Multistep electron transfer systems based on silicon phthalocyanine, [60]fullerene and trinitrofluorenone

Article abstract of DOI:10.1039/c002077j

The synthesis and photodynamics in the absence and in the presence of Mg²⁺ ions of a novel TNF-C₆₀-SiPc-C₆₀-TNF pentad are reported. The redox gradient approach allows to obtain a long-lived CS state of 160 ns and 200 μs in the absence and in the presence of Mg ²⁺ ions, respectively.

Full text of DOI:10.1039/c002077j

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Multistep electron transfer systems based on silicon phthalocyanine,
[60]fullerene and trinitrofluorenonew
a
b
a
´ ´
n-Gomis, Kei Ohkubo, Fernando Fernandez-Lazaro,
a
Luis Martı
´
Shunichi Fukuzumi*bc and Angela Sastre-Santos*
´
Received 1st February 2010, Accepted 30th March 2010
First published as an Advance Article on the web 23rd April 2010
DOI: 10.1039/c002077j
The synthesis and photodynamics in the absence and in the
presence of Mg2+ ions of a novel TNF-C60-SiPc-C60-TNF
pentad are reported. The redox gradient approach allows to
obtain a long-lived CS state of 160 ns and 200 ls in the absence
and in the presence of Mg2+ ions, respectively.
in Scheme 1) and its photodynamics in the absence and in the
presence of Mg2+ ions. Taking into account the reduction
potentials of the C60 derivative and the trinitrofluorenone,
compound 1 presents a well defined multisequential electron
transfer system, with a long distance between the TNF and the
Pc unit to avoid CR processes. Moreover the TNF moiety will
have the possibility to complex with metal ions decreasing the
back electron transfer driving force.12
Nowadays much effort is devoted to look for artificial photo-
synthetic systems,1 as donor–acceptor dyads,2 triads3 or higher
order arrays,4 presenting similar characteristics to the natural
ones. Moreover, bioinspired in the natural photosynthetic struc-
ture, the design of multistep electron transfer systems presenting
charge shift reactions in a well tuned redox gradient has received
particular attention.5 The alignment of the units along a electro-
chemical gradient promotes acceleration of forward electron
transfer and deceleration of charge recombination (CR) leading
to an efficient long-lived charge separated (CS) state.5
The synthesis of this pentad was carried out through a
convergent strategy by reaction of the bishydroxyphthalo-
cyanine 2 with the carboxylic C60-TNF derivative 3 (Scheme 1).
The bishydroxyphthalocyanine 2 was prepared by treatment
of the dichloro-substituted phthalocyanine with 4-hydroxy
benzoic acid. The synthesis of the carboxylic C60 derivative 3
was accomplished by esterification reaction of the hydroxy-
trinitrofluorenone 413 with tertbutylmonomalonate to yield a
fluorenone functionalized with a malonate group 5, which by
Bingel reaction forms TNF methanofullerene 6. Finally, the
carboxylic-TNF-C60 derivative 3 was obtained by acid treat-
ment in a very good yield (Scheme 2). (See ESI for analytical
and experimental details).w
Axially substituted silicon phthalocyanines (SiPcs) are very
attractive targets to study photophysical processes because
they are not able to aggregate due to their special structural
features, thus avoiding fluorescence quenching.6 High quantum
efficiency and low recombination speed made the [C60]-
fullerene one of the preferred electron acceptor moieties.7
Photophysical studies of trinitrofluorenone (TNF) derivatives8
due to their high electron acceptor capability have been
investigated. Different silicon phthalocyanines substituted
Semiempirical calculations place the HOMO of the pentad 1
in the phthalocyanine moiety, being the LUMO and LUMO-1
either with C60 or TNF10 have been prepared as efficient
9
systems for photoinduced intramolecular electron transfer
determining their electron transfer and back electron transfer
rate constants. Moreover, C60 has been successfully used as an
electron donor linked with an electron acceptor to attain the
longest CS lifetime in the presence of Sc3+ (23 ms at 298 K)
ever reported for electron donor–acceptor linked systems.11
Based in all the previous studies, we report herein the
synthesis of a new TNF-C60-SiPc-C60-TNF pentad 1 (as shown
a
´
´
´
Division de Quımica Organica, Instituto de Bioingenierıa,
Universidad Miguel Hernandez, Elche 03202, Spain.
´
´
E-mail: asastre@umh.es
b Department of Material and Life Science, Graduate School of
Engineering, Osaka University, SORST, Japan Science and
Technology Agency (JST), Suita, Osaka 565-0871, Japan.
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp; Fax: +81-6-6879-7370;
Tel: +81-6-6879-7368
c Department of Bioinspired Science, Ewha Womans University,
Seoul 120-750, Korea
w Electronic supplementary information (ESI) available: Experimental
details for the synthesis of TNF-C60-SiPc-C60-TNF pentad 1 and
transient absorption measurements of TNF-SiPc-TNF triad in presence
of Mg2+ ions. See DOI: 10.1039/c002077j
Scheme 1 Synthesis of the TNF-C60-SiPc-C60-TNF pentad 1.
ꢀc
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Scheme 2 Synthesis of TNF-C60 3.
Fig.
2
(a) Femtosecond transient absorption spectra of
localized in the trinitrofluorenone and the LUMO-2 and
LUMO-3 in the C60 moieties (Fig. 1).
TNF-C60-SiPc-C60-TNF pentad 1 in deaerated PhCN at 298 K after
laser excitation at 355 nm. (b) Time profile of absorbance at 950 nm.
(c) Time profile of absorbance at 880 nm.
A deaerated PhCN solution containing TNF-C60-SiPc-C60-
TNF pentad 1 gave rise upon a 355 nm femtosecond laser
pulse to transient absorption bands at 880 and 1000 nm due to
triad10 where a lifetime of the CS state of 22 ps was detected.
For the pentad 1, taking into consideration the decay of the
absorption band at 880 nm, a lifetime of 160 ns for the CS
state is observed in benzonitrile (Fig. 3b). The quantum yield
of the CS state is 23% determined from comparative method
SiPcꢁ+ and C60 À moieties, respectively. At 950 nm the decay
ꢁ
of the singlet ꢁexcited state of SiPc (1SiPc*) is overlapped with
the rise of C60 À. This clearly indicates the formation of the CS
state. From the rise in absorbance at 880 nm due to SiPcꢁ+ in
Fig. 2b the rate constant of formation of SiPcꢁ+ in the pentad
+
using SiPcꢁ absorbance at 880 nm.14 Comparison of the
was determined as 1.7 Â 109
s
À1, that agrees well with the
lifetime of the CS state (tCR) of pentad 1 (160 ns) with the tCR
of C60-SiPc-C60 triad (5.0 ns)9 indicates an increase of two
orders of magnitude of tCR in a polar solvent through the
rational design of a multistep electron transfer system by
covalently linking TNF units to the C60.
fluorescence quenching rate constant obtained from the decay
1
of the 950 nm due to SiPc* (1.7 Â 109 sÀ1). This shows that
1
intramolecular electron transfer from SiPc* to C60 occurs Àin
ꢁ
the pentad to produce the charge-separated state, TNF-C60
-
SiPcꢁ+-C60-TNF. The fluorescence from 1SiPc* is quenched
due to the electron transfer to C60 in the pentad as compared
to that of the SiPc reference compound in PhCN (Fig. S10, ESI).w
The lifetime of the CS state in PhCN was determined as
3.6 ns from the decay of absorbance at 880 nm. This photo-
dynamics totally agrees with the previously reported data for
C60-SiPc-C60 triad.9a
Upon addition of magnesium perchlorate [Mg(ClO4)2] to a
PhCN solution of pentad 1, nanosecond laser flash photolysis
was recorded with photoexcitation at 355 nm (Fig. 3c). The
bands observed at 880 and 550 nm (SiPcꢁ+) and 900 and
510 nm (TNFꢁÀ) confirm the formation of the CS state. The
decay of the band at 880 nm shows an increase of the lifetime
of the CS state till 200 ms (Fig. 3d), the higher value found to
The transient absorption spectrum obtained upon nano-
second laser pulse excitation (355 nm) of the pentad 1 in
deaerated benzonitrile gave rise to the transient absorption
bands at À880 and at 550 nm due to SiPcꢁ+ unit and 510 nm due
to TNFꢁ moiety (Fig. 3a). These data agree with the radical
pairs absorption bands previously reported for the TNF-SiPc-TNF
Fig.
3 (a) Nanosecond transient absorption spectra of
TNF-C60-SiPc-C60-TNF pentad 1 in deaerated PhCN at 298 K after
laser excitation at 355 nm. (b) Time profile of absorbance at 880 nm.
(c) Nanosecond transient absorption spectra of TNF-C60-SiPc-C60-TNF
pentad 1 in deaerated PhCN at 298 K after laser excitation at 355 nm
with Mg(ClO4)2 (10 mM). (d) Time profile of absorbance at 880 nm.
Fig. 1 Molecular orbitals of pentad 1 calculated by PM3 method.
ꢀc
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Notes and references
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´
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that has long wavelength absorption with the redox gradient
required for stepwise charge separation to attain the longer-
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This work has been supported by the Spanish Ministry
of Science and Innovation, the European FEDER funds
and the Generalitat Valenciana (grants CTQ2007-67888/
BQU, CTQ2008-05901/BQU, CONSOLIDER-INGENIO
CSD2007-00007 and ACOMP/2009/039 and ACOMP/2009/056)
and by Grants-in-Aid (No. 21750146) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan,
KOSEF/MEST through WCU project (R31-2008-000-10010-0).
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15 This indicates that Mg2+ binds to the fluorenone unit rather than
to the oligoethyleneglycol unit.
ꢀc
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3946 | Chem. Commun., 2010, 46, 3944–3946