Large photoinduced circular dichroism in chiral polyfluorene

At 77 K, thermally annealed films of chiral poly[9,9-bis((3S)-3,7- dimethyloctyl)-2,7-fluorene] show a photoinduced absorption (PIA) band near 1.5 eV due to a transition from the lowest triplet state to a higher triplet state. This photoinduced absorption is found to show circular dichroism (CD). Dividing this CD by the magnitude of the PIA, we find that the selectivity for absorption of left circular polarized light over right circularly polarized light is remarkably high and characterized by a dissymmetry ratio g = +0.2. The unusual magnitude of g is interpreted as arising from extrinsic CD, i.e., from the interaction of the local linear polarization of the triplet-triplet absorption with the linear birefringence of the surrounding polymer matrix.