Comparison of the chain length dependence of the singlet- and triplet-excited states of oligofluorenes

Using time-resolved photoluminescence and excited-state absorption spectroscopy, we investigate the chain length dependence of the energies of the singlet (S1) and triplet (T1 and Tn) excited states of a homologous series of conjugated 9,9-dihexylfluorene-2,7-diyl oligomers (nF, n = 1, 3, 5, 7, ∼50) in frozen solution. The energies of the optical transitions in absorption, fluorescence, phosphorescence, and triplet-triplet absorption initially decrease linearly with the reciprocal number of repeat units, but saturate at n⩾5. The saturation and dispersion of the optical transition energies indicate that the T1 state is only slightly more confined than the S1 state.