Chiral discrimination in the excited state of some pyrene derivatives

Abstract

Discriminatory interactions in the excited state of chiral molecules with circularly polarized light (CPL) or with other chiral molecules (chiral discrimination) have been studied using steady-state and nanosecond or picosecond time-resolved fluorescence spectroscopy. Nanosecond light pulses were generated by means of an air-gap nanosecond light pulser, while generation of picosecond light pulses employed a synchronously-pumped, mode-locked, cavity-dumped picosecond dye laser system. The circularly polarized luminescence spectra of (R)-(+)-(alpha)-pentyl-1-pyrenemethanol, R-pyr-OH, and its enantiomer, (S)-(-)-(alpha)-pentyl-1-pyrenemethanol, S-pyr-OH, have been determined in methanol at different concentrations. Dramatically enhanced optical activities associated with the excimer formation of R-pyr-OH and S-pyr-OH have been observed. Since it was found that the monomeric excited states of R- and S-pyr-OH do not exhibit measurable chirality, the large magnitude of the CPL found for the chiral excimers implies that the optical activity is configurational in nature. Differences in rates and equilibrium constants of excimer formation between (R)-(+)-(alpha)-pentyl-1-pyrenemethanol, R-pyr-OH, and its racemic mixture, RS-pyr-OH, due to chiral discrimination, were found using unpolarized light as the excitation source. The chiral discrimination is discussed in terms of differences in the reaction probability per encounter, since excimer formation is a diffusion controlled process. The rotational correlation times of (R)-(+)-(alpha)-pentyl-1-pyrene-methanol, R-pyr-OH, in (R)-(-)-2-octanol and (S)-(+)-2-octanol have been determined at several different temperatures using a single photon counting apparatus and appropriate polarizers. The difference due to chiral discrimination between the rotational correlation times at 24(DEGREES)C was found to be 0.07 ns while at -3.9(DEGREES)C the difference was 1.25 ns, rationalized in terms of hydrogen bonding interactions between the solute and solvent molecules. A reversal in the slopes of Stokes-Einstein plots was attributed to association of 2-octanol at low temperatures. The excited state acid dissociation constants (pKa('*)), of N-(1-pyrenoyl)-L-leucine, pyr-L-leu, N-(1-pyrenoyl)-D-leucine, pyr-D-leu, and N-(1-pyrenoyl)-DL-leucine, pyr-DL-leu, were measured using steady-state fluorescence techniques and the Forster-Weller equation. The difference between the pKa('*) values for pyr-D-leu or pyr-L-leu and pyr-DL-leu was 0.68, explainable in terms of long range electrostatic, dispersive and resonance interactions.