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

摘要

The internal charge transfer (ICT) type fluoroionophorenarylvinyl-bipy (bipy = 2,2′-bipyridyl) is covalently tethered to thenspirolactam form of rhodamine to afford fluorescent heteroditopicnligand 4. Compound 4 can be excited in the visible region, the emissionnof which undergoes sequential bathochromic shifts over an increasingnconcentration gradient of Zn(ClO4)2 in acetonitrile. Coordination ofnZn2+ stabilizes the ICT excited state of the arylvinyl-bipy component ofn4, leading to the first emission color shift from blue to green. Atnsufficiently high concentrations of Zn(ClO4)2, the nonfluorescentnspirolactam component of 4 is transformed to the fluorescentnrhodamine, which turns the emission color from green to orange vianintramolecular fluorescence resonance energy transfer (FRET) from thenZn2+-bound arylvinyl-bipy fluorophore to rhodamine. While this worknoffers a new design of ratiometric chemosensors, in which sequential analyte-induced emission band shifts result in the samplingnof multiple colors at different concentration ranges (i.e., from blue to green to orange as [Zn2+] increases in the current case), itnalso reveals the nuances of rhodamine spirolactam chemistry that have not been sufficiently addressed in the published literature.nThese issues include the ability of rhodamine spirolactam as a fluorescence quencher via electron transfer, and the slow kineticsnof spirolactam ring-opening effected by Zn2+ coordination under pH neutral aqueous conditions.