Systematic Modulation of the Fluorescence Brightness in Boron‐Dipyrromethene (BODIPY)‐Tagged N‐Heterocyclic Carbene (NHC)–Gold–Thiolates

Abstract

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=NMe ). 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(SC H 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.

Fluorescent Dyes in Organometallic Chemistry: Coumarin‐Tagged NHC–Metal Complexes

“…In order to understand how changes in the electron density at the transition metal affect the fluorescence, solutions of complex [AuCl( 3 )] in 1,2‐dichloroethane were treated with different aryl thiol 4‐RC 6 H 4 SH in the presence of Hünig base (Figure ). As shown previously, this is leading to the virtually quantitative formation of the respective gold thiolates [Au(SC 6 H 4 R)( 3 )]. The rate of gold thiolate formation is correlated with the electronic properties of R: electron‐deficient thiols form the respective gold thiolates faster than electron‐rich thiols.…”

“…In suitably designed complexes, changes at the metal or ligand exchange reactions in the coordination sphere result in a modulated fluorescence emission, enabling the observation of chemical reactions involving fluorophore tagged metal complexes . Concerning the origin of the fluorescence modulation, we established, that the electron density at the metal center (or more precisely: the relative HOMO–LUMO energies of fluorophore and metal) is primarily responsible for changes in the emission of fluorophore tagged complexes . This is compatible with a PET quenching mechanism .…”

“…Since the modulation of the fluorescence intensity strongly depends on the distance between fluorophore and metal, we also synthesized related coumarin substituted azolium salts, in which one coumarin is directly bonded to the azolium unit. In this way, the interaction between fluorophore and the potentially quenching metal center ought to be maximized.…”

“…The coordination of other metals to the coumarin‐tagged NHC ligand lowers φ (Table ). The origin of this decrease is not entirely clear, but it is likely, that high‐energy HOMOs in the respective metals contribute to fluorescence quenching …”

“…The electronic properties of R were modulated from ewg to edg (R = CF 3 , Cl, H, Me, t Bu, OMe, NMe 2 ). This systematic approach was developed to measure the dependence of fluorescence towards changes in the electron density at the metal , . The formation of the respective thiolates leads to a very significant increase in the electron density.…”