UV-Excited Optically Detected Magnetic Resonance (UV-ODMR) Study of ?-Conjugated Polymers

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Abstract
The photoluminescence (PL) and X-band ODMR of poly(p-phenylenevinylene) (PPV), poly(p-phenyleneacetylene) (PPA), and polythiophene (PT)derivative films excited at 254 44 λex≤400 nm is compared to those excited at 488 nm. In PPV and PT, but not in PPA, UV excitation yields a strong narrow PL-quenching ODMR in addition to the narrow PL-enhancing polaron resonance. In PPV and PT, the PL yield and the triplet exciton ODMR weaken at short λex. The nature of the narrow resonance is discussed in relation to recombination of polarons, which may yield luminescent 11Bu singlet excitons but in any case eliminates them as nonradiative singlet and triplet exciton quenching centers. The UV-excited PL-quenching resonance is then discussed in relation to several scenarios, including geminate polaron recombination and spin-dependent decay of like-charged polarons into bipolarons. The behavior of PPA derivatives is suspected to result from structural defects which trap the charged excitations into polarons.

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