Phthalocyanines
To overcome this inherrant instability of cyanines, phthalocyanine (Pc) compounds (which featured on the Bristol University "Molecule of the Month" page) were developed:
The basic molecule has been used as a dye but the (Pc)2- ligand has also been used with coordination to a metal centre.
These ligands are tetradentate and will happily coordinate to a metallic centre such as a vanandyl (V=O). Compounds such as VO-Pc are air stable at upto 350 degrees and so give much better data retention characteristics. Many such compounds are used commercially. They show virtually no toxicity, and with appropriate substitutions can be made soluble in organic solvents such as n-propanol to allow easy spin coating of discs. The basic absorption of the rigid 18 electron aromatic system of the tetra-aza-annulene ring is at about 633nm, but this can be easily modified with substituents on the ring at the indole units. Naphthalocyanines are particularly useful as they absorb with a maximum in the 800nm region. Silicon naphthaolcyanines have been patented and used commercially as CD-R dyes due to excellent dispersion characteristics and longevity of signal intensity.(Signal to Noise Ratio)