Synthesis and characterization of organic materials with conveniently accessible supercooled liquid and glassy phases: Isomeric 1,3,5-tris(naphthyl)benzenes

1,3,5-Tris(1-naphthyl)benzene (1), 1,3-bis(1-naphthyl)-5-(2-naphthyl)benzene (2), and 1-(1-naphthyl)-3,5-bis(2-naphthyl)benzene (3) easily supercool and form glasses on cooling from the melt. We synthesized 1, 2, and 3 using Suzuki's conditions to effect the cross-coupling reactions of 1,3,5-tribromobenzene with 1-naphthylboronic acid and/or 2-naphthylboronic acid. Variable-temperature C-13 NMR studies of 1 establish a barrier of ca. 12 kcal/mol for rotation about the aryl-aryl bond; this value displays good agreement with the barrier of 13 kcal/mol computed using molecular mechanics calculations (MM2). This relatively low rotational barrier is inconsistent with the previously held notion that 1,3,5-tris(1-naphthyl)benzene (1) exists as a mixture of noninterconverting rotational isomers (atropisomers) in solution at room temperature. Variable-temperature C-13 NMR studies of 2 establish barriers of ca. 12 kcal/mol for rotation about the 1-naphthylaryl bond and < 9 kcal/mol for rotation about the 2-naphthyl-aryl bond. Again, these values display good agreement with the barriers of 14 and 2 kcal/mol computed using molecular mechanics calculations (MM2). Earlier syntheses of 1,3,5-tris(1-naphthyl)benzene (1) provided materials that were poorly characterized by modem standards. H-1 and C-13 NMR spectra of one such material, a sample widely used in studies of glasses and supercooled liquids, establish the structure of the material as 2, not 1. This revised structural assignment necessitates a re-evaluation of the earlier literature. Differential scanning calorimetry (DSC) establishes that tris(naphthyl)benzenes 1, 2, and 3 melt (T-m = 182, 194, and 147 degrees C, respectively) and form glasses upon cooling (T-g = 81, 77, and 67 degrees C, respectively). Given its low melting point and glass transition temperature, 1-(1-naphthyl)-3,5-bis(2-naphthyl)benzene (3) is an attractive candidate for future studies of molecular dynamics of glassy materials.