Enhancement and Controlling the Signal of Circularly Polarized Luminescence Based on a Planar Chiral Tetrasubstituted [2.2]Paracyclophane Framework in Aggregation System

Optically active π-conjugated oligo­(p-phenylene ethynylene) dimers with a planar chiral 4,7,12,15-tetrasubstituted [2.2]­paracyclophane were synthesized. We investigated the optical and chiroptical properties of the racemic and optically active compounds by UV–vis absorption, photoluminescence (PL), circular dichroism (CD), and circularly polarized luminescence (CPL) measurements in the ground and excited states. Aggregates were prepared by the self-assembly in the mixed CHCl₃/MeOH solution, a spin-coated film, a drop-cast film, and the annealed films. In the dilute solution, the dimers exhibited good chiroptical properties, such as dissymmetry factors (g_abs and g_lum) in 10^–3 order, large absorption coefficient, and high absolute PL quantum efficiency. In the aggregation state, UV–vis absorption and PL measurements suggested that one of the dimers formed J-aggregates and the others constructed parallel H-aggregates or inclined H-aggregates as kinetically stable self-assemblies. Those differences were caused by the length of π-conjugation and the strength of the intermolecular π–π interaction. Transmission electron microscope (TEM) observations indicated the formation of the regular aggregates, and the aggregates made fiber structure. The spin-coated films and drop-cast films showed opposite CPL signal, and the values of dissymmetry factors reached 10^–2 order. The annealing process provided the thermodynamically stable forms to the films, and the g_lum values of the drop-cast thick films were drastically enhanced. Consequently, the g_lum values reached 10^–1 order (maximum: |g_lum| = 0.27). Those are one of the largest g_lum values of the organic compounds in the self-assembly system. All properties observed in this study are attributed to the planar and high symmetrical X-shaped structure of the planar chiral 4,7,12,15-tetrasubstituted [2.2]­paracyclophane framework. This is the first report realizing g_lum in 10^–1 order using the self-assembly system based on planar chirality as the only chiral source.