Meso-substituted porphyrin derivatives via palladium-catalyzed amination showing wide range visible absorption: Synthesis and photophysical studies

Article abstract of DOI:10.1021/ic300714n

In recent years, there has been a growing interest in the design and synthesis of chromophores, which absorb in a wide region of the visible spectrum, as these constitute promising candidates for use as sensitizers in various solar energy conversion schemes. In this work, a palladium-catalyzed coupling reaction was employed in the synthesis of molecular triads in which two porphyrin or boron dipyrrin (BDP) chromophores are linked to the meso positions of a central Zn porphyrin (PZn) ring via an amino group. In the resulting conjugates, which strongly absorb over most of the visible region, the electronic properties of the constituent chromophores are largely retained while detailed emission experiments reveal the energy transfer pathways that occur in each triad.

Full text of DOI:10.1021/ic300714n

Article
pubs.acs.org/IC
Meso-substituted Porphyrin Derivatives via Palladium-Catalyzed
Amination Showing Wide Range Visible Absorption: Synthesis and
Photophysical Studies
Kalliopi Ladomenou, Theodore Lazarides, Manas K. Panda, Georgios Charalambidis, Dimitra Daphnomili,
and Athanassios G. Coutsolelos*
University of Crete, Department of Chemistry, Laboratory of Bioinorganic Chemistry, PO Box 2208, Voutes Campus, 71003,
Heraklion, Greece
S
* Supporting Information
ABSTRACT: In recent years, there has been a growing interest in the
design and synthesis of chromophores, which absorb in a wide region of the
visible spectrum, as these constitute promising candidates for use as
sensitizers in various solar energy conversion schemes. In this work, a
palladium-catalyzed coupling reaction was employed in the synthesis of
molecular triads in which two porphyrin or boron dipyrrin (BDP)
chromophores are linked to the meso positions of a central Zn porphyrin
(PZn) ring via an amino group. In the resulting conjugates, which strongly
absorb over most of the visible region, the electronic properties of the
constituent chromophores are largely retained while detailed emission
experiments reveal the energy transfer pathways that occur in each triad.
often involves elaborate and multistep synthetic strategies and
tedious purification procedures. Furthermore, potential incom-
patibility between a component in the synthetic strategy and
the conditions used for cyclization greatly limits the functional
groups that can be present during synthesis. Therefore, the
functionalization on the central porphyrin was performed via
palladium-catalyzed amination to meso-brominated porphyrin.
In recent years, palladium catalyzed couplings have been used
extensively to prepare many novel mono and oligoporphyrins
for a variety of applications. For example, meso-linked oligomers
of various dimension,19−21 conjugated multichromophores,22,23
dendrimeric structures,24 and self-assembled arrays have been
reported.25 Most of these have involved carbon−carbon
couplings, but recently couplings with softer nucleophiles
have been achieved, for example, C−N, C−O, and C−S bond
formation to the porphyrin periphery.26−43 The above
approach allows the synthesis of a number of derivatives
using a single halogenated porphyrin under mild conditions and
high yields, and we expected compounds with larger absorption
bands in the visible region. All the examples of this kind of
compounds presented in the literature are focused mainly on
the synthesis and have not been photophysically studied.
With all the above in mind we synthesized C−N linked
trimeric molecules TPPH2-PZn-TPPH2 and BDP-PZn-BDP.
To the best of our knowledge, this is the first time that trimers
have been prepared and coupled with this type of linkage. This
INTRODUCTION
■
Porphyrins are one class of organic chromophores that are
widely studied due to their key roles in many biological
processes.1 Because of their attractive chemical properties, they
are used in electron and oxygen transfer,2,3 binding or transport
of small molecules,4,5 light harvesting and photosynthesis.6−9
Moreover, inspired by nature porphyrin molecules are widely
used as promising photosensitizers for dye-sensitized solar cells
(DSSCs).10−14 Researchers so far have synthesized various
substituted porphyrins and tested them as sensitizers for
DSSCs, but the conversion efficiencies remained below 8%.
Very recently, Gratzel and co-workers prepared a porphyrin dye
̈
with one of the best conversion efficiencies, greater than 12%.15
This dye was based on a donor-π-acceptor motif. In an effort to
prepare more efficient molecules we focused on changing the
peripheral substituents of the porphyrin and adding additional
chromophores to the main porphyrin core, in order to build
panchromatic dyes. Therefore, a central metalated porphyrin is
derivatized with (i) two free-base porphyrin molecules and (ii)
two boron dipyrrin (BDP) molecules forming trimeric
compounds. Free base porphyrins were selected because they
absorb light in different energy compared to metalated ones
resulting a trimer that will absorb in a wider range of the visible
region. On the other hand BDP are highly absorbing antenna
chromophores, which act as sensitizers of the porphyrin based
excited state. Moreover, BDP molecules combine a high
fluorescence quantum yield, a relatively long lifetime, a suitable
excited state energy and excellent photostability.16−18 Attach-
ment of different functional groups to the porphyrin periphery
Received: April 6, 2012
Published: September 28, 2012
© 2012 American Chemical Society
10548
dx.doi.org/10.1021/ic300714n | Inorg. Chem. 2012, 51, 10548−10556

Inorganic Chemistry
Article
Scheme 1. Synthesis of Trimer TPPH2−PZn−TPPH2
type of bond was chosen to connect other chromophores to the
meso position, which have different absorption and emission
properties of this type of porphyrins. The triads absorb light
over most of the visible region, and bear ester groups that can
be hydrolyzed to acid anchoring groups for further DSSC
studies. BDP-PZn-BDP is the most interesting of the two
trimers, where the BDP molecules supplement the porphyrin
absorption. Moreover, triad TPPH2-PZn-TPPH2 seemed
interesting because of different dynamic between the central
metalated mesosubstituted porphyrin and the two peripheral
free base porphyrins. In addition, a novel detailed photo-
physical study to understand a possible communication
between the different chromophores was performed.
modification in a 63% overall yield.44 Porphyrin PH2 was
prepared in one step by acid-catalyzed condensation of
dipyrromethane with 4-carboxymethylbenzaldehyde, followed
in situ oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoqui-
none (DDQ). Metalation with zinc acetate yielded porphyrin
PZn, and subsequent bromination with two equivalents of N-
bromosuccinimide (NBS) in the presence of pyridine gave
dibromo porphyrin Br2PZn in an excellent yield.45,46 Triad
porphyrins were successfully synthesized under palladium-
catalyzed double amination conditions with coupling of Zn-
meso-dibromoporphyrin Br2PZn (1 equiv) and three different
amines. The synthesis can be carried out under mild conditions,
typically giving high yields. All reactions were carried out in
THF at 68 °C under argon atmosphere with excess amine (4.8
equiv), and the use of 0.1 equiv of palladium acetate, 0.15 equiv
of bis(2-diphenylphosphinophenyl)ether (DPEphos) in the
presence of 2.8 equiv of cesium carbonate as a weak base. The
desired products were separated by flash column chromatog-
raphy and the excess of amines were recovered. First, Zn-meso-
dibrominated porphyrin (Br2PZn) was coupled with
NH2TPPH2 in 63% yield. The progress of the reaction was
monitored by TLC and after 4 days porphyrin Br2PZn had
been consumed forming the desired compound (TPPH2-PZn-
TPPH2). In addition a trace amount of monoamination
product was present. To the best of our knowledge, this is
the first time that a triad porphyrin has been synthesized linked
with a NH group. Then BDPNH2 (Scheme 2) was used as an
amine and triad porphyrin BDP-PZn-BDP was prepared in
high yield, 93%, the reaction was again monitored by TLC and
after 20 h all starting material had been consumed. Compound
BDPNH2 was synthesized according to the literature.47 Finally,
under the same conditions aniline was coupled with porphyrin
Br2PZn to obtain compound Ph-PZn-Ph in 91% yield. A trace
amount of monoamination byproduct was also present after the
flash column chromatography and was removed by washing the
product with CH2Cl2, since triad Ph-PZn-Ph is not soluble in
this solvent. Single crystals of this compound were obtained
suitable for diffraction analysis. All new compounds were
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The synthesis of all
compounds presented in this paper is shown in Schemes 1 and
2. NH2TPPH2 porphyrin was synthesized in two steps
following a literature procedure incorporated with our own
Scheme 2. Synthesis of Trimers BDP-PZn-BDP and Ph-
BDP-Ph
1
characterized on the basis of their H and 13C NMR spectra,
MALDI-TOF mass spectra and elemental analyses. Further
details on their synthesis and characterization can be found in
the Experimental Section.
As already mentioned all compounds were fully characterized
1
by NMR experiments including H, 13C, COSY, HMQC, and
10549
dx.doi.org/10.1021/ic300714n | Inorg. Chem. 2012, 51, 10548−10556

Inorganic Chemistry
Article
Figure 1. ORTEP view of molecular structure Ph-PZn-Ph·3THF. Thermal ellipsoids are drawn at 50% occupancy. Hydrogen atoms have been
omitted for clarity except amine hydrogen (labeled H1N) and one of the THF molecules found in the asymmetric unit. Atoms having the same label
are generated from each other by symmetry.
1
HMBC. In the H NMR (d8-THF) spectra of all trimeric
compounds the protons of the NH bridge are considerably
shifted downfield at 9.04, 9.54, and 9.19 ppm, respectively
compared to the free amino components. All synthesized
products can exist in a mixture of two atropisomers (αα- and
tetraphenyl pophyrin.48 Due to the coordination of Zn with
two THF molecules, Zn is in the plane of the four N atoms49
contrary to four or five coordinated Zn-porphyrins. The average
bond distance of Zn−N is 2.0645 Å. Two THF molecules are
weakly bound axially to Zn (Zn−O distance 2.403 Å). The
dihedral angle formed by the plane P1 (of the phenyl ring of the
aniline) to the P2 (plane of the porphyrin ring) is 81.51° (0.14)
and is slightly bigger than the other phenyl ring forms (73.99°
(0.12)). The torsion angle of aniline (C11−N3−Cmeso−Ca) is
88.77°.
Close intermolecular packing arrangement is observed,
mainly the short contact of 2.239 Å formed by the H of the
secondary amine with the carbonyl group of the ester of a
neighboring porphyrin ring. The hydrogen of the THF (H3)
form short contacts (2.366 Å) with the β-H9 and with aniline
group [H1s with C14(2.886 Å)] (Figure 2).
αβ-isomers), but in both H and 13C spectra only one set of
1
resonances were observed indicating that there is a free rotation
around the C−N bond at ambient temperature, as has been
reported in the literature.29 In the case of trimer TPPH2-PZn-
TPPH2 we observed a split of the peak corresponding to the
NH bridge whereas for compounds BDP-PZn-BDP and Ph-
PZn-Ph only a singlet peak was present (Supporting
Information). Possibly, in the case of TPPH2-PZn-TPPH2,
where three porphyrins are linked, the rotation around the
bond between the amino nitrogen atom and the porphyrin
meso-carbon atom has a higher rotation barrier compared to the
other two trimers.
Single-Crystal X-ray Diffraction Structures. An ORTEP
of Ph-PZn-Ph is depicted in Figure 1 and selected bonds and
angles in Supporting Information Table S1. The porphyrin
plane displays a slight ruffling from planarity (0.0336) and the
distances and angles of Ph-PZn-Ph are similar to Zn-
Electrochemistry. Cyclic voltammetry was used in order to
study the electrochemical properties of the trimers. For trimer
TPPH2-PZn-TPPH2 it was not possible to obtain any
voltammograms in either CH2Cl2, THF, or acetonitrile because
of stability and/or solubility difficulties. Trimer BDP-PZn-BDP
was studied in CH2Cl2 and Ph-PZn-Ph in THF, all data are
10550
dx.doi.org/10.1021/ic300714n | Inorg. Chem. 2012, 51, 10548−10556

Inorganic Chemistry
Article
Figure 2. Packing arrangement of trimeric compound Ph-PZn-Ph. The packing was viewed along a axis.
reported vs the ferrocene/ferrocenium couple (Fc/Fc+). Cyclic
voltammograms are shown in Figure 3. BDPNH2 in CH2Cl2
showed a reversible oxidation at 0.76 V due to formation of the
BDP cationic radical (Figure 3c).50,51 Reference triad Ph-PZn-
Ph in THF showed one reversible oxidation at −0.01 V,
attributed to the oxidation of the porphyrin ring1 (Figure 3b).
The cyclic voltammetry of BDP-PZn-BDP shows one
reversible process at 0.04 V and one irreversible at 0.76 V.
The first one is attributed to the first porphyrin oxidation and
the second to the oxidation of the BDP, respectively (Figure
3a). This feature is also corroborated by DFT studies. The
HOMO localized on Zn-porphyrin corresponds to the first
oxidation of BDP-PZn-BDP, while HOMO-1 localized on the
BDP unit, and corresponds to the second oxidation. In addition
triad BDP-PZn-BDP shows one reversible process at −1.74 V,
attributed to BDP reduction (Supporting Information Figure
S9 and Table S2). Porphyrin reductions in the case of BDP-
PZn-BDP and Ph-PZn-Ph could not be observed.
Photophysical Properties. Electronic Absorption Spec-
tra. The electronic absorption spectra of compounds
NH2TPPH2, TPPH2-PZn-TPPH2, and Ph-PZn-Ph are pre-
sented in Figure 4, and those of compounds BDPNH2, BDP-
PZn-BDP, and Ph-PZn-Ph in Figure 5. All absorption spectra
were measured in toluene. The absorption maxima and molar
absorption coefficients (ε) of the Soret and Q bands are listed
in Table 1. Compound NH2TPPH2 exhibits typical free-base
porphyrin absorption features1 with an intense Soret band at
422 nm and moderate Q bands at 516, 552, 594, and 650 nm.
Ph-PZn-Ph shows the characteristic bands of Zn-meso-
arylamino substituted porphyrins with a Soret at 444 nm and
two Q bands at 561 and 614 nm.29 The spectrum of triad
TPPH2-PZn-TPPH2 shows the characteristic absorptions of
Figure 3. Cyclic voltammograms of (a) BDP-PZn-BDP in CH2Cl2,
(b) Ph-PZn-Ph in THF and (c) BDPNH2 in CH2Cl2 containing 0.1
M (TBA)PF6 as supporting electrolyte. The voltages are vs Fc/Fc+.
10551
dx.doi.org/10.1021/ic300714n | Inorg. Chem. 2012, 51, 10548−10556

Inorganic Chemistry
Article
based peaks (Soret at 445 nm and Q bands at 561, 614 nm).50
Therefore, in triad BDP-PZn-BDP, as in TPPH2-PZn-TPPH2,
there is weak electronic interaction between the two side BDP
molecules and the central Zn-porphyrin in the ground state.
Emission Spectra. Steady state and time-resolved emission
spectroscopy in toluene at ambient temperature was used in
order to study all trimeric compounds. A summary of measured
spectroscopic data is presented in Table 1. BDPNH2 shows the
intense fluorescence of a BDP dye at 519 nm in toluene.
NH2TPPH2 displays two emission bands at 661 and 722 nm
(Supporting Information Figure S10). The reference porphyrin
Ph-PZn-Ph exhibits a broad fluorescence band with maximum
at 698 nm and with a lifetime of τ1 = 3.5 ns. Triad TPPH2-
PZn-TPPH2 when irradiated at 517 nm, corresponding to
selective excitation of NH2TPPH2, shows emission which
closely resembles that of the free base porphyrin chromophore
at 661 and 719 nm (Figure 6a). Also, when excited at 467 nm
Figure 4. UV−vis absorption spectra of NH2TPPH2, TPPH2-PZn-
TPPH2, and Ph-PZn-Ph in toluene and expansion of the Q bands.
Figure 5. UV−vis absorption spectra of BDPNH2, BDP-PZn-BDP,
and Ph-PZn-Ph in toluene.
Table 1. Spectroscopic Data of Trimeric Compounds and
NH2TPPH2 and BDPNH2
absorption
λmax/nm (ε/mM−1cm−1
emission
Φ
compound
)
λmax/nm
τ/ns
TPPH2-PZn- 420 (571.6), 445 sh (213.6) 661,
0.12
2.9, 9.8
TPPH2
516 (17.2), 556 (15.0),
595 (8.7), 650 (6.6)
719
BDP-PZn-
BDP
445 (119.8), 503 (113.9),
561 (11.0), 614 (8.1)
518,
684
0.073
0.11
0.11
3.8
3.5
8.8
Ph-PZn-Ph
444 (138.9), 561 (12.5),
614 (8.5)
698
NH2TPPH2
422 (258.2), 516 (12.5),
552 (6.8), 594 (3.6),
650 (2.8)
661,
722
Figure 6. (a) Emission spectra of TPPH2-PZn-TPPH2 exciting at 517
nm (peripheral porphyrin) and 467 nm (central porphyrin) and (b)
excitation spectrum of TPPH2-PZn-TPPH2 monitoring at 661 nm at
room temperature in toluene.
a
a
BDPNH2
505 (92.1)
519
0.38
3.1
a
Values were taken from ref 51.
corresponding mainly to the central Zn porphyrin, an almost
identical spectrum was observed (Figure 6a). The excitation
spectrum of TPPH2-PZn-TPPH2 monitoring at the emission
of the free base side porphyrins at 661 nm shows absorption
features from both chromophores constituting TPPH2-PZn-
TPPH2 (Figure 6b). In addition, time-resolved emission
measurements with pulsed excitation of mainly the peripheral
porphyrins at 400 nm, show that TPPH2-PZn-TPPH2 exhibits
a dual exponential decay with lifetimes of 2.9 and 9.8 ns
respectively. By comparison with the fluorescence lifetimes of
reference compound Ph-PZn-Ph and NH2TPPH2 we attribute
the short lifetime of TPPH2-PZn-TPPH2 to the central Zn-
porphyrin and the longer one to the peripheral freebase
porphyrins. Combination of the results from the excitation
spectrum and the time-resolved fluorescence measurements of
the individual macrocyclic components, namely strong
absorptions assigned to the Zn-meso-arylamino substituted
porphyrin and the NH2TPPH2 chromophores (Figure 4).
Triad TPPH2-PZn-TPPH2 displays a broad Soret band
centered at ∼420 nm with a shoulder on its low energy side
(∼445 nm) attributable to the central porphyrin of the triad.
Thus the absorption spectrum of TPPH2-PZn-TPPH2 is
largely the sum of those of its components indicating that
there is little electronic interaction between the central and the
peripheral porphyrin chromophores in the ground state.
Similarly, the absorption spectrum of triad BDP-PZn-BDP
(Figure 5) clearly shows features attributed to both of its
components namely an intense absorption at 503 nm because
of the BDP chromophores and the characteristic porphyrin
10552
dx.doi.org/10.1021/ic300714n | Inorg. Chem. 2012, 51, 10548−10556

Inorganic Chemistry
Article
TPPH2-PZn-TPPH2 show that excitation of either of the
chromophores of TPPH2-PZn-TPPH2 into its first singlet
excited state results in partial energy transfer to the first singlet
excited state of the other chromophore. This is expected
because, as shown by the emission spectra of TPPH2-PZn-
TPPH2 and Ph-PZn-Ph in toluene glass at 77 K (Supporting
Information Figure S11), the first singlet excited states of the
freebase (1.88 eV) and Zn-porphyrin chromophores (1.82 eV)
of TPPH2-PZn-TPPH2, are almost isoenergetic (energy
deference of ca. 500 cm−1). In the case of triad BDP-PZn-
BDP, irradiation at 491 nm, corresponding to selective
excitation of the two BDP units, results in emission from the
Zn-porphyrin chromophore at 684 nm, as well as residual
fluorescence from the two BDP groups at 518 nm (Figure 7a).
The BDP based emission in BDP-PZn-BDP is strongly
quenched when compared to the emission measured for
BDPNH2. Moreover, the excitation spectrum of BDP-PZn-
BDP monitoring at the fluorescence of the porphyrin moiety at
685 nm shows an intense BDP absorption feature at 499 nm
(Figure 7b). This is a clear indication of BDP to porphyrin
singlet energy transfer following excitation of the BDP unit.
DFT Results. To obtain further insight about the electronic
and orbital properties, density functional theory (DFT)
calculations were carried out on TPPH2-PZn-TPPH2, BDP-
PZn-BDP, and the reference compound Ph-PZn-Ph. The
geometry optimized structures of compounds TPPH2-PZn-
TPPH2, BDP-PZn-BDP, and Ph-PZn-Ph are shown in Figures
8 and 9 and Supporting Information Figure S12, respectively. A
Figure 9. Gas phase geometry optimized (B3LYP/6-31G*) structure
of BDP-PZn-BDP. Hydrogen atoms on the carbon have been omitted
for clarity.
Figure 7. (a) Emission spectrum of BDP-PZn-BDP exciting at 491
nm (BDP chromophore) and (b) excitation spectrum of BDP-PZn-
BDP monitoring at 685 nm at room temperature in toluene.
comparison of bond lengths in optimized TPPH2-PZn-TPPH2
and BDP-PZn-BDP with X-ray data is presented in Table S3
(Supporting Information). Coordinates of the optimized
Figure 8. Gas phase geometry optimized (B3LYP/6-31G*) structure of TPPH2-PZn-TPPH2. Hydrogen atoms on the carbon have been omitted for
clarity.
10553
dx.doi.org/10.1021/ic300714n | Inorg. Chem. 2012, 51, 10548−10556

Inorganic Chemistry
Article
Mass Spectra. High-resolution mass spectra were performed on a
Bruker ultrafleXtreme MALDI-TOF/TOF spectrometer.
structures and their energies are given in Supporting
Information Tables S4−S6. The electron density maps and
the energies of the frontier molecular orbitals (FMOs) are
shown in Supporting Information Figures S13 and S14. The
geometry optimized structure of TPPH2-PZn-TPPH2 revealed
two trans oriented terminal porphyrins in the two sides of the
central Zn-porphyrin ring (Figure 8). As observed from the
Supporting Information Figure S13, the HOMO and HOMO-1
of the triad TPPH2-PZn-TPPH2 are located on the terminal
free base porphyrin moieties, with a partial delocalization
through central Zn-porphyrin, while LUMO and LUMO + 1
orbitals are localized on the central Zn-porphyrin. This suggests
that, very weak electronic communication exists between the
chromophores in TPPH2-PZn-TPPH2.
In the case of BDP-PZn-BDP, HOMO is localized mainly on
the central Zn-porphyrin with a little delocalization into the
phenyl ring of the BDP units, while both HOMO − 1 and
HOMO-2 are localized on two BDP units. In order to gain
better insight into the electronic transitions we have carried out
TDDFT calculation adopting coordinates from geometry
optimized structures. The TDDFT calculation of BDP-PZn-
BDP exhibited peaks at 597 and 592 nm (oscillator strengths
0.108 and 0.192 respectively), which corresponds to the Q
bands of the porphyrin and assigned to a π−π* transition of the
ring having major contribution from HOMO → LUMO + 1,
HOMO → LUMO (Figure S15, Supporting Information).
BDP π−π* transitions were observed much more blue-shifted
at 474 nm (oscillator strength 0.430). Notably, the TDDFT
calculated blue shift of BDP transitions was also reported
earlier.52,53 The calculated porphyrin soret band of the BDP-
PZn-BDP appeared at 422 nm (oscillator strength 0.789)
involving major contribution from HOMO − 3 → LUMO + 1.
In the case of TPPH2-PZn-TPPH2, our approach to simulate
vertical transitions in gas phase did not produce satisfactory
results, while the solvent phase calculation was unsuccessful
even after several attempts.
X-ray Crystallography. Single crystals for Ph-PZn-Ph were
obtained by slow evaporation of a THF/pentane mixture (1:1 v/v).
The crystals were protected with paratone-N and were mounted for
data collection. Intensity data were collected at STOE IPDSII
diffractometer using graphite monochromated Mo−Kα radiation (λ
= 0.71070 Å). The data were collected at 300 K using two ω scans
with an increment of 1°, an exposure time of 2 min for each oscillation
and distance of 100 mm. Data reduction including absorption
correction and cell dimension post refinement were performed using
X-Area software package. The structure was solved using SIR 9255
program and refined by full matrix least-squares on F2 values for all
reflections using (SHELX-S97). All nonhydrogen atoms were refined
with anisotropic displacement parameters using SHELXL-97.56 All the
hydrogen atoms were introduced at calculated positions as riding on
bonded atoms. The deposited CIF remains in the Supplementary Data
Archive and is made freely available for bona fide research purposes via
http://www.ccdc.cam.ac.uk/data_request/cif.
Electrochemistry. Cyclic and square wave voltammetry experi-
ments were carried out at room temperature using an AutoLab
PGSTAT20 potentiostat and appropriate routines available in the
operating software (GPES, version 4.9). All measurements were
carried out in freshly distilled and deoxygenated dichloromethane and
THF with a solute concentration of ∼1.0 mM in the presence of
tetrabutylammonium hexafluorophosphate (0.1 M) as supporting
electrolyte. A three-electrode cell setup was used with a platinum
working electrode, a saturated calomel (SCE) reference electrode, and
a platinum wire as counter electrode. All potentials are reported versus
the ferrocene/ferrocenium couple (0.44 V versus SCE under the above
conditions).
Photophysical Measurements. UV−vis absorption spectra were
measured on a Shimadzu UV-1700 spectrophotometer using 10 mm
path-length cuvettes. The emission spectra were measured on a
JASCO FP-6500 fluorescence spectrophotometer equipped with a red-
sensitive WRE-343 photomultiplier tube (wavelength range 200−850
nm). Quantum yields were determined from corrected emission
spectra following the standard methods57 using meso-tetraphenylpor-
phyrin (TPP) (Φ = 0.11 in toluene58) or zinc meso-tetraphenylpor-
phyrin (ZnTPP) (Φ = 0.03 in toluene58) as standards. Emission
lifetimes were determined by the time-correlated single-photon
counting technique using an Edinburgh Instruments mini-tau lifetime
spectrophotometer equipped with an EPL 405 pulsed diode laser at
406.0 nm with a pulse width of 71.52 ps and pulse periods of 200 and
100 ns and a high-speed red sensitive photomultiplier tube (H5773−
04) as detector.
Computational Methods. Theoretical calculations of the trimeric
compounds were performed using density functional theory (DFT)59
on GAUSSIAN 0360 program. Gas phase geometry optimization of the
compounds was carried out employing Becke three parameter
exchange functional in conjunction with Lee−Yang−Parr correlation
functional (B3LYP).61,62 6-31G(d) basis sets was used for lighter
atoms and LANL2DZ basis set was used for Zn atom. The geometry
optimization of reference compound Ph-PZn-Ph was carried out
adopting the coordinates from the X-ray structure, while input
structure of the TPPH2-PZn-TPPH2 and BDP-PZn-BDP was
modeled on the optimized structure of the Ph-PZn-Ph using
ChemCraft software (version 1.6). The global minimum of the
optimized structure was confirmed by observation of no negative
frequencies in frequency calculation. Single point calculations of all the
compounds were carried out employing the geometry optimized
coordinates. TDDFT calculations were performed in THF solvent
using polarizable continuum model (PCM) implemented on Gaussian
03.63 All the computed structures and the orbitals were visualized by
ChemCraft software (version 1.6).
CONCLUSIONS
■
Novel triads were synthesized successfully via a palladium-
catalyzed amination reaction in high yields. To the best of our
knowledge, this is the first time that triads with an NH linkage
on the porphyrin meso position have been prepared and
photophysically studied. We believe that due to the facile
preparation and the mild conditions involved this synthetic
method can be applied to the synthesis of multichromophoric
compounds. X-ray crystallography of compound Ph-PZn-Ph
revealed the structure of this type of triads. Photophysical
studies of triad BDP-PZn-BDP showed that selective excitation
of the BDP chromophores into their first singlet excited state is
followed by energy transfer to the first singlet excited state of
the central Zn-porphyrin. Finally, the architecture of triads
TPPH2-PZn-TPPH2 and BDP-PZn-BDP makes them promis-
ing sensitizers for future studies of their photovoltaic properties
into DSSCs.
EXPERIMENTAL SECTION
■
Materials. BDPNH2 and porphyrins PH2, PZn, and Br2PZn were
prepared according to the literature.46,47 All dry solvents used were
dried by the appropriate techniques.54
5-(4-Aminophenyl)-10,15,20-triphenylporphyrin
(NH2TPPH2). Tetraphenyl porphyrin (2 g, 3.25 mmol) was dissolved
in dichloromethane (300 mL). Nitric acid 65% (4.2 mL, 61.75 mmol)
was added with a dropping funnel at 0 °C over a 2 h period. The
reaction was monitored by TLC and when it was converted to the
1
NMR Spectra. H NMR spectra were recorded on Bruker AMX-
500 MHz and Bruker DPX-300 MHz spectrometers as solutions in
deuterated solvents by using the solvent peak as the internal standard.
10554
dx.doi.org/10.1021/ic300714n | Inorg. Chem. 2012, 51, 10548−10556

Inorganic Chemistry
Article
desired product the solution was washed 3 × 150 mL with saturated
solution of NaHCO3 and then 3 × 150 mL with H2O and dried using
sodium sulfate. The solvent was removed under reduced pressure and
the crude product was dissolved in HCl (65 mL), tin chloride (2 g,
10.5 mmol) was added and the solution was refluxed overnight. The
mixture was neutralized by addition of aqueous ammonia, and washed
with ethyl acetate (5 × 150 mL) dried and solvent was removed. The
product was isolated by silica column chromatography CH2Cl2 to
6.84 (d, J = 7.5, 4 H), 6.65 (m, 2 H), 4.04 (s, 6 H). 13C NMR (75
MHz, d8-THF): δ 167.5, 155.2, 152.4, 150.2, 149.3, 135.6, 132.1,
130.5, 130.3, 129.8, 128.5, 120.5, 120.2, 118.6, 115.3, 52.5. UV/vis
(toluene) λmax, nm (ε, mM−1 cm−1): 444 (138.9), 561 (12.5), 614
(8.5). HRMS (MALDI-TOF) calcd for C48H34N6O4Zn [M]+:
822.1933, found 822.1929. Anal. Calcd for C48H34N6O4Zn: C,
69.95; H, 4.16; N, 10.20. Found: C, 69.87; H, 4.22; N, 10.12.
1
obtain the desired product as a purple solid (1.3 g, 63%). H NMR
ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic data for Ph-PZn-Ph in CIF format; H
■
(500 MHz, CDCl3): δ 8.96 (d, J = 4.5 Hz, 2 H), 8.86 (s, 6 H), 8.24
(m, 6 H), 8.00 (d, J = 8.5, 2 H), 7.77 (m, 9H), 7.05 (d, J = 8.5, 2 H),
3.99 (s, 2H), −2.71 (s, 2 H). 13C NMR (75 MHz, CDCl3): δ 146.2,
142.4, 135.8, 134.7, 132.5,131.2, 127.8, 126.8, 121.0, 120.1, 119.9,
113.6. UV/vis (toluene) λmax, nm (ε, mM−1 cm−1) 422 (258.2), 516
(12.5), 552 (6.8), 594 (3.6), 650 (2.8). HRMS (MALDI-TOF) calcd
for C44H32N5 [M + H]+ 630.2658, found 630.2662. Anal. Calcd for
C44H31N5: C, 83.92; H, 4.96; N, 11.12. Found: C, 83.85; H, 4.89; N,
11.26.
S
1
NMR and 13C NMR of NH2TPPNH2, TPPH2-PZn-TPPH2,
BDP-PZn-BDP, Ph-PZn-Ph; selected bond lengths and angles;
redox data of compounds BDP-PZn-BDP, Ph-PZn-Ph and
BDPNH2; cyclic voltammograms of BDP-PZn-BDP and
BDPNH2; normalized room temperature emission spectra of
BDPNH2, Ph-PZn-Ph, NH2TPPH2; normalized emission
spectra of Ph-PZn-Ph and TPPH2-PZn-TPPH2 at 77 K; gas
phase B3LYP/6-31G* optimized structure of Ph-PZn-Ph;
frontier orbital energy levels for compounds TPPH2-PZn-
TPPH2, BDP-PZn-BDP; TDDFT computed vertical tran-
sitions for BDP-PZn-BDP; Bond lengths obtained from of
theoretical calculation and X-ray structure for compound
TPPH2-PZn-TPPH2 and BDP-PZn-BDP; and Cartesian
coordinates of the B3LYP/6-31G* optimized structure of
TPPH2-PZn-TPPH2, BDP-PZn-BDP and Ph-PZn-Ph This
material is available free of charge via the Internet at http://
pubs.acs.org. CCDC staff scan major chemistry and crystallog-
raphy journals; printed deposition numbers CCDC 894217 are
noted and the deposited data are processed to the Cambridge
Structural Database in conjunction with the printed publication.
Synthesis of Trimer Porphyrin TPPH2-PZn-TPPH2. Zn-meso-
dibromoporphyrin Br2PZn (48 mg, 0.06 mmol), palladium acetate
(1.3 mg, 0.006 mmol), bis(2-diphenylphosphinophenyl)ether (DPE-
phos) (4.8 mg, 0.009 mmol), and cesium carbonate (55 mg, 0.17
mmol) were added in a dry Schlenk tube. Then monoaminophenyl
porphyrin (NH2TPPH2) (181 mg, 0.29 mmol) and dry THF (6 mL)
were added. The mixture was refluxed at 68 °C under argon for 4 days.
The reaction was monitored by TLC. The desired compound was
isolated by silica column chromatography THF:Hex (3/7) to obtain
1
TPPH2-PZn-TPPH2 as a purple solid (71 mg, 63%). H NMR (300
MHz, d8-THF): δ 9.77 (d, J = 4.5 Hz, 4H), 9.04 (d, 2H), 8.89 (d, J =
4.8 Hz, 4H), 8.79 (m, 16H), 8.48 (d, J = 8.1 Hz, 4H), 8.40 (d, J = 8.1
Hz, 4H), 8.19 (m, 12H), 7.97 (d, J = 8.4 Hz, 4H), 7.75 (m, 18H), 7.30
(d, J = 8.4 Hz, 4H), 4.10 (s, 6H), −2.67 (s, 4H). 13C NMR (75 MHz,
d8-THF): δ 167.5, 155.0, 152.6, 150.5, 149.3, 146.5, 143.6, 143.5,
136.7, 136.4, 135.5, 133.1, 132.6, 131.8, 130.9, 130.5, 129.6, 128.7,
127.7, 126.1, 122.5, 121.2, 120.9, 120.3, 113.6, 52.5. UV/vis (toluene)
λmax: nm (ε, mM−1 cm−1) 420 (571.6), 445 sh (213.6), 516 (17.2),
556 (15.0), 595 (8.7), 650 (6.6). HRMS (MALDI-TOF) calcd for
C124H83N14O4Zn [M + H]+: 1895.6013, found 1895.6009. Anal. Calcd
for C124H82N14O4Zn: C, 78.49; H, 4.36; N, 10.33. Found: C, 78.55; H,
4.32; N, 10.26.
AUTHOR INFORMATION
Corresponding Author
*E-mail: coutsole@chemistry.uoc.gr.
■
Notes
The authors declare no competing financial interest.
Synthesis of Porphyrin BDP-PZn-BDP. Zn-meso-dibromopor-
phyrin Br2PZn (32 mg, 0.04 mmol), palladium acetate (0.9 mg, 0.004
mmol), bis(2-diphenylphosphinophenyl)ether (DPEphos) (3.2 mg,
0.006 mmol) and cesium carbonate (36 mg, 0.11 mmol) were added
in a dry Schlenk tube. Then BDPNH2 (65 mg, 0.19 mmol) and dry
THF (5 mL) were added. The mixture was refluxed at 68 °C under
argon for 20 h. The reaction was monitored by TLC. The desired
compound was isolated by column chromatography THF/Hex (3/7)
ACKNOWLEDGMENTS
■
The European Commission funded this research by FP7-
REGPOT-2008-1, Project BIOSOLENUTI No 229927,
Heraklitos grant from Ministry of Education, and GSRT. We
thank Catherina Raptopoulou for helpful discussions for the x-
ray structure.
1
to obtain BDP-PZn-BDP as a purple solid (49 mg, 93%). H NMR
(500 MHz, d8-THF): δ 9.54 (s, 2H), 9.40 (d, J = 4.5 Hz, 4H), 8.71 (d,
J = 4.5 Hz, 4H), 8.41 (d, J = 8.0 Hz, 4H), 8.28 (d, J = 8.0 Hz, 4H),
7.01 (m, 8H), 6.00 (s, 4H), 4.06 (s, 6H), 2.45 (s, 12H), 1.69 (s, 12H).
13C NMR (75 MHz, d8-THF): δ 167.4, 155.9, 155.5, 152.1, 150.3,
148.9, 144.2, 143.6, 135.4, 133.0, 132.3, 130.5, 130.0, 129.8, 128.4,
124.7, 121.5, 120.3, 119.7, 115.6, 52.4, 15.0, 14.6. UV/vis (toluene)
λmax, nm (ε, mM−1 cm−1): 445 (119.8), 503 (113.9), 561 (11.0), 614
(8.1). HRMS (MALDI-TOF) calcd for C74H60B2F4N10O4Zn [M]+:
1314.4213, found 1314.4217. Anal. Calcd for C74H60B2F4N10O4Zn: C,
67.52; H, 4.59; N, 10.64. Found: C, 67.63; H, 4.46; N, 10.52.
DEDICATION
■
The authors would like to dedicate this manuscript in
memoriam of Ivano Bertini, who was an excellent friend,
advisor, and co-worker throughout all these years.
REFERENCES
■
(1) Kadish, K. M.; Smith, K. M.; Guilard, R. The Porphyrin Handbook;
Academic Press: San Diego, CA, 2000.
(2) Takai, A.; Chkounda, M.; Eggenspiller, A.; Gros, C. P.; Lachkar,
M.; Barbe, J.-M.; Fukuzumi, S. J. Am. Chem. Soc. 2010, 132, 4477−
4489.
(3) Jurow, M.; Schuckman, A. E.; Batteas, J. D.; Drain, C. M. Coord.
Chem. Rev. 2010, 254, 2297−2310.
(4) Collman, J. P.; Brauman, J. I.; Doxsee, K. M.; Halbert, T. R.;
Hayes, S. E.; Suslick, K. S. J. Am. Chem. Soc. 1978, 100, 2761−2766.
(5) Ladomenou, K.; Bonar-Law, R. P. Chem. Commun. 2002, 2108.
(6) Wasielewski, M. R. Chem. Rev. 1992, 92, 435−461.
(7) Garg, V.; Kodis, G.; Chachisvilis, M.; Hambourger, M.; Moore, A.
L.; Moore, T. A.; Gust, D. J. Am. Chem. Soc. 2011, 133, 2944−2954.
Synthesis of Porphyrin Ph-PZn-Ph. Zn-meso-dibromoporphyrin
Br2PZn (32 mg, 0.04 mmol), palladium acetate (0.9 mg, 0.004 mmol),
bis(2-diphenylphosphinophenyl)ether (DPEphos) (3.2 mg, 0.006
mmol), and cesium carbonate (36 mg, 0.11 mmol) were added in a
dry Schlenk tube. Then aniline (18 μL, 0.19 mmol) and dry THF (5
mL) were added. The mixture was refluxed at 68 °C under argon for
15 h. The reaction was monitored by TLC. The desired compound
was isolated by column chromatography THF:Hex (2/8) to obtain
1
Ph-PZn-Ph as a purple solid (30 mg, 91%). H NMR (500 MHz, d8-
THF): δ 9.39 (d, J = 4.5 Hz, 4 H), 9.19 (s, 2 H), 8.67 (d, J = 4.5 Hz,
4H), 8.39 (d, J = 7.5 Hz, 4H), 8.27 (d, J = 7.5 Hz, 4 H), 7.04 (m, 4 H),
10555
dx.doi.org/10.1021/ic300714n | Inorg. Chem. 2012, 51, 10548−10556

Inorganic Chemistry
Article
(8) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2009, 42,
1890−1898.
(9) D’Souza, F.; Amin, A. N.; El-Khouly, M. E.; Subbaiyan, N. K.;
Zandler, M. E.; Fukuzumi, S. J. Am. Chem. Soc. 2011, 134, 654−664.
(10) Ooyama, Y.; Harima, Y. Eur. J. Org. Chem. 2009, 2009, 2903−
2934.
(39) Pereira, A. M. V. M; Neves, M. G. P. M. S.; Cavaleiro, J. A. S.;
Jeandon, C.; Gisselbrecht, J.-P.; Choua, S.; Ruppert, R. Org. Lett. 2011,
13, 4742−4745.
(40) Khan, M. M.; Ali, H.; van Lier, J. E. Tetrahedron Lett. 2001, 42,
1615−1617.
(41) Takanami, T.; Hayashi, M.; Hino, F.; Suda, K. Tetrahedron Lett.
2003, 44, 7353−7357.
(11) Imahori, H.; Umeyama, T.; Ito, S. Acc. Chem. Res. 2009, 42,
(42) Chen, Y.; Gao, G.-Y.; Peter Zhang, X. Tetrahedron Lett. 2005,
46, 4965−4969.
1809−1818.
(12) Wang, X.-F.; Tamiaki, H. Energy Environ. Sci. 2010, 3, 94−106.
(13) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H.
Chem. Rev. 2010, 110, 6595−6663.
(43) Esdaile, L. J.; McMurtrie, J. C.; Turner, P.; Arnold, D. P.
Tetrahedron Lett. 2005, 46, 6931−6935.
(44) Kruper, W. J.; Chamberlin, T. A.; Kochanny, M. J. Org. Chem.
1989, 54, 2753−2756.
(14) Martinez-Diaz, M. V.; de la Torre, G.; Torres, T. Chem.
Commun. 2010, 46, 7090−7108.
(45) DiMagno, S. G.; Lin, V. S. Y.; Therien, M. J. J. Org. Chem. 1993,
58, 5983−5993.
(15) Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.;
Nazeeruddin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.;
(46) Mikroyannidis, J. A.; Charalambidis, G.; Coutsolelos, A. G.;
Balraju, P.; Sharma, G. D. J. Power Sources 2011, 196, 6622−6628.
(47) Imahori, H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki,
I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2000, 123, 100−110.
(48) Golder, A. J.; Povey, D. C.; Silver, J.; Jassim, Q. A. A. Acta
Crystallogr., Sect. C 1990, 46, 1210−1212.
Gratzel, M. Science 2011, 334, 629−634.
̈
(16) Maligaspe, E.; Kumpulainen, T.; Subbaiyan, N. K.; Zandler, M.
E.; Lemmetyinen, H.; Tkachenko, N. V.; D’Souza, F. Phys. Chem.
Chem. Phys. 2010, 12, 7434−7444.
(17) Rio, Y.; Seitz, W.; Gouloumis, A.; Vaz
Guldi, D. M.; Torres, T. Chem.Eur. J. 2010, 16, 1929−1940.
(18) Lazarides, T.; Charalambidis, G.; Vuillamy, A.; Reglier, M.;
́
quez, P.; Sessler, J. L.;
(49) Schauer, C. K.; Anderson, O. P.; Eaton, S. S.; Eaton, G. R. Inorg.
Chem. 1985, 24, 4082−4086.
́
Klontzas, E.; Froudakis, G.; Kuhri, S.; Guldi, D. M.; Coutsolelos, A. G.
Inorg. Chem. 2011, 50, 8926−8936.
(50) Mula, S.; Elliott, K.; Harriman, A.; Ziessel, R. J. Phys. Chem. A
2010, 114, 10515−10522.
(19) Aratani, N.; Osuka, A.; Kim, Y. H.; Jeong, D. H.; Kim, D. Angew.
Chem., Int. Ed. 2000, 39, 1458−1462.
(51) Matsuzaki, Y.; Nogami, A.; Tsuda, A.; Osuka, A.; Tanaka, K. J.
Phys. Chem. A 2006, 110, 4888−4899.
(20) Aratani, N.; Kim, D.; Osuka, A. Acc. Chem. Res. 2009, 42, 1922−
1934.
(52) Sabatini, R. P.; McCormick, T. M.; Lazarides, T.; Wilson, K. C.;
Eisenberg, R.; McCamant, D. W. J. Phys. Chem. Lett. 2011, 2, 223−
227.
(21) Holten, D.; Bocian, D. F.; Lindsey, J. S. Acc. Chem. Res. 2001, 35,
(53) Lazarides, T.; Kuhri, S.; Charalambidis, G.; Panda, M. K.; Guldi,
D. M.; Coutsolelos, A. G. Inorg. Chem. 2012, 51, 4193−4204.
(54) Armarego, W. L. F.; Chai, C. Purification of Laboratory
Chemicals, 6th ed.; Elsevier Inc.: Oxford, 2009.
(55) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A. J.
Appl. Crystallogr. 1993, 26, 343−350.
57−69.
(22) Lin, V.; DiMagno, S.; Therien, M. Science 1994, 264, 1105−
1111.
(23) Susumu, K.; Therien, M. J. J. Am. Chem. Soc. 2002, 124, 8550−
8552.
(24) Fukuzumi, S.; Saito, K.; Ohkubo, K.; Khoury, T.; Kashiwagi, Y.;
Absalom, M. A.; Gadde, S.; D’Souza, F.; Araki, Y.; Ito, O.; Crossley, M.
J. Chem. Commun. 2011, 47, 7980−7982.
(25) Milic, T.; Garno, J. C.; Batteas, J. D.; Smeureanu, G.; Drain, C.
M. Langmuir 2004, 20, 3974−3983.
(26) Esdaile, L. J.; Senge, M. O.; Arnold, D. P. Chem. Commun. 2006,
4192−4194.
(27) Matano, Y.; Matsumoto, K.; Terasaka, Y.; Hotta, H.; Araki, Y.;
Ito, O.; Shiro, M.; Sasamori, T.; Tokitoh, N.; Imahori, H. Chem.Eur.
J. 2007, 13, 891−901.
(28) Atefi, F.; McMurtrie, J. C.; Turner, P.; Duriska, M.; Arnold, D.
P. Inorg. Chem. 2006, 45, 6479−6489.
(56) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112−122.
(57) Crosby, G. A.; Demas, J. N. J. Phys. Chem. 1971, 75, 991−1024.
(58) Seybold, P. G.; Gouterman, M. J. Mol. Spectrosc. 1969, 31, 1−13.
(59) Kohn, W.; Sham, L. J. Phys. Rev. 1965, 140, A1133−A1138.
(60) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J., J. A.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Lyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;
Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J.
B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J.
J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.;
Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.; Inc., G. Gaussian 03;
Gaussian, Inc.: Wallingford CT, 2004.
(29) Chen, Y.; Zhang, X. P. J. Org. Chem. 2003, 68, 4432−4438.
(30) Gao, G. Y. Y.; Chen, Y.; Zhang, X. P. J. Org. Chem. 2003, 68,
6215−6221.
(31) Gao, G.-Y.; Colvin, A. J.; Chen, Y.; Zhang, X. P. J. Org. Chem.
2004, 69, 8886−8892.
(32) Peters, M. V.; Goddard, R.; Hecht, S. J. Org. Chem. 2006, 71,
7846−7849.
(33) Gao, G. Y.; Ruppel, J. V.; Allen, D. B.; Chen, Y.; Zhang, X. P. J.
Org. Chem. 2007, 72, 9060−9066.
(34) Liu, C.; Shen, D.-M.; Chen, Q.-Y. J. Org. Chem. 2007, 72, 2732−
2736.
(61) Becke, A. D. Phys. Rev. A 1988, 38, 3098−3100.
(62) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785−
789.
(63) Cossi, M.; Barone, V.; Cammi, R.; Tomasi. J. Chem. Phys. Lett.
1996, 255, 327−335.
(35) Chen, Y.; Fields, K. B.; Zhang, X. P. J. Am. Chem. Soc. 2004, 126,
14718−14719.
(36) Gao, G.-Y.; Colvin, A. J.; Chen, Y.; Zhang, X. P. Org. Lett. 2003,
5, 3261−3264.
(37) Gao, G. Y.; Chen, Y.; Zhang, X. P. Org. Lett. 2004, 6, 1837−
1840.
(38) Soares, A. R. M.; Martínez-Díaz, M. V.; Bruckner, A.; Pereira, A.
M. V. M; Tome,
M. G. P. M. S.; Tome,
T.; Guldi, D. M. Org. Lett. 2007, 9, 1557−1560.
́
J. P. C.; Alonso, C. M. A.; Faustino, M. A. F.; Neves,
́
A. C.; Silva, A. M. S.; Cavaleiro, J. A. S.; Torres,
10556
dx.doi.org/10.1021/ic300714n | Inorg. Chem. 2012, 51, 10548−10556