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 n5. The saturation and dispersion of the optical transition energies indicate that the T1 state is only slightly more confined than the S1 state.