Enhanced luminescence oxygen sensing property of Ru(II) bispyridine complexes by ligand modification

Phenylethynylene containing polypyridyl ruthenium (Ru) complexes 1 and 2 were prepared using chemistry-on-complex approach. The luminescent lifetimes of these complexes were extended by more than 2-fold (τ = 1.8 μs for complex 1) when compared to the reference complex 3 (i.e. Ru(Phen)(bpy)2, τ = 0.8 μs). Based on DFT/TDDFT calculations, we propose that 3LLCT/3MLCT mixed states are the emissive triplet states of 1 and 2. Luminescence oxygen sensitivity of the complexes are improved with extension of the lifetimes, e.g. I0/I100 = 15.4 for complex 3, but I0/I100 = 36.8 was observed for the complex 1 in MeCN solution (I0 is the emission in N2 and I100 is the emission intensity under O2 atmosphere). O2 sensitivity of the complexes were also studied in polymers IMPES-C, PES-C and polyvinyl chloride (PVC). The IMPES-C sensing film demonstrated very short response time and recovery time (3.4 and 7.5 s, respectively), which is ideal for practical O2 sensors. Modified Stern–Volmer equation and the two-sites model were used to fit the luminescent O2 sensing data. We found that with tuning the luminescence lifetimes, the luminescence O2 sensitivity of the Ru complexes (quenching constant, kSV) can be improved by up to 20-fold.