Supramolecular electron transfer by anion binding.
ABSTRACT Anion binding has emerged as an attractive strategy to construct supramolecular electron donor-acceptor complexes. In recent years, the level of sophistication in the design of these systems has advanced to the point where it is possible to create ensembles that mimic key aspects of the photoinduced electron-transfer events operative in the photosynthetic reaction centre. Although anion binding is a reversible process, kinetic studies on anion binding and dissociation processes, as well as photoinduced electron-transfer and back electron-transfer reactions in supramolecular electron donor-acceptor complexes formed by anion binding, have revealed that photoinduced electron transfer and back electron transfer occur at time scales much faster than those associated with anion binding and dissociation. This difference in rates ensures that the linkage between electron donor and acceptor moieties is maintained over the course of most forward and back electron-transfer processes. A particular example of this principle is illustrated by electron-transfer ensembles based on tetrathiafulvalene calixpyrroles (TTF-C4Ps). In these ensembles, the TTF-C4Ps act as donors, transferring electrons to various electron acceptors after anion binding. Competition with non-redox active substrates is also observed. Anion binding to the pyrrole amine groups of an oxoporphyrinogen unit within various supramolecular complexes formed with fullerenes also results in acceleration of the photoinduced electron-transfer process but deceleration of the back electron transfer; again, this is ascribed to favourable structural and electronic changes. Anion binding also plays a role in stabilizing supramolecular complexes between sulphonated tetraphenylporphyrin anions ([MTPPS](4-): M = H(2) and Zn) and a lithium ion encapsulated C(60) (Li(+)@C(60)); the resulting ensemble produces long-lived charge-separated states upon photoexcitation of the porphyrins.
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ABSTRACT: Multi-modular supramolecular systems capable of undergoing photoinduced energy and electron transfer are of paramount importance to design light-to-energy and light-to-fuel converting devices. Often, this has been achieved by linking two or more photo-active or redox-active entities with complementary spectral and photochemical properties. In the present study, we report a new triad made out of two entities of subphthalocyanine covalently linked to BF2-chelated azadipyrromethene ((SubPc)2-azaBODIPY). The triad was fully characterized by spectral, computational, electrochemical and photochemical techniques. The B3LYP/6-31G* calculations revealed a structure wherein the donor, SubPc, and the acceptor, azaBODIPY, were well separated with no steric crowding. The different redox states were established from the differential pulse voltammetry studies and the data were used to estimate free-energy change associated with charge separation. Such calculations revealed the charge separation from either the (1)SubPc* or (1)azaBODIPY* to be thermodynamically feasible for yielding the (SubPc)SubPc˙(+)-azaBODIPY˙(-) radical ion-pair. Steady-state fluorescence studies revealed quantitative quenching of (1)SubPc* in the triad and solvent dependent quenching of (1)azaBODIPY* indicating participation of both fluorophores in promoting photochemical events. In nonpolar toluene, singlet-singlet energy transfer from the (1)SubPc* to azaBODIPY was observed, while in polar benzonitrile, evidence of energy transfer was feeble. Femtosecond laser flash photolysis studies provided concrete evidence for the occurrence of ultrafast photoinduced electron transfer by providing spectral proof for the formation of the (SubPc)SubPc˙(+)-azaBODIPY˙(-) charge separated state. The charge recombination followed populating the (3)azaBODIPY* prior to returning to the ground state.Physical Chemistry Chemical Physics 07/2014; · 4.20 Impact Factor
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ABSTRACT: Using near-IR emitting photosensitizers, viz., zinc phthalocyanine (ZnPc), zinc naphthalocyanine (ZnNc), and BF2-chelated azadipyrromethene (azaBODIPY), donor–acceptor conjugates have been newly formed using the well-known metal–ligand axial coordination approach. To accomplish this task, the electron-deficient azaBODIPY was functionalized to possess either two pyridine or imidazole ligating entities. The X-ray crystal structure of one such derivative revealed the presence of axial binding capable pyridine entities on the azaBODIPY macrocycle. The structural integrity of the newly formed conjugates was established from optical absorption, emission, 1H NMR, electrochemical, and computational methods. The experimentally determined binding constants suggested moderately stable conjugates. The absence of excitation energy transfer from singlet excited azaBODIPY to either ZnPc or ZnNc in the conjugates was confirmed from the steady-state emission measurements. However, free-energy calculations suggested photoinduced electron transfer to be exothermic from singlet excited ZnPc or ZnNc to azaBODIPY. Femtosecond transient studies confirmed such predictions from where occurrence of ultrafast photoinduced electron transfer was observed. The measured rates of charge separation and charge recombination (kCS and kCR) revealed charge stabilization to some extent in these conjugates.The Journal of Physical Chemistry C 03/2013; 117(11):5638–5649. · 4.84 Impact Factor
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ABSTRACT: To investigate the supramolecular interactions of the mechanically interlocked rotaxane pendants and conjugated polymer backbones, four analogous polymers were systematically synthesized by copolymerization of a 9-alkylidene-9H-fluorene monomer with various monomers, which contained a diketopyrrolopyrrole unit tethered with a dumbbell unit, a metalated rotaxane, a demetalated orthogonal H-bonded rotaxane, and a simple alkyl chain, to furnish P1, P2, P3, and P4, respectively. Prevailing 1H NMR and UV–vis to NIR titration profiles indicated that the novel polyrotaxane P3 showed a sensitive and reversible acid–base molecular switch capability via supramolecular interactions in contrast to the other polymers (P1, P2, and P4). Compared with the other polymers, P3 possessed a narrower bandgap, which was also confirmed by the computational study. Prominently, the monitoring of a controllable nanoself-assembly process of P3 was obtained by reversible acid–base molecular switch approaches. The orthogonal H-bonded pendant rotaxane unit and the steric demand of P3 judiciously allowed to morph into a hierarchical nanostructure via interconvertible H-bonds, anion−π and π–π stackings, and hydrophobic interactions.Macromolecules 08/2013; 46(17):6731–6743. · 5.93 Impact Factor
Volume 48 | Number 79 | 11 October 2012 | Pages 9785–9940
Shunichi Fukuzumi, Kei Ohkubo, Francis D’Souza, Jonathan L. Sessler
Supramolecular electron transfer by anion binding
This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 9801–98159801
Citethis: Chem. Commun.,2012,48,9801–9815
Supramolecular electron transfer by anion binding
Shunichi Fukuzumi,*abKei Ohkubo,aFrancis D’Souza*cand Jonathan L. Sessler*de
Received 21st April 2012, Accepted 25th May 2012
Anion binding has emerged as an attractive strategy to construct supramolecular electron
donor–acceptor complexes. In recent years, the level of sophistication in the design of these
systems has advanced to the point where it is possible to create ensembles that mimic key aspects
of the photoinduced electron-transfer events operative in the photosynthetic reaction centre.
Although anion binding is a reversible process, kinetic studies on anion binding and dissociation
processes, as well as photoinduced electron-transfer and back electron-transfer reactions in
supramolecular electron donor–acceptor complexes formed by anion binding, have revealed that
photoinduced electron transfer and back electron transfer occur at time scales much faster than
those associated with anion binding and dissociation. This difference in rates ensures that the
linkage between electron donor and acceptor moieties is maintained over the course of most
forward and back electron-transfer processes. A particular example of this principle is illustrated
by electron-transfer ensembles based on tetrathiafulvalene calixpyrroles (TTF-C4Ps). In these
ensembles, the TTF-C4Ps act as donors, transferring electrons to various electron acceptors after
anion binding. Competition with non-redox active substrates is also observed. Anion binding to
the pyrrole amine groups of an oxoporphyrinogen unit within various supramolecular complexes
formed with fullerenes also results in acceleration of the photoinduced electron-transfer process
but deceleration of the back electron transfer; again, this is ascribed to favourable structural and
electronic changes. Anion binding also plays a role in stabilizing supramolecular complexes
between sulphonated tetraphenylporphyrin anions ([MTPPS]4?: M = H2and Zn) and a lithium
ion encapsulated C60(Li+@C60); the resulting ensemble produces long-lived charge-separated
states upon photoexcitation of the porphyrins.
The remarkable structure and function of the photosynthetic
reaction centre1,2has inspired and prompted the design and
preparation of a large number of covalently linked donor–acceptor
ensembles, including dyads, triads, tetrads and pentads, which can
mimic natural energy-transfer and electron-transfer processes.2–14
However, achieving long-lived and highly efficient charge separa-
tions, similar to what is observed in photosynthesis, has often been
hampered by the synthetic difficulties associated with installing the
covalent bonds needed to assemble the requisite covalent array. In
contrast, the use of non-covalent interactions, such as metal–ligand
coordination, electrostatic effects, hydrogen bonds and rotaxane
formation, provides a much easier means of constructing electron
donor–acceptor supramolecular ensembles that in the limit could
mimic the electron-transfer events of photosynthesis.15–20
Among the various non-covalent interactions that might
prove useful for the assembly of electron-transfer systems,
anion binding appears particularly attractive. Recently, anion
recognition has emerged as an important sub-discipline within
the broader field of supramolecular chemistry.21–31A variety
of anion receptors have been synthesized and their use in
stabilizing supramolecular complexes has been reported.21–31
In these latter complexes, the individual components (receptor,
anion) are in equilibrium with the corresponding supramolecular
ensembles. In order to use such supramolecular complexes as
models for the photosynthetic reaction centre, the rates of anion
binding and the dissociation from the supramolecular complexes
must be slower than the electron-transfer processes. Yet, few
studies have addressed the problem of anion binding dynamics in
comparison with the kinetics of photoinduced forward electron
transfer and the associated thermal back electron transfer.
aDepartment of Material and Life Science, Graduate School of
Engineering, Osaka University, ALCA, Japan Science and
Technology Agency (JST), Suita, Osaka 565-0871, Japan.
E-mail: firstname.lastname@example.org; Fax: +81 6 6879 7370;
Tel: +81 6 6879 7368
bDepartment of Bioinspired Science, Ewha Womans University,
Seoul 120-750, Korea
cDepartment of Chemistry, University of North Texas,
1155 Union Circle, #305070, Denton, Texas 76203-5017, USA.
dDepartment of Chemistry & Biochemistry, 1 University Station-A5300,
The University of Texas, Austin, Texas 78712-0165, USA.
eDepartment of Chemistry, Yonsei University, Seoul, 120-749, Korea
Dynamic Article Links
9802 Chem. Commun., 2012, 48, 9801–9815 This journal is c The Royal Society of Chemistry 2012
Given this state of development, recently we have focused
on probing the dynamics of anion binding and dissociation in
several well-defined model systems, as well as studying the
thermal and photoinduced electron-transfer behaviour of
supramolecular complexes formed via anion binding. Our
goals were to map out the kinetics of anion binding and
supramolecular complex formation relative to the rates of
both photoinduced and thermal electron transfer processes
within representative self-associated ensembles. We also
sought to analyse the associated supramolecular electron-
transfer reactions in light of the Marcus theory of electron
transfer.32In this feature article, we review these efforts and
offer suggestions for future research within the non-covalent
electron transfer and anion recognition areas.
2. Dynamics of anion binding and dissociation
The first system found to be suitable for the analysis of anion
binding and photoinduced electron-transfer dynamics is based
on the use of the expanded porphyrin, cyclopyrrole.33An
acetate anion (AcO?) binds to diprotonated form ([C8?2H]2+) to
produce a supramolecular complex, [C8?OAc?Cl], wherein strong
hydrogen bonds are formed between the AcO?substrate and the
pyrrole protons as shown in Scheme 1.34The titration curve of
[C8?2H]2+with AcO?is shown in Fig. 1a, while the corre-
sponding Job’s plot, shown in Fig. 1b, confirms a 1 : 1 binding
stoichiometry. The values of the binding constant (Ka) for the
interaction of AcO?with [C8?2H]2+were determined from the
spectral changes at various temperatures; a van’t Hoff plot then
yielded DH and DS values of 2.8 kcal mol?1and 34 cal K?1mol?1,
respectively.34The positive DH value is indicative of an
endothermic reaction, wherein there is a large positive entropy
contribution. This is common for ion-pairing events, wherein
desolvation occurs to afford a large positive entropy change.35
The kinetics of AcO?binding to [C8?2H]2+was monitored
by following the absorbance change at 425 nm in acetonitrile
(MeCN) at 243 K using stopped-flow methods.34The binding
obeyed first-order kinetics (Fig. 2a) and the observed first-order
rate constant (kobs) was found to increase with increasing AcO?
concentration (Fig. 2b).34Here, kobscould be expressed by
kobs= k1[AcO?] + k?1
where k1is the anion binding rate constant and k?1is the
dissociation rate constant of the supramolecular complex
[C8?OAc?Cl]. By fitting the data to eqn (1), the k1and k?1
values were determined to be (1.9 ? 0.2) ? 107M?1s?1and
200 ? 20 s?1, respectively.34A value for the binding constant Ka
Professor Shunichi Fukuzumi
earned his PhD degree from
Tokyo Institute of Technology
in 1978. He has been a Full
Professor of Osaka University
since 1994. He is the Director
of an ALCA (Advanced Low
Carbon Technology Research
Dr Fukuzumi is also a WCU
Professor at Ewha Womans
University in South Korea.
Kei Ohkubo earned his PhD
degree in applied chemistry
2001. He was a JSPS fellow
and JST research fellow at
Osaka University from 2001 to
2005. He has been a Designated
Associate Professor at Osaka
Francis D’Souza is a Professor
of Chemistry and Materials
Science and Engineering at
the University of North Texas,
Denton, TX. He received his
PhD from the Indian Institute
of Science in 1992, was a post-
University of Houston, USA
Bourgogne, France, and was
a Professor of Chemistry at
the Wichita State University
prior to joining University of
North Texas in 2011.
Jonathan L. Sessler
Professor Jonathan L. Sessler
Chemistry in 1977 from the
Berkeley. He obtained a PhD
from Stanford University in
1982. He was an NSF-NATO
and NSF-CNRS Postdoctoral
Fellow at Universite´ Louis
accepted a position as an
Chemistry at the University
of Texas at Austin, where he
is currently the Roland K.
Pettit Chair. Dr Sessler is also a WCU Professor at Yonsei
University in South Korea.
This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 9801–98159803
was determined from the ratio of k1/k?1and found to be
(9.5 ? 0.9) ? 104M?1, which agreed within experimental error
with that determined by the direct titration in Fig. 1a.34This
concordance, although not a ‘proof’, serves as important
check for the kinetic parameters obtained via stopped flow.
The off-rate of 200 s?1corresponds to a lifetime, t, of 5 ms.
In contrast to simple carboxylates, dihydrogen phosphate
(TBAH2PO4) forms 2 : 1 complexes with C8?2HCl as reported
for protonated expanded porphyrins.36The time profiles for
complex formation under the conditions of stopped-flow
mixing of TBAH2PO4with C8?2HCl at 253 K are shown in
Fig. 3. Based on inspection of this figure, it was inferred that
the binding of TBAH2PO4 with C8?2HCl proceeds via two
steps, namely a fast phase (Fig. 3a) and a slow phase (Fig. 3b).34
The fast process corresponds to the binding of first one,
and then a second H2PO4?(P) moiety, as shown in eqn (2)
and (3) (Fig. 3d). The rate constants for the first step were
determined to be k1= (2.65 ? 0.1) ? 105M?1s?1and k?1=
16.6 ? 0.3 s?1as deduced from a global kinetic analysis.34
These values were then used to estimate an association constant,
Ka, of (1.6 ? 0.1) ? 104M?1. The same global kinetic analysis
gave k2= (4.1 ? 0.5) ? 103M?1s?1and k?2= (3.9 ? 0.6) s?1
for the second step.34This corresponds to a lifetime of 250 ms,
which is long enough to allow intra-ensemble charge separation
and recombination without appreciable breakup of the ensemble.
These electron-transfer studies are summarized below.
3. Supramolecular photoinduced electron transfer
3.1. Pyrene–C8 complex
When a pyrene carboxylate anion (as the TBA salt) was mixed
with C8?2HCl, spectral changes were seen that are similar
to those observed in the case of AcO?(cf. Fig. 1). Such
changes reflect the formation of the supramolecular complex
[C8–Py] (Fig. 4a).37The binding constant (Ka) corresponding
to formation of C8–Py was determined from these spectral
changes (cf. Fig. 4b) to be (2.6 ? 0.3) ? 105M?1in MeCN
at 298 K.37
Transient absorption spectra obtained 10 ms after laser
excitation at 355 nm (Fig. 5) are consistent with forma-
tion of C8?+(740 and 820 nm) and Py??(480 nm) rather
than C8??and Py?+.35This finding led us to propose that
photoinduced electron transfer occurs from C8 to the singlet
excited state of Py (1Py*) thus producing the charge-separated
C8?2HCl with TBAOAc.
Model used to fit the kinetic data obtained upon mixing
(1.1 ? 10?5M) and TBAOAc in MeCN at 298 K (TBA = tetra-n-
butylammonium). (b) A Job’s plot was also constructed; it displayed a
maximum at a mole fraction of 0.5, as would be expected for a 1 : 1
(a) Steady-state spectral titration performed between C8?2HCl
post-mixing concentration) with TBAOAc at the following final
concentrations: 5, 10, 20, 30, 50 mM in MeCN at 243 K. The mixing
time (ca. 3 ms) is shown as dotted lines in these traces. The solid lines
represent the fit of a single exponential decay of the signal, giving the
observed rate constant, kobs. (b) Plot of kobsas a function of the
concentration of guest, AcO?with a solid line showing the fit to kobs=
k1[AcO?] + k?1, where k1is the slope, and k?1is the y-intercept.
(a) Kinetic traces produced by mixing C8?2HCl (4 mM final
under stopped flow conditions with 500 mM of TBAH2PO4in MeCN
at 253 K showing the evolution over the time scales of (a) 2–20 ms and
(b) 20–500 ms. (c) Change in the spectral intensity at 425 nm observed
when C8?2HCl (10 mM) is mixed with 50, 100, 200, 400, 500 mM of
TBAH2PO4in MeCN at 253 K (all concentrations are final, post-
mixing). The dotted lines are experimental data points corresponding
to the decay in the C8 absorbance at 425 nm, while the solid lines are
the best fits obtained using a global fitting procedure. (d) Equations
used for the global fitting analysis shown in part (c), wherein P
Optical spectra obtained when C8?2HCl (10 mM) is mixed
9804 Chem. Commun., 2012, 48, 9801–9815 This journal is c The Royal Society of Chemistry 2012
(CS) state, C8?+–Py??; this happens even though the energy
of the putative C8?+–Py??state (2.58 eV) is higher than that
of C8??–Py?+(1.31 eV).
The reason whyC8?+–Py??isformed instead ofC8??–Py?+is
well rationalized by the Marcus theory of electron transfer,32
which predicts that the charge separation to produce the lower
energy CS state (C8??–Py?+) with a large driving force of 2.15 eV
is deeply in the Marcus inverted region. This makes the CS rate
much slower than the CS rate required to produce the high energy
CS state of C8?+–Py??in the Marcus normal region for which
the driving force would be 0.88 eV.
The charge reversal seen in the ‘‘umpolung’’ system defined
by C8 and Py has also been reported for electron transfer from
electron donors to1Py*.38In the present instance, the CS rate
constant was determined to be 5.1 ? 106s?1from the decrease
in the fluorescence lifetime of C8–Py (140 ns) as compared to
that of the reference pyrene (480 ns). The slow CS was
confirmed by femtosecond laser flash photolysis, where little
change was observed in the transient absorption spectrum due
to the dynamic behaviour of1Py* in C8–Py.
The CS state decays to the triplet excited state (3C8*) rather
than to the ground state, as inferred from the fact that the
transient spectrum of the ensemble recorded at 900 ms in Fig. 5
agrees with that observed for
photoexcitation of C8 alone.37This mode of recombination is
also rationalized in terms of Marcus theory. Specifically, the
slower back electron transfer to the ground state as compared
to the triplet excited state reflects a process that lies in the
Marcus inverted region, wherein electron-transfer becomes
slower as the driving force of electron transfer increases.32,39
Fitting the decay at both 470 and 740 nm to a single
exponential results in a charge-recombination (CR) rate constant
of (3.3 ? 0.1) ? 103s?1, a value that corresponds to a lifetime of
the CS state of 300 ms.37Changing the excitation laser power has
no effect on the rates, as underscored by the invariance of the
first-order plots. This indicates that intramolecular electron
transfer occurs within the supramolecular complex. As expected,
the use of pyrene-1-butyric acid, instead of the base (i.e. pyridine)
in conjunction with C8 under otherwise identical conditions,
yielded none of the peaks characteristic of C8?+following
photoirradiation because the anion binding is essential for
formation of the supramolecular complex. Thus, anion binding
provides a convenient way to construct an electron donor–
acceptor supramolecular complex that gives rise to long-lived
CS states. Although long-lived, it is important to appreciate that
the lifetime of the CS state is much shorter than the dissociation
lifetime of the supramolecular complex (vide supra). As a con-
sequence, the donor–acceptor moieties remain assembled (and
hence intact) during the course of both forward photoinduced
electron transfer and subsequent back electron transfer.
3C8* species produced upon
3.2Ferrocene carboxylate–diprotonated porphyrin complexes
A different electron-transfer complex stabilized by hydrogen bonds
may be prepared from ferrocenecarboxylic acid (FcCOOH)
and H2DPP (dodecaphenylporphyrin) [H4DPP(FcCOO)2].
The resulting ensemble is shown in Fig. 6, wherein the X-ray
crystal structure of H4DPP(FcCOO)2is shown.40In the crystal
structure of H4DPP(FcCOO)2, H4DPP2+and two ferrocene-
carboxylate (FcCOO?) anions were found to be linked by
hydrogen bonds, albeit in an asymmetric fashion. On one
side of the saddle-distorted H4DPP2+macrocycle, two-point
hydrogen bonds, involving two of the pyrrole N–H protons and
two of the oxygen atoms in the bound FcCOO?carboxylate
anion, are seen. These bonds are characterized by interatomic
(N???O) distances of 2.726(5) and 2.587(4) A˚, respectively. On
the other side of the ring, hydrogen bond interactions, involving
two of the four N–H pyrrolic protons and one of the FcCOO?
carboxylate oxygen atoms, are seen. These interactions give rise
to interatomic distances of 2.773 (4) and 2.763 (5) A˚. Both
FcCOO?anions were found to reside in a cleft-like environ-
ment created by the two phenyl groups attached to the pyrroles
of the porphyrins (cf. Fig. 6).41–43
A 1 : 2 supramolecular complex between H4DPP2+and
FcCOO?[H4DPP(FcCOO)2] was also formed in benzonitrile
of tetra-n-butylammonium 1-pyrenebutyrate, Py, into cyclopyrrole,
C8, at 1.5 ? 10?5M in MeCN at 298 K. Inset: curve fit (line) to a
1 : 1 binding isotherm produced from the change in absorbance at
1155 nm (points).
(a) Proposed complex formation. (b) UV-vis absorption titration
5.0 ? 10?5M in deaeratedMeCN at 298 K taken at 10 ms (n) and 900 ms
(K) after laser photoexcitation (355 nm, 25 mJ per pulse).
Transient absorption spectra of C8 and Py, in a 1 : 1 ratio at
This journal is c The Royal Society of Chemistry 2012Chem. Commun., 2012, 48, 9801–98159805
(PhCN) as indicated by the UV-vis titration shown in Fig. 7a.40
The Job’s plot in Fig. 6b confirms the 1 : 2 binding stoichio-
metry. The observed one-step spectral change in the course of
the titration with FcCOOH is consistent with the presence of
hydrogen-bonding interactions in solution that mirror those
observed in the crystalline state. This is an important observa-
tion because formation of hydrogen bonds between the N–H
protons of pyrroles and carboxylate anions in solution is
known to facilitate diprotonation of the core H2DPP moiety.44
However, this fate is avoided in the present instance.
The formation constant of H4DPP(FcCOO)2was determined
from the spectral change to be 9.2 ? 108M?2. The presence of
hydrogen bonds was further supported by the1H NMR spectral
changes observed over the course of the FcCOOH addition.
Specifically, after adding up to 2 equiv. of FcCOOH, peaks
assigned to the meso-phenyl protons of H4DPP2+were all
downfield-shifted, which is consistent with the formation of
H4DPP2+.44–46No further shift was observed upon the addition
ofmorethan 2equiv. ofFcCOOH.In contrast,upon the addition
of increasing quantities of FcCOOH, the signals ascribed to the
ferrocene moiety exhibited upfield shifts, a finding that is ascribed
to shielding caused by the ring current of the porphyrin ring. In
CDCl3, the largest upfield shift was seen in the case of the proton
adjacent to the carboxylate moiety; this is consistent with the
presence of a hydrogen bonding interaction between the pyrrole
NH protons and the carboxylate group of FcCOO?.40
Rate constants of photoinduced electron transfer and back
electron transfer in H4DPP(FcCOO)2 were determined by
femtosecond laser flash photolysis measurements.40After
femtosecond laser excitation at 430 nm, the transient absorption
bands at 550 and 1030 nm due to the singlet excited state of the
H4DPP2+moiety,1[H4DPP2+]*, were observed at 1.5 ps. The
spectrumthen evolved to thatassigned to the one-electron reduced
species of H4DPP2+, i.e., H4DPP?+in the electron-transfer state
corresponding to the self-associated ensemble H4DPP(FcCOO)2.
From the rise and the decay of absorbance at 545 nm the rate
constants for the photoinduced electron transfer and back electron
transfer were determined to be 5.0 ? 1011s?1and 6.1 ? 1010s?1,
respectively.40Similarly, the rate constants (kET) of photoinduced
electron transfer and back electron transfer were determined for
the supramolecular complexes of H4DPP2+with other ferrocene
derivatives, which likewise contained carboxylate anions as the
anion linking motif.40The driving force dependence of kETfor
these systems was analyzed using the Marcus equation for non-
adiabatic intramolecular electron transfer (eqn (4)),32where V is
the electronic coupling matrix
V2exp ?ðDGETþ lÞ2
element, h is the Planck constant, T is the absolute temperature,
DGETis the free energy change of electron transfer and l is the
reorganization energy of electron transfer. Plots of logkETvs.
DGETare shown in Fig. 8.40The curve fit of the kinetic results
for a variety of electron-donors, obtained using eqn (4), is shown
in Fig. 8. Inspection of this figure reveals a small reorganization
energy (l = 0.68 ? 0.03 eV) and a large electronic coupling
matrix element (V = 43 ? 7 cm?1). The l value is comparable
to those of covalently linked donor–acceptor systems con-
sisting of electrically neutral porphyrins as electron donors
(0.41–0.66 eV).9,47The small l value has the consequence that
the back electron transfer falls in the Marcus inverted region,
where the CS lifetime becomes longer with increasing driving
force. The large V value indicates that the electrostatic and
directions. Gray: carbon, blue: nitrogen, red: oxygen, orange: iron.
Hydrogen atoms and solvent molecules are omitted for clarity.
Crystal structure of H4DPP(FcCOO)2viewed from different
titration of H2DPP with FcCOOH in PhCN at room temperature.
Inset: absorbance change at 470 nm (black line) and 500 nm (red line).
(b) A Job’s continuous variation plot to determine the stoichiometry of
the complex formation between H2DPP and FcCOOH: molar fraction
of H2DPP is varied while keeping the total concentration constant.
(a) Absorption spectral changes observed over the course of a
intrasupramolecular photoinduced electron transfer and back electron
transfer in supramolecular complexes of H4DPP2+with hydrogen-
bonded electron donors in PhCN at room temperature. Fits to the
Marcus equation for electron transfer (eqn (4)) are shown by the solid
line with l = 0.68 eV and V = 43 cm?1and the dotted lines with
l = 0.65 and 0.71 eV and V = 43 cm?1, respectively.
Driving force dependence of logkET(K) or kBET(E) for
9806 Chem. Commun., 2012, 48, 9801–9815 This journal is c The Royal Society of Chemistry 2012
hydrogen-bonding interactions in the supramolecular complex
established as the result of anion binding are strong enough to
establish strong electronic coupling between the electron donor
and acceptor moieties.
The dependence of kETon distance within intrasupramolecular
photoinduced electron-transferensembles was examined by using
ferrocenecarboxylic acid derivatives having linear phenylene
linker(s) between the ferrocene moiety and the carboxyl group.
This arrangement is shown in Fig. 9, where the distance between
electron-donor and -acceptor is defined as that between the
Fe atom of ferrocene and the centre of the mean plane of the
porphyrinringinH4DPP2+.40Forthesesystems, the dependence
of the rate constant for intrasupramolecular electron transfer
(kET) on distance is given by eqn (5),32
lnkET= lnk0? br (5)
where k0is the rate constant for adiabatic intrasupramolecular
electron transfer, r is the donor–acceptor centre-to-centre
distance and b is the decay coefficient factor (damping factor),
which depends primarily on the nature of the bridging molecule.48
A plot of lnkETvs. r is shown in Fig. 9. From the slope of this
linear plot, the b value was determined to be 0.64 A˚?1, which is
comparable to those of covalently phenylene-bridged multi-
porphyrin systems.49Thus, the electron donor–acceptor linkage
by anion binding in this series of supramolecular complexes
provides electron coupling interactions between the electron
donor–acceptor moieties, which are as strong as those present
in comparable covalently linked systems.
Control of supramolecular electron transfer by
4.1. Stabilization of the supramolecular CS state
complexes but also a novel way to control the electron-transfer
dynamics. In this context, supramolecular oligochromophoric
systems containing sites for binding of a reagent species and an
anionic species are particularly attractive since they may allow
the effect that the binding of an anionic cofactor has on both
the photoinduced electron-transfer and back electron-transfer
processes.50One system designed with this goal in mind is the
supramolecular complex (1) shown in Fig. 10. This bis-porphyrin-
substituted oxoporphyrinogen (1) has two different binding sites:
one site (composed of two porphyrinatozinc(II) units) capable of
binding bis(4-pyridyl)-substituted guests through coordination to
the central zinc cations,51whereas the other site (composed of two
pyrrole-type amine groups of the oxoporphyrinogen unit)
interacts with anionic species through hydrogen bonding.52
The anion binding site, the oxoporphyrinogen (OxP) unit,
prepared by the 2-electron oxidation of tetrakis(3,5-di-tert-
butyl-4-hydroxyphenyl)porphyrin, can be multiply substituted
at the central nitrogen atoms in a stepwise and regioselective
manner, thus allowing for the synthesis of 1.53
The formation of the two-host bound supramolecular guest
system was accomplished in two steps, viz., Py2C60binding to
the host system, 1, to form 1–Py2C60, and binding of the anion
to the OxP anion binding site to yield the supramolecular
complex 1(F?)–Py2C60. The unique topology of 1, which
provides for a V-like alignment of the porphyrin macrocycles,
is thought to favour supramolecular complex formation
since it permits the well-established ‘two-point’ coordinative
binding of the bis-pyridine functionalized fullerene.51The
binding constant was determined from a Benesi–Hildebrand
plot54to be 1.9 ? 105M?1; this a value that is nearly two
orders of magnitude larger than that recorded for a mono
pyridine fulleropyrrolidine binding to a zinc porphyrin.55This
difference is ascribed to the cooperative nature of the inter-
action between the bis-pyridine functionalized fullerene and
the appropriately positioned zinc porphyrin macrocycles
present in 1.51,56A Job’s plot provided support for the
proposed 1 : 1 binding stoichiometry leading to formation of
The binding of the fluoride anion (as the corresponding
tetra-n-butylammonium salt (TBAF)) to the oxoporphyrinogen
(OxP) subunit present in 1 gives rise to a red shift in its
absorbance band from 513 nm to 530 nm.50,57Notably, no
further shifts of the porphyrin absorbance bands occur on
addition of excess fluoride anions in o-dichlorobenzene solution
reagent–anion cofactor complexation (left figure, peripheral substituents
are omitted for clarity) exhibited by the combination of a bis-porphyrin-
substituted oxoporphyrinogen (1) and a bis(4-pyridyl)-substituted
fullerene. Axial coordination modes and the anion binding site are
emphasized for clarity.
Chemical structure and proposedstructural origin of two-guest
electron transfer in ensembles derived from the electron donors
((1) FcCOO?, (6) FcPhCOO?, and (7) FcbphCOO?) and the excited
state species1[H4DPP2+]* produced as the result of photoexcitation.
Here, the distance between an electron donor and an electron acceptor
is defined as that between the Fe atom of the ferrocene moiety and the
centre of a mean plane of the porphyrin ring.
Dependence of lnkET on distance for intrasupramolecular
This journal is c The Royal Society of Chemistry 2012Chem. Commun., 2012, 48, 9801–98159807
of the 1–Py2C60complex. This lack of spectral change provides
support for the notion that the fluoride anions do not interact
with the porphyrinatozinc entities. This lack of binding to these
latter sites is ascribed in part to the preferential binding of F?to
OxP as revealed by the larger binding constant values, and the
strong, competitive binding of Py2C60to the porphyrinatozinc
moieties through the two-point binding motif.51This selective
binding makes the dual-guest recognition process fully deter-
mined and ensures the integrity of the overall supramolecular
ensemble. The binding constant for F?binding to the OxP
pocket present in the preformed 1–Py2C60complex was deter-
mined to be 7.4 ? 104M?1. Moreover, the binding stoichio-
metry from a Job’s plot was found to be 1 : 1 for this
interaction. Acetate can also bind to 1 with a similar binding
constant (7.4 ? 104M?1). However, no binding to 1 was
observed when ClO4?was employed as an anion.
Anion binding of F?to OxP produces a cathodic shift of
nearly 600 mV for the first oxidation process.58Similar
potential shifts were observed for other strongly binding
anions; however, weakly binding anions, such as ClO4?, did
not produce a substantial potential shift.58This finding,
considered in light of the fact that addition of perchlorate
anions to the 1–Py2C60 complex in solution produced no
detectable spectral changes, leads us to suggest that this anion
does not bind appreciably to the OxP recognition site.
The oxidation peak of the OxP present in the 1–Py2C60
complex falls at 0.48 V in o-dichlorobenzene (DCB) containing
0.1 M TBAPF6. Although irreversible, this peak is anodically
shifted by over 600 mV upon exposure to F?. It is important to
note that in the presence of F?, neither the redox potential of
ZnP nor that of C60experienced a significant perturbation.
Again, this absence of change is taken as an indication that
these entities do not interact with the fluoride anion. Similar
voltammetric behaviour was observed for strongly interacting
acetate and dihydrogen phosphate anions but not for the
weakly interacting perchlorate anions. The fact that the anion
bound form of OxP is more readily oxidized than the ZnP
components leads us to suggest that the OxP moiety could act
as the terminal electron donor in the supramolecular complex,
1(F?)–Py2C60, upon photoexcitation of the overall ensemble.
Specifically, we propose that electron transfer from the singlet
excited porphyrinatozinc (1ZnP*) to the coordinated fullerene,
electron acceptor (Py2C60) will take place after photoexcitation.
This will then be followed by electron transfer from 1(F?) to
ZnP?+to produce the final CS state.50That is, the role of
anion bound OxP in stabilizing the charge-separated state in
the donor–acceptor conjugate is evident.
Considerable experimental evidence has now been obtained
in support of the above proposal. For instance, in the absence
of F?, excitation of the porphyrinatozinc results primarily in
charge separation leading to the formation of a ZnP?+–C60??
(energy = 1.40 eV) state that persists for a few hundred
nanoseconds before relaxing to the ground state. Complexation
of F?to the OxP centre lowers its oxidation potential by nearly
600 mV, as noted above; this creates an intermediate energy
state that allows for charge migration from the ZnP?+moiety
to the F?–OxP centre (energy = o0.83 eV). The increase in the
reorganization energy of electron transfer combined with the
decrease in CR driving force caused by F?binding results in a
decrease in the rate of the CR process. This, in turn, leads to
an increase in the lifetime of the triplet supramolecular CS
state. The CS lifetimes decrease in the order 14 ms (F?) > 11 ms
(CH3COO?) > 7.5 ms (H2PO4?) > 0.17 ms (ClO4?) D 0.16 ms
(none) s?1(Fig. 11). Generally, the stronger the binding, the
longer the lifetime of charge-separation.50
The direct effect of F?on photoinduced electron transfer
has been shown in a donor–acceptor triad that contains two
fulleropyrrolidine (C60) units substituted through 4,40-biphenyl-
methylene groups at the nitrogen atoms of an oxoporphyrinogen
(OxP) unit. This ensemble is shown schematically in Fig. 12.59In
this case, the fluoride anion (TBAF was used as the F?source)
binding constant was determined to be 5.8 ? 104M?1on the basis
of a normal spectroscopic titration, leading us to suggest that the
Both the OxP and C60subunits of the triad shown in Fig. 12
can act as electron-accepting agents.50However, in this case
the OxP core is expected to function as an electron donor given
its ease of oxidation relative to the fulleropyrrolidine groups.
o-dichlorobenzene (concentration of each component: 3.0 ? 10?5M)
recorded 1 ms after nanosecond laser excitation at 450 nm. (b) Decay of
the absorbance at 1000 nm (corresponding to fullerene anion radical)
following photoexcitation of 1(F?)–Py2C60. Inset: 0–8 ms time range.
(c) Analogous decay profile for 1(CH3COO?)–Py2C60at 1000 nm.
(d) Analogous decay profile for 1(H2PO4?)–Py2C60at 1000 nm.
(a) Transient absorption spectrum of 1(F?)–Py2C60 in
probe the effect of anion binding on charge separation.
Schematic representation of the OxP(C60)2triad, 2, used to
9808 Chem. Commun., 2012, 48, 9801–9815 This journal is c The Royal Society of Chemistry 2012
Both the OxP and C60components are fluorescent with
the doubly substituted OxP being more strongly emissive.51
Excitation of either of the entities is expected to induce
photochemical processes in the triad. The two imino hydrogens
of the OxP unit possess the ability to bind anions with high
stability52resulting in a significantly negative shift in the
one-electron oxidation potential of the OxP unit.59That
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.
The validity of this proposition was tested using model
Upon addition of F?to a solution of 2 in o-dichlorobenzene
containing 0.1 M (TBA)PF6drastic changes were found in the
oxidation potentials of the OxP entity. That is, addition of
1.1 equiv. of F?to a solution of 2 resulted in a cathodic shift of
nearly 510 mV (E1/2= ?0.14 V), presumably as the result
of F?binding to the OxP unit (Fig. 14). In contrast, the
fullerene reduction underwent a small anodic shift of 60 mV
(E1/2= ?1.22 V) due to the ion-pairing effect.60However, no
appreciable shifts were seen for OxP reduction.59
The driving forces for the charge recombination (?DGCR)
and charge separation (?DGCS) processes that were expected
to occur in the case of 2 following photoexcitation via the
singlet excited state of OxP and C60were evaluated to be 1.65
and 0.10 eV, respectively. These values take into consideration
that the energy of the singlet excited states of OxP and C60
being 1.75 eV. In the case of 2(F?), the ?DGCRvalue 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.59
As shown in Fig. 15, the transient absorption spectrum
exhibited peaks at 860 and 1000 nm that are ascribed to the
OxP?+and C60??radicals, respectively.17,50The rate of CS
from the triplet excited state was too fast to detect within the time
resolution of the nanosecond laser pulse. However, by monitoring
could be determined. This corresponds to a triplet CS state
lifetime of 6.3 ms. The photoinduced electron transfer and back
electron transfer processes that occur via the singlet and triplet
excited states of OxP are summarized in Scheme 2.59
To exploit the features of these self-assembled ET ensembles, a
supramolecular solar cell consisting of TiO2modified with a zinc
porphyrin–oxoporphyrinogen (ZnP–OxP) dyad was constructed
of added F?(0.1–0.5 equiv. as the TBA salt). (b) Benesi–Hildebrand
plot constructed to obtain the binding constant, and (c) mole ratio plot
used to obtain the molecular stoichiometry of the 2(F?) complex in
(a) Absorption spectra of 2 (7.5 mM) recorded as a function
presence (red line) of F?(as the tetra-n-butylammonium salt) in
deaerated DCB containing 0.10 M TBAClO4. Scan rate = 100 mV s?1.
Cyclic voltammograms of 2 in the absence (black line) and
DCB. Inset shows the time profile of the C60??at 1000 nm.
Nanosecond transient absorption spectra of 2(F?) in deaerated
of 2(F?) via the singlet and triplet OxP in deaerated DCB.
Energy level diagram showing the electron-transfer processes
This journal is c The Royal Society of Chemistry 2012Chem. Commun., 2012, 48, 9801–98159809
in the presence of the I3?/I?redox couple (Scheme 3).61The
sensitizing ZnP–OxP dyad was immobilized by metal–ligand
axial coordination onto a thin-film semiconducting TiO2
surface functionalized with a phenylimidazole coordinating
ligand on an FTO transparent electrode. In this set up, the
ZnP entity of the dyad is coordinated to the imidazole ligand
leaving the OxP site available for anion binding. The
ZnP–OxP:F?species formed upon F?binding is characterized
by a cathodic shift in the OxP:F?oxidation couple by about
450 mV. This makes oxidation of the bound OxP site about
380 mV more facile than oxidation of the ZnP component.
The OxP:F?moiety thus serves as a hole-transfer agent to
ZnP?+during electron-transfer events. This serves to slow
down the back electron transfer from the one-electron reduced
TiO2to the ZnP?+(Scheme 3). In other words, the relative
redox potentials of OxP:F?and ZnP permit efficient electron
transfer and hole migration, thereby improving the perfor-
mance of this proof-of-concept solar cell.61
binding of anions and cations
Reversible supramolecular electron transfer controlled by
As described above, anion binding results in a negative shift in
the one-electron oxidation potentials of the electron donors,
leading to stabilization of the CS state of the anion-bound
electron donor–acceptor dyads. Such a change in the one-
electron oxidation potential of the electron donors by anion
binding can also be used to control the thermal (i.e. ground
state) electron transfer from an electron donor to an electron
acceptor, provided these moieties are part of an appropriately
designed supramolecular electron-transfer complex. Substrate
binding can also be used to modulate the redox potentials of the
ensemble and help switch electron transfer ‘‘on’’ or ‘‘off’’. As a
proof of these design principles, we recently created a non-
covalent ensemble based on a tetrathiafulvene calixpyrrole
(TTF-C4P) donor62and a dicationic mesityl quinone (BIQ2+)
acceptor.63Using this system, we found that the addition of
selected anions or cations could be used to control the direction
of electron transfer.64
When TTF-C4P (30 mM) is mixed with 1 molar equiv. of
BIQ2+?2PF6?, no reaction was observed.64Under these condi-
tions the TTF-C4P exists in its so-called 1,3-alternate conforma-
tion whose size and shape precludes effective supramolecular
interactions with BIQ2+. When tetra-n-hexylammonium
chloride (THACl) was added to the solution of TTF-C4P
and BIQ2+?2PF6?, however, new absorption bands at 379 nm,
751 nm, and 1995 nm appeared at the expense of the original
TTF-C4P absorption band (lmax= 329 nm). A clear isosbestic
point at 340 nm is seen, as shown in Fig. 16a.64Under these
conditions, switching of the TTF-C4P unit to its so-called cone
conformation is expected. The absorption bands at ca. 751 and
1995 nm are ascribed to TTF?+and [TTF]2?+radical cations
derived from TTF-C4P, whereas the absorption band at
379 nm is assigned to the reduced radical BIQ?+. Thus,
electron transfer from TTF-C4P to BIQ2+?2PF6?occurs in
the presence of THACl and the yield of the electron-transfer
products increases with increasing concentration of THACl
(Fig. 16a).64Once produced, no further changes in these
optical features were observed under ambient conditions.
However, addition of TEACl to a CHCl3 solution of
TTF-C4P?+and BIQ?+resulted in back electron transfer
from BIQ?+to TTF-C4P?+as indicated by a decrease
in absorbance intensity at ca. 751 and 1995 nm, accom-
panied by restoration of the absorbance feature at 329 nm
ascribed to the TTF-C4P receptor (Fig. 16b).64These optical
changes are consistent with ‘‘switched on’’ anion-induced
electron transfer from TTF-C4P to BIQ2+?2PF6?and its
subsequent cation-triggered reversal (‘‘switching off’’) as
shown in Scheme 4.64
Structural analysis of the crystals of the electron-transfer
product produced by mixing TTF-C4P to BIQ2+?2PF6?in the
presence of the chloride anion revealed a supramolecular
donor–acceptor ensemble, wherein two anion-bound bowl-like
TTF-C4P moieties encapsulate a bis-imidazolium quinoneguest
in a 2 : 1 fashion (Fig. 17a). Based on the structural parameters,
this capsule was thought to consist of a tightly coupled biradical
species best described as [TTF-C4P]?2+?BIQ?+?2(Cl?).64
The existence of a tightly coupled biradical ‘‘capsule’’ was
further confirmed by low temperature (4 K) EPR analyses
of single crystals of the [TTF-C4P]?2+?BIQ?+?2(Cl?) salt.64
A typical triplet signal with zero-field splitting was observed
coordination onto the TiO2/FTO solar cell for demonstrating enhanced
photocurrent generation as a result of F?binding to OxP. The proposed
photochemical electron- and hole-transfer events are shown using arrows.
Schematic view of the ZnP–OxP surface modified via axial
is treated first with 1 molar equiv. of BIQ2+?2PF6?and then titrated
with increasing quantities of THACl in CHCl3. The final spectrum was
recorded in the presence of 10 equiv. The inset shows the EPR spectra
of TTF-C4P (1.1 ? 10?4M) in CHCl3recorded at room temperature
in the presence of 1 molar equiv. of BIQ2+?2PF6?and after incre-
mental addition of THACl. (b) Cation induced reverse electron
transfer seen upon the step-wise addition of up to 15 equiv. of TEACl
to a 1 : 1 solution of TTF-C4P and BIQ2+?2Cl?in CHCl3. The inset
shows the EPR spectra of TTF-C4P, in the presence of 1 equiv. of
BIQ2+?2Cl?at 1.1 ? 10?4M in CHCl3recorded at room temperature
upon incremental addition of TEACl up to 5 molar equiv.
(a) Spectroscopic changes observed when TTF-C4P (30 mM)
9810Chem. Commun., 2012, 48, 9801–9815This journal is c The Royal Society of Chemistry 2012
at g = 4.08 and 2.007, respectively (Fig. 17b). The zero field
splitting parameter (D) when plotted against the angle of
rotation for the single crystal, resulted in a sine curve, a
finding that is consistent with the presence of an intra-
molecular radical ion pair.64
The electron acceptor, the BIQ2+cation, used in the
supramolecular donor–acceptor complex with TTF-C4P can
be replaced by a different electron acceptor that has a similar
one-electron reduction potential. Fullerenes are attractive in
this context. To date, supramolecular complexes of C60with
electron donor hosts have attracted significant attention
because of their ability to promote efficient photoinduced
electron transfer reactions.65–69However, the ground state
C60is not a sufficiently strong electron acceptor to support a
thermal ET reaction in the case of TTF-C4P.70Lithium ion
encapsulated C60 (Li+@C60) is known to act as a more
Li+@C60is employed as an electron acceptor in conjunction
with TTF-C4P, a supramolecular ET complex is formed in the
presence of a triggering anion (vide infra).72
Upon mixing [Li+@C60]PF6and TTF-C4P in PhCN no
evidence of electron transfer is observed. This is as expected
since the one-electron oxidation potential of the TTF-C4P
receptor (Eox vs. SCE = 0.51 V) is higher than the one-
electron reduction potential of the putative guest, Li+@C60
(Eredvs. SCE = 0.14 V).72However, the addition of tetra-
n-hexylammonium chloride (THACl) to this solution of
TTF-C4P and Li+@C60induces electron transfer from the
TTF-C4P to the Li+@C60, presumably triggered by binding of
the Cl?anion to the calixpyrrole moiety. Electron transfer
was inferred from the appearance of an absorption band at
1035 nm ascribable to the one-electron reduced species produced
from Li+@C60 (Li+@C60??) as shown in Fig. 18a.72The
absorbance at 1035 nm was also seen to increase with increasing
THACl concentration to reach a constant value at the point
where electron transfer is complete (Fig. 17b).71As described
above, Cl?binds to TTF-C4P to induce a conformation change
from the so-called 1,3-alternate to the cone conformation due to
concerted NH–anion hydrogen bonding interactions, a change
that allows initial substrate binding and stabilization of the
radical ion pair consisting of the TTF-C4P?+(Cl?) host and
the bound Li+@C60??guest (Scheme 5).72In PhCN, Cl?
interacts with TTF-C4P in a 1 : 1 manner with an effective
binding constant of ca. 1.9 ? 104M?1(Fig. 18b).72
The radical ion pair between TTF-C4P?+(Cl?) and
Li+@C60??was detected by EPR (Fig. 19). Here, a four-line
signal due to TTF-C4P?+(Cl?) in which the electron spin is
localized at only one TTF moiety (aN= 0.91 G; aH= 1.10 G),
rather than delocalized within a p-dimer radical cation complex
formed by reaction with another TTF moiety, is observed.70
This finding provides support for the proposed inclusion of
Li+@C60??as a guest within the cavity provided by the cone
conformation of TTF-C4P?+(Cl?). Such a binding mode
would preclude the formation of a p-dimer radial cation.
their proposed ion-mediated electron transfer reactions.
Chemical structures of TTF-C4P and BIQ2+salts, and
donor–acceptor complex of net stoichiometry [TTF-C4P]2?[BIQ2+]?2(Cl).
(b) ESR spectra of a crystal of TTF-PQMes recorded at different
crystal angles at 4 K (upper panel). The angle variation of the D value.
(a) Single crystal X-ray structure of the supramolecular
electron transfer from TTF-C4P (5.0 ? 10?5M) to Li+@C60
(5.0 ? 10?5M) in the presence of increasing concentrations of
THACl in PhCN. (b) Plot of absorbance at 1035 nm vs. concentration
of Cl?. Inset: plot used to determine the approximate binding constant
for the interaction of THACl with TTF-C4P in PhCN at 298 K;
a = (A ? A0)/(AN? A0).
(a) Near-IR absorption spectral change in Cl?-promoted
This journal is c The Royal Society of Chemistry 2012Chem. Commun., 2012, 48, 9801–98159811
The observation of the signal at g = 4.43 (Fig. 19b)
provides support for the notion that two unpaired electrons
interact to form a triplet state. From the zero-field splitting
value (D = 100 G) in Fig. 19c, the distance (r) between two
electron spins was estimated using the relation, D = 27800/r3,
to be 6.5 A˚. This distance is commensurate with the distance
between the TTF-C4P?+(Cl?) receptor and the bound
Li+@C60??guest observed in the X-ray crystal structure of
the radical ion pair, as shown in Fig. 20.72The lack of any
additional charged species within the crystal lattice provides
further support for the suggestion that an overall neutral
complex is formed between a bowl-like TTF-C4P?+radical
species a tightly encapsulated Li+@C60??guest.
When tetraethylammonium chloride (TEACl) was added
to the electron-transfer products produced from TTF-C4P
and Li+@C60in the presence of THACl in mixed PhCN–
CHCl3solution, the absorbance due to Li+@C60??was found
to decrease as a function of increasing TEACl concentra-
tion as shown in Fig. 21.72The TEA+binds effectively to
the TTF-C4P cavity, thereby repelling Li+@C60??from the
cavity; this results in back electron transfer from Li+@C60??
to TTF-C4P?+to produce the original Li+@C60 and the
TEA+-encapsulated form of TTF-C4P (Scheme 5).72The
effective binding constant of TEA+to TTF-C4P was deter-
mined to be 3.8 M?1, as determined from the decrease in
absorbance at 1035 nm as a function of [TEACl] (a linear plot
to determine the binding constant is shown as an inset to
cule between Li+@C60and TTF-C4P.
Ion-mediated electron-transfer reactions of a supramole-
TTF-C4P (5.0 ? 10?5M) to Li+@C60(5.0 ? 10?5M) in the presence
of TEACl (3.0 ? 10?4M) in PhCN at 298 K. (b) EPR spectrum of the
radical ion pair (TTF-C4P?+–Li+@C60??) at 77 K. (c) Expanded
view of the magnetic field region highlighted by the red rectangular
frame in (b).
(a) EPR spectrum of the products of electron transfer from
transfer from TTF-C4P to Li+@C60 produced in the presence of
THACl in PhCN. Note that the Li+cation is disordered over two
positions but is seen in the difference map. Disordered solvent
molecules have been removed for clarity. However, no other charged
species are seen in the crystal lattice.
Single crystal X-ray structure of the product of electron
of increasing concentrations of TEACl to a preformed mixture of
TTF-C4P, Li+@C60and THACl in PhCN. Inset: plot for determination
of the binding constant for the interaction of TEACl with TTF-C4P;
a = (A ? A0)/(AN? A0).
Change in absorbance at 1035 nm seen upon the addition
9812 Chem. Commun., 2012, 48, 9801–9815This journal is c The Royal Society of Chemistry 2012
complexes formed by anion binding
Long-lived charge separation in supramolecular
Anion binding has also been utilized to form supramolecular
donor–acceptor systems composed of anionic sulphonated
porphyrins (H2TPPS4?and ZnTPPS4?), which function as
electron donors, and a cationic lithium ion encapsulated fullerene,
(Li+@C60), as an electron acceptor.73In PhCN, the UV-vis
spectral features of free base tetraphenylporphyrin tetra-
sulphonate [(Bu4N+)4H2TPPS4?] at 298 K undergo a discernible
change upon addition of Li+@C60. Specifically, the Soret
band undergoes a red shift to 427 nm with an isosbestic point
at 430 nm (Fig. 22a).
The absorbance change exhibits saturation behaviour with
increasing Li+@C60 concentration (Fig. 22b). From these
concentration-induced spectral changes in PhCN, the forma-
tion constant (K) corresponding to the H2TPPS4?–Li+@C60
complex was determined to be 3.0 ? 105M?1(eqn (6)).
A Job plot provided support for the expectation that the
binding stoichiometry was 1 : 1. When H2TPPS4?was replaced
by ZnTPPS4?, the K value was determined to be 1.6 ? 105M?1.
Concordant K values were obtained from fluorescence quenching
The fluorescence quenching of H2TPPS4?and ZnTPPS4?
by Li+@C60occurs via energy transfer from the singlet excited
states of H2TPPS4?and ZnTPPS4?to Li+@C60as revealed
by the femtosecond laser flash photolysis measurements.73
Nanosecond laser flash photolysis measurements have revealed
that electron transfer from H2TPPS4?and ZnTPPS4?to
the triplet excited state of Li+@C60occurs to produce the
charge-separated (CS) state as shown in Fig. 23, where the
absorption bands of [H2TPPS4?]?+(lmax= 670 nm)74and
that of the Li+@C60radical anion (lmax= 1035 nm)70are
observed. The decay of the CS state [(H2TPPS4?)?+–
Li+@C60??] in PhCN was monitored using different laser
Various first-order plots for the decay time profiles of the
CS state at different concentrations are shown in Fig. 23b. The
first-order plots afford approximately linear correlations with
the same slope irrespective of the difference in concentration of
the CS state. Thus, there is no or little contribution from the
bimolecular charge-recombination process of free [H2TPPS4?]?+
and Li+@C60??molecules. The lifetimes of the triplet CS state
of the supramolecular complex were determined to be 310 ms
for H2TPPS4?and 300 ms for ZnTPPS4?at 298 K. This
represents the longest lifetime for a CS state reported to date
for a non-covalent or supramolecular porphyrin–fullerene
system studied in solution.
Photoinduced electron transfer and back electron transfer
within donor–acceptor supramolecular complexes have been
demonstrated to occur on much faster time scales than the
anion binding and complex disassociation in a variety of
systems. Thus, as a general rule the linkage between electron
donor and acceptor moieties in the anion-bound supra-
molecular complexes can be considered as ‘‘defined’’ over
the course of the electron-transfer processes. In other words,
anion binding in the resulting supramolecular complexes
provides electronic coupling interactions between the electron
donor–acceptor moieties, which parallel those established by
covalent bonds. Anion binding to electron donors generally
produces negative shifts in the one-electron oxidation poten-
tials, leading to stabilization of the charge-separated states of
the anion-bound supramolecular complexes with electron
acceptors. Binding of anions and cations has also been demon-
strated to control ground state electron transfer from a tetra-
thiafulvalene calixpyrrole (TTF-C4P) donor to electron
acceptors within supramolecular ensembles. Long-lived charge
separation has been achieved in self-associated complexes of
sulphonated tetraphenylporphyrin anions with Li+@C60. In
conclusion, anion binding with hydrogen bonding provides
not only a structural scaffold for supramolecular electron
donor–acceptor complex formation, but also a unique way
of switching supramolecular electron-transfer reactions in
certain instances through competition with cation binding.
Such fine control of supramolecular electron transfer by anion
binding could provide the predicate for the construction of new,
of various concentrations of Li+@C60(0 to 2.5 ? 10?5M) in PhCN.
(b) Absorption change at 424 nm. Inset: plot of (a?1? 1)?1vs.
[Li+@C60] ? a[H2TPPS4?]; a = (A ? A0)/(AN? A0).
UV-vis spectra of H2TPPS4?(2.0 ? 10?6M) in the presence
in the presence of Li+@C60(5.0 ? 10?5M) in deaerated PhCN at
298 K taken at 20 and 200 ms after nanosecond laser excitation at
520 nm. (b) Decay time profiles at 1035 nm with different laser
intensities (1, 3, 6 mJ pulse?1).
(a) Transient absorption spectra of H2TPPS4?(2.5 ? 10?5M)
This journal is c The Royal Society of Chemistry 2012Chem. Commun., 2012, 48, 9801–98159813
state-of-the-art systems capable of effecting solar energy con-
version or capturing and storing charge at the molecular level.
The work accomplished to date, as summarized herein, could
thus set the stage for future advances.
The authors gratefully acknowledge the contributions of their
collaborators and coworkers mentioned in the cited references,
and financial support from the Grants-in-Aid (No. 20108010
to SF and 23750014 to KO) from MEXT, U.S. NSF grants
(CHE 1057904 to JLS and CHE 1110942 to FD), the
R.A. Welch Foundation (grant F-1018) and KOSEF/MEST
through WCU projects (R31-2008-000-10010-0, R32-2010-
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