Singlet Oxygen (1Δg) Production by Ruthenium(II) Complexes in Microheterogeneous Systems

Singlet oxygen (O 2 ( 1 Δ g )) production and quenching in homogeneous and microheterogeneous media are reported for two anionic ruthenium(II) complexes ([RuL 2 L‘] 2- (where L stands for (1,10-phenanthroline-4,7-diyl)bis(benzenesulfonate) (pbbs) and L‘ stands for N -(1,10-phenanthrolin-5-yl)acetamide (paa) or N -(1,10-phenanthrolin-5-yl)tetradecanamide (pta)) and for the cationic complex [Ru(bpy) 3 ] 2+ (bpy: 2,2‘-bipyridine). Comparative results of photosensitized 1 O 2 generation in micelles, reverse micelles, and microemulsions reveal that the nature of the ligands and the size and charge of the [RuL 3 ] complexes are important factors affecting their effective location and 1 O 2 production in these media. The rate constants of quenching of the excited state [RuL 3 ]* by molecular oxygen ( k q ) are in the range of (1−3) × 10 9 M -1 s -1 . The emission lifetimes of [RuL 3 ]* and k q values depend on the nature of the ligand and on the medium. The complexes are stable singlet oxygen sensitizers, with quantum yields of singlet oxygen production (Φ Δ ) in air-equilibrated solutions between 0.30 and 0.75. The efficiency of 1 O 2 formation (i.e., the fraction of triplet excited states quenched by oxygen yielding 1 O 2, ) is a valuable probe of the interactions of the [RuL 3 ] complexes with micelles and microemulsions. The highest values (≥0.90) were observed in micellar media based on surfactants bearing a charge opposite to that of the [RuL 3 ] complex. In the microheterogeneous systems investigated, the most probable location of the [RuL 3 ] sensitizers is the micellar interfacial region.