Photoinduced electron transfer from conjugated polymers to CdSe nanocrystals

We study photoinduced electron transfer from derivatives of poly (p-phenylenevinylene) (PPV) and nanocrystals of cadmium selenide via photoluminescence (PL) quenching and photoinduced absorption (PIA) spectroscopy. Using size-dependent quantum confinement to vary the energy levels of the nanocrystal acceptors, and chemical substitution to vary the energy levels of the polymer donors, we present a systematic investigation of charge transfer in these polymer/quantum-dot composites. We observe efficient PL quenching in blends of poly2-methoxy-5- (2^'-ethyl-hexyloxy-p-phenylenevinylene) (MEH-PPV) with nearly monodisperse CdSe samples for nanocrystal diameters from 2. 5 to 4. 0 nm. The observed PIA peaks, as well as their frequency and temperature dependence, are consistent with the formation of long-lived positive polarons on MEH-PPV following electron transfer to the nanocrystals. Both the PL quenching and the PIA features are insensitive to nanocrystal size. We have also studied blends of CdSe nanocrystals of 2. 5--4. 0 nm diameter with two high electron affinity cyano-substituted PPV derivatives. One of these polymers behaves similarly to MEH-PPV; however in the other polymer, which has different alkoxy side chains, we find neither efficient PL quenching nor any PIA features indicative of charge transfer. We explain the insensitivity of the electron transfer process to nanocrystal size in the context of the relevant polymer and nanocrystal energy levels and discuss the influence of the polymer side chains on the charge-transfer process.