Chiral power change upon photoisomerization in twisted nematic liquid crystals

• Photoisomerization induces rotatory dispersion change in nematic liquid crystals.
• Dimethylamino azobenzene is the most effective dopant tested for this purpose.
• Isomerization and chiral change display the same dynamics.
• The liquid crystal pitch change is the reason of the variation.

In this work, we use the photoisomerization of azobenzenes, a phenanthrospirooxazine, and a fulgide in a twisted nematic liquid crystalline phase to change the chiral twisting power of the system. The changes are probed by the rotatory power of linearly polarized light. Time resolved and steady state experiments are carried out. The chiral change and the photoisomerization process have similar characteristic recovery times and activation energy, thus probing that the change is induced by the modification in the chemical composition of the photochromic dopant system. The amplitude of the light twisting power change correlates with the order change in the liquid crystal (LC) but not with the modification in the absorption characteristics of the system. This indicates that the driving force of the chiral change is the microscopic order modification in the LC phase that affects the helical pitch of the phase.

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