Evidence for a Low-Lying Triplet State of the Hydroxide Ion

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Abstract
Phosphorescence with a λmax at ∼400 mm has been observed from aqueous hydroxide solutions at room temperature and at 77°K and from crystalline barium hydroxide at 77°K. Although the intrinsic absorption of the hydroxide ion appears to onset at a shorter wavelength, the phosphorescence can be excited at a photon energy as low as 350 nm which on energetic grounds eliminates the possible involvement of a quartet state of the hydroxyl radical produced by photoionization of OH−. On the basis of this observation, quenching experiments, and theoretical prediction, the phosphorescence is more plausibly assigned to emission from the lowest triplet state of the hydroxide ion.