Photochemistry and Photophysics of Coordination Compounds

80 N. Armaroli et al.
pseudo-rotaxanes [34, 35], knots [36, 37], dendrimers [38, 39], helices [40–
42], polynuclear hosts [43] etc. as originally developed by Sauvage, Dietrich-
Buchecker and coworkers [44]. Some of these fascinating structures are depicted
in Fig. 8.
Most notably, some suitably engineered supramolecular architectures
based on [Cu(NN)2] + cores are able to carry out motion at the molecular level
upon chemical [45], or electrochemical/photochemical stimulation [32, 46]
behaving as molecular machine prototypes [47, 48]. For instance a [2]-catenate
made of two different rings, one with a phenanthroline fragment the
other bearing both a phenanthroline and a terpyridine unit, undergoes spontaneous
and reversible molecular rearrangements (Fig. 9 steps (B) and (D))
upon oxidation (step A) and subsequent reduction (step C). Rearrangements
are driven by the different preferential coordination geometries of Cu(I) and
Cu(II), i.e. tetra- vs. pentacoordination [46].
Fig. 9 Electrochemically induced molecular motions in a catenane containing a [Cu(NN)2] +
center and a free tpy ligand. The spontaneous motion is driven by the different preferential
coordination geometry of Cu(I) vs. Cu(II)
More recently, Schmittel and coworkers have made new supramolecular
systems based on [Cu(NN)2] + -type building blocks such as racks [49],
grids [50], boxes [51], and macrocycles [52]. Control of the sophisticated
heteroleptic architectures has been achieved by exploiting the HETPHEN
(HETeroleptic bisPHENanthroline) concept [53]. This approach is based on
the kinetic control of the metal complexation equilibrium and, in the target
complex, the Cu(I) ionturnsouttobeboundtoasimpleandabulky
phenanthroline ligand; this concept is schematically illustrated in Fig. 10.