Inverse photochromism of substituted triphenylmethane dyes in poly(vinyl alcohol) films

.The photochemistry of substituted triphenylmethane dyes was investigated in polymer films. Malachite green and brilliant green showed thermally stable photobleaching on visible light irradiation in poly(vinyl alcohol) Alms at room temperature. UV light irradiation of photobleached films recovered the blue colour. Such inverse photochromism is due to the binding of the dye cation and its counter anion to form a leuco structure, assisted by visible light, and the dissociation of the leuco structure by UV light irradiation. UV light irradiation also caused homolytic cleavage, leading to the formation of radicals, which resulted in a gradual decrease in the extent of colour change with increasing numbers of photoreaction cycles. Although symmetrically substituted dyes, such as crystal violet and ethyl violet, showed photobleaching on visible irradiation, no recolouration occurred on UV light irradiation.