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 |glum| = ca. 2 × 10–3 at
500 nm. Moreover, an organic green light-emitting diode that showed
a maximum external quantum efficiency (ηext) of ca.
0.4% was fabricated by doping 4,4′-bis­(2,2′-diphenylvinyl)-1,1′-biphenyl
with 6.