Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

form blends with morphologies that allow a high percentage of the polymerexcitons to encounter an interface within their typical diffusion range of f10nm [71]. In addition, the charge-separation process must be fast enough tocompete with the radiative and nonradiative decay pathways of the singletexciton, which typically occur on timescales of 100–1000 ps [72].The use of TiO 2 nanocrystals as electron acceptors in this situation hasbeen briefly mentioned earlier [48–52]. The use of CdSe nanocrystalsas electron acceptors in polymer blends provides several advantages to thestudy of photoinduced charge separation. Because the nanocrystal surfacescan be modified through the addition or removal of organic ligands withoutaltering the intrinsic electronic properties of the nanocrystals, there exists thepossibility of altering the blend morphology or of introducing a controlledspatial barrier to charge transfer while still retaining the size-tunable propertiesof the quantum dots. More importantly, because the energy levels of thehost polymers can be tuned through chemical derivatization of the backbonechains and the energy levels of the nanocrystals can be tuned through sizedependentquantum-confinement effects, blends of the two materials offer thepossibility of careful and independent positioning of both donor and acceptorlevels.Greenham et al. [5] studied blends of CdS and CdSe particles in anMEH-PPV polymer host. Unlike much of the work on PVK/CdS blends, thequantum dots and polymers were synthesized separately and the compositeswere formed by spin-coating films from a common solvent, allowing forprecise control of both the nanocrystal and polymer chemistry. Photoinducedelectron transfer from MEH–PPV to 5-nm-diameter CdSe particles wasdemonstrated via quenching of the polymer photoluminescence, as well asby dramatic increases in the photoconductivity of the MEH–PPV. It wasfound, however, that charge transfer could only take place once the protectiveTOPO monolayer on the particle surfaces had been exchanged with pyridine,thereby allowing the electronically active polymer backbone and nanocrystalcore to interact closely. This surface exchange was also found to have a strongbeneficial influence on the local film morphology. At high concentrations,pyridine-treated nanoparticles also tend to form aggregates in the blends, asseen in Fig. 10.We have recently studied charge transfer between different sizes of CdSenanocrystals and various PPV derivatives with different EAs and different sidechains [27]. By monitoring quenching of the polymer fluorescence (Fig. 11), wewere able to establish that electron transfer occurs from MEH–PPV to severalsmaller sizes of CdSe, despite the lower electron affinity of the smaller nanocrystals.We also measured fluorescence quenching between CdSe nanocrystalsand two cyano-substituted PPV derivatives, MEH–CN–PPV andDHeO–CN–PPV. Grafting electron-withdrawing CN groups onto the poly-Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.