Nunchaku-like molecules containing both an azo chromophore and a biphenylene unit as a new type of high-sensitivity photo-storage material

Abstract

A series of nunchaku-like molecules containing both an azo chromophore and a rigid biphenylene unit (AZBP-X, X = CA, CN, and NT) was designed and synthesized in this study. The azo compounds were amorphous solids at room temperature with glass transition temperature (T_g) values of 98, 87, and 79 °C for AZBP-CA, AZBP-CN, and AZBP-NT, respectively. Thin solid films of the azo molecular glasses could be readily obtained by spin-coating. Photoinduced anisotropy (PIA) and surface-relief-grating (SRG) formation were investigated by irradiating the films with a linearly polarized Ar⁺ laser (488 nm) beam or interfering Ar⁺ laser beams. Upon Ar⁺ laser irradiation, photoinduced orientation occurred rapidly and the films showed significant birefringence and dichroism. When the irradiating light was switched off, the orientation did not show relaxation, as is observed for most azo polymers, and the birefringence even increased slightly. When irradiated with p-polarized interfering Ar⁺ laser beams with modest intensity (80 mW cm⁻²), SRGs were observed to be formed on the films in a high inscription rate. The grating with trough depth of 380 nm could be inscribed in 1200 s, which showed the diffraction efficiency of 27% for a He-Ne probe beam. In the initial stage of the SRG formation, a viscous flow process was visualized for the AZBP-CA film, which underwent a transition from doublet fringes on the grating crests to the final sinusoidal profile. Both PIA and SRG behavior showed a close correlation with the electron-withdrawing groups of the azo chromophores. The nunchaku-like azo molecules could be a new promising type of photo-storage materials for future applications.