A pyrene
moiety tailed by hydroxylmethylpyridinewas chosen as the electron acceptor
and the potential chelating unit, respectively, for metal ions in
the push–pull system. We speculated that this kind of ‘N–O–N’-pincer
coordination environment would result in a unique spatial geometry
for Cr3+ to bind selectively over other metal ions. This designed push–
pull system based on a phenanthroxazole moiety would offer desirable
photophysical properties, such as a large Stokes shift and a lack of sensitivity
to pH. As expected, the capture of Cr3+ by the receptor resulted
in reduced electron-donating ability for the phenanthroxazole moiety;
thus, the pyrene moiety showed a red shift of 160 nm for fluorescence
emission with high selectivity for Cr3+ over other heavy and transition
metal ions.