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

Abstract

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 (¹O₂) sensitizers in order to investigate the ¹O₂ 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 ¹O₂ 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 ¹O₂ 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 ¹O₂ 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.