Intramolecular axial ligation of zinc porphyrin cores with triazole links within dendrimers

Article abstract of DOI:10.1002/chem.200801557

A series of dendritic porphyrins 7-9 and 12, in which benzyl ether dendrons were linked to a porphyrin core through 1,2,3-triazole links, were synthesized by Cu'-catalyzed cycloaddition of azides and alkynes. Absorption and fluorescence spectra showed a s

Full text of DOI:10.1002/chem.200801557

FULL PAPER
DOI: 10.1002/chem.200801557
Intramolecular Axial Ligation of Zinc Porphyrin Cores with Triazole Links
within Dendrimers
Mutsumi Kimura,*[a, b] Yasuhiro Nakano,[a] Naoya Adachi,[a] Yoko Tatewaki,[b]
Hirofusa Shirai,[b] and Nagao Kobayashi[c]
Abstract: A series of dendritic por-
phyrins 7–9 and 12, in which benzyl
ether dendrons were linked to a por-
phyrin core through 1,2,3-triazole links,
were synthesized by CuI-catalyzed cy-
cloaddition of azides and alkynes. Ab-
sorption and fluorescence spectra
showed a stable axial ligation at the
zinc center of the porphyrin core by tri-
azole links in dendritic wedges and in-
dicated that the position of the triazole
links strongly affected the stability of
the axial ligation within the dendrimer.
When the porphyrin core was sur-
rounded by aryl ether dendrons having
anionic termini and triazole linkers, a
significant rate enhancement for photo-
induced electron transfer was observed
compared with a similar water-soluble
dendritic zinc porphyrin lacking tria-
zole linkers. These triazole links consti-
tuted a direct pathway within the den-
drimer architecture for electron trans-
fer between the zinc porphyrin core
and peripheral electron acceptors.
Keywords:
axial
ligation
·
dendrimers · porphyrins · zinc
Introduction
and other forms of intermolecular interactions. Dendrimers
have recently been counted among the most attractive mac-
romolecular compounds from the synthetic, structural and
functional points of view.[3] Construction of dendrimers from
a functional core results in their isolation from the effects of
self-association interactions, sterically shielding them from
the external medium and tuning of their surrounding micro-
environment. Porphyrins and their metal complexes have
been used to a probe of microenvironmental changes by the
encapsulation within dendritic structures.[4] Several studies
investigating the dendritic effects of embedding porphyrins
in a dendrimer core on their functionalities have reported
the use of such systems as synthetic models of natural ana-
logues. For example, Dandliker et al. have reported the con-
struction of a hydrophobic dendrimer having water-soluble
Biological systems including enzymes, hemoglobin and plant
photosynthesis[1] have provided much of the inspiration for
the development of supramolecular devices. Many supra-
molecular devices have been designed to mimic nanostruc-
tures and their functions in biological processes.[2] These bio-
logical systems are generally made up of polypeptide chains;
their amino acid sequence determine the structure and func-
tion of the resulting protein, which is folded into a unique
conformation giving a globular nanostructure incorporating
surface clefts and crevices. The special functions in biologi-
cal systems occur by three-dimensional interactions of mo-
lecular units within the globular nanostructures involving
hydrophobic effects, hydrogen-bonding, ion–ion interactions
terminal segments emanating from an ironACTHNUGRTENUNG(III)–porphyrin
core, forming an isolated and water-soluble heme ana-
logue.[5] Comparison of dendritic iron porphyrins of differ-
ent dendrimer generations revealed a remarkable shift to
positive potential in the redox potential for the Fe3+/Fe2+
couple of the iron porphyrin core. Aida et al. have reported
the dependence of intramolecular energy transduction ion
the morphology of spherical aryl ether dendritic porphyrins
that mimicked biological light-harvesting antennae.[6]
Three-dimensional interactions between a heme core and
peptide side chains have a strong influence on the function-
alities such as redox, catalytic, photochemical and molecular
recognition properties. Axial ligation at the metal center
[a] Prof. Dr. M. Kimura, Y. Nakano, N. Adachi
Department of Functional Polymer Science
Faculty of Textile Science and Technology, Shinshu University
Ueda 386-8567 (Japan)
Fax : (+81)268-51-5499
E-mail: mkimura@shinshu-u.ac.jp
[b] Prof. Dr. M. Kimura, Dr. Y. Tatewaki, Prof. Dr. H. Shirai
Collaborative Innovation Center of Nanotech Fiber (nanoFIC)
Shinshu University, Ueda 386-8567 (Japan)
[c] Prof. Dr. N. Kobayashi
Department of Chemistry, Graduate School of Science
Tohoku University, Sendai, 980-8587 (Japan)
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with coordinative imidazole or thiol units in histidine and
cysteine, respectively, enhances the functionalities of natural
biological systems. Weyermann et al. have reported the crea-
tion of a unique local microenvironment around an site-iso-
lated iron porphyrin core by the wrapping the core with a
dendrimer in conjugation with axial ligation by imidazoles
to mimic electron-transfer proteins such as cytochrome c.[7]
However, three-dimensional interactions between the por-
phyrin core and the dendritic wedges have not been as ex-
tensively explored. We report here the syntheses of the den-
dritic porphyrins 7–9 containing coordinative 1,2,3-triazole
rings using “click chemistry”[8] for the coupling of acetylene-
terminated porphryin cores with dendritic azides.[9] CuI-cata-
lyzed cycloaddition of azides and alkynes has been widely
demonstrated to be a versatile tool in the design and synthe-
sis of materials.[10] Ornelas et al. have demonstrated the se-
cessive generations of a layered structure. The dimensions
of the synthesized dendrimer 7 were estimated from the
analysis of a surface pressure versus area (p/A) isotherm on
pure water as the subphase.[13] The surface pressure began to
rise at a molecular area of about 9 nm2, increasing to
35 mNmÀ1 on further compression. The limiting surface area
per molecule of 7 was estimated at 7.8 nm2 determined by
extrapolating the slope of the p/A isotherm in the liquid-
condensed region to zero pressure. The estimated diameter
(3.2 nm) of spherical dendrimer 7 almost coincided with the
dendrimer diameter (3.5 nm) predicted by computer-gener-
ated molecular model. The other dendritic porphyrins 8, in
which the first-generation dendron was linked with the por-
phyrin core through eight triazole rings in one layer, was
synthesized by the coupling between 3 and benzyl ether
dendritic azide 6 in a similar manner. While both dendritic
porphyrins 7 and 8 contained 28 benzene rings each, the
number and position of the triazole links in these molecules
were different.
The incorporation of triazole rings in the dendrimer
framework may have affected the internal environment
around the zinc porphyrin core. Absorption and fluores-
cence spectra provided information on the internal environ-
mental within the dendritic porphyrins. Figure 1 shows the
absorption spectra of 7 and 8 in CH2Cl2 and toluene at
208C. The absorption maxima of the Soret band (lmax) and
molar extinction coefficient (e) of 3, 4, 7 and 8 in CH2Cl2
are summarized in Table 1. In CH2Cl2, the Soret and Q
bands of 8 (423, 549, 588 nm) were similar to those of the
porphyrin core 3 (421, 548, 584 nm), while the Soret band
(432 nm) of 7 with its double layer of triazole links in den-
dritic wedges was red-shifted by 11 nm from that of 3. Nei-
ther the both Soret nor Q bands in 7 and 8 shifted upon in-
creasing the concentration up to 60 mm, indicating the lack
of intermolecular aggregation among porphyrin cores in this
concentration range. The spectral shift of 7 suggested the
formation of intramolecular coordination bonds between
the zinc porphyrin core and the triazole links in the dendrit-
ic wedges.[14] Furthermore, in toluene, a broadening and hy-
pochromicity of the Soret band were observed for 7. The
Soret band of 8 was split into two peaks at 430 and 438 nm
indicating an equilibrium between coordinated and free spe-
cies. The position of the triazole links within the dendrimer
and the polarity of the environment strongly influenced the
stability of triazoles axial ligated to the zinc porphyrin core.
To investigate the most favorable positions within the den-
drimer of triazole links for the axial ligation, 9 was synthe-
sized by the coupling of acetylene-terminated 4 with tert-
butyl benzyl azide. Whereas 9 contained the same number
of benzene and triazole rings as 8, the triazole links in 9
were in the outer layer of the dendritic structure. The den-
dritic porphyrin 9 exhibited the Soret band at 431 nm and Q
bands at 559 and 600 nm in CH2Cl2, and its absorption
peaks for 9 coincided in position with those of 7, indicating
that the triazole links in the outer layer of dendritic wedges
could form a stable coordination bond with the zinc porphy-
rin core.
À
lective recognition of H2PO4 and ATP2À in triazole-contain-
ing metallodendrimers.[11] We also discuss the structural de-
pendence of the intramolecular formation of a coordination
bond of the zinc porphyrin core with triazole-containing
dendrons and the efficiency of photoinduced electron trans-
fer rate.
Results and Discussion
A series of dendritic metalloporphyrins having different
dendron units was synthesized by the formation of 1,2,3-tria-
zole links between a porphyrin core and dendritic wedges
through a “click reactions”—the Huisgen 1,3-dipolar cyclo-
addition of azides and acetylenes (Scheme 1).[12] Alkaline-
mediated coupling of 5,10,15,20-tetrakis (3’,5’-dihydroxyphe-
nyl)porphyrin with propargyl bromide gave a multivalent
porphyrin core 1, which was metalated with ZnACTHNUGTRENUNG(OAc)2. Tria-
zole-linked dendritic azide 5 was synthesized by the cop-
per(I)-catalyzed click reaction between tert-butyl benzyl
azide and 3,5-bis(propargyloxy)benzyl chloride in a mixture
of water and tert-butyl alcohol, followed by treatment with
NaN3.[9] Finally, the coupling of the dendritic azide 5 to zinc
porphyrin core 3 in THF in the presence of the organosolu-
ble catalyst [(PPh3)3CuBr][9] yielded the desired dendritic
porphyrin 7, which contained 24 1,4-disubstituted 1,2,3-tria-
zole links in two layers. Metal-free dendritic porphyrin
could not be prepared from the coupling between 1 and 5
with [(PPh3)3CuBr] due to the metallation of porphyrin
cores with copper ions. Purification of 7 was accomplished
by column chromatography followed by recycling prepara-
tive HPLC and was unambiguously characterized by means
of matrix-assisted laser desorption/ionization time-of-flight
mass spectra (MALDI-TOF-MS), gel-permeation chroma-
1
tography (GPC), and H and 13C NMR spectroscopy. Analy-
sis of 7 by MALDI-TOF-MS and GPC provided no signs of
products with defects and confirmed that the coupling of
eight terminal alkynes in 3 and a focal azidomethyl group in
5 is complete. 1H NMR resonances of the inner triazole
units nearer the porphyrin core were shifted downfield com-
pared to those of the outer triazole units, revealing the suc-
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Zinc Porphyrins
FULL PAPER
Scheme 1. Synthetic approach to a series of triazole-linked dendritic porphyrins.
quantum yield (Ff) of the porphyrin cores were independent
of structural differences in the dendron units. These results
suggested that the dendrons exerted little influence of the
dendrons on the non-radiative and radiative processes of the
porphyrin core. When excited at the Soret band in CH2Cl2,
3, 4 and 8 emitted at 598 and 645 nm, with the fluorescence
intensity ratio I598/I645 of about 0.5. On the other hand, 7 and
9 emitted at 609 and 661 nm upon the excitation at the
Soret band with a ratio I609/I661 of 1.19. The shift in fluores-
cence peaks and the change in intensity ratio also indicated
the formation of intramolecular coordination bonds between
the zinc porphyrin core and triazole-containing dendron
wedges. The fluorescence spectrum of 8 in toluene was simi-
lar to that of 7 and 9, suggesting the formation of axial liga-
tion in low-polar solvents. These changes in the absorption
and fluorescence spectra of 7 and 9 were attributable to the
axial ligation of the outer 1,2,3-triazole links in the dendritic
wedges to the zinc porphyrin core.
Table 1. Spectroscopic and photophysical data for zinc porphyrin cores
and dendritic zinc porphyrins.
Absorption[a], lmax/nm
(loge/mÀ1 cmÀ1
Emission[b]
lmax/nm (t/ns)
,
ff[c]
3
4
7
8
9
421
(5.76)
598, 645 (1.61)
597, 644 (1.58)
609, 661 (1.51)
597, 645 (1.59)
607, 660 (1.61)
0.02
0.02
0.02
0.02
0.02
423 (5.68)
432 (5.77)
423 (5.73)
431 (5.66)
Interactions within the dendrimer could also be moni-
tored by tracking change in fluorescence spectra. Figure 2
shows steady-state fluorescence spectra of 7, 8 and 9 in de-
gassed CH2Cl2 upon the excitation at the Soret band of the
porphyrin core. The fluorescence parameters for all dendrit-
ic porphyrins and porphyrin cores are also summarized in
Table 1. The fluorescence of dendritic porphyrins showed a
normal decay profile and the fluorescence lifetime (t) and
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M. Kimura et al.
Figure 1. Absorption spectra of dendritic porphyrins a) 7, b) 8 and c) 9 in
CH2Cl2 (c) and toluene (g) at 208C. [7, 8, 9]=3.0 mm.
Figure 2. Steady-state fluorescence spectra of dendritic porphyrins a) 7,
b) 8 and c) 9 upon excitation at the Soret band at 208C in CH2Cl2 (c)
and toluene (g). All spectra are normalized to a constant absorbance
at the excitation wavelength.
The stability of axial ligation in such dendrimers is evalu-
ated by titration with imidazole, which is small enough to
penetrate within the dendritic architecture and coordinates
strongly to the zinc cation of the porphyrin core. If the tria-
zole links were coordinated to the zinc cation, complexation
by imidazole would need an exchange process and the bind-
ing constant K would be diminished. The absorption spec-
trum of 8 in CH2Cl2 changed upon titration with imidazole,
and isosbestic points were clearly seen (Figure 3). Moreover,
the fluorescence spectra changed upon the titration with
imidazole as shown in the inset of Figure 3. Titration of 8
with imidazole using Jobꢁs method indicated the formation
of a 1:1 complex. The constant K was determined from the
spectral change in the Soret band. This K value for 8 was
1.9ꢂ104 mÀ1, which was comparable to that for non-dendritic
zinc porphyrins 3 and 4 (3: K=1.4ꢂ104 mÀ1, 4: K=2.4ꢂ
104 mÀ1). In sharp contrast, 7 showed no spectral changes in
the absorption or fluorescence spectra after the addition of
a large excess of imidazole. The titration of 9 with imidazole
resulted in a small red shift of the Soret band from 431 to
434 nm and a small change in the intensity ratio of the two
peaks in fluorescence spectra at high imidazole concentra-
tion. These titration results indicated that the stability of
axial ligation strongly depended on the spatial position of
the triazole rings within the dendritic structure and on the
distance between the zinc cation and these rings. A comput-
er-generated model of 7 predicted the distances of the tria-
zole links in each layer of the dendritic structure as shown
in Scheme 1. The approach of the inner triazoles to the por-
phyrin core was difficult due to the covalent linkages at the
3- and 5-positions of phenyl ring bound to the porphyrin
core. On the other hand, the flexibility of the dendritic
wedge allowed the formation of coordination bond between
the outer triazole links and the porphyrin core.
In natural heme proteins like cytochrome c, chlorophyll,
hemoglobin and metal-containing enzymes, metalloporphyr-
ins are buried within the hydrophobic peptide shell through
the formation of coordination bonds with the coordinative
side chains of surrounding peptides.[1] Such wrapping of por-
phyrins with peptide shell provides a suitable environment
for various functionalities. Spectral studies of triazole-con-
taining dendritic porphyrins have clearly demonstrated that
the dendritic wedges create a unique microenvironment
around the zinc porphyrin core. The axial ligation of triazole
links may affect the functionalities of the zinc porphyrin
core. In the electron-transfer process in natural cytochrome,
the histidine units in the globular protein shell provide the
pathway for electron transfer between two redox sites.[15]
The triazole links in the dendrimer are good mimics of the
protein histidine residue in the biotic systems. We focus on
the change in photochemical properties of the zinc–porphy-
rin core upon wrapping with triazole-containing wedges.
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Zinc Porphyrins
FULL PAPER
phyrin core was quenched through the photoinduced elec-
tron transfer through the dendrimer framework. We exam-
ined the photoinduced electron transfer from the excited
zinc porphyrin core in 12 to MV2+ in aqueous solution.
Methyl viologen is a typical one-electron transfer acceptor
unit producing the methyl viologen radical.[19] The addition
of MV2+ up to 20 mm to 12 did not change the zinc porphy-
rin absorption bands. This result is in accord with the titra-
tion behavior of the reference dendritic zinc porphyrin 13
lacking the triazole links.[16] Therefore, the absence of spec-
tral change in 12 upon addition of MV2+ revealed that the
dendritic wedges prevent access of MV2+ to the zinc por-
phyrin core.
The fluorescence decreased in intensity upon titration
with MV2+. The relative fluorescence intensity at 607 nm is
plotted as a function of MV2+ concentration (Figure 4).
Fluorescence quenching processes are described by the
[MV2+] + 1), where KSV
Stern-Volmer equation (I0/I=KSVACHTNUGTERNUNNG =
kqt0, KSV is the Stern–Volmer quenching constant (mÀ1), kq is
the quenching rate constant (mÀ1 sÀ1), and t0 is the excited
singlet lifetime of the zinc porphyrin in the absence of a
quencher. The Stern–Volmer plots of 12 showed a highly ef-
ficient fluorescence quenching at a low concentration of
MV2+. The initial slope of the Stern–Volmer plots gave the
Stern–Volmer quenching constant KSV =1.54ꢂ105 mÀ1 and kq
was 9.6ꢂ1013 mÀ1 sÀ1. This value of kq was much higher than
the diffusion-limited rate constant in water (kd =7.6ꢂ
109 mÀ1 sÀ1) for neutral species calculated by the Smoluchow-
ski equation,[20] indicating the accumulation of MV2+
around 12 through electrostatic forces. In sharp contrast, the
reference dendrimer 13 showed the saturation of I0/I at con-
centration of MV2+above 0.2 mm.[16] The rate of photoin-
duced electron transfer depends on the thermodynamic driv-
ing force for the electron-transfer reactions, the nature of in-
tervening medium, and the distance and relative orientation
of the donor and the acceptor.[21] While 12 was larger than
that of 13 due to the presence of triazole links within the
dendritic wedges,[22] the efficiency of electron transfer in 12
was higher than that in 13. Our findings may be rationalized
by considering that the increase in quenching efficiency of
12 was attributed to the triazole links within the dendritic
wedges. These links constitute an electron transfer pathway
from the excited zinc–porphyrin core buried in the dendri-
mer to MV2+ accumulated on the dendrimer surface. A
mixed solution of 12 and MV2+ exhibited the characteristic
absorption bands at 305 and 602 nm in the presence of trie-
thanolamine as a sacrificial donor under the irradiation with
visible light.[23] Furthermore, these spectral changes disap-
peared when the sample was subsequently exposed to O2.
These observations are consistent with the formation of the
methyl viologen radicals. This result implied that the tria-
zole-linked dendritic porphyrin 12 acted as a photosensitizer
in this artificial photosynthetic system.
Figure 3. Absorption changes of a) 7, b) 8 and c) 9 in CH2Cl2 at 208C on
titration with imidazole. [7, 8, 9]=3.0 mm. [imidazole]=0, 1.0, 10.0, 100.0,
1000 mm. The inset shows fluorescence spectral changes of a) 7, b) 8 and
c) 9 in CH2Cl2 at 208C on titration with imidazole; [7, 8, 9]=3.0 mm; [imi-
dazole]=0 (g), 10.0, 1000 mm.
Water-soluble dendritic zinc porphyrin 12 was synthesized
by the CuI-catalyzed coupling reaction between 4 and me-
thoxycarbonyl-terminated aryl ether dendritic azide 10, fol-
lowed by hydrolysis of the exterior ester groups with KOH
(Scheme 2).[16] The absorption spectrum of 12 in alkaline
aqueous solution (pH 10.7) exhibited the Soret band at
435 nm and the Q bands at 563 and 603 nm. Water-soluble
dendritic zinc porphyrin 12 emitted at 607 and 660 nm upon
the excitation at the Soret band with intensity ratio I607/I660
=
1.26. These absorption and fluorescence spectra were similar
to those obtained for 9 in CH2Cl2, suggesting the formation
of intramolecular axial ligation of triazole links with the
zinc–porphyrin core in aqueous solution. Self-assembled
donor–acceptor systems have shown promise for the genera-
tion of long-lived charge-separated species through photoin-
duced electron transfer and charge separation processes.[17,18]
Aida and co-workers have demonstrated the long-range
photoinduced electron transfer through an aryl ether dendri-
mer framework in a water-soluble dendritic zinc porphy-
rin.[16] In this system, the cationic electron acceptor methyl
viologen (MV2+) accumulated on the surface of anionic sur-
face of a dendrimer and the fluorescence from the zinc por-
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M. Kimura et al.
Scheme 2. Water-soluble dendritic zinc porphyrins 12 and 13.
tion of these links in the dendritic architecture. The axial li-
gation affected the efficiency of photoinduced electron
transfer from the excited zinc porphyrin core to an electron
acceptor. The construction of triazole-containing dendritic
wedges around the porphyrin core allowed the creation of a
unique microenvironment and the coordination of triazole
links created an electron transfer pathway from core to pe-
riphery within each dendrimer. The design of interactive
dendron wedges[11,24] could control the properties of the
functional core and a well-designed dendrimer could serve
as a good mimic of the natural bio-systems involved in pho-
tosynthesis, oxygen transportation and catalysis. By cova-
lently inserting the interactive segments in the dendrimer,
the functional tuning of the porphyrin core could be further
extended in future work.
*
Figure 4. Stern–Volmer plots for fluorescence quenching of 12 ( ) and 13
&
2+
( ) with MV in degassed alkaline aqueous solution at pH 10.7. The
fluorescence intensity was monitored at 607 and 660 nm for 12 and 13, re-
spectively, [12, 13]=1.5 mm.
Experimental Section
Conclusion
General: NMR spectra were recorded on a Bruker AVANCE 400 FT
NMR spectrometer at 399.65 MHz and 100.62 MHz for 1H and 13C, re-
spectively, in CDCl3 solution. Chemical shifts are reported relative to in-
ternal TMS. IR spectra were obtained on a SHIMAZU IR Prestige-21
with DuraSample IR II. UV/Vis spectra and fluorescence spectra were
measured on a JASCO V-650 and a JASCO FP-750. Fluorescence quan-
tum efficiencies were determined by HAMAMATSU Photonics absolute
PL Quantum Yield Measurement system C9920-02. MALDI-TOF mass
spectra were obtained on a PerSeptive Biosystems Voyager-De-Pro spec-
Novel triazole-linked dendritic porphyrins have been syn-
thesized though the click reaction between acetylene-termi-
nated porphyrin and dendritic azides. Absorption and fluo-
rescence spectroscopic studies demonstrated the binding
ability of the triazole links to bind as axial ligands for the
zinc porphyrin core within the single dendrimer. We also
found that the binding ability depended on the spatial posi-
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Zinc Porphyrins
FULL PAPER
trometer with dithranol as matrix resolution GPC analyses were carried
out with a JASCO HPLC system (pump 1580, UV detector 1575, refrac-
tive index detector 930) and a Showa Denko GPC KF-804 L column
(8.0ꢂ300 mm, polystyrene standards, Mw =900–400000 gmolÀ1) in THF
as an eluent at 358C (1.0 mLminÀ1).
2.7 mmol) in tetrahydrofuran (10 mL) was heated at 608C for 3 d. The
residue was purified by column chromatography on silica gel by eluting
with CH2Cl2 and recycling preparative GPC.
Compound 7: Yield: 29 mg, 53%; 1H NMR (400 MHz, CDCl3): d=8.79
(s, 8H, pyrrole), 7.59 (s, 8H, ArH), 7.33 (s, 8H, ArH), 7.27 (s, 16H,
ArH), 7.23 (d, 32H, J=8.4 Hz, ArH), 7.18 (s, 4H, ArH), 6.99 (d, 32H,
J=8.4 Hz, ArH), 6.39 (s, 8H, triazole), 6.35 (s, 16H, triazole), 5.36 (s,
16H, -CH2O-), 5.21 (s, 32H, -CH2O-), 5.16 (s, 16H, -CH2O-), 4.65 (s,
32H, -CH2O-), 1.20 ppm (s, 144H, tBu); 13C NMR (100 MHz, CDCl3):
d=31.2, 34.5, 53.7, 107.4, 122.8, 125.9, 127.8, 131.3, 143.3, 151.8,
159.7 ppm; MALDI-TOF-MS: m/z calcd for C348H372N76O24Zn: 6068.56
[M]+; found 6070.7; UV/Vis (CH2Cl2): lmax (log e)=432 (5.77), 563
(4.36), 602 nm (3.92).
Materials: Dendritic azides 5, 6, 10 and water-soluble dendritic zinc por-
phyrin 13 were synthesized from dendritic halides according to the litera-
ture method.[16,25] All chemicals were purchased from commercial suppli-
ers and used without purification. Column chromatography was per-
formed with activated alumina (Wako, 200 mesh) or Wakogel C-200. Re-
cycling preparative gel permeation chromatography was carried out by a
JAI recycling preparative HPLC using CHCl3 as an eluent. Analytical
thin-layer chromatography was performed with commercial Merck plates
Compound 8: Yield: 31 mg, 90%; 1H NMR (400 MHz, CDCl3): d=8.80
(s, 8H, pyrrole), 7.35 (s, 8H, ArH), 7.31 (s, 8H, ArH), 7.15 (m, 96H,
ArH), 6.81 (s, 4H, ArH), 6.40 (s, 8H, triazole), 6.27 (s, 16H, -CH2O-),
5.15 (s, 16H, -CH2O-), 4.74 ppm (s, 32H, -CH2O-); 13C NMR (100 MHz,
CDCl3): d=53.3, 70.0, 72.3, 102.2, 107.1, 123.3, 127.4, 127.9, 128.5, 133.4,
136.3, 142.3, 155.3, 160.2 ppm; MALDI-TOF-MS: m/z: calcd for
C236H196N28O24Zn: 3873.64 [M]+; found 3875.3; UV/Vis (CH2Cl2): lmax
(log e)=423 (5.73), 549 (4.38), 588 nm (3.60).
Compound 9: Yield: 25 mg, 52%; 1H NMR (400 MHz, CDCl3): d=8.95
(s, 8H, pyrrole), 7.41 (s, 8H, ArH), 7.20 (d, 32H, J=8.4 Hz, ArH), 6.95
(m, 36H, ArH), 6.60 (s, 16H, triazole), 6.40 (s, 8H, ArH), 5.16 (s, 32H,
-CH2O-), 4.95 (s, 16H, -CH2O-), 4.76 (s, 32H, -CH2O-), 1.17 ppm (s,
144H, tBu); 13C NMR (100 MHz, CDCl3): d=31.2, 34.5, 53.6, 70.1, 106.7,
123.3, 125.9, 127.8, 131.4, 143.75, 151.7, 160.1 ppm; MALDI-TOF-MS: m/
z: calcd for C324H348N52O24Zn: 5419.95 [M]+; found 5420.4; UV/Vis
(CH2Cl2): lmax (log e)=431 (5.66), 559 (4.29), 600 nm (3.75).
Compound 11: Yield: 77 mg, 88%. 1H NMR (400 MHz, CDCl3): d=8.86
(s, 8H, pyrrole), 7.84 (d, 64H, J=7.6 Hz, ArH), 7.40 (s, 16H, ArH), 7.27
(s, 8H, ArH), 7.22 (d, 32H, J=7.2 Hz, ArH), 6.93 (s, 4H, ArH), 6.63 (s,
16H, triazole), 6.47 (s, 8H, ArH), 6.36 (s, 16H, ArH), 6.27 (s, 32H,
ArH), 5.14 (s, 32H, -CH2O-), 4.98 (s, 16H, -CH2O-), 4.92 (s, 32H,
-CH2O-), 4.79 (s, 64H, -CH2O-), 3.77 ppm (s, 96H, -COOCH3); 13C NMR
(100 MHz, CDCl3): d=52.0, 53.6, 68.0, 69.2, 102.0, 107.1 119.3, 126.9,
129.6, 133.9, 136.9, 141.5, 131.4, 159.5, 159.9, 166.6 ppm; MALDI-TOF-
MS: m/z: calcd for C548H476N52O120Zn: 9775.31 [M]+; found 9770.9.
coated with silica gel 60 F254 or aluminum oxide 60 F254
.
Porphyrin core 1: 5,10,15,20-Tetrakis(3’,5’-dihydroxyphenyl)porphyrin
(80.0 mg, 0.11 mmol) and propargyl bromide (0.1 mL, 1.31 mmol) were
dissolved in acetone (10 mL) containing K2CO3 (0.36 g, 2.61 mmol) and
[18]crown-6 (catalytic amount), and the reaction mixture was refluxed
for 3 d with stirring. The mixture was filtrated and the filtrate was evapo-
rated to dryness. The crude product was purified using column chroma-
tography (activated alumina, petroleum ether/CH2Cl2 1:1). Yield: 84 mg,
73%. 1H NMR (400 MHz, CDCl3): d=8.95 (s, 8H, pyrrole), 7.52 (s, 8H,
ArH), 7.04 (s, 4H, ArH), 4.86 (s, 16H, -O-CH2-C=CH), 2.59 (s, 8H, -O-
CH2-C=CH), À2.84 ppm (s, 2H, -NH); 13C NMR (100 MHz, CDCl3): d=
56.3 (-O-CH2-C=CH), 75.9 (-C=CH), 78.4 (-C=CH), 102.3, 115.4, 119.4
(porphyrin), 144.0, 156.8 ppm; MALDI-TOF-MS: m/z: calcd for
C68H46N4O8: 1047.12 [M+H]+; found 1047.3; UV/Vis (CH2Cl2): lmax
(loge)=420 (5.73), 514 (4.46), 548 (4.10), 589 (4.08), 646 nm (3.91).
Porphyrin core 2: Prepared from 5,10,15,20-tetrakis(3’,5’-dihydroxyphe-
nyl)porphyrin and 3,5-bis(propargyloxy)benzyl chloride and purified by
column chromatography (activated alumina, petroleum ether/CH2Cl2
1:1). Yield: 60 mg, 38%. 1H NMR (400 MHz, CDCl3): d=8.83 (s, 8H,
pyrrole), 7.47 (s, 8H, ArH), 7.04 (s, 4H, ArH), 6.67 (s, 16H, ArH), 6.60
(s, 8H, ArH), 5.20 (s, 16H, -CH2O-), 4.64 (s, 32H, -O-CH2-C=CH), 2.38
(s, 16H, -O-CH2-C=CH), À2.92 ppm (s, 2H, -NH); 13C NMR (100 MHz,
CDCl3): d=55.9 (-O-CH2-C=CH), 70.0 (-CH2O-), 75.8 (-C=CH), 78.3
(-C=CH), 102.3, 106.9, 115.3, 119.6 (porphyrin), 139.4, 144.0, 157.8,
158.9 ppm; MALDI-TOF-MS: m/z: calcd for C148H110N4O24
: 2328.74
[M+H]+; found 2329.1; UV/Vis (CH2Cl2): lmax (log e)=422 (5.71), 516
Hydrolysis of the exterior ester groups in 11: To a solution of 11 (15 mg,
1.53 mmol) in tetrahydrofuran (5 mL) was added 1.5m KOH aqueous so-
lution (0.1 mL). The reaction was heated at reflux for 6 h. The reaction
mixture was then evaporated to dryness, and water (5 mL) was added re-
sulting in a homogeneous solution, which was then heated for 4 h. After
being cooled to room temperature, acetic acid (1 mL) was added to the
reaction solution, resulting in a purple suspension, which was collected
by centrifugation. The purple precipitate was washed with water (2 mLꢂ
3) and dried overnight in a vacuum oven at 508C to afford hydrolyzed
dendrimer 12. Yield: 11 mg, 79%; FT-IR: nmax =1692 cmÀ1 (COOH);
UV/Vis (pH 10.7 aq KOH): lmax (log e)=435 (5.58), 563 (4.22), 603 nm
(3.82).
(4.34), 551 (3.85), 589 (4.08), 646 (3.91).
Zinc porphyrin core 3 and 4: A mixture of 1 or 2 (50.0 mg, 47.8 mmol)
and ZnACHTUNGTRENNUNG(OAc)2·2H2O (16.0 mg, 71.6 mmol) were dissolved in acetic acid
(5 mL). During the reaction, UV/Vis spectrum of the reaction mixture
was monitored. The mixture was refluxed until no change in the Soret
band was observed. After aqueous workup (50 mL), the mixture was ex-
tracted with CH2Cl2 and the organic layer was washed with water for 3
times. The organic layer was dried (MgSO4) and evaporated. The residue
was purified by chromatography on activated alumina with CH2Cl2 to
afford the zinc porphyrin core.
Compound 3: Yield: 42 mg, 80%; 1H NMR (400 MHz, CDCl3): d=9.03
(s, 8H, pyrrole), 7.48 (s, 8H, ArH), 6.99 (s, 4H, ArH), 4.79 (s, 16H, -O-
CH2-C=CH), 2.57 ppm (s, 8H, -O-CH2-C=CH); 13C NMR (100 MHz,
CDCl3): d=56.2 (-O-CH2-C=CH), 75.9 (-C=CH), 78.4 (-C=CH), 102.3,
115.4, 120.4 (porphyrin), 144.7, 156.8 ppm; MALDI-TOF-MS: m/z: calcd
for C68H44N4O8Zn: 1110.49 [M]+; found 1109.0; UV/Vis (CH2Cl2): lmax
(log e)=421 (5.76), 548 (4.39), 584 nm (3.50).
Acknowledgements
Compound 4: Yield: 109 mg, 95%; 1H NMR (400 MHz, CDCl3): d=8.86
(s, 8H, pyrrole), 7.35 (s, 8H, ArH), 6.82 (s, 4H, ArH), 6.58 (s, 16H,
ArH), 6.45 (s, 8H, ArH), 4.98 (s, 16H, -CH2O-), 4.51 (s, 32H, -O-CH2-
C=CH), 2.30 ppm (s, 16H, -O-CH2-C=CH); 13C NMR (100 MHz,
CDCl3): d=54.9 (-O-CH2-C=CH), 69.0 (-CH2O-), 74.7 (-C=CH), 77.3
(-C=CH), 100.8, 105.9, 114.2, 119.5 (porphyrin), 138.3, 148.9, 156.6,
This work was supported by a project for “Innovation Creative Center
for Advanced Interdisciplinary Research Areas” in Special Coordination
Funds for Promoting Science and Technology from the Ministry of Edu-
cation, Culture, Sports, Science and Technology of Japan. We also thank
Prof. M. Ichikawa of Shinshu University for fluorescence lifetime meas-
urements.
157.8 ppm; MALDI-TOF-MS:: m/z: calcd for C148H110N4O24: 2391.84
[M]+; found 2389.6; UV/Vis (CH2Cl2): lmax (log e)=423 (5.68), 549
(4.29), 584 nm (3.38).
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Received: July 31, 2008
Published online: February 2, 2009
2624
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