Localized triplet excitations and the effect of photo-oxidation in ladder-type poly(p-phenylene) and oligo(p-phenylene)

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 photoluminescen ce (PL) detected magnetic resonance (PLDMR). The PLDMR results and quantum chemical calculations show unambiguously that the TE wave-function extent i s much smaller than that of the singlet exciton (SE). The weaker vibronic s tructure of the triplet photoinduced absorption (PA) band of m-LPPP relativ e to the singlet absorption is assigned to the small energy difference betw een the geometry of the lowest lying and excited triplet states. This is an additional indication of the strong localization of the triplet wave-funct ion as compared to that of the SE. Finally, the influence of photo-oxidatio n on the PLDMR and PA is analyzed and discussed in relation to the photocon ductivity of the materials.