cyclic voltammetry and spectroelectrochemistry as well as
the possibility to post-functionalize polythiophene films with
longer oligonucleotide strands for application as DNA
sensors (which can visualize even single-nucleotide poly-
morphisms6). Recently, Barbarella et al. published water-
soluble dinucleotide conjugates that can form aggregates both
in the solid state and aqueous solution at high ionic strength.7
This example indicated the possibility to transform the
molecular recognition of base pairs via hydrogen bonding
into a molecular recognition-driven self assembly. Meijer et
al. already demonstrated the successful application of this
principle by the formation of helical stacks from thymidine-
functionalized oligo(phenylenevinylene) and polyadenosine
strands.8
Here, we describe the synthesis of thymidine (T)- and 2′-
desoxyadenosine (A)-functionalized oligothiophenes as first
examples of a versatile protocol using Cu(I)-catalyzed
Huisgen 1,3-dipolar cycloaddition of azides and terminal
alkynes, frequently referred to as “click-reaction”.9 The
general procedure includes the connection of the important
building blocks, oligothiophene and nucleoside, through a
flexible alkyl spacer that should provide more flexibility and
might benefit the self-assembling properties. The conjugates
comprise highly amphiphilic character and can exert various
competing intermolecular forces, such as π-stacking, van der
Waals interactions, H-bonding, and solvent effects. The
combination of all of these interactions may allow fine-tuning
of the self-aggregating behavior and lead to the rational
design of supramolecular assemblies of bio-inspired organic
semiconductors.
A flexible alkyl spacer bearing a terminal azide group was
first attached to the nucleosides which subsequently were
“clicked” to ethynylated oligothiophenes. In order to couple
the spacer selectively at the 3′-OH group of the nucleoside,
thymidine (T) and desoxyadenosine (A) were protected at
the primary OH group with bulky tert-butyl-dimethylsi-
lylether (TBDMS) under common basic conditions.10 The
resulting TBDMS-protected nucleosides 1a,t were reacted
with 5-bromovaleric acid under Steglich conditions to form
esters 2a,t in 72% and 81% yield, respectively, with either
dicyclohexylcarbodiimide (DCC) or 1-ethyl-3-diisopropy-
lamino-carbodiimide (EDC) as activating agent. Azido-
functionalized “bio”-components 3a,t were obtained in 87%
and 82% yield, respectively, by nucleophilic substitution of
the terminal bromine in 2a,t with sodium azide (Scheme 1).
Scheme 1
“Semiconductor” block ethynyl-quaterthiophene 5 was
synthesized from corresponding carbaldehyde 4 via Ohira-
Bestmann reaction11 with dimethyl-diazooxopropyl-phos-
phonate in 89% yield (Scheme 2).
Scheme 2
Coupling of the two different moieties to oligothiophene-
nucleoside conjugates 6a,t was achieved by “click”-reaction
of alkyne 5 and azido-nucleosides 3a,t with the catalytic
system Cu(CH3CN)4PF6/Cu0 in THF.10 This optimized
protocol gave much better results than the standard Cu2+/
ascorbic acid system.12 2-Desoxyadenosine-quaterthiophene
6a was obtained in 77% and thymidine-quaterthiophene 6t
in 54% yield. As a reference quaterthiophene-triazole 7
without a nucleoside unit has been synthesized by the same
protocol in 79% yield by cycloaddition of alkyne 5 and
5-azidovaleric acid methylester (Scheme 3).
Investigation of the optoelectronic properties of conjugates
6a,t and reference 7 gave only small differences compared
to the parent quaterthiophene core. The absorption spectra
showed identical intense π-π* transitions at 394 nm due to
an excitation parallel to the conjugated π-system. Compared
to R-quaterthiophene this absorption is slightly red-shifted
by 5 nm due to the additional double bond in the adjacent
1,2,3-triazole ring. The second, smaller band at 257 nm which
is due to a perpendicular excitation and independent of the
length of the oligomer is only evident for 7, because for
conjugates 6a,t this absorption band is overlaid by the
specific absorption of the nucleoside. Enhanced bands with
maxima at 258 nm for adenosine 6a and at 264 nm for
thymidine 6t were found. The fluorescence behavior was
identical for all three compounds, and emission bands with
maxima at 473 and 500 nm were found (Table 1, Figure
S1).
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