Photochemically induced structure transfer from analogous azobenzenes and stilbenes onto a liquid crystalline phase
The photoinduced structure transfer from different 4-N,N-diethylamino-4′-alkoxyazobenzenes and analogous stilbenes onto a nematic liquid crystalline phase has been investigated. The structure transfer caused by the azobenzenes was found to be 7–8 times smaller than for the corresponding stilbenes. The photoinduced E→Z isomerization of the azobenzenes occurred in the liquid crystal more slowly than the thermal back reaction. In contrast, despite lower rate constants for the E→Z photoisomerization, the stilbenes under investigation brought about a greater structure transfer because no thermal back reaction was found at 25 °C.
Investigations carried out with the azobenzenes in ethanol and cyclohexane showed the great influence of the polarity of the surrounding medium on the ratio between the rate constants for the E→Z photoisomerization and the thermal Z→E back reaction. This ratio kE→Z/kZ→E,therm increased at 25 °C from ca. 0.15–0.5 in the highly polar liquid crystal to ca. 13–19 in ethanol and nearly 500 in the apolar cyclohexane.
This paper concludes that only the investigated stilbenes are suitable for photoaddressed displays using highly polar, thermotropic liquid crystals.