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

Abstract

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 .

Light Amplification Materials Based on Biopolymers Doped with Dye Molecules—Structural Insights from 15N and 13C Solid-State Dynamic Nuclear Polarization

“…for the fluorophore 1,8-naphthalimide and by some of us for linking of rhodamine B on cellulose-derived materials, which can be both applied in sensorics. On the other hand, the molecules may also be adsorbed as shown for paper materials with application in gas sensing − or for chitosan, which was applied to adsorb dyes from waste water …”

Unveiling Luminescent IrI and RhI N‐Heterocyclic Carbene Complexes: Structure, Photophysical Specifics, and Cellular Localization in the Endoplasmic Reticulum

“…In comparison to the free ligands, the complexes show a significant decrease in luminescence quantum yield. This is interpreted as a PET quenching mechanism in the complexes, [38–40] which is found to be more pronounced in the case of the rhodium metal center. When the complexes 8 a / b are compared with 9 a / b , the quantum yield is significantly higher in complexes 9 a / b as a consequence of the longer distance between the metal center and the respective 1,8‐naphthalimide core.…”

“…Short distances resulted in significant luminescence quenching, whereas those complexes with longer linkers connecting the metal center with the luminescent moiety showed enhanced luminescence quantum yields [46] . Taking into account the aforementioned phenomenon of luminescence quenching in respective Rh I or Ir I [M(COD)(NHC)X] complexes, [38–40] the second imidazolium‐functionalized 1,8‐naphthalimide derivative 7 with an ethyl linker between the two moieties was designed to reduce the probability of luminescence quenching. The desired ligand was obtained based on a literature procedure [47] .…”

“…To create such a luminescent NHC ligand, the most attractive parts for modifications are the heterocycle nitrogen atoms and the imidazolium backbone carbon atoms, offering the possibility to introduce an imidazolium moiety into virtually every possible structure [36–37] . Indeed, the attachment of a luminescent dye to an imidazolium core and its subsequent use for the synthesis of [M(COD)(NHC)X] complexes creates luminescent compounds as reported for, for example, BODIPY [38–39] and coumarin [40] functionalized [M(COD)(NHC)X] complexes. However, although the free NHC ligands showed good luminescence quantum yields, the values dropped significantly after metal ion complexation.…”

“…This loss of luminescence was explained by a metal‐related photoinduced electron transfer (PET) quenching process [38–40] . To the best of our knowledge, there are only two successful reports of luminescent [Rh(COD)(NHC)Cl] complexes used for cellular imaging within cancer cells, [24–25] which revealed mitochondrial localization of the respective complexes [25] .…”