Porphyrin-phthalocyanine/pyridylfullerene supramolecular assemblies

Article abstract of DOI:10.1002/chem.201103776

The synthesis and photophysical properties of several porphyrin (P)-phthalocyanine (Pc) conjugates (P-Pc; 1-3) are described, in which the phthalocyanines are directly linked to the β-pyrrolic position of a meso-tetraphenylporphyrin. Photoinduced energy- and electron-transfer processes were studied through the preparation of H₂P-ZnPc, ZnP-ZnPc, and PdP-ZnPc conjugates, and their assembly through metal coordination with two different pyridylfulleropyrrolidines (4 and 5). The resulting electron-donor-acceptor hybrids, which were formed by axial coordination of compounds 4 and 5 with the corresponding phthalocyanines, mimicked the fundamental processes of photosynthesis; that is, light harvesting, the transduction of excited-state energy, and unidirectional electron transfer. In particular, photophysical studies confirmed that intramolecular energy-transfer resulted from the S₂ excited state as well as from the S₁ excited state of the porphyrins to the energetically lower-lying phthalocyanines, followed by an intramolecular charge-transfer to yield P-Pc^.+·C₆₀^.-. This unique sequence of processes opens the way for solar-energy-conversion processes. Copyright

Full text of DOI:10.1002/chem.201103776

DOI: 10.1002/chem.201103776
Porphyrin–Phthalocyanine/Pyridylfullerene Supramolecular Assemblies
Ana M. V. M. Pereira,[a] Anita Hausmann,[b] Jo¼o P. C. Tomꢀ,[a] Olga Trukhina,[c]
Maxence Urbani,[c] Maria G. P. M. S. Neves,[a] Josꢀ A. S. Cavaleiro,*[a]
Dirk M. Guldi,*[b] and Tomꢁs Torres*[c, d]
Abstract: The synthesis and photophys-
ical properties of several porphyrin
(P)–phthalocyanine (Pc) conjugates
(P–Pc; 1–3) are described, in which the
phthalocyanines are directly linked to
the b-pyrrolic position of a meso-tetra-
and 5). The resulting electron-donor–
acceptor hybrids, which were formed
by axial coordination of compounds 4
and 5 with the corresponding phthalo-
cyanines, mimicked the fundamental
processes of photosynthesis; that is,
light harvesting, the transduction of ex-
cited-state energy, and unidirectional
electron transfer. In particular, photo-
physical studies confirmed that intra-
molecular energy-transfer resulted
from the S2 excited state as well as
from the S1 excited state of the por-
phyrins to the energetically lower-lying
phthalocyanines, followed by an intra-
phenylporphyrin.
Photoinduced
energy- and electron-transfer processes
were studied through the preparation
of H2P–ZnPc, ZnP–ZnPc, and PdP–
ZnPc conjugates, and their assembly
through metal coordination with two
different pyridylfulleropyrrolidines (4
molecular charge-transfer to yield P–
Keywords: electron transfer · ful-
+
C
CÀ
Pc ·C60 . This unique sequence of pro-
cesses opens the way for solar-energy-
conversion processes.
lerenes
·
photoinduced energy
transfer · phthalocyanines · supra-
molecular chemistry
Introduction
tems.[2] Porphyrins (P), and likewise phthalocyanines (Pc),
have been extensively studied owing to their roles in mim-
icking biological systems.[3,4] In addition, their high chemical
and thermal stabilities, their optical and electrochemical
properties,[5] and the possibility to tune these properties by
inserting different metals into the center of the macrocycles
or by introducing different substituents at their peripheries
are important features for those applications. Both porphyr-
ins and phthalocyanines exhibit remarkable absorption in
the visible region, which is of broad interest for optical and
photovoltaic applications.[6] In general, different possibilities
should be considered for connecting the two macrocyclic
subunits. In most cases, an efficient energy-transfer funnels
the excited-state energy from the porphyrins to the phthalo-
cyanines, when the distance between the chromophores is
small.[7] Importantly, as a consequence of a direct connec-
tion, strong excitonic couplings are formed.[8,9] In light of
the aforementioned properties, a large number of P–Pc con-
jugates have been used as electron donors that were linked
covalently or non-covalently to electroactive- or photoactive
components, including fullerenes.[1,10,11] Owing to the 3D
structure of C60, its relatively low reduction potential, and its
high symmetry, C60 features small reorganization energies in
electron-transfer reactions.[12] This property renders C60
useful as a building block in artificial-light-harvesting and
energy-conversion systems.[13–16] Non-covalently linked elec-
tron-donor–acceptor hybrids that are based on coordinating
C60 to porphyrins or phthalocyanines exhibit a number of
advantages, such as their flexible and easy synthesis as well
as their ability to monitor through-bond versus through-
space electron-transfer interactions.[1f,h,11,17–21] Such charac-
Photoinduced energy-transfer- and electron-transfer process-
es in electron-donor–acceptor models are of great interest
to researchers for designing photosynthetic systems for
light-harvesting.[1] In fact, the latter process has been the
basis for developing efficient solar-energy-conversion sys-
[a] Dr. A. M. V. M. Pereira,+ Dr. J. P. C. Tomꢀ, Dr. M. G. P. M. S. Neves,
Prof. J. A. S. Cavaleiro
Departamento de Quꢁmica e QOPNA
Universidade de Aveiro, Campus de Santiago
3810-193 Aveiro (Portugal)
E-mail: jcavaleiro@ua.pt
[b] A. Hausmann,+ Prof. D. M. Guldi
Friedrich-Alexander-Universitꢂt Erlangen-Nꢃrnberg
Institute for Physical Chemistry
Egerlandstr. 3, 91058 Erlangen (Germany)
E-mail: dirk.guldi@chemie.uni-erlangen.de
[c] O. Trukhina, Dr. M. Urbani, Prof. T. Torres
Departamento de Quꢁmica Orgꢄnica
Universidad Autꢅnoma de Madrid, Cantoblanco
28049-Madrid (Spain)
E-mail: tomas.torres@uam.es
[d] Prof. T. Torres
IMDEA-Nanociencia. Facultad de Ciencias
Cantoblanco, 28049- Madrid (Spain)
[+] Both authors contributed equally to the research.
1
Supporting information for this article, including H NMR spectra,
MALDI-TOF mass spectra, and UV/Vis absorption spectra of the
porphyrin–phthalocyanine dyads 2 and 3 (Figures S1–S14), porphyri-
nylphthalonitrile 7 (Figures S15–S17), and photophysical studies of
the reference compounds (Figures S18–S24), is available on the
WWW under http://dx.doi.org/10.1002/chem.201103776.
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FULL PAPER
teristics motivated the design of different P·C60 and Pc·C60
electron-donor–acceptor hybrids.[1f,h,22]
Recently, the synthesis and the photophysical properties
of supramolecular systems based on fused P–Pc conjugates
and the linear N-(pyridyl)fullero[c]pyrrolidine[17] have been
established. Although such P–Pc conjugates revealed a bent
configuration owing to steric hindrance, strong mutual elec-
tronic coupling caused red-shifts in the majority of the ab-
sorption spectra. The coordination of these P–Pc conjugates
with the pyridyl-based C60 gave rise to P–Pc·C60 electron-
donor–acceptor arrays, in which photophysical measure-
ments supported an initial intramolecular energy-transfer
from the porphyrins to the phthalocyanines followed by an
intramolecular electron-transfer from the phthalocyanines to
C60.[23]
Scheme 1. Structures of compounds 1 (M=ZnII), 2 (M=2H), 3 (M=
PdII), 4, and 5. R1 and R2 are defined in Scheme 2.
The direct linkage of the two chromophores through the
b-pyrrolic position of meso-tetraphenylporphyrin hinders,
for example, the free rotation in compounds 1–3 at room
temperature. Despite the loss of molecular planarity and
conjugation, strong excitonic coupling and ultrafast energy-
transfer owing to the close proximity of the porphyrins and
phthalocyanines is seen.[8] Herein, we report the extension
of our earlier studies on photoinduced electron-transfer pro-
cesses through the preparation of P–Pc conjugates that con-
tained different metals in the porphyrins (1–3; Scheme 1)
and their assembly by metal coordination with the linear
(4)[17] and non-linear pyridylfullerenes (5).[24] The various
methods used for characterizing these compounds included
steady-state absorption and fluorescence spectroscopy as
well as time-resolved fluorescence and transient absorption
spectroscopy.
were directly linked to the b-pyrrolic position of meso-tetra-
phenylporphyrin (H2TPP), is shown in Scheme 2. Com-
pounds 1a–1c were obtained by statistical cross-condensa-
tion of porphyrinylphthalonitrile 6[8,25] with an excess of
phthalonitrile, 4-tert-butylphthalonitrile, and 4,5-dibutoxyph-
thalonitrile, respectively, in a refluxing mixture of o-dichlor-
obenzene and N,N-dimethylaminoethanol (DMAE), in the
presence of zinc chloride.[8] Compounds 2a–2c were ob-
tained in excellent yields (about 95%) through the demeta-
lation of the porphyrins of compounds 1a–1c with trifluoro-
acetic acid (TFA) in CH2Cl2. The absorption spectra of
these compounds showed an additional band at 516 nm,
which is assigned to one of the Q bands of the free-base por-
phyrin. Compounds 3a–3c were obtained in moderate yields
(13–45%) through the metalation of porphyrinylphthaloni-
trile 6 with palladium, carried out in the presence of palladi-
um acetate in refluxing DMF, followed by statistical conden-
sation of palladium complex 7 with the adequate non-por-
phyrinic phthalonitriles under similar conditions to those re-
ported for compounds 1a–1c.
Results and Discussion
Synthesis of porphyrin–phthalocyanine conjugates 1–3: The
synthetic route to dyads 1–3, in which the phthalocyanines
Scheme 2. Synthesis of compounds 1, 2, and 3. Reaction conditions: i) DMAE/o-C6H4Cl2 1458C, N2, 24 h; ii) TFA/CH2Cl2, RT 30 min; iii) PdACHTUNGTRENNUNG(OAc)2,
DMF, reflux, 90 min.
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J. A. S. Cavaleiro, D. M. Guldi, T. Torres et al.
All of the new compounds were characterized by HRMS
(MALDI-TOF), which showed peaks corresponding to the
1
M+ ions. H NMR studies of compounds 2c and 3c showed
C
the structural features expected for these types of com-
pounds.[8] The singlet at dꢀ10.7 ppm, which was attributed
to the proton at the 3-position of the porphyrins, showed
that the P–Pc connection involved the b-pyrrolic position.
The presence of several sets of signals for the meso-phenyl
rings, namely those at dꢀ10.7 ppm, correspond to the reso-
nance of three ortho protons, and those at dꢀ5.5–6.2 ppm,
which were attributed the 20-phenyl ring, confirmed there
was no rotation of these groups because the ortho protons
of each phenyl ring were not equivalent. On the other hand,
the presence of the metal-free porphyrin in compound 2c
was confirmed by the appearance of two signals at dꢀ
À2.5 ppm owing to the two nonequivalent NH protons,
which can be explained by the lack of free-rotation through
Figure 1. Absorption spectra of compounds 1b (black), 2a (dark gray),
and 3c (light gray); all spectra were recorded in toluene.
À
the C C bond that connects the two macrocycles. In the
quantum yield of 0.04 and a lifetime of 2.0 ns. H2TPP
showed fluorescence maxima at 650 and 715 nm, with a
quantum yield of 0.1 and a lifetime of 10 ns, whilst PdTPP
showed fluorescence maxima at 610 and 710 nm, with a
quantum yield of 1.2ꢇ10À3 and a lifetime of 0.076 ns. Finally,
the strongest emitter, ZnPctBu4, showed a single maximum
at 685 nm, with a quantum yield of 0.3 and a lifetime of
3.6 ns. All fluorescence spectra were in excellent agreement
with their corresponding absorption spectra (see the Sup-
porting Information, Figure S19).
When comparing the phthalocyanine fluorescence of
ZnPctBu4 with those of compounds 1, 2, and 3 upon excita-
tion at 670 nm, the fluorescence maxima were also at
685 nm. From the different quantum yields, we inferred
some electronic interactions between the building blocks be-
cause they tended to be smaller than that for ZnPctBu4
(Table 1). The zinc-containing, metal-free porphyrins
showed much-weaker interactions than the palladium-con-
taining porphyrins, because palladium has a higher atomic
weight. This result led to a notable deformation of the p-
system. In addition, the heavy-metal effect increased the ra-
diationless decay rate of, for example, the intersystem cross-
ing to the triplet excited state, thereby resulting in an overall
decrease in the fluorescence quantum yields.
case of compounds 2b and 3b, which were P–Pc molecules
with tert-butyl groups on the periphery of the Pc moieties,
1
the H NMR spectra showed broad signals, probably owing
to the fact that those compounds were mixtures of positional
isomers. The same kind of behavior was observed for com-
pounds 2a and 3a, which was indicative of intermolecular
aggregation. However, even with poorly resolved spectra, it
was possible to observe the two signals (dꢀÀ2.5 ppm) that
corresponded to the resonance of the inner NH protons
present in compounds 2a and 2b.
Photophysical studies: To elucidate the electronic interac-
tions between the different photoactive components, the
model compounds (meso-tetraphenylporphyrinato)zinc(II)
(ZnTPP), meso-tetraphenylporphyrin (H2TPP), (meso-tetra-
phenylporphyrinato)palladium(II) (PdTPP), and (tetra-tert-
butylphthalocyaninato)zinc(II), ZnPctBu4, were compared
with those of compounds 1, 2, and 3 (see the Supporting In-
formation).
The model compounds were strongly emissive fluoro-
phores with strong absorptions in the UV and visible regions
(see the Supporting Information, Figure S18). Dominant
Soret-bands of ZnTPP, H2TPP, and PdTPP were observed in
the range 400–450 nm, whilst the Q-bands dominated over
the range 500–600 nm. On the other hand, ZnPctBu4 had its
Q-bands between 600–700 nm and a Soret-band with a max-
imum at 350 nm. The spectroscopic features of the Soret-
and Q-bands related to the S0–S2 and S0–S1 transitions, re-
spectively. Moreover, as has been reported for other P–Pc
conjugates,[6,8,9,23] the spectra of compounds 1, 2, and 3 in
toluene showed the characteristic absorption bands of the
individual constituents, namely strong absorption bands as-
signed to the porphyrins and phthalocyanines (Figure 1).
Only small red-shifts (about 5 nm) of the P and Pc absorp-
tions suggested the presence of electronic interactions. The
different substituent patterns at the phthalocyanines did not
have an appreciable influence on the absorption.
Table 1. Fluorescence quantum yields (FF) of compounds 1, 2, and 3 in
toluene; excitation: 670 nm.
Conjugate
FF
Conjugate
FF
Conjugate
FF
1a
1b
1c
0.26
0.20
0.19
2a
2b
2c
0.25
0.28
0.25
3a
3b
3c
0.12
0.12
0.05
In contrast to the fluorescence of ZnTPP, H2TPP, and
PdTPP, excitation of compound 1a in the range of the
Soret-band absorptions afforded the most-dominant emis-
sion in the 650–800 nm region, with quantum yields as high
as 0.24, whilst in the 600–650 nm region the emission was
quenched, with quantum yields as low as 2.75ꢇ10À3
(Figure 2, Table 2). Despite the almost-exclusive excitation
In the steady-state- and time-resolved fluorescence spec-
tra of ZnTPP, the maxima were at 600 and 650 nm, with a
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Porphyrin–Phthalocyanine/Pyridylfullerene Systems
FULL PAPER
Table 3. Fluorescence lifetimes (t) of compounds 1, 2, and 3 in toluene;
excitation: 647 nm, detection: 685 nm.
Conjugate
t [ns]
Conjugate
t [ns]
Conjugate
t [ns]
1a
1b
1c
3.34
3.31
3.47
2a
2b
2c
2.78
2.89
2.99
3a
3b
3c
1.28
1.31
1.15
look at lifetimes shorter than the instrumental limit of tran-
sient absorption spectroscopy. Upon excitation at 415 nm,
we measured time profiles at different wavelengths in the
range 575–715 nm, which were best fitted by a global-fit
function. To this end, we derived three different lifetimes
for compound 1c: 0.36, 15.61, and 2.64 ns. Reference experi-
ments with ZnTPP gave a short-lived component with a life-
time of 1.9 ps (excitation: 450 nm), which corresponded to
the deactivation of the S2 excited state, and a long-lived
component with a lifetime of 1.6 ns (excitation: 600 nm),
which was related to the deactivation of the S1 excited state.
Furthermore, the time profiles of compound 1 at 450 nm
led to lifetimes of 0.11, 0.14, and 0.18 ps for compounds 1a,
1b and 1c, respectively. Thus, we assumed that the two
shorter lifetimes reflected the intramolecular energy-transfer
from the S2 excited state as well as from the S1 excited state
of the porphyrins. The longest lifetime was assigned to the
S1 excited state of the phthalocyanines (Figure 3). These dif-
Figure 2. RT fluorescence spectra of compounds 1a (black), 2a (dark
gray), and 3a (light gray), which exhibited the same optical absorption at
an excitation wavelength of 420 nm. All spectra were recorded in tolu-
ene.
Table 2. Fluorescence quantum yields (FF) of compounds 1, 2, and 3 in
toluene; excitation: 420 nm.
Conjugate
FF (P)
FF (Pc)
1a
1b
1c
2a
2b
2c
3a
3b
3c
2.75ꢇ10À3
1.89ꢇ10À3
5.00ꢇ10À4
7.66ꢇ10À3
1.0 ꢇ10À2
2.50ꢇ10À2
5.90ꢇ10À4
5.44ꢇ10À4
5.90ꢇ10À4
0.24
0.24
0.18
0.25
0.23
0.20
0.06
0.04
0.05
of the porphyrin, compound 1a showed a fluorescence maxi-
mum at 689 nm and a fluorescence quantum yield of 0.24—
features that resembled those of ZnPctBu4. Implicit was an
efficient intramolecular energy-transfer from the photoexcit-
ed porphyrins to the covalently linked phthalocyanines. This
hypothesis was confirmed by the complementary excitation
spectra of the 689 nm maximum, which showed ground-state
features that were best described as the sum of the P and Pc
absorptions (see the Supporting Information, Figure S20).
The excitation of compound 2a revealed Pc fluorescence at
685 nm with a quantum yield of 0.25, whilst excitation of
compound 3a showed Pc fluorescence at 683 nm with a
quantum yield of 0.06 (Figure 2). The results for all of the
other conjugates are given in Table 2.
Figure 3. Fluorescence–time profiles of compound 1c at 600 nm (black)
of the porphyrin S2 excited state and at 675 nm (light gray) of the phtha-
locyanine S1 excited state; excitation: 415 nm.
In contrast to all of the reference compounds (ZnTPP,
H2TPP, and PdTPP), no detectable fluorescence decay was
found for compounds 1, 2, or 3 in the range of the porphyrin
fluorescence. This result was presumably because of a very
fast singlet-excited-state deactivation, which was out of the
range of our experimental setup. Upon excitation of com-
pounds 1, 2, and 3 at 647 nm, a different picture appeared;
in fact, at a detection wavelength of 685 nm, lifetimes were
recorded that were in good agreement with the fluorescence
quantum yields (Table 3).
ferent pathways are shown in Figure 4, where the fluores-
cence amplitude is shown as a function of wavelength. The
corresponding lifetimes of the remaining conjugates are
shown in Table 4.
Subsequently, we performed transient absorption spec-
troscopy (excitation: 420 nm) to confirm the results from
the fluorescence up-conversion, to further study the deacti-
vation processes, and to analyze their kinetics. ZnTPP
showed characteristic singlet–singlet absorptions with
maxima at 450 and 570 nm, a lifetime of 1.89 ns, and a
strong minimum at 550 nm as a reflection of the porphyrin
The lack of the aforementioned time resolution led us to
turn to fluorescence up-conversion, which enabled us to
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J. A. S. Cavaleiro, D. M. Guldi, T. Torres et al.
with the decay of the porphyrin singlet–singlet peak, which
was a reflection of the actual energy-transfer. However, the
singlet-excited state of the phthalocyanine was metastable
and decayed with a lifetime of 1.61 ns (Figure 5a). Similar
overall trends were seen for compounds 2a and 3a. For
compounds 2a and 3a, the singlet-excited-state lifetimes of
the porphyrins were 18.76 and 7.80 ps and the singlet-excit-
ed-state lifetimes of the phthalocyanines were 1.26 and
1.54 ns, respectively (Figure 5b,c). All of the lifetimes were
in good agreement with the values from the fluorescence
up-conversion experiments. The excitation of compounds
1a, 2a, and 3a at 660 nm, that is, directly into the phthalo-
cyanines, led to the exclusive and prompt formation of the
phthalocyanine singlet-excited states.
Figure 4. Decay-associated spectra obtained from the global analysis of
the fluorescence dynamics of compound 1c with lifetimes of 0.36 ps
(black), 15.61 ps (dark gray), and 2.64 ns (light gray).
Owing to the fact that coordination of the porphyrins or
phthalocyanines was feasible with, for example, pyr-
idyl ligands, we tested compounds 1, 2, and 3 in the
Table 4. Fluorescence up-conversion lifetimes (t) of compounds 1, 2, and 3 in toluene;
presence of various concentrations of fulleropyrroli-
dines 4 and 5. In the absorption spectra of com-
pounds 2 and 3, no appreciable changes were noted
with the exception that the C60 peak emerged in the
range 300–350 nm, which suggested the lack of in-
teractions between the porphyrins and the two C60
molecules. However, quite a different picture was
observed for compound 1b: a small red-shift (5 nm)
corresponded with the addition of C60 (Figure 6).
From these results, we inferred that both C60 deriva-
tives coordinated to the porphyrins, which was fur-
ther confirmed by the formation of an isosbestic
point at 420 nm. From the underlying relation-
excitation: 415 nm, detection: 600–715 nm.
Conjugate
t [s]
0.55ꢇ10À12
Conjugate
t [s]
0.57ꢇ10À12
Conjugate
t [s]
0.51ꢇ10À12
1a
2a
3a
18.20ꢇ10À12
1.63ꢇ10À9
0.43ꢇ10À12
7.12ꢇ10À12
2.38ꢇ10À9
0.36ꢇ10À12
15.61ꢇ10À12
2.64ꢇ10À9
20.59ꢇ10À12
1.36ꢇ10À9
0.08ꢇ10À12
1.98ꢇ10À12
3.00ꢇ10À9
0.11ꢇ10À12
1.66ꢇ10À12
2.06ꢇ10À9
11.66ꢇ10À12
1.16ꢇ10À9
0.46ꢇ10À12
9.24ꢇ10À12
1.73ꢇ10À9
0.41ꢇ10À12
10.08ꢇ10À12
1.04ꢇ10À9
1b
1c
2b
2c
3b
3c
ground-state absorption. The triplet-excited state, formed
through intersystem crossing, showed a maximum at around
815 nm (see the Supporting Information, Figure S21).
ship—concentration versus change in absorption—the bind-
ing constants between the porphyrins and both C60 deriva-
tives were calculated: 6.12ꢇ104 mÀ1 and 3.53ꢇ103 mÀ1 for
compound 1b with compounds 4 and 5, respectively. These
values were comparable to those reported previously.[17,26] In
terms of the difference in binding strength, the linear design
was apparently more efficient than the bent design. For
compounds 1a and 1c, the corresponding binding constants
were 2.22ꢇ104 mÀ1 (1a·4), 4.16ꢇ103 mÀ1 (1a·5), 3.77ꢇ104 mÀ1
(1c·4), and 2.49ꢇ103 mÀ1 (1c·5).
The phthalocyanine-centered fluorescence signal depend-
ed on the concentrations of compounds 4 and 5, and result-
ed in exponential quenching of 15% (4) and 60% (5;
Figure 7). The lower phthalocyanine quantum yields in com-
pounds 1, 2, and 3 in the presence of compounds 4 or 5, cor-
related to the successful formation of electron-donor–ac-
ceptor hybrids that were composed of covalently linked P–
Pc conjugates, in which the C60 derivatives were coordinated
to the phthalocyanines (see Table 5).
Likewise, the corresponding fluorescence lifetimes for
compounds 1a, 2a, and 3a in the presence of C60 were 0.40,
0.32, and 0.21 ns, respectively. From the fluorescence titra-
tion experiments, we calculated the binding constants be-
tween the phthalocyanines and C60 (Table 6). Binding to por-
phyrins was always weaker than to phthalocyanines owing
to the larger cavities in the phthalocyanines, which concur-
red well with analogous systems.[17,26,27]
Similar transient absorption measurements with H2TPP
afforded maxima at 436 nm and in the range 530–615 nm as
well as minima in the range 515–645 nm, which reflected the
singlet-excited state with a lifetime of 10.5 ns and the
ground-state absorption, respectively. In addition, a marked
signature in the NIR region, that is, at 1020 nm, correspond-
ed to the S1 excited state (see the Supporting Information,
Figure S22). For PdTPP, the maxima were at 450 and
560 nm and the minima at 523 and 610 nm. Owing to the
heavy-atom effect, the singlet-excited state became quickly
deactivated (i.e., 10 ps) into the lower lying triplet-excited
state through intersystem crossing. For the latter, maxima
were seen at 468 and 850 nm (see the Supporting Informa-
tion, Figure S23). Finally, differential absorption spectra of
ZnPctBu4 (excitation: 660 nm), gave a sharp, strong mini-
mum at 680 nm accompanied by a broad, weak maxima be-
tween 595 and 650 nm. The singlet-excited state decayed
slowly (3.17 ns), to form the corresponding triplet-excited
state, with its characteristic broad maximum at around
500 nm (see the Supporting Information, Figure S24).
In compound 1a (excitation: 420 nm), the short-lived
(21.28 ps) porphyrin singlet–singlet peak were observed in
the blue region (440–500 nm). In the red region (600–
800 nm), the phthalocyanine signal developed concomitantly
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Porphyrin–Phthalocyanine/Pyridylfullerene Systems
FULL PAPER
Figure 6. Absorption spectra of compound 1b (1.0ꢇ10À6 m) in toluene
with various concentrations of compound 4 (0–1.92ꢇ10À5 m). The spectra
show the red-shift of the P Soret-band.
Figure 5. a) Differential absorption spectra (visible and near-infrared) ob-
tained upon femtosecond flash photolysis (420 nm) of compound 1a in
toluene with time delays between 0.6 and 7.2 ps at RT. Inset: time profile
at 610 nm, legend shows the time delays. b) Differential absorption spec-
tra (visible and near-infrared) obtained upon femtosecond flash photoly-
sis (420 nm) of compound 2a in toluene with time delays between 0.6
and 7.2 ps at RT. Inset: time profile at 677 nm, legend shows the time
delays. c) Differential absorption spectra (visible and near-infrared) ob-
tained upon femtosecond flash photolysis (420 nm) of compound 3a in
toluene with time delays between 0.6 and 7.2 ps at RT. Inset: time profile
at 525 nm, legend shows the time delays.
Figure 7. a) RT fluorescence spectra of compound 1b (1.0ꢇ10À6 m) with
various concentrations of compound 4 (0–7.5ꢇ10À6 m) in toluene. The
spectra show the same optical absorptions at an excitation wavelength of
420 nm. b) RT fluorescence spectra of compound 1b (1.0ꢇ10À6 m) with
various concentrations of compound 5 (0–7.9ꢇ10À6 m) in toluene. The
spectra show the same optical absorptions at an excitation wavelength of
420 nm.
These experiments suggested a deactivation of the photo-
excited electron-donor–acceptor hybrids, i.e. an intramolecu-
lar energy-transfer from the porphyrins to the phthalocya-
nines followed by an intramolecular energy- or charge-trans-
fer to C60. To gain an insight into the second step, we turned
to transient absorption measurements. Upon excitation at
420 nm, different porphyrin singlet-excited states were ob-
served. In particular, maxima between 450 and 600 nm as
well as minima between 520 and 580 nm were observed,
which were short-lived and transformed rapidly (1a·4:
Chem. Eur. J. 2012, 18, 3210 – 3219
ꢆ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3215

J. A. S. Cavaleiro, D. M. Guldi, T. Torres et al.
Table 5. Fluorescence quantum yields (FF) of compounds 1, 2, and 3
after titration with compounds 4 or 5 in toluene; excitation: 670 nm.
Conjugate
FF (with 4)
FF (with 5)
1a
1b
1c
2a
2b
2c
3a
3b
3c
0.03
0.01
0.05
0.02
0.02
0.07
0.01
0.01
0.02
0.20
0.13
0.12
0.15
0.16
0.12
0.07
0.07
0.04
Table 6. Binding constants (k) of compounds 1, 2, and 3 with compounds
4 or 5 in toluene from fluorescence titration experiments.
Conjugate
k [mÀ1] (with 4)
k [mÀ1] (with 5)
1a
1b
1c
2a
2b
2c
3a
3b
3c
3.4ꢇ106
1.8ꢇ106
1.8ꢇ106
2.8ꢇ106
1.9ꢇ106
2.6ꢇ106
2.7ꢇ106
9.9ꢇ105
1.1ꢇ106
2.7ꢇ105
9.4ꢇ104
2.2ꢇ105
9.4ꢇ104
9.5ꢇ104
3.2ꢇ105
1.7ꢇ105
4.8ꢇ104
8.6ꢇ104
15.04 ps; 1a·5: 18.28 ps; 1b·4: 13.54 ps; 1b·5: 13.56 ps) into
the phthalocyanine singlet-excited states (Figure 8a).
For compounds 2 and 3, the lifetimes for the singlet-excit-
ed state of the porphyrins could not be determined because
they immediately formed their corresponding triplet-excited
states (Figure 8b,c). Subsequently, an intramolecular
charge-transfer occurred (Table 7), which was corroborated
by radical ion-state pairs that included the signatures of the
one-electron-oxidized form of phthalocyanine (840 nm) and
the one-electron-reduced form of C60 (1010 nm). The
charge-transfer states were deactivated to restore the
ground state without giving any evidence for any charge-
shift reaction to yield the one-electron-oxidized form of the
porphyrins. The lifetimes of the charge-transfer states are
shown in Table 8.
Figure 8. a) Differential absorption spectra (visible and near-infrared) ob-
tained upon femtosecond flash photolysis (420 nm) of conjugate 1a·4 in
toluene with time delays between 0.8 and 0.28 ps at RT. Inset: time pro-
file at 840 nm, legend shows the time delays. b) Differential absorption
spectra (visible and near-infrared) obtained upon femtosecond flash pho-
tolysis (420 nm) of conjugate 2a·4 in toluene with time delays between
0.8 and 0.28 ps at RT. Inset: time profile at 840 nm, legend shows the
time delays. c) Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (420 nm) of conjugate 3a·4
in toluene with time delays between 0.8 and 0.28 ps at RT. Inset: time
profile at 840 nm, legend shows the time delays.
Conclusion
P–Pc electron-donor–acceptor conjugates that were directly
connected through the b-pyrrolic positions of the free base
and the metallic porphyrins to the b-position of phthalocya-
nines with different substituents at the periphery were used
as photoactive compounds to study photoinduced energy-
and electron-transfer processes. The proximal location of
the two chromophores was the origin of the strong excitonic
couplings between the constituents, which caused a red-shift
of the absorption. Energy-transfer processes from the por-
phyrins to the phthalocyanines were observed for all of the
conjugates, as demonstrated by Pc-centered fluorescence
features that evolved upon the photoexcitation of P, which
showed lower fluorescence quantum yields and shorter fluo-
rescence lifetimes than their corresponding references. Fluo-
rescence up-conversion experiments confirmed that this in-
tramolecular energy-transfer resulted from the S2 excited
state as well as from the S1 excited state of the porphyrins.
Coordination of compounds 1, 2, and 3 with compounds 4
and 5 led to their corresponding electron-donor–acceptor
hybrids, in which compounds 4 and 5 were coordinated to
3216
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ꢆ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 3210 – 3219

Porphyrin–Phthalocyanine/Pyridylfullerene Systems
FULL PAPER
Table 7. Singlet-excited-state lifetimes of the Pc unit of compounds 1, 2,
and 3 with compounds 4 or 5 in toluene from transient absorption meas-
urements; excitation: 420 nm.
Experimental Section
1H NMR spectra were recorded at 300 MHz on a Bruker AMX 300 in
CDCl3. Chemical shifts (d) are expressed in parts per million (ppm) rela-
tive to tetramethylsilane. The coupling constants (J) are given in Hertz
(Hz). HRMS (MALDI-TOF) were determined by using a Bruker Reflex
III spectrometer with dithranol as the matrix. UV/Vis spectra were re-
corded with a Shimadzu UV-250 Pc spectrophotometer using THF as the
solvent (1 cm path length quartz cell). Column chromatography was car-
ried out on silica gel (Merck, 230–400 mesh). Preparative thin-layer chro-
matography was carried out on 20ꢇ20 cm glass plates coated with silica
gel (1 mm thick, Merck). Analytical TLC was carried out on pre-coated
sheets with silica gel (0.2 mm thick, Merck). CH2Cl2 was distilled from
calcium hydride, and THF was dried with sodium metal. All other sol-
vents and reagents were used without further purification.
Conjugate
*S1 lifetime of Pc
(with 4) [ps]
*S1 lifetime of Pc
ACHTUNGTRENUN(NG with 5) [ps]
U
1a
1b
2a
2b
3a
3b
11.84
7.56
19.31
17.37
12.12
17.29
11.20
7.33
20.65
17.06
14.64
12.86
Table 8. Radical-ion-pair-state lifetimes of compounds 1, 2, and 3 with
compounds 4 or 5 in toluene from transient absorption measurements;
excitation: 420 nm.
All photophysical measurements were carried out at RT and all solvents
were spectroscopic grade and were purchased from Sigma–Aldrich.
Steady-state absorption spectroscopy was performed by using a Cary
5000 spectrometer (VARIAN) in a 10ꢇ10 mm quartz cuvette. Steady-
state emission spectra were recorded on a Fluoro-Max 3 fluorometer
(Horiba JobinYvon) in a 10ꢇ10 mm quartz cuvette. Emission lifetimes
were determined by using time-correlated single-photon counting
(TCSPC) on a FluoroLog 3 emission spectrometer (Horiba JobinYvon)
Conjugate
4 [ns]
5 [ns]
1a
1b
2a
2b
3a
3b
1.35
1.45
1.25
1.90
1.50
1.40
1.65
1.10
1.05
1.35
1.40
1.50
with an MCP detector (R3809U-58). For excitation,
a laser diode
(NanoLED 647 nm) was used. Femtosecond transient absorption studies
were performed with 387, 420, and 660 nm laser pulses (1 kHz, 150 fs
pulse-width) from an amplified Ti:Sapphire laser system (Clark-MXR,
Inc.), the laser energy was around 200 nJ. Excited-state lifetime measure-
ments on shorter time scales were carried out by using fluorescence up-
conversion (FU). Excitation was achieved at 415 nm with the frequency-
doubled output of a Kerr lens mode-locked Ti:Sapphire laser (Tsunami,
Spectra-Physics). The output pulses centered at 830 nm had a duration of
100 fs and a repetition rate of 82 MHz. The polarization of the pump
beam was at the magic angle relative to that of the gate pulses at 830 nm,
except for fluorescence anisotropy measurements. Experiments were per-
formed in a 0.4 mm rotating cell. The fwhm of the IRF was about 210 fs.
the phthalocyanines in compounds 1, 2, and 3, and whose
formation was corroborated by fluorescence quenching—
lower quantum yields and shorter lifetimes—when com-
pared to conditions in the absence of C60. Transient absorp-
tion measurements helped to confirm that intramolecular
energy-transfer from the porphyrins to the energetically
lower-lying phthalocyanines was, indeed, followed by intra-
molecular charge-transfer, from which a radical ion-pair
state emerged that involved the one-electron-oxidized Pc
and the one-electron-reduced C60 (Figure 9). These results
will be important for the future design of P–Pc electron-
donor–acceptor conjugates and hybrids that feature stronger
light-harvesting properties for solar-energy conversion.
P–Pc 1a–1c,[8] porphyrinylphthalonitrile 6,[8,25] and N-pyridylfulleropyrro-
lidines 4[17] and 5[24] were synthesized accordingly literature procedures.
Typical procedure for the synthesis of P–Pc 2a–2c: Trifluoroacetic acid
(0.6 mL) was added dropwise to a vigorously stirred solution of dyad 1a
(10 mg, 8 mmol) in CH2Cl2 (2.4 mL) at RT. The reaction was monitored
by UV/Vis spectroscopy and TLC until complete demetalation of the P
moiety had occurred (ca. 30 min). The reaction mixture was then neutral-
ized with diluted NaHCO3, washed with water, and dried (Na2SO4). The
organic layer was concentrated under vacuum and dyad 2a was crystal-
lized from CH2Cl2/light petroleum in 98% yield (9.2 mg); UV/Vis (THF):
lmax (log e)=344 (4.6), 424 (5.4), 516 (4.1), 555 (3.9), 607 (4.4), 673 nm
(5.2); HRMS (MALDI-TOF): m/z calcd for C76H44N12Zn: 1188.3098;
found: 1188.3080.
Compounds 2b and 2c were prepared according to the same procedure
but using the corresponding dyads 1b and 1c instead of 1a. Each com-
pound was crystallized from CH2Cl2/light petroleum.
Data for 2b: Yield: 94% (9.0 mg); UV/Vis (THF): lmax (log e)=350
(4.8), 422 (5.4), 516 (4.2), 556 (4.0), 611 (4.4), 677 nm (5.1); HRMS
(MALDI-TOF): m/z calcd for C88H68N12Zn: 1356.4976; found: 1356.4966.
Data for 2c: Yield: 95% (9.2 mg); 1H NMR (CDCl3): d=À2.66 (s, 1H;
NH), À2.41 (s, 1H; NH), 0.87–1.37 (m, 18H; CH3), 1.65–1.98 (m, 12H;
CH2CH3), 2.19–2.33 (m, 12H; CH2CH2CH3), 3.83–4.30 (m, 3H; OCH2),
4.56–4.83 (m, 9H; OCH2), 5.53–5.56 (m, 1H; 20-Ph-H), 5.96–6.13 (m,
3H; 20-Ph-H), 6.52–8.93 (3 m, 25H; 6ꢇb-H, 6ꢇPc-a-H, 6ꢇPh-m-H, 3ꢇ
Ph-p-H, 3ꢇPh-o-H, 1ꢇ20-Ph-H), 9.07 (s, 1H; Pc-a-H), 9.27 (d, J=
7.5 Hz, 1H; 3’-H), 10.19 (s, 1H; 25’-H), 10.49–10.51 (m, 2H; Ph-o-H),
10.60 (s, 1H; 3-H), 10.69–10.72 ppm (m, 1H; Ph-o-H); UV/Vis (THF):
lmax (log e)=355 (4.8), 422 (5.5), 516 (4.1), 555 (4.0), 611 (4.4), 677 nm
(5.1); HRMS (MALDI-TOF): m/z calcd for C100H92N12O6Zn: 1620.6549;
found: 1620.6512.
Figure 9. Energy diagram that shows the excited-state-deactivation pro-
cesses in P–Pc·C60.
Chem. Eur. J. 2012, 18, 3210 – 3219
ꢆ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3217

J. A. S. Cavaleiro, D. M. Guldi, T. Torres et al.
Porphyrinylphthalonitrile 7: To a solution of porphyrinylphthalonitrile 6
(90.1 mg, 122 mmol) in DMF (7 mL) was added palladium acetate
(41.3 mg, 184 mmol, 1.5 equiv). The reaction mixture was heated to reflux
for 90 min. After cooling to RT, water was added to afford a solid that
was filtered, dissolved in CH2Cl2, washed with water, and the organic
layer was dried over Na2SO4. The solvent was removed and the crude re-
action mixture was purified by flash chromatography on silica gel (light
petroleum/CH2Cl2, 1:2). After crystallization from CH2Cl2/light petrole-
um, porphyrinylphthalonitrile 7 was obtained in 78% yield (80.3 mg);
1H NMR (CDCl3): d=7.32–7.43 (m, 3H; 20-Ph-m,p-H), 7.57 (d, J=
8.1 Hz, 1H; 5’-H or 6’-H), 7.67 (br s, 1H; 2’-H), 7.67–7.71 (m, 10H; 9ꢇ
5,10,15-Ph-m,p-H and 1ꢇH-5ꢈ or H-6’), 7.85 (d, J=7.2 Hz, 2H; 20-Ph-o-
H), 8.14–8.17 (m, 6H; 5,10,15-Ph-o-H), 8.65 (d, J=4.8 Hz, 1H; b-H),
8.74–8.81 ppm (m, 6H; b-H); UV/Vis (THF): lmax (log e)=419 (5.6), 525
(4.5), 559 nm (3.8); HRMS (MALDI-TOF): m/z calcd for C52H30N6Pd:
844.1580; found: 844.1595.
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Typical procedure for the synthesis of compounds 3a–3c: A mixture of
porphyrinylphthalonitrile 7 (25 mg, 29.6 mmol), phthalonitrile (45.5 mg,
355 mmol, 12 equiv), and ZnCl2 (32.7 mg, 240 mmol, 8 equiv) was stirred
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a short plug column of celite-545, washed with water and
MeOH, dried under reduced pressure and the crude product was dis-
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phthalocyaninatozinc(II) (ZnPc), by flash chromatography on silica gel
(light petroleum/THF, 3:1). After purification by preparative thin-layer
chromatography (light petroleum/THF, 3:1) and crystallization from
CH2Cl2/light petroleum, dyad 3a was obtained in 45% yield (17.2 mg);
UV/Vis (THF): lmax (log e)=343 (4.8), 417 (5.4), 525 (4.4), 607 (4.6),
672 nm (5.4); HRMS (MALDI-TOF): m/z calcd for C76H42N12ZnPd:
1294.1990; found: 1294.1990.
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Data for 3b: Yield: 21% (9.2 mg); UV/Vis (THF): lmax (log e)=349
(4.8), 417 (5.4), 525 (4.4), 610 (4.5), 676 nm (5.3); HRMS (MALDI-
TOF): m/z calcd for C88H66N12ZnPd: 1462.3870; found: 1462.3870.
Data for 3c: Yield: 13% (6.7 mg); 1H NMR (CDCl3): d=1.18–1.37 (m,
18H; CH3), 1.78–1.98 (m, 12H; CH2CH3), 2.12–2.34 (m, 12H;
CH2CH2CH3), 3.83–4.33 (m, 3H; OCH2), 4.57–4.81 (m, 9H; OCH2), 5.90
(t, J=7.3 Hz, 1H; 20-Ph-m-H), 6.05 (d, J=7.2 Hz, 1H; 20-Ph-o-H), 6.10–
6.18 (m, 2H; 20-Ph-m,p-H), 6.52–6.99, 7.53–8.19, 8.62–8.65, and 8.77–8.85
(4 m, 16H; 5ꢇb-H, 4ꢇPc-a-H, 3ꢇPh-p-H, 3ꢇPh-o-H, and 1ꢇ20-Ph-o-
H), 8.39 (t, J=7.8 Hz, 2H; Ph-m-H), 8.45 (t, J=7.7 Hz, 2H; Ph-m-H),
8.55 (t, J=7.5 Hz, 2H; Ph-m-H), 8.73 (d, J=5.1 Hz, 1H; b-H), 8.94, 9.00,
and 9.06 (3 s, 3H; Pc-a-H), 9.25 (d, J=7.5 Hz, 1H; 3’-H), 10.16 (s, 1H;
25’-H), 10.61–10.64 (m, 2H; Ph-o-H), 10.75 (s, 1H; 3-H), 10.80 ppm (d,
J=7.2 Hz, 1H; Ph-o-H); UV/Vis (THF): lmax (log e)=355 (5.0), 418
(5.4), 525 (4.4), 610 (4.5), 677 nm (5.2); HRMS (MALDI-TOF): m/z
calcd for C100H90N12O6ZnPd: 1726.5450; found: 1726.5431.
Acknowledgements
We thank FEDER for funding the QOPNA Research Unit and project
PTDC/QUI/65228/2006. This work was also supported by the Spanish
MICINN, the MEC (CTQ2011–24187/BQU and CONSOLIDER IN-
GENIO 2010, CSD2007–00010), the Comunidad de Madrid (MADRISO-
LAR-2, S2009/PPQ/1533), the Deutsche Forschungsgemeinschaft (SFB
583), and the FCI. M.P. thanks the FCT for her Post-doc grant (SFRH/
BPD/64693/2009). O.T. also thanks the Autonoma University of Madrid,
Spain, for a PhD fellowship (FPI UAM). The Deutsche Forschungsge-
meinschaft is acknowledged for financial support through SFB 583 and
the Excellence Cluster “Engineering of Advanced Materials”.
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Published online: February 13, 2012
Chem. Eur. J. 2012, 18, 3210 – 3219
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