Systematic Modulation of the Fluorescence Brightness in Boron-Dipyrromethene (BODIPY)-Tagged N-Heterocyclic Carbene...

Five different highly fluorescent boron-dipyrromethene (BODIPY)-tagged N-heterocyclic carbene NHC-gold halide complexes were synthesized. The substitution of the halogeno ligand by 4-substituted aryl thiolates leads to a decrease in the brightness of the complexes. This decrease depends on the electronic nature of the thiols, being most pronounced with highly electron-rich thiols (4-R=NMe₂ ). The brightness of the gold thiolates also depends on the distance between the sulfur atom and the BODIPY moiety. The systematic variation of the electron density of [(NHC-bodipy)Au(SC₆ H₄ R)] (via different R groups) enables the systematic variation of the fluorescence brightness of an appended BODIPY fluorophore. Based on this and supported by DFT calculations, a photoinduced electron-transfer quenching appears to be the dominant mechanism controlling the brightness of the appended BODIPY dye.