A Molecular Chameleon with Fluorescein and Rhodamine Spectroscopic Behaviors

A new class of fluorescein/rhodamine
hybrids with two spirolactone rings was reported to exhibit dual-output
fluorescent behaviors independently. Isolation and characterization
for two diastereomers, trans-RhOH and cis-RhOH, have been made and their X-ray crystal
structures determined. In a basic environment, the spirolactone ring
on the hydroxyl side will be opened to give a fluorescein-like optical
output with the lowest absorptions at 485 and 530 nm emission. On
the other hand, a rhodamine-like optical output, i.e., 528 nm absorption
and 575 nm emission, will be switched on by a H+ or a Hg2+ ion, attributed to the spirolactone ring opening on the
amino side. In a methanol−buffer system with different pH values,
the corresponding pKa values for the hydroxyl
and amino groups were determined as 5.7 and 2.3, respectively. Selective
Hg2+-sensing properties have also been discussed, and log Ks values of about 3.60 and 3.73 were determined.
Confocal microscopic images of Caenorhabditis elegans incubated with RhOH were found to show enhanced fluorescent
intensity with a Hg2+ ion, demonstrating the potential
application of RhOH for in vivo biological imaging.