OPTICALLY DETECTED MAGNETIC-RESONANCE (ODMR) OF POLARONS AND CHARGE-CONJUGATION SYMMETRY IN CONDUCTING POLYMERS

The main narrow photoluminescence(PL)-enhancing g = 2.0025 +/- 0.0010 X-band ODMR of poly(3-alkylthiophene) (P3AT) and poly(2,5-dialkoxyparaphenylenevinylene) (PDOPV) films, solutions, and blends is described and discussed. The resonances all closely match a sum of narrow and broad Gaussians. In P3AT the narrow Gaussian saturates at a lower power than the broad Gaussian. While this ODMR is believed to result from intrachain 'distant pair' polaron recombination, the assignment of each Gaussian to the proper polaron is undecided. The g-values of the two Gaussians are nearly identical in PDOPV, apparently reflecting a very high degree of charge-conjugation symmetry (CCS). They are, however, quite distinct (g = 2.0032, 2.0019, respectively) in P3AT, possibly due to the sulphur heteroatom on the thiophene rings. The ramifications of this CCS violation for the PL quantum yield are discussed. In all cases, the spectral dependence of the narrow resonance is similar to the PL spectrum, indicating that the polaron levels are close to the band edges and to the levels of the (presumably) short-lived geminate singlet excitons. The sensitivity of the ODMR intensity to sample processing suggests that it may possibly be affected by an unidentified native, possibly conformational, defect. The decay modes competing with polaron recombination are also discussed.