Photodegradation of cyanine and merocyanine dyes

The photodegradation kinetics for some cyanine and merocyanine dyes was studied by UV-visible spectroscopy and the results show that the fading process follows quasi-first-order or zero-order kinetics in acetonitrile. In other experiments, the principal photodegradation products of the dyes were identified with the aid of GC/MS, and it was found that those cyanine dyes holding a positive charge had higher photostability than the corresponding merocyanines. The relationship between photostability and chemical structure was established using PM3 and AM1 MO calculations. Experimental results in this regard suggest that the species associated with the photodegradation of cyanine dyes may be the semioxidized dye free radical cation, Dye+̇. The transient absorption spectra of Dye+̇ were determined by nanosecond pump and probe spectroscopy.