Porous Crystalline Ruthenium Complexes Are Oxygen Sensors

Several highly emissive, crystalline salts (ClO4-, PF6-, BF4-, B(C6F5)4-, and tfpb-; B(C6F5)4- = tetrakis(pentafluorophenyl)borate; tfpb- = tetrakis(bis-3,5-trifluoromethylphenylborate) of the Ru(pp)32+ (pp = bpy (2,2‘-bipyridine), phen (1,10-phenanthroline) or 4,7-Me2phen (4,7-dimethyl-1,10-phenanthroline) lumophore have been tested as oxygen sensors. Oxygen detection by luminescence quenching correlates with the void space in the crystalline lattice, particularly in the case of [Ru(phen)3](tfpb)2 which has channels occupying approximately 136 Å3 per Ru in the crystals. The emission intensity and lifetime quenching of [Ru(phen)3](tfpb)2 displayed strictly linear (R2 = 0.9996) Stern−Volmer behavior (plots of I0/I and τ0/τ vs mole fraction of oxygen) with a slope of 2.43. A single exponential (τ = 640 nsec, pure nitrogen; 190 nsec, pure oxygen) is observed for the emission intensity decay for all oxygen concentrations. The time dependence of the emission caused by a step function air pressure drop is significantly affected by changing the light penetration depth when different excitation wavelengths are used (at 400 nm, t1/2 = 120 ms; at 518 nm, t1/2 = 2200 ms). These experiments are consistent with the diffusion of oxygen molecules in and out of [Ru(phen)3](tfpb)2 crystals with a diffusion coefficient on the order of 10-7−10-8 cm2/s. The technological significance of these crystalline oxygen sensors was demonstrated by long-term stability studies and by the successful calibration of a ballprobe sensor coated with crystalline [Ru(phen)3](tfpb)2 against a dissolved oxygen Clark electrode using a partial least-squares (PLS) model with a single principle component.