This contribution is dedicated to the memory of Prof. Roger J. Griffin.Methylene Blue has a long history as a photochemical reagent and is known to undergo photofading on exposure to visible light in aqueous solution. Under aerobic conditions, photobleaching occurs by way of a two-step process involving intermediary formation of singlet oxygen. The first step is ascribed to regio-selective addition of singlet oxygen within the precursor complex. This geminate reaction ultimately leads to formation of the leuco-dye via a slower second step. Urea forms a weak ground-state complex with Methylene Blue which affects the optical properties of the dye but is not evident by NMR spectroscopy. This complex is weakly fluorescent and undergoes intersystem crossing to the triplet manifold. The presence of urea decreases the rate of photobleaching of the dye and, at high concentrations of urea, the bleaching kinetics are consistent with an equilibrium mixture of complexed and free dye. The complexed dye does not bleach on the timescale of the experiment. Such protection might arise from urea blocking access to the site where geminate addition of O 2 takes place.[a] S.

Indigo, a famous and well-loved dye, is highly resistant towards photobleaching but can be broken down to the colourless isatin under photocatalytic conditions.