Photoinduced hinge-like molecular motion: Studies on xanthene-based cyclic azobenzene dimers

Molecular devices incorporating azobenzene units represent active components of smart systems, as they are capable of exhibiting photoregulated cooperative molecular motion. Herein, we describe the synthesis, X-ray crystal analysis, and photochemical and thermal studies of a xanthene based cyclic azobenzene dimer and its precursor. The trans-trans isomer of the azobenzene dimer upon photoirradiation transforms to the cis-cis isomer through an intermediate trans-cis isomer. The X-ray crystal structures of the trans-trans isomer (open) and the cis-cis isomer (closed) provide unambiguous proof for the hinge-like molecular motion in this class of molecules. The inferences drawn from photochemical and thermal studies shed light on the effect of varied substitution and cyclic structures on the different transitions. The lifetime of the cis-cis isomer is estimated to be 6.43 years, whereas the trans-cis isomer is short-lived (2.73 min) at 303 K. A rational explanation for the relative stability of the different isomers is derived from the isokinetic plot and theoretical calculations.