Fluorescein-fullerene dyads: A new kind of fullerene dyad - Synthesis and their photophysical properties

Article abstract of DOI:10.1016/S0040-4039(00)01523-9

Two novel fluorescein-C₆₀ dyads have been synthesized. Fluorescence quenching in the dyads indicates a photoinduced intramolecular electron transfer from the fluorescein to the C₆₀ moiety. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01523-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 8559–8563
Fluorescein–fullerene dyads: a new kind of fullerene
dyad—synthesis and their photophysical properties
Bingwen Jing, Deqing Zhang* and Daoben Zhu
Organic Solids Laboratory, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100080, PR China
Received 22 June 2000; revised 30 August 2000; accepted 7 September 2000
Abstract
Two novel fluorescein–C60 dyads have been synthesized. Fluorescence quenching in the dyads indicates
a photoinduced intramolecular electron transfer from the fluorescein to the C60 moiety. © 2000 Elsevier
Science Ltd. All rights reserved.
Keywords: fluorescein; fullerene; photoinduced electron transfer.
Over the past decade, a great variety of dyads and more complex polyads consisting of
electron donors and acceptors have been designed to investigate photoinduced energy and
electron transfer (ET) processes and to mimic natural photosynthesis.1 Fullerenes as novel
electron acceptors, in particular the readily available C60, present a wide range of chemical and
physical properties that make them promising chromophores in photoinduced redox processes.2
Many investigations show that C60 is a good electron acceptor. One of the most remarkable
properties of C60 related to electron transfer phenomena is that it can efficiently induce a rapid
charge separation and a further slow charge recombination. These peculiar photophysical
properties are due to the small reorganization energies of C60 in ET.3 Therefore, a wide variety
of C60-donor dyads in which C60 and a donor unit are covalently attached have been recently
synthesized for investigations of their photoinduced electron transfer behavior.2 The donors in
these C60-donor dyads include porphyrins,3–10 TTFs,11,12 polypyridine ruthenium(II) complexes,13
N,N-dialkylaniline,14 ferrocene15 and carotenoid.16 Among these, the systems consisting of
porphyrins and C60 are the most prominent and have been extensively studied. However, to the
best of our knowledge, no reports concerning fluorescein (FL)–C60 systems have appeared so far,
although FL is a very important xanthene dye, which has many technical applications17 due to
its high fluorescence quantum yield, excellent redox properties as well as relatively large
* Corresponding author. E-mail: dqzhang@infoc3.icas.ac.cn
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01523-9

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extinction coefficients at visible wavelengths. Also, FL contains two active groups—the hydroxyl
and the carboxyl groups, which can be used to link FL to different electron donors or acceptors.
Thus, FL was frequently used as electron donor/acceptor and sensitizer in artificial photosyn-
thetic models.18 Based on these considerations, the FL–C60 system may be a good model for the
study of artificial photosynthesis. In order to investigate this possibility, we have synthesized two
dyads 7 and 8, consisting of a FL covalently linked to a fullerene, as well as model compounds
2 and 9.
The synthesis was carried out as outlined in Scheme 1. The synthetic approach to compounds
7–9 relies upon the 1,3 dipolar cycloaddition of azomethine ylides to C60. This methodology has
proven to be one of the most powerful procedures for the functionalization of C60 due to its
versatility and the ready availability of the starting materials. Therefore, reaction of aldehydes
5, 6 and p-anisaldehyde (0.1 mmol) with N-methylglycine (32 mg, 0.36 mmol) and C60 (72 mg,
0.01 mmol) in toluene (100 ml) under Ar at reflux for 8–12 hours, gave the corresponding
fulleropyrolidines 7–9 after column chromatographic purification (silica gel, toluene/methanol,
50:1). The aldehydes 5, 6 and dibutyl fluorescein 2 were prepared by alkylation of FL using the
corresponding alkylbromide in DMF at 90°C for 2–3 hours with anhydrous potassium carbon-
ate as base. The structures of all these compounds were verified by spectroscopic analyses.19
Scheme 1. (i) 1-Bromobutane, anhydrous K2CO3, DMF, 90°C for 2–3 hrs, 76%; (ii) 4-(2-bromoethoxyl)-benzalde-
hyde, ibid., 75%; (iii) 1-bromobutane, ibid., 85%; (iv) 4-(2-bromoethoxyl)-benzaldehyde, ibid., 71%; (v) 1-bromotane,
ibid., 77%; (vi) N-methylglycine, Ar, toluene, reflux for 8–12 hrs, 30%; (vii) ibid., 34%; (viii) ibid., 28%

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The UV–vis spectra of the dyads 7 and 8 (Fig. 1) consist simply of a superposition of the
absorption spectra of the C60 and FL subunits, indicating that there is no appreciable interaction
between the FL and C60 in the ground state. However, the steady-state fluorescence spectra of
7 and 8 (Fig. 2), taken in chloroform with the same concentration and excited at 460 nm where
Figure 1. Electronic absorption spectra of 2 (dash), 7 (dot), 8 (dash dot) and 9 (solid) in CHCl3 (2×10−5 M)
Figure 2. Fluorescence spectra of 2, 7 and 8 in CHCl3 (2×10−5 M) excited at 460 nm

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the FL part mainly absorbs, show a significant difference compared with those of 2, 5 and 6.
Whereas with 2, 5 and 6 a strong emission could be observed, the emissions of 7 and 8 are very
weak, showing rapid quenching of the excited singlet state of FL by C60. Besides, the emissions
of 7 and 8 (480–650 nm) were observed only from the FL, without detectable emission from the
C60 (680–750 nm), while the emission from C60 in model 9 was observed (680–750 nm). This
1
shows there is no evidence for the existence of singlet-singlet energy transfer from FL* to the
C60. These results imply that intramolecular photoinduced electron transfer from the FL to C60
is a main pathway for the emission quenching in chloroform. The detailed photophysical
properties of the two dyads, such as their fluorescence lifetimes and the rates of formation and
lifetime of charge separation state are under investigation.
Acknowledgements
The authors thank the CAS for the financial support (KJ-951-A1-50103).
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Becher, J. J. Chem. Soc., Perkin Trans. 2 1999, 657–665.
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Am. Chem. Soc. 1997, 119, 1400–1405.
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19. Spectroscopic data for 7: 1H NMR (300 MHz, CDCl3): l 8.28 (1H, d), 7.76–7.68 (4H, m), 7.25 (1H, d), 7.20–6.92
(5H, m), 6.90–6.65 (3H, m), 5.04–5.01 (2H, d, unresolved, 1H of CHAH 1H of CH in pyrrolidine), 4.47–4.25
+
(5H, m, 1H of CHAHB in pyrrolidine+OCH2CH2O), 3.98 (2H, m, ꢀCOOBCH2ꢀ), 2.84 (3H, s, ꢀNCH3), 1.38–1.28
(2H, m, ꢀCH2CH2CH2CH3), 1.13–1.03 (2H, m, ꢀCH2CH2CH2CH3), 0.81–0.76 (3H, t, ꢀ(CH2)3CH3). 13C NMR
(300 MHz, CDCl3): l 165.2, 163.7, 158.8, 158.2, 154.6, 147.1, 146.1, 146.0, 145.9, 145.7, 145.3, 145.2, 145.1, 145.0,
144.9, 144.5, 144.2, 144.1, 142.9, 142.5, 142.4, 142.0, 141.8, 141.7, 141.5, 141.3, 139.9, 139.7, 139.6, 139.3, 136.7,
136.4, 135.7, 135.5, 133.7, 133.6, 132.5, 131.3, 131.2, 131.0, 130.9, 130.8, 130.4, 130.1, 129.7, 129.3, 129.2, 128.9,
128.8, 128.7, 128.6, 128.4, 128.0, 127.5, 117.5, 115.2, 114.6, 105.3, 100.6, 82.8, 77.5, 76.4, 69.5, 68.6, 67.4, 65.4,
39.8, 30.0, 18.9, 13.4; FT-IR w/cm−1, 2953, 1720, 1642, 1597, 1510, 1448, 1377, 1285, 1247, 1206, 1105, 853, 758,

8563
527; MALDI-TOF m/z 1283 (M+). Spectroscopic data for 8: 1H NMR (300 MHz, CDCl3): l 8.32 (1H, d),
7.91–7.60 (4H, m), 7.25 (1H, d), 6.91–6.80 (2H, m), 6.72–6.64 (3H, m), 6.58–6.01 (3H, m), 5.03–4.94 (2H, m, 1H
of CHAH
1H of CH in pyrrolidine), 4.30–4.20 (3H, m, 1H of CHAHB in pyrrolidine and
+
COOCH2CBH2OꢀC6H4ꢀ), 4.05–3.70 (2H, m, ꢀOCH2C3H7), 3.75–3.52 (2H, m, ꢀCOOCH2CH2OꢀC6H4ꢀ), 2.83 (3H,
s, NCH3), 1.90–1.70 (2H, m, ꢀOCH2CH2C2H5), 1.60–1.40 (2H, m, ꢀOCH2CH2CH2CH3) 1.05–0.96 (3H, m,
OC3H6CH3); 13C NMR (300 MHz, CDCl3): l 165.2, 158.8, 158.7, 158.1, 156.3, 154.6, 154.5, 154.0, 153.9, 153.7,
153.5, 147.3, 146.5, 146.3, 146.2, 146.1, 145.9, 145.5, 145.4, 145.3, 145.2, 145.1, 144.7, 144.4, 143.2, 143.0, 142.7,
142.6, 142.2, 142.1, 142.0, 141.6, 140.2, 139.6, 136.4, 135.9, 135.8, 134.2, 132.9, 131.8, 130.4, 129.8, 129.5, 129.3,
129.0, 117.4, 114.8, 114.5, 105.5, 100.7, 100.6, 83.0, 77.3, 77.2, 69.9, 68.9, 65.2, 63.8, 40.0, 30.9, 19.2, 13.9; FT-IR
w/cm−1 2953, 1725, 1642, 1596, 1510, 1376, 1282, 1250, 1207, 1179, 1106, 853, 758, 527; MALDI-TOF m/z 1283
(M+). Spectroscopic data for 9: H NMR (300 MHz, CS2): l 7.87 (2H, d, H m to methyl in phenyl), 7.10 (2H,
1
d, H o to methyl in phenyl), 5.15 (1H, d, 1H of CHAHB in pyrrolidine), 5.09 (1H, s, CH in pyrrolidine), 4.48 (1H,
d, 1H of CHAHB in pyrrolidine), 4.02 (3H, s, ꢀOCH3), 3.03 (3H, s, ꢀNCH3); 13C NMR (300 MHz, CS2): 153.9,
147.5, 147.2, 146.9, 146.7, 146.5, 146.4, 146.2, 146.0, 145.9, 145.7, 145.6, 145.5, 145.4, 145.3, 145.2, 145.1, 144.9,
144.7, 144.3, 143.4, 143.3, 142.9, 142, 7, 142.5, 142.4, 142.4, 142.0, 141.9, 141.8, 141.6, 141.2, 140.5, 139.9, 137.0,
136.1, 135.0, 130.8, 128.6, 114.2 (2), 83.1, 77.7, 70.0, 69.8, 64.3, 40.0; FT-IR w/cm−1 2778, 1611, 1511, 1643, 1332,
1302, 1249, 1179, 1107, 1035, 841, 832, 765, 527; MALDI-TOF m/z 883 (M+).
.