Thermodynamics of the photoferroelectric effect in smectic-C* liquid crystals

Abstract

A general thermodynamic description of the photoferroelectric effect, where the spontaneous polarization of a ferroelectric material is changed by interaction with light, is proposed. This description leads to a distinction between primary and secondary photoferroelectricity. At a certain reduced temperature, the primary photoferroelectric effect originates from the light-induced change in the polar ferroelectric order, whereas the secondary photoferroelectric effect is a purely thermodynamic consequence of the light-induced change in the ferroelectric transition temperature T c . Using simple approximations, the general theory was applied to photoferroelectric Sm-C* liquid crystals where the photoresponse is induced by the Z,E photoisomerization of azobenzene dopant molecules. The results agree with experimental data obtained for a photoferroelectric Sm-C* mixture containing 5 mol % of the photoisomerizable dopant 4,4Ј-bis-͓(2-methyl)-butyloxy͔azobenzene dissolved in the commercial ferroelectric Sm-C* host phase abbreviated FLC 6430.

Photoresponsive Ferroelectric Liquid‐Crystalline Polymers

“…The remaining change of P S (at a constant normalized temperature) is then referred to as the primary photoferroelectric effect (DP S,1 ) in contrast to the secondary effect (DP S,2 ) that is due to the shift of T C . [12] As can be seen in Figure 5 a distinct primary photoferroelectric effect is found for our polymer. At a temperature of, for example, T = 58°C (see figure) the primary effect contributes to 56 % of the overall P S change (DP S,total ).…”

First photoresponsive liquid-crystalline materials with small layer shrinkage at the transition to the ferroelectric phase

“…1 This effect is a result of disturbing the ferroelectric ordering in the smectic C* (SmC*) phase by the light-induced trans-to-cis isomerization of the azobenzene group. [2][3][4][5][6] Such lightinduced isomerization can provide photoswitchable ferroelectric materials, which exhibit both a photoresponse and high-speed ferroelectric switching, and thus may offer numerous applications in optoelectronics, photonics, data storage, etc.…”

Theory of photoferroelectric response in SmC* liquids

“…Both changes are a route to changing P s , hence the name photoferroelectric effect. Langhoff and Giesselmann were concerned with attributing photo changes to primary (c * ) or secondary (θ) influences 10 .…”

“…Spontaneous polarization as a function of temperature calculated by using relation(10), as explained in the main text, for the values of fitting parameters δ = 0.019, τ0 = 27.76 × 10 −12 s and κ = 9765 K (solid line). For comparison, we have shown experimental results in the irradiated state (circles).…”