Multichannel Sensors Based on Biphenyl and Cyclohexane Conformational Changes - Designing Receptors for the Next Generation of Biosensors - page 23

Fig. 21 Conformation calculated by PcModel for ligand 22

other ring, and between the H (at the para position to one pyridine unit) and the
sulfur atom of the thiourea in the same ring (Fig. 21 ).
Protonation experiments were carried out because the literature establishes that
the absorption band of bipyridine systems shows a clear bathochromic shift after
monoprotonation, which takes place giving a conformational change between the
s - trans and the s - cis forms. However, ligands 22 and 23 display a different behavior;
in both cases, the UV spectra exhibited no changes upon protonation. This suggests
that protonation occurs on the sulfur atom and not on the nitrogen atom. This
suggestion agrees with the evolution observed in solution where a hydrolysis of the
ligands was observed. In contrast, complexation with NiCl 2 induces the expected
conformational change from the s - trans to the s-cis conformation with the
corresponding modification in the UV spectra. However, modifications in fluores-
cence proved more interesting; ligand 22 gives an emission band at 420 nm
( 290 nm), which undergoes a slight bathochomic shift after protonation

(426 nm), but with an enhancement of 130%, which is probably due to increased
rigidity (Fig. 22 ).
These ligands are able to complex different

l exc ¼

-dicarboxylates, which leads to
modifications in fluorescence emission. The ability of these new ligands to act as
receptors for carboxylates in DMSO was studied by UV, fluorescence, and NMR
spectroscopy. All three ligands form complexes with acetate and different
dicarboxylates, but their stoichiometries depend on the nature of both the ligand
and the carboxylate (see Table 1 ).
Ligands 22 and 24 give rise to 1:2 complexes with both dicarboxylates and
acetate, whereas ligand 23 forms 1:2 complexes with acetate and 1:1 complexes
with dicarboxylates. These observations may relate to the different substituents on
thiourea moieties and their steric hindrance (Fig. 23 ).
The different stoichiometries of the complexes allow ligand 23 to act as a sensor
which is able to discriminate between mono- and dicarboxylates. In fact, the
complexes with a 1:2 stoichiometry strongly enhance fluorescence, whereas those
with a 1:1 stoichiometry bring about minor changes (Fig. 24 ).

a , o