Nanoscale spherical architectures fabricated by metal coordination of multiple dipyrrin moieties

Article abstract of DOI:10.1021/ja0637301

Phenylethynyl-bridged dipyrrin "dimers" have performed Zn^II complexation to give coordination polymers, which provided the fluorescent colloidal spherical objects in solution as well as on the substrate according to the spacer units. Using a mi

Full text of DOI:10.1021/ja0637301

Published on Web 07/14/2006
Nanoscale Spherical Architectures Fabricated by Metal Coordination of
Multiple Dipyrrin Moieties
Hiromitsu Maeda,*,†,‡ Masahiro Hasegawa,† Takashi Hashimoto,† Takuya Kakimoto,§
Satoru Nishio,§ and Takashi Nakanishi|
Department of Bioscience and Biotechnology and Department of Applied Chemistry, Faculty of Science and
Engineering, Ritsumeikan UniVersity, Kusatsu 525-8577, Japan, Department of Applied Molecular Science,
Institute for Molecular Science (IMS), Okazaki 444-8787, Japan, and Supermolecules Group,
Organic Nanomaterials Center, and International Center for Young Scientists (ICYS),
National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan
Received May 28, 2006; E-mail: maedahir@se.ritsumei.ac.jp
Metal ion coordination enables organic ligands to form versatile
discrete or infinite architectures, such as wire structures and nano-
space materials, with potential applications in catalysis, optics, and
biosensing.1 Recently, nanoscale morphologies2 based on coordina-
tion polymers have been reported to exist as spherical and fibrous
structures.3 Of the coordinating ligands, dipyrrins (dipyrromethenes),
consisting of two pyrroles with an sp2-meso position, are essential
π-conjugated bidentate monoanionic ligands for metal ions in
natural and artificial systems.4-6 Therefore, dipyrrins are promising
planar scaffoldings for self-assemblies and would give neutral
coordination oligomers, which, in combination with various spacer
units, could be used to fabricate fine-tuned nanoscale morphologies
using bridging metal cations. In this communication, we report
nanoarchitectures based on metal coordination of dipyrrin oligomers.
Dipyrrin “dimers” (1-4, Figure 1a) with various rigid phenyl-
ethynyl spacers were prepared by cross-coupling reactions.6a
Sonogashira coupling of 4-bromobenzaldehyde and 1,4- and 1,3-
diethynylbenzenes followed by condensation with pyrrole and a
subsequent DDQ oxidation afforded dipyrrin derivatives 1 and 2
in 45 and 35% yields (from diethynylbenzene). By similar
procedures involving reaction between 3-bromobenzaldehyde and
the corresponding diethynylbenzenes, dimers 3 and 4 were also
obtained in 8.7 and 20% yields, respectively. Structures of 1-4
were confirmed by 1H NMR and FAB-MS.7 For reference, dipyrrin
5 was also prepared (Figure 1b).
Figure 1. (a) Metal complexation of dipyrrin “dimers” 1-4, (b) structure
of dipyrrin 5, and (c) X-ray single-crystal structure of 5‚ZnII as one of the
disordered conformations. Thermal ellipsoids are scaled to the 50%
probability level.
ZnII, showing the dimeric and trimeric [n + n] structures derived
from fragmented oligomers.10 In these cases and judging from the
UV/vis spectra and X-ray analysis of 5‚ZnII, ZnII coordinated by
dipyrrins does not suffer THF coordination, possibly because of
its tetrahedral geometry.
Self-assembled nanoscale structures of coordination oligomers
based on dipyrrin dimers were observed by scanning electron
microscopy (SEM). ZnII complexes of rod-like ppp 1 and crinkled
mmm 4 give randomly shaped objects from THF, wherein 1‚ZnII
is less soluble and affords precipitates. In sharp contrast, ZnII-
coordinated oligomers of partially crinkled pmp 2 and mpm 3 give
uniform nanosized spherical structures with a diameter ca. 0.3 µm
Ditopic ligands 1-4 exhibit coordination behavior indicated by
changes of solution color, and in some cases, precipitation occurs
immediately upon addition of metal salts. Of the divalent metal
cations, ZnII is known to be a useful bridging metal between two
dipyrrin units with tetrahedral geometry demonstrated by the X-ray
structure of 5‚ZnII (Figure 1c).8,9 Therefore, dipyrrins 1-4 were
treated with 1 equiv of Zn(OAc)2 in THF to give oligomeric
structures (Figure 1a). Metal coordination was confirmed by UV/
vis spectral changes in THF solution (5 × 10-5 M) upon addition
of 1 equiv of Zn(OAc)2, where the band at 434 nm due to a free
dipyrrin moiety disappeared with a concurrent emergence of a new
band at 483-485 nm due to 1-4‚ZnII. In contrast, 1 equiv of ZnCl2
provided a mixture of metal complex and free ligand, possibly due
from the same solvent with initial concentrations of ca. 10-3
M
(Figure 2a,b). In the case of 2‚ZnII, “trains” of spheres are also
observed. Submicron-sized polymer particles are normally spherical
for minimization of the interfacial free energy between the particle
and the solVents. Further, dynamic light scattering (DLS) of 2‚
ZnII and 3‚ZnII at 10-3 M in THF furnished an average diameter
of their particles of ca. 0.1 µm, which suggests the formation of
spherical structures in solution. The effects of temperature and
concentration were also observed. On the other hand, metal-free
dipyrrins 1-4 give amorphous objects from THF, which suggests
that metal bridging between organic ligands as well as both para
and meta linkages is required for well-defined nanoscale objects.
Interestingly, mixtures of THF and H2O (2:1, v/v), where the
ZnII complexes are insoluble and precipitate, afforded hemispheres
for the oligomers 2,3‚ZnII, bell-shaped objects for 3‚ZnII (Figure
2c), and spheres with “craters” for 4‚ZnII. These features suggest
that the nanospheres are colloidal and “fragile”.11 Further, 1:1 THF/
H2O solutions of 4‚ZnII provided nanoscale “golf balls” as well as
1
to partial demetalation by HCl. In addition, H NMR in THF-d8
also exhibited the ZnII-bridged oligomers of 2-4, observed as
another resonance of metal-free ligand moieties, by equimolar metal
salt. Furthermore, MALDI-TOF-MS of rather rigid polymers 1,2‚
ZnII afforded no significant peaks, in contrast with that of 3,4‚
† Department of Bioscience and Biotechnology, Ritsumeikan University.
‡ Institute for Molecular Science.
§ Department of Applied Chemistry (Ritsumeikan University).
| National Institute for Materials Science.
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J. AM. CHEM. SOC. 2006, 128, 10024-10025
10.1021/ja0637301 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
intramolecular energy transfer. In contrast to nanoscale quantum
dots consisting of inorganic compounds (e.g., CdS), particles derived
from ZnII-bridged dipyrrin oligomers show emission even at the
submicron scale.
Spherical nanoarchitectures fabricated from various metal cations
and dipyrrin-based ligands could provide functional materials.
Dimension control by solvents,13 metal cations, and peripheral
substituents will afford well-defined versatile shapes of nanoscale
objects. Further construction and application of our current system
are now being investigated.
Acknowledgment. This work was supported by the “Academic
Frontier” Project for Private Universities, namely, the matching fund
subsidy from MEXT, 2003-2008. The authors thank Prof. Atsuhiro
Osuka and Mr. Shigeki Mori, Kyoto University, for the X-ray
analysis, and Prof. Hitoshi Tamiaki, Ritsumeikan University, for
helpful discussions.
Figure 2. SEM of (a) 2‚ZnII from THF (8 × 10-4 M; inset: Zn mapping
by HRTEM-EDX), (b) 3‚ZnII from THF (3 × 10-3 M; inset: fluorescence
micrograph with 25 µm sides of each square), (c) 3‚ZnII from THF/H2O
(2:1, 1.5 × 10-3 M), and (d) 4‚ZnII from THF/H2O (1:1, 1.5 × 10-3 M;
inset: TEM with 200 nm bar).
Supporting Information Available: Synthetic procedures for
dipyrrin dimers 1-4, NMR, MS (MALDI), DLS, SEM, TEM, OM,
AFM, and solid-state FL of 1-4‚ZnII, and CIF file for the X-ray
structural analysis of 5‚ZnII. This material is available free of charge
via the Internet at http://pubs.acs.org.
objects-with-dents (Figure 2d). In contrast, using CH3CN, which
also affords precipitates, oligomers of 1-4‚ZnII form bunches of
the smaller spherical objects with ca. 100 nm diameters. In these
solvent systems and similarly to protein foldings, object formation
requires several steps: (i) formation of coordination oligomers
(primary), (ii) stacking of the oligomers (secondary), (iii) conversion
into spheres (tertiary), and (iv) assembly into larger objects without
fusion (in CH3CN) and segmentation (in THF/H2O) (quaternary).
X-ray analysis of 5‚ZnII, as a reference monomer, elucidates ordered
π-planes (average distances between phenylethynyl units ) 3.62
and 3.25 Å) and a CH-π interaction of phenylethynyl and dipyrrin
moieties9 to give structure information at subnanoscale level. In
step (iii), diameters of spheres are augmented on the substrate
compared to in solution. One of the formation mechanisms of the
dents, in the final step, could be the release of residual solvents
from the particles.11,12 Furthermore, “denaturation” by sonication
of CH3CN solutions separated some assemblies into individual
spherical objects. From the crystal packing of 5‚ZnII, ca. 105-106
dipyrrin units are estimated to be contained possibly with some
solvent molecules in a sphere with a diameter of 100 nm.
The spherical nanostructures of coordination oligomers 2-4‚
ZnII obtained from THF or CH3CN were also observed by
transmission electron microscopy (TEM) and optical microscopy
(OM). TEM also supports the formation of “dents” on the spheres
in THF/H2O solution (inset of Figure 2d). HRTEM-EDX of both
2‚ZnII (from THF, inset of Figure 2a) and 4‚ZnII (from CH3CN)
provided zinc mapping (8.63 keV derived from Zn-K level) to
suggest that Zn is contained in the core. Further, AFM of 2‚ZnII
from THF suggests the heights of 70-80 nm and widths of ca.
500 nm for the hemispheres mounted on a silicon substrate.
In THF (5 × 10-5 M), the coordination oligomers 2-4‚ZnII have
emission maxima at 510-515 nm, which can be ascribed to the
ZnII-bisdipyrrin moieties. On the other hand, in the solid state by
assembly from THF, 2-4‚ZnII give fluorescent spherical objects
(inset of Figure 2b) and emission maxima at 532-543 nm. If other
solvents, such as CH3CN, are used, no emission was observed
possibly because of the aggregation of the nanoparticles.
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Preliminary investigation of other metal cations (CoII, NiII, CuII)
gave similar spherical objects. Addition of a 1:1 mixture of Zn-
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