Studies on singlet oxygen in aqueous solution. Part 2. Water-soluble square-planar nickel complexes as quenchers

Three water-soluble square-planar nickel complexes have been investigated as quenchers of 1O2 in aqueous solution, using anthracene-9,10-bis(ethanesulphonate) as a trap. Disodium [NN′-ethylenebis(5-sulphosalicylideneiminato)] nickelate(II) and its propylene analogue are efficient quenchers, with kQ= 1.6 × 109 and 1.3 × 109 dm3 mol–1 s–1 respectively. The value of kQ, for disodium [NN′-o-phenylenebis(5-sulphosalicylideneiminato)] nickelate(II) is much less, and seems to be concentration-dependent. This is attributed to aggregation, in agreement with the electronic and 1H n.m.r. spectra.