Generating Singlet Oxygen Bubbles: A New Mechanism for Gas-Liquid Oxidations in Water

Laser-coupled microphotoreactors were developed to bubble singlet oxygen [O-1(2) ((1)Delta(g))] into an aqueous solution containing an oxidizable compound. The reactors consisted of custom-modified SMA fiberoptic receptacles loaded with 150 mu m silicon phthalocyanine glass sensitizer particles, where the particles were isolated from direct contact with water by a membrane adhesively bonded to the bottom of each device. A tube fed O-2 gas to the reactor chambers. In the presence of O-2, singlet oxygen was generated by illuminating the sensitizer particles with 669 nm light from an optical fiber coupled to the top of the reactor. The generated O-1(2) was transported through the membrane by the O-2 stream and formed bubbles in solution. In solution, singlet oxygen reacted with probe compounds (9,10-anthracene dipropionate dianion, trans-2-methyl-2-pentanoate anion, N-benzoyl-D,L-methionine, or N-acetyl-D,L-methionine) to give oxidized products in two stages. The early stage was rapid and showed that O-1(2) transfer occurred via bubbles mainly in the bulk water solution. The later stage was slow; it arose only from O-1(2)-probe molecule contact at the gas/liquid interface. A mechanism is proposed that involves O-1(2) mass transfer and solvation, where smaller bubbles provide better penetration of O-1(2) into the flowing stream due to higher surface-to-volume contact between the probe molecules and O-1(2).