Circular dichroism of host-guest complexes of achiral pyridino- and phenazino-18-crown-6 ligands with the enantiomers of chiral aralkyl ammonium salts

Circular dichroism (CD) spectroscopy was used for distinguishing different types of chiral interactions in host-guest complexes of achiral pyridino- a nd phenazino-18-crown-6 ligands with chiral aralkyl ammonium salts. The gen eral feature of the CD spectra of many homochiral (e.g., (R,R)-host and (R) -guest) and heterochiral (e.g., (R,R)-host and (S)-guest) alpha-(1-naphthyl ) ethylamine hydrogenperchlorate salt (NEA) complexes with chiral pyridino- and phenazino-18-crown-6 hosts is exciton interaction. The most interestin g example is the coupling of the transitions of the chiral guest NEA with t he energetically close transitions of the achiral phenazino-18-crown-6 host 6. The CD spectrum of the complex is predominated by exciton coupling betw een the B-1(b) transition of the chiral guest and the B-1(b), transition of the achiral host. The redshifted intense spectra of the complexes of (R)- or (S)-1-phenylethylamine hydrogenperchlorate salt (PEA) with the achiral d iester-pyridino-18-crown-6 host 4 are indicative of merging the pi. electro n systems into one joint charge transfer chromophore. The appearance of wea k bands with alternating sign in the spectrum of PEA complexes of the achir al "parent" pyridino-18-crown-6 host (1) indicates the presence of two or m ore conformers. The CD spectra of the complexes of achiral phenazino-18-cro wn-6 host 6 with PEA are also determined by pi-pi interaction. In addition to charge transfer bands, CD bands are also induced in the long-wavelength spectral region of the achiral host. The weak pi-pi interaction between the achiral phenazino-18-crown-6 host and methyl phenylglycinate hydrogenperch lorate (PGMA) or methyl phenylalaninate hydrogenperchlorate (PAMA) does not result in a definite spectral effect in the L-1(a), region of the spectrum of the chiral guest, but its existence is proven by the weak CD bands indu ced in the long-wavelength spectral region of the achiral host. Chirality 1 3:109-117 2001. (C) 2001 Wiley-Liss, Inc.