Fullerene-fluorescein-anthracene hybrids: A model for artificial photosynthesis and solar energy conversion

Article abstract of DOI:10.1016/j.tetlet.2003.10.136

Two novel C₆₀-fluorescein-anthracene hybrids have been synthesized. Fluorescence quenching in the hybrids indicates an energy transfer from the excited state of anthracene to fluorescein and photoinduced intramolecular electron transfer from the excited state of fluorescein to the C₆₀ moiety.

Full text of DOI:10.1016/j.tetlet.2003.10.136

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 221–224
Fullerene–fluorescein–anthracene hybrids: a model for artificial
photosynthesis and solar energy conversion
Bingwen Jing* and Daoben Zhu
Organic Solids Laboratory, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100080, PR China
Received 6 June 2003; revised 10 September 2003; accepted 14October 2003
Abstract—Two novel C60–fluorescein–anthracene hybrids have been synthesized. Fluorescence quenching in the hybrids indicates an
energy transfer from the excited state of anthracene to fluorescein and photoinduced intramolecular electron transfer from the
excited state of fluorescein to the C60 moiety.
ꢀ 2003 Elsevier Ltd. All rights reserved.
The chemistry of fullerenes has attracted a lot of atten-
tion and is currently under very active investigation. The
unusual properties of fullerene derivatives suggest
potential applications in many areas. A most promising
one is the artificial mimicking of photosynthesis and
related solar energy conversion. Fullerenes as novel
electron acceptors, particularly the readily available C60,
present a wide range of chemical and physical properties
that make them potential chromophores in photoin-
duced redox processes.1–3 Also a remarkable property of
C60 related to electron transfer phenomena is that it can
efficiently induce a rapid charge separation accompanied
by a slow charge recombination. The potential avail-
ability of a wide variety of dyads and triads containing
covalently bonded C60 and donors/acceptors units offer a
useful opportunity to investigate and correlate this
intrigue photoinduced electron transfer behavior. In these
dyads and triads, porphyrins,3–9 oligothiophene,10;11
pyrene,12 carbazole,13 ferrocene,14 polypyridine ruthe-
nium(II) complexes,15 N,N-dialkylaniline,16 TTFs,17;18
carotenoid,19 are usually the donors, while relatively
fewer number of acceptors such as pyromellitimide,20
nitroaromatic,21;22 benzoquinone derivatives,23 have
been used to form electron acceptor–fullerene or donor–
fullerene–acceptor dyads and triads. Among these, the
systems consisting of porphyrins and C60 are the most
prominent and have been extensively studied. It is well
known that fluorescein (FL) is a very important xanthene
dye, which has a high fluorescence quantum yield,
excellent redox properties as well as relatively large
extinction coefficients at the visible region. Thus, FL has
acquired many technical applications and has been fre-
quently used as electron donor/acceptor and sensitizer in
artificial photosynthetic models.24 Our previous study25
on FL–C60 dyad shows that intramolecular photoin-
duced electron transfer (IPET) from the singlet excited
state of FL (ꢀ1FL) to C60 takes place efficiently. Therefore
FL–C60 provides a good model to mimic artificial pho-
tosynthesis. Considering the rather low absorptions of
both C60 and FL in the range of 320–400 nm, and by
contrast, high absorption for anthracene (AN) in the
same region, as well as the fact that AN is known to be a
good electron and energy donor, which can efficiently
transfer its singlet excited energy to FL (almost 100%)
both intramolecularly and intermolecularly,26–28 intro-
duction of AN to the FL–C60 dyad to form AN–FL–C60
triad should allow favorable light harvesting as well as
charge separation. Therefore we prepared two hybrids 1
and 2, composing of a FL, AN, and fullerene, covalently
linked to each other, as well as the fullerene model
compound 3.
The synthetic approach25 to compounds 1–3 (Scheme 1)
relies upon the 1,3 dipolar cycloaddition of azomethine
ylides to C60. Thus a mixture of related aldehydes (4, 5,
or
p-anisaldehyde)
(0.1 mmol),
N-methylglycine
(0.36 mmol), and C60 (0.1 mmol) in toluene (100 mL) was
refluxed for 8–12 h under Ar, giving the corresponding
fulleropyrolidines 1–3. Purification was accomplished by
column chromatography (silica gel, toluene/methanol,
50:1). The aldehydes 4, 5 were prepared by sequential
* Corresponding author at present address: Department of Chemistry,
Lehigh University, Bethlehem, PA 18015, USA. E-mail: bij3@
lehigh.edu
0040-4039/$ - see front matter ꢀ 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.10.136

222
B. Jing, D. Zhu / Tetrahedron Letters 45 (2004) 221–224
alkylation of FL as depicted in Scheme 1 using the
corresponding alkylbromides in DMF at 90 ꢁC for 2–3 h
with anhydrous potassium carbonate as base. The
structures of all these compounds were verified by
spectroscopic analyses.
COO(CH ) OOC
2 2
O
O
O(CH ) O
2 2
N
The UV–vis spectra of hybrids 1 and 2 (Fig. 1) in
chloroform/methanol (1:1) are composed of the indi-
vidual chromophoric subunits: C60, FL, and AN, indi-
cating the absence of significant interaction among FL,
C60, and AN in the ground state. As of the steady-state
fluorescence spectra of precursor dyads (FL–AN) 4 and
5 in chloroform/methanol (1:1), when excited at 460 nm
at which the absorption of FL is dominant, no obvious
change of FLÕs fluorescence intensity occurs compared
to 6-ethoxyl fluorescein ethylester (FLdiEt), which
means that there is no energy/electron transfer between
ꢀ1FL and AN. The electron transfer from AN to ꢀ1FL
was not observed as expected, though this process is
thermodynamically allowed (DG ¼ À0:33 eV < 0).29 The
same result had previously been obtained by ShenÕs
group28 and this was attributed to the conformation
mismatch of the chromophoric planes. By contrast,
when AN was excited at 364nm, the emission from the
singlet excited state of AN (ꢀ1AN) was completely
quenched, while the fluorescence from ꢀ1FL was
observed. These results indicate that intramolecular
1
CH
3
CH
N
3
COO(CH ) O
2 2
O
COO(CH ) O
2 2
O
2
COO(CH ) OOC
2 2
O
C
O
O
H
O(CH ) O
2 2
4
O
C
COO(CH ) O
2 2
H
O
COO(CH ) O
2 2
O
5
The Structures of Hybrids 1, 2 and Precursors 4, 5
All new compounds gave satisfactory spectroscopic and analytical
data consistent with the assigned structures. All 1H NMR spectra
were recorded on a Bruker 300 MHz instrument; chemical shifts are
reported in ppm and are referenced to residual solvent. All UV
spectra were recorded using a U-3010 spectrophotometer (HIT-
COOH
1. ( i )
( iii )
4
1
2. ( ii )
O
HO
O
ACHI). Emission spectra were recorded on
a Hitachi F-450
fluorescence spectrometer. Solvents used for UV and fluorescence
measurements were redistilled and the related samples were prepared
in aerated methanol/chloroform (1:1, v/v) and the concentration is
about 2 · 10À5 M. IR spectra were recorded on KBr pellets with a
Perkin Elmer System 2000FTIR. Mass spectra were obtained on
MALDI-TOF MS, Bruker BIFLEX III, or AEI-MS 50 mass
spectrometers.
Hybrid 1: 1H NMR (CDCl3, d, ppm): 2.97 (s, 3H), 3.44–3.58 (m, 2H),
4.21–4.45 (m, 5H), 4.95–5.05 (m, 4H), 6.00–6.50 (m, 2H), 6.61–6.65
(m, 3H), 6.75–7.10 (m, 3H), 7.25–7.40 (m, 1H), 7.54 (d, 4H), 7.70–
7.90 (m, 4H), 8.04–8.15 (m, 4H), 8.37 (d, 1H), 8.59 (d, 1H); 13C NMR
(CDCl3, d, ppm): 39.9, 63.0, 63.1, 65.1, 66.7, 68.8, 77.2, 77.3, 82.7,
82.8, 100.9, 105.7, 115.4, 117.9, 125.0, 125.6, 127.2, 128.7, 128.8,
129.1, 129.8, 130.0, 130.3, 130.4, 131.0, 131.9, 133.0, 140.1, 141.7,
142.0, 142.1, 142.2, 142.5, 144.4, 145.2, 145.3, 145.5, 145.9, 146.1,
146.3, 147.3, 153.1, 153.7, 154.1, 158.0, 158.6, 165.3, 168.9, 169.1,
169.2; FT-IR: 1725, 1643, 1597, 1512, 1447, 1377, 1286, 1246, 1203,
1105, 853, 527; MALDI-TOF, m=z: 720.24, 756.36, 868.17, 1474.36
(M)1).
COOH
1. ( ii )
2. ( i )
( iii )
5
2
O
HO
O
CH
3
N
O
C
( iii )
C60
H
OCH
+
3
OCH
3
3
Scheme 1. Reactions and conditions: (i) (2-bromoethyl) anthracene-
9-carboxylate ibid; (ii) 4-(2-bromoethoxyl) benzaldehyde, anhydrous
K2CO3, DMF, 90 ꢁC for 2–3 h; (iii) N-methylglycine, Ar, toluene,
reflux for 8–12 h.
singlet energy transfer occurs from ꢀ1AN moiety to FL
moiety. It is known that singlet excited-state energies of
9-anthracene carboxylic acid ethyl ester (ANCO2Et) and
FLdiEt are 3.21 and 2.43 eV, respectively.26 Moreover,
the fluorescence spectrum of AN overlaps well with the
absorption spectrum of FLdiEt. Hence the energy
transfer from ꢀ1ANCO2Et to the FLdiEt is feasible.
Though the electron transfer from ꢀ1AN to FL is also
thermodynamically allowed (DG ¼ À1:09 eV < 0), no
electron transfer from ꢀ1AN to FL was observed in this
case as reported by others.26;28 This is probably because
the energy transfer of Ôemission–absorptionÕ is a more
Hybrid 2: 1H NMR (CDCl3, d, ppm): 2.80 (s, 3H), 4.22–4.32 (m, 5H),
4.52 (s, 2H), 4.58–4.75 (m, 2H), 4.91 (s, 1H), 4.98 (d, 1H), 6.35–6.58
(m, 3H), 6.66 (s, 1H), 6.77–6.83 (q, 2H), 7.00 (d, 2H), 7.26 (d, 1H),
4.40–7.51 (hexa, 4H), 7.66–7.78 (hexa, 4H), 7.94–7.97 (d, 2H), 8.03–
8.06 (d, 2H), 8.34(d, 1H), 8.58 (s, 1H); 13C NMR (CDCl3, d, ppm):
39.9, 62.8, 63.3, 65.8, 67.3, 68.8, 77.2, 82.9, 96.1, 100.7, 105.4, 114.1,
114.2, 114.5, 114.7, 115.0, 117.6, 124.8, 125.5, 127.0, 128.4, 128.7,
128.9, 129.7, 129.8, 130.2, 130.5, 130.9, 131.3, 133.0, 134.5, 139.8,
141.6, 141.9, 142.0, 142.1, 142.2, 142.5, 142.6, 145.1, 145.4, 145.5,
145.8, 146.0, 146.1, 146.2, 147.2, 153.3, 153.9, 154.4, 156.1, 158.3,
158.8, 163.3, 164.7, 169.0; FT-IR: 1727, 1642, 1597, 1510, 1206, 1106,
853, 527; MALDI-TOF, m=z: 1474.36 (M)1).

B. Jing, D. Zhu / Tetrahedron Letters 45 (2004) 221–224
223
fluorescence, while IPET from the ꢀ1FL to C60 (DG ¼
À0:97 eV29) is a main pathway for the emission
quenching in chloroform/methanol (1:1, v/v).
In summary, two hybrids consisting of AN, fullerene and
FL were prepared in which AN, as an energy antenna
collects light in the range of 320–400 nm, transfers
its energy to reaction center FL. FL itself can absorb the
light in the visible region (400–600 nm), and transfers an
electron to C60, generating a charge separated state. This
model (AN–FL–C60) may find applications in the areas
such as artificial photosynthesis and solar energy con-
version. Detailed photophysical properties of the two
hybrids are under investigation.
Acknowledgments
The authors thank Natural Science Foundation of
China for financial support.
References and Notes
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Figure 1. Electronic absorption (A, B) and fluorescence (C, D) spectra
of 1, 2 and their model compounds 3, 4, and 5. (A) is for 1, 3, 4 and (B)
is for 2, 3, and 5. (C, D) are the fluorescence spectra of the related
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length (364or 460 nm). Both electronic and fluorescence spectra were
measured in CH3OH/CHCl3 (1:1) (2 · 10À5 M).
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Y.; Zhu, D. Synth. Commun. 2002, 32, 2507–2512.
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favorable pathway than electron transfer. Whereas
dyads AN–FL 4, 5 have strong fluorescence with either
364or 460 nm as excitation wavelength, emissions from
hybrids 1 and 2 are very weak. Also, the emissions of 1
and 2 (480–650 nm) were observed only from the FL,
without detectable emission from the AN (380–480 nm)
and C60 (680–750 nm) moieties. Furthermore, the weak
emission from C60 in model 3 was observed (680–
750 nm). Thus there is no evidence for the existence of
singlet–singlet energy transfer from ꢀ1FL to the C60, and
these results indicate that intramolecular energy transfer
from ꢀ1AN to FL is responsible for a lack of ANÕs

224
B. Jing, D. Zhu / Tetrahedron Letters 45 (2004) 221–224
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29. According to the Rehm–Weller equation: DG ¼ EoxðDÞÀ
EredðAÞ À E0À0 À C, DG is the free energy change of the
electron transfer process, EoxðDÞ is the ground oxidation
potential of donor, EredðAÞ is reduction potential of
acceptor, E0À0 is the zero–zero excitation energy (equal
to singlet energy ES or triplet energy ET) and C is ca. 0.06
in polar solvent. So DGðAN to ꢀ1FLÞ ¼ EoxðANÞÀ
EredðFLÞ À ESðFLÞ À 0:06 ¼ 0:95 À ðÀ1:2ÞÀ 2:42À0:06 ¼
À0:33
eV;
DGðꢀ1AN to FLÞ ¼ EoxðANÞ À EredðFLÞ
ÀESðANÞ À 0:06 ¼ 0:95 À ðÀ1:2Þ À 3:18 À 0:06 ¼ À1:09
eV; DGðꢀ1FL to C60Þ ¼ EoxðFLÞ ÀEredðC60Þ À ESðFLÞÀ
0:06 ¼ 0:90 À ðÀ0:61Þ À 2:42 À 0:06 ¼ À0:97 eV.
The
photophysical and electrochemical data about FL,
ANCO2Et, and C60 are summarized in the following table:
Eox (eV) Ered (eV) ES (eV)
ET (eV)
FLa
0.90
0.95
––
)1.20
––
)0.61,
)1.00,
)1.46,
)1.74
2.42
3.18
51.741.4
1.94
1.85
ANCO2Eta
b
C60
a
Cited from Refs. 26 and 28.
Cited from Ref. 1.
b