Abstract
Interfaces provide the structural basis for function as, for example, encountered in nature in the membrane-embedded photosystem or in technology in solar cells. Synthetic functional multilayers of molecules cooperating in a coupled manner can be fabricated on surfaces through layer-by-layer self-assembly. Ordered arrays of stimulus-responsive rotaxanes undergoing well-controlled axle shuttling are excellent candidates for coupled mechanical motion. Such stimulus-responsive surfaces may help integrate synthetic molecular machines in larger systems exhibiting even macroscopic effects or generating mechanical work from chemical energy through cooperative action. The present work demonstrates the successful deposition of ordered mono- and multilayers of chemically switchable rotaxanes on gold surfaces. Rotaxane mono- and multilayers are shown to reversibly switch in a coupled manner between two ordered states as revealed by linear dichroism effects in angle-resolved NEXAFS spectra. Such a concerted switching process is observed only when the surfaces are well packed, while less densely packed surfaces lacking lateral order do not exhibit such effects.
Highlights
The idea of fabricating synthetic molecular machines[1−5] has fascinated a whole generation of supramolecular chemists
The present study examines the importance of ordered arrays for coupled molecular motions in stimulus-responsive shuttle rotaxanes
Starting from suitably functionalized tetralactam macrocycles MCn,[47] a templated one-step rotaxane synthesis has been achieved by efficiently trapping diketopiperazine 1 inside the wheels to form hydrogen-bonded pseudorotaxanes pRotn (Scheme 1; for experimental details, see the Supporting Information)
Summary
The idea of fabricating synthetic molecular machines[1−5] has fascinated a whole generation of supramolecular chemists. Several seminal steps in this direction have been made,[40,41] e.g., Leigh’s monolayers of axle-fluorinated, light-switchable [2]rotaxanes moving diiodomethane droplets uphill,[42] Rapenne’s unidirectionally rotating surface-bound “piano-stool” rotors,[43] Feringa’s surface-anchored overcrowded double bond motors,[44] or Stoddart’s electrochemically switched cantilever-bending “molecularmuscle” [3]rotaxanes.[45,46] In the latter example, the finding that a cantilever can be bent, when the rotaxanes are switched, suggests that the rotaxanes deposited on the cantilever operate in a directional way This aspect, has so far not been further investigated in greater detail
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