Orientation of triplet and singlet transition dipole moments in polyfluorene, studied by polarised spectroscopies

The orientation of the transition dipole moments for four commonly studied optical transitions of the conjugated polymer polyfluorene have been deduced using polarised spectroscopies, including fluorescence, phosphorescence, and photoinduced absorption in oriented polymer films. The data show that all the transitions except from the phosphorescence are highly polarised parallel to the chain orientation. The phosphorescence in this polyfluorene homopolymer however, has its dominant component orthogonal to the chain orientation. The data are interpreted by regarding shape and electron distribution of the excited states. It has been deduced that the singlet states and higher triplet states are strongly delocalised along the chain whereas the T1 excited state is more localised and has its electron distribution oriented orthogonal to the chain.