A light-controlled molecular brake with complete ON-OFF rotation.

A light-controlled molecular machine based on cyclic azobenzenophanes consisting of a dioxynaphthalene rotating unit and a photoisomerizable dioxyazobenzene unit bridged by methylene spacers is reported. In compounds 1 and 2, 1,5- and 2,6-dioxynaphthalene moieties, respectively, are linked to p-dioxyazobenzene by different methylene spacers (n=2 in 1a and 2; n=3 in 1b), whereas a 1,5-dioxynaphthalene moiety is bonded to m-dioxyazobenzene by bismethylene spacers in 3. In 1b and 2, the naphthalene ring can rotate freely in both the trans and cis states at room temperature.

Aggregation properties of heavy atom substituted squaraine dyes: evidence for the formation of J-type dimer aggregates in aprotic solvents.

The squaraine dye bis(2,4,6-trihydroxyphenyl)squaraine (SqH) was earlier reported to form J-type dimer aggregates in acetonitrile solutions at higher concentrations. Subsequent studies have suggested that concentration-dependent changes in the absorption spectrum of SqH in acetonitrile could be attributed to shifts in the acid-base equilibrium due to the presence of water as an impurity. In this work, we describe our studies on the effect of varying acid and dye concentration on the absorption spectra of the bromo and iodo substituted dyes, bis(3,5-dibromo-2,4,6-trihydroxyphenyl)squaraine (SqBr) and bis(3,5-diiodo-2,4,6-trihydroxyphenyl)squaraine (SqI).