Abstract: A multicomposite P₂Mo₁₈ /DODA self-assembled multilayer film was fabricated by casting the chloroform solutions of ionic complex between a dimethyldidecylammomium and a phosphomolybdate anion onto the borosilicate glass and calcium fluoride substrates. Infrared spectra revealed that the Dawson strcucture characteristic of the P₂Mo₁₈ O₆₂^6- anion was presented in the composite film. Awell ordered superlattice structure, d-spacing 3.59 nm, was identified by X-ray diffraction. The composite film shows good photochromic properties. Under the irradiation with UVlight, the transparent film changed from yellow to blue. Then, bleaching occurred when the film was in contact with ambient air or O₂ in the dark. If the UVirradiated films were stored in nitrogen, helium, argon or vacuum conditions, it could retain blue coloration for a long time. It suggests that oxygen plays an important role during the bleaching process. The photochromic mechanism of the composite film was studied by UV-Vis spectra, FTIRspectra and ESR. The appearance of IVCTbands shows that electron transfer occurs between the organic substrate and heteropolyanion, converting heteropolyanions to heteropolyblues with simultaneous oxidation of the organic substrate. Observation of UVinduced ESRsignal suggests that electron transfer takes place in the lattice and P₂Mo₁₈ O₆₂^6- acting as an acceptor gets electrons during UVirradiation. The changes of IRspectra for the composite film after UVirradiation are another powerful proof to identify the DODA was oxidized and P₂Mo₁₈ O₆₂^6- was reduced in the photochronic process.

Key words: 18-Molybdophosphoric acid, Dimethyldioctadecylamine, Photochromism, Charge-transfer