Synthesis of [60]fullerene-coumarin polyads

Article abstract of DOI:10.1016/j.tetlet.2004.07.079

New polyads were obtained by covalently linking a coumarin dye to a fullerene (C₆₀) following two different strategies: 1,3-dipolar cycloaddition reaction of azomethine ylides or by cyclopropanation reaction. The new polyads are spectroscopically characterized and their fluorescence properties, including the observation of efficient singlet-singlet energy transfer, reported.

Full text of DOI:10.1016/j.tetlet.2004.07.079

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 6927–6930
Synthesis of [60]fullerene–coumarin polyads
a
Maria Joa˜o Brites, Celia Santos, Susana Nascimento, Barbara Gigante
a
a,b
a,
*
´
and Mario N. Berberan-Santos
´
b,
*
´
a
´
´
INETI––Departamento de Tecnologia de Industrias Quımicas, Estrada do Pac¸o do Lumiar, 22, 1649-038 Lisboa, Portugal
b
´ ´ ´
Centro de Quımica-Fısica Molecular, Instituto Superior Tecnico 1049-001 Lisboa, Portugal
Received 19May 2004; revised 13 July 2004; accepted 19July 2004
Available online 5 August 2004
Abstract—New polyads were obtained by covalently linking a coumarin dye to a fullerene (C60) following two different strategies:
1,3-dipolar cycloaddition reaction of azomethine ylides or by cyclopropanation reaction. The new polyads are spectroscopically
characterized and their fluorescence properties, including the observation of efficient singlet–singlet energy transfer, reported.
ꢀ 2004 Elsevier Ltd. All rights reserved.
Over the past decade, a large number of C60-based
donor–acceptor dyads, and more complex polyads, in
which C60 is covalently linked to photoactive groups,
were obtained, aiming at exploring their excited-state
donor–acceptor interactions, including photoinduced
electron transfer.1–11 Understanding of the energy and
electron transfer processes involved is important to-
wards practical applications such as photovoltaic de-
vices for solar energy conversion.6 For this purpose,
several synthetic methodologies have been developed,
which allow covalent linkages of C60 with other photo-
and electroactive molecules.2,3 Different donors and lin-
kers to C60 have been used. In fact, the length, chemical
constitution or the presence of rigid or saturated hydro-
carbon bridges is known to play an important role on
the electronic coupling between the donor and the
acceptor chromophores.1,6,7
We report here the synthesis of novel fullerene–couma-
rin polyads, consisting of one or two coumarin moieties
covalently linked to fullerene by different approaches
(Scheme 1 and 2) and then compare their photophysical
properties (Table 1).
Commercially available coumarins, 4-(bromomethyl)-7-
methoxycoumarin 1 and 7-hydroxy-4-methylcoumarin 2
containing active groups (bromoalkyl or hydroxyl
groups) in different carbons (C-4 or C-7), were used as
starting materials.
From 1 or 2, following 1,3-dipolar cycloaddition
reaction of azomethine ylides generated in situ,
from the reaction of an aldehyde group with N-methyl-
glycine to fullerene,19 [60]fulleropyrrolidines derivatives
5 or 6 were obtained (Scheme 1). To this end, 2 was
alkylated with equimolar amount of 4-(2-bromoeth-
oxyl)-benzaldehyde or 1 by reaction with 4-hydroxy-
benzaldehyde both in dimethylformamide at 90ꢁC for
2–4h using anhydrous potassium carbonate as base to
obtain, respectively, the intermediates 4 and 3 with
the aldehyde functionality at C-7 or C-4 of the couma-
ryl moiety. [60]Fulleropyrrolidines 5 or 6 were sepa-
rated from the reaction mixtures by flash
chromatography using gradients of toluene and ethyl
acetate as eluent. After the unchanged C60, the mono-
adducts 5 or 6 were separated. Minor products
with higher polarity, probably bis-adducts, were
discarded. After purification by precipitation from
chloroform and methanol, each of the fullerene deriva-
tives was isolated with moderated yields: 5 (23%) and
6 (24%).
Coumarins are attractive molecules due to their
extended spectral range, high emission quantum yields
and photostability.12–15 Moreover coumarin derivatives
are frequently encountered as signalling units in sensors
and in sophisticated photophysical systems.16–18 How-
ever, to the best of our knowledge, no reports concern-
ing coumarins–C60 systems have appeared so far.
Keywords: Coumarin; C60; Dyads; Polyads.
*
Corresponding authors. Tel.: +351 210924600; fax: +351 217168100
(B.G.); tel.: +351 218419254; fax: +351 218464455 (M.N.B.-S.);
e-mail addresses: barbara.gigante@ineti.pt; berberan@ist.utl.pt
0040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.07.079

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M. J. Brites et al. / Tetrahedron Letters 45 (2004) 6927–6930
Scheme 1. Reagents and conditions: (i) 4-(2-bromoethoxyl)-benzaldehyde for 4 and 4-hydroxybenzaldehyde, for 3, K2CO3, DMF, 90ꢁC; (ii) C60
,
MeNHCH2CO2H, N2, toluene, reflux.
Scheme 2. Reagents and conditions: (i) 6-bromo-hexanol, K2CO3, DMF, 60ꢁC; (ii) methyl-3-chloro-3-oxopropionate, Et3N, CH2Cl2, N2, 0ꢁC–rt;
(iii) malonyl dichloride, Et3N, CH2Cl2, N2, 0ꢁC–rt; (iv) C60, DBU, I2, toluene, rt.
From 2, methano[60]fullerene dyad 10 and triad 11 were
also obtained by cyclopropanation20 of C60 with the
appropriate malonate derivative 8 or 9 (Scheme 2).
6-bromo-1-hexanol, followed by the reaction of the
resulting alcohol 7 with the appropriate malonyl chlo-
ride (methyl malonyl chloride or malonyl dichloride)
in high yield (85%). Cyclopropanation reaction of C60
with malonates 8 or 9 afforded 10 or 11, respectively.
Separation from the reaction mixture by column chro-
matography (toluene–ethyl acetate 4:1) gave the mono-
The malonate derivatives 8 and 9 containing one and
two coumarin moieties were obtained, respectively, by
alkylation of the 7-hydroxyl group of coumarin 2 with

M. J. Brites et al. / Tetrahedron Letters 45 (2004) 6927–6930
6929
Table 1. Absorption and fluorescence peak wavelengths, and fluorescence quantum yields of C60, coumarin intermediates 3, 4, 8 and 9, and of
fullerene derivatives 5, 6, 10 and 11.
Compound
kabs (nm)
kem (nm)
UF/10ꢀ3
C60
3
328, 408 (sh), 404, 689712
322
320, 337(sh)
––
0.2
398, 468 (sh), 498 (sh)
382
18
18
16
––
––
––
––
––
4
8
322, 337 (sh)
321
384
383
––
9
––
5
322, 434, 706
320, 434, 704
327, 429, 496, 688
326, 429, 490, 687
382, 710
378, 712
380, 700
384, 700
1.7a
1.8a
1.0a
0.77a
6
10
11
1.7
1.3
a Direct excitation of the fullerene moiety (excitation wavelength: 400nm).
adducts 10 (35%) and 11 (19%). Either the intermediates
as final products were characterized by UV–vis, FT-IR,
1H and 13C NMR and mass spectroscopy including
MALDITOF-MS for fullerene derivatives.
standard for fullerene derivatives. The excitation wave-
length was 320nm for coumarin compounds 3, 4, 8, 9
and 300nm for all fullerene derivatives. FT-IR spectra
were recorded on a Perkin–Elmer 1725 spectrometer.
NMR spectra were run on a General Electric QE-300
spectrometer with resonance frequency of 300.13MHz
for 1H and 75.47MHz for 13C, with appropriate solvent.
The chemical shifts (d) are reported in parts per million,
and coupling constants (J) in hertz. MALDITOF-MS
spectra were determined on a LAZARUS II, using
DCTB (3-methyl-4-(4-tert-butylphenyl)butadiene-1,1-
dinitrile) as a matrix.
The fluorescence quantum yields of the coumarin com-
pounds (3, 4 and 8), at room temperature in toluene,
are of the order of 0.02. The absorption spectra of 10
and 11 are the sum of the absorption spectra of the
C60 and coumarin subunits, respectively, 8 and 9. On
the other hand, the absorption spectra of 5 and 6, when
compared to those of 3, 4 and of [60]fulleropyrrolidines
without coumarin moiety, reveal the existence of ground
state interactions.
1.1. [60]Fulleropyrrolidine 5
The fluorescence of the fullerene derivatives 5, 6, 10 and
11, obtained at room temperature in toluene and with
300nm excitation, differs significantly from that of 3, 4
and 8, as shown in Table 1. Apart from the coumarin
emission that occurs in the 340–520nm range, a second
emission band, characteristic of the fullerene moiety, is
observed in the 670–800nm region. Furthermore, in
the polyads (5, 6, 10 and 11), there is a dramatic de-
crease of the coumarin moiety quantum yield when com-
pared with that of the coumarin intermediates (3, 4 and
8). This implies a strong intramolecular quenching of the
coumarin fluorescence by the fullerene moiety. Simulta-
neously, an increase in the emission quantum yield of
the C60 moiety (680–705nm) compared to that of di-
rectly excited fullerene mono-adduct, is observed, indi-
cating the existence of fast and efficient singlet–singlet
energy transfer from the coumarin moiety to the fulle-
rene. This process is consistent with a dipole–dipole
mechanism, as will be discussed elsewhere.
UV–vis (toluene) kmax 322, 434, 706nm; Fluorescence
(toluene) kmax 382, 710nm; FT-IR (KBr) mmax 2778,
1720, 1611, 1509, 1207, 834, 802, 527, 481cmꢀ1 1H
;
NMR (CDCl3, TMS): d 2.80 (s, 3H, 100-H), 3.87 (s,
3H, 7-OCH3), 4.26 (d, J = 9.0, 1H, 500-H), 4.91 (s, 1H,
200-H), 4.97 (d, J = 9.0, 1H, 500-H), 5.18 (s, 2H, 9-H),
6.48 (s, 1H, 3-H), 6.84 (d, J = 2.4, 1H, 8-H), 6.85 (dd,
1H, J = 9.0 and 2.4, 6-H), 7.03 (d, J = 8.7, 2H, 20-H
and 60-H), 7.46 (d, J = 9.0, 1H, 5-H), 7.74 (br s, 2H,
30-H and 50-H) ppm; 13C NMR (CDCl3, TMS): d 40.3
(C-100), 55.9(7-O CH3), 65.9(C-9), 66.0 (C-3 00), 69.2 (C-
500), 83.2 (C-200), 77.1 (C-400), 101.4 (C-8), 109.9 (C-4),
0
111.0 (C-3), 112.9(C-6), 114.9(C-2
and (C-60), 124.7
(C-5), 130.7 (C-30 and C-50); 131.0.0 (C-40), 130-148
(C(sp2)–C60), 149.8 (C-4a), 156.0 (C-8a), 160.7 (C-7),
163.0 (C-2), MALDITOF-MS (DCTB) m/z 1057 (Mꢀ).
1.2. [60]Fulleropyrrolidine 6
UV–vis (toluene) kmax 320, 434, 704nm; Fluorescence
(toluene) kmax 378, 712nm; FT-IR (KBr) mmax 2778,
1. Experimental
1718, 1611, 1509, 1248, 848, 804 528, 466cmꢀ1 1H
;
NMR (CDCl3/CS2, TMS): d 2.40 (s, 3H, 9-H), 2.80 (s,
3H, 100-H), 4.25 (d, J = 9.3, 1H, 500-H), 4.38 (m, 4H, 10-
H and 20-H), 4.90 (s, 1H, 200-H), 4.98 (d, J = 9.3, 1H,
500-H), 6.15 (s, 1H, 3-H), 6.87 (d, J = 2.4, 1H, 8-H),
6.90 (dd, J = 8.7 and 2.4, 1H, 6-H), 7.02 (d, J = 8.7,
2H, 40-H and 80-H), 7.50 (d, J = 8.7, 1H, 5-H), 7.75
(br s, 2H, 50-H and 70-H) ppm; 13C NMR (CDCl3/
CS2, TMS): d 18.7 (C-9), 40.0 (C-100), 66.1 (C-10 or C-
20), 67.1 (C-10 or C-20), 69.0 (C-300), 70.0 (C-500), 77.3
All spectral measurements were made in quartz cells of
1cm path length, at room temperature and all solvents
used were of spectroscopic grade. Electronic absorption
spectra were measured using a Shimadzu 3101 UV/vis/
NIR and a Hitachi 150-20 spectrophotometers. Fluores-
cence quantum yields at room temperature in toluene
were determined by a comparative method, using an-
thracene (Fluka) in air-saturated ethanol (/F = 0.27)21
as the fluorescence standard for coumarins and C60
(C-400), 83.2 (C-200), 101.6 (C-8), 112.2 (C-3), 112.7
(99.5%, Aldrich) in toluene (/F = 2 · 10ꢀ4
)
as the
(C-6), 113.9(C-4), 114.9(C-4
and C-80), 125.6 (C-5),
22
0

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M. J. Brites et al. / Tetrahedron Letters 45 (2004) 6927–6930
129.7 (C-60), 130.6 (C-50 and C-70), 130-148 (C(sp2)C60),
152.4 (C-4a), 154.1 (C-30), 155.2 (C-8a), 161.2 (C-7),
161.6 (C-2) ppm; MALDITOF–MS (DCTB) m/z 1071
(Mꢀ).
MS spectra. This work was carried out within projects
POCTI 34400/QUI/2000 and POCTI 34836/FIS/2000
ˆ
from Fundac¸a˜o para a Ciencia e a Tecnologia (Portu-
gal).
1.3. [60]Methanofullerene 10
References and notes
UV–vis (toluene) kmax 327, 429, 496, 688nm Fluores-
cence (toluene) kmax 380, 700 nm; FT-IR (KBr) mmax
2928, 2848, 1742, 1734, 1719, 1610, 1387, 1266, 1235,
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1144, 1069, 849, 527cmꢀ1; H NMR (CDCl3, TMS): d
1.56–1.61 (m, 4H, 30-H and 40-H), 1.82-1.92 (m, 4H,
20-H and 50-H), 2.39(s, 3H, 9-H), 4.03 (t, J = 6.3, 2H,
0
10-H), 4.09(s, 3H, 9 -OCH3), 4.53 (t, J = 6.3, 2H, 60-
H), 6.12 (s, 1H, 3-H), 6.79(d, J = 2.4, 1H, 8-H), 6.84
(dd, J = 2.4 and 8.7, 1H, 6-H), 7.47 (d, J = 8.7, 1H, 5-
H) ppm; 13C NMR (CDCl3, TMS): d 18.7 (C-9), 25.7
(C-30 or C-40), 25.8 (C-30 or C-40), 28.5 (C-50), 28.97
(C-20), 52.2 (methano bridge, C-80), 54.0 (C–OCH3),
67.3 (C-60), 68.3 (C-10), 71.5 (C(sp3)–C60), 101.4 (C-8),
111.9(C-3), 112.7 (C-6), 113.6 (C-4), 125.6 (C-5),
138.0-146.0 (C(sp2)–C60), 152.5 (C-4a), 155.3 (C-8a),
161.3 (C-7), 162.1 (C-2), 163.6 (C–CO), 164.1 (C–CO)
ppm; MALDITOF-MS (DCTB) m/z 1094 (M+.).
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1.4. [60]Methanofullerene 11
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UV–vis (toluene) kmax 326, 429, 490, 687nm; Fluores-
cence (toluene) kmax 384, 700nm; FT-IR (KBr) mmax
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1
802, 787, 527cmꢀ1: H NMR (CDCl3, TMS) d 1.45–
1.70 (m, 8H, 30-H and 40-H), 1.83–1.91 (m, 8H, 20-H
and 50-H), 2.39( s, 6H, 9-H), 4.02 (t, J = 6.3, 4H, 10-
H), 4.54 (t, J = 6.3, 4H, 60-H), 6.12 (s, 2H, 3-H), 6.77
(d, J = 2.4, 2H, 8-H), 6.84 (dd, J = 2.6 and 9.3, 2H, 6-
H), 7.47 (d, J = 8.7, 2H, 5-H) ppm; 13C NMR (CDCl3,
TMS): d 18.7 (C-9), 25.7 (C-030 or C-40), 25.9(C-3 or
0
C-40), 28.6 (C-50), 28.9(C-2 ), 52.4 (methano bridge,
C-80), 67.3 (C-60), 68.3 (C-10), 71.6 (C(sp3)–C60), 101.4
(C-8), 111.9(C-3), 112.6 (C-6), 113.5 (C-4a), 125.6 (C-
5), 138.0-146.0 (C(sp2)–C60), 152.6 (C-4a), 155.3 (C-
8a), 161.3 (C-7), 162.1 (C-2), 163.7 (C-70 and C-90)
ppm; MALDITOF-MS (DCTB) m/z 1339(MH +.).
Acknowledgements
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To Dr. H. Luftmann, Institute of Organic Chemistry,
University of Muenster, Germany, for MALDITOF-