Browsing by Author "Mullen, K."
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- Direct evidence for singlet-triplet exciton annihilation in pi-conjugated polymersList, 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.
- 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.