The importance of the linking bridge in donor-C60 electroactive dyads

Article abstract of DOI:10.1039/b106012k

The synthesis, spectroscopic characterization, electrochemistry and photophysical measurements of three 2-methoxyphenylfullerene derivatives are described and their properties compared. Cyclic and Osteryoung square-wave voltammetry studies indicate that t

Full text of DOI:10.1039/b106012k

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The importance of the linking bridge in donor–C60 electroactive dyads
Fernando Langa,*a Pilar de la Cruz,a Juan L. Delgado,a Marıa J . Gomez-Escalonilla,a
Araceli Gonzalez-Cortes,b Antonio de la Hozc and Vicente Lopez-Arzad
a
Facultad de Ciencias del Medio Ambiente, Universidad de Castilla-La Mancha, 45071 Toledo,
Spain. E-mail: flanga@amb-to.uclm.es; Fax: þ 34 9 2526 8840; Tel: þ 34 9 2526 8843
b
Facultad de Qu´ımica, Universidad Complutense, 28040 Madrid, Spain
Facultad de Ciencias Qu´ımicas, Universidad de Castilla-La Mancha,
c
45071 Ciudad Real, Spain
Escuela de Ingenieros Te´cnicos Industriales, Universidad de Castilla-La Mancha,
45071 Toledo, Spain
d
Received (in Montpellier, France) 6th July 2001, Accepted 17th September 2001
First published as an Advance Article on the web 20th December 2001
The synthesis, spectroscopic characterization, electrochemistry and photophysical measurements of three 2-
methoxyphenylfullerene derivatives are described and their properties compared. Cyclic and Osteryoung square-
wave voltammetry studies indicate that the pyrazolino[60]fullerene shows improved electron affinity in comparison
to the parent C60 . Preliminary photophysical studies suggest the occurrence of an electron transfer process.
The dramatic increase in research into fullerenes since 1991
continues unabated1 and intense effort is nowfocused on new
fullerene-based materials and devices.2 Due to their unique
electrochemical3 and photophysical properties,4 the design of
covalently linked donor–[60]fullerene systems that are capable
of undergoing photoinduced electron transfer5 has been
extensively studied in recent years. C60 is an excellent electron
acceptor, as demonstrated by cyclic voltammetry experiments
in which six reversible one-electron reduction waves were
observed.6 However, the majority of C60 derivatives have a
lower electronegativity than the parent system and attempts to
increase the electron affinity of C60 have only met with partial
success. Indeed, only a fewexamples of derivatives that exhibit
better reduction potentials than the parent C60 have been
described.7 An improvement in the electron-accepting prop-
erties of fullerene derivatives may therefore lead to more effi-
cient behavior in charge transfer processes.8
first developed by Prato, has for many years been the most
widely used approach to link a donor to the fullerene cage.9
The popularity of this method is due to the simplicity of the
procedure and the good yields of isolated products. However,
derivatization of C60 through this spacer causes a decrease in
the electron affinity of the C60 sphere and leads to a dis-
placement of reduction potentials to more negative values (by
about 150 mV).10 In this respect, the inductive effect of organic
groups attached to the C60 core has proven to be the most
important factor in determining the redox properties of
organofullerenes. Indeed, we11 and others12 have shown that
isoxazolo[60]fullerenes (B) are similar, or even better, acceptor
systems than the parent C60 , depending on the nature of the
organic addend. These fullerene derivatives can be easily syn-
thesized by 1,3-dipolar cycloaddition reactions involving nitrile
oxides, which can in turn be prepared from oximes. In con-
trast, the functionalization of C60 with 1,3-nitrile imines to give
pyrazolino[60]fullerenes (C) has not been widely exploited.13
We recently described the facile one-pot synthesis of such
pyrazolyl-pyrazolino[60]fullerene systems from the corre-
sponding hydrazones under microwave irradiation.14
We decided to compare the electrochemical and photo-
physical properties of cycloadducts formed by the incorpora-
tion of the same substituent into the molecule by different
approaches. The work reported here describes the synthesis of
three 2-methoxyphenylfullerene derivatives: the pyrrolidino-
[60]fullerene 1, the isoxazolo[60]fullerene 2 and the pyr-
azolino[60]fullerene 3. The spectroscopic and electrochemical
properties of these donor–C60 derivatives are also described in
order to assess howthe spacer affects the properties of the
molecule when a weak donor, such as the 2-methoxy group, is
incorporated within the system.
Results and discussion
The syntheses of the target molecules 1, 2 and 3 were per-
formed according to Scheme 1. Pyrrolidino[60]fullerene 1 was
prepared by a 1,3-dipolar cycloaddition reaction between the
appropriate azomethine ylide (generated in situ from glycine
The 1,3-dipolar cycloaddition between azomethine ylides
and C60 to afford pyrrolidino[60]fullerenes (A), a technique
76
New J. Chem., 2002, 26, 76–80
DOI: 10.1039/b106012k
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Scheme 1 (i) C60 , H2NCH2COOH, toluene, microwave irradiation, (ii.a) H2NOH Á HCl, CH3COONa, EtOH, H2O; (ii.b) NBS, C60 , Et3N,
benzene, microwave irradiation; (iii.a) 4-NO2–C6H4–NHNH2 , EtOH, AcOH, reflux; (iii.b) NBS–benzene, C60 , Et3N, microwave irradiation.
and 2-methoxybenzaldehyde) and C60 according to the
well-established procedure first reported by Prato et al.15 The
cycloaddition proceeded in moderate yield (35% overall, 92%
based on reacted C60) by reaction of 2-methoxybenzaldehyde
with glycine and [60]fullerene (Scheme 1, path i). Isox-
azolo[60]fullerene 2 was synthesized (32% overall yield, 75%
based on reacted C60) in one pot by reacting 2-methoxy-
benzaldoxime (4; prepared in 81% yield by the reaction of 2-
methoxybenzaldehyde with hydroxylamine hydrochloride;
Scheme 1, path ii.a) with NBS in the presence of Et3N. The
resulting product was reacted in situ with C60 under focused
microwave irradiation16 according to our previously described
procedure (Scheme 1, path ii.b).11a Finally, pyr-
azolino[60]fullerene 3 was prepared by a 1,3-dipolar cycload-
dition between the corresponding nitrile imine and C60 under
microwave irradiation according to our previously published
method for other pyrazolino derivatives.14 Firstly, 4-nitro-
phenylhydrazone (5; prepared in 80% yield by reaction of 2-
methoxybenzaldehyde with 4-nitrophenylhydrazine; Scheme 1,
path iii.a) was reacted with NBS in benzene. Et3N and C60 were
then added to the reaction mixture, which was irradiated for 40
min (210 W power in a focused microwave reactor) to afford 3
(Scheme 1, path iii.b) in 18% yield (82% based on reacted C60).
In all cases, cycloadducts were purified by column chromato-
graphy (silica gel, toluene–chloroform 2 : 1 for 1 and toluene–
hexane 9 : 1 and 2 : 1, respectively, for 2 and 3), followed by
centrifugation in hexane, methanol and diethyl ether.
wave voltammetry (OSWV) at room temperature. The samples
were dissolved in 1,2-dichlorobenzene–acetonitrile (4 : 1) and
tetrabutylammonium perchlorate (TBAP) was used as the
supporting electrolyte. The results of the OSWV studies are
summarized in Table 1 and Fig. 1 along with, for the sake of
comparison, the data for C60 itself.
The cyclic voltammogram of pyrrolidino[60]fullerene 1
shows four reversible reduction peaks and retains the electro-
chemical pattern of C60 . However, as previously described for
other fullereno derivatives, the reduction potentials are shifted
to more negative values when compared to those of C60 . In
contrast, a different trend is found for isoxazolo[60]fullerene 2.
In this case, the reduction potentials are slightly shifted to
more positive values in comparison to the parent C60 and this
behavior is similar to that described recently, by ourselves and
others, for this kind of fullerene derivative.11,12
Finally, pyrazolo[60]fullerene
3 shows four reversible
reduction potentials, which are assigned to the C60 cage, as
well as a fifth reduction potential situated between the second
and the third potentials of C60 . This fifth potential is assigned
to the reduction of the nitrophenyl group. Interestingly,
compound 3 shows an anodic shift relative to both C60 and
a
Table 1 Redox potentials (OSWV) of organofullerenes 1–3 and C60
E1red
E2red
E3red
E4red
1
The novel compounds 1, 2 and 3 were characterized by H
NMR, 13C NMR, FT-IR, and UV-vis spectroscopy as well as
by FAB-MS. The FAB mass spectra of adducts 1, 2 and 3
C60
1
2
À0.99
À1.10
À0.97
À0.95
À1.42
À1.51
À1.37
À1.38
(À1.64)
À1.87
À2.04
À1.88
À1.80
À2.41
À2.36
À2.36
À2.30
þ
showthe MH
peaks together with several fragmentation
3
peaks. All of the organofullerenes synthesized showed a weak
absorption band at around 430 nm in their UV-vis spectra,
which is a typical feature of 1,2-dihydrofullerenes.
The electrochemical properties of compounds 1–3 were
studied by cyclic voltammetry (CV) and Osteryoung square-
a
V vs. Ag=AgNO3 ; GCE as working electrode; 0.1 M TBAP; ODCB–
.
MeCN (4 : 1); scan rate: 100 mV sÀ1
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Fig. 2 Fluorescence spectra of compounds 1–3 and 6 in (a) toluene
and (b) benzonitrile.
When benzonitrile was used as the solvent [Table 2,
Fig. 2(b)], the maximum in 1 was red-shifted to 722 nm and the
emission intensity remained practically unaffected (95%) with
respect to that found in toluene (considered as 100%). The
reference compound 6 shows the same behavior (relative
intensity 96%). Previous studies on the photochemical prop-
erties of 6 indicated that the pyrrolidine ring nitrogen is not
involved in charge transfer17 and this fact explains why com-
pound 1 shows similar emission behavior in toluene and
benzonitrile. A similar red shift, with the maximum at 710 nm,
was observed for compound 2 and a slight decrease in the
fluorescence was observed, being 85% with respect to that
found in toluene. In contrast, the maximum for compound 3
was blue-shifted to 705 nm in benzonitrile. In addition to this
blue shift, a substantially lower fluorescence intensity (60%
with respect to that in toluene) was observed.
The quenching of the fluorescence in pyrazolino[60]fullerene
3 as the polarity of the solvent is increased, suggests the
occurrence of an electron transfer (ET) process from the lone
pair of the sp3 nitrogen atom to the C60 cage in benzonitrile.
Such a process should be facilitated by the close proximity of
these two moieties. Sun et al. observed a similar effect in a
number of aminofullerene derivatives.17
Fig. 1 Osteryoung square-wave voltammograms of C60 (a), 1 (b), 2
(c) and 3 (d).
compound 2. The inductive effect of the nitrogen atom, which
is close to the C60 cage, is thought to be responsible for this
improved electron affinity.
The fluorescence spectra of fullerene derivatives 1, 2, 3 and,
for the sake of comparison, the model N-methylpyrrolidino-
[60]fullerene (6),15 were measured at room temperature in
toluene and benzonitrile with excitation at 430 nm (Table 2,
Fig. 2). Solutions with the same absorbance were employed so
that the fluorescence intensity and the quantum yields could be
correlated.
When toluene was used as the solvent [Fig. 2(a), Table 2], the
reference compound 6 presented a maximum at 717 nm and
while compound 1 showed a maximum at 718 nm, which is in the
expected region for this type of system.8 The emission of 1 shows
a similar intensity to that of the model compound 6, indicating
that 1 does not undergo photoinduced charge transfer under
these conditions. On using solutions with the same absorbance,
the fluorescence intensities were also found to be similar, indi-
cating that the quantum yield should be almost equal.
A similar pattern was observed for the isoxazolo dyad 2 but
the intensity of the emission was slightly lower and the max-
imum appeared at 706 nm. Finally, pyrazolino[60]fullerene 3
showed a maximum at 709 nm and a stronger reduction in the
fluorescence emission with respect to the model compound 6
was observed [see Fig. 2(a) and Table 2].
Addition of TFA to benzonitrile solutions of 1 and 3 (ca.
10À7 M) produces opposite effects: for compound 1, the
addition of TFA ([TFA]=[1] ¼ 530) caused a weak quenching
of the fluorescence (by 30%) and the maximum was blue-
shifted from 723 to 713 nm [Fig. 3(b)]. This phenomenon can
be explained in terms of the formation of the pyrrolidinium
salt of 1 (pyrrolidinium salts have shown better electron affi-
7a
nities than C60 and ET from donors in donor–pyrrolidi-
nium[60]fullerene salts has been observed)8 and the possibility
Table 2 Fluorescence data of compounds 1, 2, 3 and 6
l=nm (Rel. int.)
Toluene
Benzonitrile
1
2
3
6
718 (0.462)
706 (0.372)
709 (0.307)
717 (0.481)
722 (0.438)
710 (0.320)
705 (0.205)
722 (0.462)
Fig. 3 Fluorescence spectra of a solution of (a) 3 and (b) 1 upon
addition of increasing amounts of trifluoroacetic acid: (——) 0 mL,
(------) þ 0.5 mL ( þ 6.5 Â 10À6 equiv.), ( Á Á Á Á Á ) þ 1 mL ( þ 1.3 Â 10À5
equiv.).
78
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voltammetry measurements were carried out using a BAS 100
potentiostat with a BAS MF-2062 Ag=0.01 M AgNO3 , 0.1 M
TBAP in MeCN reference electrode, an auxiliary electrode
consisting of a Pt wire and a Metrohm 6.0805.010 conven-
tional glassy carbon electrode (3 mm o.d.) as the working
electrode, which was directly immersed in the solution. A 10
mL electrochemical cell from BAS (model VC-2) was also
used. The reference potential was shifted by 290 mV towards a
more negative potential compared with the Ag=AgCl scale.
E1=2 values were taken as the average of the anodic and
cathodic peak potentials. The scan rate used was 100 mV sÀ1
.
Syntheses
2-Methoxybenzaldoxime (4). A solution of 2-methoxybenz-
aldehyde (1 g, 7 mmol) in ethanol (5 mL) was added to a
solution of hydroxylamine hydrochloride (0.647 g, 7 mmol)
and sodium acetate (0.574 g, 7 mmol) in water (5 mL). The
solution was heated under reflux for 10 min. The crude pro-
duct was purified by recrystallization from ethanol to afford
the product 4 in 81% yield. FT-IR (KBr) n=cmÀ1 3168.1,
2870.5, 1627.4, 1599.7, 1496.2, 1465.1, 1439.0, 1320.2, 1304.9,
1258.4, 1199.3, 1171.9, 1111.3, 1051.1, 972.1, 949.1, 875.4,
786.4; 1H NMR (CDCl3) d 3.88 (s, 3H), 6.95 (dd, J ¼ 8.1,
J ¼ 7.7 Hz, 2H), 7.35 (t, J ¼ 8.1 Hz, 1H), 7.67 (d, J ¼ 7.7 Hz,
1H), 8.49 (s, 1H); 13C NMR (CDCl3) d 157.8, 146.9, 131.4,
127.4, 120.6, 111.3, 55.7; Anal. calc. for C8H9NO2 : C, 63.56,
H, 6.00, N, 9.27; found: C, 63.50, H, 5.99, N, 9.15%.
Scheme 2 Proposed mechanism for the quenching of the ET process
in compound 3.
that a weak ET process from the 2-methoxyphenyl moiety can
occur (Scheme 2). On the other hand, addition of TFA to a
solution of 3 ([TFA]=[3] ¼ 450) in benzonitrile resulted in a
slight increase in the fluorescence [Fig. 3(a)], which suggests
quenching of the ET process by virtue of the fact that the
nitrogen lone pair is no longer available (Scheme 2). The
addition of base (pyridine) reversed this process and quenching
of the fluorescence was observed once again.
3-(2-Methoxyphenyl)-1,N-(4-nitrophenyl)hydrazone (5). A so-
lution of 2-methoxybenzaldehyde (1 g, 7 mmol), 4-nitrophe-
nylhydrazine (1.124 g, 7 mmol) and two drops of acetic acid in
ethanol (10 mL) was heated under reflux for 10 min. The crude
product was purified by recrystallization from ethanol to
afford compound 5 in 80% yield. FT-IR (KBr) n=cmÀ1 3270.0,
1586.7, 1493.9, 1460.8, 1314.9, 1235.4, 1102.8, 1016.6, 824.3,
744.7, 697.7, 532.6; 1H NMR (CDCl3) d 3.88 (s, 3H), 6.93
(d, J ¼ 7.7 Hz, 1H), 7.02 (dd, J ¼ 7.7, J ¼ 7.3 Hz, 1H), 7.11 (d,
J ¼ 9.1 Hz, 2H), 7.36 (d, J ¼ 8.1, J ¼ 7.3 Hz, 1H), 7.99
(d, J ¼ 8.1 Hz, 1H), 8.1 (br s, 1H), 8.18 (d, J ¼ 9.1 Hz, 2H),
8.25 (s, 1H); 13C NMR (CDCl3) d 157.6, 149.8, 137.8, 131.1,
126.5, 126.3, 122.8, 121.2, 111.8, 55.8; Anal. calc. for
C14H13N3O3: C, 61.99, H, 4.83, N, 15.49; found: C, 61.78, H,
5.06, N, 15.65%.
Conclusions
We have shown that the use of pyrazolino[60]fullerenes as
building blocks for donor–C60 systems has several advantages
over other fullereno derivatives: (i) they can be prepared in one
step from hydrazones by a simple procedure; (ii) the deriva-
tives showimproved electron affinities, even when compared to
C
60 ; (iii) the preliminary photophysical studies suggest that an
electron transfer process from the nitrogen atom to the C60
cage occurs in the pyrazolino[60]fullerene system.
20-(2-Methoxyphenyl)pyrrolidino[30,40 : 1,2][60]fullerene (1).
A solution of C60 (100 mg, 0.138 mmol), glycine (21 mg, 0.277
mmol) and 2-methoxybenzaldehyde (47 mg, 0.345 mmol) in
toluene (50 mL) was stirred under reflux for 18 h. The solvent
was removed in vacuo and the residue was purified by flash
chromatography on silica gel using a toluene–chloroform
(2 : 1) mixture as eluent to yield 42 mg of 1 (35%, 92.5% based
on reacted C60). Further purification of the solid was achieved
by centrifuging three times with methanol and once with di-
ethyl ether. FT-IR (KBr) n=cmÀ1 2919.3, 2839.8, 1633.9,
1460.8, 744.7, 526.0; 1H NMR (CDCl3) d 3.82 (s, 3H), 4.87 (d,
J ¼ 11.3 Hz, 1H), 5.12 (d, J ¼ 11.3 Hz, 1H), 6.08 (s, 1H), 6.95
(d, J ¼ 8.06 Hz, 1H), 7.06 (dd, J ¼ 7.33, J ¼ 7.33 Hz, 1H), 7.33
(dd, J ¼ 8.06, J ¼ 8.06 Hz, 1H), 7.72 (d, J ¼ 7.33 Hz, 1H);
13C NMR (CDCl3–CS2 , 1 : 1) d 157.8, 156.7, 154.4, 147.3,
146.4, 146.2, 145.5, 144.6, 142.8, 142.4, 142.3, 140.4, 136.2,
134.4, 131.1, 129.8, 128.5, 125.5, 121.4, 111.6, 63.0, 55.3; UV-
vis (CH2Cl2) lmax=nm (log e) 705.0 (3.1), 662.5 (3.1), 431.0
Experimental
General
All cycloaddition reactions were performed under argon.
Reactions under microwave irradiation were carried out in a
focused reactor—Maxidigest MX-350 from Prolabo—equip-
ped with an infrared temperature detector. The irradiation
power and temperature were controlled with the program
MPX-2 from PACAM. C60 was purchased from MER Cor-
poration (Tucson, AZ) and the other starting materials were
purchased from ACROS. Cycloaddition reactions were mon-
itored by TLC using Merck silica gel 60-F254 .
1H and 13C,
NMR spectra were recorded on a Varian Mercury 200 spec-
trometer. UV-vis absorption spectra were obtained using a
Shimadzu spectrophotometer. FT-IR spectra were recorded on
a Nicolet Impact 410 spectrophotometer using KBr disks.
Fluorescence spectra were obtained on a JASCO FP-750
spectrophotometer.
(3.5), 308.0 (4.32), 256.5 (4.7); MS m=z 870 [M þ H]þ
,
720 [C60]þ
.
30-(2-Methoxyphenyl)isoxazolo[40,50 : 1,2][60]fullerene (2).
C60 (50 mg, 0.069 mmol) and Et3N (21 mg, 0.21 mmol) were
added to a stirred solution of anisaldoxime 4 (32 mg,
FAB mass spectra were obtained on a VG AutoSpec
instrument, using 3-nitrobenzyl alcohol as a matrix. Cyclic
New J. Chem., 2002, 26, 76–80
79

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0.21 mmol) and NBS (38 mg, 0.21 mmol) in benzene (45 mL).
The solution was irradiated for 10 min at 210 W power. The
product was purified by flash chromatography on silica gel
using a mixture of toluene–hexane (9 : 1) as eluent to yield 20
mg of 2 (32%, 75% based on reacted C60). Further purification
of the solid was achieved by centrifuging three times with
methanol and once with diethyl ether. FT-IR (KBr) n=cmÀ1
2919.3, 2839.8, 1525.4, 1235.4, 751.4, 526.0; 1H NMR (CDCl3)
d 3.82 (s, 3H), 7.05 (d, J ¼ 8.4 Hz, 1H), 7.13 (dd, J ¼ 7.7,
J=7.7 Hz, 1H) 7.51 (dd, J ¼ 8.4, J ¼ 7.7 Hz, 1H), 7.67 (d,
J ¼ 8.4 Hz, 1H); 13C NMR (CDCl3–CS2 , 1 : 1) d 149.8, 147.9,
147.3, 146.3, 145.9, 145.5, 144.4, 142.9, 142.5, 141.8, 140.6,
140.2, 137.1, 135.7, 132.2, 131.7, 121.1, 111.7, 55.4; UV-vis
(CH2Cl2) lmax=nm (log e) 686.0 (3.4), 457.0 (4.0), 425.0
(4.14), 316.0 (5.4), 255.5 (5.04); MS m=z 870 [M þ H]þ , 720
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mg, 0.21 mmol) and NBS (38 mg, 0.21 mmol) in benzene
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1
n=cmÀ1 2916.5, 2843.8, 1985.7, 1640.9, 784.6, 522.9; H NMR
(CDCl3) d 3.08 (s, 3H), 7.05 (d, J ¼ 8.4 Hz, 1H), 7.15 (dd,
J ¼ 7.3, J ¼ 6.8 Hz, 1H), 7.52 (dd, J ¼ 6.8, J ¼ 8.4 Hz, 1H),
7.67 (d, J ¼ 7.3 Hz, 1H), 8.24 (d, J ¼ 9.5 Hz, 1H), 8.31 (d,
J ¼ 9.5 Hz, 1H); 13C NMR (CDCl3–CS2 , 1 : 1) d 158.4, 150.1,
147.7, 142.1, 141.9, 137.3, 135.0, 132.0, 131.9, 125.4, 120.9,
118.9, 111.7, 55.4; UV-vis (CH2Cl2) lmax=nm (log e) 689.0
(2.7), 426 8 (3.3), 324.0 (4.1), 256.5 (4.7); MS m=z 990
[M þ H]þ , 720 [C60]þ
.
15 N. Maggini, G. Scorrano and M. Prato, J. Am. Chem. Soc., 1993,
115, 9798.
16 A. de la Hoz, A. Dıaz-Ortız, A. Moreno and F. Langa, Eur. J.
Org. Chem., 2000, 3659.
Acknowledgement
Support of this work by the DGESIC of Spain (PB 97-0429) is
gratefully acknowledged.
17 Y. P. Sun, B. Ma and C. E. Bunker, J. Phys. Chem. A, 1998, 102,
7580.
80
New J. Chem., 2002, 26, 76–80