Photoconductivity and fading mechanisms of dyes

Abstract

The semiconductive and photoconductive properties of the triphenylmethane dyes used in the first three standards of the ISO fastness to light test have been measured in solid dye films. Although zwitterionic, they behave as n-type semiconductors and all have slow photoconductive response times. The product of the photobleaching process of each dye in vacuo is the leuco dye. It is proposed that electrons contributing to semiconductivity and photoconductivity are in different levels, in order to account for the connection of the latter with the photobleaching process. Photobleaching is considered to occur by a free-racidal mechanism. The proposed fading mechanism can account for the differences between the fastness to light of the three dyes and also for the increased fastness found when dyes are used in both very polar and non-polar media.