Browsing by Author "Scherf, U."

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    Direct evidence for singlet-triplet exciton annihilation in pi-conjugated polymers
    List, E. J. W.; Scherf, U.; Mullen, K.; Graupner, W.; Kim, C. H.; Shinar, J. (American Physical Society, 2002-12-06)
    A blend of two conjugated polymers with different optical band gaps and different triplet exciton (TE) magnetic resonance features was studied by means of photoinduced absorption (PA) and photoluminescence detected magnetic resonance. From the latter we find that a reduction of TE's on either of the two polymers enhances the radiative singlet exciton (SE) decay on both polymers nearly identically. The PA results rule out other possible mechanisms to yield this signal, except one: a long-range SE-TE annihilation due to a dipole-dipole transfer mechanism. Based on this finding, we calculate the transfer radius and quantify the annihilation process for this system and for other conjugated polymers.
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    Geometry-dependent electronic properties of highly fluorescent conjugated molecules
    Yang, S. C.; Graupner, W.; Guha, S.; Puschnig, P.; Martin, C.; Chandrasekhar, H. R.; Chandrasekhar, M.; Leising, G.; Ambrosch-Draxl, C.; Scherf, U. (American Physical Society, 2000-09-11)
    We present a combined experimental/theoretical study of the electronic properties of conjugated para-phenylene type molecules under high pressure up to 80 kbar. Pressure is used as a tool to vary the molecular geometry and intermolecular interaction. The influence of the latter two on singlet and triplet excitons as well as polarons is monitored via optical spectroscopy. We have performed bond structure calculations for the planar poly(para-phenylene) and calculated the dielectric function. By varying the intermolecular distances and the length of the polymer repeat unit the observed pressure effects can be explained.
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    Interaction of singlet excitons with polarons in wide band-gap organic semiconductors: A quantitative study
    List, E. J. W.; Kim, C. H.; Naik, A. K.; Scherf, U.; Leising, G.; Graupner, W.; Shinar, J. (American Physical Society, 2001-10-15)
    The steady-state photoinduced absorption (PA), photoluminescence (PL), PL-detected magnetic resonance (PLDMR), and PA-detected magnetic resonance (PADMR) of poly- and oligo-(para-phenylenes) films is described. In particular, the excitation density (laser power) No dependence of the PA, PL, and PLDMR signals is analyzed by means of a rate equation model, which describes the dynamics of singlet excitons (SE's) and polarons in all three experiments quantitatively with the same set of parameters. The model is based on the observations that mobile SE's are quenched by trapped and free polarons and that the spin-1/2 magnetic resonance conditions reduce the total polaron population. Since the sublinear N-0 dependences of the positive (PL-enhancing) spin-1/2 PLDMR and the polaron PA band are essentially the same, we conclude that PLDMR is due to a reduced quenching of SE's by polarons. The agreement between the model, the current results, and results from other spectroscopic techniques provides strong evidence for this quenching mechanism. This also suggests that it is a very significant process in luminescent pi -conjugated materials and organic light-emitting devices. Consequently, the quenching mechanism needs to be taken into account, especially at high excitation densities, which is of great importance for the development of electrically pumped polymer laser diode structures.
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    Localized triplet excitations and the effect of photo-oxidation in ladder-type poly(p-phenylene) and oligo(p-phenylene)
    List, E. J. W.; Partee, J.; Shinar, J.; Scherf, U.; Mullen, K.; Zojer, E.; Petritsch, K.; Leising, G.; Graupner, W. (American Physical Society, 2000-04-15)
    The photophysics of methyl-bridged poly(para-phenylene)-type ladder polymer (m-LPPP) and oligomer films and solutions is described and discussed. The spin sensitive properties, such as the formation and properties of polaron pairs and triplet excitons (TE's) were studied using X-band photoluminescence (PL) detected magnetic resonance (PLDMR). The PLDMR results and quantum chemical calculations show unambiguously that the TE wave-function extent is much smaller than that of the singlet exciton (SE). The weaker vibronic structure of the triplet photoinduced absorption (PA) band of m-LPPP relative to the singlet absorption is assigned to the small energy difference between the geometry of the lowest lying and excited triplet states. This is an additional indication of the strong localization of the triplet wave-function as compared to that of the SE. Finally, the influence of photo-oxidation on the PLDMR and PA is analyzed and discussed in relation to the photoconductivity of the materials.