Azobenzene Side-Chain Liquid Crystalline Polyesters with Outstanding Optical Storage Properties

A flexible azobenzene side-chain liquid crystalline (SCLC) polyester architecture employed for reversible optical storage is described. The modular design allows four structural parameters to be individually modified. These parameters: ithe methylene side-chain spacer length, iithe substituent on azobenzene, iiithe methylene main-chain segment length, and ivthe polyester molecular mass, all influence the optical storage properties. A general synthetic route to novel mesogenic azobenzene diols comprising parameters i and ii is outlined. Polyesters with molecular masses (parameter iv) up to 100,000 are routinely obtained by melt transesterification of the novel diols and selected diacid precursors (parameter iii). Prominent storage features include no prealignment of thin SCLC polyester films prior to the writing process, and sensitivity in a broad laser wavelength window (415-532 nm). Additionally, information can be recorded either through polarization holography or as direct computer generated pattern (grey tones). Thus polarization holography results in high diffraction efficiency (> 50%) and high storage density (> 5000 lines/mm interference gratings) lasting presently well over 5 years without any sign of fatigue. The non-destructive read out is performed with red light (600-750 nm). Finally, erasing the information can be achieved by heating the polyester film to 80◦C or irradiating it briefly with UV-light. In the latter case at least 10,000 write, read and erase cycles are possible. Atomic force and scanning near-field optical microscopic investigations of gratings prepared with orthogonally polarized overlapping beams have demonstrated that the anisotropy is preserved in the film despite extensive mass transport and surface corrugation after the irradiation process. However, the observed surface roughness is strongly dependent on the laser polarization. Polarization Fourier-Transform infrared studies of laser induced segmental motion in selectively deuterated SCLC cyanoazobenzene polyesters have revealed that not only the azobenzene chromophores but also main-chain and side-chain spacers align preferentially perpendicular to laser polarization.