Photoinduced orientation in nunchaku-like azo molecular glass studied by birefringence characterization and FT-IR spectroscopy

Photoinduced orientation of amorphous azo materials based on nunchaku-like molecules was studied by birefringence characterization, polarized infrared spectroscopy, and 2D-IR correlation spectroscopy. The nunchaku-like molecules (AZBP-CA and AZBP-CN) contained both an azo chromophore and a rigid biphenylene unit linked with the flexible spacer. Upon irradiation with a linearly polarized Ar+ laser beam (488 nm), the birefringence growth and unique relaxation behavior were investigated for solid thin films of the materials. The results show that the high birefringence of AZBP-CA upon the light irradiation is related with the hydrogen-bonding in the material as indicated by IR spectroscopy. The nunchaku-like molecule films show the unique relaxation behavior when switching off the exciting laser, i.e. the photoinduced birefringence further increases instead of partial decay, which is typically observed for most azo polymer films. The amount of the post-irradiation birefringence growth is positively correlated with the illumination time during the writing process. Polarized infrared spectroscopic investigation indicates that the orientations of the azo chromophore and biphenylene unit are strongly correlated with each other. As revealed by the two-dimensional infrared correlation spectroscopy, the azo chromophore and biphenylene unit move cooperatively almost at the same time. The flexible spacer of the molecules prefers to align in the same direction as the azo chromophore after the light irradiation, which suggests that the photoinduced orientation of the azo chromophore transfers through the flexible spacer to the biphenylene unit.