Methylene Blue Encapsulation in Cucurbit[7]uril: Laser Flash Photolysis and Near-IR Luminescence Studies of the Interaction with Oxygen

The effect of methylene blue (MB) encapsulation in cucurbit[7]uril (CB[7]) on triplet excited-state behavior and singlet oxygen (¹O₂) generation has been studied by using laser flash photolysis (LFP) and time-resolved near-IR luminescence spectroscopy. The lifetime of the triplet excited state of MB is longer in the CB[7] cavity (140 μs for MB-CB[7] vs 79.5 μs for aqueous MB). Cucurbituril also protects the dye triplets from quenching by oxygen, reducing the quenching rate constant [k_q(O₂)] from 2.6 × 10⁹ M⁻¹ s⁻¹ to 0.2 × 10⁹ M⁻¹ s⁻¹. The quantum yield of ¹O₂ production in the air-equilibrated D₂O solutions is similar for free MB and for MB−CB[7], and the singlet oxygen lifetime is ∼70 μs, suggesting its decay occurs in the aqueous (D₂O) phase. The generation of singlet oxygen is delayed by CB[7]; this is attributed to the time required for oxygen to access the CB[7] nanocavity and react with the MB triplet. Thus, the rate-limiting step for sensitization is the entry of oxygen into the CB[7] cavity. Encapsulation inside CB[7] increases the relative efficiency of photoinduced MB^2+• dication-radical generation, for which a modest yield is observed.