Chemistry Reference
In-Depth Information
4.5 DENDRONIZED POLYMERS FOR SINGLE MOLECULE
CHEMISTRY
Synthetic chemists in academia are used doing their experiments all the way from the
500 g to 1 kg scale in a small reactor to a fewmilligrams in anNMR tube or nanograms
in biochemical assays. All these scales have their own specifics, challenges, and
fascinations. An ultimate goal of synthetic chemistry would, of course, be to work on
the smallest possible scale that is using individual molecules and do specific reactions
with them. Obviously this poses localization, handling, and analytical problems.
Among the pioneers in this field are Hla and Rieder, who back in 2000 reported a
covalent reaction between individual small molecules on a solid substrate. They
combined two iodo benzene molecules to biphenyl on a Cu surface by using the
interactions between the scanning tunneling microscope (STM) tip and the mole-
cules [42]. Other bimolecular reactions [43] and even polymerizations [44] initiated
by tip molecule interactions followed. Related are experiments in which chemical
reactions on solid substrates were initiated by other means (e.g., electrochemical or
photochemical ones) but the successful course of which was monitored by the
STM [45]. This is the point where dendronized polymers and the AFM come into
play. This fruitful symbiosis of a specific chemical system and a specificmanipulation
and analysis tool laid the foundation for a series of interesting experiments on the
single macromolecule level and brought them to ripeness. Nowadays this technology
can in principle be used by everybody. What are the key factors of both object and tool
that made this symbiosis a success story? The first to mention regarding object
certainly is thickness. Compared to other linear polymers, dendronized polymers have
a considerable thickness even when in the collapsed state adsorbed on a substrate.
Qualitatively this can nicely be seen from Figure 4.4 (Section 4.2). This allows
the AFM tip to actually grab the chains and drag them across when scanning near to
the substrate surface. The tip neither snaps over the object nor scratches into the
substrate. Thus, if the adsorption forces are properly tuned (see below), two entities
can be moved together to create a situation in which an intermolecular bond can be
formed at a predetermined position. In this context it is another important feature of
dendronized polymers that they can easily be equipped with groups such as azides,
capable of initiating covalent cross-linking when properly triggered, for example, by
photochemical means. Many of these azide groups will be located near or at the
object's “surface” and, thus, ideally positioned to bring about interchain cross-linking
events if the other chain is in sufficiently tight contact. Finally, for dendronized
polymers prepared on highly oriented pyrolytic graphite (HOPG) or carpet-modified
HOPG [46] the adsorption forces are in a very attractive intermediate range. Neither
do the chains adhere too strongly (e.g., as on mica) that the tip would tear them apart,
nor do they adhere too weakly to complicate imaging before and after manipulation
due to an uncontrolled lateral diffusion.
The key factor that comes to mind regarding the tool AFM are the ambient
conditions under which it can be operated. No ultrahigh vacuum conditions are
required as they are typically applied for the STM. It is fully sufficient to spin-cast a
polymer solution onto the substrate, and the AFM experiment can be started. Though
