Optically induced effects on the nematic-smecticA and smecticA-smecticC phase transitions critical behaviour in photochromic liquid crystal mixtures with different nematic range

The effect of the UV light irradiation on the characteristics of the nematic-smectic-A and smecticA-smecticC phase transitions is investigated by the photopyroelectric calorimetric technique in mixtures of photochromic naphtopyran molecules with the 8¯S5 liquid crystal and the results are compared with those previously obtained in mixtures with the 8CB liquid crystal having a smaller nematic range. In 8¯S5, where the transition shows 3D-XY-like characteristics, the photoisomerization of the naphtopyran molecules to a more planar and elongated “open” form did not affect significantly the critical behaviour over the nematic-smecticA transition, while in 8CB it shifted it from a non universal to a the 3D-XY-like one. The results are discussed in terms of the effects caused by the naphtopyran molecules in their “open” form being able to be hosted in the smectic layers, while remaining confined between the planes when in their “closed” form. Similar arguments are used to also justify the dramatic increase of the transition temperature induced at the smecticA-smecticC phase transition by the UV irradiation of the 8¯S5 sample. Results obtained also in 8¯S5 -silica nanoparticles mixtures are also reported for comparison and discussed.