Synthesis of extremely π-Extended porphyrin tapes from hybrid meso-meso linked porphyrin arrays: An approach towards the conjugation length

Article abstract of DOI:10.1002/asia.200900125

Directly meso-meso, β-β, β-β triply linked porphyrin arrays are exceptional π-conjugated molecules exhibiting remarkably red-shifted absorption bands extending deeply in the IR region. In order to determine the effective conjugated length (ECL), we embark

Full text of DOI:10.1002/asia.200900125

FULL PAPERS
DOI: 10.1002/asia.200900125
Synthesis of Extremely p-Extended Porphyrin Tapes from Hybrid meso-meso
Linked Porphyrin Arrays: An Approach Towards the Conjugation Length
Toshiaki Ikeda, Naoki Aratani, and Atsuhiro Osuka*[a]
Abstract: Directly meso-meso, b-b, b-b
triply linked porphyrin arrays are ex-
ceptional p-conjugated molecules ex-
hibiting remarkably red-shifted absorp-
tion bands extending deeply in the IR
region. In order to determine the effec-
tive conjugated length (ECL), we em-
barked on the synthesis of the porphy-
rin tapes far beyond the 12-mer, which
is the longest we have prepared so far.
In this study, to find the compromise
between the feasibility of the meso-
meso coupling reaction up to longer
arrays and the sufficient solubility and
chemical stability of the resultant por-
phyrin tapes, we prepared hybrid meso-
meso linked porphyrin arrays BOn up
to 24-mer, which have two different
aryl groups, a 2,4,6-tris(3,5-di-tert-butyl-
phenoxy) phenyl group (Ar1) and a
3,5-dioctyloxy phenyl group (Ar2). All
these arrays were effectively converted
into the corresponding triply linked
porphyrin tapes TBOn by oxidation
low energy Q-band-like absorption
bands of TBOn are progressively red-
shifted with an increase in the number
of porphyrins n until 16 but the red-
shift is saturated at n=16, indicating
the ECL of the porphyrin tape to be
around 14–16. The regularly introduced
meso-aryl bulky substituents impose
facial encumbrance, hence leading to
the effective suppression of p–p inter-
actions as well as improvement of the
chemical stabilities of TBOn.
with DDQ-ScACTHUNRGTNEUNG(OTf)3. Importantly, the
Keywords: conjugation · molecular
wires · oligomerization · porphyrin
tapes · porphyrinoids
Introduction
tionally, for one-dimensional p-systems, there is an intrinsic
problem of saturation as characterized by the effective con-
jugated length (ECL). ECL defines the extent of p-conju-
gated systems in which the electronic delocalization is limit-
ed and at which point the optical, electrochemical, and
other physical properties reach a saturation level that is
common with the analogous polymer.[1a,3c]
Porphyrin, a tetrapyrrolic pigment with an 18p-electron
conjugated system, has been used in a variety of fields in-
cluding reaction catalyst, artificial photosynthesis, photody-
namic therapy, sensors, and so on.[4] Electronic properties of
porphyrins are susceptible to chemical modifications at the
periphery. These characteristics have been used, through the
attachments of unsaturated segments to a porphyrinic p-net-
work, to create conjugated porphyrins that exhibit rather al-
tered optical and electrochemical properties.[5–12] To extend
the p-conjugation of porphyrin oligomers, it is important to
enforce a planar structure by installing multiple direct link-
ages between the porphyrin moieties. Along with this strat-
egy, we have recently explored directly meso-meso, b-b, b-b
triply linked porphyrin arrays that are so-called as porphyrin
tapes. These porphyrin arrays are unprecedented in respect
of the extent of p-conjugation, in that their absorption spec-
tra exhibits the progressive red-shift, reaching an exception-
Organic molecules with extended p-conjugated systems
have attracted considerable interest because of their possi-
ble applications to organic conducting materials, non-linear
optical (NLO) materials, near-infrared (NIR) dyes, and mo-
lecular wires.[1–3] Therefore, extensive synthetic efforts have
been made towards such conjugated molecular systems but
have often encountered serious problems such as synthetic
difficulty, chemical instability, and poor solubility with in-
creasing the size of p-conjugated systems. For the use of
these extended p-conjugated systems to electronic and opti-
cal devices, such problems need to be solved. In this respect,
discrete molecules with extended p-conjugation should be
equipped with chemical stability and good solubility. Addi-
[a] Dr. T. Ikeda, Dr. N. Aratani, Prof. Dr. A. Osuka
Department of Chemistry
Graduate School of Science
Kyoto University
Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto 606-8502 (Japan)
Fax : (+81)75-753-3970
E-mail: osuka@kuchem.kyoto-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.200900125.
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ally red-shifted absorption band at 2800 nm for the dodeca-
meric porphyrin tape 1a (Figure 1).[13] These features are
quite intriguing, since they do not exhibit a saturation be-
havior up to the dodecamer, indicating that ECL of these
arrays is at least larger than 12. Additionally, these porphy-
rin tapes have been also shown to be promising as non-
linear optical materials, as recent studies revealed their ex-
ceptionally large two-photon absorption (TPA) values.[14,15]
Despite these promises, triply linked porphyrin tapes have
serious problems such as poor solubility, strong stacking ten-
dency, and chemical instability, all of which become more
serious with increasing number of porphyrin units. In order
to solve these problems, we prepared doubly-strapped por-
phyrin tapes[16] and insulated porphyrin tapes by bulky
meso-aryl substituents.[17] These porphyrin tapes were syn-
thesized up to dodecamer 1b and tetramer 1c, respectively.
Chemical instability was certainly improved in the doubly-
strapped porphyrin tapes but their solubility became seri-
ously dropped upon an increase in the number of the array.
Both the chemical stability and solubility were considerably
improved in the insulated porphyrin tapes, but the meso-
meso coupling reactions became increasingly difficult for
longer arrays, probably as a consequence of a continuous
buttressing effect. Accordingly, the longest array in the
latter series is the tetramer 1c and we could not find any
suitable conditions for the coupling of 1c.
Scheme 1. Schematic representation for making hybrid porphyrin arrays.
The triangle shows Ar1 and the stick shows Ar2.
cient space to mitigate the serious steric buttressing effect
for further coupling. In this paper, we report the synthesis
of a new series of hybrid porphyrin arrays, which have 2,4,6-
tris(3,5-di-tert-butylphenoxy)phenyl group (Ar1) and 3,5-dio-
ctyloxyphenyl group (Ar2).
Results and Discussion
Synthesis of meso-meso Linked Porphyrins
Meso-meso linked hybrid diporphyrin BO2 was synthesized
as shown in Scheme 2. Mono meso-borylated ZnII porphyrin
with Ar1 (BB1) and mono meso-bromo porphyrin with Ar2
(BrO1) were synthesized following the reported methods.[18]
Suzuki–Miyaura cross coupling reactions of usual porphyrins
have been already developed with satisfactory yields.[19] In
contrast, the Suzuki–Miyaura coupling reaction of BB1 and
In the present study, to overcome these problems, we em-
ployed the hybrid strategy to use, as a construction motif, a
meso-meso linked tetraporphyrin unit that consists of two
terminal porphyrins, bearing bulky meso-aryl substituents,
and two internal porphyrins, bearing soluble long meso-aryl
substituents (Scheme 1). The idea is to provide substantial
facial encumbrance against p–p stacking yet provide suffi-
BrO1 (Cs2CO3, PdACTHUNTRGNE(UNG PPh3)4, toluene/DMF (2:1)) and follow-
Figure 1. Molecular structures of 1a, 1b, and 1c
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Scheme 2. Synthesis of BO2. (i) NBS (1.1 equiv), pyridine, CHCl3. (ii) pinacolborane, PdCl2ACHTUNGTRNE(UNNG PPh3)2, triethylamine, 1,2-dichloroethane, 62% for 2 steps.
(iii) NBS (1.1 equiv), pyridine, CHCl3, 53%. (iv) Pd
for 2 steps.
2CAHTNUGTRNEUN(GN dba)3, tri-2-furylphosphine, Cs2CO3, CsF, THF, DMF, H2O. (v) ZnACHTUNGTERN(NUNG OAc)2, methanol, CHCl3, 23%
ing zinc(II) insertion, gave meso-meso linked hybrid dipor-
phyrin BO2 in only 9% yield. The poor yield may be ascri-
bed mainly to the severe steric hindrance of Ar1. The yield
was improved up to 23% by using tri-2-furylphosphine in-
stead of triphenylphosphine and THF/DMF/H2O (10:2:1) as
solvent. Diporphyrin BO2 exhibited the parent molecular
ion peak at m/z=2782.31 (calcd for C180H222N8O10Zn2,
2783.57) in the MALDI-TOF mass spectrum. The 600 MHz
1H NMR spectrum of BO2 in CDCl3 exhibited two singlet
peaks for meso protons at 10.39 and 10.23 ppm, four sets of
mutually coupled doublets for the peripheral b-protons at
9.50 and 9.27 ppm, 9.47 and 9.44 ppm, 8.95 and 8.07 ppm,
and 8.73 and 8.09 ppm in line with the assigned meso-meso
linked hetero porphyrin dimer structure (see Figure S1 in
the Supporting Information).
The silver(I)-promoted meso-meso coupling reaction is se-
riously influenced by steric hindrance of the meso-aryl sub-
stituent.[17] Reflecting this trend, meso-meso linked porphy-
rin tetramer BO4 was formed as a result of regioselective
coupling of BO2 at the Ar2 side in 35% yield under mild
conditions (3 equivalent of AgPF6 at room temperature for
12 h) along with recovered BO2 (56%). A trace amount of
meso-meso linked porphyrin hexamer BO6 was also ob-
tained as a byproduct in this reaction, as a result of further
coupling of BO4 and BO2 (Scheme 3). These two oligomers
were characterized by 1H NMR, MALDI-TOF mass, and
UV/Vis absorption spectroscopy. Each compound exhibited
the expected parent molecular ion peaks in the MALDI-
TOF mass spectroscopy. The H NMR spectrum of BO4 in
CDCl3 provided a singlet at 10.23 ppm for the meso proton,
and four sets of mutually coupled doublets for the b-protons
at 9.48 and 9.45 ppm, 9.03 and 8.24 ppm, 8.89 and 8.32 ppm,
and 8.75 and 8.13 ppm, which indicates BO4 has S4 symme-
try.
Under the same mild conditions, meso-meso coupling re-
action of BO4 did not proceed at all, probably arising from
the steric hindrance at the edge porphyrins that bear large
Ar1 substituents. We, thus, attempted the coupling reaction
of BO4 under stronger conditions (Scheme 4). Coupling
products BO8 (9%) and BO12 (2%) were obtained when
BO4 was refluxed with 10 equivalents of AgPF6 in CHCl3
for 72 h. Coupling yields were further improved by the addi-
tion of N,N-dimethylacetamide (DMA) (1 mol%),[21] which
gave BO8 (15%), BO12 (5%), BO16 (2%), 20-mer (1%),
and 24-mer (trace). These oligomers were all separated in a
pure form and fully characterized by 1H NMR, MALDI-
TOF mass, and UV/Vis absorption spectroscopy. MALDI-
TOF mass spectrometry was very effective for the detection
of the parent molecular ion peaks of these oligomers. The
1H NMR spectra of these oligomers in CDCl3 provided a
singlet around 10.2 ppm for the end meso-protons, and sev-
eral peaks of doublets from 8.1 to 9.5 ppm for the b-protons,
which indicates these oligomers
1
also have S4 symmetry. In the
1H NMR spectra of 20-mer and
24-mer, some minor peaks were
observed after purification by
silica-gel column chromatogra-
phy and recrystallization, and
the UV/Vis absorption spectra
of these compounds showed ad-
ditional peaks around 700 to
1700 nm, which are attributed
Scheme 3. AgI-promoted oxidation coupling of BO2. (i) AgPF6 (3 equiv), CH3CN, CHCl3, room temperature,
12 h.
to triply linked porphyrin dimer
and trimer (see Supporting In-
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Synthesis of Extremely p-Extended Porphyrin Tapes
parent ion peaks for TBO4 at
m/z=5550.21
(calcd
for
C360H430N16O20Zn4;
m/z=
5553.03), but the parent ion
peaks of higher porphyrin tapes
could not be detected by this
1
method. Although the H NMR
spectrum of TBO4 in CDCl3
showed broadening of peaks,
which may be a result of aggre-
gation character,
a relatively
clear spectrum could be ob-
tained at low concentration
(see Supporting Information).
Scheme 4. AgI-promoted oxidation coupling of BO4. (i) AgPF6 (10 equiv), DMA (1 mol%), CH3CN, CHCl3,
reflux, 72 h.
1
The H NMR spectrum shows a
singlet for the meso proton at
8.24 ppm, a set of mutually cou-
formation). These observations imply that the fractions of
20-mer and 24-mer concomitantly include partially fused
parts in their molecular structure. In our previous studies,
these partially fused porphyrin arrays were not detect-
ed.[10,16,17] The formation of such partially fused porphyrin
arrays may be ascribed to the rather strong reaction condi-
tions used. Since it is difficult to remove these partially
fused oligomers and it does not influence the next fusion re-
action of these oligomers, 20-mer and 24-mer were used for
the next fusion reaction without further purification.
pled doublets for the outer b-protons at 8.06 and 7.99 ppm,
and three singlets for the inner b-protons at 7.37, 6.58, and
6.56 ppm. These spectral data are in accord with the previ-
ously reported insulated porphyrin tape tetramer.[17] The
1H NMR spectrum of TBO6 in CDCl3 displays seriously
broadened peaks in the range of 7.8 to 8.5 ppm, which may
correspond to the meso and b protons. Further assignments
1
of these H NMR spectra were attempted by adding axial li-
gands, such as n-butylamine, using a variety of solvents
(C2D2Cl4, [D5]pyridine, and [D8]toluene), or increasing the
temperature; up to 1408C in C2D2Cl4. But these attempts
failed owing to the significant aggregation. The observed
broadening arises from the p–p stacking of porphyrin tapes.
Therefore it is conceivable that the regularly introduced
bulky aryl groups (Ar1) are certainly effective in the sup-
pression of p–p stacking for shorter porphyrin tapes but are
not sufficient enough for the longer porphyrin tapes. The
1H NMR spectra of longer porphyrin tapes were quite broad
and almost useless for the structural characterizations.
Transformation into Porphyrin Tapes
In general, the oxidation of terminal-meso-free meso-meso
linked porphyrins with DDQ-Sc
polymeric products as a result of intermolecular coupling. In
contrast, the DDQ-Sc(OTf)3 oxidation of BOn in toluene at
ACHTUNGRTEN(NUNG OTf)3 gives complicated
AHCTUNGTRENNUNG
808C for 2 h afforded meso-free porphyrin tapes without
any intermolecular coupling.[17] In this way, terminal-meso-
free porphyrin tapes TBOn were synthesized from BOn
under the same conditions in the following yields; TBO4
(63%), TBO6 (68%), TBO8 (56%), TBO12 (52%),
TBO16 (54%), TBO20 (44%), and TBO24 (38%)
(Scheme 5). The molecular length of the 24-mer reached
20 nm long. MALDI-TOF mass spectroscopy revealed the
Optical Properties
The absorption spectra of meso-meso linked porphyrin olig-
omers BOn in CHCl3 are shown in Figure 2. Similarly to the
previously reported meso-meso linked porphyrin oligomers,
BOn exhibit split Soret bands
and red-shifted Q-bands. These
split Soret bands can be under-
stood in terms of the exciton
coupling theory.[21,22] Of the
split two Soret bands, the high-
energy band remains in the
same position as that of a ZnII
porphyrin monomer, and the
other low-energy band is pro-
gressively red-shifted as the
number of porphyrins increases.
The UV/Vis/NIR absorption
spectra of porphyrin tapes
Scheme 5. DDQ-Sc
G
G
TBOn in CHCl3 are shown in
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Figure 4 shows the plot of the QACTHNUGTRNEUNG(0,1) absorption peak of
band III of TBOn versus the number of porphyrin units n.
In the previous report,[16] it is indicated that the observed
Figure 2. UV/Vis absorption spectra of BOn in CHCl3.
Figure 3. Similarly to the previously reported porphyrin
tapes,[13] the absorption spectra of TBOn have three main
bands; band I (~400 nm), band II (500~1000 nm), and band
Figure 4. Plots of the QACTHNUTRGENUG(N 0,1) absorption peak of band III of TBOn from
the UV/Vis/NIR spectra versus the number of porphyrin units n.
absorption maxima in band III of the broadened absorption
spectra of porphyrin tapes is attributed to a Q
tion, so the observed peaks in band III of TBO12, TBO16,
TBO20, and TBO24 are Q(0,1) absorptions. In a range of n
ACHTUNGTREN(NUNG 0,1) absorp-
AHCTUNGTRENNUNG
smaller than 12, the plot is linear, but the red-shift of ab-
sorption peak is saturated at n=16. Because theoretical
analysis suggest that the plot would give rise to a well-corre-
lated straight line if the plot was based on the free electron
model,[23a] saturation of the red-shift means saturation of the
extension of p-conjugation, which indicates that the ECL of
TBOn is about 14-mer (ca 12 nm). This is the first experi-
mental observation to reveal the ECL of a triply-linked por-
phyrin tape.
Figure 3. UV/Vis/NIR absorption spectra of TBOn in CHCl3.
III (>1000 nm). Band I is commonly observed around
400 nm at the same position of the Soret band of porphyrins,
and band II is shifted to lower energy with an increase in
the number of porphyrins, and band III is the lowest energy
bands and is most red-shifted with an increase in the
number of porphyrins. The bands I and II have been inter-
preted as split Soret bands and band III has been interpret-
ed as Q-like bands on the basis of several theoretical analy-
ses.[23] Band III of TBO4 and TBO6 shows typical vibronic
Since the electronic absorption bands of TBOn reach the
infrared (IR) region, their IR spectra in a KBr pellet were
examined (Figure 5). In these spectra, the electronic absorp-
structure of porphyrins consisting of QACTHNUGRTENNU(G 0,0) and QAHCUTNGTREN(NGUN 0,1) ab-
sorption bands, and that of TBO8 partially preserves such
vibronic structures. Those of longer oligomers are very
broad without such vibronic structure. Importantly, the ab-
sorption maxima of band I is observed at 422 nm for TBO4
and TBO6, while that of TBO8 is observed at 480 nm, and
those of the longer porphyrin tapes (TBO12, TBO16,
TBO20, and TBO24) are observed at 495 nm. These obser-
vations may indicate the existence of J-aggregate-like aggre-
gations for the longer porphyrin tapes. This interpretation
seems in accord with the shapes of band III, considering
that aggregation causes the loss of vibronic structure of the
porphyrin tape. On the basis of this consideration, it may be
concluded that TBO4 and TBO6 form no aggregates, TBO8
partially forms aggregates, and the longer tapes aggregate
effectively in CHCl3.
Figure 5. IR absorption spectra of TBOn in KBr. Arrow indicates the
band top position.
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Synthesis of Extremely p-Extended Porphyrin Tapes
tions were actually observed in addition to the stretching
and rotational absorptions. Like the UV/Vis/NIR absorption
spectra, a red-shift of band III was observed as the number
of porphyrin units increases, and the red-shift is saturated at
3800 cmÀ1 at about n=16. This result accords with the con-
clusion obtained from the UV/Vis/NIR spectra.
Electrochemical Properties
The first oxidation potential of triply linked porphyrin tapes
were reported up to the octamer,[13d] but details of electro-
chemical character of porphyrin tapes have not been re-
vealed, mainly owing to the low solubility of porphyrin
tapes. The improved solubility of the present porphyrin
tapes, caused by the introduction of bulky meso-aryl sub-
stituents, encourages us to investigate the electrochemical
properties of porphyrin tapes. The oxidation and reduction
potentials of porphyrin monomer B1 and porphyrin tapes
TB2, TB3, TB4, TBO4, TBO6, and TBO8 (for the struc-
tures of TB2, TB3, and TB4, see Supporting Information)
were measured by cyclic voltammetry (CV) and differential
pulse voltammetry (DPV) (Figure S12 in the Supporting In-
formation). Porphyrin tapes showed two reversible peaks
for one-electron oxidation and two reversible peaks for one-
electron reduction. Unfortunately, the potentials of longer
tapes could not be determined because of serious peak
broadening. The first oxidation potentials (Eox1) and the first
1
reduction potentials (Ered1) are summarized in Table 1. Eox
1
gradually decrease and Ered increase as the number of por-
1
^
Figure 6. a) The first oxidation potential Eox
( ) and the first reduction
1
1
phyrin units n increases. Each of Eox and Ered are propor-
tional to nÀ1 (Figure 6a).[23a] The reduction potentials of por-
phyrin tapes were observed for the first time, and the
HOMO–LUMO energy gaps were estimated from
1
potential Ered (~) of B1, TB2, TB3, TB4, TBO4, TBO6, and TBO8
versus the inverse number of porphyrin units n. b) HOMO–LUMO
^
energy gap estimated by electrochemical method ( ) and optical method
(~) of B1, TB2, TB3, TB4, TBO4, TBO6, and TBO8 versus the inverse
number of porphyrin units n.
1
1
Eox ÀEred (Table 2), which decrease as n increases. Table 2
also lists the HOMO–LUMO energy gap estimated by lmax
of band III in the UV/Vis/NIR absorption spectra. These
HOMO–LUMO gaps estimated from the electrochemical
method and optical method show good agreement and each
of them is indirectly proportional to n. These linear depend-
encies of oxidation potential, reduction potentials, and also
HOMO–LUMO gaps are in line with the view that the elec-
tronic delocalization of the porphyrin tapes is fully expand-
ed over the whole array, in which the behaviors of p-elec-
trons are like that of a particle in a box model within ECL
(Figure 5b).
Table 2. HOMO–LUMO energy gap estimated by electrochemical
method and optical method.
B1
TB2 TB3 TB4 TBO4 TBO6 TBO8
electrochemical [eV] 2.48 1.18 0.85 0.63 0.60
0.43
0.60
0.25
0.52
optical [eV]
2.28 1.18 0.89 0.76 0.75
Summary
Hybrid meso-meso linked porphyrin arrays and porphyrin
tapes, which bear two meso-aryl substituents, a bulky group
Ar1 and a non-bulky group Ar2,
were prepared and character-
ized. Long porphyrin tape 16-
mer, 20-mer, and 24-mer were
Table 1. The oxidation and reduction potentials of porphyrin tapes.
B1[a]
TB2[a]
TB3[a]
TB4[b]
TBO4[a]
TBO6[a]
TBO8[b]
synthesized for the first time,
and the ECL of porphyrin tape
was estimated to be about n=
14, which revealed that the por-
phyrin tape has an extremely
extended p-conjugation length
Eox1 [V]
Ered1 [V]
0.37
À0.03
À1.21
1.18
À0.14
À0.99
0.85
À0.27
À0.90
0.63
À0.28
À0.88
0.60
À0.32
À0.75
0.43
À0.40
À0.65
0.25
À2.11
1
Eox ÀEred1 [V]
2.48
Potentials were measured in anhydrous CH2Cl2 containing 0.1m TBABF4 on Pt working electrode (scan rate:
0.1 Vs)À1. Potential versus Fc/Fc+. [a] Potentials were determined by CV. [b] Potentials were determined by
DPV.
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A. Osuka et al.
FULL PAPERS
10.23 ppm (s, 2H, meso); MS (MALDI-TOF): found m/z=5573.11, calcd
for C360H442N16O20Zn4, m/z=5575.00. BO6: UV/Vis (CHCl3): lmax (e)=
417 (529000), 495 (422000), and 579 (216000) nm; 1H NMR (600 MHz,
CDCl3): d=0.77 (m, 36H, -CH3), 0.82 (s, 72H, tBu), 0.88 (s, 72H, tBu),
0.89 (s, 72H, tBu), 1.17 (s, 36H, tBu), 1.25 (m, 72H, tBu), 1.14–1.32 (m,
96H, -CH2-), 1.40 (m, 24H, -CH2-), 1.77 (m, 24H, -CH2-), 4.05 (m, 24H,
OCH2-), 6.54 (m, 8H, Ar-o-H), 6.55 (m, 8H, Ar-o-H), 6.56 (m, 8H, Ar-o-
H), 6.61 (s, 4H, Ar-m-H), 6.65 (s, 4H, Ar-m-H), 6.66 (s, 4H, Ar-m-H),
6.73 (m, 12H, Ar-o-H), 6.75 (m, 4H, Ar-p-H), 6.78 (m, 2H, Ar-p-H), 6.94
(m, 4H, Ar-p-H), 7.02 (m, 8H, Ar-p-H), 7.08 (m, 2H, Ar-p-H), 7.14 (m,
4H, Ar-p-H), 7.47 (m, 8H, Ar-o-H), 7.51 (m, 4H, Ar-o-H), 8.17 (br, 4H,
b), 8.31–8.38 (m, 16H, b), 8.78 (m, 4H, b), 8.85 (d, J=4.1 Hz, 2H, b),
8.87 (d, J=4.1 Hz, 2H, b), 8.93 (d, J=4.1 Hz, 2H, b), 8.95 (d, J=4.1 Hz,
2H, b), 9.06 (m, 4H, b), 9.11 (m, 4H, b), 9.48 (m, 4H, b), 9.51 (m, 4H,
b), and 10.26 ppm (m, 2H, meso); MS (MALDI-TOF): found m/z=
8349.28, calcd for C540H660N24O30Zn6, m/z=8359.48.
reaching about 12 nm. Additional favorable features are the
improved chemical stability and solubility of TBOn as com-
pared with the conventional porphyrin tapes. From a stabili-
ty viewpoint, the conventional porphyrin tapes 8-mer and
12-mer were reasonably stable for their separation and ma-
nipulations at ambient temperature in air, but bleached
slowly during storage in a refrigerator after one or two
months. On the other hand, such decompositions were not
observed for TBOn under similar conditions for more than
one year, demonstrating that the bulky aryl group improves
the chemical stabilities of higher porphyrin tapes. The solu-
bility is also considerably improved, as seen by the nice sol-
ubility of TBO24 in CH2Cl2, CHCl3, and toluene, which
allow the measurements of the UVVis/NIR absorption spec-
tra in these solvents. These properties are favorable for po-
tential uses of these porphyrin tapes in single molecular
spectroscopy and molecular electronics and devices. Efforts
along these lines are actively in progress in our laboratory.
Meso-meso linked porphyrin oligomers BOn: Meso-meso linked tetramer
BO4 (150 mg, 27 mmol) was dissolved in dry chloroform (27 mL). To the
solution was added a stock solution of AgPF6 in acetonitrile (0.12m,
1.1 mL, 134 mmol) and N,N-dimethylacetamide (DMA, 27 mL, 0.27 mmol)
under N2 atmosphere in the dark and the reaction mixture was stirred
under reflux for 72 h and the reaction was quenched by addition of
water. The organic layer was separated, washed with brine, dried over
Na2SO4, and evaporated. Zn insertion was carried out, and the residue
was passed through a short silica gel column and separated by GPC. Fur-
ther purification by silica gel column chromatography (Wakogel C-400)
and recrystallization from CHCl3/acetonitrile provided meso-meso linked
oligomers BO8 (15%), BO12 (5%), BO16 (2%), 20-mer (1%), 24-mer
(0.6%), and recovered BO4 (51%). BO8: UV/Vis (CHCl3): lmax (e)=419
(669000), 492 (438000), and 578 (251000) nm; 1H NMR (600 MHz,
CDCl3): d=0.77 (m, 48H, -CH3), 0.82 (s, 144H, tBu), 0.88 (s, 144H, tBu),
1.18 (s, 72H, tBu), 1.25 (s, 72H, tBu), 1.14–1.32 (m, 128H, -CH2-), 1.40
(m, 32H, -CH2-), 1.77 (m, 32H, -CH2-), 4.05 (m, 32H, OCH2-), 6.55 (m,
32H, Ar-o-H), 6.66 (s, 16H, Ar-m-H), 6.73 (m, 8H, Ar-o-H), 6.75 (m,
8H, Ar-o-H), 6.78 (m, 8H, Ar-p-H), 6.95 (m, 8H, Ar-p-H), 7.01 (m, 8H,
Ar-p-H), 7.08 (m, 4H, Ar-p-H), 7.14 (m, 4H, Ar-p-H), 7.47 (m, 8H, Ar-
o-H), 7.51 (m, 8H, Ar-o-H), 8.16 (br, 4H, b), 8.31–8.40 (m, 20H, b), 8.78
(m, 4H, b), 8.85 (d, J=4.1 Hz, 2H, b), 8.92–8.96 (m, 8H, b), 9.04–9.11
(m, 8H, b), 9.48 (m, 4H, b), 9.51 (m, 4H, b), and 10.26 ppm (s, 2H,
meso); MS (MALDI-TOF): found m/z=11142.87, calcd for
Experimental Section
Meso-meso linked porphyrin dimer BO2: 10-Borylated porphyrin BB1
(100 mg, 53 mmol), 10-brominated porphyrin BrO1 (67 mg, 64 mmol), Pd2
ACHTUNGTRENNUNG(dba)3 (2.5 mg, 2.6 mmol), PFu3 (5.0 mg, 10.4 mmol), Cs2CO3 (26 mg,
80 mmol), and CsF (12 mg, 80 mmol) were dissolved in THF/DMF/H2O
(5.0:1.0:0.1 mL) and the mixture was stirred under N2 atmosphere at
1108C for 48 h. The resulting mixture was washed with water and brine,
dried over Na2SO4, and evaporated. The residue was passed through a
short silica gel column and separated by GPC. Zn insertion was carried
out, and further purification by silica gel column chromatography (Wako-
gel C-400) and recrystallization from CHCl3/acetonitrile provided meso-b
linked porphyrin dimer BO2 as a reddish purple powder. Yield 33 mg
(23%). UV/Vis (CHCl3): lmax (e)=419 (252000), 451 (190000), and 559
(45300) nm1H NMR (600 MHz, CDCl3): d=0.80 (t, J=6.9 Hz, 12H,
-CH3), 0.83 (s, 72H, tBu), 1.20 (s, 36H, tBu), 1.18–1.33 (m, 32H, -CH2-),
1.43 (m, 8H, -CH2-), 1.80 (m, 8H, -CH2-), 4.07 (m, 8H, OCH2-), 6.48 (m,
8H, Ar-o-H), 6.59 (s, 4H, Ar-m-H), 6.69 (m, 4H, Ar-o-H), 6.81 (m, 2H,
Ar-p-H), 6.96 (m, 4H, Ar-p-H), 7.10 (m, 2H, Ar-p-H), 7.42 (m, 4H, Ar-
o-H), 8.07 (d, J=4.1 Hz, 2H, b), 8.09 (br, 2H, b), 8.73 (d, J=4.1 Hz, 2H,
b), 8.95 (d, J=4.1 Hz, 2H, b), 9.27 (d, J=4.1 Hz, 2H, b), 9.44 (d, J=
4.1 Hz, 2H, b), 9.47 (d, J=4.1 Hz, 2H, b), 9.50 (d, J=4.1 Hz, 2H, b),
10.23 (s, 1H, meso), and 10.39 ppm (s, 1H, meso); MS (MALDI-TOF):
found m/z=2786.68, calcd for C180H224N8O10Zn2, m/z=2788.51.
C
720H882N32O40Zn8, m/z=11147.98. BO12: UV/Vis (CHCl3): lmax (e)=418
(919000), 495 (628000), and 580 (399000) nm; 1H NMR (600 MHz,
CDCl3): d=0.77 (m, 72H, -CH3), 0.82 (s, 144H, tBu), 0.88 (s, 288H, tBu),
1.18 (s, 72H, tBu), 1.25 (s, 144H, tBu), 1.14–1.32 (m, 192H, -CH2-), 1.40
(m, 48H, -CH2-), 1.77 (m, 48H, -CH2-), 4.05 (m, 48H, OCH2-), 6.55 (m,
48H, Ar-o-H), 6.67 (m, 24H, Ar-m-H), 6.73 (m, 8H, Ar-o-H), 6.75 (m,
16H, Ar-o-H), 6.80 (m, 12H, Ar-p-H), 6.96 (m, 16H, Ar-p-H), 7.02 (m,
8H, Ar-p-H), 7.09 (m, 8H, Ar-p-H), 7.14 (m, 4H, Ar-p-H), 7.48 (m, 8H,
Ar-o-H), 7.51 (m, 16H, Ar-o-H), 8.16 (br, 4H, b), 8.31–8.40 (m, 32H, b),
8.78 (m, 4H, b), 8.85–9.01 (m, 16H, b), 9.04–9.11 (m, 16H, b), 9.48 (m,
4H, b), 9.51 (m, 4H, b), and 10.26 ppm (s, 2H, meso); MS (MALDI-
Meso-meso linked porphyrin tetramer BO4: Meso-meso linked dimer
BO2 (200 mg, 72 mmol) was dissolved in dry chloroform (70 mL). To the
solution was added a stock solution of AgPF6 in acetonitrile (0.12m,
3.0 mL, 360 mmol) under N2 atmosphere in the dark and the reaction mix-
ture was stirred at room temperature for 12 h and the reaction was
quenched by addition of water. The organic layer was separated, washed
with brine, dried over Na2SO4, and evaporated. Zn insertion was carried
out, and the residue was passed through a short silica gel column and
separated by GPC. Further purification by silica gel column chromatog-
raphy (Wakogel C-400) and recrystallization from CHCl3/acetonitrile
provided meso-meso linked tetramer BO4 (70 mg, 35%) along with hexa-
mer BO6 (1.0 mg, 0.5%) and starting material BO2 (112 mg, 56%).
BO4: UV/Vis (CHCl3): lmax (e)=419 (430000), 484 (290000), and 572
(121000) nm; 1H NMR (600 MHz, CDCl3): d=0.75 (t, J=6.9 Hz, 24H,
-CH3), 0.87 (s, 144H, tBu), 1.23 (s, 72H, tBu), 1.14–1.32 (m, 64H, -CH2-),
1.39 (m, 16H, -CH2-), 1.77 (m, 16H, -CH2-), 4.01 (m, 16H, OCH2-), 6.53
(m, 16H, Ar-o-H), 6.64 (s, 8H, Ar-m-H), 6.71 (m, 8H, Ar-o-H), 6.72 (m,
4H, Ar-p-H), 6.99 (m, 8H, Ar-p-H), 7.13 (m, 4H, Ar-p-H), 7.45 (m, 8H,
Ar-o-H), 8.13 (br, 4H, b), 8.24 (d, J=4.1 Hz, 4H, b), 8.33 (d, J=4.1 Hz,
4H, b), 8.76 (d, J=4.1 Hz, 4H, b), 8.89 (d, J=4.1 Hz, 4H, b), 9.04 (d, J=
4.1 Hz, 4H, b), 9.45 (d, J=4.1 Hz, 4H, b), 9.49 (d, J=4.1 Hz, 4H, b), and
TOF): found m/z=16712.89, calcd for
C1080H1222N48O60Zn12, m/z=
16720.97. BO16: UV/Vis (CHCl3): lmax (e)=417 (1110000), 500
(776000), and 585 (518000) nm; 1H NMR (600 MHz, CDCl3): d=0.77
(m, 96H, -CH3), 0.82 (s, 144H, tBu), 0.88 (s, 432H, tBu), 1.18 (s, 72H,
tBu), 1.25 (s, 216H, tBu), 1.14–1.32 (m, 256H, -CH2-), 1.40 (m, 64H,
-CH2-), 1.77 (m, 64H, -CH2-), 4.05 (m, 64H, OCH2-), 6.55 (m, 64H, Ar-
o-H), 6.67 (m, 32H, Ar-m-H), 6.73 (m, 8H, Ar-o-H), 6.75 (m, 24H, Ar-o-
H), 6.80 (m, 16H, Ar-p-H), 6.96 (m, 24H, Ar-p-H), 7.02 (m, 8H, Ar-p-
H), 7.09 (m, 12H, Ar-p-H), 7.14 (m, 4H, Ar-p-H), 7.48 (m, 8H, Ar-o-H),
7.51 (m, 24H, Ar-o-H), 8.16 (br, 4H, b), 8.31–8.40 (m, 56H, b), 8.78 (m,
4H, b), 8.85–9.01 (m, 28H, b), 9.04–9.11 (m, 28H, b), 9.48 (m, 4H, b),
9.51 (m, 4H, b), and 10.26 ppm (s, 2H, meso); MS (MALDI-TOF):
found m/z=22279.91, calcd for C1440H1762N64O80Zn16, m/z=22293.94. 20-
mer: MS (MALDI-TOF): found m/z=27848.75, calcd for
C
1800H2202N80O100Zn20 (BO20), m/z=27866.22. 24-mer: MS (MALDI-
TOF): found m/z=33215.24, calcd for
m/z=33237.46.
C2160H2442N96O120Zn24 (BO24),
1254
www.chemasianj.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2009, 4, 1248 – 1256

Synthesis of Extremely p-Extended Porphyrin Tapes
General procedure for DDQ-Sc
meso linked porphyrin array (10 mg) in dry toluene (10 mL) was added 4-
(n-1) equivalent of DDQ and 4(n-1) equivalent of Sc(OTf)3. The result-
ing reaction mixture was stirred at 808C for 2 h under N2 atmosphere in
the dark and the reaction was quenched by the addition of THF. The re-
sulting mixture was passed through a short alumina column and the sol-
vent was evaporated. Further purification by silica gel column chroma-
tography (Wakogel C-400) and recrystallization from CHCl3/acetonitrile
provided triply linked porphyrin tape.
G
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Porphyrin tape tetramer TBO4: Following the general procedure, TBO4
was prepared from BO4 in 63% yield as green solids. MS (MALDI-
TOF): found m/z=5550.21, calcd for C360H430N16O20Zn4; m/z=5553.03;
UV/Vis (CHCl3): lmax =423, 744, 1401, and 1646 nm.
Porphyrin tape hexamer TBO6: Following the general procedure, TBO6
was prepared from BO6 in 68% yield as black solids. UV/Vis (CHCl3):
lmax =422, 598, and 2110 nm.
Porphyrin tape octamer TBO8: Following the general procedure, TBO8
was prepared from BO8 in 56% yield as black solids. UV/Vis (CHCl3):
lmax =480, 910, 2108, and 2410 nm.
Porphyrin tape dodecamer TBO12: Following the general procedure,
TBO12 was prepared from BO12 in 52% yield as black solids. UV/Vis
(CHCl3): lmax =494, 1018, and 2580 nm.
Porphyrin tape hexadecamer TBO16: Following the general procedure,
TBO16 was prepared from BO16 in 54% yield as black solids. UV/Vis
(CHCl3): lmax =490, 1008, and 2800 nm.
Porphyrin tape icosamer TBO20: Following the general procedure,
TBO20 was prepared from BO20 in 44% yield as black solids. UV/Vis
(CHCl3): lmax =490, 982, and 2800 nm.
Porphyrin tape tetracosamer TBO24: Following the general procedure,
TBO24 was prepared from BO24 in 38% yield as black solids. UV/Vis
(CHCl3): lmax =500, 1072, and 2800 nm.
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Acknowledgements
The work was supported by Grants-in-Aid for Scientific Research (No.
19205006, and 20108001 “pi-Space“) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan. N.A. thanks a Grant-in-
Aid for Young Scientists (B) and GCOE program “Integrated Materials
Science” for financial support. T.I. thanks the JSPS Research fellowships
for Young Scientists.
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Received: April 2, 2009
Published online: June 30, 2009
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