Full-conjugated layer-by-layer self-assembly ultrathin functional films fabricated from terpyridine and transition metal ions based on coordinate interaction

A novel conjugated light-sensitive diazonium-containing compound, 4-(2, 2':6', 2″-terpyrid-4'-yl) benzenediazonium tetrafluoroborate (Diazo-tpy) was firstly synthesized and used to modify substrate by electrostatic interaction. Following the decomposition of diazonium group of Diazo-tpy under UV irradiation, the ionic bonds between the diazonium salt and substrate were converted to the covalent bonds. The terpyridine-containing monolayer film anchoring on the substrate is very stable. Specially, the π-conjugated system of the terpyridyl groups is linked to the surface of the substrate, which can greatly facilitate the transport of charges between them. Furthermore, the layer-by-layer (LBL) self-assembled (SA) films of 1, 4-bis(2, 2':6', 2″-terpyrid-4'-yl) phenylene (Bi-tpy) and different transition metal ions (Mⁿ⁺: Pt⁴⁺, Ru³⁺, Rh³⁺, Pd²⁺) were fabricated on the tpy-modified substrate, and well characterized using UV-Vis absorption spectroscopy. The UV-Vis analytic results indicate that the LBL Bi-tpy/Mⁿ⁺ SA multilayer films, having an axial ligand between the metal atom and terpyridyl group, have been successfully fabricated and a progressive assembly runs regularly with almost equal amount of deposition in each cycle. Under illumination, the SA films on ITO show an effective photoinduced charge transfer and changed the current density of the ITO. The result revealed that the photoelectric conversion property of Bi-tpy/Ru film was higher than that of Bi-tpy/Pt film. With an increase in the layer number, the photocurrent rose and reached its maximum at 6 bilayers. Further increment led to a current drop due to the increase in probability of objection and the cell resistance. The results allow us to design new materials for more optoelectronic applications.