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 micros for MB-CB[7] vs 79.5 micros for aqueous MB). Cucurbituril also protects the dye triplets from quenching by oxygen, reducing the quenching rate constant [kq(O2)] from 2.6x10(9) M(-1) s(-1) to 0.2x10(9) 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 approximately 70 micros, 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 MB2+* dication-radical generation, for which a modest yield is observed.
González-Béjar
María
M
Department of Chemistry, Centre for Catalysis Research and Innovation, University of Ottawa, Ottawa, Ontario, Canada, K1N 6N5.
Montes-Navajas
Pedro
P
García
Hermenegildo
H
Scaiano
J C
JC
D000388:Air; D004396:Coloring Agents / Q000737:chemistry; D004583:Electrons; D007700:Kinetics; D007834:Lasers; D008163:Luminescent Measurements; D047028:Macrocyclic Compounds; D008751:Methylene Blue / Q000737:chemistry*; D009584:Nitrogen; D010084:Oxidation-Reduction; D010782:Photolysis; D017319:Photosensitizing Agents / Q000737:chemistry*; D026082:Singlet Oxygen / Q000737:chemistry*; D012996:Solutions; D013055:Spectrophotometry, Infrared; D000099246:Cucurbit(n)urils
D004396:Coloring Agents; D047028:Macrocyclic Compounds; D008751:Methylene Blue; D009584:Nitrogen; D017319:Photosensitizing Agents; D026082:Singlet Oxygen; D012996:Solutions; D000099246:Cucurbit(n)urils