Chemistry Reference
In-Depth Information
FIGURE 4.13
Tapping-mode AFM images of two individual dendronized polymers
PG3
,
moved toward each other (“move”; a
c), and
challenged mechanically (“prove”; d, e, f). The arrows indicate the movement of the AFM tip
during manipulation. Reprinted from Ref. [47] with permission of Wiley-VCH.
!
b), irradiated by UV light (“connect”; b
!
one admittedly never knows the exact state of the substrate when working under
ambient conditions, the ease of operation has its attraction. A sample is ready for
manipulation and analysis after a few minutes time! Additionally, dendronized
polymers are such huge objects that adsorbed molecules, for example, water, will
alter the adsorption forces but cannot render unreliable the interpretation of the
experiments dealing with covalent bond formation between these objects.
In 2003, the first experiment was reported in which two dendronized polymers
were prepared as single entities on a solid substrate, moved together by the AFM and
connected by photochemical treatment. The connection was then proven again with
the help of the AFM, by dragging the combined macromolecules across the
surface [47].
5
These experiments, which were coined move-connect-prove sequence,
were done with a third-generation dendronized polymer
[Y mostly NH
3
þ
CF
3
CO
2
but also NHCO(CH
2
)
5
NHC
6
H
3
(NO
2
)(N
3
)
3
] in which some of the periph-
eral ammonium groups had been converted into azide derivatives (Figure 4.13). The
actual “gluing” process was achieved through these azide groups, which are known to
decompose to the highly reactive nitrene intermediates upon irradiation. At the site
where the two chains were in tight contact, these nitrenes caused a structurally ill-
defined intermolecular cross-linking.
Because of the potential importance of such protocols for the bottom-up approach
to the nanosciences, in the following years this experiment was developed to
maturity [48]. The key steps were (i) the introduction of a dense monolayer (“carpet”)
of long-chain fatty acids between the solid substrate (HOPG) and the deposited
macromolecules to be subjected to such a “move-connect-prove” sequence and
(ii) the use of the structurally precisely defined dendronized polymer
PG3
PG3A
5
This initial experiment was carried out with a not fully characterized azidified polymer that had been
prepared on site and not purified. It is therefore likely that the substratewas covered with small molecules of
different nature. They of course cannot have altered the key conclusion namely the successful execution of a
move-connect-prove sequence but will have influenced the lateral mobility of all components involved.
