Transition Metal Dithiolene Near-IR Dyes and Their Applications in Liquid Crystal Devices

Abstract

Numerous commercial and military applications 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 transition metal dithiolenes are zerovalent, they can exhibit high solubility in LC hosts (up to 10 wt%). The λ_max in these materials can range from 600 to 1500 nm, depending on structure. Depending on their overall molecular geometry and the choice of terminal functional groups, transition metal dithiolenes that show either positive or negative dichroism from the same basic core structure can be readily synthesized. When enantiomerically enriched terminal substituents are employed, 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.

In this paper, we overview our past and present activities in the design and synthesis of transition metal dithiolene dyes, show some specific applications examples for these materials as near-IR dyes in LC electro-optical devices, and present our most recent results in the computational modeling of physical and optical properties of this interesting class of organometallic optical materials.

Keywords:

• chirality
• dyes
• liquid crystals
• transition metal complexes

Acknowledgments

This work is supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460 and the University of Rochester. The support of DOE does not constitute an endorsement by DOE of the views expressed in this article.

Notes

^a E7;

^b Toluene.

*The flexible correlation was used.