Dynamic broadening alters triplet extinction coefficients in fluorene oligomers and polymers

We report T_n ← T₁ spectra and extinction coefficients, ε, and other properties as functions of chain length for a series of fluorene oligomers, oF_n, and polymers, pF_n, with n = 2-84 repeat units. We find that ε increases with length, peaking at 159 400 M^-1 cm^-1 for oF₃ and then decreases for longer chains. ε does not scale with 1/n or e^-n to reach a constant value at long length, as predicted by the commonly applied oligomer extrapolation approximation, although spectral shifts, oscillator strengths, and transition dipole moments do reach limiting values for chains near 10 units long. While computations describe the triplet in oF₂ and oF₃ as having similar geometries with a single flattened dihedral angle between units, computations and simulations suggest that in longer oligomers motion along the chains of the short 2-3 unit, the long T₁ state is probably the source of the unusual changes in ε. These occur because hopping along the chain is sufficiently fast that the dihedrals between fluorene units cannot fully relax. At a length near 10 units, hopping and dihedral angle changes produce a steady state distribution of geometries with only small changes from the ground state, which persist for longer chains. Additional decreases in ε from pF₂₈ to pF₈₄ are plausibly due to a small number of chain defects which result in loss of triplets.