Credit: © 2006 ACS

In the cascade of chemical transformations that comprise photosynthesis, the first step is one in which a light-absorbing molecule (chlorophyll) captures the Sun’s rays. This absorbed energy excites the photoactive part of the molecule — a type of porphyrin — and starts the electron-transfer process that ultimately produces the chemical fuel for a plant to survive. Researchers at the University of California, Los Angeles have now studied an artificial electron donor–acceptor system in which single-walled carbon nanotubes (SWNTs) have been coated with porphyrin molecules.

George Grüner and colleagues1 made a field-effect transistor (FET) device from SWNT networks grown on silicon oxide wafers. Using lithographic and plasma-etching techniques, nanotube channels of known length (500 µm) and width (1 mm) were defined between Pd/Cr electrodes. Porphyrin molecules were then carefully added to the middle two-thirds of the nanotube channel, avoiding any contact with the electrodes. It was shown that the resistance of the device decreased when exposed to light. Moreover, the rate and magnitude of the electron-transfer processes was seen to depend upon the intensity as well as the wavelength of the light source. Although porphyrins are usually electron donors, surprisingly, electrons were shown to move from the nanotubes to the porphyrin in this device.

This work offers further insight into the photophysics of artificial photosynthesis and may be useful for tunable light detection or other applications that require light-harvesting materials.