Molecular switching: A fully reversible, optically active photochemical switch based on a tetraethynylethene-1,1 '-binaphthalene hybrid system

The synthesis, characterization, and physical properties of a novel, fully reversible, light-driven molecular switch, (R,R)-1/(R,R)-2, based on a tetr aethynylethene-1,1'-binaphthalene hybrid system are presented. trans-Config ured (R,R)-1 was synthesized in 57% yield by Stifle cross-coupling between stannylated tetraethynylethene 3 and 3-iodo-1,1'-binaphthalene derivative ( R)-4 (cf. Scheme 2). The cis-isomer (R,R)-2 was prepared from (R,R)-1 by ph otoisomerization. X-Ray crystal-structure analyses were obtained for both c is- and trans-forms of the photoswitch (Figs. 1 and 2). In the crystalline state, molecules of the cis-isomer (R,R)-2 exhibit intramolecular edge-to-f ace (C-H ... pi) interactions between naphthalene rings of the two 1,1-bina phthalene moieties (Fig.3). The switching properties were investigated by e lectronic absorption spectroscopy (Table and Fig. 4): irradiation at lambda = 398 nm converts trans-isomer (R,R)-1 into cis-isomer (R,R)-2, whereas sw itching occurs in the opposite direction upon irradiation at lambda = 323 n m. No thermal interconversion between the two isomers was observed in CH,CI , at room temperature over a period of 2-3 months, and the system possesses good resistance against photofatigue (Fig. 5). Investigations of the circu lar dichroism of (R,R)-1 and (R,R)-2 in CH2Cl2 solution showed that the chi ral binaphthalene moieties induce a weak Cotton effect in the achiral tetra ethynylethene core (Fig. 6). System (R,R)-1/(R,R)-2 represents one of the r are switches allowing two-way photochemical interconversions, not perturbed by thermal-isomerization pathways.