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

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)2 (bdp-NHC-bdp')] and [IrCl(CO)2 (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 (IFl [Ir(CO)2 ]]/IFl [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 RC6 H4 S. Depending on the nature of the R substituent in [Ir(SC6 H4 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(SC6 H4 R)(cod)(bdp-NHC)]. DFT calculations on closely related [Ir(SC6 H4 R)(cod)(bdp-NHC)] complexes indicate that a photoinduced electron transfer mechanism is the dominant quenching pathway for the iridium thiolates with R=COMe, CF3 , Cl, H, Me, tBu, OMe, NEt2 . The CO-responsive FRET complex was immobilized on paper, displaying a red fluorescent color upon exposure to CO.