Effects of local dipoles on the chiroptical properties of chiral oligo(phenyleneethynylene)s: Large g values for annealed films of chiral oligo(phenyleneethynylene) bearing alkoxy and bromo-substituents

The effects of local dipole on the chiroptical properties of chiral conjugated oligomers were investigated by comparative studies on two analogous chiral oligo(phenyleneethynylene)s: Myr-OPE-Br and Myr-OPE. Myr-OPE-Br carries local dipoles introduced by electron-accepting Br-groups and electron-donating chiral alkoxy-groups, while Myr-OPE only carries chiral alkoxy groups. The DSC thermogram of Myr-OPE-Br showed a glass transition at ∼80 °C and a cold crystallization centered at 120 °C, while Myr-OPE only showed a glass transition at ∼90 °C. Thermal annealing treatment showed significant effects on the absorption and CD spectra of spin-coated film of Myr-OPE-Br: with annealing above 100 °C, the absorption maximum exhibited an obvious red shift; the absorption bands displayed well-resolved vibronic structures; the CD spectrum was inversed compared to those with no annealing or annealing at 80 °C; and the gabs value of the annealed film increased with increasing annealing temperature and time. After annealing at 140 °C for 1 h, a maximum gabs value above 0.117 was observed for Myr-OPE-Br film. While for Myr-OPE, the thermal annealing treatment showed no obvious effect on both the absorption and CD spectra. The CD spectra of both the as-coated film and annealed film of Myr-OPE showed no obvious Cotton effect. The observed large g values as well as the thermochromic changes in the absorption and CD spectra of annealed Myr-OPE-Br films were attributed to the presence of local dipoles.