Third-order NLO properties of ultrathin films containing cationic phthalocyanine and Keggin polyoxometalates fabricated using layer-by-layer deposition from aqueous solution

Abstract

Composite films with alternating layers (PAH/PSS)₃(AB/POM)_n (PAH = polyallylamine hydrochloride, PSS = polystyrene sulfonate) derived from Alcian Blue-tetrakis(methyl-pyridinium) chloride (abbreviated as AB) and Keggin type polyoxometalates (POM = H₄SiW₁₂O₄₀, H₃PW₁₂O₄₀, H₃PMo₁₂O₄₀, H₅PMo₁₀V₂O₄₀) have been fabricated and characterized. Under laser irradiation at 532 nm with a pulse duration of 6–7 ns, a repetition rate of 10 Hz and the intensity of the light at focus E₀ being 10 μJ, the composite films exhibit self-defocusing behavior and saturated absorption effects, and the nonlinear optical absorption coefficient β (m W⁻¹) and refractive index n₂ (m² W⁻¹) of the films are 4–5 times larger than those of pure AB solution in view of their magnitude. Interestingly, it is found that third-order NLO susceptibilities χ⁽³⁾ of films are in the order of χ⁽³⁾(AB/PSS) < χ⁽³⁾(AB/SiW₁₂) < χ⁽³⁾(AB/PW₁₂) < χ⁽³⁾(AB/PMo₁₂) < χ⁽³⁾(AB/PMo₁₀V₂), which is reversed to the order of E_LUMO(SiW₁₂) > E_LUMO(PW₁₂) > E_LUMO(PMo₁₂) > E_LUMO(PMo₁₀V₂), affirming that the incorporation of different Keggin type POMs of a lower LUMO energy level with phthalocyanine molecules into multilayers could tune the NLO responses of phthalocyanine films. It is concluded that the lower LUMO level of POM than that of the phthalocyanine can stimulate the easier transformation of excited electrons from AB to the POM within the donor–acceptor system (phthalocyanine molecules acted as an electron donor while POMs as an electron acceptor in the composite films) when flashed with the laser, which is thought to be accountable for the profound third-order optical nonlinearity of the composite films.