Boron-Dipyrromethene-Functionalized Hemilabile Ligands as “Turn-On” Fluorescent Probes for Coordination Changes in Weak-Link Approach Complexes

Herein we report
a new class of hemilabile ligands with boron-dipyrromethene (Bodipy)
fluorophores that, when complexed to Pt­(II), can signal changes in
coordination mode through changes in their fluorescence. The ligands
consist of phosphino-amine or phosphino-thioether coordinating moieties
linked to the Bodipy’s meso carbon via a phenylene spacer.
Interestingly, this new class of ligands can be used to signal both
ligand displacement and chelation reactions in a fluorescence “turn-on”
fashion through the choice of weakly binding heteroatom in the hemilabile
moiety, generating up to 10-fold fluorescence intensity increases.
The Pt­(II) center influences the Bodipy emission efficiency by regulating
photoinduced electron transfer between the fluorophore and its meso
substituent. The rates at which the excited Bodipy-species generate
singlet oxygen upon excitation suggest that the heavy Pt­(II) center
also influences Bodipy’s emission efficiency by affecting intersystem
crossing from the Bodipy excited singlet to excited triplet states.
This signaling strategy provides a quantitative read-out for changes
in coordination mode and potentially will enable the design of new
molecular systems for sensing and signal amplification.