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.

Complexes of the general formula [IrCl(cod)(bdp-NHC-bdp')] and [IrCl(cod)(bdp-NHC)] (bdp=bodipy=4,4-difluoro-4-bora-3a,4a-diaza-s-indacene, cod=1,5-cyclooctadiene) were synthesized. The substitution reaction of cod with two molecules of CO converts weakly fluorescent into strongly fluorescent complexes [IrCl(CO) (bdp-NHC-bdp')] and [IrCl(CO) (bdp-NHC)]. Bdp and bdp" form a fluorescence resonance energy transfer (FRET) pair and the excitation of bdp leads to a strong emission from bdp" with a virtual Stokes shift of 98 nm. The fluorescence gain (I [Ir(CO) ]]/I [Ir(cod)]=1.7) upon reaction with CO in this complex is modest. To increase the fluorescence gain, the quenching capacity of the transition metal was improved by increasing the electron density at iridium. This was achieved by substituting the metal-bound chloride with an electron-rich thiolate RC H S. Depending on the nature of the R substituent in [Ir(SC H R)(cod)(bdp-NHC-bdp')], an improved fluorescence gain in the cod/CO substitution reaction of up to 4.3 was observed and up to 26 (from gain=5) in [Ir(SC H R)(cod)(bdp-NHC)]. DFT calculations on closely related [Ir(SC H R)(cod)(bdp-NHC)] complexes indicate that a photoinduced electron transfer mechanism is the dominant quenching pathway for the iridium thiolates with R=COMe, CF , Cl, H, Me, tBu, OMe, NEt . The CO-responsive FRET complex was immobilized on paper, displaying a red fluorescent color upon exposure to CO.

7-NEt 2 -and julolidene-coumarin-substituted azolium salts were synthesized and used to prepare the respective coumarin-substituted NHC-metal complexes: [AuCl(NHC)], [PdCl(allyl)(NHC)], [MCl(CO) 2 (NHC)] (M = Rh, Ir), [MCl(cod)(NHC)] (M = Rh, Ir), [PdCl 2 (Cl-py)(NHC)], [PdC(allyl)] 2 , and [RuCl 2 -(ind)(NHC)(SIMes)] -sorted according to the respective fluorescence quantum yields of between = 0.70 [Au] and = 0.005 [Ru]. The fluorescence intensity of the complexes depends on the electron density at the metal and can be increased by elec- [a] Nonetheless, Bodipy fluorophores also possess a few less favorable properties: [24] a) often small Stokes shifts are observed, as little as 10 nm are not unusual; b) treatment with strong bases causes problems with acidic protons, nucleophiles can react with the BF 2 unit; and c) typically excitation requires > 500 nm light, excitation maxima approaching 400 nm are rare. [25] It therefore appears rewarding, to look for alternatives, to complement the weaknesses of Bodipy. Coumarin is an established lipophilic fluorophore whose fluorescence properties can be designed and which has found use in numerous applications and it can be easily modified to allow UV-light excitation of the fluorescence. [26] We therefore set out to synthesize, coumarintagged organometallic complexes and to study the fluorescence properties during ligand-substitution reactions. Results and Discussion Synthesis of Coumarin-Tagged Azolium Salts and NHC-Metal ComplexesIt was decided to use 7-NEt 2 coumarins since these fluorophores are characterized by blue to ultraviolet excitation Scheme 1. Synthesis of a coumarin-substituted azolium salt (Mes = 2,4,6trimethylphenyl). Reagents and conditions: a) coumarin acid chloride 1, azolium salt 2, Et 3 N, CH 2 Cl 2 , room temp., 24 h; yield 78 %; b) 3·HBF 4 and Amberlite IRA910 Cl resin, 1:1 MeOH/CH 2 Cl 2 , room temp., overnight, yield 99 %.Scheme 2. Synthesis of coumarin-tagged NHC-metal complexes. Reagents and conditions: a) [AuCl(Me 2 S)], 3·HCl, K 2 CO 3 , 60°C, acetone; b) 3·HCl, [PdCl(allyl)] 2 and K 2 CO 3 acetone, 60°C, 4 h; c) PdCl 2 , 3·HCl, 3-Cl-pyridine solvent, K 2 CO 3 , 80°C, 24 h; d) 3·HCl, Ag 2 O, 40°C, CH 2 Cl 2 then [(SIMes)RuCl 2 (ind)(py)], (ind = 2phenylindene-1-ylidene), 60°C, toluene; e) [RhCl(cod)] 2 , acetone, 60°C, 3·HCl, K 2 CO 3 or [IrCl(cod)] 2 , CH 2 Cl 2 , 40°C, 3·HCl, Ag 2 O; f) CO, 30 min, room temp., CH 2 Cl 2 .

Fluorescence resonance energy transfer (FRET) in appropriately tagged NHC-gold complexes can be utilized to in situ observe the formation of digold species, which are known to be catalytically relevant intermediates in various gold catalyzed reactions.