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

We have studied polaron pair photoexcitations in a variety of π-conjugated polymer films by photoinduced absorption (PA) and optically-detected magnetic resonance (ODMR). An exchange interaction between polarons broadens the ODMR spectrum, permitting the unambiguous identification of the polaron pair species. Both isotropic and anisotropic exchange interactions have been observed. An isotropic exchange interaction broadens the spin-1/2 resonance associated with polarons, whereas an anistropic exchange interaction gives rise to both full-field and half-field powder patterns. We have successfully modelled the observed spin-1/2 ODMR spectra and estimate the strength of the isotropic exchange interaction J. Both polarons and polaron pairs are characterized by two absorption bands. The high energy PA band of polaron pairs is blueshifted with respect to that of isolated polarons.