Photooxidation of 1,5-dihydroxynaphthalene with iridium complexes as singlet oxygen sensitizers

Photooxidation reactions of 1,5-dihydroxynaphthalene (DHN) have been carried out in the presence of cyclometalated neutral and cationic iridium (Ir) complexes 1-6 as singlet oxygen ((1)O(2)) sensitizers in order to investigate the (1)O(2) generation quantum yield and photosensitizing durability of the complexes. The reactions allowed a successful kinetic study to provide the pseudo-first-order rate constants and the initial rates of DHN consumption, which in turn led to the (1)O(2) generation quantum yields. The results revealed that cationic Ir complexes [Ir(ppy)(2)(phen)(+) (4) and Ir(ppy)(2)(bpy)(+) (5), where ppy = 2-phenylpyridine, phen = 1,10-phenanthroline, bpy = 2,2'-bipyridyl] have high (1)O(2) generation quantum yields (Φ(Δ) = 0.93, 0.97). On the other hand, neutral complexes with lower oxidation potentials were considered to have a more efficient charge-transfer (CT) interaction with molecular oxygen, which decreased the efficiency of singlet oxygen formation. Additionally, a steric factor of the ligands was reflected in (1)O(2) generation quantum yield. High yields of the oxidized product for the photoreactions using the cationic complexes indicated their excellent photosensitizing durability, originating from the high photochemical stability upon irradiation.