The photoinduced birefringence, Δnind, in 19–35 μm thick films has been studied for two series of chiral photochromic liquid crystalline side chain copolymers. The birefringence measured is induced by excitation of the n‐π* band only. The Δnind value induced in 10 min is linear upon concentration of dye moieties but achieves saturation with increase of illumination intensity. Evolution of the birefringence can be described by double exponent growth; the faster exponent is the same for both copolymer series and should relate to the photoselection process, while the slower one should relate mainly to photo‐orientation and differs strongly dependent on chemical structure of photochromic groups. The different behaviour of two copolymer series during the growth governed by the second exponent is interpreted in terms of the “frictional model”. Due to elevated thickness of the films, rather moderate Δnind values result in huge phase retardation up to 270° and more. The materials are promising therefore for holography applications.
Chemical structure of the synthesised chiral photochromic copolymers.
