Exceptional Redox and Photophysical Properties of a Triply Fused Diporphyrin-C60 Conjugate: Novel Scaffolds for Multicharge Storage in Molecular Scale Electronics

Article abstract of DOI:10.1002/anie.200352265

Up to fifteen electrons are reversibly accommodated in a triply fused porphyrin dimer conjugated to two [60]fullerene moieties. Its photophysical properties differ completely from those of the many known porphyrin-fullerene dyads: Photo-excitation of the

Full text of DOI:10.1002/anie.200352265

Communications
interest.[1] Fascinating examples are the molecular tapes
consisting of triply fused porphyrins, reported by Osuka and
co-workers, which exhibit exceptionally low-energy electronic
states and hold great potential as molecular wires in nanoscale
electronics devices.[2]
The past decade has seen the construction of a great
variety of electron donor–acceptor dyads, frequently featur-
ing C60 as the electron-accepting component because of its
three-dimensional structure and six easily accessible reduc-
tion states in solution.[3] In a biomimetic fashion, many of
these dyads feature porphyrin donors which lose their
characteristic luminescence properties as a result of light-
induced energy and/or electron transfer to the fullerene
acceptor.[4,5]
Since the chemical functionalization of the triply fused
porphyrin sheets is largely unexplored[6] and the nature of
their interactions with redox- and photoactive chromophores,
such as fullerenes, unknown, we decided to prepare conjugate
1 with two C60 spheres covalently attached to a triply fused
porphyrin dimer (Scheme 1). Here, we describe the synthesis
and the exceptional physical properties of 1. The first full
electrochemical study on triply fused porphyrin sheets shows
that 1 is capable of undergoing as many as fifteen reversible
electron-transfer processes. Moreover, photophysical inves-
tigations demonstrate that 1 does not exhibit at all the
classical[7] behavior documented for numerous porphyrin–
fullerene dyads. Rather, photoexcitation of the fullerene units
results in quantitative sensitization of the weakly emitting
lowest singlet level of the porphyrin sheet while the fullerene
emission is quenched.
Porphyrin–C60 Conjugates
Exceptional Redox and Photophysical Properties
of a Triply Fused Diporphyrin–C60 Conjugate:
Novel Scaffolds for Multicharge Storage in
Molecular Scale Electronics**
Davide Bonifazi, Markus Scholl, Fayi Song,
Luis Echegoyen,* Gianluca Accorsi, Nicola Armaroli,*
and François Diederich*
Dedicated to Professor Manfred T. Reetz
on the occasion of his 60th birthday
The expansion and functionalization of p-conjugated molec-
ular architectures to enhance the optoelectronic properties of
the resulting chromophores is a topic of broad current
The functionalized triply fused porphyrin dimer 2 on the
way to 1 was obtained following two routes (Scheme 1).
Dimerization of diarylporphyrin 3 with AgPF6, followed by
silver-mediated iodination and subsequent Suzuki coupling
with phenylboronic ester 4,[8] afforded the biaryl-type dimer 5.
Dimer 5 was converted into 2 (11% yield over four steps)
following the protocol by Tsuda and Osuka[2c] for oxidative
ring closure (DDQ and Sc(OTf)3). As a better alternative,
monoiodination of 3 afforded 6 and Suzuki coupling with 4
gave 7. Oxidative ring closure of 7 directly led to 2 (39% yield
over three steps). Reduction of 2 (DIBAL-H) provided
dialdehyde 8 which was reduced (NaBH4) to the correspond-
ing bis(benzyl alcohol). Esterification with EtO2CCH2COCl
yielded bismalonate 9, and Bingel reaction with C60 gave the
target conjugate 1.
[*] Prof. Dr. L. Echegoyen, Dr. F. Song
Department of Chemistry
Clemson University
219 Hunter Laboratories, Clemson SC 29634 (USA)
Fax: (+1)864-656-6613
E-mail: luis@clemson.edu
Dr. N. Armaroli, Dr. G. Accorsi
Istituto per la Sintesi Organica e la Fotoreattività
40129 Bologna (Italy)
Fax: (+39)051-6399844
E-mail: armaroli@isof.cnr.it
Prof. Dr. F. Diederich, D. Bonifazi, M. Scholl
Laboratorium für Organische Chemie
ETH-Hönggerberg, HCI
8093 Zürich (Switzerland)
Fax: (+41)1-632-1109
The redox properties of 1, 2, 5, and comparison com-
pounds 10,[9] 11,[9] and 12 were examined by cyclic (CV) and
differential pulse (DPV) voltammetry (Table 1, Figures 1 and
2).
E-mail: diederich@org.chem.ethz.ch
The cyclic voltammogram of 5 (Table 1) features two
partially overlapped reduction couples at ꢀ1.75 and ꢀ1.86 V
(difference of 0.11 V). The first 1-eꢀ oxidation of the two
porphyrin moieties appears as separated couples at 0.38 and
0.51 V whereas the second oxidation of both rings also gives
two couples at 0.78 and 1.10 V. Seven reversible redox couples
with identical current for each peakwere observed for the
triply fused porphyrin dimer 2 in CH2Cl2 (Figure 1b). The
first and second reduction couples correspond to two-step,
1-eꢀ per porphyrin ring reductions. Likewise, the first and the
[**] We thank Rolf Häfliger for the mass spectrometric measurements.
This research was supported by the Swiss National Science
Foundation and the NCCR “Nanoscale Science”, the Spinner
Agency (Regione Emilia-Romagna, Italy), and the US National
Science Foundation (CHE-0135786).
Supporting information for this article (selected physical and
spectral data of 1 and 2, cyclic voltammograms of 1, 2, and 11,
luminescence spectra of 11 compared to reference compounds 5
and 13, UV/Vis data of 1, 2, 9, and 13) is available on the WWW
under http://www.angewandte.org or from the author.
4966 ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200352265
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Scheme 1. Synthesis of conjugate 1. a) AgPF6, CH3CN/CHCl3 1:6, RT, 15 h; b) AgPF6, I2, pyridine/CHCl3 1:60, RT; c) 4, Cs2CO3, [Pd(PPh3)4], toluene, reflux,
18 h; d) DDQ, Sc(OTf)3, toluene, reflux, 30 min; e) AgPF6, I2, pyridine/CHCl3 1:60, RT; f) 4, Cs2CO3, [Pd(PPh3)4], toluene, reflux, 18 h; g) AgPF6, CH3CN/
CHCl3 1:6, RT, 15 h; h) DDQ, Sc(OTf)3, toluene, reflux, 30 min; i) DIBAL-H, CH2Cl2, ꢀ788C!RT, 2.5 h; j) NaBH4, THF/EtOH 1:1, 0 8C!RT, 4 h;
k) EtO2CCH2COCl, Et3N, CH2Cl2, RT, 1 6 h; l) C , I2, DBU, toluene, RT, 1.5 h. DDQ=2,3-dichloro-5,6-dicyano-p-benzoquinone; DIBAL-H=di(iso-butyl)alumi-
60
num hydride; DBU=1,8-diazabicyclo[5.4.0]undec-7-ene; Tf=trifluoromethanesulfonyl.
Table 1: Cyclic voltammetry data in CH2Cl2 (+0.1m nBu4NPF6).[a]
1=2
red;1
1=2
red;2
1=2
red;3
1=2
red;4
1=2
ox;1
1=2
ox;2
1=2
ox;3
1=2
ox;4
E
E
E
E
E
E
E
E
12
5
ꢀ1.71(1eꢀ)
ꢀ1.75(1eꢀ)
ꢀ1.06(1eꢀ)
ꢀ1.01(1eꢀ)
ꢀ1.06(1eꢀ)
ꢀ0.98
ꢀ2.09(1eꢀ)
ꢀ1.86(1eꢀ)
ꢀ1.40(1eꢀ)
ꢀ1.26(1eꢀ)
ꢀ1.41(1eꢀ)
ꢀ1.41(2eꢀ)
–
–
–
0.44(1eꢀ)
0.38(1eꢀ)
0.15(1eꢀ)
0.09(1eꢀ)
0.36(1eꢀ)
0.37(1eꢀ)
0.74(1eꢀ)
0.51(1eꢀ)
0.47(1eꢀ)
0.37(1eꢀ)
0.74(1eꢀ)
0.50(1eꢀ)
–
–
ꢀ2.20(1eꢀ)
ꢀ2.29(1eꢀ)
ꢀ2.18(1eꢀ)
ꢀ1.84(2eꢀ)
ꢀ1.84(4eꢀ)
0.78(1eꢀ)
0.92(1eꢀ)
0.83(1eꢀ)
–
1.10(1eꢀ)
2[b]
ꢀ2.59(1eꢀ)
–
2
10
11
–
–
–
1.10(1eꢀ)
–
0.73(1eꢀ)
0.84(1eꢀ)
ꢀ1.06(2eꢀ)
ꢀ0.99
1
ꢀ1.40(3eꢀ)
ꢀ1.87(2eꢀ)
ꢀ2.29(3eꢀ)
0.03(1eꢀ)
0.34(1eꢀ)
0.82(1eꢀ)
1.09(1eꢀ)
ꢀ1.09(3eꢀ)
[a] Potentials versus Fc/Fc+. Working electrode: glassy carbon electrode; counter electrode: Pt; reference electrode: Ag/AgCl. Scan rate: 0.1Vs ꢀ1
.
E1/2 =(Epa +Epc)/2, where Epc and Epa =cathodic and anodic peak potentials, respectively. [b] Solvent: THF.
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measurements in THF revealed the presence of the fourth
reduction peak(Figure 1c). These results confirm that each
redox couple for 12 splits into two couples for 2.
Conjugation of two C60 moieties to the triply bridged
porphyrin dimer in 1 results in nine reversible redox processes
involving a total of fifteen electrons (Figure 2b). The four
oxidation peaks correspond to the four 1-eꢀ oxidation steps
centered at the porphyrin units, and the first oxidation peakis
cathodically shifted relative to 2 (Figure 2a) by 60 mV. The
two partially overlapped peaks at ꢀ1.0 V correspond to the
first reduction of the two C60 units and a 1-eꢀ process for the
porphyrin system (a 3-eꢀ net process). The peakat ꢀ1.38 V
corresponds, likewise, to the second reduction of the two
C
60 units and a second reduction of the porphyrin sheet
(again, a net 3-eꢀ process). The smaller peakat ꢀ1.85 V is
attributed to the third 1-eꢀ reduction of each of the two
C
60 moieties (a 2-eꢀ process). The peakat ꢀ2.28 V corre-
sponds to the fourth 1-eꢀ reduction of the fullerene moieties
and the third 1-eꢀ reduction of the porphyrin (a 3-eꢀ net
process). The cyclic voltammogram of biaryl-type dimer 11
features the first porphyrin-based oxidation peakpotential
anodically shifted by 300 mV relative to 1 (Figure 2c).
Furthermore, the first two porphyrin-centered reduction
steps in 11 are shifted cathodically by 850 mV relative to
the corresponding first reduction couple in 1.
Figure 1. Cyclic voltammograms of a) 12 in CH2Cl2, b) 2 in CH2Cl2, and
c) 2 in THF at 20ꢁ28C (+0.1m nBu4NPF6).
second oxidation peaks each correspond to a 1-eꢀ process, one
per porphyrin ring. Relative to 12, the first 1-eꢀ reduction
potential of 2 is shifted positively by 0.7 V whereas the first
1-eꢀ oxidation potential is shifted negatively by 0.35 V. The
fourth reduction peakwas not observed because of the
limited potential window in CH2Cl2. However, CV and DPV
The photophysical properties of the two conjugates 1 and
11 also differ dramatically. The absorption spectrum (toluene)
Figure 2. Differential pulse voltammograms of a) 2, b) 1, and c) 11 in CH2Cl2 at 20ꢁ28C.
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of conjugate 11 with the biaryl-type por-
phyrin dimer does not match the sum
spectrum of its component units (5 and 2 ”
between the two fused porphyrin units and a shift of the Q-
band envelope into the NIR region (Figure 4).[13] The
comparison of the absorption spectrum of 1 with the sum of
its component units (9 + 2 ” 13) shows two small but relevant
differences: a) a 3-nm red shift and a very small decrease in
13; Figure 3), particularly in the Soret-
band region, which is split as a result of
exciton coupling.[10] This result is attrib-
uted to strong face-to-face electronic inter-
actions between the porphyrin and full-
erene chromophores.[11] Only the 630–800-nm region of the
spectrum of 11 is more intense than its calculated counterpart,
which suggests the occurrence of sizeable interchromophoric
charge transfer interactions.[12] An emissive charge-transfer
Figure 4. Absorption and (from 960 nm) fluorescence spectra
(lexc =420 nm, A=0.20) of 9 (blue) and 1 (red). Inset: fluorescence
spectra of 13 (black) and 1 (red), lexc =330 nm, A=0.30. All fluores-
cence spectra were recorded from air-purged samples.
the intensity of the highest energy Soret band and b) a 10-nm
shift to lower energy of the most intense Q-band in the NIR
domain. These spectral changes are smaller than those
observed for 11 or other conjugates with face-to-face align-
ment of porphyrin and C60 moieties,[12] thus suggesting weaker
electronic interchromophoric interactions.
Figure 3. Red line: sum of the electronic absorption spectra of 5 and
two fullerenes 13. Black line: Absorption and (from 600 nm) emission
spectra (lexc =330 nm) of 11. Inset: Transient absorption spectrum of
11 at lexc =532 nm and spectral time decay at 660 nm. All spectra
were recorded in toluene at 298 K.
(CS) state is detected in the NIR region (lmax = 950 nm,
Figure 3). The transient absorption spectrum of 11 in toluene
(lexc = 532 nm, Figure 3) exhibits the diagnostic broad band of
the ZnII–porphyrin radical cation with a maximum at 640 nm,
thus indicating that photoinduced electron transfer from the
porphyrin to the fullerene moieties occurs.[12] The lifetime of
the CS state is 630 ps. Porphyrin singlet and triplet excited
states are dramatically quenched as shown from fluorescence
and phosphorescence spectra at 298 and 77 K, respectively. In
contrast, some residual excited-state features of the fullerene
are detected (that is, a tail of fullerene fluorescence above
700 nm and a very weakfullerene triplet absorption trace at
lmax = 740 nm), which are accompanied by a very minor
amount of singlet oxygen luminescence (Figure 3). All the
above results indicate that the local excited states of 11
centered on the porphyrin and fullerene moieties are
deactivated to a lower lying luminescent charge-separated
state that is localized at about 1.4 eV, as derived from
electrochemical (Table 1, CH2Cl2) and luminescence data
(Figure 3, toluene). The residual localized excited states of
the fullerene are attributed to a minority (< 10%) of loose
molecules where strong donor–acceptor porphyrin–fullerene
attractions are not established.[12b]
Dimer 9 exhibits an emission band in the NIR region
(lmax = 1080 nm) at all excitation wavelengths. This is a mirror
image of the profile of the Q bands (Figure 4), and is
unambiguously assigned to fluorescence from the lowest
singlet excited state. The NIR emission band is observed also
for 1 upon selective excitation of the porphyrin chromophore
(420 or 585 nm). This band is shifted by 12 nm (lmax
=
1092 nm) relative to that in 9, and is in line with the trend
seen in the absorption spectra. Preferential excitation of the
C
60 chromophores in 1 at 330 nm (ꢂ 80%) shows a dramatic
quenching of the fullerene fluorescence relative to 13,
whereas a pure porphyrin fluorescence band is detected in
the NIR region (Figure 4, inset). According to the electro-
chemical data in CH2Cl2, the charge-separated state, which
corresponds to the reduction of a fullerene unit and oxidation
of the porphyrin moiety, is located at about 1.0 eV.[14] This
energy value is comparable to the singlet level of the
porphyrin, as estimated from the wavelength of the fluores-
cence band (Figure 4). Thus, in principle, both energy and
electron transfer may be responsible for the observed
quenching of the fullerene luminescence. Two experimental
results helped to unravel the quenching mechanism: 1) the
picosecond transient absorption of 1 with a time resolution of
35 ps shows only (at early times) the spectral features of the
fused porphyrin dimer, and 2) the porphyrin fluorescence
The steady-state absorption spectrum of 9 is characterized
by splitting of the Soret band as a result of exciton coupling
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Photobiol. Sci. 2003, 2, 73 – 87; c) D. Gust, T. A. Moore, A. L.
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[10] N. Aratani, A. Osuka, H. S. Cho, D. Kim, J. Photochem.
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[11] The interchromophoric ground-state interaction in 10 and 11 was
quantified as about 12 kJmolꢀ1 (toluene, 298 K), see ref. [9]. In
contrast, interchromophoric interactions in 1 are weaker (see
text). Thus, the conformation of 1 depicted in Scheme 1 is only
one of many possible; conformers with the carbon spheres
nesting on opposite faces of the porphyrin dimer or turned away
from the heterocycle are of equal probability.
[12] a) N. Armaroli, G. Marconi, L. Echegoyen, J.-P. Bourgeois, F.
Diederich, Chem. Eur. J. 2000, 6, 1629 – 1645; b) N. Armaroli in
Fullerenes: From Synthesis to Optoelectronic Properties (Eds.:
D. M. Guldi, N. Martin), Kluwer, Dordrecht, 2002, pp. 137 – 162.
[13] H. S. Cho, D. H. Jeong, S. Cho, D. Kim, Y. Matsuzaki, K. Tanaka,
A. Tsuda, A. Osuka, J. Am. Chem. Soc. 2002, 124, 14642 – 14654.
[14] The use of the electrochemical data in CH2Cl2 to estimate the
quantum yield is identical, within experimental uncertainty,
for 1 and 9 (FF = 3.5 ” 10ꢀ4, lexc = 330 nm). We conclude that
excitation of the fullerene chromophore results in quantita-
tive sensitization of the lowest singlet state of the porphyrin,
and that the role of the nearly isoenergetic charge-separated
state in toluene solution, if any, is not relevant.
In summary, despite the fact that triply fused porphyrin
dimers are much better electron donors than simple porphyr-
ins, their low-lying and very short lived (4.5 ps)[13] singlet level
offers an extremely competitive deactivation pathway and
acts as a sinkfor the higher energy electronic levels. Hence,
the photoinduced process in 1 (C60!porphyrin energy trans-
fer) is completely different from that in the biaryl-type
bisporphyrin conjugate 11 (porphyrin!C60 electron transfer)
while, notably, both molecules are NIR emitters.
Received: June 30, 2003 [Z52265]
Keywords: electrochemistry · electron transfer · fullerenes ·
.
luminescence · porphyrinoids
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energy of the CS state of
1 in toluene is a reasonable
approximation in light of the good accord found between the
energy of the CS state calculated by luminescence data (toluene)
and electrochemistry (CH2Cl2) for the similar system 11.
[4] Recent reviews: a) H. Imahori, Y. Mori, Y. Matano, J. Photo-
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4970
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