Multifunctional Octamethyltetrasila[2.2]cyclophanes: Conformational Variations, Circularly Polarized Luminescence, and Organic Electroluminescence

Both symmetrical and unsymmetrical cyclophanes containing disilane units, tetrasila[2.2]­cyclophanes 1–9, were synthesized. The syn and anti conformations and the kinetics of inversion between two anti-isomers were investigated by X-ray diffraction and variable-temperature NMR analysis, respectively. The flipping motion of two aromatic rings was affected by the bulkiness of the aromatic moiety (1 vs 6), the phase (solid vs solution), and the inclusion by host molecules (1 vs 1⊂[Ag₂L]²⁺). The photophysical, electro­chemical, and structural properties of the compounds were thoroughly investigated. Unsymmetrical tetrasila[2.2]­cyclophanes 5–8 displayed blue–green emission arising from intramolecular charge transfer. Compound 6 emitted a brilliant green light in the solid state under 365 nm irradiation and showed a higher fluorescence quantum yield in the solid state (Φ = 0.49) than in solution (Φ = 0.05). We also obtained planar chiral tetrasila[2.2]­cyclophane 9, which showed interesting chiroptical properties, such as a circularly polarized luminescence (CPL) with a dissymmetry factor of |g_lum| = ca. 2 × 10^–3 at 500 nm. Moreover, an organic green light-emitting diode that showed a maximum external quantum efficiency (η_ext) of ca. 0.4% was fabricated by doping 4,4′-bis­(2,2′-diphenylvinyl)-1,1′-biphenyl with 6.