Functionalization of covalent organic frameworks

Covalent Organic Frameworks (COFs) are a novel class of highly
stable, purely organic crystalline frameworks made of molecular
building blocks. For example, the condensation of boronic acids
with appropriate polyols in principle allows the design of
precisely controllable structures since their chemical and physical
properties can be easily tuned through the selection of the
building blocks. The young research field of COFs has attracted
scientists due to their extraordinary and versatile properties,
however, strategies to control the topology and the properties of
the backbone as well as the inner surface are still not well
established. With support of Prof. Knochel and his group, who
contributed numerous new organic COF linkers, this thesis aims to
extend the functionalization strategies for the design of Covalent
Organic Frameworks. Investigation of the structural modification
and the associated change in physical and chemical properties
should lead to progress regarding the applicability of these
materials. Employing the concept of reticular chemistry in
combination with High Throughput Synthesis Techniques, the
formation of a very large Covalent Organic Framework BTP-COF with 4
nm open pores was successfully carried out. The solvothermal
co-condensation of 1,3,5-benzenetris(4-phenylboronic acid) (BTPA)
and 2,3,6,7-tetrahydroxy-9,10-dimethyl-anthracene (THDMA) was
carried out using microwave irradiation instead of conventional
synthesis in an oven, thus synthesis time of BTP-COF was reduced
from initially 72 h to 5 min. Extending the open pore diameter of a
crystalline material to 4 nm, in combination with the resulting
high accessible surface area of 2000 m2/g offers great potential to
exploit organic reactions in the pores and enables the
incorporation of large functional guests, such as polymers or dyes.
Bearing these results in mind the scope of functionalization
possibilities was expanded from the geometric extension to the
chemical modification of the inner surface of COFs. Decorating the
organic building blocks with small functional active groups, such
as methyl-, -methoxy- and hydroxy- allowed for the successful
synthesis of several organic frameworks. Chemical and physical
properties of the backbone and the inner surface can be precisely
tailored by chemical modification of the building blocks. In order
to investigate post-synthetic modification strategies, the methyl-
and hydroxy-groups were used as reaction anchor points to
covalently attach molecules after framework formation. The
co-condensation of benzene-1,3,5-triyltriboronic acid (BTBA) and
the 9,10-dimethyl-anthracene-2,3,6,7-tetraol (DMAT) succeeded in
the formation of AT-COF-Me. In a radical bromination reaction the
methyl groups of an anthracene linker were successfully brominated
giving AT-COF-Br without degrading the crystalline framework of
AT-COF-Me. The formation of the resulting benzylic bromine was
monitored with IR spectroscopy and solid state NMR, respectively.
Elemental analysis results correspond to the bromination of half
the -CH3 groups. Reaction of
(2',5'-dihydroxy-[1,1':4',1''-terphenyl]-4,4''-diyl)diboronic acid
(HTDBA) and 2,3,6,7,10,11-hexahydroxytri-phenylene (HHTP) The
terphenyl-based hydroxyl substituted T-COF-OH, formed by
(2',5'-dihydroxy-[1,1':4',1''-terphenyl]-4,4''-diyl)diboronic acid
(HTDBA) and 2,3,6,7,10,11-hexahydroxytri-phenylene (HHTP), was
tested in several nucleophilic substitution reactions.
Esterification of the –OH group was achieved with either
acetylchloride or in a Steglich type reaction with 4-pentynoic
acid. X-ray diffraction analysis after the post-synthetic
modification shows that the crystallinity of the framework was
preserved. This indicates that T-COF-OH is compatible with the
reaction conditions. The detection of the newly formed ester
moieties in IR and in solid state NMR spectra proves the successful
post-synthetic esterification of the –OH groups. Another approach
to tailor functionality in COFs is to assemble monomers with
distinct properties in COF synthesis. Modification of the backbone
of the framework was realized with two heterocyclic building
blocks. Benzothiadiazole (BTD) and thienothiophene (TT) monomers
are known as building blocks of semiconducting polymers. These
molecules were equipped with boronic acid or boronate ester
moieties in para position. The linkers were then used in
co-condensation reactions with HHTP. The synthesis of BTD-COF was
carried out in a two step microwave synthesis procedure: first the
pinacolboronate
4,7-Bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)benzo[c][1,2,5]thiadiazole
(BTDA) was cleaved with HCl, in a second step addition of HHTP
resulted in the crystalline product in only 60 min. TT-COF was
synthesized in a conventional co-condensation reaction of
thieno[3,2-b]thiophene-2,5-diyldiboronic acid (TTBA) with HHTP; the
black TT-COF showed aborbance over the whole spectrum of the
visible light. Upon irradiation with light the system showed
significant photoconductivity. The 3 nm pores of the
hole-transporting TT-COF offer enough space to incorporate large
fullerene-based electron-transporting materials such as PCBM. This
inclusion leads to a significant quenching of the luminescence of
TT-COF, indicating light-induced charge transfer at the interface
of these two materials. The oriented growth of thin films of porous
COF-10, a product of the condensation of 4,4’-biphenyldiboronic
acid(BPBA) and HHTP, and TT-COF on self-assembled monomer
(SAM)-functionalized gold surfaces is shown. Films grown on boronic
acid terminated SAMs result in a parallel orientation of the pores
along the substrate. Scanning electron microscopy was used to
investigate the morphology of the films. Homogenous films with
thicknesses of around 150 nm and a total coverage of the substrates
were obtained. In summary, several functionalization strategies are
shown to control or tune the topology and properties of Covalent
Organic Frameworks. Tuning the topology and functionality to large
open pore systems or intrinsic semiconductivity allows
incorporation of large functional molecules and study the
host-guest interactions. The post-synthetic modification of COFs
offers a synthetic pathway to integrate organic functionalities,
which cannot be synthesized directly by co-condensation. These
strategies provide the means necessary for a precise control of the
pore environment and design a porous material for specific
applications. A facile and rapid method to produce thin oriented
COF films will pave the way for this material to fabricate
technological devices, such as photovoltaic devices, sensors of
OFETs.