MODULATION OF THE LIGATIONAL PROPERTIES OF A NEW CYLINDRICAL MACROTRICYCLE BY COUPLING OF PHOTOCHEMICAL-SWITCHING AND PH-SWITCHING PROPERTIES

The new cylindrical molecule L containing two tetraazamacrocyclic ring s linked by two azobenzene pillars displays photoelastic properties, L ight absorption at 366 nm gives rise to trans --> cis isomerization of the azobenzene moieties producing two isomers containing one or two c is-azobenzenes, respectively, The three trans-trans (E-E), trans-cis ( E-Z) and cis-cis (Z-Z) isomers have been identified and characterized by H-1 NMR spectroscopy, allowing the dependence of their formation pe rcentages with irradiation time to be determined, The sequence of phot ochemical reactions E-E --> E-Z --> Z-Z allows almost complete convers ion of the E-E into the Z-Z isomer at 366 nm and 298 K, Both thermal ( k = 1.75 x 10(-5) s(-1) at 313 K) and photo-induced (at 436 and 313 nm ) back-isomerization reactions have been studied, The protonation cons tants of the three isomers in equimolar solutions of water-DMSO indica te a decreasing basicity in the order E-E > E-Z > Z-Z, in agreement wi th increasing electrostatic repulsion between the positive charges cau sed by a reduction in the separation between the protonation sites occ urring upon Z --> E isomerization.