Helical polymer-anchored porphyrin nanorods

Article abstract of DOI:10.1002/chem.200390204

Well-defined arrays of porphyrins attached to a rigid polyisocyanide backbone have been synthesized and their physical and optical properties studied. The helical polymers are rigidified by an inter-side chain hydrogen-bonded network and have an average m

Full text of DOI:10.1002/chem.200390204

FULL PAPER
Helical Polymer-Anchored Porphyrin Nanorods
Pieter A. J. de Witte,[a] Mariangela Castriciano,[b] Jeroen J. L. M. Cornelissen,[a]
Luigi Monsu Scolaro,*[b] Roeland J. M. Nolte,[a] and Alan E. Rowan*[a]
¡
Abstract: Well-defined arrays of porphyrins attached to a rigid polyisocyanide
backbone have been synthesized and their physical and optical properties studied.
The helical polymers are rigidified by an inter-side chain hydrogen-bonded network
Keywords:
and have an average mass of 1.1 Â 106 Daltons and a polydispersity indexof 1.3. Each
structures
chirality
¥
helical
¥
nanostructures
¥
of the polymer strands contains four columns of around 200 stacked porphyrins and
polymers ¥ porphyrin arrays
has an overall length of 87 nm. The chromophores are arranged in a left-handed
helical fashion along the polymer backbone. Photophysical studies show that at least
25 porphyrins within one column are excitationally coupled.
routes which usually lead to low yields, although more
recently an elegant procedure has been developed by Osuka
et al. which allows the covalent synthesis of porphyrin arrays
through a AgI-promoted meso meso-coupling, resulting in
linear arrays of up to 128 porphyrins (ca. 106 nm).[5]
Introduction
Well-defined arrays of chromophoric molecules are of great
interest not only as mimics of natural antenna systems but also
because they can display unique optical and physical proper-
ties.[1] The beautiful examples found in nature, for example
the photosynthetic light-harvesting antenna complexes and
reaction centers in bacteria and green plants, contain organ-
ized assemblies of bacteriochlorophyll molecules held togeth-
er by protein scaffolds.[2] The precise organization and
orientation of the chromophores results in efficient absorp-
tion and transportation of light energy and its conversion into
chemical energy.[3]
Inspired by these naturally occurring architectures, much
attention is currently focused on the design of nanometer-
sized chromophoric assemblies, which may find application in
the fields of molecular photonics and electronics. In order to
construct such assemblies several methods have been tried.
Stepwise covalent coupling is one of the most commonly used
approaches.[4] This approach is, however, limited, because
large structures can only be obtained by multi-step synthetic
The introduction of molecular recognition motifs into the
porphyrin building blocks, such as hydrogen bonding, p
p
stacking, electrostatic interactions, and metal-ligand bonds
and the use of self-assembly, offers an easy way to access well-
defined arrays such as fibers, sheets, grids, cubes and rings.[6]
In this paper we describe an alternative approach to obtain
well-defined nanometer-sized arrays of porphyrins by anchor-
ing these molecules to precisely defined polymers, for
example polymers of isocyanides. Many porphyrin pendant
polymers and porphyrin coordination polymers have been
synthesized, but they possessed limited structural definition.[7]
We previously attempted to graft porphyrins onto polyiso-
cyanides but obtained only incomplete substitution of the
polymeric backbone and an ill-defined arrangement of the
chromophores.[8] To overcome this problem we decided to
synthesize monomeric isocyano peptide porphyrins, which
can be subsequently polymerized to give highly organized
architectures. Recently, we reported that polyisocyanides
prepared from isocyanodipeptides form exceptionally stable
b-helical architectures due to the presence of hydrogen-
bonding arrays along the helical polymer backbone.[9] This
secondary hydrogen-bonding network rigidifies the helixand
prevents unwinding, a problem previously encountered with
polyisocyanides.[10] The chiral centers in the side chains
control the handedness of the polymeric backbone, which
allows one in principle to tune the architectures and
consequently the macromolecular properties. The resulting
helical polymers are very rigid, have a precise architecture
[a] Dr. A. E. Rowan, P. A. J. de Witte,
Dr. J. J. L. M. Cornelissen, Prof. Dr. R. J. M. Nolte
University of Nijmegen
Department of Organic Chemistry, Toernooiveld1
6525ED Nijmegen (The Netherlands)
Fax: ( ‡31)24-3652929
E-mail: rowan@sci.kun.nl
¡
[b] Prof. L. Monsu Scolaro, Dr. M. Castriciano
University of Messina
Dipartimento di Chimica Inorganica
Chimica Analitica e Chimica Fisica
Salita Sperone n.3198166, V.ll S.Agata, ISMN-CNR
Sezione di Messina and INFM, Unita' di Messina, Messina (Italy)
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and a persistence length of the order of 70 nm. Using
improved polymerization procedures, the isocyanopeptides
can be polymerized to reach single polymer lengths of up to
12 mm.[11] It was therefore decided to decorate these rigid
peptido polyisocyanides with porphyrins and synthesize
nanometer-long porphyrin wires.
In this paper we describe the synthesis of porphyrin
monomer 1 and its polymerization with a nickel(ii) catalyst
to give polymers in which the chromophores are arranged in
precisely defined stacks that run parallel to the polymer
axis.[12]
dodecyloxy tails was synthesized according to standard
procedures and coupled to 3-bromopropylamine. The result-
ing amino compound 5 was coupled to N-formyl-l-alanine
using dicyclohexylcarbodiimide and N,N-dimethylaminopyr-
idine, and the product 6 was converted into the isocyanide
using diphosgene as dehydrating agent and N-methylmorpho-
line as base. Polymerization of 1 was carried out in CH2Cl2
under air in a reaction vessel protected from light using 3 Â
10À3 equivalents of Ni(ClO4)2 ¥ 6H2O as catalyst. The yield of
À
polymer 2 was 68%. The IR spectrum showed an N H
stretching vibration at 3265 cmÀ1, which implies a structure in
which the side chains are in a hydrogen-bonding arrangement,
in agreement with previous studies on polyisocyanides
prepared from isocyanodipeptides.[13] Additionally, the posi-
tion of the amide carbonyl vibration (1655 cmÀ1) was proof of
its participation in a hydrogen-bonding array.
Results and Discussion
Synthesis: The preparation of porphyrin 1 is shown in
Scheme 1. The monomer is derived from the amino acid l-
alanine in order to provide a peptide bond, which leads to the
b-helical structure of the polymer. In addition the porphyrin is
functionalized with aliphatic tails to increase the solubility of
the resulting polymer. Monohydroxyporphyrin 4 having three
Photophysical studies: The UV/Vis absorption spectra of
monomer 1 and polymer 2 are depicted in Figure 1. The
observed spectra are identical in chloroform and in aromatic
Scheme 1. a) C12H25Br/K2CO3, cyclohexanone; b) p-hydroxybenzaldehyde/pyrrole, propionic acid, reflux 1 h; c) bromopropylamine hydrobromide/NaOH,
DMF; d) N-formyl-l-alanine, DCC/DMAP, CH2Cl2; e) N-methylmorpholine/diphosgene, CH2Cl2 ; f) Ni(ClO4)2 ¥ 6H2O (2 Â 10À3 equiv), CH2Cl2.
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Porphyrin Nanotubes
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blue-shifted Soret bands. No CD signal was observed for the
porphyrin monomer 1. The intense bisignate signal at 431 nm
is slightly asymmetric and blue-shifted by 6 nm due to overlap
of other transitions. It stems from coupling interactions
between the 1st and the 5th porphyrin molecule in the
polymer backbone, which are located on top of each other
(see Figure 6). From the negative sign of this couplet a left-
handed helical arrangement of the porphyrins can be inferred,
according to the exciton chirality method for porphyrins.[14]
The smaller bisignate signals are due to other intramolecular
couplings, as mentioned above. The positive couplet at 400 nm
is assigned to an exciton coupling between the 1st and 4th
porphyrin, which have a right-handed orientation with respect
to each other in the stack (see Takahashi et al.)[12]
* * * *
Figure 1. Absorption spectrum of monomer 1 (
, solvent CHCl3, 2.4 Â
10À6 m) and polymer 2 (––, CHCl3, 4.3 Â 10À6 m), and emission spectrum
Atomic force microscopy: Single molecules of polymer 2 were
visualized by atomic force microscopy (AFM) using spin-
coated solutions of 2 on mica. The results are presented in
Figure 3. At high concentration (10À5 m) monolayer islands of
aggregated polymers are visible (Figure 3a) as is often seen
for rigid rod polymers. Upon dilution, isolated single polymer
strands can be observed (10À6 m, Figure 3b). Further dilution
did not result in any change of the features. The images reveal
that polymer 2 has a rod-like character as was expected for
polyisocyanides with hydrogen-bonding amino acid side
chains.[11] The measured height of the single molecules was
4.2(Æ0.3) nm and of the islands was 4.7(Æ0.2) nm. CPK
models show that the diameter of a polymer chain with
stretched side groups is 8.6 nm, and without tails the diameter
(
, CHCl3, 4.3 Â 10À6 m) of polymer 2.
solvents such as toluene. Dramatic changes are observed upon
going from the monomer spectrum to that of the polymer. The
monomer displays a Soret band at 421 nm, whereas, in the
polymer this band has disappeared and two new Soret bands
are present at 413 and 437 nm. The splitting of the bands and
the very sharp absorption observed at 437 nm are indicative of
the presence of well-defined and excitationally-coupled stacks
of porphyrin molecules. We attribute the red-shifted Soret
band to an offset stacking of the 1st and 5th porphyrin and the
blue shift is attributed to a combination of interactions
between the 1st and 4th porphyrin and the 1st and 2nd
porphyrin.
The emission spectrum of monomer 1 showed a typical two-
band profile of a free-base porphyrin. Polymer 2 displayed
essentially the same two bands (Figure 1), but the maxima
were hypsochromically shifted by 3 nm and the intensity had
decreased significantly. This is attributed to quenching by
neighboring porphyrins.
The circular dichroism spectrum of a solution of 2 in CHCl3
(Figure 2) showed a strong bisignate Cotton effect originating
from the porphyrin Soret band at 437 nm and smaller
bisignate signals with lower intensities corresponding to the
Figure 3. AFM images of polymer 2: a) monolayer islands at 10À5 m (inset:
enlargement of island); b) isolated single polymer fibers at 10À6 m (inset:
enlargement of fibers). (barˆ 100 nm).
Figure 2. CD spectum of polymer 2 (solvent CHCl3, conc. 4.0 Â 10À6 m) and
the corresponding absorption spectrum (
).
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A. E. Rowan, L. Monsu Scolaro, et al.
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amounts to 5.6 nm. It is known that, due to the indentation of
the tip, the measured height can be less than the real height.[15]
The measured values are in good agreement with the
calculated heights and further confirm the presence of single
molecules.
information about the relative orientation of the stacked
porphyrin molecules.[19]
The RLS effect is an enhancement of the scattered light
intensity in the red edge of an absorption band. In turn this is
related to an electronic coupling between adjacent chromo-
phores, to the size and the geometry of the aggregates in
which the chromophores are present, and to the molar
absorbance of the individual chromophores.[17] This phenom-
enon allows identification of the precise orientation of the
chromophores in an aggregate and their interactions even
when they are present in complexmatrices. A simple quantum
mechanical model for RLS, based on exciton-coupling theory,
has recently been developed, addressing the relationship
between the intensities of the observed RLS features and the
electronic and geometrical properties of the aggregates.[19]
The RLS spectrum of 2 (Figure 5) showed the presence of
an intense peak (at least two orders of magnitude larger than
the peak of the neat solvent) in the region of the Soret band
(444 nm), accompanied by a deep well in the region 408
438 nm, due to photon absorption. The enhancement ob-
served for the scattered light intensity of the polymer is
consistent with the presence of large domains of more than 25
(n ꢀ 25) interacting chromophores.
Molecular weight determination: Since it was not possible to
determine the molecular weight and polydispersity of 2 by
MALDI-MS or GPC due to the rod-like nature of the
polymers we turned to the AFM images to calculate these
parameters. Several images of the type shown in Figure 3b
were used to evaluate the polymer contour lengths. The
results are shown in Figure 4. From this histogram the weight-
average and number-average apparent length (Lw and Ln) and
the length polydispersity PD were calculated using the
Equations (1) and (2).[16]
X
X
NiÁL2i
NiÁL
i
i
L ˆ
w
,
Ln ˆ
(1)
X
X
Ni Á Li
Ni
i
i
Lw
PD ˆ
(2)
Ln
These values amounted to: Lw ˆ (87 Æ 7) nm and Ln ˆ (69 Æ
5) nm, giving PD ˆ 1.27 Æ 0.14. Assuming that every mono-
mer segment adds 1.05 ä to the polymer chain,[9] a weight-
average length of 87 nm corresponds to a degree of polymer-
ization of about 830 and a weight-average molecular mass of
1.1 Â 106 Daltons. Therefore these porphyrin polyisocyanides
are probably the longest well-defined porphyrin pendant
polymers described to date.
Figure 5. RLS spectrum of 2 (
). The dispersion profile of the RLS
depolarization ratio 1v(90) is reported as a full line.
In addition to obtaining information concerning the num-
ber of interacting chromophores, depolarized RLS studies can
also give details about the precise orientation of the chromo-
phores in the polymeric nanorods. Figure 5 shows the
dispersion profile of the depolarization ratio 1V(90) at the
absorption feature for polymer 2. The value of 1V(90)
provides information about the geometry of the excited state
of an aggregated species.[19] The absorption feature at 437 nm
has the appearance of a J band and the corresponding 1V(90)
is 0.185. From this an angle a  308 is calculated[20] between
porphyrin molecules 1 and 5 which are stacked 4.2 ä apart
(see below).[9] From this value the twist angle b (Figure 6c) is
calculated to be 228.[21] This angle b is in complete agreement
with that predicted using previous experimental results.
Crystal structures of the monomeric peptido isocyanides
indicate that the hydrogen-bonding network places the
dipeptides 4.7 ä apart. In contrast, studies on simpler poly-
Figure 4. Histogram of the length distribution of 2, as calculated from the
AFM images.
Resonance light scattering: In order to obtain more insight
into the precise geometry of the porphyrins in these long rigid
polymers we used the technique of resonance light scattering
(RLS). This technique has proven to be a sensitive and
selective tool for the investigation of electronic and geo-
metrical properties of aggregated chromophores, such as
synthetic porphyrins[17] and natural chlorophyll a arrays.[18] In
particular, depolarized RLS measurements can give further
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Porphyrin Nanotubes
1775 1781
À
tapping-mode tips (Digital Instruments) were used with a typical resonance
frequency of around 300 kHz.
isocyanides predict an imine C1 C5 length of 4.2 ä. In order to
accommodate this difference, the helixhas to twist by a
calculated angle b of 208. This value is in good agreement with
the value of 228 that is derived from the RLS experiments. In
addition, it is known that porphyrins have a preference for an
off-set stacking.[22]
Light scattering: Resonance light scattering experiments were performed
on a Jasco mod. FP-750 spectrofluorimeter, using a synchronous scan
protocol with a right angle geometry.[17] Depolarized resonance light
scattering measurements were carried out on the same spectrofluorimeter
equipped with linear polarizers (Sterling Optics 105UV). The depolariza-
tion ratio is defined as 1V(90) ˆ IVH/IVV, where IVH and IVV are the scattered
light intensities with horizontal and vertical polarization, respectively. In
order to correct the value of IVH for the difference in transmission efficiency
of polarized light from both excitation and emission monochromators, we
used Equation (3) where G ˆ IHV/IHH is a correction factor.[19]
Conclusions
The combined results presented in this paper indicate that
nanometer-long, well-defined arrays of porphyrin molecules
can be constructed using peptido isocyanide polymers as
scaffolds. These polymers have a rigid chiral core to which the
porphyrin chromophores are attached in four parallel stacks.
In one stack the porphyrins are precisely 4.2 ä apart with a
slip angle of 308 (see Figure 6). Resonance light scattering
shows that the arrays contain at least 25 interacting porphyrin
molecules in one stack, with the overall stack having an
average length of 87 nm.
IVH
1V (90) ˆ G Â
(3)
IVV
Depolarized dynamic light scattering measurements were made with a
Malvern 4700 submicron particle analyzer. Elastic light scattering experi-
ments were performed with a home-built goniometer apparatus in the
range 20 1508 (5.8 32.5mmÀ1 in the scattered wavevector range). The
exciting light source was a 50 mW polarized Nd/YAG laser (532 nm). A
Glan-Thompson polarizer was placed before the photomultiplier.
Molecular weight determination with AFM: For the analysis of the
polymer several images were evaluated, taken from different samples and
different places on the mica substrate. Only molecules that were separated
and had a consistent height and phase appearance were measured. In total
1058 molecules were analyzed. It was assumed that the overestimation of
the length due to the shape of the AFM tip was equal to the overestimation
of the width.
To the best of our knowledge the present polymer system is
the only system displaying a unique well-defined face-to-face
porphyrin architecture involving hundreds of chromophores
arranged over tens of nanometers.[12]
Synthesis
4-(Dodecyloxy)benzaldehyde (3): This compound was synthesized using a
modified literature procedure, that is dodecyl bromide was used instead of
hexadecyl bromide in cyclohexanone.[23] 1H NMR (CDCl3, 300.13 MHz):
d ˆ 9.88 (s, 1H; HC(O)), 7.82 (d, 2H; ArH ortho to formyl), 6.98 (d, 2H;
ArH ortho to formyl), 4.04 (t, 2H; OCH2), 1.81 (p(pentet), 2H; OCH2CH2),
1.46 (p, 2H; OCH2CH2OCH2), 1.27 (m, 16H; aliphatic), 0.88 (t, 3H; CH3).
Porphyrin 4: A suspension of 3 (15 g, 51.7 mmol) and 4-hydroxybenzalde-
hyde (2.09 g, 17.3 mmol) in propionic acid was heated until the compounds
were dissolved. As soon as the temperature had reached 1108C freshly
distilled pyrrole (4.66 g, 69.2 mmol) was added at once to the solution,
whereupon the mixture was refluxed for 1 h. After cooling, the obtained
slurry was filtered and washed with ethanol until most of the polypyrrole
side product was removed. After column chromatography (2 Â ) (silica gel,
0
2% MeOH in CHCl3) compound 3 was isolated as a purple solid (0.95 g,
1
4.6%). H NMR (CDCl3, 300.13 MHz): d ˆ 8.86 (m, 8H; b-pyrrole), 8.10
(d, 6H; ArH meta to OR), 8.06 (d, 2H; ArH meta to O-aminopropyloxy),
7.28 (d, 6H; ArH, ortho to OR), 7.19 (d, 2H; ArH ortho to O-amino-
propyloxy), 5.01 (brs, 1H; OH), 4.25 (t, 6H; OCH2), 1.98 (p, 6H;
OCH2CH2), 1.63 (p, 6H; OCH2CH2CH2), 1.37 (m, 48H; aliphatic), 0.89 (t,
9H; CH3), À2.77 (s, 2H; NH).
Figure 6. a) Schematic drawing of a molecule of polymer 2, one column is
shown in yellow; b) side view of a detail of the stack showing that the fifth
porphyrin has a slip angle of 308 with respect to the first porphyrin; c) top
view showing a twist angle b of 228.
Porphyrin 5: For the synthesis of this compound a modified literature
procedure was followed.[24] To a solution of 4 (350 mg, 0.30 mmol) in DMF
(10 mL) and toluene (15 mL) was added crushed sodium hydroxide
(0.245 g). After stirring for 20 min at room temperature, 3-bromopropyl-
amine hydrobromide (74 mg, 0.34 mmol) was added and the reaction was
continued for 3 h, upon which another 50 mg (0.23 mmol) of 3-bromopro-
pylamine hydrobromide was added. The suspension was stirred for an
additional 1 h and the reaction mixture was poured into water and
extracted with dichloromethane (2 Â ). The combined organic layers were
washed with water, dried (MgSO4) and evaporated to dryness. The
resulting purple solid was subjected to column chromatography (silica
gel, 5% MeOH in CHCl3) to yield 5 as a purple solid (276 mg, 74.2%).
1H NMR (CDCl3, 300.13 MHz): d ˆ 8.86 (m, 8H; b-pyrrole), 8.09 (d, 8H;
ArH meta to OR), 7.26 (d, 8H; ArH ortho to OR), 4.35 (t, 2H; OCH2), 4.24
(t, 6H; OCH2), 3.10 (t, 6H; NHCH2), 2.16 (p, 2H; NHCH2CH2), 1.96 (p,
6H; OCH2CH2), 1.61 (m, 6H; OCH2CH2CH2), 1.30 (m, 48H; aliphatic),
0.90 (t, 9H; CH3), À2.75 (s, 2H; NH); 13C NMR (CDCl3, 75.47 MHz): d ˆ
158.9 (ArC ipso to OCH3), 158.7, 135.6, 134.7, 134.4 (all ArC), 130.9 (br;
ArC next to ArN), 119.8, 119.6, 113.8, 112.7 (all ArC), 68.3, 66.2 (ArOCH2),
These rigid rod-like porphyrin polymers may be of interest
as a synthetic antenna system, which will be further optimized
and equipped with an energy gradient to measure energy
transfer properties. Work along this line is in progress.
Experimental Section
Atomic force microscopy: AFM experiments were performed using a
Nanoscope IIIa instrument from Digital Instruments. A solution of 2 in
CHCl3 was spincoated onto freshly cleaved Muscovite mica. All images
were taken in tapping mode in air at room temperature. Commercial
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39.4 (CH2NH2), 31.9 (CH2CH2CH3), 29.6 (m; CH2), 26.2 (CH2CH2NH2),
22.7 (CH2CH3), 14.2 (CH3); FAB-MS: m/z: 1240 [M] .
Acknowledgement
‡
Dr. P. Thordarson is acknowledged for performing MALDI-MS experi-
ments and J. Gerritsen for help with the AFM experiments. This work was
supported by the European Science Foundation through the SMARTON
programme.
Porphyrin 6: N-Formyl-l-alanine (31.8 mg, 0.168 mmol) was suspended in a
solution of 5 (190 mg, 0.153 mmol) in dichloromethane (10 mL) and the
mixture was cooled with an ice bath. To this mixture was added
dicyclohexylcarbodiimide (34.9 mg, 0.168 mmol) and a catalytic amount
of dimethylaminopyridine. The reaction mixture was stirred for 1 h at 08C
under a nitrogen atmosphere and stirred overnight at room temperature.
The mixture was concentrated and subjected to column chromatography
(silica gel, 2% MeOH in CHCl3). The collected purple solid was dissolved
in CHCl3 and precipitated by addition of MeOH. The compound was
filtered, washed with MeOH and dried to give 6 as a purple solid (142 mg,
69.3%). 1H NMR (CDCl3, 300.13 MHz): d ˆ 8.86 (m, 8H; b-pyrrole), 8.22
(s, 1H; formyl), 8.12 (d, 2H; ArH meta to O-propyl), 8.10 (d, 2H; ArH
meta to OC12H25), 7.26 (d, 8H; ArH ortho to OR), 6.50 (t, 1H; NHCH2),
6.36 (d, 1H; HC(O)NH), 4.62 (p, 1H; CH), 4.33 (t, 2H; OCH2), 4.24 (t, 6H;
OCH2), 3.65 (q, 2H; NHCH2), 2.22 (p, 2H; NHCH2CH2), 1.97 (p, 6H;
OCH2CH2), 1.63 (p, 6H; OCH2CH2CH2), 1.50 (d, 3H; alanine CH3), 1.30
(m, 48H; aliphatic), 0.89 (t, 9H; CH3), À2.75 (s, 2H, NH); 13C NMR
(CDCl3, 75.47 MHz): d ˆ 171.2 (CO; alanine), 160.3 (HCO; formyl), 158.5
(ArC ipso to OC12H25), 157.9 (idem), 135.2 (ArC meta to OC12H25 and O-
propyl), 134.7 (CH; b-pyrrole), 134.0 (C; meso), 130.6 (br; ArC next to
ArN), 119.5 (ArC ipso to meso C), 119.0 (idem), 112.3 (ArC ortho to OR),
67.9 (OCH2), 66.1 (OCH2), 47.3 (CH; alanine), 37.4 (H2NCH2), 31.5 22.3
(CH2; aliphatic), 25.8 (CH2; propyl), 18.1 (CH3; alanine), 13.7 (CH3); MS
(HR-MALDI-TOF, dithranol): m/z: calcd for C87H114N6O6: 1338.880;
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‡
found: 1338.882 [M] .
Isocyanoporphyrin 1: N-Methylmorpholine (5.3 mL, 47.4 mmol) was added
to a solution of the formamide 6 (30 mg, 22.4 mmol) in CH2Cl2 (6 mL). The
mixture was cooled on an ice bath at 08C and over a period of 30 min
diphosgene (1.4 mL, 11.3 mmol in 1 mL CH2Cl2) was added. After 10 min
the ice bath was removed and the mixture was dropped into a vigorously
stirred saturated aqueous NaHCO3 solution (100 mL). The resulting
mixture was extracted with CH2Cl2 (2 Â ), and the combined organic layers
were washed with water (2 Â ), dried (MgSO4), and concentrated. The
resulting solid was purified by preparative HPLC (reversed phase, 1%
MeOH in CHCl3) to yield the title compound as a purple solid (20 mg,
67.6%). 1H NMR (CDCl3, 300.13 MHz): d ˆ 8.86 (m, 8H; b-pyrrole), 8.13
(d, 2H; meta to O-propyl), 8.10 (d, 6H; meta to OC12H25), 7.34 (d, 2H; ortho
to O-propyl), 7.27 (d, 6H; ortho to OC12H25), 7.15 (t, 1H; NHCH2), 4.39 (t,
2H; OCH2), 4.25 (t, 6H; OCH2), 4.34 (q, 1H; CH), 3.71 (q, 2H; NHCH2),
2.26 (p, 2H; NHCH2CH2), 1.98 (p, 6H; OCH2CH2), 1.74 (d, 3H; alanine
CH3), 1.63 (p, 6H; OCH2CH2CH2), 1.31 (m, 48H; aliphatic), 0.90 (t, 9H;
CH3), À2.75 (s, 2H; NH); 13C NMR (CDCl3, 75.47 MHz): d ˆ 166.2 (CO;
alanine), 158.9 (ArC ipso to OC12H25), 158.3 (idem), 135.7 (ArC meta to
OC12H25 and O-propyl), 135.1 (CH; b-pyrrole), 134.1 (C; meso), 131.0 (br;
ArC next to ArN), 120.0 (ArC ipso to meso C), 119.5 (idem), 112.8 (ArC
ortho to OR), 68.2 (OCH2), 67.1 (OCH2), 53.8 (CH; alanine), 38.7
(H2NCH2), 32.0 22.5 (CH2; aliphatic), 26.1 (CH2; propyl), 20.0 (CH3;
alanine), 14.2 (CH3); FTIR (KBR): nÄ ˆ 3313 (NH), 2134 (NC), 1685 cmÀ1
(amide); MS (HR-MALDI-TOF, dithranol) m/z: calcd for C87H112N6O5:
‡
1320.869; found: 1320.876 [M] .
Polymer 2: In a vessel protected from light monomer 1 (19 mg, 14 mmol)
was dissolved in CH2Cl2 (1.5 mL). To this solution was added 0.003 equiv
Ni2‡ catalyst (0.35 mL of a solution of Ni(ClO4)2 ¥ 6H2O (2.5 mg) in 97 mL
CH2Cl2 and 3 mL EtOH). The mixture was stirred for 1 h, after which it was
poured into methanol/water 1:1 v/v (50 mL). The precipitate was filtered
and the residue washed with acetone until the filtrate was colorless,
followed by washing with dichloromethane. The residue was dissolved in
CHCl3 and precipitated in EtOAc filtered and dried, resulting in a purple/
red solid (13 mg, 68%). 1H NMR (CDCl3, 400.15 MHz): d ˆ 9.0 6.7 (br
with maxima at 8.7, 8.0, 7.2), 4.1 (br; OCH2), 2.5 1.0 (br with maxima at 1.6,
1.5, 1.3, 0.9, 0.7, 0.6, 0.3); 13C NMR (CDCl3, 100.62 MHz): d ˆ 32.2, 30.1,
23.1, 14.5 (aliphatic tails); FTIR (KBR): nÄ ˆ 3265 (NH), 1655 (amide),
√
Monsu Scolaro, R. Lauceri, S. Gurrieri, A. Romeo, J. Am. Chem. Soc.
1605 cmÀ1 (C N).
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ˆ
For further characterization see text. It was not possible to determine the
molecular weight of polymer 2 by GPC due to severe tailing on the column.
MALDI-TOF MS experiments were also unsuccessful. AFM was used
instead (see text).
1780
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Chem. Eur. J. 2003, 9, No. 8

Porphyrin Nanotubes
1775 1781
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¬
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1
porphyrin planes. The expected range for this ratio is 1³8 ꢁ 1V(90) ꢁ ³3,
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the group of Takahashi indepently prepared aryl isocyanides bearing
À
porphyrins, which have been polymerized using a Pd Pt m-ethynediyl
dinuclear complexas initiator. They have used the porphyrin side
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Received: September 11, 2002 [F4409]
Chem. Eur. J. 2003, 9, No. 8
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1781