Synthesis of a double-stranded spiroborate helicate bearing stilbene units and its photoresponsive behaviour

Abstract

A novel spiroborate-based double-stranded helicate bearing photoresponsive cis-stilbene units in the middle (cis-3) was successfully synthesised from the corresponding cis-stilbene-bound tetraphenol strand in the presence of NaBH₄, whereas the tetraphenol strands with a trans-stilbene or trans-azobenzene unit did not form such a double-stranded helicate. The ¹H NMR and NOESY experiments revealed that cis-3 adopted contracted (cis-3_C) and extended (cis-3_E) forms under equilibrium in CD₃CN at 25 °C. The contracted cis-3_C that accommodated a Na⁺ ion in the center showed almost reversible extension and contraction motions by removal and addition of a Na⁺ ion. The cis-to-trans photoisomerisation of the extended cis-3_E with UV light (295 nm) further induced an extension of the helicate, producing a mixture of cis,trans-3_E and trans-3_E helicates in the photostationary state. However, trans-to-cis photoisomerisation of the trans-mixtures using UV light (360 nm) was irreversible in this system and produced the photooxidated aldehyde species (trans-4), resulting from the photo-cleavage of the trans-stilbene moieties of trans-3_E.