Chemistry

Practically Perfect Pi Chains

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Science  18 Mar 2011:
Vol. 331, Issue 6023, pp. 1367
DOI: 10.1126/science.331.6023.1367-b

In principle, a planar π-bond network in a conjugated polymer chain should provide a perfect highway for delocalized charge carriers created by photoexcitation. In practice, what should be a rigid molecule has bends and defects that usually cause the polymer chain to segment into localized chromophores. Da Como et al. performed broad-band single-molecule photoluminescence (PL) and PL excitation (PLE) spectroscopy on β-phase poly(9,9-dioctylfluorene) oligomers (nine repeat units) and polymers (about 500 repeat units), in which all of the repeat units lie in one plane. Previous studies revealed a narrow emission transition, suggesting single-chromophore behavior, but the emission was not completely polarized, implying that bending defects were present. The authors isolated the molecules at low concentration and obtained spectra at 4 K, and under these conditions, the spectra of the oligomer and polymer were virtually the same—a single 0-0 transition peak at 445 nm, with mirror symmetry between excitation and emission and some excitation of vibrational modes. Thus, only a single exciton is created in the polymer, and it can travel more than 50 times its length along the chain.

J. Am. Chem. Soc. 133, 10.1021/ja109342t (2011).

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