Synthesis and photophysical investigation of new porphyrin derivatives with β-pyrrole ethynyl linkage and corresponding dyad with [60] fullerene

Article abstract of DOI:10.1021/jp062735h

Two new β-substituted arylethynyl meco-tetraphenylporphyrins, 2-[(4′-formyl)phenyl]ethynyl-5,10,15,20-tetraphenylporphyrin (system A) and 2-[(4′-methyl)phenyl]ethynyl-5,10,15,20-tetraphenylporphyrin (system B) and their zinc derivatives were synthesized by palladium catalysis, using a synthetic approach that affords high yields of the target systems. Comparative ultraviolet-visible (UV-vis), NMR, and cyclic voltammetry studies of such macrocycles reveal the presence of an extensive conjugation between the tetrapyrrolic ring and the linker, through π-π orbital interaction. This interaction was observed in the form of a push-pull effect that moves the electronic charge between the porphyrin and the aldehyde group of system A. System B, bearing a methyl group instead of the formyl group, was synthesized in order to evaluate the effect of the substitution on the charge delocalization, which is necessary to corroborate the push-pull mechanism hypothesis. The new porphyrin, system A, was also used as a starting material for the synthesis of new porphyrin-fullerene dyads in which the [60]fullerene is directly linked to the tetrapyrrolic rings by ethynylenephenylene subunits. Fluorescence and transient absorption measurements of the new dyads reveal that ultrafast energy and electron transfer occur, respectively, in nonpolar and polar solvents, with high values of the rate constant. The UV-vis, NMR, and cyclic voltammetry results show that it is possible for both energy and electron transfer between porphyrin and fullerene to take place through the π-bond interaction. Such results evidence that the coupling between the donor and acceptor moieties is strong enough for possible photovoltaic applications.

Full text of DOI:10.1021/jp062735h

11424
J. Phys. Chem. A 2006, 110, 11424-11434
Synthesis and Photophysical Investigation of New Porphyrin Derivatives with â-Pyrrole
Ethynyl Linkage and Corresponding Dyad with [60] Fullerene
Angelo Lembo,† Pietro Tagliatesta,*,† and Dirk M. Guldi*,‡
Institute for Physical Chemistry, Friedrich-Alexander UniVersity Erlangen-Nu¨rberg, Egerlandstrasse 3,
91058 Erlangen, Germany, and Dipartimento di Scienze e Tecnologie Chimiche, UniVersity of Rome-Tor
Vergata, Via della Ricerca Scientifica, 00133 Rome, Italy
ReceiVed: May 4, 2006; In Final Form: July 13, 2006
Two new â-substituted arylethynyl meso-tetraphenylporphyrins, 2-[(4′-formyl)phenyl]ethynyl-5,10,15,20-
tetraphenylporphyrin (system A) and 2-[(4′-methyl)phenyl]ethynyl-5,10,15,20-tetraphenylporphyrin (system
B) and their zinc derivatives were synthesized by palladium catalysis, using a synthetic approach that affords
high yields of the target systems. Comparative ultraviolet-visible (UV-vis), NMR, and cyclic voltammetry
studies of such macrocycles reveal the presence of an extensive conjugation between the tetrapyrrolic ring
and the linker, through π-π orbital interaction. This interaction was observed in the form of a “push-pull”
effect that moves the electronic charge between the porphyrin and the aldehyde group of system A. System
B, bearing a methyl group instead of the formyl group, was synthesized in order to evaluate the effect of the
substitution on the charge delocalization, which is necessary to corroborate the push-pull mechanism
hypothesis. The new porphyrin, system A, was also used as a starting material for the synthesis of new
porphyrin-fullerene dyads in which the [60]fullerene is directly linked to the tetrapyrrolic rings by
ethynylenephenylene subunits. Fluorescence and transient absorption measurements of the new dyads reveal
that ultrafast energy and electron transfer occur, respectively, in nonpolar and polar solvents, with high values
of the rate constant. The UV-vis, NMR, and cyclic voltammetry results show that it is possible for both
energy and electron transfer between porphyrin and fullerene to take place through the π-bond interaction.
Such results evidence that the coupling between the donor and acceptor moieties is strong enough for possible
photovoltaic applications.
Introduction
Despite the fact that chlorin and bacteriochlorin derivatives
generate long-lived charge separated states, they are very
difficult to synthesize. Moreover, often natural compounds must
be used as the starting material. Therefore, the application of
simple synthetic pathways, able to afford new functionalized
porphyrin derivatives, emerges as an important aspect of such
studies. Here, we wish to report the synthesis of new â-substi-
tuted porphyrins, which exhibit interesting photochemical
properties.
During the last 10 years, porphyrins and fullerenes have been
widely used to build different molecular networks1 able to mimic
the natural photosynthetic processes.2 The main goal of such
studies was to understand the structural parameters that govern
these phenomena in order to achieve the storing of solar energy
and its conversion into chemical potential.3 In such types of
molecules, porphyrins and fullerenes interact by self-assembly4
or through covalent bonds.1,5
The possibility to introduce different substituents at the
â-pyrrole positions constitutes a potent possibility to modify
the photophysical properties of the tetrapyrrolic ring (i.e., to
modulate the porphyrin-fullerene electronic interaction) at will.
It appears plausible to obtain, in the new â-subtituted
porphyrins and metalloporphyrins, a complete charge delocal-
ization, that is, along the carbon-carbon bond that links the
ethynylenephenylene subunit and the tetrapyrrole ring. This is
known as the typical push-pull effect of the oligoethynyle-
nephenylene subunit.18-20 Such electronic communication gives
rise to a very fast photoinduced charge separation in the
porphyrin-fullerene dyads.
The principal effort for obtaining long-lived charge separation
has been focused toward the production of multistep electron
transfer6-10 steps along different subunits, that are not strongly
electronically coupled. In a great variety of these systems,
porphyrins act as light harvesting systems,11,12 while ferrocene
and C60sthat are covalently linked to the phenyl groups of a
meso-tetraphenylporphyrinsconstitute the donor and acceptor
groups, respectively. Such a covalent arrangement prevents,
however, a modification of the electronic properties of porphy-
rins. Notably, only a few papers are present in the literature
that are concerned with the synthesis and photochemical studies
of similar structures having a C60 directly linked to the
tetrapyrrolic ring.13-17 In most of these papers, pyrrole reduced
porphyrinsschlorin16 or bacteriochlorin derivatives15sare used
to synthesize C60 containing dyads.
The structures of the systems studied in this work are reported
in Figure 1.
Experimental Section
* To whom correspondence should be addressed. E-mail:
pietro.tagliatesta@uniroma2.it (P.T.); dirk.guldi@chemie.uni-erlangen.de
(D.M.G.).
General Methods. 1H NMR spectra were recorded as CDCl3
solutions on a Bruker AM-400 instrument using tetramethyl-
silane (TMS) as an internal standard. Chemical shifts are given
† University of Rome-Tor Vergata.
‡ University of Erlangen.
10.1021/jp062735h CCC: $33.50 © 2006 American Chemical Society
Published on Web 09/21/2006

New Porphyrin Derivatives
J. Phys. Chem. A, Vol. 110, No. 40, 2006 11425
Synthesis. 4-(Trimethylsilyl)ethynylbenzaldehyde.21 A 16 g
(86 mmol) portion of 4-bromobenzaldehyde and 14 g (142
mmol) of trimethylsilylacetylene were dissolved in 100 mL of
dried (KOH) triethylamine. The solution was deareated using
argon bubbling for 30 min; after that, 227 mg (1 mmol) of Pd-
(OAc)2 and 450 mg (1.7 mmol) of PPh3 were added to the
solution. The solution was then refluxed under argon, with
vigorous stirring. After 4 h, the crude mixture was cooled and
filtered and the solid salts were washed with triethylamine. The
organic solution was concentrated under vacuum, diluted with
dichloromethane (100 mL), and washed with water (3 × 100
mL). The organic layer was separated, dried over anhydrous
Na2SO4, and filtered. The solvent was then evaporated under
vacuum. The pale yellow residue was purified by column
chromatography on silica gel, eluting with petroleum ether (40-
70°)/diethyl ether, 98:2, giving the desired product as white
crystals, 10.8 g (62% yield). MS (GC): m/z 202 [M+]. 1H NMR
(400 MHz, CDCl3): δ (ppm) 10.02 (s, 1H), 7.84 (d, 2H; J )
8.2 Hz), 7.63 (d, 2H; J ) 8.2 Hz), 0.29 (s, 9H).
4-Ethynylbenzaldehyde. A 4 g (20 mmol) portion of 4-(tri-
methylsilyl)ethynylbenzaldehyde was dissolved under nitrogen
in 50 mL of CHCl3, and then, a saturated solution (10 mL) of
K2CO3 in methanol was added dropwise. After 2 h, water was
added to the organic solution. The organic phase was separated,
and the aqueous phase was extracted with chloroform. The
combined organic solutions were washed with water and dried
over anhydrous Na2SO4. The solvent was evaporated under
vacuum to give 2.4 g of pure 4-ethynylbenzaldehyde (92%
yield). MS (GC): m/z 130 [M+]. 1H NMR (400 MHz,
CDCl3,): δ (ppm) 9.98 (s, 1H), 7.70 (d, 2H; J ) 8.2 Hz), 7.50
(d, 2H; J ) 8.2 Hz), 3.22 (s, 1H).
2-Bromo-5,10,15,20-tetraphenylporphyrin. N-Bromosuccin-
imide (251 mg, 1.4 mmol) was slowly added to a refluxing
solution of meso-tetraphenylporphyrin (500 mg, 0.8 mmol) in
CHCl3 (150 mL) under nitrogen. After 2 h, the crude mixture
was cooled to room temperature and 2 mL of pyridine was
added. The solvent was evaporated under vacuum, and the crude
product was purified by column chromatography on silica gel,
eluting with xilene/petroleum ether (40-70°), 3:2, obtaining 260
mg of the desired porphyrin (yield 46%). MS (FAB): m/z 693
[M+], 613 [M - 80]+. 1H NMR (400 MHz, CDCl3): δ (ppm)
8.94-8.91 (br m, 2H), 8.89 (s, 1H), 8.88-8.83 (br m, 2H),
8.79-8.77 (br m, 2H), 8.22 (br m, 6H), 8.11 (br m, 2H), 7.82-
7.72 (br m, 12H), -2.82 (br s, 2H).
2-[(4′-Formyl)phenyl]ethynyl-5,10,15,20-tetraphenylporphy-
rin (System A). 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-tetraphe-
nylporphyrin (260 mg, 0.37 mmol) and 4-ethynylbenzaldehyde
(70 mg, 0.54 mmol) in toluene/triethylamine, 5:1 (60 mL), was
added. The solution was deareated for 30 min with argon
bubbling, and then, Pd(dba)2 (40 mg, 0.07 mmol) and AsPh3
(170 mg, 0.55 mmol) were added. The solution was deareated
for a 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 50 °C. After 16 h,
the mixture was cooled at room temperature and the solvent
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 evaporated under
vacuum. The product was purified by column chromatography
on silica gel eluting with CH2Cl2/petroleum ether (40-70°), 2:3.
The first compound eluted from the column was AsPh3; after
that, the polarity of the eluting mixture was gradually increased
Figure 1. Compounds investigated in this study. M ) 2H, Zn.
as δ values. FAB mass spectra were measured on a VG-4
spectrometer using m-nitrobenzyl alcohol (NBA) as a matrix.
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
instrument (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.
Ultraviolet-visible (UV-vis) spectra were recorded on a
Varian Cary 50 Scan spectrophotometer in toluene solution.
Steady state fluorescence studies were carried out on a Fluo-
romax 3 (Horiba) instrument, and all the spectra were corrected
for the instrument response. Time resolved fluorescence spectra
were recorded on a Photon Technology International 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 an Autolab electro-
chemical 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 a saturated calomel reference electrode (SCE).
This reference electrode was separated from the bulk of the
solution by a fritted-glass bridge filled with a 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 concen-
trated H2SO4 and distilled over P2O5 under vacuum prior to
use. Tetra-n-butylammonium hexafluorophosphate (TBAPF6),
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.

11426 J. Phys. Chem. A, Vol. 110, No. 40, 2006
Lembo et al.
up to CH2Cl2/petroleum ether (40-70°), 3:2. The desired
porphyrin obtained from the column was recrystallized from
dichloromethane/methanol to give 247 mg of purple powder
(s, 2H), 8.93 (s, 2H), 8.90 (d, 1H; J ) 5.1 Hz), 8.80 (d, 1H; J
) 5.1 Hz), 8.23 (br m, 5H), 7.78-7.66 (br m, 15H), 7.25 (d,
2H; J ) 7.3 Hz), 7.16 (d, 2H; J ) 7.3 Hz), 2.42 (s, 3H). UV-
vis (toluene): λmax (10-4ꢀ) ) 434 (23.9), 558 (1.75), 592 (0.63).
ZnP-C60. Yield: 98%. MS (FAB): m/z 1552 [M+], 832 [M
1
(yield 90%). MS (FAB): m/z 742 [M+]. H NMR (400 MHz,
CDCl3): δ (ppm) 10.06 (s, 1H), 9.14 (s, 1H), 8.92 (s, 2H), 8.86
(d, 1H; J ) 5.1 Hz), 8.80 (br s, 3H), 8.25 (br m, 5H), 7.86 (d,
2H; J ) 8.2 Hz), 7.80-7.66 (br m, 15H), 7.52 (d, 2H; J ) 8.2
Hz), -2.64 (s, 2H). UV-vis (toluene): λmax (10-4ꢀ) ) 432
(20.9), 525 (2.03), 560 (0.62), 602 (0.58), 659 (0.46).
1
- 720]+, 720 [M - 832]+. H NMR (400 MHz, CDCl3): δ
(ppm) 9.24 (s, 1H,), 8.95 (s, 2H), 8.93 (s, 2H), 8.88 (d, 1H; J
) 5.1 Hz), 8.77 (d, 1H; J ) 5.1 Hz), 8.22 (br m, 3H), 8.16 (br
d, 2H; J ) 7.2 Hz), 7.80 (br m, 10H), 7.60 (m, 3H), 7.45-7.40
(br m, 4H), 7.20 (d, 2H; J ) 7.2 Hz), 4.95 (d, 1H; J ) 9.2 Hz),
4.85 (s, 1H), 4.20 (d, 1H; J ) 9.2 Hz), 2.87 (s, 3H). UV-vis
(toluene): λmax (10-4ꢀ) ) 311 (4.8), 434 (26.1), 558 (1.94),
593 (0.82).
2-[(4′-Methyl)phenyl]ethynyl-5,10,15,20-tetraphenylporphy-
rin (System B). A solution of 2-bromo-5,10,15,20-tetraphe-
nylporphyrin (40 mg, 57 µmol) and 4-ethynyltoluene (9 mg,
77 µmol) in toluene/triethylamine, 5:1 (36 mL), with Pd(dba)2
(6 mg, 10 µmol) and AsPh3 (24 mg, 80 µmol) was allowed to
react under a nitrogen atmosphere for 16 h at 50 °C. The mixture
was cooled at room temperature, the solvent evaporated under
vacuum, and the residue redissolved in dichloromethane. The
organic solution was washed with water (3 × 50 mL), dried
over anhydrous Na2SO4, and evaporated under vacuum. The
residue was purified on a silica gel column, eluting with CH2-
Cl2/petroleum ether (40-70°), 25:75, to obtain 33 mg of
porphyrin (yield 79%) which was recrystallized from dichlo-
romethane/methanol to give the desired compound as purple
powder. MS (FAB): m/z 728 [M+]. 1H NMR (400 MHz,
CDCl3): δ (ppm) 9.07 (s, 1H), 8.88 (s, 2H), 8.82 (d, 1H; J )
5.1 Hz), 8.80 (s, 2H), 8.74 (d, 1H; J ) 5.1 Hz), 8.23 (br m,
5H), 7.80-7.66 (br m, 15H), 7.25 (d, 2H; J ) 7.3 Hz), 7.16 (d,
2H; J ) 7.3 Hz), 2.42 (s, 3H), -2.66 (s, 2H); UV-vis
(toluene): λmax (10-4ꢀ) ) 428 (22.5), 523 (2.15), 558 (0.609),
600 (0.59), 657 (0.36).
N-Methyl-2-(4′-ethynyl)phenyl-3,4-fulleropyrrolidine, (C60-
ref.). A mixture of 4-ethynylbenzaldehyde (10 mg, 77 µmol),
C60 (56 mg, 77 µmol), and N-methylglicine (54 mg, 0.6 mmol)
was refluxed for 24 h in anhydrous toluene under a nitrogen
atmosphere. After cooling to room temperature, the solvent was
evaporated under vacuum and the residue was purified on a
silica gel column, eluting with toluene/petroleum ether (40-
70°), 3:2, recovering as the first band the unreacted [60]fullerene,
followed by the reaction product (29 mg, yield 42%). MS
1
(MALDI): m/z 877 [M+], 720 [M - 157]+. H NMR (400
MHz, CDCl3): δ (ppm) 7.81 (d, 2H; J ) 7.9 Hz), 7.58 (d, 2H;
J ) 7.9 Hz), 5.0 (d, 1H; J ) 8.9 Hz), 4.96 (s, 1H), 4.28 (d, 1H;
J ) 9.9 Hz), 3.11 (s, 1H), 2.82 (s, 3H). UV-vis (toluene): λmax
(10-4ꢀ) ) 330 (5.0).
Results and Discussion
Synthesis. We report in Scheme 1 the synthetic pathways
for obtaining the porphyrin derivatives. The bromination of
meso-tetraphenylporphyrin was carried out using N-bromosuc-
cinimide as the brominating agent, slightly modifying the
literature procedures22 to obtain a higher yield of the mono-
brominated derivative. The two following cross-coupling
reactions between the halogenated porphyrin and the two
different substituted ethynylbenzenes, 4-ethynylbenzaldehyde
for system A and 4-ethynyltoluene for system B, were carried
out using the catalytic system Pd(dba)2/AsPh3, developed by
Lindsey.23 Avoiding the use of copper iodide as a cocatalyst,
the homocoupling side reaction between the alkynes was
suppressed and high yields of the desired compounds were
obtained.
Next, the aldehydic group of system A was used to covalently
link the [60]fullerene to the porphyrin by the reaction with
N-methylglicine in toluene.24 Zinc insertion for all of the
compounds was carried out by adding a saturated methanol
solution of Zn(OAc)2 to the porphyrin products dissolved in
chloroform.
H2P-C60. A mixture of 2-[(4′-formyl)phenyl]ethynyl-5,10,-
15,20-tetraphenylporphyrin (35 mg, 47 µmol), C60 (150 mg, 200
µmol), and N-methylglicine (185 mg, 0.2 mmol) was refluxed
for 24 h in anhydrous toluene under a nitrogen atmosphere. After
cooling, the solvent was evaporated and the residue purified
on a silica gel column, eluting with toluene, recovering the
unreacted [60]fullerene as the first fraction, the reaction product
as the second (42 mg, yield 59%), and the unreacted porphyrin
as the third one. MS (FAB): m/z 1490 [M + H]+, 769 [M -
720]+, 720 [M - 769]+. 1H NMR (400 MHz, CDCl3): δ (ppm)
9.07 (s, 1H), 8.89 (s, 2H), 8.82 (d, 1H; J ) 5.1 Hz), 8.79 (s,
2H), 8.73 (d, 1H; J ) 5.1 Hz), 8.22 (br m, 5H), 7.79 (br m,
15H), 7.61 (br, m, 2H), 7.43 (br, m, 2H), 4.96 (d, 1H; J ) 9.2
Hz), 4.89 (s, 1H), 4.22 (d, 1H; J ) 9.2 Hz), 2.86 (s, 3H), -2.68
(s, 2H). UV-vis (toluene): λmax (10-4ꢀ) ) 300 (4.88), 429
(21.6), 523 (2.1), 558 (0.71), 601 (0.62), 657 (0.38).
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 and eluting with chloroform.
The fullerene derivative, N-methyl-2-(4′-ethynyl)phenyl-3,4-
fulleropyrrolidine (C60-ref), was synthesized by previously
reported procedures.24 All of the porphyrin compounds, H2P-
C60, and ZnP-C60 were characterized by FAB-MS, MALDI,
2-[(4′-Formyl)phenyl]ethynyl-5,10,15,20-tetraphenylporphy-
rin Zn(II), (Zn-System A). Zinc porphyrin was obtained
following the general procedures previously described. Yield
95%. MS (FAB): m/z 805 [M+]. 1H NMR (400 MHz, CDCl3):
δ (ppm) 10.06 (s, 1H), 9.30 (s, 1H), 8.96 (s, 2H), 8.95 (s, 2H),
8.91 (d, 1H; J ) 5.1 Hz), 8.80 (d, 1H; J ) 5.1 Hz), 8.25 (br m,
5H), 7.88 (d, 2H; J ) 8.2 Hz), 7.80-7.66 (br m, 15H), 7.55 (d,
2H; J ) 8.2 Hz). UV-vis (toluene): λmax (10-4ꢀ) ) 311 (2.17),
438 (23.8), 559 (1.86), 596 (0.96).
1
and H NMR spectra.
1
In particular, the H NMR spectra of H2P-C60 and ZnP-
C60 show the presence of the characteristic signals of the
N-methylpyrrolidine derivatives: one singlet at 2.86 ppm due
to the methyl substituent on the nitrogen atom, one singlet at
4.89 ppm due to the hydrogen in position 2 of the pyrrolidinic
ring, and two doublets at 4.22 and 4.96 due to the methylene
protons.
2-[(4′-methyl)phenyl]ethynyl-5,10,15,20-tetraphenylporphy-
rin Zn(II), (Zn-System B). Yield: 95%. MS (FAB): m/z 791
[M+]. 1H NMR (400 MHz, CDCl3): δ (ppm) 9.24 (s, 1H), 8.95
Cyclic Voltammetry Studies. In Table 1, the potentials of
the free bases and the corresponding zinc derivatives of all the
compounds are reported. The cyclic voltammograms, in DCB

New Porphyrin Derivatives
J. Phys. Chem. A, Vol. 110, No. 40, 2006 11427
SCHEME 1: Synthetic Pathways for System A, System B, and H2P-C60
TABLE 1: Half-Wave Potentials (V vs SCE) for Reduction
and Oxidation of Different Compounds, in DCB Containing
0.05 M TBAPF6
system B, while the E1/2 values for the first and fourth reductions
of H2P-C60 are almost identical to those for the first and third
reductions of C60-ref.
oxidation
reduction
II III
1.55a -0.68 -1.08 -1.68
The two-electron process centered at -1.11 V derives from
the overlapping of two reductions, one from system B and one
from C60-ref. As shown in Figure 2, the E1/2 value for the first
and second oxidations of H2P-C60 (E1/2 ) 1.08 and 1.43 V)
occur at virtually identical potentials for the first and second
oxidations of system B (E1/2 ) 1.08 and 1.44 V). Such data
suggest the porphyrin and fullerene as two different and non-
interacting chromophores, retaining their own electronic and
physical properties. From the reported data, it is evident that
the highest occupied molecular orbital (HOMO)-lowest unoc-
cupied molecular orbital (LUMO) gap of the porphyrin systems
compound
C60-ref
H2-TPP
system A
system B
H2P-C60
Zn-TPP
VI
V
I
IV
1.48 1.05
-1.24 -1.55
-1.10 -1.34
-1.16 -1.44
1.43 1.10
1.44 1.08
1.43 1.08
1.25 0.91
-0.66 -1.11 -1.44 -1.67
-1.45
Zn-system A 1.21 0.95
Zn-system B 1.17 0.93
-1.28
-1.33
ZnP-C60
1.19 0.94
-0.66 -1.05 -1.32 -1.57
a Irreversible.
(solvent) and with TBAPF6 (supporting electrolyte), reveal the
influence of the â-substituent on the oxidation potentials of the
porphyrins. In fact, the free base porphyrins of system A and
system B are characterized by two reversible oxidation processes
centered at 1.10, 1.43 and 1.08, 1.44 V, respectively, while in
general H2-TPP shows two reversible oxidations at 1.05 and
1.48 V.25
The two systems also present two reversible reductions
centered at -1.10 and -1.34 V for system A and -1.16 and
-1.44 V for system B. The difference of 60 and 100 mV,
respectively, for the first and second reduction potentials
between the two systems reflects the electron-withdrawing
effect26 of the aldehydic group of system A. The zinc derivatives
of both systems show two reversible oxidations at 0.95 and 1.21
V for system A and 0.93 and 1.17 V for system B, while it is
possible to detect only one reversible reduction for both of the
compounds at -1.28 and -1.33, respectively.
Figure 2 illustrates the room-temperature cyclic voltammo-
grams of system B, H2P-C60, and C60-ref in DCB, 0.05 M
TBAPF6. H2P-C60 undergoes three reversible one-electron and
one reversible two-electron reductions and two reversible one-
electron oxidations between +1.50 and -2.00 V. The reductions
of the H2P-C60 occur at E1/2 ) -0.66, -1.11, -1.44, and -1.67
V. The E1/2 values for the second and third reductions are very
close to the E1/2 values for the first and second reductions of
Figure 2. Cyclic voltammograms at 200 mV/s in DCB with 0.05 M
TBAPF6 as the supporting electrolyte of system B, H2P-C60, and C60-
ref.

11428 J. Phys. Chem. A, Vol. 110, No. 40, 2006
Lembo et al.
TABLE 2: UV-vis Spectral Data of All the Investigated Compounds in Toluene and THF
λ
max (nm) (ꢀ10-4
)
compound
toluene
311(2.17), 432(20.9), 525(2.03), 560(0.62), 602(0.58), 659(0.46) 428(20.2), 522(1.92), 558(0.59), 601(0.54), 658(0.40)
428(22.5), 523(2.15), 558(0.60), 600(0.59), 657(0.36) 424(20.1), 521(1.89), 556(0.62), 599(0.52), 656(0.32)
300(4.88), 429(21.6), 523(2.1), 558(0.71), 601(0.62), 657(0.38) 300(4.6), 425(19.1), 521(1.85), 556(0.65), 600(0.54), 656(0.34)
THF
system A
system B
H2P-C60
Zn-system A 311(2.17), 438(23.8), 559(1.86), 596(0.96)
Zn-system B 434(23.9), 558(1.75), 592(0.63)
ZnP-C60
437(22.6), 566(1.72), 603(0.87)
434(24.7), 564(1.67), 600(0.60)
311(4.5), 435(21.6), 565(1.46), 601(0.62)
311(4.8), 434(26.1), 558(1.94), 593(0.82)
is lowered going from H2-TPP, 2.29 V to system B and system
A, 2.24 and 2.20 V, respectively. Such an observation is justified
by postulating an extension of conjugation along the linker.
By adding the corresponding reduction and oxidation poten-
tials of the electron-accepting C60 and electron-donating por-
phyrins, respectively, we estimate the following radical ion pair
energies: H2P-C60, 1.74 eV; ZnP-C60, 1.6 eV.
overall electron-withdrawing effect due to the C60 moiety can
be observed. Furthermore, the Q-bands of all the examined
compounds are slightly shifted. The introduction of zinc into
the macrocycles produces an increase in the electron density of
the rings, giving very similar values of molar extinction
coefficients in both porphyrins, 2.38 × 105 and 2.39 × 105 M-1
cm-1 for systems A and B, respectively, despite the 4 nm
difference in the Soret band, 434 nm for system B and 438 nm
for system A. The spectra of the free bases and zinc complexes
of system B and dyad are virtually superimposable, as illustrated
in Figures 3 and 4. From Figure 4, it is also discernible that the
ZnP-C60 spectrum is the sum of the Zn-system B spectrum
and of the N-methyl-2-(4′-ethynyl)phenyl-3,4-fulleropyrrolidine
compound (C60-ref) which was synthesized as a reference.
NMR Studies. Both porphyrin systems were investigated by
1H NMR spectroscopy. In addition, system A was probed with
two different bidimensional NMR methodologies, that is, HSQC
and HMBC. HSQC allows mapping of the direct coupling of
each proton with its relative bonded carbon atom, and HMBC
sheds light onto J2 and J3 1H-13C couplings.28 These method-
ologies allowed us to assign the NMR signals concerning the
(4′-formyl)phenylethynyl substituent on the pyrrolic ring (Figure
5). From the inspection of the HMBC spectrum of system A, it
is possible to see different signals arising from J2 and J3
hydrogen-carbon couplings. The two couplings at 135.5 and
129.0 ppm of the aldehydic proton at 10.06 correspond to carbon
atoms 2 and 3, respectively.
UV-Visible Studies. The UV-vis spectra of all the com-
pounds show interesting characteristics concerning the porphyrin
systems. In Table 2, the spectral features of all the new
compounds are reported. The spectra of systems A and B in
toluene present a broad Soret band, shifted toward higher
wavelengths compared with the Soret band of H2-TPP.27 In
toluene, the Soret band of system B is located at 428 nm, while
that of system A is at 432 nm, with values of the molar
extinction coefficients of 2.25 × 105 and 2.09 × 105 M-1 cm-1
,
respectively. H2P-C60 shows the Soret band located at 429 nm,
with a molar extinction coefficient of 2.16 × 105 M-1 cm-1
.
Furthermore, in this last spectrum, a broad band, located between
300 and 350 nm, due to the C60 moiety is present (Figure 3).
In our opinion, the position of the Soret bands and the ꢀ values
of such absorptions, compared with those of H2-TPP, are
influenced by the electronic conjugation of the substituents in
the â-pyrrole position. The smaller value of ꢀ in system A
derives from the electron-withdrawing effect of the aldehydic
group. The extension of conjugation based on the resonance
effect also justifies the shift of the Soret band to higher
wavelengths, if compared with the shift of the band of system
B. This observation can be justified by the presence of an
electron-donating effect of the porphyrin toward the electron-
withdrawing aldehydic group of system A, generating a “push-
pull” effect that is transmitted through the carbon-carbon
skeleton of the ethynylenephenylene subunit. This effect lowers
the electron density of the tetrapyrrole ring, in turn lowering
the ꢀ value of system A. On the contrary, in H2P-C60, only an
The most diagnostic signals are those that relate to the
ethynylene carbons, which are coupled with protons 4 and 3.
Their relative resonance frequencies at 9.14 ppm for proton 4
and 7.52 ppm for proton 3 correspond to the resonance
frequency of carbon 7 at 91.0 ppm and carbon 6 at 98.3 ppm.29
In the HSQC spectrum, it is possible to see five different pyrrolic
proton signals coupled with four magnetically different carbon
atoms: the first one at 9.14 ppm coupled with the carbon signal
Figure 3. UV-vis spectra of system A (red dotted line), system B
(blue solid line), and H2P-C60 (black dashed line). Q-bands are reported
in the inset.
Figure 4. UV-vis spectra of ZnP-C60 (red line), Zn-system B (blue
line), and C60-ref (black line).

New Porphyrin Derivatives
J. Phys. Chem. A, Vol. 110, No. 40, 2006 11429
the substituent. Figure 8 depicts the three fundamental resonance
structures30 of porphyrin system A, in which it is clear how
only three of the four pyrrolic rings are subjected to the
extension of the conjugation. On the other hand, the two protons
on the ring C (H4) are not affected by this charge delocalization
and in this case it is possible to attribute these two protons to
the singlet at 8.80 ppm in both systems. The â-pyrrole carbon
atoms of rings B and D are magnetically equivalent, and we
can attribute the carbon signal at 130.4 ppm to the carbon with
the positive charge, while the other one corresponds to the signal
at 129.2 ppm. Since the resonance effect depends on the distance
relative to the aldehydic group, it is possible to visualize the
proton H2 (8.87 ppm) and proton H3 (8.80 ppm) as doublets.
Notably, such an impact is not discernible for the proton H5
(8.92 ppm), also because the symmetry of porphyrin is lost.
The signal at 133.6 ppm is due to the â-pyrrolic carbon atoms
of pyrrolic ring C.
Figure 5. Carbon and proton assignments of the â-substituent of
system A.
These results are in good agreement with the effect of the
electron-withdrawing substituents on the â-positions of the
tetrapyrrole ring31 and with the fact that the free base porphryins
tend to retain their aromaticity, excluding, however, the two
external double bonds. In our case, only one double bond is
excluded from the delocalization. Still, in the HSQC spectrum
of system B, five different pyrrolic proton signals seem to couple
with four magnetically different carbon atoms, but all of the
carbon signals are shifted toward higher fields. In particular,
while in system A the carbon with the positive charge in the
ring A resonances at 141.0 ppm, in system B, such signal is
shifted to 138.0 ppm. This constitutes further evidence of the
electron-withdrawing effect of the aldehydic group. Finally, it
should be noted that the â-pyrrole proton pattern in the 1H NMR
spectrum of the free base dyad is completely similar to that of
system B.
1
The H NMR spectra of the corresponding zinc derivatives
present an identical â-pyrrolic proton pattern (Figure 9). In
particular, the singlet found at 8.80 ppm for the free base is
largely shifted to lower field and this fact is due to the
coordination of the zinc atom to the nitrogen atom of the pyrrole
ring C. This causes a decrease in electron density on the
aforementioned pyrrole ring, and in addition, the average ∆ppm
between pyrrole proton signals of system A and system B is
less pronounced in the zinc derivatives relative to the free base
porphyrins.
All of the aspects discussed above point to the presence of
an extension of conjugation from the porphyrin ring along the
â-substituent; especially when an aldehydic group is present
on the phenyl ring of the substituent, a push-pull effect takes
place from the porphyrin to the aldheyde.
Figure 6. HSQC of system A. 1H-13C couplings of â-pyrrolic protons
and carbons.
at 141.0 ppm, the second one the singlet at 8.92 ppm, which
correspond two different carbon signals at 129.2 and 130.4 ppm,
the third one the doublet at 8.87 ppm coupled with a carbon
signal at 129.2, then the singlet at 8.80 ppm coupled with a
carbon signal at 133.6 ppm, and finally the doublet at 8.80 ppm
coupled with the carbon signal at 130.4 ppm (Figure 6).
Moreover, since Zn-system A and Zn-system B have more
or less the same molar extinction coefficient, it is possible that
the zinc atom turns off this phenomenon, because the metal
insertion in the tetrapyrrolic ring renders all of the â-pyrrole
positions, in terms of resonance structures, equivalent.
1
Comparing the H NMR of system A and system B, it is
clear that between 8 and 9 ppm different patterns concerning
the pyrrole protons are present (Figure 7). In system B, a doublet
at 8.75 ppm is clearly visible, while, in system A, this is
superimposed with the singlet at 8.80 ppm. Changing the
substituent on the phenyl ring linked to the â-position, a shift
toward lower frequencies is evident when going from a methyl
to an aldehydic group. Moreover, the NMR shifts do not all
have the same magnitude and so the peak at 9.14 ppm in system
A (H1) is shifted in system B to 9.07 ppm (∆ppm ) 0.07),
while all other peaks are shifted by about 0.04 ppm, except the
singlet at 8.80 ppm in system A.
Photophysical Studies. In Table 3, the fluorescence proper-
ties of all the examined compounds are reported.
Steady state fluorescence spectra were recorded to analyze
the photophysical properties of the new porphyrins and dyads.
The steady state fluorescence studies were recorded in four
different solvents: toluene, chloroform, tetrahydrofuran (THF),
and benzonitrile. The excitation wavelength was 525 nm for
the free base derivatives and 560 nm for the zinc derivatives.
In toluene, system A presents two emission bands at 666 and
734 nm, while system B, at 663 and 731 nm. Such a difference
of 3 nm remains also in the other solvents. In the corresponding
It is possible to understand all of these aspects by postulating
an extension of the conjugation from the tetrapyrrole ring along

11430 J. Phys. Chem. A, Vol. 110, No. 40, 2006
Lembo et al.
Figure 7. 1H NMR signals pattern of pyrrolic protons of (a) system A and (b) system B.
Figure 8. Three fundamental resonance structures of system A.
Figure 9. 1H NMR spectra in CDCl3 of Zn-system A (a) and ZnP-C60 (b). Pyrrolic proton pattern.
TABLE 3: Fluorescence Propertiesa,b in Different Solvents
zinc derivatives, the emission maxima are located at 607 and
661 nm for Zn-system A, while they are at 605 and 658 nm
for Zn-system B.
toluene
THF
compound λemission
τ
Φ
λemission
τ
Φ
Steady state fluorescence spectra of zinc derivatives in THF
and benzonitrile show a substantial red shift due to the
coordinating effect of the solvents. In fact, in THF, the first
emission maximum of Zn-system A is at 620 nm and, in
benzonitrile, it is at 628 nm.
In toluene solution, the fluorescence of the porphyrin part in
H2P-C60 is quenched for 93% by the C60 moiety. The
fluorescence quenching is calculated taking system B as a
reference. On the other hand, for ZnP-C60, the quenching is
99%. In addition to the higher quenching of the porphyrin
fluorescence, in the spectra of ZnP-C60, a third emission band
appears at 715 nm (Figure 10). An excitation spectrum collected
system A
system B
H2P-C60
666 11 ns 0.11
664 11 ns 0.14
661 11.3 ns 0.138
663 10.9 ns 0.109
663 20.5 ps 2.0 × 10-4 c 661 50.7 ps 6.0 × 10-4 c
c
Zn-system A 607 2.3 ns 0.05
Zn-system B 605 2.4 ns 0.05
620 2.1 ns 0.044
615 2.1 ns 0.047
ZnP-C60
605 15.9 ps 3.5 × 10-4 c 615 5.8 ps 1.0 × 10-4 c
a λmax in nanometers. b At room temperature. c Theoretical quantum
yield related to the lifetime of the single excited state of porphyrin in
the dyad.
at 715 nm (i.e., exciting from 400 to 650 nm) confirms the origin
of that emission band: the porphyrin moiety. Further analysis

New Porphyrin Derivatives
J. Phys. Chem. A, Vol. 110, No. 40, 2006 11431
Figure 10. Steady state fluorescence spectra of system A, system B, and H2P-C60 (a) and corresponding zinc derivatives (b) in toluene solution,
respectively, upon excitation at 525 and 560 nm.
it is possible to correlate the fluorescence quantum yield and
the lifetime with the following equation:
ΦZn-systemB - Φ
ZnP-C60
) kdeact
Φ
τZn-systemB
ZnP-C60
where ΦZn-systemB is the zinc porphyrin quantum yield, while
ΦZnP-C is the zinc porphyrin quantum yield in the ZnP-C60
60
and kdeact is the rate of the porphyrin singlet excited state
deactivation process. From these analyses, we succeeded in
estimating the kdeact value in toluene as 1.0 × 1011 s-1 which
corresponds to a lifetime of 10 ps, while in THF a kdeact value
of 2.0 × 1011 s-1 yields a lifetime of 5 ps. Such results are in
agreement with the singlet excited state lifetimes measured by
transient absorption spectroscopy, vide infra. Similar results were
gathered for the corresponding H2P-C60.
Figure 11. Steady state fluorescence spectra of C60-ref and ZnP-C60
in toluene solution upon excitation at 400 nm. The relative absorbance
match (0.05 au) at excitation wavelength.
Femtosecond transient absorption spectroscopy was used to
complement the aforementioned fluorescence experiments. In
particular, the references (i.e., Zn-system B and system B) and
the electron donor-acceptor dyads (i.e., ZnP-C60 and H2P-
C60) were probed upon 387 nm excitation. This guarantees the
predominant photoexcitation of the Zn porphyrin and free base
porphyrin chromophores. At first, our attention should be
directed to the references. For system B, for example, we see
the nearly instantaneous formation of the singlet excited states
only a fast internal conversion (i.e., ∼2 ps) from a higher lying
excited state precedes the singlet excited state formationsfor
which we gather the following spectral characteristics: minima
at 521, 557, 600, and 660 nm and a maximum in the near-
infrared at 700 nm. The minima correspond to maxima seen in
the system B ground state absorption spectrum. Similar ground
state bleachings (i.e., 555 and 595 nm) and new maxima in the
near-infrared (i.e., 710 nm) were noted for Zn-system B. In
contrast, the singlet excited state lifetimes of system B (i.e.,
10.5 ns) and Zn-system B (i.e., 2.3 ns) differ fundamentally.
Mainly, the same singlet excited state formations of Zn-
system B and system B were registered in ZnP-C60 and H2P-
C60, respectively, despite the presence of the electron-accepting
C60. This affirms in toluene and THF the successful photoex-
citation of the chromophores in the electron donor-acceptor
dyads. At longer delay times, however, the electron donor-
acceptor dyads revealed a behavior that is vastly different from
the reference systems. In particular, ultrafast deactivations of
the singlet excited state are derived from kinetic analyses of
the ZnP-C60 (i.e., toluene, 15.9 ps; THF, 5.8 ps) and H2P-C60
(i.e., toluene, 20.5 ps; THF, 50.7 ps) transient species in the
different solvents; see Figures 13-15.
on the reference compound N-methyl-2-(4′-ethynyl)phenyl-3,4-
fulleropyrrolidine identifies this band as the singlet excited state
emission of C60 (Figure 11). The same behavior has also been
observed in chloroform.
Such data demonstrate that in toluene solution the principal
deactivation pathway of the porphyrin fluorescence is a near
quantitative energy transfer from the singlet excited state of zinc
porphyrin to the singlet excited state of C60. In THF and
benzonitrile, the porphyrin fluorescence quenching in H2P-C60
is much stronger relative to what has been seen in toluene and
chloroform. In ZnP-C60 fluorescence spectra, on the other hand,
1*
we do not observe the fluorescence emission of
nm.
C
at 715
60
In general, we observe a solvent effect on the fluorescence
quenching; in particular, the porphyrin fluorescence in H2P-
C60 tends to decrease passing from less polar to more polar
solvent (Figure 12). To gather a deeper evaluation of the
photophysical phenomena, time resolved fluorescence spectra
were carried out in the nanosecond time domain with a 337 nm
nitrogen laser.
Unfortunately, for both the free base and the zinc derivatives,
the deactivation pathway of the singlet excited state of porphy-
rins in the dyads is faster than the time resolution. Only the C60
singlet excited state deactivation in the ZnP-C60, following the
energy transfer reaction in toluene, was registered.
The measured lifetime is 1.30 ( 0.01 ns, a result that is
consistent with the singlet excited state lifetime of fulleropyr-
rolidine derivatives.32 From the steady state fluorescence spectra
of ZnP-C60 in toluene and THF, it was possible to estimate
the lifetimes of the zinc porphyrin singlet excited state. In fact,

11432 J. Phys. Chem. A, Vol. 110, No. 40, 2006
Lembo et al.
Figure 12. Fluorescence emission spectra of H2P-C60 (a) and ZnP-C60 (b) in different solvents, respectively, upon excitation at 525 and 560 nm.
The absorbance values match at excitation wavelength for both the free base and zinc derivative.
Figure 13. (left) Differential absorption spectra (visible and near-infrared) obtained upon femtosecond flash photolysis (387 nm) of ZnP-C60
(∼1.0 × 10-6 M) in toluene with 0.05, 1.4, and 50 ps time delays at room temperature. (right) Time-absorption profile of the spectra shown above
at 476 nm, reflecting the intramolecular energy transfer dynamics.
Figure 14. (left) Differential absorption spectra (visible and near-infrared) obtained upon femtosecond flash photolysis (387 nm) of ZnP-C60
(∼1.0 × 10-6 M) in THF with 0.05, 1.4, 50, and 1525 ps time delays at room temperature. (right) Time-absorption profile of the spectra shown
above at 520, 666, and 1000 nm, reflecting the intramolecular charge separation and charge recombination dynamics.
Figure 15. (left) Differential absorption spectra (visible and near-infrared) obtained upon femtosecond flash photolysis (387 nm) of H2P-C60
(∼1.0 × 10-6 M) in THF with 0.05, 1.4, and 200 ps time delays at room temperature. (right) Time-absorption profile of the spectra shown above
at 550, 675, and 1000 nm, reflecting the intramolecular charge separation and charge recombination dynamics.

New Porphyrin Derivatives
J. Phys. Chem. A, Vol. 110, No. 40, 2006 11433
Spectral characteristics, which are taken immediately at the
conclusion of the Zn-system B and system B singlet excited
state decays, bear no resemblance with the singlet or triplet
features of either chromophore. On the contrary, in toluene, the
distinct absorption of the C60 singlet excited state evolves around
900 nm (see Figure 13), within the first 100 ps, with kinetics
(i.e., ZnP-C60, 17.4 ps; H2P-C60, 23 ps) that resemble the Zn-
system B or system B singlet deactivations.
resonance structures of system A; a comparison between the
UV-vis spectra of H2-TPP, system A, and system B; and
cyclic voltammograms of H2-TPP, system A, and system B in
DCB. This material is available free of charge via the Internet
at http://pubs.acs.org.
References and Notes
(1) Guldi, D. M. Chem. Soc. ReV. 2002, 31, 22-36.
Therefore, we postulate that in toluene a transduction of sing-
let excited state energy is responsible for the kinetic and spectral
observations. On the longer time scale, namely, up to 1500 ns,
the C60 singlet/C60 triplet intersystem crossing takes place, which
leads ultimately to the quantitative triplet formation, which is
spectroscopically confirmed by the growth of a new transient
that maximizes at 700 nm. In THF, on the other hand, it is not
the C60 singlet feature in the near-infrared that is seen for both
electron donor-acceptor dyads but the C60 radical anion bands,
as gathered in Figures 14 and 15, with maxima at 1000 nm.
The C60 radical anion band in the near-infrared is comple-
mented by the Zn-system B and system B radical cation
absorptions in the visible with transient maxima at 635 and 690
nm, respectively. Both the radical cation and the radical anion
features are formed with the same kinetics, which leads us to
conclude that in THF an intramolecular electron transfer
converts the initial excited state into a radical ion pair state.
Moreover, the radical cation and radical anion transitions also
decay with the same kinetics. From the corresponding decay
dynamics, we derive lifetimes of the radical ion pair states of
398 and 1175 ps for the ZnP-C60 and H2P-C60, respectively.
The different charge recombination dynamics (i.e., 2.5 × 109
s-1 versus 8.5 × 108 s-1) are well in agreement with kinetics
that are located in the inverted region of the parabolic
dependence of electron transfer rate on the thermodynamic
driving force.
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Conclusions
Two new â-ethynyl porphyrins were synthesized, and the
substituent effect has been studied. The new compounds show
an extension of conjugation along the â-substituent, that leads
to a push-pull effect when the aldehydic group is present on
the phenyl ring of the substituent. These aspects are outlined
by UV-vis, NMR, and cyclic voltammetry studies. Although
the substituent affects the fundamental state properties of the
porphyrins, it has been shown that â-substitution on the pyrrole
ring deeply affects the physicochemical properties of the
porphyrins, giving us a further tool to modulate porphyrin-
fullerene interaction in the electron donor-acceptor systems.
The new porphyrin-fullerene dyad presents a moderate charge
separation state lifetime that could be improved by increasing
the porphyrin-fullerene distance. This prompts us to study the
“photonic wire” behavior of an enlarged spacer.
(14) Kuciauskas, D.; Liu, S.; Seely, G. R.; Moore, A. L.; Moore, T. A.;
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Acknowledgment. The financial support from the Italian
MIUR for a Ph.D. grant (A.L.), the Deutsche Forschungsge-
meinschaft (SFB 583), FCI, and The Office of Basic Energy
Sciences of the U.S. Department of Energy is gratefully
acknowledged. We also thank Dr. Guido Sauer for femtosecond
transient absorption measurements, Giuseppe D’Arcangelo and
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Supporting Information Available: HSQC and HMBC
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1
(29) All of the heteronuclear H-13C NMR spectra are reported in the
Supporting Information.
(30) The three depicted resonance structures are those significative to
explain the magnetical differences occurring in the proton and carbon signals
of the â-pyrrolic positions. In fact, there are many different resonance
structures induced by the â-substituent, as it’s possible to see in the
Supporting Information.
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