Lutetium phthalocyanine

Lutetium phthalocyanine (LuPc2) is a coordination compound derived from lutetium and two phthalocyanines. It was the first known example of a molecule that is an intrinsic semiconductor. It exhibits electrochromism, changing color when subject to a voltage.

Structure
LuPc2 is a double-decker sandwich compound consisting of a Lu(3+) ion coordinated to two the conjugate base of two phthalocyanines. The rings are arranged in a staggered conformation. The extremities of the two ligands are slightly distorted outwards. The complex features a non-innocent ligand, in the sense that the macrocycles carry an extra electron. It is a free radical with the unpaired electron sitting in a half-filled molecular orbital between the highest occupied and lowest unoccupied orbitals, allowing its electronic properties to be finely tuned.

Properties
LuPc2, along with many substituted derivatives like the alkoxy-methyl derivative Lu[(C8H17OCH2)8Pc]2, can be deposited as a thin film with intrinsic semiconductor properties; said properties arise due to its radical nature and its low reduction potential compared to other metal phthalocyanines. This initially green film exhibits electrochromism; the oxidized form is red, whereas the reduced form  is blue and the next two reduced forms are dark blue and violet, respectively. The green/red oxidation cycle can be repeated over 10,000 times in aqueous solution with dissolved alkali metal halides, before it is degraded by hydroxide ions; the green/blue redox degrades faster in water.

Electrical properties
LuPc2 and other lanthanide phthalocyanines are of interest in the development of organic thin-film field-effect transistors.

LuPc2 derivatives can be selected to change color in the presence of certain molecules, such as in gas detectors; for example, the thioether derivative Lu[(C6H13S)8Pc]2 changes from green to brownish-purple in the presence of NADH.