Tunable circularly polarized luminescence from molecular assemblies of chiral AIEgens

Abstract

Circularly polarized luminescence (CPL) is important to chiral photonic technologies. In molecular systems, besides their intrinsic chemical structures, architectures of molecular assemblies at the mesoscopic scale also account for the final macroscopic CPL properties. Herein, tunable CPL responses can be induced through architectural regulation of these molecular assemblies in suspension and solid states. A liquid crystalline assembled system of DPCE-ECh exhibiting a smectic C* phase with a high dissymmetry factor (g_CD = −0.20 and g_lum = +0.38) is reported. The intense and apparent CD and CPL of the film stem from the intrinsic helical structure of the molecular assembles with a weak contribution of Bragg reflection, where the helical axis is perpendicular to the optical axis and parallel to the direction of the glass substrate. To the best of our knowledge, this large g_lum factor is very rare for organic compounds even in the assembled state formed by annealing at the smectic liquid crystalline temperature. Interestingly, a strong CPL signal with a g_lum value of +0.18 is still recorded when DPCE-ECh is annealed in a chiral isotropic liquid (Iso*) state. On the other hand, DPCE-ACh can form two coexisting phases of chiral hexagonal and smectic liquid-crystalline phases due to intermolecular hydrogen bonding. The non-periodic molecular orientations of DPCE-ACh break its helical structure to give a weak negtive CPL signal of the order of 10⁻³. This work thus provides a new insight for developing efficient chiroptical materials in the aggregate state that have profound implications for high-performance CPL-based devices.