Charge-transfer localization in molecularly doped thiophene-based donor polymers

We provide evidence for highly localized charge-transfer complex formation between a series of thiophenetetrafluorobenzene donor copolymers and the molecular acceptor tetrafluorotetracyanoquinodimethane (F₄TCNQ). Infrared absorption spectra of diagnostic vibrational bands in conjunction with theoretical modeling show that one acceptor molecule undergoes charge transfer with a quaterthiophene segment of the polymers, irrespective of the macroscopic polymer ionization energy and acceptor concentration in thin films. The interaction is thus determined by the"local ionization potential" of quaterthiophene. Consequently, using materials parameters determined on a macroscopic length scale as a guideline for making new charge-transfer complex materials for high electrical conductivity turns out to be oversimplified, and a reliable material design must take into account property variations on the nm scale as well.