Change in the site of electron-transfer reduction of a zinc- quinoxalinoporphyrin/gold-quinoxalinoporphyrin dyad by binding of scandium ions and the resulting remarkable elongation of the chargeshifted-state lifetime

Article abstract of DOI:10.1002/chem.200901105

The site of electron-transfer reduction of AuPQ⁺ (PQ = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b']porphyrin) and AuQPQ⁺ (QPQ = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl) bisquinoxalino[2,3-b':12, 13-b'']porphyri

Full text of DOI:10.1002/chem.200901105

FULL PAPER
DOI: 10.1002/chem.200901105
Change in the Site of Electron-Transfer Reduction of a Zinc–
Quinoxalinoporphyrin/Gold–Quinoxalinoporphyrin Dyad by Binding of
Scandium Ions and the Resulting Remarkable Elongation of the Charge-
Shifted-State Lifetime
Kei Ohkubo,[a] Rachel Garcia,[b] Paul J. Sintic,[c] Tony Khoury,[c] Maxwell J. Crossley,*[c]
Karl M. Kadish,*[b] and Shunichi Fukuzumi*[a]
Abstract: The site of electron-transfer
reduction of AuPQ+ (PQ=5,10,15,20-
tetrakis(3,5-di-tert-butylphenyl)quino-
ined by femto- and nanosecond laser
flash photolysis measurements. The ob-
served transient absorption bands at
630 and 670 nm after laser pulse irradi-
ation in the absence of Sc3+ in benzoni-
trile are assigned to the charge-shifted
250 ps in benzonitrile. Addition of Sc3+
to a solution of ZnPQ–AuPQ+ in ben-
zonitrile caused a drastic lengthening
of the CS lifetime that was determined
to be 430 ns, a value 1700 times longer
than the 250 ps lifetime measured in
the absence of Sc3+. Such remarkable
prolongation of the CS lifetime in the
presence of Sc3+ results from a change
in the site of electron transfer from the
AuIII center to the quinoxaline part of
the PQ macrocycle when Sc3+ binds to
the quinoxaline moiety, which deceler-
ate BET due to a large reorganization
energy of electron transfer. The change
in the site of electron transfer was con-
firmed by ESR measurements, redox
potentials, and UV/Vis spectra of the
singly reduced products.
xalinoACHTUNGTRENNUNG
(QPQ=5,10,15,20-tetrakis(3,5-di-tert-
butylphenyl)bisquinoxalino[2,3-b’:12,-
A
(CS) state (ZnPQ +–AuPQ). The CS
C
AuIII center to the quinoxaline part of
the PQ macrocycle in the presence of
Sc3+ in benzonitrile because of strong
binding of Sc3+ to the two nitrogen
atoms of the quinoxaline moiety.
Strong binding of Sc3+ to the corre-
sponding nitrogen atoms on the qui-
noxaline unit of ZnPQ also occurs for
the neutral form. The effects of Sc3+
on the photodynamics of an electron
donor–acceptor compound containing
state decays through back electron
transfer (BET) to the ground state
rather than to the triplet excited state.
The BET rate was determined from the
disappearance of the absorption band
due to the CS state. The decay of the
CS state obeys first-order kinetics. The
CS lifetime was determined to be
Keywords: donor–acceptor
sys-
tems · electron transfer · macrocy-
cles · porphyrinoids · scandium
a
linked ZnII and AuIII porphyrin
([ZnPQ–AuPQ]PF6) have been exam-
[a] Dr. K. Ohkubo, Prof. Dr. S. Fukuzumi
Department of Material and Life Science
Graduate School of Engineering
Osaka University and
Introduction
In the photosynthetic reaction center, charge separation
(CS) is faster than charge recombination (CR).[1] The fast
CS is made possible due to a large CR driving force that is
located in the Marcus inverted region,[2] in which the driving
force (ꢀDGET; ET=electron transfer) is larger than the re-
organization energy of electron transfer (l), that is,
ꢀDGET >l. The CR rate decreases as the driving force of
electron transfer increases in the Marcus inverted region,
whereas the CS rate increases in the Marcus normal region
(ꢀDGET <l).[2] In such a case, the smaller the l value, the
faster the CS rate and the slower becomes the CR rate.[2]
Thus, the same strategy as used in natural photosynthesis
has so far been chosen to optimize the efficiency of the
charge separation processes in biomimetic electron donor–
acceptor ensembles, that is, the use of components which
SORST (JST)
2–1 Yamada-oka, Suita, Osaka 565-0871 (Japan)
Fax : (+81)6-6879-7370
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
[b] R. Garcia, Prof. Dr. K. M. Kadish
Department of Chemistry, University of Houston
Houston, TX 77204-5003 (USA)
Fax : (+1)713-743-2745
E-mail: kkadish@uh.edu
[c] Dr. P. J. Sintic, Dr. T. Khoury, Prof. Dr. M. J. Crossley
School of Chemistry, The University of Sydney
NSW 2006 (Australia)
Fax : (+61)2-9351-3329
E-mail: m.crossley@chem.usyd.edu.au
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901105.
Chem. Eur. J. 2009, 15, 10493 – 10503
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10493

have small reorganization energies of electron transfer in
order to accelerate the forward electron transfer in the
Marcus normal region, and to decelerate the back electron
transfer in the Marcus inverted region.[3–10]
A totally opposite approach to attain long-lived CS states
would be to use a component that has a large l value, which
may result in slow CR in the Marcus normal region
(ꢀDGET <l). In such a case, the CS driving force should be
much larger than the CR driving force to make the CS rate
faster than the CR rate. Metalloporphyrins, which are fre-
quently used in photosynthetic models for light-harvesting
transfer reduction of AuPQ+ and AuQPQ+, which not only
changes the site of electron transfer from the AuIII metal to
the fused quinoxaline part of the PQ macrocycle, but also
leads to a remarkable elongation of the lifetime in the
charge-shifted state of a ZnPQ–AuPQ dyad in benzonitrile
due to the strong binding of Sc3+ to the ZnPQ and AuPQ+
moieties.
The present study provides an alternative and convenient
way to attain long-lived CS states by the simple binding of
metal ions to porphyrin-based donor–acceptor dyads. The
investigated compounds, ZnPQ, [AuPQ]PF6, [AuQPQ]PF6,
and [ZnPQ–AuPQ]PF6, are shown here.
and electron-donor molecules, have relatively small
l
values,[3–10] and thus these metallomacrocycles cannot be
used by themselves as the component with a large l value.
On the other hand, extremely large l values have com-
monly been associated with ET reduction of electron accept-
ors when metal ions bind strongly to radical anions of elec-
tron acceptors.[11–18] The scandium ion (Sc3+) is known to be
most effective for binding to radical anions.[15–18] Thus, if
metal-ion binding sites are introduced into metalloporphyr-
ins, they would be good candidates as a component in the
construction of electron donor–acceptor (D–A) dyads that
can afford long-lived CS states with large l values due to
strong binding of metal ions. In this context, the fusion of a
quinoxaline group to the b,b’-pyrrolic position of the por-
phyrin macrocycle[19,20] merits special attention, as it can act
as a metal-ion binding site. The quinoxaline group also leads
to easier reduction and a greater degree of delocalization
throughout the molecule,[19,20] resulting in more effective
electron transfer of the entire complex.
We previously reported the initial site of electron transfer
in ’’simple’’ AuIII porphyrins (AuP+) to be at the central
metal ion, giving a AuII complex when the measurement was
carried out in nonaqueous solvents such as benzonitrile,
CH2Cl2, pyridine, or THF.[21] Gold
ACTHNUTRGNE(UNG III) quinoxalinoporphyr-
ins (AuPQ+) were also shown to undergo reduction at the
central metal ion to give the gold(II)–porphyrin complex,[22]
overturning the long-held assumption that reduction of such
complexes only occurs at the macrocycle.[23–25] The fused qui-
noxaline group on the porphyrin macrocycle is typically re-
duced at potentials more negative than ꢀ1.9 V versus SCE
as compared to the reversible reduction of quinoxaline itself
which occurs at E1/2 =ꢀ1.60 V in THF containing 0.1m
TBAP.[26] We have also reported the synthesis and photoin-
duced electron-transfer dynamics of electron D–A linked
Results and Discussion
Positive shift of one-electron reduction potential of AuPQ+:
A cyclic voltammogram of [AuPQ]PF6 in PhCN before and
after the addition of two equivalents of ScACTHNURTGNENG(U OTf)3 is shown in
Figure 1. Before the addition of Sc3+, the porphyrin exhibits
three reductions, the first of which is metal-centered. This
behavior is comparable with that of other AuIII–porphyrins
previously examined by our groups.[21,22] No oxidations are
observed for gold quinoxalinoporphyrins up to positive
range of the PhCN solvent (about 1.8 V vs SCE).
systems involving AuPQ+.[27] Gold
ACTHNUTRGNEUNG(III)–porphyrin com-
plexes have frequently been used as electron acceptors in
donor–acceptor ensembles due to their ability to be easily
reduced.[28–34] However, neither the effect of metal ions on
the ET reduction of quinoxalinoporphyrins, nor those on the
photoinduced electron transfer of D–A linked systems in-
volving AuPQ+, have so far been investigated.
The second and third reductions of [AuPQ]PF6 afford the
AuII–porphyrin p-radical anion and the porphyrin dianion,
respectively. The potential separation between the first and
second one-electron reduction of [AuPQ]PF6 is 500 mV, and
the separation between the second and the third processes is
680 mV in PhCN. These latter values of DE1/2 are compara-
ble to what was reported for [AuP]PF6 under the same solu-
This is now undertaken in the present study in which the
effect of scandium ion (Sc3+) on the electron-transfer reduc-
tion of goldACHTUNGTRENNUNG(III)–quinoxalino- and –bisquinoxalinoporphyrin
complexes (AuPQ+ and AuQPQ+) is elucidated. As will be
shown, there is a dramatic effect of Sc3+ on the electron-
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Electron Transfer Reduction of a ZnPQ–AuPQ Dyad
FULL PAPER
Figure 2. UV/Vis spectral change upon addition of ScACTHUNRGTNEUNG(OTf)3 (0–2.8ꢁ
10ꢀ5 m) to a solution of [AuPQ]PF6 in PhCN (1.4ꢁ10ꢀ5 m) in the presence
of Fc* (1.7ꢁ10ꢀ3 m).
Figure 1. Cyclic voltammograms of [AuPQ]PF6 (3.7ꢁ10ꢀ4 m) a) before
the absence of Sc3+ to the macrocycle, and, more specifical-
ly, to the quinoxaline part of the compound as discussed in
detail in later sections of the manuscript. It is significant to
point out that the UV/Vis spectrum of the electrochemically
generated AuIIPQ[22] is quite different than the spectrum of
the chemically reduced product in Figure 2.
and b) after the addition of ScACHTNUGRTNEUNG
(OTf)3 (7.4ꢁ10ꢀ4 m) to a PhCN solution
containing 0.10m TBAPF6.
tion conditions.[21] All three processes are positively shifted
by 210 mV upon going from [AuP]PF6 to [AuPQ]PF6, consis-
tent with what has been reported for the other quinoxalino-
porphyrins.[19,20,22,26]
The titration curve in Figure 3a, illustrating a plot of ab-
ACTHNUTRGNEUNG
sorbance at 439 nm due to AuPQ versus [Sc3+]/[AuPQ+],
Four redox processes are observed for [AuPQ]PF6 in
ACTHNGUTERNNUG
shows an end point at [Sc3+]/[AuPQ+]=2, thus indicating
PhCN containing two equivalents of ScACTHNUTRGENUG(N OTf)3 (Figure 1b),
the first of which occurs at E1/2 =+0.22 V, which is signifi-
cantly shifted to the positive direction as compared to that
in the absence of ScACTHNUTRGNEUNG
(OTf)3.[35] The following two reversible
reduction waves at E1/2 =ꢀ0.67 and ꢀ1.18 V and the reduc-
tion peak at Ep =ꢀ1.63 V, are potentials almost identical to
the E1/2 values for the reduction of [AuP]PF6 in PhCN not
[21,36]
containing Sc3+
.
The large positive shift in the first re-
duction potential of [AuPQ]PF6 allows the electron-transfer
reduction to be carried out by decamethylferrocene (Fc*) in
the presence of Sc3+ (vide infra).
No electron transfer from Fc* to [AuPQ]PF6 occurs in the
absence of Sc3+, because the electron transfer is highly en-
dergonic judging from the one-electron oxidation potential
of Fc* (Eox =ꢀ0.11 V vs. SCE)[37,38] and the one-electron re-
duction potential of [AuPQ]PF6 (Ered =ꢀ0.45 V vs. SCE).
However, the addition of Sc3+ to a PhCN solution of
[AuPQ]PF6 in the presence of Fc* results in a homogenous
electron transfer from Fc* to [AuPQ]PF6 to produce Fc*+
Cꢀ
and AuPQ as shown in Figure 2. Several isosbestic points
Figure 3. Plots of a) absorbance at 439 nm for [AuPQ]PF6 versus [Sc3+]/
[AuPQ+] ratio with a fixed concentration of Fc* (1.65ꢁ10ꢀ4 m) and b) ab-
ACHTUNG-
are observed, and the 418 and 515 nm bands of the product
can be compared with the spectrum of the same AuIII–por-
phyrin not having the fused quinoxaline group, that is,
[AuP]PF6 in PhCN.[21]
TERNNUG
sorbance at 439 nm versus [Fc*]/ACHTUNGTRNEUNG
[AuPQ+] with a fixed concentration of
Sc3+ (7.12ꢁ10ꢀ4 m) in electron transfer from Fc* to [AuPQ]PF6 (1.37ꢁ
10ꢀ5 m) in the presence of Sc3+ in PhCN.
There is also a broad absorption band at 790 nm in the
one-electron reduced product, and this can be assigned to
the decamethylferricenium cation (Fc*+: lmax =790 nm,
emax =240mꢀ1 cmꢀ1),[37,38] which would be overlapped with
any long-wavelength absorptions of AuPQ. Such long-wave-
length absorption extending into the NIR region is diagnos-
tic of the absorption band due to the PQ p-radical anion,
and clearly indicates that the site of the electron-transfer re-
duction of [AuPQ]PF6 is changed from the AuIII metal in
that two Sc3+ ions are required for the electron-transfer re-
duction of [AuPQ]PF6 by Fc*. The one-electron reduction
of [AuPQ]PF6 is confirmed by the titration curve in Fig-
ure 3b, in which the plot of absorbance at 439 nm due to
AuPQ+ versus [Fc*]/[AuPQ+] exhibits an end point at [Fc*]/
ACHTUNGTRENNNUG ACHTUNG-
TERNNUG
[AuPQ+]=1. It should be emphasized that the spectral titra-
Chem. Eur. J. 2009, 15, 10493 – 10503
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10495

S. Fukuzumi, M. J. Crossley, K. M. Kadish et al.
tion of a solution of AuPQ+ in PhCN with Sc3+ (see the
Supporting Information Figure S1) indicates that two Sc3+
ions bind to [AuPQ]PF6 prior to the electron-transfer reduc-
tion. Judging from the spectral titration in Figure S1, the for-
mation of the 1:2 complex between AuPQ+ and Sc3+ is
quantitative when the binding constant is too large to be de-
termined accurately.
When AuPQ+ is replaced by AuQPQ+ containing two
quinoxaline units, a similar spectral change is observed for
the electron-transfer reduction by Fc* as shown in Figure 4a.
In this case, however, a two-electron reduction of AuQPQ+
by Fc* occurs as indicated by the spectral titration data in
Figure 4b, in which four equivalents of Sc3+ ions are re-
quired for the reaction (see the Supporting Information Fig-
ure S2).
The number of Sc3+ ions required for the electron-trans-
fer reduction of AuPQ+ and AuQPQ+ corresponds to the
total number of nitrogen atoms on the quinoxaline units,
that is, two and four for AuPQ+ and AuQPQ+, respectively.
Thus, the electron-transfer reductions of AuPQ+ and
AuQPQ+ are made possible by binding of Sc3+ to the nitro-
gen atoms of the quinoxaline units of AuPQ+ and AuQPQ+,
as shown in Scheme 1. The site of electron-transfer reduc-
tion of AuPQ+ and AuQPQ+ is not the AuIII center but the
fused quinoxaline group on the compound, which binds with
Sc3+ at its nitrogen atoms. Additional proof that AuIII is not
reduced is given by the fact that similar spectral changes are
observed for the electron-transfer reduction of ZnPQ by
Fc* in the presence of Sc3+ (see the Supporting Information
Figure S3).
Figure 4. a) UV/Vis spectral change upon addition of Fc* (0–2.8ꢁ10ꢀ5 m)
to a solution of AuQPQ+ in PhCN (1.37ꢁ10ꢀ5 m) in the presence of Sc3+
ACTHNUTRGNEUNG
(7.12ꢁ10ꢀ4 m). b) Absorbance at 455 nm versus [Fc*]/[AuPQ+] with a
fixed concentration of Sc3+ (7.12ꢁ10ꢀ4 m) in electron transfer from Fc*
to AuQPQ+ (1.37ꢁ10ꢀ5 m) in the presence of Sc3+ in PhCN.
ESR measurements of the electron-transfer products also
confirm a change in the site of electron transfer, from the
AuIII metal of AuPQ+ in the absence of Sc3+, to the fused
quinoxaline unit in the presence of Sc3+. Figure 5 shows an
Scheme 1. Electron-transfer reduction of AuPQ+ (left) and AuQPQ+ (right) with decamethylferrocene in the presence of Sc
ACTHNUTRGNEN(UG OTf)3.
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Electron Transfer Reduction of a ZnPQ–AuPQ Dyad
FULL PAPER
Figure 5. ESR spectrum for the products of electron-transfer reduction of
AuPQ+ (7.0ꢁ10ꢀ4 m) by Fc* (7.0ꢁ10ꢀ4 m) in PhCN containing Sc
(5.0ꢁ10ꢀ3 m) at 298 K. Asterisk denotes Mn2+ marker.
ACHTUNGERTN(NUNG OTf)3
ESR spectrum of the products formed after electron-trans-
fer reduction of AuPQ+ by Fc* in the presence of Sc3+ in
PhCN. The g value of 2.0031 is consistent with the reported
g value of the Sc3+ complex of the acridine radical anion
(g=2.0031), in which Sc3+ binds to the nitrogen of the acri-
dine radical anion.[17a] The measured ESR signal due to the
one-electron reduced species of the quinoxaline unit is in
sharp contrast to the broad ESR signal having a much larger
g value (2.06) for the AuII–porphyrin observed after the
one-electron reduction of AuPQ+ by the naphthalene radi-
cal anion.[22]
The change in site of the electron-transfer reduction of
AuPQ+ resulting from binding of Sc3+ to the nitrogen of the
quinoxaline unit is also supported by DFT calculations.
Figure 6 (top) shows the LUMO also changed to the fused
quinoxaline unit upon protonation. When Sc3+ binds to the
nitrogen atoms of the quinoxaline unit of AuPQ+, however,
the LUMO orbital is localized on the Sc3+ nucleus. Thus,
the solvation orbital initially delocalized on the AuIII metal,
but also to a lesser extent on the macrocycle, whereas
Figure 6 (bottom) shows that the LUMO orbitals of the
Sc3+ and the counter anion (OTfꢀ) may be important factors
in determining the site of electron transfer.[39]
Figure 6. LUMO orbitals of AuPQ+ (top) and diprotonated AuPQ+
(bottom) calculated by the DFT method at the B3LYP/lanl2dz level.
If the change in site of electron-transfer reduction of
AuIIIPQ results from the stronger binding of Sc3+ to the qui-
III
Cꢀ
noxaline unit of Au PQ (one-electron reduced species) as
compared to that of AuIIIPQ, the one-electron reduction po-
tential of quinoxaline should also exhibit a large positive
shift. This is indeed the case, as shown in Figure 7 in which
E1/2 (ꢀ1.66 V vs. SCE) is shifted to a more positive poten-
tial: one at E1/2 =0.25 V, and the other at E1/2 =ꢀ0.17 V
versus SCE in the presence of two equivalents of Sc3+. The
first quasi-reversible process at 0.25 V is located at nearly
the same potential as for AuPQ+ reduction in PhCN con-
taining two equivalents of Sc3+ (Figure 1).
Figure 7. Cyclic voltammograms of a) quinoxaline (1.18ꢁ10ꢀ3 m) in
PhCN, TBAPF6 (0.1m) only and b) the same solution in the presence of
ScACTHNUTRGNEUNG
(OTf)3 (2.45ꢁ10ꢀ3 m).
value for electron transfer from Me2Fc to AuPQ+ is
changed by the addition of Sc3+ (+0.04 eV), judging from
ACHTUNGTRENNUNG
the one-electron reduction potential of the AuPQ+/(Sc3+)2
Reorganization energy of electron transfer for reduction of
AuPQ+: The large positive shift in the one-electron poten-
tial of AuPQ+, from ꢀ0.45 to +0.22 V versus SCE, caused
species (Ered =0.22 V vs. SCE). Thus, in the presence of two
equivalents of Sc3+, an efficient electron transfer from
Me2Fc to AuPQ+/
C
(Sc )2
3+
C
Cꢀ
by the much stronger binding to PQ than to PQ (Figure 1)
and Me2Fc+ [Eq. (1)].
makes the electron transfer from Fc* to AuPQ+ occur as
shown in Figure 2. No electron transfer from 1,1’-dimethyl-
ferrocene (Me2Fc; Eox =0.26 V vs. SCE) to AuPQ+ occurs
in PhCN at 298 K, because the free energy change of elec-
tron transfer (DGet) is largely positive (0.71 eV). The DGet
þ
Me2Fc þ AuPQþ=ðSc3þÞ2 ! Me2Fc þ AuPQ =ðSc3þÞ2
C
ð1Þ
Figure 8a shows UV/Vis spectral changes as a function of
time for the electron transfer from Me2Fc to AuPQ+ in the
Chem. Eur. J. 2009, 15, 10493 – 10503
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10497

S. Fukuzumi, M. J. Crossley, K. M. Kadish et al.
metal-centered electron-transfer reactions of gold–porphyrin
(l= ꢂ1.23 eV).[22]
Photoinduced electron transfer in ZnPQ–AuPQ+ dyad in
the presence of Sc3+: We previously reported the formation
of a long-lived CS state of the ZnPQ–AuPQ+ dyad in non-
polar solvents such as cyclohexane.[27a] However, the long
CS lifetime (10 ms) in cyclohexane becomes much shorter in
PhCN (250 ps).[27b] Such a short-lived CS state could only be
measured by using femtosecond laser flash photolysis. The
short CS lifetime in PhCN results from the large solvent re-
organization energy for the back electron transfer in con-
trast to the much smaller solvent reorganization energy in
cyclohexane. This is because the larger the reorganization
energy, the faster the rate of back electron transfer, as ex-
pected from the Marcus theory of electron transfer, provid-
ed that the driving force of back electron transfer is larger
than the reorganization energy of electron transfer.[2] This
region is referred to as the Marcus inverted region.[2]
Time-resolved transient absorption spectra of ZnPQ–
AuPQ+ in the absence and presence of Sc3+ were measured
by femtosecond laser photolysis. A transient absorption
spectrum observed after the femtosecond laser pulse excita-
tion of a solution of ZnPQ–AuPQ+ in PhCN in the absence
of Sc3+ is shown in Figure 9a. The observed transient ab-
sorption band at 670 nm and the shoulder at 760 nm at 1 ps
Figure 8. a) UV/Vis spectral change in electron transfer from Me2Fc
(5.8ꢁ10ꢀ4 m) to AuPQ+ (5.8ꢁ10ꢀ5 m) in the presence of Sc
ACTHUNTRGNE(UGN OTf)3 (1.1ꢁ
10ꢀ3 m) in deaerated PhCN at 298 K. Inset: Decay and rise time profiles
at 418 nm and 439 nm. b) Plots of pseudo-first-order rate constants versus
concentration of Me2Fc and Fc in electron transfer from Me2Fc (5.8ꢁ
ACTHNUTRGNEUNG
10ꢀ4 m) to AuPQ+ (5.8ꢁ10ꢀ5 m) in the presence of Sc(OTf)3 (1.1ꢁ10ꢀ3 m).
after femtosecond laser excitation agree with those observed
+
C
in the one-electron oxidation of ZnPQ to ZnPQ (Support-
presence of Sc3+ in PhCN. The decay of AuPQ+ (lmax
=
ing Information S4a). There is no significant absorption at
600–800 nm due to AuIIPQ and thus the observed transient
absorption spectrum in Figure 9a indicates that electron
C
439 nm) and the formation of AuPQ (lmax =418 nm) are
also shown in the inset of Figure 8a. The decay and rise time
profiles obey pseudo-first-order kinetics, and the pseudo-
first-order rate constant (kobs) increases linearly with in-
creasing concentration of Me2Fc (Figure 8b). The observed
second-order rate constant (ket) is determined as 13mꢀ1 sꢀ1.
The dependence of the ket value on the DGet value for
outer-sphere electron transfer has been well-established by
Marcus as given by Equation (2), in which l is the reorgani-
zation energy of electron transfer, kB is the Boltzmann con-
1
+
transfer from ZnPQ* to AuPQ+ occurs to afford ZnPQ
–
C
AuIIPQ.[27a] The first fast-decay component at 590 nm with
the rate constant of 3.7ꢁ1012 sꢀ1 (lifetime of 270 fs) agrees
+
C
with the rise in rate at 760 nm for the formation of ZnPQ
(inset of Figure 9b). This process corresponds to the electron
transfer from the singlet excited state of ZnPQ (1ZnPQ*) to
+
AuPQ+ to form the charge-shifted (CS) state, ZnPQ
–
C
AuIIPQ. The second component with the rate constant of
6.7ꢁ1010 sꢀ1 (lifetime of 15 ps) seen in Figure 9b corresponds
to the electron transfer from ZnPQ to the triplet excited
stant and
Z
is the collision frequency, taken as 1ꢁ
1011 mꢀ1 sꢀ1.[2]
state of AuPQ+ (3AuPQ+*), in which AuPQ+* is produced
3
2
by extremely fast intersystem crossing of the 1AuPQ+*
moiety upon the 410 nm laser pulse illumination of the
AuPQ+ unit.[40] The ratio of laser excitation of the ZnPQ
and AuPQ+ units is 15:1, determined from their absorption
coefficients at 410 nm. The final slow component with the
rate constant of 4.0ꢁ109 sꢀ1 (lifetime of 250 ps) corresponds
ð2Þ
ket ¼ Z exp½ꢀðl=4Þð1 þ DGrt=lÞ =kBTꢁ
The l value is determined as 2.26 eV for the electron
transfer from Me2Fc to AuPQ+/(Sc3+)2 from the ket and
AHCTUNGTRENNUNG
DGet values using Equation (3), which is derived from Equa-
tion (2).
II
+
C
to the back electron transfer from Au PQ to ZnPQ to the
ground state.
l ¼ꢀDGetꢀ2 RT lnðket=ZÞþ
ð3Þ
In contrast to the short-lived CS state of ZnPQ–AuPQ+
in the absence of Sc3+ in PhCN (Figure 9), the transient ab-
sorption spectrum observed after the femtosecond laser
pulse excitation of a solution of ZnPQ–AuPQ+ in PhCN in
the presence of Sc3+ (5 mm) remains the same at 3000 ps, as
shown in Figure 10a (dark gray circles). The observed transi-
ent absorption bands at 630 nm and the shoulder at 670 nm
2
2 1=2
½fDGet þ 2 kBT lnðket=ZÞg ꢀðDGetÞ ꢁ
Virtually the same l value (2.29 eV) is obtained from the
ket (1.1mꢀ1 sꢀ1) and DGet (0.15 eV) values of electron transfer
ACTHNUTRGNEUNG
from Fc (Eox =0.37 V vs. SCE) to AuPQ+/(Sc3+)2. The l
value obtained for electron-transfer reduction of AuPQ+/
(Sc3+)2 is significantly larger than the reported l values of
ACHTUNG-
TRENNUNG
10498
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Chem. Eur. J. 2009, 15, 10493 – 10503

Electron Transfer Reduction of a ZnPQ–AuPQ Dyad
FULL PAPER
Figure 9. a) Transient absorption spectra of ZnPQ–AuPQ+ in the absence
Figure 10. a) Transient absorption spectra of ZnPQ–AuPQ+ in the pres-
of Sc
G
ence of ScACTHNUGTERNU(NG OTf)3 (5.0 mm) in PhCN, taken by femtosecond laser excita-
b) Time profile of absorbance at 670 nm; inset: Time profiles at the
shorter time range at 590 and 760 nm. The solid curve represents the best
fit to the exponential rise or decay.
tion at 410 nm. b) Time profile of absorbance at 670 nm. The gray curve
represents the best fit to the two-exponential decay.
at 1000 ps after femtosecond laser excitation agree well with
what is observed in the one-electron oxidation of ZnPQ in
decay of the CS state could not be determined because the
limit of time range of our femtosecond laser flash photolysis
is within 3.2 ns (see Experimental Section). This was deter-
mined by nanosecond laser flash photolysis (vide infra).
No transient absorption was observed by nanosecond
laser flash photolysis of ZnPQ-AuPQ+ in PhCN, because
the photoinduced event is completely finished in the femto-
to nanosecond timescale. In the presence of Sc3+, however,
a transient absorption spectrum is observed by nanosecond
laser flash photolysis of ZnPQ–AuPQ+ in PhCN, as shown
in Figure 11.
the presence of Sc3+ (Supporting Information S4b). These
+
absorption bands assigned to ZnPQ
/
(Sc3+)2 are clearly
C
blue-shifted as compared to those of ZnPQ without Sc3+
.
+
C
Since two Sc3+ ions bind to the ZnPQ and AuPQ moieties
and the site of the electron-transfer reduction is changed
from AuIII to PQ in AuPQ+ (vide supra), photoinduced
electron transfer from 1ZnPQ*/
occurs to afford the CS state, ZnPQ
(Sc3+)2.
(Sc3+)2 to AuPQ+/(Sc3+)2
ACHUTGTNRENNUG CAHTUNGTRENNUGN
+
3+
C
C
ACHTUNG-
/
N
TRENNUNG
The time profile of absorbance at 670 nm prior to the
The transient absorption maxima at 630 and 670 nm are
the same as those observed at 3000 ns after femtosecond
laser excitation in Figure 10a. This clearly indicates that the
CS state is still observed by nanosecond laser flash photoly-
sis of ZnPQ–AuPQ+ in the presence of Sc3+ in PhCN.
The CS lifetime is determined from the single exponential
decay of transient absorption to be 430 ns (Figure 12). Thus,
the addition of Sc3+ to a solution of ZnPQ–AuPQ+ in
PhCN results in a 1700 times longer CS lifetime. Such re-
markable elongation of the CS lifetime by the binding of
back electron transfer in the presence of Sc3+ has two com-
ponents (Figure 10b). The fast component with the rate con-
stant of 7.6ꢁ109 sꢀ1 (lifetime of 130 ps) corresponds to the
1
electron transfer from ZnPQ*/
G
E
+
3+
III
3+
C
afford the CS state, ZnPQ
/
N
slow component with the rate (1.0ꢃ0.3)ꢁ108 sꢀ1 (lifetime of
10ꢃ3 ns) corresponds to the electron transfer from ZnPQ/
ACHTUNG-
(Sc3+)2 to AuPQ +*/
(Sc3+)2 to afford the CS state. The rate
3
of electron transfer from ZnPQ to AuPQ+* is decelerated
by three orders of magnitude after addition of Sc3+. The
Cꢀ
Sc3+ to the PQ moiety of the CS state can be explained by
Chem. Eur. J. 2009, 15, 10493 – 10503
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10499

S. Fukuzumi, M. J. Crossley, K. M. Kadish et al.
Figure 11. Transient absorption spectrum of ZnPQ–AuPQ+ (1.0ꢁ10ꢀ5 m)
in the presence of Sc3+ (5.0ꢁ10ꢀ3 m) in PhCN at 150 ns taken by nanosec-
ond laser excitation at 430 nm.
Scheme 2. Energy diagrams of photoinduced electron transfer in ZnPQ–
AuPQ+ in the a) absence and b) presence of Sc
ACTHNUTRGNE(UNG OTf)3. Solid arrows
denote main pathways. Broken arrows denote minor pathways.
potential of the quinoxaline moiety by binding of Sc3+
(Figure 7), which causes the change in the site of electron
transfer from the AuIII center to the quinoxaline moiety.
Such a process is known to require a large reorganization
energy. The lowering of the CS state energy due to the
Figure 12. Decay time profile at 630 nm due to the CS state of ZnPQ–
AuPQ+ in the presence of Sc3+ in PhCN. The solid curve represents the
best fit to the exponential decay.
Cꢀ
strong binding of Sc3+ to the PQ moiety together with a
large reorganization energy results in slower back electron
transfer as well as photoinduced electron transfer in the
Marcus normal region. Thus, the photodynamics of ZnPQ–
AuPQ+ are remarkably changed by binding of Sc3+ to the
quinoxaline moiety of ZnPQ–AuPQ+. The stronger binding
of Sc3+ to the one-electron reduced quinoxaline moiety re-
sults in significant stabilization of the CS state and an in-
crease in the reorganization energy of electron transfer,
leading to remarkable lengthening of the CS lifetime.
the change in driving force of the back electron transfer and
reorganization energy of the electron transfer, as shown in
the energy diagrams of Scheme 2 in which the energies and
photodynamics described above are summarized a) in the
absence of Sc3+ and b) in the presence of Sc3+. The energies
of the singlet excited states of ZnPQ-AuPQ+ in the absence
and presence of Sc3+ were determined by using the absorp-
tion and fluorescence maxima of the corresponding refer-
ence compounds.
The triplet energies were determined by using phosphor-
escence in EPA with ethyl iodide glass at 77 K (see Experi-
mental Section). The CS energies were determined from the
difference in the redox potentials (vide supra). The stronger
binding of Sc3+ to the PQ moiety following photoinduced
electron transfer from ZnPQ to AuPQ+ in PhCN results in
a lowering of the CS state energy (Scheme 2b) as compared
to the CS energy in the absence of Sc3+ (Scheme 2a). Since
the initial one-electron oxidation potential of ZnPQ is shift-
ed by only 40 mV due to the binding of Sc3+ to the quinoxa-
line moiety of ZnPQ (see the Supporting Information Fig-
ure S5), the large stabilization of the CS state mainly results
from the large positive shift of the one-electron reduction
Conclusions
The site of the first electron-transfer reduction in AuPQ and
AuQPQ is changed from the AuIII center to the quinoxaline
part of the PQ and QPQ macrocycles in PhCN containing
Sc3+, because of the strong binding of two and four Sc3+
ions to the two and four nitrogen sites of PQ and QPQ ,
respectively. The photodynamics of an electron donor–ac-
ceptor linked compound (ZnPQ–AuPQ+) are also changed
drastically, particularly in terms of the CS state lifetime,
which is 1700 times longer due to the strong binding of Sc3+
Cꢀ
Cꢀ
Cꢀ
to PQ . Such a remarkable elongation of the CS lifetime in
10500
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Chem. Eur. J. 2009, 15, 10493 – 10503

Electron Transfer Reduction of a ZnPQ–AuPQ Dyad
FULL PAPER
the presence of Sc3+ provides a promising new strategy to
lengthen the CS state of electron donor–acceptor ensembles,
potentially proving useful in a number of areas, including
photovoltaic devices.
The working electrode was a glassy carbon electrode, the counter elec-
trode was a Pt wire, and the reference was either a commercially avail-
able or homemade SCE. The homemade SCE was separated from the
bulk of the solution by a fritted glass bridge of low porosity that con-
tained PhCN, 0.1m TBAPF6. UV/Vis spectroelectrochemical experiments
were performed with a homemade thin-layer cell with a platinum net
working electrode. Time-resolved UV/Vis spectra were recorded with a
Hewlett Packard 8453 diode array spectrophotometer. Phosphorescence
spectra were measured in a deaerated EPA/ethyl iodide (5:5:2:12 isopen-
tane: diethyl ether: ethanol: ethyl iodide) glass at 77 K. Near-IR emission
spectra of singlet oxygen were recorded on a SPEX Fluorolog t3 spectro-
photometer. A photomultiplier (Hamamatsu Photonics, R5509–72) was
used to detect emission in the near-infrared region (band path 2 mm).
Experimental Section
Chemicals: Scandium triflate (ScACTHUNTRGNEU(GN OTf)3), ferrocene (Fc), 1,1’-dimethylfer-
rocene (Me2Fc), and decamethylferrocene (Fc*) were purchased from
Aldrich and used as received without further purification. Benzonitrile
(PhCN), obtained from Tokyo Chemical Industry or Aldrich was distilled
over phosphorous pentaoxide (P2O5) under vacuum prior to use. Tetra-n-
butylammonium hexafluorophosphate (TBAPF6) was purchased from
Sigma Chemical or Fluka Chemika, was recrystallized from ethyl alcohol,
and was dried under vacuum at 408C for at least one week prior to use.
The synthesis of 1-benzyl-1,4-dihydronicotinamide dimer (BNA)2 was re-
ported previously.[41] Synthesis of ZnPQ, [AuPQ]PF6, and the dyad
[ZnPQ–AuPQ]PF6 are reported in the literature.[42] The synthesis of
[AuQPQ]PF6 is reported below. The reduced metal complexes of PQ
Laser flash photolysis: Time-resolved fluorescence decays were measured
by a Photon Technology International GL-3300 with a Photon Technolo-
gy International GL-302, nitrogen laser/pumped dye laser system,
equipped with a four channel digital delay/pulse generator (Stanford Re-
search System Inc. DG535) and a motor driver (Photon Technology In-
ternational MD-5020). The excitation wavelength was 431 nm using
POPOP (Wako Pure Chemical Ind. Ltd., Japan) as a dye. Fluorescence
lifetimes were determined by an exponential curve fit using a microcom-
puter.
were prepared by reduction with Fc* in the presence of Sc
Synthesis of hexafluorophosphate{5,10,15,20-tetrakis(3,5-di-tert-butyl-
phenyl) bisquinoxalino[2,3-b’:12,13-b’’]porphyrinato}aurate(III)
ACHTUNGERTN(NUNG OTf)3.
Femtosecond transient absorption spectroscopy experiments were con-
ducted using an ultrafast source: Integra-C (Quantronix Corp.), an opti-
cal parametric amplifier: TOPAS (Light Conversion Ltd.) and a commer-
cially available optical detection system: Helios provided by Ultrafast
Systems LLC. The source for the pump and probe pulses were derived
from the fundamental output of Integra-C (780 nm, 2 mJ/pulse and
fwhm=130 fs) at a repetition rate of 1 kHz. 75% of the fundamental
output of the laser was introduced into TOPAS, which has optical fre-
quency mixers, resulting in a tunable range from 285 to 1660 nm, with
the rest of the output used for white light generation. Prior to generating
the probe continuum, a variable neutral density filter was inserted in the
path in order to generate a stable continuum, then the laser pulse was
fed to a delay line that provides an experimental time window of 3.2 ns
with a maximum step resolution of 7 fs. In our experiments, a wavelength
at 410 nm of TOPAS output, which is the fourth harmonic of signal or
idler pulses, was chosen as the pump beam. As this TOPAS output con-
sists of not only the desirable wavelength but also unnecessary wave-
lengths, the latter were filtered using a wedge prism with a wedge angle
of 188. The desirable beam was irradiated at the sample cell with a spot
size of 1 mm diameter, where it was merged with the white probe pulse
in a close angle (<108). The probe beam, after passing through the 2 mm
sample cell, was focused on a fiber optic cable that was connected to a
CCD spectrograph for recording the time-resolved spectra (410–800 nm).
Typically, 2500 excitation pulses were averaged for 5 s to obtain the tran-
sient spectrum at a set delay time. Kinetic traces at appropriate wave-
lengths were assembled from the time-resolved spectral data. All meas-
urements were conducted at room temperature, 295 K.
AHCTUNGTRENNUNG
([AuQPQ]PF6): General procedures for the synthesis of [AuQPQ]PF6
are the same as those reported previously for other bisquinoxalinopor-
phyrins.[42] The 31P NMR spectrum of the compound was acquired on a
Bruker DPX-400 (162 MHz) spectrometer. 31P NMR chemical shifts are
referenced to external neat trimethyl phosphite, taken to be 140.85 ppm
at room temperature.
5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b’:12,13-
b’’]porphyrin[41] (71.0 mg, 0.0560 mmol), potassium tetrachloroaurate
ACTHNUTRGNE(NUG III)
(105 mg, 0.278 mmol), and sodium acetate (170 mg, 2.17 mmol) were dis-
solved in a mixture of toluene (13 mL) and glacial acetic acid (18m,
13 mL). The reaction mixture was heated at reflux for 4 h. The mixture
was then diluted in dichloromethane (30 mL), washed with water (2ꢁ
100 mL), sodium carbonate solution (10%, 2ꢁ100 mL), and water (2ꢁ
100 mL), dried over anhydrous sodium sulfate, and filtered; the filtrate
was evaporated to dryness. The residue was dissolved in chloroform
(12 mL) and stirred with a saturated solution of potassium hexafluoro-
phosphate (1.20 g, 6.52 mmol) in water (12 mL) for 18 h. The mixture
was then diluted in chloroform (40 mL), washed with water (2ꢁ200 mL),
dried over anhydrous sodium sulfate, and filtered; the filtrate was evapo-
rated to dryness. The mixture was then purified by column chromatogra-
phy over silica (chloroform/methanol; 100:4). The polar green band was
collected and the solvent removed. The product was redissolved in
chloroform (10 mL), and stirred with a saturated solution of potassium
hexafluorophosphate (1.00 g, 5.43 mmol) in water (10 mL) for 18 h. The
mixture was then washed with water (2ꢁ200 mL), dried over anhydrous
sodium sulfate, and filtered; the filtrate was evaporated to dryness. The
major green band was collected and the solvent was removed to afford
pure [AuQPQ]PF6 (20.0 mg, 22%) as a green solid. M.p. >3008C; IR
For nanosecond laser flash photolysis experiments, deaerated solutions of
the dyad were excited by a Panther OPO equipped with a Nd:YAG laser
(Continuum, SLII-10, 4–6 ns fwhm) at l=430 nm with a power of 10 mJ
per pulse. The photochemical reactions were monitored by continuous
exposure to a Xe lamp (150 W) as a probe light and a photomultiplier
tube (Hamamatsu 2949) as a detector. The transient spectra were record-
ed using fresh solutions in each laser excitation. The solution was deoxy-
genated by argon purging for 15 min prior to the measurements.
˜
(CHCl3): n=3059 (w), 2964 (s), 2905 (m), 2868 (m), 1595 (s), 1491 (w),
1477 (m), 1466 (w), 1421 (w), 1364 (s), 1329 (w), 1265 (m), 1248 (m),
1223 (s), 1217 (s), 1207 (m), 1136 (w), 1122 cmꢀ1 (m); UV/Vis (CHCl3): l
(log e)=345 sh (4.53), 384 (4.65), 451 (5.18), 537 (4.02), 601 (4.09),
1
644 nm (4.35); H NMR (400 MHz, CDCl3, 258C, TMS): d=1.49 (s, 72H;
Theoretical calculations: Density functional calculations were performed
with Gaussian 03[43] using the spin-restricted B3LYP functional[44] on an
8-process Quantum CubeTM developed by the Parallel Quantum Solu-
tions. All calculations were performed using GAUSSIAN-03. Graphical
outputs of the computational results were generated with the Gauss
View software program (version 3.09) developed by Semichem, Inc.[45]
tert-butyl H), 7.94 (d, J=1.8 Hz, 8H; Ho), 7.95–7.96 (m, 8H; quinoxaline
H), 8.015 (t, J=1.8 Hz, 4H; Hp), 9.36 ppm (s, 4H; b-pyrrolic H);
31P NMR (162 MHz, CDCl3): d=ꢀ146.42 (septet, J=714 Hz, 1P, PF6);
MS (ESI): m/z: 1462.0 ([MꢀPF6]+ requires 1461.7); HR-ESI-FT/ICR:
m/z: [MꢀPF6]+ 1461.7408; C88H96AuF6N8P requires 1461.7418.
Instrumentation: Spectral measurements were performed using a Hewlett
Packard 8453 diode array spectrophotometer. ESR measurements were
recorded on a JEOL JES-RE1XE spectrometer. Cyclic voltammetry was
performed on either an ALS electrochemical analyzer or an EG&G
Princeton Applied Research (PAR) 173 potentiostat/galvanostat in dea-
erated PhCN, 0.1m TBAPF6 using a conventional three-electrode cell.
Chem. Eur. J. 2009, 15, 10493 – 10503
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S. Fukuzumi, M. J. Crossley, K. M. Kadish et al.
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Acknowledgements
This work was partially supported by the Ministry of Education, Culture,
Sports, Science and Technology of Japan, with a Grant-in-Aid (Nos.
19205019, 19750034, 21750146) to S.F. and K.O. and a Global COE pro-
gram, ’’The Global Education and Research Center for Bio-Environmen-
tal Chemistry” to S.F. and KOSEF/MEST through WCU project (R31-
2008-000-10010-0). The support of the Robert A. Welch Foundation
(KMK, Grant E-680) and the Texas Advanced Research Program to
K.M.K. under Grant No. 003652-0018-2001 and from the Rice-Houston
Alliance for Graduate Education and Professoriate for Minorities are
also gratefully acknowledged. This work was also partially supported by
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Chem. Eur. J. 2009, 15, 10493 – 10503

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reduced species of AuPQ+/
portionation between two AuPQ /
(Sc3+
)
[AuPQꢀ/
G
2
AHCTUNGTERNNUNG )
2
AuPQ/
N
/ACHTUNGTRENNUNG
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Received: April 27, 2009
2465.
Published online: August 26, 2009
Chem. Eur. J. 2009, 15, 10493 – 10503
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10503