the mechanism for the photoinduced electron transfer reaction between a polymer semiconductor and C60 is shown in Fig. 13.37. As proposed [13.164,168], the incident photon is absorbed across the semiconducting gap of the polymer. This excitation drives a rapid structural relaxation of the polymer (< 10~13 s) due to electron-phonon coupling, thereby creating self-trapped polarons whose energy levels are in the semiconducting gap. The upper polaron level strongly couples to the LUMO of C6(1, and electron transfer on a time scale of ~10-12 s produces a metastable charge separation. The hole (positive polaron) then is free to drift away from the C60 anion that was produced, provided that the interaction between the hole and C60 anion is sufficiently screened. Thus this process represents a metastable photodoping process.
Whereas most optical studies on fullerenes have dealt with C«,, some studies have been carried out on C70, and a very few studies have been reported for higher-mass fullerenes (C„c; nc = 76,78,82,84,90,96) [13.174-177], Many similarities are found between the observed spectra for all fullerenes because the spectra relate to the fundamental molecular electronic structure of fullerenes, which are all closed cage molecules of similar basic design. For each of these fullerene species, the solution and solid-state spectra are very similar, and all involve excitons near the absorption edge. However, with increasing nc, the HOMO-LUMO gap decreases, as expected since in the limit nc -* oo, the 2D zero-gap graphene semiconductor is reached.
13.6. Optical Properties of Higher-Mass Fullerenes 537
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