Charge transfer dynamics in squaraine-naphthalene diimide copolymers

The synthesis of an alternating squaraine-naphthalene diimide donor-acceptor low band gap polymer (1.14-1.40 eV) as well as its monomolecular analogue is presented. Spectroelectrochemistry experiments and transient absorption spectroscopy in the fs-time regime reveal an ultrafast population of a charge separated state for both polymer and monomer. Local excitation of the squaraine moiety is followed by population of intermediate states, presumably charge transfer states, followed by full charge separation, which occurs within a ca. 2 ps. Charge recombination takes place within 5.2 ps, probably because the system is close to the Marcus optimal region for barrierless ET. For the polymer, measurements of the transient absorption anisotropy show that neither charge nor does energy transfer take place within the lifetime of the charge separated state, indicating that this state is essentially confined within one donor-acceptor pair.