PHOTOINDUCED ABSORPTION FROM TRIPLET EXCITATIONS IN POLY(2-METHOXY, 5-(2'-ETHYL-HEXYLOXY)-P-PHENYLENE VINYLENE) ORIENTED BY GEL-PROCESSING IN POLYETHYLENE

We use the technique of steady state photoinduced absorption to study the long-lived photoexcitations in the conjugated polymer poly(2-methoxy, 5-(2'-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) as a dilute component in ultrahigh molecular weight polyethylene (PE) oriented by gel-processing. In contrast to the three states found previously in films of alkoxy derivatives of poly(para-phenylene vinylene), the photoinduced absorption spectrum of MEH-PPV/PE blends has only one subgap absorption with peak at 1.4 eV. The single peak in the photoinduced absorption spectrum and the relatively long Lifetime, as determined from the dependence of the signal on the modulation frequency, are consistent with the identification of the photoexcitation as a long-lived triplet exciton. We conclude that the formation of charged bipolarons is suppressed in the aligned blends due to the increased order and dilution. By using a modified double-modulation technique to separate the triplet-to-excited-triplet-state (T-T*) absorption from the intense luminescence, we are able to determined the lineshape of the T-T* absorption. We find an asymmetric lineshape which is well described as a broadened square root singularity, indicative of a system with a one-dimensional electronic structure. This lineshape lacks well-defined vibronic structure indicating tittle configurational relaxation on exciting from T to T*.