Circularly Polarized Luminescence and Circular Dichroisms in Small Organic Molecules: Correlation between Excitation and Emission Dissymmetry Factors

Abstract

Prompted by the recent rapid growth of interest in circularly polarized luminescence (CPL) of organic molecules, we have collected all the reliable CPL, as well as the corresponding circular dichroism (CD), data measured in fluid solutions. To analyze the correlation between CPL and CD, we employed the absorption and luminescence dissymmetry factors (g abs and g lum) of the π–π* transition reported for chiral organic molecules of various categories, including planar chiral cyclophanes and helicenes, axially chiral biaryls and spiro compounds, and point‐ and axially chiral BODIPY derivatives. In rigid π‐systems, the absorption and fluorescence spectra are often mirror images of each other with a small Stokes shift, reflecting the minimal conformational relaxation in the emissive excited state, which should also affect the chiroptical properties in the excited state and be better sensed by CPL. However, no comprehensive efforts have hitherto been made to correlate the two relevant chiroptical properties, i. e. CPL versus CD, and also to quantitatively elucidate the effects of conformational relaxation in the excited state on the CPL behavior. The global linear regression analysis of all the reported g abs and g lum values, though fairly scattered (see TOC), led us to a quantitative relationship: |g lum|=0.81×|g abs| (r2=0.60), which demonstrates that the CPL dissymmetry factor is proportional to, and smaller than, the CD dissymmetry factor. A closer look revealed that the slope of the plot, or the proportional coefficient, is a critical function of the class of compounds, varying from 0.99 for cyclophanes to 0.93 for biaryls, to 0.77 for BODIPYs, and then to 0.61 for helicenes/helicenoids. The scattered g lum–g abs plot and the general trend g lum≤g abs appear to be inherent to the CPL of organic molecules in their isolated states, originating from the conformational flexibility, vibrational contribution, and Stokes shift that differ in each category.