Keeping the Charges in Line

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Science  08 Dec 2006:
Vol. 314, Issue 5805, pp. 1516-1517
DOI: 10.1126/science.314.5805.1516d

Materials that convert sunlight into electrical current not only form electron-hole pairs upon light absorption but also must efficiently transport the carriers to prevent charge trapping and recombination. Disc-shaped liquid crystalline materials such as the contorted hexabenzocoronenes, which naturally form columnar conduit structures, have been studied for use in photovoltaic devices.

Cohen et al. find that photoconduction in films of these molecules is exclusively onedimensional. Optical absorption spectroscopy indicates that the puckered molecular geometry disrupts full delocalization of the π-bonding network. As the molecules stack, the six phenyl rings at the edges interact weakly; only the nearly planar core regions overlap sufficiently for effective π-conjugation. Density functional calculations were used to quantify this observation and indicated a 3.2-eV gap between the highest-energy occupied and lowestenergy unoccupied molecular orbitals in the core, in contrast to a 5.6-eV gap in the outer rings. As a result, these outer rings form an insulating cladding that promotes onedimensional conductivity in the encircled radialene core. The high charge separation observed in these molecules renders them exciting candidates for applications. — MSL

Nano Lett. 6, 10.1021/nl0620233 (2006).

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