Differentiation of enantiomeric anions by NMR spectroscopy with chiral bisurea receptors.

Chiral anionic species are ubiquitous and play important roles in biological systems. Despite the recent advancements in synthetic anion receptors bearing urea functionalities, urea-based chiral solvating agents (CSAs) that can separate the NMR signals of racemic anions remain limited. Herein, three dibenzofuran-based C2-symmetric chiral bisureas were synthesized from the reaction of (R,R)-4,6-bis(1-aminopropyl)dibenzo[b,d]furan with phenyl isocyanate, phenyl thioisocyanate, or tosyl isocyanate. The chiral anion recognition properties of these bisureas were examined by 1H NMR spectroscopy using dl-tetrabutylammonium mandelate (TBAM) as a model substrate. A clear baseline separation of the enantiomeric signals of the benzylic proton of TBAM was achieved upon mixing with 0.5 equivalents of bis(phenylurea). In contrast to previous urea-based chiral anion receptors that differentiate the enantiomers of chiral anions by forming 1 : 1 host-guest complexes, a high chiral recognition ability of chiral bis(phenylurea) was achieved owing to the generation of an equilibrium between free guests, 1 : 1 host-guest complexes, and 1 : 2 host-guest complexes. Chiral bis(phenylurea) was also successfully employed in the separation of the enantiomeric 1H NMR signals of various racemic anions.

Concentration-dependent circularly polarized luminescence (CPL) of chiral N,N'-dipyrenyldiamines: sign-inverted CPL switching between monomer and excimer regions under retention of the monomer emission for photoluminescence.

Herein, an unprecedented switching of circularly polarized luminescence (CPL) is described for chiral 4,6-bis(1-(pyren-1-ylamino)propyl)dibenzo[b,d]furan (1). The CPL band of chiral diamine 1, which contains two pyrene rings, can be switched between the monomer and excimer emission regions under concomitant inversion of the handedness, simply by changing the concentration of the fluorophore. In contrast, the maximum photoluminescence (PL) intensity is always observed in the monomer region, regardless of the concentration. The reversal of the intensity ratio of monomer and excimer emission between PL and CPL was attributed to a stronger CPL (|gem| = ∼3-4 × 10-3) contribution from the minor excimer component, which should exhibit an efficient chiral environment around the dimeric pyrenes.

Two-Step Mechanochromic Luminescence of N,N'-Bis-Boc-3,3'-di(pyren-1-yl)-2,2'-biindole.

Despite recent advances in the development of mechanochromic luminescent organic molecules, the mechano-responsive properties of intramolecularly stacked fluorophores are not well documented. Herein, the synthesis and emission properties of three N,N'-bis-Boc-3,3'-diaryl-2,2'-biindole (Boc=tert-butoxycarbonyl) derivatives as a new class of solid-state emissive fluorophores are described. For a 3,3'-dipyrenyl derivative in the crystalline state, partial intramolecular stacking of the dipyrenyl groups was observed, which results in a two-step mechanochromic luminescence from blue (λem =462 nm) to green (λem =516 nm) and yellow (λem =535 nm).

N-Boc-Indolylbenzothiadiazole Derivatives: Efficient Full-Color Solid-State Fluorescence and Self-Recovering Mechanochromic Luminescence.

Herein, the solid-state emission with good fluorescence quantum yields of N-Boc-indolylbenzothiadiazoles as a new class of fluorophores is described. Their solid-state emission covers the wide range of the visible spectrum and the emission color can be tuned easily by changing the substituents on the two heteroaromatic rings. Among these, 3-methylindolyl derivatives exhibit moreover autonomously self-recovering mechanochromic luminescence, whereby the original solid-state emission could be recovered spontaneously at room temperature after exposure to a mechanical stimulus. The emission color, as well as the recovery time for the color change could be tuned via the introduction of different substituents on the benzothiadiazole ring. We propose that the mechanism of the autonomously self-recovering mechanochromic luminescence of 3-methylindolylbenzothiadiazoles is based on a partial amorphization of the crystals upon exposure to the mechanical stimulus, followed by autonomous recovering in the form of recrystallization.

Molecular mechanics (MM3) calculations on lithium amide compounds.

The MM3 force field has been extended to deal with the lithium amide molecules that are widely used as efficient catalysts for stereoselective asymmetric synthesis. The MM3 force field parameters have been determined on the basis of the ab initio MP2/6-31G* and/or DFT (B3LYP/6-31G*, B3-PW91/6-31G*) geometry optimization calculations. To evaluate the electronic interactions specific to the lithium amides derived from the diamine molecules properly, the Lewis bonding potential term for the interaction between the lithium atom and the nonbonded adjacent electronegative atom such as nitrogen was introduced into the MM3 force field. The bond dipoles were evaluated correctly from the electronic charges on the atoms calculated by fitting to the electrostatic potential at points selected. The MM3 results on the molecular structures, conformational energies, and vibrational spectra show good agreement with those from the quantum mechanical calculations.