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Functional Molecular Assemblies.

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Meeting Reviews
Functional Molecular Assemblies**
Jim A. Thomas
In the last decade, nanotechnology has
moved from science-fiction novels into
the research lab. As a result, “designer”
molecular materials have become
increasingly important. To construct
and understand these systems requires
an approach that often stretches over
the whole of chemistry and into physics,
biology, or materials science. However,
judging from the excitement communicated by the participants of a recent
meeting, it seems that although navigating this terra incognita is daunting,
the intellectual rewards more than make
up for it.
The organizing committee of the
Royal Society of Chemistry Dalton
Discussion 9 Meeting (DD9), held on
April 19–21, 2006, interpreted its theme,
Functional Molecular Assemblies, very
widely. So, although this was an inorganic-chemistry meeting, participants
from a wide range of molecular-based
science attended.
The meeting was divided into four
specific sub-themes, each introduced by
a leading researcher. An unusual aspect
was the time allocated to discussion—
after several presentations, the floor was
opened for contributions from the audience leading to exchanges that lasted for
up to 45 minutes.
The opening session was on surface
chemistry, and topics ranged from the
use of rotaxane dyes as photographic
[*] Dr. J. A. Thomas
Department of Chemistry
University of Sheffield
Brook Hill, Sheffield S3 7HF (UK)
Fax: (+ 44) 114-273-8673
E-mail: james.thomas@sheffield.ac.uk
[**] Dalton Discussion 9: Functional Molecular Assemblies, April 19–21, 2006, Manchester (UK)
4396
inks to the synthesis of dendrimerencapsulated quantum dots. Highlights
included a fascinating talk by Bart Jan
Ravoo from the Reinhoudt group (University of Twente) who explained a
“molecular print-board” approach[1] for
precisely positioning molecules in two
dimensions. By using cyclodextrins
immobilized on a surface as a substrate,
the group “prints” molecules—through
multivalent noncovalent interactions,
stable positioning of guests such as
ferrocene-terminated dendrimers is achieved. Ravoo explained how this
method is being used to construct devices such as field-effect transistors.
One question that arose during the
associated discussion was “when do
noncovalent
interactions
between
phases become more important than
the interaction between molecules of a
phase?” Another issue was the problem
of reading and writing at the molecule
scale. Since it was raised by Richard
Feynman in his agenda-setting “Room
at the Bottom” talk,[2] this problem has
remained a great challenge to functional
nanotechnology. A paper in this session
outlined one approach to meeting this
challenge. Mario Ruben of the Institute
of Nanotechnology in Karlsruhe discussed the construction of coordination
complex based nanoarchitectures and
demonstrated that by employing current
imaging tunneling spectroscopy (CITS),
it is possible to image individual metal
centers in 2 > 2 CoII-based grids (Figure 1 a). Later, during the second session
on magnetic materials, Laurie Thompson (Memorial University, Newfoundland) showed images of a 3 > 3 MnIIbased grid obtained by the same technique (Figure 1 b). He also discussed
bulk studies revealing that individual
grid redox states are switchable, leading
to changes in the magnetic properties of
the grid. Furthermore, both researchers
described successful strategies to produce extended structural arrangements.
Given these observations it seems clear
that high-density storage media based
upon single molecules are possible.
Several presentations during the
second session involved oligonuclear
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1. A) A 2 2 CoII grid and B) a 3 3
MnII grid imaged with CITS. Adapted from
references [3 and 4]; reproduced with permission of the Royal Society of Chemistry.
metal complexes and their possible use
in molecular-based devices, for example,
as candidates for quantum bits in quantum
computational
architectures.
During his keynote lecture and the
resulting discussion, Joel Miller of the
University of Utah pointed out that
although these latter systems are frequently called single molecular magnets
they are often weakly coupled in the
long range and hence are not true (bulk)
magnets. In his talk, Miller elucidated
how molecule-based bulk magnets can
be designed and why such materials
often have strikingly different properties from those of traditional atombased systems. This point is illustrated
by just one of the many systems described. Magnets based on second-row
transition-metal complexes are comparatively rare; consequently Miller and
colleagues have been investigating
structures containing “paddle wheels”
based on [Ru2(O2CMe)4]+ units. By
Angew. Chem. Int. Ed. 2006, 45, 4396 – 4398
Angewandte
Chemie
using [Cr(CN)6]3 as a second chargecompensating unit, a 3D structure of
interpenetrated cubic lattices was
obtained. This system magnetically
orders below 100 K and displays magnetic hysteresis. Interestingly, the hysteresis loop takes an unusual constricted
form (Figure 2), and magnetic saturation occurs at magnetic-field strengths
30 % higher than that of a related noninterpenetrated structure.
Figure 2. Constricted magnetic hysteresis for
the interpenetrated cubic lattice structure of
[Ru2(O2CMe)4]3[Cr(CN)6]. Adapted from reference [5]; reproduced with permission of the
Royal Society of Chemistry.
A third session, on self-assembly,
began with Makoto Fujita of the University of Tokyo explaining how squareplanar metal centers template the
assembly of a wide range of discrete
architectures with hydrophobic cavities;
these cavities provide a nanospace for a
number of unusual phenomena: For
example, mixed-valence tetrathiafulvalene dimers can be generated and studied, common reactions such as Diels–
Alder cyclization produce entirely new
products, and previously short-lived
intermediates in photochemical reactions of metal-carbonyl complexes can
be structurally characterized.
After this, Markus Albrecht
(RWTH-Aachen) described the selfassembly of large [Ti4L4]8+ tetrahedral
cages, and Rolf W. Saalfrank of the
University of Erlangen–NHrnberg illustrated how the interactions of simple
ligands with FeIII that usually lead to
metallacoronands assembly[6] can produce tetranuclear “stars”. Two talks
then illustrated how additional interactions can be used in assembly processes.
Lee Cronin (University of Glasgow)
described triangular isopolyoxotungAngew. Chem. Int. Ed. 2006, 45, 4396 – 4398
state analogues of crown ethers,
hypothesizing that the assembly of the
structures is a result of symmetry transfer from the triethanolamine cations
used in their preparation. Ed Constable
of the University of Basel discussed the
self-assembly of metal complexes driven
by p stacking in the bulk phase and on
surfaces. This latter study reinforced
observations previously raised by
Mario Ruben that self-assembly at surfaces lowers the symmetry and introduces new weak interactions, leading to
new structures not found in crystals.
In this session, Sijbren Otto of the
University of Cambridge introduced a
thought-provoking concept—that secondary interactions within a receptor
reinforce substrate binding. Suppose
binding of a substrate by a protein is
enthalpically favored but, owing to the
loss of freedom in the final assembly, is
entropically disfavored (Figure 3 a); if
binding to the substrate is accompanied
by creation of intrareceptor interactions
that only form on assembly of the host–
guest complex (Figure 3 b and c) then
the entropic cost of creating two favorable interactions is only paid once. Otto
then outlined how this effect could be
used in synthetic receptors.
The subject of the last half day was
photoactive materials. Papers by David
Parker and Stephen Faulkner concerned
the distinctive luminescent properties of
lanthanide complexes. In his keynote
lecture, Parker discussed his groupJs
work at the University of Durham on
complexes that sense specific ions and
biomolecules. A point that clearly
emerged from discussion was that the
commonly made assumption that
charged species are only appropriate
for in vitro studies is incorrect. For
example, Parker described how tricationic complexes that sense citrate or BDNA in vitro are localized within nuclei
in cellulo. More surprisingly, neutral and
anionic analogues of one of these complexes show almost no cellular uptake.
Faulkner then outlined how lanthanide
centers can be photosensitized by coordination of suitable chromophores. He
showed that phosphonate ligands yield
complexes that are substitutionally
robust, even in the presence of competing phosphate. Raymond Ziessel of
ULP Strasbourg then showed that fluorescent boron dipyrromethane systems
can be used for a variety of device
applications including as hybrid materials such as luminescent zeolites.
Both the final two papers involved
self-assembly and optical studies. In the
first, Rainer Glaser of the University of
Missouri-Columbia demonstrated how
highly anisotropic materials with perfect
polar order and layer stacking could be
constructed from relatively simple aromatic azine building blocks. These materials have very high nonlinear optical
responses with second-harmonic generation so efficient that it can be detected
by the naked eye. In the last paper, Jim
Thomas (University of Sheffield) discussed a general procedure for the
assembly of kinetically robust mixedvalence molecular bowls constructed
from ruthenium centers and hindered
nucleobases.
In concluding remarks, Richard
Winpenny applauded the speakersJ
readiness to present “work in progress”
that was perhaps still not fully understood. As he and his fellow organizer
Faulkner pointed out, this added fuel to
the informal and stimulating conversations that took place within (and out-
Figure 3. Thermodynamics of a protein binding to a ligand. A) Ligand binding with no intrareceptor interactions. B) Ligand binding with intrareceptor interactions. C) Intrareceptor interactions that cannot be formed without binding. Adapted from reference [7].
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
4397
Meeting Reviews
side) the lecture theater. A forthcoming
special edition of Dalton Transactions
will include all the DD9 presentations.
[1] T. Auletta, B. Dordi, A. Mulder, A.
Santori, S. Onclin, C. M. Bruinink, M.
PLter, C. A. Nijhuis, H. Beijleveld, H.
SchMnherr, G. J. Vancso, A. Casnati, R.
Ungaro, B. J. Ravoo, J. Huskens, D. N.
Reinhoudt, Angew. Chem. 2004, 116, 373;
Angew. Chem. Int. Ed. 2004, 43, 369.
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www.angewandte.org
[2] “ThereJs plenty of room at the bottom”:
R. P. Feynman in Miniaturization (Ed.:
H. D. Hilbert), Reinhold, New York,
1961, p. 282.
[3] N. Lin, S. Stepanow, F. Vidal, K. Kern,
M. S. Alam, S. StrMmsdMrfer, V. Dremov,
P. MHller, A. Landa, M. Ruben, Dalton
Trans. 2006, 2794.
[4] V. A. Milway, S. M. T. Abedin, V. Niel,
T. L. Kelly, L. N. Dawe, S. K. Dey, D. W.
Thompson, D. O. Miller, M. S. Alam, P.
MHller, L. K. Thompson, Dalton Trans.
2006, 2835.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
[5] J. S. Miller, Dalton Trans. 2006, 2742.
[6] See for example: R. W. Saalfrank, C.
Deutcher, H. Maid, A. M. Ako, S.
Dperner, T. Nakajima, W. Bauer, F.
Hampel, B. A. Heß, N. J. R. van Eikema
Hommes, R. Puchta, F. W. Heiemann,
Chem. Eur. J. 2004, 10, 1899.
[7] S. Otto, Dalton Trans. 2006, 2861.
DOI: 10.1002/anie.200602068
Angew. Chem. Int. Ed. 2006, 45, 4396 – 4398
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