Spin and spectral signatures of polaron pairs in pi-conjugated polymers

We have studied polaron pair photoexcitations in a variety of rr-conjugated polymer films by photoinduced absorption (PA) and optically detected magnetic resonance (ODMR). Both luminescent and nonluminescent polymers have been studied. An exchange interaction between polarons broadens their magnetic resonance spectrum, permitting the unambiguous identification of the polaron pair species by ODMR, Both isotropic and anisotropic exchange interactions have been observed. An isotropic exchange interaction broadens the spin-1/2 resonance associated with polarons, whereas an anisotropic exchange interaction gives rise to both full-field and half-field powder patterns. We have successfully modeled the observed spin-1/2 ODMR spectra and estimate the strength of the isotropic exchange interaction I. An anisotropic exchange interaction has been observed in C-60-doped 2,5-dioctyloxy poly(p-phenylene vinylene) and cia-rich polyacetylene. Both polarons and polaron pairs are characterized by two absorption bands. However, the high-energy PA band of polaron pairs is blueshifted with respect to that of isolated polarons and the low-energy PA band of polaron pairs is relatively weak. We have also observed neutral soliton pair excitations in polymers with degenerate ground states.