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Title: Dynamic broadening alters triplet extinction coefficients in fluorene oligomers and polymers

Journal Article · · Journal of Chemical Physics
DOI: https://doi.org/10.1063/1.5132798 · OSTI ID:1583112

We report Tn ← T1 spectra and extinction coefficients, ε, and other properties as functions of chain length for a series of fluorene oligomers, oFn, and polymers, pFn, 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. Furthermore, 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.

Research Organization:
Brookhaven National Laboratory (BNL), Upton, NY (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES); USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Grant/Contract Number:
AC02-98CH10886; SC0012704
OSTI ID:
1583112
Report Number(s):
BNL--213532-2020-JAAM
Journal Information:
Journal of Chemical Physics, Journal Name: Journal of Chemical Physics Journal Issue: 2 Vol. 152; ISSN 0021-9606
Publisher:
American Institute of Physics (AIP)Copyright Statement
Country of Publication:
United States
Language:
English

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