Effect of anion binding on charge stabilization in a bis-fullerene- oxoporphyrinogen conjugate

Article abstract of DOI:10.1039/c0cc03167d

Presence of strongly binding anion, F^- stabilizes the photo-induced charge-separated states of a bis-fullerene-substituted oxoporphyrinogen due to the large shift in the oxidation potential of the oxoporphyrinogen moiety upon anion binding through hydrogen bonding at its core.

Full text of DOI:10.1039/c0cc03167d

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COMMUNICATION
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Effect of anion binding on charge stabilization
in a bis-fullerene–oxoporphyrinogen conjugatewz
Jonathan P. Hill,*ab Mohamed E. El-Khouly,c Richard Charvet,a Navaneetha K. Subbaiyan,d
Katsuhiko Ariga,ab Shunichi Fukuzumi*ce and Francis D’Souza*d
Received 10th August 2010, Accepted 6th September 2010
DOI: 10.1039/c0cc03167d
Presence of strongly binding anion, Fꢀ stabilizes the photo-
induced charge-separated states of a bis-fullerene-substituted
oxoporphyrinogen due to the large shift in the oxidation
potential of the oxoporphyrinogen moiety upon anion binding
through hydrogen bonding at its core.
and C60 of the triad are usually considered electron-accepting
agents2,6 except that in this case the OxP unit is expected to be
an electron donor given its ease of oxidation relative to the
fulleropyrrolidine groups. Both OxP and C60 components are
fluorescent with the doubly substituted OxP being more
strongly emissive.2,7 Excitation of either of the entities is
expected to induce photochemical processes in the triad.
Significantly, the two imino hydrogens of the OxP unit possess
the ability to bind anions with high stability8a resulting in
modulation of the OxP unit redox properties.8b That is, the
one-electron oxidation potential of the OxP unit, with four
hemiquinone entities, is expected to exhibit large cathodic
shifts upon binding of an anion. Consequently one should
expect substantial changes in the energy levels which should
alter the overall photochemical quenching processes. This
phenomenon has been tested in the present study by using
the newly synthesized model compound 1.
Control of electron-transfer processes is an important issue in
molecular species that may lead to their application in
synthetic organic light-harvesting systems or molecular electronic
devices.1–3 Control of electron-transfer processes is often made
possible by adding external ionic cofactors, which favor either
charge separation or recombination.4 For example, in the O2
evolving complex of photosystem II (PSII), Ca2+ and Clꢀ are
known to be essential cofactors for efficient water oxidation.5
In this context, we have previously reported a supramolecular
oligochromophoric model system containing sites for binding
of a reagent species and an anionic species (Fꢀ) in order to
probe the effect of the binding of Fꢀ on the identities of
products from photoinduced electron-transfer processes that
occur within the resulting complex.6 However, the effects of
anionic species on photoinduced electron-transfer processes
via the singlet excited state in covalently linked electron
donor–acceptor systems, which are more chemically robust,
have yet to be clarified.
We report herein the effect of binding of Fꢀ to an appropriately
structured tetrapyrrole chromophore, which is covalently
linked with electron acceptor units, on the photoinduced electron-
transfer (PET) processes via both the singlet and triplet excited
states. The chemical structure of the subject molecule 1 con-
tains two fulleropyrrolidine (C60) units substituted through
4,40-biphenylmethylene groups at the nitrogen atoms of an
oxoporphyrinogen (OxP) unit as depicted in Scheme 1. Both OxP
The synthetic details of 1 are given in ESIz. Synthesis
of bis(4-bromobenzyl)-substituted-OxP8 was followed by
Suzuki–Miyaura coupling with 4-formylphenylboronic acid
to yield the bisformyl compound.9 The latter was used
as a substrate for the Prato reaction10 yielding a bis-fullero-
pyrrolidino-OxP triad, 1.
Fig. 1a shows the optical absorption spectral changes of 1
during increasing addition of Fꢀ in o-dichlorobenzene (DCB).
The main peak of 1 corresponding to OxP entity located at
511 nm revealed diminished intensity with the concurrent
appearance of two new bands at 603 and 756 nm. Isosbestic
points at 309, 348 and 426 nm are observed suggesting the
existence of only one equilibrium process in solution.
The sharp peak at 430 nm of fulleropyrrolidine showed no
visible effect upon addition of Fꢀ in solution. Plots of mole
ratio against absorbance change (Fig. 1c) suggest 1 : 1 complex
a WPI-Center for Materials Nanoarchitectonics,
National Institute of Materials Science (NIMS), Namiki 1-1,
Tsukuba, Ibaraki, 305-0044, Japan.
E-mail: Jonathan.Hill@nims.go.jp
b JST-CREST, Namiki 1-1, Tsukuba, Japan
c Graduate School of Engineering, Osaka University, Suita,
Osaka 565-0871, Japan. E-mail: Fukuzumi@chem.eng.osaka.u.ac.jp;
Tel: +81-6-6879-7368
d Department of Chemistry, Wichita State University, 1845,
Fairmount, Wichita, KS 67260, USA.
E-mail: Francis.DSouza@Wichita.edu; Tel: 316-878-3431
e Department of Bioinspired Science, Ewha Womans University, Seoul,
120-750, Korea
w This article is part of the ‘Carbon Nanostructures’ web-theme issue
for ChemComm.
z Electronic supplementary information (ESI) available: Details of
synthesis of 1 and general experimental techniques. See DOI: 10.1039/
c0cc03167d
Scheme 1 Structure of the newly synthesized OxP(C60 2
) triad, 1 to
probe anion binding effect on charge separation.
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 7933–7935 7933

Fig. 3 Cyclic voltammograms of 1 in the absence (dark line)
and presence (red line) of Fꢀ in deaerated DCB containing 0.1 M
(TBA)ClO4. Scan rate = 100 mV sꢀ1
.
respectively. Interestingly, addition of Fꢀ revealed drastic
changes in the oxidation potentials of the OxP entity. That
is, addtion of 1.1 eq. of Fꢀ to a solution of 1 resulted in a
cathodic shift of nearly 510 mV (E1/2 = ꢀ0.14 V) as a result
of Fꢀ binding to the OxP unit. On the reduction side, the
fullerene reduction underwent a small anodic shift of 60 mV
(E1/2 = ꢀ1.22 V) due to ion-pairing effect.13 However, no
appreciable shifts are seen for OxP reduction.
Fig. 1 (a) Absorption spectra of 1 (7.5 mM) upon increasing addition
of Fꢀ (0.1–0.5 eq.). (b) Benesi–Hildebrand plot constructed to obtain
the binding constant, and (c) mole ratio plot to obtain the molecular
stoichiometry of the 1 : Fꢀ complex in DCB.
formation. The binding constant evaluated by constructing
a Benesi–Hildebrand plot11 (Fig. 1b) is found to be 5.8 ꢁ
104 Mꢀ1, suggesting stable complex formation.
Free-energy calcuations for charge separation (CS) and
charge recombination (CR) were performed according to
Weller’s approach.2b The driving forces for charge recombination
(ꢀDGCR) and charge separation (ꢀDGCS) processes of 1 via
the singlet excited state of OxP and C60 are evaluated as 1.65
and 0.10 eV, respectively, taking into consideration that
the energy of the singlet excited state of OxP and C60 are
1.75 eV.7,15 In case of 1 : Fꢀ, the ꢀDGCR value was determined
to be 1.14 eV. The PET processes are exothermic from the
singlet (ꢀDGCS = 0.61 eV) and triplet (ꢀDGCS = 0.41 eV)
excited states of the OxP unit.
Both OxPR2 (R = alkyl or aryl)7 and fulleropyrrolidine2 are
known to emit in the 720 nm region and this is also the case for
the investigated compound 1, albeit with much lower quantum
yields. That is, a broad emission spanning the 600–850 nm
range was observed for 1 (Fig. 2). The weak emission of
fullerene is hidden under the relatively strong emission of
OxP. Addition of Fꢀ further decreased the emission intensity
resulting in formation of an almost non-emitting complex
(>98% quenching) likely caused by occurrence of very efficient
photochemical events.
A spectroscopic signature for the formation of radical ion
pair species and kinetics of CS and CR are obtained from the
transient absorption studies in the femtosecond and nano-
second time regimes by utilizing the 430 nm laser light, which
selectively excites the OxP entity (Fig. 4). The femtosecond
transient absorption spectrum of 1 and 1 : Fꢀ revealed absorp-
tion bands in the visible and near infrared (NIR) reꢂgions with
maxima at 870 and 1000 nm due to OxPꢂ+ and C60 ꢀ species,
Electrochemical studies are performed to probe the effect of
anion binding on the redox potentials of 1 and their subsequent
effect on the energy levels of photoinduced processes. The first
oxidation of 1 is a reversible process located at E1/2 = 0.37 V
vs. Fc/Fc+ in DCB (Fig. 3). The potential value is similar to
that reported earlier for OxP(bz)2 (bz = benzyl) derivative12
lacking the fullerene suggesting that the site of oxidation
involves the OxP unit. A second oxidation is also observed
at higher anodic potentials (E1/2 = 0.58 V). The fullerene
reduction of 1 is located at E1/2 = ꢀ1.28 V vs. Fc/Fc+,
whereas the first reduction of OxP and the second reduction
of fullerene are close and appeared at ꢀ1.56 and ꢀ1.64 V,
Fig. 4 Femtosecond transient absorption spectra of 1 : Fꢀ in deaerated
DCB; lex = 430 nm. The figure inset shows decay of the 1000 nm
transient band corresponding to fullerene radical anion.
Fig. 2 Fluorescence spectra of 1 (7.5 mM) on increasing addition of
Fꢀ (0.1–0.5 eq.) in DCB; lex = 509 nm.
c
7934 Chem. Commun., 2010, 46, 7933–7935
This journal is The Royal Society of Chemistry 2010

leading to a large cathodic shift of its oxidation potential. As a
result, the donor–acceptor pair reveals very efficient PET from
both the singlet and triplet excited states of OxP. Presence of
Fꢀ in the OxP pocket slows down the CR process thus
generating the much desired long-lived CS state. Further
studies to prolong the charge-separated state by means of
multi-step charge migration in anion bound OxP containing
polyads are in progress.
This work was financially supported by the World Premier
International Research Center Initiative from MEXT, Japan,
the National Science Foundation (Grant Nos. 0804015 and
EPS-0903806) and matching support from the State of
Kansas through KTEC, Grant-in-Aid (No. 20108010) from
Ministry of Education, Culture, Sports, Science and Tech-
nology, Japan and KOSEF/MEST through WCU project
(R31-2008-000-10010-0).
Fig. 5 Nanosecond transient absorption specꢀtra of 1 : Fꢀ in deaerated
ꢂ
DCB. Inset shows the time profile of the C60 at 1000 nm.
respectively, offering clear evidence for the occurrence of PET
from the electron donating OxP to the electron accepting
C60.6,14 The rate of CS via the singlet excited state is found
to be very fast (B1012 sꢀ1 ps). From fitting the decay of the
radical ion pair species with clean first-order kinetics, the rates
of charge recombination (kCR) were found to be 6.6 ꢁ 1010 and
1.40 ꢁ 1010 sꢀ1 for 1 and 1 : Fꢀ, respectively. Using these kCR
Notes and references
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)
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,
T
T
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of OxP are summarized as shown in Fig. 6.
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Fig. 6 Energy level diagram showing the electron transfer processes
of 1 : Fꢀ via the singlet and triplet OxP in deaerated DCB.
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 7933–7935 7935