Modulation of the ligational properties of a new cylindrical macrotricycle by coupling of photochemical- and pH-switching properties

The new cylindrical molecule L containing two tetraazamacrocyclic rings linked by two azobenzene pillars displays photoelastic properties. Light absorption at 366 nm gives rise to trans → cis isomerization of the azobenzene moieties producing two isomers containing one or two cis-azobenzenes, respectively. The three trans-trans (E-E), trans-cis (E-Z) and cis-cis (Z-Z) isomers have been identified and characterized by ¹H NMR spectroscopy, allowing the dependence of their formation percentages with irradiation time to be determined. The sequence of photochemical reactions E-E → E-Z → Z-Z allows almost complete conversion of the E-E into the Z-Z isomer at 366 nm and 298 K. Both thermal (k = 1.75 × 10^-5 s^-1 at 313 K) and photo-induced (at 436 and 313 nm) back-isomerization reactions have been studied. The protonation constants of the three isomers in equimolar solutions of water-DMSO indicate a decreasing basicity in the order E-E > E-Z > Z-Z, in agreement with increasing electrostatic repulsion between the positive charges caused by a reduction in the separation between the protonation sites occurring upon Z → E isomerization.