Chemistry Reference
In-Depth Information
Based on this concept, Krebs and coworkers [
197
] reported a synthesis of a light-harvesting
material that consists of three structural domains. Two of them are conjugated homopolymers that
are linked with a zinc porphyrin:
A
B
A
n
m
A
=
n
N
N
B
=
Ze
NN
Sections A and B of the above block copolymer are illustrated above. The polymer has a constant
ratio of the zinc porphyrin to the incorporated monomer units, regardless of the molecular weight.
The ratio of zinc porphyrin to the polymer blocks can be varied in the material by varying the size of
the blocks A. Studies of energy transfer from the polymer to the zinc porphyrin showed that there
is actually very little energy transfer when the material is in solution. On the other hand, there is
quantitative energy transfer in the solid state. Also, it was observed that the light-harvesting
properties of the three-domain structures depend on the chain lengths of the conjugated polymers.
Cheng and Luh reported that they are also trying to develop polymers that would mimic natural
photosynthesis with synthetic polymers [
198
]. They point out that silylene moieties have been used
extensively as insulating spacers [
198
]. In general, when the silylene spacer contains only one silicon
atom, no conjugative interactions between the
systems and the silicon moiety are observed [
199
]. They
believe, therefore, that introduction of an energy gradient with three well-designed chromophores into a
silylene-spaced polymeric chainmay lead to sequential energy transfer. To achieve this goal, they carried
out preparations of regioregular silylene-spaced copolymers composed of energy gradients with three
different chromophores. One of the polymers prepared in this way can be illustrated as follows:
p
Si Si Si
A
n
Their synthesis utilized rhodium-catalyzed hydrosilylation of bis-vinylsilanes and bis-alkynes.
The ratio of the three chromophores in the above polymer is 1:2:1, corresponding to D1, D2, and A
chromophores, respectively. Cheng and Luh found that upon excitation of the donor chromophore
D1, only emission from the acceptor A was observed [
199
].
Si
D
2
D
2
10.6.2 Photo-Isomerization of Polymeric Materials
