Chiral Eutectic Mixtures and Deep Eutectic Solvents for Induced Circularly Polarized Luminescence

Abstract

Deep eutectic solvents (DES) or eutectic mixtures prepared with a chiral component can lead to new chiral solvents with applications that include asymmetric synthesis and chiral light emitting materials. DES have low melting points, because of strong interactions, such as hydrogen bonding, between components of the mixture. Mixtures are prepared with ammonium salts, tetrabutylammonium chloride ([TBA]Cl) and choline chloride ([Ch]Cl), as hydrogen bond acceptor (HBA) and L‐lactic acid, L‐leucic acid, L‐ascorbic acid, R/S‐acetoxypropionic acid, and methyl‐(S)‐lactate as chiral hydrogen bond donors (HBD). Eight combinations of the HBAs and HBDs were prepared, and a racemic mixture of dissymmetric chiral europium complexes was dissolved in the mixtures. The circularly polarized luminescence (CPL) spectra were measured to determine the chiral discrimination by these chiral solvents. The CPL spectra show that the handedness of the chiral HBD is important to the chiral discrimination exhibited. However, the inversion of the sign of the CPL spectra in 1 : 3 [TBA]Cl:L‐lactic acid vs. 1 : 3 [Ch]Cl:L‐lactic acid, and 1 : 1.5 [Ch]Cl:L‐leucic acid vs. 1 : 1 [TBA]Cl:L‐leucic acid shows that the achiral HBA also plays a critical role in the handedness of the chiral discrimination by the chiral solvent.

Chiral Solvent Discovery: Exploring Chiral Eutectic Mixtures and Deep Eutectic Solvents

“…The exceptions are the two higher viscosity mixtures with >28000 mPa-s, and the mixture, 2:1 thymol: (S)-(+)-1-cyclohexylethylamine, with the lowest viscosity in the table, 57 mPa-s. To the authors knowledge, only three of the mixtures in Table 6 have been utilized in DES or eutectic mixtures prior to this study. 39,51,52 Therefore, it is impossible to make direct comparisons with similar mixtures, but the viscosities in Table 6 are all much higher than those reported for ChCl based DES. 1,6,53 The temperature dependent viscosities (283-313 K range) for seven of the mixtures from Table 6 are shown in Figure 2.…”

“…ChCl:L-lactic acid (g L-lacticacid = 0.94) and 2:1 TBACl:(S)-acetoxypropionic acid (g TBACl = 1.1) have g ³ 1. 39 The 1:1 TBACl:L-leucic acid and 1:1 TBACl: dimethyl-D-malate only show glass transitions, and the remaining mixtures in table 5 have g A < 1 at the measured molar ratios indicating that the interactions are consistent with the presence of a DES. a uncertainty in density is ± 0.001 g/mL, and the uncertainty in viscosity is ± 3-5 mPa-s Density and Viscosity.…”

“…For example, incorporating a chiral HBA or HBD results in a chiral DES solvent which can be utilized in asymmetric synthesis, [33][34][35][36][37] chiral separations, 24,38 and circularly polarized light emitting materials. [39][40][41] The role of chirality in chemistry and biochemistry highlights the importance of developing new chiral DES solvents However, the large number of possible chiral HBA and HBDs and lack of predictive capability make finding new chiral DES a time-consuming process of trial and error.…”

Chiral Solvent Discovery: Exploring Chiral Eutectic Mixtures and Deep Eutectic Solvents

“…The exceptions are the two higher viscosity mixtures with >28000 mPa-s, and the mixture, 2:1 thymol: (S)-(+)-1-cyclohexylethylamine, with the lowest viscosity in the table, 57 mPa-s. To the authors knowledge, only three of the mixtures in Table 5 have been utilized in DES or eutectic mixtures prior to this study. 39,51,52 Therefore, it is impossible to make direct comparisons with similar mixtures, but the viscosities in Table 5 are all much higher than those reported for ChCl based DES. 1,6,53 The temperature dependent viscosities (283-313 K range) for seven of the mixtures from Table 6 are shown in Figure 2.…”

“…ChCl:L-lactic acid (g L-lacticacid = 0.94) and 2:1 TBACl:(S)-acetoxypropionic acid (g TBACl = 1.1) have g ³ 1. 39 The 1:1 TBACl:L-leucic acid and 1:1 TBACl: dimethyl-D-malate only show glass transitions, and the remaining mixtures in table 4 have g A < 1 at the measured molar ratios indicating that the interactions are consistent with the presence of a DES. a uncertainty in density is ± 0.001 g/mL, and the uncertainty in viscosity is ± 3-5 mPa-s Density and Viscosity.…”