Ordered Langmuir-Blodgett films of a substituted lutetium bisphthalocyanine

Abstract

High-quality LB multilayers have been prepared from the Lu(III) sandwich complex of 2,3,9,10,16,17,23,24-octa ( n-butoxy)phthalocyanine (LuPc 2(OBu) 16). Surface pressure-area isotherms were characterized and indicate that a stable monolayer is formed corresponding to an area per molecule of 2.4 nm 2 at 30 mN m −1. The LB films were highly birefringent, and polarized spectra gave dichroic ratios of 3.3 for the 670 nm absorption band and between 0.5 and 2.8 for infrared absorptions. The results indicate that the phthalocyanine rings were highly oriented perpendicular to the dipping direction but somewhat tilted from the substrate normal. The order was shown to be absent when (i) unsubstituted LuPc 2 was used for LB films, or (ii) the horizontal lifting method of film deposition was used, or (iii) the surface pressure was increased to 50 mN m −1, causing a molecular rearrangement. The ordering was improved at 100 °C and finally lost at 280 °C by annealing on a hot stage. The d.c. electrical conductivity of LB films of LuPc 2(OBu) 16 was low (σ ≈ 2 × 10 −7 Ω −1 m −1), in contrast with unsubstituted LuPc 2 (σ ≈ 10 −1 Ω −1 m −1) and showed no evidence for anisotropy. The findings are in broad agreement with related studies and illustrate some of the many factors involved in improving the structure of phthalocyanine LB films for possible applications.