Novel photochromic 2,2′-bithiophene azo dyes

The photochromic properties and colour constancy of two series of 2,2′-bithiophene azo dyes in THF solution were strongly dependent on the substitution pattern of the dyes. Under visible irradiation (>420 nm) while some dyes exhibited a significant change in colour intensity others exhibited an almost stable absorption. The photokinetic parameters of the systems are also described.