Photoreduction of 1,10-phenanthroline

Abstract

The effects of solvent polarity on the photophysical properties of 1,10-phenanthroline indicate the close proximity of (n, π^*) and (π, π^*) excited singlet states, whilst the lowest triplet state is (π, π^*) in all solvents. In hydrocarbon solvent, the first excited singlet state is of (n, π^*) character but in water a (π, π^*) is situated ∼ 700 cm^–1 below this state. Even in water, the (n, π^*) state is capable of abstracting a hydrogen atom from the solvent forming an intermediate semidiaza radical. The radical can be detected by flash photolysis and e.s.r. techniques and, in organic solvents, it decays with second order kinetics due to a disproportionation reaction. The major product of reaction is dihydrophenanthroline which, in hydrocarbon solvents, is formed with a quantum efficiency of ∼ 25 %. In aqueous solvent, the yield and rate of decay of the semidiaza radical are pH dependent, and the pK for protonation of the radical is 7.3. Thus, irradiation in water at pH 5 results in formation of the stable diprotonated radical cation.