Spectroscopic studies of photochromic N-[4-(4-alkoxybenzoyloxy)-2-hydroxy-benzylidene]-4-alkylanilines

Abstract

Much attention has been paid to molecular systems that exhibit photochromism from fundamental and optical application viewpoints [1, 2]. N-(2-hydroxybenzylidene) aniline (HBA) (more commonly called salicylideneaniline) is known to exhibit photochromism in the crystal and glass states [3±5]. It is known that, in photochromic compounds, the colourless ground-state enol form, on illumination with ultraviolet (UV) light, changes to the keto form through the hydrogen bridge with a concomitant rotation around the central C3⁄4N bond [2±5]. The possibility of using liquid crystals in various technical applications has been successfully exploited [6, 7]. Photochromic liquid crystal compounds representing both the liquid crystal behaviour and the photochromic properties have been the subject of research [1, 8]. However, attempts to synthesize such materials failed because the combination of photochromic and mesogenic moieties in one molecule led to the impairment of one or both of the properties [9]. It is well known that the change in molecular structure for HBAs through photoisomerization is small compared to that of azobenzenes and spiropyrans which are typical compounds exhibiting the photochromism and the liquid crystal phases. It can be expected that photochromism is observed in the liquid crystal phases of HBAs. However, this has scarcely been studied. We synthesized N-[4-(4-alkoxybenzoyloxy-2hydroxy-benzylidene)]-4-alkylanilines (mAHBAn, m ˆ 1y8, n ˆ 2, 4, 8):