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HL: Fachverband Halbleiterphysik

HL 41: Organic semiconductors

HL 41.5: Talk

Thursday, February 28, 2008, 16:00–16:15, ER 270

How strain controls the exciton linewidth in single polyfluorene nanowires — •Enrico Da Como, Klaus Becker, and Jochen Feldmann — Photonics and Optoelectronics Group, Dep. of Phys. and CeNS, LMU, Munich

In this contribution, by low temperature single molecule fluorescence spectroscopy, we show a correlation between intramolecular order and the exciton linewidth in single chains of poly(9,9-dioctylfluorene) (PFO). According to the degree of planarization in the adjacent fluorene repeat units, PFO single chains can be distinguished between beta- (planar) or glassy (twisted) phase. By single molecule polarization anisotropy in excitation we have compared the overall shape of the chain in both phases. The results demonstrate that beta-phase chains are characterized by higher polarization values than the glassy ones, reflecting a structure comparable to a one-dimensional (1D) crystalline nanowire [1]. A detailed investigation of beta-phase chains shows a strong correlation between the exciton zero-phonon-linewidth and the polarization anisotropy of the emitting chromophore. While linewidth provides a lower limit for the exciton dephasing time, which reaches remarkable values up to 3 ps for straight chromophores, low anisotropy is symptomatic of the degree of strain which acts on the 1D structure. Bent chromophores show systematically larger linewidths suggesting an accelerated dephasing in the exciton wavefunction or an increased spectral diffusion. The results give a picture on how structure in a 1D organic semiconductor correlates with the electronic properties. [1] E. Da Como et al. Nano Lett. 7, 2993 (2007).