Variation of the energy gap in fullerene-based dendrons: Competitive versus sequential energy and electron transfer events

Article abstract of DOI:10.1039/b100397f

Artificial molecular models, based on rigid dendrimeric meta-substituted phenylenevinylene moieties bearing different electron donating groups and the electron acceptor C₆₀, were probed in an effort to gain control over a sequential versus a co

Full text of DOI:10.1039/b100397f

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Variation of the energy gap in fullerene-based dendrons: competitive versus
sequential energy and electron transfer events
José L. Segura,a Rafael Gómez,a Nazario Martín,*a Chuping Luo,b Angela Swartzb and Dirk M. Guldi*b
a Departamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Complutense, E-28040-Madrid,
Spain. E-mail: nazmar@eucmax.sim.ucm.es
b Radiation Laboratory, University of Notre Dame, IN 46556, USA. E-mail: guldi.1@nd.edu
Received (in Cambridge, UK) 9th January 2001, Accepted 15th February 2001
First published as an Advance Article on the web 29th March 2001
Artificial molecular models, based on rigid dendrimeric
meta-substituted phenylenevinylene moieties bearing differ-
ent electron donating groups and the electron acceptor C60,
were probed in an effort to gain control over a sequential
versus a competitive scenario in energy and electron transfer
events.
cycloaddition of the respective rigid dendron (7a,b) to C60 in the
presence of sarcosine in refluxing toluene (Fig. 1).11
The redox properties of the fullerene-containing dendrons
8a,b were studied by cyclic voltammetry in toluene/acetonitrile
(4+1 v/v) solutions and revealed several oxidation and reduction
steps. In particular, anodic processes with peaks at 0.72 V (8a)
and 1.30 V (8b) refer to the oxidation of the dibutylaniline and
dodecyloxynaphthalene moieties, respectively. In the cathodic
region three quasireversible reduction waves were recorded (8a:
20.65; 21.07, 21.70 V, 8b: 20.64, 21.03; 21.62 V), which
correspond to the stepwise reduction of the fullerene moiety and
resemble those observed for a fulleropyrrolidine reference.
Replacement of the poly(arylenevinylene) cores (8b) with the
analogous dibutylaniline functionalities (8a) imposes important
consequences on the dendron’s physico-chemical properties:
red-shifted ground-state maxima in combination with better
electron donor character create quite different energy gaps
between the dendron’s singlet excited state and that of the
C60·2–dendron·+ radical pair in 8a and 8b.
In general, the strongly fluorescing dendrons 7a and 6b with
quantum yields as high as 0.47, give rise to an almost entirely
quenched emission in dyads 8a and 8b (Table 1). For example,
in non-polar toluene the dendrimer emission in the visible
(400–450 nm) is reduced by more than two orders of
magnitude.
From this observation we reach the conclusion that in toluene
the dendrimer singlet excited state in 8a,b undergoes a rapid
singlet–singlet energy transfer to the energetically lower-lying
The design of supramolecular ensembles for use in energy
conservation and conversion is a long-standing objective.1
Understanding the complexity of efficient energy migration and
unidirectional electron transfer occurring, for example, in the
photosynthetic reaction center2 offers powerful guidelines for
developing artificial devices. A particularly challenging aspect
is to explore the role of a sequence versus a competition in
energy/electron transfer reactions, especially upon altering the
energy gaps.
The choice of the connecting spacer in donor–bridge–
acceptor (D–B–A) assemblies is fundamental, since it ensures
the control over distance, angle and coupling between the donor
and acceptor. These parameters govern in large character, rate
and efficiency of long distance energy and/or electron transfer
processes.3,4 Dendritic structures provide key criteria such as
rigidity and conjugation, which, most importantly, can be
chemically fine-tuned.5 In turn, these unique molecular archi-
tectures emerged as versatile building blocks for the preparation
of artificial systems.6
Previous work has focused mainly on encapsulating flexible
dendron units.7 The flexibility prohibits, however, a detailed
comprehension of effects stemming from donor–acceptor
separation and/or orientation, which drew our attention to the
unique features of a stiff dendritic macromolecule.8
Here we report on the synthesis and photophysical studies of
D–B–A systems in which four dibutylaniline (8a) or dodec-
yloxynaphthalene (8b) electron donors are located at the
peripheral positions of well-defined phenylenevinylene-based
dendrons and an acceptor fullerene at the focal point of the
dendrimer.
Yu and coworkers9a and Meier and Lehman9b have reported
synthetic routes towards formyl-substituted rigid poly(pheny-
lenevinylene) dendrimers starting from different AB2 type
building blocks. A remarkable advantage of the convergent
route, presented here is the use of a nitrile group in the readily
available AB2 starting material 1.10 This functionality remains
unaffected during the Arbuzov and the Wittig–Horner reaction
(i.e. with aldehydes 3a,b) to yield bisphosphonate 2 and the
first-generation dendrons 4a,b with an all-trans configuration,
respectively.
Subsequent treatment of the nitrile-substituted systems 4a,b
with DIBAL-H in dichloromethane transforms them in good
yield to the corresponding formyl-substituted analogues 5a,b.
In the next step, a second Wittig–Horner reaction of 5a,b with
bisphosphonate 2 affords the nitrile-substituted second-genera-
tion dendrons 6a,b. Additional DIBAL-H treatment allowed us
to prepare the formyl functionalized dendrons 7a,b. Dyads 8a,b
have been prepared in good yields (30%) by 1,3-dipolar
Fig. 1
DOI: 10.1039/b100397f
Chem. Commun., 2001, 707–708
This journal is © The Royal Society of Chemistry 2001
707

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Table 1 Photophysical properties of dendrons 6b/7a and dyads 8a/8b
therefore, prevents a meaningful kinetic analysis of the
intramolecular reaction. On a longer time-scale the fate of the
fullerene singlet excited state is identical to that known for a
fulleropyrrolidine: intersystem crossing, driven by a strong
spin-orbit coupling, governs the transformation (ca. 6.0 3
108 s21) of the singlet into the triplet excited state. The latter
was identified by a long-lived (ca. 20 ms) and strongly
absorbing triplet–triplet maximum (700 nm).
Upon probing more polar THF and benzonitrile, the fullerene
fluorescence in 8b reveals a gradual decrease of up to 50%,
while the dendron emission continued to be almost unchanged
relative to that in toluene. This observation is consistent with an
assumption that implies an energy transfer scenario, which is in
competition with an activated electron transfer. Crucial support
for this competitive synopsis evolves from the fullerene singlet
lifetimes (Table 1), which are, despite the emission quenching,
identical to those of a fulleropyrrolidine in all the solvents
investigated.
8a, on the other hand, reveals much stronger reductions of the
fullerene emission (up to 94%). Most importantly, much shorter
lifetimes of the fullerene singlet excited state, the product of the
initial energy transfer, were seen. This suggests, in sharp
contrast to 8b, a sequential energy and electron transfer starting
from the initially excited dendron. In its final instance this
sequence generates restrictively a radical pair, C60·2–den-
dron·+.
Formation of the charge-separated state was established for
both donor–acceptor systems by means of transient absorption
spectroscopy. The fullerene p-radical anion displayed its typical
near-IR absorption at 1000 nm, while the one-electron oxidized
forms of the donors were evidenced through their absorption in
the visible with lmax ca. 480 nm (8a) and 540 nm (8b). It is
important to note that for the dibutylaniline-containing dyad 8a
the radical pair, with lifetimes of 350 ns and 725 ns, in THF and
benzonitrile, respectively is the sole product, corroborating the
sequence of energy and electron transfer. Quite different is the
situation for the poly(arylenevinylene) derivative 8b: both
energy transfer and electron transfer products were noted as
superimposed spectral features.
6b
7a
8b
8a
Dendron
FFL (toluene)
FFL (THF)
0.47e
1.99e
0.31f
3.15 3 1023e
3.07 3 1023e
2.5 3 1023f
2.2 3 1023f
tFL (toluene)/ns
2.05f
2.01
tsinglet (toluene)/ns 1.97
2DGET°a/eV
1.34
1.49
1.24
1.67
1.42
0.99
la/eV
2DGEnergyT°/eV
Fullerene
FFLb (toluene)
FFLb (THF)
5.4 3 1024
4.0 3 1024
3.0 3 1024
1.51
1.49
1.51
1.42
0.60
0.43
0.30
3.9 3 1024
0.6 3 1024
0.4 3 1024
1.48
1.40
0.16
FFLb (BzCN)
tFLb (toluene)/ns
tsingletc (toluene)/ns
tsingletc (THF)/ns
tsingletc (BzCN)/ns
Ftripletd (toluene)
Ftripletd (THF)
Ftripletd (BzCN)
2DGET°a/eV
0.09
0.29
g
—
g
—
0.10
0.68
a In benzonitrile; radius: (r+) dibutylaniline = 3.7 Å; (r+) naphthalene = 3.5
Å; (r2) fullerene = 4.4 Å; donor–acceptor separation: (rD–A) 8a = 16.7 Å;
(rD–A) 8b = 19.1 Å. b Measured at the 715 nm maximum, excitation at
dendron ground state maximum. c Measured at the 880 nm maximum.
d Measured at the 700 nm maximum. e Measured at the 413 nm maximum;
excitation at 340 nm. f Measured at the 450 nm maximum; excitation at 380
nm. g Radical pair.
fullerene singlet (Fig. 2). Evidence that this energy transfer
indeed takes place, comes from a set of decisive emission and
excitation measurements.
The species evolving from energy transfer, namely, the
fullerene singlet excited state (1.76 eV), has been identified via
the characteristic fluorescence pattern12 of a fulleropyrrolidine
produced in nearly comparable quantum yields. Importantly,
the complementary excitation spectrum of the fullerene emis-
sion in 8a,b is virtually identical with the ground-state
absorption and the excitation spectra of the reference com-
pounds. To further test the above assignment, that is, a rapid
singlet–singlet energy transfer, transient absorption spectros-
copy was deemed necessary. Despite the unequivocal and
nearly quantitative excitation (355 nm) of the dendrimer
moieties no spectral evidence for the dendrimer’s singlet–
singlet absorption (ca. 650 nm) was found after the 18 ps laser
pulse. Instead the spectral features are identical with the texture
of the fullerene singlet excited state. Specifically, a character-
istic maximum at 880 nm is a clear attribute of the fullerene
singlet–singlet absorption. The rise time of the 880 nm
absorption, representing the actual energy transfer event in
8a,b, is, however, masked by the instrument response time and,
This work has been supported by the DGESIC of Spain
(Project PB98-0818) and by the European Comission (Contract
JOR3CT980206). Part of this work has been supported by the
Office of Basic Energy Sciences of the US Department of
Energy. This is document NDRL-4286 from the Notre Dame
Radiation Laboratory.
Notes and references
1 M. R. Wasielewski, Chem. Rev., 1992, 92, 435; M.-J. Blanco, M. C.
Jiménez, J.-C. Chambron, V. Heitz, M. Linke and J.-P. Sauvage, Chem.
Soc. Rev., 1999, 28, 293.
2 W. F. J. Vermaas, Science, 1994, 263, 1511; M. A. Stefen, K. Q. Lao and
S. G. Boxer, Science, 1994, 264, 810.
3 M. A. Fox and M. Chanon, Photoinduced Electron Transfer, Elsevier,
Amsterdam, 1988.
4 M. H. B. Stowell, T. M. McPhillips, D. C. Rees, S. M. Soltis, E. Abresch
and G. Feher, Science, 1997, 276, 812.
5 For a review on light-harvesting dendrimers, see: A. Adronov and
J. M. J. Fréchet, Chem. Commun., 2000, 1701.
6 A. Adronov, S. L. Gilat., J. M. J. Frechet, K. Ohta, F. V. R. Neuwahl and
G. R. Fleming, J. Am. Chem. Soc., 2000, 122, 1175 and references
therein.
7 J.-F. Nierengarten, Chem. Eur. J., 2000, 6, 3667.
8 Z. Xu and J. S. Moore, Acta Polym., 1994, 45, 83; C. Devadoss, P.
Bharathi and J. S. Moore, J. Am. Chem. Soc., 1996, 118, 9635; A. G.
Avent, P. R. Birkett, F. Paolucci, S. Roffia, R. Taylor and N. K.
Wachter, J. Chem. Soc., Perkin Trans. 2, 2000, 1409.
9 (a) S. K. Deb, T. M. Maddux and L. Yu, J. Am. Chem. Soc., 1997, 119,
9079; (b) H. Meier and M. Lehmann, Angew. Chem. Int. Ed., 1998, 37,
643.
10 G. J. Bodwell, J. N. Bridson, T. J. Houghton and B. Yarlagadda,
Tetrahedron Lett., 1997, 38, 7475.
11 M. Prato and M. Maggini, Acc. Chem. Res., 1998, 31, 519.
12 D. M. Guldi and M. Prato, Acc. Chem. Res., 2000, 33, 695.
Fig. 2
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