ABSTRACT
Well-studied examples of nanoscale systems include semiconductor nanocrystals, carbon nanotubes (CNTs), organic conjugated polymers, and molecular aggregates, shown in Figure 23.1. ›e excitons in quantum dots and CNTs are closely related to molecular excited states; however, the size of the nanoscale systems has forced researchers to make severe approximations in their quantum-mechanical descriptions. Nonetheless, the excited states of nanoscale systems tend to be more amenable to approximate descriptions than molecules because the wavefunction delocalization reduces the importance of the electron correlation for an accurate calculation of the energies of electronic states. Such electronic excited states are o§en described as WannierMott excitons (Banyai and Koch 1993; Basu 1997; Gaponenko 1998; Jorio et al. 2008). Other systems, like molecular aggregates, crystals, and certain proteins-namely, those involved in photosynthetic energy transduction-have optical properties that are better described with reference to the molecular building blocks of the aggregate. ›e lowest electronic excited states of these
aggregates are called Frenkel excitons (Kasha 1976). Conjugated polymers, now used in organic light-emitting diodes and displays (OLEDs), have excited states somewhere between these limits (Sarici§ci 1997; Hadziioannou and Malliaras 2006). ›is relationship is illustrated in Figure 23.2.
