Porphyrin-β-01igo-ethynylenephenylene-[60]fullerene triads: Synthesis and electrochemical and photophysical characterization of the new porphyrin-oligo-PPE-[60]fullerene systems

Article abstract of DOI:10.1021/jp809557e

The synthesis and electrochemical and photophysical studies of new electron donor-acceptor arrays, bearing porphyrins covalently linked to fullerene, are described. In the reported investigation, phenyleneethynylene subunits were chosen as a linking bridg

Full text of DOI:10.1021/jp809557e

J. Phys. Chem. A 2009, 113, 1779–1793
1779
Porphyrin-ꢀ-Oligo-Ethynylenephenylene-[60]Fullerene Triads: Synthesis and
Electrochemical and Photophysical Characterization of the New
Porphyrin-Oligo-PPE-[60]Fullerene Systems
Angelo Lembo,† Pietro Tagliatesta,*,† Dirk M. Guldi,*,‡ Mateusz Wielopolski,‡ and
Marzia Nuccetelli§
Dipartimento di Scienze e Tecnologie Chimiche, UniVersity of Rome “Tor Vergata”, Via della Ricerca
Scientifica, 00133 Rome, Italy, Department of Chemistry and Pharmacy, Friedrich-Alexander UniVersity
Erlangen-Nuremberg, Egerlandstrasse 3, 91058 Erlangen, Germany, and Dipartimento di Medicina Interna,
UniVersity of Rome “Tor Vergata”, Via della Ricerca Scientifica, 00133 Rome, Italy
ReceiVed: October 29, 2008; ReVised Manuscript ReceiVed: December 17, 2008
The synthesis and electrochemical and photophysical studies of new electron donor-acceptor arrays, bearing
porphyrins covalently linked to fullerene, are described. In the reported investigation, phenyleneethynylene
subunits were chosen as a linking bridge to guarantee a high conjugation degree between the donor (i.e.,
porphyrin), the molecular bridge (i.e., oligo-phenyleneethynylenes), and the acceptor (i.e., fullerene). To enhance
the electronic interactions through the extended π-system, the molecular bridge has been directly linked to
the ꢀ-pyrrole position of the porphyrin ring, generating a new example of donor-bridge-acceptor systems
where, for the first time, the meso-phenyl ring of the macrocycle is not used to hold the “bridge” between
porphyrin and fullerene moieties. This modification allows altering the chemical and physical properties of
the tetrapyrrole ring. Steady-state and time-resolved fluorescence studies together with transient absorption
measurements reveal that in nonpolar media (i.e., toluene) transduction of singlet excited-state energy governs
the excited-state deactivation, whereas in polar media (i.e., tetrahydrofuran) charge transfer prevails generating
a long-lived radical ion pair state. The lifetimes hereof range from 300 to 700 ns. The study also sheds light
onto the wirelike behavior of the oligo-phenyleneethynylene bridges, for which a damping factor (ꢀ) of 0.11
( 0.05 Å-1 has been determined in the current study.
Introduction
phenylenevinylene,6 oligo-phenyleneethynylenes,7 oligo-acety-
lenes,8 oligo-thiophenes,9 and oligo-phenylenes.10 Similarly,
Organic materials for electronic and photonic applications
are under continuous and fervent studies. Great interest has been
focused on the design and synthesis of molecular materials on
the nanoscale size to produce novel electron donor-acceptor
systems, in which charge transfer is realized over distances
comparable to those known in natural photosystems. Such
systems harvest solar energy in order to shuttle charges along
distances of approximately 35 Å at remarkable ratessin a few
picoseconds1sand with impressiVe efficienciessnearly quantita-
tive quantum yields.2 In this context, the focal point is on
molecular structures capable of mediating charges between the
donor and the acceptor moieties. These offer, for example, the
opportunity to control charge transfer/shifts over nanometer
distances. An imperative requirement that these molecular
structures must fulfill is an effective wirelike behavior.3 A more
practical figure of merit is the attenuation factor (ꢀ), which
assists in correlating the charge-transfer rate constants with
distance.4 In fact, ꢀ should be as low as possible to facilitate
fast and efficient charge transport between the donor and the
acceptor sites.5
porphyrin oligomers have also been considered as molecular
wires.11 In all these cases the wirelike behaviorsefficient or
inefficientsdepends on the combination of several factors
including the degree of conjugation in the bridge, orbital
matching, donor-bridge energy gap, etc.12 The ꢀ values that
have been reported so far for the different molecular bridges
range from rather low, i.e., around 0.4 Å-1 10d
small with a value of 0.01 Å-1 6b
As a comparison the
corresponding value for vacuum is 1.0 Å-1
,
to exceptionally
.
.
In this investigation we focused our attention on the oligo-
p-phenyleneethynylenes (oligo-PPEs) as molecular bridges,
because of their synthetic versatility and their outstanding
physicochemical properties. It has been previously reported that
these molecules assist in a good conduction of the charges due
to their high electron density and the extended π-system.13 In
particular, as soon as these systems are self-assembled on metal
surfaces (i.e., gold surface) they are able to transport and store
negative charges via localized molecular orbitals.14 Another
important feature of oligo-PPEs, namely, their extended π-con-
jugation, guarantees reasonable contactssover intermediate and/
or long distancessbetween one electron donor and one electron
acceptor moiety covalently linked to the two termini of the
oligomer.15
To this purpose, many π-conjugated organic bridges were
investigated and tested as molecular wires including oligo-
* To whom correspondence should be addressed. Fax: +39-06-72594754
(P.T.). E-mail: pietro.tagliatesta@uniroma2.it (P.T.); dirk.guldi@
chemie.uni-erlangen.de. (D.M.G.).
In the newly synthesized systems, the ꢀ-pyrrole position
affords the chemical junction between the bridge and the
tetrapyrrole macrocycle. A carbon-carbon triple bond is chosen
to afford a completely extended π-π conjugation, which starts
† Dipartimento di Scienze e Tecnologie Chimiche, University of Rome
“Tor Vergata”.
‡ Friedrich-Alexander University Erlangen-Nuremberg.
§ Dipartimento di Medicina Interna, University of Rome “Tor Vergata”.
10.1021/jp809557e CCC: $40.75
2009 American Chemical Society
Published on Web 02/05/2009

1780 J. Phys. Chem. A, Vol. 113, No. 9, 2009
Lembo et al.
Figure 1. Porphyrin-oligo-PPE-fullerene systems and related oligo-PPE-fullerene reference compounds studied in this work. M ) 2H, Zn.
on the porphyrin residue and ends in close proximity to the
fullerene network, where, nevertheless, the sp3 carbon of the
pyrrolidine ring interrupts the conjugation pathwayssee
Figure 1.
In view of previously studied, similar exTTF-oligo-PPE-C60
systems,7e,f the influence of the exchanging the exTTF donor
moiety by a porphyrin donor on the electron-transfer properties
was of particular interest.
Owing to the direct connection of the triple bond and the
ꢀ-pyrrole position, charges (i.e., electrons) will be conducted
through the bridge toward the electron-accepting fullerene.
Implicit is a dependency on the conformation of the bridge itself.
A completely planar configuration, for example, might lead to
an excellent wirelike behavior, whereas a rearrangement of the
phenyl rings of the bridge in an orthogonal fashion would
certainly weaken the wirelike behavior.5b,12 In fact, such a
conformation might perturb the π-π conjugation between the
single subunits of the molecular bridge. Equally important is
the matching of the orbital energy levels of the single compo-
nents, which in such a case facilitate the electron transfer through
the bridge enabling a charge-hopping mechanism. Alternatively
the photoinduced charge-separation process may be governed
by a superexchange mechanism.
To gain thorough insight into the reaction pathwaysstarting
with photoexcitationsboth free-base porphyrin and the corre-
sponding zinc complex were studiedssee Figure 1.
Absorption measurements, cyclic voltammetry, fluorescence,
and transient absorption measurements were employed to
elucidate the different energy and electron-transfer reactions both
in nonpolar and in polar solvents.
Results and Discussion
Synthesis. In order to gather deeper insight into the electronic
interaction between the single components in the molecular
triads, the reference compounds 8a,b, reported in Figure 1, and
6a,b, reported in Supporting Information Figure S1, were also
synthesized, while the oligomers 4a,d (see Supporting Informa-

Porphyrin-Oligo-PPE-[60]Fullerene Systems
J. Phys. Chem. A, Vol. 113, No. 9, 2009 1781
SCHEME 1: Synthesis of 4-(Trimethylsilylyethynyl)benzaldehyde (1) and 1-Iodo-4-(trimethylsilylethynyl)benzene (2)
SCHEME 2: Synthesis of Different Molecular Bridgesa
a (a) K2CO3 in CH3OH/CHCl3 solution; (b) 2, PdCl2(PPh3)2/CuI in THF/NEt3 50 °C.
tion Figure S1) were used as model “wires” in the present
investigation.
reveals the presence of two well-resolved doublets. Such
resonances are due to the protons in ortho position to the
aldehydic group (δ ) 7.90 ppm) and the terminal-protected triple
bond (δ ) 7.70 ppm). The inner protons, on the other hand,
give rise to unresolved signals. 4a shows signals that correspond
to the protons in ortho position relative to the internal triple
bond as an unresolved double doublet (δ ) 7.50 ppm). In 4b,
the two and four protons as part of the inner phenyl rings and
of those in ortho position with respect to the two internal triple
bonds, respectively, are evidenced as two slightly broadened
singlets (δ ) 7.55 and 7.48 ppm).
The synthetic pathway for obtaining 6a,b, and 7a,b, as
reported in Scheme 3, involved the terminally deprotected triple
bond compounds 5a and 5b. The use of the catalytic system
Pd(dba)2/AsPh3 in the coupling reaction between the bromopor-
phyrin and the terminal alkyne compounds afforded the desired
products in good yields (65-70%).
The first step of the synthetic procedure to obtain 7a,b
consisted of the synthesis of the molecular bridges with variable
length. The well-known Sonogashira coupling emerged as a
suitable method for obtaining the desired compounds.16 How-
ever, in the current context it was necessary to endow the
molecular “wires” with formyl groups at one of their ends to
use them in the Prato reaction with C60.17 Following the synthesis
of 4-(trimethylsilylethynyl)benzaldehyde18 and 1-iodo-4-(tri-
methylsilylethynyl)benzene19 (Scheme 1) it became necessary
to adopt a stepwise procedure for obtaining the appropriate
oligomers (Scheme 2). In particular, the synthesis of 1-iodo-4-
(trimethylsilylethynyl)benzene was carried out in the presence
of a slight excess of 1,4-diiodobenzene, 1.6:1, compared to
trimethylsilylacetylene. This last precaution promotes the ex-
clusive formation of the monocoupling product. Trimethylsi-
lylacetylene was added dropwise over a period of 1 h, giving a
62% yield of the desired compound after silica gel column
purification, i.e., the separation from the bis-coupling adduct.
The sequence of quantitative deprotection of the terminal
ethynyl groupscarried out using K2CO3 in CH3OH/CHCl3
solutionsand subsequent condensation reaction led to the
oligomers bearing either two or three ethynylenephenylene
Despite the tetrapyrrole ring asymmetry, the 1H NMR analysis
reveals a quite simple ꢀ-pyrrole proton pattern for both
porphyrin-oligo-PPE systems (6a and 6b): one singlet (1H) at
δ ) 9.10 ppm corresponding to the proton of the substituted
pyrrole, two singlets (each one 2H) at δ ) 8.90 and 8.70 ppm
due to the protons on the two remaining pyrrole rings, one
adjacent and the other one opposite to the substituted pyrrole
and two doublets (each one 1H) at δ ) 8.85 and 8.77 ppm
1
subunits (Scheme 2). H NMR spectroscopy of 4a and 4b

1782 J. Phys. Chem. A, Vol. 113, No. 9, 2009
Lembo et al.
SCHEME 3: Synthesis of 6a,b and 7a,b Systems
arising from the fourth pyrrole ring adjacent to the substituted
ones.20 The compounds 4a and 4b together with 6a and 6b,
were subsequently used for the cycloaddition reaction with
fullerene, affording, respectively, the reference compounds 8a
and 8b (see Figure 1) and the desired ZnP/H2P systems 7a and
7b.
In the 1H NMR analysis, all the fulleropyrrolidine derivatives
exhibit the typical proton pattern of 2-substituted pyrrolidines.
In particular, two doublets at δ ) 4.90 and 4.13 ppm, originating
from the methylene protons, one singlet at δ ) 4.80 ppm, due
to the hydrogen in the position two of pyrrolidinic ring, and
finally one additional singlet at δ ) 2.80 ppm, arising from the
methyl substituent on the nitrogen atom.
extended conjugation arising from the insertion of an additional
ethynylenephenylene subunit (Supporting Information Figure
S2).
The absorption spectra of 8a,b attest a blue shift of the
absorption bands relative to 4a,b. This trend is ascribed to the
loss of aldehyde groups upon functionalization with C60. A
reduction of the conjugation length is implicit as it may arise
due to the introduction of sp3 carbon atom at the termini of the
wires. In addition, the fullerene absorption, which enters into
the visible region, is clearly discernible (Figure 2).
A comparison of the UV-vis spectra of the ZnP and H2P
derivatives of 7a,b to those of the other reference compounds,
namely, the oligo-PPE-C60 reference compounds 8a,b and
porphyrin-oligo-PPE systems 6a,b, reveals a lack of ap-
preciable electronic interactions in the ground state. The Soret
bands of ZnP and H2P, for example, remain fixed at 436 and
430 nm, respectively, even upon functionalization with C60
(Figure 2).
Cyclic Voltammetry Studies. The electrochemical charac-
terization of the compounds was carried out at room temperature
in a 0.05 M tetra-n-butylammonium hexafluorophosphate
(TBAPF6) o-dichlorobenzene (o-DCB) solution, using a satu-
rated calomel electrode (SCE) as reference electrode, a platinum
button working electrode, and a platinum wire counter electrode.
4a,b reveal in the investigated electrochemical window, i.e.,
between -2.0 and 1.8 V, only a single but irreversible reduction
peak at -1.75 V. However, no particular oxidation processes
are visible.21 For 8a,b, on the other hand, three reduction
processes are discernible in the potential range between -2.25
and 0 V. These three reduction steps occur for 8a at -0.64,
-1.10, and -1.70 V, whereas in 8b slightly shifts toward less
negative potentials (i.e., -0.62, -1.06, and -1.65 V) are
present.
Finally, the respective zinc(II) porphyrinate complexes of all
compounds 6a,b and 7a,b were obtained following the well-
known procedure for the zinc insertion. A saturated solution of
Zn(OAc)2 was added dropwise to a chloroform solution of the
desired compound, and the resulting mixture was left to react
at room temperature for 2 h. After that the zinc complex was
recovered by filtration through a silica gel plug eluted with
chloroform. The matrix-assisted laser desorption ionization time-
of-flight (MALDI-TOF) analysis of ZnP/H2P 7a,b confirms the
presence of the molecular peaks for both compounds at [m/z]+
) 1589 uma for 7a and [m/z]+ ) 1689 uma for 7b with M )
2H and at [m/z]+ ) 1652 uma and [m/z]+ ) 1752 uma,
respectively, for Zn7a and Zn7b with M ) Zn. All the reported
compounds were characterized by 1H NMR, mass spectrometry
(MS), and UV-vis analysis.
UV-Vis Study. The UV-vis spectra of all compounds were
recorded in toluene. 4a exhibits two distinct peaks at 325 and
347 nm, whereas 4b shows a broadened, red-shifted and less
defined absorption band with a maximum at 345 nm due to the

Porphyrin-Oligo-PPE-[60]Fullerene Systems
J. Phys. Chem. A, Vol. 113, No. 9, 2009 1783
Figure 3. Complete scan of 6b in o-DCB solution with 0.05 M
TBAPF6.
TABLE 1: Oxidation and Reduction Potentials in DCB
Solution, with 0.05 M TBAPF6, for the Investigated
Compounds
oxidation (V)
reduction (V)
compd
I
II
I
II
III
IV
6a
4a
Zn6a
8a
7a
Zn7a
6b
4b
Zn6b
8b
7b
1.12
1.45
-1.10
-1.33
-1.74a
-1.74a
0.96
1.26
-1.31b -1.47b -1.87b
-0.64
-0.63
-0.63
-1.10
-1.10
-1.09
-1.02
-1.34
-1.70
-1.39
-1.30
-1.60
-1.76a
1.10
0.96
1.11
1.46
1.23
1.45
-1.63
-1.59
Figure 2. UV-vis absorption spectra of compounds 6b, 7b, and 8b
in toluene solution (n ) 3) with M ) 2H (upper part). UV-vis
absorption spectra of compounds 4b and 8b in toluene solution (n )
3) (lower part).
0.98
1.28
-1.30
-0.62
-0.65
-0.64
-1.59
-1.06
-1.08
-1.02
-1.65
-1.34
-1.31
1.20c
0.98
1.52c
1.24
-1.59
-1.58
Zn7b
However, the differences of 20, 40, and 50 mV for the first,
second, and third reduction process, respectively, are not
sufficient to propose electronic interactions between the elec-
troactive units. 6a and 6b give rise to a pattern that is typically
found in the electrochemistry of porphyrins. In particular, two
oxidation steps between 1 and 1.5 V, two reduction steps
between -1 and -1.35 V, plus one irreversible reduction step
at -1.75 V, are seen. The latter is, nevertheless, due to the
oligomer-aldehyde functionality (Figure 3).
In the cyclic voltammograms of 7a and 7b the redox
processes for all the subunits (i.e., H2P, oligo-PPE, and C60)
are detected. The two oxidation processes are associated
exclusively with H2P, whereas the four reduction processes are
spread over H2P and C60. One of them, namely, the third one,
is a superimposition of the first H2P reduction and the second
C60 reduction. Moreover, the first and fourth waves are attributed
to the first and third C60 reduction. The third wave can be
justified by the second H2P reduction (Supporting Information
Figure S11). In general, the features of the ZnP analogues (i.e.,
6a,b and 7a,b) resemble those discussed for the related H2P
derivatives. The only noticeable exception is the irreversibility
of the reduction processes in both ZnP derivatives 6a,b
(Supporting Information Figures S13 and S14). A closer
inspection of Table 1, which summarizes all the relevant
potentials, infers the lack of significant electronic interactions
between the redox- and photoactive moietiessH2P, oligo-PPE,
and C60 and ZnP, oligo-PPE, and C60, respectively. In conclu-
sion, both cyclic voltammetry and absorption measurements
reveal the absence of electronic interactions between the
a Irreversible. b Potential values obtained by square wave
voltammetry. c Values corresponding to the oxidation wave vertex.
different constituents. Nonetheless, the irreversibility of some
of the investigated peaks is ascribed to the local conformational
effects or to the adsorption phenomena at the electrode surfaces.
Quantum Chemical Investigation. Previous quantum chemi-
cal studies of oligo-PPE molecular wire systems led to the
conclusion that rotations around the phenyl rings of these
π-conjugated oligomers are not restricted due to very low
rotational barriers (<2 kcal/mol).22 However, the dihedrals
between the phenyl rings tend to adopt nearly planar configura-
tions in vacuo, which implies an improved π-conjugation
throughout the entire oligo-PPE linker. Taking these findings
into account, we have carefully looked at the rotational barrier
between the phenyl rings in the monomer 920 (see Figure 4 and
12), dimer 7a, and trimer 7b. The geometries of the zinc
porphyrin (ZnP) and the free-base porphyrin (H2P) derivatives
were optimized at different levels of theory, namely, semiem-
pirically (using the VAMP 10.023 program package with the
AM1*24 Hamiltonian) and by density functional theory (DFT)
methods. The electronic ground states were optimized at the
DFT level with the B3PW9125 functional and a 6-31G(d) basis
set as implemented in the Gaussian0326 program package. To
evaluate the rotational barriers, further single-point DFT calcula-
tions with improved basis sets including polarization and diffuse
functions, i.e., 6-31+G(d,p) and 6-31++G(3df,2pd), were
employed on structures with systematically varying dihedral

1784 J. Phys. Chem. A, Vol. 113, No. 9, 2009
Lembo et al.
Figure 4. Optimized geometries (B3PW91/6-31G*) of the investigated triads 9, 7a, and 7b including the dihedral angles between the phenyl rings
and the bridge and the porphyrin moiety. Left column: free-base porphyrin derivatives. Right column: zinc porphyrin derivatives.
angles and compared to the optimized geometries. Interestingly,
both monomeric derivatives of 9, i.e., ZnP and H2P, show a
dihedral angle of approximately 20°, leading to close contacts
between the porphyrins and the C60 cage (Figure 4). This
explains the faster excited-state deactivation of the monomer20
in comparison to the dimer and trimer.
Significant overlap between the donor and the bridge suggests
strong electronic coupling even in the ground state and facilitates
electron injection from the excited porphyrin to the oligo-PPE
connector. However, in the ZnP trimer coefficients of the
HOMO are significantly present on the bridge structure and,
hence, imply a stronger donor character of the ZnP derivatives
in comparison to H2P. Simultaneously, faster charge-separation
dynamics, as evidenced throughout the photophysical study,
confirm these findings. One possible explanation is that the
linker of the ZnP derivative needs to accommodate higher
electron density from the central metal ion. For that reason,
electron density is redistributed into the adjacent vacant levels
of the bridge. As a consequence, Coulomb repulsive interactions
force the zinc porphyrin into an orthogonal arrangement to the
phenyl units of the bridge. In the monomer, on the other hand,
the short separation distance allows a facile transfer of electron
density onto the LUMOs of the fullerene core. Noticeably, the
electron-accepting features of the fullerene, as incorporated into
the triads, are perfectly preserved. All oligomers exhibit a
localization of the LUMO exclusively surrounding the sym-
metrical C60 cage. Furthermore, the short distance between donor
and acceptor in the monomer 9 leads to significant overlap
between HOMO and LUMO giving very fast electron-transfer
processes. Important evidence for the interactions between the
donor, bridge, and acceptor arises from an analysis of HOMO/
LUMO orbital energies of the particular building blocks of the
triads. Figure 6 relates the HOMOs and LUMOs of the building
blocks to the corresponding orbitals in the triads. Herein, the
significance of the matching of the orbital energy levels becomes
apparent.
These interactions allow a rapid through-space electron-
transfer reaction, which was implied by photophysical measure-
ments. The remaining oligomers contain nearly planar bridge
configurations with dihedral angles in the range of (14°, which
is in a good agreement with our previous results.22 As expected,
the calculated barriers of rotation turned out to be lower than
1.4 kcal/mol, which proves the accuracy of the chosen DFT
model according to other experimental and theoretical studies.
These have shown that rotational barriers in oligo-PPE structures
are between 0 and 0.96 kcal/mol.27 Enthrallingly, in the trimer
7b the angle between the adjacent phenyl ring and the porphyrin
framework is roughly orthogonal, which might have a strong
impact on the conjugation between the bridge and the porphyrins
(Figure 4). Replacing the free base by the metallated porphyrin
enhances this effect on account of increasing electron density
stemming from the metal ion. Torsions around the ethynylene
bond attached to the porphyrin apparently become more and
more restricted with increasing length of the oligo-PPE linker
which may impact the electron-transfer properties of these triads.
Thus, a lack of coplanarity between the porphyrin and oligo-
PPE, as can be evidenced in the trimer 7b, might per se lead to
a loss of electronic communication between donor and acceptor
and is reflected in the longer charge-recombination rates.
In order to complement these findings, frontier orbitals
schemes were analyzed. In particular, the highest occupied
molecular orbitals/lowest unoccupied molecular orbitals (HOMO/
LUMO) clearly reflect the donor-acceptor character of the
systems. In line with our expectations, the HOMO and LUMO
orbitals are strongly localized on the donor and the acceptor,
i.e., the porphyrin and the fullerene, respectively (Figure 5).
The most interesting subject of concern are the LUMO
energies in the triads and C60-oligo-PPE references, namely,
3.28 eV, which exactly matches the LUMO energy of the
C60-methyl fulleropyrolidine reference and clearly demonstrates
the role of the C60 core as the predominant electron acceptor.
Importantly, this feature is evident in all triads independently

Porphyrin-Oligo-PPE-[60]Fullerene Systems
J. Phys. Chem. A, Vol. 113, No. 9, 2009 1785
Figure 5. HOMO/LUMO orbital schemes as resulted from DFT optimizations (B3PW91/6-31G*) displaying the donor-acceptor character of the
triads. The HOMOs are represented in orange-red and LUMOs in green-blue.
Figure 6. HOMO/LUMO orbital energies as resulted from DFT calculations (B3PW91/6-31++G(3df,3pd)) of the building blocks of the triads in
relation to the triads. The HOMO energies are represented in red and LUMO energies in blue.
from the distance between donor and acceptor. On the other
hand, a comparison of the HOMO energies of the triads and
porphyrin-oligo-PPE conjugates to the HOMO energies of
pristine H2P and ZnP reveals the electron-donating features of
the porphyrins. Here, the porphyrin HOMOs can be energetically
compared to the corresponding HOMOs in the triads and the
porphyrin-oligo-PPEs. Thus, attaching the oligo-PPE linker to
the porphyrin derivatives does not alter the HOMO energy at
all. Equally, the HOMO/LUMO energies of the donor and
acceptor are preserved in all triads leading to the assumption
that the electronic communication between both does not depend
on the bridge length. Apparently, the bridge orbital energies do
not mix with the energy levels of the donor and the acceptor
which implies perfect conditions for a superexchange rather than
a through-bond hopping mechanism for the electron-transfer
processes. Nevertheless, the presence of significant overlap
between the oligo-PPE linkers and the electron donor and
acceptor, respectively, render the possibility of a thermally

1786 J. Phys. Chem. A, Vol. 113, No. 9, 2009
Lembo et al.
induced hopping mechanism involving the bridge orbitals upon
higher temperatures. Up the front, the energy difference of the
orbitals of the oligo-PPE oligomers vanishes upon covalent
binding to the porphyrins. Implicit extension of the π-system
of the porphyrins onto the bridge is a possible explanation. On
the other hand, in case of the fullerene, the HOMO and LUMO
orbitals remain strongly localized on C60 implying a lack of
electronicperturbationbythebridge.Moreover,theHOMO-LUMO
gap of the oligo-PPE units is approximately 0.2 and 1.0 eV
higher than in the corresponding porphyrin-oligo-PPE conju-
gates and the triads, respectively. This implies strong electronic
coupling between the porphyrins and C60 which does not depend
on the length of the oligo-PPE linker and leads to nearly distant-
independent electron-transfer processes, as reflected by the
extremely low ꢀ values of 0.11 Å-1
.
Furthermore, the HOMO/LUMO energies are in perfect
agreement with the oxidation and reduction potentials obtained
by the cyclic voltammetry study. Noteworthy, electrochemical
measurements suggest that the reduction of the C60 moiety in
8a,b does not depend on the oligo-PPE bridge and thus neglect
considerable interactions between these two moieties. Our DFT
calculations corroborate these features revealing that the reduc-
tion potentials of 8a,b which appear around -3.8 eV comply
with the LUMO energies of C60 (-3.3 eV). These values exceed
the LUMO energies of oligo-PPE by more than 1.0 eV. Along
with the assumption that the calculated orbital energies match
the potentials obtained by electrochemistry, these findings imply
a lack of electronic interactions between the redox-active
moieties. For instance, the oxidation and reduction of 6a,b are
separated by an energy gap in the range of 2.2 and 2.3 eV which
perfectly resembles the energy required for a HOMO to LUMO
transition obtained in the calculations, namely, 2.6-2.7 eV.28
Similarly, the potential gaps for the redox processes in the triads
7a,b that appear between 1.6 and 1.9 eV are ascribed to the
calculated HOMO to LUMO transitions at 1.8 eV.
Figure 7. Local electron affinity maps of the exTTF-oligo-PPE3-C60
trimer compared to the porphyrin-oligo-PPE3-C60, 7b, triads, scaling
high to low in red to blue.
typically observed in the 400-500 nm range, ZnP/H2P emits
in the 600-700 nm range and C60 emits with a maximum at
715 nm. The quantum yields vary substantially with values that
range between 0.2 and 6 × 10-4 for the H2P and C60 reference,
respectively.
Also characteristic features were recorded in our ultrafast and
fast transient absorption experiments. We see, for example,
singlet excited states that are formed instantaneously: in case
of the oligo-PPE singlet-singlet absorptions develop with
maxima in the 500-700 nm range. Characteristics for ZnP/
H2P are 480 nm. For the C60 reference, on the other hand, the
maximum is in the red at 880 nm. A fast intersystem crossing
process (i.e., in the range of 108 s-1) governs the fate of the
metastable singlet excited states in all references (i.e., C60, oligo-
PPE, ZnP/H2P). The corresponding formed triplet-triplet
absorptions are all located in the range between 500 and 900
nm.
C60-Oligo-PPE. Relative to the strong and long-lived emis-
sion of the oligo-PPE references, 4a,b, the oligo-PPE emission
in the C60-oligo-PPE systems, 8a,b, is nearly quantitatively
quenched (Supporting Information Figure S7, top). Instead, a
familiar fullerene fluorescence spectrum was found with a 0 f
0 emission at 715 nm, despite exclusive excitation of the oligo-
PPE moiety (Supporting Information Figure S8, bottom). To
reveal the mechanism of producing the emission, an excitation
spectrum was taken. The excitation spectra of the 8a,b references
exactly matches the ground-state absorption of the oligo-PPE
moieties. This implies a rapid transfer of singlet excited-state
energy from the photoexcited oligo-PPE to the covalently linked
fullerene.
Regarding the transient absorption measurements, immedi-
ately, after the laser excitation of 8a,b at 387 nm, broadly
absorbing transients with maxima between 400 and 500 nm (i.e.,
singlet-singlet absorptions of the oligo-PPE) were found
(Supporting Information Figure S8). The spectral features
recorded right after the laser pulse clearly confirmedsdespite
the presence of C60sthe successful formation of the oligo-PPE
singlet excited state. However, the excited-state absorption was
short-lived with lifetimes (i.e., <14 ps) that corroborated the
efficient emission quenching.
Finally, local electron affinity mappings, as computed with
Parasurf 0729 and visualized using Tramp 1.1d,30 corroborate
the electron-transfer properties of 7a,b. Previously studied oligo-
PPE linkers between exTTF (extended-tetrathiafulvalene) as
electron donor and C60 as electron acceptor revealed an
attenuation factor ꢀ of 0.21 Å-1 7e,f,22
The systems presented in
.
this work exhibit lower attenuation factors, namely, a value of
0.11 Å-1. In our foregoing studies the electron-transfer pathway
of the linkers, i.e., the areas of high local electron affinity, was
disrupted by the triple bonds due to their strong polarizing
features. In addition, electron density minima appeared between
the phenyl rings. Currently, the electron density in the por-
phyrin-oligo-PPE-C60 systems is more homogeneously dis-
tributed throughout the whole bridge due to the more electron-
rich porphyrin donors in comparison to exTTF. In line with the
electron density, the local electron affinity exhibits a much more
uniform distribution. Nevertheless, minima are localized at the
triple-bond sites but the discrepancy between values of high
(orange) and low (yellow to green) electron affinity is less
pronounced due to higher electron density of these systems
(Figure 7). These findings nicely comply with the lower ꢀ values
obtained for the oligo-PPEs. Interestingly, the ZnP derivatives
exhibit notably high electron affinity values at the central metal
ion that reflect the inhomogeneous distribution of the electrons
around the Zn atom facilitating electron injection into the bridge.
Photophysics. References. In steady-state experiments, all
reference compounds (i.e., C60, oligo-PPE, ZnP/H2P) emit singlet
excited-state energy in, however, different spectral regions of
the solar spectrum. Although the fluorescence of oligo-PPE is
Once the rapid disappearance of the excited oligo-PPE
absorption comes to an end (i.e., ∼20 ps after the laser pulse),
only characteristics of the fullerene singlet excited-state absorp-
tion remain. The noted maximum at 880 nm is reminiscent to
that found for the reference C60. In line with an energy transfer

Porphyrin-Oligo-PPE-[60]Fullerene Systems
J. Phys. Chem. A, Vol. 113, No. 9, 2009 1787
TABLE 2: Selected Spectroscopic Date for the Investigated
Triads 7a,b and the Appropriate Reference Compounds
Φfl
τ1 [ns]
kCS [s-1
]
kCR [s-1
]
C60 reference 6.0 × 10-4
H2P reference 1.1 × 10-1
ZnP reference 4.0 × 10-2
1.2
9.8
2.4
7a
toluene: 6.1 × 10-2 4.18
THF: 4.4 × 10-2
2.39 × 108 1.15 × 106
1.68 × 108 0.84 × 106
2.22 × 109 1.44 × 106
0.85 × 109 1.10 × 106
PhCN: 3.2 × 10-2
7b
toluene: 9.1 × 10-2 5.97
THF: 8.2 × 10-2
PhCN: 7.9 × 10-2
Zn7a
Zn7b
toluene: 7.4 × 10-3 0.45
THF: 2.7 × 10-3
PhCN: 2.1 × 10-3
toluene: 2.7 × 10-2 1.18
THF: 1.7 × 10-2
PhCN: 1.3 × 10-2
8a
8b
<5.0 × 10-4
<5.0 × 10-4
1.4 ( 0.02
1.4 ( 0.02
mechanism the singlet-singlet absorption revealed a two-step
grow-in dynamics. (Supporting Information Figure S9, top) The
faster process stems from the direct excitation of the C60 core,
while the slower component is ascribed to the actual transfer
of excited-state energy. The latter assignment is based on the
nearly identical dynamics (i.e., <18 ps) observed for the second
component relative to those of the decays at 500 nm (time
profiles at those wavelengths). Interestingly, the singlet-singlet
excited-state deactivation in the dimer 8a occurs on a faster
time scale compared to the trimer 8b. Yet, another process
follows the conclusion of the energy transfer reaction in 8a,b,
whose outcome on the time scale of our femtosecond experi-
ments is the formation of a distinct, new maximum at 700 nm
(Supporting Information Figure S9, bottom). This absorption
is in excellent agreement with the triplet excited-state absorption
of the C60 reference, which infers that the underlying reaction
involves intersystem crossing from the C60 singlet to the
energetically lower-lying triplet manifold.
C60-Oligo-PPE-ZnP/C60-Oligo-PPE-H2P. The aforemen-
tioned energy transfer is in all C60-oligo-PPE (8a and b) nearly
quantitative and helps to concentrate the singlet excited-state
energy at the fullerene end. Notable, the contribution from an
exothermic electron transfer is inappreciably small. This general
pattern is not affected when linking ZnP/H2P to the other end
of the wire, C60-oligo-PPE-ZnP/C60-oligo-PPE-H2P. In case
of exciting the oligo-PPE part in C60-oligo-PPE-ZnP/C60-oligo-
PPE-H2P (i.e., 300-500 nm range), a rapid intramolecular
transduction of energysas evidenced by fluorescence quantum
yields of <5.0 × 10-4 (see Supporting Information Figure
S7)sfunnels the excited-state energy to the C60 core, generating
1*C60 with nearly unity quantum yields (Table 2). Drawing the
attention to the porphyrin emission, please note that in contrast
to previously studied C60-oligo-PPE-donor systems, ZnP and
H2P not only absorb notably in the current C60-oligo-
PPE-donor systems, but their strong fluorescence dominates
large parts of the spectrum. Interestingly, the ZnP/H2P centered
fluorescence is appreciably quenched in all C60-oligo-PPE-ZnP/
C60-oligo-PPE-H2Psregardless of the excitation wavelength
(i.e., Soret- or Q-band region)sprompting to the fact that C60
must enhance the fluorescence deactivation (vide infra). Figure
8 illustrates the fluorescence quenching of the ZnP and H2P
oligomers in tetrahydrofuran (THF).
Figure 8. Fluorescence spectra of C60-oligo-PPE-ZnP and ZnP
(upper part) and C60-oligo-PPE-H2P and H2P (lower part) in THF
upon 440 and 460 nm excitation, respectively, with solutions displaying
the same optical density at the excitation wavelengths (i.e., 0.1).
cence lifetimes are seen in C60-oligo-PPE-ZnP/C60-oligo-
PPE-H2P (Table 2). Upon modifying the solvent polarity from
toluene (ε ) 2.4) over THF (ε ) 7.6) to benzonitrile (ε ) 24.8)
a gradual intensification of the quenching is discernible, as
evidenced by the decreasing quantum yields in Table 2. We
hypothesize that the underlying solvent dependence is due to
an intramolecular electron transfer between the photoexcited
ZnP/H2P donors and C60 to yield C60•--oligo-PPE-ZnP•+/
C60•--oligo-PPE-H2P•+.
In transient absorption measurements, we focused exclusively
on the generation of the singlet excited states of ZnP/H2P. The
overlapping absorptions of all components renders any other
analysis ambiguous, though we must assume that in accordance
with previous work that starting with the C60 singlet excited
state a similar reaction pattern will evolve. Instead of seeing,
however, the slow intersystem crossing dynamics, as the ZnP/
H2P references, the singlet-singlet absorption decays in the
presence of C60 acceptors with accelerated dynamics. An
important aspect is that the singlet excited-state lifetimes match
quantitatively those values derived in the fluorescence experi-
ments. Spectroscopically, the transient absorption changes, taken
after the completion of the decay, bear no resemblance with
any triplet excited state (vide infra). Again, varying the solvent
polarity from THF to benzonitrile leads to an acceleration of
the singlet deactivation. This supports our earlier hypothesis
that an intramolecular electron transfer, yielding C60•--oligo-
PPE-ZnP•+/C60•--oligo-PPE-H2P•+, governs the ZnP/H2P
singlet excited-state deactivation.
To shed light on the nature of the product, evolving from
this intramolecular deactivation, complementary time-resolved
fluorescence and transient absorption measurements were neces-
sary. Let us first direct our attention to the fluorescence lifetime
measurements. A notable shortening of the ZnP/H2P fluores-
Spectroscopic evidence for the radical pair formation was lent
from the features developing in parallel with the disappearance

1788 J. Phys. Chem. A, Vol. 113, No. 9, 2009
Lembo et al.
Figure 9. Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (387 nm, 200 nJ) of
C60-oligo-PPE2-ZnP in argon-saturated THF with several time delays
between 0 and 1000 ps at room temperature, illustrating the charge
transfer.
of the ZnP/H2P singlet-singlet absorption. In the visible region,
the observed maxima between 600 and 700 nm as well as 480
nm correspond to the one-electron-oxidized π-radical cations
of ZnP (ZnP•+) and H2P (H2P•+), respectively, whereas in the
near-infrared region the 1000 nm maximum resembles the
signature of the one-electron-reduced form of C60 (C60•-)s
Figure 9.
Since no charge-transfer was evidenced in the C60-oligo-
PPE references, we exclude the involvement of a transient
oxidized oligomer, C60•--oligo-PPE•+-ZnP/C60•--oligo-
PPE•+-H2P throughout the charge-separation reaction. Hence,
we imply that in the dimer- and trimer-based systems charge
separation occurs within a single step. Figure 10 compares the
charge-transfer dynamics of the dimer- to the trimer-based ZnP
system.
Figure 10. Upper part: time-absorption profiles at 480, 640, and 1000
nm of the differential absorption measurements with C60-oligo-
PPE2-ZnP in THF solutions. Lower part: time-absorption profiles at
530, 630, and 1000 nm of the differential absorption measurements
with C60-oligo-PPE3-ZnP in THF solutionssmonitoring the charge-
transfer processes.
To examine the charge-recombination dynamics the spectral
fingerprints of the C60 π-radical anion and that of the ZnP/H2P
π-radical cations are useful probes. Both spectral attributes are
persistent on the picosecond time scale and only start to decay
slowly in the nanosecond regime. Time absorption profiles
illustrate that C60•--oligo-PPE-ZnP•+/C60•--oligo-PPE-H2P•+
decay via a single step. The charge-recombination dynamics
within these systems were determined accurately by fitting the
decays of both fingerprints to a monoexponential rate law in
complementary nanosecond experiments.
Importantly, comparing solvents of different polarity, namely,
THF and benzonitrile, stabilizing effects for all C60•--oligo-
PPE-ZnP•+/C60•--oligo-PPE-H2P•+ radical pairs were seen
toward the more polar solvents. This trend prompts to charge-
recombination dynamics that are in the normal region of the
Marcus parabola. For that reason, we have calculated the driving
forces for charge separation and charge recombination in each
of the three solvents according to the method by Weller.31
Supporting Information Table S1 represents the calculated ∆G
values for the charge separation and recombination placing the
back electron transfer onto the normal regime of the Marcus
parabolas. On the basis of these values it was possible to
illustrate the different possible reaction pathways in a state
diagram (Figure 11).
and transient absorption kinetic measurements (femto- and
nanosecond time scales) we determine attenuation factors (ꢀ)
for our ensembles as 0.11 Å-1, which are exceptionally small
(Figure 13).
Conclusion
A series of novel porphyrin-wire-fullerene systems in which
the wire component is covalently bound to the ꢀ-pyrrole carbon
atom of the porphyrin macrocycle has been synthesized and
characterized. Upon photoexcitation both free-base and related
zinc derivatives systems are capable of electron-transfer reac-
tions from the electron-donating porphyrin to the electron-
accepting C60 moiety in polar solvent environment (i.e., THF).
Long-lived charge-separated states with lifetimes in the order
of hundred of nanoseconds have been demonstrated. Further-
more, the lifetimes of the radical ion pair tend to increase with
increasing distance between the two photoactive units. Since
the charge-separation reaction occurs over a long distance,
namely, up to 23 Å, when n ) 3, a through-bond mechanism,
where the bridge plays an important role, is plausible; in addition
to that, transient absorption and cyclic voltammetry studies did
not reveal accessible oxidized states of the linker. Therefore,
electron transfer via a superexchange mechanism seems to be
the most probable operative mode. The studied systems (n )
2, 3) indicate a linear distance dependence of the electron-
transfer rate constantsconsidering the edge-to-edge distance
between the two photoactive chromophoresswith an attenuation
factor ꢀ equal to 0.11 Å-1. This value complements those
Plotting the electron-transfer behaviorsboth charge-separation
rate and charge-recombination ratesas a function of donor-
acceptor separation including the results from the previously
published “monomer”20 (Figure 12) led to linear dependences
for THF and benzonitrile as solvents. From fluorescence lifetime

Porphyrin-Oligo-PPE-[60]Fullerene Systems
J. Phys. Chem. A, Vol. 113, No. 9, 2009 1789
Figure 13. Edge-to-edge distances (REE) dependence of charge-
separation (ln kCS) and charge-recombination (ln kCR) rate constants of
H2P-oligo-PPEn-C60 (upper part) and ZnP-oligo-PPEn-C60) (lower
part) triads (n ) 1, 2, 3) in nitrogen-saturated THF at room temperature.
Figure 11. Schematic illustration of the reaction pathways in photex-
cited C60-oligo-PPE2-H2P (top) and C60-oligo-PPE2-ZnP (bottom)
with the state energies as determined in THF. The different deactivation
pathways are indicated by arrows.
The slope represents ꢀ with the value of 0.11 Å-1
.
the oligo-vinylenephenylene ꢀ-substituted porphyrin,32 (2) the
oxidation potential of the porphyrin macrocycle is nearly
unaffected by the length of the linker during cyclic voltammetry.
These findings imply that the conformation of the porphyrin
relative to the phenyl rings of the bridge prevents the full
conjugation between donor and the linker. On the other hand,
the oligo-ethynylenephenylenes demonstrate a rather effective
wirelike behavior resulting from a possible coplanarity of the
phenyl rings.5
Quantum chemical investigations provided further under-
standing for the influence of electronic and structural properties
of the investigated triads 7a,b and 6a,b on their charge-transfer
features. Geometry optimizations suggest a nearly planar bridge
configuration of the oligo-PPEs, which supports the extended
π-conjugation of these systems. Differences between the H2P
and ZnP derivatives have been found regarding the dihedral
angle between the porphyrins and the bridge, which might lead
to a loss of electronic communication between donor and
acceptor in 7 and reflects the longer charge-recombination rates.
Further analysis of the electronic structure revealed a strong
localization of the HOMOs and LUMOs on the electron-
donating porphyrin and electron-accepting fullerene, respec-
tively, proposing an increased donor character of the Zn
derivatives. Regardless of the incorporation of C60 into the triads,
its electron-accepting features turned out to be perfectly
preserved. Furthermore, the matching of the HOMO/LUMO
energy levels of the single components of the triads plays a
major role for efficient electronic communication between donor
and acceptor, which was also proved by the electrochemical
investigation. Thus, the mechanism of the charge-transfer
Figure 12. Porphyrin-fullerene system with one ethynylenephenylene
subunit (n ) 1). M ) 2H, Zn.
reported in previous studies,5 taking the damping factor ꢀ of
0.21 Å-1 into account, which was found in similar systems
bearing exTTF7e as donor moiety. Thus, at this point, we would
like to underline the dependence of the wirelike behavior of
the linker on the donor features.
From UV-vis absorption and cyclic voltammetry studies we
can exclude a complete extension of the porphyrin HOMO along
the whole length of the linker for compounds 7a,b and 6a,b
for two reasons: (1) no red shift of the Soret band of the
porphyrin upon an increase of the linker length is observed in
the UV-vis spectra, which has been previously observed for

1790 J. Phys. Chem. A, Vol. 113, No. 9, 2009
Lembo et al.
processes seems to remarkably depend on the structural proper-
ties of the linker between donor and acceptor and its conjugation
into the porphyrin/C60 moieties. These findings have been
corroborated by local electron affinity mappings.
needles in a 62% yield. MS (GC): m/z 300 [M+], 285 [M -15]+,
1
158 [285-127]+. H NMR (300 MHz, CDCl3,): δ (ppm) 7.66
3
3
(d, J ) 8.8 Hz, 2H), 7.20 (d, J ) 8.8 Hz, 2H), 0.26 (s, 9H).
General Procedure for the Sonogashira Coupling in the
Synthesis of the Oligo-Phenyleneethynylenes. 1-Iodo-4-(tri-
methylsilyl)ethynyl-benzene and the appropriate aldehyde de-
rivative were dissolved in a 1:1 molar ratio in 60 mL of dry
THF (distilled over LiAlH4); 3 mL of dry triethylamine (KOH)
was added with 0.03 equiv of PdCl2(PPh3)2 and 0.07 equiv of
CuI. The mixture was left to react under a nitrogen atmosphere
at 40 °C for 6 h, then the solvent was removed under vacuum,
and the crude product was purified over silica gel column
chromatography eluting with CH2Cl2/petroleum ether (40-70°),
30:70.
Experimental Section
General Methods. 1H NMR spectra were recorded as CDCl3
solutions on a Bruker AM-300 instrument using residual solvent
signal as an internal standard. Chemical shifts are given as δ
values. Fast atom bombardment (FAB) mass spectra were
measured on a VG-4 spectrometer using m-nitrobenzyl alcohol
(NBA) as a matrix. Gas chromatography (GC) mass spectra were
recorded on a VG-4 spectrometer equipped with a 30 mt Supelco
SPB-5 capillary column. MALDI mass spectra were performed
with a MALDI-TOF Reflex IV (Bruker-Daltonics) in reflector
mode, using a 337 nm nitrogen laser (8 Hz). A 2 mg/mL 2,5-
dihydroxybenzoic acid (gentysic acid) solution in CH3CN/TFA
(0.1% solution) was used as a matrix. UV-vis spectra were
recorded on a Varian Cary 50 Scan spectrophotometer in toluene
solution. Steady-state fluorescence studies were carried out on
a Fluoromax 3 (Horiba) instrument, and all the spectra were
corrected for the instrument response. Time-resolved fluores-
cence spectra were recorded on a Photon Technology Interna-
tional Company lifetime spectrofluorometer in the nanosecond
time domain, with a 337 nm nitrogen laser and elaborated with
Felix software. The femtosecond transient absorption studies
were performed with 387 nm laser pulses (1 kHz 150 fs pulse
width) from an amplified Ti:Sapphire laser system (model CPA
2101, Clark-MXR Inc.). Nanosecond laser flash photolysis
experiments were performed with 532 nm laser pulses from a
Quanta-Ray CDR Nd:YAG system (6 ns pulse width) in a front
face geometry. Cyclic voltammetry was carried out with a
Autolab electrochemical system, Eco Chemie equipped with PG
Stat-12. Current-voltage curves were recorded using GPES,
Eco Chemie software. A three-electrode system was used and
consisted of a platinum button working electrode, a platinum
wire counter electrode, and an SCE reference electrode. This
reference electrode was separated from the bulk of the solution
by a fritted-glass bridge filled with solvent/supporting electrolyte
mixture. All potentials are referenced to the SCE.
Chemicals. Silica gel 60 (70-230 and 230-400 mesh,
Merck) was used for column chromatography. 1,2-Dichloroben-
zene (DCB) for electrochemistry was purchased from Aldrich
Chemical Co. and purified by several washings with concd
H2SO4 and distilled over P2O5 under vacuum prior to use. Tetra-
n-butylammonium hexafluorophosphate from Aldrich Chemical
Co. was recrystallized from ethyl acetate and dried under
vacuum at 40 °C for at least 1 week prior to use. High-purity-
grade nitrogen gas was purchased from Rivoira. C60 was
purchased from Term-USA. All other reagents and solvents were
from Fluka Chem. Co., Aldrich Chem. Co., or Carlo Erba and
were used as received. 4-Ethynylbenzaldehyde18 and 2-bromo-
5,10,15,20-tetraphenylporphyrin20 were synthesized following
the procedures reported in the literature.
4-[4′-(Trimethylsilyl)ethynyl]phenyl-ethynylbenzalde-
hyde, (TMS-Oligo-PPE2-CHO) (4a). Starting reagents:
1-iodo-4-(trimethylsilyl)ethynyl-benzene (700 mg, 2.3 mmol)
and 4-ethynylbenzaldehyde (305 mg, 2.3 mmol). Yield 65%
(450 mg). MS (GC): m/z 302 [M+]. 1H NMR (300 MHz,
3
CDCl3,): δ (ppm) 10.05 (s, 1H), 7.89 (d, J ) 8.3 Hz, 2H),
7.69 (d, 3J ) 7.5 Hz, 2H), 7.51 (m, 4H), 0.28 (s, 9H). UV-vis
(toluene): λmax (nm) (ε) ) 325 (31 300), 347 (28 200 mol-1 dm3
cm-1).
4-{4′-[4′′-(Trimethylsilyl)ethynylphenyl]ethynylphenyl}-
ethynylbenzaldehyde, (TMS-Oligo-PPE3-CHO) (4b). Start-
ing reagents: 4-[(4′-ethynyl)phenyl]-ethynylbenzaldehyde (157
mg, 6.8 × 10-4 mol), 1-iodo-4-(trimethylsilyl)ethynyl-benzene
(205 mg, 6.8 × 10-4 mol). The desired product was recrystal-
lized from CHCl3/petroleum ether (40-70°), affording 170 mg
of pale yellow-white needles, yield 65%. MS (FAB): m/z 402
1
[M+]. H NMR (300 MHz, CDCl3,): δ (ppm) 10.05 (s, 1H),
7.90 (d, 3J ) 7.98 Hz, 2H), 7.70 (d, 3J ) 7.98 Hz, 2H), 7.55 (s,
4H), 7.48 (s, 4H), 0.28 (s, 9H). UV-vis (toluene): λmax (nm)
(ε) ) 345 (65 800 mol-1 dm3 cm-1).
General Procedure for the Terminal Triple-Bond Depro-
tection Reaction. Oligomers having terminal-protected triple
bond were dissolved in 50 mL of CHCl3, and then a K2CO3-
saturated methanol solution (10 mL) was added dropwise under
nitrogen atmosphere. After 2 h, water (50 mL) was added to
the organic solution. The organic phase was separated, and the
aqueous phase was twice extracted with 25 mL of chloroform.
The combined organic solutions were dried over anhydrous
Na2SO4, filtered, and the solvent was evaporated under vacuum.
4-[(4′-Ethynyl)phenyl]-ethynylbenzaldeide (5a). The reac-
1
tion is almost quantitative. H NMR (300 MHz, CDCl3,): δ
3
3
(ppm) 10.05 (s, 1H), 7.90 (d, J ) 7.8 Hz, 2H), 7.70 (d, J )
7.8 Hz, 2H), 7.52 (s, 4H), 3.22 (s, 1H).
4-{4′-[4′′-(Ethynyl)phenyl]ethynylphenyl}-ethynylbenzal-
dehyde (5b). The reaction is almost quantitative.1H NMR (300
3
MHz, CDCl3,): δ (ppm) 10.05 (s, 1H), 7.90 (d, J ) 7.98 Hz,
3
2H), 7.70 (d, J ) 7.98 Hz, 2H), 7.55 (s, 4H), 7.50 (s, 4H),
3.20 (s, 1H).
General Procedure for the Sonogashira Coupling in the
Synthesis of Porphyrin-Oligo-PPE Systems. A three-neck
round-bottom flask, fitted with a condenser, was carefully
deoxygenated with a strong stream of dry argon; after that a
solution of 2-bromo-5,10,15,20-tetraphenylporphyrin (1 equiv)
and the appropriate aldehyde oligomer (1.5 equiv) in toluene/
triethylamine, 5:1 (60 mL), was added. The solution was
deaerated for 30 min with argon bubbling, and then Pd(dba)2
(0.2 equiv) and AsPh3 (1.5) were added. The solution was
deaerated for further 5 min; after that the argon inlet was placed
1 cm above the solution. The argon flow rate was turned up
slightly and the reaction was left under nitrogen at 60 °C. After
1-Iodo-4-(trimethylsilyl)ethynyl-benzene (2). 1,4-Diiodo-
benzene (7.6 g, 23 mmol), PdCl2(PPh3)2 (212 mg, 3 × 10-4
mol), and CuI (115 mg, 6 × 10-4 mol) were dissolved in dry
THF (50 mL), then dry triethylamine (KOH, 3 mL) was added.
The solution was left under nitrogen atmosphere at 40 °C, and
a THF solution (10 mL) of trimethylsilylacetylene (1.40 g, 14
mmol) was added dropwise over 1 h. After 6 h, the reaction
was stopped and solvent evaporated under vacuum The residue
was chromatographed on silica gel column eluting with petro-
leum ether (40-70°). The desired product was obtained as white

Porphyrin-Oligo-PPE-[60]Fullerene Systems
J. Phys. Chem. A, Vol. 113, No. 9, 2009 1791
16 h, the mixture was cooled at room temperature and the
solvent was evaporated. The crude product was dissolved in
dichloromethane and washed several times with water. The
organic solution was dried over anhydrous Na2SO4, and the
solvent was evaporated under vacuum. The desired product was
purified by silica gel column chromatography.
7.80-7.50 (br m, several signals difficult to resolve and
3
3
integrate), 7.48 (d, J ) 7.8 Hz, 2H), 7.34 (d, J ) 8.3 Hz,
2H), 4.90 (d, 2J ) 9.8 Hz, 1H), 4.79 (s, 1H), 4.13 (d, 2J ) 9.8
Hz, 1H), 2.78 (s, 3H), -2.67 (s, 2H). UV-vis (toluene): λmax
(nm) (ε) ) 334 (90 900), 430 (239 000), 524 (25 000), 560
(9400), 603 (6900), 659 (4300 mol-1 dm3 cm-1).
H2P-Oligo-PPE2-CHO (6a). The crude product was
purified by column chromatography on silica gel eluting with
CHCl3/petroleum ether (40-70°), 1:1. The first compound eluted
from the column was AsPh3; after that the polarity of eluting
mixture was gradually increased up to CHCl3/petroleum ether
(40-70°), 70:30. The desired porphyrin obtained from the
column was recrystallized from dichloromethane/methanol to
give a purple powder (yield 65%). MS (FAB): m/z 842 [M+].
1H NMR (300 MHz, CDCl3): δ (ppm) 10.06 (s, 1H), 9.10 (s,
1H), 8.90 (s, 2H), 8.85 (d, 3J ) 5.9 Hz, 1H), 8.79 (s, 2H), 8.77
TMS-Oligo-PPE2-C60 (8a). Starting reagents: C60 (55 mg,
7.6 × 10-5 mol), 4-[4′-(trimethylsilyl)ethynyl]phenyl-ethynyl-
benzaldeide (16 mg, 5.2 × 10-5 mol) and N-methylglicine (70
mg, 7.6 × 10-4 mol). Eluant mixture: petroleum ether (40-70°)/
toluene, 3:2. Obtained 33 mg of brown powder as final product
(yield 60%). MS (MALDI): m/z 1049 [M+], 720 [M - 329]+.
1H NMR (300 MHz, CDCl3): δ (ppm) 7.82 (br d, 2H), 7.61 (d,
3J ) 7.6 Hz, 2H), 7.45 (s, 4H), 5.02 (d, 2J ) 9.8 Hz, 1H), 4.98
2
(s, 1H), 4.29 (d, J ) 9.8 Hz, 1H), 2.84 (s, 3H), 0.26 (s, 9H).
UV-vis (toluene): λmax (nm) (ε) ) 312 (63 800), 332 (64 900
3
3
mol-1 dm3 cm-1).
(d, J ) 5.9 Hz, 1H), 8.23 (br m, 5H), 7.91 (d, J ) 7.8 Hz,
3
2H), 7.80-7.66 (br m, 17H), 7.54 (d, J ) 7.8 Hz, 2H), 7.38
TMS-Oligo-PPE3-C60 (8b). Starting reagent: TMS-oligo-
PPE3-CHO (20 mg, 5 × 10-5 mol), C60 (54 mg, 7.5 × 10-5
mol), and N-methylglicine (67 mg, 7.5 × 10-4 mol). Eluant
mixture: petroleum ether (40-70°)/toluene, 3:2. Obtained 40
mg of brown-reddish powder as final product (yield 70%). MS
(d, 2H; J ) 7.8), -2.65 (s, 2H). UV-vis (toluene): λmax (nm)
(ε) ) 333 (28 900), 378 (30 600), 430 (194 000), 524 (20 000),
560 (6900), 603 (5400), 659 (3500 mol-1 dm3 cm-1).
H2P-Oligo-PPE3-CHO (6b). The crude product was
purified by column chromatography on silica gel eluting with
CHCl3/petroleum ether (40-70°), 1:1. The first compound eluted
from the column was AsPh3 after that the polarity of eluting
mixture was gradually increased up to CHCl3/petroleum ether
(40-70°), 3:2. The desired porphyrin obtained from the column
was recrystallized from dichloromethane/methanol to give a
purple powder (yield 70%). MS (FAB): m/z 942 [M+]. 1H NMR
(300 MHz, CDCl3): δ (ppm) 10.04 (s, 1H), 9.10 (s, 1H), 8.90
1
(MALDI): m/z 1149 [M+], 720 [M - 429]+. H NMR (300
3
MHz, CDCl3): δ (ppm) 7.85 (br d, 2H), 7.63 (d, J ) 8.3 Hz,
2H), 7.50 (s, 4H), 7.46 (s, 4H) 5.02 (d, 2J ) 9.1 Hz, 1H), 4.98
2
(s, 1H), 4.29 (d, J ) 9.1 Hz, 1H), 2.84 (s, 3H), 0.27 (s, 9H).
UV-vis (toluene): λmax (nm) (ε) ) 336 (90 500), 362 (54 200
sh, mol-1 dm3 cm-1).
General Procedure for Zinc Insertion. To a solution of
starting compound (porphyrins or dyad) in chloroform a
saturated solution of Zn(AcO)2 in methanol was added, and the
mixture was left to react at room temperature under nitrogen
for 2 h. The solvent was evaporated, and the product was
purified using a plug of silica gel, eluting with chloroform.
ZnP-Oligo-PPE2-CHO (Zn6a). Yield: 97%. MS (MALDI):
3
3
(s, 2H), 8.85 (d, J ) 4.5 Hz, 1H), 8.79 (s, 2H), 8.77 (br d, J
3
) 5.2 Hz, 1H), 8.23 (br m, 5H), 7.91 (d, J ) 8.3 Hz, 2H),
7.80-7.66 (br m, 15H), 7.70 (d, J ) 8.3 Hz, 2H), 7.58 (s,
4H), 7.53 (d, J ) 8.3 Hz, 2H), 7.38 (d, J ) 8.3 Hz, 2H),
-2.67 (s, 2H). UV-vis (toluene): λmax (nm) (ε) ) 344 (57 000),
373 (54 000), 430 (239 000), 524 (24 600), 560 (9300), 603
(6700), 659 (4400 mol-1 dm3 cm-1).
3
3
3
1
m/z 905 [M+]. H NMR (300 MHz, CDCl3): δ (ppm) 10.04 (s,
1H), 9.26 (s, 1H), 8.95 (s, 2H), 8.93 (s, 2H), 8.90 (d, 3J ) 5.3 Hz,
3
3
1H), 8.79 (d, J ) 5.3 Hz, 1H), 8.23 (br m, 10H), 7.90 (d, J )
7.6 Hz, 2H), 7.78 (b m, 10H), 7.69 (d, 3J ) 6.0 Hz, 2H), 7.55 (d,
3J ) 8.3 Hz, 2H), 7.40 (d, 3J ) 8.3 Hz, 2H). UV-vis (toluene):
λmax (nm) (ε) ) 333 (39 300), 436 (220 800), 559 (17 400), 594
(9100 mol-1 dm3 cm-1).
General Procedure for the Synthesis of Porphyrin-Oligo-
PPE-Fullerene and Oligo-PPE-Fullerene Systems. A mix-
ture of appropriate aldehyde compound (1 equiv), C60 (1.5
equiv), and N-methylglicine (10 equiv with respect to C60) was
refluxed for 24 h in anhydrous toluene (distilled from Na) under
nitrogen atmosphere. After cooling, the solvent was evaporated
and the residue was directly purified on silica gel column,
recovering the unreacted [60]fullerene as first fraction, the
desired reaction product as second one, and the unreacted
porphyrin as the third one.
ZnP-Oligo-PPE3-CHO (Zn6b). Yield: 95%. MS (MALDI):
m/z 1005 [M+]. 1H NMR (300 MHz, CDCl3): δ (ppm) 10.04 (s,
1H), 9.26 (s, 1H), 8.95 (s, 2H), 8.93 (s, 2H), 8.90 (d, 3J ) 5.3 Hz,
3
3
1H), 8.80 (d, J ) 5.3 Hz, 1H), 8.24 (br m, 10H), 7.90 (d, J )
8.3 Hz, 2H), 7.78 (b m, 10H), 7.71 (br d, 3J ) 8.3 Hz, 2H), 7.58
(s, 4H), 7.53 (d, J ) 8.3 Hz, 2H), 7.39 (d, J ) 8.3 Hz, 2H).
UV-vis (toluene): λmax (nm) (ε) ) 345 (60 000), 436 (298 000),
559 (24 300), 594 (12 700 mol-1 dm3 cm-1).
3
3
H2P-Oligo-PPE2-C60 (7a). Eluant mixture: toluene/petro-
leum ether (40-70°), 70:30. The product was recovered as
black-purple powder (yield 57%). MS (MALDI): m/z 1590 [M
+ H]+, 869 [M - 720]+, 720 [M - 869]+. 1H NMR (300 MHz,
ZnP-Oligo-PPE2-C60 (Zn7a). Yield: 97%. MS (MALDI):
1
m/z 1652 [M+], 932 [M - 720]+, 720 [M - 932]+. H NMR
3
CDCl3): δ (ppm) 9.09 (s, 1H), 8.89 (s, 2H), 8.83 (d, J ) 5.8
3
(300 MHz, CDCl3): δ (ppm) 9.26 (s, 1H), 8.94 (s, 2H), 8.91 (s,
2H), 8.88 (d, 3J ) 4.5 Hz, 1H), 8.77 (d, 3J ) 4.5 Hz, 1H), 8.19
(br m, 10H), 7.80-7.64 (series of broadened multiplets, 14H),
Hz, 1H), 8.77 (s, 2H), 8.75 (d, J ) 5.8 Hz, 1H), 8.21 (br m,
3
5H), 7.80-7.66 (br m, 19H), 7.48 (d, J ) 7.8 Hz, 2H), 7.34
3
2
(d, J ) 7.8 Hz, 2H), 4.92 (d, J ) 9.8 Hz, 1H), 4.84 (s, 1H),
4.16 (d, 2J ) 9.8 Hz, 1H), 2.80 (s, 3H), -2.67 (s, 2H). UV-vis
(toluene): λmax (nm) (ε) ) 323 (66 700), 430 (215 000), 524
(22 300), 560 (8100), 603 (6200), 659 (3800 mol-1 dm3 cm-1).
H2P-Oligo-PPE3-C60 (7b). Eluant mixture: toluene/petro-
leum ether (40-70°), 70:30. The product was recovered as black
powder (yield 52%). MS (MALDI): m/z 1690 [M + H]+, 969
[M - 720]+, 720 [M - 969]+. 1H NMR (300 MHz, CDCl3): δ
3
3
7.48 (d, J ) 8.3 Hz, 2H), 7.39 (d, J ) 8.3 Hz, 2H), 4.83 (d,
2J ) 9.8 Hz, 1H), 4.68 (s, 1H), 4.04 (d, 2J ) 9.8 Hz, 1H), 2.79
(s, 3H). UV-vis (toluene): λmax (nm) (ε) ) 324 (68 100), 436
(235 000), 559 (21 600), 594 (11 500 mol-1 dm3 cm-1).
ZnP-Oligo-PPE3-C60 (Zn7b). Yield: 96%. MS (MALDI):
m/z 1752 [M+], 1032 [M - 720]+, 720 [M - 1032]+. 1H NMR
(300 MHz, CDCl3): δ (ppm) 9.27 (s, 1H), 8.94 (s, 2H), 8.91 (s,
2H), 8.88 (d, 3J ) 4.5 Hz, 1H), 8.77 (d, 3J ) 4.5 Hz, 1H), 8.19
(br m, 10H), 7.80-7.63 (series of broadened multiplets, 14H),
3
(ppm) 9.10 (s, 1H), 8.89 (s, 2H), 8.85 (d, J ) 5.3 Hz, 1H),
3
8.78 (s, 2H), 8.75 (d, J ) 5.3 Hz, 1H), 8.23 (br m, 5H),

1792 J. Phys. Chem. A, Vol. 113, No. 9, 2009
Lembo et al.
3
3
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7.57 (d, J ) 8.3 Hz, 2H), 7.50 (d, J ) 7.5 Hz, 4H), 7.40 (d,
3J ) 8.3 Hz, 2H), 4.85 (d, 2J ) 9.8 Hz, 1H), 4.70 (s, 1H), 4.06
(d, 2J ) 9.8 Hz, 1H), 2.79 (s, 3H). UV-vis (toluene): λmax (nm)
(ε) ) 333 (96 100), 436 (281 000), 559 (23 800), 594 (12 900
mol-1 dm3 cm-1).
Acknowledgment. This work was supported by the Deutsche
Forschungsgemeinschaft DFG (SFB 583: Redoxaktive
MetallkomplexesReaktivita¨tssteuerung durch molekulare Ar-
chitektur), the FCI, and the Office of Basic Energy Sciences of
the U.S. Department of Energy.
1
Supporting Information Available: UV-vis, H NMR,
FAB MS and MALDI-TOF MS spectra, and cyclic voltammety
results. This material is available free of charge via the Internet
at http://pubs.acs.org.
(11) Winters, M. U.; Dahlstedt, E.; Blades, H. E.; Wilson, C. J.;
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(12) Pettersson, K.; Wiberg, J.; Ljungdahl, T.; Mårtensson, J.; Albinsson,
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