Singlet Oxygen Quantum Yields (φΔ) in Water using Beetroot Extract and an Array of LEDs

It is proposed a simple and inexpensive strategy to determine singlet oxygen (O-1(2)) quantum yields (phi(Delta)) of photosensitizers (PS) in water using beetroot extract containing betacyanin (Bc) and a set of light emitting diodes (LEDs) for excitation. Bc, a cationic natural dye, was obtained by flash chromatography purification from the red beet extract (Beta vulgaris) and employed as a convenient probe for O-1(2) detection. Solutions of Bc and PS were illuminated with an array of LEDs adapted in the cuvette compartment of a commercial spectrophotometer, and the decrease in Bc absorbance was followed as a function of time. Bc photobleaching decreased in de-aerated solution and increased in D2O, indicating the involvement of O-1(2). The observed photobleaching rate constant (k(obs)) was proportional to the LED intensity, concentration and phi(Delta) of the PS. By keeping the light source constant we could estimate the overlap integral (R) between the LED emission and PS absorbance for different PS concentrations. The slope of R versus k(obs) is the value of the photobleaching rate constant (k), which was shown to be proportional to phi(Delta). Values of phi(Delta) obtained by this method were compared with those obtained by measuring NIR (near infrared) emission for a series phenothiazine dyes.