Color-tunable anisotropic optical films fabricated using perylene diimide mixed with naphthalene benzimidazole

Naphthalene benzimidazole compound (NBI) was synthesized and mixed with perylene diimide (PDI) in order to modulate the absorbance wavelength of an optically anisotropic film between 400 and 550 nm. Equimolar mixture of NBI and PDI could form an ordered lyotropic liquid crystal (LLC) phase in 16.5 N formic acid solution when the solid content was larger than 20 wt.%, even though NBI salt itself did not show any LLC behaviors in all range of concentration. With increasing the content of NBI in NBI/PDI solution, the intensity of absorbance bands between 400 and 450 nm increased but polarization efficiency (P_eff) of the oriented films decreased. Based on the polarized optical microscopy morphological observations, it was realized that the orientation of PDI was hindered by the formation of NBI aggregates. When the molar ratio of NBI and PDI was 0.25, P_eff of the oriented film exhibited 84.1% at 475 nm and 49.8% at 400 nm, which can be compared with those (90.4% at 486 nm and 30.2% at 400 nm) of the oriented film without NBI.