Fluorescent and Circular Dichroic Detection of Monosaccharides by Molecular Sensors: Bis[(Pyrrolyl)ethynyl]naphthyridine and Bis[(Indolyl)ethynyl]naphthyridine

The push−pull conjugated molecules 2,7-bis-(1H-pyrrol-2-yl)ethynyl-1,8-naphthyridine (BPN) and
2,7-bis(1H-indol-2-yl)ethynyl-1,8-naphthyridine (BIN) adopting daad relays of proton donors (d) and acceptors
(a) form multiple hydrogen-bonding complexes with various monosaccharides that possess complementary
adda sequences. Although the free BPN emits blue light at λmax = 475 nm in CH2Cl2, its complexation with
octyl β-d-glucopyranoside gives green fluorescence at λmax = 535 nm. The excellent photophysical properties
make BPN a highly sensitive probe for monitoring glucopyranoside to a detection limit of ∼100 pM. On the
other hand, the CD-silent BIN molecule binds with monosaccharides to form the CD-active multiple hydrogen-bonding complexes, which exhibit the remarkable chirality dependent helicities consistent with the prediction
by the ab initio approaches. On the basis of the similar daad cleft and hence the binding property, the
fluorescence and CD absorption methods in BPN and BIN, respectively, are complementary, which, in
combination with computational molecular modeling, not only give a detailed insight into the structures of
the receptor-saccharide complexes in solution, but also differentiate octyl β-d-glucopyranoside from its
enantiomer and other monosaccharides.