Structure, electrical conductivity and electrochromism in thin films of substituted and unsubstituted lanthanide bisphthalocyanines

The structure, electrical conductivity and electrochromic properties of thin films of lutetium bisphthalocyanine, prepared mostly by the Langmuir-Blodgett technique, have been investigated with particular reference to the differences between unsubstituted (LuPc2) and substituted (LuPc2 (OBu)(16)) materials. Scanning tunnelling microscopy images on graphite show the difference in the two structures and give molecular dimensions of 1.5 x 1.0 nm and 2.8 x 1.1 nm respectively, in agreement with previous estimates. The in-plane d.c. conductivity was studied as a function of film thickness and temperature. LuPc2 is approximately 10(6) times more conductive (sigma approximate to 5 x 10(-1) Omega(-1) m(-1), E-a = 0.25 eV) than the substituted material. The conduction normal to the film plane and the electrochromism were very sensitive to the choice of bottom electrode. The electrochromism on indium-tin oxide coated glass was studied in LuPc2, ErPc2 and LuPc2 (OBu)(16) using cyclic voltammetry and spectroscopy. The green/red oxidative step is seen in all cases but the green/blue reductive step is abolished in LuPc2 (OBu)(16). The importance of molecular packing and n-orbital overlap in these effects, and the possible relevance to more complex layer structures and devices, is discussed. (C) 1997 Elsevier Science S.A.