Enantiomeric discrimination of 1,1'-binaphthol by room temperature phosphorimetry using gamma-cyclodextrin as chiral selector

In the presence of a small amount of 1,2-dibromopropane (1,2-DBP), 1,1'-binaphthol (BINOL) displays strong room temperature phosphorescence in gamma-cyclodextrin (gamma-CD) solution without deoxygenation. The phosphorescence intensity as well as the phosphorescence lifetime of (S)-BINOL is greater than that of (R)-BINOL, indicating a distinct chiral discrimination of gamma-CD toward this pair of enantiomers. Both (R)-BINOL and (S)-BINOL exhibit a double exponential phosphorescence decay with lifetimes of 5.89 ms and 17.3 ms for (R)-BINOL and 7.58 ms and 23.6 ms for (S)-BINOL, respectively. The association constant obtained for (S)-BINOL/gamma-CD/1,2-DBP ternary complex is larger than that for (R)-BINOL/gamma-CD/1,2-DBP complex. Thus, the observation of RTP lifetime differences between (R)-BINOL and (S)-BINOL can be attributed to their different ability to form complexes with chiral gamma-CD, which is further supported by an analysis of the proton NMR chemical shift differences between (R)-BINOL and (S)-BINOL.