We present an experimental method, which provides information on the steric and electronic properties of ligands in metal complexes. This approach is based on the equilibrium of ionpairing of a cationic [(NHC)Ir(cod)] + and a fluorescent bodipysulfonate (bdpSO 3 À ). The close ion-pair of [(NHC) Ir(bdpSO 3 )(cod)] in toluene solution is weakly fluorescent, but dissociates into solvent-separated ion pairs upon exposure to slightly more polar solvents. This spatial separation leads to a very pronounced increase of the fluorescence. The separation into distinct ions depends on the polarity of the solvent, but more importantly also on the stereoelectronic properties of the NHC ligand. 26 different NHC ligands with varying steric and electronic properties were probed in 1,2-dichloroethane solvent. The electronic properties of the ligands were examined via the established descriptors (redox potential and ν(CO)). The systematic variation of NHC donation enables the deconvolution of electronic and steric contributions of the respective ligand.

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A new imidazolium salt 3•HI with 8-Bodipy as one N-aryl substituent was synthesized from 8-chloro-Bodipy and imidazole followed by alkylation with iPrI. NHC-metal complexes [CuCl( 3)], [AuCl(3)], [Pd(allyl)Cl(3)] and [MCl(cod)(3)] and [MCl(CO) 2 (3)] (M = Rh, Ir) were synthesized and the X-ray crystal structure of [IrCl(CO) 2 (3)] determined. The donation of NHC 3 was determined via IR (ν(CO) and cyclic voltammetry (Ir(I/II) redox potential). The photosensitizing properties of the complexes for the generation of 1 O 2 were quantified observing singlet oxygen quantum yield of up to Φ s.o. = 0.63. [IrCl(cod)(3)] displays poor Φ s.o. = 0.09, but, following a simple ligand exchange reaction of cod by two CO [IrCl(CO) 2 (3)], a pronounced increase to Φ s.o. = 0.63 is observed.

FRET pairs involving up to three different Bodipy dyes are utilized to provide information on the assembly/disassembly of organometallic complexes. Azolium salts tagged with chemically robust and photostable blue or green or red fluorescent Bodipy, respectively, were synthesized and the azolium salts used to prepare metal complexes [(NHC_blue)ML], [(NHC_green)ML] and [(NHC_red)ML] (ML = Pd(allyl)Cl, IrCl(cod), RhCl(cod), AuCl, Au(NTf2), CuBr). The blue and the green Bodipy and the green and the red Bodipy, respectively, were designed to allow the formation of efficient FRET pairs with minimal cross-talk. Organometallic dyads formed from two subunits enable the transfer of excitation energy from the donor dye to the acceptor dye. The blue, green and red emission provide three information channels on the formation of complexes, which is demonstrated for alkyne or sulfur bridged digold species and for ion pairing of a red fluorescent cation and a green fluorescent anion. This approach is extended to probe an assembly of three different subunits. In such a triad, each component is tagged with either a blue, a green or a red Bodipy and the energy transfer blue green  red proves the formation of the triad. The tagging of molecular components with robust fluorophores can be a general strategy in (organometallic) chemistry to establish connectivities for intermediates in homogeneous catalysis

FRET pairs involving up to three different Bodipy dyes are utilized to provide information on the assembly/disassembly of organometallic complexes. Azolium salts tagged with chemically robust and photostable blue or green or red fluorescent Bodipy, respectively, were synthesized and the azolium salts used to prepare metal complexes [(NHC_blue)ML], [(NHC_green)ML] and [(NHC_red)ML] (ML = Pd(allyl)Cl, IrCl(cod), RhCl(cod), AuCl, Au(NTf2), CuBr). The blue and the green Bodipy and the green and the red Bodipy, respectively, were designed to allow the formation of efficient FRET pairs with minimal cross-talk. Organometallic dyads formed from two subunits enable the transfer of excitation energy from the donor dye to the acceptor dye. The blue, green and red emission provide three information channels on the formation of complexes, which is demonstrated for alkyne or sulfur bridged digold species and for ion pairing of a red fluorescent cation and a green fluorescent anion. This approach is extended to probe an assembly of three different subunits. In such a triad, each component is tagged with either a blue, a green or a red Bodipy and the energy transfer blue green  red proves the formation of the triad. The tagging of molecular components with robust fluorophores can be a general strategy in (organometallic) chemistry to establish connectivities for intermediates in homogeneous catalysis