No abstract

A marked difference is observed in the photochemistry of [2]catenanes containing a complexed ruthenium(II) center. The photochemical reactions result in a decoordination process and large amplitude motions through the formation of a strongly dissociative ligand‐field excited state (see example, L=CH3CN or Cl−), with the rate dependent on the ring size. The back reaction is performed thermally.

In this work we describe a rotaxane in which a new type of motion, pirouetting of the wheel around its axle, can be electrochemically triggered. The rotaxane was synthesized by using the three-dimensional effect of copper(i). It incorporates both a hetero-biscoordinating ring (containing both 2,9-diphenyl-1,10-phenanthroline and 2,2':6',2''-terpyridine units) and a molecular string which contains only one 2,9-diphenyl-1,10-phenanthroline. Both ends of the string are functionnalized by a bulky stopper (tris(p-tert-butylphenyl)(4-hydroxyphenyl)methane). Large molecular motions have been induced electrochemically in this molecule and were detected by using cyclic voltammetry. The driving force of the two rearrangement processes observed is the high stability of two markedly different coordination environments for copper(i) and copper(ii) ions. In the copper(i) state, two phenanthroline units (one of the ring, one of the string) interact with the metal in a tetrahedral geometry (Cu I (4) ), whereas in the copper(ii) state the fivecoordinate geometry (Cu II (5) ) is due to the phenanthroline of the string and to the terpyridine of the ring. The kinetic rate constants of the two molecular motion processes (from Cu I (5) ) to Cu I (4) ) and fom Cu II (4) ) to Cu II (5) )) have been determined and both rates are much faster than the ones in previously studied analoguous systems. In addition, the rate of the pirouetting motion depends greatly on the copper oxidation state. The divalent four-coordinate copper complex (Cu II (4) )) rearranges in tens of seconds, whereas the monovalent five-coordinate copper system leads to the four-coordinate complex in the millisecond time scale.

Addition of trifluorsacetic acid to CH,CI, solutions of 1 ,lo-phenanthroline (l), 2,9-diphenyl-1 ,lo-phenanthroline (2) and 2,9-dianisyI-l,lO-phenanthroline (3) causes strong changes in the absorption and fluorescence spectra at room temperature. The low-energy absorption bands move to the red, while isosbestic points are maintained.The intensity of the structured, short-lived m* fluorescence band decreases and a new, unstructured longerlived fluorescence band arises at longer wavelength. The number of equivalents of acid needed to obtain 50% protonation in the ground state increases in the series 3 < 2 c 1. The spectral changes can be fully reversed by addition of base. The proton affinity in the singlet excited state is higher than that in the ground state (ApK, x 14 units for 3), but the protonation and deprotonation processes are too slow to compete with the excited-state decay. In a rigid CH,CI, matrix at 77 K the shift of the fluorescence band upon protonation is smaller than that observed at room temperature. The effect of protonation on the phosphorescence band, which can be observed in the rigid CH,CI, matrix at 77 K, is very small.

A new bistable rotaxane, consisting of a 2,2'-bipyridine-containing thread and a ring incorporating both a bidentate chelate and a tridentate fragment, has been prepared; this complex undergoes an electrochemically driven pirouetting motion of the ring around the axis which takes place on the millisecond timescale, i.e. several orders of magnitude faster than the other copper-based machines previously described.

No abstract

No abstract

No abstract