Small molecule helical emitters

Abstract

The development of materials exhibiting circularly polarized luminescence (CPL) is a key area of research for next-generation optical technologies, including 3D displays and secure communications. The central goal in this field is to create chiral emitters with a high luminescence dissymmetry (g_CPL) factor, a measure of the emission's chirality. While theoretically reaching ±2, practical values in small organic molecules have historically been much lower, on the order of 0.001 or less. This summary outlines the core strategies in molecular design focusing on helical emitters that have recently enabled significant breakthroughs, pushing g values beyond the 0.01 threshold. The magnitude of g factor is determined by the cosine of the angle between the electric (μ_e) and magnetic (μ_m) dipole transition moments, as well as their respective magnitudes. Consequently, the most successful research has moved beyond simple screening and has focused on rationally engineering molecules to optimize this relationship. One of the most direct strategies has been to design rigid, helical molecules where high symmetry forces the μ_e and μ_m to be parallel. By enforcing D₂ and other symmetry in certain helicenes, helical nanographenes and related structures, researchers have minimized the angle between the moments, thus maximizing the cosine term and leading to a significant enhancement in the g factor value. A second, distinct approach targets the magnitude of the μ_m. In most organic chromophores, μ_m is inherently small, limiting the potential g factor intensity. To overcome this, researchers have designed for example belt-shaped macrocyclic molecules that function as molecular-scale solenoids. The cyclic arrangement of chromophores induces a large, circulating electric current in the excited state, which in turn generates a powerful μ_m along the cylinder's axis. A third innovative strategy circumvents the limitation of a small intrinsic μ_m by leveraging exciton coupling between two and more chromophores. In these systems, two π-conjugated units such as pyrene are held in a fixed, chiral arrangement. Upon photoexcitation, they form an intramolecular excimer, a transient excited-state complex with a well-defined helical geometry. The resulting CPL signal originates from the chiral interaction of the two strong electric transition moments, generating a large rotational strength and a high g factor without relying on the weak magnetic moment of the individual units. The progress in CPL-active materials is a testament to the power of targeted molecular engineering. As seen in the state-of-the-art examples in the review, the field has matured to a point where the fundamental photophysical principles governing CPL are being directly translated into synthetic molecular designs. While current high-performing materials are often complex and synthetically challenging, these proof-of-concept molecules validate the core design strategies.