Red to Near-Infrared Isoindole BODIPY Fluorophores: Synthesis, Crystal Structures, and Spectroscopic and Electrochemical Properties

A series
of high-performance fluorophores named isoindole boron
dipyrromethenes (BODIPYs) containing either symmetrical or unsymmetrical
alkyl substitution patterns on pyrrole rings were synthesized by an
efficient process and were characterized by X-ray diffraction and
spectroscopic and electrochemical analyses. Most of these dyes show
strong, sharp absorption and bright fluorescence emission in the red
to near-infrared (NIR) region (up to 805 nm in acetonitrile). Pyrrolic
alkyl substitutions lead to increases in the HOMO and LUMO energy
levels and an overall decrease in the energy band gaps of the dye.
Among the 23 isoindole BODIPY dyes synthesized, solvent-dependent
fluorescence emission and lifetime decay were only observed for those
containing a 3-methyl substituent on the uncoordinated pyrrole ring,
whereas little variation in the fluorescence intensity was observed
for the rest of the dyes upon changing the polarity of the solvent.
These resultant dyes can be further functionalized via the Knoevenagel
condensation on the α-methyl substituent of the chromophore
to install a variety of functionalities, including a dimethylamine
group demonstrated in this work. This dimethylamine-functionalized
isoindole BODIPY shows weak fluorescence at 805 nm in acetonitrile
and a ratiometric “turn-on” NIR fluorescence response
to decreasing pH.