Liquid Crystalline Nonconjugated Open-Shell Organic Molecules

Recently, nonconjugated radical polymers, organic radicals have shown specific conductive, electrochemical, and magnetic properties that make them potential candidates to supplement, or replace, more established electronic materials. Moreover, liquid crystals (LCs) and single crystals are already well-known as suitable materials for optoelectrical devices due to their intriguing nanostructural features. However, radical-containing LCs have been infrequently reported. This work focuses on understanding the crystalline and LCs structures shown by a class of 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) derivatives. Depending on the length of the carbon chain on the tail of TEMPO LCs, various crystalline phases and liquid crystalline (e.g., the nematic and smectic phases) were observed. For the crystalline materials, the structure was calculated from the unit cell of TEMPO-C_S and the nearest TEMPO-TEMPO distance were 6.14 Å and 6.12 Å. Specific phases and the growth of the LCs were determined using polarized optical microscopy (POM). One radical crystal shows electrical conductivity of ~ 16.97 S m^-1, which is the highest value reported for nonconjugated radical conductors over a 1 mm scale. However, the other material shows 10,000 times lower conductivity despite the similar molecular structure. These results present a structure-property relationship, the design rules, and the charge transport mechanism for a new set of radical-based small-molecule materials.