Design, synthesis and photophysical studies of phenylethynyl-bridged phthalocyanine-fullerene dyads

Article abstract of DOI:10.1142/S1088424615500662

A zinc and a free base phthalocyanine-fulleropyrrolidine dyad in which the chromophores are linked by a phenylethynyl group have been prepared using a new synthetic route, and their photoelectrochemical properties have been investigated. The zinc dyad is

Full text of DOI:10.1142/S1088424615500662

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2015; 19: 1–12
DOI: 10.1142/S1088424615500662
Published at http://www.worldscinet.com/jpp/
Design, synthesis and photophysical studies
of phenylethynyl-bridged phthalocyanine-fullerene dyads
Jaro Arero, Gerdenis Kodis, Robert A. Schmitz, Dalvin D. Méndez-Hernández,
Thomas A. Moore*, Ana L. Moore* and Devens Gust*◊
Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287, USA
Received 24 February 2015
Accepted 17 April 2015
ABSTRACT: A zinc and a free base phthalocyanine-fulleropyrrolidine dyad in which the chromophores
are linked by a phenylethynyl group have been prepared using a new synthetic route, and their
photoelectrochemical properties have been investigated. The zinc dyad is readily soluble in a variety of
solvents, and its spectroscopic properties have been determined in toluene and benzonitrile. In toluene,
excitation of the zinc phthalocyanine is followed by rapid establishment of an equilibrium between the
phthalocyanine and fullerene excited states. These excited states decay mainly to the ground state and
the respective triplet states. The fullerene triplet then transfers its energy to form the phthalocyanine
triplet. About 20% of the phthalocyanine excited states lead to formation of a charge-separated state. In
benzonitrile, the same decay pathways are observed, but photoinduced electron transfer is much faster,
and generates the charge separated state with a quantum yield of ≥85%. The charge separated state has
a lifetime of 2.8 ns in toluene and 94 ps in benzonitrile.
KEYWORDS: phthalocyanine, fullerene, dyad, synthesis, electron transfer.
and their structures modified to tune properties. However,
INTRODUCTION
porphyrins do have drawbacks, and one of these is the
The initial chemical event in photosynthetic solar
relatively weak absorption by the Q-bands in the red
energy conversion is photoinduced electron transfer
spectral region. Because of relatively low extinction
from an excited state donor to an acceptor. Since the
coefficients in this region, where there is abundant
late 1970’s artificial photosynthetic reaction centers
sunlight, porphyrins alone are not ideal light harvesters
consisting of donor chromophores covalently linked
for efficient artificial photosynthesis.
to acceptors have been investigated as models for the
Phthalocyanines(Pc),ontheotherhand,haveverystrong
natural process, and for possible applications in solar
absorption in the Q-band region, typically 670–690 nm.
energy conversion [1–10]. Many of these donor–acceptor
Thus, they have the potential to absorb sunlight efficiently
dyads, triads, etc. have featured porphyrins as the light
absorbing electron donor. This is due to the resemblance
of porphyrins to natural chlorophylls, their reasonably
at those wavelengths, as do chlorophylls. In addition,
although the presence of substituents at the b-positions of
phthalocyanines generally has little effect on the Q-band
good light absorption, excited state lifetimes and redox
absorption maxima, the position of these bands may be
properties, their high stability and lack of aggregation in
solution, and the ease with which they can be synthesized
tuned by a-substitution [11]. The strong, long-wavelength
Q-band absorption makes phthalocyanines attractive
candidates for artificial photosynthesis. However, less
◊ SPP full member in good standing
research has been done in the area, relative to porphyrins,
due to difficulties in synthesis and the tendency of
phthalocyanines to aggregate and be insoluble.
In recent years, several research groups have begun
investigating the use of phthalocyanines in artificial
*Correspondence to: Devens Gust, email: gust@asu.edu, tel: +1
480-965-4547; Thomas A. Moore, email: tmoore@asu.edu, tel:
+1 480-965-3308; Ana L. Moore, email: amoore@asu.edu, tel: +1
480-965-2953
Copyright © 2015 World Scientific Publishing Company

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J. ARERO ET AL.
photosynthesis [12–40]. Since our initial synthesis and
studyofaporphyrin-fullerenedyadasanartificialreaction
center [41], fullerenes (e.g. C60) have been very popular
as electron acceptor components in donor–acceptor
assemblies, and the vast majority of the work with
phthalocyanines has employed this acceptor. In general,
phthalocyanine-fullerene dyads display photoinduced
electron transfer to generate a Pc·+-C60·- charge separated
state. In most Pc-C60 dyads the lifetime of this state is
rather short (on the time scale of some ps). This is due in
part to the relatively short chemical linkers which have
been employed between the moieties, which favor rapid
recombination. Longlifetimesforsuchstatesaredesirable
if one wishes to harvest the redox potential stored within
them to make electricity or useful compounds such as
fuels. Recently, Quintiliani and coworkers [14] reported a
dyad consisting of a zinc phthalocyanine linked through
a b-position to a fulleropyrrolidine via an ethynyl group.
Upon excitation in benzonitrile, this molecule generated
a charge-separated state with a lifetime of 36 ps and a
quantum yield of 0.36. Longer lifetimes were observed
in other solvents.
is actually an inseparable mixture of isomers with the
t-butyl groups distributed among the 6 b-positions. The
hydroxyl functionality on 3 was oxidized to a formyl
group by 2-iodoxybenzoic acid dissolved in a dimethyl
sulfoxide/tetrahydrofuran mixture at room temperature
to yield ZnPc 4 (79%). Synthesis of ZnPc-C60 dyad 1Zn
from 4 was achieved by a 1,3-dipolar cycloaddition of
azomethine yield generated in situ in a Prato reaction.
Reaction between 4, C60 and sarcosine in refluxing
toluene afforded a 53% yield of dyad 1Zn after silica gel
column purification.
For the synthesis of 1, 4 was heated in a mixture of
pyridine and pyridine hydrochloride at 120°C for 24 h
to yield Pc 5 essentially quantitatively (98%) without
the need for column purification. Then, a 1,3-dipolar
cycloaddition between 5, C60 and sarcosine was carried
out to obtain Pc-C60 dyad 1 in 69% yield, after silica gel
column purification. Note that 1 and 1Zn are racemic
mixtures due to the presence of the stereocenter on the
pyrrolidine moiety.
Model free base phthalocyanine 6 (Fig. 2) was prepared
by removal of the zinc from 3. A portion of 3 was heated
in pyridine and pyridine-HCl at 120°C for 21 h, followed
by addition of water to the reaction mixture, cooling,
centrifugation and filtration (38% yield).
Fulleropyrrolidine 7 (Fig. 2) was prepared by the
Prato reaction of 4-anisaldehyde, sarcosine and C60.
Compound 7 was then demethylated by treatment with
boron tribromide in a mixture of dichloromethane and
toluene to give model fullerene 8. Details of all syntheses
and characterization of the products are given in the
Experimental section.
Electron transfer theory predicts that decreasing the
electronic coupling between donor and acceptor (for
example, by increasing the length of a linker between
them) will reduce the rate constant for electron transfer.
·-
With the goal of increasing the lifetime of a Pc·+-C60
charge-separates state while maintaining a respectable
quantum yield for its formation, we have synthesized
Pc-C60 dyad 1 and its zinc analog 1Zn (Fig. 1), wherein
the phthalocyanine and fulleropyrrolidine are linked
by a phenylacetylene, and studied the photochemical
properties of these molecules.
Electrochemistry
RESULTS AND DISCUSSION
The redox potentials of the dyads and model
compounds were measured by cyclic voltammetry in
order to allow estimation of the energies of charge
separated states and driving force values for electron
transfer reactions. Cyclic voltammetry was carried
out in benzonitrile solution containing 0.10 M tetra-
n-butylammonium hexafluorophosphate as supporting
electrolyte. The experiments were performed in a one-
compartment cell with a three-electrode configuration
consisting of a glassy carbon working electrode, a
Pt counter electrode, and an Ag+/Ag quasireference
electrode.Voltammograms were measured in benzonitrile
containing 100 mM tetrabutylammonium hexafluoro-
phosphate after deoxygenation with argon, and using a
scan rate of 20 mV/s. Potentials were converted to SCE
by using ferrocenium/ferrocene (0.45 V vs. SCE) as an
internal reference.
Synthesis
The synthesis of dyad 1Zn by a significantly
different method was previously reported by Bottari
et al. [42]. The synthesis of the dyads employed here is
diagrammed in Fig. 1. Phthalonitrile 2 was synthesized
by a literature method [18] involving Sonogashira
coupling of 4-iodophthalonitrile with 4-ethynylbenzyl
alcohol in the presence of [Pd(PPh3)2Cl] and CuI as
the catalyst. Unsymmetrical ZnPc 3 was obtained by
a statistical condensation reaction between 2 and 4-t-
butylphthalonitrile in N,N-dimethylaminoethanol, which
were heated at reflux in the presence of Zn(OAc)2•2H2O.
Thin-layer chromatography (TLC) of the crude product
showed two main products: the less polar zinc(II) tetra-t-
butylphthalocyanine side product and the desired ZnPc 3.
Due to the significant polarity difference between the two
products, separation was easily achieved by using silica
column chromatography to yield 3 (17%). As is the case
with all phthalocyanines of this general type, the product
The results of the cyclic voltammetry are shown in
Table 1. Useful results could not be obtained for dyad 1
due to aggregation and lack of solubility in benzonitrile.
Table 1 indicates that the first oxidation potential for
1Zn is 0.54 V vs. SCE, whereas that for model Pc 6 is
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DESIGN, SYNTHESIS AND PHOTOPHYSICAL STUDIES OF PHENYLETHYNYL-BRIDGED
3
NC
NC
NC
[Pd(PPh3)Cl2], CuI
+
OH
TEA, 70 °C, 12 h
I
NC
2
OH
NC
N
NC
N
N
N
N
DMAE, Zn(OAc)2•2H2O
Zn
N
N
N
1Zn
N
N
N
N
N
N
C60, sarcosine,
toluene, reflux, 22 h
Zn
N
N
N
3
N
Zn
N
N
N
N
N
N
N
IBX, THF/DMSO, rt
4
HO
pyridine HCl, pyridine
120 °C
O
N
N
NH
N
NH
N
N
N
C60, sarcosine,
N
N
N
N
toluene, reflux, 22 h
1
HN
HN
N
N
N
5
O
Fig. 1. Synthesis of dyads 1 and 1Zn. TEA = triethylamine, DMAE = N,N-dimethylaminoethanol, IBX = 2-iodoxybenzoic acid,
DMSO = dimethylsulfoxide, THF = tetrahydrofuran
0.59 V. Thus, this oxidation is ascribed to formation of
the phthalocyanine radical cation. Similarly, the first
reduction potential of 1Zn is -0.66 V, whereas that of
model C60 8 is -0.63 V. Thus the -0.66 V feature in the
CV of 1Zn is due to formation of the fullerene radical
anion. These results show that linking the two moieties
has little effect on their redox potentials. These data also
permit estimation of the energy of the ZnPc·+-C60·- charge
separated state of 1Zn in benzonitrile as 1.20 eV above
the ground state. The very small perturbations of the
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J. ARERO ET AL.
N
O
OH
N
N
N
N
H
N
N
H
N
N
N
6
7
8
HO
Fig. 2. Structures of model zinc phthalocyanine 6, pyrrolidinofullerene intermediate 7 and model pyrrolidinofullerene 8
Table 1. Cyclic voltammetric data in benzonitrile
Compound
1st oxidation in V vs. SCE
1st reduction in V vs. SCE
and (peak width in mV)
and (peak width in mV)
Pc model 6
0.62 (160)
—
-0.78 (54)
-0.63 (80)
-0.99 (90)
-0.66 (74)
C60 model 8
ZnPc model 3
ZnPc-C60 1Zn
0.59 (145)
0.54 (210)
redox potentials observed upon linking the donor and
acceptor moieties to form 1Zn suggest that although the
potentials for 1 could not be measured directly, they will
be similar to those measured for model Pc 6 and model
fullerene 8. Thus, the energy of Pc·+-C60·- in 1 is estimated
to be ca. 1.25 eV above the ground state.
moiety has absorbance out to 706 nm, it contributes
significantly to the spectrum only in the UV region. The
spectrum of 1Zn in toluene solution (not shown) is very
similar to that in benzonitrile, but with a small shift to
shorter wavelengths; it has phthalocyanine maxima at
346, 613, 634, 675 and 689 nm and a small fullerene
maximum at 702 nm.
Figure 4a shows the fluorescence emission spectra
of the molecules in benzonitrile. The amplitudes
at the maxima have been normalized. Model zinc
phthalocyanine 3 shows an emission maximum at 711 nm
with a weak shoulder around 735 nm and another
maximum at 776 nm. Fullerene 8 has a somewhat similar
emission, but with the long-wavelength maximum further
into the infrared (maxima at 719 nm and ~756 nm).
Dyad 1Zn emission shows a maximum at 713 nm, a
weak shoulder around 735 nm and another maximum at
774 nm. Figure 4b shows the emission spectra of 1Zn
in both benzonitrile and toluene; the absorbance of the
two solutions was identical at the excitation wavelength.
The spectral shapes are similar in both solvents, but the
emission in benzonitrile is quenched by a factor of ca.
13. This indicates that in benzonitrile, a nonradiative
pathway for decay of the excited state(s) has become
more important.
Spectroscopy
Steady-state spectroscopy. Due to the tendency of 1 to
aggregate in solution, spectroscopic studies were carried
out only on the zinc dyad and related model compounds.
Figure 3 shows the absorption spectra of ZnPc 3, model
fullerene 8, and ZnPc-C60 dyad 1Zn in benzonitrile. The
two ZnPc spectra have been normalized at the Q-band
maxima. The fullerene absorption spectrum is shown
at a similar concentration. Model ZnPc 3 has a B-band
absorption at 356 nm and Q-band maxima at 617, 641,
680 and 692 nm. In 1Zn¸ the B-band appears at 352 nm
and Q-band maxima at 617, 639, 680 and 694 nm. Model
fullerene 8 shows broad absorption throughout the
visible region, with strong absorption in the UV around
320 nm and a long wavelength maximum at 706 nm. The
spectrum of the dyad is similar to a linear combination of
the spectra of the two components with small differences
in the 320 nm region that indicate weak interactions
between the chromophores. Although the fullerene
The emission spectrum of 1Zn resembles that of 3,
but is relatively more intense at wavelengths between
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DESIGN, SYNTHESIS AND PHOTOPHYSICAL STUDIES OF PHENYLETHYNYL-BRIDGED
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Fig. 3. Absorption spectra in benzonitrile of dyad 1Zn (solid),
model zinc phthalocyanine 3 (dashes) and model fullerene
8 (dots). A portion of the fullerene spectrum is also shown
expanded by a factor of 83 along the absorbance axis
ca. 725 nm and 830 nm (Fig. 4a). The fullerene emits
in this spectral region, but has very little absorption
at the excitation wavelength, relative to that of the
phthalocyanine. These facts suggest that the emission
spectrum of the dyad features fluorescence from both
the fullerene and the phthalocyanine, and the fullerene
excited singlet state is populated mainly by singlet-
singlet energy transfer from the phthalocyanine. This
is reasonable, because the two excited states are nearly
isoenergetic, and the chromophores are separated by a
short distance that should permit energy transfer by the
Förster mechanism [43].
Transient absorption spectroscopy. In order to
learn more about the excited state behavior of 1Zn, ps
transient absorption measurements were performed on
1Zn in a toluene solution that was deoxygenated by
bubbling with argon. Excitation was at 695 nm, near
the ZnPc maximum, with ca. 100 fs laser pulses. At
this wavelength, essentially all of the excitation leads
Fig. 4. Corrected fluorescence emission spectra for (a) model
phthalocyanine 3 (dash) with excitation at 300 nm, and model
fullerene 8 (dot) and dyad 1Zn (solid) with excitation at 430 nm
in benzonitrile, with all spectra normalized at the maxima,
and (b) dyad 1Zn in toluene (solid) and in benzonitrile (dash),
obtained with equal absorption at the excitation wavelength
(547 nm). The spectrum in benzonitrile is also shown multiplied
by 13 along the emission axis
1
to initial formation of ZnPc-C60. The results obtained
are shown in Fig. 5a as decay-associated spectra, where
each spectrum is associated with a particular decay time.
Four decay components were observed. The shortest,
with a time constant of 1.1 ps, has a negative amplitude
in the 690 nm region and positive amplitude around
700 nm, and is ascribed to a bathochromic shift of the
ZnPc stimulated emission due to excited singlet state
relaxation and solvation. The decay lifetimes of 52 ps
and 1.24 ns both show similar ground state bleaching
and stimulated emission in the 660–780 nm range. These
decays reflect both the ZnPc and fullerene excited singlet
states. These transients are associated with the singlet–
singlet energy transfer between the phthalocyanine and
the fullerene discussed above; following excitation of the
ZnPc moiety, a pseudoequilibrium is established by the
two excited states, which then both decay concurrently to
yield the ZnPc·+-C60·- charge separated state (giving rise
to the ZnPc·+ induced absorption at ~550 and ~860 nm)
and the ground and triplet states of both moieties. The
1
energy of ZnPc in toluene is estimated to be 1.78 eV
above the ground state from the wavenumber average
of the longest-wavelength absorption maximum and the
shortest-wavelength emission maximum. Similarly, the
fullerene first excited singlet state lies at 1.75 eV. These
values give an equilibrium constant of 3.1 for singlet–
singlet energy transfer between the phthalocyanine and
fullerene excited singlet states.
The longest decay time is longer than could be
determined using the conventional fs-ns apparatus
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J. ARERO ET AL.
Fig. 6. Energy diagram showing ground and transient high
energy states of dyad 1Zn and their interconversion pathways.
Values for the rate constants for the various steps indicated are
given in the text for two solvents
induced absorption at ~700 nm, which is due to decay
of the C60 triplet state, and the formation of ground state
bleaching at ~690 nm and induced absorption at ~500 nm
are due to formation of the ZnPc triplet state. Thus, this
component represents triplet-triplet energy transfer from
the fullerene triplet to the zinc phthalocyanine. Finally,
the 2.5 μs spectrum represents decay of 3ZnPc.
The excited state behavior of 1Zn in toluene can be
discussed with reference to Fig. 6, which shows the
various states of interest and pathways for interconversion
between them. The energies of the fullerene triplet state
(~1.50 eV) [44] and the zinc phthalocyanine triplet state
(1.13 eV) [45] are based on literature values for similar
molecules. When two excited states are in equilibrium
with energy transfer rate constants k3 and k-3 (for
steps 3 and -3 in Fig. 6) and each decays by the usual
photophysical pathways to the ground state with rate
constants k1 and k2 and the triplet state with k7 and k8,
transient behavior will feature two time constants tA and
tB with associated rate parameters gA and gB [46]. As
indicated in Equations 1 and 2, these rate parameters may
be related to the rate constants in Fig. 6, where the values
of X and Y are as given in Equations 3 and 4.
Fig. 5. (a) Transient absorption decay associated spectra of dyad
1Zn in toluene on the fs-ns time scale, with excitation at 695 nm.
The time constants associated with the decays are 1.1 ps
(squares), 52 ps (circles), 1.24 ns (triangles) and a value too long
to determine on the time scale of this measurement (inverted
triangles). (b) Transient absorption decay associated spectra of
dyad 1Zn in toluene on the ns-μs time scale. The time constants
associated with these decays are 1.24 ns (hollow squares), 2.81 ns
(hollow circles), 46.6 ns (triangles) and 2.5 μs (diamonds)
YA = {(X +Y)+[(X −Y)2 + 4k3k-3 ]1/2} 2
(1)
(2)
(IRF ~150 fs, optical delay line ≤ 3 ns). We therefore
investigated the material on the ns-ms time scale using an
EOS spectrometer from Ultrafast Systems (IRF ~800 ps).
Excitation at 670 nm of an argon-bubbled toluene
solution of 1Zn with ca. 100 fs pulses gave the decay
associate spectra shown in Fig. 5b. The 1.24 ns decay
associated spectrum is the same decay that was observed
on the ps time scale. The 2.81 ns decay shows ground
state bleaching at ~690 nm and induced absorption
at ~550 nm and ~860 nm that are characteristic of the
ZnPc radical cation, and therefore can be associated
with the decay of the ZnPc·+-C60·- charge-separated state.
The 46.6 ns decay associated spectrum shows decay of
/
YB = {(X +Y)+[(X −Y)2 + 4k3k-3 ]1/2} 2
/
X = k1 + k7 + k4 + k3+
Y = k2 + k8 + k5 + k-3
(3)
(4)
The values of gA and gB are the reciprocals of the 52 ps
and 1.24 ns decays observed in the decay-associated
spectra.Thesumofk1 andk7 maybetakenasthereciprocal
of the excited singlet state lifetime of model ZnPc 3. This
lifetime was measured in toluene by time correlated
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DESIGN, SYNTHESIS AND PHOTOPHYSICAL STUDIES OF PHENYLETHYNYL-BRIDGED
7
single photon counting fluorescence measurements, and
equals 2.91 ns. Similarly, the lifetime of 1C60 in model 8
was found to be 1.34 ns, and its reciprocal equals k2 + k8.
The equilibrium constant for singlet energy transfer, K,
is 3.1, as mentioned above, and thus k-3 = k3/3.1. We are
therefore left with two equations and three unknowns, k3,
k4 and k5. Electron transfer steps 4 and 5 involve the same
two chromophores and the same linkage between them.
The driving force for electron transfer is very nearly the
same for the two reactions, given the small difference in
energies of the initial states, and the fact that the final state
is the same in both cases. However, the orbitals involved
are different, as step 4 involves transfer of an electron
between LUMOs, and step 5 is “hole transfer” involving
HOMOs. In the absence of other information, we will
assume that the electronic coupling is approximately
the same for the two reactions, and that k4 = k5. Making
this assumption allows us to solve Equations 1–4 [47] to
yield values of k3 = 1.4 × 1010 s-1 (t3 = 71 ps), k-3 = 4.6 ×
109 s-1 (t-3 = 220 ps), and k4 = k5 = 1.6 × 108 s-1 (t4 =
t5 = 6.3 ns). Based on these values, the total quantum
yield of charge separation in toluene is ca. 20%. From the
time-resolved data above, we find that k6, rate constant
Fig. 7. Decay associated spectra from excitation of 1Zn in
benzonitrile at 695 nm. The time constants are 1.3 ps (squares),
10 ps (circles) and 94 ps (triangles)
associated spectrum shows ground state bleaching at
·-
~600–700 nm and <ꢀ540 nm due to the ZnPc·+ and C60
radical ions. There is also induced absorption at ~560 nm
that is characteristic of ZnPc·+, and no stimulated emission
is observed. Thus, this decay is associated with the decay
·-
for charge recombination of ZnPc·+-C60 is 3.6 × 108 s-1
(the reciprocal of 2.81 ns). Thus, charge recombination is
slightly faster than charge separation. The rate constant
for transfer of triplet energy from the fullerene to the
phthalocyanine, k9, is 2.2 × 107 s-1 (t = 46.6 ns), and
k10, the rate of decay of the phthalocyanine triplet to the
ground state, is 4.0 × 105 s-1 (t = 2.5 ms). It is likely that
this lifetime is shortened due to small amounts of residual
oxygen in the sample. In summary, in toluene 1ZnPc-C60
is in equilibrium with ZnPc-1C60 by singlet–singlet
energy transfer, and both excited states decay to yield the
·-
of the ZnPc·+-C60 charge-separated state to the ground
state. The 1.3 ps component has negative amplitude at
~690 nm and positive amplitude at ~700 nm due to the
bathochromic shift of the ZnPc stimulated emission that
results from excited state relaxation and solvation, as was
observed in toluene. This component also shows some
negative amplitude at ~560 nm, which may be ascribed
·-
to some very rapid formation of ZnPc·+-C60 from the
unrelaxed ZnPc excited state.
·-
ZnPc·+-C60 charge-separated state, their corresponding
We will discuss this behavior with reference to Fig. 6,
as was done for the results in toluene. The energies of
1ZnPc-C60 and ZnPc-1C60 are estimated to be 1.76 and
1.74 eV, respectively, in benzonitrile. Photoinduced
triplet states, and their ground states. ZnPc-3C60 decays
by triplet energy transfer to yield 3ZnPc-C60, which lies at
significantly lower energy.
1
·-
electron transfer from ZnPc-C60 to yield ZnPc·+-C60
The steady-state fluorescence results above show that
in benzonitrile, the excited state lifetimes of both the zinc
phthalocyanine and fullerene chromophores of 1Zn are
significantly shorter than they are in toluene. In order to
investigate the reasons for this behavior, we obtained the
transient absorption decay associated spectra for 1Zn in
benzonitrile with excitation at 695 nm, and the results are
plotted in Fig. 7. Three time constants were observed:
1.3 ps, 10 ps and 94 ps. The 10 ps component shows decay
of phthalocyanine stimulated emission in the 690–780 nm
spectral region and decay of phthalocyanine excited
singlet state induced absorption in the 600–680 nm
(step 4) is much more rapid in benzonitrile than in
1
toluene, and dominates the decay of ZnPc-C60. The
rate constant for step 4 may be estimated according to
Equation 5, where t1Zn = the 10 ps lifetime of the first
excited singlet state of 1ZnPc-C60;
1
1
k4 =
−
− k3
(5)
t1Zn t1
1
t1 is taken as the excited state lifetime of ZnPc in
model compound 3, which has been found to be 2.64 ns
in benzonitrile via fluorescence decay measurements,
and k3 is the rate constant for singlet–singlet energy
transfer from 1ZnPc-C60 to yield ZnPc-1C60. Because rate
constants for energy transfer show only a weak solvent
dependence, we will take the value of k3 as 1.4 × 1010 s-1,
as calculated above for toluene. This yields a value for
k4 of 8.5 × 1010 s-1, and a quantum yield of photoinduced
1
region, all of which are consistent with decay of ZnPc.
This decay associated spectrum also features negative
amplitude around 560 nm, which is due to the formation
of induced absorption due to ZnPc·+. Thus, the 10 ps
component is associated mainly with formation of the
·-
ZnPc·+-C60 charge-separated state by photoinduced
electron transfer from 1ZnPc-C60. The 94 ps decay
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J. ARERO ET AL.
·-
electron transfer of 85% from this process. Equation 5
ignores any contribution from k-3. The fate of the small
amount of ZnPc-1C60 excited state that results from
energy transfer is unknown, but it is likely that it decays
mainly to ZnPc·+-C60·-, for the reasons mentioned above.
In any case, no time constant corresponding to the
unperturbed lifetime of the fullerene excited state (1.40 ns
in benzonitrile) was observed. The rate constant for
charge recombination step 6 is k6 = 1.06 × 1010 s-1, as
calculated from the 94 ps decay time.
and the energy of ZnPc·+-C60 as estimated from cyclic
voltammetry in the same solvent (1.20 eV) as 0.56 eV.
The driving force for charge recombination to the ground
state is 1.20 eV. Charge recombination in benzonitrile
is slower than charge separation even though the
driving force for recombination is greater because
the photoinduced electron transfer reaction lies in the
normal region of the Marcus rate constant vs. free energy
change relationship, whereas charge recombination
likely lies in the inverted region. In toluene, the driving
force for photoinduced electron transfer is expected to
decrease, due to loss of solvent stabilization of the very
polar charge-separated state. This will decrease the rate
of charge separation because the reaction lies in the
normal region. But the solvent reorganization energy will
decrease, which will act to increase the rate of charge
separation. In 1Zn, the reorganization effect is evidently
outweighed by the contribution from the free-energy
change. In charge recombination, changing the solvent
from benzonitrile to toluene increases the driving force
for charge recombination because the charge separated
state is less stable, and this is expected to slow the rate
of charge recombination in the inverted region. This is
observed. The decrease in solvent reorganization energy
in the less polar toluene is also expected to slow charge
recombination in the inverted region, as observed.
Discussion
A new synthetic approach for the preparation of
phenylethynyl-bridgedzincandfreebasephthalocyanine-
fullerene dyads has been developed. Statistical
condensation of functionalized phthalonitrile 2 with
4-tert-butylphthalonitrile yields zinc phthalocyanine
3, which bears a 4-ethynylbenzyl alcohol group. This
zinc phthalocyanine can be easily separated from the
reaction mixture. The benzyl alcohol functionality is then
converted to the corresponding aldehyde by use of a mild
oxidant, 2-iodoxybenzoic acid, to afford 4, which serves
as a common intermediate for both the zinc and free base
dyads. Dyad 1Zn was prepared directly from 4 in a Prato-
typereactionwithsarcosineandC60. Synthesisoffreebase
dyad 1 was achieved via a quantitative demetallation of 4
using pyridine-HCl/pyridine to afford free base aldehyde
5, which also underwent a 1,3-dipolar cycloaddition with
C60 and sarcosine to form 1. This is the first report of
the preparation of 1. Dyad 1Zn was prepared previously
by a different route in which aldehyde 4 was made from
a tri-tert-butyliodophthalocyanine using a Sonogashira
coupling reaction with 4-ethynylbenzaldehyde [42].
The spectroscopic results show that in toluene,
1ZnPc-C60 decays into an equilibrium with ZnPc-1C60,
and that the resulting excited states decay mainly to the
ground and triplet states, although about 20% of the
excited states undergo photoinduced electron transfer
to yield ZnPc·+-C60·-. In benzonitrile, on the other hand,
1ZnPc-C60 decays to give mainly ZnPc·+-C60·-. About 14%
of the decay is by energy transfer to yield ZnPc-1C60¸
which likely also decays to ZnPc·+-C60·-. Overall the yield
of the charge-separated state is ≥85%.
Inarecentverybriefnoteconcernngthephotochemistry
of 1Zn by Guldi, Torres and coworkers [24], it was stated
that excitation of 1Zn in toluene yielded ZnPc-C60,
1
which decayed with a time constant of 30 ps to give the
ZnPc·+-C60 charge separated state, which decayed with
·-
a time constant of approximately 30 ns. Detailed spectra
were not provided. As mentioned above, our study also
reveals some photoinduced electron transfer in toluene,
but indicates that the vast majority of 1ZnPc-C60 decays to
an equilibrium with ZnPc-1C60 by singlet–singlet energy
transfer, and that the two excited states then decay mainly
to the ground state and a long-lived triplet state in ~2.8 ns.
The complex photochemical behavior of 1Zn in toluene
is not apparent unless a thorough study of the spectra at
many wavelengths and time scales is available.
A report by Guldi,Torres, Hieringer and coworkers [14]
has reported results for a zinc phthalocyanine-fullerene
dyad with a structure similar to that of 1Zn, but lacking
the phenyl group between the fulleropyrrolidine and the
triple bond. They report that in that molecule, excitation
of the phthalocyanine moiety in benzonitrile leads rapidly
to the formation of a ZnPc·+-C60·- charge-separated state.
The time constant for photoinduced electron transfer is
not given, but the quantum yield was reported as 0.56. The
The rate constant for photoinduced electron transfer
in benzonitrile is 53 times that in toluene, whereas the
rate constant for charge recombination in benzonitrile is
30 times faster than that in toluene. The electron transfer
rates can be discussed in terms of the Marcus–Hush theory
of electron transfer [48–52]. For a given dyad molecule
in different solvents, differences in rate constants are
due mainly to differences in solvent reorganization
energy and to differences in solvent stabilization of the
charge-separated state, which affect the thermodynamic
driving force for electron transfer. The driving force
for photoinduced electron transfer in benzonitrile may
lifetime of ZnPc·+-C60 was 36 ps. The shorter lifetime
·-
for the charge-separated state, as compared to the 94 ps
found in 1Zn, is consistent with the shorter chemical
linkage joining the chromophores, which in turn would
lead to stronger electronic coupling and faster electron
transfer. The thermodynamic driving force parameters
for the two molecules are expected to be almost identical.
1
be estimated from the energy of ZnPc-C60 (1.76 eV)
Copyright © 2015 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2015; 19: 8–12

DESIGN, SYNTHESIS AND PHOTOPHYSICAL STUDIES OF PHENYLETHYNYL-BRIDGED
9
As might be expected, reducing the electronic coupling
(Millennia/Tsunami/Spitfire, Spectra Physics). Part of
the laser pulse was sent through an optical delay line and
focused on to a 3 mm sapphire plate to generate a white
light continuum for the probe beam. The remainder of
the pulse energy was used to pump an optical parametric
amplifier (Spectra Physics) to generate excitation pulses,
which were selected using a mechanical chopper. The
white light generated was then compressed by prism
pairs (CVI) before passing through the sample. The
polarization of pump beam was set to the magic angle
(54.7°)relativetotheprobebeamanditsintensityadjusted
using a continuously variable neutral density filter. The
white light probe was dispersed by a spectrograph (300
line grating) onto a charge-coupled device (CCD) camera
(DU420, Andor Tech.). The final spectral resolution was
about 2.3 nm for over a nearly 300 nm spectral region.
The instrument response function (IRF) was ca. 150 fs.
Transient absorption spectra and kinetics on the ps-ms
time scale were obtained using EOS spectrometer from
Ultrafast Systems (IRF ~800 ps); excitation was from the
same optical parametric amplifier as descibed above.
Data analysis was carried out using locally written
software (ASUFIT) developed under a MATLAB
environment (Mathworks Inc.). Decay-associated spectra
(DAS) were obtained by fitting the transient absorption
or fluorescence change curves over a selected wavelength
region simultaneously as described by Equation 6
(parallel kinetic model),
between the phthalocyanine and fullerene in different
dyad architectures leads to an increase in the lifetime of
the charge-separated state [16, 33].
EXPERIMENTAL
General analytical
The UV-visible spectroscopy employed a Shimadzu
UV-3101PCUV-visiblespectrophotometer.Massspectro-
metry was performed by the MALDI-TOF method using
a Voyager DE STR from Applied Biosystems in reflector
1
mode. H NMR spectra were obtained using a Varian
400 MHz instrument with tetramethylsilane as internal
reference. Proton assignments were aided by the use of
COSY experiments. Electrochemistry was carried out
using CH Instruments 650C or 760D electrochemical
workstations and a standard 3-electrode cell setup.
Parameters were as described above.
Fluorescence
Steady-state fluorescence spectra were measured using
a Photon Technology International MP-1 spectrometer
and corrected for detection system response as a function
of wavelength. Excitation was provided by a 75 W
xenon-arc lamp and a single-grating monochromator.
Fluorescence was detected at 90° to the excitation
beam via a single-grating monochromator and an R928
photomultiplier tube having S-20 spectral response and
operating in the single photon counting mode.
Fluorescence decay measurements were performed
on optically dilute (ca. 1 × 10-5 M) samples by the time-
correlated single-photon-counting method. The excitation
source was a mode-locked Ti:Sapphire laser (Spectra
Physics, Millennia-pumped Tsunami) with a 130-fs pulse
duration operating at 80 MHz. The laser output was sent
through a frequency doubler and pulse selector (Spectra
Physics Model 3980) to obtain 370–450 nm pulses at
4 MHz. Fluorescence emission was detected at the magic
angle using a double grating monochromator (JobinYvon
Gemini-180) and a microchannel plate photomultiplier
tube (Hamamatsu R3809U-50). The instrument response
function was 35–55 ps. The spectrometer was controlled
by software based on the LabView programming
language and data acquisition was done using a single
photon counting card (Becker-Hickl, SPC-830).
n
DA(l,t) =
A (l)exp(-t / ti )
(6)
∑
i
i=1
whereDA(l,t)istheobservedabsorption(orfluorescence)
change at a given wavelength at time delay t and n is
the number of kinetic components used in the fitting. A
plot of Ai(l) vs. wavelength is called a decay-associated
spectrum, and represents the amplitude spectrum of the
ith kinetic component, which has a lifetime of ti. The
global analysis procedures described here have been
extensively reviewed in literature [53]. Random errors
associated with the reported lifetimes obtained from
fluorescence and transient absorption measurements
were typically ≤ꢀ5%.
Synthesis
4-(2-(4-(Hydroxymethyl)phenyl)ethynylphthalo-
nitrile (2). A 30-mL portion of Et3N was placed in a
round-bottomed flask immersed in ice-water and
deoxygenated with argon for 40 min. A mixture of
4-ethynylbenzyl alcohol (126 mg, 0.953 mmol) and
4-iodophthalonitrile (219 mg, 0.862 mmol) was dissolved
in the Et3N. Next, Pd(PPh3)2Cl2 (63 mg, 0.0898 mmol)
and CuI (21 mg, 0.11 mmol) were added. The
reaction mixture was stirred for 15 h at 70°C under
the argon atmosphere. The solvent was then removed
by distillation under reduced pressure and the crude
product was purified by flash column chromatography on
Transient absorption
The conventional femtosecond transient absorption
apparatus consisted of a kilohertz pulsed laser source
(adjustable up to sub-Hertz frequencies for EOS
applications, vide infra) and a pump-probe optical setup.
Laser pulses of 100 fs at 800 nm were generated from an
amplified, mode-locked titanium sapphire laser system
Copyright © 2015 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2015; 19: 9–12

10
J. ARERO ET AL.
silica gel (CH2Cl2/EtOAc, 9/1) to yield 207 mg (93%)
of 2 as a white solid. H NMR (CDCl3, 400 MHz):
d, ppm 7.83 (s, 1 H), 7.75–7.69 (m, 2 H), 7.48–7.46
(d, J = 8 Hz, 2 H), 7.35–7.33 (d, J = 8 Hz, 2 H), 4.68
(s, 2 H), 1.85 (br s, 1 H).
were stirred under argon for 24 h. After this time stirring
and heating were stopped and water (10 mL) was added
to the hot mixture. The mixture was allowed to cool and
the resulting precipitate was collected by centrifugation
at 3,250 rpm. The bluish green precipitate was washed
several times with water and then MeOH and dried under
high vacuum to obtain phthalocyanine 5 (91 mg, 98%).
1H NMR (THF-d8, 400 MHz): d, ppm 10.05 (s, 1 H),
8.70–7.19 (m, 16 H), 1.71–1.88 (m, 27 H). UV-vis
(CH2Cl2): lmax, nm 693, 677, 646, 620, 338. MALDI-TOF
MS (terthiophene): m/z calcd. for C53H46N8 [M]+ 810.38,
found 810.55.
1
Zinc phthalocyanine 3. A mixture of 4-tert-
butylphthalonitrile (715 mg, 3.88 mmol), phthalonitrile
.
2 (200 mg, 0.775 mmol) and Zn(OAc)2 2H2O (825 mg,
3.76 mmol) in 30 mL of DMAE was stirred and heated
at 130°C for 24 h. The mixture was cooled to room
temperature, the solvent was removed by distillation at
reduced pressure, the residue was washed with MeOH/
H2O (5/1) and the dark green crude product obtained was
purified by flash column chromatography on silica gel
(toluene/THF, 9/1). The blue symmetrical phthalocyanine
eluted first, followed by the greenish blue phthalocyanine
3 (119 mg, 17%). 1H NMR (d8-THF, 400 MHz): d, ppm
10.02 (s, 1 H), 9.45–9.20 (m, 8 H), 8.33–8.27 (m, 3 H),
7.86–7.72 (m, 4 H), 4.71 (s, 2 H), 4.35 (br s, 1 H), 1.85–
1.73 (m, 27 H). UV-vis (THF): lmax, nm 683, 611 and
350. MALDI-TOF MS (terthiophene): m/z calcd. for
C53H46N8OZn [M]+ 874.38, found 874.75.
Phthalocyanine-C60 dyad 1.
A
solution of
phthalocyanine 5 (20 mg, 0.025 mmol), C60 (59 mg,
0.082 mmol), and sarcosine (15.8 mg, 0.177 mmol) in
anhydrous toluene (25 mL) was heated at reflux with
stirring under an argon atmosphere for 22 h.After this time
the solvent was removed by distillation under reduced
pressure and the crude product was purified by column
chromatography (toluene/CS2, 1/1) to obtain 1 as a green
1
solid (26.7 mg, 69%). H NMR (toluene-d8, 400 MHz):
d,ppm9.14–8.65(m,9H),7.96–7.68(m,7H),4.77(s,1H),
4.67–4.64 (d, J = 12 Hz, 1 H), 3.99–3.97 (d, J = 8 Hz,
1 H), 2.62 (s, 3 H, N-CH3), 1.66–1.63 (m, 27 H), 2.29 (br
s, 2 H). UV-vis (THF): lmax, nm 694, 672, 644, 613, 343.
MALDI-TOF MS (dithranol): m/z calcd. for C115H51N9
[M]+ 1558.43, found 1558.80.
Zinc phthalocyanine 4. A mixture of phthalocyanine
3 (20 mg, 0.023 mmol) and iodoxybenzoic acid (20 mg,
0.0714 mmol) in THF (3 mL) and DMSO (3 mL) was
stirred under an argon atmosphere at room temperature
for 48 h. Stirring was stopped, 10 mL of brine was added
to the mixture and the resulting solution was extracted
with ether. The solvent was removed from the organic
layer by distillation under reduced pressure and the crude
product was purified by column chromatography toluene/
THF (19/1) to yield green phthalocyanine 4 (16 mg,
Phthalocyanine 6. Compound 3 (36 mg, 0.41 mmol),
pyridine (10 mL) and pyridine-HCl (0.089 g) were
heated at 120°C while stirring under argon for 21 h. After
brief cooling, water (5 mL) was added to the mixture.
The mixture was allowed to cool, and the resulting
precipitate was collected by centrifugation at 3,250 rpm.
The green precipitate was washed several times with
water and then MeOH and dried under high vacuum
1
79%). H NMR (THF-d8, 400 MHz): d, ppm 9.99 (s,
1 H), 9.40–9.28 (m, 4 H), 9.22–911 (m, 4 H), 8.22–8.15
(m, 4 H), 7.96–7.86 (m, 4 H), 1.75–1.62 (m, 27 H).
UV-vis (THF): lmax, nm 687, 672, 611, 352. MALDI-
TOF MS (terthiophene): m/z calcd for C53H44N8OZn [M]+
872.29, found 874.49.
1
to obtain phthalocyanine 6 (12.7 mg 38%). H NMR
(toluene-d8, 400 MHz): d, ppm 8.94–8.37 (m, 10 H),
8.22–8.05 (m, 6 H), 4.67 (s, 2 H), 4.34 (br s, 1 H), 1.95–
1.72 (m, 27 H). UV-vis (THF): lmax, nm 694, 667, 644,
611, 344. MALDI-TOF MS (terthiophene): m/z calcd.
for C53H48N8O [M]+ 812.40, found 812.45.
Zinc phthalocyanine-C60 dyad (1Zn). A mixture
of phthalocyanine 4 (15 mg, 0.017 mmol), C60 (41 mg,
0.057 mmol) and sarcosine (11 mg, 0.12 mmol) in
anhydrous toluene (25 mL) was stirred at reflux for 22 h
under an argon atmosphere. After this time the mixture
was cooled and the solvent was removed by distillation
under reduced pressure. The crude product was purified
by column chromatography on silica gel (toluene/THF,
49/1)toobtainthephthalocyanine-C60 dyad1Znasadark-
blue powder (15 mg, 53%). 1H NMR (THF-d8, 400 MHz):
d, ppm 9.41–9.49 (m, 4 H), 9.31–9.23 (m, 4 H),
8.31–8.22 (m, 4 H), 7.98 (br s, 2 H), 7.87 (br s, 2 H),
5.06–5.01 (m, 3 H), 4.28–4.25 (d, J = 12 Hz, 2H), 2.86
(s, 3 H, N-CH3), 1.94–1.76 (m, 27 H). UV-vis (THF):
N-methyl-2-(p-methoxyphenyl)-3, 4-fulleropyrro-
lidine 7. A mixture of C60 (216 mg, 0.30 mmol), 4-
anisaldehyde (20 mg, 0.15 mmol) and sarcosine (136 mg,
1.50 mmol) in toluene (60 mL) was warmed to reflux
under an argon atmosphere and stirred for 26 h. The
solvent was removed by distillation under reduced
pressure. The crude product was purified by silica
column chromatography (toluene/EtOAc, 99/1) to obtain
fullerene 7 as a brown powder (78.8 mg, 60%). 1H NMR
(CDCl3, 400 MHz): d, ppm 7.69 (br s, 2 H), 6.95–6.93 (d,
J = 8 Hz, 2 H), 4.97–4.95 (d, J = 8 Hz, 1 H), 4.87 (s, 1 H),
4.25–4.21 (d, J = 8 Hz, 1 H), 3.80 (s, 3 H, N-CH3), 2.77
(s, 3 H, O-CH3). MALDI-TOF MS (terthiophene): m/z
calcd. for C53H48N8O [M]+ 883.10, found 883.18.
l
max, nm 685, 672, 632, 610, 350. MALDI-TOF MS
(dithranol): m/z calcd. for C115H49N9Zn [M]+ 1620.34,
found 1620.48.
Phthalocyanine 5. Phthalocyanine 4 (100 mg, 0.115
mmol), pyridine (10 mL) and pyridine-HCl (0.30 g)
Fulleropyrrolidine 8. To a solution of fullerene 7
(32 mg, 0.036 mmol) in toluene (20 mL) immersed in
Copyright © 2015 World Scientific Publishing Company
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DESIGN, SYNTHESIS AND PHOTOPHYSICAL STUDIES OF PHENYLETHYNYL-BRIDGED
11
ice-cold water, 1.5 mL of 1 M BBr3 (1.5 mmol) was added
2. Gust D and Moore TA. In The Porphyrin Handbook,
Kadish KM, Smith KM and Guilard R. (Eds.) Aca-
demic Press: San Diego, 2000; pp 153–190.
3. Meyer TJ. Acc. Chem. Res. 1989; 22: 163–170.
4. Redmore NP, Rubtsov IV and Therien MJ. J. Am.
Chem. Soc. 2003; 125: 8769–8778.
5. Wasielewski MR. Chem. Rev. 1992; 92: 435–461.
6. Gust D, Moore TA and Moore AL. Acc. Chem. Res.
2001; 34: 40–48.
7. Falkenstrom M, Johansson O and Hammarstrom L.
Inorg. Chim. Acta 2007; 360: 741–750.
8. Fukuzumi S and Imahori H. Electron Transfer in
Chemistry 2001; 2: 927–975.
dropwise. The mixture was stirred at room temperature
for 24 h under an argon atmosphere. The mixture was
then transferred to a separatory funnel and washed with
water (30 mL × 3) and then the solvent was removed by
distillation under reduced pressure. The crude product
was purified by column chromatography on silica gel
(toluene/MeOH, 19/1) to obtain fullerene 8 as a brown
solid (20 mg, 65%). 1H NMR (CDCl3, 400 MHz): d, ppm
7.65 (br s, 2 H), 6.85–6.83 (d, J = 8 Hz, 2 H), 4.97–4.95
(d, J = 8 Hz, 1 H), 4.86 (s, 1 H), 4.25–4.23 (d, J = 8 Hz,
1 H), 2.97 (s, 3 H). MALDI-TOF MS (terthiophene): m/z
calcd. for C69H11NO [M]+ 869.08 found 869.15.
9. Flamigni L, Armaroli N, Barigelletti F, Balzani V,
Collin J-P, Dalbavie J-O, Heitz V and Sauvage J-P.
J. Phys. Chem. B 1997; 101: 5936–5943.
CONCLUSION
10. Gust D, Moore TA and Moore AL. Acc. Chem. Res.
2009; 42: 1890–1898.
11. Kobayashi N, Nakajima S and Osa T. Inorg. Chim.
Acta 1993; 210: 131–133.
12. Tkachenko NV, Efimov A and Lemmetyinen H. J.
Porphyrins Phthalocyanines 2011; 15: 780–790.
13. Isosomppi M, Tkachenko NV, Efimov A, Vahasalo
H, Jukola J, Vainiotalo P and Lemmetyinen H.
Chem. Phys. Lett. 2006; 430: 36–40.
14. Quintiliani M, Kahnt A, Woelfle T, Hieringer W,
Vazquez P, Goerling A, Guldi DM and Torres T.
Chem. Eur. J. 2008; 14: 3765–3775.
We report a new synthetic route to covalently linked
phthalocyanine-fullerene dyads 1 and 1Zn, and have
1
shown that in toluene, ZnPc-C60 rapidly equilibrates
with ZnPc-1C60 by singlet–singlet energy transfer.
These excited states decay mainly to the ground states
of both chromophores and the phthalocyanine triplet
state, as the fullerene triplet state transfers its triplet
energy to the phthalocyanine. About 20% of the singlet
excited states transfer an electron to the fullerene to
generate ZnPc·+-C60·-, which has a lifetime of 2.8 ns.
In the more polar benzonitrile, the rate constant for
photoinduced electron transfer increases dramatically,
·-
giving ZnPc·+-C60 with a quantum yield of ≥85%.
15. Kahnt A, Quintiliani M, Vazquez P, Guldi DM and
Torres T. Chemsuschem 2008; 1: 97–102.
The lifetime of the charge separated state decreases in
this solvent, in accord with Marcus theory. The 94 ps
lifetime of ZnPc·+-C60·- in benzonitrile is almost 3 times
longer than that for the corresponding state of a similar
triad lacking the phenyl ring in the phthalocyanine-
fullerene linkage. This is still short compared to lifetimes
of charge separation in many porphyrin-fullerene and
similar dyads, and longer lifetimes would be useful for
the application of such dyads in solar energy conversion.
Both charge separation and recombination are facilitated
by the conjugation in the linkage joining the donor and
the acceptor.
16. Quintiliani M, Kahnt A, Vazquez P, Guldi DM and
Torres T. J. Mater. Chem. 2008; 18: 1542–1546.
17. Lemmetyinen H, Kumpulainen T, Niemi M, Efimov
A, Ranta J, Stranius K and Tkachenko NV. Photo-
chem. Photobiol. Sci. 2010; 9: 949–959.
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19. Ranta J, Kaunisto K, Niskanen M, EfimovA, Hukka
TI and Lemmetyinen H. J. Phys. Chem. C 2014;
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20. El-Khouly ME, Kim JH, Kay KY and Fukuzumi S.
J. Porphyrins Phthalocyanines 2013; 17: 1055–1063.
21. Urbani M, Osati S, Kuhri S, Guldi DM and Torres T.
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22. Al-Subi AH, Efimov A, Niemi M, Tkachenko NV
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Acknowledgements
This work was supported by the Office of Basic
Energy Sciences, Division of Chemical Sciences,
Geosciences, and Energy Biosciences, Department of
Energy under contract DE-FG02-03ER15393 and the
Center for Bio-Inspired Solar Fuel Production, an Energy
Frontier Research Center funded by the U.S. Department
of Energy, Office of Science, Office of Basic Energy
Sciences Under Award Number DE-SC0001016.
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Zheng J. Chin. J. Chem. 2012; 30: 1766–1770.
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