PyrrolylBODIPYs: Syntheses, Properties, and Application as Environment-Sensitive Fluorescence Probes

Four
pyrrole B-ring-functionalized pyrrolylBODIPYs
and their B-ring unsubstituted analogues were synthesized
from easily accessible starting 5-halo-2-formylpyrroles and were characterized
by nuclear magnetic resonance, high-resolution mass spectrometry,
X-ray analysis, and optical/electronic properties. In great contrast
to the substitution(s) at the other two pyrrolic units, electron-donating
substituent(s) at pyrrole B-ring bring significant blue
shift of the absorption and emission bands. Cyclic voltammetry and
density functional theory calculations indicate that this blue shift
may be attributed to the increased highest occupied molecular orbital
and the lowest unoccupied molecular orbital energy levels and the
overall increase in the energy band gaps. These pyrrolylBODIPYs generally
show intense absorption (centered at 570–624 nm) and fluorescence
emission (582–654 nm) in nonpolar solvents. A gradual decrease
in the fluorescence intensity was observed for these dyes with the
increase in solvent dipolar moment, which combines with the red to
far-red absorption/emission, rendering these pyrrolylBODIPYs potential
applications as environment-sensitive fluorescence probes as demonstrated
in this work for bovine serum albumin.