Planar chiral asymmetric naphthalenediimide cyclophanes : synthesis, characterization and tunable FRET properties

A series of planar chiral asymmetric naphthalenediimide (NDI) cyclophanes (1-4) consisting of two different NDIs interlinked by rigid meta-dimethylenebenzene spacers was synthesized. A modular synthetic strategy based on the dichloro cyclophane 1 as common precursor gave a straightforward access to racemic mixtures of the NDI cyclophane dyes 2-4. In particular the substitution of both chlorine atoms of 1 by tert-butylsulfanyl-, methoxy-and piperidinyl-groups provided racemic mixtures of the fluorescent NDI cyclophane dyes 2, 3 and 4 respectively. Fluorescence spectroscopy revealed chemically tuneable intramolecular FRET properties, as the spectral overlap between the emission of the unsubstituted NDI and the absorption of the core-substituted NDI subunit of the cyclophane could be adjusted by the core substituents. The racemic nature of the samples of cyclophanes 2-4, and in the case of the di-tert-butylsulfanyl functionalized cyclophane 2 the enrichment of both enantiomers, was demonstrated by HPLC using a chiral stationary phase. The solid state structure of 2 was determined by X-ray analysis. A periodic arrangement of both enantiomers of the planar chiral cyclophane 2 in pillars such that NDI- and 2,6-core substituted NDI chromophores alternate periodically was observed.