Photostability of Luminescent Ruthenium (II) Complexes in Water and Acid

Luminescence spectroscopy is widely used as an analytical technique in many fields due to its high sensitivity, simple instrumentation, and low cost. Luminophores as sensors are used to measure various analytes such as carbon dioxide, oxygen, glucose, anions, metal ions, pressure, temperature and pH, to name a few. When used as a probe, the photostability of a luminophore is important for validating the accuracy of measurements to develop a better sensor.

However, measurements of quantitative photodecomposition yield of luminophores require either complicated instrumentation or long experimental times. Herein, an instrument was developed to rapidly and accurately measure the photostability of quite stable luminophores.

The accuracy of the developed instrument was confirmed with fluorescein isothiocyanate in water. The instrument has a good reproducibility of less than 2% relative standard deviation on three consecutive measurements of the identical sample of tris(bipyridine)ruthenium(II) in water. The instrument measures decomposition yield of relatively very stable Ru(II) polypyridyl complexes with the yield of 10-6 to 10-7, in less than 30 minutes.

Photodecomposition yield and photooxidation yield of Ru(II) polypyridyl complexes in water were obtained. Tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) is one of the most photostable complexes among Ru(II) polypyridyl complexes investigated. Tris(bipyridine)ruthenium(II) compound is less photostable than tris(1,10-phenanthroline)ruthenium(II) in general. Tentative rules on the design of stable sensor molecules are given.

Under acidic conditions, the emission intensity of the luminophores decreased drastically in the first minute unlike typical photolysis decay in water. This unusual shape of photolysis decay curves is due to oxidation of the excited state Ru(II) to Ru(III) by an oxidizer present in the acid and additional unknown kinetics. The behavior can be prevented by addition of a reducing agent, such as ascorbic acid to promptly reduce the Ru(III) back to Ru(II). The rate of photolysis under acidic conditions is about ten times faster than in water. However, it is yet unknown what oxidizer causes an oxidation of Ru(II) in acid. It is highly likely that it is caused by impurities, as both sulfuric acid and trichloroacetic acid have the same phenomenon. A possible oxidant is a proton; however, this cannot explain all the data.

In summary, an instrument to measure the quantitative photostability of luminescent complexes was successfully developed. The measurements made by the instrument are reproducible and accurate. Photolysis time to decompose luminophores is less than 30 min, which is significantly less than in the previous work.