Fine‐Tuning the Fluorescence Gain of FRET‐Type (Bodipy)(Bodipy′)‐NHC‐Iridium Complexes for CO Detection with a Large Virtual Stokes Shift

Abstract

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.

Using coligands to gain mechanistic insight into iridium complexes hyperpolarized with para-hydrogen

“…Such reactivity is well known for metal surfaces65 and bimetallic complexes 66,67. Similar iridium N-heterocyclic carbene complexes containing bound thiolates have been prepared from the displacement of Ir–Cl under basic conditions 68. Hence there is a clear rational for this behaviour and related products are expected to account for the catalyst deactivation seen in SABRE catalysis upon scavenging with a supported thiol 55,69…”

Redox Chemistry of BODIPY Dyes

“…Plenio and coworkers have incorporated the BODIPY core into N-heterocyclic carbene scaffolds coordinated to iridium and rhodium centers for use in carbon monoxide detection. Plenio and coworkers have incorporated two different BODIPY cores into a N-heterocyclic carbene scaffold, where each of the BODIPY cores are connected to the carbene through a five carbon linker [23]. With the free BODIPY dyes having oxidation potentials of 0.90 and 1.01 V, coordination to an IrCl(COD) fragment, where COD = cis-cyclooctadiene, shifts the oxidation potentials to 0.92 and 1.02 V, respectively.…”

Iridium complexes with monodentate N-heterocyclic carbene ligands

“…More recently, the scope of bodipy-tagged NHC-Ir complexes for CO detection was extended with further examples. 157 In these (NHC)Ir(COD)X complexes the X = Cl ligand was replaced by electron-rich thiolate RC 6 H 4 S. A fluorescence gain (defined as a ratio of fluorescence intensities of the complexes) was observed upon conversion of (NHC)Ir(COD)X to (NHC)Ir(CO) 2 X. While this gain was equal to 5 for X = Cl, an excellent gain of 26 for X = p-NO 2 C 6 H 4 S was measured.…”