Synthesis and Investigation of Linker-Free BODIPY–Gly Conjugates Substituted at the Boron Atom

An efficient synthesis of boron-functionalized cyclic BODIPY–Gly conjugates, using commercially available N-protected glycine amino acids and a BF2–BODIPY moiety as starting materials, is reported. The existence of two conformers (up and down) is revealed through comprehensive DFT calculations and 1H and 11B NMR analyses. The experimental and computational results indicate that all BODIPYs are stable in aqueous solutions at neutral pH and that Fmoc-BODIPY (4) is more stable than Ac-BODIPY (6) in the presence of trifluoroacetic acid (TFA). In part due to their enhanced rigidity, all BODIPY–Gly conjugates display increased fluorescence quantum yields (0.6 < Φ < 0.9) relative to the corresponding BF2–BODIPY, making them excellent candidates for fluorescence imaging applications.