Tricolor Emission of a Fluorescent Heteroditopic Ligand over a Concentration Gradient of Zinc(II) Ions

The internal charge transfer (ICT) type fluoroionophore
arylvinyl-bipy
(bipy = 2,2′-bipyridyl) is covalently tethered to the spirolactam
form of rhodamine to afford fluorescent heteroditopic ligand 4. Compound 4 can be excited in the visible region,
the emission of which undergoes sequential bathochromic shifts over
an increasing concentration gradient of Zn­(ClO4)2 in acetonitrile. Coordination of Zn2+ stabilizes the
ICT excited state of the arylvinyl-bipy component of 4, leading to the first emission color shift from blue to green. At
sufficiently high concentrations of Zn­(ClO4)2, the nonfluorescent spirolactam component of 4 is transformed
to the fluorescent rhodamine, which turns the emission color from
green to orange via intramolecular fluorescence resonance energy transfer
(FRET) from the Zn2+-bound arylvinyl-bipy fluorophore to
rhodamine. While this work offers a new design of ratiometric chemosensors,
in which sequential analyte-induced emission band shifts result in
the sampling of multiple colors at different concentration ranges
(i.e., from blue to green to orange as [Zn2+] increases
in the current case), it also reveals the nuances of rhodamine spirolactam
chemistry that have not been sufficiently addressed in the published
literature. These issues include the ability of rhodamine spirolactam
as a fluorescence quencher via electron transfer, and the slow kinetics
of spirolactam ring-opening effected by Zn2+ coordination
under pH neutral aqueous conditions.