Self-assembled multilayers of CdSe nanocrystals and carboxylate-handled phenylene-based molecules: optical, electrochemical and photoconductive properties

A series of linear phenylene-based molecules, bearing carboxylic acid or carboxylate functionalities at both ends, were reacted with (hexadecylamine/stearate)-capped CdSe nanocrystals (7.5 nm diameter) dispersed in CHCl3 to form multilayers on ITO-glass surfaces via layer-by-layer (LBL) alternation. The new multilayered materials were investigated by UV-Vis spectroscopy, cyclic voltammetry, photoelectrochemistry and photoconductivity. The conjugated linkers, with reversible reduction potentials in the range -1.7 to -2.8 V vs. Ag/Ag*, ease charge transport between dots (LUMO at -1.4 V). Photoconduction is dominated by interdot distance and linker characteristics rather than by LUMO energy levels. The logarithm of photoconductivity and linker length are linearly related, as in conduction of molecular wires, and the tunnelling attenuation factor b value is 2.4 1/nm. Comparison of n-type aromatics and p-type oligothiophenes is discussed.