronic properties of C60 have generated sig-
nificant research activities focused on its use as an electron
and/or energy acceptor in photoactive multicomponent sys-
tems.[1] Of particular interest is the combination of the
carbon sphere with p-conjugated oligomers for the construc-
[a] Dr. M. Gutierrez-Nava, Dr. P. Masson, Dr. J.-F. Nierengarten
Institut de Physique et Chimie des MatØriaux de Strasbourg
UniversitØ Louis Pasteur et CNRS (UMR 7504)
23 rue du Loess, 67037 Strasbourg (France)
E-mail: jfnierengarten@chimie.u-strasbg.fr
[b] Dr. G. Accorsi, Dr. N. Armaroli
Instituto Per la Sintesi Organica e la Fotoreattività
Laboratorio di Fotochimica, Consiglio Nazionale delle Ricerche CNR
Via Gobetti 101, 40129 Bologna (Italy)
Fax : (+39)051-639-9844
E-mail: armaroli@isof.cnr.it
[c] Dr. J.-F. Nierengarten
New address:
Groupe de Chimie des Fuller›nes et des Syst›mes ConjuguØs
Ecole EuropØenne de Chimie, Polym›res et MatØriaux (ECPM)
UniversitØ Louis Pasteur et CNRS, 25 rue Becquerel
67087 Strasbourg Cedex 2 (France)
Fax : (+33)390-24-27-06
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5076 – 5086
than the fullerene singlet.
Thus, the cascade of photoin-
duced processes can be easily
modulated by changing the do-
nating ability of the conjugated
system.[2c] Interestingly, when
the energy level of the charge
separated state is close to the
fullerene singlet, the deactiva-
tion pathways are sensitive to
the polarity of the solvent.[2–7]
For example, Hummelen, Jans-
sen and co-workers have re-
ported the photophysical prop-
erties of an oligophenylenevi-
nylene (OPV)–C60 conjugate
in solvents of different polari-
ty.[3a] In apolar solvents such as
toluene, the charge separated
state is higher in energy than
the first fullerene singlet and
triplet excited states. There-
fore, the light energy absorbed
by the OPV fragment prompt-
ly conveyed to the fullerene
lowest singlet excited state via
energy transfer cannot yield
charge separation anymore. In
more polar solvents, the situa-
tion changes dramatically. Ef-
fectively, the energy of the
charge-separated state drops
below that of the first fullerene
singlet excited state allowing
occurrence of electron transfer
after the initial energy transfer event. The latter two-step
process was confirmed by kinetic studies revealing a close
correspondence of the experimental results with the relative
ordering of the various excited states as derived from theo-
retical calculations.[3a] Similar findings have been reported
by Martin, Guldi and co-workers[7b] for a series of oligo-
naphthylenevinylene–fullerene dyads and by us for fullerene
derivatives covalently linked to OPV simple fragments[2d] or
dendritic wedges.[7a]
In this paper, we report the synthesis and the photophysi-
cal properties of dendrimers 1–3 with an OPV central core
and C60 terminal units. Whereas the first generation com-
pound 1 shows the expected solvent dependent photophysi-
cal behavior, the last generation compound is almost not af-
fected by changes in polarity as a result of dendritic encap-
sulation.
bis-adduct was obtained in ten steps according to a previous-
ly reported procedure.[8] G1CO2tBu is actually easily pre-
pared on a multi-gram scale and is highly soluble in
common organic solvents owing to the presence of the four
dodecyl chains. Therefore, it appears to be an excellent can-
didate for the preparation of fullerene-containing dendrons
by using the synthetic strategy developed by some of us.[9]
The latest is based on the successive cleavage of a tert-butyl
ester moiety under acidic conditions followed by a N,N’-di-
cyclohexylcarbodiimide (DCC)-mediated esterification reac-
tion. Thus, treatment of G1CO2tBu with CF3CO2H in
CH2Cl2 gave G1CO2H in a quantitative yield and subse-
quent reaction with diol 4[8] under esterification conditions
using DCC, 1-hydroxybenzotriazole (HOBt) and 4-dimethy-
laminopyridine (DMAP) led to the tert-butyl protected den-
dron G2CO2tBu of second generation in 67% yield. The
structure of G2CO2tBu was easily established based on its
1H and 13C NMR spectra. In addition, the MALDI-TOF
mass spectrum showing the expected molecular ion peak
also confirmed the structure of G2CO2tBu.
Results and discussion
1
Synthesis: The synthesis of fullerodendrimers 1–3 was achiev-
ed by a convergent approach. To this end, dendritic branch-
es with peripheral fullerene subunits were prepared first
from G1CO2tBu (Scheme 1). This Cs symmetrical fullerene
The H NMR spectrum of G2CO2tBu is shown in Figure 1
together with the spectrum of the corresponding first gener-
ation derivative G1CO2tBu. Both spectra show the charac-
teristic features of Cs symmetrical 1,3-phenylenebis(methy-
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Scheme 1. i) CF3CO2H, CH2Cl2, rt (99%); ii) DCC, DMAP, HOBt, CH2Cl2, 08C to rt (67%); iii) CF3CO2H, CH2Cl2, rt (99%); iv) 4, DCC, DMAP,
HOBt, CH2Cl2, 08C to rt (85%); v) CF3CO2H, CH2Cl2, rt (96%).
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Photophysics of Dendrimers
5076 – 5086
lene)-tethered fullerene cis-2 bis-adducts.[10] Effectively, in
addition to the signals arising from the 3,5-didodecyloxyben-
zyl units, an AB quartet is observed for the diastereotopic
benzylic CH2 group (HA-B) and an AX2 system for the aro-
matic protons of the 1,3,5-trisubstituted bridging phenyl ring
(H3-4). For G2CO2tBu, the signals corresponding to the cen-
tral tert-butyl [3,5-bis(methylene)phenoxy]acetate are clearly
observed: an AX2 system is revealed for the aromatic pro-
tons (H5-6), two singlets for the methylene units HF and HG
as well as a singlet for the methyl groups of the tert-butyl
moiety at d 1.51 ppm. Finally, it can be noted that the reso-
nance of the methylenic protons HE is slightly down-field
shifted when the corresponding acetate unit is connected to
a benzylic moiety (G2CO2tBu) rather to a tert-butyl subunit
(G1CO2tBu).
The preparation of the next generation compound was
achieved by repeating the same reaction sequence. Treat-
ment of G2CO2tBu with CF3CO2H in CH2Cl2 afforded
G2CO2H in high yield (99%) and subsequent esterification
with diol 4 (DCC/DMAP/HOBt) gave G3CO2tBu in 85%
yield. This compound was characterized by 1H and 13C NMR,
MALDI-TOF mass spectrometry and elemental analysis. As
shown in Figure 1, the 1H NMR spectrum of G3CO2tBu
clearly reveals the signals corresponding to three [3,5-bis-
(methylene)phenoxy]acetate subunits in a 4:2:1 ratio in full
agreement with the proposed dendritic structure. Finally, hy-
drolysis of the tert-butyl ester group under acidic conditions
afforded the third generation carboxylic acid G3CO2H.
The preparation of the central OPV core bearing two al-
cohol units[11] allowing further attachment of G1–3CO2H
under DCC-mediated esterification conditions is described
in Scheme 2. Treatment of tetraethyleneglycol (TEG) with
Scheme 2. i) TBDMSCl (1 equiv), imidazole, DMF, 08C (48%); ii) TsCl,
pyridine, DMAP, CH2Cl2, 08C (60%); iii) K2CO3, DMF, 808C (76%);
iv) Pd(OAc)2, TOP, Et3N, xylene, 808C (61%); v) TBAF, THF, 08C
(90%); vi) GnCO2H, DCC, DMAP, HOBt, CH2Cl2, 08C to rt (1: 87%
from G1CO2H; 2: 59% from G2CO2H; 3: 70% from G3CO2H).
1 equiv
tert-butyldimethylsilyl
chloride
(TBDMSCl) afforded the mono-protected deriv-
ative 5 which after reaction with tosyl chloride
(TsCl) in the presence of pyridine and DMAP
yielded 6. 2,5-Diiodohydroquinone (7) was pre-
pared in two steps from 1,4-dimethoxybenzene
as previously described by Weder and Wright-
on.[12] Treatment of 7 with tosylate 6 in DMF at
808C in the presence of K2CO3 gave the alkyl-
ation product 8 in 76% yield. The preparation of
the OPV derivative 10 was then based on Heck-
type chemistry.[13] Thus, reaction of 8 with sty-
rene 9[14] in Et3N/xylene in the presence of tri-o-
tolylphosphine (POT) and a catalytic amount of
Pd(OAc)2 afforded 10 in 61% yield (Scheme 2).
Figure 1. 1H NMR spectra (CDCl3, 200 MHz) of G1CO2tBu (A); G2CO2tBu (B) and
G3CO2tBu (C).
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Subsequent treatment with tetra-n-butylammonium fluoride
(TBAF) in THF gave diol 11 in 90% yield.
assignment was possible on the basis of 2D-COSY and
NOESY spectra. The spectrum of 2 reveals all the signals
arising from the OPV core unit as well as the typical reso-
nances of the G2CO2 groups. Similarly, the characteristic
signals of the core unit as well as those of the surrounding
The first generation derivative 1 was obtained in 87%
yield by esterification of diol 11 with G1CO2H (Scheme 2).
1
Fullerodendrimer 1 was characterized by H and 13C NMR,
1
MALDI-TOF mass spectrometry and elemental analysis.
The mass spectrum of 1 displays only one peak at m/z 5978
corresponding to the expected molecular ion peak (calcd m/
dendrons are clearly seen in the H NMR spectrum of 3.
Photophysical properties
1
z 5977.9). The H NMR spectrum of fullerodendrimer 1 re-
corded in CDCl3 is in full agreement with the proposed cen-
trosymmetric structure (Figure 2). Unambiguous assignment
was achieved on the basis of 2D-COSY and NOESY spectra
recorded at room temperature in CDCl3. The spectrum of 1
shows all the characteristic features of the G1CO2 moieties
already discussed for G1–3CO2tBu. The spectrum is also
characterized by three sets of signals for the aromatic pro-
tons of the central OPV core unit: an AA’XX’ system for
Hl-m and two singlets corresponding to Hi and Hp. Two AB
quartets are observed for the vinylic protons of the OPV
backbone (Hj-k and Hn-o). Importantly, coupling constants of
about 17 Hz confirmed the all-E stereochemistry of the p-
conjugated system in 1.
Fullerodendrimers 2 and 3 were prepared by DCC-medi-
ated esterification of diol 11 with G2CO2H and G3CO2H,
respectively. The structure of both 2 and 3 was confirmed by
mass spectrometry. The expected molecular ion peak is ob-
served at m/z 10368 for 2 (calcd m/z 10367.2) and m/z
19144 for 3 (calcd m/z 19145.7). In both cases, characteristic
fragments are also observed. In particular, hydrolysis of one
or two 3,5-didodecyloxybenzylic ester followed by a decar-
boxylation is observed, leading to signals at [M +ÀC32H55O4]
and [M +À2”(C32H55O4)]. However, no signals correspond-
ing to defected dendrimers could be detected, thus showing
the monodispersity of both 2 and 3. In spite of the high mo-
lecular weights of dendrimers 2 and 3, their 1H NMR spectra
are well resolved (Figure 3). For both compounds, complete
Reference compounds: The electronic absorption spectrum
of the bis-methanofullerene G1CO2tBu (Figure 4) shows
very intense bands in the UV side and much weaker fea-
tures in the Vis spectral region with the peak corresponding
to the lowest allowed singlet transition[15] at 437 nm (e =
3100mÀ1 cmÀ1). The shape of the absorption spectra of
G2CO2tBu and G3CO2tBu are identical to that of
G1CO2tBu with molar extinction coefficients two and four
times larger, respectively (Figure 4). Absorption shapes of
the dendronic fullerene reference units Gn-3CO2tBu are
substantially unchanged in toluene (PhMe) and benzonitrile
(PhCN), relative to CH2Cl2, over the whole spectral range.
In CH2Cl2 G1CO2tBu exhibits
peaked, after correction for the instrumental response, at
the border between the Vis and the IR region (lmax
a fluorescence band
=
778 nm, F=0.0003, t=1.6 ns), inset of Figure 4. Within ex-
perimental uncertainty (see Experimental Section), these
parameters are unaffected in G2CO2tBu and G3CO2tBu
even by changing to PhMe and PhCN solvents.
An intense triplet–triplet transient absorption spectrum is
recorded for G1–3CO2tBu in CH2Cl2, with a maximum at
720 nm. The spectral decay is monoexponential for
G1CO2tBu and biexponential for G2CO2tBu and
G3CO2tBu (Figure 5).
The intensity of the shorter-lived component (about
100 ns) increases with the laser energy and, at the same exci-
tation energy, is more pronounced for G3CO2tBu than for
Figure 2. 1H NMR spectrum (CDCl3, 400 MHz) of fullerodendrimer 1.
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Photophysics of Dendrimers
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Figure 3. 1H NMR spectra (CDCl3, 400 MHz) of 2 (A) and 3 (B).
Figure 5. Transient absorption spectrum of G1CO2tBu at 298 K in CH2Cl2
air-purged solution upon laser excitation at 355 nm (2 mJpulseÀ1). The
spectra were recorded at delays of 2, 10, 20, 30, 50 ms following excita-
tion. The inset shows decay time profiles (720 nm) of G1CO2tBu,
G2CO2tBu and G3CO2tBu at high energy laser input (20 mJpulseÀ1, A=
0.50, air equilibrated samples). This evidences the size-dependent short-
lived component, in parallel with the decrease of the signal of the “regu-
lar” triplet decay, attributed to T–T annihilation processes.
Figure 4. Absorption spectra of G1CO2tBu (G1); G2CO2tBu (G2) and
G3CO2tBu (G3) in CH2Cl2; in the region above 400 nm a multiplying
factor of 10 is applied. Inset: a) absorption and fluorescence spectra of 10
(lexc =425 nm); b) fluorescence spectrum of G1CO2tBu (lexc =530 nm)
and sensitized singlet oxygen luminescence band (lmax =1270 nm). Lumi-
nescence spectra are corrected for the instrumental response.
G2CO2tBu. These observations suggest the occurrence of in-
tramolecular triplet–triplet annihilation processes[16] in
G2CO2tBu and G3CO2tBu which bear dimeric and tetra-
meric fullerene arms. The probability of such excited state
interactions, which are observable only under intense laser
light irradiation, is higher when the number of carbon
spheres per dendrimer molecule is larger. The long-lived
component of the triplet decays of G2CO2tBu and
G3CO2tBu are coincident and identical to that of
G1CO2tBu (t=600Æ30 ns and 18Æ1 ms in air-equilibrated
and air-free solution, respectively). Similar excitation inten-
sity dependent triplet-triplet annihilation processes have
been observed earlier in C60 fine particles.[17]
The absorption and emission spectra in CH2Cl2 of the
OPV reference derivative 10 are depicted in Figure 4. As is
typical for OPV-type molecules, it exhibits intense and
short-lived fluorescence (lmax =483 nm, F=0.52, t=1.0 ns),
which can be conveniently used to monitor the occurrence
of OPV/C60 excited state interactions (see below).[2b,d,3a,5,18,19]
Dendrimers: The absorption spectra of 1–3 in CH2Cl2 solu-
tion are shown in Figure 6. Both the features of the fuller-
ene moiety (UV side and above 500 nm) and those of the
OPV chromophore (maximum or shoulder around 425 nm)
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are observable. The spectra of the multicomponent arrays
are not well superimposed to the profiles obtained by sum-
ming the contribution of each chromophoric moiety. Specifi-
cally, the absorption of the OPV core is less than expected
and the effect is more and more pronounced by increasing
the dendrimer size. As an example, in Figure 6 the spectrum
of 3 is compared to the profile calculated for its component
subunits (2G3CO2tBu + 10). On the contrary, in the regions
where the absorption of the fullerene moiety is prevalent or
exclusive experimental and calculated spectra are in excel-
lent agreement. This trend may suggest a modification of
the solvation environment for the central OPV core by in-
creasing the dendrimers size. A similar effect has been
found for fullerodendrimers having the carbon sphere as
central unit.[10d,20,21]
quenched lifetime of 6.7 ps, well below our instrumental
time resolution.[2b,18]
ꢀ
ꢁ
Fref
F
À1
ð1Þ
kEnT
¼
tref
In Equation (1) F and Fref are, respectively, the OPV fluo-
rescence quantum yield of the dendrimers and of the refer-
ence compound, whereas tref is the singlet lifetime of the
latter.
Photoinduced electron transfer in OPV–C60 arrays has
been obtained in solution from the lowest fullerene singlet
excited state (~1.7 eV), directly or indirectly populated fol-
lowing light absorption.[2d,3a] On the contrary, population of
upper lying OPV singlet levels (>2.8 eV) does not result in
electron transfer since competitive ultrafast energy-transfer
prevails.[2b,3a,5] Thus electron transfer in OPV–C60 systems
can be conveniently signaled by the quenching of C60 fluo-
rescence and can be observed or not depending on the sol-
vent polarity (Figure 7).[2d,3a]
Figure 6. Absorption spectra in CH2Cl2 of 1–3 (c). The dotted spec-
trum represents the sum of (2”G3CO2tBu + 10); in the region above
350 nm a multiplying factor of 10 is applied.
Relative to the reference OPV, the fluorescence band of
the central chromophore is strongly quenched in dendrimers
1–3 (CH2Cl2, lexc =425 nm, c ꢀ5”10À6 m). Excitation spectra
taken at 800 nm (fullerene fluorescence) exhibit the intense
fullerene band at 260 nm but also the contribution of the
OPV absorption at 425 nm indicating that the OPV quench-
Figure 7. Typical energy-level diagram of rigid OPV–C60 arrays describing
intrinsic and intercomponent processes following light excitation of the
OPV subunit. Electron transfer only occurs from the fullerene singlet
state and may be switched on/off by solvent polarity, which allows energy
tuning of the charge separated state below/above the fullerene singlet.
1
ing is due to singlet–singlet (1OPV*! C60*) energy transfer,
in line with previous reports on OPV–C60 hybrids.[2b,d,3a,5,18,19]
Model calculations to evaluate the rate of energy transfer[2b]
are hampered by the fact that 1–3 are flexible structures
where the distance between the single central unit and the
multiple external carbon cages is hardly predictable. One
cannot exclude, especially in more polar solvents (see
below), that there is close vicinity (through space) between
OPV and C60 subunits. Quenching factors of the OPV fluo-
rescence in CH2Cl2 (Fref/F) are 125, 150, and 210 for 1,2,
and 3, respectively. In the latter case the quenching factor is
the highest despite the fact that in an ideal extended struc-
ture the OPV–C60 distance is the longest. This supports the
view of folded compact structures for the largest dendrimer
with short intercomponent distances between the energy
transfer couples.
In Figures 8–10 (top) are reported the fluorescence spec-
tra of 1–3, compared with those of the corresponding refer-
ence fullerene systems G1–3CO2tBu under the same condi-
tions in solvent of increasing polarity. We used toluene
(PhMe), dichloromethane (CH2Cl2), and benzonitrile
(PhCN), which exhibit static relative permittivity e of 2.4,
8.9, and 25.2 respectively.[22]
In PhMe solution (Figure 8) the fullerene-type fluores-
cence is unquenched relative to the model compounds for
the whole 1–3 family. This signals the absence of OPV!C60
electron transfer upon population of the fullerene singlet
state in apolar toluene, as observed in OPV–C60 arrays in-
vestigated earlier.[2d,3a] It is interesting to note that, in any
solvent, the quantum yield of singlet oxygen sensitization of
the dendrimers is about 15% lower than that of the corre-
sponding reference compounds. In principle this could indi-
cate that, to some extent, electron transfer is promoted by
the fullerene triplet level. This process, never reported for
For 2, the dendrimer exhibiting intermediate quenching
factor, we estimate a rate constant for the energy transfer
process of 1.5”1011 sÀ1 [Eq. (1)]. This corresponds to a
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Photophysics of Dendrimers
5076 – 5086
Figure 8. Top, from left to right: Corrected emission spectra of 1–3 (a)
Figure 9. Top, from left to right: Corrected emission spectra of 1–3 (a)
and their reference compounds G1–3CO2tBu (c) in PhCH3; lexc
=
and their reference compounds G1–3CO2tBu (c) in CH2Cl2; lexc
=
425 nm or lexc =530 nm. Bottom, in the same order: Sensitized singlet
425 nm or lexc =530 nm. Bottom, in the same order: Sensitized singlet
*
oxygen emission in PhCH3 of 1–3 ( ) and their reference compounds
*
oxygen emission in CH2Cl2 of 1–3 ( ) and their reference compounds
*
G1–3CO2tBu ( ) ; lexc =500 nm. A=0.30 for all compounds.
*
G1–3CO2tBu ( ); lexc =500 nm. A=0.30 for all compounds.
rigid OPV–C60 systems, might take place in the present flexi-
ble arrays where interchromophoric distance is difficult to
predict and short distance between the carbon sphere and
the central OPV chromophore could be achieved, especially
in polar solvents. However this hypothesis is discarded since
electron transfer from the fullerene triplet is thermodynami-
cally not allowed (see below) and this is confirmed by the
absence of triplet lifetime quenching for 1–3 relative to the
fullerene model compounds, whatever the solvent. Thus we
attribute the decrease of the singlet oxygen sensitization
yield to peculiar intramolecular contacts occurring in the
dendrimer structures, which decrease the surface of the ful-
lerene cages available for dioxygen bimolecular interac-
tions.[23]
are identical to 1. Practically no electron transfer from ful-
lerene singlet occurs for 3 in CH2Cl2 (Figure 9, top right),
whereas some of it is still detected in the more polar PhCN
(Figure 10, top right). These trends can be rationalized by
considering increasingly compact dendrimer structures in
more polar solvents.[26] This implies that the actual polarity
experienced by the involved electron transfer partners, par-
In CH2Cl2, from electrochemical potentials,[24] the energy
À
of the OPV+–C60 charge separated state of 1 is estimated
to be 1.68 eV,[25] that is, almost isoenergetic to the fullerene
singlet (Figure 7). Similar energetics do not allow electron
transfer in rigid OPV–C60 arrays, owing to an activation bar-
rier of about 0.2 eV.[2d] Clearly, in the present flexible sys-
tems where electron transfer partners can get in tighter vi-
cinity, such barrier is lower and electron transfer can occur,
as suggested by the decrease of fullerene fluorescence for 1
(Figure 9, top left). A more marked effect on fullerene fluo-
rescence in polar PhCN supports this interpretation
(Figure 10, top left).[6] The lifetime of the fullerene moiety
of 1 is 750 ps, compared to 1600 ps of the reference moiety
G1CO2tBu, corresponding to an electron transfer rate con-
stant of 7.1”108 sÀ1 in benzonitrile.
Figure 10. Top, from left to right: Corrected emission spectra of 1–3
(a) and their reference compounds G1–3CO2tBu (c) in PhCN;
lexc =425 nm or lexc =530 nm. Bottom, in the same order: Sensitized sin-
Interestingly, for larger dendrimers 2 and 3 there is recov-
ery of fluorescence intensity in both in CH2Cl2 and in PhCN,
suggesting a decreased electron transfer efficiency, despite
the fact that electron transfer partners and thermodynamics
*
glet oxygen emission in PhCN of 1–3 ( ) and their reference compounds
*
G1–3CO2tBu ( ); lexc =500 nm. A=0.30 for all compounds.
Chem. Eur. J. 2004, 10, 5076 – 5086
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5083

N. Armaroli, J. F. Nierengarten et al.
FULL PAPER
ticularly the central OPV, is no longer that of the bulk sol-
vent. This strongly affects electron transfer thermodynamics
which, being reasonably located in the normal region of the
Marcus parabola,[2d,3a] becomes less exergonic and thus
slower and less competitive towards intrinsic deactivation of
the fullerene singlet state. This dendritic effect is in line
with the molecular dynamics studies which suggest that the
central OPV unit is more and more protected by the den-
dritic branches when the generation number is increased.
Actually, the calculated structure of 3 shows that the two
dendrons of third generation are able to fully cover the cen-
tral OPV core (Figure S1).
The trend of sensitized singlet oxygen luminescence re-
sembles that of fullerene fluorescence, since the detected
signal is more intense by increasing the dendrimer size, con-
firming the above mentioned polarity effects. However, as
in the case of PhMe solvent, the amount of fullerene triplet
(indirectly monitored via NIR singlet oxygen emission)[2d,23]
is comparably lower than that of singlet (probed by fuller-
ene fluorescence). This behavior is in contrast to what was
observed in rigid OPV–C60 arrays where the relative amount
of fullerene fluorescence intensity and sensitized singlet
oxygen are identical.[2d] Similarly to what discussed on
PhMe solvent (see above) we exclude that any electron
transfer from the triplet state occurs, since triplet lifetimes
of 1–3 are unchanged relative to the fullerene models G1–
3CO2tBu. Thus we attribute reduced singlet oxygen sensiti-
zation to structural factors which, in polar solvents where
the compactness of the dendrimers structure is probably en-
hanced, are expected to play an even major role.
Experimental Section
General: Reagents and solvents were purchased as reagent grade and
used without further purification. THF was distilled over sodium/benzo-
phenone. Compounds G1CO2tBu,[8] G1CO2H,[8] 4,[8] 7,[12] and 9[14] were
prepared according to previously reported procedures. The preparation
of G2–3CO2H and 11 is described in the Supporting Information. All re-
actions were performed in standard glassware under an inert Ar atmos-
phere. Evaporation and concentration were done at water aspirator pres-
sure and drying in vacuo at 10À2 Torr. Column chromatography: silica gel
60 (230–400 mesh, 0.040–0.063 mm) was purchased from E. Merck. Thin-
layer chromatography (TLC) was performed on glass sheets coated with
silica gel 60 F254 purchased from E. Merck, visualization by UV light.
NMR spectra were recorded on a Bruker AC 200 (200 MHz) or a Bruker
AM 400 (400 MHz) with solvent peaks as reference. Mass measurement
were carried out on a Bruker Biflex matrix-assisted laser desorption
time-of-flight mass spectrometer (MALDI-TOF) equipped with Scout
High Resolution Optics, an X–Y multi-sample probe and a gridless re-
flector. Ionization is accomplished with the 337 nm beam from a nitrogen
laser with a repetition rate of 3 Hz. All data were acquired at a maximum
accelerating potential of 20 kV in the linear positive ion mode. The
output signal from the detector was digitized at a sampling rate of
1 GHz. A saturated solution of 1,8,9-trihydroxyanthracene (dithranol
ALDRICH EC: 214-538-0) in CH2Cl2 was used as a matrix. Typically, a
1:1 mixture of the sample solution in CH2Cl2 was mixed with the matrix
solution and 0.5 mL of the resulting mixture was deposited on the probe
tip. Calibration was performed in the external mode with insulin
(5734.6 Da) and ACTH (2465.2 Da). Elemental analyses were performed
by the analytical service at the Institut Charles Sadron, Strasbourg.
Compound 1: DCC (58 mg, 0.28 mmol) was added to a stirred solution of
G1CO2H (300 mg, 0.145 mmol), 11 (140 mg, 0.07 mmol), DMAP (9 mg,
0.07 mmol) and HOBt (10 mg, 0.07 mmol) in CH2Cl2 (10 mL) at 08C.
After 1 h, the mixture was allowed to slowly warm to room temperature
(within 1 h), then stirred for 24 h, filtered and evaporated. Column chro-
matography (silica gel, CH2Cl2/0.2% MeOH) yielded 1 (369 mg, 87%) as
a dark orange glassy product. IR (CH2Cl2): n˜ = 1745 (C=O); 1H NMR
(CDCl3, 400 MHz): d=0.89 (t, J=6 Hz, 42H), 1.26 (m, 252H), 1.70 (m,
16H), 1.83 (m, 12H), 3.64 (m, 8H), 3.66 (m. 8H), 3.79 (m, 4H), 3.83 (t,
J=6 Hz, 16H), 3.93 (m, 4H), 3.98 (t, J=6 Hz, 4H), 4.03 (t, J=6 Hz,
8H), 4.24 (t, J=5 Hz, 4H), 4,36 (t, J=5 Hz, 4H), 4.67 (s, 4H), 5.04 (d,
J=13 Hz, 4H), 5.28 (s, 8H), 5.72 (d, J=13 Hz, 4H), 6.34 (t, J=2 Hz,
4H), 6.45 (d, J=2 Hz, 8H), 6.72 (s, 4H), 6.77 (s, 4H), 6.99 (AB, J=
17 Hz, 4H), 7.10 (d, J=17 Hz, 2H), 7.12 (s, 2H), 7.17 (s, 2H), 7.46 (d,
J=17 Hz, 2H), 7.49 (AA’XX’, J=7 Hz, 8H); 13C NMR (CDCl3,
50 MHz): d=14.1, 22.6, 25.7, 26.0, 26.1, 29.2, 29.3, 29.4, 29.6, 29.7, 29.8,
30.4, 31.9, 49.0, 63.1, 64.3, 65.3, 66.8, 67.1, 68.0, 68.7, 68.8, 69.1, 69.9, 70.5,
70.6, 70.7, 70.9, 73.5, 101.6, 105.2, 107.1, 111.2, 112.5, 116.0, 122.9, 126.6,
126.9, 127.3, 128.7, 132.5, 134.3, 135.7, 136.0, 136.5, 136.6, 137.0, 137.7,
138.3, 139.9, 141.0, 141.1, 142.2, 142.6, 143.15, 143.5, 143.7, 143.9, 144.1,
144.2, 144.5, 144.8, 144.9, 145.1, 145.3, 145.5, 145.6, 145.7, 146.0, 147.3,
147.4, 148.6, 151.1, 153.3, 157.8, 160.3, 162.4, 162.5, 168.4; MALDI-TOF-
MS: calcd for C402H442O44: 5977.9; found: 5978; elemental analysis calcd
(%) for C402H442O44: C 80.77, H 7.45; found: C 80.54, H 7.54.
Conclusion
A new series of dendrimers with an OPV core and peripher-
al fullerene subunits have been prepared. Their photophysi-
cal properties have been systematically investigated in dif-
ferent solvents and singular polarity effects resulting from
the dendritic structure evidenced. Actually, the photophysi-
cal properties of the first generation compound are similar
to those already reported for related OPV–fullerene sys-
tems.[2d,3a] Specifically, photoinduced electron transfer origi-
nating from the fullerene singlet excited state has been found
dramatically solvent dependent,[2d,3a] because the energy of
the charge separated state can be finely tuned around the
energy value exhibited by the fullerene singlet excited state
(~1.7 eV). In other words, solvent polarity can affect the
electron transfer energetics and transform an endoergonic
process in a moderately exergonic one, by switching from
apolar (e.g. PhMe) to more polar media (e.g. PhCN). In
contrast, for the highest generation dendrimer, the strong
solvent effects on the OPV–C60 charge separation processes
are severely limited. For a given solvent, the extent of elec-
tron transfer is reduced with dendrimers size because pro-
gressive isolation of the central OPV electron donor by the
surrounding dendrons tends to disfavor solvent induced sta-
bilization of the transient ionic species. The dendritic archi-
tecture is therefore not only able to isolate the central core
unit but it can also influence dramatically its properties.
Compound 2: DCC (36 mg, 0.17 mmol) was added to a stirred solution of
G2CO2H (401 mg, 0.095 mmol), 11 (85 mg, 0.043 mmol), DMAP (9 mg,
0.07 mmol) and HOBt (10 mg, 0.07 mmol) in CH2Cl2 (10 mL) at 08C.
After 1 h, the mixture was allowed to slowly warm to room temperature
(within 1 h), then stirred for 48 h, filtered and evaporated. Column chro-
matography (silica gel, CH2Cl2/0.2% MeOH) yielded 2 (264 mg, 59%) as
a dark orange glassy product. IR (CH2Cl2): n˜ = 1745 (C=O); 1H NMR
(CDCl3, 400 MHz): d=0.89 (t, J=6 Hz, 66H), 1.26 (m, 396H), 1.70 (m,
32H), 1.83 (m, 12H), 3.64 (m, 8H), 3.70 (m, 8H), 3.78 (m, 4H), 3.83 (t,
J=6 Hz, 32H), 3.92 (m, 4H), 3.98 (t, J=6 Hz, 4H), 4.03 (t, J=6 Hz,
8H), 4.22 (m, 4H), 4.35 (m, 4H), 4.66 (s, 4H), 4.70 (s, 8H), 5.06 (d, J=
13 Hz, 8H), 5.21 (s, 8H), 5.28 (s, 16H), 5.71 (d, J=13 Hz, 8H), 6.34 (t,
J=2 Hz, 8H), 6.45 (d, J=2 Hz, 16H), 6.72 (s, 4H), 6.78 (s, 8H), 6.88 (s,
4H), 6.98 (s, 2H), 7.00 (d, J=17 Hz, 4H), 7.10 (d, J=17 Hz, 2H), 7.13 (s,
4H), 7.45 (d, J=17 Hz, 2H), 7.48 (AA’XX’, J=7 Hz, 8H); 13C NMR
(CDCl3, 50 MHz): d=14.0, 22.6, 26.0, 29.2, 29.3, 29.4, 29.6, 30.3, 31.8,
48.96, 63.0, 64.2, 65.1, 65.3, 66.3, 66.7, 67.0, 68.0, 68.6, 68.7, 69.1, 69.8,
70.4, 70.5, 70.6, 70.8, 73.4, 101.5, 105.1, 107.0, 111.1, 112.4, 114.5, 116.0,
5084
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Chem. Eur. J. 2004, 10, 5076 – 5086

Photophysics of Dendrimers
5076 – 5086
121.2, 122.9, 126.6, 126.8, 127.2, 128.7, 132.4, 134.3, 135.6, 135.9, 136.4,
136.6, 136.9, 137.2, 137.7, 138.3, 138.4, 138.8, 139.9, 140.9, 141.0, 142.1,
142.6, 143.0, 143.4, 143.6, 143.8, 144.0, 144.2, 144.5, 144.8, 144.9, 145.0,
145.2, 145.4, 145.6, 145.9, 147.2, 147.3, 148.5, 151.0, 153.2, 157.7, 158.1,
tract FIRB RBNE019H9K, Molecular Manipulation for Nanometric Ma-
chines) and M.G.N. the CONACyT for their reasearch fellowships. We
further thank L. Oswald for technical help, Dr. C. Bourgogne for his help
for the molecular modeling, Prof. M. Gross and Dr. J.-P. Gisselbrecht for
the CV measurements, J.-M. Strubfor the MALDI-TOF mass spectra
and M. Schmitt for high field NMR measurements.
160.3, 162.4, 162.5, 168.2, 168.3; MALDI-TOF-MS: calcd for C698H698O80
:
10367.2; found: 10368 [M +]; elemental analysis calcd (%) for
C698H698O80: C 80.87, H 6.79; found C 80.84, H 6.77.
Compound 3: DCC (22 mg, 0.11 mmol) was added to a stirred solution of
G3CO2H (515 mg, 0.060 mmol), 11 (53 mg, 0.027 mmol), DMAP (9 mg,
0.07 mmol) and HOBt (10 mg, 0.07 mmol) in CH2Cl2 (10 mL) at 08C.
After 1 h, the mixture was allowed to slowly warm to room temperature
(within 1 h), then stirred for 48 h, filtered and evaporated. Column chro-
matography (silica gel, CH2Cl2/0.2% MeOH) yielded 3 (364 mg, 70%) as
a dark orange glassy product. IR (CH2Cl2): n˜ =1745 (C=O); 1H NMR
(CDCl3, 400 MHz): d=0.89 (t, J=6 Hz, 114H), 1.27 (m, 684H), 1.70 (m,
64H), 1.82 (m, 12H), 3.65 (m, 8H), 3.70 (m, 8H), 3.77 (m, 4H), 3.82 (t,
J=6 Hz, 64H), 3.91 (m, 4H), 3.98 (t, J=6 Hz, 4H), 4.02 (t, J=6 Hz,
8H), 4.21 (m, 4H), 4.33 (m, 4H), 4.61 (s, 4H), 4.69 (s, 24H), 5,06 (d, J=
13 Hz, 16H), 5.17 (s, 8H), 5.20 (s, 16H), 5.28 (s, 32H), 5.71 (d, J=13 Hz,
16H), 6.33 (t, J=2 Hz, 16H), 6.44 (d, J=2 Hz, 32H), 6,72 (s, 4H), 6.78
(s, 16H), 6.86 (s, 4H), 6.88 (s, 8H), 6.92 (s, 2H), 6.97 (s, 4H), 7.00 (AB,
J=17 Hz, 4H), 7.12 (m, 10H), 7.47 (m, 10H); 13C NMR (CDCl3,
50 MHz): d=14.1, 22.6, 26.0, 29.2, 29.3, 29.4, 29.6, 31.9, 48.9, 65.3, 66.3,
66.7, 67.0, 68.0, 68.7, 69.1, 70.5, 70.8, 73.5, 101.5, 105.1, 107.1, 112.4, 114.5,
115.9, 121.3, 126.6, 126.8, 127.2, 128.7, 132.4, 134.3, 135.6, 135.9, 136.4,
136.6, 137.2, 137.3, 137.7, 138.4, 139.9, 140.9, 141.0, 142.1, 142.6, 143.1,
143.5, 143.6, 143.8, 144.0, 144.2, 144.5, 144.8, 144.9, 145.0, 145.2, 145.5,
145.6,; 145.9, 147.2, 147.3, 148.5, 151.0, 153.2, 157.7, 158.0, 160.3, 162.5,
162.6, 168.2; MALDI-TOF-MS: calcd for C1290H1210O152: 19145.7; found:
19144 [M +]; elemental analysis calcd for C1290H1210O152: C 80.93, H 6.37;
found: C 80.70, H 6.62.
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ried out in toluene, dichloromethane, and benzonitrile solutions (Carlo
Erba or Aldrich spectrofluorimetric grade). The path of fluorimetric cu-
vettes was 1 cm. Absorption spectra were recorded with a Perkin-Elmer
l 45 spectrophotometer. Uncorrected emission spectra were obtained
with a Spex Fluorlog II spectrofluorimeter (continuous 150 W Xe lamp),
equipped with Hamamatsu R-928 photomultiplier tube. The corrected
spectra were obtained via a calibration curve determined with a proce-
dure described earlier.[27] The steady-state NIR luminescence spectra
were obtained with an Edinburgh FLS920 spectrometer equipped with
Hamamatsu R5509-72 supercooled photomultiplier tube (193 K) and a
TM300 emission monochromator with NIR grating blazed at 1000 nm.
An Edinburgh Xe900 450 W Xenon arc amp was used as light source;
the emission calibration curve was supplied by the manufacturer. Emis-
sion lifetimes were determined with (i) an IBH single photon counting
spectrometer equipped with a thyratron gated nitrogen lamp (2–40 kHz,
lexc =337 nm, 500 ps time resolution) and a Hamamatsu 3237-01 photo-
multiplayer tube (185–850 nm) or ii) an Edinburgh FLS920 spectrometer
equipped with laser diode heads (1 MHz, lexc =407 or 635 nm, 100 ps
time resolution) and a peltier-cooled Hamamatsu R928 photomultiplayer
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scribed in detail earlier.[28] Excitation was performed with the second or
third harmonic (532/355 nm) of a Nd:YAG laser (J. K. Lasers Ltd.) with
20 ns pulse duration and 1–2 mJ of energy per pulse (up to 20 mJ for
energy-dependent triple-triplet annihilation processes). Triplet lifetimes
were obtained by averaging at least five different decays recorded
around the maximum of the absorption peak (720 nm). When necessary,
oxygen was removed by at least four freeze-thaw-pump cycles by means
of a diffusive vacuum pump at 10À6 Torr. Experimental uncertainties are
estimated to be 7% for lifetime determinations, Æ20% for emission
quantum yields, and 2% for absorption and emission peaks.
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Acknowledgements
This work was supported by the CNR, the CNRS, the French Ministry of
Research (ACI Jeunes Chercheurs to J.-F.N.) and EU (RTN Contract
“FAMOUS”, HPRN-CT-2002-00171). G.A. thanks Italian MIUR (con-
Chem. Eur. J. 2004, 10, 5076 – 5086
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5085

N. Armaroli, J. F. Nierengarten et al.
FULL PAPER
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Received: February 17, 2004
Revised: June 23, 2004
Published online: August 20, 2004
5086
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www.chemeurj.org
Chem. Eur. J. 2004, 10, 5076 – 5086