Synthesis, cyclic voltammetry, and photophysical properties of a bridged o-phenylenediamine-C60 dyad

Article abstract of DOI:10.1021/ol005992s

The synthesis, complete spectroscopic characterization, cyclic voltammetry, and photophysical measurements of a new o-phenylenediamine-C₆₀ dyad are described. By using a tether strategy, only a single regioisomer was obtained. Cyclic voltammetr

Full text of DOI:10.1021/ol005992s

ORGANIC
LETTERS
2000
Vol. 2, No. 18
2741-2744
Synthesis, Cyclic Voltammetry, and
Photophysical Properties of a Bridged
o-Phenylenediamine−C Dyad
60
,†
,‡
Michael Diekers, Andreas Hirsch,* Chuping Luo,‡ Dirk M. Guldi,*
Klaus Bauer, and Ulrich Nickel*
,§
Institut fu¨r Organische Chemie, Henkestrasse 42, D-91054 Erlangen, Germany,
Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556, and
Institut fu¨r Physikalische Chemie, Egerlandstrasse 3, D-91058 Erlangen, Germany
hirsch@organik.uni-erlangen.de
Received April 27, 2000
ABSTRACT
The synthesis, complete spectroscopic characterization, cyclic voltammetry, and photophysical measurements of a new o-phenylenediamine−
60 dyad are described. By using a tether strategy, only a single regioisomer was obtained. Cyclic voltammetry measurements indicate that
C
the two electroactive groups do not interact in their singlet ground states. Photophysical investigations reveal a rapid photoinduced electron
transfer between the singlet excited state of the fullerene acceptor and the o-phenylenediamine donor, yielding a charge-separated radical
pair.
Buckminsterfullerene C60 and its derivatives1 exhibit a
number of unique electronic and photophysical properties.
In particular, the low reduction potential of C60 (E1/2 ) -0.44
V versus SCE) and its singlet ground-state absorption renders
them interesting candidates for inter- and intramolecular
electron-transfer reactions.2 The low reorganization energy
of C60 in electron-transfer reactions stimulated the use of
fullerenes as a new building block for the design of
multicomponent arrays, which give rise to light-induced
electron-transfer processes and yield long-lived charge-
separated states. Furthermore, the recent advances in the
chemistry of fullerenes provide a powerful tool to change
parameters, which are essential to the efficiency of electron-
transfer processes. For example, the relative donor-acceptor
orientation as well as the distance between the chromophores
can be modulated systematically with relative ease. In this
context, a variety of donor and acceptor molecules have been
covalently attached to C60, and several remarkable electron-
transfer phenomena have been disclosed.
† Institut fu¨r Organische Chemie.
Other groups as well as our group have focused on
developing chinoid systems as acceptor moieties and por-
phyrins or o-phenylenediamines as sacrificial electron do-
nors.3 Recently, we have carried out extensive investigations
regarding the regiochemistry of multiple addition to C60 and
have unraveled several principles of fullerene regioselectiv-
ity.4
‡ University of Notre Dame.
§ Institut fu¨r Physikalische Chemie.
(1) (a) Hirsch, A. The Chemistry of the Fullerenes; Georg Thieme-
Verlag: Stuttgart, 1994. (b) Hirsch, A. Synthesis 1995, 895-913. (c)
Diederich, F.; Thilgen, C. Science 1996, 271, 317-323.
(2) (a) Imahori, H.; Sakata, Y. AdV. Mater. 1997, 9, 537-546. (b)
Imahori, H.; Sakata, Y. Eur. J. Org. Chem. 1999, 2445-2457. (c) Guldi,
D. M. Chem. Commun. 2000, 321-327. (d) Martin, N.; Sanchez, L.; Illescas,
B.; Perez, I. Chem. ReV. 1998, 98, 2527-2547, and references therein.
10.1021/ol005992s CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/09/2000

To develop a new C60-donor dyad with the o-phenylene-
diamine moiety as a strong donor, we used N,N ′-dimethyl-
o-phenylenediamine, 1, which is readily available from
o-phenylenediamine in three steps5 (Scheme 1). The alkyl-
technique.8 Formation of this hexaadduct pattern can only
be obtained starting from equatorial bisadducts, which, in
turn, leads to the conclusion that bisadduct 4 is an equatorial
isomer.
Compounds 3, 4, and 5 were studied electrochemically
using a glassy carbon electrode in a dichloromethane solution
(supporting electrolyte: TBAPF6) under argon. Well-resolved
and reversible cyclic voltammetric waves were observed in
all cases. The results were compared with those of the parent
C60 and the equatorial bisadduct e-C62(COOC2H4COOC4H9)4
6, carrying alkyl groups, as reference compounds (Table 1).
Scheme 1a
Table 1. E1/2 Values Measured as the Average of the Anodic
and Cathodic Peak Potentials vs the Potential for the Redox
Couple of Internally Added Fc/Fc+
compound
E1
E2
E3
E4
C60
6
-1.08
-1.21
-1.48
-1.58
3
4
5
+0.56
+0.65
+0.67
+0.20
+0.23
+0.21
-1.19
-1.77a
-1.57
a Irreversible.
The reduction potentials of the equatorial bisadduct 6
compared with those of dyad 4 and the oxidation potentials
of bismalonate 3 compared with those of dyad 4 and
hexaadduct 5 are quite similar. This suggests that no
electronic coupling prevails between the two electroactive
groups of dyad 4 in their singlet ground state.
The fluorescence quantum yields of the o-phenylenedi-
amine-C60 dyad 4 were measured in solvents of different
polarity and compared with that of the corresponding
equatorial reference isomer (Φ ) 4.2 × 10-4). Two dominant
features are derived from the data in Table 2: First a strong
quenching of the fluorescence is noted in all the solvents
(3) (a) Liddell, P. A.; Sumida, J. P.; Macpherson, A. N.; Noss, L.; Seely,
G. R.; Clark, K. N.; Moore, A. L.; Moore, T. A.; Gust, D. Photochem.
Photobiol. 1994, 60, 537-541. (b) Imahori, H.; Hagiwara, K.; Aoki, M.;
Akiyama, T.; Taniguchi, S.; Okada, T.; Shirakawa, M.; Sakata, Y. J. Am.
Chem. Soc. 1996, 118, 11771-11782. (c) Lawson, J. M.; Oliver, A. M.;
Rothenfluh, D. F.; An, Y.-Z.; Ellis, G. A.; Ranasinghe, M. G.; Khan, S. I.;
Franz, A. G.; Ganapathi, P. S.; Shephard, M. J.; Paddon-Row: M. N.; Rubin,
I. J. Org. Chem. 1996, 61, 5032-5054. (d) Liddell, P. A.; Kuciauskas, D.;
Sumida, J. P.; Nash, B.; Nguyen, D.; Moore, A. L.; Moore, T. A.; Gust, D.
J. Am. Chem. Soc. 1997, 119, 1400-1405. (e) Cheng, P.; Wilson, R.;
Schuster, D. I. Chem. Commun. 1999, 89-90. (f) Diekers, M.; Hirsch, A.;
Pyo, S.; Rivera, J.; Echegoyen, L. Eur. J. Org. Chem. 1998, 1111-1121.
(g) Dietel, E.; Hirsch, A.; Zhou, J.; Rieker, A. J. Chem. Soc., Perkin Trans.
2 1998, 1357-1364. (h) Dietel, E.; Hirsch, A.; Eichhorn, E.; Rieker, A.;
Hackbarth, S.; Ro¨der, B. Chem. Commun. 1998, 1981-1982. (i) Bourgeois,
J.-P.; Diederich, F.; Echegoyen, L.; Nierengarten, J.-F. HelV. Chim. Acta
1998, 81, 1835-1844. (j) Camps, X.; Dietel, E.; Hirsch, A.; Pyo, S.;
Echegoyen, L.; Hackbarth, S.; Ro¨der, B. Chem. Eur. J. 1999, 5, 2362-
2373. (k) Guldi, D. M.; Chuping, L.; Prato, M.; Dietel, E.; Hirsch, A. Chem.
Commun. 2000, 373-374. (l) Guldi, D. M.; Chuping, L.; da Ros, T.; Prato,
M.; Dietel, E.; Hirsch, A. Chem. Commun. 2000, 375-376.
a (a) Cl(CH2)3OH, CaCO3, H2O, 100 °C; (b) ClCOCH2COOCH3,
CH2Cl2; (c) C60, CBr4, DBU, toluene; (d) dimethylanthracene,
diethyl bromomalonate, DBU, toluene.
ation of the amino groups leads to diol 2,6 which was
converted into the corresponding bismalonate 3 and serves
as a bifunctional addend for a subsequent tether-function-
alization. The nucleophilic cyclopropanation governed the
reaction between bismalonate 3 and C60. Only the equatorial
isomer 4 was formed upon bisaddition. Bisadduct 4 was
isolated by flash chromatography (SiO2, toluene:triethylamine
19:1 v/v) followed by preparative HPLC (grom-sil amino,
toluene). The NMR and UV spectra of 4 reveal the
characteristics of an equatorial adduct.7 Additional proof for
the equatorial structure of bisadduct 4 stems from the follow-
up synthesis of hexaadduct 5, using our template mediation
(4) (a) Hirsch, A. Top Curr. Chem. 1999, 199, 1-65. (b) Diederich, F.;
Kessinger, R. Acc. Chem. Res. 1999, 32, 537-545.
(5) Cheeseman, G. W. H. J. Chem. Soc. 1955, 3308-3310.
(6) Kyba, E. P.; Davis, R. E.; Hudson, C. W.; John, A. M.; Brown, S.
B.; McPaul, M. J.; Liu, L.-K.; Glover, A. C. J. Am. Chem. Soc. 1981, 103,
3838-3875.
2742
Org. Lett., Vol. 2, No. 18, 2000

Table 2. Photophysical Properties of o-Phenylenediamine-C60 Dyad 4 in Various Solvents at rta
fluorescence
lifetime (τ)
lifetime (τ)
quantum yield
solvent
toluene
1-chlorobutane
THF
benzonitrile
dimethylformamide
ꢀ
quantum yield (Φ)
singlet excited state
radical pair
charge separation (Φ)
2.39
7.39
7.6
24.8
36.7
3.6 × 10-5
1.8 × 10-5
8.5 × 10-6
7.4 × 10-6
5.9 × 10-6
383 ps
144 ps
142 ps
110 ps
78 ps
b
c
620 ns
447 ns
683 ns
822 ns
0.21
0.48
0.21
0.13
a Excitation wavelength: 337 nm. b τ < 50 ns. c No measurable electron-transfer products on the nanosecond time scale.
investigated. Far more important is the second observation,
namely, the solvent dependence, giving rise to a marked
increase of the quenching with increasing solvent polarity
from toluene (ꢀ ) 2.39) to DMF (ꢀ ) 36.7). Since polar
solvents decrease the energy of the charge-separated state,
a more exothermic driving force (-∆G) characterizes an
intramolecular electron transfer in, for example, benzonitrile
and DMF relative to toluene or THF. Thus, the observed
trend is a first indication for an electron-transfer mechanism
being responsible for the rapid deactivation of the fullerene
singlet excited state. On the other hand, an intramolecular
energy transfer mechanism from the fullerene singlet excited
state (ESINGLET ) 1.77 eV) to the o-phenylenediamine moiety
(ESINGLET ) 3.97 eV; ETRIPLET ) 3.08 eV)9 can be ruled out,
based on the unfavorable thermodynamics.
In the case of the o-phenylenediamine-C60 dyad 4, the
transient absorption changes, recorded immediately after the
completion of the picosecond laser pulse, are virtually
identical to those of the reference. In particular, the singlet-
singlet absorption at 870 nm is evident in all solvents. The
lifetime of the singlet excited state absorption is, however,
markedly shortened in toluene and decreases further in
solvents of higher polarity (see Table 2). These changes are
in good resemblance with the steady-state fluorescence
measurements and corroborate the electron-transfer mecha-
nism.
Spectral evidence for the hypothesis that an intramolecular
electron transfer from the electron-donating o-phenylenedi-
amine moiety to the fullerene governs the fate of the
photoexcited fullerene systems is derived from complemen-
tary nanosecond photolytic experiments. In particular, a sharp
maximum found at 1065 nm, shown in Figure 1, resembles
To shed further light onto the deactivation processes of
the fullerene singlet excited state, time-resolved transient
absorption measurements following an 18 ps or a 50 ns laser
pulse were carried out. Following the 18 ps laser pulses, the
equatorial reference reveals the grow-in of a transient
absorption, maximizing around 870 nm. These spectral
characteristics are ascribed to the singlet excited-state
absorption of the equatorial isomer. On a longer time scale
(i.e., 10 ns) the singlet-singlet absorption at 870 nm decays
with a lifetime of 3.1 ns. In parallel with this decay, the grow-
in of a 650 nm absorption was observed, which corresponds
to the fullerene triplet excited-state absorption.10
(7) Spectroscopic data of 4. 1H NMR (400 MHz, CDCl3, 25 °C): δ [ppm]
) 1.88 (m, 4 H, CH2), 2.70 (s, 3 H, NCH3), 2.80 (s, 3 H, NCH3), 3.15 (t,
2 H, NCH2), 3.31 (m, 2 H, NCH2), 3.99 (s, 3 H, OCH3), 4.04 (s, 3 H,
OCH3), 4.18 (m, 1 H, OCH2), 4.27 (m, 1 H, OCH2), 4.44 (m, 1 H, OCH2),
4.63(m, 1 H, OCH2), 6.92 (m, 4 H, CH). 13C NMR (100.50 MHz, CDCl3,
25 °C): δ [ppm] ) 25.35 (1 C, CH2), 25.55 (1 C, CH2) 39.36 (1 C, NCH3),
40.08 (1 C, NCH3), 50.22 (1 C, NCH2), 50.44 (1 C, NCH2), 51.37 (1 C,
bridgehead-C), 53.85 (1 C, bridgehead-C), 53.92 (1 C, OCH3), 53.98 (1 C,
OCH3), 65.87 (1 C, OCH2), 66.00 (1 C, OCH2), 70.35 (1 C, sp3-C60-C),
71.50 (1 C, sp3-C60-C), 71.61 (2 C, sp3-C60-C), 119.44 (1 C, CH), 120.43
(1 C, CH), 121.85 (1 C, CH), 122.60 (1 C, CH), 138.64, 138.90, 139.63,
140.71, 141.27, 141.51, 141.66, 141.86, 141.93, 142.08, 142.33, 142.53,
143.06, 143.26, 143.45, 143.56, 143.70, 143.81, 143.85, 143.94, 144.03,
144.09, 144.31, 144.47, 144.60, 144.69, 144.76, 144.87, 144.93, 145.07,
145.18, 145.24, 145.33, 145.55, 145.73, 146.08, 146.13, 146.32, 146.44,
146.50, 146.74, 147.28, 148.44 (sp2 C), 163.16, 163.78, 164.04 (CdO).
UV/Vis (CH2Cl2): λmax (ꢀ) [nm] ) 252 (108000), 306 (41300), 396 (4200),
421 (2400), 478 (3000). MS (FAB): m/z (%) ) 1170 (M+). IR (KBr): ν
[cm-1] ) 3053, 2950, 2844, 2796, 1749, 1494, 1433, 1384, 1266, 1237,
1212, 1102, 1061, 1024, 746, 708.
Figure 1. Transient absorption spectrum (near-IR part) recorded
50 ns after flash photolysis of 2.0 × 10-5 M dyad 4 at 337 nm in
deoxygenated benzonitrile.
an earlier pulse-radiolytic investigation11 and, in turn,
confirms the formation of the one-electron-reduced fullerene
π-radical anion.
On the other hand, a transient maximum in the visible (i.e.,
at 440 nm, see Figure 2) corroborates the oxidation of the
(8) Lamparth, I.; Maichle-Mo¨ssmer, C.; Hirsch, A. Angew. Chem. 1995,
107, 1755-1757.
(9) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochem-
istry; Marcel Dekker Inc.: New York, 1993.
(10) Guldi, D. M.; Asmus, K.-D. J. Phys. Chem. A 1997, 101, 1472-
1481.
(11) Guldi, D. M.; Hungerbu¨hler, H.; Asmus, K.-D. J. Phys. Chem. 1995,
99, 9380-9385.
Org. Lett., Vol. 2, No. 18, 2000
2743

were also derived from the decay of the one-electron-
oxidized aniline absorption in the visible.
The observation that the back electron transfer is appar-
ently located in the normal region, but, nevertheless, slower
than forward electron transfer suggests additional stabilization
of the charge-separated radical pair. A possible rational
explanation implies the formation of a three-electron bond
between the two nitrogens of the o-phenylenediamine moiety,
similar to the case of the radical cation of diazabicyclo-
octane.12 This, in turn, helps to slow the back electron transfer
dynamics.
The quantum efficiency of the charge separation using
1-chlorobutane as a solvent leads to the highest yields,
although the lifetime of the radical pair is apparently not
optimized. This observation is consistent with earlier litera-
ture reports.13
Figure 2. Transient absorption spectrum (UV-vis part) recorded
50 ns after flash photolysis of 2.0 × 10-5 M dyad 4 at 337 nm in
deoxygenated benzonitrile.
Acknowledgment. This work was supported by the
Stiftung Volkswagenwerk and the Office of Basic Energy
Sciences of the Department of Energy. This is document
NDRL-4230 from the Notre Dame Radiation Laboratory.
o-phenylenediamine moiety and completes the characteriza-
tion of the charge-separated radical pair, namely, C60•--(o-
phenylenediamine)•+. Toluene is the only exception, and on
the time scale investigated no electron-transfer products were
identified. This indicates the low stabilization of the radical
pair in this nonpolar solvent and implies a fast back electron
transfer.
The strong fullerene π-radical anion absorption at 1065
nm was employed as a reliable probe for (i) measuring the
lifetime of the charge-separated state and (ii) determining
the overall quantum yield of the charge-separation process.
Interestingly, polar solvents slow down the back electron
transfer and, consequently, help to stabilize the radical pair.
This indicates that the back electron transfer lies in the
normal region of the “Marcus Parabola”. Similar lifetimes
Supporting Information Available: Experimental pro-
cedures and full characterization for compounds 3-5, a
figure of compound 6, and cyclic voltammograms of C60
and compounds 3-6. This material is available free of charge
via Internet at http://pubs.acs.org.
OL005992S
(12) Alder, R. W.; Bonifacic, M.; Asmus, K.-D. J. Chem. Soc, Perkin
Trans. 2 1986, 277-284.
(13) Gust, D.; Moore, T. A.; Moore, A. L.; Macpherson, A. N.; Lopez,
A.; DeGraziano, J. M.; Gouni, I.; Bittersmann, E.; Seely, G. R.; Gao, F.;
Nieman, R. A.; Ma, X. C.; Demanche, L. J.; Hung, S. C.; Luttrull, D. K.;
Lee, S.-J.; Kerrigan, P. K. J. Am. Chem. Soc. 1993, 115, 11141-11152.
2744
Org. Lett., Vol. 2, No. 18, 2000