A new class of dyes for use as the recording layer in DVDs has been developed by a group of scientists from the Fuji Photo Film Company in Japan. The team, led by Yoshio Inagaki, worked to improve the light stability of dyes based on oxonol. Dyes used for DVDs need to be resistant both to ambient light and to the laser beam used to read the disk.

The dyes they developed consist of an oxonol anion and an electron-accepting bipyridinium cation. When the film absorbs a photon, it is passed quickly from the oxonol ion to the bipyridinium ion and back again, allowing the molecules to quickly return to their ground state. Shortening the time the molecules spend in the excited state minimises damage done to the film by the laser.

'We have actually introduced new dyes based on the present design principle into a DVD-R that has recently been put on the market,' said Inagaki. The oxonol dyes absorb light very strongly and so can be used in applications which require very thin films. Unusually among this type of dye, they do not contain heavy metal ions, meaning they are safer to make and dispose of than many conventional dyes. It should also be possible to tune the colour of the dyes. Inagaki is optimistic: 'the present design principle applies to any oxonol dye, and properties of the oxonol dyes can be easily and widely changed by modifiying their molecular structure,' he said.

Clare E Boothby