Chirality transfer from chiral solvents and its memory in an azobenzene derivative exhibiting photo-switchable racemization

Abstract

The transfer and dynamic fixation of chirality in cyclic azobenzenes using R-(+)-1-phenylethylalcohol (R-PEA) and S-(−)-1-phenylethylalcohol (S-PEA) as solvents or additives are investigated. The cyclic azobenzenes used in this study carry a 1,5-dioxynaphthalene moiety as rotating unit, connected to the photoisomerizing (E–Z) azobenzene unit with spacers of varying lengths. With suitable lengths of the spacers the molecules exhibit stable enantiomers originated from the element of planar chirality in the E form due to the stopped rotation of the rotor, while in the Z form the allowed rotation results in racemization. The CD spectra of racemic compounds in the E form in chiral solvents were inert or almost negligible before irradiation, while 366 nm irradiation causing E–Zphotoisomerization resulted in induction of clear CD bands. The thermal or photochemical reverse Z–Eisomerization causes a change in the CD spectra to new ones which are reasonably matching with the spectra of the pure enantiomers recorded in non-chiral solvents. The obtained new CD spectra are maintained even in a racemic solvent system attained by the dilution with an equal amount of chiral solvent of opposite stereostructure. These results indicate that the chirality is transferred from the chiral solvents or additives to the racemizing Z form of cyclic azobenzene and it is fixed in the non-racemizing E form. The molecule without racemization in both E and Z forms did not show any significant induced CD bands irrespective of E–Zisomerizations. The molecule showing racemization in E and Z forms just shows the non-fixed induced CD. The property of photo-switchable racemization is necessary for the effective transfer and temporal fixation of the chirality in this type of chirality sensors.