In situ optical absorption spectra and electron spin resonance measurements of poly(1,4-naphthalene vinylene) during electrochemical n-type doping

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Abstract
The electrochemical, optical and magnetic properties in poly(1,4-naphthalene vinylene) (PNV) during electrochemical n-type doping have been investigated by cyclic voltammetry, optical absorption spectroscopy, and electron spin resonance (ESR) measurements. Evidence for electrochemical n-type doping of this polymer and evolution of localized states is obtained from measurements of a cyclic voltammogram and is demonstrated by the spectral change associated with doping. In the lightly doped state only two absortpion peaks originating from the transitions between two gap states and the conduction band appear within the gap region at 1.1 and 1.7 eV. The spin susceptibility increases by about two orders of magnitude from 5.9*10-8 to 6.1*10-6 emu mol-1 upon doping up to a dopant concentration of about 2.8 mol.%. With a further increase in dopant concentration the spin susceptibility decreases slightly. The spin density evaluated from the susceptibility was inconsistent with the polaron density assumed to be formed by dopants of 2.8 mol.%. That is, even at lightly doped slates the newly developed species turn into n-type spinless bipolarons. The ESR linewidth decreases by doping from about 9.9 G to 0.4 G. The g factor of 2.0031 at the neutral state shifts upon doping to the smaller value 2.0028, which corresponds to the nearly free electron g factor. These results are discussed in terms of bipolaron models.