Photophysical insights and guidelines for blue “turn‐on” fluorescent probes for the direct detection of nitric oxide (NO•) in biological systems

Abstract

AbstractSynthesis and photophysical properties of a family of five blue fluorescent “turn‐on” probes for the direct detection of nitric oxide (NO•) is reported. The probes CB1‐5 feature a substituted 7‐hydroxy coumarin chromophore coupled to 2‐methyl‐8‐aminoquinoline, which act as tridentate ligand for Cu(II) and active site for monitoring NO• using a replacement strategy. The UV/vis absorption and fluorescence emission characteristics of the probes are significantly influenced by the substitution pattern on the coumarin ring, as well as by solvent polarity and pH. Time‐dependent density functional theory (TD‐DFT) calculations for CB4 and CB5 showed that the absorptions are due to π‐π* transitions localised on coumarin, with a small charge transfer contribution from the quinoline system at higher pH where the 7‐hydroxy coumarin moiety is deprotonated, which leads to a bathochromic shift of the absorption. Complexation of the probes with Cu(II) quenches the fluorescence, which is turned back on upon reaction with NO•, allowing selective detection of this important signalling molecule in μM concentrations. Preliminary experiments revealed that CB4 and CB5 enable to monitor endogenously produced NO• in living bacterial cells in multi‐dye imaging experiments.