Jin Bakalis, Andrew R. Cook, Sadayuki Asaoka, Michael Forster, Ulrich Scherf, and John R. Miller
J. Phys. Chem. C 118, 114-125 (2014).
[Find paper at ACS Publications]
Abstract:
Extensive reductive chemical doping in four conjugated polymers showed evolution of optical spectra for negative polarons and more reduced species. Delocalization lengths of the polarons, which ranged from 2 to 6 nm, were determined from measurements of bleaching of the neutrals, with the extinction coefficients measured by pulse radiolysis. A particular advantage of reductive doping is the ability to encapsulate the Na+ counterions in the C222 cryptand to control the interaction of charges with counterions. For lightly doped chains C222 had little effect on the delocalization lengths or spectra of the two polaron transitions P1 and P2, perhaps because most were completely dissociated to free ions. C222 did strongly alter the spectra when many electrons were added to a chain. For the shortest polarons, 2 nm in poly(phenylene-vinylene) (PPV), energies of the P1 and P2 transitions increased with the extent of reduction. The effect on the P1 transition was greater in the absence of C222 indicating ion-pairing equilibria for the short PPV polarons. Highly reduced ions formed upon injection of multiple electrons included polarons compressed by factors of four or more from their normal lengths to 1 charge/nm: a highly reduced 60 nm long chain contained 60 electrons. For compressed polarons the transitions shifted with increasing reduction indicating sensitivity to counterions: ion pairing is an important determinant of the behavior upon multiple reductions.