Fig. 13-37. Schematic energy level diagram for the photo-induced electron transfer from a semiconducting polymer on to CM [13.168]. The left column shows the carrier occupation prior to photoexcitation and the right column shows the formation of polaron states in the band gap by photoexcitation. Transfer of the electron from the upper polaron state produces a C^, anion and a mobile hole in the polymer.

The color differences between the C60 and C70 solutions are consistent with a smaller gap for C70 than for C60. The deeper, reddish-orange color observed for similarly concentrated C70 solutions stems from both a red-shifted HOMO-LUMO gap for C70 relative to C60 and stronger optical absorption from electronic transitions across this gap, associated with the lower symmetry of C70. Molar extinction coefficients for C60 and C70 have also been reported at several wavelengths [13.178], The availability of reliable molar extinction data is essential for quantitative high-performance liquid chromatography (HPLC) analysis of C60/C70 mixtures.

13.6.1. Optical Properties of C70

To date, most of the optical studies on pristine and doped C70 solids have been carried out in transmission on thin solid films deposited on various substrates such as quartz, Si, or KBr. Typically C70 films have been prepared by sublimation of microcrystalline C70 powder onto a substrate without careful attention given to the choice of substrate, nor to lattice matching issues.

The room temperature (T = 300 K) dielectric function of nanocrystalline solid C70 films (i.e., grain size on the order of 10-50 nm) was first reported

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