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

Abstract

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 p-p* transition reported for chiral organic molecules of various categories, including planar chiral cyclophanes and helicenes, axially chiral biaryls and spiro compounds, and pointand axially chiral BODIPY derivatives. In rigid p-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: j g lum j = 0.81 j g abs j (r 2 = 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.The absorption and emission intensities are proportional to the relevant dipole strength D, which is defined by Equation (7):In isotropic solutions, the electric quadrupole moment is cancelled out in R and generally small and negligible for D. The dissymmetric factors g abs and g lum are thus given by [Eq. (8)]:

Azabora[5]helicene Charge‐Transfer Dyes Show Efficient and Spectrally Variable Circularly Polarized Luminescence

“…Interesting is the comparison between the observed values of the dissymmetry factors with those of other helicenes and helicenoids . The g abs factors are well in the range of those observed for small [ n ]helicenes ( n =4–6).…”

“…As part of our own research program we have developed borylated arylisoquinoline (BAI) dyes, which in their N,C‐chelate variant are strongly emitting, solvatochromic, and two‐photon‐absorbing charge‐transfer fluorophores with application potential in bioimaging . With the idea to further enrich the photophysics of this fascinating platform we have built on their biaryl structure, which provides a means to arrive at dyes with helical chirality and the corresponding observation of interesting chiroptical properties, including the detection of circularly polarized luminescence (CPL) . The phenomenon is well established for supramolecular assemblies or nanostructures .…”

An Enantiopure Propeller‐Like Trityl‐Brominated Radical: Bringing Together a High Racemization Barrier and an Efficient Circularly Polarized Luminescent Magnetic Emitter

“…In that case, the differential absorption of the left‐ and right‐handed circularly polarized light in terms of molar absorptivities ( ϵ ) were considered, according to [ ϵ L ( λ )− ϵ R ( λ )]/[ ϵ L ( λ )+ ϵ R ( λ )]. As a result, efficient chiral emissions for both samples were set, since | g abs ( λ )|/| g lum ( λ )| ratios close to unity were obtained. Full stability against racemization of the two possible enantiomers, Plus (P ) and Minus (M) , was proved at the selected working conditions (i.e.…”

Circularly Polarized Luminescence of Aluminum Complexes for Chiral Sensing of Amino Acid and Amino Alcohol

“…And the correction of g lum =1.36×g abs +0.288 (r 2 =0.622) between CPL (g lum ) and CD (g abs ) was established, though fairly scattered,which demonstrated that the CPL dissymmetry factors were larger than, the CD dissymmetry factor, as shown in Figure . This relationship was different from that of small organic molecules, in which the general trend is g lum ≤g abs . The g lum /g abs ratio is above 1, which may promise metal ion complexes as CPL emitting materials a bright future.…”