Platinum-acetylide dendrimers possessing a porphyrin core

Article abstract of DOI:10.1039/b206254b

Intramolecular energy transfer from platinum-acetylide moieties to the porphyrin core was observed in novel organometallic dendrimers that were prepared from a tetra(4-ethynylphenyl)porphyrin-bridged tetranuclear platinum-acetylide core and platinum-acety

Full text of DOI:10.1039/b206254b

Platinum–acetylide dendrimers possessing a porphyrin core
Kiyotaka Onitsuka, Hotaka Kitajima, Masanori Fujimoto, Asako Iuchi, Fumie Takei and Shigetoshi
Takahashi*
The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka
567-0047, Japan. E-mail: takahashi@sanken.osaka-u.ac.jp
Received (in Cambridge, UK) 28th June 2002, Accepted 23rd September 2002
First published as an Advance Article on the web 9th October 2002
Intramolecular energy transfer from platinum–acetylide
moieties to the porphyrin core was observed in novel
organometallic dendrimers that were prepared from a
tetra(4-ethynylphenyl)porphyrin-bridged tetranuclear plati-
num–acetylide core and platinum–acetylide dendrons by a
convergent method.
The key intermediate in our convergent approach to the target
dendrimers is a tetranuclear platinum complex (ZnPC) bridged
by Zn(II) tetra(4-ethynylphenyl)porphyrin, which is the core of
the present convergent approach. The reaction of 4-(trimethylsi-
lylethynyl)benzaldehyde and pyrrole in refluxing propionic
acid for 3 h gave tetra[4-(trimethylsilylethynyl)phenyl]por-
phyrin (1) in 33% yield. After metallation with Zn(OAc)2,
trimethylsilyl groups were removed by treatment with tetra-
butylammonium fluoride (TBAF) to give Zn(II) tetra(4-ethynyl-
phenyl)porphyrin (3). Platinum moieties were introduced by the
reaction of 3 with 6 equiv. of Pt(PEt3)2Cl2 in the presence of
catalytic amounts of CuCl in a mixture of piperidine/dioxane
(1/3 v/v) under reflux for 1 week to give ZnPC in 29% yield.
Simple 1H-, 13C- and 31P NMR spectra of ZnPC are consistent
with a highly symmetric structure.
A series of organometallic dendritic porphyrins was synthe-
sized by the reaction of ZnPC with platinum–acetylide
dendrons up to the third generation (Scheme 2). Platinum
acetylide dendrons GD1–GD3 were prepared as reported
previously.6 Reaction of ZnPC with 4 equiv. of first-generation
dendron GD1 catalyzed by CuCl in a mixture of THF/
diethylamine (1/3 v/v) at room temperature produced the first-
generation dendrimer G1 along with small amounts of by-
products, which were estimated to be defective dendrimers with
less than three GD1 based on 31P NMR and gel permeation
chromatography (GPC) analyses. Therefore, G1 was isolated in
Dendrimers are regularly hyperbranched macromolecules that
adopt well-defined nanostructures. Recently, increasing atten-
tion has been focused on the incorporation of functional units
into such dendritic structures.1 Dendritic porphyrins are promis-
ing molecules, since porphyrins are representative functional
organic molecules and the local environment often affects their
functionalities. Several types of dendritic porphyrin with
organic dendrons have already been prepared, and some of them
exhibit interesting properties due to the encapsulation of a
porphyrin core in the interior of dendritic structures.2 On the
other hand, organometallic dendrimers are also promising as a
new type of functional dendrimer due to their magnetic,
electronic and photo-optical properties, as well their reactivity
as catalysts.3 Although dendritic porphyrins surrounded by an
organometallic sphere are fascinating, only one example of such
dendritic molecules has been prepared so far.4 In the course of
our study on organometallic dendrimers,5 we developed an
efficient route to platinum–acetylide dendrimers by a con-
vergent method.6,7 We present here the synthesis and properties
of novel organometallic dendrimers composed of platinum–
acetylide units and a porphyrin core.(Scheme 1)
Scheme 1 Reagents and conditions: i, propionic acid, reflux, 3 h, 33%; ii,
Zn(OAc)2, CHCl3, reflux, overnight, 97%; iii, TBAF, 278 °C to r.t., 1.5 h,
90%; iv, Pt(PEt3)2Cl2 (6 eq.), CuCl cat., piperidine/dioxane (1+3 v/v),
reflux, 1 week, 29%.
Scheme 2 Reagents and conditions: i, CuCl cat., Et3N/THF (1+3 v/v), r.t.,
73% (G1); 67% (G2); 41% (G3).
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73% yield by preparative GPC followed by reprecipitation of a
CH2Cl2 solution in hexane. Second- and third-generation
dendrimers G2 and G3 were prepared by similar reactions of 4
with second- and third-generation dendrons GD2 and GD3 in
yields of 67 and 41%, respectively. Although mass spectrome-
try is a powerful tool for the characterization of dendrimers, the
molecular ions of platinum–acetylide dendrimers with more
than four platinum moieties have never been observed, even in
MALDI-TOF and ESI mass spectrometry. However, the
platinum–acetylide dendrimers prepared in this study were fully
characterized by NMR and GPC analyses in the same manner as
was used for other platinum–acetylide dendrimers.5–7 Since
dendrimers G1–G3 were synthesized by reaction between large
molecules, the presence of products with a different composi-
tion is easily detected by GPC analysis. Narrow GPC profiles of
G1–G3 showed the absence of impurities with different
molecular sizes. In the 31P NMR spectra, no signals assignable
to the C·CPt(PEt3)2Cl moieties were detected. The 1H and 13
C
NMR spectra are also consistent with the proposed structure of
G1–G3.
Fig. 2 Fluorescence spectra of G1 (excited at 341 nm), G2 and G3 (excited
at 344 nm) in THF. The spectra were nomalized to the concentration of 8.9
3 1028 M. The spectra were measured through a cut-off filter (l 5 220 nm)
in the region of l 5 550 nm, and that (l 5 450 nm) in the region of l 4 550
nm.
The electronic absorption spectra of dendrimers G1–G3 in
THF are shown in Fig. 1 along with those of 3 and ZnPC. The
higher energy bands (l = 300 nm) are due to p–p* transitions
of the triethynylbenzene-bridges, and the absorption at about
lmax = 341 nm is attributable to the MLCT band of platinum–
acetylides.8 The molar absorptivity (e) of these absorptions
remarkably increased with an increase in the generation of
dendrimers. This phenomenon is due to the increase in platinum
moieties in the molecules, since the spectra in this region based
on platinum moieties resemble each other very closely.
Dendritic porphyrins G1–G3 and the core ZnPC showed a
Soret band at about lmax = 435 nm, whereas that of 3 was
observed at lmax = 426 nm. The e value of the Soret band
decreased with an increase in the generation. Although the
reason for this phenomenon is still unclear, it may be of interest
since the Soret band is scarcely influenced by dendrons in
dendritic porphyrins.2,4 Two absorptions at about 560 and 600
nm characteristic of the Q band of zinc porphyrins were
observed in the spectra of all of the samples.
acetylide units by excitation at 343 nm, suggesting that energy
transfer takes place intramolecularly. The intramolecular en-
ergy transfer was confirmed by the fluorescence spectra of G2
and G3. Fluorescence from the porphyrin core at 617 nm
appreciably decreased as the generation of dendrimers in-
creased, while another fluorescence peak at 648 nm relatively
increased. This is also a characteristic phenomenon of plati-
num–acetylide dendrimers, and studies on the reason of this
phenomenon are in progress.
This work was supported by Grant-in-Aid for Scientific
Research on Priority Areas from the Ministry of Education,
Science, Sports and Culture. We thank The Material Analysis
Center, ISIR, Osaka University, for support of spectral
measurements.
When G1 was excited at the MLCT band of platinum–
acetylide units (341 nm), fluorescence peaks were observed at
381 and 617 nm (Fig. 2). Upon excitation at the Soret band (435
nm), G1 emitted fluorescence at 617 nm, which is characteristic
of the zinc porphyrin nucleus. Platinum–acetylide dendron
GD1 showed fluorescence at 384 nm upon excitation at 341 nm.
Thus, the fluorescence spectrum of G1 indicates energy transfer
from platinum–acetylide dendrons to the porphyrin core. A
mixture of ZnPC and GD1 in a ratio of 1+4 did not show
fluorescence from the porphyrin but rather from platinum–
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Fig. 1 Electronic absorption spectra of 3, ZnPC, G1, G2 and G3 in THF.
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