Irreverent Nature of Dissymmetry Factor and Quantum Yield in Circularly Polarized Luminescence of Small Organic Molecules

Abstract

Recently, a rational modification of small organic molecules has attracted considerable attention for designing advanced materials with enhanced circularly polarized luminescence (CPL) activity. A particular emphasis has been placed on fully allowed π-π^* transition of rigid aromatic systems, due to their relatively superior emission properties or quantum yields of luminescence (Φ_lum). However, their dissymmetry factors (g _lum), differential left and right CPL intensities, are typically disappointingly low at least in one to two orders of magnitude. Truly useful organic CPL materials, rated by a circular polarization luminosity index (Λ_CPL) per single molecule, possess both |g _lum| and Φ_lum values high. However, how to improve these two factors simultaneously with a proper molecular design is an open question. Here, we addressed this issue by theoretical and statistical inspection on a possible relation of the g _lum and Φ_lum values. According to the analysis, we propose simple, unpretentious, yet pertinent guidelines for designing superior organic CPL materials for the future with large Λ_CPL values.

Keywords: allowed π-π* transition; circularly polarized luminescence; dissymmetry factor; luminescence quantum yield; structure-chiroptical property relationship.

Figure 1

Conceptual diagram for two-dimensional improvement of CPL materials.

Figure 2

(A) Log–log plot between luminescence (g_lum) and absorption (g_abs) dissymmetry factors. (B) Log plot between g_lum and square of 1—luminescence quantum yield (Φ_lum). Blue, helicenes and derivatives; red, planar chiral cyclophanes; orange, binaphthyls with axial chirality; green chiral BODIPY derivatives. Data are taken from Tanaka et al. (2018b).