A photo-responsive molecular wire composed of a porphyrin polymer and a fullerene derivative

Dendron-protected porphyrin polymers (M_n 43000 g mol⁻¹; M_w 111000 g mol⁻¹) were deposited using the Langmuir–Blodgett method on glass surfaces modified with N-phenyl-3-aminopropyltriethoxysilane. These deposited films were soaked in a solution of a fullerene derivative bearing a pyridinyl moiety. Topographical images of the surface obtained by atomic force microscopy showed that the height of the porphyrin polymers was 3.1 ± 0.5 nm as deposited, and clearly increased to 3.9 ± 0.3 nm upon soaking. The increase in height (about 0.8 nm) corresponded to the diameter of the fullerene, indicating that the pyridinyl moieties of the fullerene molecules bound to the zinc atoms of the porphyrin units. Porphyrin polymers having thiol groups at both ends bridging nano-gapped gold electrodes were fabricated on silicon chips. The chips were soaked in the fullerene derivative solution to make devices containing porphyrin polymers whose central zinc atoms were coordinated by the fullerene derivative. The current–voltage (I–V) characteristics of these devices were measured, and higher conductivity was observed during photoirradiation only when the devices contained both the porphyrin polymer and the fullerene derivative; no photo-response was observed when either component was absent.