Effects of molecular structures on the near-infrared optical properties of polyimide derivatives and their corresponding optical waveguides

Polyimides have been demonstrated to be a potential candidate as optical waveguide materials. Previous studies were focusing on the reducing the optical loss by decreasing the C-H bonding density of the molecular structure. The tuning of the near-infrared (NIR) overtone absorption position from the polarizability of the bridge group in polyimides is rarely studied. In this study, a series of polyimides prepared from pyromellitic dianhydride (PMDA) and diamines with various bridge groups were used to investigate the structure-NIR optical property relationships. The bridge groups include -SO2-, -S-, -O-, and -CH2-. Both the experimental and ab initio calculation on the model compounds and polyimide derivatives revealed that the stronger the electron-withdrawing ability of the neighboring bridge group, the shorter the aromatic C-H bond length. This results in the shifting of the overtone frequencies of the C-H bond to higher frequencies. Such bridge effects played an important role in the optical loss of the corresponding polyimide waveguides. It was found that the optical loss was resulted from both the C-H bonding density and the shifting of the overtone absorption by the bridge group. It indicates that tuning of NIR optical loss from the molecular structures of polyimides could be achieved.