“…However, for this use, the lightfastness of the dye to ambient light must be sufficient to ensure memory retention for tens of years. 3) To enhance the lightfastness of oxonol dyes, electron acceptors that can quench the lowest excited singlet state (S 1 ) of anionic oxonol dyes have been investigated, using 4,4 0bipyridinium dication (4,4 0 -bpy) as a counter cation. 4,5) Introducing 4,4 0 -bpy to the oxonol dye yielded the oxonolsalt dye that is free of heavy metals, soluble, lightfast, and highly suitable for use as an ideal DVD material.…”