Undoped Photorefractive Ferroelectric Liquid Crystal

Abstract

Light-sensitive media constitute a class of materials that are extensively studied because of their technological relevance for a variety of applications. In particular, the non-local reversible refractive index control by light, which is possible in substances exhibiting photorefractive properties, is of wide interest for real-time holography.[1] Photorefractivity is observed in photoconductors when a spacially non-uniform light exposure generates charge carriers which redistribute over macroscopic distances. This causes an electric field, which affects the refractive index of the material. Driven by the aim of obtaining low-cost and easily processable media, many efforts have been devoted to the understanding of the photorefractive properties of organic amorphous glasses [2±4] and polymers, [5±9] followed more recently by the interest in photorefractive liquid crystals. [10] Mesophases are attractive for photorefractivity because of their high spontaneous birefringence and the wide variety of field-induced refractive index control mechanisms.[11] The first liquid crystals studied for their photorefractive properties were bulk [12±14] and dispersed [15,16] nematic materials, but in the last years a number of investigations have showed that smectic phases can be photorefractive as well. [17±19] In particular, studies were carried out on chiral smectic A and C phases, where the mechanisms of field-induced fast molecular reorientation are connected with an induced or permanent polarization, respectively. [20] In all the reported investigations, the liquid-crystalline phase is not intrinsically photoconducting, so that the space±charge field set up is achieved either by dissolving a light sensitizer (and often a charge-transport agent as well) within its bulk, or by placing the mesophase within (like in dispersed systems), or between layers of, a photoconducting material. In this work the first example of a smectic mesophase that exhibits photorefractive behavior without any doping will be presented. It will also be shown that the observed photorefractive performance is larger than that previously reported for doped systems. Our approach is based on two recent results: the observation of photorefractivity in amorphous glasses formed by cyclopalladated complexes [21] and the synthesis of a new complex, with a closely related molecular structure, which exhibits a chiral smectic C phase. [22] The mesomorphic behavior of this complex, with a structure shown in Figure 1a and called for simplicity AzPdL, was investigated by optical microscopy, differential scanning calorimetry (DSC) and low-angle X-ray diffraction measurements. The observed transition temperatures between the isotropic (I), cholesteric (N*), chiral smectic C (SmC*) and crystalline (Cr) phases are included in Figure 1a.

Photorefractive organic material for optical thresholding

“…In contrast with photorefractive FLCs developed previously, [11][12][13][14][15] the prepared SSFLCs were bistable; i.e., both director orientations induced by electric fields of different polarities were stable after the field removal. A schematic illustration of the prepared samples is shown in the left side of Fig.…”

Organic Photorefractives: Mechanisms, Materials, and Applications

“…Recently, the PR effect in chiral smectic LC phases, including ferroelectric SmC* phases, was observed and launched extensive studies directed toward understanding the mechanisms and improving the materials. − Ferroelectric SmC* LCs exhibit a net polarization ( P s ) that leads to fast (∼μs) and efficient molecular reorientation in the space-charge field ( E sc ) as a result of the coupling between P s and E sc and in principle can exhibit an electronic electro-optic effect (not possible in nematic phases) …”

Factors affecting the lability of the σ(M–X) bond in cycloplatinated and cyclopalladated complexes containing [C(sp2, ferrocene),N,X]− or [C(sp2, phenyl),N,X]− (X=S, N) terdentate ligands

“…Cyclometallated complexes containing platinum(II) or palladium(II) ligands [1] have generated considerable interest since long ago mainly due to their applications in homogeneous catalysis or for the design of new materials with outstanding properties [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Their utility as precursors in organic and organometallic synthesis [13][14][15][16][17][18] is also well-known.…”