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

quantum dots. Furthermore, because of its large bandgap, TiO 2 is almostalways found as the charge acceptor, with absorption taking place in theadsorbed sensitizer. Lower-bandgap nanocrystals can be used both as tunable,sensitizing chromophores, as well as electron or hole acceptors.B. II–VI Nanocrystal SystemsCdSe and CdS are prototypical colloidal quantum-dot materials, and much ofthe work on photoinduced charge transfer in chemically synthesized quantumdots has occurred in these two systems. Although many charge transferexperiments involving these particles are driven by applications, others haveused charge transfer as a tool to understand the fundamental properties ofthe nanoparticles. In this regard, one useful probe of charge transfer fromluminescent quantum dots (or from luminescent molecules to quantum dots)is photoluminescence quenching. In the absence of a nearby electron or holeacceptor, a high percentage of photoexcitations can relax through radiativerecombination. However, if charge transfer from a photoexcited dot to anearby acceptor is fast enough, the majority of electron–hole pairs will beseparated before they recombine and the photoluminescence of the samplewill be quenched.An elegant example of photoluminescence quenching as a probe ofcharge transfer was provided by Weller and co-workers in an experiment tomeasure the trap distributions in CdS quantum dots (Fig. 8) [59]. Theirsamples exhibited both band-edge and trap luminescence and they usednitromethane and methylviologen as electron acceptors. With a relativelyhigh electron affinity, methylviologen was found to quench both the excitonicand trap luminescence in samples of both large- and small-sized CdS dots.However, they found that although nitromethane quenched the excitonicphotoluminescence of both sizes of particles, it quenched the trap luminescenceonly in the smaller nanocrystals. From the difference in EAs of thequantum-dot samples and the known reduction potentials of the electronacceptors, they were able to estimate the depth and width of the electron trapdistribution. They found that the energy difference between the excitonic andtrap photoluminescence originated from deep trapping of the hole.Electron transfer from photoexcited CdS to methylviologen is known tooccur on ultrafast timescales (200–300 fs), with the charge-separated statethen persisting for microseconds; El-Sayed and co-workers used this phenomenonto isolate the hole trapping dynamics from those of the electron inthe CdS system [60]. Similar experiments showed that ultrafast (200–400 fs)electron transfer occurs from photoexcited CdSe nanocrystals to adsorbedquinones, but with the quinones acting as electron shuttles that facilitateback-electron transfer in a few picoseconds, faster than the native CdSeCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.