Transition metal dithiolene complexes as near-IR dyes for liquid crystal device applications

Both commercial and military applications (e.g., free-space IR communications and sensor protection) exist for guest-host liquid crystal (LC) devices operating in the near- to mid-IR region. Progress in this area has been hindered by the severe lack of near-IR dyes with good solubility in the LC host, low impact on the inherent order of the LC phase, good thermal and chemical stability, and a large absorbance maximum tunable by structural modification over a broad range of the near-IR region. Transition metal complexes based on nickel, palladium, or platinum dithiolene cores show substantial promise in meeting these requirements. These new dye complexes are extraordinarily stable, possess liquid crystalline phases in their own right with the proper terminal functional groups, and can have melting points below room temperature. The latter property is especially significant for producing liquid crystal/dye mixtures with both high dye concentration and good resistance to phase separation. Because they are zerovalent, they can exhibit high solubility in LC hosts (up to 10 wt%). The λmax in these materials can range from 600 nm to 1600 nm, depending on structure. With enantiomerically enriched terminal substituents, nickel dithiolenes can induce a chiral mesophase in a nonchiral nematic host. This finding opens the possibility of generating novel LC mixtures with two degrees of tunability: an electronic absorbance band tunable by synthesis, and a selective reflection band tunable by temperature or applied electric field. Such a materials system would be particularly advantageous in sensor protection for dealing with frequency-agile laser threats.