# Fluorescence Spectroscopy of Enantiomeric Amide Compounds Enforced by Chiral Light
## 3 Results  
We investigated the following combinations of the \(S\)- and \(R\)-solutions in DMF solvent: 100% \(S\), 75% \(S\)/25% \(R\), 50% \(S\)/50% \(R\), 25% \(S\)/75% \(R\), 100% \(R\), and pure DMF. In Figure 4, we show the luminescence properties when these substances were excited with 400 nm light with linear polarization. As expected, DMF showed no fluorescence. All mixtures had a broad green fluorescence, starting around 430 nm and peaking at around 510 nm. It can be noted that the broadband fluorescence spectral shape almost did not depend on the relative enantiomer concentrations in the solutions; this is in accordance with chirality being the property of the absence of mirror symmetry, while keeping the molecular constituents equal. We next rotated the QWP and measured the intensity of the fluorescence for enantiomer solutions (100% \(S\) and 100% \(R\)). In Figure 5a, slight differences can be noted in the fluorescence signal under the excitations with LCP and RCP states of light. Such a difference  
Figure 4: Fluorescence for different relative concentrations of enantiomer solutions in DMF, under linearly polarized excitation at 400 nm. The fluorescence intensities are normalized to their own maximum value and translated by a factor of 0.2 in order to increase the curve visibility. As a reference, we also report the measurements for DMF alone, which, as expected, did not produce any luminescence. Figure 3: Experimental FDCD setup. Laser light at 800 nm impinges on the BBO crystal, which generates the second harmonic. The high-intensity fundamental beam is filtered by two short-pass filters, while the second harmonic is polarized by a linear polarizer and quarter waveplate. This beam excites the cuvette with molecules. Fluorescence is laterally caught by a fiber and sent to the spectrometer. reversed sign by changing the enantio-pure solutions from 100% \(S\) to 100% R. Finally, we calculated FDCD as the normalized difference in intensities emitted with LCP and RCP excitation (\(I_{LCP}\) and \(I_{RCP}\), respectively). \[FDCD[\%]=100\frac{I_{LCP}-I_{RCP}}{I_{LCP}+I_{RCP}}. \tag{1}\]  
Figure 5b plots the FDCD for the two pure enantiomers. Due to the low luminescence signal in the spectral region where the wavelengths were shorter that 470 nm or larger than 570 nm, we report the FDCD only in the spectral region 470-570 nm, where the signal-to-noise ratio resulted to be larger than one. While the solution with 100% \(R\) molecule gave a rather flat negative FDCD (\(-1.1\%\)\(\pm\) 0.3%) signal from 470 nm to 570 nm, the opposite enantiomer gave a positive FDCD in this range. The difference in CD for opposite enantiomers was around 1.5%, well beyond the sensitivity of the system of 0.3%. Figure 6a plots spectra of the mixtures, i.e., 75% \(S\)/25% \(R\) solution, 25% \(S\)/75% \(R\) solution, and 50% \(S\)/50% \(R\) racemic solution. The difference in luminescence between LCP and RCP excitation was noticeably smaller with respect to the pure solutions, as can be evidenced by calculating the FDCD (Figure 6b). As expected, the 50% \(S\)/50% R racemic solution gave a negligible FDCD, whereas \(S\)- and \(R\)-enantiomer excess in a mixture led to a negative and positive FDCD signal, respectively. Lastly, it can be noted that FDCD signals of 10 mmol enantiomer concentrations reached an absolute value of 1.1% in the green range. As previously stated, chiral detection  
Figure 5: (**a**) Fluorescence spectra of DMF solutions with (red, blue curves) 100% \(S\) and 100% \(R\) (cyan, magenta curves), under LCP and RCP excitation. (**b**) FDCD spectra of the two solutions. Figure 6: (**a**) Fluorescence spectra of DMF solutions with relative enantiomer concentrations of 75% \(S\)/25% \(R\) (blue, red), 50% \(S\)/50% \(R\) (black, green), and 25% \(S\)/75% \(R\) (cyan, magenta), under LCP and RCP excitation. (**b**) FDCD spectra of the three solutions.