Conjugated porous polymers for visible-light photocatalysis
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Abstract
In this thesis, two different aspects of porous conjugated polymers as visible-light-active and
heterogenous photocatalysts have been discussed: 1.) Synthesis strategies for control of
hydrophilicity 2.) Metal-free photoredox mediated reaction design as replacement for
traditional transition metal-catalyzed carbon-carbon bond formation reactions.
The pristine hydrophobic polymers were modified by a specific functionalization in order to
conduct photocatalysis in water. Two different molecular design strategies for modification of
cross-linked conjugated polymers are presented. Moreover, the photocatalysts were applied to
aqueous and non-aqueous reactions. The photocatalytic reaction mechanisms were elucidated
by various methods.
The first design strategy of the hydrophilicity control was demonstrated via a simple
protonation step of a series of azulene-based conjugated microporous polymers (CMPs). Here,
the protonation of the azulene units led to the formation and permanently stabilization of
tropylium cations. This concept of the controlled change from hydrophobic to hydrophilic
networks showed that highly functional CMPs can be used in both organic and aqueous
solvent systems. After post-modification, the change on porosity, morphology, and opticalelectronical properties were analyzed. The photocatalytic efficiency of the modified polymers
in water were demonstrated by the reduction of chromium(VI) to chromium(III), whereas the
efficiency was comparable to established inorganic heterogeneous photocatalysts.
The second example of hydrophilicity control was demonstrated by the combination of
photoactive conjugated donor-acceptor polymers with cross-linkable ionic side groups, which
led to the formation of a series of highly porous conjugated polyelectrolytes (CPEs). This
concept merged multiple material properties: 1.) The irradiation with visible light led to a selfinitiation and cross-linkage process of the polymers; 2.) The amphiphilic properties allowed
one to use the polymer as stabilization agent for colloids and emulsions; 3.) The irradiation
with visible light of those aggregates leads to permanent cross-linkage, and therefore the
formation of stable heterogeneous photocatalysts with defined macrostructure inside the
materials. The material properties, the morphology, and the optical-electronical properties of
the materials were analyzed. Finally, the photocatalytic performance by means of known
reactions such as photodegradation of organic dyes or the photoreduction of metal-ions were
demonstrated.
The third example deals with the photocatalytic Stille-type C-C coupling reaction, which was
demonstrated by the employment of azulene based conjugated polymers. In order to
determine the range of C-C coupling reactions, a series of aromatic halides and aromatic
stannanes were tested. The photoredox mediated reaction mechanism was investigated and the
key steps have been identified.
In addition, the elucidation of the photocatalytic reaction mechanism was conducted via
electrochemical methods as well as indirect chemical methods or trapping and detection of
reactive species.