Threshold-like Complexation of Conjugated Polymers with Small Molecule Acceptors in Solution within the Neighbor-Effect Model

In some donor-acceptor blends based on conjugated polymers, a pronounced charge-transfer complex (CTC) forms in the electronic ground state. In contrast to small-molecule donor-acceptor blends, the CTC concentration in polymer: acceptor solution can increase with the acceptor content in a threshold-like way. This threshold-like behavior was earlier attributed to the neighbor effect (NE) in the polymer complexation, i.e., next CTCs are preferentially formed near the existing ones; however, the NE origin is unknown. To address the factors affecting the NE, we record the optical absorption data for blends of the most studied conjugated polymers, poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and poly(3-hexylthiophene) (P3HT), with electron acceptors of fluorene series, 1,8-dinitro-9,10-antraquinone (DNAQ), and 7,7,8,8-tetracyanoquinodimethane (TCNQ) in different solvents, and then analyze the data within the NE model. We have found that the NE depends on the polymer and acceptor molecular skeletons and solvent, while it does not depend on the acceptor electron affinity and polymer concentration. We conclude that the NE operates within a single macromolecule and stems from planarization of the polymer chain involved in the CTC with an acceptor molecule; as a result, the probability of further complexation with the next acceptor molecules at the adjacent repeat units increases. The steric and electronic microscopic mechanisms of NE are discussed

Charge Transfer Dynamics in Donor-Acceptor Complexes between a Conjugated Polymer and Fluorene Acceptors

We report on ground and excited state charge transfer in charge-transfer complexes in films formed between a semiconducting polymer, MEH-PPV (poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene]), and a series of fluorene electron acceptors. The latter were designed to vary systematically the electron affinity (EA) over 1.5 eV by attachment of various electron withdrawing groups to the fluorene core. The EAs of the acceptors are determined by cyclic voltammetry and compared with those from density functional theory calculations. The charge-transfer dynamics are studied using an ultrafast visible-pumpIR-probe photoinduced absorption technique. We demonstrate that the acceptor EA is the keybut not the onlyparameter that governs charge recombination rates that scale exponentially with the acceptor EA. From the time-resolved data we deduced a model that describes charge dynamics for acceptors with low and high EAs. The two opposite trendshigher acceptor EA increases the driving force for charge separation but also inevitably increases the rate of undesirable charge recombinationshould be carefully counterbalanced in designing novel polymerfullerene bulk heterojunctions