Aligned and unaligned thin polyfluorene (PF) films doped with varying degrees of the dyes tetraphenyl porphyrin (TPP) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) have been investigated via polarised steady-state and time-resolved fluorescence anisotropy measurements. In the aligned films, the PF and DCM emission have dichroic ratios of approximately 10 and 4 respectively, while the TPP emission is completely depolarised. The polarised dopant emission in the case of the DCM is likely to arise from its linear shape and possible orientation parallel to the polymer chains, whereas the TPP is planar and exhibits no preferential orientation. These experiments show the possibility of polarised, red-shifted luminescence from polymer films.
We present a structural study of poly(9,9-bis(2-ethylhexyl)-fluorene-2,7-diyl) (PF2/6) in aligned thin films on a rubbed polyimide substrate. PF2/6 forms 5/2-helical hairy-rodlike molecular structure which is self-organized in the hexagonal structure. In thin films, the aligned rigid polymer chains are parallel to the substrate in the direction corresponding to the molecular backbone, c axis. The cells are flattened in the direction of the surface normal and -in particular- reveal a multiple orientation where the greater proportion of the crystallites have one crystal axis a perpendicular to the substrate surface but a small proportion are aligned with the crystal axis a parallel to surface. We find further that by reducing the polymer chain length to approximately twenty repeat units the degree of axial alignment of PF2/6 is considerably improved while the local order changes from hexagonal packing towards nematic phase with no sign of multiple orientation.