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.