UV-EXCITED OPTICALLY DETECTED MAGNETIC-RESONANCE (UV-ODMR) STUDY OF PI-CONJUGATED POLYMERS

The photoluminescence (PL) and X-band ODMR of poly(p-phenylenevinylene ) (PPV), poly(p-phenyleneacetylene) (PPA), and polythiophene (PT) deri vative films excited at 254 less than or equal to lambda(ex), less tha n or equal to 400 nm is compared to those excited at 488 nm. In PPV an d PT, but not in PPA, UV excitation yields a strong narrow PL-quenchin g ODMR in addition to the narrow PL-enhancing polaron resonance. In PP V and PT, the PL yield and the triplet exciton ODMR weaken at short la mbda(ex). The nature of the narrow resonance is discussed in relation to recombination of polarons, which may yield luminescent 1(1)B(u) sin glet excitons but in any case eliminates them as nonradiative singlet and triplet exciton quenching centers. The UV-excited PL-quenching res onance is then discussed in relation to several scenarios, including g eminate polaron recombination and spin-dependent decay of like-charged polarons into bipolarons. The behavior of PPA derivatives is suspecte d to result from structural defects which trap the charged excitations into polarons.